Sol-gel Synthesis of Materials in the System ZnO – SnO2

The effect of the anionic composition on the preparation of powder material in the ZZZZZZ − SSZZZZ2 system by the sol-gel method has been investigated. It was found that the best composition of zinc orthostannate is obtained using zinc sulfate and tin (IV) chloride. According to the data of X-ray phase analysis, the optimum temperature for the synthesis of the material is 1050 oC.


Introduction
Binary system − 2 , depending on the ratio of the initial components, can represent two compounds: 3 -zinc metastannate with a perovskite structure and 2 4zinc orthostannate with a reverse spinel structure [1,2]. Zinc orthostannate is an electron semiconductor due to its low toxicity, high temperature stability and high electron mobility. Zinc orthostannate material ( 2 4 ) can be used as lithium batteries [3], photocatalysts [4] and gas sensors [5].
The most promising methods for preparing zinc orthostanate are the sol-gel method, solid-phase synthesis from oxides [6], and co-precipitation of salts [7]. The sol-gel method is a well-controlled, low-temperature, and high-performance method for obtaining homogeneous ultradispersed nanostructures.
The sol-gel process is the best chemical route used for the low-temperature synthesis of one-and multicomponent ceramic systems in the form of ultradisperse powders, thin-films, and dense abrasive materials.
The study is a comparison of the results of the interaction of various zinc salts with tin (IV) chloride and the further preparation of zinc orthostannate by sol-gel synthesis.

Experimental procedure
The synthesis of the sol-gel method of − 2 powder was carried out using tin (IV) chloride and zinc salts of different anionic compositions: The ratio of the starting salts was 2 ∶ . Dissolution of salts was carried out with isobutanol at low-temperature. The main goal at this stage was to obtain 2 zinc and tin salts dissolved in isobutanol.
Then the solutions of 2 salts were mixed using a magnetic stirrer. Next, an aqueous solution of ammonia was poured in a deficiency, until a curdled precipitate appeared and from the environment to the alkaline side (Equation 1, 2).
The phase composition of the obtained powders was investigated using X-ray phase analysis.

X-ray diffraction analysis (XRD)
The phase composition of the obtained powders was investigated using X-ray diffraction analysis. When zinc and tin (IV) chloride are used as starting components ( Fig. 1), no product of the co-reaction is formed (Equation 6.). This may be due to the reactivity of zinc chloride, which forms a complex compound tetraamminezinc (II) chloride [8], which further decomposes into zinc chloride, which does not interact with tin (IV) oxide (Equation 7). In fig. 2 and 3, nitrate and acetate are used as starting zinc salts. In fig. 3, the formation of a product is observed at 1000 ºС. The presence of a nitrate and acetate group in the zinc compound promotes the formation of a complex compound tetraamminezinc hydroxide (II), which upon heating forms zinc oxide and then reacts with tin oxide (IV) to form zinc orthostannate.  Based on the results obtained, the synthesis of orthostannate from zinc and tin oxides occurs at 1050 ºC using zinc sulfate and tin chloride as starting salts. In fig. 4, the best crystallization of the orthostannate phase is observed. Zinc sulfate is more reactive compound than other salts.  The best crystallization of the product is observed at 1050 ºC and using zinc sulfate and tin (IV) chloride as starting salts. This may be due to the activity of the anionic composition of the zinc salt. It is assumed that the more active the acid forming the acidic residue of the salt, the better the reaction of interaction between 2 и в 2 4 .

Scanning Electron Microscopy (SEM)
In fig. 5 (a), noticeable recrystallization of grains is observed. When the temperature of the material rises to 1200 ºC, the grains in the polycrystalline structure begin to coalescence of particles. Good crystal growth steps are also noticeable. The structure of the polycrystal is represented by intergrown cubic crystals with a size of 2 to 5 µm, which form screw dislocations (Fig. 5 (b)). In some crystals, the distribution of cracks between crystals and along the grain structure is noticeable ( Fig. 5 (b)). As the temperature rises, the crystals grow very actively. A photograph of the microstructure of a material synthesized from zinc chloride and tin (IV) chloride is shown in Fig. 6. The structure is presented in the form of unbound grains of MATEC Web of Conferences 346, 02008 (2021) ICMTMTE 2021 https://doi.org/10.1051/matecconf /202134602008 crystals of zinc and tin oxide, cylindrical, with an average size of 0.5 -1 μm. This material is less reactive than others. In fig. 7. shows the material synthesized from zinc acetate and tin (IV) chloride. Significant crystal growth is observed. The polycrystalline structure is composed of spherical grains with an average size of 0.5 to 1.5 μm. In fig. 8. shows the material synthesized from zinc nitrate and tin (IV) chloride. Good crystallization of the material is observed. A stepwise growth of crystals is observed. The polycrystalline structure of the material is presented in the form of prismatic and tetrahedral crystals with an average size of 0.5 to 1.5 μm, which are added together to form screw dislocations.

Conclusions
When zinc and tin (IV) chloride is used as starting components, the product of the joint reaction is not formed. This may be due to the reactivity of zinc chloride, which forms a complex compound tetraamminezinc (II) chloride.
Materials synthesized from zinc nitrate and zinc acetate form reaction products at 1000 °C. The presence of a nitrate, sulfate and acetate group in the zinc compound promotes the formation of a complex compound tetraamminezinc (II) hydroxide, which in turn, upon heating, forms zinc oxide and then reacts with tin (IV) oxide to form zinc orthostannate.
The microstructure of the material, synthesized from sulfate, nitrate, zinc acetate and tin (IV) chloride, is represented by well-defined crystals. The polycrystalline structure consists of bound grains of various shapes, depending on the anionic composition of the initial components. A clear recrystallization is noticeable after a temperature of 1200 ºС.
When using zinc chloride and tin (IV) chloride as starting components, poor crystal growth is observed. The structure is represented by unbound grains consisting of zinc oxide and tin (IV). Zinc chloride is less reactive than other salts.
The best crystallization of the product is observed at 1050 ºC and the use of sulfatac and tin (IV) chloride as starting salts.