Electrocatalytic Performance Regulation Of Two-dimensional Transition Metal Sulfide Heterojunction

Two-dimensional transition metal sulfides（TMDs）have potential application value in the fields of solar cells,photoelectric decomposition water and photodetectors,due to the matching energy level structure and excellent catalytic activity.Different sulfide kinds and preparation methods of metal sulfides heterostructure have attracted lots of attention.In this paper,the deposition of two TMDs in different sequences were taken as the subject,the influence of the combination of energy bands on the direction and electron transfer resistance were explored.For two band-matched TMDs,electrons rapidly move from high conduction band（CB）to low conduction band under light excitation.It was difficult to move from low conduction band to high conduction band in turn.This phenomenon was called energy level resistance.Two kinds of TMDs were prepared on the conductive glass（FTO）by pulsed laser deposition（PLD）and chemical vapor deposition（CVD）.In the electrocatalytic reduction reaction,I₃^-was reduced to I^-,further analyzing and comparing the electrocatalytic properties.The major research contents and results were shown as followings:（1）Firstly,MoO₃（WO₃）was deposited on the FTO substrate by PLD method,then WO₃（MoO₃）was deposited to obtain FTO/MoO₃/WO₃（FTO/WO₃/MoO₃）.FTO/MoS₂/WS₂ and FTO/WS₂/MoS₂ were synthesised by CVD method.Under illumination excitation,the electron transport direction was from WS₂ to MoS₂,and redox catalysis of FTO/WS₂/MoS₂was more capable than FTO/MoS₂/WS₂,due to the conduction band of MoS₂ is-4.38 eV and the CB of WS₂ is-4.13 eV.The structure was analyzed by X-ray diffraction（XRD）,Raman,transmission electron microscopy（TEM）,scanning electron microscopy（SEM）,and X-ray photoelectron spectroscopy（XPS）.The films were prepared by PLD and CVD.The films were flat and stable,thus it was advantageous for contact with the electrolyte as a counter electrode.FTO/MoS₂/WS₂（FTO/WS₂/MoS₂）and the photoanode were assembled into a dye-sensitized solar cell（DSSC）for current-voltage characteristic curve test（I-V）.The photoelectric conversion efficiency（η）of FTO/WS₂/MoS₂ was measured to be 6.83%,which was equivalent to Pt（6.90%）and higher than FTO/MoS₂/WS₂（4.47%）.（2）Combining PLD method with CVD method to prepare FTO/WS₂/NiS₂ and FTO/NiS₂/WS₂ by regulating deposition sequence.In the two structures of FTO/WS₂/NiS₂and FTO/NiS₂/WS₂,the CB of NiS₂ is-5.39 eV,the CB of WS₂ is-4.13 eV.The FTO/WS₂/NiS₂ was easily to collecte electrons and promote the redox reaction on the surface than FTO/NiS₂/WS₂,because of efficient separation of electrons and hole.The direction of electron transfer and the magnitude of resistance can be characterized by electrochemical performance.Cyclic voltammetry（CV）,electrochemical impedance（EIS）,Tafel curve and I-V curve were used to compare the catalytic performance of FTO/WS₂/NiS₂ and FTO/NiS₂/WS₂.The results show that under the same conditions,with Pt as the reference,theηof FTO/WS₂/NiS₂ was higher than FTO/NiS₂/WS₂.Compared with the CV curves,the E_ppp value was ranked as FTO/WS₂/NiS₂（598 mV）<FTO/NiS₂/WS₂（873 mV）.The R_ct of FTO/WS₂/NiS₂ was 2.33Ωcm²,which was much smaller than FTO/NiS₂/WS₂（15.43Ωcm²）.