Crystal Orientation And Defect Control Of Antimony Sulfide Thin Film Photocathode

Solar-driven photoelectrochemical（PEC）water splitting is an ideal artificial photosynthesis process that converts highly intermittent solar energy into storable and transportable clean hydrogen fuels,which is a significant research focus in the field of new energy at present.Antimony sulfide（Sb₂S₃）has recently emerged as a promising photocathode material for PEC water reduction due to its appropriate band gap（～1.7 e V）,band position,and high absorption coefficient(α>10⁴ cm^-1),satisfying the fundamental requirement for achieving high efficiency in PEC water splitting application.However,the reported high half-cell solar-to-hydrogen efficiency（HC-STH）of the Sb₂S₃photocathode is still very low（0.64%）.Due to its unique quasi-1D crystal structure,the optoelectronic properties of Sb₂S₃ strongly depend on its crystallographic orientation.In general,the photogenerated carriers transfer more efficiently along the[hk1]orientation than along the[hk0]orientation.In addition,there are a significant number of intrinsic defects present within the Sb₂S₃ film.Furthermore,when it is used to construct a heterojunction with a buffer layer,a substantial number of interface defects are generated within the heterojunction.These defects are also another important reason for the low separation and transport of photo-generated charge carriers.Therefore,the dissertation mainly aims to improve the separation and transport of photo-generated charge carriers in Sb₂S₃ photocathode,and first adjusts the crystal orientation of Sb₂S₃ films to accurately synthesize Sb₂S₃ films with complete[hk1]orientation.In addition,the effect of Se doping on the defect properties and energy band structure in Sb₂S₃ films was also studied.Finally,the separation and transport efficiency of the photogenerated carriers of the Sb₂S₃photocathode is gradually improved by constructing the heterojunction and designing the atomic interface layer to achieve an increase in the water reduction efficiency of the PEC.The main research content of the dissertation is as follows:（1）The Sb₂S₃ film is prepared by sulfurizing an Sb metal precursor film deposited using dual-source electron beam evaporation.By optimizing the deposition rate and deposition thickness of the Sb precursor film,and sulfidation temperature,a high-quality compact Sb₂S₃ film was successfully prepared.Compared to traditional chemical bath and electrodeposition methods,the electron beam evaporation method utilized in this study avoids the introduction of certain impurities and defects and provides conditions for controlled doping in subsequent processes.Furthermore,the type of heterojunction material and hydrogen evolution reaction cocatalyst loaded on the surface of the Sb₂S₃film was selected and optimized.It is found that when the CdS buffer layer and Pt catalyst are loaded on the surface of Sb₂S₃ film,it is most beneficial for PEC water reduction reaction on an Sb₂S₃-based photocathode.（2）Although the[hk1]crystallographic orientation of Sb₂S₃ films has the potential for highly efficient separation and transport of photogenerated carriers,the synthesis of an Sb₂S₃ film with precisely controlled[hk1]orientation is still very challenging.Herein,a completely[hk1]-oriented Sb₂S₃ film is prepared by sulfurizing an Ag/Sb bimetallic precursor film.A sliver-induced crystal growth model is proposed to elucidate the formation mechanism of the[hk1]-oriented Sb₂S₃ film.Sb₂S₃ film exhibit significantly enhanced higher conductivity and faster carrier mobility due to the favorable tailor of crystal orientation.Because the surfaces orthogonal to the[hk1]orientation usually have no dangling bonds,suppressing the[hk0]orientation of Sb₂S₃ also appropriately reduces the trap density of the Sb₂S₃ film.The Sb₂S₃ film with a completely[hk1]oriented crystal structure has a higher separation and injection efficiency of photogenerated carriers in comparison to those of the randomly oriented Sb₂S₃ film.As a result,a photocathode based on the[hk1]-oriented Sb₂S₃ film delivers a high photocurrent density of 10.0 m A cm^-2 at 0 V versus RHE and a high HC-STH of 1.68%in a neutral electrolyte.（3）There are a large number of S vacancies in Sb₂S₃ films prepared by electron beam evaporation,and these S vacancies will form deep-level defects in the Sb₂S₃ film,resulting in severe recombination of photogenerated carriers.In the dissertation,Se-doped Sb₂S₃ films were successfully prepared by adding a certain amount of selenium powder in the sulfurization process,and the influence of Se doping on the defect concentration and energy band structure of Sb₂S₃ films was studied.It is found that when the atomic ratio of Se/S was 1:7.35,the prepared Sb₂S₃ film had the best stoichiometric ratio,and the doping of Se atoms into the Sb₂S₃ film could effectively passivate the deep-level defects formed by S vacancies,thereby improving the separation efficiency of photogenerated carriers in the film.In addition,Se doping also leads to an elevated valence position of Sb₂S₃,which makes the Sb₂S₃/Mo interface have a better band alignment,which is conducive to the transfer of holes from Sb₂S₃ to Mo substrate.Finally,the Se-doped Sb₂S₃-based photocathode exhibits great photogenerated carrier separation efficiency,thus it delivers a high photocurrent density of 11.4 m A cm^-2 at 0 V versus RHE and a high HC-STH of 1.48%in a neutral electrolyte.（4）The construction of a heterojunction with a CdS buffer layer has been demonstrated as an excellent strategy to significantly improve the separation and transport efficiency of photo-generated charge carriers in Sb₂S₃-based photocathodes.However,the cation diffusion between Sb₂S₃/CdS heterojunction during the chemical bath deposition process of CdS could generate detrimental interfacial defects,which are often overlooked.In this article,an ultrathin amorphous Al₂O₃ be used as an interlayer to address these defects issues of the Sb₂S₃-based photocathode for improving its PEC water reduction performance.Mechanistic studies reveal that the Al₂O₃ interlayer successfully eliminates the interfacial defects between Sb₂S₃/CdS interface by blocking the Cd²⁺cations between the Sb₂S₃/CdS interface.Meanwhile,the surface states of Sb₂S₃ were efficiently passivated by Al₂O₃.As a result,the Sb₂S₃/Al₂O₃/CdS/Pt photocathode delivers a high photocurrent density of 13 m A cm^-2 at 0 V versus RHE and a high HC-STH of 2.78%in a neutral electrolyte,which compares favorably with the previous literature on Sb₂S₃-based photocathodes,to the best of our knowledge.