Control over photophysical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) is the key to advance their applications in next-generation optoelectronics. Although chemical doping and surface modification with plasmonic metals have been reported to tune the photophysical and catalytic properties of 2D TMDs, there have been few reports of tuning optical properties using dynamic electrochemical control of electrode potential. Herein, we report (1) the photoluminescence (PL) enhancement and red-shift in the PL spectrum of 2D MoS2, synthesized by chemical vapor deposition and subsequent transfer onto an indium tin oxide electrode, upon electrochemical anodization and (2) spatial heterogeneities in its photoelectrochemical (PEC) activities. Spectroelectrochemistry shows that positive electrochemical bias causes an initial ten-fold increase in the PL intensity followed by a quick decrease in the enhancement. The PL enhancement and spectrum red-shift are associated with the decrease in nonradiative decay rates of excitons formed upon electrochemical anodization of 2D MoS2. Additionally, scanning electrochemical cell microscopy (SECCM) study shows that the 2D MoS2 crystal is spatially sensitive to PEC oxidation at positive potentials. SECCM also shows a photocurrent increase caused by spatially heterogeneous edge-type defect sites of the crystal.

Understanding the electrochemical properties at a localized scale is critically important to comprehend the origin of corrosion and develop multifunctional materials with robust corrosion resistance, particularly at conjoined metal interfaces typically encountered in automobile manufacturing. Scanning electrochemical cell microscopy (SECCM) is an emerging technique which enables to study the corrosion of metal surfaces to be visualized at the microscopic level. In this work, we developed scanning electrochemical cell impedance microscopy (SECCIM) by combining SECCM with electrochemical impedance spectroscopy (EIS) and explored the unique advantages of using SECCIM to measure the corrosion kinetics on single-crystal Mg (0001) as the model surface using direct current and alternating current polarization techniques. Specifically, a theta capillary with a tip diameter of 10 μm filled with a 0.01 M NaCl electrolyte was used as a probe to perform spatially resolved potentiodynamic Tafel polarization and EIS. The combination of traditional SECCM with EIS led to the development of SECCIM and enabled us to study small interfacial events such as charge transfer, adsorption, and emergence of resistive oxide films on the surface using the distribution of relaxation time analysis. Furthermore, by comparing localized SECCIM measurements with bulk electrochemical measurements, we establish the reliability of SECCIM for the mapping of corrosion potential and associated charge-transfer resistance on the Mg (0001) surface. Our results indicate that SECCIM measurement with Tafel and EIS analysis will provide an unparalleled ability to characterize the pitting corrosion mechanism on the heterogeneous surface of mixed-metal alloys and metal joints.

Close-spaced vapor transport is a plausibly low-cost, high-rate method to grow III−V materials for photovoltaic and photoelectrochemical device applications. We report the first homoepitaxial growth of GaAs microstructures on (100)-and (111)B-oriented GaAs substrates using patterned SiO x and Al 2 O 3 masks and show that the resulting microstructured GaAs is an efficient semiconductor absorber for photovoltaic and photoelectrochemical applications. Cross-sectional transmission electron microscopy reveals an unusually low density of twin-plane defects in the (111)-oriented microstructures and the occurrence of stacked twin-plane defects in the (100)-oriented microstructures. Nonaqueous photoelectrochemical measurements show similar short-circuit currents of 9.7 and 9.1 mA cm −2 for (100)-and (111)-oriented microstructures, respectively, with promising external quantum efficiencies. Together, the low twin density and good electronic properties indicate that micro-or nanostructures grown by selective area epitaxy in close-spaced vapor transport are promising for device applications that take advantage of their three-dimensional structure.

Artificial photosynthesis can potentially address the global energy challenges and environmental issues caused by fossil fuels. Photoelectrochemical heterojunction structures of new photonic structures have been developed for efficient sunlight absorption, charge generation and separation and transport, and selective reduction of CO2 and water splitting. In this review, an overview of several recently developed heterojunction model systems comprised of low-cost photonic materials such as transition metal dichalcogenides (TMDs), perovskite semiconductor nanocrystals, and plasmonic nanostructures is presented to rationalize the potential benefits of utilizing heterojunction structures for efficient and selective CO2 reduction with renewable energy resources. Recent advances in electroanalytical methods for CO2 reduction such as scanning electrochemical microscopy (SECM) are reviewed. These techniques can potentially resolve local CO2 reduction kinetics and their spatial heterogeneities of a heterojunction photoelectrochemical structure.

Hydrogen is a promising alternative to gasoline due to its energy density and ability to burn cleanly. However, to make hydrogen a more viable alternative energy, affordable and durable electrocatalysts must be developed to replace expensive catalysts such as platinum. Transition metal dichalcogenides (TMDs) are a promising alternative to platinum because they are abundant, inexpensive, and have a tunable structure. There are various ways to produce TMD films including chemical and mechanical exfoliation, chemical vapor deposition (CVD), and electrodeposition. Exfoliation and CVD techniques often require transfer from the growth substrate to an electrode, which introduces impurities to the film. Electrodeposition, however, provides a way to produce TMDs directly on the electrode without the need for transfer with excellent surface coverage. This work uses electrodeposition to produce TMD and TMD-bi-layer electrodes for electrocatalytic hydrogen evolution reaction. The results presented include cost-effective deposition techniques along with enhanced proton reduction activity for the bi-layer TMD structure consisting of MoS2 and MoSe2, which suggests that the electron transfer kinetics from the conductive glass substrate to the top-layer are enhanced with a MoS2 layer. Furthermore, the bilayer structures are characterized via XPS, XPS depth-profiling, and SEM-EDS for enhanced understanding of the structure synthesized.