Study On Surface Modification And Photoelectric Properties Of TiO₂ And WO₃ Photoanodes

Photoelectrochemical（PEC）technology of hydrogen production by fractionating water is one of the promising hydrogen production methods.However,the photoelectric conversion efficiency of the PEC technology remains below the design value.In the process of water decomposition by PEC,Light absorption,separation and transfer of federated carriers and the interfacial catalytic reaction are the three important factors that determine light conversion efficiency.Therefore,how to enhance light absorption performance,improve the separation and transport efficiency and enhance the surface catalytic reaction activity are the core issues to achieve efficient photoelectric decomposition of water.Semi-conductor inorganic materials have become the preferred material for photoanodes due to their advantages of easy synthesis,diverse nano-morphological structures and good stability.However,the current inorganic semiconductor photoanodes represented by TiO₂ and WO₃ still have narrow light absorption range and low surface catalytic activity,which limit the improvement of the performance of inorganic semiconductor photoanodes.On this basis,this article took typical metallic inorganic semiconducting photoanodes TiO₂ and WO₃ as research objects,and altered their surface by in-situ construction of the heterojunction and in-situ etching,in order to expand the range of light absorption and enhance the surface catalytic activity.The photoelectric performance of the modified photoanodes was examined.The effects of the interfacing features of heterojunction and in situ surface attack on photoelectrochemical performance were proposed.The key research content was as follows:（1）Taking TiO₂ nanorod array as substrate,the TiO₂/BiVO₄ heterojunction photoanode had been prepared through in situ transformation（TiO₂→TiO₂/Bi₄Ti₃O12→TiO₂/BiVO₄）.The formation of heterojunction could change the edge of the absorptive energy band of photoanode from the ultraviolet visible region（420 nm）to the visible region（500 nm-520 nm）.By optimizing the Bi source concentration and calciner temperature,when the source of Bi source concentration is 0.1 g/m L and the calcining temperature was 550 degrees centigrade,the TiO₂/BiVO₄ heterojunction had the best photoelectric performance.In visible light（λ>400nm）,the photocurrent density was around 1.1 m A/cm~2（vs.RHE）,compared with the TiO₂photocurrent increased by about 15 times.Compared to the TiO₂/BiVO₄ heterojunction photoanode prepared by soaking,the photocurrent density of TiO₂/BiVO₄ photoanode prepared by in-situ conversion method was significantly higher than that of TiO₂/BiVO₄ photoanode prepared by soaking（0.2 m A/cm~2）.By further characterizing heterojunction structure and photoelectrochemical analysis,The TiO₂/BiVO₄ heterojunction prepared using the in-situ conversion method was found to exhibit high photoelectrochemical performance because the BiVO₄ particles growth site was fixed,the heterojunction bond was firm,the orientation of the interfacial crystalline plane（202）was unified,and the effective separation and transfer of photogenerated carriers in the photoanode could be promoted.However,the BiVO₄ particles in the TiO₂/BiVO₄ heterojunction prepared by immersion method show poor photochemical properties due to the random orientation of crystal plane at the interface of heterojunction and the mismatch of energy levels between TiO₂ and BiVO₄.（2）The in-situ etched TiO₂ photoanode was prepared by in situ etching（TiO₂→TiO₂/Bi₄Ti₃O12→TiO₂/BiVO₄→in situ etched TiO₂）.By adjusting the thickness of TiO₂substrate and etching depth,when the substrate hydrothermal temperature was 180 degrees centigrade and the Bi source concentration was 0.85 g/m L,the photoelectric properties of TiO₂prepared by in-situ surface etching were best.The photocurrent density was approach 1.0m A/cm~2（vs.RHE）at 1.23V,which is nearly double the value of unetched TiO₂.Analysis of the crystal plane showed that after etching two crystal planes（110）and（101）were been shown on the surfaces of TiO₂,the oxygen vacancies on the surface of the photoanode were increasing,which reduced the over-potential of oxygen production in the photoanode,and made the open-circuit potential of TiO₂ shift from 0.45V（vs.RHE）to 0.31 V（vs.RHE）.By studying E-TiO₂with different oxygen content,it appeared that oxygen vacancy formation was beneficial for reducing the overpotential oxygen production of photoanode,which provided an efficient strategy for modifying the photoanode.（3）Using WO₃ nanosheet arrays as substrates,The in-situ etched WO₃ photoanode was prepared by in situ etching（WO₃→WO₃/Bi2WO6→WO₃/Bi2S3→in situ etched WO₃）It was found that the etched WO₃ had a large specific surface area,and more active sites were provided for the photoanode.By adjusting the etching depth,the high photoelectric performance of WO₃prepared by in-situ etching were found when the concentration of Bi source is 0.3 g/m L and the concentration of S source is 0.0172 mol/L.Under the irradiation of simulated sunlight,the photocurrent density is about 1.07 m A/cm~2（vs.RHE）at 1.5 V.