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Parallel arrays of either Au or Pd nanowires were fabricated on glass substrates via the electrochemical process of lithographically patterned nanowire electrodeposition (LPNE) and then characterized with scanning electron microscopy (SEM) and a series of optical diffraction measurements at 633 nm. Nanowires with widths varying from 25 to 150 nm were electrodeposited onto nanoscale Ni surfaces created by the undercut etching of a photo-resist pattern on a planar substrate. With the use of a simple transmission grating geometry, up to 60 diffraction orders were observed from the nanowire gratings, with separate oscillatory intensity patterns appearing in the even and odd diffraction orders. The presence of these intensity oscillations is attributed to the LPNE array fabrication process, which creates arrays with alternating interwire spacings of distances d +[DELTA] and d -[DELTA], where d = 25 [micro]m and the asymmetry A varied from 0 to 3.5 [micro]m. The amount of asymmetry could be controlled by varying the LPNE undercut etching time during the creation of the nanoscale Ni surfaces. The Fourier transform of a mathematical model of the nanowire array was used to predict the diffraction intensity patterns and quantitatively determine A for any grating. Additional sensitivity and an expanded diffraction order range were obtained through the use of external reflection (ER) and total internal reflection (TIR) diffraction geometries.

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. [ABSTRACT FROM AUTHOR]

Eine integrierte Zelle für die solargetriebene Wasserspaltung besteht aus mehreren funktionellen Komponenten und kombiniert verschiedene photoelektrochemische (PEC ‐ )Prozesse auf unterschiedlichen Längen ‐ und Zeitskalen. Der Gesamtwirkungsgrad eines derartigen Systems für die Umwandlung von Solarenergie in Wasserstoff (solar ‐ to ‐ hydrogen; STH) wird von der Leistung und den Materialeigenschaften seiner Einzelkomponenten sowie ihrer Integration, von seiner Gesamtarchitektur und den Betriebsbedingungen bestimmt. Dieser Aufsatz beschäftigt sich mit der Implementierung von solargetriebenen Prototypen für die Wasserspaltung auf der Basis von Modellierungen und Simulationen, welche die Wechselwirkungen zwischen den Einzelbauteilen in ihrer Gesamtheit berücksichtigen. Physik und Wechselwirkungen innerhalb der Zelle werden diskutiert, und die Anwendung des Zellmodells für die Bestimmung der benötigten Materialeigenschaften sowie beim Design von herkömmlichen und ungewöhnlichen Architekturen wird erörtert. Die durch Modellierung und Simulation geleitete Entwicklung integrierter solargetriebener Wasserspaltungszellen hat in den letzten Jahren große Fortschritte gemacht. Mehrdimensionale Multiphysikmodelle haben Leitlinien für das Design von Halbleitern, Elektrokatalysatoren, flüssigen Elektrolyten und Membranelektrolyten geliefert. Dieser Aufsatz diskutiert die hieraus resultierenden Leitprinzipien und wesentlichen Erkenntnisse. [ABSTRACT FROM AUTHOR]

An integrated cell for the solar-driven splitting of water consists of multiple functional components and couples various photoelectrochemical (PEC) processes at different length and time scales. The overall solar-to-hydrogen (STH) conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as on the component integration, overall device architecture, and system operating conditions. This Review focuses on the modeling- and simulation-guided development and implementation of solar-driven water-splitting prototypes from a holistic viewpoint that explores the various interplays between the components. The underlying physics and interactions at the cell level is are reviewed and discussed, followed by an overview of the use of the cell model to provide target properties of materials and guide the design of a range of traditional and unique device architectures. [ABSTRACT FROM AUTHOR]

Light-driven generation of H2O2 only from water and molecular oxygen could be an ideal pathway for clean production of solar fuels. In this work, a mixed metal oxide/graphitic-C3N4 (MMO@C3N4) composite was synthesized as a dual-functional photocatalyst for both water oxidation and oxygen reduction to generate H2O2. The MMO was derived from a NiFe-layered double hydroxide (LDH) precursor for obtaining a high dispersion of metal oxides on the surface of the C3N4 matrix. The C3N4 is in the graphitic phase and the main crystalline phase in MMO is cubic NiO. The XPS analyses revealed the doping of Fe3+ in the dominant NiO phase and the existence of surface defects in the C3N4 matrix. The formation and decomposition kinetics of H2O2 on the MMO@C3N4 and the control samples, including bare MMO, C3N4 matrix, Ni- or Fe-loaded C3N4 and a simple mixture of MMO and C3N4, were investigated. The MMO@C3N4 composite produced 63 μmol L−1 of H2O2 in 90 min in acidic solution (pH 3) and exhibited a significantly higher rate of production for H2O2 relative to the control samples. The positive shift of the valence band in the composite and the enhanced water oxidation catalysis by incorporating the MMO improved the light-induced hole collection relative to the bare C3N4 and resulted in the enhanced H2O2 formation. The positively shifted conduction band in the composite also improved the selectivity of the two-electron reduction of molecular oxygen to H2O2. [ABSTRACT FROM AUTHOR]

An efficient, stable, and intrinsically safe solar water‐splitting device is demonstrated using a III–V tandem junction photoanode, an acid‐stable, earth‐abundant hydrogen evolution catalyst, and a bipolar membrane. The integrated photoelectrochemical cell operates under a steady‐state pH gradient and achieves ≈10% solar‐to‐hydrogen conversion efficiency, >100 h of stability in a large (>1 cm2) photoactive area in relation to most previous reports. [ABSTRACT FROM AUTHOR]

A fully integrated solar-driven water-splitting system comprised of WO3/FTO/p+n Si as the photoanode, Pt/TiO2/Ti/n+p Si as the photocathode, and Nafion as the membrane separator, was simulated, assembled, operated in 1.0 M HClO4, and evaluated for performance and safety characteristics under dual side illumination. A multi-physics model that accounted for the performance of the photoabsorbers and electrocatalysts, ion transport in the solution electrolyte, and gaseous product crossover was first used to define the optimal geometric design space for the system. The photoelectrodes and the membrane separators were then interconnected in a louvered design system configuration, for which the light-absorbing area and the solution-transport pathways were simultaneously optimized. The performance of the photocathode and the photoanode were separately evaluated in a traditional three-electrode photoelectrochemical cell configuration. The photocathode and photoanode were then assembled back-to-back in a tandem configuration to provide sufficient photovoltage to sustain solar-driven unassisted water-splitting. The current-voltage characteristics of the photoelectrodes showed that the low photocurrent density of the photoanode limited the overall solar-to-hydrogen (STH) conversion efficiency due to the large band gap of WO3. A hydrogen-production rate of 0.17 mL hr−1 and a STH conversion efficiency of 0.24 % was observed in a full cell configuration for >20 h with minimal product crossover in the fully operational, intrinsically safe, solar-driven water-splitting system. The solar-to-hydrogen conversion efficiency, ηSTH, calculated using the multiphysics numerical simulation was in excellent agreement with the experimental behavior of the system. The value of ηSTH was entirely limited by the performance of the photoelectrochemical assemblies employed in this study. The louvered design provides a robust platform for implementation of various types of photoelectrochemical assemblies, and can provide an approach to significantly higher solar conversion efficiencies as new and improved materials become available. [ABSTRACT FROM AUTHOR]

A protocol is described for forming reconnectable sub-5 nm nanogaps in single ultralong (>100 μm) gold nanowires fabricated by lithographically patterned nanowire electrodeposition (LPNE). During an initial computer-controlled electromigration process, gold nanowires with a rectangular cross-section were transformed by the formation of a constriction at a single point along the 250 μm length of the nanowire, and within this constriction a nanogap of width <5 nm. After this initial nanogap formation, 42% (19 of 45) of the gaps could be reconnected by applying a voltage ramp, restoring the electrical resistance of the original nanowire to within 10%. The voltage threshold for nanogap reconnection was narrowly distributed across multiple wires and nanogaps and in the range from 2 to 3 V. Using voltage programming, it was possible to cycle between the open and closed states for some nanogaps more than 100 times. We propose that the mechanism for reconnection involves the field evaporation of gold, qualitatively as observed previously for metal transfer from the tip of a scanning tunneling microscope. [ABSTRACT FROM AUTHOR]