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.

Organometallic halide perovskites have gained immense scientific interest because of their unique optoelectronic properties that can benefit applications such as solar cells, lasers, and light-emitting diodes. Here, we develop a facile method to form a density gradient of MAPbI 3 (MA = methylammonium) crystal particles as a function of the electrochemical potential gradient across an indium-doped tin oxide (ITO) electrode. At the highpotential end of the ITO, a high density of nucleation sites is formed, yielding high-density MAPbI 3 crystals with a broad particle size distribution. Less density of MAPbI 3 crystals can be optically resolved at the low-potential end of the ITO. Spatial distributions of fluorescence and photoelectrochemical properties of the perovskite films show the same clear trends as the particle density gradients. Mass spectrum imaging of PbO 2 and MAPbI 3 gradients shows a spatial distribution of PbO 2 , MAPbI 3 of fresh sample and PbI 2 for aged sample. Mass spectrum imaging also reveals a poor structural stability of MAPbI 3 crystals formed at the high-potential side of the gradient because of their fast growth kinetics producing smaller nucleation sites.

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.

Lead-based perovskite MAPbX3 (MA = CH3NH3, X = Cl and Br) has shown great potential benefits to advance modern optoelectronics and clean energy harvesting devices. Poor structural stability is one of the major challenges of MAPbX3 perovskite materials to overcome to achieve desired device performance. Here, we present the electrochemical stability study of CH3NH3PbCl1.08Br1.92 quantum dots (QDs) by electrogenerated chemiluminescence (ECL) and photoluminescence (PL) spectroelectrochemistry methods. Electrochemical anodization of pristine MAPbX3 QD film results in the disproportionate loss of methylammonium and halide ions (X = Cl and Br). ECL efficiency and stability of perovskite QDs in the presence of coreactant tripropyl amine (TPrA) can be greatly improved after being incorporated into a polystyrene (PS) matrix. Mass spectrum and X-ray photoelectron spectroscopy (XPS) measurements were used to provide chemical composition variation details of QDs, which are responsible for the ECL and PL characteristics (e.g., wavelength redshift) of perovskite QDs in an electrochemical cell.

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