Effects of bubbles on the electrochemical behavior of hydrogen-evolving Si microwire arrays oriented against gravity

The size-distribution, coverage, electrochemical impedance, and mass-transport properties of H₂ gas-bubble films were measured for both planar and microwire-array platinized n⁺-Si cathodes performing the hydrogen-evolution reaction in 0.50 M H₂SO₄ (aq). Inverted, planar n⁺-Si/Ti/Pt cathodes produced large, stationary bubbles which contributed to substantial increases in ohmic potential drops. In contrast, regardless of orientation, microwire array n⁺-Si/Ti/Pt cathodes exhibited a smaller layer of bubbles on the surface, and the formation of bubbles did not substantially increase the steady-state overpotential for H₂ (g) production. Experiments using an electroactive tracer species indicated that even when oriented against gravity, bubbles enhanced mass transport at the electrode surface. Microconvection due to growing and coalescing bubbles dominated effects due to macroconvection of gliding bubbles on Si microwire array cathodes. Electrodes that maintained a large number of small bubbles on the surface simultaneously exhibited low concentrations of dissolved hydrogen and small ohmic potential drops, thus exhibiting the lowest steady-state overpotentials. The results indicate that microstructured electrodes can operate acceptably for unassisted solar-driven water splitting in the absence of external convection and can function regardless of the orientation of the electrode with respect to the gravitational force vector

Light-directed electrochemical patterning of copper structures

A method creating a patterned film with cuprous oxide and light comprising the steps of electrodepositing copper from a solution onto a substrate; illuminating selected areas of said deposited copper with light having photon energies above the band gap energy of 2.0 eV to create selected illuminated sections and non-illuminated sections; and stripping non-illuminated sections leaving said illuminated sections on the substrate. An additional step may include galvanically replacing the copper with one or more noble metals

Dynamics of confined water reconstructed from inelastic x-ray scattering measurements of bulk response functions

Nanoconfined water and surface-structured water impacts a broad range of fields. For water confined between hydrophilic surfaces, measurements and simulations have shown conflicting results ranging from “liquidlike” to “solidlike” behavior, from bulklike water viscosity to viscosity orders of magnitude higher. Here, we investigate how a homogeneous fluid behaves under nanoconfinement using its bulk response function: The Green's function of water extracted from a library of S(q,ω) inelastic x-ray scattering data is used to make femtosecond movies of nanoconfined water. Between two confining surfaces, the structure undergoes drastic changes as a function of surface separation. For surface separations of ≈9 Å, although the surface-associated hydration layers are highly deformed, they are separated by a layer of bulklike water. For separations of ≈6 Å, the two surface-associated hydration layers are forced to reconstruct into a single layer that modulates between localized “frozen’ and delocalized “melted” structures due to interference of density fields. These results potentially reconcile recent conflicting experiments. Importantly, we find a different delocalized wetting regime for nanoconfined water between surfaces with high spatial frequency charge densities, where water is organized into delocalized hydration layers instead of localized hydration shells, and are strongly resistant to `freezing' down to molecular distances (<6 Å)

Light-directed electrochemical patterning of copper structures

A method creating a patterned film with cuprous oxide and light comprising the steps of electrodepositing copper from a solution onto a substrate; illuminating selected areas of said deposited copper with light having photon energies above the band gap energy of 2.0 eV to create selected illuminated sections and non-illuminated sections; and stripping non-illuminated sections leaving said illuminated sections on the substrate. An additional step may include galvanically replacing the copper with one or more noble metals

Effects of bubbles on the electrochemical behavior of hydrogen-evolving Si microwire arrays oriented against gravity

The size-distribution, coverage, electrochemical impedance, and mass-transport properties of H₂ gas-bubble films were measured for both planar and microwire-array platinized n⁺-Si cathodes performing the hydrogen-evolution reaction in 0.50 M H₂SO₄ (aq). Inverted, planar n⁺-Si/Ti/Pt cathodes produced large, stationary bubbles which contributed to substantial increases in ohmic potential drops. In contrast, regardless of orientation, microwire array n⁺-Si/Ti/Pt cathodes exhibited a smaller layer of bubbles on the surface, and the formation of bubbles did not substantially increase the steady-state overpotential for H₂ (g) production. Experiments using an electroactive tracer species indicated that even when oriented against gravity, bubbles enhanced mass transport at the electrode surface. Microconvection due to growing and coalescing bubbles dominated effects due to macroconvection of gliding bubbles on Si microwire array cathodes. Electrodes that maintained a large number of small bubbles on the surface simultaneously exhibited low concentrations of dissolved hydrogen and small ohmic potential drops, thus exhibiting the lowest steady-state overpotentials. The results indicate that microstructured electrodes can operate acceptably for unassisted solar-driven water splitting in the absence of external convection and can function regardless of the orientation of the electrode with respect to the gravitational force vector

Hierarchically nanostructured films and applications thereof

In one aspect, nanostructured films are described herein comprising controlled architectures on multiple length scales (e.g. .gtoreq.3). As described further herein, the ability to control film properties on multiple length scales enables tailoring structures of the films to specific applications including, but not limited to, optoelectronic, catalytic and photoelectrochemical cell applications. In some embodiments, a nanostructured film comprises a porous inorganic scaffold comprising particles of an electrically insulating inorganic oxide. An electrically conductive metal oxide coating is adhered to the porous inorganic scaffold, wherein the conductive metal oxide coating binds adjacent particles of the insulating inorganic oxide

Crystallographic refinement of collective excitations using standing wave inelastic x-ray scattering

We propose a method for realizing true, real-space imaging of charge dynamics in a periodic system, with angstrom spatial resolution and attosecond time resolution. In this method, inelastic x-ray scattering (IXS) is carried out with a coherent, standing wave source, which provides the off-diagonal elements of the generalized dynamic structure factor, S(q_1,q_2,\omega), allowing complete reconstruction of the inhomogeneous response function of the system, \chi(x_1,x_2,t). The quantity \chi has the physical meaning of a propagator for charge, so allows one to observe - in real time - the disturbance in the electron density created by a point source placed at a specified location, x_1 (on an atom vs. between atoms, for example). This method may be thought of as a generalization of x-ray crystallography that allows refinement of the excited states of a periodic system, rather than just its ground state.Comment: 28 pages, 8 figures, submitted to Chemical Physic

Photoelectrochemical Behavior of Hierarchically Structured Si/WO_3 Core–Shell Tandem Photoanodes

WO_3 thin films have been deposited in a hierarchically structured core–shell morphology, with the cores consisting of an array of Si microwires and the shells consisting of a controlled morphology WO_3 layer. Porosity was introduced into the WO_3 outer shell by using a self-assembled microsphere colloidal crystal as a mask during the deposition of the WO_3 shell. Compared to conformal, unstructured WO_3 shells on Si microwires, the hierarchically structured core–shell photoanodes exhibited enhanced near-visible spectral response behavior, due to increased light absorption and reduced distances over which photogenerated carriers were collected. The use of structured substrates also improved the growth rate of microsphere-based colloidal crystals and suggests strategies for the use of colloidal materials in large-scale applications