Photocatalytic And Photoelectrochemical Properties Of Monoclinic Bismuth Vanadate

Photocatalytic and photoelectrochemical water splitting under irradiation by sunlight has received much attention for production of renewable hydrogen from water on a large scale. Many challenges still remain in improving energy conversion efficiency, such as utilizing longer-wavelength photons for hydrogen production, enhancing the reaction efficiency at any given wavelength, and increasing the lifetime of the semiconductor materials. Controlling the semiconducting properties of photocatalysts and photo-electrode materials is the primary concern in developing materials for solar water splitting, because they determine how much photoexcitation occurs in a semiconductor under solar illumination and how many photoexcited carriers reach the surface where water splitting takes place. Given a specific semiconductor material, surface modifications are important not only to activate the semiconductor for water splitting but also to facilitate charge separation and to upgrade the stability of the material under photoexcitation. In add ition, reducing resistance loss and forming p-n junction have a significant impact on the efficiency of photoelectrochemical water splitting.In this paper, due to monoclinic phase bismuth vanadate could been excited under visible light irradiation, we prepared BiVO4-based ternary composite nanosheets photocatalyst and nanoporous photoanode with or without cocatalyst modification on its surface. The physical and chemical properties of the as-prepared samples were characterized by various techniques. Finally, the photocatalytic activities were studied by degradation of organic dyes under visible light irradiation and the photocurrent density, activity of H2 production of photoanode were invesitigated. The detail researches and conclusions are listed as follow:(1) We firstly developed the BiOCl/BiVO4 nanosheets photocatalysts using a hydrothermal substitution method by using ethanolamine as a morphology control agent. Then, metallic Bi was in situ depo sited on the BiOCl/BiVO4 nanosheets by a poloalcohol reduction method and the ternary Bi/BiOCl/BiVO4 nanosheets were obtained finally. More importantly, ternary nanosheets possess p-n heterojunction and metal surface plasmon resonance structure, exhibitting greatly enhanced activities in the visible- light driven photocatalytic degradation of RhB and photoelectrochemical characterization.(2) We demonstrate a general and templateless method for one-step construction of pore-size controllable BiVO4 nanoporous films. Benefiting from unique structural features, the as-prepared BiVO4 with a pore size distribution of 300-400 nm could show enhanced light adsorption, charge separation and photo-electrochemical properties. Importantly, the origin of highly efficient photoelectric conversion in nanoporous BiVO4 photoanodes has been explored and clarified. More specifically, the visualized evidence for more efficient electron-hole separation in the nanoporous BiVO4 film than traditional particle films has been achieved by using a powerful in situ XPS technique. Moreover, the optical tests confirmed the evident light confinement effect in nanoporous structures for effectively trapping visible-light.(3) We demonstrate a simple ethylene glycol dispersion and impregnation method for uniform coating ultrathin g-C3N4 nanosheets on nanoporous BiVO4 photoanodes. Synchronous illumination X-ray photoelectron spectroscope(SIXPS) clearly reveals that as compared with pure BiVO4 photoanode, more efficient charge separa tion and holes transfer have been achieved. More specifically, ultrathin g-C3N4 nanosheets could effectively suppress the surface charge recombination on the BiVO4, and photogenerated holes could be effectively stored on their N atoms for water oxidation. Mott-Schottky plots and Nyquist curves further demonstrated that the ultrathin g-C3N4 structure could greatly increase the charge-carrier density and facilitate more efficient electron-hole separation. As expected, the ultrathin g-C3N4 nanosheets loading BiVO4 photoelectrodes exhibit superior photoelectrochemical water oxidation as compared with Co-Pi and FeOOH modified BiVO4 electrodes.