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.

Interfacial electric fields play crucial roles in electrochemistry, catalysis, and solar energy conversion. Understanding of the interfacial electric field effects has been hindered by the lack of a direct spectroscopic method to probe of the interfacial field at the molecular level. Here, we report the characterization of the field and interfacial structure at Au/diisocyanide/aqueous electrolyte interfaces, using a combination of in situ electrochemical vibrational sum frequency generation (SFG) spectroscopy, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. For 1,4-phenylene diisocyanide (PDI), 4,4′-biphenyl diisocyanide (BPDI), and 4,4″-p-terphenyl diisocyanide (TPDI), our results reveal that the frequency of the gold-bound NC stretch mode of the diisocyanide self-assembled monolayer (SAM) increases linearly with the applied potential, suggesting that SFG can be an in situ probe of the strength of the electric field at electrode/electrolyte interfaces. Using DFT-computed Stark tuning rates of model complexes, the electric field strength at the metal/SAM/electrolyte interfaces is estimated to be 108–109 V/m. The linear dependence of the vibrational frequency (and field) with applied potential is consistent with an electrochemical double-layer structure that consists of a Helmholtz layer in contact with a diffused layer. The Helmholtz layer thickness is approximately the same as the molecular length for PDI, suggesting a well-ordered SAM with negligible electrolyte penetration. For BPDI and TPDI, we found that the Helmholtz layer is thinner than the monolayer of molecular adsorbates, indicating that the electrolyte percolates into the SAM, as shown by molecular dynamics simulations of the Au/PDI/electrolyte interface. The reported analysis demonstrates that a combination of in situ SFG probes and computational modeling provides a powerful approach to elucidate the structure of electrochemical interfaces at the detailed molecular level.

The bond dissociation energies of a set of 44 3d transition metal-containing diatomics are computed with phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) utilizing a correlated sampling technique. We investigate molecules with H, N, O, F, Cl, and S ligands, including those in the 3dMLBE20 database first compiled by Truhlar and co-workers with calculated and experimental values that have since been revised by various groups. In order to make a direct comparison of the accuracy of our ph-AFQMC calculations with previously published results from 10 DFT functionals, CCSD(T), and icMR-CCSD(T), we establish an objective selection protocol which utilizes the most recent experimental results except for a few cases with well-specified discrepancies. With 1 arXiv:1901.09464v1 [physics.chem-ph] 27 Jan 2019 the remaining set of 41 molecules, we find that ph-AFQMC gives robust agreement with experiment superior to that of all other methods, with a mean absolute error (MAE) of 1.4(4) kcal/mol and maximum error of 3(3) kcal/mol (parenthesis account for reported experimental uncertainties and the statistical errors of our ph-AFQMC calculations).In comparison, CCSD(T) and B97, the best performing DFT functional considered here, have MAEs of 2.8 and 3.7 kcal/mol, respectively, and maximum errors in excess of 17 kcal/mol for both methods. While a larger and more diverse data set would be required to demonstrate that ph-AFQMC is truly a benchmark method for transition metal systems, our results indicate that the method has tremendous potential, exhibiting unprecedented consistency and accuracy compared to other approximate quantum chemical approaches.

Attaching molecular catalysts to metal and semiconductor electrodes is a promising approach to developing new catalytic electrodes with combined advantages of molecular and heterogeneous catalysts. However, the effect of the interfacial electric field on the stability, activity, and selectivity of the catalysts is often poorly understood due to the complexity of interfaces. In this work, we examine the strength of the interfacial field at the binding site of CO 2 reduction catalysts including Re(S-2,2′-bipyridine)(CO) 3 Cl and Mn(S-2,2′-bipyridine)(CO) 3 Br immobilized on Au electrodes. The vibrational spectra are probed by sum frequency generation spectroscopy (SFG), showing pronounced potential-dependent frequency shifts of the carbonyl stretching modes. Calculations of SFG spectra and Stark tuning rates based on density functional theory allow for direct interpretation of the configurations of the catalysts bound to the surfaces and the influence of the interfacial electric field. We find that electrocatalysts supported on Au electrodes have tilt angles of about 65−75°relative to the surface normal with one of the carbonyl ligands in direct contact with the surface. Large interfacial electric fields of 10 8 −10 9 V/m are determined through the analysis of experimental frequency shifts and theoretical Stark tuning rates of the symmetric CO stretching mode. These large electric fields thus significantly influence the CO 2 binding site.

A pentamethylcyclopentadienyl (Cp*) iridium water-oxidation precatalyst was modified to include a silatrane functional group for covalent attachment to metal oxide semiconductor surfaces. The heterogenized catalyst was found to perform electrochemically driven water oxidation at an overpotential of 462 mV with a turnover number of 304 and turnover frequency of 0.035 s–1 in a 0.1 M KNO3 electrolyte at pH 5.8. Computational modeling of the experimental IR spectra suggests that the catalyst retains its Cp* group during the first hour of catalysis and likely remains monomeric.