Research On Construction And Electrocatalytic Performances Of MoS₂ Based Nanodevices

Water splitting using electricity generated by wind or solar power has been proposed as an effective strategy for sustainable hydrogen production.MoS₂ is considered to be a promising alternative to Pt-group metals for use in an electrocatalytic hydrogen evolution reaction?HER?owing to its noble metal-like moderate hydrogen adsorption Gibbs energy.However,the limitation of the number of active sites and the electrical properties affect the overall electrocatalytic activity.For a semiconductive electrocatalyst,electrocatalytic activity can be affected by various factors,such as charge transport,band structure.Chemical doping and the intentional incorporation of defects have been employed to modulate the Fermi level of 2H-MoS₂ by introducing impurity energy level.However,these tactics ultimately relate to complex processes of controlling the doping species and the defects concentration,which obfuscates our understanding of the function of energy band structure in HER catalysis.In this work,a series of individual MoS₂ nanosheets,including 2H-MoS₂,2H-MoS₂ contact and 1T-MoS₂ channel,1T-MoS₂,1T-MoS₂ contact and 2H-MoS₂ channel,are used to create electrocatalytic nanodevices to address the function of the contact resistance and the band structure in HER catalysis.The results are as follows:?1?For nanodevices with 2H phase contact,the overpotenticals are 285 and 282mV respectively at a current density of 10 mA cm^-2.For nanodevices with 1T phase contact,the overpotentials are 230 and 187 mV respectively at a current density of 10mA cm^-2.After the contact phase changes from 2H phase to 1T phase,the electrocatalytic performance is obviously improved.Through testing the electrical transport performance of devices with 2H channel,it was found that the electrical conductivity before and after the contact phase transition are 20 S m^-1 and 410 S m^-1respectively.The results indicate that the change from schottky contact to ohmic contact greatly improves the performance of electrocatalytic hydrogen evolution.It can be inferred that effective charge injection is very important for evoked electrocatalytic activity.?2?Based on the above research,a strategy that can mono-tuning the Fermi level position for elucidating HER performance with the minimum impact of contact resistance is proposed.Under the vertical electric field,the open-circuit voltage of 2H phase MoS₂ becomes more and more negative,which indicates that the electrons have a more negative electrochemical potential,the Fermi level position is constantly close to the conduction band.At the back gate voltages of 0,1,2 and 3 V,the Fermi level positions are located at 5.13,4.3,3.93 and 3.86 eV,respectively.?3?Electrocatalytic performance of 2H-MoS2 improves with an increase in back-gate voltage.The overpotential at 10 mA cm^-2 decreases from 228,218,192 to 74 mV with the increase of back-gate voltage from 0,1,2 to 3 V.Electrocatalytic performance of 1T-MoS₂ changes little,which can be attributed to the intrinsic metallic electronic property.Interestingly,when the back-gate voltage is increased from 2 V to 3 V,the overpotential of 2H-MoS₂ is significantly reduced by 118 mV.In addition,at the back-gate voltage of 3 V,the HER performance of the 2H-MoS₂ is superior to that of 1T-MoS₂?187 mV?.This unexpected excellent HER performance is ascribed to that electrons are injected into the conduction band under the condition of back-gate voltage,which leads to the increased Fermi level of 2H-MoS₂ and a shorter Debye screen length.Hence,the required energy to drive electrons from the electrocatalyst surface to reactant will decrease,which activates the 2H-MoS₂ thermodynamically.