WO3/TiO2 Hybrids and Their Applications for the Photocatalytic Degradation of P-Substituted Phenols

Abstract

Tungsten trioxide (WO3) has received a great attention in last decade because of its commercial applications in smart windows, display devices, mobile phones, photocatalysis, solar cells and hydrogen generation. Due to a bandgap around 2.8 eV, WO3 is a semiconductor suitable for photocatalytic and photovoltaic applications using sunlight1, in contrast to most popular TiO2 of the bandgap of about 3.2 eV.2 However, a combination of WO3 with TiO2 leads to significantly enhancement of photoactivity of the system mainly due to increased separation efficiency of photogenerated electron-hole pairs3. When these charge carriers reach the semiconductor/solution interface, they may be involved in the surface reactions leading to decomposition of pollutants. These reactions occur via formation of highly reactive intermediates, such as hydroxyl radicals and superoxide radicals that break down organic pollutants into safe, smaller compounds such as carbon dioxide, water, and short-chain acids.5 Photocatalysis is important in the degradation of phenolic compounds, for example 4-chlorophenol that are products of industrial waste.4 This method is both eco-friendly and inexpensive. Tungsten trioxide and its composites were synthesized on indium-doped tin oxide (ITO) plates using sol-gel, electrochemical and hydrothermal methods. The morphology and composition of the synthesized WO3 and WO3/TiO2 hybrid were characterized using SEM, XRD, UV-Vis and an optical microscope. Electrochemical studies including cyclic voltammetry and chronoamperometry were performed to test their photoactivity. Lastly, the samples were used in the photocatalytic degradation of 4-chlorophenol monitored by UV-vis spectroscopy. Chronoamperometry studies showed that the WO3 by itself had a rapid electron-hole recombination rate. After combination with TiO2, the charge recombination rate decreased and the system was rapidly stabilized under illumination. The photoactivity of WO3 and WO3/TiO2 hybrid was measured with the use of cyclic voltammetry. HPLC analysis of the degradation products of 4-chlorophenol over time using the TiO2/WO3 composite indicated that the hybrid was able to degrade the pollutant more effectively, in a shorter time than WO3 alone. The addition of TiO2 to WO3 deposited on ITO resulted in increase of photoactivity of the system due to slowing down the electron-hole recombination process. Secondly, the degradation of 4-chlorophenol was more efficient using the WO3/TiO2 system. Future experiments will be focused on explanation of differences in the degradation mechanisms of 4-chlorophenol with the use of WO3 alone and WO3/TiO2 hybrid. The effect of the ratio of TiO2 and WO3 in the hybrid on photoactivity and photocatalytic degradation will also be explored. Tada et. al, Langmuir. 20, 4665-4670 (2004)Dette et. al, Nano Lett. 14, 6533-6538 (2014)DohLevil-Mitrovil et. al, Materials Research Bulletin. 83, 217-224 (2016)Meng et. al, Materials Science and Engineering: B. 180, 20-26 (2014)Linsebigler et. al, Chem. Rev. 95, 735-758 (1995)