Preparation Of SrTaO₂N Photoanode And Its Photoelectrochemical Performance

Photoelectrochemical (PEC) hydrogen production via water splitting is a highly promising strategy to solve the environment and energy problems through converting solar energy into storable chemical fuel for 21 century. The development of photoanode materials is crucial for the practical application of N-type photoelectrochemical cell. The ideal photoanode materials must own the proper bandgap and conduction band、valence band level, efficient carrier transport property, and be stable and inexpensive. However, none of the developed photoanode materials so far satisfied the aforementioned demands at the same time. In this case, it is necessary to improve the photoelectrochemical activity of the current semiconductor photoanode and develop new photoanode martial.The improvement of photoelectrochemical activity can be achieved by improving the light harvesting efficiency, charge separation efficiency or interfacial charge transfer efficiency. Specific ways for implementation:(1) designing nanostructure, inhibiting the bulk recombination by increasing the surface area and improving the carrier transport property; (2) doping, since the electronic band structure and carrier density will be changed, the light absorption and electronic transport property can be enhanced; (3) designing heterojunction photoelectrode, the matched band structure of the heterojunction will enhance the built-in electricfield which is beneficial for carrier separation; (4) surface modification, the interfacial charge transfer efficiency can be improved and the corrosion of the photoelectrode can be restrained; (5) substrate modification, the electronic transport from the electrode to substrate can be improved; (6) other methods.In the last decades, oxide and nitride photoanodes have been widely researched. However, the relatively large bandgap limited the light absorption of oxide photoanode which is stable and inexpensive. While the nitride photoanode shows poor stability even though its favorable light absorption. On the contrary, oxynitride photoanode shows better light absorption property than oxide and better stability than nitride. Several oxynitrides had been developed successfully as photoanodes so far, such as TaON. SrNbO2N、LaTiO2N、LaTaON2、BaTaO2N.In this paper, we aimed at developing new photoanode material. We set SrTaO2N as the research target which has the proper bangap and band level and is been able to produce H2 as photocatalyst in sacrificial system. Based on the photoelectrochemical principle and specific implementation ways for the improvement of photoelectrochemical activity, we researched the photoelectrochemical property of SrTaO2N photoanode. Further, the photoelectrochemical activity of SrTaO2N photoanode was improved by optimizing the synthesis condition, SrTaO2N powder, electronic transport property and the photoanode surface. The SrTaO2N photoanode had been researched in this paper to set an example for new photoanode material development. The main conclusions are as follows:(1) The highly crystallized SrTaO2N particle with lower defect density had been synthesized by the one-step flux method. The two-step solid-state reaction method and one-step flux method were used to synthesize SrTaO2N powder. Compared with the two-step solid-state reaction method, the one-step flux method simplified the synthesis condition. SrTaO2N synthesized by the solid-state reaction method showed the badly aggregated and irregular flake-like morphology, the defect density increased with the increasing sintering temperature of Sr2Ta2O7. On the contrary, SrTaO2N from flux method show highly crystallized particles with smooth surface and lower defect density.(2) The SrTaO2N photoanode shows good PEC activity when TaCl5 necking was replaced by TiCl4. For the photoanode prepared by EPD, isolated particles connect with each other through physical contact, which is inconvenient for electron transport and therefore restrained its PEC performance. The inter-particle electron transport will be ameliorated by necking treatment. The result suggests that TiCl4 necking exhibits better performance than TaCl5.(3) The PEC performance of SrTaO2N photoanode was improved after SrTaO2N powder was heated under H2 flow. The SrTaO2N(H) photoanode was obtained by heating SrTaO2N powder from flux method under H2 flow. We quantitative analyzed the light harvesting efficiency, charge separation efficiency and interfacial charge transfer efficiency. The result proved that the improved PEC performance of SrTaO2N(H) photoanode was attributed to the increased charge separation efficiency from the increased carried density.(4) When Co-Pi was deposited onto SrTaO2N(H) photoanode, the PEC performance was further improved. To improved the interfacial charge transfer efficiency of SrTaO2N(H) photoanode, Co-Pi was deposited onto SrTaO2N(H) photoanode by electrodeposition. The photocurrent density of Co-Pi/SrTaO2N(H) reached to 1.2 mA cm-2 at 1.23 VRHE under AM 1.5 G and the IPCE between 400-470 nm is 20%.