A Spatially Resolved Study of the Role of Surface Motifs in the Photoelectrochemical Conversion on Prospective 2-D Layered Chalcogenide Photoelectrodes for Solar-Fuel Generation
Authors: John Jimmy , Velazquez Jesus M , Esposito Daniel , Pieterick Adam , Pala Ragip , Saive Rebecca , Ardo Shane , Brunschwig Bruce , Lewis Nathan S. 
Published: February 2020

2-D Layered chalcogenide materials, with the general formula, MX2(where, generally, M= Mo, W and X=S, Se), have been of sustained research interest, over the past few decades, in the area of solar-fuel generation. They have been looked at as light absorbers as they exhibit favorable bandgaps, high absorption coefficients for capturing solar energy and have good photoelectrochemical stability. More recently, they have been also found to be highly active electrocatalysts for hydrogen evolution reaction (HER). The surfaces of these materials are highly heterogeneous displaying macroscopic terraces and step edges; along with other micro/nano-scale defects. The optimal role of these materials in solar-energy conversion is decisively determined by their surface morphology. For HER catalysis, material forms such as nanoparticles and amorphous powders that have high concentration of step edges are preferable as the dangling bonds/exposed atoms at the step edges are instrumental in catalyzing the reaction. On the other hand, in their role as light absorbers, the surfaces of these materials should, ideally, be composed of large flat terraces since the absorption of light and the separation of carriers at the semiconductor/electrolyte interface occur most efficiently at the terraces. The step edges and other defects are thought to be detrimental as they act as sites for carrier recombination. It is clear that, in employing these materials for solar-fuel generation, it is of fundamental importance to delineate the specific roles of the surface motifs of 2-D layered chalcogenides in the photoelectrochemical conversion processes.

Addressing the above-mentioned knowledge gap, we were able to spatially probe the local photoelectrochemical response of the surface motifs on the layered chalcogenide photoelectrodes by employing the technique of laser beam induced current microscopy (LBIC). LBIC involves the photogeneration of carriers locally using a laser beam and the resulting photocurrent could then be spatially correlated to the excitation spot on the surface of the electrode. Hence, a detailed spatial map of the local photoconversion efficiencies on the photoelectrodes could be obtained. An important observation of the spatially resolved photocurrent measurements was that there were significant variations in the photoconversion efficiencies of different terraces and that, more than the macroscopic step edges, it was the presence of low performing terraces that crucially determined the overall performance of these materials as light absorbers.

 The source of the low efficiencies on some terraces were further explored by studying and comparing the topography of the various terraces at the micro/nano-scale using AFM and STM. These studies revealed that the terraces were invariably textured at the micro/nano-scale and that the texturing differed from terrace to terrace. Further, local spectral response measurements at terraces of differing photo-conversion efficiencies provided evidence for the existence of sub-bandgap surface states on the low-performing terraces; possibly, explaining the variations in the efficiencies between terraces.  Differences in work-function values across terraces measured by Kelvin probe force microscopy also supported the prevalence of surface states on the low performing terraces.

 In conclusion, employing a suite of spatially resolved techniques to study in-depth the surface motifs of layered chalcogenide photoelectrodes, we have been able to construct a detailed picture of the photoconversion processes occurring on these materials at the microscopic level. The results from our study crucially highlight the sources of performance losses in these materials and will serve to guide efforts in optimization of this material class for application in solar-fuel generation.