Surface And Interface Engineering Of Silicon Photoanode For Photoelectrochemical Water Splitting

With the continuous growth of the global economy,the escalating demand for energy faces severe challenges due to the non-renewable nature of traditional fossil fuels and the environmental issues they cause.This situation highlights the urgency of finding clean and sustainable energy alternatives.Utilizing artificial photosynthesis to convert solar energy into storable fuels is an ideal solution to replace fossil fuels.Among these,photoelectrochemical（PEC）water splitting technology has attracted considerable attention.It utilizes the excitation of electrons and holes in semiconductors to drive the hydrogen evolution reaction（HER）and the oxygen evolution reaction（OER）,thereby achieving direct conversion of solar energy into green hydrogen energy.However,the core challenge in developing efficient and stable PEC water splitting systems lies in enhancing the conversion efficiency and long-term stability of photoelectrodes.Single-crystal silicon,as the mainstream semiconductor material in the market,has wide applications in microelectronics and photovoltaics.Its solar cells demonstrate high saturation current density(about 40 m A·cm^-2)and high open-circuit voltage（about 700m V）,with a photovoltaic conversion efficiency exceeding 25%.Additionally,single-crystal silicon possesses numerous advantages such as abundant terrestrial reserves,broad solar spectrum absorption range（with E_g～1.1 e V）,and scalability for industrial production,making it a strong candidate material in the field of PEC water splitting.However,during the PEC water oxidation process,n-type silicon（n-Si）semiconductor exhibits limited photoelectrochemical performance due to its unfavorable band position for water oxidation reaction and susceptibility to corrosion in extreme electrolyte environments,resulting in lower photovoltage,higher photocurrent onset potential,and poorer photoelectrochemical stability.To address these shortcomings of n-Si photoanodes in PEC water oxidation,it is typically necessary to deposit a protective layer with hole extraction capability and a highly catalytic auxiliary catalyst layer on their surfaces.Unfortunately,the inherent instability of surface auxiliary catalyst activity and the mismatch in interface energy between the semiconductor light absorption layer and the extraction layer limit further enhancement of the stability and efficiency of n-Si photoanodes in water splitting.Therefore,the key to improving the performance of n-Si photoanodes lies in optimizing the extraction effect of the surface hole extraction layer,enhancing the catalytic activity and stability of surface auxiliary catalysts,and regulating the interface energy between the semiconductor light absorption layer and the extraction layer.The main research contents of this thesis around the above problems are as follows:（1）To overcome the intrinsic limitations of n-Si semiconductors in terms of hole extraction and stability,a uniform,dense,and lattice-ordered NiO film was deposited on the surface of n-Si photoanodes using electron beam evaporation（EB）technology.The formed PN heterojunction between the film and n-Si resulted in favorable band bending on the surface of the n-Si semiconductor,conducive to the separation of photo-generated charge carriers and hole extraction.Simultaneously,the extracted holes were efficiently transported through the lattice-ordered NiO layer,significantly enhancing the efficiency of hole injection into the photoelectrode surface and participation in the water oxidation reaction.Compared to NiO/n-Si photoanodes prepared by reactive sputtering（which showed no photocurrent response at 1.23 V vs.RHE）,the NiO/n-Si photoanodes constructed by electron beam evaporation in this study exhibited superior PEC activity(with 29.0 m A·cm^-2 at 1.23 V vs.RHE)and achieved stable saturation photocurrent density for up to 60 h,despite an inevitable positive shift in the photocurrent onset potential.（2）To address the degradation of PEC activity caused by the increase in photocurrent onset potential while maintaining stable saturation photocurrent in stability tests of NiO/n-Si photoanodes,effective coupling was made by incorporating NiCo Fe-B_iauxiliary catalysts with high intrinsic catalytic activity,ultra-long self-repairing stability,compatibility with light-absorbing bodies,high light transmission,and unique film thickness self-limitation.This strategy not only achieved stable PEC efficiency for up to100 h but also increased the half-cell solar-to-hydrogen efficiency（HC-STH）of the photoelectrode from 1.54%to approximately 2.00%.In-depth exploration revealed that the improvement in PEC activity and stability of NiO/n-Si photoanodes was mainly attributed to the unique self-healing mechanism of NiCo Fe-B_i auxiliary catalysts.To further elucidate this mechanism,direct detection of Fe^II pre-deposited ions and Fe^VIactive intermediates in aqueous solution under operating conditions was achieved for the first time.A detailed explanation and improvement of the self-repair mechanism that enhances the stability and activity of NiO/n-Si photoanodes were provided.Specifically,the stability enhancement of the NiO/n-Si photoelectrode was attributed mainly to the oxidation and deposition process of Fe^II ions by Co,while the enhancement of its activity relied on the generation of high-valence Fe^VI species.（3）To further enhance the PEC activity and stability,the interface energy of the NiO/n-Si heterojunction was optimized by introducing a Cu_xO intermediate layer.Direct detection of the Cu_xO interface layer buried in the NiO/n-Si interface was achieved using advanced Hard X-ray Photoelectron Spectroscopy（HAXPES）technology.The results showed an in-situ transformation of the Cu_xO intermediate layer from Cu₂O to Cu O after prolonged exposure to air.This transformation was closely related to the gradual improvement in the PEC activity of NiCo Fe-B_i/NiO/Cu_xO/n-Si photoanodes.Further studies indicated that the introduction and in-situ transformation of the Cu_xO intermediate layer resulted in a higher band bending barrier in the NiO/n-Si heterojunction,thereby significantly enhancing the photovoltage under illumination conditions.Based on this important finding,a reaction electron beam evaporation deposition technique was further developed for direct deposition of Cu O intermediate layers,and high-performance NiCo Fe-B_i/NiO/Cu O/n-Si photoanodes were successfully prepared.The HC-STH efficiency of this photoanode reached a record-breaking 4.56%,and it maintained efficiency stability for up to 100 hours,providing crucial support for the development of efficient and stable solar-driven water splitting technology.