Preparation Of Molybdenum Disulfide Based Heterojunctions And Their Photoelectrode Properties Study

Molybdenum disulfide(MoS₂)has many potential photoelectric applications due to its excellent optoelectronic properties,such as broadband optical absorption(its band gap can be adjusted between 1.2-1.9 eV with different layers),high carrier mobility and environmental stability.Photoelectrochemical(PEC)cell is a renewable energy device that can produce hydrogen from water by solar energy.It is expected to alleviate the energy crisis and improve the environmental degradation.In the PEC cell,the oxygen evolution reaction of photoanode involves four electron transfer process,which has higher kinetic requirements than the hydrogen evolution reaction of photocathode.Therefore,the design and synthesis of highly efficient photoanode materials is the key to the development of PEC cell.The excellent optoelectronic characteristic of MoS₂ make it become a potential photoanode material with high efficiency.However,the photoelectric performance of MoS₂ based photoanode is limited by the ultrafast photogenerated carrier recomnination,the limited number of active sites and weak in-plane electrical conductivity.The construction of heterojunction between MoS₂ and appropriate semiconductor is an effective method to reduce the recombination rate of MoS₂ photogenerated carriers and improve its optoelectronic performance.In this paper,the above-mentioned problems were improved by the construction of MoS₂-based heterojunction,the regulation of MoS₂morphology(thickness and size),and the modification of MoS₂ by highly conductive materials.The main work and innovation points are as follows:(1)Preparation of MoS₂/ZnSnO₃ heterojunction photoanode and its photoelectric properties study.A novel MoS₂/ZnSnO₃ heterojunction photoanode was prepared by two-step electrophoretic deposition(EPD)method.The optimal thickness of MoS₂/ZnSnO₃ film as photoanode was determined by adjusting the electrophoretic time.The type?band arrangement of MoS₂/ZnSnO₃ was determined by UV-vis spectroscopy and X-ray photoelectron spectroscopy analysis.Herein,the photocurrent density of MoS₂/ZnSnO₃heterojunction photoanode at the bias voltage of 0.8 V(vs.Hg/Hg₂Cl₂)reaches 21.9?A/cm²,which is 27.3 times and 2.3 times larger than that of MoS₂ and ZnSnO₃photoanodes,respectively.In this work,the MoS₂/ZnSnO₃ heterojunction photoanode with enhanced photoelectric performance was prepared by EPD method with low cost,short deposition time and controllable film thickness,which provides an experimental reference for the preparation of new two-dimensional layered semiconductor heterojunction.(2)Preparation of 2D-2D MoS₂/WS₂ Van der Waals heterojunction photoanode and its photoelectric properties studyReducing the thickness of MoS₂ film is an effective method to reduce the bulk recombination rate of MoS₂ and enhance its PEC activity.Na Cl assisted chemical vapor deposition(CVD)method was used to grow few-layer of MoS₂ and WS₂ films on sapphire substrate,and the prepared films were firstly transferred to indium tin oxide(ITO)substrate by clean water-assisted transfer and stacking to obtain ITO/MoS₂/WS₂ and ITO/WS₂/MoS₂ photoanodes.ITO/MoS₂/WS₂ photoanode shows enhanced PEC performance due to its enhanced light absorption capacity and efficient carrier separation efficiency,while ITO/WS₂/MoS₂ photoanode shows lower PEC performance than ITO/MoS₂/WS₂ photoanode due to its transport path of photogenerated electrons is blocked.This work provides a new method for the fabrication of van der Waals heterojunction,compared with wet transfer technology,this transfer method is easier to operate and can produce non-polluting and high-quality heterojunction films.Furthermore,this transfer method also has potential applications in other electronic and optoelectronic devices.(3)Preparation of 2D/1D MoS₂/ZnO heterojunction photoanode and its photoelectric properties studyExposing the edge active site of MoS₂ is an effective method to improve the its PEC activity.The MoS₂/ZnO heterojunction was prepared by using the ZnO nanorods prepared by hydrothermal method as the growth skeleton.Then,few-layer MoS₂ nanosheets were grown on the surface of the ZnO nanorods by CVD method.The existence of few-layer MoS₂ nanosheets not only enhances the utilization of sunlight,but also exposes more edge active sites,which promotes the kinetic process of water oxidation on the surface of electrode,reduce the accumulation of holes,and further prevent photocorrosion.Herein,the photocurrent density of MoS₂/ZnO photoanode at the bias voltage of 0.1 V(vs.Ag/AgCl)is 2.1 and 53.8 times larger than that of ZnO and MoS₂ photoanodes,respectively.After 6 h J-t cycle test,the photocurrent density of MoS₂/ZnO photoanode is only reduced by 7.5%compared with that in the stationary state,indicating its high stability.This work provides a new experimental basis for the construction of highly efficient and stable MoS₂ based heterojunction photoanode.(4)Preparation of 2D-2D-1D MoS₂/rGO/ZnO heterojunction photoanode and its photoelectric properties studyThe combination between MoS₂ and high conductivity material is an effective way to improve its electrical conductivity.The reduction of graphene oxide(rGO)with high conductivity was introduced into the MoS₂/ZnO photoanode by the spin coating method.In this procedure,the conductivity of MoS₂ was modified and the interface of MoS₂/ZnO heterojunction was optimized,which furtherromoted the transmission and separation efficiency of photogenerated carriers.Based on these above-mentioned advantages,the electron lifetime of MoS₂/rGO/ZnO heterojunction photoanode is greatly improved compared with that of MoS₂/ZnO photoanode.Therefore,the photocurrent density of MoS₂/rGO/ZnO heterojunction photoanode at the bias voltage of 0.1 V(vs.Ag/AgCl)is0.17 m A/cm² higher than that of MoS₂/ZnO heterojunction photoanode.