The dissertation is devoted to the obtaining of photocatalytically active materials based on titanium dioxide modified with carbon, sulfur and carbon and sulfur simultaneously. It has been implemented the systematic investigations of influence of modification methods, the nature of the modifier and its amount on the morphological, structural, textural, electronic, optical, adsorption and photocatalytic properties of the obtained composites. The chemical state of the elements, the phase composition of the powders, and their photocatalytic activity in the reaction of Safranin T destruction and photocatalytic hydrogen evolution from alcohol-water solution were studied.
A method of synthesis of mesoporous nanosized photocatalysts based on titanium dioxide doped with carbon and sulfur has been developed and substantiated, which provides for the optimization of the existing synthesis processes by reducing the temperature and the absence of an inert atmosphere.
Analysis of SEM-images of the samples shows that they consist of roundish and fragmented agglomerates in the range of 5–30 m. The agglomerates of titanium dioxide consist of the particles of 14 nm in size. Modification with carbon leads to an increase in particle size: as the amount of carbon increases from 1 to 21 mass. %, the particle size increases from 14 to 19 nm. Sulfur additives inhibit grain growth of TiO2 and form the particles of 9–10 nm in S/TiO2 composites. Simultaneous modification of TiO2 with carbon and sulfur leads to the formation of particles with a size of 7–8 nm. The change of TiO2 particle sizes due to the doping with carbon and sulfur was confirmed by the TEM study.
It was found that additives of carbon into the binary samples lead to anatase formation, in the case of S/TiO2 XRD show the reflexes of anatase and rutile phases regardless of sulfur concentration. The addition of carbon (1.5 mass. %) and sulfur (from 0.6 to 9 mass. %) into ternary composites leads to anatase formation, but an increase of sulfur amount (12 and 15 mass. %) in the samples leads to appearance of rutile reflexes. Thus, an increase of the sulfur amount in the ternary samples leads to the formation of rutile structure. During doping and co-doping of titanium dioxide with carbon and sulfur using mechanochemical treatment, the formation of rutile does not occur.
All samples show the presence of a hysteresis loop which is the evidence for mesoporous structure of the powders. The isotherms correspond to type IV of IUPAC classification for mesoporous materials with H1 type for C/TiO2 and H2 type for S/TiO2, and C/S/TiO2 of hysteresis loop. The modification of TiO2 by carbon and sulfur leads to increase of specific surface area (of about 3.3 times for S/TiO2 and about 4.7 times for C/S/TiO2), average pore volume and decrease of radius pore volume compared with TiO2. It was identified C–C, C–O, O–C=O, С–Н і –СН2 bonds on the surface of C/TiO2, S/TiO2 and C/S/TiO2 samples and also S=O, S–O on the surface of S/TiO2 and C/S/TiO2 powders which obtained by developed method and C=O bonds on the surface of C/TiO2, S/TiO2 and C/S/TiO2 samples and S2– and -NH2 on the surface of S/TiO2 and C/S/TiO2 powders which obtained by mechanochemical treatment. Due to comparison of XPS and EDS data it was found that the content of carbon and sulfur additives is higher on the surface of obtained samples in all cases.
Absorption spectra of nanocomposites showed a bathochromic shift as compared with the absorption band of pure TiO2 and band gap narrowing which explained by rutile formation. Nanocomposite samples showed higher adsorption and photocatalytic activity compare with pure titanium dioxide and P25 (in the presence of the best samples in 1 hour there is a destruction of 99 % of the dye), which is associated with a decrease in particle size, increase in specific surface area and narrowing of the band gap. Correlation of photocatalytic activity with adsorption capacity indicates that photocatalytic transformations occur on the surface of the powder. It was found that samples of titanium dioxide modified with carbon and sulfur show photocatalytic activity under visible irradiation in the destruction of Safranin T, which is explained by their sensitization to visible light by the adsorbed dye. It is shown, that titanium dioxide modified with activated carbon by a patented method shows higher photocatalytic activity in photocatalytic hydrogen evolution from alcohol-aqueous solutions compared to pure TiO2 and P25.
Key words: titanium dioxide, modification, photocatalysis, sulfur, carbon, photocatalytic activity, Safranine T, hydrogen, photocatalyst
