Effect of indium and niobium segregation on the surface vs. bulk chemistry of titanium dioxide (rutile)

Since the landmark paper in 1972 by Fujishima and Honda [1], TiO2 has become one of the most promising candidates of a new generation of solar energy materials capable of generating clean hydrogen fuel using only sunlight (photo-electrochemically) to dissociate water. TiO2 has both bulk properties and surface properties which contribute to its functional performance. Considering that all of the electrochemical reactions induced by light occur at the surface of TiO2, it becomes clear that understanding the surface properties of TiO2 is of crucial importance for its performance; specifically the conversion of solar energy into chemical energy. The surface phase of TiO2 can be substantially different from that of the bulk phase as a result of a phenomenon known as segregation. Segregation involves the transport of certain lattice species from the bulk phase to the surface, driven by excess surface energy. To date, developments in the understanding of TiO2 solid solutions and related properties have mainly been centred on bulk properties. In comparison, relatively little work has been reported on segregation in TiO2 solid solutions and its influence on functional properties, such as reactivity and photoreactivity. The present work has studied the effect of indium (acceptor-type ion) and niobium (donor-type ion) segregation on the surface chemistry of well-defined In-doped and Nb-doped TiO2 solid solutions. Specifically, examining the relationship between imposed sample processing conditions, such as the gas phase oxygen activity, on segregation-induced surface enrichment. This was achieved using a range of complimentary analysis techniques including X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), Rutherford backscattering (RBS) and proton-induced X-ray emission (PIXE)

Effect of oxygen activity on surface composition of in-doped TiO2 at elevated temperatures

The present work reports the effect of oxygen activity on the segregation-induced surface concentration of indium for In-doped TiO2 at different time intervals (5-120 h) of annealing at 1273 K. It is shown that equilibrium segregation of indium in oxidizing conditions, p(O2) = 21 kPa, is established within 20 h. However, annealing in reducing conditions, p(O2) = 10-10 Pa, does not lead to equilibrium segregation due to indium evaporation, which becomes substantial at p(O2) < 102 Pa. In these conditions, annealing results in an initial rise of surface concentration due to segregation and subsequent decrease due to evaporation. These data may be used for the modification of surface vs bulk composition of In-doped TiO2 in a controlled manner

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 TiO2. 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 TiO2 containing 0.3 atom % In at 1273 K in the gas phase of controlled oxygen activity [p(O2) = 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 TiO2-based semiconductors with enhanced performance in solar energy conversion

Reactivity between In2O3 and TiO2 (rutile) studied using secondary ion mass spectrometry (SIMS)

The present work considers the reaction kinetics between titanium dioxide, TiO2 (rutile) single crystal and a thin layer of indium oxide, In2O3 deposited on its surface. The reported experimental data are reflective of the chemical reaction involving the diffusive transport of In3+ in the rutile phase. The reaction progress at 1173 K is monitored using the secondary ion mass spectrometry (SIMS) technique. It is shown that the SIMS depth profiles may be considered in terms of two distinctly different components, related to the surface layer of In2O3, and the rutile single crystal phase beneath. The depth profile of the rutile phase involves the region related to bulk diffusion of indium as well as the background composition. The bulk diffusion coefficient of indium, In3+ in single crystal TiO2 (rutile) at 1173 K and p(O2) = 21 kPa was determined to be 4.4(±0.2) × 10−18 m2 s−1

Gold nanoparticle incorporation into porous titania networks using an agarose gel templating technique for photocatalytic applications

Porous titania networks containing gold nanoparticles have been synthesized and tested in photocatalytic applications. The porous structure was controlled using a templating technique, while a range of gold concentrations and a variety of routes were investigated to incorporate the gold nanoparticles. The influence of these parameters on the final structure (surface area and pore size), the gold crystal size, distribution, and content, and the photocatalytic activity of the porous materials were investigated. UV-vis diffuse reflectance spectra of the Au/TiO2 materials showed strong absorbance at approximately 580 nm, indicating the successful incorporation of the gold species. X-ray diffraction analysis ascertained that the titania materials were crystalline (anatase phase) with gold peaks observed only when the gold content was greater than 0.25 wt %. Gold distribution and content in the materials were measured using secondary ion mass spectrometry and inductively coupled plasma mass spectrometry. From transmission electron microscopy analysis, the gold particle size and distribution varied with both the material preparation method and the concentration of gold used in the synthesis. Photocatalytic activity was dependent on the gold particle size and gold quantity. The highest photocatalytic activity under UV light irradiation as monitored by the photodecomposition of methylene blue was obtained for the Au/TiO2 sample containing 2.0 wt % gold prepared by the deposition of gold onto prefabricated porous TiO2

Diffusion kinetics of indium in TiO2 (rutile)

This work determines the self-diffusion coefficients of indium in TiO 2 single crystal (rutile). Diffusion concentration profiles were imposed by deposition of a thin surface layer of InCl3 on the TiO2 single crystal and subsequent annealing in the temperature range 1073-1573 K. The diffusion-induced concentration profiles of indium as a function of depth were determined using secondary ion mass spectrometry (SIMS). These diffusion profiles were used to calculate the self-diffusion coefficients of indium in the polycrystalline In2TiO5 surface layer and the TiO2 single crystal. The temperature dependence of the respective diffusion coefficients, in the range 1073-1573 K, can be expressed by the following formulas: DIn-In2TiO5=1. 9×10-13exp(-142kJ/mol/RT)[m2s-1] and DIn-TiO2=7.4×10-4exp(-316kJ/mol/RT) [m2s-1] The obtained activation energy for bulk diffusion of indium in rutile (316 kJ/mol) is similar to that of zirconium in rutile (325 kJ/mol). The determined diffusion data can be used in selection of optimal processing conditions for TiO2-In2O3 solid solutions

Defect engineering of photosensitive oxide materials. Example of TiO2 solid solutions

The imperative to protect the environment from increasingly apparent climate change imposes the urgent need to reduce the emissions of greenhouse gases to the atmosphere. This, consequently, results in intensification of research in the development of new materials and devices for the generation of energy that is environmentally clean. This work considers photosensitive oxide semiconductors for solar energy conversion by light-induced water oxidation. It has been documented that the performance of oxide semiconductors for solar-to-chemical energy conversion is determined by a range of defect-related properties, including the concentration of surface active sites, Fermi level, charge transport, electronic structure, and alignment of band edges with the energy level of the redox couple. The present work considers the research strategy in processing TiO2-based semiconductors, which are the promising candidates for a new generation of solar materials. It is shown that the performance-related properties of TiO2 and its solid solutions are determined by surface versus bulk defect disorder and the associated semiconducting properties. Therefore, the development of TiO2-based materials with enhanced performance could be based on using defect engineering for imposing optimized bulk versus surface properties. In this work, we discuss a range of defect-related properties of TiO2 and its solid solutions, such as electrical and optical properties and the related photocatalytic performance. We show that the phenomenon of segregation may be used as the technology for imposition of controlled surface versus bulk defect disorder that is required for processing the systems with optimized properties

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 105 Pa results in: (i) increase in the band gap from 2.42 eV to 2.91 eV (direct transitions) or 2.88 eV 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