Primary Corrosion Processes for Polymer-Embedded Free-Standing or Substrate-Supported Silicon Microwire Arrays in Aqueous Alkaline Electrolytes.

Solar fuel devices have shown promise as a sustainable source of chemical fuels. However, long-term stability of light absorbing materials remains a substantial barrier to practical devices. Herein, multiple corrosion pathways in 1 M KOH(aq) have been defined for TiO2-protected Si microwire arrays in a polymer membrane either attached to a substrate or free-standing. Top-down corrosion was observed in both morphologies through defects in the TiO2 coating. For the substrate-based samples, bottom-up corrosion was observed through the substrate and up the adjacent wires. In the free-standing samples, uniform bottom-up corrosion was observed through the membrane with all wire material corroded within 10 days of immersion in the dark in 1 M KOH(aq).

Genesis and Propagation of Fractal Structures During Photoelectrochemical Etching of n-Silicon.

The genesis, propagation, and dimensions of fractal-etch patterns that form anodically on front- or back-illuminated n-Si(100) photoelectrodes in contact with 11.9 M NH4F (aqueous) have been investigated during either a linear potential sweep or a constant potential hold (E = +6.0 V versus Ag/AgCl). Optical images collected in situ during electrochemical experiments revealed the location and underlying mechanism of initiation and propagation of the structures on the surface. X-ray photoelectron spectroscopic (XPS) data collected for samples emersed from the electrolyte at varied times provided detailed information about the chemistry of the surface during fractal etching. The fractal structure was strongly influenced by the orientation of the crystalline Si sample. The etch patterns were initially generated at points along the circumference of bubbles that formed upon immersion of n-Si(100) samples in the electrolyte, most likely due to the electrochemical and electronic isolation of areas beneath bubbles. XPS data showed the presence of a tensile-stressed silicon surface throughout the etching process as well as the presence of SiOxFy on the surface. The two-dimensional fractal dimension, Df,2D, of the patterns increased with etching time to a maximum observed value of Df,2D = 1.82. Promotion of fractal etching near etch masks that electrochemically and electronically isolated areas of the photoelectrode surface enabled the selective placement of highly branched structures at desired locations on an electrode surface.

Spontaneous Formation of >90% Optically Transmissive, Electrochemically Active CoP Films for Photoelectrochemical Hydrogen Evolution.

Earth-abundant catalysts for the hydrogen-evolution reaction require increased mass loadings, relative to Pt films, to achieve comparable activity and stability in acidic electrolytes. We report herein that spontaneous nanostructuring of opaque, electrodeposited CoP films, 40-120 nm in thickness, leads to transparent electrocatalyst films that exhibit up to 90% optical transmission in the visible spectrum. The photocurrent density under simulated sunlight at a representative n+p-Si(100)/CoP photocathode increases by 200% after exposure to 0.50 M H2SO4(aq) and remains stable for 12 h of continuous operation. Atomic force microscopy and scanning electron microscopy of the film before and after exposure to 0.50 M H2SO4(aq) validate an optical model for transparent CoP films as probed with spectroscopic ellipsometry.

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.

Nanoelectrical and Nanoelectrochemical Imaging of Pt/p-Si and Pt/p+ -Si Electrodes.

The interfacial properties of electrolessly deposited Pt nanoparticles (Pt-NPs) on p-Si and p+ -Si electrodes were investigated on the nanometer scale using a combination of scanning probe methods. Atomic force microscopy (AFM) showed highly dispersed Pt-NPs with diameters of 20-150 nm on the Si surface. Conductive AFM measurements showed that only approximately half of the particles exhibited measurable contact currents, with a factor of 103 difference in current observed between particles at a given bias. Local current-voltage measurements revealed a rectifying junction with a resistance ≥10 MΩ at the Pt-NP/p-Si interface, whereas the Pt-NP/p+ -Si samples formed an ohmic junction with a local resistance ≥1 MΩ. The particles were strongly attached to the sample surface in air. However, in an electrolyte, the adhesion of the particles to the surface was substantially lower, and most of the particles had tip-contact currents that varied by a factor of approximately 10. Scanning electrochemical microscopy (SECM) showed smaller but more uniform electrochemical currents for the particles relative to the currents observed by conductive AFM. In accord with the conductive AFM measurements, the SECM measurements showed conductance through the substrate for only a minority of the particles. These results suggest that the electrochemical performance of the electrolessly deposited Pt nanoparticles on Si can be ascribed to: 1) The high resistance of the contact between the particles and the substrate, 2) the low (<50 %) fraction of particles that support high currents, and 3) the low adhesion of the particles to the surface when in contact with the electrolyte.

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).

Scanning tunneling microscopy studies of monolayer templates: alkylthioethers and alkylethers.

Scanning tunneling microscopy has been used to determine the molecular ordering in stable, ordered monolayers formed from long-chain normal and substituted alkanes in solution on highly oriented pyrolytic graphite surfaces. Monolayers were initially formed using an overlying solution of either a symmetrical dialkylthioether or a symmetrical dialkylether. Initially pure thioether solutions were then changed to nearly pure solutions of the identical chain-length ether, and vice versa. The direct application of a pure solution of long-chain symmetrical ethers onto graphite produced a lamellate monolayer within which the individual molecular axes were oriented at an angle of approximately 65 degrees to the lamellar axes. In contrast, a pure solution of long-chain symmetrical thioethers on graphite produced a monolayer within which the molecular axes were oriented perpendicular to the lamellar axes. When ethers were gradually added to solutions overlying pure thioether monolayers, the ethers substituted into the existing monolayer structure. Thus, the ether molecules could be forced to orient in the perpendicular thioether-like manner through the use of a thioether template monolayer. Continued addition of ethers to the solution ultimately produced a nearly pure ether monolayer that retained the orientation of the thioether monolayer template. However, a monolayer of thioether molecules formed by gradual substitution into an ether monolayer did not retain the 65 degrees orientation typical of dialkylethers, but exhibited the 90 degrees orientation typical of dialkylthioether monolayers. The thioethers and ethers were easily distinguished in images of mixed monolayers, allowing both an analysis of the distribution of the molecules within the mixed monolayers and a comparison of the monolayer compositions with those of the overlying solutions. Substitution of molecules into the template monolayer did not proceed randomly; instead, a molecule within a monolayer was more likely to be replaced by a molecule in the overlying solution if it was located next to a molecule that had already been replaced.

Use of alkane monolayer templates to modify the structure of alkyl ether monolayers on highly ordered pyrolytic graphite.

Scanning tunneling microscopy (STM) has been used to investigate the structure of pure and mixed monolayers formed by adsorption of long-chain alkanes and/or ethers on highly ordered pyrolytic graphite. Application of a pure phenyloctane solution of simple alkanes, such as tritriacontane, CH3(CH2)31CH3, produced a monolayer within which the individual molecular axes were oriented perpendicular to the lamellar axes. In contrast, a pure solution of symmetrical long-chain ethers, such as di-n-hexadecyl ether, CH3(CH2)15O(CH2)15CH3, produced a monolayer within which the molecular axes were oriented at an angle of approximately 65 degrees relative to the lamellar axes. The compositions of the overlying solutions were then gradually changed either from pure alkanes to nearly pure ethers or from pure ethers to nearly pure alkanes. When ethers replaced alkanes in the monolayer, the ethers conformed to the orientation within the existing alkane layer, rather than adopting the characteristic orientation of pure ether monolayers. However, when alkanes were incorporated into monolayers that had been formed from pure ether solutions, the orientation of the molecules within the monolayer converted to that characteristic of pure alkanes. Alkane monolayers thus acted as templates for subsequent ether layers, but ether monolayers did not act as templates for alkane layers.