Tin oxide nanostructured materials: an overview of recent developments in synthesis, modifications and potential applications

Abstract

Tin oxide nanostructures represent an important class of crystalline semiconducting nanomaterials. Being wide band gap (3.6 eV) n-type semiconductors, these materials have the inherent potential to be used as catalysts, sensors, anode materials etc. Moreover, these materials have permitted rational structure design and control over the band gap by suitable modifications. This structure–property relationship can be readily explored by taking advantage of the knowledge of their detailed electronic environment, which enables fine-tuning of their functionalities for desired applications.

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Dipyaman Mohanta

Dipyaman Mohanta is a senior research scholar in the Dept. of Chemistry, NIT silchar. He has passed M.Sc. in Chemistry from Assam University. He has published several papers in international journals as well as in various national and international conferences.

M. Ahmaruzzaman

Md. Ahmaruzzaman is currently working as a senior Assistant Professor in the Department of Chemistry, National Institute of Technology Silchar. He received his postgraduate degree in M.Sc. (Chemistry) from North Bengal University after his graduation from the same University. He secured first class first position (Gold Medalist) in M.Sc. (Chemistry). He also received his postgraduate degree in M.Tech. (Energy Science & Technology) from Jadavpur University. He obtained his Ph.D. degree from Indian Institute of Technology Delhi. He is Associate Editor of the Frontiers in Environmental Science Journal. He is also the Editorial Board Member of the various international Journals. He has produced five PhDs and supervised more than 20 projects at the post graduate level. He has published more than 120 research papers in international journals as well as in various national and international conferences. He has more than 2700 Citations in International Journals. He is a reviewer of more than 50 peer-reviewed International and National journals. He was awarded 3 year honorary membership to prestigious American Chemical Society.

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1. Introduction

Nanotechnology is an emerging concept in the field of science and technology. The tremendous applicability of nanoscale materials has been crucial towards many uprising technological and environmental challenges in the areas of solar energy conversion, catalysis, energy storage and water treatment.¹ Semiconducting nanomaterials like TiO₂,^2,3 MnO₂,^4,5 NiO,⁶ ZnO,^7,8 SnO₂ (ref. 9 and 10) etc. have attracted much attention because of their potential applications in electronics,¹¹ optoelectronics,¹² photovoltaics,¹³ photocatalysis,¹⁴ sensing^14,15 and so forth. Among the other metal oxide semiconductors, nanocrystals of SnO₂ are considered to be technologically important materials and have been investigated for a wide range of applications such as high energy density rechargeable lithium batteries,^16–19 storage of solar energy,²⁰ gas sensors,^21–23 electrocatalysis²⁴ and photocatalysis.^25,26 This diversity in the application is found to be a function of the size, morphology, phase, and crystallinity of the nanocrystals. Various geometrical morphologies of tin oxide have been produced, for example, spherical particles²⁷ networks of ribbons,²⁸ hollow microspheres,²⁹ sheets,³⁰ flowers³¹ and belts.³² Among all the shapes and morphologies, small spherical nanoparticles with high crystallinity have a great significance because of their higher colloidal stability in aqueous solution and higher surface area than other structures.

Under ambient pressure condition, SnO₂ crystallizes in the tetragonal rutile structure with space group P4₂/mnm. However a sequential pressure driven transitions to other possible polymorphs like rutile-type → CaCl₂-type → α-PbO₂-type → pyrite-type → ZrO₂-type orthorhombic phase I → fluorite-type → cotunnite-type orthorhombic phase II have also been reported.³³ Fig. 1 shows the diagram of the all the crystal phases of SnO₂ and the observed diffraction patterns.

Fig. 1 (a) Crystal structures of the SnO₂ polymorphs (gray and red colors represent Sn and O atoms, respectively). (a) Rutile (P4₂/mnm) and CaCl₂-type (Pnnm), (b) R-PbO₂-type (Pbcn), (c) pyrite-type (Pa3̄), (d) ZrO₂-type (Pbca), (e) fluorite-type (Fm3̄m), and (f) cotunnite-type (Pnam) and the diffraction pattern for different crystal phases of SnO₂. Reproduced from ref. 33 with permission from Elsevier.

A thorough study of literature revealed that the electronic structure of semiconducting nanomaterials intrinsically controls the light absorbance, redox potential, charge-carrier mobility, exciton generation and recombination and hence, the reactivity of semiconductor nanocatalysts. The conventional approach of modifying the electronic structure of a semiconductor is doping. Extensive studies on doped tin oxide nanomaterials such as indium tin oxide^34–38 fluorine-doped tin oxide,^34,39,40 antimony-doped tin oxide,^41–47 Eu·SnO₂ (ref. 48), Fe·SnO₂,⁴⁹ Pt·SnO₂ (ref. 50 and 51) etc. have been reported and found to have splendid applications in H₂ detection,^52,53 organic photovoltaics,^54,55 dye sensitized solar cells,⁵⁶ water oxidation,^57–60 ethanol sensing,⁶¹ cysteine oxidation⁶² etc.

Further, development of newer hybrid materials by impinging remarkable physical and chemical properties of novel metals, carbonaceous compounds like CNTs, graphene etc. have opened a new dimension in nanoparticle research. Thin films,^63,64 core shells,^65–68 carbon nanotubes–SnO₂ nanocomposites,^69,70 graphene–SnO₂ nanocomposites^71–75 etc. have also been extensively investigated for wide range of applications like anode material for Li-ion batteries,^73–77 CO oxidation⁶⁵ heterojunction solar cells⁷⁸ and chemical sensors.^79,80

Understanding the immense potential and innumerable possibilities of tin oxide nanomaterials towards material science and technology, here an attempt has been made to summarise different methods of synthesis of SnO₂ nanomaterials, various possible modifications and potential applications towards sensing, photocatalysis, photovoltaics, electrocatalysis, energy storage etc.

2. Synthesis

Synthesis of nanomaterials with controlled morphology, size, chemical composition and crystal structure, in large quantity is a key step towards nanotechnological applications. The different synthetic methodologies adopted for tin oxide nanostructures can be broadly classified as solution processing method, vapour phase synthesis and solid state synthesis.

2.1. Solution processing methods

The ‘solution processing methods’, often referred as the ‘chemical synthesis methods’ involve an initial solution preparation step followed by solvent removal and then decomposition of the dried product to the final desired nanopowders. The solution processing methods offer the advantages of easy preparation of nearly any composition maintaining compositional homogeneity and high purity. The important solution based chemical synthesis methods include: sol–gel method, hydrothermal method, precursor compound method, emulsion technique and so forth.

2.1.1. Sol–gel method. Sol–gel method is a novel technique for the synthesis of metal oxide nanoparticles. It involves gelling of a solution containing the metal salts (viz., tin alkoxides,⁸¹ SnCl₄·5H₂O^82–84 or SnCl₂·2H₂O^85,86) and subsequent calcination, resulting in the formation of tin oxide nanoparticles with a large surface area. In a typical synthetic procedure, calculated amount of 25% ammonia solution is added dropwise to an aqueous solution of SnCl₄·5H₂O under vigorous stirring and under a controlled feed rate (0.01–0.1 mL min⁻¹).^87,88 After 2 h of stirring, the sol thus formed is allowed to age at room temperature for 24 h. The resulting gel, after washing (to remove excess ammonia and Cl⁻ ions) and drying, calcined at 400 °C for 2 h to get the nanoparticles.

SnCl₄·5H₂O + 4NH₄OH + 4H₂O → Sn(OH)₄ + 4NH₄Cl + 6H₂O

The particle size of the nanocrystals thus formed is found to be dependent on the feed rate of the dropping ammonia solution and annealing temperature. It has been reported that with decreasing ammonia feed rate, particle size decreases⁸⁸ and the increase in annealing temperature, results in the increase in particle size⁸⁹ (Fig. 2) (Table 1). Further, the key step is the conversion of sol into gel and is accompanied by adjusting the activity of some species like H⁺, OH⁻ and other ions. In principle, the pH, ionic strength and temperature of the precursor mixture control the gelation of the sol.⁹⁰

Fig. 2 (A) TEM micrographs of SnO₂ at different ammonia feed rate (a) 0.1 mL min⁻¹, (b) 0.05 mL min⁻¹, (c) 0.017 mL min⁻¹ and (d) 0.01 mL min⁻¹ and (B) at different reaction temperature (a) 30 °C (b) 50 °C (c) 70 °C (d) 90 °C. Reproduced from ref. 88 with permission from Wiley.

Table 1 Tin oxide nanoparticles synthesized via various methods

Methods	Solvents	Precursors/surfactants	Temperature	Morphology	Average crystallite size	Reference
Solution processing method
Sol–gel	Water		200 °C	Spherical	2.5 nm	48, 89
			400 °C	Spherical	3.5 nm	48
			800 °C	Spherical	18.2 nm	48
	Ethanol		450 °C	Spherical	20 nm	94
	Ethylene glycol		195 °C	Spherical	20 nm	95
Solvothermal	Water		200 °C	Spherical	5.5 nm	97
	Isopropanol		600 °C	Spherical	10–30 nm	99
Precursor compound method		Organometallic	135 °C	Spherical	20 nm	105
		Carboxylate precursor	295 °C	Spherical	5.2 nm	107
		Polyacrylate precursor	600 °C	Spherical	15 nm	110
		Poly(propyleneimine)		Flower like	2.5 nm	113
		Poly(amidoamine)		Flower like	4–5 nm	113
Amino acid mediated synthesis	Water		200 °C	Nearly spherical	10–20	128
Microemulsion method		SDS surfactant		Floral petal	8 nm	124
		CTAB surfactant		Florets of cauliflower	11 nm
		PEG surfactant		Sheets and rods	13 nm
Microwave irradiation	Acidified water		200 °C	Spheres irregular shape and size	6 nm	129
			400 °C		9 nm
			600 °C		21 nm
			800 °C		33 nm
Sonochemical	Water		80 °C	Irregular shape	3 nm	137
Vapour phase synthesis
Laser ablation			900 °C	Nanowire	20 nm (diameter)	141
Arc plasma				Nearly spherical particles of irregular size	5 nm	142
Flame spray			200 °C	Nearly spherical particles of irregular size	20 nm	144
Solid state synthesis
Mechanical pressing			25 °C	Nearly spherical	3–15 nm	147
Ball milling, 4 h, 600 rpm			600 °C	Nearly spherical	25–40 nm	149
Focused solar irradiation				Nearly spherical	20–60 nm	152
Bio-synthesis	Water	Plant/bacteria extract	150 °C	Spherical	∼30 nm	159

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Sol–gel fabrication has gained much interest because of its simplicity, inexpensive precursors, low processing temperature, better homogeneity and flexibility of forming dense monoliths, thin films, or nanoparticles,⁹¹ but there are two major limitations as well. First, it is difficult to control the stoichiometry as NH₃ has a tendency to form soluble complexes with the metal ions which on repeated washing may depart thereby altering stoichiometry of the intended metal oxides.⁹² Second, residual Cl⁻ ions not only affect the stability of the sol but also retard the kinetics of gelation.⁹³ The use of alcoholic solvent is preferred to water as sol solution is more stable in alcoholic medium, and gelation process is not susceptible to residual Cl-ions in the solution.⁹² Accordingly, Chen and his co-workers suggested a synthetic route where SnCl₂·2H₂O in ethanol was used as a precursor.⁹⁴ Wherein, a certain amount of SnCl₂·2H₂O in ethanol was acidified with HCl to maintain pH 2–3 and stirred for 24 h at room temperature to get white gel. Addition of deionised water and stirring for >24 h yielded light yellow coloured gel which on subsequent heat treatment at 450 °C for 2 h in an oxygen atmosphere resulted nano-SnO₂ with average particle size 20 nm.

SnCl₂ + 2OHCH₂CH₃ → Sn(OCH₂CH₃)₂ + 2HCl

Sn(OCH₂CH₃)₂ + H₂O → Sn(OH)₂ + 2OHCH₂CH₃

Further, Ng and colleagues reported polyol mediated synthetic process which involves heating of sufficient precursors in a multivalent and high-boiling point alcohol (e.g., ethylene glycol) where alcohol itself acts as a stabilizer, limiting particle growth and prohibiting agglomeration.⁹⁵

Recently, Soltan et al.⁹⁶ synthesized mesoporous nanocrystalline SnO₂ materials by polyol method with pore size ∼8.1 nm and pore volume ∼0.11 cm³ g⁻¹.

2.1.2. Hydrothermal synthesis. Reactions in aqueous medium above its boiling points and therefore under pressure are called hydrothermal reactions. The process involves precipitation of hydrated tin oxide (SnO₂·nH₂O) by mixing aqueous solutions of tin chloride and ammonium hydrogen carbonate and subsequent hydrothermal treatment in an autoclave at ∼200 °C for 3 h thereby resulting uniform, monodispersed tin oxide nanoparticles of average particle size less than 10 nm.^97,98 A major advantage with the hydrothermal processing is that high-temperature calcination is not required for the formation of oxide thus the formation of hard agglomerates can easily be avoided and the dispersity of the particles is greatly improved, unlike the sol–gel process.⁹⁹ Furthermore, the hydrothermal process is desirable for the large-scale fabrication of other ultrafine oxide nanoparticles. Use of various precipitants like hydrazine hydrate, hexamethylenetetramine, trisodium citrate etc.^100–102 provides control over particle size and aggregation of the nanoparticles. Solvothermal process is similar to hydrothermal method, the only difference being that the precursor solution is usually not aqueous. The medium can be anything like ammonia, alcohol, or any other organic or inorganic solvent.¹⁰³ Fig. 3 shows the morphology of ITO nanoparticles obtained via solvothermal method.

Fig. 3 FESEM images of the ITO particles prepared via the solvothermal method at 600 °C using (a) water (b) isopropanol (c) 1,4-butandiol as solvents. Reproduced from ref. 99 with permission from Elsevier.

2.1.3. Precursor compound method. This method involves preparation of precursor compounds which are usually complex combination of metal ions in the proper ratio in their crystalline structures, together with ionic and molecular species as a source of necessary oxygen and the remainder being volatile or decomposable into volatile elements.¹⁰⁴ The pyrolysis of the complex combination in an appropriate inert, oxidizing or reducing atmosphere results nanoscale particles with desired stoichiometry¹⁰⁵ (Fig. 4). Literature reveals various organometallic precursors,^106,107 carboxylate precursors,¹⁰⁸ acac precursors,^109,110 polymer precursors^111–115 for the synthesis of tin oxide nanomaterials. This method is superior to other conventional methods as it provides control over reproducibility and high crystallinity of the synthesised tin oxide nanomaterials.¹¹⁶

Fig. 4 TEM micrographs of SnO₂ nanoparticles using (a) carboxylate precursor (b) organometallic precursor (scale 100 nm) (c) dendrimeric precursors. Reproduced from ref. 105, 107 and 113 respectively with permission from Wiley and ACS publications.

2.1.4. Amino acid mediated synthesis. Various amino acids like glycine,¹¹⁷ serine,¹¹⁸ tyrosine,¹¹⁹ lysine¹²⁰ etc. have been used for the synthesis of SnO₂ nanoparticles as amino acids act as good complexing or capping agents. Amino acid mediated synthesis provides elimination of toxic chemicals and solvents during the synthesis and the size, morphology and properties also get modified. The average crystallite size of the nanoparticles were reported to be in the range on 10 to 20 nm. The most probable mechanistic pathway¹¹⁷ for the formation of SnO₂ nanoparticles is

Sn(OOC–CH₂–NH₂)_n + xH₂O → Sn(OOC–CH₂–NH₂)_n−x(OH) + xNH₂–CH₂–COOH

2Sn(OOC–CH₂–NH₂)_n−x(OH) → [(OH)_x−1(OOC–CH₂–NH₂)_n−xSn]₂O + H₂O

[(OH)_x−1(OOC–CH₂–NH₂)_n−xSn]₂O → SnO₂

2.1.5. Microemulsion method. Microemulsions are colloidal ‘nano-dispersions’ of water in oil (or oil in water) stabilized by a surfactant film. These thermodynamically stable dispersions can be considered as truly nanoreactors which can be used to carry out chemical reactions and, in particular, to synthesize nanomaterials.¹²¹ Based on the ratio of oil and water, there are three basic types of microemulsions, namely oil-in-water (o/w), water-in-oil (w/o) and bicontinuous. The basis of this method is the incorporation of soluble metal salt in the aqueous phase of the microemulsion where the aqueous microdroplets are surrounded by oil. Chemical reactions can take place when droplets containing the desirable reactants collide with each other. Each of these aqueous droplets thus acts as a nanosized reactor thereby resulting in the formation of nanosized solid particles. Song and Kim¹²² reported formation of tin oxide nanoparticles in the range of 2–3 nm in diameter together with a narrow particle size distribution by the water-in-oil microemulsion method using n-heptane as the oil phase and sodium dioctylsulfosuccinate as surfactant. Use of other surfactants like CTAB,¹²³ SDS, PEG,¹²⁴ Tergitol 15-S-5 (ref. 125) HTMAB¹²⁶ etc. (Fig. 5) are also reported for the synthesis of tin oxide nanoparticles. The main idea behind this technique is that by appropriate control of the synthesis parameters one can use these microemulsion nanoreactors to produce tailor-made products down to a nanoscale level with a wide range of morphological diversity and special properties which are otherwise difficult.

Fig. 5 SEM images of SnO₂ nanoparticles (a) SDS (b) CTAB (c) PEG. Reproduced from ref. 124 with permission from Chalcogen.

2.1.6. Microwave irradiation method. Fabrication of high quality nanoparticles using microwave heating has been on the increase in recent years because of its homogeneous and fast-heating characteristics. Using microwave irradiation it is possible to synthesize nanoparticles with exact parameter control in a short time.¹²⁷ Microwave reactors allow easy access to high temperatures and pressures and the exact temperature and pressure control significantly govern the particle size, morphology and hence properties^128,129 (Fig. 6). Further, it is also an eco-friendly process because it operates under dry condition without the use of solvents or toxic chemicals.^130,131 Cirera et al.¹³² prepared tin dioxide nanoparticles by microwave technique after conventionally treating at higher temperatures of order of 450 °C to 1000 °C for 8 h. Later, Krishnakumar et al.¹²⁸ reported rapid synthesis of SnO₂ nanoparticles via microwave technique without any post-synthesis annealing. Further, Krishna and coworkers¹³³ compared the conventional hydrothermal technique and the microwave hydrothermal route for the synthesis of tin oxide nano crystals and concluded that the latter process led to higher yield in a shorter time with enhanced product performance.

Fig. 6 TEM micrographs of SnO₂ (a) as prepared (b) calcined at 600 °C. Reproduced from ref. 129 with permission from Elsevier.

2.1.7. Sonochemical route. Ultrasonication is an effective technique for preparing nanoparticles with attractive properties in a short period of reaction time. The enhanced chemical effect of ultrasound is due to acoustic cavitation process, that is, rapid formation, growth, and the collapse of microbubbles in liquid solution.¹³⁴ The implosive collapse of the bubbles gives rise to localized hotspot through adiabatic compression or shock wave formation within the gas phase of the collapsing bubble. The extreme conditions generated within these hotspots have been exploited for the synthesis of metal oxide nanoparticles.^135,136 Zhu et al.¹³⁷ have reported synthesis of SnO₂ nanoparticles of size approximately 3–5 nm (Fig. 7), by the ultrasonic irradiation of an aqueous solution of SnCl₄ and azodicarbonamide in air. As, cavitation is a quenching process, the composition of particles, so formed, is identical to the composition of the vapor in bubbles and hence the atomic level mixing of the constituent ions is possible in sonochemical route.¹³⁸ This becomes advantageous as the contamination level is negligible and particle growth and aggregation can be controlled.

Fig. 7 TEM micrographs of SnO₂ nanoparticles obtained via sonochemical method. Reproduced from ref. 137 with permission ACS publications.

2.2. Vapour phase synthesis

Vapour-phase syntheses of nanoparticles involve vapourization of primary reactants followed by vigorous quenching of the resultant vapours on to a cold metal substrate. Supersaturated conditions are created where the vapour phase mixture is thermodynamically unstable relative to the solid material and hence under controlled condensation condition, the vapours nucleate homogeneously to nanoparticulate form.^139,140

The most common method of achieving supersaturation is to heat the solid to evaporate into a background gas. The temperature required for vaporization can be via thermal plasma, laser ablation, spark plasma, arc discharge etc. Liu et al.¹⁴¹ presented a detailed synthesis of SnO₂ nanowires where the tin target was ablated with Nd:YAG laser to form tin vapour which was carried down by the oxygen–argon mixture. At high supersaturation, the vapors rapidly nucleate, forming very large numbers of nanowires with extremely small diameter (Fig. 8a). There are many factors that affect the product such as the type of laser, number of pulses, pulsing time and type of solvent. Further, Lu et al.¹⁴² introduced simple, convenient, and low-cost mini-arc plasma source to synthesize tin oxide nanoparticles at atmospheric pressure. They used tungsten and graphite as electrodes and thin tin wire as the source of tin. The high temperature in the arc discharge melts and vaporizes the solid tin and cold nitrogen flow quenches the tin vapor and nucleates tin nanoparticles, which were then oxidized to form tin oxide nanoparticles by introducing purified air immediately at the exit of the mini-arc reactor.

Fig. 8 TEM micrographs of tin oxide nanostructures obtained by vapour phase synthesis (a) laser ablation (b) arc plasma (c) spray pyrolysis. Reproduced from ref. 141, 142 and 144 with permission from Wiley, Hindawi Publishing Corporation and Elsevier publications respectively.

An alternate means of achieving the supersaturation may be direct spraying of liquid precursor into flame called flame spray pyrolysis. This method allows use of precursors that do not have sufficiently high vapor pressure to be delivered as a vapor.¹⁴³ Sahm et al.¹⁴⁴ have prepared single crystalline tin oxide particles of about 20 nm size from ethylhexanoate precursor in ethanol via spray pyrolysis method.

Other advances in this method have been in preparing composite tin oxide nanoparticles and in controlling the morphology by controlled sintering after particle formation.^145,146

2.3. Solid state synthesis. The term solid-state synthesis is used to describe interactions where neither a solvent medium nor controlled vapor-phase interactions are utilized. Li et al.^147,148 reported simple solid state synthesis of tin oxide nanoparticles by grounding SnCl₄·5H₂O and KCl in the ratio 1 : 1 for 30 min followed by addition of solid KOH. The product on calcination yielded SnO₂ nanoparticles of size approximately 3–15 nm. Yang et al.¹⁴⁹ introduced SnCl₂, anhydrous Na₂CO₃ and NaCl in definite proportion into planetary mill for 4 h at 600 rpm. The as-milled powder was then annealed at 600 °C in air to get the desired nanoparticles of size in the range 25–40 nm. The size and morphology of the nanostructures thus synthesized by mechanical processing are very sensitive to the grinding conditions and may be unpredictably affected by unwanted contamination from the milling media and atmosphere. In addition, extensive long periods of milling time may be required to obtain particles smaller than 20 nm.^150,151

Recently, Sinha and colleagues¹⁵² have reported an interesting pseudo solid state synthesis of Sn:SnO₂ using focused solar irradiation. The first step is the precipitation of blue black SnO microplates by treating SnCl₂·2H₂O with NaOH. The solid state photo decomposition of the as formed SnO powders yielded highly crystalline Sn:SnO₂ of size less than 50 nm (Fig. 9c and d).

Fig. 9 TEM micrographs of SnO₂ nanoparticles obtained by solid phase synthesis (a) mechanical pressing (b) ball milling (c and d) focused solar irradiation. Reproduced from ref. 147 and 152 respectively with permission from Elsevier and ACS publications.

2.4. Bio-synthesis

Apart from the aforementioned chemical and physical methods for fabrication of SnO₂ nanoparticles, researchers have developed new methodologies with clean, nontoxic, and environmentally friendly, green chemistry approaches by utilizing biological entities such as plants and microorganisms. Srivastava et al.¹⁵³ reported facile, green, biogenic method for the synthesis of SnO₂ nanoparticles using Gram negative bacteria Erwinia herbicola. It was reported that the bacterial protein and biomolecules act as the template for reduction and stabilization of SnO₂ and control the size and aggregation of nanoparticles. In a typical synthesis, fresh and clean bacterial cells (0.4 g) were added directly into 100 mL of an aqueous solution of 1 mM SnCl₂·2H₂O. The sample was incubated at 30 °C, 120 rpm, for 48 h and finally annealed at 150 °C for 2 h to get the nanoclusters. The average crystallite size of biosynthesized SnO₂ nanoparticles was reported to be 28.89 nm.

Further, various plant extracts such as Daphne alpine,¹⁵⁴ Ficus benghalensis, Baringtoria acutagularis, Cyclea peltata,¹⁵⁵ Plectranthus amboinicus,¹⁵⁶ Aloe vera,¹⁵⁷ Ficus carica¹⁵⁸ etc. have also been used for the biomediated green synthesis of SnO₂ nanoparticles. Gattu et al.¹⁵⁹ reported bio-green synthesis of Ni-doped tin oxide nanoparticles (Fig. 10) using Bengal gram bean (Cicer arietinum L.) extract. The as synthesized particles were nearly spherical with average particle size ranging from 30–60 nm. It was reported that, pectin (complex polysaccharide present in plant extract) acts as a complexing and stabilizing agent which binds to the tin ions and inhibits the growth of the nanoparticles (Fig. 11).

Fig. 10 TEM micrographs (a), (b) and particle size distribution histogram plots (c), (d) of tin oxide nanoparticles and Ni-doped tin oxide nanoparticles biosynthesized from Cicer arietinum L. extract. Reproduced from ref. 159 with permission from RSC publishing.

Fig. 11 Schematic representation role of pectin in biosynthesis of tin oxide nanoparticles. Reproduced from ref. 159 with permission from RSC publishing.

Moreover, S. M. Roopan and coworkers¹⁶⁰ synthesized stable SnO₂ nanoparticles from waste dried peel of sugar apple (Annona squamosa) and examined cytotoxicity of the as synthesized materials against hepatocellular carcinoma cell line (HepG2). It was reported that as synthesized SnO₂ nanoparticles inhibited the cell proliferation in a dose and time-dependent manner with an IC₅₀ value of 148 μg mL⁻¹.

3. Modifications

3.1. Doping

Doping is the intentional introduction of impurities into an extremely pure semiconductor to alter its electronic arrangement. A survey of experimental literatures reveals that impurities doped into tin oxide semiconductor nanocrystals could strongly influence the electronic and optical properties.^38–51 So, much effort has been devoted into the research of impurity atoms such as F, Sb, Ni, Fe, Pt etc., doped tin oxide nanomaterials that can effectively complement the properties of the bulk material.

Han et al.¹⁶¹ synthesized nano-crystalline F-doped SnO₂ having better sensitivity and selectivity for the hydrogen gas sensing then commercially available SnO₂. Yin et al.⁵⁷ suggested that F-doped SnO₂ an efficient material for water-splitting whereas Mu et al.⁶² found electrocatalytic and photocatalytic potentials of F:SnO₂ towards cysteine oxidation. Chakraborty and co-workers⁴⁹ reported Fe-doped tin oxide sensors for selective detection for methane and butane by temperature modulation while Neri and team⁵⁰ prepared ethanol sensors based on Pt-doped tin oxide nanopowders. Further Pd/SnO₂ was found to be an efficient CO gas sensor.¹⁶²

Recently, much attention has been paid to rare-earth (RE) ion doped tin oxide nanomaterials for photonic applications. The quantum confinement effects of semiconductor nanoparticles generate photogenerated carriers that may have an interaction with 4f-electrons of lanthanides thereby influencing the optical properties.⁴⁸ Fu et al.¹⁶³ found that the Eu³⁺ luminescence of Eu:SnO₂ matrix is strongly enhanced by energy transfer from the SnO₂ to Eu³⁺ ions. Further Wang et al.¹⁶⁴ reported fluorinated europium doped tin oxide nanocrystals with improved photoluminescence.

Doped tin oxide materials (ATO, FTO, ITO etc.) were commonly synthesized by sol–gel or co-precipitation method and solvothermal method where the dopant ions of appropriate stoichiometric amounts mixed homogenously with the tin chloride solution and precipitated simultaneously so as to acquire lattice positions in SnO₂ lattice.^165–172 These synthetic strategies provide greater homogeneity and control over the doping concentration thereby directing desired properties in the doped nanomaterials.

3.2. Carbon–SnO₂ nanocomposites

Carbon–SnO₂ nanohybrid materials have gained much attention owing to their profound electrochemical and photochemical applications. Carbonaceous material matrix can provide effective cushion against the specific volume changes in the nanostructure region thereby imparting greater capacity as an electrode material.¹⁷³ Electrochemical testing of the nano-SnO₂/carbon composites by Courtel and team¹⁷⁴ showed enhanced lithium storage capacity up to 32% compared to the neat carbon and the neat SnO₂ electrodes. Moreover, Xie et al.¹⁷⁵ and Du et al.¹⁷⁶ suggested SnO₂–CNTs (Fig. 12a) as a better choice for lithium ion batteries as it was found that SnO₂/MWCNT composites show very stable cyclic retention up to 100 cycles. Furthermore, it delivers reasonable capacity when the current density is changed back from a high to a smaller value. Also, reduced graphene oxide/SnO₂ nanocomposites (Fig. 12b) were found to have good capacity retention with a capacity of 649 mA h g⁻¹ after 30 cycles.¹⁷⁷

Fig. 12 TEM micrographs of (a) SnO₂–f-CNTs (b) SnO₂–f-graphene sheets reproduced from ref. 175 and 177 respectively with permission from Elsevier.

Tin oxide nanoparticles coated on multi-walled carbon nanotube were also reported to have the higher photocatalytic activity compared to pure SnO₂ and TiO₂ owing to the effective electron transfer between SnO₂ and MWNT.¹⁷⁸

3.3. Thin films/core shells/hollow spheres

Modification in surface properties of semiconducting nanomaterials leads to detectable changes in its conductivity thereby giving rise to positive changes in the properties. Transparent semiconducting thin films of tin oxide were reported to be good materials for optoelectronic applications¹⁷⁹ and were found to be advantageous as gas sensors owing to their high sensitivity, low fabrication cost, enhanced thermal stability and applicability for microsensors.^21,180 Recently, indium tin oxide (ITO) thin films, antimony doped tin oxide thin film (ATO), Er:SnO₂ thin films etc. have been thoroughly investigated as gas sensors, to detect NO_x ethanol, methanol, acetone, CO, H₂ etc.^181–185

Hollow spheres and core shell fabrications as structural modifications are the recent trends in nanoengineering. Yu et al.¹⁸⁶ synthesized Au@SnO₂ core/shell nanocomposites (Fig. 13a) and it was found that due to synergetic confinement effect of Au@SnO₂, the well-encapsulated core/shell nanoparticles show efficient and high-temperature-stable CO catalytic activity. Likewise, crystalline SnO₂ hollow nanospheres have been synthesized extensively and found to exhibit good electrochemical performance as an anode material in lithium ion batteries.¹⁸⁷ Hollow SnO₂ nanospheres (Fig. 13b) exhibited high discharge capacities and higher coulombic efficiency than the theoretical value of SnO₂.¹⁸⁸ Because these hollow nanospheres have large surface area, stable structure, and a particular inner environment, such materials are believed to have great applications in gas sensors, heterogeneous catalysis, optical devices, and microreactors.^189–192

Fig. 13 (a and b) FE-SEM image of hollow SnO₂ reproduced (c and d) TEM micrograph of Au@SnO₂ core/shell from ref. 186 and 187 respectively with permission from ACS publications.

3.4. Mesoporous tin oxide materials

Recently, the fabrication of mesoporous tin oxide materials has drawn considerable attention due to their large internal surface area and narrow pore size distribution, considerably large amount of active sites and diverse composition, which make them ideal candidate for catalyst, sensors, electrodes in solid state ionic devices etc.^193,194 The surfactant templating pathway with variety of surfactants like CTAB, HMDS, TEOS, long-chain RTILs 1-hexadecyl-3-methylimidazolium bromide, poly(isobutylene)-b-poly(ethylene oxide) etc. have been commonly applied for the synthesis of mesostructured materials.^195–197 It was reported that porous SnO₂/C composites (Fig. 14) act as high performance anode materials for lithium ion due to the designed porous carbon matrix, which alleviate the volume expansion of SnO₂ NPs during cycling, provide a large contact area between electrode and electrolyte, and shorten the ionic transport and diffusion.¹⁹⁸

Fig. 14 (a) TEM micrographs of mesoporous SnO₂/C composites showing interconnected carbon particles with SnO₂ NPs homogeneously dispersed inside them. (b) HRTEM image of SnO₂/C composites showing particle size of SnO₂ NPs. Reproduced from ref. 198 with permission from Elsevier.

Further, electrical conductivity was found to increase with mesoporosity of the tin oxide materials.¹⁹⁹ Moreover, Marakatti et al.²⁰⁰ showed the mesoporous tin oxide as a simple, efficient and eco-friendly solid acid catalyst for the activation of carbonyl group in selected organic transformations, Gan et al.²⁰¹ reported mesoporous tin oxide-functionalized rGO for ultrasensitive detection of guaiacol in red wines. Mesoporous tin oxide materials were also found to be an efficient oxidation-resistant catalyst for proton exchange membrane fuel cells and dye-sensitized solar cells.^202–204

4. Applications

4.1. Gas sensing

Selective detection of various gases is very crucial for monitoring environmental pollution, public safety and human health.^205,206 Metal oxides in general and SnO₂, in particular, have attracted much scientific interests in gas sensing under atmospheric conditions due to the high selectivity, high sensitivity, easy designing and implementation, good reversibility and low manufacturing costs.^21–23,207 The SnO₂ sensor response is because of the physical and chemical changes on its surface due to the adsorption of a chemical stimulant and the sensitivity can be fine-tuned by reducing the size up to nano dimensions, varying the crystal structure and morphology and/or adding dopants (typically noble metals and other metal oxides) to create nanocomposite materials etc.^{21–23,208–210} Fig. 15 demonstrates Leite and coworkers²² work on sensing response of SnO₂ sensors as a function of doping.

Fig. 15 Experimental observation showing sensor response of undoped and doped SnO₂ nanoparticles towards ethanol sensing. Reproduced from ref. 22 with permission from Wiley.

Millions of carbon monoxide (CO) alarms utilize SnO₂ as the active sensing element and it has been found that Pd-loaded SnO₂ nanostructures exhibit great response, high stability, selectivity, sensitivity and fast recover properties when detecting carbon monoxide gases.¹⁶² Further, Maeng et al.²¹¹ recently reported highly sensitive SnO₂ nano-slab network for NO₂ sensing. Fig. 16 shows the schematic representation of the sensor device and the experimental observations including conductance dependence of SnO₂ nano-slab sensor upon the cyclic exposure to NO₂. Various hybrid materials like SnO₂/reduced graphene oxide composites, reduced graphene oxide–multiwalled carbon nanotubes–tin oxide nanoparticles hybrids etc. have also been employed for high performance low temperature NO₂ sensing.^212,213

Fig. 16 Graphical representation and experimental observations of SnO₂ nano-slab network towards NO₂ sensing. Reproduced from ref. 211 with permission from ACS publications.

Over the last 15 years, considerable research has been conducted on understanding the chemical and electronic mechanisms that govern the sensing behavior of SnO₂ sensors towards various gases (NO_x ethanol, methanol, ammonia, acetone, CO, H₂ etc.) including trace amounts of toxic gases^{162,182–186,214–218} thereby establishing the great possibility and huge potential of SnO₂ towards gas sensing applications.

4.2. Bio-sensing

Recently doped tin oxide nanomaterials gained tremendous popularity in electrical and photoelectrochemical biosensing applications. Liu et al.²¹⁹ studied the fluorescently labeled single-stranded DNA adsorption by indium tin oxide nanoparticles (ITO) and demonstrated the average DNA adsorption behavior of ITO compared to SnO₂ and In₂O₃. ITO has got just the enough DNA binding affinity so that desorption can occur easily in presence of complementary DNA (cDNA), which is very unlikely for other metal oxide nanoparticles, thereby making it a unique surface to directly detect cDNA (Fig. 17). This reversible DNA–ITO interaction facilitates it as a very useful transparent electrode material for detection of various other targets like DNA,²²⁰ cDNA,²²¹ DNA methylation,²²² pathogen²²³ etc.

Fig. 17 Schematic representation of DNA desorption in the presence of cDNA from ITO. Reproduced from ref. 219 with permission from ACS publications.

Further, FTO materials have been proved to have immense potential for next generation cancer sensing devices. Masuda and colleagues²²⁴ suggested fluorine doped tin oxide nanosheets modified with dye-labeled monoclonal antibody as a suitable electrode of cancer sensing. Photoluminescence and photocurrent conversion due to reaction of monoclonal antibody with human alpha-fetoprotein (present in blood serum of hepatocellular cancer patient) formed the basis of cancer sensing. Furthermore, latest developments like graphene-modified fluorine-doped tin oxide substrates as opto-electrochemical sensor to detect insulin (Fig. 18) thereby yielding high performance, low cost protein sensors, reduced graphene–gold nano particle composite on indium tin oxide for label free immuno sensing of estradiol, Pd doped SnO₂ for glucose sensing, SiNWs/AuNPs-modified indium tin oxide for DNA sensing and tin oxide QDs based DNA sensor for pathogen detection etc.^225–231 give insight of the immense possibilities of tin oxide based nanomaterials towards biosensing application.

Fig. 18 Schematic representation of opto-electrochemical biorecognition of insulin. Reproduced from ref. 225 with permission from ACS publications.

4.3. Catalytic applications

Solid acid catalysts have emerged as potential alternate catalysts to homogeneous liquid counterparts due to their non-hazardous nature, requirements in catalytic amounts, enhanced selectivity, reusability, activity in moderate temperatures and the ease of separation.^232,233 Ahmed and coworkers²³⁴ reported the use of sulfated tin oxide nanomaterials as an efficient solid acid catalyst for the preparation of 7-hydroxy-4-methyl coumarin by Pechmann condensation reaction. Further, Marakatti et al.²⁰⁰ showed that mesoporous tin oxide as an efficient solid acid catalyst for the activation of carbonyl group in selected organic transformations. The mesoporous tin oxide showed higher catalytic performance in the Prins, glycerol acetylation, ketalization and carbonylation reactions compared to the nano and bulk tin oxides with more than 80% conversion rate and excellent reusability. Collins et al.²³⁵ demonstrated high performance of hierarchically porous tin dioxide palladium nanomaterials as catalyst in Suzuki coupling reactions. The SnO₂–Pd showed excellent reactivity in cross coupling of aryl iodides and bromides with phenylboronic acid obtaining yields >90% under air and at room temperature. Furthermore, the catalyst does not require any separation procedure after the reaction and showed excellent recyclability over 3 cycles showing no loss in catalytic activity with yields of 95% ± 3%.

The tin oxide based nanocatalyst exhibit superior catalytic activity, high stability after high temperature calcination, and excellent recycling and reusability in gas- and solution-phase reactions which is evident from various reports on catalytic hydrogenation of p-nitrophenol to p-aminophenol, catalytic oxidation of CO, methanol oxidation and oxygen reduction reaction etc.^236–240

4.4. Photocatalytic degradation of dyes and organic compounds

Rapid industrialization and dissolution of toxic chemicals, textile dyes, pesticides etc. into running water may result a number of health hazards. Monitoring and easy degradation of these poisonous chemicals in waste water is very critical for environmental security purpose. Tin oxide nanomaterials play a crucial role for photo catalytic degradation of a number of common textile dyes and organic compounds. Bhattacharjee and coworkers^241–246 reported degradation of common textile dyes like eosin Y, methylene blue, rose bengal, methyl violet 6B, etc. using direct sunlight which is a very cost effective method and scalable to large extent compared to common UV assisted photodegradation technique.

The recombination of photogenerated electrons and holes is detrimental for efficient photocatalytic activity and the anti-recombination of carriers can be endorsed by modifying tin oxide nanostructures with cation and anion doping, composite formation, and nanostructures supported over different substrates (Fig. 19). Pan et al.²⁴⁷ reported that the indium tin oxide nanoparticles show higher photodegradation efficiency of rhodamine B than the commercial P25 TiO₂ within the given concentration range. Further, Sinha and team¹⁵² investigated the light-assisted degradation of MB, malachite green, rhodamine B, methyl violet, methyl green, and rose bengal using Sn–SnO₂ NPs and claimed that the presence of Sn(0) nanoparticles in spherical SnO₂ nanoparticles improves the charge (electrons and holes) separation efficiency thereby enhancing the rate of degradation.

Fig. 19 Schematic representation of degradation of dyes. Reproduced from ref. 243 with permission from Elsevier.

4.5. Dye sensitized solar cells

Dye-sensitized solar cells (DSSCs) are green energy devices consisting of metal oxide semiconductor photoanodes, redox electrolytes, and dye molecules incorporated onto photoanode and electrolyte.^248,249 The DSSCs utilize photon energy to excite dye molecules which results in rapid injection of electrons into the conduction band of the metal oxide semiconductors thereby resulting in photocurrent. DSSCs employing TiO₂ NPs photoanodes exhibited record efficiencies of about 11%.²⁵⁰ Nevertheless, tin dioxide is believed to be a potential candidate to replace TiO₂ thereby improving the photovoltaic performance due to its wider band gap and higher electron mobility.²⁵¹ Ramasamy et al.²⁵² demonstrated ordered mesoporous SnO₂ photoanode to have improved efficiency, greater dye loading capability and enhanced light scattering compared to conventional randomly oriented nanoparticle photoanode. Further, Ga doped tin oxide NPs photoanodes were reported to have greater performance due to moderate up-shifting of the band edge by Ga-doping which facilitated electron injection from the sensitizer to the electrode, and hindered electron recombination with the electrolyte (Fig. 20).²⁵¹ Numerous reports on ITO, FTO DSSCs^253–262 etc. firmly establish the importance of tin oxide based photoanodes towards dye sensitized solar cells.

Fig. 20 Schematic representation of band structure and electron transfer of Ga–SnO₂ photoanode. Reproduced from ref. 251 with permission from ACS publications.

4.6. Electrocatalytic applications

Tin oxide nanostructures find extensive applications in electrocatalytic processes owing to high effective surface area, enhancement of mass transport, and control over electrode microenvironment. Carbon-supported PtSnO₂ catalysts, platinum–tin oxide core–shell catalysts, Pd–SnO₂/graphene, Pt–ITO catalysts etc. were extensively utilized for the electro-oxidation of ethanol and methanol, antimony doped tin oxide modified carbon nanotubes were used as electro catalyst for methanol oxidation and oxygen reduction reactions.^263–267 Mu et al.²⁶⁸ found bare FTO very effective towards cysteine oxidation, Zhang and colleagues²⁶⁹ demonstrated selective electrochemical reduction of carbon dioxide to formate and so forth. Interestingly, quantum dots modified indium tin oxide electrodes were found to be an efficient photoelectrochemical bioanode for catalytic oxidation by human sulfite oxidase.²⁷⁰

Among all these electrocatalytic processes, water oxidation is of the greatest importance as it facilitates clean energy in the form of hydrogen. Yin and his coworkers²⁷¹ reported metal oxide-based tree-like heterostructures on fluorine-doped tin oxide as an efficient light-harvesting photoanode to generate H₂ through the water-splitting process. Also, ATO nanorod scaffolds supported over FTO were used to improve hematite's photoelectrochemical water oxidation performance.²⁷² Recently, mesoporous indium tin oxide electrodes and cobalt modified fluorine doped tin oxide electrodes were utilized photoelectrochemical water oxidation and oxygen monitoring purpose.^273,274

4.7. Lithium ion batteries

Rechargeable lithium ion batteries are considered as the most favored power sources for portable electronic devices due to their high capacity, high cyclability and flexible design. SnO₂ based materials have attracted much interest as anode material owing to their high theoretical lithium storage capacity (781 mA h g⁻¹) which is more than twice than that of conventional graphite anodes (372 mA h g⁻¹).⁷⁶ However, practical applicability of tin based anodes is significantly impeded due to poor capacity retention over large cycles, resulting from large volume changes and agglomeration of tin nanocrystals during charge/discharge processes.¹⁷⁶ Use of tin oxides with specific structures like hollow spheres, nanowires, nanoflowers, nanosheets, etc. have been proposed to address the problem.^{17,76,275,276} Further, hybridization of SnO₂ with carbonaceous materials furnished improved cyclability as the carbon-matrix can cushion the mechanical strain during volume changes thereby overcoming the effect of volume change. SnO₂/carbon composites, SnO₂/CNTs, SnO₂/graphenes etc. showed much better performances in terms of storage capacity, coulombic efficiency and cycling life.^{19,72,73,175,176,277} Moreover, among the other carbonaceous materials, graphene–SnO₂ composites were found to be the most promising anode materials due to their high surface area, excellent electronic mobility, and mechanical support which are bonus to active SnO₂ nanoparticles. Wang et al.⁷² reported the charge capacities of SnO₂/graphene composites as 590 mA h g⁻¹ and 270 mA h g⁻¹, upto 50 cycles, at current densities 400 mA g⁻¹ and 1000 mA g⁻¹ respectively. Fig. 21 demonstrates the increased capacity and enhanced cyclability of SnO₂/graphene anodes for lithium ion batteries.

Fig. 21 Schematic representation of increased capacity and enhanced cyclability of SnO₂/graphene composites. Reproduced from ref. 277 with permission from ACS publications.

Recently, various new materials with diverse combination of carbon and tin oxide such as Sn/SnO₂@C composite, CNTs@SnO₂/SnO/Sn, hollow core–shell SnO₂/C fibers, double-shell SnO₂@C hollow nanospheres, carbon coated SnO₂/CNTs, carbon-coated hierarchical SnO₂ hollow spheres, CNTs@SnO₂@C coaxial nanocables, SnO₂–graphene–CNT mixture, carbon-coated SnO₂/graphene nanosheets etc., have been utilized in order to get advanced anode material showing superior result.^278–288 Further, introduction of dopants in carbon–SnO₂ materials showed positive impact as chemical functionalization of carbonaceous materials could potentially produce localized highly reactive regions and thus exhibit unexpected properties. N-doped SnO₂/graphene, SnO₂@N-doped carbon hollow nanoclusters, SnO₂/N-doped carbon nanofibers, SnO₂–TiO₂@graphene ternary composites, SnO₂@TiO₂ double-shell nanotubes on carbon cloths sandwich-stacked SnO₂/Cu hybrid nanosheets etc. were reported to have high flexibility, superior rate capacity and long life cycle as anode materials in lithium ion batteries.^289–296

5. Discussion and future prospects

It has become evident that the ease of synthesis and the exceptional tunability of properties made tin oxide nano structures compelling for various remarkable applications. Irrespective of the mode of synthesis, each synthetic methodology provides unique characteristics, morphological diversity, tunability to develop various modified and hybrid structures, thereby, endorsing low cost, highly capable materials towards various applications. It is worth mentioning that, among the other synthetic routes, sol–gel method is the most popular and mostly studied method because of the obvious reasons of simplicity, homogeneity, and flexibility. However, some of its limitations can be addressed by hydrothermal method, precursor mediated synthesis and microemulsion technique. It is pertinent here to mention that, equipped with appropriate control over the reaction parameters, microemulsion technique can give rise to tailor-made products of desired morphology. Advanced techniques like microwave irradiation, ultrasonication etc. yielded high quality nanomaterials with desired size/shape in a very short span of time. Need for greener approach have led to develop vapour phase and solid state synthesis by avoiding hazardous organic solvents and precursor compounds. All these synthetic strategies have tremendous flexibility thereby opening door for variety of nanostructures/hybrid materials of all possible shape, size, geometry and composition. The control over the band structure of the tin oxide nanoforms by various structural modifications like doping, carbon–SnO₂ composites, hollow spheres, thin films, core–shells etc. have led to unusual properties which in turn brought about revolution in the field of technological and environmental engineering.

The implausible potentials and innumerable possibilities of tin oxide nanostructures demand immense scientific interests. Development of bio-mediated green synthetic strategies, exploration of newer hybrid materials and their properties, surface functionalization and controlled aggregation etc. may give rise to new dimensions to the tin oxide nanodevice research. Advanced research on tin oxide based materials can provide solutions to some of the serious concerns like pollution monitoring and control, public safety and health care, clean energy generation and storage. Moreover, biosensing potentials of tin oxide based materials provide numerous possibilities towards molecular diagnostics and therapeutic applications.

6. Conclusion and outlook

In this account, we tried to investigate the recent progresses on tin oxide nanoparticles/nanostructured materials, connecting various methods of synthesis with their utilities & drawbacks, various modifications and their effects, and potential applications towards sensing, catalysis, photovoltaics, energy storage etc. There are number of ways to prepare tin oxide nanostructures and to bring about structural modifications, each method has its own characteristics and limitations. The simplicity of synthesis and the ease with which the electronic environment and hence the properties can be controlled, have led to these materials to have standout applications in gas sensing, biosensing, photocatalysis, electrocatalysis, optoelectronics, dye-sensitized solar cells, rechargeable Li ion batteries etc. Further, the potentiality is augmented by the low cost of manufacturing and multidimensional utility. Though these materials seemed to provide promising platforms which may lead to potentially practical solutions to some of the critical problems like clean energy generation, energy storage, pollution monitoring and control, public safety and human health assurance etc., it is still in theoretical domain. In order to move from curiosity-driven discoveries to commercialization of some of the prototypes, close collaboration among scientists and engineers of different disciplines are essential which will eventually lead some practically useful materials having positive impact on our daily-life and environment.

Acknowledgements

Authors would like to acknowledge Dr Sudip Choudhury and Dr Abhijit Nath for insightful discussion and help throughout the work. The Director NIT Silchar is acknowledged for financial support through STIS Research Grant (STIS project sanction No. PA/254/23130).

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