Optical Properties and Photocatalytic Activity of the CeО2/SnO2 Thin-Film Heterostructure

Abstract

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.