Chromium-modified Bi4Ti3O12 photocatalyst: Application for hydrogen evolution and pollutant degradation

Novel chromium-modified Bi4Ti3O12 photocatalyst (Bi4Ti3 - xCrxO12, x = 0-0.5) was developed by a facile sol-gel hydrothermal process with low reaction temperature. The XRD, SEM, TEM, and XPS analyses indicated the formation of the single-crystalline, rectangular, and {001} facet-exposed Bi4Ti3 - xCrxO12 nanosheets. Their average side lengths and thickness progressively decreased with increase in the Cr content, and the minimal side length and thickness for Bi4Ti2.6Cr0.4O12 are 95 and 15 nm, respectively. With more addition of Cr content, sample showed significant particle agglomeration even though some of small nanosheets were observed. The UV-vis study revealed a reduction in the band gap of Bi4Ti3 - xCrxO12 and enhanced photoabsorption in the visible light region, compared to the calcined Bi4Ti3O12. The photoluminescence and electrochemical impedance spectroscopy results displayed a decreased recombination of photogenerated charges in the Cr-modified Bi4Ti3O12 compared to Cr-free one. The photocatalysts were optimized for both maximum hydrogen generation from alcohol-water mixture and methyl orange (MO) degradation in an aqueous solution under visible light irradiation. As a result, addition of Cr effectively improves photocatalytic performance of Bi4Ti3O12. Bi4Ti2.6Cr0.4O12, the optimal photocatalyst, showed highly stable reusability as a hydrogen generator. The photocatalyst reached as much as 117 mu mol g(-1) h(-1) hydrogen evolution rate under 240 min of irradiation, which was similar to 2.85 times that of the Bi4Ti3O12 nanosheet. The hydrogen yields of the Bi4Ti2.6Cr0.4O12 using various alcohol-water mixtures decreased in the order: isopropanol > ethanol > methanol. The optimal photocatalyst also showed up to 0.01003 min-1 of average apparent rate constant in the MO degradation, about 3 times that of the Cr-free nanosheet. The improved photocatalytic performance of the Cr-modified Bi4Ti3O12 is attributed to its strong absorption in visible light region, small nanosheet size, exposed {001} facets as well as the low recombination rate or the high separation efficiency for photogenerated electron-hole pairs. (C) 2016 Elsevier B.V. All rights reserved.