Hydrogenation Of 2-Dimensional Transition-metal Oxides（TiO₂,WO₃） Photocatalysts For Enhanced Photocatalytic Performance

As a kind of cheap, efficient, and stable photocatalyst, transition metal oxides have drawn dense interest owing to its wide applications in solar cells, solar-driven hydrogen production, photocatalytic degradation of dyes and so forth. Because the band gap energy of most transition metal oxides is large, which limits their photocatalytic activity in ultraviolet range, great efforts have been dedicated to extend the light absorption of TiO2 into the visible range of the spectrum to allow more efficient use of solar light. Recently, fabrication of “black” TiO2 as the breakthrough approach reported by Chen et al., has attracted great attention owing to its high-efficiency enhanced photocatalytic properties. It is suggested that the hydrogenated TiO2 can boost the harvesting solar light impressively by introducing disorder on the surface.Compared with bulk materials, the ultrathin nanosheets own extremely high percentage of surfaces(nearly 100 %), exotic electronic properties and significant quantum confinement effect in the thinnest dimension. Theoretically, the ultrathin 2Dnanosheets are more prone to being hydrogenated, because metal atoms are on the exposed surface possessing higher electron densities. From the point view of application, the quantum confinement effects and the big specific surface area of ultrathin nanosheets are also expected to improve the photocatalytic properties. So exploring hydrogenation of 2D nanosheets to obtain strong and stable photocatalyst as well as investigating the synergistic effect of structure and electron, is an important and attractive research topic. In this work, our main research content and results are as follows:(1) The surfactant was used as soft template to form the inverse lamellar micelles and confine the hydrated inorganic oligomers. Hydrothermal or solvothermal treatment was then carried out to induce complete condensation and crystallization and the two-dimension TiO2 was thus obtained. Because of poor crystalline degree, the TiO2 was annealed in the air. NaBH4 was employed as reducing agent to hydrogenate the TiO2 nanosheets. As the transmission electron microscope photographs suggests, The TiO2 maintained the nanosheets structure during the hydrogenation and the hydrogenation first occurred on the edges of the nanosheets, and then occurred on the inner surface at higher hydrogenation temperatures. With the increase of the degree of hydrogenation, the hydrogenated samples significantly enhanced the absorbance of visible light, and the band gaps of the nanosheets also decreased obviously. The Raman spectrum suggested that the typical peaks of TiO2 underwent red shifts, and became both broader and lower. Besides these modes, an additional active mode was observed after hydrogenation. All of these changes may be caused by the emergence of oxygen vacancy, Ti3+ and Ti-OH during hydrogenation, which created the new mid-gap states and reduced the required excitation energy. The samples were characterized by photocurrent, photocatalytic hydrogen generation and photocatalytic degradation of methyl orange, which demonstrated that the photocurrents were improved and the properties of photocatalytic hydrogen generation and MO degradation were also enhanced after hydrogenation.(2) Two-dimensional WO3 nanosheets were prepared by the same process and hydrogenated under the condition of 1%H2/Ar at 300 oC. Both the influence of the hydrogenation for the properties and the influence of anneal for hydrogenation were studied in this work. After hydrogenation, the color of all samples became deepening. And the results suggested that the reaction of hydrogenation in the annealed samples was more difficult to happen, which is mainly due to the more perfect crystal structure of WO3 and the H atom was difficult to be inserted in the WO3 crystal structure. UV-Vis absorption spectrum was conducted to characterize of the absorbance, which indicates the enhanced absorption for the visible light after hydrogenation and the narrow band gaps also could be concluded. The X-ray photoelectron spectroscopy(XPS) was employed to characterize the binding energy of the samples. It also showed that the binding energy of W 4f underwent a low energy shift, while the O 1s underwent a high energy shift and shoulder peak emerged. The changes of the binding energy are mainly attributed to the emergence of W5+ and W-OH during the hydrogenation. The photocurrent of WO3 before and after hydrogenation was conducted to evaluate the influence of hydrogenation on the photoelectrochemistry properties of the samples. And the result indicated that the WO3 which was hydrogenated after being annealed has the highest photocurrent. Furthermore, photocatalytic degradation of methyl orange also suggested it has the best photocatalytic properties.