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

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 cm_(ECSA)⁻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

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

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 (K_4[Fe(CN)_6] •3H_2O) were measured and shown to compare favorably with calculated values from the Levich equation and with data obtained using more typical, non-gastight 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

The Influence of Structure and Processing on the Behavior of TiO_2 Protective Layers for Stabilization of n-Si/TiO_2/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 TiO_2 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 TiO_2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO_2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO_2 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/TiO_2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO_2 in reducing or oxidizing environments. All of the polycrystalline TiO_2 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 TiO_2 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 TiO_2, and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance

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 cm_(ECSA)⁻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

An Electrochemical, Microtopographical and Ambient Pressure X-Ray Photoelectron Spectroscopic Investigation of Si/TiO_2/Ni/Electrolyte Interfaces

The electrical and spectroscopic properties of the TiO_2/Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Specifically, the energy-band relations of the p+-Si/ALD-TiO_2/Ni interface have been determined for a selected range of Ni thicknesses. AP-XPS measurements using tender X-rays were performed in a three-electrode electrochemical arrangement under potentiostatic control to obtain information from the semiconductor near-surface region, the electrochemical double layer (ECDL) and the electrolyte beyond the ECDL. The degree of conductivity depended on the chemical state of the Ni on the TiO2surface. At low loadings of Ni, the Ni was present primarily as an oxide layer and the samples were not conductive, although the TiO_2 XPS core levels nonetheless displayed behavior indicative of a metal-electrolyte junction. In contrast, as the Ni thickness increased, the Ni phase was primarily metallic and the electrochemical behavior became highly conductive, with the AP-XPS data indicative of a metal-electrolyte junction. Electrochemical and microtopographical methods have been employed to better define the nature of the TiO_2/Ni electrodes and to contextualize the AP-XPS results

Electrical, Photoelectrochemical, and Photoelectron Spectroscopic Investigation of the Interfacial Transport and Energetics of Amorphous TiO_2/Si Heterojunctions

Solid-state electrical, photoelectrochemical, and photoelectron spectroscopic techniques have been used to characterize the behavior and electronic structure of interfaces between n-Si, n^+-Si, or p^+-Si surfaces and amorphous coatings of TiO_2 formed using atomic-layer deposition. Photoelectrochemical measurements of n-Si/TiO_2/Ni interfaces in contact with a series of one-electron, electrochemically reversible redox systems indicated that the n-Si/TiO_2/Ni structure acted as a buried junction whose photovoltage was independent of the formal potential of the contacting electrolyte. Solid-state current–voltage analysis indicated that the built-in voltage of the n-Si/TiO_2 heterojunction was ∼0.7 V, with an effective Richardson constant ∼1/100th of the value of typical Si/metal Schottky barriers. X-ray photoelectron spectroscopic data allowed formulation of energy band-diagrams for the n-Si/TiO_2, n^+-Si/TiO_2, and p^+-Si/TiO_2 interfaces. The XPS data were consistent with the rectifying behavior observed for amorphous TiO_2 interfaces with n-Si and n^+-Si surfaces and with an ohmic contact at the interface between amorphous TiO_2 and p^+-Si

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 (K_4[Fe(CN)_6] •3H_2O) were measured and shown to compare favorably with calculated values from the Levich equation and with data obtained using more typical, non-gastight 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

Direct observation of the energetics at a semiconductor/liquid junction by operando X-ray photoelectron spectroscopy

Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces provide a method of converting solar energy to electricity or fuels. Currently, the understanding of semiconductor/liquid interfaces is inferred from experiments and models. Operando ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has been used herein to directly characterize the semiconductor/liquid junction at room temperature under real-time electrochemical control. X-ray synchrotron radiation in conjunction with AP-XPS has enabled simultaneous monitoring of the solid surface, the solid/electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, U. The observed shifts in binding energy with respect to the applied potential have directly revealed ohmic and rectifying junction behavior on metallized and semiconducting samples, respectively. Additionally, the non-linear response of the core level binding energies to changes in the applied electrode potential has revealed the influence of defect-derived electronic states on the Galvani potential across the complete cell