METAL OXIDE HETEROSTRUCTURES FOR EFFICIENT PHOTOCATALYSTS

Abstract

Photocatalytic processes over semiconducting oxide surfaces have attracted worldwide attention as potentially efficient, environmentally friendly and low cost methods for water/air purification as well as for renewable hydrogen production. However, some limitations to achieve high photocatalytic efficiencies have been found due to the fast recombination of the charge carriers. Development of heterostucture photocatalysts by depositing metals on the surface of semiconductors or by coupling two semiconductors with suitable band edge position can reduce recombination phenomena by vectorial transfer of charge carriers. To draw new prospects in this domain, three different kinds of heterostructures such as n-type/n-type semiconductor (SnO2/ZnO), metal/n-type semiconductor (RuO2/TiO2 and RuO2/ZnO) and p-type/n-type semiconductor (NiO/TiO2) heterojunction nanomaterials were successfully prepared by solution process. Their composition, texture, structure and morphology were thoroughly characterized by FTIR, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and N2 sorption measurements. On the other hand, a suitable combination of UV–visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) data provided the energy band diagram for each system. The as-prepared heterojunction photocatalysts showed higher photocatalytic efficiency than P25 TiO2 for the degradation of organic dyes (i.e. methylene blue and methyl orange) and the production of hydrogen. Particularly, heterostructure RuO2/TiO2 and NiO/TiO2 nanocomposites with optimum loading of RuO2 (5 wt %) and NiO (1 wt %), respectively, yielded the highest photocatalytic activities for the production of hydrogen. These enhanced performances were rationalized in terms of suitable band alignment as evidenced by XPS/UPS measurements along with their good textural and structural properties. This concept of semiconducting heterojunction nanocatalysts with high photocatlytic activity should find industrial application in the future to remove undesirable organics from the environment and to produce renewable hydrogen