The behavior of crystalline nanoparticles depends strongly on which facets are exposed. Some facets are more active than others, but it is difficult to selectively isolate particular facets. This study provides fundamental insights into photocatalytic and photoelectrochemical performance of three types of TiO(2) nanoparticles with predominantly exposed {101}, {010}, or {001} facets, where 86-99% of the surface area is the desired facet. Photodegradation of methyl orange reveals that {001}-TiO(2) has 1.79 and 3.22 times higher photocatalytic activity than {010} and {101}-TiO(2), respectively. This suggests that the photochemical performance is highly correlated with the surface energy and the number of under-coordinated surface atoms. In contrast, the photoelectrochemical performance of the faceted TiO(2) nanoparticles sensitized with the commercially available MK-2 dye was highest with {010}-TiO(2) which yielded an overall cell efficiency of 6.1%, compared to 3.2% for {101}-TiO(2) and 2.6% for {001}-TiO(2) prepared under analogous conditions. Measurement of desorption kinetics and accompanying computational modeling suggests a stronger covalent interaction of the dye with the {010} and {101} facets compared with the {001} facet. Time-resolved THz spectroscopy and transient absorption spectroscopy measure faster electron injection dynamics when MK-2 is bound to {010} compared to other facets, consistent with extensive computational simulations which indicate that the {010} facet provides the most efficient and direct pathway for interfacial electron transfer. Our experimental and computational results establish for the first time that photoelectrochemical performance is dependent upon the binding energy of the dye as well as the crystalline structure of the facet, as opposed to surface energy alone.

In this study, we overview resonance energy transfer between molecules in the presence of plasmonic structures and derive an explicit Förster-type expression for the rate of plasmon-coupled resonance energy transfer (PC-RET). The proposed theory is general for energy transfer in the presence of materials with any space-dependent, frequency-dependent, or complex dielectric functions. Furthermore, the theory allows us to develop the concept of a generalized spectral overlap (GSO) J̃ (the integral of the molecular absorption coefficient, normalized emission spectrum, and the plasmon coupling factor) for understanding the wavelength dependence of PC-RET and to estimate the rate of PC-RET W. Indeed, W = (8.785 × 10 mol) ϕτJ̃, where ϕ is donor fluorescence quantum yield and τ is the emission lifetime. Simulations of the GSO for PC-RET show that the most important spectral region for PC-RET is not necessarily near the maximum overlap of donor emission and acceptor absorption. Instead a significant plasmonic contribution can involve a different spectral region from the extinction maximum of the plasmonic structure. This study opens a promising direction for exploring exciton transport in plasmonic nanostructures, with possible applications in spectroscopy, photonics, biosensing, and energy devices.

The production of molecular hydrogen by catalyzing water splitting is central to achieving the decarbonization of sustainable fuels and chemical transformations. In this work, a series of structure-making/breaking cations in the electrolyte were investigated as spectator cations in hydrogen evolution and oxidation reactions (HER/HOR) in the pH range of 1 to 14, whose kinetics was found to be altered by up to 2 orders of magnitude by these cations. The exchange current density of HER/HOR was shown to increase with greater structure-making tendency of cations in the order of Cs + < Rb + < K + < Na + < Li + , which was accompanied by decreasing reorganization energy from the Marcus–Hush–Chidsey formalism and increasing reaction entropy. Invoking the Born model of reorganization energy and reaction entropy, the static dielectric constant of the electrolyte at the electrified interface was found to be significantly lower than that of bulk, decreasing with the structure-making tendency of cations at the negatively charged Pt surface. The physical origin of cation-dependent HER/HOR kinetics can be rationalized by an increase in concentration of cations on the negatively charged Pt surface, altering the interfacial water structure and the H-bonding network, which is supported by classical molecular dynamics simulation and surface-enhanced infrared absorption spectroscopy. This work highlights immense opportunities to control the reaction rates by tuning interfacial structures of cation and solvents.

First and second dissociation constants (pKa values) of oxalic acid, malonic acid, and adipic acid were computed by using a number of theoretical protocols based on density functional theory and using both continuum solvation models and mixed discrete-continuum solvation models. We show that fully implicit solvation models (in which the entire solvent is represented by a dielectric continuum) fail badly for dicarboxylic acids with mean unsigned errors (averaged over six pKa values) of 2.4-9.0 log units, depending on the particular implicit model used. The use of water-solute clusters and accounting for multiple conformations in solution significantly improve the performance of both generalized Born solvation models and models that solve the nonhomogeneous dielectric Poisson equation for bulk electrostatics. The four most successful models have mean unsigned errors of only 0.6-0.8 log units.

This paper presents a new real-time electrodynamics approach for determining the rate of resonance energy transfer (RET) between two molecules in the presence of plasmonic or other nanostructures (inhomogeneous absorbing and dispersive media). In this approach to plasmon-coupled resonance energy transfer (PC-RET), we develop a classical electrodynamics expression for the energy transfer matrix element which is evaluated using the finite-difference time-domain (FDTD) method to solve Maxwell's equations for the electric field generated by the molecular donor and evaluated at the position of the molecular acceptor. We demonstrate that this approach yields RET rates in homogeneous media that are in precise agreement with analytical theory based on quantum electrodynamics (QED). In the presence of gold nanoparticles, our theory shows that the long-range decay of the RET rates can be significantly modified by plasmon excitation, with rates increased by as much as a factor of 10 leading to energy transfer rates over hundreds of nm that are comparable to that over tens of nm in the absence of the nanoparticles. These promising results suggest important future applications of the PC-RET in areas involving light harvesting or sensing, where energy transfer processes involving inhomogeneous absorbing and dispersive media are commonplace.

A mechanism for electronic rectification under low bias potentials is elucidated for the prototype molecule HS-phenyl-amide-phenyl-SH. We apply density functional theory (DFT) combined with the nonequilibrium Green's function formalism (NEGF), as implemented in the TranSIESTA computational code to calculate transport properties. We find that a single frontier orbital, the closest to the Fermi level, provides the dominant contribution to the overall transmission and determines the current. The asymmetric distribution of electron density in that orbital leads to rectification in charge transport due to its asymmetric response, shifting toward (or away from) the Fermi level under forward (or reverse) applied bias voltage. These findings provide a simple design principle to suppress recombination in molecular assemblies of dye-sensitized solar cells (DSSCs) where interfacial electron transfer is mediated by frontier orbitals with asymmetric character.

This paper describes how metal-organic frameworks (MOFs) conformally coated on plasmonic nanoparticle arrays can support excitonplasmon modes with features resembling strong coupling but that are better understood by a weak coupling model. Thin films of Znporphyrin MOFs were assembled by dip coating on arrays of silver nanoparticles (NP@MOF) that sustain surface lattice resonances (SLRs). Coupling of excitons with these lattice plasmons led to an SLR-like mixed mode in both transmission and transient absorption spectra. The spectral position of the mixed mode could be tailored by detuning the SLR in different refractive index environments and by changing the periodicity of the nanoparticle array. Photoluminescence showed mode splitting that can be interpreted as modulation of the exciton line shape by the Fano profile of the surface lattice mode, without requiring Rabi splitting. Compared with pristine Znporphyrin, hybrid NP@MOF structures achieved a 16-fold enhancement in emission intensity. Our results establish MOFs as a crystalline molecular emitter material that can couple with plasmonic structures for energy exchange and transfer. metal-organic framework | plasmonic nanoparticle arrays | conformal coating | surface lattice resonance | photoluminescence P lasmonic nanoparticles (NPs) are driving advances in nanophotonics (1, 2) and photochemistry (3, 4) because of their ability to concentrate and convert optical excitations into heat production, hot-electron generation, and light localization (5). Interactions between the localized surface plasmons (LSPs) (6) of isolated metallic NPs and excitons from molecular emitters can both enhance light absorption and emission (7) and engineer radiative and nonradiative decay pathways (8). Compared with LSPs, surface lattice resonances (SLRs), also referred to as lattice plasmons, are collective modes supported in arrays of NPs that exhibit stronger near-electric field enhancements and higher quality factors Q (9, 10). Because SLRs result from hybridization of LSPs on individual NPs with Bragg diffraction modes from the array (11, 12), their resonance wavelength can be tuned by changing local dielectric environment and lattice periodicity (13,14). Strong coupling between excitons and surface plasmons has been demonstrated by integrating emitters such as single-walled carbon nanotubes (15), monolayer MoS 2 (16), organic dyes (17)(18)(19), and light-harvesting complexes (20, 21) with plasmonic NP arrays.Plasmon-exciton coupling is sensitive to the location of emitters (22): those very close to the NPs are quenched (8), whereas the ones beyond the near-field region show little effect (23). Reports of coupling between organic dyes and plasmonic NP arrays have limited control over emitter locations, and hence, exciton contributions to the mixed SLR-exciton states cannot be tuned (17)(18)(19). Although strong exciton-SLR coupling can be obtained at high dye concentrations (24), such densities typically result in aggregation-induced photoluminescence quenching (25). Thus, the...

High performance dye-sensitized solar cells (DSSCs) rely upon molecular linkers that allow efficient electron transport from the photoexcited dye into the conduction band of the semiconductor host substrate. We have studied photoinduced electron injection efficiencies from modular assemblies of a Zn-porphyrin dye and a series of linker molecules which are axially bound to the Zn-porphyrin complex and covalently bound to TiO2 nanoparticles. Experimental measurements based on terahertz spectroscopy are compared to the calculated molecular conductance of the linker molecules. We find a linear relationship between measured electron injection efficiency and calculated single-molecule conductance of the linker employed. Since the same chromophore is used in all cases, variations in the absorptivities of the adsorbate complexes are quite small and cannot account for the large variations in observed injection efficiencies. These results suggest that the linker single-molecule conductance is a key factor that should be optimized for maximum electron injection efficiencies in DSSCs. In addition, our findings demonstrate for the first time the possibility of inferring values of single molecule conductance for bridging molecules at semiconductor interfaces by using time-resolved THz spectroscopy.