Preparation And Photoelectrochemical Performance Of Cu₂ZnSnS₄-based Photocathodes

Photoelectrochemical catalysis is a kind of efficient and potential catalytic technique,which can be used in the fields of H2 evolution from water splitting,reduction of CO₂ and solar cell.It possesses environmentally-friendly feature.Photoelectrode is the main component of reaction system for photoelectrochemical catalysis,and thus,searching suitable electrode materials and fabricating high-performance electrodes are prerequisites to achieve practical application of this technique.Cu₂ZnSnS₄?CZTS?is a typical p-type semiconductor and an ideal photocathode material because of its proper band gap of 1.5 eV and a large absorption coefficient?over 104 cm-1?.Furthermore,all of the element compositions in CZTS are nontoxic and abundant on the earth.However,the defects of decomposition and elements volatilization of CZTS during synthesis and the rapid recombination rate of photogenerated carriers hinder the enhancement of photoelectrochemical performance over CZTS.Aiming to overcome the above-mentioned two problems,we proposed two programs as following.?1?By introducing a semi-closed synthetic system,sulfur partial pressure could be increased properly in order to restrain the decomposition and elements volatilization of CZTS.?2?An approach of composition segregation-in situ synthesis was used to fabricate p-p type CZTS-SnS heterojunction,which can achieve efficient separation of photogenerated electrons and holes by controlling their directional transfer.The main results are as below.1.Modulation of sulfur partial pressure in sulfurization to significantly improve photoelectrochemical performance over Cu2 ZnSn S4 photocathode.The CZTS photocathodes were prepared on FTO conductive substrate by electrodepositing metal precursors followed by sulfurization.The sulfur partial pressure was modulated by changing the windows size of the semi-closed system and three samples under different sulfur partial pressures were fabricated.The characterizations of X-ray diffraction,Raman spectra,UV-Visible-IR absorption spectra and scanning electron microscope showed little difference among the three samples.Photoelectrochemical test demonstrated that CZTS-WS1 sample?prepared in semi-closed system with window size of 1 mm?exhibited the best photoelectrochemical performance;the photocurrent density of CZTS-WS1 was 1.8 mA/cm² at-0.50 V vs.Ag/AgCl reference and could be kept stable for a long time.The incident photon-to-electron conversion efficiency?IPCE?could reach 4%-7% in visible-light region.Element-composition analysis for all CZTS samples provided two reasonable explanations to the enhancement of photoelectrochemical performance over CZTS-WS1 sample.On one hand,relatively high sulfur partial pressure could increase the penetration of S and thus hinder the decomposition of CZTS,which suppressed the volatilization of Sn to ensure the accuracy of stoichiometric ratio of CZTS.On the other hand,the compositions of CZTS-WS1 sample were close to a Zn-rich and Cu-poor characteristic.2.Composition segregation-in situ synthesis to prepare p-p type CZTS-SnS heterojunction towards enhanced photoelectrochemical performance.The CZTS-SnS photocathode was prepared on FTO conductive glass via composition segregation-in situ synthesis.By electrodepositing a little excess amount of Sn,the SnS could be segregated from CZTS in sulfuration process to form CZTS-SnS heterojunction.The measurements of X-ray diffraction and UV-Visible-IR absorption spectra demonstrated that a little amount of SnS was generated on CZTS,which confirmed the formation of CZTS-SnS heterojunction.The observation from scanning electron microscope indicated that,in contrast to the CZTS-WS1 sample,the particle size of CZTS in CZTS-SnS heterojunction decreased because of the existence of SnS,but the thickness of thin film didn't changed.Photoelectrochemical test demonstrated that CZTS-SnS showed a promoted photoelectrochemical performance;the photocurrent density of CZTS-SnS reached 3.6 mA/cm² at-0.50 V vs.Ag/AgCl reference which was two-folds magnification of that of CZTS-WS1.The IPCE could be enhanced to 8%-15% in UV-visible region.Then,the transfer rate of photocarrier was studied by electrochemical impedance spectroscopy.The diameter of semicircular Nynquist curve of CZTS-SnS was significantly smaller than that of CZTS-WS1,revealing that the formation of CZTS-SnS heterojunction enhanced the separation of photogenerated electrons and holes.This was the primary reason for obvious promotion of photoelectrochemical performance over CZTS-SnS photoelectrode.