Modification of Surface Properties of Layered Compounds by Chemical and (Photo)Electrochemical Processes

Removing Defects in WSe2 via Surface Oxidation and Etching to Improve Solar Conversion Performance

“…Layered transition metal dichalcogenide (MX 2 ) materials, such as WSe 2 and MoS 2 , were extensively studied in the 1970s and 1980s for their promising photovoltaic performance when grown as single crystals. − In particular, n-type WSe 2 single crystals have shown solar conversion efficiencies as high as 14% in a photoelectrochemical (PEC) liquid-junction cell . In order to achieve such high efficiency, some surface treatment of this material was required, achieved through a variety of methods with various levels of success and reproducibility: (photo)­electrochemical etching, ,, ligand modification, ,, and surface polymerization. , However, the underlying mechanisms for these treatments and improvements have not been well understood. It was hypothesized that these surface treatments or modifications are necessary because step edges act as recombination sites for the photogenerated carriers. , Because of this problem, polycrystalline thin films of these materials were never able to achieve the high efficiencies of their single-crystal counterparts, although improvements could be made via polymerization of the surface .…”

“…More recent work has shown promising performance for PEC solar water splitting using both single crystals and exfoliated thin films of WSe 2 − as the photoabsorber, with improved performance using ligand modification, but more work is needed to increase efficiency. Etching or corrosion of single-crystal MX 2 materials, often preferentially along the step edges or dislocation defects, was commonly observed throughout the literature. − PEC etching has been reported to significantly increase the solar efficiency of n-type WSe 2 single crystals. However, it requires the use of strong acid or base at elevated temperatures. ,, One additional oddity is that the etching process actually appears to increase the number of edges, which would seem counterintuitive for the increased performance.…”

“…Etching or corrosion of single-crystal MX 2 materials, often preferentially along the step edges or dislocation defects, was commonly observed throughout the literature. − PEC etching has been reported to significantly increase the solar efficiency of n-type WSe 2 single crystals. However, it requires the use of strong acid or base at elevated temperatures. ,, One additional oddity is that the etching process actually appears to increase the number of edges, which would seem counterintuitive for the increased performance. In fact, if the MX 2 materials are etched in a different way, these edges will degrade the PEC performance and can enhance the electrocatalytic activity for the hydrogen evolution reaction (HER) .…”

“…While a previous study showed that annealing MX 2 monolayers in a chalcogen-rich atmosphere at high temperature can increase the conductivity of the material as the chalcogen vacancies are healed, the new approach that we describe here perhaps yields similar results but under ambient conditions with a common laboratory instrument (a UV lamp). Prior studies of photoetching and PEC etching on WSe 2 single crystals have used treatments that likely remove a significant amount of material, especially along step edges and screw dislocation defects, ,,, which could be problematic when using nanomaterials. In contrast, because only some of the surface material is removed by our mild oxidation and etching treatment, we anticipate that it is better suited for thin films and the very thin, two-dimensional MX 2 materials that are currently of intense interest. − We believe that this oxidation and etching treatment method could be a general approach to improve the solar performance of p-type MX 2 materials that are either S- or Se-based and have preliminarily demonstrated that this treatment also leads to improved PEC performance for p-type MoS 2 single crystals found naturally, as shown in SI Figure S11.…”

“…1−3 In particular, ntype WSe 2 single crystals have shown solar conversion efficiencies as high as 14% in a photoelectrochemical (PEC) liquid-junction cell. 2 In order to achieve such high efficiency, some surface treatment of this material was required, achieved through a variety of methods with various levels of success and reproducibility: (photo)electrochemical etching, 2,4,5 ligand modification, 4,6,7 and surface polymerization. 4,6 However, the underlying mechanisms for these treatments and improvements have not been well understood.…”