Study On Gas-phase Photoelectrocatalysis And Photocurrent Characterization Of Metal Oxide Composite

Photocatalytic technology is an effective pathway for solving the problems of energy crisis and environmental pollution. The semiconductor as a photocatalyst can absorb solar energy to generate photoinduced electron-hole pairs with powerful redox ability. Through a series of degradation reactions initiated by electrons and holes, the gaseous pollutants can be decomposed into harmless carbon dioxide and water. The research of photocatalytic technology can be divided into three levels, which contain photocatalyst, characterization and application. The hetero structured composite, photocurrent and photoelectrocatalysis for VOCs are the focuses in the corresponding levels. In this paper, , the research focuses on the inherent relations between the three aspects based on the foundation of charge transfer. The aim is to explore the new perspectives in the gas-phase photoelectrocatalysis, and to try to promote its development process in the application.This paper started from the metal oxide compound system. Based on the thoughts of combinational material science, a material library of the TiO2/WO3/MnO2 ingredient triangle was parallelly synthesized, which consisted of 66 ingredient points. All the ingredient points were high-throughput screened by photocurrent measurements. It was attempted to explore the relationships between the band match of composite and the photo-response. It was found that the TiO2/WO3 composite with the stagger type of matched potentials enhanced the charge separation and improved the photocurrent. In contrary, the TiO2/MnO2 and WO3/MnO2 composites with the straddle type of matched potentials enhanced the charge recombination and inhibited the photocurrent. Futhermore, the potential barrier at grain boundaries was also an important influence factor of the photocurrent. The largest photocurrent under UV, blue and white light irradiation was observed when the mole ratio of TiO2/WO3 was 2/8 in the composite system.Then, the screening TiO2/WO3 composite was used to investigate the gas-phase photoelectrocatalysis. The results showed that the TiO2/WO3 composite was able to show a significant photoelectric synergy effect by applying only a low bias of 0.2V. However, the single TiO2 could not present any improvement of degradation effciency at the low bias. This is a meaningful discovery in the view of low carbon economy. We expect that the realization of low bias PEC degradation in gas phase will promote the rapid development of this technology in practical application.To further improve the photocatalytic activity, we designed a system with the holes enriched in the surface, which was named heterojunctional-electrical layered (HEL) system. The HEL system consists of interdigital electrode, WO3 layer and TiO2 layer, and they are orderly printed onto the alumina substrate from bottom to top using the technology of screen printing. It is surprising that the synergistic effect of layered heterojunction and external low bias can strengthen the separation of electron-hole pairs in both TiO2 and WO3, and enrich the TiO2 surface layer with photogenerated holes to degrade the gaseous pollutants. In comparison with the pure TiO2 film, a 6-fold enhancement in photocatalytic activity was observed using the HEL system by applying a very low bias of 0.2 V.Under the inspirations of the strengthened separation of electrons and the harmful effect of high bias, it is considered to add a frequency parameter to the low bias. It was found that the photocatalytic activity of TiO2 and WO3 could be obviously raised by applying low bias with high frequency. What is more, the the high frequency of the bias affected the photocatalytic reaction path. The the formation of formaldehyde as a more poisonous intermediate could be prevented in the degradation process.Finally, we disproved this general viewpoint about the relationship between photocatalytic activity and photocurrent, and tried to explore the comprehensive relations. We pronounced that the photocurrent is not corresponded with the e-h separation efficiency, and the e-h separation efficiency is also not corresponded with the photocatalytic activity. We recommend that, to explore the valid relationship, the degradation efficiency ((Co-C)/Co), initial reaction rate (ln(Co/C)/Δt) and mineralization ability (evolved CO2) are all needed to display the phtocatalytic activity. The results confirmed that the photocurrent combined with the carrier mobility is essential in the analysis of the e-h separation efficiency. In addition, the introduction of pollutant ambience is needed in photocurrent test. The ratio of photocurrent in air to that in pollutant ambience can be an index to evaluate the initial reaction rate of photocatalytic activity. We expect that, with the deep understanding of the correlations between them, the photocurrent measurement cannot only be just a characterization method, but also can be utilized as a technique for rapid testing and screening of new composite photocatalysts.