Leigh R Sheppard

Reactivity of titanium dioxide with oxygen at room temperature and the related charge transfer.

The measurements of electron work function were applied for in situ monitoring of the charge transfer during oxidation and reduction for well-defined titanium dioxide, TiO 2, at room temperature. The TiO 2 specimen was initially standardized at 1173 K in the gas phase of controlled oxygen activity, at p(O 2) = 10 Pa, and then cooled down in the same gas phase. The work function changes were monitored during oxidation at room temperature at p(O 2) = 75 kPa and subsequent reduction at p(O 2) = 10 Pa. It is shown that oxidation of TiO 2 at room temperature results in fast oxygen chemisorption, involving initially the formation of singly ionized molecular oxygen species, followed by the formation of singly ionized atomic oxygen species, and subsequent slow oxygen incorporation. Although all these processes lead to work function increase, the components of the work function changes related to the individual processes may be distinguished based on different kinetics. The obtained work function data indicate that oxidation results in rapid surface coverage with singly ionized molecular oxygen species, which are subsequently dissociated leading to the formation of singly ionized atomic species. The related chemisorption equilibria are established within 2 and 5 h, respectively. Oxygen incorporation leads to slow work function changes, which achieve a maximum within 100 h. The determined work function data were assessed by using a theoretical model that describes the electrical effects related to different mechanisms of TiO 2 oxidation. The work function data indicate that oxygen incorporation leads to structural changes of the outermost surface layer resulting, in consequence, in a change of the external work function component. Reimposition of the initial gas phase, p(O 2) = 10 Pa, leads to partial desorption of weakly adsorbed molecular species formed during oxidation.

Materials for photoelectrochemical energy conversion

Abstract The main driving forces of the hydrogen economy are considered and its impact on the environment is discussed. The present work is focused on hydrogen generation from water using solar energy through photoelectrochemical energy conversion. Progress in this area, which is determined by the development of photosensitive materials and devices, is overviewed. The effect of materials selection and cell structures on the performance of photoelectrochemical cells is discussed. It is argued that TiO2 and TiO2 based oxide systems exhibit the most promising functional properties and, therefore, are the top candidates for photoelectrodes. The major challenges in the development of commercial photoelectrodes and photoelectrochemical energy conversion devices are discussed.

A Sinusoidally Architected Helicoidal Biocomposite.

A fibrous herringbone-modified helicoidal architecture is identified within the exocuticle of an impact-resistant crustacean appendage. This previously unreported composite microstructure, which features highly textured apatite mineral templated by an alpha-chitin matrix, provides enhanced stress redistribution and energy absorption over the traditional helicoidal design under compressive loading. Nanoscale toughening mechanisms are also identified using high-load nanoindentation and in situ transmission electron microscopy picoindentation.

Niobium segregation in TiO2

Abstract The present paper reports the effect of oxygen activity p(O2) on Nb segregation in a polycrystalline specimen of Nb doped TiO2 (2·91 at.-%). The determined segregation induced enrichment factor following annealing at 1773 K in p(O2) = 10 Pa and p(O2) = 21 kPa is 3·1 and 7·3 respectively. The data on the effect of p(O2) on the segregation induced Nb enrichment factor are considered in terms of defect disorder of Nb doped TiO2 and the related lattice charge neutrality requirements. The reported segregation data may be used for controlled modification of surface concentration gradients in TiO2 through the imposition of well defined gas phase composition during processing procedures.

Reactivity of ideal and defected rutile TiO2 (110) surface with oxygen

Abstract First principles pseudopotential density functional calculations have been performed to investigate the reactivity of O2 on ideal and defected surface of rutile TiO2 (110). All ionic positions are allowed to relax on slab geometry with periodic boundary conditions. The results reveal that both the molecular and dissociated adsorption of O2 is most favourable on a defective surface in the presence of titanium vacancy and a substantial change in surface relaxation was observed. This remarkable surface ionic displacement and titanium vacancy induced acceptor-like environment favoured in stable adsorption of oxygen for both molecular and dissociated states. The least favourable adsorption or no adsorption of O2 is observed on an ideal surface.

The Formation of Defect-Pairs for Highly Efficient Visible-Light Catalysts

Highly efficient visible-light catalysts are achieved through forming defect-pairs in TiO2 nanocrystals. This study therefore proposes that fine-tuning the chemical scheme consisting of charge-compensated defect-pairs in balanced concentrations is a key missing step for realizing outstanding photocatalytic performance. This research benefits photocatalytic applications and also provides new insight into the significance of defect chemistry for functionalizing materials.

Study of gamma irradiation effect on commercial TiO2 photocatalyst

The aim of this work is to understand the effect of gamma irradiation on commercial TiO2 photocatalyst for water treatment applications. Previous studies concluded that gamma-irradiation is able to modify the electronic properties of TiO2 based photocatalysts and consequently their photocatalytic performance. However, there are some discrepancies in the literature where on one hand a significant enhancement of the material properties is reported and on the other hand only a weak effect is observed. In this study a surface effect on TiO2 is confirmed by using low and medium gamma irradiation doses.

The Effect of Defect Disorder on the Electronic Structure of Rutile TiO2-x

The purpose of this work is to study the effect of bulk point defects on the electronic structure of rutile TiO2. The paper is focused on the effect of oxygen nonstoichiometry in the form of oxygen vacancies, Ti interstitials and Ti vacancies and related defect disorder on the band gap width and on the local energy levels inside the band gap. Ab initio density functional theory is used to calculate the formation energies of such intrinsic defects and to detect the positions of these defect induced energy levels in order to visualize the tendency of forming local mid-gap bands. Apart from the formation energy of the Ti vacancies (where experimental data do not exist) our calculated results of the defect formation energies are in fair agreement with the experimental results and the defect energy levels consistently support the experimental observations. The calculated results indicate that the exact position of defect energy levels depends on the estimated band gap and also the charge state of the point defects of TiO2.

Surface photovoltage studies of nonstoichiometric rutile titanium dioxide

The aim of the present investigation was to assess how processing at elevated temperatures impacts upon the room temperature electronic structure of rutile TiO2. Consequently, strongly reduced and oxidized rutile TiO2 pellets were processed under controlled conditions and studied using surface photovoltage spectroscopy. Under illumination with photon energies above the band gap energy, reduced and oxidized rutile, respectively, displayed positive and negative surface photovoltages. This result indicates that undoped TiO2 may be tailored to promote either photoreduction or photo-oxidation reactions. Several transitions involving surface and bulk electronic states within the band gap were observed in oxidized rutile, but in contrast, similar states could not be readily distinguished in reduced rutile. A band diagram of oxidized rutile is proposed.

Effect of sintering on microstructure of TiO2 ceramics

Abstract The present paper reports experimental data on the effect of processing of undoped polycrystalline TiO2 on the microstructure. The processing included the precipitation of TiO2 powder, obtained by adding water to Ti isopropoxide (99·999%)–ethanol mixture, drying, pressing and sintering. The applied procedures of pressing included both cold isostatic pressing and high temperature isostatic pressing. The obtained SEM micrographs are considered in terms of the effect of the applied processing procedure on microstructure and defect chemistry. The processing including sintering at 1688 K and subsequent high temperature isostatic pressing at low oxygen activity resulted in maximum density.