Papers by Author: Janusz Nowotny

Abstract: Rutile is a good candidate for total water oxidation and hydrogen production due mainly to its corrosion resistance in water. In its native form, the direct band gap of 3.02eV, with the absorption edge in the visible part of the solar spectrum at 416nm, but close to the UV region, results in poor photo-catalytic efficiency of around 3%, well below the commercial threshold of around 10%. One option to enhance the photo-catalytic efficiency is band gap engineering, through the reduction of the Fermi level, which may be achieve by either changing the oxygen activity, or the incorporation of aliovalent foreign ions. The incorporation of donor and acceptor-type of ions may lead to a shift in the Fermi level close to the conduction or valence bands. Single crystals of TiO₂-Rutile (SG: P4 2/mnm), with (001) orientation were implanted along <001> at 10keV and 2MeV, where the range of C in TiO₂ is 21nm and 1.6μm, respectively. The ion implantation was carried out at various equivalent C concentrations, and characterised in-situ by ERDA with 82MeV Iodine, and PIXE with 2.5MeV protons. Optic band gap was assessed by ground state band structure calculations based on CASTEP code, with geometry optimisation by general gradient approximation and PBE functional, and energy optimisation by local density approximation and CA-PZ functional.

Abstract: 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.

Abstract: The present work considers the performance-related properties of TiO2-based photosensitive semiconductors as photocatalysts for water purification and photoelectrodes for water splitting. These properties, including electronic structure, charge transport, surface properties, and near-tosurface properties, have an effect on light absorption and its conversion into chemical energy, and are closely related to defect disorder. Therefore, defect chemistry may be used as a framework for their modification in order to achieve enhanced performance. The present work considers the relationships between these specific performance-related properties and defect disorder.

Abstract: Titanium dioxide (TiO2) nanotubes have been reported one decade ago and have proven to be of a great interest in photocatalytic water splitting, as well as gas sensing and anti-bacterial/cancer treatment. This paper presents an overview on general preparation approaches (chemical treatment, template method and anodic oxidation) of tubular TiO2 nanoarchitectures and their characterization. Current applications of the nanotubes as photocatalysts are also reviewed.

Abstract: 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.