Svetlana A. Kuznetsova

Tin(II) oxide is a promising material for photocatalytic wastewater treatment. However, the established relationships between particle size, shape, and photocatalytic activity of SnO are contradictory, indicating the influence of other factors. In this work, the effect of the SnO crystallographic texture on its band gap and photocatalytic activity was shown for the first time. The relationship between the methods (microwave and hydrothermal microwave) and synthesis conditions (time, pressure, and chemical composition of the suspension) of polycrystalline tin oxide(II) and the crystallographic texture was studied. The crystallographic texture was estimated by the Harris method using the repeatability factor and the Lotgering coefficient. The formation of crystallites oriented in the growth plane (00l) was facilitated by the carbonate medium of the suspension. In the ammonia medium, crystallites were preferably formed in the plane (h0l). Increasing the time and pressure leads to the recrystallization of SnO. The band gap energy of the SnO increases from 3.0 to 3.6 eV, and the rate of photodestruction of methyl orange decreases with the growth of crystallites in the (00l) plane from 17 to 40%.

The discharge of wastewater without proper treatment gives rise to environmental problems. One of the ways to solve them is photocatalytic decomposition of organic pollutants in wastewater. This method attracts researchers’ attention as being simple and cheap. Powdered semiconductor oxide materials exhibiting good adsorption properties toward organic compounds, photochemical stability, and nontoxicity are mainly used as photocatalysts. Tin(IV), titanium(IV), zinc(II), and cerium(IV) oxides exhibit such properties. The possibility of using as photocatalysts thin-film materials, which can be readily separated from the treated water, has been demonstrated recently. In addition, it is known that heterostructures enhance the photocatalytic activity of the material, in particular, by decreasing the bandgap, which allows more efficient absorption of visible light by the substance. This study compares the optical properties of the SnO₂, CeO₂, and CeO₂/SnO₂ thin-film materials on quartz supports. The materials were prepared by the sol–gel method from film-forming solutions based on cerium(III) nitrate and/or tin(IV) chloride with salicylic acid. The film thickness and refractive index were studied by ellipsometry, and the transmittance was examined by spectrophotometry. The photocatalytic properties of CeO₂/SnO₂ thin-film materials were demonstrated in the model reaction of the decomposition of an organic dye, Methylene Blue, under UV irradiation (312 nm) in the daylight and in the dark. Films with the CeO₂/SnO₂ heterostructure having the thickness of 124 nm, refractive index of 1.33, and transmittance higher than 70% in the wavelength range 440–1000 nm absorb the visible light more efficiently than the SnO₂ and CeO₂ thin-film materials do. The photocatalytic activity of CeO₂/SnO₂ films under UV irradiation in the daylight is higher than that under UV irradiation in the dark. The amount of Methylene Blue decomposing under UV irradiation in the daylight is 1.2 times larger than that decomposing under UV irradiation for the same time in the dark.

The sol-gel process for making thin-film oxide materials is a low-cost and versatile way of creating nanostructures. This method provides a uniform distribution of elements of multicomponent systems over the surface of various solids. The thickness and morphology of the surface of oxide films are largely determined by the composition, structure, and processes occurring in the sols from which the films are obtained. The selection of precursors is essential in this method to acquire the desired composite oxide materials. In this paper, we present an overview of the results of studies on the sol-gel production of thin-film oxide materials based on SiO₂–E_xO_y (E = a rare earth element (REE), Sn, Mn, Co, Ni, Ca, P), TiO₂–E_xO_y (E = Si, Sn, Co, Ni), SnO₂–E_xO_y (E = In, Sb, Ce), and ZrO₂. In the main part of the work, we consider sol-gel processes (hydrolysis, polycondensation, complex formation) involving tetraethoxysilane, tetrabutoxytitanium, antimony(III), and tin(II, IV) acetylacetonate complexes, as well as polynuclear zirconium(IV) clusters. We explore the processes of obtaining a sol, leading to its film-forming ability, the composition of micelles, the size of colloidal particles, and the change in the composition of micelles when an additive is introduced to the tetraethoxysilane-based sol. We discuss the effect of adding salts of various natures, organic ligands, and solvents on the time stability of sols. We then consider the effect of the hydrolyzing ability of doubly charged nickel, manganese, and cobalt cations on the rate of hydrolysis and polycondensation of tetraethoxysilane. Based on the studies mentioned above, we propose a general technological approach for creating sols that are resistant to film formation, using the example of tetraethoxysilane and tetrabutoxytitanium sols. We consider the relationship between the network structure of a tetraethoxysilane sol and the network structure of oxide films by the example of SiO₂–CeO₂, SiO₂–NiO, SiO₂–Mn₂O₃, and SiO₂–Co₃O₄. The effect of the rate of thermal treatment of gels on the morphology of oxide films is also studied.

Anion exchange resins are widely used in template synthesis of oxide composites. The present work is aimed to study the thermal destruction of anion exchangers with divinyl‐benzene and polyacrylate matrices in order to obtain hollow spherical MoO₃/TiO₂–SiO₂ composites from these resins preliminarily saturated with paramolybate ion and coated with tetrabutoxytitanium sol with tetroethoxysilane. It is shown that the processes of resins decomposition affect the strength of spherical composites. Decomposition of the resin with a polyacrylate matrix, accompanied by random nucleation and burnout of the organic matrix at 350°C, leads to the formation of spherical oxide composites with cracks on the surface. The decomposition reactions of the resin with a divinylbenzene matrix occurring at the boundaries of the phases of cylindrical and spherical symmetry, as well as a lower burnout temperature of the organic matrix (290°C), contribute to the formation of dense MoO₃/TiO₂–SiO₂ spheres. Hollow spherical composites 0.3–0.5 mm in size are orthorhombic α‐MoO₃ coated with a mixture of titanium dioxide with anatase structure and amorphous silicon dioxide.

The effect of the precipitant nature of the tin(II) hydroxoform on the composition, structure, and surface morphology of samples based on tin(II) oxide prepared by precipitation under the same conditions with various precipitants: NaOH, a mixture of NaOH with hexamethylenetetramine, and NH₃ solution, was studied. It was found by X-ray phase analysis with Rietveld refinement that the SnO structure is sensitive to the composition of the precipitant. The addition of hexamethylenetetramine to NaOH leads to the formation of SnO with a distorted tetragonal lattice due to a decrease in the distance between the Sn–Sn atoms. It is shown that the temperature of SnO formation determines the shape of particles, porosity, size of agglomerates, and the width of the forbidden zone of the oxide. The possibility and conditions of preparing SnO samples, which exhibit photocatalytic activity in the model reaction of methyl orange decomposition, have been revealed. It was found that the photocatalytic activity is exhibited by SnO samples without SnO₂ impurities with a specific surface area of 5.4, 5.9, 6.8 m² g^–1 and a pore volume of 0.026, 0.034, 0.025 cm³ g^–1, respectively.