John S. Foord

Shape‐Dependent Acidity and Photocatalytic Activity of Nb2O5 Nanocrystals with an Active TT (001) Surface

Nb(2)O(5) nanorods and nanospheres were synthesized, and their photocatalytic activity for methylene blue decomposition in water compared. Nb(2)O(5) nanorods clearly displayed higher activity, despite their comparable surface area. With a shape-dependent surface acidity, hydrothermal stability, and high photoactivity, these Nb(2)O(5) nanorods are a unique and exciting nanomaterial for non-classical photocatalytic mineralization of organic compounds in water.

High carrier mobility in polycrystalline thin film diamond

Polycrystalline diamond films have been found to display p-type surface conductivity. No bulk impurity is added to the films; the p-type characteristics of the undoped diamond are thought to be due to a surface or near surface hydrogenated layer. Carrier concentrations within the range 1017–1019 cm−3 have been measured; control over the carrier concentration can be achieved by annealing the “as-grown” films in air. For a given annealing temperature a stable carrier concentration arises. The Hall carrier mobility has been explored and a value of >70 cm2/Vs has been measured for a film with a carrier concentration of ∼5×1017 cm−3, the highest reported for polycrystalline thin film diamond and equivalent to boron doped single crystal diamond.

Sono-cathodic stripping voltammetry of manganese at a polished boron-doped diamond electrode: application to the determination of manganese in instant tea

Ultrasonically assisted cathodic stripping voltammetry at a boron-doped diamond electrode was developed for the detection of manganese. Differential-pulse voltammetry was used to give the analytical signal from a cathodic strip of electrodeposited MnO2; linearity was observed from 10(-11) M to at least 3 x 10(-7) M, with 10(-11) M being the detection limit for a 2 min deposition. The procedure involves both ultrasonic-anodic deposition of MnO2 and ultrasonic-cathodic stripping. This novel analytical tool is robust, reproducible, mercury free, oxygen insensitive and highly specific towards manganese. The differential-pulse sono-cathodic stripping voltammetric technique was used to determine successfully the manganese content of two instant tea samples, giving excellent agreement with independent AAS analyses.

Surface chemical processes in metal organic molecular‐beam epitaxy; Ga deposition from triethylgallium on GaAs(100)

The adsorption of triethylgallium on the GaAs (100) (4×1) surface has been studied using the techniques of low energy electron diffraction, x‐ray photoelectron and Auger spectroscopies, high resolution electron energy loss spectroscopy and temperature‐programmed desorption. Condensed multilayers of the organometallic compound formed following adsorption at 150 K desorb from the surface at ∼170 K to leave a chemisorbed molecular monolayer of triethylgallium. Upon further heating this layer partially desorbs and partially decomposes to form diethylgallium in two competing processes. The diethylgallium so formed can also desorb or otherwise decompose ultimately to adsorbed Ga atoms in a reaction which results in the formation of hydrogen, ethene, and ethane. The temperature‐programmed desorption characteristics of these latter species are found to be similar to those observed for a dissociated layer of ethyl bromide. A reaction scheme is proposed to account for the observations and kinetic parameters are obt...

Growth of In2O3(100) on Y-stabilized ZrO2(100) by O-plasma assisted molecular beam epitaxy

Thin films of In2O3 have been grown on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy with a substrate temperature of 650°C. Ordered epitaxial growth was confirmed by high resolution transmission electron microscopy. The position of the valence band onset in the x-ray photoemission spectra of the epitaxial films is found to be inconsistent with the widely quoted value of 3.75eV for the fundamental bandgap of In2O3 and suggests a revised value of 2.67eV.

An STM study of surface structures on WO3(001)

Structure on the (001) surface of monoclinic WO3 has been studied by scanning tunnelling microscopy. Electrostatic instability at the polar surface of WO3(001) is avoided by termination of the outer WO2 layer with half a monolayer of oxygen ions to produce a (2 × 2)R45° arrangement of on-top oxygen. An additional periodicity is introduced in some terrace areas by tilting of WO6 octahedra to produce alternately long and short on-top OO separations along the [110] direction. This produces a (2 × 2) superstructure approximating to the space group c2mm. Prominent defect troughs on oxygen annealed surfaces are attributed to missing oxygen ions. Topographically resolved I-V curves show that significant filled state electron density resides in the trough bottoms. Argon ion bombardment and subsequent annealing leads to the appearance of defects in which on-top oxygen is removed from adjacent W ions to generate linear features three unit cells wide running along the [100] direction. These new defects aggregate with the troughs to produce characteristic branched structures. Prolonged bombardment and annealing leads eventually to large terrace areas displaying a new (2 × 2) superstructure in which only a 14 the W ions carry on-top oxygen.

Chemical vapour deposition on silicon: In situ surface studies

Abstract The decomposition of Fe(CO) 5 to produce Fe films on Si(100) has been examined in situ using Auger electron spectroscopy. Processes occurring during pyrolytic, UV photolytic and electron beam deposition are determined. The nature of the surface films produced by these different methods is compared and contrasted.

Electrochemistry at boron doped diamond films grown on graphite substrates: redox-, adsorption and deposition processes

Highly boron-doped (atomic concentration ∼1021 cm−3) conducting diamond films were grown on graphite substrates by microwave assisted vapor deposition from a gaseous feed of hydrogen and methane and solid boron. These diamond films of ca. 5 to 10 μm thickness composed of crystals of up to 10 μm size were characterized by both surface analytical techniques such as Raman spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM), and electrochemical techniques. The reduction of Ru(NH3)63+, the oxidation of chlorpromazine, and the reduction of Pb2+ in aqueous media were studied in order to investigate the processes involving oxidation and reduction as well as adsorption and deposition. The one-electron oxidation of chlorpromazine in aqueous 0.1 M KCl and the one electron reduction of Ru(NH3)63+ in aqueous 0.1 M KCl gave well defined cyclic voltammetric responses at diamond electrodes with peak currents proportional to the square root of the scan rate consistent with diffusion control at a macroscopically uniformly active electrode. For the oxidation of chlorpromazine at concentrations less than 4 mM adsorption of the neutral chlorpromazine at the diamond electrode was additionally detected. At very low scan rates a transition to sigmoidally shaped responses occurred which may be attributed partly to the presence of areas of low or no conductivity on a microscopic level. The reduction of Pb2+ in aqueous 0.1 M HClO4 allowed the deposition of metallic lead on the diamond surface. Studies by ex-situ SEM and in-situ AFM show that Pb nuclei are formed and distributed inhomogeneously over the polycrystalline diamond film in the active surface areas, irrespective of grain boundaries. However, little or no Pb deposition occurred in some less active areas with dimensions of 2 to 100 μm. This effect may be attributed to regions of poor conductivity between the graphite substrate and the diamond film coupled to low lateral conductivity of the film itself. The size of the regions of lower activity decreases with higher applied over-potential. A characteristic delay in the re-dissolution process of the Pb deposit was observed and attributed to both poor adhesion and a resistive contact between deposit and electrode surface.

AN INSIGHT INTO THE MECHANISM OF SURFACE CONDUCTIVITY IN THIN FILM DIAMOND

Abstract Diamond containing hydrogen at or near the surface displays p-type conductivity. The origin of this effect has been controversial. We have used I–V, Hall effect, SIMS, Raman, UPS and XPS to study hydrogenated polycrystalline CVD diamond films. The direct formation of acceptor states by hydrogen, which resides within the top 20 nm of the film, is the origin of the carriers present rather than surface band bending. Up to 1019 holes cm−3 can be measured and mobilities as high as 70 cm2 Vs−1 recorded. H-termination of the surface is important for the formation of high quality metal-diamond interfaces.

Voltammetry at carbon nanofiber electrodes

Carbon nanofibers with diameters in the range 10–500 nm have been evaluated as novel electrode materials for electrochemical applications. Compared with other forms of nanostructured carbons, such as aerogels or activated charcoal, carbon nanofibers exhibit low BET surface areas, 50 vs. , because their surfaces are not readily penetrated by gaseous nitrogen. But somewhat surprisingly, they exhibit higher electrochemical capacitances (ca. 60 vs. ) because the spaces between the fibers are readily penetrated by electrolyte solution. As a result, capacitive currents tend to mask voltammetric currents during cyclic voltammetry. The situation is quite different when the spaces between carbon nanofibers are impregnated by an inert dielectric material, such as high-melting paraffin wax. Then the carbon nanofibers form a high-density composite electrode with good conductivity and low capacitance. Indeed, well-defined voltammetric responses are readily observed for the reduction of Ru(NH3)63+ in aqueous solution, even in the absence of supporting electrolyte. Metal deposition and anodic stripping processes can also be observed for the reduction of Pb2+ in aqueous nitric acid. This suggests that carbon nanofibers represent a new class of material suitable for electroanalytical applications.