Abstract
The objective of this research was to investigate how the photocatalytic activity of pure TiO2 can be improved by SnO2 modification. Different molar ratios of tin to titanium were prepared. The correlation between tin concentration and structural properties was investigated to explain the mechanism of photocatalytic efficiency and to optimize the synthesis conditions to obtain enhanced activity of the SnO2-modified TiO2 photocatalysts under UV-irradiation. The SnO2-modified TiO2 photocatalysts were prepared by a low-temperature sol-gel method based on organic tin and titanium precursors. The precursors underwent sol-gel reactions separately to form SnO2-TiO2 sol. The sol-gels were deposited on a glass substrate by a dip-coating technique and dried at 150 °C to obtain the photocatalysts in the form of a thin film. To test the thermal stability of the material, an additional set of photocatalysts was prepared by calcining the dried samples in air at 500 °C. The photocatalytic activity of the samples was determined by measuring the degradation rate of an azo dye. An increase of up to 30% in the photocatalytic activity of the air-dried samples was obtained when the TiO2 was modified with the SnO2 in a concentration range of 0.1–1 mol.%. At higher SnO2 loadings, the photocatalytic activity of the photocatalyst was reduced compared to the unmodified TiO2. The calcined samples showed an overall reduced photocatalytic activity compared to the air-dried samples. Various characterization techniques (UV-Vis, XRD, N2-physisorption, TEM, EDX, SEM, XAS and photoelectrochemical characterization) were used to explain the mechanism for the enhanced and hindered photocatalytic performances of the SnO2-modified TiO2 photocatalysts. The results showed that the nanocrystalline cassiterite SnO2 is attached to the TiO2 nanocrystallites through the Sn-O-Ti bonds. In this way, the coupling of two semiconductors, SnO2 and TiO2, was demonstrated. Compared to single-phase photocatalysts, the coupling of semiconductors has a beneficial effect on the separation of charge carriers, which prolongs their lifetime for accessibility to participate in the redox reactions. The maximum increase in activity of the thin films was achieved in the low concentration range (0.1–1 mol.%), which means that an optimal ratio and contact of the two phases is achieved for the given physical parameters such as particle size, shape and specific surface area of the catalyst.
Highlights
In photocatalysis, the positions of the energy levels of the band gap of a semiconductor determine the chemical potentials of the photo generated electrons and holes to participate in redox reactions [1]
The best fit values of the rate constants k1 obtained for the unmodified and SnO2-modified TiO2 catalysts with different SnO2 loadings are presented in Table S3 (Supplementary Materials), together with the relative photocatalytic activities, calculated with respect to the value of the rate constant k1 of the unmodified TiO2 photocatalyst dried at 150 ◦C
The photocatalysts dried at 150 ◦C show up to 30% improved rela tive photocatalytic activities when the TiO2 is modified with low SnO2 concentrations (0.1–1 mol.%)
Summary
The positions of the energy levels of the band gap of a semiconductor determine the chemical potentials of the photo generated electrons and holes to participate in redox reactions [1]. In this respect, TiO2 has particular electronic structure as it allows simul taneously the oxidation of hydrogen and the reduction of oxygen. Catalysis Today xxx (xxxx) xxx resistance [2]. For these reasons, titanium dioxide is used in numerous applications: water purification, degradation of air pollutants, removal of pesticide residues, self-cleaning coatings, dye-sensitized solar cells, water splitting, etc. There is a continuous search for novel, more efficient photocatalysts with improved beneficial properties or avoiding their weaknesses
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