A study of the oxygen electrochemistry of ruthenium and iridium

This thesis is concerned with an investigation of the anodic behaviour of ruthenium and iridium in aqueous solution and particularly of oxygen evolution on these metals. The latter process is of major interest in the large-scale production of hydrogen gas by the electrolysis of water. The presence of low levels of ruthenium trichloride ca. 10-4 mol dm-3 in acid solution give a considerable increase in the rate of oxygen evolution from platinum and gold, but not graphite, anodes. The mechanism of this catalytic effect was investigated using potential step and a.c. impedance technique. Earlier suggestions that the effect is due to catalysis by metal ions in solution were proved to be incorrect and it was shown that ruthenium species were incorporated into the surface oxide film. Changes in the oxidation state of these ruthenium species is probably responsible for the lowering of the oxygen overvoltage. Both the theoretical and practical aspects of the reaction were complicated by the fact that at constant potential the rates of both the catalysed and the uncatalysed oxygen evolution processes exhibit an appreciable, continuous decrease with either time or degree of oxidation of the substrate. The anodic behaviour of iridium in the oxide layer region has been investigated using conventional electrochemical techniques such as cyclic voltammetry. Applying a triangular voltage sweep at 10 Hz, 0.01 to 1.50V increases the amount of electric charge which the surface can store in the oxide region. This activation effect and the mechanism of charge storage is discussed in terms of both an expanded lattice theory for oxide growth on noble metals and a more recent theory of irreversible oxide formation with subsequent stoichiometry changes. The lack of hysteresis between the anodic and cathodic peaks at ca. 0.9 V suggests that the process involved here is proton migration in a relatively thick surface layer, i.e. that the reaction involved is some type of oxide-hydroxide transition. Lack of chloride ion inhibition in the anodic region also supports the irreversible oxide formation theory; however, to account for the hydrogen region of the potential sweep a compromise theory involving partial reduction of the outer regions of iridium oxide film is proposed. The loss of charge storage capacity when the activated iridium surface is anodized for a short time above ca. 1.60 V is attributed to loss by corrosion of the outer active layer from the metal surface. The behaviour of iridium at higher anodic potentials in acid solution was investigated. Current-time curves at constant potential and Tafel plots suggested that a change in the mechanism of the oxygen evolution reaction occurs at ca. 1.8 V. Above this potential, corrosion of the metal occurred, giving rise to an absorbance in the visible spectrum of the electrolyte (λ max = 455 nm). It is suggested that the species involved was Ir(O2)2+. A similar investigation in the case of alkaline electrolyte gave no evidence for a change in mechanism at 1.8 V and corrosion of the iridium was not observed. Oxygen evolution overpotentials were much lower for iridium than for platinum in both acidic and alkaline solutions.

The optical response of nanostructured surfaces and the composite diffracted evanescent wave model

Investigations of the optical response of subwavelength-structure arrays milled into thin metal films have revealed surprising phenomena, including reports of unexpectedly high transmission of light. Many studies have interpreted the optical coupling to the surface in terms of the resonant excitation of surface plasmon polaritons (SPPs), but other approaches involving composite diffraction of surface evanescent waves (CDEW) have also been proposed. Here we present a series of measurements on very simple one-dimensional subwavelength structures to test the key properties of the surface waves, and compare them to the CDEW and SPP models. We find that the optical response of the silver metal surface proceeds in two steps: a diffractive perturbation in the immediate vicinity (2–3 mu m) of the structure, followed by excitation of a persistent surface wave that propagates over tens of micrometres. The measured wavelength and phase of this persistent wave are significantly shifted from those expected for resonance excitation of a conventional SPP on a pure silver surface.

Quantum chemical study of the effect of precursor stereochemistry on dissociative chemisorption and surface redox reactions during the atomic layer deposition of the transition metal copper

Using quantum chemical calculations, we investigate surface reactions of copper precursors and diethylzinc as the reducing agent for effective Atomic Layer Deposition (ALD) of Cu. The adsorption of various commonly used Cu(II) precursors is explored. The precursors vary in the electronegativity and conjugation of the ligands and flexibility of the whole molecule. Our study shows that the overall stereochemistry of the precursor governs the adsorption onto its surface. Formation of different Cu(II)/Cu(I)/Cu(0) intermediate complexes from the respective Cu(II) compounds on the surface is also explored. The surface model is a (111) facet of a Cu55 cluster. Cu(I) compounds are found to cover the surface after the precursor pulse, irrespective of the precursor chosen. We provide new information about the surface chemistry of Cu(II) versus Cu(I) compounds. A pair of CuEt intermediates or the dimer Cu2Et2 reacts in order to deposit a new Cu atom and release gaseous butane. In this reaction, two electrons from the Et anions are donated to copper for reduction to metallic form. This indicates that a ligand exchange between the Cu and Zn is important for the success of this transmetalation reaction. The effect of the ligands in the precursor on the electron density before and after adsorption onto the surface has also been computed through population analysis. In the Cu(I) intermediate, charge is delocalized between the Cu precursor and the bare copper surface, indicating metallic bonding as the precursor densifies to the surface.

Langmuir-Blodgett assembly of colloidal crystals combined with controlled infiltration of conducting metal oxides

Colloidal photonic crystals (PhCs) possess a periodic dielectric structure which gives rise to a photonic band gap (PBG) and offer great potential in the ability to modify or control light at visible wavelengths. Although the refractive index contrast between the void or infill and the matrix material is paramount for photonics applications, integration into real optoelectronics devices will require a range of added functionalities such as conductivity. As such, colloidal PhCs can be used as templates to direct infiltration of other functional materials using a range of deposition strategies. The work in this thesis seeks to address two challenges; first to develop a reproducible strategy based on Langmuir-Blodgett (LB) deposition to assemble high quality colloidal PhCs based on silica with precise film thickness as most other assembly methods suffer from a lack of reproducibility thickness control. The second is to investigate the use of LBdeposited colloidal PhCs as templates for infiltration with conducting metal oxide materials using vapor phase deposition techniques. Part of this work describes the synthesis and assembly of colloidal silica spheres with different surface chemical functionalities at the air-water interface in preparation for LB deposition. Modification of surface funtionality conferred varying levels of hydrophobicity upon the particles. The behaviour of silica monolayer films at the air-water interface was characterised by Brewster Angle Microscopy and surface pressure isotherms with a view to optimising the parameters for LB deposition of multilayer colloidal PhC films. Optical characterisation of LB-fabricated colloidal PhCs indicated high quality photonic behaviour, exhibiting a pseudo PBG with a sharp Bragg diffraction peak in the visible region and reflectance intensities greater than 60%. Finally the atomic layer deposition (ALD) of nominally undoped ZnO and aluminium “doped” ZnO (Al-doped ZnO) inside the pores of a colloidal PhC assembled by the LB technique was carried out. ALD growth in this study was performed using trimethyl aluminium (TMA) and water as precursors for the alumina and diethyl zinc (DEZn) and water for the ZnO. The ZnO:Al films were grown in a laminate mode, where DEZn pulses were substituted for TMA pulses in the sequences with a Zn:Al ratio 19:1. The ALD growth of ZnO and ZnO:Al in colloidal PhCs was shown to be highly conformal, tuneable and reproducible whilst maintaining excellent photonic character. Furthermore, at high levels of infiltration the opal composite films demonstrated significant conductivity.

Loading effect in copper(II) oxide cluster-surface-modified titanium(IV) oxide on visible- and UV-light activities

Cu(acac)2 is chemisorbed on TiO2 particles [P-25 (anatase/rutile = 4/1 w/w), Degussa] via coordination by surface Ti–OH groups without elimination of the acac ligand. Post-heating of the Cu(acac)2-adsorbed TiO2 at 773 K yields molecular scale copper(II) oxide clusters on the surface (CuO/TiO2). The copper loading amount (Γ/Cu ions nm–2) is controlled in a wide range by the Cu(acac)2 concentration and the chemisorption–calcination cycle number. Valence band (VB) X-ray photoelectron and photoluminescence spectroscopy indicated that the VB maximum of TiO2 rises up with increasing Γ, while vacant midgap levels are generated. The surface modification gives rise to visible-light activity and concomitant significant increase in UV-light activity for the degradation of 2-naphthol and p-cresol. Prolonging irradiation time leads to the decomposition to CO2, which increases in proportion to irradiation time. The photocatalytic activity strongly depends on the loading, Γ, with an optimum value of Γ for the photocatalytic activity. Electrochemical measurements suggest that the surface CuO clusters promote the reduction of adsorbed O2. First principles density functional theory simulations clearly show that, at Γ < 1, unoccupied Cu 3d levels are generated in the midgap region, and at Γ > 1, the VB maximum rises and the unoccupied Cu 3d levels move to the conduction band minimum of TiO2. These results suggest that visible-light excitation of CuO/TiO2 causes the bulk-to-surface interfacial electron transfer at low coverage and the surface-to-bulk interfacial electron transfer at high coverage. We conclude that the surface CuO clusters enhance the separation of photogenerated charge carriers by the interfacial electron transfer and the subsequent reduction of adsorbed O2 to achieve the compatibility of high levels of visible and UV-light activities.

Dissociative adsorption of methane on the Cu and Zn doped (111) surface of CeO2

The development of economical heterogeneous catalysts for the activation of methane is a major challenge for the chemical industry. Screening potential candidates becomes more feasible using rational catalyst design to understand the activity of potential catalysts for CH4 activation. The focus of the present paper is the use of density functional theory to examine and elucidate the properties of doped CeO2. We dope with Cu and Zn transition metals having variable oxidation state (Cu), and a single oxidation state (Zn), and study the activation of methane. Zn is a divalent dopant and Cu can have a +1 or +2 oxidation state. Both Cu and Zn dopants have an oxidation state of +2 after incorporation into the CeO2 (111) surface; however a Hubbard +U correction (+U = 7) on the Cu 3d states is required to maintain this oxidation state when the surface interacts with adsorbed species. Dissociation of methane is found to occur locally at the dopant cations, and is thermodynamically and kinetically more favorable on Zn-doped CeO2 than Cu-doped CeO2. The origins of this lie with the Zn(II) dopant moving towards a square pyramidal geometry in the sub surface layer which facilitates the formation of two-coordinated surface oxygen atoms, that are more beneficial for methane activation on a reducible oxide surface. These findings can aid in rational experimental catalyst design for further exploration in methane activation processes.

Development and characterisation of solution processed vanadium oxide and transparent metal oxide thin films

Metal oxide thin films are important for modern electronic devices ranging from thin film transistors to photovoltaics and functional optical coatings. Solution processed techniques allow for thin films to be rapidly deposited over a range of surfaces without the extensive processing of comparative vapour or physical deposition methods. The production of thin films of vanadium oxide prepared through dip-coating was developed enabling a greater understanding of the thin film formation. Mechanisms of depositing improved large area uniform coverage on a number of technologically relevant substrates were examined. The fundamental mechanism for polymer-assisted deposition in improving thin film surface smoothness and long range order has been delivered. Different methods were employed for adapting the alkoxide based dip-coating technique to produce a variety of amorphous and crystalline vanadium oxide based thin films. Using a wide range of material, spectroscopic and optical measurement techniques the morphology, structure and optoelectronic properties of the thin films were studied. The formation of pinholes on the surface of the thin films, due to dewetting and spinodal effects, was inhibited using the polymer assisted deposition technique. Uniform thin films with sub 50 nm thicknesses were deposited on a variety of substrates controlled through alterations to the solvent-alkoxide dilution ratios and employing polymer assisted deposition techniques. The effects of polymer assisted deposition altered the crystallized VO thin films from a granular surface structure to a polycrystalline structure composed of high density small in-plane grains. The formation of transparent VO based thin film through Si and Na substrate mediated diffusion highlighted new methods for material formation and doping.

Patterning optically clear films: co-planar transparent and color-contrasted thin films from interdiffused electrodeposited and solution-processed metal oxides

Transparent thin films can now be site-selectively patterned and positioned on surface using mask-defined electrodeposition of one oxide and overcoating with a different solution-processed oxide, followed by thermal annealing. Annealing allows an interdiffusion process to create a new oxide that is entirely transparent. A primary electrodeposited oxide can be patterned and the secondary oxide coated over the entire substrate to form high color contrast coplanar thin film tertiary oxide. The authors also detail the phase formation and chemical state of the oxide and how the nature of the electrodeposited layer and the overlayer influence the optical clearing of the patterned oxide film.

Engineering interfacial silicon dioxide for improved metal-insulator-semiconductor silicon photoanode water splitting performance

Silicon photoanodes protected by atomic layer deposited (ALD) TiO2 show promise as components of water splitting devices that may enable the large-scale production of solar fuels and chemicals. Minimizing the resistance of the oxide corrosion protection layer is essential for fabricating efficient devices with good fill factor. Recent literature reports have shown that the interfacial SiO2 layer, interposed between the protective ALD-TiO2 and the Si anode, acts as a tunnel oxide that limits hole conduction from the photoabsorbing substrate to the surface oxygen evolution catalyst. Herein, we report a significant reduction of bilayer resistance, achieved by forming stable, ultrathin (<1.3 nm) SiO2 layers, allowing fabrication of water splitting photoanodes with hole conductances near the maximum achievable with the given catalyst and Si substrate. Three methods for controlling the SiO2 interlayer thickness on the Si(100) surface for ALD-TiO2 protected anodes were employed: (1) TiO2 deposition directly on an HF-etched Si(100) surface, (2) TiO2 deposition after SiO2 atomic layer deposition on an HF-etched Si(100) surface, and (3) oxygen scavenging, post-TiO2 deposition to decompose the SiO2 layer using a Ti overlayer. Each of these methods provides a progressively superior means of reliably thinning the interfacial SiO2 layer, enabling the fabrication of efficient and stable water oxidation silicon anodes.