M. K. Nowotny

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.

Charge transfer at oxygen/zirconia interface at elevated temperatures: Part 1: Basic properties and terms

Abstract The purpose of the present study is to consider the mechanism of oxidation of zirconia and related charge transfer at the oxygen/zirconia interface, focusing on yttria-stabilised cubic zirconia. The study considers unresolved problems concerning the electrochemistry of the oxygen/zirconia interface at elevated temperatures and formulates the relevant questions to be addressed. The present paper outlines the research strategy for addressing these questions. The basic properties of yttria stabilised zirconia, such as electrical properties and oxygen diffusion, are summarised.

Charge transfer at oxygen/zirconia interface at elevated temperatures Part 2: Oxidation of zirconia

Abstract The objective of the present paper is to review the literature on the mechanism and kinetics of oxidation of yttria-stabilised zirconia (YSZ) and related models of the transfer of charge and mass at the oxygen/zirconia interface at elevated temperatures. There is general agreement that oxygen may be incorporated efficiently into the lattice of zirconia at the triphase boundary (TPB), which is formed by the gas phase, zirconia oxygen ion conductor and electronic conductor. Thus, a number of efforts have been made to understand the effect of the TPB structure on the oxidation mechanism. Although it is clear that the TPB is required for efficient charge transfer at high current, there is a growing number of reports to the effect that oxygen may be incorporated into zirconia in the absence of the TPB. As a result, attempts have been made to develop zirconia for use in electrodeless zirconia-based electrochemical devices.

Charge transfer at oxygen/zirconia interface at elevated temperatures Part 9: Room temperature

Abstract The present paper reports the reactivity between the surface of zirconia and oxygen at room temperature. The experimental studies, performed with polycrystalline yttria-stabilised zirconia (YSZ), include in situ monitoring of charge transfer at the oxygen/zirconia interface using work function measurements and determination of mass transport at the oxygen/zirconia interface using the incorporation of oxygen isotope 18O and its analysis using secondary ion mass spectrometry. Oxidation of YSZ at room temperature is found to result in oxygen chemisorption, leading to the formation of singly ionised molecular oxygen species, and subsequent oxygen incorporation into the lattice. Oxygen incorporation takes place by lattice diffusion from the entire surface exposed to the gas phase, resulting in oxygen penetration of depths of the order of 2 nm over 60 days, and diffusion of oxygen species along the surface from the adsorption sites towards grain boundaries with subsequent rapid diffusion of oxygen into the bulk along grain boundaries. These processes take place in the absence of an electronic conductor. The present findings contradict the widely held perceptions that YSZ at room temperature is quenched and not reactive, and that oxygen incorporation into YSZ requires the presence of a triphase boundary (TPB) formed by the gas phase (oxygen), electronic conductor and oxygen ionic conductor (zirconia).

Titanium Dioxide Photocatalyst - Unresolved Problems

The present work considers the performance of TiO2-based photosensitive oxide semiconductors as photocatalysts for water purification. This paper brings together the concepts of solid state chemistry for nonstoichiometric compounds and the concepts of photocatalysis in order to discuss the reactivity between TiO2 and water including microorganisms (bacteria and viruses). The performance of TiO2 photocatalysts are considered in terms of a model of photoelectrochemical cell. The experimental data on photocatalytic removal of microorganisms from water are considered in terms of the effect of several properties, including pH, dispersion, light intensity, and temperature. It is argued that correct understanding of the performance of TiO2 photocatalysts requires recognition that properties of TiO2, which is a nonstoichiometric compound, are determined by defect disorder and the related ability to donate or accept electrons. The photocatalytic properties of TiO2 are considered in terms of the reactivity of both anodic and cathodic sites with water and the related charge transfer at the TiO2/H2O interface. It is shown that the formation of well defined photocatalysts requires knowledge of mass and charge transfer during processing and performance, respectively. The main hurdles in the development of high-performance photocatalysts are discussed.

Electrical conductivity of TiO2 within n-p transition Part I -Verification of defect disorder model

Abstract The present paper reports semiconducting properties and the related defect disorder for undoped TiO2 single crystal using measurements of the electrical conductivity. Isothermal changes of the electrical conductivity as a function of oxygen activity were determined within oxygen activities ranging between 10 Pa and 75 kPa and temperatures between 1073 and 1323 K. The electrical conductivity data, involving both n and p type regimes, are considered in terms of defect disorder assuming that the predominant mobile defects are oxygen vacancies, which are compensated by immobile titanium vacancies. Application of this model led to the determination of the electrical conductivity components related to electrons, electron holes and ions. The determined empirical relationships between electrical conductivity and oxygen activity may be used for predicting the effect of experimental conditions on semiconducting properties of TiO2. The activation energy for the standard conductivity component of electrons and holes is 226 and 102 kJ mol−1 respectively.

Photoreactivity models for titanium dioxide with water

Abstract The present work considers the mechanisms of the reactivity of water with titanium dioxide, which, due to its high chemical stability, is one of the most promising photoelectrode materials for photoelectrochemical water splitting. The reactivity is considered in terms of two models. The first model considers the reaction between a water molecule and a Ti vacancy at the surface of an ideal rutile structure. The second reactivity model considers the charge transfer between a bi-dimensional surface layer and water molecules.

Electrical conductivity of TiO2 within n–p transition Part II – Electrical conductivity components

Abstract The present paper reports the electrical properties of undoped TiO2 single crystal in terms of the conductivity components for electrons, holes and ions and the related transference numbers within the n–p transition regime. These data indicate that ionic conductivity in TiO2 may assume a substantial value, which cannot be ignored. The minimum of the electrical conductivity at the n–p transition and its electronic component, were used for the determination of the band gap, which is 3·13 and 3·1 eV respectively.

Defect disorder and semiconducting properties of titanium dioxide

The semiconducting properties of TiO2 single crystal and their changes during oxidation and reduction at elevated temperatures (1073 - 1323 K) under controlled oxygen activity (10-9 - 105 Pa) were monitored using measurements of electrical conductivity and thermoelectric power. The experimental data obtained in equilibrium led to a TiO2 defect disorder model. According to this model, oxygen vacancies are the predominant defect species in TiO2 across a wide range of oxygen activities. This work has discovered the diffusion of Ti vacancies, which are formed during prolonged oxidation at elevated temperatures and in a gas phase of high oxygen activity. Observations indicate that appreciable concentrations of Ti vacancies are formed on the TiO2 surface and then are very slowly incorporated into the bulk. The obtained diffusion data has shown that in the commonly studied temperature range (1000-1400 K) the Ti vacancy concentration is quenched and can be considered as constant. Prolonged oxidation involves two kinetic regimes that are related to the transport of defects of different mobilities. The defect disorder model derived in this work may be beneficial for engineering TiO2 for enhanced water splitting through the selection of optimal processing conditions, including temperature and oxygen activity.