Abstract
The exfoliation of layered materials into two-dimensional (2D) semiconductors creates new structural domains, for example, basal planes, defect-rich in-planes, and edge sites. These surface species affect the photoelectrochemical (PEC) performance, which in turn determines their applicability in solar energy conversion technologies. In this study, a custom-designed microdroplet cell-based spatially resolved PEC approach was employed to identify the structural parts and to measure the PEC activity of the mechanically exfoliated MoSe2 and WSe2 nanosheets for bulk, few-layer, and monolayer specimens. The PEC performance decreased with the decreasing thickness of nanoflakes, and the relative PEC activity (photo/total current) reduced by introducing more defects to the 2D flakes: 1–3% loss was found for in-plane defects and 30–40% for edge defects. While edge sites act as charge carrier recombination centers, their electrocatalytic activity is higher than that of the basal planes. The comparison of PEC activity of micromechanically and liquid phase exfoliated bulk and few-layer MoSe2 and WSe2 flakes further confirmed that the PEC performance of 2D flakes decreases with an increasing number of edge sites.
Highlights
IntroductionTransition metal dichalcogenides (TMDCs) have become attractive candidates in search for new materials for nanoelectronics and catalysis
Among inorganic layered species, transition metal dichalcogenides (TMDCs) have become attractive candidates in search for new materials for nanoelectronics and catalysis
(i) the sample area is selected under the microscope; (ii) a microdroplet is deposited and maintained using a microinjection and manipulator system; subsequently, (iii) the electrochemical experiment is performed with a potentiostat−galvanostat; (iv) the illumination is provided with a fiber-optic light source with controlled wavelength and intensity
Summary
Transition metal dichalcogenides (TMDCs) have become attractive candidates in search for new materials for nanoelectronics and catalysis. The activity of the basal- (smooth) and defect-rich (stepped) plane was investigated in the 1980s and 1990s already, focusing on bulk single crystals.[4−7] It was proposed that defects and edge sites attract adsorbates, which create surface states within the band gap, acting as recombination centers for the photogenerated charge carriers These surface states increase the photogeneration of charge carriers by photons with lower energy compared to the band gap (i.e., midgap states).[8] The defect-rich plane has higher EC and lower PEC activity than the basal plane. These states behave as recombination centers for the photogenerated charge carriers, which explains the lower PEC activity.[4]
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