Tunable Interlayer Interactions in Exfoliated 2D van der Waals Framework Fe(SCN)2(Pyrazine)2.

2D materials can be isolated as monolayer sheets when interlayer interactions involve weak van der Waals forces. These atomically thin structures enable novel topological physics and open chemical questions of how to tune the structure and properties of the sheets while maintaining them as isolated monolayers. Here, this work investigates 2D electroactive sheets that exfoliate in solution into colloidal nanosheets, but aggregate upon oxidation, giving rise to tunable interlayer charge transfer absorption and photoluminescence. This optical behavior resembles interlayer excitons, now intensely studied due to their long-lived emission, but which remain difficult to tune through synthetic chemistry. Instead, the interlayer excitons of these framework sheets can be modulated through control of solvent, electrolyte, oxidation state, and the composition of the framework building blocks. Compared to other 2D materials, these framework sheets display the largest known interlayer binding strengths, attributable to specific orbital interactions between the sheets, and among the longest interlayer exciton lifetimes. Taken together, this study provides a microscopic basis for manipulating long-range opto-electronic behavior in van der Waals materials through molecular synthetic chemistry.

Nanoscale Measurements of Charge Transfer at Cocatalyst/Semiconductor Interfaces in BiVO4 Particle Photocatalysts.

Semiconductor photocatalyst particles convert solar energy to fuels like H2. The particles are often assumed to provide crystalline-facet-dependent electron-hole separation. A common strategy is to deposit a hydrogen evolution reaction (HER) electrocatalyst on electron-selective facets and an oxygen evolution reaction (OER) electrocatalyst on hole-selective facets. A precise understanding of how charge-carrier-selective contacts emerge and how they are rationally designed, however, is missing. Using a combination of ex situ and in situ conducting atomic force microscopy (AFM) experiments and new ionomer/catalyst-semiconductor test structures, we show how heterogeneity in charge-carrier selectivity can be measured at the nanoscale. We discover that the presence of the water/electrolyte interface is critical to induce hole selectivity between the CoOx water-oxidation catalyst and the BiVO4 light absorber. pH-dependent measurements suggest that negative surface charge on the semiconductor is central to inducing hole selectivity. The work also demonstrates a new approach to control local pH and introduce water using thin-film ionomers compatible with conductive AFM measurements.