Recent advances in nanoflowers: compositional and structural diversification for potential applications

Abstract

In recent years, nanoscience and nanotechnology have emerged as promising fields in materials science. Spectroscopic techniques like scanning tunneling microscopy and atomic force microscopy have revolutionized the characterization, manipulation, and size control of nanomaterials, enabling the creation of diverse materials such as fullerenes, graphene, nanotubes, nanofibers, nanorods, nanowires, nanoparticles, nanocones, and nanosheets. Among these nanomaterials, there has been considerable interest in flower-shaped hierarchical 3D nanostructures, known as nanoflowers. These structures offer advantages like a higher surface-to-volume ratio compared to spherical nanoparticles, cost-effectiveness, and environmentally friendly preparation methods. Researchers have explored various applications of 3D nanostructures with unique morphologies derived from different nanoflowers. The nanoflowers are classified as organic, inorganic and hybrid, and the hybrids are a combination thereof, and most research studies of the nanoflowers have been focused on biomedical applications. Intriguingly, among them, inorganic nanoflowers have been studied extensively in various areas, such as electro, photo, and chemical catalysis, sensors, supercapacitors, and batteries, owing to their high catalytic efficiency and optical characteristics, which arise from their composition, crystal structure, and local surface plasmon resonance (LSPR). Despite the significant interest in inorganic nanoflowers, comprehensive reviews on this topic have been scarce until now. This is the first review focusing on inorganic nanoflowers for applications in electro, photo, and chemical catalysts, sensors, supercapacitors, and batteries. Since the early 2000s, more than 350 papers have been published on this topic with many ongoing research projects. This review categorizes the reported inorganic nanoflowers into four groups based on their composition and structure: metal, metal oxide, alloy, and other nanoflowers, including silica, metal–metal oxide, core–shell, doped, coated, nitride, sulfide, phosphide, selenide, and telluride nanoflowers. The review thoroughly discusses the preparation methods, conditions for morphology and size control, mechanisms, characteristics, and potential applications of these nanoflowers, aiming to facilitate future research and promote highly effective and synergistic applications in various fields.

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Su Jung Lee

Su Jung Lee received her PhD degree in Chemistry from Yonsei University, South Korea in 2015. She pursues her research work at Ingenium College of Liberal Arts (Chemistry), Kwangwoon University, Korea under the supervision of Prof. D. N. Lee. Her research focus is design and synthesis of silicon-based nanomaterials and its applications: optical materials for display and solar cells, and anti-bacterial and anti-cancer materials for biomedical application.

Hongje Jang

Hongje Jang is an Associate Professor in the Department of Chemistry at Kwangwoon University in the Republic of Korea. During his postdoctoral research in laser dynamics systems in nanomaterials at the Georgia Institute of Technology, he worked on real-time in vitro autophagy tracking platforms. His current research at KWU focuses on the design and synthesis of metal or metal oxide nanoparticles by controlling their size, morphology, and compositions. Expanding on this, he focuses on the precise physicochemical property control of nanosystems.

Do Nam Lee

Do Nam Lee received her B.S. and M.S. in chemistry from Yonsei University in South Korea. She earned her PhD from the same university in 1992 under the supervision of Prof. Chang Whan Kim and completed postdoctoral research as a member of the group of Prof. Robert West at University of Wisconsin–Madison, USA. She worked as a visiting scholar at Peking University, China in 2003. She is currently an associate professor at Kwangwoon University in South Korea and is mostly focusing on research of functional metal–organic frameworks, nanoflowers, and drug delivery.

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1. Introduction

Dr Richard Feynman introduced the idea and concepts of nanotechnology in his talk titled “There's Plenty of Room at the Bottom” at the American Physical Society's annual meeting in 1959, initiating the exploration of nanoscience and nanotechnology as new materials-science fields.¹ In the 1980s, various spectroscopic techniques, such as scanning tunneling microscopy (STM)^2–4 and atomic force microscopy (AFM),^5,6 were developed for the characterization and tuning of nanomaterials, facilitating the investigation of various types of nanomaterials, such as fullerenes,^7,8 graphene,^9,10 nanotubes,^11,12 nanofibers,^13,14 nanorods,^15,16 nanowires,^17,18 nanoparticles,^19,20 nanocones,^21,22 and nanosheets.^23,24 Recently, the applications of these nanomaterials have expanded to encompass a wide range of commercial fields, including electronics, energy-related fields, cosmetics, and biomedicine.²⁵

Among them, flower-shaped hierarchical 3D nanostructures, called nanoflowers,²⁶ have attracted immense attention because of their higher surface-to-volume ratio compared to that of spherical nanoparticles, along with their low cost and eco-friendly preparation methods. Additionally, they enhance the stability of many catalytic reactions by the immobilization of enzymes and proteins.^27–31 Numerous publications report the potential utilities of these 3D nanostructures with peculiar morphologies, which are fundamentally based on nanoflowers. As we have explained in the previous paper, nanoflowers can be classified according to their compositions: inorganic, organic, and organic–inorganic hybrid.³² First, inorganic nanoflowers are defined as being composed only of inorganic materials such as metals, metal oxides, alloys, and metalloids.^33–36 In addition, as introduced later in this paper, metalloid, carbon, nitride, sulfide, phosphide, selenide, and telluride coated or doped inorganic materials are also defined as inorganic nanoflowers.^37–44 Second, nanostructures that include inorganic elements as part of a medium composed of only organic molecules or in which organic molecules are the main components are called organic nanoflowers.^45–47 Reported findings have indicated that organic nanoflowers, characterized by an open structure, provide a greater number of accessible active sites and enable easy movement of ions.⁴⁸ Additionally, the intersecting extended nanopetals within these structures promote the efficient transportation of electrons. These nanoflowers have demonstrated promising outcomes in applications related to energy storage and electrocatalysis.^48,49 For example, one-pot synthesis of polyacrylonitrile (PACN) and its copolymer nanostructured particles with various superstructures (flower, pompom, hairy leave, and petal shapes) has been reported. PACN flowers (PACN-F) are prepared with acrylonitrile monomer (ACN) in acetone through free radical polymerization initiated by azobis(isobutyronitrile) (AIBN). Through stabilization and carbonization steps, these PACN flowers are converted to carbon flowers. The morphology is controlled by the incorporation of different co-monomers or by employing various solvents. These carbon flower materials may be applied as promising candidates for various energy, environment, or electronic applications.⁴⁸ Zheng's group introduced nitrogen, phosphorus, and fluorine doped carbon NFs (NPF@CNFs) by ultrasound-induced polycondensation and pyrolysis. The synthesized NPF@CNF exhibited enhanced activity (E_1/2 = 0.85 V (oxygen reduction reaction, ORR), E_j=10 = 1.56 V (oxygen evolution reaction, OER) vs. RHE) and cycling stability (retention above 90% after cycling for 50 000 s) as a bifunctional electrocatalyst toward the ORR and OER during the discharging and charging process of zinc–air batteries, which is even superior to those of commercial Pt/Ir-based catalysts.⁵⁰ Third, hybrid nanoflowers are described as all components of inorganic nanostructures associated with organic materials.⁵¹ Hybrid nanoflowers, which combine biomolecules and inorganic components, have found extensive applications in biosensors, bioassays, biocatalysis, biomedicine, and diagnostics.^52–55 This is attributed to their straightforward synthesis, remarkable effectiveness, and ability to improve enzyme stability.⁵² The fabrication process of hybrid organic–inorganic nanoflowers under mild conditions involves utilizing copper(II) ions as the inorganic element and natural amino acids as the organic element. Asparagine (Asn)-incorporated nanoflowers with diameters in the range of 10–15 μm closely resemble the shape of Dahlia in nature and have hierarchical structures with high surface-to-volume ratios. The nanoflowers have a Brunauer–Emmett–Teller (BET) surface area of 32.0 m² g⁻¹ and an average pore size of 112.5 nm. The nanoflowers incorporating amino acids exhibit catalytic activity based on a reaction principle like Fenton's reagents. The presence of copper ions enables self-assembly between amino acids and copper phosphate, resulting in intrinsic peroxidase-like activity in the amino acid-incorporated nanoflowers. These amino acid–inorganic hybrid nanoflowers are expected to have significant applications in biosensors, bioanalytical devices, pharmaceuticals, and industrial biocatalysis.⁵⁶ In particular, inorganic nanoflowers are attracting researchers' attention because of their catalytic efficiency and optical characteristics, depending on the composition, crystal structure, and local surface plasmon resonance (LSPR).³² Thanks to these properties, inorganic nanoflowers are widely used in electro, photo, and chemical catalysts, sensors, supercapacitors and batteries. Moreover, in the synthesis of inorganic nanoflowers, as will be discussed in detail later, the availability of diverse synthesis methods and the ability to regulate reaction conditions make it easier to achieve precise control of form and composition. However, there have been very few reviews of inorganic nanoflowers, and this is the first review of inorganic nanoflowers for catalyst, sensor, supercapacitor, and battery applications.

To the best of our knowledge, more than 350 such reports on inorganic nanoflowers have been published since the early 2000s, with an increasing number of studies being published currently. As shown in Fig. 1, many of the published papers are on metal and metal oxide nanoflowers, with the number of papers on metal oxides increasing rapidly. Additionally, numerous studies on alloys, core–shell structures, doped systems, and so on, have been published recently. They investigate the preparation method, morphology, size-control conditions, characteristics, and potential applications of nanoflowers composed of metals, metal oxides, alloys, core–shell, and doped systems. Recent research has primarily focused on optimizing their morphologies and composition ratios for practical applications.

Fig. 1 Publications on flower-shaped nanostructures since 2000. (a) The number of articles, categorized by the material composition and structures of the nanostructures reported, (b) the number of such publications in the past 20 years, and (c) the number of papers published on flower-shaped hierarchical 3D nanostructures in the past 20 years.

In this review, nanoflowers have been categorized as metal, metal oxide, alloy, and other nanoflowers, depending on their compositions. Synthetic methodologies, experimental parameters' effects on the morphology and size, mechanisms, physical, chemical, and biological properties derived from the physical properties of inorganic materials, synergistic effects among composition ratios, morphologies, and structural effects, with regard to potential applications as photocatalysts,⁵⁷ electrocatalysts,⁵⁸ anode materials for lithium-ion storage,⁵⁹ supercapacitor electrodes,⁶⁰ and gas sensors⁶¹ are primarily considered.

2. Methods for preparing nanoflowers

Continuous research efforts have been made to develop new synthesis methods for the nanoflowers.^62–66 According to Kulkarni's book: Nanotechnology: Principles and Practices, nanomaterials are prepared by four types of methods, namely, physical, chemical, biological, and hybrid.⁶⁷ The preparation of inorganic nanoflowers also uses the same or modified technology as the conventional nanomaterial synthesis technology, as depicted in Fig. 2.

Fig. 2 Commonly used synthesis technology of inorganic nanoflowers: physical, chemical, biological, and hybrid.

First, the physical technique is mainly represented by physical vapour deposition in the inorganic nanoflower synthesis. For example, Bi₂S₃ nanoflowers are prepared on a Si substrate via a simple vapour deposition method.⁶⁸ The morphology of the Bi₂S₃ nanoflowers is modulated by tuning the partial pressure of the reactant. The second technology is the chemical synthesis method, such as hydrothermal,⁶⁹ solvothermal,⁷⁰ sonochemical,³⁵ colloids,⁷¹ sol–gel⁷² and microwave synthesis.⁷³ TiO₂ nanoflowers are synthesized by a hydrothermal method.⁷⁴ The solvothermal reaction is used to synthesize Pd nanoflowers in the presence of oleic acid.⁷⁵ IrCo and IrNi nanoflowers are ultrasonicated with IrCl₃·xH₂O, Co(acac)₂ or Ni(acac) and oleylamine and heated in a hot oil bath.³⁵ Hierarchical NiCo₂O₄ nanoflowers are synthesized using the microwave synthesis method.⁷³ Third, the biological strategy is an environmentally benign and biomimetic approach using natural material extracts, such as Kalanchoe daigremontiana,⁷⁶Dodonaea angustifolia,⁷⁷Ocimum sanctum (Tulsi) leaves,⁷⁸ and Azadirachta indica leaves,⁷⁹ without utilizing any toxic materials.

The last synthesis technology is the hybrid synthesis strategy that combines physical, chemical, and biological methods: chemical vapour⁸⁰ and electrochemical⁸¹ deposition, high energy ball-milling hydrothermal treatment,⁶⁰ and the solution-immersion RF-sputtering method.⁸²

As we introduced above, inorganic nanoflower synthesis can utilize various techniques and methods. This can be used differently depending on the material and application of the nanoflower to be synthesized. We will illustrate some commonly used nanoflower synthesis methods with examples for each.

2.1. Physical method

2.1.1. Physical vapor deposition. Physical vapor deposition (PVD) is a process used to deposit thin films and coatings of nanomaterials onto a substrate. The PVD method is useful to prepare metal- and metal oxide- based nanomaterials.^68,83 However, only a few studies on nanoflower synthesis through the PVD method have been reported so far, mainly due to the high cost and elevated reaction temperatures. The controlled growth of nanoflowers using PVD techniques is an area that holds great promise and potential for further exploration.

For instance, Yu et al. have described a vapor deposition technique for the synthesis of exquisite Bi₂S₃ nanoflowers on a silicon substrate.⁶⁸ The researchers utilize a horizontal tube furnace for the vapor deposition process. Bi₂S₃ powder and S powder are used as the starting materials. In the furnace tube, a ceramic boat containing Bi₂S₃ powder is positioned at the center, while sulfur powder is placed between the Bi₂S₃ and the gas inlet. Silicon wafers are positioned downstream, a few centimeters away from the Bi₂S₃ powder. The tube is tightly sealed and flushed with high purity argon gas. The temperature is then gradually increased to 650 °C at a heating rate of 8 °C min⁻¹ and maintained at that temperature for 2 hours. Subsequently, the temperature is gradually decreased to room temperature. Throughout the thermal treatment, argon gas is continuously allowed to flow at approximately 100 sccm (standard cubic centimeter per minute) to provide both protection and carrier gas functions. Additionally, they have demonstrated the ability to adjust the shape of the Bi₂S₃ nanostructures, ranging from flower-like structures to bundles of nanorods, by controlling the partial pressure of the reactants.

There is a need for research aimed at exploring novel methods and improving existing PVD approaches to achieve precise control over the formation and arrangement of nanoflowers. This involves investigating various parameters such as deposition conditions, precursor materials, and substrate properties to optimize PVD processes specifically tailored for nanoflower growth.

2.2. Chemical method

2.2.1. Hydrothermal and solvothermal methods. The hydrothermal method is one of the most well-known and widely employed methods for the synthesis of diverse nanostructured materials, including nanoflowers. This method is simple, cost-effective, eco-friendly, and easily controllable.^84,85 These characteristics make it highly advantageous for the large-scale synthesis of nanoflowers. The precise control of temperature and pressure in the hydrothermal process enables the formation of hierarchical 3D nanostructures with isotropic growth in all directions. Moreover, facile manipulation of reaction parameters such as composition and concentration of precursors and reducing agents and the reaction time, and the ability to adjust the presence or type of templates or surfactants enable precise control over the morphology and size of the nanoflowers (Table 1). However, it is important to address the aggregation tendency of nanomaterials, as it can negatively impact the uniform distribution and morphology of the resulting structure.^84,85 The solvothermal method is similar to the hydrothermal method, with the only distinction being that it is conducted in a non-aqueous medium (Table 2).

Table 1 Nanoflowers are synthesized by a hydrothermal process under diverse reaction conditions

Nanostructure/morphology	Size	Reaction conditions	Time and temperature in the autoclave	Surfactant or template	Ref.
TiO2 nanoflowers on a Ti substrate	360–420 nm	10 mL H2O2 (37%) + 60 mL NaOH (1 M)	5 h, 120 ± 2 °C	—	92
Anatase and brookite TiO2 nanoflowers	—	7.8 mL tetrabutyl titanate (TBOT) + 1.10 g NaCl + 68 mL aqueous NH3·H2O	24 h, 180 °C	—	93
ZnO nanoflowers	3 μm	0.549 g, C4H6O4Zn·2H2O (2.5 mmol) + 0.735 g, sodium citrate (2.5 mmol)+ 4 mL NaOH (4.0 M)	8 h, 120 °C	—	94
ZnO nanoflowers	4 μm	(Zn(CH3COOH)2_2H2O) + (1 mM) + NaOH (10 mM)	8 h, 140 °C	PEG-20000	95
Porous NiO nanoflowers	15.0 (PEG), 16.7 (PVP) nm	2.9080 g of Ni(NO3)2·6H2O or 2.4884 g of Ni(CH3COO)2·4H2O + HMTA solution (0.5 M)	6 h, 110 °C	PEG, PVP	96
Sparse-type and close-type NiO nanoflowers	5 (CTAB), 2 (SDS) μm	0.475 g NiCl2·6H2O + 0.300 g urea	8 h, 160 °C	CTAB, SDS	97
CuO nanoflowers	1 μm	6 mL NH3·H2O (1 M) + 10 mL Cu(NO3)2 (0.01 M)	6 h, 100 °C	—	98
Flower-like CuO nanostructures with porous nanosheets on the alumina tube	—	0.3 M Cu(CH3COO)2·H2O + 25 mL NH3·H2O (17 M) + 1.0 g NaOH	24 h, 180 °C	—	99

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Table 2 Nanoflowers are synthesized by a solvothermal process under diverse reaction conditions

Nanostructure/morphology	Size	Reaction conditions	Time and temperature in the autoclave	Surfactant or template	Ref.
TiO2 nanoflowers	47.56–274.36 nm	NaOH (0.024 M) + 8 mL TiCl3 + 60 mL ethanol	18 h, 130 °C	—	34
Porous NiO nanoflowers	—	0.5 g Ni(ac)2·4H2O + 0.25 g urea + 30 mL ethanol	3–24 h, 190 °C	—	100
Zn-doped CuS nanoflowers	1 μm	0.25 mmol CuCl + 0.25 mmol Zn(Ac)2·2H2O + 0.75 mmol S powder + 15 mL ethanol	24 h, 180 °C	—	101
Mo-doped Bi2Se3 nanoflowers	100–200 nm	0.85 mmol Bi(NO3)3·5H2O + 0.15 mmol (NH4)6Mo7O24·4H2O+ 18 mL N,N-dimethylformamide + ammonia + Se powder + Na2SO3	20 h, 160 °C	—	102
Bi2O2.33 nanoflowers	—	0.11 g NaBiO3 + 20 mL ethanol, + 5.0 mL distilled water + 2.0 mL 1 M HNO3 solution	3–5 h, 100 °C	PVP	103

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For instance, in the first step, 5 mM of Cu(NO₃)₂·3H₂O powder is dissolved in 100 mL of deionized water. In the second step, a 1 mM aqueous solution of hexamethylenetetramine is mixed with 100 mL of deionized water. In the third step, the solutions from steps 1 and 2 are combined and 1 mL of 30% NaOH is added. The resulting solution is continuously stirred for 60 minutes at room temperature. Subsequently, the suspended mixture is transferred into an autoclave and securely sealed. The autoclave is then placed in an oven and maintained at 110 °C for 3 hours. After completing the entire process and allowing the oven to cool to room temperature, the resulting solution is centrifuged and washed with deionized water. Finally, the product, characterized by its black color (CuO nanoflowers), is obtained after drying in a vacuum oven at 60 °C for 2 hours.⁸⁶

In recent studies, it has been frequently reported that a combination of the hydrothermal method with other synthesis techniques is employed to synthesize nanoflowers with diverse structures. Qu et al. reported a novel approach for the controlled synthesis of three types of 3D fluffy ZnO nanoflowers with distinct nanostructures. This method involves a simple ultrasonic treatment followed by a hydrothermal process.⁸⁷ One type, denoted as ZnO-0, is directly formed through the hydrothermal reaction without prior ultrasonic treatment, resulting in smooth edges. The other two types, namely ZnO-250 and ZnO-950, are obtained by subjecting the samples to ultrasonic treatment at 250 W and 950 W, respectively, prior to the hydrothermal process, resulting in jagged margins. The experimental findings revealed that increasing the intensity of ultrasonic treatment leads to a decrease in the size, specific surface area, crystallite dimension, intrinsic donor defects, and the signals of reactive radicals in the ZnO nanoflowers.

2.2.2. Colloidal synthesis. Colloidal synthesis offers the advantage of combined control over the morphology and composition of nanoflowers, making it a relatively inexpensive and scalable method. To demonstrate, the synthesis of M_1−xW_xSe₂ nanoflowers with precise control over both composition and morphology is demonstrated through the utilization of colloidal synthesis.⁷¹ Interestingly, a wide range of nanoflowers with diverse compositions and complex structures: MoSe₂,⁸⁸ Ru-doped MoSe₂,⁸⁹ homogeneously Co-doped and Co-edge doped MoSe₂,⁹⁰ and Au nanoparticles decorated Bi₂Se₃ (ref. 91) nanoflowers have been synthesized through colloidal synthesis methods.

Here is an overview of the experimental procedure for synthesizing MoSe₂ nanoflowers using the colloidal method: in a 100 mL three-neck flask, a mixture of 20 mg (0.1 mmol) of Na₂MoO₄, 8 mL of oleic acid, and 2 mL of 1-octylamine is prepared and degassed under vacuum at 120 °C for approximately 10 minutes. The reaction vessel is then purged with Ar(g) and slowly heated to around 240 °C at a rate of approximately 5 °C per minute, resulting in the formation of a clear, dark, red-brown solution. Subsequently, 2 mL of the ODE-Se stock solution (0.1 M), which is 1-octadecene and Se solution, is continuously injected at a rate of 0.1 mL min⁻¹ using a syringe pump. After the injection is completed, the solution is aged for an additional 30 minutes at a final temperature of approximately 300 °C. The reaction mixture is then rapidly cooled by removing the flask from the heating mantle. The resulting MoSe₂ particles are precipitated by adding 10 mL of toluene and 10 mL of ethanol, followed by centrifugation. The black precipitate is washed three times using a 1 : 1 mixture of toluene and ethanol (with centrifugation steps in between washes), and finally suspended in ethanol to form a dark purple colloidal suspension.⁸⁸

2.2.3. Microwave synthesis. The microwave synthesis method offers notable advantages due to its rapidity, simplicity, environmental friendliness, and cost-effectiveness.¹⁰⁴ To the best of our knowledge, microwave reactions are utilized for producing various nanoflowers based on Ni through the interaction of molecules, which is facilitated by localized indirect heating within the solution.^73,105,106

For example, 40 mmol of Ni(NO₃)₂·6H₂O is dissolved in 50 mL of distilled water. Subsequently, 5 wt% of the cationic surfactant cetyltrimethyl ammonium bromide (CTAB) is added, and then 3 mL of ammonia is introduced into the solution. The resulting solution is stirred for 10 minutes. The solution, contained within a polypropylene-capped autoclave container, is subjected to microwave irradiation at a power of 300 watts for a duration of 15 minutes. The green precipitates obtained are washed multiple times with distilled water and ethanol to eliminate soluble impurities. Subsequently, the precipitates are dried in a hot oven at 80 °C for 12 hours to obtain Ni(OH)₂ samples. The as-prepared powder samples are then subjected to calcination in air at 400 °C for 2 hours to yield NiO nanoflowers.¹⁰⁵ The researcher also employed a hydrothermal method using the same amount of precursor, surfactant, and ammonia, with the reaction conducted at 140 °C for 15 minutes. In scanning electron microscopy (SEM) analysis, the microwave treatment process results in the formation of nanostructures that exhibit the same flower-like morphologies but with a higher degree of agglomeration compared to the samples grown through the hydrothermal method. This can be attributed to the indirect heating of molecules through microwave radiation.¹⁰⁵

2.3. Biological method

2.3.1. Green synthesis. Green synthesis, also known as biosynthesis, of metal nanoparticles using plants, herbs, or microorganisms has garnered significant attention,¹⁰⁷ particularly in the field of medicine. The use of plant extracts for the production of metal nanoparticles has gained popularity due to its straightforward, scalable, and non-toxic method.⁷⁶ However, it should be noted that the synthesis of nanoflowers using this method is currently in its nascent stages, with only a scant number of research papers dedicated to this particular approach.

Ag nanoflowers are obtained using natural materials extract: Kalanchoe daigremontiana, as a natural reducing agent.⁷⁶ The green synthesis of nanospindles of CuO and their subsequent assembly into nanoflowers has been successfully achieved by modifying the solvents used, such as water and ethanol, in combination with the extract obtained from Dodonaea angustifolia.⁷⁷ Also, CuO nanoflowers are synthesized using eugenol (4-allyl-2-methoxyphenol), which is extracted from the leaves of Ocimum sanctum (Tulsi), serving as a natural reducing agent.⁷⁸ A simple, environmentally friendly, biomimetic method is employed to fabricate Au/ZnO hybrid nanoflowers using Azadirachta indica (neem) leaf extract as both a reducing agent and a capping agent.⁷⁹

Upon closer examination of one of these methods, the synthesis of ZnO nanoflowers has been achieved by employing Withania coagulans extract as a reducing agent. For the synthesis of ZnO nanoflowers, a solution is prepared by dissolving 0.2 g of zinc acetate in 50 mL of distilled water. The pH of the solution is adjusted to 12 using a 2 M solution of NaOH. Subsequently, 5 mL of Withania coagulans plant extract is added to the solution, which is then heated at 90 °C for 5 hours with continuous stirring. To obtain the final product, the reaction mixture is subjected to centrifugation at 6000 rpm for 15 minutes. The resulting ZnO nanoflowers are collected and further incubated at 37 °C overnight.¹⁰⁸

2.4. Hybrid method

2.4.1. Electrochemical deposition. Electrochemical deposition is a highly popular technique employed for the synthesis of nanoflowers. Synthesis by electrochemical deposition has been recognized as a potentially superior method compared to other techniques due to its advantageous features. These include: (1) the capability of being performed in a single step at room temperature, resulting in reduced process time; (2) effective control over the size and shape of the particles, leading to the formation of uniform deposits with high purity; (3) ease of securely anchoring the particles onto the substrate; (4) environmentally friendly characteristics.^109,110

A simple and reproducible technique for producing 3D Pt nanostructures on silicon substrates has been reported at ambient temperature using potentiostatic pulse plating.¹¹¹ An aqueous solution containing 1 M H₂PtCl₆ and 1 M H₂SO₄ is mixed and stirred for 5 hours at room temperature. The Pt catalyst is then electrodeposited onto a flat silicon substrate using potentiostatic pulse plating in a three-electrode cell system with a saturated calomel reference electrode (SCE). The positive potential pulse (+0.05 V) lasted for 5 ms, while the negative potential pulse (−0.02 V) lasted for 1 ms. This bipolar pulse electrodeposition method facilitates the synthesis of 3D Pt nanoflowers on the Si substrate. Following Pt electrodeposition, the sample is washed with deionized (DI) water to remove surface contamination and dried under ambient conditions.

Also, the galvanic replacement method is used to create monomorphic single-crystalline Pt nanoflowers by transforming Te nanowires into a fully-formed Pt nanostructure.¹¹² The template-free ultrasonic electrodeposition method is employed to produce Pt nanoflowers directly on the bare Au electrodes.¹¹⁰ This method is utilized for synthesizing not only Pt nanoflowers but also Ag, Pd, AuPt, and AuAgCu nanoflowers.^113–116

2.4.2. Multi-step synthesis for complex structures. With the increasing diversity of composition and complexity in nanoflower structures, there have been reports of employing a combination of various synthesis methods to fabricate them. Zhao et al. reported the synthesis of Ag@NiO core–shell nanoflower arrays through a one-step solution-immersion process followed by the RF-sputtering method.⁸² In the initial stage, the bulk ilmenite FeTiO₃ undergoes ball-milling, followed by a mild hydrothermal treatment in an aqueous NaOH solution. This process leads to the formation of nanostructured FeTiO₃, which exhibits a distinctive nanoflower morphology.⁶⁰ MnO₄ nanoflowers are fabricated utilizing a microwave-assisted hydrothermal synthesis under alkaline conditions.¹¹⁷

3. Experimental parameters' effect

The morphology of nanoflowers plays a crucial role in various applications and scientific studies. The unique and intricate structures of nanoflowers offer distinct advantages: a large surface area-to-volume ratio and enhanced catalytic properties. By understanding the morphology and controlling the growth conditions and parameters, researchers can manipulate the size, shape, and arrangement of nanoflower structures, leading to improved performance in sensing devices, energy storage systems, drug delivery platforms, and more. Here, we aim to introduce how various experimental conditions can influence the size and shape of nanoflowers, using several examples.

Porous nanoflower- and nanourchin-like structures are synthesized using the hydrothermal method, employing either hexamethylenetetramine (HMTA) or urea as a precipitant, with or without the addition of surfactants such as poly(ethylene glycol) (PEG) or poly(vinyl pyrrolidone)(PVP).⁹⁶ Based on the information provided in Table 3, it can be observed that NiO precursors synthesized using HMTA as a precipitant exhibited a morphology resembling nanoflowers, while those prepared with urea as a precipitant displayed a shape similar to that of nanourchins. NiO-HMTA, NiO-HMTA-PEG, NiO-HMTA-PVP, and NiO-urea samples exhibit a high surface area ranging from 31 to 66 m² g⁻¹. Also, NiO-HMTA-PVP possesses the highest BET surface area and pore volume. As a highly active catalyst, NiO-HMTA-PVP exhibits the highest surface area, highest concentration of oxygen adspecies (O_ads), and the best reducibility. It demonstrates superior catalytic performance with a T_50% of 253 °C and T_90% of 266 °C.⁹⁶

Table 3 Preparation parameters, crystal phases, crystallite sizes, morphologies, BET surface areas, average pore sizes, and pore volumes of the as-prepared NiO samples. Reprinted with permission from ref. 96. Copyright © 2012 Elsevier B.V. All rights reserved

Sample code	Precipitating agent	Ni/precipitating agent molar ratio	Surfactant	Crystal phase	Crystallite size^a (nm)	Morphology	BET surface area (m^2 g^−1)	Average pore size (nm)	Pore volume (cm^3 g^−1)
a The crystallite sizes are calculated according to Scherrer's equation using the FWHM of the (200) line of the XRD patterns.
NiO-HMTA	HMTA	1 : 1	—	Cubic	19.1	Irregular nanoflower-like spheres	31.5	26.4	0.202
NiO-HMTA-PEG	HMTA	1 : 1	PEG	Cubic	15.0	Nanoflower-like spheres	56.1	26.2	0.296
NiO-HMTA-PVP	HMTA	1 : 1	PVP	Cubic	16.7	Nanoflower-like spheres	66.3	22.9	0.414
NiO-urea	Urea	1 : 1	—	Cubic	25.2	Nanourchin-like spheres	30.5	9.6	0.103
NiO-bulk	—	—	—	Cubic	78.6	—	5.6	—	—

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Sparse-type and close-type NiO nanoflowers are successfully synthesized using a hydrothermal process with the assistance of CTAB and sodium dodecyl sulfate (SDS) as surfactants, respectively. The performance of the close-type NiO nanoflowers is found to be superior, and this can be attributed to several factors. Firstly, the close-type nanoflower architecture exhibits a higher degree of porosity and compactness. Additionally, there are more nanojunctions between adjacent nanosheets, creating a well-connected structure. Furthermore, the ultrathin nanosheets assemble abundant micro reaction rooms. Finally, the thickness of the petal structures is smaller than twice the Debye length.⁹⁷

In another study, in the hydrothermal synthesis, nanorods and nanoneedles assembled NiO nanoflowers are synthesized using different surfactants, namely CTAB and ethylene glycol (EG), respectively. Based on TEA and SEM analysis, it is determined that the nanorods have an average diameter of approximately 900 nm and a length of approximately 6 μm. On the other hand, the nanoneedles exhibit a thicker structure at the roots with sharper emanative ends. Each needle has a length of around 2.5 μm and a diameter of approximately 80 nm at the middle.¹¹⁸

Borbón et al. conducted a hydrothermal study to examine how the reaction time affects the size and morphology of ZnO nanoflowers. They observe that well-defined flower-like structures are formed within 5 minutes of the reaction. Regardless of the reaction time (10, 15, and 30 minutes), the nanoflowers exhibit similar high-density morphologies. The dimensions of the petals in each nanoflower increase as the crystal growth continued over time, with the nanorods becoming wider and larger (Table 4). However, between 10 and 15 minutes, there is no significant change observed. The research also showed a decrease in sheet resistance from 38.5 MΩ sq⁻¹ at 5 minutes to 0.00344 MΩ sq⁻¹ at 30 minutes. The reduction in the resistivity of the samples could be attributed to a decrease in crystal defects and an increase in crystallite size as the reaction time increased, as confirmed by XRD results. This suggests that the electrical resistance can be adjusted by varying the reaction time.¹¹⁹

Table 4 Dimensions, band gap and sheet resistance of ZnO nanoflowers synthesized at different times. Reprinted with permission from ref. 119. Copyright © 2019 Elsevier Ltd. All rights reserved

Reaction time (min)	Crystallite size (nm)	Petal width (nm)	Petal length (nm)	Single flower length (nm)	Band-gap energy (eV)	Sheet resistance (MΩ sq^−1)
05	14.73	288 ± 27	714 ± 81	1523 ± 151	3.21	38.5000
10	22.22	345 ± 45	740 ± 90	1674 ± 180	3.27	18.5000
15	24.11	327 ± 48	772 ± 57	1653 ± 154	3.10	0.01020
30	24.06	370 ± 50	772 ± 85	1785 ± 150	3.28	0.00344

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A hydrothermal method is employed to synthesize well-defined and uniformly sized spherical mesoporous nanoflowers. The synthesis process involves using a solvent mixture of cyclohexane and water, with cetylpyridinium bromide (CPB) acting as the template, tetraethyl orthosilicate (TEOS) as the source of inorganic silica, and urea as the hydrolysis additive. The study aims to investigate the effects of solvent composition (V_cyclohexane/V_water), hydrothermal temperature, and Si to CPB molar ratio on the morphology and structure of the nanoflowers.¹²⁰

First, various solvent compositions are examined, including higher cyclohexane content (7 : 5, 1 : 1) and lower cyclohexane content (5 : 7, 1 : 2). Results show that a higher cyclohexane content leads to uniform flowers with a loose flower structure, while a relatively lower cyclohexane content results in a more compact structure and uniform-sized flowers. However, changing the ratio to 1 : 3 or 0 caused the spherical morphology of the nanoflowers to be severely disrupted. Nonetheless, the XRD patterns of the samples still indicate favorable mesoporous characteristics. This suggests that the mesoporous structure is primarily determined by the template, while cyclohexane plays a crucial role in shaping the spherical morphology of the nanoflowers.

Second, spherical nanoflowers are synthesized at various hydrothermal temperatures ranging from 80 °C to 140 °C. Interestingly, all four samples, synthesized at 80 °C, 100 °C, 120 °C, and 140 °C, respectively, displayed uniform flower sizes and maintained the fundamental spherical morphology, indicating that temperatures within the investigated range do not significantly affect the basic shape of the nanoflowers. When comparing the samples synthesized at 80 °C and 100 °C, there is no noticeable variation in flower size, but the petals of the latter appeared more elongate and exhibit better overall appearance. At 120 °C, there is a significant change in both flower size and petal shape. The diameter decreases to less than 200 nm, and the nanostructure becomes looser. However, when the temperature is increased to 140 °C, the flower size increased to 400 nm, but the petals do not fully expand.

Third, the synthesis of the samples involves using different molar ratios of TEOS to CPB. The results clearly demonstrate that this ratio has a significant impact on the morphology of the materials. When the TEOS/CPB ratios are 4.37 and 6.02 (sample B and D), the nanoflowers exhibit a diameter of approximately 200 nm. On the other hand, when the ratios are 3.58 and 5.21 (samples A and C), the diameter increased to approximately 500 nm. Additionally, the mesostructures or petal structures of the materials varied noticeably across the different TEOS/CPB ratios in the four samples. For instance, the wrinkled structure observed in the sample with a ratio of 4.37 (sample B) stretched further compared to the sample with a ratio of 6.02 (sample D). Furthermore, the BET measurement results revealed that the surface area of sample B is 501 m² g⁻¹, which is higher than that of sample D (352 m² g⁻¹).

ZnO nanoflowers, referred to as ZnO-0, are synthesized through a hydrothermal process. Additional types of ZnO nanoflowers, namely ZnO-250 and ZnO-950, are created by subjecting them to ultrasonic treatments using different ultrasonic devices prior to the hydrothermal procedure. The nanosheets within the ZnO-0 nanoflowers have smooth edges, while the edges of ZnO-250 and ZnO-950 nanoflowers display jagged margins resembling carnations.⁸⁷ Analysis of the materials yielded the following findings: the average diameter of the ZnO nanoflowers decreases from 2.31 μm to 1.95 μm as the intensity of ultrasonic treatment increased. This reduction in size is also observed in the nanosheets constituting the nanoflowers. Specifically, the average thickness of the nanosheets decreases from 40.32 nm to 18.72 nm, and their average length varied from 350.35 nm to 270.21 nm with higher ultrasonic intensity. As a result, ZnO-950 nanoflowers exhibit the smallest size, primarily attributed to the more pronounced acoustic cavitation induced by the ultrasonic treatment.

Ag nanoflowers (Ag NFs) with hierarchical structures consisting of large buds (250–580 nm) and thin petals (9–22 nm) are synthesized.¹²¹ The size of the buds increases significantly with higher concentrations of ascorbic acid (0.1–1 M), as a reducing agent, resulting in the formation of Ag NFs named Ag NF I, II, III, IV, and V. The use of higher concentrations of ascorbic acid leads to a more spontaneous reduction and larger bud size. The specific surface area of the Ag NFs, measured from the BET adsorption isotherm,^122,123 also increases with higher concentrations of ascorbic acid (2.08, 3.82, and 5.15 m² g⁻¹ for Ag NF I, III, and V). However, when the concentration of ascorbic acid is too high (2 M), the flower structure with thin petals cannot be maintained. Changing the reaction time (10 s ∼120 min) does not affect the bud size of Ag NF III (0.3 M of AgNO₃ and 0.3 M of ascorbic acid), confirming its spontaneous anisotropic growth. The bud size of Ag NF III is not affected by changes in temperature below 80 °C, but the petal structure of the flower changes when the reaction temperature exceeds 90 °C.

Ag hierarchical assemblies are formed in a solution with the help of small acid molecules, without the use of polymer surfactants or capping agents. As shown in Fig. 3a, irregular Ag particles with Ag+ reduced by ascorbic acid (no extra acid added) are observed, with approximately 1 μm size. A magnified image of the Ag particle shows that it comprises 50–100 nm thick Ag nanosheets (Fig. 3b), with the structure resembling Ag nanoparticles obtained through a similar procedure. The addition of citric acid forms perfect microspheres with a diameter of ∼2 nm composed of numerous close-packed 20 nm thick Ag nanosheets (Fig. 3c and d). In the presence of mandelic acid, flower-like Ag particles are formed from loosely packed 50 nm thick Ag nanosheets (Fig. 3e and f), while using toluene sulfonic acid forms microscale Ag yarn-balls assembled from cross-linked 50 nm thick Ag nanosheets (Fig. 3g and h).¹²⁴

Fig. 3 SEM images of Ag structures prepared through a chemical reduction of Ag+ ions (AgNO₃) by ascorbic acid as the reducing agent, without using any polymersurfactant or capping agent. Before ascorbic acid is added to initiate the reduction reaction, (a and b) no acid, (c and d) citric acid, (e and f) mandelic acid, and (g and h) toluenesulfonic acid are added to the reaction system to control the assembly of the produced Ag nanostructures. Adapted with permission from ref. 124. Copyright © the Royal Society of Chemistry 2011.

Flower-shaped NiO nanostructures are currently being synthesized using microwave and hydrothermal methods. The hydrothermally reacted sample exhibits cubic phase NiO nanostructures with a reduced particle size of 8 nm, as observed in the XRD patterns. In contrast, the microwave-reacted sample has a larger particle size of 12 nm. The UV band gap curves confirm a blue shift, indicating the quantum confinement effect of the NiO nanostructures, and this is further supported by the photoluminescence spectrum. The SEM analysis of the hydrothermally processed samples reveal an abundance of flowers with minimal agglomeration. These flowers form clusters with a diameter ranging from 1 to 2.5 μm. Conversely, the nanostructures produced through the microwave treatment process exhibit similar flower-like morphologies but show higher levels of agglomeration compared to the hydrothermally grown samples due to indirect heating of molecules caused by microwave radiation.¹⁰⁵

The researcher conducts an investigation into the diameter distribution of Pd–Ag nanoflowers, taking into account the composition ratio of Pd to Ag. The Pd₁Ag₂ nanoflowers obtained in their original state exhibited a narrow size distribution, with an average diameter of approximately 29.27 ± 3.74 nm. This size is significantly smaller than that of Pd₁Ag₁ (33.20 ± 4.23 nm) and Pd₁Ag₃ (32.48 ± 4.27 nm).¹²⁵

Uniform-sized silica nanoflowers with a large accessible surface area are successfully synthesized using a visible light-driven green chemistry route. The synthesis process involves the use of CTAB as a structure-directing agent in combination with various co-solvents. The size, BET surface area, pore volume, and pore diameter of the resulting SiO₂ nanoparticles are analyzed and summarized in Table 5. CTAB molecules within a mixture of water, anisole, and hexanol self-assemble into micelles that play a crucial role in providing the necessary template derived from these micellar aggregates. This template subsequently controls the shape and size of the nanoparticles. Therefore, due to the interaction between co-solvents and CTAB, the shape and size of Si nanoparticles can be influenced. As the number of alcohol co-solvents increases from C-3 to C-8, the size of the nanoparticles decreases, while the BET surface area and pore volume increase.¹²⁶

Table 5 Summary of the BET surface area, pore volume, pore diameter and size of SiO₂ NPs formed in a number of co-solvents, varying from C-3 to C-8 alcohols. Reprinted with permission from ref. 126. Copyright © 2019 Elsevier B.V. All rights reserved

Co-solvent	Solvent	Size of NPs (nm)	BET surface area (m^2 g^−1)	Pore diameter (nm)	Pore volume (cm^3 g^−1)
Propanol	Methyl phenyl ether (anisole)	505	567	3.26	0.564
Butanol		480	580	3.27	0.603
Pentanol		440	621	3.27	0.768
Hexanol		385	705	3.44	0.996
Heptanol		385	702	3.45	1.015
Octanol		380	711	3.45	1.021

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4. Mechanism

Nanoflowers are complex nanostructures composed of nanoscale petals or branches arranged in a flower-like pattern. They are typically synthesized using various techniques, as introduced in Section 2. Methods for preparing nanoflowers. The specific mechanism of nanoflower formation depends on the materials and fabrication techniques employed. Here, we will provide a general overview of commonly used approaches with representative examples.

The first mechanism involves synthesizing surfactant-free, template-free, and seedless nanoflowers. The synthesis of NiO nanoflowers is initiated with the chemical reaction and the subsequent hydrothermal process, leading to the formation of Ni(OH)₂ nuclei. The growth process of nanoflowers typically commences with the formation of small nuclei. The chemical reaction can be described as follows:

CO(NH₂)₂ + 3H₂O → NH₄HCO₃ + NH₃·H₂O → OH⁻ + NH₄⁺ (1)

Ni²⁺ + NH₃ + H₂O ↔ [Ni(H₂O)_6−x(NH₃)_x]²⁺ (2)

Ni²⁺ + OH⁻ → Ni(OH)₂↓ → NiO (3)

Subsequently, the nuclei grow into nanosheets. With an extended hydrothermal time, the surface energy decreases, leading to a gradual self-orientation of thin nanosheets. This self-orientation process ultimately results in the formation of flower-like hierarchical structures composed of Ni(OH)₂. Following that, the obtained Ni(OH)₂ structures are subjected to calcination at 500 °C. This calcination process transforms the Ni(OH)₂ into NiO nanoflowers, characterized by the assembly of smooth nanosheets.¹²⁷

The following mechanism encompasses the creation of nanoflowers through a process that eliminates the use of surfactants and templates, relying on seed mediation. The Au@CdSe and Ag@CdSe core–shell nanoflowers are synthesized via a one-pot hot-injection method.¹²⁸ Initially, gold clusters act as heterogeneous nucleation sites for the growth of CdSe nanocrystals. As CdSe clusters with a magic-size, which refers to a specific size of stable CdSe clusters due to their bulk crystalline atomic packing and fully closed outer shell, and zinc blende crystal structure begin to form, they diffuse towards the gold nanocrystals and bind to surface defect sites. Through a multiple injection process involving the addition of more Cd and Se ions, additional CdSe species attach to the CdSe core clusters on the gold surface. These attached species undergo a transformation into wurtzite and adopt a multiple-branched structure, ultimately resulting in the formation of nanoflower shapes. The proposed mechanism is illustrated in Scheme 1. Third, seedless NiO nanoflowers assembled with nanorods or nanoneedles are synthesized using two surfactants: CTAB and EG.¹¹⁸ The synthesis process involves the following steps. In the case of nanorod-assembled NiO nanoflowers, ammonia aqueous is used as an alkaline reagent to release OH⁻ ions. CTAB, a surfactant with a hydrophobic part, interacts with Ni(OH)₂ and preferentially absorbs onto the CTA⁺ heads, causing the Ni(OH)₂ nanoparticles to connect and form nanorods. CTAB acts as an adhesive, facilitating the gathering of the nanorods. Finally, driven by the minimum surface energy theory, the nanorods self-assemble into flower-like architecture. For the formation of nanoneedle-assembled hierarchical NiO nanoflowers, Ni²⁺ and C₂O₂⁻⁴ form a NiC₂O₄·2H₂O polymer type ribbon due to the complexation of Ni+ and C₂O₂⁻⁴. EG, a surfactant with symmetrical structures and functional group-OH, acts as a ligand to Ni and blocks the crystal surface parallel to the [0,1,1] direction. The microstructures formed by NiC₂O₄·2H₂O are connected along the [0,1,1] direction, resulting in needle-like structures. Over time, the NiC₂O₄·2H₂O nanoneedles aggregate and assemble into hierarchical needle-flowers. Finally, NiO nanoflowers are obtained through thermal calcination.

Scheme 1 Proposed mechanism for the formation of the Au@CdSe nanoflowers. Reprinted with permission from ref. 128. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The growth mechanism of seed mediated flower-like CuO nanostructures utilizing PEG 4000 as a surfactant and template can be explained as follows.¹²⁹ Initially, the formation of CuCO₃ precipitate occurs through the following chemical reaction:

2Cu(NO₃)₂ + NH₄HCO₃ → 2CuCO₃ + 5NO + NO₂ + 5H₂O (4)

Under hydrothermal conditions, CuCO₃ undergoes a transformation into CuO through the following reaction:

CuCO₃ → CuO + CO₂ (5)

The formation of flower-like CuO nanostructures is likely guided by the proposed mechanism depicted in Fig. 4. According to this mechanism, PEG plays a role as a soft template for the development of CuO nanoflowers. In the initial stage, the combination of Cu²⁺ aqueous solution and PEG leads to the generation of initial nucleation seeds, serving as the starting point for particle growth. PEG, being a nonionic surfactant with a molecular structure of H(–O–CH₂–CH₂)_n–OH, tends to form chain-like structures when dissolved in water. When the nucleation seeds reach a critical size, the PEG chains act as templates by absorbing smaller particles through the end OH bonds for the formation of the CuO flower nanostructure. Subsequently, these absorbed particles aggregate around the larger CuO seeds. Following a 2-hour annealing process at 600 °C, the PEG templates are removed, resulting in the formation of flower-like CuO nanostructures.

Fig. 4 Formation mechanism of flower-like CuO hierarchical nanorods. Reprinted with permission from ref. 129. Copyright © 2014 Elsevier Ltd. All rights reserved.

Finally, the fabrication of hollow-channel Au nanoflowers is achieved through a simple one-step synthesis method, utilizing a bifunctional template composed of an auto-degradable nanofiber.¹³⁰ This templated synthesis involves three primary steps (Fig. 5). (1) In situ nucleation: the initial formation of primary Au nanocrystals occurs on the template composed of methyl orange–FeCl₃ (MO–FeCl₃). This is facilitated by strong interactions, including the coordination between Au(III) and azobenzene in MO molecules, as well as the electrostatic interaction between AuCl₄⁻ ions and Fe³⁺. The absorption of AuCl₄⁻ onto the MO–FeCl₃ nanofiber takes place due to these interactions, achieved through the reduction of HAuCl₄ by ascorbic acid (AA). After the introduction of the reducing agent AA, Au nuclei form on the surface of the template, resulting in the generation of primary Au nanocrystals. (2) Anisotropic growth: the primary nanoparticles experience anisotropic growth, resulting in the formation of flower-like structures. This growth process leads to the development of the hollow-channel Au nanoflowers. (3) Auto-degradation of the template: the presence of AA catalyzes the auto-degradation of the template. As a result, the as-formed Au nanoflowers contribute to the degradation process, leading to the dissolution of the nanofiber template.

Fig. 5 Schematic illustration of the proposed mechanism for the formation of hollow gold nanoflowers: stage (I) Au nucleates on the template surface; stage (II) Au primary crystal undergoes anisotropic growth to form branches; stage (III) the as-formed Au nanoflowers catalyze the degradation of the template in the presence of AA, leaving the open hollow channel and releasing the individual HAuNFs. Adapted with permission from ref. 130. Copyright © The Royal Society of Chemistry 2016.

5. Synthesis methods and characterization of inorganic nanoflowers

5.1. Nanoflowers with metallic composition

In this section, the synthesis methods and characteristics of nanoflowers having a metal constituent, such as Au, Pt, Pd, and Ag, are mainly reviewed. Various shapes, surfaces, densities of corners and edges of nano-petals, particle size distribution, and growth mechanisms are also described.

5.1.1. Au nanoflowers. Hierarchical branched Au nanostructures (labeled nanoflowers), with sharp peaks and valleys on the surface of multibranched Au nanoparticles, have recently received considerable attention because of their high total surface-to-volume ratios, forming potential hotspots for localized near-field enhancements.¹³¹ Depending on their structural features, they also exhibit interesting optical properties.^130,132 Thus, branched nanomaterials can be applied in several fields, including catalysis,¹³³,¹³⁴ sensors,¹³² drug delivery,¹³⁵ and photothermal therapy.¹³⁶

The branched Au^137,138 nanomaterials are obtained in the presence of (1-hexadecyl) trimethyl ammonium chloride (CTAC), or gemini amphiphiles, as capping agents.

In addition, the anisotropic Au flower-shaped nanomaterials are synthesized using a surfactant-free biocompatible Good's buffer, such as 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (HEPES).¹³¹ Synthesis of hollow-channel Au nanoflowers using MO–FeCl₃ nanofiber as a bifunctional template is also reported.¹³⁰

Interestingly, unlike other synthetic strategies, relatively monodisperse Au nanoflowers¹³⁹ without seeds and other surfactants have been formed using the nontoxic chemical hydroxylamine (NH₂OH) without stabilizers or adjustment of the pH environment. First, 50 μL of aqueous HAuCl₄ solution (wt 1%) is added to 2.5 mL of deionized water. Subsequently, 10 μL of aqueous NH₂OH solution (wt 50%) is injected into the solution with stirring and reacted for an additional minute. An Au nanoflower dispersion is obtained, with Au nanoflowers of approximately 100 nm diameter, with relatively narrow size distribution, based on SEM and high-resolution transmission electron microscopy (HR-TEM) analyses (Fig. 6A–C). These Au nanoflowers can be used as a nitroaromatic pesticide degradation platform, to catalyze the otherwise impossible reduction of pendimethalin by NaBH₄. This novel degradation platform could be beneficial for environmental protection, including the remediation of soil contamination and sewage treatment.

Fig. 6 (A and B) SEM and (C) TEM images of gold nanoflowers that are synthesized; mass spectral data of (D) show the process of degradation of pendimethalin (above) to N-(1-ethylpropyl)-2,6-diamino-3,4-xylidine (below). Reprinted with permission from ref. 139. Copyright © 2014, American Chemical Society. All rights reserved.

5.1.2. Pt nanoflowers. Pt nanomaterials have extensive research potential because of their unique physical and chemical properties. Their catalytic activity has been utilized in many applications, such as the production of hydrogen gas from cyclohexane,¹⁴¹ control of vehicle emissions,^142,143 sensing,¹⁴⁴ and fuel cell applications (as electrocatalysts).¹⁴⁵ The catalytic efficiency and selectivity of Pt nanomaterials are highly dependent on their size, shape, composition, crystallinity, and surface structures.¹⁴⁶ Therefore, many research groups have developed Pt nanostructures of different shapes.^147–152 The synthesis of flower-shaped Pt nanoparticles, described in detail in this section, has attracted considerable attention.

According to Table 7, although the liquid phase synthesis¹⁵³ has also been reported for preparing flower-shaped Pt nanomaterials, one of the most widely used methods is electrodeposition, such as potentiostatic¹¹¹ and galvanic methods,¹¹² as well as sonoelectrodeposition.¹¹⁰

The 3D ultra-fine Pt nanoflowers deposited on the surface of a carbon-coated gas diffusion layer electrode (C-GDL) are also prepared by a one-step electrodeposition method (Scheme 2).¹⁵⁴ The morphological features of the Pt nanomaterials are modified from nanospheres to 3D nanoflowers by a difference in current density. The Pt nanoflowers electrodeposited at a current density of −24 mA cm⁻² for 15 min exhibit high peak power density of 660 mW cm⁻² at 0.6 V in polymer electrolyte fuel cells (PEFCs).

Scheme 2 Illustration of Pt metal nanoparticles, directly deposited on carbon-coated GDL, and its surface morphology. Reprinted with permission from ref. 154. Copyright © 2020 Elsevier Inc. All rights reserved.

Zuo et al. used the galvanic replacement reaction of Te nanowires (Te NWs) to synthesize monomorphic single-crystalline Pt dendritic flowers with enriched edge and corner atoms.¹¹² As shown in Scheme 3, after the addition of the as-prepared Te NWs to the CTAB aqueous solution, H₂PtCl₆ (pH = 7) is injected into the mixture of Te NWs and CTAB to generate a metastable Te@Pt structure. Subsequently, H₂PtCl₆ (pH = 7) is injected for a second time when the color of the solution turns amber, to form Pt nanoflowers. The Pt nanoflowers exhibit excellent catalytic activity for glycerol electro-oxidation under acidic conditions, with higher mass activity and better structural stability than commercial Pt/C (20% Pt), indicating their potential utility in direct-glycerol fuel cells.

Scheme 3 Schematic illustration of the evolution from Te NWs to Pt nanoflowers with well-defined morphologies. Reprinted with permission from ref. 112. Copyright © 2015, American Chemical Society. All rights reserved.

5.1.3. Pd nanoflowers. Pd is often considered to be an alternative catalyst to Pt for the oxygen reduction reaction and direct alcohol oxidation due to their similar properties (they belong to the same group in the periodic table, exhibit FCC (face-centered cubic) crystal structures, and have similar atomic sizes). Furthermore, Pd is approximately 50 times more abundant than Pt and much less expensive.¹⁶¹

In recent years, Pd nanostructures have attracted immense research attention because of their larger surface-area-to-volume ratio and numerous active centers due to their unique morphology.^162–165 The key factors affecting the properties and functions of a material are architecture, crystallographic features, size, shape, and specific crystal planes.^166–169 Among the various nanostructures, 3D Pd nanomaterials, with a porous nature, large surface areas, and numerous active centers, have been considered as promising catalytic materials.^166,170 This section introduces the fabrication methodologies and applications of Pd nanoflowers.

The Pd nanoflowers have been synthesized by liquid phase synthetic methods¹⁷¹ and electrodeposition¹⁷² (Table 8), like the Au and Pt nanomaterials.

For instance, Maniam and Chetty reported the electrodeposition of shape-controlled Pd nanoflowers on an electrochemically activated carbon-black substrate (Vulcan XC-72R) using a potentiostatic technique, with/without different concentrations of PEG-6000 as an additive.¹⁷² The PEG polymer surrounding the crystal nuclei hinders crystal growth and generates a steric hindrance effect to reduce particle–particle aggregation.¹⁷³ Additionally, the concentration of PEG strongly influences particle aggregation. Electrodeposition of Pd with 10⁻² mM PEG changes the morphology from spherical to flower-like, forming well-dispersed nanoflowers with 0.2 mM PEG. The PEG acts as a scaffold for the nucleation sites at an optimum concentration (0.2 mM), to form flower-like shapes without any nucleation. In terms of the electrochemical surface area (ESA) and mass-specific current density, the electrodeposited Pd nanoflowers exhibit three- to four-fold enhanced electrocatalytic activities compared to spherical Pd deposits in the oxygen reduction reaction, formic acid oxidation, and CO stripping reaction.

What is noteworthy is the development of green chemistry research. Pd nanoflowers are synthesized using carrageenan as the capping agent, which is a natural macromolecule from the ocean, and L-ascorbic acid as the reducing agent (Fig. 7).¹⁷¹ Experimental data indicate that the critical factor for the synthesis of Pd nanoflowers is the use of carrageenan, while the concentration of L-ascorbic acid affects the length of thorns grown on Pd nanoflowers. To investigate the effect of concentration on the surface morphology of Pd nanoflowers, Pd products are prepared using 20, 40, 60, 120, and 180 mg L-ascorbic acid in synthetic solutions and labeled Pd-AA20, Pd-AA40, Pd-AA60, Pd-AA120, and Pd-AA180, respectively. Pd-AA60(0) indicates the synthesis of Pd-AA60 without carrageenan. As shown in the SEM image (Fig. 8), the average particle size of Pd-AA60 is approximately 280 ± 30 nm, with Pd-AA60 exhibiting the longest and thinnest thorns (approximately 132 ± 15 nm long and 25 ± 7 nm thick). All Pd products using carrageenan exhibit flower-like morphology, except Pd-AA60(0). The electrocatalytic activity of the synthesized Pd nanoflowers toward ethanol oxidation is investigated in an alkaline medium.

Fig. 7 Schematic illustration of the formation mechanism of Pd nanoflowers. Reprinted with permission from ref. 171. Copyright © 2017, American Chemical Society. All rights reserved.

Fig. 8 SEM images of Pd-AA20 (a and e), Pd-AA60 (b and f), Pd-AA180 (c and g), and Pd-AA60(0) (d and h). Reprinted with permission from ref. 171. Copyright © 2017, American Chemical Society. All rights reserved.

Pd-AA60, with the longest thorns, exhibits higher catalytic current density (1160 mA mg⁻¹) than commercial Pd/C, the reported Pd/C samples,¹⁷⁴ and other prepared Pd products (Pd-AA20, Pd-AA180, and Pd-AA60(0)) with short thorns exhibit better cycle stability, with catalytic activity maintenance of above 96% after 300 cycles.

5.1.4. Ag nanoflowers. Ag has been employed as an electrically conductive component in electronic devices. Ag particles with various morphologies have been investigated, with special emphasis on their size and structural effects on their electrical properties.^178–181 The Ag nanoparticles and their potential technological applications, such as water remediation, catalysis, and medicine have also been intensively studied owing to their chemical and physical properties, which are different from those of bulk systems.^182–186 At present, various Ag nanomaterials have been studied, such as, nanowires,¹⁸⁷ nanocubes,¹⁸⁸ nanoclusters,¹⁸⁹ nanoflowers,¹⁹⁰ and so on.¹⁹¹ The 3D hierarchical nanostructures (with many external curved regions and nanogaps) could be promising candidates for highly sensitive SERS substrates for sensing applications.^124,192 Additionally, flower-shaped Ag nanoparticles with high electrical conductivity could be used for printable and flexible electronics.¹²¹

In this section, since Ag nanoflowers (Table 9) are also synthesized with the aforementioned strategies for manufacturing Au, Pt, and Pd nanoflowers, we would like to introduce some unique generation methods for Ag nanoflowers.

Zhang et al. reported the fabrication of novel self-assembled silver hierarchical structures by incorporating a small amount of acid in conventional solution chemistry, without using any polymer surfactant or capping agent.¹²⁴ This acid-directed synthesis inhibits the sorption of polymer molecules on metal surfaces, forming Ag particles with coarse morphology, which can be utilized as substrates to detect chemical or biological molecules through SERS. As we explained in Section 3. Experimental parameters' effect, the SEM images in Fig. 3 show the Ag assemblies prepared with the assistance of different acids at low and high concentrations, with uniform-structure Ag particles also being produced, corresponding to the acid incorporated by the acid-directed process. The irregular Ag particles, composed of Ag+ ions reduced by ascorbic acid, have an approximate size of 1 μm. They are made up of Ag nanosheets with a thickness of 50–100 nm. Adding citric acid forms perfect microspheres with a diameter of about 2 nm, which are composed of closely packed Ag nanosheets measuring 20 nm in thickness. In the presence of mandelic acid, flower-like Ag particles are formed, consisting of loosely packed Ag nanosheets with a thickness of 50 nm. Using toluenesulfonic acid results in the formation of microscale Ag yarn-balls assembled by cross-linked Ag nanosheets with a thickness of 50 nm. The obtained Ag particles with rough surfaces are promising SERS platforms for sensing applications, and the fabricated core–shell Ag wires exhibit high SERS sensitivity toward melamine, with a detection sensitivity of 5 ppm.

Long-range ordered Ag nanoflower arrays on patterned wafers are fabricated using a lithographic template-guided method based on the optical interference method and the difference in hydrophilicity between silicon and the photoresist.¹⁹² As shown in Fig. 9a, 1D and 2D ordered array templates are obtained for supplying alternate relative hydrophilic and hydrophobic regions, using holographic technology. The photoresist part is more hydrophilic than silicon, as shown by the contact angle of the droplet in Fig. 9b.¹⁹³ As shown in Fig. 9c, the prepared photolithographic template is immersed in the reaction solution containing AgNO₃, PVP, ascorbic acid, and citric acid, to form Ag nanoparticles. Ag seeds are induced on the photoresist surface owing to the difference in hydrophilicity between the photoresist and silicon. Finally, the seeds grow into silver nanoflowers with abundant hotspots.

Fig. 9 Schematic diagram of fabricating the LROSNA structure. (a) Preparation of lithographic templates. (b) Optical image of the contact angle for hydrophilicity difference between silicon (the top) and photoresist (the bottom). (c) Schematic diagram of preparing the LROSNA structure. Reprinted with permission from ref. 192. Copyright © 2019 Elsevier B.V. All rights reserved.

As SERS substrates, the 1D and 2D pattern structures demonstrate high sensitivity with low detection limits of 5 × 10⁻¹⁰ M and 5 × 10⁻⁹ M, respectively, using rhodamine 6G as a probe molecule to investigate the hotspot effect of the nanogaps within/between the Ag nanoflowers.

5.1.5. Other-metal nanoflowers. Metal nanoflowers composed of Ni, Co, and Cu are summarized in Table 10.

5.1.6. Conclusion. Metal nanoflowers are one of the main topics of nanoflower research. Metal nanoflowers composed of Au, Pt, Pd, Ag, Ni, Co, and Cu are synthesized using various methods, such as liquid phase synthesis, electrodeposition, and green synthesis (also called biosynthesis) (Tables 6–10). As summarized in Tables 6–10, metal nanoflowers can be utilized as electro-, photo-, and chemical-catalysts, gas- and biosensors, antibacterial agents, and so on, because of their unique morphologies and elemental effects. Initially, research mainly focused on new synthetic methods, shapes, formation mechanisms, and the application-based chemical, physical, and optical property testing of elemental nanoflowers. Many studies have been recently conducted to control the morphology and size of nanoflowers, to optimize their properties for applications.

Table 6 Synthesis methods and applications of flower-like Au nanostructures. (Interestingly, all reactions used HAuCl₄ as a precursor.)

Variety of formed nanostructures	Synthesis method	Application	Ref./year
Fluorescent Au nanoflowers	Seedless, surfactantless room temperature synthesis using 5-hydroxyindole-3-acetic acid	SERS and electrocatalyst	134/2008
Au nanoflowers	Vesicle-directed generation using gemini amphiphiles	SERS	137/2010
Au nanoflowers	Melanin assisted seedless, template free synthesis	Catalysis, biochemistry, and Raman fields	140/2012
Hydrangea flower-like hierarchical Au nanostructures	One-pot, seedless synthesis using ascorbic acid and CTAC	SERS-based sensors	138/2015
Au nanoflowers with sheet-like petals	Seedless, reduction synthesis using ascorbic acid and silver nitrate	SERS substrates and sensors	132/2016
Hollow-channel Au and Ag nanoflowers	One step fabrication using an auto-degradable MO–FeCl3 nanofiber as a bifunctional template	Catalysts and SERS probes	130/2016
Au nanoflowers, nanostars, and nanosnowflakes	Seed-less and surfactant-free approach using catechol as the reducing and structure-inducing agent	Catalyst and photothermal therapy	136/2019

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Table 7 Synthesis methods and applications of flower-like Pt nanostructures

Variety of formed nanostructures	Synthesis method	Application	Ref./year
Pt nanoflowers with thin projections (petals)	Stabilizing reagent-free synthesis using NaBH4	Surface-assisted laser desorption/ionization mass spectrometry of biomolecules	155/2007
Pt nanoflowers on a Si substrate	Potentiostatic pulse plating method	Electrocatalysts toward methanol and CO oxidation	111/2009
Porous Pt nanoflowers on clean indium tin oxide (ITO)	One-step and template-free electrodeposition method	Electrocatalysts for the methanol electro-oxidation of direct methanol fuel cells (DMFCs)	142/2010
Pt nanoflowers on the bare Au electrodes	Template-free ultrasonic electrodeposition method	Nonenzymatic glucose sensors	110/2012
Pt nanoflowers composed of an ordered array of nanoparticles	Ethanol reduction of H2PtCl6	Electrocatalysts for DMFCs and catalyst in C–C coupling reaction	153/2013
3D Pt nanoflowers on porous silicon	Electroless plating method	Electrocatalysts for monolithic integrated micro DMFCs	156/2013
Pt nanoflowers	Sonoelectrodeposition method	Electrocatalysts and nonenzymatic sensors	157/2014
Pt needle-like nanoflowers on a glassy carbon electrode (GCE) substrate	Template-free electrochemically potentiostatic method	Electrocatalysts for methanol oxidation reaction in DMFCs	158/2015
Pt nanoflowers	Chemical reduction and seed-mediated growth using tri-sodium citrate and ascorbic acid	—	159/2016
Nanoflower-shaped Pt on GCE	One-step electrochemical deposition method	Sensor for simultaneous detection of lead and cadmium at trace levels	160/2019

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Table 8 Synthesis methods and applications of flower-like hierarchical Pd nanostructures

Variety of formed nanostructures	Synthesis method	Application	Ref./year
Pd porous single-crystalline nanoflowers	Seed mediated growth method using ascorbic acid and CTAB	Electrocatalysts for ethanol electro-oxidation	174/2014
Porous Pd nanoflowers	Polyol process using Pd(acac)2 and oleylamine	Electrocatalysts for methanol electro-oxidation	175/2009
Spinous flower-like, cone-like, flower and coral reef-like Pd dendrites on glassy carbon plates	Electrochemical methods using cyclic voltammetry	Electrocatalysts toward formic acid electro-oxidation of microfluidic fuel cell channels	81/2013
Pd nanoflowers	Green synthesis (seedless chemical reduction method) using HEPES	Electrocatalysts for methanol electro-oxidation in alkaline media	170/2013
3D Pd nanoflowers on Ni-YSZ (yttria-stabilized zirconia) anode	Galvanic displacement reaction	Catalysts for direct ethanol solid oxide fuel cells	114/2016
Pd nanoflowers	Radiolytic reduction of Pd^II(acac)2 in ethanol under a CO atmosphere	Plasmonic photocatalysts for the Suzuki–Miyaura cross-coupling reaction	176/2019
Pd nanoflowers on Ti film or on GCE	Electrodeposition process	Electrocatalysts in the reduction of Cr(VI)	177/2019

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Table 9 Synthesis methods and applications of flower-like Ag nanostructures

Variety of formed nanostructures	Synthesis method	Application	Ref./year
Rose-, spike-, and snowflake-shaped Ag nanostructures	One-pot reduction method using ascorbic acid, poly(ethylene glycol), sodium acetate, and sodium citrate	SERS matrices	194/2007
Branched Ag nanoflowers	Biosynthesis using biofriendly molecule-rutin	SERS and antibacterial activities against Pseudomonas aeruginosa, S. Faecalis, and Escherichia coli bacterium	195/2009
Ag nanoflowers with multiple petals on ITO glass	Double-potentiostatic electrodeposition method	SERS	196/2012
Ag nanoflowers	Trisodium citrate-assisted biosynthesis using trisodium citrate and Canarium album foliar broths	SERS substrates	197/2013
Ag nanoflowers with nanogaps	Ice-water bath method using citric acid, and PVP	Broadband scatterers for high performance random lasers	198/2017
Ag nanoflowers on fluorine-doped tin oxide (FTO) glass (nanorods, dendrites, decahedrons, and icosahedrons are also formed)	Cyclic scanning electrodeposition method	Catalysts of non-enzymatic electrochemical glucose biosensors	199/2017
3D and 2D Ag nanoflowers	Seedless green liquid reduction method using AgNO3, ascorbic acid, and sodium citrate	Multiple phase SERS-based molecule detection of food additives and environmental pollutants	200/2019
Ag nanoflowers	Green synthesis using Kalanchoe daigremontiana extract	Photocatalysts in the dye degradation of methylene blue and antibacterial activities against the Gram-negative bacteria Escherichia coli and the Gram-positive Staphylococcus aureus	76/2019
Ag nanoflowers with nanoleaves	Electrochemical deposition approach	SERS sensing substrates	113/2019

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Table 10 Synthesis methods and applications of flower-like metal nanostructures

Compound	Variety of formed nanostructures	Synthesis method	Application	Ref./year
Ni	Ni nanospheres, nanowires, and nanoflowers	Surfactant-free non-aqueous sol–gel approach	—	201/2008
	Ni nanoflowers	Capping agent assisted and Ag-catalyzed growth	Catalysts	202/2010
Co	Co nanoflowers	Solvothermal synthesis using RuCl3·xH2O (35 wt% Ru) and hexadecylamine	Catalysts for hydrogenolysis of glycerol to propylene glycol	33/2009
Cu	Cu nanoflowers	Capping agent assisted reduction process using oleylamine and CTAB	Catalysts in oxygen reduction reactions	203/2018

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5.2. Nanoflowers with metal oxide composition

Here, the unique properties of each metal-oxide material are introduced, and methods and properties of synthesizing nanoflowers using these materials are summarized.

5.2.1. TiO₂ nanoflowers. Here, TiO₂ has three crystalline phases: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic).²⁰⁴ Rutile TiO₂ is the most stable form, and the anatase and brookite phases can be transformed into the rutile phase at high temperatures (∼750 °C).²⁰⁴ However, bulk TiO₂ has little photocatalytic ability compared to nano TiO₂ due to its low dimensionality and quantum size effect.²⁰⁵ The TiO₂ nanostructures have been utilized not only for photocatalytic materials,^206–208 but also for solar cells,²⁰⁹ gas sensors,^210,211 molecular sensors,²¹² and lithium batteries^213,214 because of their high oxidation activity,²¹⁵ low toxicity,²¹⁶ non-polluting nature,²¹⁷ and low cost.²¹⁷

Over the past decade, various TiO₂ nanostructures have been investigated and used to form nanomaterials, such as nanosheets,²¹⁸ nanowalls,²¹⁹ nanorods,²²⁰ nanowires,²²¹ nanotubes,²²² and nanobelts.²²³ In particular, TiO₂ nanostructures with 3D dimensions, such as flowers and urchins, are promising candidates for many applications because of their properties.²²⁴

Our investigation (Table 11) shows that the most commonly used method for synthesizing TiO₂ nanoflowers is a hydrothermal method to synthesize nanomaterials by chemical reaction in aqueous media within a pressurized vessel (known as an autoclave).²²⁵ Anatase and brookite TiO₂ nanoflowers are prepared using tetrabutyl titanate, NaCl and NH₃·H₂O.⁹³ Hierarchical 3D TiO₂ nanoflowers on Ti substrate are processed by the low temperature hydrothermal method.⁹² TiO₂ and reduced TiO₂ nanoflowers on FTO glass are fabricated using hybrid technology combining hydrothermal and immersion reduction processes.²²⁶

Table 11 Synthesis methods and applications of flower-like hierarchical TiO₂ nanostructures

Variety of formed nanostructures	Synthesis method	Application	Ref./year
Anatase and brookite TiO2 nanoflowers	Hydrothermal process using tetrabutyl titanate, NaCl and NH3·H2O	Photocatalysts in the photodegradation of methyl orange	93/2009
Flower-like TiO2 nanostructures	Low-temperature hydrothermal process using Ti powder and HF	Photocatalysts for the degradation of methylene blue dye	228/2010
Rutile TiO2, stacks of multilayered nanoflowers with hexagonal nanopetals	Acid vapor oxidation method	Catalysis, solar cells, and electronic devices	229/2011
Hierarchically flower-like TiO2 superstructures	Surfactant-free alcohothermal strategy in a HF–H2O–C2H5OH mixed solution using titanate nanotubes	Photocatalysts for decomposition of acetone in air and methyl orange in aqueous solution under UV illumination	230/2011
TiO2 nanoflowers	Hydrothermal method using titanium(IV) butoxide and acetic acid	Photoelectrodes for dye-sensitized solar cells (DSSCs), photocatalysts for aqueous methylene-blue photo-oxidation	231/2011; 232/2020
Rutile-TiO2 nanowires and nanoflowers on FTO glass and glass	Hydrothermal method using titanium(IV) butoxide and HCl	Solar cells, gas sensors, optoelectronics etc.	233/2015
Rutile/anatase TiO2 heterojunction nanoflowers	Hydrothermal approach using titanium tetrachloride, urea, and CTAB	Photocatalysts for the photodegradation of real-dye waste water	234/2015
TiO2 nanoflowers on ITO	PVP surfactant assisted liquid phase deposition technique using (NH4)2TiF6, PVP, and boric acid	Photovoltaic materials of the photoelectrochemical cell	62/2015
TiO2 nanoflowers on TiO2 seed layer coated p-Si substrates	Hydrothermal method using titanium butoxide and HCl	Electron field emission and self-powered ultraviolet photodetector	235/2016
Hierarchical 3D TiO2 nanoflowers on Ti substrate	Low temperature hydrothermal process using Ti plate and H2O2, and NaOH	Volatile organic compound sensors (acetone, methanol, 2-butanone, toluene and 2-propanol)	92/2016
Rutile TiO2 nanoflower films on FTO glass	Hydrothermal process using tetrabutyl titanate and HCl	Photoanodes for DSSCs	236/2017
TiO2 and reduced TiO2 nanoflowers on FTO glass	Hydrothermal process and dipping reduction process with NaBH4	Photoanodes for superior photoelectrochemical water splitting	226/2017
Rutile-TiO2 nanoflower thin film on FTO substrate	Hydrothermal method using titanium(IV) isopropoxide and HCl	Solar cells	69/2020
Rutile- phase TiO2 nanoflowers	Hydrothermal method using titanium(IV) butoxide, HCl, and CTAB	—	237/2020
3D TiO2 hierarchical nanoflowers	Template-free solvothermal alcoholysis reaction using titanium trichloride, NaOH, and ethanol	Photocatalysts in the degradation of organic pollutants in wastewater	34/2021

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Rutile TiO₂ 3D nanoflowers are synthesized by the hydrothermal method in a saturated sodium chloride solution⁷⁴ and characterized by X-ray diffraction (XRD), whereby all the diffraction peaks indicate that the product is pure rutile TiO₂. The effects of the solvent on NF synthesis are investigated using FE-SEM. sample 1, synthesized in saturated NaCl solution, consists of partly agglomerating TiO₂ nanoflowers, with a diameter of 2–3 μm. The powder (sample 2) prepared by the hydrothermal method in anhydrous ethanol is stacked with uneven nanospheres. Furthermore, using deionized water as the solvent produces nanobulks with irregular shapes (sample 3). Sensors are fabricated using the as-synthesized TiO₂ samples and tested, to evaluate their ethanol gas-sensing ability and to investigate the influence of morphology on it. The gas response of the sensor, fabricated using partly agglomerated TiO₂ nanoflowers (sample 1) with a large surface area, is higher than that of the others.²²⁷

5.2.2. CuO nanoflowers. CuO, a p-type metal oxide semiconductor with a narrow bandgap (indirect bandgap of 1.2 eV (ref. 238) and direct bandgap of 3.2–3.3 eV (ref. 239–241)), has been intensively studied because of its unique characteristics,^242–244 and used in a wide range of applications as catalysts,²⁴⁵ gas sensors,^246,247 lithium-ion battery electrodes,²⁴⁸ and so on.^249,250 The microstructures and morphology of CuO nanomaterials significantly affect their physical and chemical properties.^251,252 Therefore, numerous CuO nanostructures with controlled morphologies have been researched, including nanoparticles,^253,254 nanorods,²⁵⁵ nanowires²⁵⁶ nanosheets,^257,258 nanotubes,^259,260 and nanoneedles.²⁶¹ Recently, nanoflowers have received considerable attention because of their unique morphologies.²⁶²

CuO nanoflowers are manufactured by various synthesis strategies: liquid phase synthesis, chemical deposition, hydrothermal method, photo-synthesis, and green synthesis (Table 12).

Table 12 Synthesis methods and applications of flower-like hierarchical CuO nanostructures

Variety of formed nanostructures	Synthesis method	Application	Ref./year
CuO nanoflowers on Cu plates	Direct reaction between a Cu plate and a KOH solution at room temperature	Field emission for nano-electronic devices	263/2008
CuO nanoflowers	Hydrothermal method using Cu(NO3)2·3H2O and NH3·H2O	H2O2 sensor	98/2010
CuO nanoflowers on the glass substrate	Low-temperature chemical bath method using copper nitrate and hexamethylenetetramine	Electrochemical pH sensor	264/2011
CuO nanoribbons and nanoflowers	Low-temperature, one-pot water bath method using Cu(OAc)2·H2O and NaOH	Supercapacitors	265/2013
CuO nanoflowers	Solution combustion method using cupric nitrate and glycine	Photocatalysts for the photodegradation of methyl orange	266/2013
CuO nanoplatelets and nanoflowers	Hydrothermal method	Catalysts for the degradation of methylene blue	267/2014
Hierarchical 3D-flower-like CuO nanostructure on copper foil	Chemical bath deposition method	Supercapacitors	268/2015
Hierarchical CuO nanoflowers on the surface of flexible Cu foil	Chemical deposition method	Supercapacitor electrodes	269/2016
Flower-like CuO nanostructures with porous nanosheets on an alumina tube	Hydrothermal method using Cu(CH3COO)2·H2O, NH3·H2O and NaOH	H2S sensors	99/2017
CuO nano-flowered surfaces	Submerged photo-synthesis of crystallites	Antibacterial agents against Gram-positive (Staphylococcus aureus) bacteria and Gram-negative (Escherichia coli K12) bacteria	270/2017
CuO nanoflowers	Low temperature solution process using copper nitrate, HMTA, and NaOH	Adsorbent for the removal of Pb^2+ in waste-water treatment	271/2017
Hierarchical flower-like CuO film on Cu foil	One-step solution route at room temperature using Cu foil, NaOH, and (NH4)2S2O8	Photocatalysts for the degradation of methylene blue	272/2017
CuO nanospindles and CuO nanoflowers	Green synthesis process using Dodonaea angustifolia extract	Anti-microbial agents against Gram-negative E. coli and Gram-positive S. aureus	77/2018
Hierarchical porous CuO/Cu nanoflowers	Alkaline solution oxidation process in a water bath using Cu powder, sodium hydroxide, and ammonium peroxydisulfate	Non-enzymatic H2O2 sensors	273/2018
Hierarchical CuO nanoflowers	Hydrothermal method using CuSO4·5H2O, CTAB, sodium citrate, and KOH	Non-enzymatic sensors for H2O2 and glucose	85/2019
Flower-shaped CuO nanostructures	Biosynthesis using Cu(CH3COO)2 and Ocimum sanctum (Tulsi) leaves-extracted eugenol (4-allyl-2-methoxyphenol)	Photocatalysts and antibacterial agents	78/2020
Co3O4, NiO and CuO nanoflowers	Hydrothermal method	Photodiode, photodetector, sensor, battery and supercapacitor applications	274/2020
CuO nanoflowers on a glass slide	Wet chemical method using Cu foil, (NH4)2S2O8, and NaOH	Inflammable gas sensors (acetone, ethanol, methanol, isopropyl alcohol, toluene, chloroform and ammonia gas)	275/2020
CuO nanoflowers	Hydrothermal method	Catalysts for the degradation of methylene blue in the presence of H2O2 in waste water	286/2021

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Their applications are also very diverse from hydrazine, H₂O₂, H₂S, and pH sensors to field emission for nano-electronic devices, supercapacitors, photocatalysts and absorbent for waste-water treatment, and antibacterial agents (Table 12).

For example, flower-like CuO nanostructures are synthesized using a wet chemical-assisted hydrothermal reaction,¹²⁹ with the formation mechanism illustrated in Fig. 4. The first nucleation seeds, formed by a mixed aqueous solution of Cu²⁺ and PEG, provide the nuclei for particle growth. PEG chains, nonionic surfactants composed of H(–O–CH₂–CH₂)_n–OH, absorb small seeds using the terminal OH bonds as soft templates when the nucleation seeds reach a critical dimension. Subsequently, they aggregate around the large seeds to form CuO flower nanostructures. Finally, the annealing process removes the PEG templates, forming flower-like CuO hierarchical nanorods. Fig. 10 shows FE-SEM images of the CuO nanoflowers obtained through hydrothermal treatment and calcination. Highly uniform nano-sized flowers (petals with 60–100 nm diameter and 500–700 nm length), consisting of many nanorods, are formed. The CuO nanoflowers are coated on SiO₂/Si substrates attached to Pt interdigitated electrodes to fabricate volatile gas sensors. The fabricated sensors show high selectivity and sensitivity toward ethanol vapor at approximately 230 °C (the optimized working temperature).

Fig. 10 FE-SEM images of CuO nanoflowers prepared at 170 °C for 24 h with different magnifications of: (a) 100 00×, (b) 200 00×, and (c) 1 000 00×. Reprinted with permission from ref. 129. Copyright © 2014 Elsevier Ltd. All rights reserved.

5.2.3. NiO nanoflowers. NiO nanostructures have been widely studied for decades and are considered promising candidates for many applications, such as gas sensors,²⁷⁶ lithium-ion batteries,²⁷⁷ magnetic materials,²⁷⁸ and others.²⁷⁹ The NiO nanostructures exist with various morphologies, such as, nanoflakes,²⁸⁰ nanoparticles,²⁸¹ nanoplatelets,²⁸² nanowires,²⁸³ nanowalls,²⁸⁴ nanotubes,²⁸⁵ and nanoflowers.²⁸⁶ Among them, NiO nanoflowers have been extensively studied owing to their interesting architecture. Various morphologies of NiO nanoflowers have been fabricated using hydrothermal, microwave-assisted, solution plasma, solvothermal, and hard and soft template methods (Table 13). The NiO nanoflowers have been synthesized using various precursors in the presence/absence of surfactants/templates because the morphologies (petal size, thickness, porous/nonporous surface, and hollow structures) of the flower-shaped NiO nanostructures are influenced by the reaction conditions, including the type and amount of reagents (precursors, reducing agents, surfactants, template agents, and so on), reaction time, and temperature (Table 13). Cao et al. reported the synthesis of flower-like NiO nanostructures with three different morphologies using a template-free hydrothermal method by changing the volume of ammonia (25–28 wt%).¹²⁷ Sparse-type and close-type NiO nanoflowers are synthesized using a surfactant-assisted hydrothermal method with CTAB and SDS.⁹⁷ Additionally, porous NiO nanoflowers with different morphologies are synthesized by microwave-assisted fabrication¹⁰⁶ or solvothermal methods.^100,287 As shown in Table 13, these nanoflowers are primarily used for wastewater treatment, and as sensors, catalysts, and Li-ion batteries.

Table 13 Synthesis methods and applications of flower-like hierarchical NiO nanostructures

Variety of formed nanostructures	Synthesis method	Application	Ref./year
NiO nanoparticles and nanoflowers	Soft synthesis without organics at low temperature using Ni powder and water	—	291/2008
NiO with novel flower-like morphology	Calcination in a muffle furnace under a nitrogen atmosphere	Catalysts for CO oxidation	292/2009
Hierarchical porous NiO nanoflowers	Microwave-assisted fabrication and calcination	Supercapacitor	106/2010
NiO nanoflowers	Wet chemical process with a subsequent thermal treating process	Electrochemical sensors	293/2011
NiO nanodiscs and nanoflowers	Hydrothermal synthesis in a non-basic solution and calcination	Electrochemical test and water treatment	294/2011
NiO nanoflowers	Surfactant-free hydrothermal process and thermal decomposition	Li-ion batteries	295/2011
Porous NiO nanoflowers and nanourchins	Hydrothermal method using HMTA or urea as the precipitator in the absence or presence of a surfactant (PEG or PVP)	Catalysts for toluene combustion	96/2012
NiO nanoflowers	Solution plasma process	NO2 gas sensor	296/2012
Flower shaped NiO nanostructures	Microwave and hydrothermal methods	—	105/2014
NiO hierarchical bundle-like nanoflowers	Hydrothermal-route assisted EG and subsequent thermal calcination	Gas sensors	297/2015
NiO nanoflowers	Template-less surfactant-free hydrothermal synthesis using nickel nitrate and HMTA	Photoelectrochemical hydrogen production	298/2015
3D hierarchical porous NiO nanoflowers	Solvothermal and post-calcination	Li-ion batteries	287/2016
3D flower-like NiO	Surfactant-assisted hydrothermal method	Ethanol gas sensors	299/2016
Flake–flower NiO architecture	Hydrothermal process and calcination using Ni(NO3)2·6H2O and PVP	Ethanol gas sensors	300/2016
Rose-like, sphere-like, and plate–flower-like NiO nanostructures	Surfactant-assisted hydrothermal method and calcination using Ni(NO3)2·6H2O, or NiCl2·6H2O, and PVP, or CTAB	Ethanol gas sensors	301/2017
Nanoneedle-assembled hierarchical and nanosheet-assembled hierarchical NiO nanoflowers	Hydrothermal method and subsequent calcination	Ethanol gas sensors	302/2017
Sparse-type and close-type NiO nanoflowers	Hydrothermal process assisted by CTAB or SDS	Ethanol gas sensors	97/2017
Porous NiO nanoflowers	Solvothermal method and calcination using nickel acetate tetrahydrate (Ni(ac)2·4H2O) and urea	Li-ion batteries	100/2017
3D flower-like NiO hierarchical structures	Hydrothermal synthesis and calcination using Ni(NO3)2·6H2O, cetyltrimethyl ammonium bromide (CTAB) and ammonia, or NiCl2·6H2O, Na2C2O4, and ethylene glycol (EG)	Acetylene gas sensors	118/2018
3D hierarchical flower-like NiO nanostructures with three different morphologies	Template-free and low-cost hydrothermal method and subsequent calcinations with different volumes of ammonia (25%–28% wt%)	Ethanol gas sensors	127/2020
NiO nanoflowers	Polymer directed hydrothermal method and calcination using Pluronic F-127 block copolymer, NiNO3·6H2O, and urea	Electrostatic adsorption of Congo red dye for wastewater treatment	303/2021

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As shown in Scheme 4,²⁸⁶ Pluronic F127 acts as a micellar template for the fabrication of a lamellar structure. Ni(NO₃)₂·6H₂O is ultrasonically dissolved in water and mixed evenly under stirring in a water bath to form a homogeneous suspension. Subsequently, 1,3,5-trimethyl benzene (TMB), which acts as an interface-adjusting agent that avoids piling up of the lamellar structure, is slowly injected into the homogeneous suspension. The hydrophobic poly(phenylene oxide) segment of F127 can interact with TMB molecules through van der Waals interactions, while Ni²⁺ undergoes oriented attachment and self-assembles with the hydrophilic segment of F127. Addition of concentrated NH₃·H₂O to the emulsion system promotes crystal growth. An Ni-based hydroxide precursor is generated through the hydrothermal process, and F127 and TMB are removed on decomposition to generate numerous mesopores during the high-temperature sintering process, forming NiO nanoflowers with mesoporous nanosheets. The synthesized samples are labeled NiO–NF-x, where x (0.5, 1, 3, 5, and 7 mL) is the volume of NH₃·H₂O. The FE-SEM images of the NiO–NF samples in Fig. 11 show the influence of the amount of NH₃·H₂O on their morphology. The size range of the nanoflowers is 1–2 μm. On increasing the amount of NH₃·H₂O to 3 mL, flower-like NiO–NF-3.0, composed of mesoporous hexagonal nanosheets, is formed (Fig. 11c). With increasing alkalinity, the stacked nanosheets become thicker (Fig. 11d and e) due to the faster formation of Ni hydroxide facilitated by higher NH₃·H₂O content. Moreover, the size of the mesopores on the nanosheets remains around 10 nm. Notably, NiO–NF-3.0 exhibits the smallest mesopores (primarily 9.5 nm) and the thinnest nanosheets (approximately 15 nm), indicating a higher number of active sites and promising electrocatalytic performance. In the electrochemical nitrogen reduction reaction, the NiO–NF-3.0 electrode materials exhibit excellent electrochemical properties in 0.1 M Na₂SO₄.

Scheme 4 Schematic illustration of the formation mechanism of NiO–NF-0.5, NiO–NF-1.0, NiO–NF-3.0, NiO–NF-5.0, and NiO–NF-7.0. Adapted with permission from ref. 286. Copyright © the Royal Society of Chemistry 2021.

Fig. 11 FE-SEM images of (a) NiO–NF-0.5, (b) NiO–NF-1.0, (c) NiO–NF-3.0, (d) NiO–NF-5.0, (e) NiO–NF-7.0, and (f) NiO–NF-3.0 before annealing. Adapted with permission from ref. 286. Copyright © The Royal Society of Chemistry 2021.

Multifunctional nanoflowers composed of partial hydroxide nickel oxide (NiO_x) are grown on cotton fabric using a chemical bath deposition technique to develop UV filters and flexible gas/chemical sensors.²⁸⁸ Nickel sulfate hexahydrate and potassium persulfate are mixed at room temperature, followed by the addition of an ammonium hydroxide solution to the prepared precursor solution under vigorous stirring. Pre-treated cotton cellulose samples are dipped in the prepared precursor solutions and kept undisturbed for 96 h, followed by hot-air-oven drying at 80 ± 6 °C for 1 h. At the beginning of the growth process, the Ni dissolved in potassium persulfate is immediately hydrolyzed and reacts with ammonium hydroxide to generate Ni(OH)₂ nuclei with an electrostatic driving force, minimizing the surface energy of cotton cellulose, promoting the growth of nanosheets.^289,290 A high saturation level facilitates the formation and enhancement of nucleation sites, whereas a low saturation level facilitates crystal growth. On increasing the reaction time, the growth of nanosheets increases through further nucleation and assembly, forming nanostructured NiO_x-3D-green-button chrysanthemum flowers on the cotton fabric (Fig. 12). The ultraviolet protection factor (UPF) indicates the number of UV rays (both UV-A and UV-B) blocked by the material.²⁸⁸ The UPF of the nanoflower modified on cotton fabric, measured using a UV transmittance analyzer by an in vitro method according to the AATCC 183:2004 standard, is 2000. According to chemical/gas sensing measurements, the selectivity response of the NiO_x-modified fabric is 12 431 toward trimethylamine at room temperature. Additionally, it exhibits good thermal stability and increased hydrophobicity compared to unmodified materials. Owing to these excellent features, the nanostructured cotton fabric can be used as a flame retardant, hydrophobic screen, and protective suit against gas leaks and UV rays.

Fig. 12 Schematic representation of the growth process of the NiO_x on cotton cellulose. Reprinted with permission from ref. 288. Copyright © 2019, American Chemical Society.

5.2.4. Other-metal-oxide nanoflowers. There are many other-metal-oxide nanoflowers, including Sn, Fe, Ce, Mn, Bi, Mg, Zn, and Cd. They have been summarized in Table 14 according to their synthetic methods, structures, and applications.

Table 14 Synthesis methods and applications of flower-like hierarchical other metal oxide nanostructures

Compound	Variety of formed nanostructures	Synthesis method	Application	Ref./year
SnO2	Super-hydrophobic 3D SnO2 flowers with nanoporous petals	Controlled shape-preserving thermal oxidation process	—	304/2004
SnO	Nanoparticle-attached SnO nanoflowers	Free cation-induced decomposition of an intermediate product tin oxide hydroxide (Sn6O4(OH)4)	Anodes for high-capacity lithium-ion rechargeable batteries	305/2009
α-Fe2O3 (hematite)	Porous α-Fe2O3 flower-like nanostructures	Solvothermal reactions and sequential calcinations	Lithium-ion batteries and photocatalysts	306/2008
CeO2	CeO2 nanoflowers with controlled shape (cubic, four-petaled, and starlike)	Rapid thermolysis of (NH4)2Ce(NO3)6 in oleic acid/oleylamine	Catalysts in CO oxidation	307/2008
MnO2	Monodisperse 3D manganese oxide nanoflowers	Reduction of potassium permanganate (KMnO4) in deionized water with formamide (HCONH2) at 40 °C	Supercapacitor electrode material	308/2009
MnO4	Homogenous manganate nanoflowers	Microwave-assisted hydrothermal synthesis	Nuclear decontamination uses	117/2019
Bi2O3	Large-area arrays of 1D nanowires and nanoflowers of Bi2O3	Oxidative metal vapor phase deposition technique	—	83/2007
	Nonstoichiometric Bi2O2.33 nanoflowers	One-step solvothermal route using NaBiO3 precursor in aqueous solution with PVP	—	103/2014
MgO	Magnesium oxide nanoflowers	Chemical precipitation method, sequential calcinations, and surface modification with acacia gum	Adsorbents for the removal of divalent metallic species from synthetic waste water	309/2015
ZnO	Flower-like ZnO nanostructures	Hydrothermal method in the absence of surfactants or organic solvents	—	310/2007
	ZnO nanoflowers on Si substrate	Seed-layer assisted solution route	Optical, optoelectronic and sensing devices	311/2010
	ZnO nanoflowers	Solution plasma	—	312/2011
	Flower-like ZnO-on-ZnO nanorod arrays on a zinc substrate	Low-temperature hydrothermal synthesis	—	313/2013
	ZnO nanoflowers	Biosynthesis using Bacillus licheniformis MTCC 9555	Photocatalysts	314/2014
	Hierarchical ZnO nanoflowers	PEG-20000 assisted hydrothermal synthesis	H2S gas sensors	95/2015
	Three types of 3D fluffy ZnO nanoflowers	Direct hydrothermal reaction with/without ultrasonic treatment	Photocatalysts	87/2020
	Single-crystalline ZnO nanoflowers	Hydrothermal method with post-annealing treatment	Ultraviolet photodetectors	315/2021
	ZnO nanoflowers	Bioinspired synthesis using Withania coagulans extract as a reducing agent	Antibacterial agents and bioethanol production	108/2021
CdO	CdO nanoflowers on soda lime glass, fused silica glass, Si and ITO-coated glass	Sol–gel method	Photodetection application	316/2019

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5.2.5. Conclusion. The reported articles on metal oxide nanoflowers, the most published topic in nanoflower research, are mainly focused on TiO₂, CuO, NiO, and ZnO. Many other metal-oxide nanostructures have also been studied. The ZnO nanoflowers synthesized by surface treatment have been reported in many publications, such as, ZnO–TiO₂ core–shell,³¹⁷ Ag-/Pt-doped ZnO,^318,319 and C-coated ZnO nanoflowers.³²⁰ Metal oxide nanoflowers have been synthesized using various methods; one-third of the preparation routes use hydrothermal or solvothermal methods (Tables 11–14) under mild reaction conditions. According to Tables 11–14, metal oxide nanoflowers can be applied for wastewater treatment, catalysis, nuclear decontamination, and used as gas sensors, antibacterial agents, photodetectors, and supercapacitors. Research on metal oxide nanoflowers has focused on controlling their morphology and size under various reaction conditions, including the types and amounts of reagents (precursors, reducing agents, surfactants, and template agents), reaction time, and temperature, to determine the optimal structures for different applications.

Flower-like NiO nanostructures with three different morphologies are synthesized by a modified hydrothermal method by changing the volume of ammonia (25–28% wt%).¹²⁷ Each product exhibits unique morphology and different gas sensing ability. Ni et al. compared the electrochemical performances of the synthesized CuO nanoflowers with previously reported CuO-based nanomaterials, including nanoflowers prepared under different synthesis conditions, for H₂O₂ determination and glucose biosensing.⁸⁵

5.3. Alloy nanoflowers

Bimetallic nanoparticles composed of two different metal elements have recently attracted significant attention as catalysts owing to their alloy effects or synergistic effects, resulting in enhanced catalytic performance compared to that of their corresponding monometallic counterparts.^321–325

Pd is the most commonly used anode catalyst for formic acid oxidation in direct formic acid fuel cells,^326–328 and according to a size-effect study of the electro-oxidation of formic acid, Pd nanoparticles with optimal binding energy show the highest activity.^329,330

Hybrid catalysts, such as, PdNi,³³¹ PdCo,^332,333 PdCu,³³⁴ and PdIr³³⁵ exhibit enhanced formic acid oxidation compared to single-component catalysts because of their synergistic effects that can manipulate the binding energy of Pd. A similar effect is expected for bimetallic-alloy nanoflowers.

5.3.1. Au-based nanoflowers. Multibranched Au–Pd bimetallic nanoflowers have been synthesized through a seeded reduction method by Ma and co-workers,³³⁶ whereby seed-mediated growth proceeds via two steps: (i) the reduction of a metal precursor to form uniform small seeds; and (ii) the reduction of another metal on the as-prepared seeds. The Au nanospheres are prepared as seeds in Au cores (∼16 nm size),³³⁷ Au and Pd bimetallic nanoparticles (Au₁Pd₁ core, ∼20 nm size),³³⁸ and Au nanostars (∼40 nm size).^337,339 Subsequently, Pd is deposited on these seed nanocrystals by a reduction process, forming Au–Pd nanoflowers with three different morphologies, as shown in Scheme 5. According to a recent study,³³⁷ Au nanostars exhibit better catalytic activity than Au nanocages and nanoantennas, when 4-nitrophenol is reduced to its amino derivative, 4-aminophenol, with sodium borohydride (NaBH₄) at room temperature. The degradation of 4-nitrophenol is a crucial catalytic reaction in the fine-chemical and pharmaceutical industries owing to its convenient monitoring by UV-vis absorption spectroscopy.³⁴⁰

Scheme 5 Schematic illustration of the formation of three morphology-controlled Au–Pd nanoflowers (Au–Pd NFs) by seed-mediated growth. Adapted with permission from ref. 336. Published by MDPI. Copyright@ 2017 by Tao Ma et al.

At a low potential of −0.30 V (vs. Pt) and a low temperature of 30 °C, flower-like AuPt alloy nanoparticles are synthesized by the one-step electrochemical reduction method.³⁴¹ In this method, deep eutectic solvents are used as shape-directing agents and solvents. Through the potentiostatic method, a standard three-electrode cell with a Pt wire counter electrode, a Pt quasi-reference electrode, and a GCE plate (15 mm × 10 mm × 1 mm) working electrode are used for the electrochemical-deposition synthesis of flower-like AuPt alloy nanoparticles in ethaline with 10% water (Fig. 13). Another promising strategy for electro-organic synthesis is mentioned next. The electrode modified with the as-prepared nanomaterials is utilized as the anode in the electrochemical oxidation reaction, using the GCE modified with AuPt nanoflowers, which is directly used to electrooxidize xanthene (XT) to xanthone (XO) with a high yield under a constant low potential (0.80 V vs. Ag/AgCl) at room temperature.

Fig. 13 Schematic illustration of the preparation of AuPt nanoflowers (AuPt NFs) and the electrochemical synthesis of xanthone from xanthene. Adapted with permission from ref. 341 Published by Springer Nature. Copyright © 2018, Aoqi Li et al.

Imura et al. reported the preparation of bimetallic Au–Ag nanoflowers by reducing HAuCl₄ in a solution containing melamine (as the capping agent) and ascorbic acid (as the reducing agent), which are weakly adsorbed on the surface of the metal.³⁴² Bimetallic Au–Ag nanoflowers are obtained with a clean surface using a support (g-Al₂O₃)^38,343–347 and an extracting method (extracting the surface ligands with water).^348,349 A weakly adsorbing capping agent, adequate washing processes to remove capping agents from the metal surface, and a supporting method to inhibit aggregation are vital factors in obtaining surface-clean Au nanoflowers.³⁴⁷ Bimetallic nanoflowers with clean surfaces are applied for the aerobic oxidation of 1-phenylethyl alcohol. Surface-clean Au nanoflowers and spherical Au–Ag nanoparticles of almost the same size and Au/Ag ratio exhibit lower rates for the oxidation of acetophenone than bimetallic nanoflowers.³⁴⁷

5.3.2. Pt-based nanoflowers. Bimetallic alloyed Pt₇₁Co₂₉ lamellar nanoflowers (LNFs) with abundant active sites are fabricated via a one-pot solvothermal method in the presence of CTAC.³⁵⁰ 1-Nitroso-2-naphthol (1-N-2-N) acts as a co-structure directing agent and coordinates with metal precursors to form stable complexes by controlling the size and morphology of nanomaterials,³⁵¹ whereas oleylamine acts as the solvent and reducing agent. A large number of PtCo nanoflowers with an average particle size of 20.4 nm are observed in the TEM image. The medium-magnification TEM images show each crystal assembled by many ultrathin nanoflakes with a flower-like architecture. The Pt₇₁Co₂₉ LNFs exhibit higher mass activity (MA, 128.29 mA mg⁻¹) for oxygen reduction compared to those of home-made Pt₄₈Co₅₂ nanodendrites, Pt₇₉Co₂₁ nanodendrites, and commercial Pt black, with values of 39.46, 49.42, and 22.91 mA mg⁻¹, respectively. The Pt₇₁Co₂₉ LNFs exhibit higher methanol oxidation (MA, 666.23 mA mg⁻¹ and specific activity (SA), 2.51 mA cm⁻²) than Pt₄₈Co₅₂ nanodendrites (MA, 213.91 mA mg⁻¹; SA, 1.99 mA cm⁻²), Pt₇₉Co₂₁ nanodendrites (MA, 210.09 mA mg⁻¹; SA, 1.12 mA cm⁻²), and Pt black (MA, 57.03 mA mg⁻¹; SA, 0.25 mA cm⁻²).

A novel atmospheric microplasma induced liquid chemistry (AMILC) method has been applied for the controlled synthesis of 2D and 3D binary Pt₃Co nanoflowers by Wang et al.³⁵² As shown in the atmospheric microplasma (AMP) setup in Fig. 14, the electrolyte consists of 0.002 M chloroplatinic acid hydrate (H₂PtCl₆·xH₂O), 0.5 M sodium sulfate (Na₂SO₄), and variable molar concentrations of cobalt(II) sulfate heptahydrate (CoSO₄·7H₂O) dissolved in distilled water (40 mL). Bimetallic Pt–Co nanostructures are generated on the cathodic Si substrate, half-inserted in the aqueous electrolyte and separated by at least 3 cm from the anodic capillary.

Fig. 14 Schematic diagram of the atmospheric microplasma-induced liquid chemistry synthesis method. Reprinted with permission from ref. 352. Copyright © 2020 IOP Publishing Ltd.

PtRu alloy nanoflower catalysts are synthesized by controlling their compositions through a solvothermal method and using oleylamine as a coordinating solvent, reductant, and surfactant, through the co-reduction of H₂PtCl₆·6H₂O and RuCl₃·xH₂O, without additional surfactants, reducing, or structure-directing agents.³⁵³ These PtRu nanocatalysts exhibit different morphologies depending on the Pt/Ru ratio. The PtRu nanoflowers composed of nanodendrites exhibit excellent electrocatalytic properties toward methanol electro-oxidation through the normalization of the electrochemically active surface area for direct methanol fuel cells; these properties are derived from the synergetic effect between Pt and Ru and their unique 3D interconnected nanostructures.

Hierarchical nanoporous (hnp) PtCu alloy nanoflowers with different components are fabricated from a PtCuAl precursor alloy by selectively etching Al atoms, with different Pt and Cu ratios, by dissolving the Cu atoms partially.³⁵⁴ In particular, the hnp–Pt₃₅Cu₆₅ sample exhibits superior catalytic activity compared to other PtCu catalysts with different Pt contents. The SEM images (Fig. 15a and b) reveal that each nanoflower consists of a cluster of nanorod-like leaves, approximately 50 nm in width and 100 nm in length. These leaves exhibit uniform interconnected nanopores measuring approximately 5 nm in size, spanning the entire sample. The TEM and HR-TEM images show flower-like structures with bimodal size distributions (Fig. 15c). Pores of various sizes corresponding to the bright region, confirm the formation of hierarchical nanoporous channels, whereas the dark skeleton is attributed to the interconnected nanoscale backbone. The HR-TEM image of hnp–Pt₃₅Cu₆₅ and the ordered lattice fringes are well-resolved in Fig. 15d; the hnp–Pt₃₅Cu₆₅ sample shows good structural stability in the hierarchical nanoporous flower-like architecture. The lattice spacing is determined to be 0.226 nm, indicating that it corresponds to the (1 1 1) crystal plane of the PtCu alloy.

Fig. 15 SEM (a and b), TEM (c), and HR-TEM (d) images of the hnp–Pt₃₅Cu₆₅ alloy. Reprinted with permission from ref. 354. Copyright © 2017 Elsevier Inc.

5.3.3. Pd-based nanoflowers. A novel class of self-supported worm-like PdAg nanoflowers are fabricated with the assistance of CTAB under mild conditions.¹²⁵ Interestingly, the PdAg nanoflowers assemble to form a worm-like structure that can serve as a self-supported material to protect the relative supporters from severe corrosion and oxidation. The as-prepared Pd₁Ag₂ nanoflowers exhibit a narrow size distribution (29.27 ± 3.74 nm), which is much smaller than the average diameters of Pd₁Ag₁ (33.20 ± 4.23 nm) and Pd₁Ag₃ (32.48 ± 4.27 nm). Its homogeneous structure contains approximately 10 petals, exposing more surface-active area and increasing its activity towards ethylene glycol oxidation (EGOR).³⁵⁸ Owing to its high active surface area of 42.36 m² g⁻¹, Pd₁Ag₂ nanoflowers display special electrocatalytic performances toward EGOR with mass and specific activities of 5545 mA mg⁻¹ and 12.82 mA cm⁻², respectively, for direct ethylene glycol fuel cells, with 6.9 and 2.1 times higher activity than that of commercial Pd/C, respectively.

Through a facile wet-chemical method, ultrafine 2D alloyed PdCu nanosheet-constructed 3D flowers are fabricated from a mixture of Na₂PdCl₄ and CuCl₂·2H₂O (metal precursors) in DMF (solvent), acetic acid (reducing agent), and W(CO)₆ (structural regulator).³⁵⁹ As shown in TEM images, the flower-like Pd₂Cu nanoflowers consist of numerous ultrafine nanosheets. The optimal Pd₂Cu nanoflowers with a mass/specific activity value of 4714.1 mA mg⁻¹/13.7 mA cm⁻² show significantly increased activity toward EGOR, with 4.4 and 6.85-fold increases relative to commercial Pd/C catalysts (1066.8 mA mg⁻¹/2.0 mA cm⁻²). Pd₂Cu nanoflowers exhibit high long-term stability and are a novel class of cost-efficient electrocatalysts that are advantageous for fuel-cell reactions.

An Ir-alloyed ultrathin ternary PdIrCu nanosheet-composed flower (NCF) structure has been synthesized through a one-pot solvothermal reduction in the absence of any surfactant.³⁶⁰ The Cu and Ir atoms together modify the electronic structure of Pd during synthesis, forming an unparalleled ultrathin NCF structure. The PdIrCu/C NCF catalyst exhibits significantly enhanced electrocatalytic activity compared to that of commercial Pd/C for formic acid electro-oxidation. Fig. 16 illustrates the preparation procedures and catalytic mechanism of PdCu and PdIrCu NCF for enhanced formic acid electro-oxidation. Initially, the precursor reactants are transformed into 2D NCFs, which are then cross-stacked into 3D NCFs. The enhanced electrocatalytic performance of PdIrCu/C NCF is attributed to the following: (1) a change in the electronic structure of Pd due to Pd, Ir, and Cu; easy formation of Ir–OH species at a lower potential by the unique surface chemistry of Ir, facilitating the oxidation of CO_ad and its removal from the active sites of catalysts, reducing the poisoning effect; (2) an increase in the specific surface area and formation of more available catalytic active sites by the ultrathin-nanosheet structure of PdIrCu/C; and (3) the enhancement of electroconductivity and promotion of the mass transport process in the electrocatalysis reaction by the unique 3D NCF structure.

Fig. 16 Schematic illustration of the synthesis procedures and enhanced electrocatalytic performance mechanisms of the PdIrCu NCF catalyst. Reprinted with permission from ref. 360. Copyright © 2018, American Chemical Society.

5.3.4. Other-alloy nanoflowers. Other-alloy nanoflowers are fabricated using the reduction method, colloidal synthesis, and ultrasonication. The nanostructures and their applications as electrocatalysts for water splitting and fuel cells are summarized in Table 17.

5.3.5. Conclusion. Alloy nanoflowers are a recent research topic, with publications dating back to only 10 years; approximately 20–30 studies have been reported on Au-, Pt-, Pd-, Ni-, Mo-, and Ir-based alloy nanoflowers. The nanoflowers have been synthesized by various reduction methods,^{341,342,354,355,361} the AMILC method,³⁵² solvothermal methods,^{70,350,353,360} and so on.^{115,116,336,357} Alloy nanoflowers primarily exhibit potential applications as electrocatalysts for water splitting and fuel cells (Tables 15–17) due to their alloy and synergistic effects.^321–325 Interestingly, research focuses on the morphologies and ratios of metals in the alloy system, because the latter can affect the former, thereby influencing catalyst performance. For instance, Zhang et al. reported that the formation of highly branched nanoflowers, can be induced by varying the Pt/Co ratio and controlling their size,³⁵⁰ forming Pt₇₁Co₂₉ lamellar nanoflowers exhibiting higher mass activity for oxygen reduction than Pt₄₈Co₅₂ and Pt₇₉Co₂₁ nanodendrites.

Table 15 Synthesis methods and applications of flower-like hierarchical Au-based alloy nanostructures

Compound	Variety of formed nanostructures	Synthesis method	Application	Ref./year
AuPt	AuPd nanoflowers on a polyamidoamine dendrimer-modified surface	Electrodeposition	Electrode materials, electrocatalysts	115/2006
AuAgCu	Hollow Au–Ag–Cu nanoflowers	Combined seed mediated and galvanic replacement method	Electrocatalysts for direct ethylene glycol fuel cells	116/2017

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Table 16 Synthesis methods and applications of flower-like hierarchical Pt based alloy nanostructures

Compound	Variety of formed nanostructures	Synthesis method	Application	Ref./year
PtPd	Porous Pt–Pd alloy nanoflowers	One-pot cochemical reduction method in a poly(allylamine hydrochloride) based aqueous solution	Cathode electrocatalyst in direct alcohol fuel cells	355/2013
Pt3Co	Pt3Co nanoflowers	Solvothermal method without seeds and templates	Electrocatalysts in DMFCs	70/2014
PtAg	Multi-branched AgPt alloyed dendritic nanoflowers	One-pot successive aqueous coreduction strategy with the assistance of 3-aminopyrazine-2-carboxylic acid (Apzc) as a structure-directing agent	SERS	356/2017
PtPdAg	Porous ternary Pt–Pd–Ag alloy nanoflowers	Pd-seed mediated co-reduction	Cathode electrocatalyst in advancing fuel cell technology	357/2017

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Table 17 Synthesis methods and applications of flower-like hierarchical other-alloy nanostructures

Compound	Variety of formed nanostructures	Synthesis method	Application	Ref./year
NiCo	NiCo nanoflowers	An amphiphilic polymer(Jeffamine)-assisted homogeneous polyol reduction method	Magnetorheological fluids	361/2014
MoWSe	Mo1−xWxSe2 nanoflowers	Colloidal synthesis	Electrocatalysts for the hydrogen evolution reaction in both acidic and alkaline aqueous solutions	71/2017
IrCo	IrCo nanoflowers	Ultrasonication with IrCl3·xH2O, Co(acac)2 or Ni(acac) and oleylamine, heated in a hot oil bath	Proton exchange membrane water electrolyzers	35/2019
IrNi	IrNi nanoflowers

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5.4. Other nanoflowers

5.4.1. Silica nanoflowers. Porous silica (SiO₂) nanostructures have been intensely studied because of their unique features: high surface area, precise pore structure, tunable pore size, stability, high permeability, and biocompatibility.^362–369 However, research on the synthesis of silica nanomaterials with well-defined morphology and controllable size still has a long way to go. In particular, silica nanoflowers have been rarely reported.

Hierarchically structured spherical mesoporous silica nanoflowers (HSMNFs) with well-defined morphologies and uniform sizes are synthesized hydrothermally in a mixture of cyclohexane and water using CPB as the template. TEOS and urea are used as the source of inorganic silica and the hydrolysis additive, respectively.¹²⁰ The flower, with 200–500 nm size, approximately 10 nm petal thickness, contained 4.0 nm and ca. 40 nm mesopores and macropores, respectively, with up to 502 m² g⁻¹ surface area. The nanoflower size, structure, and petal thickness strongly depend on the hydrothermal temperature and the molar ratio of Si to CPB; the optimal conditions are 120 °C (hydrothermal temperature) and 4.37 (molar ratio of Si to CPB). This material could be used for catalysis, adsorption, and controlled drug release in medicine because of its hierarchical porous structure, nanoflower morphology, and large surface area.

Dandelion flower-like silica nanoflowers with a uniform size, containing an accessible large surface area, have been synthesized through a visible-light-driven green chemistry method.¹²⁶ CTAB and urea are dissolved in water, while TEOS is added to a mixture of methyl phenyl ether and hexanol. Subsequently, both the solutions are mixed in a vial by stirring, followed by stirring under visible light (tungsten bulb) for 20 h at 90 °C. After cooling the mixed solution to room temperature, the products are collected and purified by filtration and washing, followed by air-drying and calcination (at 575 °C for 5 h under atmospheric conditions) to remove the structure-directing agent, CTAB. The TEM images indicate that the synthesized silica nanoflowers have a relatively narrow size distribution (with a diameter of 385 ± 5 nm), and the edges of the nanoflower are composed of elongated spikes oriented outwards in three dimensions (Fig. 17D–F).

Fig. 17 (A–C) FE-SEM micrographs of SiO₂ nanomaterials at different magnifications, representing their hierarchical dandelion flower-like morphology. Photograph of a dandelion flower is presented in the inset of Fig. (B). TEM images of (D and E) a bunch of SNF and (F) a single SiO₂ nanoflower containing elongated spikes which assemble in a divergent way to form dandelion-like flowery morphology. Reprinted with permission from ref. 126. Copyright © 2019 Elsevier B.V. All rights reserved.

The SiO₂ nanoparticles are synthesized by modulating the chain length of alcohols from propanol to octanol, while maintaining the other reaction conditions, to investigate the effect of co-solvents on nanoflower morphology.

The TEM analysis indicates that the morphologies of SiO₂ nanoparticles are affected by the alcohol chain length. A noticeable flowery architecture encompassing elongated spikes is formed in higher chain-length alcohol co-solvents, which becomes persistent for hexanol to octanol. Additionally, the specific surface area gradually increases with increasing chain length of the alcohols (propanol to octanol), remaining almost constant beyond hexanol (Table 5). Amine-functionalized SiO₂ nanoflowers have been synthesized using (3-aminopropyl)triethoxysilane (SNF-1), N-[3-(trimethoxysilyl) propyl]ethylenediamine (SNF-2), and N1-(3-trimethoxysilyl propyl)diethylenetriamine (SNF-3), respectively, for use as a CO₂ adsorbent. Furthermore, chromogenic-functionalized SiO₂ nanoflower composites have been fabricated from amine-functionalized nanoflowers for the detection of CO₂.

5.4.2. Metal–metal oxide nanoflowers. Au/ZnO hybrid nanoflowers are fabricated by an easy, eco-friendly, and low-cost synthetic method employing A. indica (neem) leaf extract as a reducing and capping agent.⁷⁹ Some biomolecules in A. indica leaves (such as ascorbate, a natural reducing agent) reduce Au³⁺ ions to Au⁰, attracting the negatively charged zincate ions to the active surface of Au⁰. They deposit on the Au surface and grow as multiple ZnO petals via the thermal decomposition of sodium zincate [Na₂Zn(OH)₄] (Scheme 6).

Scheme 6 Schematic chart of the fabrication procedure of Au/ZnO hybrid nanoflowers using A. indica (neem) leaf extract. Reprinted with permission from ref. 79. Copyright © 2021, American Chemical Society.

The BET surface area of catalysts toward ZnO, Au, and Au/ZnO hybrid nanoflowers have been measured; the BET surface area of Au/ZnO nanoflowers is 19.23 m² g⁻¹ for the reduction of 4-nitrophenol to 4-aminophenol, which is wider than that of the others (Table 18). Additionally, the activation energy of the Au/ZnO hybrid nanoflowers (32.46 ± 1.62 kJ mol⁻¹) is lower than that of ZnO (57.75 ± 2.85 kJ mol⁻¹) and Au (48.40 ± 2.41 kJ mol⁻¹). Thus, Au/ZnO hybrid nanoflowers exhibit higher catalytic activity for nitro reduction compared to ZnO and Au. Other metal–metal oxides have been developed as catalysts, electrode materials, and theranostic agents (Table 19).

Table 18 Summary of the activation energy (E_a), pre-exponential factors (A), entropy of activation (ΔS), and surface area from the BET study for ZnO, Au, and Au/ZnO nanoflowers for the reduction of 4-nitrophenol to 4-aminophenol. Reprinted with permission from ref. 79. Copyright © 2021, American Chemical Society

System	E a (kJ mol^−1)	A (min^−1)	ΔS (J mol^−1 K^−1)	Surface area (m^2 g^−1)
ZnO	57.75 ± 2.85	2.36–2.54 (×10^8)	160.76 ± 4.82	7.79
Au	48.40 ± 2.41	1.39–1.62 (×10^7)	137.2 ± 4.11	8.64
Au/ZnO nanoflowers	32.46 ± 1.62	1.15–1.74 (×10^5)	98.5 ± 2.94	19.23

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Table 19 Synthesis methods and applications of flower-like other metal–metal oxide nanostructures

Compound	Structure	Synthesis method	Application	Ref./year
FeTiO3	Ilmenite FeTiO3 nanoflowers	High-energy ball-milling of ilmenite and subsequent mild hydrothermal treatment in 1 M NaOH aqueous solution	Electrode materials for supercapacitors	60/2011
FeMnO	Fe0.6 Mn0.4O nanoflowers	High-temperature thermal–decomposition reaction	Theranostic agent with T1–T2 dual-mode MRI for diagnostic purposes, and combined with magnetic hyperthermia for effective therapeutics	370/2016
NiCo2O4	NiCo2O4 nanoflowers	Microwave synthesis	Catalysts for the electro-oxidation reaction of methanol in DMFCs	73/2019

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5.4.3. Nanoflowers with core–shell structure. Different physical and chemical procedures have been used to fabricate core–shell nanoflower materials, including various modified hydrothermal/solvothermal processes, and templating methods, as shown in Tables 20 and 21. Most research has focused on the influence of constituents and active sites of the surface shell of nanoflowers and core–shell nanostructural features on the electrochemical, catalytic, and optical properties, to utilize them in gas sensors, catalysts, supercapacitors, lithium-ion batteries, biosensors, antibacterial agents, and for water splitting (Tables 20 and 21). This section introduces the synthetic strategies, morphologies, physical and chemical properties, and applications of core–shell nanoflowers.

Table 20 Synthesis methods and applications of flower-like Au core nanostructures

Core	Shell	Structure	Synthesis method	Application	Ref./year
Au	Pd	Nanoflowers with an Au core and Pd petals	Seed-mediated growth method	Catalysts for Suzuki coupling reaction	371/2011
		Au@Pd core–shell mesoporous nanoflowers	Seed-mediated growth method	Catalysts of methanol oxidation reaction for DMFCs	372/2021
AuPd	Pd	Au–Pd@Pd core–shell nanoflowers	Polyallylamine-assisted water-based synthesis	Electrocatalyst for ethanol electro-oxidation	373/2015
Au	AgAu	Au@AgAu alloy nanoflowers	Deposition of HAuCl4 on the surface of Au@Ag core–shell nanoparticles	Antibacterial agents	375/2018
Au	Rh	Au@Rh core–shell nanoflowers	Seeded growth	Electrocatalysts for the hydrogen evolution reaction	376/2021

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Table 21 Synthesis methods and applications of flower-like other core–shell nanostructures

Core	Shell	Structure	Synthesis method	Application	Ref./year
Pd	Pt	Pd-core Pt-shell nanoflowers supported on carbon	Coreduction method	Catalysts for the oxygen reduction reaction of fuel cells	377/2018
		Pt deposited on Pd nanoflowers	Coreduction of Pd and Pt precursors with N,N-dimethylformamide and CTAC as the structure directing agent	Catalysts for methanol oxidation reaction of fuel cells	378/2020
Ag	NiO	NiO layer deposited on Ag nanoflower arrays on a Cu substrate	One-step solution-immersion process and subsequent RF-sputtering method	Anode materials for lithium-ion batteries	82/2015
Fe3O4	MnO2	Fe3O4@MnO2 core–shell nanoflowers	Solvothermal method	Colorimetric detection of phenol in wastewater	379/2015
MnCo2O4	Ni(OH)2	MnCo2O4@Ni(OH)2 multicomponent core–shell nanoflowers on Ni foam	Stepwise hydrothermal process	Supercapacitor electrodes	380/2016
Co3O4	NiCo2S4	Ultrathin NiCo2S4 shell on porous Co3O4 nanoflower array on a Ni-foam substrate	Stepwise hydrothermal process	Supercapacitors	381/2017
NiCo2O4	MnO2	MnO2@NiCo2O4 core–shell nanoflowers on Ni-foam	Hydrothermal process and calcination	Supercapacitors	382/2021

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5.4.3.1. Au-core nanoflowers. As mentioned in Section 5.3.1, Au-based Nanoflowers, Au@AuPd (Au–Pd NFs with Au cores, shown in Scheme 5), and Au@Pd (Au@Pd NFs) core–shell nanoflowers are fabricated by a seed-mediated reduction method, whereby Au nanoparticles as seeds induce the growth of a layer of Au and Pd petals and Pd nanoparticles decorated Au nanoflowers as seeds, respectively (Scheme 5).³³⁶ The catalytic activities for 4-nitrophenol reduction are in the order of Au@Pd core–shell nanoflowers (Au@Pd NFs) > Au–Pd alloy nanoflowers (Au–Pd NFs (Au₁Pd₁ core)) > Au@AuPd (Au–Pd NFs (Au core)) core–shell nanoflowers. Thus, the content and reactive surfaces of Pd in the alloys affect their catalytic activities.³³⁶

Xu et al. reported the synthesis of Au@Pd core–shell nanoflowers through the reduction of Pd ions by hydroquinone on gold seeds.³⁷¹ Yin et al. synthesized Au@Pd core–shell mesoporous nanoflowers with Au nanoparticles coated with mesoporous Pd petals, using poly(styrene)-b-poly(ethylene oxide) (PS-b-PEO) as a polymeric micelle-assembled template to induce mesopores in Pd petals by the seed-growth method.³⁷² A facile one-pot synthesis of Au–Pd@Pd core–shell nanoflowers has also been performed in an aqueous polyallylamine solution.³⁷³

The Au@CdSe NFs are synthesized via one-pot heterogeneous nucleation and growth. Au nanoparticles are produced by injecting an HAuCl₄ solution into an alkyl amine (oleylamine or hexadecylamine) at room temperature, followed by increasing the temperature gradually to 150 °C or 300 °C,¹²⁸ adding trioctylphosphine oxide to the reaction mixture, and injecting CdO in oleic acid and Se in trioctylphosphine multiple times under stirring. As shown in Scheme 1, the CdSe clusters are adsorbed on the surface defects of the Au core and grow as multiple arms and branches after the injection. Using hexadecylamine instead of oleylamine or increasing the reaction temperature to 300 °C forms larger Au cores, improving the interface between the Au and CdSe domains to produce a core–shell structure. The Ag@CdSe nanoflowers are also prepared using the same synthetic strategy.

The Au@AgPd core–shell nanoflowers are synthesized by a seed-mediated reduction method. First, Au seeds with a narrow size distribution of 13.5 ± 1.0 nm are synthesized by the citrate reduction of HAuCl₄·4H₂O. Subsequently, AgNO₃ and H₂PdCl₄ are co-reduced by L-ascorbic acid to grow on the surface of the Au seeds using CTAB, which assists the AgPd to bloom on the Au seed.³⁷⁴ The size distribution of Au@AgPd nanoflowers is narrow, with a diameter of 38.4 ± 2.1 nm, branch length of 10.5 ± 1.3 nm, and width of 5.4 ± 0.7 nm. According to inductively coupled plasma mass spectrometry (ICP-MS) analysis, the weight percentages of Au, Ag, and Pd in the Au@AgPd nanoflowers are 14.2%, 40.6%, and 45.3%, respectively, consistent with the ratio of Au/Ag/Pd in the precursors. Compact branches are rarely observed on large Au cores, especially over 60 nm, whereas tiny Au seeds exhibit dense branches on their surface. The Au@AgPd nanoflowers could be applied for the in situ SERS monitoring of the catalytic reaction of 4-nitrothiophenol to 4-aminothiophenol.

5.4.3.2. ZnO-core nanoflowers. Harito et al. reported PbO₂-decorated ZnO@TiO₂ core–shell nanoflowers. First, a bicarbonate electrolyte produces flower shaped ZnO nanowires on the Zn plate surface by an electrochemical anodizing process³¹⁷ performed at a cell voltage of 5 V in 30 mM sodium bicarbonate, to produce the optimized uniform ZnO nanoflowers, according to FE-SEM analysis. Second, a layer of TiO₂ is coated on the surface of the ZnO nanoflowers via dip coating and calcination to enhance its properties for the photocatalytic degradation of organic dyes. Third, PbO₂ is deposited over the ZnO@TiO₂ core–shell nanoflowers using anodic electrodeposition, which is applied for the electrochemical oxidation of the dye. These nanoflowers can be used as catalysts for wastewater treatment.
5.4.3.3. Other core–shell nanoflowers. Other core–shell nanoflowers composed of Pd Ag, Fe, Mn, Ni, and Co have been reported for applications in catalysts, anode materials, sensors, and supercapacitors, as shown in Table 21.
5.4.3.4. Conclusion. Core–shell nanoflowers have been investigated in the past decade, with a small number of published papers. The introduced core–shell nanostructures have been fabricated using various modified hydrothermal or solvothermal processes, solution phase synthetic methods, and so on (see Tables 20 and 21). The components of the reported core–shell nanoflowers become increasingly complex from the simple bimetallic Au@Pd^371,372 and Pd@Pt^377,378 core–shell nanoflowers to the NiCo₂O₄@MnO₂ core–shell nanoflowers.³⁷⁴ The composition ratio of the core–shell nanoflowers is one of the leading factors that affect the shape of the nanomaterial and its catalytic performance. For example, closely spaced and uniformly branched Au@AgPd core–shell nanoflowers are synthesized when the ratio of Pd/Ag is 1 : 1, exhibiting high catalytic activity owing to its small size, highly dispersed alloy shell, and defect abundance.³⁷⁴ The size of Au@Rh nanoflowers increases with an increase in the amount of RhCl₃ added as the precursor. The Au₆₈Rh₃₂ nanoflowers with high Rh content exhibit high performance in the hydrogen evolution and plasmon-enhanced hydrogen evolution reactions.³⁷⁶

5.4.4. Doped nanoflowers. Yang et al. synthesized nickel-doped walnut-like MnO₂ nanoflowers using a template-less and surfactant-less hydrothermal method.³⁸³ A strong Ni peak is observed in energy-dispersive spectroscopy (EDS) analysis, indicating the doping of Ni cations in the prepared nanoflowers. According to SEM images, the Ni-doped MnO₂ nanoflowers are composed of many interleaving thin nanosheets, and the diameter of the nanoflowers is approximately 5 μm. Their cyclic voltammograms, galvanostatic charge/discharge, and electrochemical impedance spectroscopy experiments indicate high specific capacitance (153.6 F g⁻¹ at 1 A g⁻¹) and outstanding cycling stability (a capacitance retention of 103% after 1000 cycles). These properties are primarily attributed to their hierarchical walnut-like architecture. The Ni-doped MnO₂ nanomaterials have potential applications in supercapacitors owing to their excellent electrochemical performances (Table 22).

Table 22 Synthesis methods and applications of doped flower-like metal or metal-oxide-based nanostructures

Core	Doping	Structure	Synthesis method	Application	Ref./year
ZnO	Pt	Pt-doped on ZnO nanoflowers	One-step hydrothermal method	Solar cells and gas sensors	319/2010
	Ag	Ag-doped ZnO nanoflowers on the substrate	Low-temperature hydrothermal process	NO gas sensors	318/2018
SnO2	Zn	Zn-doped SnO2 nanoflowers	Hydrothermal method	Photoanode materials for dye-sensitized solar cells	384/2011
TiO2	Ni	Ni-doped rutile TiO2 nanoflowers	Acid treatment of the precursor hydrogen titanate nanowires and immersing the nanowires in an aqueous H2SO4 solution containing NiSO4	Photocatalysts for wastewater treatment	37/2016
CeO2	Fe	Porous Fex-doped Ce1−xO2 with flower-like morphology	Citric acid-assisted hydrothermal process	Catalysts for H2S-selective oxidation	385/2020
Mo2C	N	N-doped porous molybdenum carbide nanoflowers	Templating method and calcination using Pluronic F127, dopamine hydrochloride, and ammonium molybdate	Acetylcholinesterase (AChE) biosensors for detecting organophosphorus pesticides	386/2021

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5.4.5. Coated nanoflowers. Liu and colleagues have fabricated ZnO@C nanoflowers via a facile, fast, and low-cost synthetic route.³²⁰ In the approach, Zn(NO₃)₂·6H₂O and C₆H₅Na₃O₇·2H₂O are mixed in an aqueous solution with controlled pH, followed by hydrothermal processing at 100 °C, assembling ZnO nanosheets into flower-shaped ZnO nanoparticles. Dopamine hydrochlorides are added to the suspension of ZnO nanoflowers and Tris-buffer (pH = 8) under magnetic stirring to form polydopamine (PDA)-coated ZnO nanoflowers (Scheme 7). The coated nanoflowers are carbonized under an Ar atmosphere to obtain C-layer-encapsulated ZnO nanoflowers. The SEM and TEM images (Fig. 18) indicate ZnO@C nanoflowers with diameters in the range of 2–6 μm, nanosheet petals with ∼27 nm thickness, and a carbon layer with ∼46 nm thickness. The ZnO@C nanoflower electrodes in lithium-ion batteries show a high capacity (approximately 1200 mA h g⁻¹ at 0.1 A g⁻¹ after 80 cycles), good cycling stability (638 and 420 mA h g⁻¹ at 1 and 5 A g⁻¹, respectively, after 500 cycles), and excellent rate capability because of their micro–nano secondary architecture and chemical interactions between ZnO and the C layer (Table 23).

Scheme 7 Schematic illustration for the synthesis of ZnO@C nanoflowers. Reprinted with permission from ref. 320. Copyright © 2018 Elsevier B.V. All rights reserved.

Fig. 18 SEM images at different magnifications: (a) low, (b) medium, and (c) high magnification images of pure ZnO nanoflowers; (d) low, (e) medium, and (f) high magnification images of ZnO@C nanoflowers. (g and h) TEM images and (i) HR-TEM image of ZnO@C nanoflowers. Reprinted with permission from ref. 320. Copyright © 2018 Elsevier B.V. All rights reserved.

Table 23 Synthesis methods and applications of coated flower-like metal or metal oxide nanostructures

Core	Coating	Structure	Synthesis method	Application	Ref./year
Au	SiO2	Silica-coated Au nanoflowers on alumina	(1) Melamine assisted surfactant-free synthesis^134	—	38/2017
			(2) Addition of α-Al2O3 (ref. 38 and 347)
			(3) Silica coating using TEOS^387,388
Fe3O4	C	Fe3O4 nanoflowers coated by a black C layer	One-pot solvothermal route	Biosensors	39/2020

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5.4.6. Nitride, sulfide, phosphide, selenide, and telluride nanoflowers. Nitride, sulfide, phosphide, selenide, and telluride nanoflowers combined with metals (Ti, Bi, Cd, V, Mo, and Zn) and bimetals (MnIn, NiCo, FeCo, and CuCo) are summarized in Table 24. Zn, Nd, Mo, Ru, and Co-doped or Au nanoparticles decorated with sulfide and selenide nanostructures are summarized in Tables 25 and 26, respectively.

Table 24 Synthesis methods and applications of flower-like nitride, sulfide, phosphide, selenide, and telluride nanostructures

Compound	Structure	Synthesis method	Application	Ref./year
TiN	TiN nanoflower buds	Ammonialization of hydrothermally prepared titanium dioxide nanobuds	Pt-free counter electrodes for DSSC	40/2021
Bi2S3	Bi2S3 nanoflowers on Si substrates	Vapor deposition process	Nanoscale photodetectors and optical switches	60/2008
CdS	CdS and CdSe nanoflowers, branched nanowires, and nanotrees	Solvothermal method using Cd(Ac)2·2H2O and CS(NH2)2 or Na2SeO3 in a mixed solution made of diethylenetriamine and deionized water	Branched CdS nanowires can be applied as photocatalysts for the degradation of acid fuchsin	57/2006
CdSe
VS2	1T-VS2 nanoflowers	Hydrothermal method using Na3VO4·12H2O and CH3CSNH2	Electrocatalysts for the hydrogen evolution reaction	58/2017
MoS2	MoS2 nanoflowers	Green hydrothermal synthesis from ammonium molybdenum hydrate and thiourea without using any acids or bases	Photocatalysts for the degradation of methylene blue and crystal violet dyes under natural sunlight	389/2018
	MoS2 nanoflowers	Hydrothermal method	Phase-change materials for thermal energy storage	390/2021
MnInS	Mn0.4In1.6S3 nanoflower solid solutions	Hydrothermal method	Photocatalysts	41/2019
NiCoP	Ni0.7Co0.3P nanoflowers	Hydrothermal–phosphorization method	Electrocatalysts for water splitting in alkaline media	42/2019
NiCoP	NiCoP nanoflowers or FeCoP nanoflowers on Ni foam	Hydrothermal and low-temperature phosphating method	Supercapacitors	391/2021
FeCoP
CuCoP	Hydrangea-like CuCoPx nanoflowers	Solvothermal and phosphating method	Electrocatalysts for the methanol oxidation reaction	392/2021
ZnSe	Particle- and flower-shaped ZnSe nanocrystals	Green chemical approach	—	393/2008
	ZnSe rose-like nanoflowers or microspheres on Zn foil	Hydrothermal method	—	394/2014
MoSe2	Colloidal MoSe2 nanoflowers	Colloidal synthesis	—	88/2015
	MoSe2 nanoflowers on carbon cloth	Chemical vapor deposition	Electrocatalysts	80/2018
Bi2Se3	Hierarchical Bi2Se3 nanoflowers	Hot-solution injection method at relatively low temperature (130 °C)	Photocatalysts in the presence of H2O2	43/2018
ZnTe	ZnTe nanoflowers, nanodots, and nanorods	High temperature-injection method using zinc stearate as precursors and trioctylphosphine or tributylphosphine as activating agents	—	44/2007

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Table 25 Synthesis methods and applications of doped flower-like sulfide and selenide nanostructures

Core	Doping	Structure	Synthesis method	Application	Ref./year
CuS	Zn	Zn-doped CuS nanoflowers	Ethanol solvothermal process	Photoelectric and photocatalytic applications	101/2016
Bi2Se3	Nd	Nd^III doped Bi2Se3 nanoflowers	Hydrothermal synthesis	—	395/2009
	Mo	Mo-doped Bi2Se3 nanoflowers	Solvothermal method	Absorbents for the removal of organic dyes from wastewaters	102/2013
MoSe2	Ru	Ru-doped MoSe2 nanoflowers	Colloidal process	Catalysts of the hydrogen evolution reaction	89/2019
	Co	Homogeneously Co-doped and Co-edge doped MoSe2 nanoflowers	Colloidal synthesis	Catalysts for the hydrogen evolution reaction at the cathode of an electrochemical water splitting cell	90/2020
			(1) Addition of the dopant atoms to the mixture at the beginning of the reaction
			(2) Hot injection method

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Table 26 Synthesis methods and applications of nanoparticle-decorated flower-like sulfide and selenide nanostructures

Core	Surface	Structure	Synthesis method	Application	Ref./year
MoS2	Au	MoS2 nanoflowers decorated with Au nanoparticles	Hydrothermal process	Gas sensors	61/2021
Bi2Se3	Au	Au nanoparticles decorated Bi2Se3 nanoflowers	Colloidal synthesis method	Catalyst for the hydrogen evolution reaction	91/2019

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6. Applications of nanoflowers

Several studies on the properties of nanoflowers have demonstrated promising potential in various applications as catalysts, sensors, supercapacitors, wastewater treatments, chemical reactions, and green energy sources.

6.1. Nanoflowers for catalysis

Nanoflowers with highly branched structures exhibit a reasonably large specific surface area and high specific activity (i.e., activity per unit surface area) because of the high densities of the edges, corners, and stepped atoms on their branches.¹³⁹ Thus, the study of catalysis by nanoflowers is of special interest.

6.1.1. Chemical catalysts. Au nanoflowers are effective catalysts for the reduction of nitroaromatic pesticides, including pendimethalin, trifluralin, and p-nitrophenol, in the presence of NaBH₄. They show high efficiency and fast reaction.¹³⁹ Additionally, the morphologies of the Au nanomaterials strongly affect their catalytic activity. According to catalytic studies of Au nanoflowers, nanostars and nano-snowflakes show high catalytic activity toward the reduction of p-nitrophenol in aqueous solutions. The catalyst capacity varies according to the shape, in the following order: Au nanoflowers < Au nanostars < Au nanoflakes; Au nanoflakes, with sharp tips, have the highest reducing power due to the availability of more surface atoms for catalysis.¹³⁶

The catalytic activities of Au–Pd nanoflowers for the reduction of 4-nitrophenol exhibit the following order: Au@Pd nanoflowers (AuPd alloy) > Au–Pd nanoflowers (Au₁Pd₁ core) > Au–Pd nanoflowers (Au core). This could be attributed to the different contents and exposed reactive surfaces of Pd in the alloys.³³⁶

Bimetallic Au–Ag nanoflower catalysts with a clean surface exhibit higher reaction rates for acetophenone formation than those with Au nanoflowers and spherical nanoparticles of almost the same size and Au/Ag ratio.³⁴² Additionally, the catalytic activities of flower-like NiO for the oxidation of CO²⁹² and Au@Pd core–shell nanoflowers for the Suzuki coupling reaction³⁷¹ have been studied.

Zhou et al. have reported on the utilization of nanoporous Pt–Cu alloy nanoflower catalysts for the hydrolytic dehydrogenation of NH₃BH₃, a promising hydrogen storage material.³⁵⁴ The as-prepared hnp–PtCu catalysts exhibit better catalytic performance than pure Pt and Cu catalysts. Also, the hnp–Pt₃₅Cu₆₅ sample, characterized by its stable hierarchical nanoporous flower-like architecture, exhibits superior catalytic performance compared to other PtCu catalysts with different Pt contents. This confirms the significant influence of the Pt-to-Cu ratio on catalytic activity. Table 27 provides the turnover frequency (TOF) values and activation energy (E_a) of all catalysts in the hydrolytic dehydrogenation of NH₃BH₃. The values for the Pt₃₅Cu₆₅ sample are comparable to or higher than those reported for most Pt-based or other noble metal-based catalysts.

Table 27 Catalytic activities and the E_a of catalysts in the hydrolysis of NH₃BH₃. Reprinted with permission from ref. 354. Copyright © 2017 Elsevier Inc^a

Catalyst	TOF (mol H2 molM^−1 min^−1) M = Pt, Ru and Ag	E a (kJ mol^−1)	Ref.
a PVP: poly(N-vinyl-2-pyrrolidone), PAMAM:multifunctional poly(amidoamine), RGO: reduced graphene oxide.
RuCu nanoparticles/graphene	135	30.39	396
Pd–Pt@PVP nanoparticles	125	51.7	397
hnp–Pt35Cu65 nanoflowers	108	40.5	354
Ag0.3Co0.7 nanoparticles/PAMAM	52.8	35.66	398
Pd–Pt (1 : 1) nanoparticles	51.9	—	399
PdPt cubic nanoparticles	50.02	21.8	400
Pt0.65Ni0.35 nanoparticles	44.3	39.0	401
PtPd spherical nanoparticles	22.51	57.3	400
RuCu (1 : 1)/γ-A12O3	16.4	52	402
Cu nanoparticles/RGO	3.61	38.2	403

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6.1.2. Electrocatalysts. Fuel cells are considered to be an efficient green energy source.⁴⁰⁴ Their performance mainly relies on the electrocatalysis of the oxygen reduction reaction and fuel electro-oxidation (e.g., methanol or ethanol oxidation reactions) by the cathode and anode.⁴⁰⁵ The electrocatalytic activity and stability of Pt–Pd alloy nanoflowers for the ORR have been investigated by rotating disk electrode voltammetry, whereby the nanoflowers exhibit superior ORR activity, long-term durability, and methanol tolerance in acidic media due to their unique porous structure and alloy properties.³⁶⁸ Additionally, the electrocatalytic mass-activity, specific activity, and durability of AuPd@Pd core–shell nanoflowers toward the ethanol oxidation reaction are significantly higher than that of commercial Pd black.³⁷³

Mesoporous NiO nanoflowers consisting of crossed nanosheets, are synthesized using Pluronic F127 as a micellar template (synthesis details can be found in Scheme 4).²⁸⁶ The process of assembling the interface is controlled by adjusting the amount of NH₃·H₂O, which led to the ability to modify the thickness and dimensions of the mesoporous NiO nanosheets. These nanoflowers are utilized for electrochemical nitrogen reduction reaction (NRR), a sustainable method for NH₃ synthesis that offers a viable alternative to the industrial energy and Haber–Bosch process under ambient conditions.^406,407 NRR measurements demonstrate that NiO–NF-3.0, having the smallest mesopores and the thinnest nanosheets in all NiO–NF samples, exhibits excellent NRR performance, with an NH₃ yield of 16.16 μg h⁻¹ mg⁻¹_cat. and faradaic efficiency (FE) of 9.17% at −0.4 V vs. reversible hydrogen electrode in 0.1 M Na₂SO₄. X-ray photoelectron spectroscopy (XPS) confirms the presence of Ni²⁺ and Ni³⁺ in all NiO–NF samples, with NiO–NF-3.0 displaying the highest Ni³⁺ content, correlating with its superior electrochemical NRR performance. This can be attributed to the electropositivity promotion of NiO-NFs by Ni³⁺, facilitating the adsorption of N₂ gas and enhancing the electrocatalytic properties. The NRR performances of the five different NiO–NF electrodes follow the order: NiO–NF-3.0 > NiO–NF-5.0 > NiO–NF-7.0 > NiO–NF-1.0 > NiO–NF-0.5. Interestingly, the abundance of Ni³⁺ in NiO-NFs also follows the same order, indicating that the activity in the NRR is influenced by the abundance of Ni³⁺, enabling easier nitrogen adsorption compared to Ni²⁺. These improved NRR performances are comparable to or even surpass those of reported catalysts containing noble metals, transition metal oxides, and NiO composite materials. Table 28 provides a summary of NRR performances for various recently reported catalysts, and the prepared NiO–NF-3.0 demonstrates comparable or superior performance to most of these catalysts.

Table 28 Performance comparison of the other NiO electrodes for electrocatalytic ammonia synthesis. Adapted with permission from ref. 286. Copyright © the Royal Society of Chemistry 2021

Catalyst	Electrolyte	Yield (μg h^−1 mg^−1cat.)	FE (%)	Ref.
Pd nanoparticles	0.1 M PBS	4.5	8.2%	408
Pd–Ag–S nanosponges	0.1 M Na2SO4	9.73	18.41%	409
Mn3O4 nanocubes	0.1 M Na2SO4	11.6	3%	410
Fe2O3 nanorods	0.1 M Na2SO4	15.9	0.94%	411
NiO nanodots on graphene	0.1 M Na2SO4	18.6	7.8%	412
N-C@NiO nanosheets on graphite paper	0.1 M HCl	14.022	30.43%	413
NiO nanoflowers	0.1 M Na2SO4	16.16	9.17%	286

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6.1.3. Photocatalysts. Synthetic dyes are widely used in the industry to dye fabrics, food, leather, plastics, and cosmetic products. It is estimated that 2–15% of the dye produced is eventually discharged into streams and rivers as wastewater.²²⁰ Water pollution is a serious issue all over the world, especially in developing countries. To mitigate this problem, several researchers have recently synthesized photocatalysts with different types of nanomaterial structures to decompose organic pollutants in water bodies.

The photocatalytic activities of TiO₂ nanoflowers (TNF-T-t, T = calcination temperature, t = time of calcination) for the photo-oxidation of aqueous methylene blue under UV irradiation have been evaluated. The TNF-T-t (T = 400–800 °C, t = 1–5 h) photocatalysts show remarkable activity for the photo-oxidation at pH 6 under UV excitation (365 nm), with activities in the following order: TNF-700-1 > TNF-600-1 > TNF-500-1 > TNF-400-1∼P25 TiO₂ (the commercially available photocatalyst) TNF-800-1. Excessive rutile formation by prolonged calcination of the TNFs at 700 °C is detrimental to their dye-degradation performance, as it reduces the photocatalyst surface area and inhibits OH˙ formation.²²⁰ The MoS₂ nanoflowers exhibit almost 100% degradation of methylene blue and crystal violet under natural sun light.⁴¹⁵

Three types of fluffy ZnO nanoflowers with different nanostructures: one with smooth edges (ZnO-0 generated via a hydrothermal process without ultrasonic treatment) and two with different jagged margins (ZnO-250 and ZnO-950 produced via ultrasonic treatment at 250 W and 950 W, respectively), have been evaluated as photocatalysts for the degradation of methyl orange. ZnO-0 exhibits the highest photocatalytic activity with a kinetic constant of 0.0478 min⁻¹.³²⁴ The photocatalytic behavior of Bi₂Se₃ nanoflowers has been explored by degrading rhodamine B in the presence of H₂O₂, whereby the degradation rate of rhodamine B increases up to 93% after 2 h of visible-light irradiation.⁴³ The Ni-free rutile TiO₂ nanoflowers and Ni-doped rutile TiO₂ nanoflowers exhibit two and four times the photocatalytic activity of Degussa P25 TiO₂ powder, respectively, for the photodegradation of rhodamine B in water under Xe lamp illumination. This could be attributed to their higher specific surface areas (115 and 118 m² g⁻¹ for Ni-free rutile TiO₂ nanoflowers and Ni-doped rutile TiO₂ nanoflowers, respectively) than Degussa P25 TiO₂ powder (50 m² g⁻¹), and Ni-doping that favors light harvesting and charge separation.³⁷

Plasmonic Pd nanoflowers exhibit high photocatalytic activity for the Suzuki–Miyaura cross-coupling reactions under visible light irradiation.¹⁵⁶ Hierarchical flower-like TiO₂ superstructures show significantly higher photocatalytic activity than commercial Degussa P25 and tabular-shaped anatase TiO₂, due to a hierarchically porous structure, exposed {001} facets, and increased light harvesting ability.²¹⁸

6.2. Sensors

Studies on the detection of industrially produced volatile organic compounds and toxic gases have attracted considerable attention because they cause environmental pollution, which seriously affects human safety. Semiconducting nanomaterials with large surface-area-to-volume ratios are potential candidates for gas-sensing applications.

A CuO nanoflower-based gas-sensing device has been reported to exhibit a very strong response for acetone vapor (250–2250 ppm concentration). The stability of the sensing device is maintained with a very small change (3.3% change in response) on repeating measurements for one month.²⁷⁵ A hierarchical 3D TiO₂ nanoflower sensor exhibits high selectivity toward acetone at 60 °C with response magnitudes of 3.45% and 66.58% corresponding to response times of 33 s and 19 s and recovery times of 11 s and 37 s for 1 ppm and 700 ppm acetone, respectively.⁹² Additionally, N-doped porous Mo₂C nanoflowers have been used to fabricate acetylcholinesterase biosensors for detecting organophosphorus pesticides.³⁸⁶

The NiO nanoflowers show excellent responses to NO₂ (among various gases, such as, H₂, NO₂, CO, toluene, NH₃, and H₂S) at 150 °C and 100 ppm due to the enhanced interaction between NO₂ and NiO owing to their mesoporous flower structure and large surface area.³⁷⁹ The 530 nm-light-assisted Au@MoS₂ gas sensor utilizing the localized surface plasmon resonance (LSPR) effect of Au nanoparticles has been developed to detect NO₂ gas with a ppb-level detection limit and robust resistance to humidity interference.⁶¹ By utilizing 530 nm light irradiation, the gas sensor can achieve a limit of detection as low as 10 ppb for NO₂, without requiring a specific operating temperature. Additionally, it demonstrates robust resistance to humidity, maintaining steady response values with only a slight drift of 6.5% across a wide range of humidity variations (10–90% relative humidity) because the illumination of light can decelerate the adsorption of H₂O molecules while facilitating the desorption of H₂O molecules.⁴¹⁴ The effective utilization of the LSPR enhances the absorption of visible light by MoS₂, leading to an increased surface carrier concentration. Table 29 provides a comparison of the gas-sensitive properties between the Au–MoS₂ gas sensor assisted by 530 nm light and other previously reported NO₂ sensors that utilize visible-light assistance. The prepared nanoflowers exhibit superior performance in terms of response value, detection limit, and recovery time when compared to other gas sensing materials.

Table 29 Comparison of the gas-sensitive properties of visible-light-assisted NO₂ gas sensors. Reprinted with permission from ref. 61. Copyright © 2021, American Chemical Society. All rights reserved

Gas-sensing material	NO2 (ppm)	Response value	Detection limit (ppb)	Recovery time	Excitation source	Ref.
ZnO–Ag nanoparticles	5	4.5	500	No	430 nm LED	415
CdS nanoflakes	5	89%	20	113 s	530 nm LED	416
WO3	0.16	2.9	160	18.3 min	480 nm LED	417
CdSe quantum dots/ZnO	1	20	200	>60 min	535 nm LED	418
Au nanoparticles-ZnO nanorods	1	1.093	10 000	2500 s	495 nm LED	419
Black NiO	0.372	31.04%	57	No	480 nm LED	420
WO3 nanofibers	0.4	1.49	160	>60 min	430 nm LED	421
Au–MoS2 nanoflowers	1	8.1	10	27 s	530 nm LED	61

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6.3. Supercapacitors

The widespread applications of supercapacitors (also called electrochemical capacitors or ultracapacitors) in transportable electronics, uninterruptible power supplies, hybrid electric vehicles, and renewable energy sources have received significant attention from researchers owing to their high power densities, long cycle life, low cost, and fast charge–discharge rates compared to those of lithium-ion batteries and conventional capacitors.^422,423 Transition metal oxides with nanostructures exhibit immense potential as electrodes for supercapacitors because of their excellent pseudocapacitive behavior and relatively high electrical conductivity.^424–426

Electrochemical tests of CuO nanoflowers fabricated on the surface of flexible Cu foil exhibit a high specific capacitance of 284.5 F g⁻¹ at a current density of 0.5 mA cm⁻² and high electrode cyclability (with a 20% loss in capacitance over 1000 cycles).²⁶¹ Among the three different morphologies of NiO nanomaterials (particle, honeycomb, and 3D nanoflower), NiO nanoflowers show the highest specific capacitance (364 F g⁻¹ at 1 A g⁻¹), rate capability (64% capacitance retention from 1 A g⁻¹ to 10 A g⁻¹), and cycling stability.⁴²⁷ The Co₃O₄@NiCo₂S₄ core–shell nanoflower array show excellent electrochemical performance: 1234 F g⁻¹ specific capacitance at a current density of 1 A g⁻¹, with a maximum energy density of 168 W h kg⁻¹, and approximately 11% loss of initial specific capacitance after 5000 cycles, owing to the unique configuration and strong synergistic effect of the porous Co₃O₄ core and the ultrathin NiCo₂S₄ shell.³⁸¹

Nanoflower-like MCoP (M = Ni and Fe) composites on Ni foam, denoted as NF@MCoP, are synthesized through hydrothermal and low-temperature phosphating methods. NF@MCoP provides a larger electroactive area, shorter electron transfer distance, and faster electron conductivity due to its unique 3D structure, nanoflower morphology, and the conductive substrate, and exhibits enhanced electrochemical performance.³⁹¹ Optimization of the feeding molar ratio of M/Co, the amount of dimethyl sulfoxide (DMSO), and the hydrothermal time of Co precursors is carried out to improve the electrochemical capacitive performance of as-prepared NF@MCoP electrodes. The samples with a M/Co feeding molar ratio of 1 : 2 exhibits a higher specific capacitance compared to the other samples (1 : 1, 2 : 1, 1 : 1.5, and 1.5 : 1). The sample synthesized using 8 mL of DMSO demonstrates the highest specific capacitance, which aligns well with the cyclic voltammetry result. Extending the hydrothermal time to 6 hours results in a decrease in the electrochemical performance of nanoflower-like MCoP composites on Ni foam. The nanoflower-like composites show remarkable energy density, power density, and long-term cycling stability. The area-specific capacitance values of NF@FeCoP and NF@NiCoP composites are measured to be 6.2 and 6.9 F cm⁻², respectively.

According to chronopotentiometry and cyclic voltammetry investigations, the hybrid battery–supercapacitor (HBS) devices display excellent capacitive performance, with 48.1 W h kg⁻¹ at 2400 W kg⁻¹ for NF@FeCoP and 33.3 W h kg⁻¹ at 3746 W kg⁻¹ for NF@NiCoP, along with high retention rates of 88% and 89% after 5000 cycles for NF@FeCoP and NF@NiCoP, respectively. The excellent performance of the NF@MCoP nanostructure can be attributed to several factors, including the highly reactive surface and short diffusion path provided by the nanoflower structure, the significant influence of DMSO on the morphology of NF@MCoP, and the low series resistance resulting from direct deposition on Ni foam.

6.4. Li-ion battery

Due to its high voltage, high capacity, long cycle life, and excellent safety performance,^428,429 the lithium-ion battery is an efficient energy storage device that has gained widespread use in portable electronics and is increasingly becoming the primary power source for electric vehicles.^280,430 To enhance the rate at which lithium ions intercalate and deintercalate in electrode materials, as well as to improve the capacity, cyclability, and safety, there has been significant focus on nano-sized electrode materials in recent years. Particularly, extensive research has been conducted on constructing 3D hierarchical nanostructures.^{305,431–433} SnO nanoflowers have the potential to serve as effective anode materials in lithium-ion rechargeable batteries, exhibiting a high capacity of approximately 800 mA h g⁻¹, which is close to the theoretical value of 875 mA h g⁻¹.³⁰⁵ In addition, the reversible capacities of metal oxide materials with 3D hierarchical nanostructures have been found to be significantly high when used as anode electrode materials in Li-ion batteries. This is primarily due to their large surface areas.^434,435 Hence, it is crucial to have precise control over the morphology during the preparation of the nanoflowers to ensure their effectiveness in applications related to lithium-ion rechargeable batteries.

Cao et al. studied the electrochemical properties of NiO electrodes in a lithium-ion battery. They found that the initial discharge capacity of the NiO nanoflowers is approximately 1300 mA h g⁻¹.²⁹⁵

The high surface area (71.9 m² g⁻¹) of the porous flower-like α-Fe₂O₃ nanostructures, as determined by the BET test, significantly enhances its performance in Li-ion battery applications.³⁰⁶ Increasing the surface area or porosity of the hematite crystals improves the lithium intercalation performance.³⁰⁶ In Fig. 19, the charge/discharge curves of the flower-like α-Fe₂O₃ nanostructures are compared for the first, tenth, and twentieth cycles. The initial discharge capacity is 974.43 mA h g⁻¹. Even after 10 and 30 cycles, the discharge capacities remain higher than that of graphite, with values of 705.5 and 548.47 mA h g⁻¹, respectively. These flower-like nanostructures exhibit superior cycling performance compared to the previously reported α-Fe₂O₃ hollow spindles and microspheres (1313 and 1298 mA h g⁻¹) due to their higher surface area.⁴³⁶

Fig. 19 The first (solid line), 10th (dashed line), and 30th (dotted line) charge–discharge curves of flower-like α-Fe₂O₃ nanostructures at a current density of 0.2 mA cm⁻². Reprinted with permission from ref. 306. Copyright © 2008, American Chemical Society. All rights reserved.

Interestingly, the electrochemical properties of porous NiO nanoflowers are compared to those of similar NiO nanostructures (Table 30). The NiO nanoflower electrode demonstrates a favorable and consistent discharge capacity. Also, at a high current rate of 1C, the electrode exhibited an initial discharge capacity of 671.2 mA h g⁻¹ and maintained a stable capacity of 306.8 mA h g⁻¹ even after 80 cycles with a high coulombic efficiency of 98%. The produced NiO nanoflowers demonstrated promising outcomes as anode materials for Li-ion batteries, attributed to their high surface area and short diffusion length.¹⁰⁰

Table 30 Summaries of the electrochemical properties of the NiO nanoflowers prepared in this work with other reported NiO nanostructures. Reprinted with permission from ref. 100. Copyright © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved

Sample	Current density (mA g^−1)	Potential range (V)	Initial capacity (mA h g^−1)	Capacity retention (mA h g^−1)	Ref.
NiO nanotubes	25	0.01–3.0	610	200 after 200 cycles	285
NiO microspheres	50	0.01–3.0	1570	80 after 30 cycles	437
Hollow NiO microspheres	100	0.02–3.0	1100	560 after 50 cycles	438
NiO nanospheres	100	0.01–3.0	1200	518 after 60 cycles	439
NiO–Ni nanocomposite	286	0.02–3	503	420 after 100 cycles	440
Three-dimensional porous NiO	200	0.05–3.0	800	520 after 30 cycles	441
Flower-like NiO microspheres	50	0.01–3.0	1104	105 after 30 cycles	442
Urchin-like NiO microspheres	50	0.01–3.0	1295	134 after 50 cycles	442
Electrospun NiO nanofibers	100	0.005–3.0	1280	583 after 100 cycles	443
Porous NiO nanoparticles	15	0.01–3.0	900	380 after 100 cycles	444
Hierarchical NiO nano/microspheres	100	0.01–3.0	1165	720 after 100 cycles	445
Hierarchical NiO microspheres	700	0.005–3.0	1452	598 after 100 cycles	446
Hollow NiO nanotubes	200	0.01–3.0	1072	600 after 100 cycles	447
NiO nanoflowers	100	0.01–3.0	1936.9	552 after 50 cycles	100

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7. Conclusion and future perspectives

Research related to flower-like hierarchical nanomaterials has developed considerably, with the exploration of synthetic technologies, growth mechanisms, particle size distributions, morphologies, chemical, physical, and optical properties to enhance catalytic, energy-related, and biomedical applications. Numerous types of inorganic-based nanoflowers have been synthesized through physical, chemical, biological, and hybrid methods: physical vapour deposition, solvothermal and hydrothermal processes, chemical vapour deposition, sol–gel, plant extract (green or bio synthesis), electrochemical deposition, galvanic displacement, microwave-assisted synthesis, high-energy ball-milling hydrothermal treatment, and the solution-immersion RF-sputtering method. In the last two decades, research related to flower-shaped hierarchical nanomaterials has progressed rapidly, and the number of related publications has gradually increased owing to their merits, such as high surface-to-volume ratio, strong adsorption capacity, high loading efficiency, and excellent catalytic activity.

Research on alloy and core–shell nanoflowers has particularly gained prominence, and flower-shaped nanostructure alloys and metal/metal oxide core–shell structures have been investigated for potential applications in the photo- and electro-catalysis of water splitting and fuel cells, owing to their composition and synergistic effects.

The modification of synthetic pathways, not by using surfactants, templates, or stabilizing agents, but rather by promoting reactions at lower temperatures and for shorter times has led to the development of simple, cost-effective, and eco-friendly synthetic routes for industrial-scale applications. Additionally, numerous studies have been conducted by controlling the reaction conditions (including the type and amount of reagents (precursors, reducing agents, surfactants, template agents), reaction time, temperature, and ratio of metals) to optimize the morphology and size distribution for specific applications. Numerous unique functionalities and potential applications have been reported.

Future research could focus on the development of uniform and monodisperse nanoflowers with high electrochemical, optical, and physical properties, suitable for various applications. Additionally, research on alloy, core–shell, and surface-treated nanoflowers is expected to expand multi-metallic compositions with modified structures. Furthermore, future studies on nanomaterials will potentially encompass the combination of multiple nanostructured materials, such as CoS nanoflowers wrapped in reduced graphene oxides,⁴⁴⁸ nitrogen-doped graphene-encapsulated Ni–Cu alloy nanoflowers,⁴⁴⁹ Pt nanoflower monolayers on single-walled carbon nanotube membranes,⁴⁵⁰ and amorphous carbon nanotube–nickel oxide nanoflower hybrids.⁴⁵¹

Author contributions

Conceptualization, H. J. and D. N. L.; methodology, S. J. L., H. J. and D. N. L.; validation, S. J. L. and H. J.; formal analysis, S. J. L.; investigation, S. J. L.; data curation, S. J. L. and D. N. L.; writing—original draft preparation, S. J. L.; writing—review and editing, H. J. and D. N. L.; supervision, D. N. L.; funding acquisition, S. J. L., H. J., and D. N. L. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (grants 2021R1A2C1004285 and 2019R1A2C2086770), by the Nano Material Technology Development Program through the NRF funded by the Ministry of Science, ICT, and Future Planning (2009-0082580), and by a Center for Women in Science, Engineering and Technology (WISET) grant funded by the Ministry of Science and ICT (MSIT) under the program for returners into R&D.

Notes and references

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