Surface States of Photoelectrodes by Surface-Specific Steady-State and Time-Resolved Sum Frequency Spectroscopies
The surface states of photo-electrodes as catalysts heavi-ly influence their perfor-mance in pho-tocatalysis and photoelectro-catalysis appli-cations.These catalysts are necessary for developing ro-bust solutions to the climate and global energy crises by promoting CO2 reduction,N2 reduc-tion,contaminant degradation,and water splitting.The semiconductors that can fill this role are beholden as photoelectrodes to the processes of charge generation,separation,and utiliza-tion,which are in turn products of surface states,surface electric fields,and surface carrier dy-namics.Methods which are typically used for studying these processes to improve semicon-ductors are indirect,invasive,not surface specific,not practical under ambient conditions,or a combination thereof.Recently,nonlinear optical processes such as electronic sum-frequency generation(ESFG)and second-harmonic generation(ESHG)have gained popularity in inves-tigations of semiconductor catalysts systems.Such techniques possess many advantages of in-situ analysis,interfacial specificity,non-invasiveness,as well as the ability to be used under any conditions.In this review,we detail the importance of surface states and their intimate relationship with catalytic performance,outline methods to investigate semiconductor sur-face states,electric fields,and carrier dynamics and highlight recent contributions to the field through interface-specific spectroscopy.We will also discuss how the recent development of heterodyne-detected ESHG(HD-ESHG)was used to extract charged surface states through phase information,time-resolved ESFG(TR-ESFG)to obtain in-situ dynamic process moni-toring,and two-dimensional ESFG(2D-ESFG)to explore surface state couplings,and how further advancements in spectroscopic technology can fill in knowledge gaps to accelerate photoelectrocatalyst utilization.We believe that this work will provide a valuable summary of the importance of semiconductor surface states and interfacial electronic properties,inform a broad audience of the capabilities of nonlinear optical techniques,and inspire future original approaches to improving photocatalytic and photoelectrocatalytic devices.
Surface statePhotoelectrodeSum frequency generationPhotocatalysisPho-toelectrocatalysis
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