The one-electron reduction of [CpRu(bpy)NCCH3]PF6 (Cp = cyclopentadienyl; bpy = 2,2′-bipyridine), abbreviated as [Ru-S]+, where S = CH3CN, in CO2-saturated acetonitrile initiates a cascade of rapid electrochemical and chemical steps (ECEC) at an electrode potential of ca. 100 mV positive of the first reduction of the ruthenium complex. The overall two-electron process leads to the generation of a CO-bound ruthenium complex, [Ru-CO]+, and carbonate, as independently confirmed by NMR spectroscopy. Simulations of the cyclic voltammograms using DigiElch together with density functional theory based calculations reveal that the singly reduced ruthenium complex [Ru-S]0 binds CO2 at a rate of ca. 105 M–1 s–1 at almost zero driving force. Subsequent to CO2 binding, all of the steps leading up to deoxygenation are highly exergonic and rapid. A model of the potential energy profile of the CO2 approach to the Ru center in the singly reduced manifold reveals a direct correlation between the reactivity toward CO2 and the nucleophilicity at the metal center influenced by different ligand environments. Through the binding of CO2 after the first reduction, overpotentials associated with consecutive electrochemical reductions are avoided. This work therefore provides an important design principle for engineering transition-metal complexes to activate CO2 under low driving forces.

A series of square-planar nickel hydride complexes supported by bis(phosphinite) pincer ligands with varying substituents (–OMe, –Me, and –But) on the pincer backbone have been synthesized and completely characterized by NMR spectroscopy, IR spectroscopy, elemental analysis, and X-ray crystallography. Their cyclic voltammograms show irreversible oxidation peaks (peak potentials from 101 to 316 mV vs. Fc+/Fc) with peak currents consistent with overall one-electron oxidations. Chemical oxidation by the one-electron oxidant Ce(NBu4)2(NO3)6 was studied by NMR spectroscopy, which provided quantitative evidence for post-oxidative H2 evolution leading to a solvent-coordinated nickel(II) species with the pincer backbone intact. Bulk electrolysis of the unsubstituted nickel hydride (3a) showed an overall one-electron stoichiometry and gas chromatographic analysis of the headspace gas after electrolysis further confirmed stoichiometric production of dihydrogen. Due to the extremely high rate of the post-oxidative chemical process, electrochemical simulations have been used to establish a lower limit of the bimolecular rate constant (kf > 107 M−1 s−1) for the H2 evolution step. To the best of our knowledge, this is the fastest known oxidative H2 evolution process observed in transition metal hydrides. Quantum chemical calculations based on DFT indicate that the one-electron oxidation of the nickel hydride complex provides a strong chemical driving force (−90.3 kcal mol−1) for the production of H2 at highly oxidizing potentials.

We synthesized nanoscale TiO2–RuO2 alloys by atomic layer deposition (ALD) that possess a high work function and are highly conductive. As such, they function as good Schottky contacts to extract photogenerated holes from n-type silicon while simultaneously interfacing with water oxidation catalysts. The ratio of TiO2 to RuO2 can be precisely controlled by the number of ALD cycles for each precursor. Increasing the composition above 16% Ru sets the electronic conductivity and the metal work function. No significant Ohmic loss for hole transport is measured as film thickness increases from 3 to 45 nm for alloy compositions ≥ 16% Ru. Silicon photoanodes with a 2 nm SiO2 layer that are coated by these alloy Schottky contacts having compositions in the range of 13–46% Ru exhibit average photovoltages of 525 mV, with a maximum photovoltage of 570 mV achieved. Depositing TiO2–RuO2 alloys on nSi sets a high effective work function for the Schottky junction with the semiconductor substrate, thus generating a large photovoltage that is isolated from the properties of an overlying oxygen evolution catalyst or protection layer.Keywords: atomic layer deposition; MIS junctions; photoanodes; photovoltage; Schottky junctions; TiO2 alloys

The ruthenium hydride [RuH(CNN)(dppb)] (1; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) reacts rapidly and irreversibly with CO2 under ambient conditions to yield the corresponding Ru formate complex 2. In contrast, the Ru hydride 1 reacts with acetone reversibly to generate the Ru isopropoxide, with the reaction free energy ΔG°298 K = −3.1 kcal/mol measured by 1H NMR in tetrahydrofuran-d8. Density functional theory (DFT), calibrated to the experimentally measured free energies of ketone insertion, was used to evaluate and compare the mechanism and energetics of insertion of acetone and CO2 into the Ru–hydride bond of 1. The calculated reaction coordinate for acetone insertion involves a stepwise outer-sphere dihydrogen transfer to acetone via hydride transfer from the metal and proton transfer from the N–H group on the CNN ligand. In contrast, the lowest energy pathway calculated for CO2 insertion proceeds by an initial Ru–H hydride transfer to CO2 followed by rotation of the resulting N–H-stabilized formate to a Ru–O-bound formate. DFT calculations were used to evaluate the influence of the ancillary ligands on the thermodynamics of CO2 insertion, revealing that increasing the π acidity of the ligand cis to the hydride ligand and increasing the σ basicity of the ligand trans to it decreases the free energy of CO2 insertion, providing a strategy for the design of metal hydride systems capable of reversible, ergoneutral interconversion of CO2 and formate.

A CuI complex of 3-ethynyl-phenanthroline covalently immobilized onto an azide-modified glassy carbon surface is an active electrocatalyst for the four-electron (4-e) reduction of O2 to H2O. The rate of O2 reduction is second-order in Cu coverage at moderate overpotential, suggesting that two CuI species are necessary for efficient 4-e reduction of O2. Mechanisms for O2 reduction are proposed that are consistent with the observations for this covalently immobilized system and previously reported results for a similar physisorbed CuI system.

A convenient, laboratory-scale method for the vapor deposition of dense siloxane monolayers onto oxide substrates was demonstrated. This method was studied and optimized at 110 °C under reduced pressure with the vapor of tetradecyltris(deuteromethoxy)silane, (CD3O)3Si(CH2)13CH3, and water from the dehydration of MgSO4·7H2O. Ellipsometric thicknesses, water contact angles, Fourier transform infrared (FTIR) spectroscopy, and electrochemical capacitance measurements were used to probe monolayer densification. The CD3 stretching mode in the FTIR spectrum was monitored as a function of the deposition time and amounts of silane and water reactants. This method probed the unhydrolyzed methoxy groups on adsorbed silanes. Excess silane and water were necessary to achieve dense, completely hydrolyzed monolayers. In the presence of sufficient silane, an excess of water above the calculated stoichiometric amount was necessary to hydrolyze all methoxy groups and achieve dense monolayers. The excess water was partially attributed to the reversibility of the hydrolysis of the methoxy groups.

The close proximity of two individually addressable electrodes in an interdigitated array provides a unique platform for electrochemical study of multicatalytic processes. Here, we report a “plug-and-play” approach to control the underlying self-assembled monolayer and the electroactive species on each individually addressable electrode of an interdigitated array. The method presented here uses selective anodic desorption of a monolayer from one of the individually addressable electrodes and rapid formation of a different self-assembled monolayer on the freshly cleaned electrode. We illustrate this strategy by introducing variations in the length of the linker to the electroactive species in the self-assembled monolayer, which determines the rate of electron transfer. In order to separate the assembly of the monolayer from the choice of the electroactive species, we use CuI-catalyzed triazole formation (“click” chemistry) to covalently attach an acetylene-terminated electroactive species to an azide-terminated thiol monolayer selectively on each electrode. The resulting variations in the electron-transfer rate to surface-attached ferrocene and in the rate of catalytic oxidation of ascorbate by the ferrocenium/ferrocene couple demonstrate an application of this approach.

We report the selective removal of gold from the tips of germanium nanowires (GeNWs) grown by chemical vapor deposition on gold nanoparticles (AuNPs). Selective removal was accomplished by aqueous hydrochloric acid solutions containing either potassium triiodide or iodine. Measurement of the residual number of gold atoms on the GeNW samples using inductively coupled plasma−mass spectrometry shows that 99% of the gold was removed. Photoemission spectroscopy shows that the germanium surfaces of these samples were not further oxidized after treatment with these liquid etchants. Auger electron spectroscopy shows that AuNPs that did not yield GeNWs contain germanium and also that the addition of gaseous HCl to GeH4 during GeNW growth increased the selectivity of germanium deposition to the AuNPs.

A series of square-planar nickel hydride complexes supported by bis(phosphinite) pincer ligands with varying substituents (–OMe, –Me, and –But) on the pincer backbone have been synthesized and completely characterized by NMR spectroscopy, IR spectroscopy, elemental analysis, and X-ray crystallography. Their cyclic voltammograms show irreversible oxidation peaks (peak potentials from 101 to 316 mV vs. Fc+/Fc) with peak currents consistent with overall one-electron oxidations. Chemical oxidation by the one-electron oxidant Ce(NBu4)2(NO3)6 was studied by NMR spectroscopy, which provided quantitative evidence for post-oxidative H2 evolution leading to a solvent-coordinated nickel(II) species with the pincer backbone intact. Bulk electrolysis of the unsubstituted nickel hydride (3a) showed an overall one-electron stoichiometry and gas chromatographic analysis of the headspace gas after electrolysis further confirmed stoichiometric production of dihydrogen. Due to the extremely high rate of the post-oxidative chemical process, electrochemical simulations have been used to establish a lower limit of the bimolecular rate constant (kf > 107 M−1 s−1) for the H2 evolution step. To the best of our knowledge, this is the fastest known oxidative H2 evolution process observed in transition metal hydrides. Quantum chemical calculations based on DFT indicate that the one-electron oxidation of the nickel hydride complex provides a strong chemical driving force (−90.3 kcal mol−1) for the production of H2 at highly oxidizing potentials.