T. Bak

Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects

Abstract The present work considers hydrogen generation from water using solar energy. The work is focused on the materials-related issues in the development of high-efficiency photo-electrochemical cells (PECs). The property requirements for photo-electrodes, in terms of semiconducting and electrochemical properties and their impact on the performance of PECs, are outlined. Different types of PECs are overviewed and the impact of the PEC structure and materials selection on the conversion efficiency of solar energy are considered. Trends in research in the development of high-efficiency PECs are discussed. It is argued that very sophisticated materials engineering must be used for processing the materials that will satisfy the specific requirements for photo-electrodes. An important issue in the processing of these materials is the bulk vs. interface properties at the solid/solid interfaces (e.g., grain boundaries) and solid/liquid interfaces (e.g., electrode/electrolyte interface). Consequently, the development of PECs with the efficiency required for commercialization requires the application of up-to-date materials processing technology. The performance of PECs is considered in terms of: • excitation of electron–hole pair in photo-electrodes; • charge separation in photo-electrodes; • electrode processes and related charge transfer within PECs; • generation of the PEC voltage required for water decomposition. This work also gives empirical data on the performance of PECs of different structures and materials selection. It is argued that PEC technology is the most promising technology for hydrogen production owing to several reasons: • PEC technology is based on solar energy, which is a perpetual source of energy, and water, which is a renewable resource; • PEC technology is environmentally safe, with no undesirable byproducts; • PEC technology may be used on both large and small scales; • PEC technology is relatively uncomplicated. According to current predictions, the production of hydrogen will skyrocket by 2010 (Morgan and Sissine, Congressional Research Service, Report for Congress. The Committee for the National Institute for the Environment, Washington, DC, 20006-1401, 28 April 1995). Consequently, seed funding already has been allocated to several national research programs aiming at the development of hydrogen technology. The countries having access to this PEC technology are likely to form the OPEC of the future.

Photo-electrochemical properties of the TiO2-Pt system in aqueous solutions

Abstract Electrochemical characteristics of photo-electrochemical cell (involving photo-anode made of TiO2 and Pt as cathode) were investigated during the light-on and the light-off regimes using the sunlight. Maximum photo-voltage of the open cell involving undoped TiO2 as photo-anode is 0.7 and 1.6 V without and with chemical bias (Δ pH=14.6), respectively. Application of ceramic Cr-doped TiO2 (involving 1 wt % of Cr2O3) as photo-anode results in a substantial decrease of the cell voltage (to 0.8 V ) despite the application of chemical bias.

Defect chemistry and semiconducting properties of titanium dioxide: I. Intrinsic electronic equilibrium☆

Abstract The present work describes defect chemistry and semiconducting properties of TiO 2 within the n–p transition regime. Quantitative considerations on the relationships between the concentration of ionic and electronic defects at the minimum of electrical conductivity vs. oxygen partial pressure resulted in the derivation of a theoretical model. The model is based on the empirical data of electrical conductivity for Cr-doped TiO 2 (exhibiting n–p transition). This model was then applied for the determination of the intrinsic electronic equilibrium constant which is the following function of temperature: (1) K i =3.74×10 −2 exp − 3.039±0.053 eV kT A good agreement between this equilibrium constant and the experimental data of electrical conductivity for TiO 2 doped with donors (Cr) was revealed. It was observed that the concentration of oxygen vacancies determined using the derived model is only slightly dependent on the value of the electronic intrinsic equilibrium constant.

Defect chemistry and defect engineering of TiO2-based semiconductors for solar energy conversion

This tutorial review considers defect chemistry of TiO2 and its solid solutions as well as defect-related properties associated with solar-to-chemical energy conversion, such as Fermi level, bandgap, charge transport and surface active sites. Defect disorder is discussed in terms of defect reactions and the related charge compensation. Defect equilibria are used in derivation of defect diagrams showing the effect of oxygen activity and temperature on the concentration of both ionic and electronic defects. These defect diagrams may be used for imposition of desired semiconducting properties that are needed to maximize the performance of TiO2-based photoelectrodes for the generation of solar hydrogen fuel using photo electrochemical cells (PECs) and photocatalysts for water purification. The performance of the TiO2-based semiconductors is considered in terms of the key performance-related properties (KPPs) that are defect related. It is shown that defect engineering may be applied for optimization of the KPPs in order to achieve optimum performance.

Defect chemistry and semiconducting properties of titanium dioxide: III. Mobility of electronic charge carriers☆ ☆

Abstract The present work performs quantitative analysis of the electrical conductivity data reported in the literature in terms of defect chemistry models. The analysis results in the determination of the mobility terms for electrons and electron holes leading to the following respective forms: μ n =0.106 ( cm 2 V −1 s −1 ) μ p =(1.05±0.89) 10 6 T exp − 0.853±0.073( eV ) kT The mobility data determined in this work were then used for verification of defect chemistry disorder models and a good agreement was revealed.

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.

Effect of indium segregation on the surface versus bulk chemistry for indium-doped TiO2

This work reports the effect of indium segregation on the surface versus bulk composition of indium (In)-doped TiO(2). The studies are performed using proton-induced X-ray emission (PIXE), secondary-ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectroscopy (RBS). The results of XPS analysis indicate that annealing of In-doped TiO(2) containing 0.3 atom % In at 1273 K in the gas phase of controlled oxygen activity [p(O(2)) = 75 kPa and 10 Pa] results in a surface enrichment of 2.95 and 2.61 atom % In, respectively. The obtained segregation data are considered in terms of the transport of indium ions from its titanium sites in the bulk phase to the surface where these ions are incorporated into interstitial sites. The effect of oxygen activity on the segregation-induced surface enrichment is considered in terms of the formation of a low-dimensional surface structure and a sublayer, which are charged negatively. The latter is formed as a result of strong interactions between titanium vacancies and interstitial indium ions, leading to the formation of defect complexes. The data obtained in this work may be used for engineering of TiO(2)-based semiconductors with enhanced performance in solar energy conversion.

Photocatalytic Properties of TiO2: Evidence of the Key Role of Surface Active Sites in Water Oxidation

Photocatalytic activity of oxide semiconductors is commonly considered in terms of the effect of the band gap on the light-induced performance. The present work considers a combined effect of several key performance-related properties (KPPs) on photocatalytic activity of TiO2 (rutile), including the chemical potential of electrons (Fermi level), the concentration of surface active sites, and charge transport, in addition to the band gap. The KPPs have been modified using defect engineering. This approach led to imposition of different defect disorders and the associated KPPs, which are defect-related. This work shows, for the first time, a competitive influence of different KPPs on photocatalytic activity that was tested using oxidation of methylene blue (MB). It is shown that the increase of oxygen activity in the TiO2 lattice from 10(-12) Pa to 10(5) Pa results in (i) increase in the band gap from 2.42 to 2.91 eV (direct transitions) or 2.88 to 3 eV (indirect transitions), (ii) increase in the population of surface active sites, (iii) decrease of the Fermi level, and (iv) decrease of the charge transport. It is shown that the observed changes in the photocatalytic activity are determined by two dominant KPPs: the concentration of active surface sites and the Fermi level, while the band gap and charge transport have a minor effect on the photocatalytic performance. The effect of the defect-related properties on photoreactivity of TiO2 with water is considered in terms of a theoretical model offering molecular-level insight into the process.

Defect chemistry and semiconducting properties of calcium titanate

The present paper considers the effect of oxygen partial pressure on the presence of point defects in calcium titanate (CaTiO3) at elevated temperatures at which a gas/solid equilibrium is reached. Defect models of undoped (CaTiO3) are considered within several regimes of oxygen partial pressures involving (i) extremely reducing conditions, (ii) reducing conditions, and (iii) oxidizing conditions, which are described by different charge-neutrality conditions. The mechanism of donor incorporation is considered in terms of both ionic and electronic charge compensation. It is shown that electronic and ionic charge compensations prevail at low and high p(O2), respectively.

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.