Co-Catalyst And Electronic Structure Modification Of Semiconductor Promotes Interfacial Charge Transfer In Photocatalysis

In recent years,the nearly unity monochromatic quantum yield of photocatalytic water splitting has greatly stimulated the exploration of novel reactions.In the photocatalytic reaction system,the transfer of photogenerated electrons from semiconductor nanostructures to reduction cocatalysts dominates the photocatalytic hydrogen evolution reaction.In some photocatalytic reactions,despite the thermodynamic feasibility,because of the improper selection of co-catalysts,the energy dissipation is very large due to the very high barrier formed between the semiconductor and the metal cocatalysts,which kinetically prevents the interfacial charge transfer,intensifies the recombination of photogenerated charge carriers finally.Under weak illumination,trap-state-intermediated charge recombination dominates the interfacial electron transfer process,while under high-intensity irradiation,thermionic emission becomes the major electronic process,and the energy utilization would encounter a minimum along the variation of irradiation intensity which has an impact on the overall photocatalytic reaction efficiency.In order to improve the photocatalytic hydrogen evolution reaction rate,we managed to reduce the activation energy of interfacial charge transfer by reducing the contact barrier height between semiconductor and cocatalyst,so as to increasing the hydrogen evolution reaction rate.We adopt platinum and WO_Xdeposited semiconductors as the model to demonstrate the energy dissipation for the interfacial charge transfer.Compared with platinum,WO_Xwith a relatively small work function would form and low Schottcky barrier when contacted to the semiconductor,which can effectively save the energy from excessive dissipation,which kinetically facilitates the transfer of photogenerated electrons from semiconductor nanostructures to cocatalyst and promotes the hydrogen evolution.In order to completely eliminate the contact barrier between the semiconductor and cocatalyst and reduce the apparent activation energy of the photocatalytic reaction,we constructed cocatalyst-free photocatalysts W-CdS.We demonstrate here that electronic doping not only provides catalytically active sites in cocatalyst-free photocatalysts but also plays certain additional roles.These electronic states can efficiently channel the trapped electrons to the semiconductor surface without suffering from time-consuming detrapping and can facilitate the extraction of photogenerated holes.These features endow our demonstrated tungsten-doped CdS with evident superiority in photocatalytic performance over conventional counterparts loaded with platinum cocatalysts.The simultaneous increase in the reduction and oxidation half-reaction rates further improves the efficiency of the entire photocatalytic reaction.The main findings are as follows:（1）WO_Xdeposited on CdS semiconductor films was synthesized by hydrothermal method.Characterization by SEM,ACTEM and XPS showed that a small amount of WO_Xwas indeed successfully loaded on the CdS surface.The electrochemical experiments show that the performance of 0.5%WO_X/CdS is better than that of traditional Pt-loaded photocatalysts.This is because the work function of WO_Xis smaller than that of Pt,and the contact barrier is small,which is beneficial to the transfer of charges between CdS and WO_x,thus promotes the electrocatalytic hydrogen evolution on the cocatalyst WOx.At light intensity close to sunlight（AM 1.5G,100 m W/cm²）,the hydrogen evolution rate of0.5%WO_X/CdS was 9.456 mmol g^-1_cath^-1,which was 12.65 times as that of 1%Pt/CdS.Light-intensity-dependent hydrogen evolution behaviors of 1%Pt/CdS and 0.5%WO_X/CdS photocatalysts showed that the photon utilization of 1%Pt/CdS photocatalyst was the smallest when the light intensity was close to sunlight,while that of 0.5%WO_X/CdS photocatalyst,increasing with the incident light intensity,exhibits approximately constant photon utilization.This also demonstrates that lowering the contact barrier can effectively save the excessively dissipated energy,which kinetically facilitates the transfer of photogenerated electrons from semiconductor nanostructures to cocatalyst and promotes the hydrogen evolution.（2）A cocatalyst-free photocatalyst 5%W-CdS was synthesized by a one-step solvothermal method.Characterization by SEM,ACTEM,XRD,XPS,UPS,and ICP showed that the tungsten dopant in W-CdS has a uniform distribution,and evident modification of the electronic states occurred in the semiconductor.When the irradiated intensity close to sunlight（AM 1.5G,100 m W/cm²）,the hydrogen production rate of 5%W-CdS is 10.8 mmol g^-1_cath^-1,which is 14.4 times that of 1%Pt/CdS.Light-intensity-dependent hydrogen evolution behaviors showed that the photon utilization rate of 1%Pt/CdS photocatalyst has a minimum value when the irradiated intensity is close to sunlight.Compared with 1%Pt/CdS,5%W-CdS has no obvious change in photon utilization with the increase of incident light intensity.This is because the decrease in the apparent activation energy kinetically facilitates the transport of electrons from the interior to the surface catalytically active sites.The linear scanning test,the open-circuit potential decay test,and the decay of the light-induced potential test showed that elementary doping can simultaneously channel the transport of electrons from the interior to the surface and catalytically transfer these charges to the solution to promote the photocatalytic reduction half reaction.Besides,elementary doping can also promote the oxidation half-reaction by facilitating the hole extraction.The demonstration of the multiple roles of elementary doping can provide important hints for further enhancing the performance of cocatalyst-free photocatalysts.