Preparation And Enhanced Photoelectrochemical Properties Of MoS₂ Nanosheets/ferroelectric Thin Films

Semiconductor photoelectrochemical?PEC?systerm transforms low-density solar energy into high-density electrical or hydrogen energy by integrating semiconductor photoelectrode,solar energy and water/electrolyte,which is one of the most promising ways to obtain green renewable energy.The most important part of PEC system is semiconductor materials of photoelectrode.Traditional semiconductor materials,such as TiO₂ and ZnO,are generally low cost and fairly stable for the PEC reaction.However,the low efficiency due to the narrow light absroption and poor charge transport property exclude them from pratical application.Therefore,exploring the semiconductor material with visible light response,expanding the optical response range of the existing photoelectrode materials and promoting the separation of photogenerated electrons and holes are key issues in PEC studies.It has been shown that the spontaneous polarization of ferroelectric materials can effectively promote the separation of photogenerated carriers,however,most of the ferroelectric perovskite oxides have large band gap,which can only absorb a small part of the sunlight,which limits the development of the ferroelectric materials as photoelectrode materials.Recently,the single-element thickness of two-dimensional materials have been paid more attention in the photoelectric conversion area because of its unique properties.Molybdenum disulde?MoS₂?,a member of the layered transition metal dichalcogenides family,has attracted great attention for its application in sensing,lithium battery,phototransistor and photocatalytic area.As a noble metal free co-catalysts,MoS₂ has be used as electrocatalyst for the hydrogen evolution reaction due to its high H₂ activation ability.Most importantly,MoS₂ is a narrow band gap semiconductor,which can be coupled with other semiconductor materials to improve efficient charge separation as well as expand the light absorption.In this paper,we try to prepare the composite photoelectrode by combining ferroelectric thin films and MoS2 nanosheets,the expanding optical absorption range of ferroelectric thin films and the promoting separation of photogenerated carriers by the heterojunction are expected which resulting the improvement of the PEC properties of composite photoelectrode.The main contents of this thesis include the following two aspects:?1?Preparation and enhanced photoelectrochemical properties of MoS₂/BTO thin filmsPolycrystalline BaTiO₃?BTO?films are fabricated on Pt/Ti/SiO₂/Si?100?substrates by pulsed laser deposition.By changing the deposition temperature and oxygen pressure,the appropriate growth condition of BTO film was obtained,and the effect of thickness on the photoelectrochemical properties of BTO thin films was further studied.Few layers MoS₂nanosheets were fabricated by a moderate disintegration-exfoliation approach using diluted H₂SO₄ as an‘‘efficient knife'',and the composite photoelectrode were then prepared by by dip-coating technique.Our results show that under the xenon lamp irradiation,the enhanced PEC performance is achieved using the BTO decorated with 5?L MoS₂,where the photocurrent density of the composite film is about 30%higher than that of the pure BTO film.It should be noted that although the decorating of MoS₂ nanosheets can increase the photoelectrochemical properties of BTO films,the BTO film can only absorb ultravioletlight,which the PEC properties of the composite films are still dissapointment.?2?Preparation and enhanced photoelectrochemical properties of MoS₂/BFO thin filmsIn order to find more efficiency MoS₂/ferroelectric thin film heterojunction photoelectrode,we choosed BiFeO₃?BFO?,a ferroelectric material with small band gap,as the photoetrode material.BFO polycrystalline thin films ware prepared on Pt/Ti/SiO₂/Si?100?substrate using the sol-gel method,and MoS₂ nanosheets were then decorated on top of BFO film through dip-coating method.Our results shows the optimized MoS₂/BFO composite film exhibits a 300%improvement of the short-circuit photocurrent density compared to that of the pristine BFO.Furthermore,the incident photon-to-current efficiency of the optimized MoS₂/BFO sample shows three-fold enhancement in the range of visible wavelength.Such improvement can be ascribed to the improved light absorption at visible light range and faster charge separation with lower recombination rate in the composite films.Part of the work of the study has been published in Small,2017,13,1603457.