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Synthesis and applications of porous non-silica metal oxide submicrospheres

Yash Boyjoo a, Meiwen Wang a, Vishnu K. Pareek a, Jian Liu *a and Mietek Jaroniec *b
aDepartment of Chemical Engineering, Curtin University, Perth, WA 6845, Australia. E-mail: jian.liu@curtin.edu.au
bDepartment of Chemistry & Biochemistry, Kent State University, Kent, Ohio 44242, USA. E-mail: jaroniec@kent.edu

Received 22nd January 2016

First published on 12th September 2016


Nowadays the development of submicroscale products of specific size and morphology that feature a high surface area to volume ratio, well-developed and accessible porosity for adsorbates and reactants, and are non-toxic, biocompatible, thermally stable and suitable as synergetic supports for precious metal catalysts is of great importance for many advanced applications. Complex porous non-silica metal oxide submicrospheres constitute an important class of materials that fulfill all these qualities and in addition, they are relatively easy to synthesize. This review presents a comprehensive appraisal of the methods used for the synthesis of a wide range of porous non-silica metal oxide particles of spherical morphology such as porous solid spheres, core–shell and yolk–shell particles as well as single-shell and multi-shell particles. In particular, hydrothermal and low temperature solution precipitation methods, which both include various structure developing strategies such as hard templating, soft templating, hydrolysis, or those taking advantage of Ostwald ripening and the Kirkendall effect, are reviewed. In addition, a critical assessment of the effects of different experimental parameters such as reaction time, reaction temperature, calcination, pH and the type of reactants and solvents on the structure of the final products is presented. Finally, the practical usefulness of complex porous non-silica metal oxide submicrospheres in sensing, catalysis, biomedical, environmental and energy-related applications is presented.


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Yash Boyjoo

Yash Boyjoo received his PhD in 2013 from Curtin University, Australia. After two years as a research associate at Curtin University working with Dr Jian Liu and Prof. Vishnu Pareek, he currently works as a postdoctoral research fellow at Université Lille 1 in France, where his studies deal with the synthesis of materials for VOC elimination from polluted air. His other research interests include photocatalytic wastewater treatment, synthesis of activated carbons for application as CO2 adsorbents and supercapacitors, and the design and modelling of photocatalytic reactors.

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Meiwen Wang

Meiwen (Sharon) Wang received her bachelor's degree with first class honours in Chemical Engineering at Curtin University, where she is now pursuing PhD research under the supervision of Dr Jian Liu. She is interested in the design and fabrication of yolk–shell particles and their use as nanoreactors.

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Vishnu K. Pareek

Vishnu Pareek is a Professor in the Department of Chemical Engineering at Curtin University, Australia. He received his PhD degree from the University of New South Wales, an MTech from IIT Delhi and a BE from the University of Rajasthan, Jaipur, all in Chemical Engineering and with high distinction. Prof. Pareek's expertise lies in the simulation and design of chemical processes with particular emphasis on the use of these tools in industrial-scale processes. He is an ardent cricket fan and an enthusiastic reader especially on subjects related to politics, environment and economy. In his spare time, he is a bird watcher with special interest in understanding the behaviour of birds of prey.

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Jian Liu

Jian Liu obtained his PhD degree in Physical Chemistry from the Dalian Institute of Chemical Physics, Chinese Academy of Science, in 2008. Subsequently, he moved to Australia and worked as a Postdoctoral Research Fellow at AIBN, the University of Queensland. He started as a Lecturer at Curtin University in 2013 and was promoted to Senior Lecturer in 2014. Dr Liu has published more than 120 peer reviewed journal articles with over 6300 citations (H-index of 43). He has been honoured with a prestigious UQ Foundation Research Excellence Award and Australian Postdoctoral Fellowship. His current research interests are nanoreactor design, green chemical processes, and utilization of CO2.

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Mietek Jaroniec

Mietek Jaroniec received his MS and PhD from M. Curie-Sklodowska University, Poland, in 1972 and 1976, respectively. Since 1991 he has been a Professor of Chemistry at Kent State University, Kent, Ohio (USA). Before joining Kent State he was a Professor of Chemistry at M. Curie-Sklodowska University, Poland. His research interests revolve primarily around inter-disciplinary topics of interfacial chemistry, and chemistry of materials, including physical adsorption at the gas/solid and liquid/solid interfaces, adsorbents, and catalysts. At Kent State he has established a vigorous research program in the area of ordered nanoporous materials such as ordered mesoporous silicas, organosilicas, inorganic oxides and carbons, focusing on their synthesis and environmental and energy-related applications.


1. Introduction

The design of functional nano- and microsized particles is a hot topic due to their application in a wide range of fields such as energy conversion and storage, catalysis, biomedicine, and environmental remediation. These tiny particles feature a high surface area to volume ratio, which is beneficial for diffusion and transport of reactants and products. Furthermore, depending on the material used, the particle size, crystal size, surface area, pore size distribution, and morphology can be tailored for specific applications by adjusting the chemical composition and synthesis conditions.

Metal oxides (MeO) exhibit several attractive features such as high mechanical strength, thermal stability, chemical inertness, non-toxicity, biocompatibility, oxygen vacancies, semiconductor properties and high isoelectric point, and can act as supports for noble metals such as Au, Pt and Pd, or rare earth metals to achieve synergistic catalytic activity toward specific chemical reactions. Furthermore, for nanosized crystals, quantum effects become important as reflected by significantly different optical and electronic properties from those observed for the bulk phases,1 which can have favorable outcomes for their application in catalysis and photocatalysis; for example, new physicochemical phenomena such as ferromagnetism and paramagnetism can be achieved for otherwise antiferromagnetic systems, as in the case of NiO spheres.2 As a result, countless studies have been performed to synthesize submicrosized porous non-silica metal oxide particles with different morphologies, phases, sizes, crystal sizes and pore size distributions. These particles are useful for a variety of industrial, biomedical, environmental and energy-related applications as illustrated in Fig. 1.


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Fig. 1 Spherical metal oxide particles and their potential applications.

The porous non-silica metal oxide submicrospheres can be designed to have large surface areas and well-developed porosity to enhance interfacial interactions with reactants and facilitate the transport and diffusion of reactants and products. Also, the crystal size and growth directions need to be considered for specific catalytic and semiconductor applications. Furthermore, a proper balance between micropores, mesopores and macropores that respectively act as reaction sites, distribution/evacuation pathways and reservoirs is desired. The spherical morphology is the most stable shape that is achieved in nature. In the case of metal oxides, the advantages offered by spherical structures are high mechanical strength, short pathways for diffusion of species, dispersion enhancement due to the stabilization of electrostatic charges, high surface area to volume ratios, easy coating with other species or metal oxides, minimization of viscous effects and predictable hydrodynamics. Typical spherical morphologies of metal oxides discussed in this review can be classified into three groups: porous solid spheres (Fig. 2Aa), core–shell spheres (Fig. 2Ab) and hollow spheres (Fig. 2Ac). The core–shell spheres shown in Fig. 2Ab can be further extended into single-core particles with a multi-particle (raspberry-like) shell (Fig. 2Ab1), multi-core particles with a single shell (Fig. 2Ab2), single-core particles with a multi-shell (Fig. 2Ab3), or a combination of particles shown in Fig. 2Ab and b1 (see Fig. 2Ab4). Also, hollow spheres can be further classified into yolk–shell spheres (Fig. 2Ac1) and multi-shell hollow spheres (Fig. 2Ac2). In principle, each compartment of these spheres can be nonporous or porous with different pore sizes (micropores, mesopores, and macropores). More complex metal oxide spheres can be proposed and synthesized by combining and modifying the aforementioned nine patterns as shown in Fig. 2A.


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Fig. 2 (A) Graphically illustrated classification of metal oxide spheres: (a) porous spheres; (b) core–shell spheres: (b1) raspberry-like core–shell spheres; (b2) multi-core spheres with a single shell; (b3) single-core spheres with multi-shells; (b4) raspberry-like single-core with multi-shells; and (c) hollow spheres: (c1) yolk–shell spheres; (c2) multi-shell hollow spheres. (B) SEM and TEM images of various metal oxide spherical particles: (a) dense spheres; (b) porous spheres; (c) core–shell spheres; (d and e) yolk–shell spheres; (f) hollow spheres. Panel B reproduced with permission from ref. 3. Copyright © 2007, American Chemical Society.

Several excellent reviews have been published previously focusing on the synthesis and applications of metal oxides. However, these reviews consider either a specific metal oxide such as TiO24,5 and iron oxides,6 or a specific application such as gas sensors,7 energy storage and conversion applications8,9 or a specific morphology such as hollow spheres10 and multi-shell structures.11 Indeed, a concise and up-to-date review on the synthesis strategies of different types of porous non-silica metal oxide submicrospheres with various spherical morphologies as well as their emerging applications is timely as this research field continuous to rapidly grow. This review is focused on the synthesis and applications of non-silica-based metal oxide spheres based on the literature for the past five years. One of the main sections of this review is devoted to the major strategies frequently used for the preparation of MeO particles such as hydrothermal/solvothermal synthesis at elevated temperatures (>100 °C), solution precipitation synthesis at low temperatures (<100 °C), and the aerosol-type synthesis. Other methods are only briefly mentioned. The hydrothermal and low temperature precipitation syntheses are general methods that allow for the development of more sophisticated structures by hard templating, soft templating or controlled hydrolysis, or by taking advantage of Ostwald ripening and the Kirkendall effect.11,12 The aforementioned section on the synthesis methods is supplemented by a critical appraisal of the effect of different experimental parameters such as reaction time, reaction temperature, calcination, pH and the type of reactants and solvents on the structure of the resulting metal oxide particles. Finally, the last section presents the major applications of metal oxide spheres in industrially relevant sensing and catalysis, in the development of biomedically relevant photoluminescent devices and drug delivery vehicles, in environmentally relevant photocatalysis and adsorption, and in energy-relevant applications such as lithium-ion batteries, supercapacitors and dye sensitized solar cells.

2. Synthesis methods

Metal oxide particles have been generated by using different synthetic strategies, for example, colloidal synthesis, sol–gel process, aerosol process, precipitation, hydrothermal/solvothermal synthesis, hot injection, and non-aqueous and non-hydrolytic chemistry method (e.g., solid-state reactions, solid–gas reactions). In this section we summarize the main methods for the preparation of MeO particles with spherical structures such as the hydrothermal synthesis at elevated temperatures (>100 °C), low temperature solution precipitation synthesis (<100 °C) and the spray method. The first two methods include structure defining approaches such as hard templating, soft templating, hydrolysis, Ostwald ripening and the Kirkendall effect. This section ends with a brief overview of other methods involving electrodeposition, laser irradiation or ultrasonic irradiation.

2.1 Hydrothermal and solution precipitation methods

The hydrothermal/solvothermal synthesis of metal oxide particles involves the use of a batch reactor at elevated temperatures (above 100 °C) and pressures. It offers a controlled environment for interactions of the salts used as metal oxide with other reactants in solution to form nanosized crystallites. The elevated pressure and temperature conditions facilitate dissolution of precursors and recrystallization of materials that can be insoluble under ambient conditions. As a result, high purity, homogeneous, metastable and often crystalline products are formed with unique properties and narrow particle size distribution.13 This synthesis method is flexible and well suited for the control of morphology and crystallinity of the porous metal oxide spheres by varying experimental parameters such as reaction temperature and time, type of reactants and solvents, and the chemical composition of the synthesis mixture. During the hydrothermal process, the nanosized crystallites self-assemble (with or without additives such as polymers and surfactants) into more complex architectures with optimal stability and lowest surface energy; hence, spherical shapes are favored. Also, metal oxide spherical structures can be synthesized via solution precipitation using milder conditions (i.e., temperature below 100 °C and atmospheric pressure) as reported elsewhere.13–20

Tables 1 and 2 present the pertinent experimental conditions and characteristic features of solid spheres and shell-type particles obtained by hydrothermal synthesis together with relevant references.21–119 As can be seen from these tables, the hydrothermal method is very popular for the synthesis of various categories of metal oxides such as alkaline earth metals (MgO), rare-earth metals (CeO2, Y2O3), transition metals (TiO2, V2O5, Cr2O3, MnO2, Fe2O3, Co3O4, NiO, CuO, ZnO, ZrO2, Nb2O5, MoO2, Ta2O5 and WO3) and post-transition metals (Al2O3, Ga2O3, In2O3, Bi2O3 and SnO2). Composites, perovskites and doped metal oxides can also be created by controlling the ratio of the precursors.

Table 1 Porous solid spheres synthesized by the hydrothermal method
Ref. Particle type Reactants Solvents Hydrothermal conditions Calcination conditions Particle size BET SA (m2 g−1) Pore volume (cm3 g−1) Pore size (nm)
a Microwave heating. b By varying the concentration of NbCl5 between 0.3 g and 0.5 g in 25 ml solution. c Prepared by an antisolvent method. d For nanowire-, flower- and urchin-like spheres respectively by increasing urea concentration. e For nanowire-, flower- and urchin-like spheres respectively. f For W(CO)6 concentration between 4.26 mM and 28.4 mM. g For WO3/TiO2 between 2% and 10%. h Supercritical temperature. NR: not reported.
22 Bi2O3 spheres Bi(NO3)3·5H2O, PVP EG 180 °C, 0.17 ha 400 °C, 3 h in air 10 μm NR NR NR
23 Bi2O3 spheres Bi(NO3)3·5H2O, HNO3, urea Water, EG 150 °C, 3 h None 350 nm 8 0.018 NR
Bi(NO3)3·5H2O, NaOH, HNO3, PVP 100 nm 23 0.15
24 Bi2WO6 spheres Na2WO4·2H2O, Bi(NO3)3·5H2O, PVP K30 Water 180 °C, 12 h None 4 μm NR NR NR
25 Bi2WO6 perovskite spheres Bi(NO3)3·5H2O, Na2WO4·2H2O, NaHCO3, citric acid Water 200 °C, 18 h None 2 μm 24 NR NR
26 CeO2 spheres Ce(NO3)3·6H2O Water, C2H5COOH, EG 180 °C, 3.3 h None 130 nm 216 NR 3.8
27 Co3O4 spheres Co(CH3COO)2·4H2O, NH3 Water, EG 180 °C, 12 h 500 °C, 4 h in air 2–5 μm 13 NR NR
28 Co3O4 spheres Co(NO3)2·6H2O, urea Water 160 °C, 6 h 300 °C, 2 h in air 8–20 μm 30 0.245 17
29 CoFe2O4 spheres CoCl2·6H2O, FeCl3·6H2O, urea Water, ethanol 170 °C, 0.42 ha 500 °C in air 1 μm 25 0.18 25
30 Cr2O3 spheres C15H21CrO6, NH4HCO3 Ethanol 250 °C, 2 h 500 °C, 4 h 1–1.2 μm 15 NR 20–80
31 Cr2O3 spheres Cr(NO3)3·9H2O, H2C2O4, urea Ethanol, PEG 180 °C, 5 h 500 °C, 2 h in air 2–3 μm NR NR NR
32 CuO spheres Cu(CH3COO)2 Water, EG 160 °C, 1 h None 412 nm 168 NR 5
33 CuO spheres Cu(CH3COO)2, NH3, sodium alginate Water 160 °C, 2 h None 500 nm 21 NR NR
34 α-Fe2O3 spheres FeCl3·6H2O, ascorbic acid, urea Water 160 °C, 4 h 500 °C, 4 h in air 0.5–5 μm 20 0.11 2–50
35 α-Fe2O3 spheres Fe(NO3)3·9H2O Water, 2-butanone 140 °C, 12 h None 100 nm NR NR NR
36 α-Ga2O3 spheres Ga(NO3)3, oxalic acid Water 200 °C, 10 h 450 °C, 3 h 0.5–4 μm 62 0.193 12.3
37 Cubic-In2O3 spheres InCl3·4H2O, citric acid Water, ethylenediamine 180 °C, 7 h 400 °C, 0.17 h in air 150–200 nm 88 NR NR
Hexagonal-In2O3 spheres InCl3·4H2O, tartaric acid Water, ethylenediamine 180 °C, 7 h 400 °C, 0.17 h in air 150–200 nm 85 NR NR
38 In2O3 spheres InCl3·4H2O, urea, sodium citrate Water, EG 200 °C, 16 h 400 °C, 2 h in air 600–700 nm 19 NR NR
39 Nb2O5 spheres NbCl5 Ethanol 200 °C, 24 h 550 °C, 2 h in air 200–900 nmb 23–68b NR NR
40 Nb2O5 spheres Glycolated Nb2O5 spheresc Water 180 °C, 12 h None 400–500 nm 312 0.567 2
41 NiO spheres Ni(NO3)2·6H2O, Na2SO4, NaOH, glycine Water 180 °C, 0.5 ha 300 °C, 3 h in air 2 μm 202 NR 25
42 NiO spheres Ni(NO3)2·6H2O, NaCl, sodium acetate EG 190 °C, 8 h 300 °C, 2 h in air 600 nm 222 NR 4–10
2 NiO spheres NiCl2, sodium acetate, polyethyleneimine Water, triethanolamine 200 °C, 8 h 270 °C, 0.5 h in air 500 nm 60 NR 10–30
43 NiO spheres NiCl2·H2O, urea Water 100 °C, 20 h 300 °C, 2 h in air 3–4 μm 200–240d NR 3.2, 8.9, 4e
44 La doped NiO spheres Ni(NO3)2·6H2O, La(NO3)2·6H2O, NH3, glucose Water 140 °C, 12 h 550 °C, 4 h in air 1–2 μm 278 0.79 2–50, >50
45 SnO2 spheres Na2SnO3·3H2O, sodium alginate EG, water 180 °C, 24 h None 200–400 nm 29 NR 15
46 SnO2 spheres SnCl2·2H2O, NaClO, HCl Ethanol 180 °C, 12 h None 150 nm 62 NR 4
47 SnO2 spheres SnCl4·5H2O, PVP Methanol 180 °C, 3 h 500 °C, 2 h in air 400–700 nm 78 NR 10
48 SnO2 spheres SnCl4·5H2O, PVP Methanol 180 °C, 3 h 500 °C, 2 h in air 500–700 nm 78 NR 10
49 SnO2@C spheres K2SnO3·3H2O, glucose Water 180 °C, 4 h 450 °C, 4 h in N2 100 nm NR NR NR
50 C–V2O3 spheres NH4VO3, citric acid Ethanediol, water 180 °C, 24 h 600 °C, 3 h in N2 2 μm 45 NR 20
51 V2O5 spheres VO(OiPr)3 Acetic acid 200 °C, 1.5 h 350 °C, 0.5 h in air 4–10 μm 42 NR NR
52 WO2 spheres W(CO)6 Ethanol 200 °C, 24 h None 0.7–1.5 μmf 78–114f NR NR
53 WO3 spheres WCl6, carbon microspheres Dimethylformamide 120 °C, 4 h 420 °C, NR 150–220 nm 22 0.0447 NR
54 WO3/TiO2 spheres (NH4)10H2(W2O7)6, TiOSO4, P123 Water, ethanol 140 °C, 16 h 500 °C, 6 h in air NR 45–64g 0.21–0.26g 12.2–15.2g
55 Eu3+:Y2O3 spheres Y(NO3)3·6H2O, Eu(NO3)3, KOH Water, 1-propanol 400 °Ch, 0.17 h 1000 °C, 1 h in air 2–3 μm NR NR NR
56 ZnO spheres Zn(NO3)2·6H2O, L-asparagine, urea Water 100 °C, 3 h 300 °C, 0.5 h in air Several μm 194 NR 5
57 ZnO spheres Zn(NO3)2, urea Water 120 °C, 2 h 450 °C, 2 h in air 10 μm 38 NR 8.67
58 ZnO spheres Zn(NO3)2·6H2O, trisodium citrate, urea Water 120 °C, 6 h 300 °C, 2 h in air 4–6 μm 40 NR 20–60
59 ZnO spheres Zn(CH3COOH)2·2H2O, MEA, urea Water 120 °C, 12 h 450 °C, 2 h in air 1–2 μm 40 NR 5–50
60 ZnO spheres Zn(CH3COO)2·2H2O, NaOH, citric acid Water, ethanol 120 °C, 24 h None 2–3 μm 42 NR 2–30
61 ZnO spheres Zn(CH3COO)2, thiourea Water 180 °C, 10 h 500 °C, 3 h in air 3–5 μm 21 NR 22.6
60 Ag loaded ZnO spheres Zn(CH3COO)2·2H2O, AgNO3, NaOH, citric acid Water, ethanol 120 °C, 24 h None 2–3 μm 37 NR NR
62 ZrO2 spheres ZrOCl2·8H2O, HCl, urea Ethanol, water 160 °C, 2 h None 1–2 μm 102 0.09 2–102


Table 2 Porous shells synthesized by the hydrothermal method
Ref. Particle type Reactants Hollowing mechanism Solvents Hydrothermal conditions Calcination conditions Particle and shell dimensions BET SA (m2 g−1) Pore volume (cm3 g−1) Pore size (nm)
a Microwave heating. b For F/Fe fractions between 0 and 1. c By varying the fraction of glycerol in water between 0.05 and 0.125. d (acac) = CH3COCH[double bond, length as m-dash]C(O−)CH3. e Ratio SiO2[thin space (1/6-em)]:[thin space (1/6-em)]Ta2O5 = 1[thin space (1/6-em)]:[thin space (1/6-em)]0.85. f Ratio SiO2[thin space (1/6-em)]:[thin space (1/6-em)]Ta2O5 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.7. g Nanosheet shell. h Porous shell. NR: not reported.
63 γ-Al2O3 hollow spheres KAl(SO4)2·12H2O, urea Ostwald ripening Water 170 °C, 3 h 600 °C, 2 h in air 4–6 μm, shell thickness 700–900 nm 149 0.45 12.3
64 Perovskite BaZrO3 hollow spheres Ba(NO3)2, ZrOCl2·8H2O, KOH Ostwald ripening Water 200 °C, 24 h None 160 nm, shell thickness 15 nm NR NR NR
65 Bi2O3/Co3O4 hollow spheres Bi(NO3)3·5H2O, Co(NO3)3·6H2O, PEG, NaAc Ostwald ripening EG 180 °C, 12 h 500 °C, 2 h in air 2–6 μm 46 0.16 NR
66 BiFeO3 hollow spheres Bi(NO3)3·5H2O, Fe(NO3)3·9H2O, citric acid Ostwald ripening Glycerol, ethanol 160 °C, 24 h 500 °C, 24 h in air 1.5 μm, shell thickness 0.2 μm 15 NR NR
67 CeO2 hollow spheres Ce(NO3)3·6H2O, HCl, citric acid Ostwald ripening Water 160 °C, 24 h 365 °C, 1.5 h in air 2–4 μm 56 NR NR
68 CeO2 hollow spheres Ce(NO3)3·6H2O, PVP, H2O2, urea Ostwald ripening Water 180 °C, 24 h None 126 nm 21 NR 4
69 CeO2 hollow spheres Ce(NO3)3·6H2O, adipic acid Ostwald ripening Water, EG 180 °C, 5 h None 135 nm 145 NR 4
70 CeO2 hollow spheres CeCl3·7H2O, H2O2, urea Ostwald ripening Water 180 °C, 10 h None 300 nm, shell thickness 50 nm 85 0.23 3–10
71 CeO2 hollow spheres Ce(NO3)3·6H2O, PVP Ostwald ripening EG, ethanol, water 180 °C, 24 h None 160 nm 66 0.181 3–30
72 CeO2 hollow spheres CeCl3·7H2O, urea Ostwald ripening Water 180 °C, 4 h None 300 nm, shell thickness 30 nm 37 NR 36
73 Yolk–shell CeO2 Ce(NO3)3·6H2O, PVP, NH4Ac·2H2O Carbon spheres Ethanol 180 °C, 12 h 600 °C, 3 h in air 180 nm NR NR NR
74 Multi-yolk–shell Pd@CeO2 spheres Pd@SiO2, Ce(NO3)3·9H2O SiO2 etching EG, CH3COOH, water 130 °C, 12 h 350 °C, 2 h in H2 150–200 nm 104 0.078 2–25
75 Co3O4 hollow spheres Co(NO3)2·6H2O, sodium citrate, HMT, sucrose In situ carbon from sucrose Water 140 °C, 24 h 500 °C, 5 h in air Shell thickness 130 nm 60 NR 7.8
76 Co3O4 hollow spheres Co(NO3)2 Ostwald ripening Glycerol, isopropanol 180 °C, 6 h 200 °C, 2 h in air 1 μm 180 NR 2–150
77 CoFe2O4 double shell spheres CoSO4·7H2O, (NH4)2Fe(SO4)2·6H2O, sucrose In situ carbon from sucrose Water 180 °C, 24 h 600 °C, 2 h in air 0.5–1.5 μm, 200–500 nm hollow core NR NR NR
Same as above with half sucrose concentration NR 38 NR 30
33 CuO hollow spheres Cu(CH3COO)2, NH3, sodium alginate Ostwald ripening Water 160 °C, 6 h None 500 nm 72 NR NR
78 CuO hollow spheres Cu(NO3)2, urea Ostwald ripening Water 180 °C, 18 h 400 °C, 2 h in air 4.5–6.5 μm NR NR NR
79 CuO hollow spheres Cu(CH3COO)2·H2O Ostwald ripening Water 120 °C, 24 h None 3.5 μm, shell thickness 1.25 μm NR NR NR
32 Cu2O hollow spheres Cu(CH3COO)2, glucose In situ carbon from glucose Water, EG 160 °C, 1 h None 1.5 μm, shell thickness 400 nm 37 NR 50
15 Cu/Cu2O hollow spheres Cu(Oac)2·H2O, PVP Ostwald ripening Ascorbic acid 100 °C, 0.5 ha None 150–500 nm 19 0.118 2–100
80 CuO/Cu2O composite hollow spheres Cu(NO3)2·3H2O, ethanolamine Ostwald ripening Water 180 °C, 12 h None 1.5–3 μm 16 NR NR
31 Cr2O3@C core shell spheres Cr(NO3)3·9H2O, H2C2O4, urea Controlled annealing Ethanol, PEG 180 °C, 5 h 750 °C, 6 h in 5% Ar and 95% H2 2–3 μm NR NR NR
81 α-Fe2O3 hollow spheres FeCl3·6H2O Ostwald ripening Water, DMF, TFA 180 °C, 24 h None 2 μm 4 NR 65.8
82 α-Fe2O3 hollow spheres K3[Fe(C2O4)3] In situ gas bubbles Water, EG 150 °C, 48 h 450 °C, 3 h in air 190 nm 41 NR 4–12
83 α-Fe2O3 hollow spheres FeSO4·7H2O Quasi-emulsion droplets Water, glycerol 145 °C, NR None 1 μm, shell thickness 100–200 nm 103 NR <30
84 Double-shelled α-Fe2O3 spheres K3[Fe(CN)6], NH4H2PO4 Ostwald ripening Water 200 °C, 30 h None 350 nm, 200 nm core, 20 nm outer shell, 40 nm inner shell 98 NR 11.2
85 Fe3O4 hollow spheres FeCl3·6H2O, NaOH, SDBS Precursor templated EG 200 °C, 1.5 ha 300 °C, 1 h in N2 2–4 μm 62 0.131 10.2
86 Fe3O4@TiO2 double shelled yolk–shell spheres Fe3O4@SiO2@TiO2, NaOH Ostwald ripening + NaOH etching Water 150 °C, 24 h None 560 nm 150 0.27 7.5
85 γ-Fe2O3 hollow spheres FeCl3·6H2O, SDBS, NaOH Precursor templated EG 200 °C, 1.5 ha 300 °C, 1 h in air 2–4 μm 56 0.159 16.3
87 α-Fe2O3 four shelled hollow spheres Fe(NO3)3·9H2O, L-histidine Amino acid templated Water 180 °C, 12 h 600 °C, 2 h in air 3 μm 14 0.07 NR
88 γ-Fe2O3 hollow spheres FeCl3·6H2O, NH4F, ethylenediamine Ostwald ripening EG 200 °C, 20 h 250 °C, 5 h in air 250 nm, shell thickness 20–40 nmb 9–19b NR 13.3–34.5b
89 Fe3O4 hollow spheres Fe(NO3)3·6H2O Kirkendall mechanism Glycerol, isopropanol, water 190 °C, 12 h 350 °C, 3 h in N2 900 nm, shell thickness 10 nm 89 NR 4, 5, 7
90 α-FeOOH hollow spheres FeSO4·7H2O Quasi-emulsion Water, glycerol 120 °C, 24 h None 1 μm, varied shell thicknessc 54–97c 0.28–0.36c <20
91 Perovskite LaFeO3 hollow spheres La(NO3)3·6H2O, Fe(NO3)3·9H2O, citric acid Ostwald ripening Water 180 °C, 24 h 800 °C, 2 h in air 2–5 μm, shell thickness 40–60 nm 49 NR 30–80 and 100–300
92 β-Ga2O3 hollow spheres Metallic Ga, HCl, urea In situ gas bubbles Acetone 200 °C, 4 h 700–800 °C, 2 h in air 1–2 μm 22 NR 3
γ-Ga2O3 hollow spheres 500–600 °C, 2 h in air 1–2 μm 31 NR 7
93 Er doped In2O3 hollow spheres InCl3·4H2O, Er(NO3)·7H2O Carbon spheres Water 180 °C, 6 h 500 °C, 3 h in O2 300 nm, shell thickness 40 nm NR NR NR
94 Rh-loaded In2O3 hollow spheres In(NO3)3·xH2O, RhCl3·xH2O, D(+) glucose monohydrate In situ carbon from glucose Water 180 °C, 24 h 500 °C, 2 h in air 2.1 μm, shell thickness 180 nm NR NR 40
95 MgO hollow spheres MgCl2·6H2O, urea Ostwald ripening Water, EG 120 °C, 10 h 450 °C, 1 h in air 3–4 μm 130 0.414 7
96 MgO hollow spheres Mg(Oac)2·4H2O, PVP K-30, NH4OH Ostwald ripening EG 185 °C, 5 h 500 °C, 1 h in Ar + 1 h in air 1 μm 343 1.9 <30
97 MnO2 hollow spheres KMnO4, SiO2 spheres, Pluronic F127 SiO2 etching Water 150 °C, 48 h None 210 nm 233 NR NR
98 MnO2 hollow spheres KMnO4 Hollow carbon spheres Water 160 °C, 5 h None 316 nm, shell thickness 69 nm 30 0.112 19.4
99 MnO2 hollow spheres KMnO4, Ce(NO3)3·6H2O, HNO3 Ostwald ripening Water 140 °C, 3 h None 3–4 μm 29 0.3 2
100 C@MnO2 spheres MnSO4·H2O, (NH4)2S2O8, glucose Ostwald ripening Water 180 °C, 3 h None 1.5 μm 142 0.27 3–4
101 MoO2 hollow spheres MoO3, diethylenetriamine Ostwald ripening Water 200 °C, 144 h 700 °C, 4 h in Ar 3–5 μm NR NR NR
102 MoO2@MoO2 yolk–shell particles MoO2(acac)2d, HNO3 Ostwald ripening Isopropanol, water 180 °C, 24 h 350 °C, 2 h in N2 1 μm, shell thickness 80 nm 31 NR 3–4
103 NiO hollow spheres Ni(NO3)2·6H2O, NH3, L-cysteine Ostwald ripening Water 120 °C, 10 h 600 °C, 1 h in air 2–3 μm, shell thickness 400 nm 66 0.442 10–50
104 NiO multi-shelled spheres Ni(NO3)3·6H2O, NH3, D-glucose In situ carbon from glucose Water 150 °C, 15 h 500 °C, 6 h in air 2–3.5 μm, shell thickness 50 nm 29 NR NR
105 Core-in-double shell NiCo2O4 spheres Ni–glycerate spheres prepared hydrothermally Kirkendall mechanism None NA 350 °C, 2 h@1 °C min−1 400 nm outer shell, 200 nm inner shell, 40 nm core, 70 nm and 40 nm outer and inner shell thickness 61 NR <10
81 SnO2 hollow spheres SnCl4·5H2O Ostwald ripening Water, DMF, TFA 180 °C, 48 h None 2 μm 108 NR 6.04
106 SnO2 hollow spheres SnSO4 Ostwald ripening Water 120 °C, 48 h None 100–200 nm 69 NR 4
107 SnO2 hollow spheres SnF2, H2O2 Ostwald ripening Water 180 °C, 12 h None 100–200 nm, shell thickness 40–50 nm 156 NR NR
108 SnO2 hollow spheres K2SnO3·3H2O, urea Ostwald ripening Water, ethanol 150 °C, 24 h None 150–400 nm 110 NR 4
109 SnO2 hollow spheres SnCl2·2H2O, HCl, urea Hollow polystyrene spheres Mercaptoacetic acid 120 °C, 6 h 400 °C, 2 h in air 650 nm, shell thickness 100 nm 62 NR 3–8
73 Yolk–shell SnO2 SnCl2·2H2O, HCl Carbon spheres DMF, water 180 °C, 12 h 600 °C, 3 h in air 420 nm 43 0.073 6.8
110 SnO2 multishell spheres SnCl4·5H2O, sucrose In situ carbon from sucrose Water 190 °C, 24 h 600 °C, 3 h in air 0.5–2 μm 36 0.197 2.50
111 SnO2/C hollow spheres Sn spheres, glucose Kirkendall mechanism Water 180 °C, 3 h 500 °C, 3 h in N2 100 nm NR NR NR
108 SnO2/C hollow spheres SnO2 hollow spheres, glucose In situ carbon from glucose Water 180 °C, 3 h 550 °C, 3 h in N2 150–400 nm NR NR NR
112 Perovskite SrTiO3 hollow spheres Anatase TiO2, SrCl2·6H2O, NaOH Kirkendall mechanism Water 180 °C, 6 h None 3–5 μm, shell thickness 700 nm NR NR NR
113 SiO2–Ta2O5 hollow spherese Tantalum isopropoxide, CTAB, TEOS, NH3 Ostwald ripening Water, ethanol 120 °C, 48 h 550 °C, 5 h in air 100–250 nm, shell thickness 50 nm 249 0.48 14.8
SiO2–Ta2O5 hollow spheresf 200 nm, shell thickness 60 nm 225 0.26 13.5
73 Yolk–shell Tb4O7 Tb(NO3)3, NH4Ac·2H2O Carbon spheres Ethanol 180 °C, 12 h 600 °C, 3 h in air 200 nm NR NR NR
114 V2O5 hollow spheres NH4VO3 In situ gas bubbles EG 180 °C, 24 h 500 °C, 2 h in air 3 μm, shell thickness 1.125 μm 22 NR 5–8
115 V2O5 hollow spheres VO(C5H7O2)2, PVP PVP micelles templated EG 140 °C, 12 h 350 °C, 2 h in air 800 nm NR NR NR
116 V2O5@V2O5 yolk–shell spheresg Vanadium oxytriisopropoxide Carbon spheres Isopropanol 200 °C, 12 h 350 °C, 2 h in air 1 μm, shell thickness 200 nm NR NR NR
V2O5@V2O5 yolk–shell spheresh Vanadium oxytriisopropoxide Carbon spheres Isopropanol, water 200 °C, 12 h 350 °C, 2 h in air 2 μm, shell thickness 100 nm NR NR NR
117 V2O5 yolk–shell spheres V2O5, oxalic acid Ostwald ripening Water, isopropanol 200 °C, 2.5 h 350 °C, 2 h in air 1 μm, shell thickness 100 nm 28 0.15 NR
118 ZnO hollow spheres ZnCl2, glucose In situ carbon from glucose Water 180 °C, 24 h 500 °C, 4 h 0.8 μm 63 0.17 <5, 9–90
119 ZnO single shell hollow spheres ZnSO4·7H2O, glucose In situ carbon from glucose Water 180 °C, 12 h 550 °C directly, 3 h in air 1 μm 10 0.04 NR
ZnO double shell hollow spheres 550 °C, 5 °C min−1, 3 h in air 1 μm 19 0.07 NR
ZnO triple shell hollow spheres 550 °C, 2 °C min−1, 3 h in air 1 μm 25 0.09 NR
62 ZrO2 hollow spheres ZrOCl2·8H2O, HCl, urea Ostwald ripening Ethanol, water 160 °C, 24 h None 1–2 μm 136 0.1 2–105
120 Yolk–shell ZnCo2O4 ZnAc2·2H2O, CoAc2·4H2O Carbon spheres EG 180 °C, 12 h 600 °C, 3 h in air 300–500 nm 16 0.063 20


Tables 3 and 4 provide a summary of metal oxide particles obtained by a solution precipitation method together with relevant references.13,16–20,120–159 However, the range of metal oxide particles that can be made by this method is not as extensive as in the case of hydrothermal synthesis, probably due to the lower solubility of metal oxide precursors at lower temperatures and the smaller flexibility at the temperatures used. Nevertheless, such mild conditions are attractive in terms of green technology and cost effectiveness.

Table 3 Porous solid spheres synthesized by the solution precipitation method
Ref. Particle type Reactants Solvents Reaction conditions Calcination conditions Particle size BET SA (m2 g−1) Pore volume (cm3 g−1) Pore size (nm)
a Sonicated. b When the ratio of Al2(SO4)3·16H2O[thin space (1/6-em)]:[thin space (1/6-em)]Al(NO)3·9H2O is between 0.33 and 0.167. c To obtain V2O3 phase. d To obtain V2O5 phase. e Depending on water or pyridine concentration. f Mol ratio of Na2WO4·2H2O[thin space (1/6-em)]:[thin space (1/6-em)]concentrated HCl = 1[thin space (1/6-em)]:[thin space (1/6-em)]50. g Removed by NaOH etching. h By varying the Ce/HMT ratio. i Assumed room temperature. NR: not reported.
19 Ag2O–MnO2 spheres MnSO4, (NH4)2S2O8, Ag nanoparticles Water 50 °C, 1 ha None 2.2 μm NR NR NR
121 α-Al2O3 spheres Al2(SO4)3·16H2O, Al(NO3)3·9H2O, urea Water 98 °C, 1.5 h 1100 °C, 1 h in air 125–430 nmb 76b NR 2–20
γ-Al2O3 spheres Al2(SO4)3·16H2O, Al(NO3)3·9H2O, urea Water 98 °C, 1.5 h 900 °C, 1 h in air NR 102b NR 2–20
18 CuO spheres Cu powder, NaOH, (NH4)2S2O8 Water 25i °C, 20 h None 1–2 μm 8 NR NR
122 CuO spheres Cu(NO3)2·H2O, NH3, NaOH Water, glycol 100 °C, 2 h 300 °C, 4 h in air 1–3 μm 88 NR NR
17 MnO2 spheres (CH3COO)2Mn·4H2O, AgNO3, H2SO4, oxone monopersulfate Water 25i °C, 36 h None 1–3 μm 163 NR 65
21 MnO2 spheres MnSO4, (NH4)2S2O8, FeSO4 Water 50 °C, 1.5 ha None 700 nm NR NR NR
20 α-MnO2 spheres MnSO4·H2O, K2S2O8, K2SO4, H2SO4, AgNO3 Water 40 °C, 12 h None 2 μm 150 NR 2, 10–20
60 °C, 12 h None 2 μm 106 NR 2, 10–20
80 °C, 12 h None 2 μm 83 NR 2, 10–20
123 Nb2O5 spheres NbCl5, HNO3, resol, PEO-b-PS diblock copolymer THF 50 °C, 24 h + 100 °C, 24 h 350 °C, 3 h and 550 °C, 2 h in N2 + 400 °C, 3 h in air 0.2–1 μm 131 0.26 11.4
124 NiO spheres Ni(NO3)2·6H2O, NH3 Water 97 °C, 1 h 300 °C, 2 h in air 5 μm 216 0.38 64.3
125 SnO2 spheres Na2SnO3·3H2O, D-glucose monohydrate Water 50 °C, 12 h None 50 nm 160 0.196 2.55
150 °C in air 50 nm 146 NR NR
300 °C in air 50 nm 103 NR NR
500 °C in air 50 nm 75 NR NR
126 SnO2 spheres SnSO4 Water, ethanol 25i °C, 1 h 500 °C, 2.5 h in air 100–800 nm 29 NR 4
127 V2O5 spheres Vanadium isopropoxide Acetone, pyridine, water 25i °C, 0.5 h 400 °C, 2 h in H2c + 300 °C, 1 h in aird 150–1000 nme 31 NR <30
128 V2O5 spheres NH4VO3, HCl, hydrazine Water 25i °C, 0.5 h 350 °C, 2 h in air 400 nm 12 NR <50
129 WO3 spheres Na2WO4·2H2O, HClf Water, EG 75 °C, 12 h 450 °C, 2 h in air 3–5 μm 13 NR 3.3–5.4
130 WO3 spheres Na2WO4, HCl, oxalic acid Water 25i °C, 1 ha 500 °C in air 1–3 μm 13 NR 28.1
131 WO3·H2O spheres Na2WO4·2H2O, HCl Water 70 °C, 10 h 400 °C, 2 h in air 2–3 μm 11 NR 1.7–30
132 Y2O3:Er spheres Mesoporous SiO2 spheresg, Y(NO3)3, Er(NO3)3, urea Water 90 °C, 2 h 700 °C, 3 h in air 560 nm 85 0.196 5.7
133 ZnCo2O4@CeO2 core–shell spheres ZnCo2O4, Ce(NO3)3, hexamethylenetetramine Water, ethanol 60 °C, 2 h None 1.55–1.68 μmh 34–57h NR NR
134 ZnO spheres Zn(CH3COO)2, TEA Water 25i °C, 2 ha None 520 nm 17 NR 25, 180
135 ZnO spheres Zn(CH3COO)2·2H2O, hexamine, sodium citrate Water 90 °C, 6 h 600 °C in air 2.5 μm NR NR NR
136 ZrO2 spheres ZrOCl2·8H2O, porous polymer spheres Water, ethanol 25i °C, 0.17 ha 600 °C, 6 h in air 2.6 μm 22 0.17 31


Table 4 Porous shells synthesized by the solution precipitation method
Ref. Particle type Reactants (mol/mass ratios) Hollowing mechanism Solvents Reaction conditions Calcination conditions Particle and shell dimensions BET SA (m2 g−1) Pore volume (cm3 g−1) Pore size (nm)
a Hollow-microporous organic network (H-MON). b By varying the solvent ratio or the SiO2 amount. c For H-MON prepared with the ratio of toluene[thin space (1/6-em)]:[thin space (1/6-em)]triethylamine = 1[thin space (1/6-em)]:[thin space (1/6-em)]1. d Micrococcus lylae. e Sonicated. f Mol ratio of Na2WO4·2H2O[thin space (1/6-em)]:[thin space (1/6-em)]concentrated HCl = 1[thin space (1/6-em)]:[thin space (1/6-em)]15. g Polyvinylalcohol@glucose derived carbon rich polysaccharide spheres. h Poly(styrene-acrylic acid). i When the molar ratio of ZrOCl2·8H2O[thin space (1/6-em)]:[thin space (1/6-em)]ethanol is between 0.011 and 0.032. j Assumed room temperature. NR: not reported.
137 Al2O3 hollow spheres Al2O3 spheres, PVP, NaOH NaOH etching/Kirkendall Water 25j °C, few minutes 400 °C in air 190 nm, shell thickness 23–30 nm 292 0.442 6.3
138 Al2O3, ZrO2, ZnO shell Metal salt Solid core Buffer solution 70 °C, 2 h 450 °C, 2 h in air Shell thickness tuneable 1–20 nm NR NR NR
139 CdO hollow spheres Cd(CH3COO)2, NaOH Yeast Water 25j °C, 12 h 500 °C, 4 h in air 2.3 μm, shell thickness 250–280 nm 5 0.009 3–30
140 CeO2 hollow spheres Ce(NO3)3·6H2O, HMT PS spheres Water 75 °C, 2 h 600 °C, 2 h in air 190 nm, shell thickness 15 nm 66 0.19 NR
141 Co3O4 hollow spheres Co(NO3)2 Untreated carbon spheres Water 25j °C, 1 h 450 °C, 2 h in Ar + 450 °C in air 240 nm, shell thickness 40 nm 223 0.29 15.3
Co(NO3)2 Acid treated carbon spheres 240 nm, shell thickness 15 nm 301 0.36 9.9
Co(NO3)2 Alkali treated carbon spheres 240 nm, shell thickness 70 nm 174 0.2 23
142 Co3O4 hollow spheres Co2(CO)8 H-MONa spheres Toluene 100 °C, 12 h 500 °C, 5 h in air 500 nm, shell thickness 20–80 nmb 64c 0.32c NR
14 Co3O4 hollow spheres CoCl2·6H2O, NaBH4 Bacterial suspensiond Water 25j °C, 12 h None 1 μm 149 0.26 7.7
143 CuO hollow spheres CuSO4, KOH, NH3 Ostwald ripening Water 68 °C, 24 h None 3–5 μm, shell thickness 500 nm NR NR 1–2.2, 5–30
144 CuO hollow spheres Cu(CH3COO)2·H2O, urea In situ gas bubbles Water 80 °C, 2 he None 400–500 nm, shell thickness 45 nm 60 0.104 3.6
145 Gd2O3 hollow spheres Gd(NO3)3, urea Carbon spheres Water, ethanol 90 °C, 6 h 800 °C, 2 h in air 200–250 nm, shell thickness 20 nm 33 0.17 10.9
146 In2O3 hollow spheres InCl3 Polymer spheres C2Cl4 55 °C, 6 h 600 °C, in air 720 nm, shell thickness 110 nm 329 NR 3
75 °C, 6 h 600 °C, in air 950 nm, shell thickness 140 nm 28 NR 3
95 °C, 6 h 600 °C, in air 1180 nm, shell thickness 220 nm 27 NR 3
147 MnO2 hollow spheres MnSO4 CH2Cl2/H2O interface Water, CH2Cl2 25j °C, 48 h 300 °C, 2 h in air 200–500 nm 219 0.451 5.9
148 NiO hollow spheres Ni(NO3)2·6H2O, urea Sulfonated polystyrene hollow spheres Water, ethanol 80 °C, 12 h 450 °C, 2 h in air 500 nm, shell thickness 100 nm 62 NR 2–4
149 NiO hollow spheres NiCl2·6H2O, (NH4)2C2O4 Calcination of organic species Water 25j °C, 0.67 he 500 °C, 1 h in air 1.7 μm 32 NR 3–20
150 SnO2 hollow spheres SnCl2, HCl Ostwald ripening Water 90 °C, 12 h None 100–300 nm, shell thickness 10 nm 89 NR NR
151 SnO2 hollow spheres Tin butoxide Microemulsion template CTAB, hexanol, n-dodecane, methanol, water 20 °C, 12 h None 15–25 nm, shell thickness 3–5 nm 417 NR NR
152 SnO2 hollow spheres SnCl2·2H2O Hollow SiO2 spheres None 80 °C, 24 h 700 °C, 2 h in air 340 nm, shell thickness 50 nm 46 NR 2–5
129 WO3 hollow spheres Na2WO4·2H2O, HClf Ostwald ripening Water, EG 75 °C, 12 h 450 °C, 2 h in air 3–4 μm 16 NR 5.4–89.6
153 WO3 hollow spheres with multiple shells WCl6 PVA@GCPg Ethanol 0 °C, 12 h 450 °C, 1 h in air 500 nm 124 0.14 4.3
154 WO3/WO3·H2O hollow spheres Na2WO4·2H2O, HCl, oxalic acid Ostwald ripening Water, isopropyl alcohol 80 °C, 12 h 200 °C, 2 h in air 2 μm, shell thickness 200 nm 22 0.079 14.2
155 Y2O3 hollow spheres Y(NO3)3, urea Melamine formaldehyde spheres Water 85 °C, 3 h 800 °C, 2 h in air 1.8 μm, shell thickness 100 nm NR NR NR
156 Y2O3:Ln3+ hollow spheres Y(NO3)3, Eu(NO3)3, urea PS spheres Water 90 °C, 4 h 800 °C, 2 h in air 2.1 μm, shell thickness 70 nm 62 0.313 20.7
157 Y2O3:Tb3+ hollow spheres Y(NO3)3, Tb(NO3)3, urea PS spheres Water, ethyl alcohol 85 °C, 3 h 800 °C, 2 h in air 1.3 μm, shell thickness 50 nm NR NR NR
158 ZnO hollow spheres Zn(NO3)2·6H2O, (CH2)6N4, sodium citrate Ostwald ripening Water 95 °C, 5 h 400 °C, 2 h in air 2 μm 42 NR 5–8
159 ZnO hollow spheres Zn(CH3COO)2, HMT, sodium citrate Calcination of organic species Water 95 °C, 3 h 400 °C, 2 h in air 2–3 μm 138 NR NR
160 ZrO2 hollow spheres ZrOCl2·8H2O, NH3 vapour PSAh spheres Ethanol 50 °C, NR 700 °C, 4 h in air 3.2–3.4 μm, shell thickness 80–200 nmi NR NR NR


In general, metal salts are used for the synthesis of metal oxide spheres, often supplemented by complexing/structure directing agents and basic reactants in the case of precipitation method. A reducing or oxidizing agent can also be used. For instance, NaBH4 was used to reduce Co2+ into Co nanoparticles, which subsequently were spontaneously oxidized in air onto bacterial templates to form Co3O4,14 ascorbic acid was employed to reduce Cu(OH)2 to form composite Cu/Cu2O spheres under microwave hydrothermal conditions,15 and N2H4·H2O was used to reduce Cu2+ in solution to Cu2O in the reduction-induced precipitation method,16 while oxidizing agents such as oxone monopersulfate17 or (NH4)2S2O818,19 were employed to increase the valence states of metal cations. Moreover in some cases, metal ions were used as catalysts. For the synthesis of MnO2 spheres, Ag+ was used as a catalyst to help the reaction proceed at low temperature,17,20 while in another work, Fe2+ was used to oxidize Mn2+ to MnO2 crystals.21

The hydrothermal and solution precipitation syntheses are general methods that include hard templating, soft templating, hydrolysis, Ostwald ripening and the Kirkendall effect, which are briefly discussed in the following subsections.

2.1.1 Hard templating.
2.1.1.1 Shell structures. Hard templating is the most common method for production of hollow spheres. In hard templating, nanoparticles are attached to the surface of a solid sphere and they aggregate to form a shell. A decade ago, the commonly used hard templates for the creation of spherical shells were carbon,161–163 silica164 or polymer spheres.165 As can be seen from Tables 2 and 4 these templates are still very popular, while bio-organisms have also been used. The aforementioned templates have a large number of reactive oxygen functional groups that are electron donors and therefore attract positively charged metal cations, as depicted in Fig. 3. The size of the inner hollow space can be tailored by selecting solid templates with appropriate sizes, while the thickness of the shell can be adjusted by changing the concentration of the reactants160 or by changing the hydrophilicity of the template's surface by alkaline or acid treatment.141 Just recently, hollow TiO2 nanospheres with a thin single layer of TiO2 nanoparticles were synthesized by using quasi-nanosized carbonaceous spheres as a template.166 In addition, hollow mesoporous organic networks (H-MONs), a new class of functional materials prepared through various carbon–carbon coupling reactions between organic building blocks, were prepared with varying shell thickness and used as templates to generate Co3O4 shells of different thicknesses and having surface areas between 60 and 67 m2 g−1.142
image file: c6cs00060f-f3.tif
Fig. 3 Hard templating method for synthesis of hollow metal oxide spheres.

However, the use of templates does not restrict the final shape to hollow spheres. Lou and co-workers116 found that the deposition of vanadium species on carbon sphere (CS) templates via a non-hydrolytic hydrothermal reaction between vanadium oxytriisopropoxide and isopropanol produced core–shell CS@V particles with shells made from interconnected nanosheets. Interestingly, when a small amount of water was added to the process, yolk–shell structures with rough surfaces were obtained. The VO2 species formed by sol–gel reaction on the carbon spheres underwent an Ostwald ripening process to form a well-defined gap between the core and the shell. Yolk–shell particles of V2O5@V2O5 were also created from C@V2O5 core–shell particles116 whereby the vanadium species bound to the carbon core surface would shrink into a core during annealing. Similarly, yolk–shell ZnCo2O4,120 SnO2, CeO2 and Tb4O773 were generated by hydrothermal loading of the metal precursor into the carbon template pores followed by calcination in air. TGA studies revealed that the removal of the template occurred in two steps to give the shell first and then the core. Zeng et al.167 reported the synthesis of multi-shell ZnO with single, double or triple shells by simply using carbon templates of different diameters for loading of the ZnO precursor.

Elsewhere, a penetration–solidification–annealing method was used to synthesize multi-shell spheres of CoxMn3−xO4 by using carbon spheres as templates and controlling the molar ratios of Co and Mn oxide precursors.168 Based on the anion-adsorption mechanism and usage of carbon spheres as templates, Wang and co-workers developed a sequential templating approach for production of multi-shell hollow spheres with different composition including ZnO, TiO2, SnO2, Co3O4, α-Fe2O3, Mn2O3, V2O5, etc.166,169–176 For example, multi-shell V2O5 hollow spheres were synthesized by a novel method involving competitive anion adsorption on carbon sphere templates followed by a Trojan catalytic combustion procedure (Fig. 4).176 The carbon spheres were pre-treated to create a negative charge, which could facilitate adsorption of metal anions from a solution of ammonium salt. The NH4+ cations also penetrated the templates, neutralizing the negative charges and stimulating further anion adsorption. As a result, single- or multi-shell structures were formed either by using different precursor concentrations or by performing multiple adsorption processes. The method was shown to be flexible and could be extended to the synthesis of MnO2, MoO3, Cr2O3 and WO3 multi-shell hollow spheres.


image file: c6cs00060f-f4.tif
Fig. 4 Multi-shell metal oxides prepared via an anion-adsorption mechanism: (A) schematic representation of two synthesis routes to obtain multi-shell hollow microspheres. (i) Cation-adsorption process. (ii) Anion-adsorption process. (B) Effects of synthesis conditions on the morphology of products. (C) Morphological and structural analysis of V2O5 spheres: (a–f) TEM images of the as-prepared samples. (g–i) SEM images of the as-prepared samples. Reproduced with permission from ref. 176. Copyright © 2016, Nature Publishing Group.

However, hard templating has a few disadvantages. An additional synthesis step is required to remove the templates, which is by either calcination in air for carbon-rich templates or alkaline or HF etching for SiO2 templates. The use and subsequent removal of the solid templates represents a waste in resources, which goes against “green” processing. Furthermore, calcination can lead to the partial or full collapse of the shell architectures, while alkaline etching can implicate formation of unwanted crystalline phases, such as sodium titanate (by reacting with TiO2) or impurities,152,177 whereas HF is a very toxic chemical to deal with. Nevertheless, a recent study led to the successful synthesis of hollow spheres with sandwich-type heterostructured shells via SiO2 templating, whereby hydrothermal treatment resulted in crystallization of metal oxide and simultaneous etching of SiO2 in the superhot water.178

Carbon templates can also be produced in situ during hydrothermal synthesis by adding glucose, sucrose or other organics to the metal oxide precursors during the one-pot hydrothermal process. For instance, this method afforded carbon-supported amorphous and crystalline V2O3 microspheres consisting of assembled ultrathin nanosheets of ca. 10 nm thickness by using NH4VO3 and citric acid in a mixed water and ethanediol solvent, followed by calcination in a N2 atmosphere.50 The carbon was generated by carbonization of citric acid and ethanediol, which resulted in its uniform distribution in the resulting composite having 10.6% and 8.1% of carbon in the amorphous and crystalline V2O3 respectively. Hollow spheres and multiple shells of α-Fe2O3, Cr2O3, Co3O4, NiO and ZnO were also successfully prepared by hydrothermal heating of metal chlorides in a solution of fructose at a moderate temperature of 135 °C for 6 h, followed by calcination in air.179 The carbonaceous core of partially dehydrated fructose contains functional groups such as –C[double bond, length as m-dash]O and –OH that can attract positive metal ions forming a metal oxide–carbonaceous composite. In another work,180 composite carbonaceous and Y2O3 spheres produced under hydrothermal conditions were used to generate hollow Y2O3 spheres with 1 to 4 shells by controlled calcination at different heating rates. Other multi-shell structures were successfully produced via this method such as ZnO,119 NiO,104 SnO2110 and CoFe2O4.77 However, the metal oxide shells produced this way sometimes tend to aggregate and may be non-uniform in size.75,110,181

Nevertheless, hard templates could be environmentally friendly through the use of biotemplates such as yeast139 and bacteria.14 The utilization of bacteria (Micrococcus lylae) as a template to synthesize flower-like hierarchical Co3O4 hollow spheres with a uniform size of 1 μm, a high surface area of 149 m2 g−1 and a pore volume of 0.26 cm3 g−1 was achieved via a one-pot reduction/oxidation reaction at room temperature.14 The bacterial surface is naturally covered with carboxyl, ester, amine and hydroxyl groups, which could readily capture cations. The bio-templating method is very attractive as it is facile, scalable and cost effective and could be extended to fabricate other materials and composites.

The choice of the template is important in determining the quality of the produced metal oxide nanoparticles. This has been shown when TiO2 shells were prepared by coating a core with a TiO2 layer followed by adding an outer protective layer around the TiO2.182 The core and the protective layer were made of either SiO2 or a resorcinol–formaldehyde resin-derived carbon, which was then removed by either etching or calcination in air. It was found that better crystallization occurred with the use of resorcinol–formaldehyde resin as a template due to enhanced flexibility and volume shrinkage of the resin during carbonization, while the growth of TiO2 crystals was inhibited by impregnated silicate species.


2.1.1.2 Solid spheres. The solid templates have been mainly used in the synthesis of metal oxide spheres for the creation of large mesopores. In this case, the metal species were infiltrated into mesoporous templates, which upon template removal created a porous spherical structure. Some reported examples are porous ZrO2 spheres obtained by using the EDA-functionalized poly(GMA-co-EGDMA) microspherical templates, Y2O3:Er mesoporous spheres fabricated by using mesoporous SiO2 spheres as templates, and porous WO3 spheres created by employing carbon microsphere templates.53,132,136 In another work, Chen et al.74 prepared multi-core–shell Pd@SiO2@mesoporous–CeO2 by assembling CeO2 nanoparticles around multiple Pd@SiO2 cores via a hydrothermal method. Once the silica was removed with concentrated NaOH, a multi-yolk–shell Pd@mesoporous–CeO2 structure was obtained with a size of 150–200 nm and a BET surface area of ∼103 m2 g−1.
2.1.2 Soft templating.
2.1.2.1 Shell structures. Soft templating is also a commonly used method for production of hollow spheres and yolk–shell spheres. In this method the gas–liquid, liquid–liquid or dissolved long chained organic micelles are used as soft templates for the formation and crystallization of metal oxide nanoparticles. Gas–liquid interfaces are in situ formed around gas bubbles that are generated during chemical reactions; for example, CO2 bubbles are formed from urea, NH4HCO3183 and ferric potassium oxalate82 during decomposition at high temperatures or N2 bubbles are formed from the reaction of NH4VO3 with EG.114 Nevertheless, the hollowing of structures on gas bubbles is difficult to prove and it could be more likely that Ostwald ripening is responsible for the generation of hollow spheres.

A liquid–liquid interface occurs between immiscible liquids such as water and oil. Hierarchical mesoporous MnO2 spheres were formed at the droplet interface between CH2Cl2 and H2O.147 Similarly, SnO2 hollow spheres were prepared in a water-in-oil microemulsion of CTAB–hexanol-n-dodecane–methanol–water.151 The calcined SnO2 particles had a surface area of 119 m2 g−1. In another work, a mixture of TBOT and PS in toluene was emulsified in formamide containing the triblock polymer Pluronic P123.184 After removal of toluene and PS spheres via heat treatment, macroporous TiO2 spheres of sizes between 500 nm and 2000 nm were obtained. Finally, a glycerol, water and Fe oxide precursor mixture was hydrothermally heated at 145 °C to obtain mesoporous α-Fe2O3via a quasi-emulsion templating mechanism. The product had a surface area of 103 m2 g−1 when using a glycerol/water volume ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]7;83 however a different phase of α-FeOOH90 was produced at 120 °C.

A long-chained polymer such as PVP K30 (Mw ∼ 40[thin space (1/6-em)]000) can form micelles in solution with a hydrophobic core and a hydrophilic shell. These micellar structures were used as soft templates to attach vanadium species and upon calcination to form uniform V2O5 hollow spheres with sizes of ∼800 nm115 and with exposed [110] facets, as shown in Fig. 5. During the formation of hollow spheres, VO(acac)2 accumulated on the hydrophilic PVP micelles in the EG solvent to form VO(acac)2@PVP core–shell particles. The C5H8O2 ligand subsequently was replaced by EG to form VEG, which oligomerized via the LaMer process into a tight layer on the surface of micelles. Upon calcination, V2O5 hollow spheres were formed. On the other hand, α-Fe2O3 with multi-shell morphology of ∼3 μm in size and with a surface area and pore volume of 14 m2 g−1 and 0.07 cm3 g−1, respectively, was obtained with the L-histidine amino acid as a template.87 The Fe(OH)3L-histidine–H+–NO3– complex obtained under hydrothermal conditions underwent size shrinkage and phase separation during the calcination stage to form porous multi-shell particles. In another work,97 a double templating method was employed by decorating soft micelles of the F127 copolymer onto SiO2 spheres to obtain MnO2 hollow particles following hydrothermal treatment of the template with KMnO4. The morphology and surface area could be varied from urchin-like (233 m2 g−1) to flower-like (201 m2 g−1) and non-hierarchical (120 m2 g−1) by varying the F127/SiO2 mass ratio to 0.2, 0.4 and 0.6 respectively.


image file: c6cs00060f-f5.tif
Fig. 5 Hierarchical orthorhombic V2O5 hollow nanospheres prepared via soft-templating: (a) schematic illustration of the evolution of V2O5 hollow nanospheres. (b) FESEM and (c) TEM images of V2O5 nanospheres; (d) atomic resolution HRTEM image, from which the interlayer structure of V2O5 was directly observed. Reproduced with permission from ref. 115. Copyright © 2014, Royal Society of Chemistry.

2.1.2.2 Solid spheres. In the synthesis of metal oxide spheres, long chained organics are mostly used as soft templates for surface stabilization of building blocks. These organics accumulate on the nanocrystallites favoring their growth in certain directions or planes, which affects the spatial orientation of crystal nanoparticles and growth of hierarchical structures. Eventually, the removal of these organics by calcination frees hidden pores in the assembled metal oxide structures. Some examples of the organic compounds used as structure directing agents are amino acids such as L-asparagine56 and glycine,41 sugars such as D-glucose monohydrate,125 carbohydrates such as starch185 and polymers such as sodium alginate,33,45 polyethyleneimine2 and also PVP, which has been commonly employed.22–24,47,48 To avoid unnecessary use of polymers and surfactants, alcohols and carboxylic acids have been proved to be very competent capping/structure directing agents by chelating with metal ions. Moreover when mixed together, they react at elevated temperatures to produce esters, which can further influence the growth rate and self-assembly process of nanocrystallites.26,69

Some researchers have used micelles to create mesoporous structures. Luo et al.123 examined a resol-assisted solvent evaporation method in the presence of THF, PEO-b-PS block copolymer and NbCl5. Upon evaporation of THF, the block copolymer aggregated into cylindrical micelles covered by the resol/Nb5+ composite. Upon further evaporation, the micelles tended to bend and aggregate into spherical particles, while resol was acting as a binder. Subsequent pyrolysis and calcination produced Nb2O5 spheres of diameter 0.2–1 μm having uniform mesopores with an average size of 11.4 nm, a high surface area of 131 m2 g−1, and a pore volume of 0.26 cm3 g−1. This solvent evaporation-driven self-assembly was also recently used to create mesoporous TiO2 microspheres with [101] exposed facets from spherical composite micelles consisting of PEO–PPO–PEO and titania oligomers.186 Similarly, surfactants such as Pluronic P123, due to its long hydrophobic chains, can be used to create large mesopores between 12 and 15 nm in the WO3/TiO2 composite spheres.54 Elsewhere, Wang's group187 used a water in oil system in the presence of acrylamide and azobisisobutyronitrile to synthesize hierarchically mesoporous hematite microspheres with high surface area and bimodal structure with mesopores of 2.5 nm and 9 nm.

2.1.3 Sol–gel and controlled hydrolysis. The sol–gel process is a viable method for the synthesis of metal oxides from alkoxides such as butoxides, ethoxides and isopropoxides. Alkoxides vigorously react with water molecules in the presence of an excess of alcohol and ammonia as catalyst to form an inorganic network or gel of a polymer-like metal oxide that condenses into particles shown in Fig. 6. Titanium alkoxides such as titanium butoxide and titanium isopropoxide and halides such as TiCl4 are very well known and commonly used precursors for the synthesis of TiO2 spheres. Because of the nature of these TiO2 precursors, the reaction can be controlled by adding a small amount of water. Some of the techniques used to control the amount of water are discussed in the previous section.
image file: c6cs00060f-f6.tif
Fig. 6 Schematic illustration of the formation of metal oxide spheres by a sol–gel process.

Controlled hydrolysis has been used to prepare TiO2 spheres,188–196 core–shell,197 yolk–shell,198,199 and hollow177,183,200–204 particles. A more detailed discussion on the synthesis of TiO2 has been presented elsewhere; the reader is encouraged to refer to a comprehensive review on the synthesis of spherical TiO2 nanostructures by Chen et al.5

Nevertheless, other types of metal alkoxides have also been used to produce the respective oxides, such as vanadium oxiisopropoxide,51,116 vanadium(V) oxytriisopropoxide127 and tin tert-butoxide.151 As shown in Fig. 7, V2O5 mesoporous spheres were synthesized at room temperature by reacting vanadium isopropoxide in a mixture of acetone, pyridine and water at a volume ratio of 983[thin space (1/6-em)]:[thin space (1/6-em)]500[thin space (1/6-em)]:[thin space (1/6-em)]1.127 The average size of the particles was tuned between ∼1 μm and ∼150 nm by increasing the amount of water while maintaining the ratio of pyridine/acetone. The reduction of particle size with increasing water content was attributed to the increased number of sites for nucleation of particles.


image file: c6cs00060f-f7.tif
Fig. 7 SEM image of the V2O5 porous microspheres; the inset shows the porous structure of a single sphere. Reproduced with permission from ref. 127. Copyright © 2011, Royal Society of Chemistry.

Silica supported Ta2O5 (SiO2–Ta2O5) composite shells were produced by sol–gel synthesis using TEOS, tantalum isopropoxide, CTAB, H2O, NH3 and ethanol.113 NH3 catalyzed the reaction but also assisted in the dissolution of cores at higher temperature. The diameter and shell thickness were tuned by changing the molar ratio of Si[thin space (1/6-em)]:[thin space (1/6-em)]Ta. The BET surface area of the calcined particles increased from 225 to 610 m2 g−1 with increasing Ta content.

The sol–gel method can be also used to manufacture templates for the synthesis of hollow and porous spheres. The one-pot sol–gel polymerization of formamide–resorcinol was employed to create vesicle templates for the synthesis of hollow In2O3 spheres,205 while the porosity in SnO2 spheres was created upon removal of the carbon template from composite Sn–resorcinol–formaldehyde resin particles.206

2.1.4 Ostwald ripening. According to the IUPAC's terminology, the process of Ostwald ripening refers to “the growth of larger crystals from those of smaller size which have higher solubility than the larger ones”.207 As can be seen from Tables 2 and 4 the Ostwald ripening represents the major process of hollowing particles under hydrothermal conditions. Once a metal oxide amorphous precursor is formed, crystallization proceeds with increasing time; namely, smaller nanocrystals from the central core dissolve and recrystallize into loosely packed crystallites at the surface of spheres that act as seeds. The resulting void enlarges until a hollow structure is obtained, as shown in Fig. 8a. For example, flower-like hollow MgO spheres having a size of about 1 μm, cavities of ∼500 nm, a high surface area of 343 m2 g−1 and a pore volume of 1.9 cm3 g−1 were fabricated by ripening magnesium glycolate under hydrothermal conditions.96
image file: c6cs00060f-f8.tif
Fig. 8 Ostwald ripening initiated by dissolution of the middle core (panel a), and localized Ostwald ripening (panel b).

In some cases, due to the localized Ostwald ripening around a dense core, an intermediate yolk–shell architecture is formed (Fig. 8b), as in the case of MoO2@MoO2,102 CeO2@CeO2,70 SnO2@SnO2,106 TiO2@TiO2208 and V2O5@V2O5,117 which subsequently is converted to a multi-shell structure having 2–3 shells after a prolonged hydrothermal process, and finally to hollow spheres. Similarly, double-shell CoO and Co3O4,209 double-shell α-Fe2O3,84 as well as perovskite-type BaZrO3@BaZrO364 and LaFeO3@LaFeO391 structures were obtained. In a different work, Li et al.86 found that the two interfaces created by hydrothermal etching of silica from Fe3O4@SiO2@TiO2 in 1 M NaOH at 150 °C for 24 h allowed for the dissolution of TiO2 crystals and their subsequent growth in an opposite direction to eventually form Fe3O4@TiO2 double-shell spheres with flower-like morphology and with a uniform size of ∼560 nm and a high surface area of 150 m2 g−1. Ostwald ripening involving selective etching of crystals is favored in basic and acidic solutions and it is apparent from Tables 2 and 4 that bases such as urea, KOH, NaOH and amines or acids such as HCl, HNO3 and other organic acids facilitate this process.

2.1.5 Kirkendall effect. The Kirkendall effect is an atomic diffusion phenomenon that occurs at elevated temperatures between two species separated by an interface (solid–solid, solid–liquid or solid–gas) and having different diffusion rates. The net directional flow of matter is balanced by an opposite flux of vacancies, which can result in the formation of voids, normally at the interface because of its high energy and defect density. Consequently, the Kirkendall effect has been utilized for the preparation of hollow nanocrystals,210 yolk–shell particles211 as well as metal oxide hollow spheres.111,112,212,213Fig. 9 shows the formation of a shell by the exchange of ions between the solid core and its surrounding.
image file: c6cs00060f-f9.tif
Fig. 9 Hollowing by the Kirkendall effect.

The thermal treatment in air of a film decorated with the Cu(II) complex showed that the bulk diffusion of atoms/ions at the Cu–O interface gave Cu2O-rich and CuO-poor spheres at 200 °C.212 As the temperature increases, the Cu from the core moves outwards through the oxide shell to react with oxygen, leaving a hollow space (since the outward diffusion of Cu ions is much faster than the inward diffusion of O ions) until a hollow sphere of pure CuO is formed at 400 °C. Core-in-double-shell hollow NiCo2O4 spheres were obtained by slow annealing of NiCo–glycerate spheres in air due to a combination of the Kirkendall effect and the contraction and adhesion forces during the oxidative degradation of organic species.105 This method can also be extended to the synthesis of ZnCo2O4 and CoMn2O4 with complex interior structures.

The Kirkendall effect can also occur during the hydrothermal reaction stage. The reaction of TiO2 microspheres with a solution of strontium chloride hexahydrate at 180 °C for 6 h generated perovskite-type SrTiO3 hollow spheres having a size of 3–5 μm and a shell thickness of ∼700 nm.112 With the assistance of NaOH, the Ti–O–Ti bonds can be broken to form Ti–O–Na on the surface of the sphere. Then, the Sr2+ ions can react with the sodium titanate to form a thin layer of SrTiO3 shell, separating the inner TiO32− ions from the Sr2+ ions in solution. Hence, the concentration gradient between these two types of ions permitted TiO32− to diffuse out and the Sr2+ ions to diffuse in through the shell, resulting in hollow SrTiO3 spheres. Additionally, composite SnO2–C hollow spheres were prepared by Wu et al.111 under hydrothermal conditions by reacting Sn spheres in a solution of glucose at 180 °C.

Interestingly, a simple solution route by mixing hydrothermal carbon spheres in a solution of KMnO4 at room temperature produced MnO2 spheres of different morphologies.214 Solid MnO2 spheres were produced with 100 ml of 25 g L−1 KMnO4, C@MnO2 yolk–shell spheres were obtained with 100 ml of 2.5 g L−1 KMnO4 and finally MnO2 hollow shell spheres were obtained with 200 ml of 2.5 g L−1 KMnO4 solution. The formation of different morphologies was achieved due to different stages of the Kirkendall effect occurring through the soft surface of the hydrothermally synthesized carbon spheres by varying MnO4 concentration.

The Kirkendall mechanism provides a pathway for the selective etching of the surface-protected metal oxides to produce hollow structures. For instance, the PVP-protected TiO2 solid spheres were selectively etched by fluoride ions to form hollow or yolk–shell TiO2.215 Similarly, it was also reported that NaOH and HCl were used to etch the PVP-protected colloidal Al2O3 and ZnO into hollow spheres, respectively.137

It is apparent that during the Kirkendall mechanism, a solid core acts as a sacrificial template by reacting with its surrounding environment to form different hollow structures. This method could therefore be extremely useful to synthesize a variety of complex hollow compounds and composites from various solid templates.

2.2 Spray method

Spray methods include electrospray ionization216–219 and gas phase processes such as aerosol220–225 and flame or ultrasonic spray pyrolysis (USP).226–229 These methods use high temperatures to evaporate the liquid from the colloidal precursor solution released by the spray nozzle to form solid or hollow spherical structures. Several types of metal oxides such as CeO2 spheres,216 TiO2 spheres,217 WO3 spheres,228 ZnO spheres,221,225 α-Fe2O3 microspheres,230 CuO hollow spheres,222 Mn3O4 hollow spheres,224 WO3 hollow spheres,226 hollow TiO2 and ZrO2 spheres,223 TiO2 core–shell particles,218 Bi2WO6 spheres,229 Fe3O4–carbon composite spheres,220 Li2O–CuO–SnO2 multi-deck cage-type spherical composites219 and α-Fe2O3 multi-shell hollow spheres231 have been produced via the spray method. Some morphologies including porous spheres,221 hollow spheres,222,223 yolk–shell spheres,232 yolk–multi-shell spheres,233 and “ant-cave” spherical structure234 are shown in Fig. 10.
image file: c6cs00060f-f10.tif
Fig. 10 Different types of metal oxide structures prepared by the aerosol method: (a) porous sphere (reproduced with permission from ref. 221; Copyright © 2014, Royal Chemical Society); (b) hollow sphere loaded with nanometals (reproduced with permission from ref. 223. Copyright © 2013, Wiley); (c) hollow shell (reproduced with permission from ref. 222. Copyright © 2013, Wiley); (d) yolk–shell sphere (reproduced with permission from ref. 232. Copyright © 2013, Wiley); (e) yolk–multi-shell sphere (reproduced with permission from ref. 233. Copyright © 2013, Wiley); and (f) “ant-cave” spherical structure (reproduced with permission from ref. 234. Copyright © 2013, American Chemical Society).

In the electrospray method, the liquid is evaporated via the potential difference between the nozzle and the metal receptor, while in the gas phase processes, the colloids pass through a flame or horizontal furnace (Fig. 11). Kang and co-workers have published numerous works on spray pyrolysis for the synthesis of hollow spheres, yolk–shell particles, multi-shell spheres and porous microspheres. Hollow WO3 spheres with thin and porous shells were produced by USP using citric acid as the carbon source.226 Multi-shell structures were created from the precursor dissolved in sucrose solution. During decomposition, a dense carbon–metal oxide composite was formed, which upon further heating resulted in contraction and combustion of the carbon to form the multi-shell structures. This method was used to prepare yolk–shell TiO2 and composite multi-component systems (composed of up to 5 components including TiO2, Al2O3, ZrO2, CeO2 and Y2O3),232 double-shell LiNi0.5Mn1.5O4 particles,235 Pd loaded double-shell SnO2 particles233 and double-shell SnO2 spheres.234 Alternatively, the spray-pyrolysis method has been extended to the synthesis of yolk–shell structured metal oxide with 10 kinds of metal components in one step as shown in Fig. 11b.236 The method could also be modified to produce metal sulfide multi-shell spheres. SnO2 yolk–double shell spheres were indeed treated with H2S gas to produce SnS yolk–double shell spheres.237 Another work reported the synthesis of a new structured material named “ant-cave microball”, where polystyrene nanobeads were used as templates to create MoO3–C composite spheres.234 The decomposition of these nanobeads resulted in unique morphology of porous composite spheres with nanochannels, effectively resembling an ant-cave.


image file: c6cs00060f-f11.tif
Fig. 11 Spray pyrolysis for the formation of yolk–shell-structured LiNi0.5Mn1.5O4 spheres (panel a; reproduced with permission from ref. 235. Copyright © 2013, Royal Chemical Society) and yolk–shell ten-component transition metal oxide powder (panel b; reproduced with permission from ref. 236. Copyright © 2014, Royal Chemical Society).

ZnO spheres were synthesized via an aerosol method using an organometallic precursor dissolved in toluene and Brij 58 as the structure directing agent.221 The ZnO spheres had a BET surface area, crystal size and maximum pore volume of 61 m2 g−1, 8.6 nm and 13 nm, respectively. Al and S could be easily incorporated into the ZnO matrix by adding similar organometals into the precursor solution. Unfortunately, the dopants reduced the crystal size and hence the maximum pore size of the resulting ZnO spheres but this could be counteracted by using the triblock copolymer P123. Recently, very high surface area α-Fe2O3 microspheres with an average size of 560 nm, a BET surface area of 301 m2 g−1 and an average pore size of 2.1 nm were synthesized by USP using Fe(NO3)3 and Na2CO3 as precursors.230 The average particle size could be tuned by changing the concentration of the precursors.

Hollow or macroporous structures could be synthesized by using hard templates,223,228 furnace synthesis at elevated temperatures,225in situ bubble reactions222,224 or non-equilibrium air calcination.231 Au nanorods, Pd nanocubes and Au core/Pd shell nanorods were successfully introduced into hollow TiO2 and ZrO2 spheres by initially embedding these nanometals in PS nanospheres. The PS spheres were dispersed in solution containing metal alkoxides and then sprayed by using N2 through a tube furnace. Subsequently, calcination was performed to remove the PS template, leaving hollow TiO2 and ZrO2 spheres of average diameters of 0.8 μm and 0.6 μm, respectively, and containing nanometals inside the hollow space. In another work,225 ZnO spheres of various shapes were produced simply by changing the furnace temperature. Amorphous porous spheres were obtained between 40 °C and 100 °C, solid spheres at 400 °C, yolk–shell spheres at 600 °C and hollow spheres with different crystallite sizes between 700 °C and 1200 °C. The hollowing process was induced by the Kirkendall effect. An interesting in situ bubble hollowing method was devised by Jian et al.222 to prepare hollow CuO spheres by adding sucrose and H2O2 to Cu(NO3)2 solution. The decomposition of the sucrose into CO2 and H2O (with H2O2 acting as a catalyst) within the aerosol at high temperature inflated the spheres like balloons to produce particles with an average size of ∼85 nm and very thin walls of 5–10 nm. The same strategy was used to prepare hollow Mn3O4 spheres.224 α-Fe2O3 multi-shell hollow spheres231 were synthesized by spray drying a mixture of Fe(III) citrate and sucrose. The obtained Fe(III)–sucrose composite was then calcined in air to remove the carbon template. From the effect of non-equilibrium heating in air, the number of shells could be varied between 2 and 4 by simply changing the Fe(III) citrate/glucose ratio between 0.25 and 1.5.

The spray method is a simple and continuous process with a short residence time (a few seconds) of particles at a high temperature, which produces high purity products and can be easily implemented on an industrial scale. Moreover, other constituents can be included in the precursor solution allowing the preparation of composite or doped metal oxide particles. However, due to the low residence time of the particles at high temperature, further annealing may be required to improve the crystallinity of the products. Furthermore, the method has not yet been able to create hierarchical structures and the surface areas of the particles are often in the low to moderate range.

2.3 Other methods

Besides the methods presented above, there are a variety of other routes for the preparation of colloidal metal oxide structures such as template- and solvent-free methods, ultrasonic irradiation- and microwave-assisted syntheses, electrodeposition, direct printing, and methods involving lasers or taking advantage of gas–liquid diffusion.

A template- and solvent-free method was devised by Wang et al.238 for the synthesis of hierarchical metal oxide spheres (HMOS) of TiO2, Fe2O3, ZrO2 and their composites. This method involves grind milling of the metal oxides in the presence of PEG and some water to create a paste, which was spread into a film and annealed. The process generated microspheres via PEG modification and self-assembly and has great potential for large scale production of HMOS.

During ultrasonic irradiation, the formation and collapse of bubbles in the aqueous phase results in localized extremely high temperatures (>5000 K), high pressures (>20 MPa) and very high cooling rates (1010 K s−1), which can supply enough energy to drive the formation of spherical metal oxide structures.144 Various metal oxide spherical particles such as ZnO hollow nanospheres of size ∼80 nm,239 mesoporous NiO hollow spheres,149 MnO2 spheres made of interconnected nanoflakes,21 ZnO spheres with bimodal pores at 25 nm and 180 nm,134 CuO hollow spheres,144 composite Ag2O–MnO2 spheres with Ag2O residing at the end of MnO2 nanowires,19 and WO3 spheres130 were synthesized that way.

The electrodeposition method was used to prepare uniformly distributed 100–500 nm sized MnO2 spheres with a very high surface area of 129 m2 g−1 and mesopores in the range of 5–12 nm, which were composed of randomly oriented nanorod-like structures.240 This synthesis is inexpensive, operates at room temperature and the deposition potential is an extra parameter that can be varied to achieve different morphologies. However, the synthesized oxides have low crystallinity and require further annealing. Other examples of particles prepared via electrodeposition are Y(OH)3 and Y2O3 nanospheres,241 Co3O4 hollow spheres deposited on PS spheres and organized into a close-packed monolayer array242 and SnO2 spheres.243

The microwave-assisted synthesis is analogous to the hydrothermal method but offers a much faster heating rate of the solution. An enormous advantage of this method is the very short time which is reduced to minutes instead of hours (as shown in Tables 1 and 2) as compared to the hydrothermal method. Some examples of metal oxides prepared by this method are Fe3O4 and γ-Fe2O3 hollow spheres,85 NiO spheres41 and hollow spheres244 with very high surface areas reaching 200 m2 g−1, TiO2 spheres195,245 and Bi2O3 spheres.22

Laser irradiation is another powerful and versatile way to obtain CuO246 and ZrO2 spheres.247 This method was even used to make hollow spheres of metals and semiconductors such as Fe, Co, Ni, TiO2, Co3O4, NiO, WO3 and Fe2O3.248 The method offers control over the size of particles and high crystallinity, and therefore no annealing step is required. The high energy dispersed during laser heating was successful in producing single-crystalline rutile TiO2 at room temperature with an average size of 540 nm248 from commercial anatase TiO2 nanoparticles dispersed in acetone. The hollowing was attributed to the Kirkendall effect. The size of spheres could be tuned by controlling the laser beam and irradiation time; however the size of the void space could not be controlled. In another light-driven approach, UV irradiation was used to decompose titanium glycolate spheres into highly uniform mesoporous TiO2 spheres with amorphous structure.249

Some other methods include thermal decomposition of various precursors such as Ni(CH3COO)2·4H2O at 500 °C for 10 h to produce NiO mesoporous spheres,250 synthesis of hollow CuO spheres212 involving direct printing of metal–ion complex ink on a substrate followed by thermal heating, and a gas–liquid diffusion method to prepare Co3O4 hollow spheres by controlled precipitation from a solution of Co(NO3)2 in the presence of a vapor from crushed ammonium carbonate.251

2.4 Methods for controlling the precipitation process

To prevent the uncontrolled precipitation of the metal oxide into large aggregates, the release of reactants in solution can be regulated. The commonly used procedures to control the precipitation process include metal complexation, pH control and controlled release of other reactants. Very low concentrations of metal precursors (usually in mmol amounts) are used as the first preventive measure.
2.4.1 Complexation. Metal ions present in solutions have a high positive charge to radius ratio, which makes them highly electronegative, i.e., they have a high affinity for electrons. As a result, the metal cations tend to attract polar molecules (or ligands) in solution by chelation to form stable dissolved metal complexes. Examples of ligands that form metal complexes are water, NH3,122 simple organic compounds such as organic acids (e.g. citric acid,37 ascorbic acid34 and tartaric acid37), alcohols (methanol48 and EG82,88), amines,59,252 acetylacetonates,30 acetates,2,32,42 citrates38,58 and oxalates.31 The metal complexes prevent the uncontrolled precipitation of the metal oxides into aggregates by slowly combining with hydroxide ions via multiple steps, as shown in eqn (1) and (2) for a hexa-aqua 2+ complex, until a neutral complex is precipitated out of the solution.
 
[M(H2O)6]2+ + OH → [M(H2O)5(OH)]+ + H2O(1)
 
[M(H2O)5(OH)]+ + OH → [M(H2O)4(OH)2] + H2O(2)
Cations with high positive charge such as Ce3+,26 Fe3+,35 In3+,253 Sn4+[thin space (1/6-em)]47,48 and W6+[thin space (1/6-em)]52,53 can precipitate from the solution via forced hydrolysis at elevated temperatures without the need of a basic additive. The soluble hydroxyl complexes created by hydrolysis reaction (as per eqn (3)) form precursors for the nucleation of crystallites.
 
[M(H2O)n]z+ ⇌ [M(H2O)ny(OH)y](zy)+ + yH+(3)
Ligands behave like surfactants or capping agents, since they adhere to preferred planes of the grains/crystallites, which then prevent their growth in specific directions. This gives rise to hierarchical spherical structures, built from differently shaped building blocks, such as nanocubes, nanosheets, nanopetals, nanowires, etc.
2.4.2 pH control. While the availability of metal ions is controlled via chelation, there are other methods to limit the availability of OH ions for precipitation. A slow release of OH ions for controlled precipitation of metal hydroxide complexes can be realized in solution via degradation of amides (such as urea) and amines (such as HMT, hydrazine and ethanolamine) at moderate temperatures (above 80 °C). Tables 1 and 4 show that this method is popular for the synthesis of solid and hollow spheres at moderate temperatures.

To successfully coat a homogeneous and very thin layer of metal oxide on a substrate can be a very challenging task. This subject has been studied by Zhang et al.,138 who reported a simple but interesting method of controlling the amount of OH ions available for precipitation of metal hydroxides, which could be converted to metal oxides upon calcination. For controlled precipitation, the ionic product of the metal hydroxide precursor (Kmp) has to be equal to or slightly higher than its solubility constant (Ksp). For a metal hydroxide M(OH)n, Kmp can be related to pH as expressed by eqn (4) and (5):

 
Kmp = [Mn+aq][OHaq]n(4)
 
Kmp = [Mn+aq][Kw × 10pH]n(5)
Hence for precipitation,
 
KmpKsp(6)
where n is the valence number of the metal ion and Kw is the water dissociation constant, which has the value of 1.00 × 10−14 mol2 L−2. A buffer solution within the required pH range can then be used to provide a constant pH during reaction for controlled precipitation. Consequently, for Al2(SO4)3 in a formic acid/ammonium formate buffer solution within the pH range of 3.8 and 4.6, a nanolayer of Al2O3 could be coated on different kinds of substrates such as Au, Si, SiO2 and PS and different shapes could be created such as core–shell structures of multi-walled carbon nanotubes. The thickness of the shell could be controlled between 1 and 20 nm by adjusting the concentration of Al2(SO4)3 salt. This method can be easily extended to other metal oxide coatings.

2.4.3 Controlled hydrolysis. The reaction of alkoxides with water during the sol–gel process is extremely fast. A few protocols are adopted to avoid uncontrolled precipitation of particles. Commonly, very small amounts of water are used in high purity alcoholic solvents (purity very close to 100%). Also, alkoxide or hydrated precursors are added very slowly, e.g., by vapor deposition193 or in a dropwise fashion.177,199,201 Gyger et al.,151 on the other hand, minimized the contact of Sn(Ot-Bu)4 with water by slowly adding a mixture of Sn(Ot-Bu)4 in dodecane to a water-in-oil microemulsion system. The hydrolysis of tin tert-butoxide occurred slowly on water droplets at the organic–water interface, eventually leading to the formation of SnO2 hollow spheres.

The deposition of a very thin layer of TiO2via the modified Stöber sol–gel method is normally difficult. However, it was successfully achieved by controlling the hydrolysis rate of the TiO2 precursor by varying the amount of ammonia, which has a crucial role in controlling the reaction kinetics for the formation of TiO2 shells.204 Accordingly, the thickness of the TiO2 shell could be changed from 25 nm to 70 nm by varying the ammonia concentration from 0.25 vol% to 0.4 vol%.

Some researchers used the in situ generated water during chemical reactions for the synthesis of metal oxide particles. Liu et al.254 used the aldol condensation reaction between titanium isopropoxide and acetone to generate water molecules for the formation of TiO2 spheres. Similarly, the water produced during the reaction of vanadium(V) oxytriisopropoxide with acetic acid catalyzed the hydrolysis–condensation process that led to the formation of vanadium oxide spheres.51 Guo et al. synthesized TiO2 spheres by using water generated during esterification reaction of ethanol and acetic acid.255 In another example, TiO2 shells were prepared on hydrated sulfate templates of ZnSO4·7H2O in ethanol.256 The spherical templates of ZnSO4·7H2O in ethanol that were formed acted as sites for deposition of titanium species. Moreover, water present in hydrated crystals acted as a supplier of water molecules for hydrolysis of titania precursors.

3. Effect of synthesis parameters

In this section, we examine the effects of the different experimental parameters such as reaction time, reaction temperature, calcination, pH and reactant and solvent types on the structure and phase of metal oxide spherical particles.

3.1 Reaction time

During initial stages of the reaction, nanocrystallites are formed and start aggregating into spheres to reduce their surface energy. The spheres are amorphous in nature but their size, long range ordering and the degree of crystallinity improve with increasing synthesis time.

Solution phase reactions at high temperatures favor hollowing via Ostwald ripening as time increases. The hollowing can be initiated at the central core whereby the relatively small crystallites dissolve and migrate to recrystallize on larger crystals on the surface of spheres. This mechanism has been used to obtain hollow spheres of different metal oxides, such as CeO2,67,69,72 CuO,33,78,79,143 CuO/CuO2,80 ZnO,158 MgO,95 ZrO2,62 SnO2,150 NiO,244 TiO2257 and In2O3.94 Some examples are presented in Tables 2 and 4. The shell thickness67 or hollow core size94 could be increased by increasing the reaction time. However, the synthesis time should be carefully adjusted because an excessive reaction time can corrode and collapse the formed hollow structures.33,158

Another hollowing Ostwald ripening mechanism occurs at localized spots within the spheres, at fracture points or low crystal densities. Such cases can result in the formation of yolk–shell particles such as MoO2@MoO2,102 TiO2@TiO2,81,203 CeO2@CeO2,70 γFe2O3@γFe2O3,88 BaZrO3@BaZrO364 and LnFeO3@LnFeO3 (Ln = La, Pr–Tb),91 multi-shell particles such as Fe3O4@TiO2 double-shell structure86 and V2O5 with up to three shells117 or porous In2O3 particles.37 However these structures are, in general, eventually transformed into hollow shells due to further dissolution of the core if reaction time is further increased.

3.2 Reaction temperature

The reaction temperature is an important parameter, which affects the rate of crystal nucleation and therefore the size, morphology and polymorph of metal oxide particles.

Temperature affects the crystallization rate, which limits the quantity of crystals formed. This in turn affects the shape32,36,64,79 and size21,32,146 of the synthesized particles. As the nanocrystals continuously form, they aggregate into hierarchical structures. During hydrothermal growth of CuO spheres, the particle shape evolved from irregular CuO nanoparticles at 80 °C to uniform spindle-shaped CuO nanorods with sharp ends at 100 °C, small amounts of nanospheres at 140 °C and large scale monodisperse nanospheres with a wave-like surface at 160 °C.32 On the other hand, hollow flower-like spheres of α-GaOOH changed into microspheres and finally into rods as the temperature increased from 175 °C to 225 °C.36 Therefore, the optimum temperature for achieving a maximum amount of the required structures needs to be experimentally determined. The diameters of solid spheres21 or shells32,146 can also increase with temperature. The ultrasound-assisted solution precipitation of MnO2 produced spherical particles with an average size that could be tuned between 0.4 and 1.28 μm by varying the temperature from 30 °C to 70 °C.21 High temperature was shown to favor the hollowing process of solid spheres via Ostwald ripening.64,79,244 Conversely, the synthesis of hollow structures via bubble82 and emulsion templating83 is not recommended at high temperatures due to reduced stability of the liquid medium.

Changes in temperature can also affect the growth patterns of the crystals, giving rise to different crystal phases. As an example, the Cu2O content in CuO spheres increased with temperature79,80 while α-Fe2O3 was favored at elevated temperatures instead of FeOOH.35,90

3.3 Calcination

Calcination improves the crystallinity of metal oxides by densifying the crystallites and also removes organic impurities bound to the precursor during the hydrothermal process. As a result, the grain size of the crystals increases and the removal of water and CO2 creates additional pores and interspaces. The sum of these effects generally decreases the surface area of particles (although an increase in the surface area has been observed in some cases56) and increases the pore volume.

Calcination in an oxygen-containing atmosphere is required to remove carbon templates in the synthesis of porous or hollow spheres. Sometimes, multi-step calcination involving a pyrolysis step followed by oxidative calcination is required. During the synthesis of mesoporous Nb2O5,123 the PEO-b-PS copolymer was used as a pore forming template and resol as a “glue” for the niobia composite following high temperature polymerization. The composite was pyrolyzed first at 350 °C to selectively decompose PEO-b-PS, then at 550 °C to crystallize Nb2O5 crystals without collapsing the polymer skeleton. Finally, the carbon skeleton was removed by calcination in air at 400 °C to obtain mesoporous crystalline Nb2O5 particles.

Some researchers reported the formation of yolk–shell structures of ZnCo2O4,120 SnO2, CeO2 and Tb4O773via simple calcination of metal oxide precursors at 600 °C (using a rate of 5 °C min−1) that were hydrothermally loaded into the pores of carbon spheres. The template removal occurred in two steps with the burning off of the first layer closest to the surface, separating the oxide shell and the composite core. Further calcination removed the carbon template from the core to produce the yolk–shell metal oxide structures.

In some circumstances, the heating rate was found to have some interesting effects on the final structure of calcined particles. A zinc oxide precursor–carbon composite was calcined in air via three methods.119 The first sample was calcined for 3 h in a preheated furnace at 550 °C, the second sample was calcined at a heating rate of 5 °C min−1 to 550 °C and maintained for 3 h and the third sample was calcined at a heating rate of 2 °C min−1 to 550 °C and maintained for 3 h. Single-shell hollow spheres were obtained from the first sample, while double- and triple-shell spheres were formed from the second and third samples, respectively. The formation of multi-shell particles was explained by the occurrence of a temperature gradient between the exterior and interior of the precursors, giving rise to an inside out Ostwald ripening process. Dong et al.170 went a step further by proposing an easy way to control the number of shells and inter-shell spacing in hollow ZnO microspheres by controlling the heating processes (heating rate and final temperature) and precursor concentration on the carbon sphere templates. In another work, a gradual removal of the carbon core from C@V composite microspheres was successfully used to obtain V2O5@V2O5 yolk–shell particles.116 The core was much larger when the C@V sample was annealed at 350 °C at a rate of 3 °C min−1 as compared to a rate of 1 °C min−1, due to the relatively fast removal of the carbon core at the highest rate, leaving behind a V2O5 core formed from the vanadium species that were initially bound to the carbon core. However, annealing at 400 °C at a rate of 1 °C min−1 gave porous single-shell spheres. In a similar fashion, the control of heating rate could generate core-in-double-shell NiCo2O4 spheres105 and multi-shell Y2O3 spheres.180

The calcination temperature can also govern the metal oxide polymorphism. For example, anatase TiO2 forms between 300 °C and 600 °C and the more crystalline rutile phase starts to form at temperatures >600 °C,182,191,258 the perovskite LaFeO3 would not form below an annealing temperature of 750 °C,91 β-Bi2O3 was obtained at 350 °C while α-Bi2O3 was formed at 450 °C,259 the change of phase from γ-Ga2O3 to β-Ga2O3 occurred at T > 700 °C92 and γ-Al2O3 and α-Al2O3 were formed at 900 °C and 1100 °C, respectively.121

3.4 pH

We have already mentioned how pH alters the degree of precipitation and induces Ostwald ripening in the particles. Nevertheless, the solution pH can affect particle growth in various ways, for instance, by controlling adsorption of the precursor on carbon templates,175 changing the degree of ionization of the ligands,67 modifying the electric charge of the surface of nanoparticles253 and altering the degree of complexation of the metal ions in solution.104

In the synthesis of Mn2O3, multi-shell structures were formed with the help of carbon sphere templates; the amount of metal cations that can be adsorbed on the carbon spheres was varied by using different solution pH (since high precursor concentration results in its accumulation on the template surface rather than in its infiltration into template pores).175 This strategy was shown to be well suited to alter the zeta potential of carbon spheres and hence their electrostatic interaction with metal ions. As a result, depending on pH, single-, double- or triple-shell Mn2O3 structures were formed upon calcination.

CeO2 particles of different shapes were respectively obtained at pH of 1, 2 and 3.5 in the presence of citric acid used as a ligand, namely, solid spheres, hollow spheres and microplates.67 Due to the three levels of ionization of citric acid (pKa = 3.13, 4.76 and 6.4), different ligands were formed at different pH. At pH below 1, most of the citric acid was in solution as H3Cit without ionization, which coordinated with Ce3+ to give solid microspheres. At pH = 3.5, H2Cit concentration was higher than H3Cit concentration, giving rise to microplates. At pH = 2, the ligands were a mix of H3Cit and H2Cit, which resulted in the formation of coordination polymers (metal organic frameworks) in a metastable stage between solid spheres and microplates, compromising the shape into hollow spheres.

The pH of a microemulsion system was found to alter the surface energy and the electric charge distribution on the surface of In(OH)3 nanoparticles as well as the amount of adsorbed CTAB on the different crystal faces.253 As a result, the particles aggregated and grew in different directions to form bundles of nanorods at pH = 5 and spheres at pH = 3.

The hydrothermal synthesis of NiO in the presence of D-glucose was performed at different ammonia concentrations104 to investigate its effect on the shape of final particles. A high concentration of ammonia decreased the concentration of free Ni2+ by forming stable [Ni(NH3)x]2+ complexes in solution and thereby controlling the rate of precipitation of Ni(OH)2. Therefore, aggregated porous structures were obtained at pH < 10.5, and at pH 10.5 a controlled precipitation of the Ni species on the in situ formed carbon spheres followed by calcination afforded hollow microspheres. A further increase of pH to 10.9 reduced the rate of precipitation of nickel hydroxide, allowing formation of carbon layers on the surface of Ni(OH)2 particles, which upon calcination were converted to multi-shell NiO microspheres. Therefore, pH is an important parameter to consider for controlling the morphology of particles.

3.5 Solvents and reactants

The type and composition of solvents is also an important factor that can be used to manipulate the morphology and surface area of metal oxide particles.55,83,90,95,122,251,260

A mixture of water and ethanol was used to synthesize multi-shell Co3O4 when using carbon spheres as templates as opposed to single shells when only water was used.171 The effect of ethanol was used to decrease the number of aqua groups coordinated to Co ions, hence to decrease the size of the hydrated Co ions. This strategy was used to control the diffusion of ions through the carbon template, allowing the production of spherical structures, which upon calcination gave multi-shell structures. The same principle was applied to synthesize α-Fe2O3 multi-shell hollow microspheres.172

The quasi-emulsion template was used for the formation of α-Fe2O3 hollow spheres;83 the concentration of the soft template was varied by changing the glycerol/water ratio at 145 °C. As a result, the morphology was evolved from solid flower-like particles consisting of densely packed needle-like subunits to hollow spheres and finally solid spheres made up of nanosheets at the solvent ratios of 1[thin space (1/6-em)]:[thin space (1/6-em)]19, 1[thin space (1/6-em)]:[thin space (1/6-em)]7 and 1[thin space (1/6-em)]:[thin space (1/6-em)]4, respectively. The effect of the EG/water ratio was investigated during hydrothermal synthesis of MgO microspheres.95 It was found that at a high EG volume, spheres were obtained, while at a high water volume, nanoplates were formed.

In another work, the glycol concentration had no effect on the morphology but rather affected the surface area of the synthesized particles.122 The BET surface area of CuO spheres (before calcination) decreased from 157 m2 g−1 to 104 m2 g−1 as the glycol content in water increased.

The solvent ratio can also influence the particle size. The solution phase precipitation at room temperature of V2O5 spheres in a mixed solvent of acetone, pyridine and water gave particles of different sizes (between ∼150 nm and ∼1000 nm) simply by varying the water or pyridine concentration.127

An increase in the ratio of the structure directing agent (SDA) to the metal precursor improved the sphericity of particles such as ZnO,56,185 SnO2,125 and Bi2WO6 spheres24 as well as α-Fe2O3 multi-shell hollow spheres87 because SDA acted as a capping agent and prevented uncontrolled aggregation, although an excess of SDA can be detrimental to the crystallinity118 or development of the structures.24 Moreover, this ratio has also been used to control the size and morphology of particles such as NiO,43 Y2O3,261 ZnO,134 ZnCo2O4@CeO2,133 Bi2O3,23 SnO248 and Co3O4.28 Conversely, the effect of the change in the precursor concentration while keeping the overall ratio constant has been examined in the low temperature precipitation of ZnO hollow spheres using HMT at a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio.159 Hollow fluffy-like spheres having a size of 2–3 μm and a surface area of 138 m2 g−1 were produced; however a decrease in the concentration of the reactants (but keeping the same ratio) gave solid spheres with a net-like surface and a much higher surface area of ∼368 m2 g−1.

The type of reactant can also impact the shape or phase of the final products. CuO/Cu2O hollow spheres were obtained via a hydrothermal method using ethanolamine but CuO flower-like spheres were obtained when ammonia was used.80 In the hydrothermal synthesis of NiO spheres, the β-Ni(OH)2 phase was formed when NaOH or ethanolamine was used while α-Ni(OH)2 was formed with urea instead.43 This study indicates that the phase and structure of the final products depend on the special properties and structure of the reactants used.

4. Applications of spherical metal oxide particles

Metal oxides are used in diverse and multidisciplinary areas ranging from industrially relevant sensing and catalytic uses to biomedical, environmental and energy-related applications. This section shows how the characteristics of different metal oxides affect their respective applications.

4.1 Sensing and catalytic applications

4.1.1 Sensors. Sensors are used to measure and detect slight changes of a chemical compound or molecule in their surroundings, resulting in important applications in fields such as environmental monitoring, biomedical detection, oil and gas industry, metallurgy, food industry, etc. Some typical chemicals that require sensing are flammables such as ethanol,27,45,56,57,59,93,251,262–264 acetone131,262 and hydrogen,47,48,125 strong oxidizing agents such as H2O2,123 health hazards such as 2-chloroethanol,46 ammonia,158 H2S,143 methanol,183 toluene,30,131 NO2,130 trimethylamine,226 formaldehyde82,205 and CO47,48,151 as well as glucose239,240 to monitor its level in blood. While the most common materials that have been used are SnO2 spheres and hollow spheres45,47,48,125,151,262,264 and ZnO spheres and hollow spheres56,57,59,158,239 due to their low cost and availability, other metal oxides have been tested too, such as Nb2O5 spheres,123 In2O3 hollow spheres,93,94,205 MnO2 spheres,240 Co3O4 spheres,27 Co3O4 hollow spheres,251 α-Fe2O3 hollow spheres,82 yolk–shell α-Fe2O3 spheres,84 CuO hollow spheres,143 WO3·H2O spheres,131 WO3 spheres,130 WO3 hollow spheres,226 Cr2O3 spheres30 and TiO2 hollow spheres.183

Basically sensors operate in two ways. Biosensors make use of immobilized enzymes that can break down a biomolecule to give an electrochemical signal. On the other hand, n-type oxide semiconductors such as SnO2, ZnO, In2O3, TiO2 and WO3 operate by the chemoresistive detection of reducing gases at their surface, involving the diffusion of the analyte gas towards the sensor surface and its electrochemical oxidation with a negatively charged adsorbed oxygen.94 Sensors should have a high sensitivity (at the ppb or ppm level), very fast response (a few seconds), good selectivity towards similar substances, stability, a wide response range, a linear dependency with respect to gas concentration, good repeatability and reusability. These qualities are imparted by the proper choice of sensor material with a microstructure that has been tailored to have specific properties such as a high surface area, porosity, thermal and chemical stability, a short diffusion length for efficient mass transfer, non-toxicity and biocompatibility (for biosensors). From morphology viewpoint, porous or hollow particles have been found to be well suited as sensing materials since their pores allow for rapid mass diffusion and can accommodate numerous active sites, while hollow particles offer short diffusion lengths and an empty structure that can act as a reservoir to continuously replenish active sites with the monitored substance. To improve their performance, some sensors have been loaded with precious metals such as Au,47,56,262 Er93 and Rh,94 made as composites, for example α-Fe2O3/In2O3 hollow spheres263 or as hierarchical structures with high surface area such as nanosheet-assembled WO3 microspheres130 or Co3O4 microspheres composed of large and thin nanoplatelets.27

Kim et al.94 observed that 1.67 atomic% Rh loaded into In2O3 hollow spheres showed an excellent response to 2–100 ppm ethanol, which was up to 180 times higher than in the case of unloaded In2O3 hollow spheres. Also, Rh additionally decreased the optimum operating temperature from 475 °C to 371 °C and enhanced the selectivity to ethanol 15–25 times. The sensing time was very short (0.4 s) but recovery time was relatively large (∼200 s) due to thermal promotion of the surface reactions. A higher Rh loading could decrease the recovery time at the expense of a lower response, which was attributed to the formation of unwanted rhodium and indium phases.

Hierarchical MnO2 spheres consisting of nanorod subunits were synthesized via electrodeposition and loaded with the glucose oxidase enzyme for the mediator-less detection of glucose.240 The particles exhibited a high surface area of 129 m2 g−1 and pore size between 5 and 12 nm, which could easily accommodate the enzyme molecules. The sensor had a high sensitivity of 31.6 μA mM cm−2, a large linear range up to 3.15 mM and a low detection limit of 0.35 μM. Moreover, no interference was observed with species that coexist with glucose in blood, such as ascorbic acid, uric acid and acetaminophen, indicating good selectivity of the sensor.

With the advent of more and more chemicals that are classified as toxic and the implementation of more stringent laws for their detection, the development of more versatile and sensitive sensors will be required in the future and metal oxides can effectively offer a solution.

4.1.2 Catalysts. The attributes that make metal oxide nanoparticles attractive as catalysts are their high surface area, easy accessibility of pores (for reactants as well as for loading noble metals), thermal resistance (since many catalytic reactions are performed at elevated temperatures), toughness (to avoid damage during multiple use), chemical stability, environmental friendliness, special inherent properties that are useful for specific applications and ability to interact synergistically with loaded nanometals for superior catalytic performances. Fig. 12A presents a simplified illustration of the operating principle of a supported catalyst.
image file: c6cs00060f-f12.tif
Fig. 12 (A) Operating principle of a metal oxide support loaded with nanoparticles of a metal catalyst. (B) Catalytic reduction of 4-NP to 4-AP. (a) Reaction equation. (b) Time-dependent absorption spectra of the reaction solution in the presence of the Pd nanocube-embedded hollow mesoporous TiO2 microspheres. (c) Plot of ln[C(t)/C(0)] against the reaction time. The R2 = 0.9956 is the coefficient of determination obtained from the linear fitting. (d) Plot of C(t)/C(0) against the reaction time for five successive cycles of the reduction reaction catalyzed by Pd nanocube-embedded hollow mesoporous ZrO2 microspheres. (e) TEM image of the ZrO2 microspheres after five cycles of the reduction reaction (panel B reproduced with permission from ref. 223. Copyright © 2013, Wiley-VCH).

The environmentally friendly room temperature degradation of HCHO to CO2 and H2O was achieved by using Pt loaded γ-Al2O3 hollow spheres.63 As shown in Fig. 12B, the hierarchical macro–mesoporous structure allowed for high dispersion of Pt and accessible pores facilitated diffusion of reactive molecules and products to/from reaction sites, respectively. In another work,223 the embedded Pd nanoparticles inside the hollow space of mesoporous TiO2 and ZrO2 spheres synthesized via the PS nanospherical template-assisted aerosol method prevented the aggregation of metal nanoparticles and reduced the loss of catalyst during recycling, enhancing the reduction reaction of 4-nitrophenol. Similarly, high CO oxidation at low temperatures was possible via the use of nanosized Au in Au@TiO2 yolk–shell particles due to the unique synergy between Au and TiO2 and the protection of nanometal by the shell.199

Porous ZrO2 microspheres have strong Lewis acidity and poor Brønsted sites but their treatment with phosphoric acid could improve the acidity of the Brønsted sites.136 The synthesized catalyst was tested for the catalytic Friedel–Crafts alkylation of indoles with chalcones, giving 98% yield in 6 h and showed a negligible loss in its activity even after 22 reuses. Similarly, Friedel–Crafts reactions were carried out with mesoporous Nb2O5 spheres40 and Fe3O4@TiO2 double-shell yolk–shell particles86 once the Lewis and Brønsted sites were improved by acidification with sulfuric acid and hydrothermal treatment in basic solution, respectively.

CeO2 on the other hand shows strong oxygen storage and release capacity via facile conversion between Ce3+ and Ce4+. This makes ceria a model catalyst for oxidation reactions. Hollow CeO2 spheres converted 43% CO at 295 °C,70 while 10% Cu doped CeO2 spheres showed >98% CO conversion at 210 °C68 and a complete oxidation was achieved on the core–shell ZnCo2O4@CeO2 particles at less than 200 °C.133 However, loading CeO2 with noble nanometals such as Pd,74 Ag26 and Au69 drastically improved its performance at much lower temperatures. For instance, Pd@CeO2 multi-yolk–shell particles achieved 100% CO oxidation at 110 °C.74 The catalyst also performed excellently in the case of aerobic oxidation of cinnamyl alcohol into cinnamaldehyde with >99.9 conversion after 1.5 h. In another case, CeO2 doped into MnO2 hollow spheres could catalytically oxidize benzene with 90% oxidation at 252 °C and completely oxidize it at 340 °C.99

Mesoporous NiO spheres and flowers having a size of 500 nm and mesopores in the range of 10–30 nm showed high catalytic activity for the transformation of toxic phenolic pollutants.2 Furthermore, the NiO nanostructures with large scale nanocrystal domains and well-shaped morphologies imparted magnetic properties to an otherwise antiferromagnetic system due to the quantum confinement effect, allowing easy magnet-assisted separation.

Other types of metal oxides and their combinations have been investigated for several other reactions. Perovskite LnFeO3 (Ln = La, Pr–Tb) hollow spheres showed an excellent catalytic performance for NO + CO reaction at high temperatures between 200 °C and 500 °C due to their outstanding thermal and chemical stability.91 MgO spheres and flowers performed much better than bulk MgO in the Claisen condensation of benzaldehyde and acetophenone under solvent-free conditions.265 Co3O4 hollow spheres were used for methane conversion141 and H2O2 oxidation.142 Fe3O4@SiO2@void@TiO2 particles were employed as a catalyst for epoxidation of styrene,198 giving a high conversion and selectivity of 90.2% and 88.5% respectively. In the latter work, the Fe3O4 core was well protected by the SiO2 layer and the void space was successfully loaded with Au nanometals, which respectively provided magnetic separability and high reactivity and reusability.

4.2 Biomedical applications

4.2.1 Photoluminescence. Rare earth doped materials have been widely used as high performance upconversion or downconversion luminescent devices. Upconversion is an optical process in which the interaction of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength such that energy is conserved, while in downconversion, one high energy (short wavelength) photon is converted to two or more, low energy (longer wavelength) photons. This behavior arises due to their unique electronic and optical properties resulting from their 4f electrons. The host matrix for the rare earth elements needs to have good chemical durability, thermal stability and low phonon energy. Lanthanide oxide spheres such as Y2O3 solid and hollow spheres55,132,155–157,261 or Lu2O3 hollow spheres266 have been used in upconversion devices. For example, under UV or low voltage electron beam excitation, the Ln3+ (Ln3+ = Eu3+ and Tb3+) ion doped Y2O3 samples showed strong red and green emission, respectively,156 whereas multi-color upconversion emission was realized under 980 nm excitement when using Er doped Y2O3 spheres.132 Furthermore, multi-shell structures revealed that the upconversion luminous intensity increased as the number of shells increased due to multiple reflections and the enhanced light harvesting.180 Luminescent materials could find application in the field of lighting, optoelectronic devices, biological labeling and drug delivery. Recently, hollow structured Y2O3:Yb/Er–CuxS nanospheres with multifunctionalities were created.267 Aside from upconversion luminescence properties, the material showed good biocompatibility and exhibited high ablation efficiency with regard to tumor cells because of the synergetic therapeutic effect between chemotherapy and photothermal therapy arising from loaded DOX and attached CuxS nanoparticles.

Some semiconductors were shown to have photoluminescence capabilities. This arises from oxygen vacancy defects that can accommodate photogenerated electrons, which release photons upon recombination with free photogenerated holes. Some examples of semiconductors that have been found to possess photoluminescence properties are perovskite BaZrO3 hollow spheres,64 In2O3 spheres37,253 and γ-Ga2O3 hollow nanoflowers,92 with the latter also showing excellent solar blind detection performance.

4.2.2 Drug delivery and antimicrobial properties. Metal oxide spheres are also good candidates as drug delivery vehicles. TiZr oxide spheres showed high loading of drugs such as IBU, DEX and ERY as well as hydrolytic stability and sustained release profiles in simulated body fluids.268 Gd2O3 nanoparticles have a combination of magnetic and optical properties, which can be advantageous in the field of biomedicine. Indeed, Gd2O3:Yb/Er hollow spheres could simultaneously serve as a dual-imaging agent for optical/MR imaging and as a vehicle for drug storage/release.145 Magnetic-based drug distribution was also a possibility with PEG-coated Fe3O4 and γ-Fe2O3 hollow spheres.85 Both structures had high ibuprofen storage capacities of 297 mg g−1 and 237 mg g−1, respectively. Furthermore, a sustained drug release could be observed with most of the loaded drug released to the simulated body fluid in 136 h; the process of loading and releasing drugs is illustrated in Fig. 13A.
image file: c6cs00060f-f13.tif
Fig. 13 (A) Schematic illustration of loading and unloading a drug to/from the PEG-coated hollow metal oxide particles. (B) (a) HRSEM image, (b) TEM image of hematite HNS, (c) HRSEM image, (d) TEM image of magnetite HNS, (e) HRTEM images of hematite HNS showing lattice fringes of the (006) plane, and (f) magnetite HNS showing lattice fringes of the (311) plane. (C) (a) Dynamics of extracellular dissolution of total Fe throughout the course of microbial growth in the absence and presence of bacteria and (b) intracellular content of Fe in the bacteria after 4 h incubation with nanoparticles of 500 μg ml−1 concentration in LB broth (the inset illustrates the enhancement of the antimicrobial process). (Panels B and C reproduced with permission from ref. 269. Copyright © 2016, Wiley-VCH.)

Nor et al. reported hematite hollow nanospheres (HNS) synthesized by the hard templating method; reduction of hematite HNS by H2 led to magnetite HNS (Fig. 13B). It was reported that magnetite HNS (hematite HNS and C-magnetite) shows superior antibacterial performance towards both E. coli and S. epidermidis (Fig. 13C). In comparison to hematite HNS, magnetite HNS allows for a multiple-fold increase in the generated soluble iron ions, showing that the control over both the composition and nanostructure is crucial for tuning the antimicrobial activity of iron oxides.

4.3 Environmental applications

4.3.1 Photocatalysis. Semiconductors can be photoexcited by light of suitable energy to generate electrons and holes, which can be used as mediators for the photocatalytic removal of pollutants (commonly dyes) and even for bacterial deactivation.23,107 The pertinent features of a semiconductor nanomaterial that make it an attractive photocatalyst are its morphology, phase, size of crystals, chemical and physical stability and reduced amount of defects in the crystal lattice. These allow for increased lifetime of photogenerated electrons and holes and their fast migration towards the particle's surface for redox reactions, while reducing the probability of their recombination. Additionally, a proper engineering of semiconductor nanocrystallites can effectively reduce the band gap energy (hence the required energy of incident photons) with respect to the bulk sample, due to the quantum confinement phenomena, making them responsive to visible light. Numerous materials have been employed as photocatalysts using UV light such as SnO2 spheres,107,125 ZnO spheres and hollow spheres,58,60,118,270,271 ZnO multi-shell hollow spheres,119 γ-Fe2O3 hollow spheres,88 In2O3 spheres,38 α-Ga2O3 spheres,36 CuO/Cu2O composite hollow spheres,80 SiO2 supported Ta2O5 hollow spheres,113 perovskite type SrTiO3 hollow spheres112 and Fe3O4@TiO2 double-shell yolk–shell spheres.86 Catalysts that performed well under visible light due to their narrow band gap energy were Bi2O3 spheres,23,259 WO3 spheres and hollow spheres,129,153 α-Fe2O3 spheres34 and CeO2 hollow spheres,71 perovskite types such as BiFeO3 spheres66 and Bi2WO6 spheres25,229 and composites such as CeO2/Bi2O3 nanospheres,272 Bi2O3/Co3O4 spheres,65 WO3/TiO2 spheres54 and WO3/WO3·H2O hollow spheres.154

The morphology of the material is of utmost importance for designing a photocatalyst. A large surface area affords numerous reaction sites and a high crystallinity favors photocatalytic activity due to effective charge migration. Indeed, the annealing process improves the crystallinity of the material but at the expense of the total surface area. Therefore, a compromise needs to be reached for the best crystal size and the largest possible surface area for optimum photocatalytic performance. SnO2 spheres calcined at 150 °C had the best activity under UV light for the degradation of methyl orange as compared to uncalcined samples and samples calcined at 200 °C, 300 °C and 500 °C respectively.125 Similarly, Bi2O3 spheres calcined at 350 °C degraded 99% of methyl orange under visible light after 3.5 hours as compared to only 25% degradation for samples calcined at 500 °C. It was also shown that hollow structures performed better than solid ones due to multiple light reflection and absorption within the structure cavities (Fig. 14A) and stability of the photogenerated charge carriers.


image file: c6cs00060f-f14.tif
Fig. 14 (A) Schematic illustration of multiple reflection and absorption of UV light in a hollow shell photocatalyst. (B) SEM image and schematic illustration of photoexcitation electrons and holes migrating to the opposite sides of the interface in hollow ZnO spheres (panel B reproduced with permission from ref. 225. Copyright © 2014, American Chemical Society).

Dilger et al.225 studied the effect of nanoarchitecture on the photoconductivity of ZnO particles (Fig. 14B). They found that the time needed for the current from photogenerated charge carriers to drop down to 10% of its maximum value (T0.1) was <60 s for bulk and porous ZnO, whereas for yolk–shell and hollow spheres, T0.1 was substantially higher reaching 521 s and 1150 s, respectively. This structural effect is duly reflected by photocatalytic experiments using various materials. Porous CeO2 hollow spheres degraded 92% of gas phase acetaldehyde within 24 h, four times better than in the case of CeO2 nanoparticles, despite the fact that the latter had a smaller band gap (2.88 eV as compared to 3.01 eV for hollow CeO2 spheres) and better crystallinity.71 WO3 hollow spheres had better performance than flower-like spheres, with first order kinetics of 0.056 min−1 as compared to 0.0151 min−1 for the removal of rhodamine B under UV light.129 While both structures possessed similar surface areas (13 m2 g−1 and 16 m2 g−1 for the microspheres and hollow spheres, respectively), the improvement in the catalytic activity of the hollow spheres was attributed to the large textural porosity between 3.3 nm and 89.6 nm. Increasing the number of shells also resulted in better photoactivity. For example, ZnO hollow spheres with 1, 2 and 3 shells degraded 84.1%, 88.3% and 99% of rhodamine B respectively,119 while the required time for complete degradation of the same pollutant under visible light was 1.5 h for WO3 with multiple shells as compared to 2.5 h for single shell particles.153

The crystal phase, due to its geometric structure, is another important aspect that is related to photoactivity. Hou et al.273 showed that the β-phase of Ga2O3 exhibited superior photocatalytic activity towards the gas phase degradation of aromatic compounds as compared to α- and γ-Ga2O3. Also, studies of Fe2O3 as catalyst showed that α-Fe2O3 spheres could completely degrade rhodamine 6G after 1 h under visible light irradiation,34 while F doped γ-Fe2O3 needed UV light to degrade rhodamine B.88

TiO2 has been the most extensively studied semiconductor for photocatalytic purposes due to its abundance, low cost, non-toxicity and high photoactivity accompanied by a high oxidation and reduction potential. TiO2 can oxidize a large variety of organic compounds in water or air182,189,193,200,203,254–257,274–278 and has also been successful for reducing metal ions such as Cr6+[thin space (1/6-em)]193 and for H2 production.254 The major factors affecting the degree of photoactivity are surface area, porosity, crystallinity and morphology. A high crystallinity is desired rather than a large surface area to increase the separation between the photogenerated electrons and holes, and reduce their instantaneous recombination. However, the rutile phase despite its higher crystallinity and lower band gap energy has a lower activity than the anatase phase because of the lower surface area of the rutile particles. As a result, the surface area and the crystal size of particles have to be finely tuned due to their inverse role in photocatalysis, which is the reason why a wide discrepancy has been observed for the optimum calcination temperatures of the synthesized TiO2 particles (between 450 °C and 800 °C for grain sizes varying between ca. 12 and 30 nm), depending on their sizes, morphologies and treatment methods.177,182,200,278 The activity of TiO2 under visible light has been enhanced by doping with Pt,254 CO32−,254 N,193,279 metal oxides,256 CdS184 and even trace organics from the solvent or organic precursor,255 since the inclusion of a dopant within the TiO2 matrix provides a sink for photogenerated electrons, thereby reducing the degree of electron–hole recombination. Similar to other semiconductors discussed in this section, the hollow shell morphology TiO2 performed better than its other counterparts203 because of multi-reflection of light.

4.3.2 Adsorption. Due to their isoelectric point and porosity, semiconductors have been useful as adsorbents for removal of pollutants from wastewater. MgO spheres had excellent adsorption properties towards fluoride and arsenic in water, with 120 mg g−1[thin space (1/6-em)]95 and 569.7 mg g−1 respectively.96 ZrO2 solid and hollow spheres,62 CuO spheres and Cu2O hollow spheres,32 NiO spheres,42,43 perovskite BaZrO3 hollow spheres64 and CeO2 hollow spheres70 were used for the removal of anionic dyes, while urchin-like α-FeOOH hollow spheres90 had high removal capacities towards both organic dyes and heavy metal ions. MnO2 spheres with a hierarchical dandelion-like surface structure, a high surface area of 163 m2 g−1 and main mesopores of about 65 nm showed both oxidation and adsorption properties over lethal As(III) species.17 The As(III) species could be effectively oxidized by MnO2 particles to the less toxic As(V) species, followed by their adsorption.

4.4 Energy storage and conversion applications

4.4.1 Lithium-ion batteries. Transition metal oxides have been actively investigated as electrodes for lithium-ion batteries because they can reach high theoretical reversible capacities of more than 700 mA h g−1,280 while the graphite used in commercial batteries has a relatively low Li storage capacity of 370 mA h g−1.18 The energy density and life-cycle of the existing Li-ion batteries are unsatisfactory for many applications involving large devices such as electric vehicles and large scale energy grid storage,102 and better alternatives are being explored. The two major advantages offered by metal oxide colloidal structures that make them attractive as electrodes in lithium-ion batteries are high surface area for better contact of the active material with the electrolyte and porosity, which accommodates the volume change associated with the insertion and extraction of Li ions during charging and discharging processes, as shown in Fig. 15 for yolk–shell type particles.
image file: c6cs00060f-f15.tif
Fig. 15 (A) Charging and discharging of Li ions in a yolk–shell type Li-ion battery; (B) schematic illustration of the formation process of yolk–shell MoO2 microspheres; (C) (a) discharge–charge voltage profiles of the MoO2 electrode at different current densities of 50, 500 and 2000 mA g−1; and (b) cycling performance of the as-synthesized MoO2 microspheres at 50, 500 and 2000 mA g−1. (Panels B and C reproduced with permission from ref. 102. Copyright © 2013, Royal Chemical Society.)

V2O5 microspheres synthesized by an additive-free hydrothermal method involving a hierarchical assembly of nanoporous fibers possessed a moderate surface area of 42 m2 g−1.51 The structure endowed the material with shorter diffusion pathways for easier Li and electron transport hence enhancing electrochemical performance. The microspheres displayed a very stable capacity retention of 130 mA h g−1 over 100 cycles at a current rate of 0.5C and showed an excellent rate capability with a capacity of 105 mA h g−1, even at a high rate of 30C. Also, TiO2 anatase spheres consisting of ultrathin nanosheets formed via crystals growing in the [001] direction188 performed well at low and high rates. In another work,250 NiO spheres with loosely connected crystals, a surface area of 30 m2 g−1 and pore size distribution in the range of 10–60 nm provided suitable pathways for efficient transport of electrolyte ions, large surface to volume ratios, and good structural stability to deliver a reversible capacity of 800 mA h g−1 after 100 cycles at a current density of 500 mA g−1. The material was also tolerant to various charge and discharge currents, indicating high rate performance for high power applications. Aside from the morphology, crystal polymorphs can also be a deciding factor in the fabrication of metal oxide-based Li-ion batteries. It was shown that the anatase phase exhibited higher capacity than either brookite or rutile due to the more favorable Li ion insertion mechanism.188,191,281,282

Hollow structures have also attracted great attention due to the increased contact area between the electrode material and electrolyte as well as their mechanical flexibility. Hierarchical orthorhombic V2O5 hollow spheres showed good performance in Na-ion batteries (an alternative to Li-ion batteries) due to the predominantly exposed [110] crystal planes, which provided channels for easy Na+ insertion and extraction as well as high tolerance to the deformation imparted by voids in the shells of hollow spheres.115

Mn3O4 hollow spheres synthesized by an aerosol method featured a high surface area of 96 m2 g−1 and thin shells of 5–10 nm. These hollow spheres exhibited good stability with a high capacity retention of 980 mA h g−1 over 140 cycles and exceptional rate capability by retaining a capacity of 300 mA h g−1 at an ultra-high current density of 10[thin space (1/6-em)]000 mA g−1.224 These values represent the best electrochemical performance for Mn3O4 anode materials to date and were attributed to the unique thin wall hollow structure, which provided considerably reduced diffusion paths for electrons and Li ions. On the other hand, multi-shell hollow α-Fe2O3 showed a superior capability of 1203 mA h g−1 at a current density of 100 mA g−1 after 128 cycles and an excellent cycling stability of 870 mA h g−1 at a current density of 400 mA g−1 after 300 cycles due to the material's hierarchical porosity and structure, which assured shorter diffusion pathways for efficient transport of electrons and Li ions.87 An excellent performance of multi-shell hollow α-Fe2O3 spheres was indeed recently confirmed by another work,172 reporting up to 1702 mA h g−1 at a current density of 50 mA g−1, which was due to the enhanced volumetric capacity of the structure that allows for maximum lithium storage. Recently, multi-shell V2O5 hollow microspheres, synthesized by a simple method involving adsorption of anions on carbon templates, were found to exhibit an exceptionally high specific capacity of 447.9 mA h g−1 (at a high current density of 1000 mA g−1), exceptional rate capability and cycling stability, due to the ample charge storage sites, short transport paths and good structural stability of the material.176 These attributes also belong to yolk–shell structures such as MoO2@MoO2102 and V2O5@V2O5.116,117 The superiority of the yolk–shell structure over solid spheres was demonstrated for the V2O5@V2O5 yolk–shell particles retaining 89% of its specific discharge capacity after 50 cycles as compared to around 60% only for solid V2O5 spheres.117

Composites or multi-component structures are attractive for studies due to the synergetic effect between the different components. Carbon-supported amorphous and crystalline V2O3 microspheres both showed excellent high rate and electrochemical performance due to the uniform distribution of partly graphitized carbon within the framework and also due to the stable structure of spheres, which assured a low charge resistance, fast electronic transport, a large surface area and excellent stability.50 The amorphous spheres showed 95% retention in the discharge capacity after 7000 cycles at a high current density of 2 A g−1, while the crystalline ones retained 98% after 9000 cycles. Ant-cave structured MoO3–C composite microspheres234 synthesized by USP exhibited high initial discharge and charge capacities of 1109 and 724 mA h g−1 and still delivered a discharge capacity of 733 mA h g−1 after 300 cycles. The high performance was attributed to the combination of ant-cave channel structure and conductivity of the carbon in the composite. A yolk–shell structure of CuO@NiO spheres exhibited much higher capacity than the theoretical value of 1061 mA h g−1 after 200 cycles due to the unique multilayer hollow structure, which provided a large electrochemically active surface, more active sites for Li ion storage and facilitated Li diffusion.283 In another example, the carbon coated triple-shell hollow spheres of CoMn2O4 possessed a high specific capacity of 726.7 mA h g−1 and nearly 100% capacity retention after 200 cycles.168

Numerous other metal oxides and composites of various shapes and accommodating space for lithium have been successfully used in Li-ion batteries. The list of metal oxides used in Li-ion batteries is quite long and includes the following: CuO spheres and hollow spheres,18,78,79,122 Fe2O3 hollow spheres,83,88 α-Fe2O3 multi-shell hollow spheres,172,231 Co3O4 spheres,28 Co3O4 multiple shells,171 Gd2O3 hollow spheres,145 NiO spheres and hollow spheres,41,103,149,284 TiO2 hollow spheres,285 TiO2 yolk–shell spheres,281 multi-shell TiO2 hollow microspheres,174 Fe2O3@TiO2 core–shell spheres,204 V2O5 spheres and hollow spheres,114,127,128 SnO2 spheres and hollow spheres,106,109,206,264 SnO2 multi-shell spheres,110 Li2O–CuO–SnO2 multi-deck cage spheres,219 CoFe2O4 and other metal ferrite spheres,29,77 core-in-double-shell NiCo2O4 particles,105 multi-shell LiMn2O4 hollow microspheres,286 Cr2O3–C core–shell spheres,31 SnO2@C spheres,49 SnO2/C composite hollow spheres108,111 and Fe3O4–C composite spheres.220

4.4.2 Supercapacitors. Supercapacitors are energy storage devices that provide a large specific energy density, a high specific power density, fast charging capability and a long lifetime due to the excellent cycling stability. Metal oxides are promising materials because of their high theoretical capacitance, low cost and environmental friendliness. To enhance the rate of ion and electron transport in the electrodes and at the electrode/electrolyte interface, the structural aspects such as short diffusion path lengths, a high surface area and porosity to accommodate the strain from the high rate of insertion and extraction of ions are required.

The surface area of MnO2 hollow spheres prepared by a double templating method was varied by changing the ratio of the F127 surfactant and SiO2 spheres used as soft and hard templates, respectively.97 The urchin-like hollow spheres showed the best capacitance of 266.6 F g−1 within the potential range of 1 V at a current density of 0.1 A g−1 due to their high surface area of 233 m2 g−1 relative to the other hollow structures. Co3O4 hollow spheres with thin shells of 130 nm, a moderate surface area of 60 m2 g−1 and mesopores centered at 7.8 nm, prepared hydrothermally using sucrose as a precursor for carbon sphere templates, performed well giving a specific capacitance of 470 F g−1 at 1 A g−1 with no obvious capacitance decrease observed over 1000 cycles.75 On the other hand, bacteria-templated Co3O4 hollow spheres prepared by a one-pot mineralization method at room temperature featured a similar average pore size of 7.7 nm but a larger surface area of 149 m2 g−1 due to their fluffy-like surface.14 These particles showed a high capacitance of 214 F g−1 at 2 A g−1, a Coulombic efficiency averaging over 95% and excellent cycling stability that showed a capacitance retention of about 95% after 4000 cycles. Mn2O3 triple-shell hollow microspheres with thin porous shells and a large pore volume of 0.52 cm3 g−1 but a moderate surface area of 37 m2 g−1 showed a record high specific capacitance up to 1651 F g−1 at 0.5 A g−1, an excellent rate capability of 1422 F g−1 at 10 A g−1 and a cycling stability retention of 92% after 2000 cycles.175 These findings show the importance of porosity, surface texture and high surface area for the performance of metal oxide particles as supercapacitors.

However, the particle morphology may surpass the effect of surface area in some cases, as it was shown in the case of NiO spheres prepared by a hydrothermal method.43 It was found that NiO spheres made of ultrathin nanowires performed much better than urchin-like spheres with nearly 100% capacity retention after 200 cycles at a current density of 10 A g−1, despite the higher surface area of the latter (243 m2 g−1vs. 215 m2 g−1). The high capacity was attributed to the bimodal pore size distribution of the ultrathin nanowire-assembled spheres at 3.2 nm and 8.9 nm as opposed to the monomodal 4 nm pores in the urchin-like spheres as well as to their network-like surface texture, which assured efficient diffusion paths for OH ions and greatly enhanced intercalation of the electrolyte ions, ensuring sufficient Faradic reactions.

Doped or composite structures can also perform well as supercapacitors as it was shown for Fe3O4 doped MnO2 microspheres,21 Ag2O–MnO2 composite spheres19 and MnO2/C composite spheres.100 Core–shell C@MnO2 had a high specific capacitance of 583 F g−1 at a current density of 1 A g−1 in 0.1 M Na2SO4 electrolyte,214 which was attributed to the inner graphitized carbon core coupled with porous interconnected MnO2 nanorods for enhanced electrolyte accessibility, short ion diffusion length and charge transfer pathways. Ternary metal oxides with two different metal cations also exhibited high electrochemical performance as it was demonstrated for core-in-double-shell hollow NiCo2O4 spheres105 with a complex interior, porous shells and consisting of small nanocrystalline particles. This material delivered high pseudocapacitance values of 1141, 1048, 965, 862 and 784 F g−1 at current densities of 1, 2, 5, 10 and 15 A g−1. Furthermore, after 4000 cycles at 5 A g−1, 94.7% of the specific capacitance was retained, proving the high stability of the spheres.

Other examples of metal oxides that have been used as supercapacitors are NiO spheres and hollow spheres,41,124,148,244,287 La doped NiO spheres,44 Co3O4 hollow spheres242 and MnO2 hollow spheres.98

4.4.3 Dye sensitized solar cells (DSSCs). In a DSSC, light is absorbed by dye molecules attached to the surface of a wide band gap metal oxide (usually TiO2) to generate free electrons, which are then injected into the conduction band of the metal oxide to be transported to an external circuit, thereby generating a current. Generally, the photoanodes in DSSCs are in the form of a film consisting of anatase TiO2 nanocrystals of size ca. 20 nm.192 These films have high surface area, allowing for high dye adsorption and high energy conversion efficiency. However, these films have poor separability of electrons and holes due to various defects, grain boundaries and non-homogeneity. Moreover, the small particle size means that most of the visible light of longer wavelengths passes through without being absorbed.

To address this issue, an additional layer of metal oxide particles has been deposited to enhance the light to current capability of DSSCs. The engineered particles have increased dye loading, interconnected crystals to improve the current transport, improved crystallization to intensify electron–hole separation and more importantly, enhanced back-scattering of light of longer wavelength (visible and near infrared region) onto the primary layer of DSSCs, which augments absorption of light, as shown in Fig. 16. There are numerous reports on the use of TiO2 particles of varying architecture in DSSCs such as spheres,184,190,192,195,217,245,288–290 hollow spheres,227,248,291 and yolk–shell structures.208


image file: c6cs00060f-f16.tif
Fig. 16 Scattering of long wavelength light (red light) due to the layer of larger particles on the top of TiO2 film in a DSSC.

Macroporous TiO2 spheres were prepared by an emulsion templating method using PS particles as templates for the macropores,184 and were formed via calcination of the PS–TiO2 composite spheres. These spheres had size in the range of 500–2000 nm, pore size in the range of 200–300 nm, a surface area of 76 m2 g−1 and were composed of crystals with sizes of 6–8 nm. The high surface area assured a high amount of dye loading, which was about 1.5 times higher than that adsorbed on a commercial nano-TiO2, and the proper size of spheres improved the scattering of light in the wavelength range of 500–800 nm. Hence, the geometry of particles assured a significant improvement in light scattering, and increased dye loading as well as interconnected pores (fewer grain boundaries) enhanced light to current efficiency as compared to conventional TiO2 materials.

TiO2 spheres of size 2.1 μm consisting of nanorods were prepared by a hydrothermal method.289 The large surface area (64 m2 g−1) and size of the particles resulted in a high power conversion efficiency of 10.34%. The high performance was also due to the crystal size and structure, which increased the electron transport rate and slowed down the recombination rate of electrons and holes. On the other hand, TiO2 mesoporous microspheres of size ca. 800 nm, created by an evaporation-driven self-assembly method,186 possessed a large surface area of 112 m2 g−1, a large pore volume of 0.164 cm3 g−1, and highly crystalline walls with [101] exposed facets. These attributes resulted in DSSCs with high energy conversion efficiency of up to 12.1%.

The wavelength of the scattered light is closely related to the size of particles. Xu et al.202 showed that TiO2 particles with a size of 380 nm exhibited strong resonance with light of wavelength 366 nm. However, Yu et al.288 studied the effect of light scattering by the particles with sizes in the range of 260–800 nm and found that particles with a size of 450 nm showed the highest scattering of light in the range 600–750 nm and therefore the best photon to current efficiency; this was attributed to the size uniformity and long range ordering when the particles were applied as a light scattering layer in DSSCs.

Other semiconductors such as SnO2,126,243 quintuple shell SnO2173 and Nb2O539 have also been used in DSSCs. However, the power conversion efficiencies did not reach those of TiO2-based electrodes.

5. Conclusions and perspectives

We have reviewed the synthesis methods and potential applications of metal oxide spherical nanoparticles. These particles were engineered to have a high surface area to volume ratio, a high pore volume, and high mechanical strength, which endorse them for a variety of applications in the catalytic, environmental, biomedical and energy fields. They can show synergistic interactions with noble metals when used as supports, making them potentially attractive catalysts. Furthermore, for specific crystal size, the quantum effects can impart semiconducting or magnetic properties to the particles, which can be beneficial in the field of photocatalysis and separation. The most common methods currently used for the synthesis of metal oxide spheres are the high temperature hydrothermal and low temperature solution precipitation methods. However, these methods are time consuming and suitable for batch production only. Thus, the emerging methods such as aerosol and spray drying or spray pyrolysis methods are well suited for continuous large scale production, and can be used to synthesize porous spheres and shells, allowing high loading of noble metals. However, further studies are required toward controlling the uniformity and size of spheres and the thickness of shells. For multi-shell hollow spheres, the finely controlled synthesis of concentric hollow spheres is still a challenge and more research in this direction is needed. Further studies on the design and synthesis of metal oxide structures, especially core–shell and yolk–shell particles with cores and shells of tailored surface properties, porosity and chemical composition for advanced catalytic, biomedical and energy-related applications, are highly desirable. Since chemistry of metal oxide nanoparticles is a rapidly growing research area, new discoveries in the synthesis and applications of these materials are anticipated.

In perspective, synergetic interactions of metal oxide composites need to be further investigated to possibly avoid the use of precious metals as active agents in catalysis, and enhance the energy storage capacities of the materials used in the energy field. Multiple functionalization within the same structure could generate catalysts that can cope with tandem reactions, hence avoiding the use of two or more catalysts. Furthermore, more work on the synthesis of hollow metal oxide spheres in the nanometer regime needs to be carried out for use as nanoreactors in colloidal solutions. The new generation of porous non-silica metal oxide submicrospheres with precisely controlled functionality can be envisioned to be created by taking advantage of prosperous nanochemistry synthesis methods under mild conditions. A further development of the computationally assisted design and fabrication of complex porous non-silica metal oxide particles is needed for structure prediction and reduction of the number of synthesized samples. In the past decade, there has been a remarkable evolution in porous non-silica metal oxide submicrospheres with unique properties as platforms for emerging applications, which envisages a bright and exciting future for these unique materials.

Abbreviations

BETBrunauer–Emmett–Teller
CSCarbon spheres
CTABCetyltrimethylammonium bromide
DEXDexamethasone
DMF N,N-Dimethylformamide
EDAEthylenediamine
EGEthylene glycol
ERYErythromycin
GMA-co-EGDMAGlycidyl methacrylate-co-ethylene glycol dimethacrylate
HMTHexamethylenetetramine
IBUIbuprofen
IUPACInternational Union of Pure and Applied Chemistry
MEAMonoethanolamine
PEGPoly-ethylene glycol
PEO-b-PSPolyethylene oxide-block-polystyrene
PEO–PPO–PEOPoly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide)
PSPolystyrene
PVPPolyvinyl pyrrolidone
SDBSSodium dodecylbenzenesulfonate
TBOTTitanium butoxide
TEATriethanolamine
TEOSTetraethyl orthosilicate
TFATrifluoroacetic acid
THFTetrahydrofuran

Acknowledgements

The authors would like to thank Prof. Shaobin Wang for fruitful discussions.

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