ATO nanoparticles have attracted significant attention due to their well-known infrared light insulation quality, electrically conducting oxide and optical transparency. Antimony as the n-type dopant of tin oxide (SnO2) modifies the band structure of the tin oxide. Extra electron donation to the conduction band upon a cation substitutional replacement is carried on via a dopant as an impurity. The band structure of antimony doped tin oxide shows analytical data of Sb-5s-like band in the tin oxide’s band gap by a characteristic free electron at the G point. Additionally, it is elicited that this band gap is basically a half filled metallic ion with extra thermal excitation into bands similar to those of tin capable of increasing the conductivity 1. Antimony, which is the doped agent in antimony tin oxide has two ionic states based on its two valence electrons as Sb3+ and Sb5+ with a feasible switch Sb3+↔Sb5+ regarded as the redox enzymes catalytic cycle analogue in which the metal ions serve as cofactors in order to promote the possible reversing redox reactions against intracellular oxidizing agents.
Generally, transparent conducting oxides (TCOs) like antimony tin oxide and its nanoscale crystals combine transparency quality in the visible range of electromagnetic wave with high electrical conductivity making them considerable materials for several optoelectrochemcial applications. TCOs like antimony tin oxide nanoparticles are produced predominantly and extensively as thin layers and coatings. However, there has been a fast-growing trend and interest on obtaining nanosized crystals as small as possible in order to take the advantages of nanoparticles novel and improved properties that add to the bulky mother materials. The colloidal dispersion of antimony tin oxide nanoparticles are of a great interest for wet chemical deposition purposes. As the chemical and physical properties of materials like ATO nanoparticles directly depends of the size and morphology, synthesis of this class of agents is of a significant step for humidity sensing applications.
Considering the predominant applications of antimony tin oxide nanoparticles, numerous methods and techniques have been introduced to obtain them. Basically, it has to be taken into consideration that the properties of ATO is influenced by the synthesis and preparation process. The goal is always to obtain crystals with controlled size and an appropriate monodispersity so that high performance material is achieved. On the contrary, it is still challenging to synthesize antimony tin oxide nanoparticles combining all the required standards and determining factors like adequate conductivity, desirable particle size crystallinity, narrow particle size distribution and favorable dispersibility in a particular solvent. The most common and known methods include simple thermal evaporation, microemulsions, sol-gel, hydrothermal methods, co precipitation, mechanochemical, laser ablation, screen printing, combustion route, microwave assisted synthesis, thermal decomposition, hot injection and DC arc plasma jet synthesis.
In a synthesis procedure based chemical precipitation method, a solution of hydrochloric acid (HCl) is prepared and a mixture of SnCl2.H2O and SbCl2 with the ratio of 1:4 is added to acid solution with a consecutive dropwise addition of ammonium hydroxide to them. In the next step, an amount of polyethylene glycol as a capping agent is added to the mixture while vigorous magnetic stirring for around 24 hours until all the ingredients engage in a complete reaction. The resulting crystals are dried at 80°C for 8 hours and calcined at 400°C 2.
In a different method, the synthesis of antimony tin oxide nanoparticles is done via dissolving granulated tin (Sn) in nitric acid (HNO3). Separately, Antimony trioxide (Sb2O3) is dissolved in melted citric acid and is added to the previous solution and stirred for 3 hours at room temperature. After that, antimony hydroxide is added to the solution in order to process the precipitation of Sb3+ and Sn4+ metal ions and the pH of the solution is adjusted at 7. The hydroxide precipitate is washed in double distilled water until the color of the samples turn yellow. The resulting precipitates are dried at 100°C for 4 hours and calcined at 600°C for 2 hours in a muffle furnace essential to ensure the particles conductive property 3. A number of studies has focused on the assembly of antimony tin oxide nanoparticles into aerogels as big as a few centimeters with increasing applications as catalysis, photo catalysis and ferroelectric agents with promising properties like high porosity, high surface area to low density magnetic and optical quality. The assembly procedure is carried out by microwave heating and the assistance of non-aqueous sol gel reaction of antimony acetate Sb(CH3COO)3 with tin chloride (SnCl2) in the solvent made by mixing benzyl alcohol and toluene. The resulting mixture is heated up to 150°C for 9 minutes to obtain a brown precipitate 4.
Referring to the unique electrical and conductive properties, antimony tin oxide (ATO) nanoparticles have been used extensively as transparent electrodes and thin films, support material for electro catalysis, energy storage devices, gas sensors and photoelectrocatalysis. It has been employed as a supporting agent in composite materials, radioactive waste management and optoelectronics. The antimony doped tin oxide nanoparticles have found broad applications in solar cells and heat reflection coatings too. Since ATO nanoparticles are transparent in visible region, they can be used as heat mirrors and transparent electrodes as well.
Antimony tin oxide nanoparticles quality as a transparent agent with favorable electrical conductivity has made it interesting nanoscale crystals in thin film electrode fabrication. In fact, most of the research has focused on optimizing the synthetic methods with applications in thin films production. The electron collecting quality of ATO nanoparticles has been drawing attention to their use in developing efficient solar cells.
1. Krishnakumar, T. et al. Structural, optical and electrical characterization of antimony-substituted tin oxide nanoparticles. J. Phys. Chem. Solids 70, 993–999 (2009).
2. Yadav, B. C., Singh, R., Singh, S., Kumar, R. & Srivastava, R. Nanostructured antimony tin oxide synthesized via chemical precipitation method : its characterization and application in humidity sensing. 1–16.
3. Zhang, J. & Gao, L. Synthesis and characterization of antimony-doped tin oxide (ATO) nanoparticles. Inorg. Chem. Commun. 7, 91–93 (2004).
4. Rechberger, F., Ilari, G. & Niederberger, M. Assembly of antimony doped tin oxide nanocrystals into conducting macroscopic aerogel monoliths. Chem. Commun. 50, 13138–13141 (2014).
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