Over 75% incident-photon-to-current efficiency without solid electrodes.

The efficiency of photoelectrochemical reactions is conventionally defined in terms of the ratio between the current responses arising from the collection of carriers at electrical contacts and the incident photon flux at a given wavelength, i.e. the incident-photon-to-current-efficiency (IPCE). IPCE values are determined by a variety of factors such as the absorption constant of the active layer, bulk and surface recombination of photogenerated carriers, as well as their characteristic diffusion length. These parameters are particularly crucial in nanostructured photoelectrodes, which commonly display low carrier mobility. In this article, we examine the photoelectrochemical responses of a mesoporous TiO2 film in which the IPCE is enhanced by fast extraction of carriers via chemical reactions. TiO2 films are spontaneously formed by destabilisation of colloidal particles at the polarisable interface between two immiscible electrolyte solutions. The photocurrent arises from hole-transfer to redox species confined to the organic electrolyte, which is coupled to the transfer of electrons to oxygen in the aqueous electrolyte. The dynamic photocurrent responses demonstrate that no coupled ion transfer is involved in the process. The interplay of different interfacial length scales, molecularly sharp liquid/liquid boundary and mesoporous TiO2 film, promotes efficiencies above 75% (without correction for reflection losses). This is a significant step change in values reported for these interfaces (below 1%), which are usually limited to sub-monolayer coverage of photoactive molecular or nanoscopic materials.

Growth of epitaxial Pt1-xPbx alloys by surface limited redox replacement and study of their adsorption properties.

The surface limited redox replacement (SLRR) method has been used to design two-dimensional Pt-Pb nanoalloys with controlled thickness, composition, and structure. The electrochemical behavior of these alloys has been systematically studied as a function of alloy composition. A single-cell, two-step SLRR protocol based on the galvanic replacement of underpotentially deposited monolayers of Pb with Pt was used to grow epitaxial Pt1-xPbx (x < 0.1) alloys of up to 10 ML thickness on Au substrates. It is shown that by varying the terminating potential of the galvanic replacement step, the Pb atomic content can be controlled in the films. Electrochemical analysis of the alloys showed that the adsorption of both H and CO exhibits similar, and systematic, decreases with small increases in the Pb content. These measurements, commonly used in electrocatalysis for the determination of active surface areas of Pt, suggested area values much lower than those expected based on the net Pt composition in the alloy as measured by XPS. These results show that Pb has a strong screening effect on the adsorption of both H and CO. Moreover, changes in alloy composition result in a negative shift in the potential of the peaks of CO oxidation that scales with the increase of Pb content. The results suggest electronic and bifunctional effects of incorporated Pb on the electrochemical behavior of Pt. The study illustrates the potential of the SLRR methodology, which could be employed in the design of 2-dimensional bimetallic Pt nanoalloys for fundamental studies of electrocatalytic behavior in fuel cell reactions dependent on the nature of alloying metal and its composition.

Porous boron-doped diamond/carbon nanotube electrodes.

Nanostructuring boron-doped diamond (BDD) films increases their sensitivity and performance when used as electrodes in electrochemical environments. We have developed a method to produce such nanostructured, porous electrodes by depositing BDD thin film onto a densely packed "forest" of vertically aligned multiwalled carbon nanotubes (CNTs). The CNTs had previously been exposed to a suspension of nanodiamond in methanol causing them to clump together into "teepee" or "honeycomb" structures. These nanostructured CNT/BDD composite electrodes have been extensively characterized by scanning electron microscopy, Raman spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. Not only do these electrodes possess the excellent, well-known characteristics associated with BDD (large potential window, chemical inertness, low background levels), but also they have electroactive areas and double-layer capacitance values ∼450 times greater than those for the equivalent flat BDD electrodes.

Charge transport across high surface area metal/diamond nanostructured composites.

High surface area composites featuring metal nanostructures and diamond particles have generated a lot of interest in the fields of heterogeneous catalysis, electrocatalysis, and sensors. Diamond surfaces provide a chemically robust framework for active nanostructures in comparison with sp(2) carbon supports. The present paper investigates the charge transport properties of high surface area films of high-pressure, high-temperature diamond particles in the presence and absence of metal nanostructures, employing electrochemical field-effect transistors. Oxygen- and hydrogen-terminated surfaces were generated on 500 nm diamond powders. Homogeneously distributed metal nanostructures, with metal volume fractions between ca. 5 and 20%, were either nucleated at the diamond particles by impregnation or incorporated from colloidal solution. Electrochemical field-effect transistor measurements, employing interdigitated electrodes, allowed the determination of composite conductivity as a function of electrode potential, as well as in air. In the absence of metal nanostructures, the lateral conductivity of the diamond assemblies in air is increased by over one order of magnitude upon hydrogenation of the particle surface. This observation is consistent with studies at diamond single crystals, although the somewhat modest change in conductivity suggests that charge transport is not only determined by the intrinsic surface conductivity of individual diamond particles but also by particle-to-particle charge transfer. Interestingly, the latter contribution effectively controls the assembly conductivity in the presence of an electrolyte solution as the difference between hydrogenated and oxygenated particles vanishes. The conductivity in the presence of metal nanoparticles is mainly determined by the metal volume fraction, while diamond surface termination and the presence of electrolyte solutions exert only minor effects. The experimental trends are discussed in terms of the electrochemical formation of charge carriers in the diamond particles, percolation theory, and charge screening at the double layer.