Mohammad A. Alim

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.

An In-house-Built Thermally Stimulated Current Measurement Setup: Strontium Titanate as a Test System

A semiautomated thermally stimulated current (TSC) measurement setup was developed in-house, which is capable of measuring low currents (<10-15 A) in a wide temperature range. This setup was used to study defects (bulk traps) in strontium titanate ceramics and single crystals. Strontium titanate was chosen as a test system because of the availability of data in the literature. Analysis of the TSC curves using various methods revealed traps with activation energies of approximately 0.64 eV and 0.80 eV, which can be attributed to acceptor impurities in single crystal and undoped SrTiO3, respectively. In polycrystalline SrTiO3 doped with 0.5, 1.0 and 1.5 mole% Nb2O5, traps with activation energy of ~0.95 eV were found. These traps are believed to arise from Nb2O5 dissolved in the SrTiO3 matrix, as confirmed by scanning electron microscope with energy dispersive X-ray spectrometry (SEM/EDX) and electrical conductivity measurements.

Electrical Conductivity, Thermoelectric Power, and Equilibration Kinetics of Nb-Doped TiO2

This work considers the equilibration kinetics of Nb-doped TiO2 single crystal (0.066 atom % Nb) during oxidation and reduction within a wide range of temperature (1073-1298 K) and oxygen activity (10(-14)-10(5) Pa). The associated semiconducting properties were determined using simultaneous measurements of both electrical conductivity and thermoelectric power. It is shown that the chemical diffusion coefficient in the strongly reducing regime, p(O2) < 10(-5) Pa, is 4 orders of magnitude larger than that in the reducing and oxidizing regimes, 10 Pa < p(O2) < 22 kPa. The derived theoretical model considers the gas/solid kinetics for the TiO2/O2 system in terms of two diffusion regimes: the fast regime related to fast defects (oxygen vacancies and titanium interstitials) and leading to quasi-equilibrium, and the slow regime associated with slow defects (titanium vacancies) resulting in the gas/solid equilibrium. It has been shown that incorporation of donor-type elements, such as niobium, and imposition of oxygen activity above a certain critical value, results in a substantial reduction in the concentration of high mobility defects and leads to slowing down the equilibration kinetics. In consequence, the fast kinetic regime is not observed. Comparison of the kinetic data for Nb-doped TiO2 single crystal (this work) and polycrystalline Nb-doped TiO2 (reported before) indicates that the gas/solid kinetics for the polycrystalline specimen at higher oxygen activities is rate controlled by the transport of oxygen within individual grains.

Electrical properties and defect chemistry of In-doped TiO2 in terms of Jonker formalism

The present work considers the semiconducting properties of In-doped TiO2 in terms of the Jonker formalism applied for both electrical conductivity and thermoelectric power data determined simultaneously in equilibrium with the gas phase of controlled oxygen activity. It is shown that the electrical properties of In-doped TiO2 annealed in oxidizing conditions [p(O2) > 10 Pa] can be described by the Jonker formalism very well. However, annealing of In-doped TiO2 in strongly reducing conditions [p(O2) < 10(-10) Pa], imposed by the gas phase involving hydrogen, results in a deviation of the experimental data from the Jonkers theoretical model derived for the Maxwell-Boltzmann statistics. This departure is considered in terms of the effect of hydrogen on the formation of structural domains, which are expected to be entirely different from those of oxidized TiO2 in terms of its electronic properties. It is argued that In-doped TiO2 annealed in the gas phase involving hydrogen exhibits a high concentration of donor-type ionic defects, which lead to the formation of high concentration of electrons. The related semiconducting properties are inconsistent with the model of classical semiconductor where the electrons are described by the Maxwell-Boltzmann statistics. It is concluded that strong interactions within the electron gas lead, in consequence, to the behavior resembling correlated transport of electrons. The obtained results suggest that indium incorporation into the rutile structure of TiO2 results in the formation of structural properties that exhibit extraordinary charge transport.

Equilibration Kinetics and Chemical Diffusion of Indium-Doped TiO2

The present work reports the gas/solid equilibration kinetics for In-doped TiO2 (0.4 atom % In) at elevated temperatures (1023-1273 K) in the gas phase of controlled oxygen activity [10(-13) Pa < p(O2) < 10(5) Pa]. Thus, the determined chemical diffusion coefficient is considered in terms of a microdiffusion coefficient that is reflective of the transport kinetics within very narrow ranges of oxygen activities. In analogy to pure TiO2, the chemical diffusion coefficient for In-doped TiO2 exhibits a maximum at the n-p transition point. The activation energy of the chemical diffusion exhibits a decrease with temperature from 200 kJ/mol at 1023 K to an insignificant value at 1273 K. This effect is reflective of a segregation-induced electrical potential barrier blocking the transport of defects. The absolute value of the chemical diffusion coefficient for In-doped TiO2 is larger from that of pure TiO2 by a factor of approximately 10. The effect of indium on the diffusion rate is considered in terms of the associated concentration of oxygen vacancies, which are formed in order to satisfy the charge neutrality for In-doped TiO2.

Basic Concepts of Solar-to-Chemical Energy Conversion by Oxide Semiconductors

The purpose of this work is to consider the basic concepts on the present state of understanding of photocatalytic energy conversion using oxide semiconductors. This work also considers the approaches in derivation of theoretical models that allow explanation of the effect of properties on the performance of oxide-based photocatalysts in photocatalytic water oxidation. In this work we show that the performance of photocatalytic systems must be considered in terms of a range of the key performance-related properties (KPPs) that, in addition to the band gap, include the concentration of surface active sites, charge transport and Fermi level. Taking into account that all these KPPs are related to defect disorder, defect engineering may be applied in processing oxide semiconductors with optimal properties that are required to exhibit maximised performance in solar-to-chemical energy conversion.