Influence of Substrates on the Long-Range Order of Photoelectrodeposited Se-Te Nanostructures.

The long-range order of anisotropic phototropic Se-Te films grown electrochemically at room temperature under uniform-intensity, polarized, incoherent, near-IR illumination has been investigated using crystalline (111)-oriented Si substrates doped degenerately with either p- or n-type dopants. Fourier-transform (FT) analysis was performed on large-area images obtained with a scanning electron microscope, and peak shapes in the FT spectra were used to determine the pattern fidelity in the deposited Se-Te films. Under nominally identical illumination conditions, phototropic films grown on p+-Si(111) exhibited a higher degree of anisotropy and a more well-defined pattern period than phototropic films grown on n+-Si(111). Similar differences in the phototropic Se-Te deposit morphology and pattern fidelity on p+-Si versus n+-Si were observed when the deposition rate and current densities were controlled for by adjusting the deposition parameters and illumination conditions. The doping-related effects of the Si substrate on the pattern fidelity of the phototropic Se-Te deposits are ascribable to an electrical effect produced by the different interfacial junction energetics between Se-Te and p+-Si versus n+-Si that influences the dynamic behavior during phototropic growth at the Se-Te/Si interface.

Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films.

Reactively sputtered nickel oxide (NiOx) films provide transparent, antireflective, electrically conductive, chemically stable coatings that also are highly active electrocatalysts for the oxidation of water to O2(g). These NiOx coatings provide protective layers on a variety of technologically important semiconducting photoanodes, including textured crystalline Si passivated by amorphous silicon, crystalline n-type cadmium telluride, and hydrogenated amorphous silicon. Under anodic operation in 1.0 M aqueous potassium hydroxide (pH 14) in the presence of simulated sunlight, the NiOx films stabilized all of these self-passivating, high-efficiency semiconducting photoelectrodes for >100 h of sustained, quantitative solar-driven oxidation of water to O2(g).

Gastight Hydrodynamic Electrochemistry: Design for a Hermetically Sealed Rotating Disk Electrode Cell.

Rotating disk electrodes (RDEs) are widely used in electrochemical characterization to analyze the mechanisms of various electrocatalytic reactions. RDE experiments often make use of or require collection and quantification of gaseous products. The combination of rotating parts and gaseous analytes makes the design of RDE cells that allow for headspace analysis challenging due to gas leaks at the interface of the cell body and the rotator. In this manuscript we describe a new, hermetically sealed electrochemical cell that allows for electrode rotation while simultaneously providing a gastight environment. Electrode rotation in this new cell design is controlled by magnetically coupling the working electrode to a rotating magnetic driver. Calibration of the RDE using a tachometer shows that the rotation speed of the electrode is the same as that of the magnetic driver. To validate the performance of this cell for hydrodynamic measurements, limiting currents from the reduction of a potassium ferrocyanide (K4[Fe(CN)6]·3H2O) were measured and shown to compare favorably with calculated values from the Levich equation and with data obtained using more typical, nongastight RDE cells. Faradaic efficiencies of ∼95% were measured in the gas phase for oxygen evolution in alkaline media at an Inconel 625 alloy electrocatalyst during rotation at 1600 rpm. These data verify that a gastight environment is maintained even during rotation.

Well-defined vanadium organoazide complexes and their conversion to terminal vanadium imides: structural snapshots and evidence for a nitrene capture mechanism.

We report the synthesis and structural characterization of a family of well-defined organoazide complexes supported by a three-fold-symmetric pyrrolide scaffold and their conversion to the corresponding terminal imido congeners. Kinetic measurements on a series of structurally homologous but electronically distinct vanadium organoazide complexes reveal that the azido-to-imido transformations proceed via a process that is first-order in the metal complex and has a positive entropy of activation. Further studies suggest that these reactions may involve metal-mediated generation and capture of nitrene fragments.

A structurally characterized nitrous oxide complex of vanadium.

Nitrous oxide (N(2)O), a widespread greenhouse gas, is a thermodynamically potent and environmentally green oxidant that is an attractive target for activation by metal centers. However, N(2)O remains underutilized owing to its high kinetic stability, and the poor ligand properties of this molecule have made well-characterized metal-N(2)O complexes a rarity. We now report a vanadium-pyrrolide system that reversibly binds N(2)O at room temperature and provide the first single-crystal X-ray structure of such a complex. Further characterization by vibrational spectroscopy and DFT calculations strongly favor assignment as a linear, N-bound metal-N(2)O complex.

Enhancing the activity of oxygen-evolution and chlorine-evolution electrocatalysts by atomic layer deposition of TiO2.

We report that TiO2 coatings formed via atomic layer deposition (ALD) may tune the activity of IrO2, RuO2, and FTO for the oxygen-evolution and chlorine-evolution reactions (OER and CER). Electrocatalysts exposed to ~3-30 ALD cycles of TiO2 exhibited overpotentials at 10 mA cm-2 of geometric current density that were several hundred millivolts lower than uncoated catalysts, with correspondingly higher specific activities. For example, the deposition of TiO2 onto IrO2 yielded a 9-fold increase in the OER-specific activity in 1.0 M H2SO4 (0.1 to 0.9 mA cmECSA -2 at 350 mV overpotential). The oxidation state of titanium and the potential of zero charge were also a function of the number of ALD cycles, indicating a correlation between oxidation state, potential of zero charge, and activity of the tuned electrocatalysts.

The influence of structure and processing on the behavior of TiO2 protective layers for stabilization of n-Si/TiO2/Ni photoanodes for water oxidation.

Light absorbers with moderate band gaps (1-2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aqueous solution. Amorphous TiO2 deposited by atomic-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidation, with the TiO2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO2 in reducing or oxidizing environments. All of the polycrystalline TiO2 films with compact grain boundaries investigated herein protected the n-Si photoanodes against photocorrosion in pH = 14 KOH(aq). Hence, in these devices, conduction through the TiO2 layer is neither specific to a particular amorphous or crystalline structure nor determined wholly by a particular extrinsic dopant impurity. The coupled structural and energetic properties of TiO2, and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance.