We report a facile chemical vapor deposition (CVD) growth of vertical heterostructures of layered metal dichalcogenides (MX2) enabled by van der Waals epitaxy. Few layers of MoS2, WS2, and WSe2 were grown uniformly onto microplates of SnS2 under mild CVD reaction conditions (<500 °C) and the heteroepitaxy between them was confirmed using cross-sectional transmission electron microscopy (TEM) and unequivocally characterized by resolving the large-area Moiré patterns that appeared on the basal planes of microplates in conventional TEM (nonsectioned). Additional photoluminescence peaks were observed in heterostructures of MoS2-SnS2, which can be understood with electronic structure calculations to likely result from electronic coupling and charge separation between MoS2 and SnS2 layers. This work opens up the exploration of large-area heterostructures of diverse MX2 nanomaterials as the material platform for electronic structure engineering of atomically thin two-dimensional (2D) semiconducting heterostructures and device applications.

In this Article, we investigate the effects of binding geometry and intermolecular interactions in monolayers of a rhenium-based dye adsorbed to TiO 2 . We combine two-dimensional infrared (2D IR) spectroscopy of samples prepared with different dye loadings with density functional theory (DFT) calculations of dye binding energies and vibrational frequencies. Our 2D IR spectra reveal splitting of the CO symmetric stretch mode into two peaks of unequal intensity at high surface coverages, which persists even when samples are washed to remove unadsorbed aggregates. Our DFT calculations indicate that it is unlikely that dye binding geometries account for the shifts in peak frequency observed in our experimental spectra. Instead, we find that the shifts in vibrational frequency and 2D IR peak structure are consistent with coupling of dyes associated on the TiO 2 surface. The relative peak intensities in our 1D and 2D spectra indicate different transition dipole strengths, also a signature of molecular coupling. We show that aggregation of dyes on the surface is energetically favorable. Adsorbate−adsorbate interactions may play an important role in defining surface structure and electronic properties of dyesensitized solar cells and related organic/inorganic interfaces. Infared spectroscopy is a good means to identify its occurrence, and to begin exploring its effects on phenomena like electron injection kinetics.

Sn-substituted zeolite Beta (Sn-Beta) is a promising catalyst for efficient aldose to ketose isomerization, a key step in the conversion of biomass to platform chemicals such as 5-(hydroxymethyl)furan and furfural. Recent experimental studies probing the mechanism and active site for glucose isomerization (to fructose) and competing epimerization (to mannose) have found that isomerization proceeds via a 1,2 intramolecular hydride transfer (HT) and epimerization by either two subsequent HT steps (on pure Sn-Beta; water and methanol) or one 1,2 intramolecular carbon shift (CS) step (on Na-exchanged Sn-Beta; water and methanol). In order to address remaining atomic-level mechanistic questions raised by this data, we investigate the various pathways with computational methods using several sizes of cluster models of Sn-Beta and density functional theory. First, we find an energetically plausible pathway for mannose formation via two subsequent HT steps that is consistent with experimental observations. Additionally, we conclude that Na exchange influences the mechanism by electrostatic stabilization of CS relative to HT, and that this effect is relatively independent of geometric and flexibility constraints (even though the exact details of the mechanism are not). Finally, we find that the experimentally observed increase in glucose conversion when methanol is used as a solvent instead of water can be explained by the difference in solvation of the hydrophobic pores.

Recently a novel approach to the photocatalytic reduction of molecular nitrogen under ambient conditions was reported in which hydrated electrons generated from ultraviolet illumination of diamond served as the reducing agent [Zhu, D.; Zhang, L.; Ruther, R. E.; Hamers, R. J. Photo-Illuminated Diamond as a Solid-State Source of Solvated Electrons in Water for Nitrogen Reduction. Nat. Mater. 2013, 12, 836-841]. This surprising reduction of N2 by aqueous solvated electrons is absent from the vast existing radiolysis literature and thus has little mechanistic precedent. In this work, a combination of experimental and computational approaches is used to elucidate the detailed molecular-level mechanistic pathway from nitrogen to ammonia. A variety of approaches, including electronic structure calculations, molecular dynamics simulations, kinetic modeling, and pH-dependent experimental measures of NH3 and competing H2 production, implicate a hydrogen atom addition mechanism at early reduction steps and sequential protonation/direct reduction by a solvated electron at later steps, thus involving both direct and indirect reactions with solvated electrons. This work provides a framework for understanding the possible application of solvated electrons as energetic reducing agents for chemically inert species under mild conditions.