Advances in Applied Ceramics: Structural,Functional and Bioceramics

Water disinfection (removal of microbial agents) using sunlight is an emerging technology, which has the capacity to address the global shortage of drinking water. Therefore, intensive investigations in many laboratories aim to develop photocatalyst for water disinfection. The research is focused on titanium dioxide (TiO₂), which is the most promising candidate for high performance photocatalyst able to address the commercial requirements. The present work (Part 1) considers the effect of defect disorder on semiconducting and photocatalytic properties of TiO₂ (rutile) in water disinfection using solar energy. It is shown that photocatalytic properties of TiO₂ in water are closely related to the light induced reactivity of TiO₂ with water leading to the formation of active species, such as OH*, H₂O₂ and , which have the capacity to oxidise microorganisms. It is also shown that the ability of TiO₂ to form the active radicals is closely associated with the presence of point defects in the TiO₂ lattice and the related semiconducting properties. Therefore, photocatalytic properties of TiO₂ may be modified in a controlled manner by changes in its defect disorder. Consequently, defect chemistry may be used as the framework in the development of TiO₂ with controlled properties that are desired for solar water disinfection. The following work (Part 2) considers the structure of bacteria and their reactivity/photoreactivity with TiO₂ in aqueous environments. Both Part 1 and 2 bring together the concepts of TiO₂ photocatalysis and the concepts of microbiology in order to derive the theoretical models that are needed for the development of high performance photocatalysts for solar water disinfection.

Worldwide, waterborne diseases resulting from ingestion of contaminated drinking water carrying infectious microbial agents kill some 2 million people annually. Disinfection of drinking water aimed at the removal and inactivation of microbiological agents typically requires the consumption of energy, which to date is mainly generated from fossil fuels. The titanium dioxide (TiO₂) based photo-induced oxidising system (POS) is one of the most promising candidates for the disinfection of water using solar energy. Progress in photocatalytic water disinfection requires understanding both concepts of TiO₂ semiconductors as well the biochemistry of microorganisms. The aim of the present study is to introduce workers in the solid state sciences to relevant basic concepts and terminology of microbial biochemistry. We review the structure of the bacterial cell envelope, which is the major target of the TiO₂ based POS. We outline how attack by the POS leads to the production of additional reactive species commonly referred to in the biological literature as reactive oxygen species both at the surface and the inside of the cell and develop a unifying model of the molecular mechanisms underlying the biocidal activity of the TiO₂ POS.

Mesoporous C/N doped TiO₂ (MCNT) samples were prepared, and Pt was deposited on their surfaces. The hydrogen production capability of this material was investigated by irradiating it with UV light at two different wavelengths. It was found that MCNT could be used to produce hydrogen gas. The highest hydrogen production rate was obtained when 0·003 mol Pt was deposited on the surface of 1 mol MCNT. Since this optimal Pt concentration is the same as that for P-25, it was concluded that the mesopore surface was not directly deposited with Pt. More hydrogen was produced when Pt deposited MCNT was irradiated with 350 nm wavelength UV light than with 370 nm wavelength UV light at similar intensity. This implies that the wavelength of UV light strongly affects hydrogen production.

The development of novel oxides, nanocomposites and architectures is in demand for the direct conversion of solar energy into other forms of energy, such as chemical and electrical energy. Especially, those oxides and devices, which can be used for photolysis of water (hydrogen and oxygen evolution) and for water purification, are of interest. So far, semiconducting oxides such as TiO₂, Fe₂O₃, WO₃, SrTiO₃, tantalates and niobates are the only class of materials which have shown high stability as photoelectrodes towards corrosion in the rate limiting step in the oxygen evolution reaction (OER) under illumination at the electrode/electrolyte interface during photolysis of water. Oxides have to be developed to be highly conductive and have a bandgap, which can be achieved by tailoring the defect chemistry of the oxides or by formation of a suited mixed oxide phase. Of special interest is the preparation of highly conductive p- and n-type TiO₂ and WO₃ as well as their alloys as corrosion stable and photoelectrocatalytically active electrodes. Bandgap reduction of pure TiO₂ involved the formations of solid solutions of TiO₂–FeO and TiO₂–Fe₂O₃, which were reported to have a bandgap of ∼2·2 eV.¹ Besides TiO₂, WO₃ also has a superior stability as a photoelectrode material in the OER. Alloying with FeO also leads to lowering of the bandgap. Alternatively, ternary oxides of the systems Ni–Co–O and Ni–Fe–O are known for their high catalytic activity in the OER. They are considered as potential cocatalysts in the process of water oxidation. The materials can also be used in hybrid photoelectrodes consisting of a photovoltaic structure to absorb the sunlight with a corrosion stable and catalytically active window layer, which is in contact with the electrolyte.

The point defect diagrams in non-stoichiometric titanium (IV) oxide TiO_2−δ, pure and doped with M³⁺ and M⁵⁺ metal ions, are presented in this work. A new method was used for the calculations of the diagrams. This method is based on derived relations between standard Gibbs energy of formation of oxygen vacancies and interstitial cations, intrinsic ionic and electronic defects and oxygen pressure at which the oxide has stoichiometric composition, and it uses experimental values of deviation from the stoichiometry. The calculations were performed using the results of studies obtained by many authors in the temperature range of 1073–1573 K.

Titanium dioxide (rutile) is known as n-type semiconductor. Recent studies show that prolonged oxidation of pure n-type TiO₂ may lead to its conversion into a p-type semiconductor. It has been documented that the conversion is associated with the formation of titanium vacancies. The present work derives the defect disorder model of TiO₂, which explains the effect of oxygen activity on the concentration of all point defects, including titanium vacancies, and the related semiconducting properties. The derived defect diagram, plotting the concentration of all ionic and electronic defects in TiO₂ within a wide range of oxygen activity [10^–15 Pa<p(O₂)<10⁵ Pa], allows to predict the effect of oxygen activity on semiconducting properties of rutile within both n- and p-type properties. This diagram may be used in the selection of processing conditions of p-type TiO₂ from n-type TiO₂. The present work also considers the kinetic aspects related to the imposition of defect equilibria associated with titanium vacancies. The real chemical formula of rutile, representing the semiconducting properties within both n- and p-type regimes, is derived.

This review paper describes primarily recent theoretical calculations with some supporting experimental findings on titania nanotubes. Nanotubes with different types and sizes are discussed in detail in terms of existing theoretical and experimental achievements. Both classical and quantum mechanical simulations are focused on. The properties of these nanotubes have been treated within first principle density functional electronic structure simulation methods. In this paper, we pay particular attention to computational aspects, but when appropriate, relationships with experimental results on titania nanostructures will be mentioned. First, the structural properties of titania nanotubes are reviewed, focusing from experimental growth mechanism to possible theoretical stable structure and orientation. Second, the electronic structure of nanotubes is discussed in terms of band gap modifications of titania and photocatalytic efficiencies in photoelectrochemical devices. Finally, current computational limitations and future directions are described with respect to the performances of nanotube titania based photosensitive devices.

Whereas doping CaTiO₃ or SrTiO₃ with ∼0·05 formula units (f.u.) of trivalent rare earth ions substituted for Ca can yield majority positron annihilation lifetime τ₁ increases of up to 30% due to the presence of cation vacancies, similar doping of BaTiO₃ produces increases of only ∼10%. The difference is attributed to hole trapping around cation vacancies in BaTiO₃. The absence of significant changes to τ₁ for La doped CaTiO₃ when oxidised samples are reduced by sintering in a hydrogenous atmosphere is attributed to the formation of oxygen vacancies that co-exist with the original cation vacancies. Solid state nuclear magnetic resonance showed that a significant fraction of the La ions in BaTiO₃ containing 0·04 f.u. of La were in cubic coordination, irrespective of the charge compensation scheme employed. Doping of sintered SrTiO₃ with La to produce up to 0·2 f.u. of Sr cation vacancies had no significant effect on Sr leaching in deionised water at 90°C.

In this review, microstructural properties, such as porosity, phase content, thermal expansion coefficient data, oxygen uptake and release data, and conducting properties of cermets consisting of Ag and LaGaO₃ doped with Sr and Mg are presented and discussed with respect to their potential application in solid oxide fuel cells operating at temperatures of about 500–600°C.

As new environmentally friendly techniques, hydride materials have been proposed to be introduced to fast reactor (FR) cores in this paper. Hydrogen atoms in metal hydride can efficiently moderate fast neutrons. Based on this fact, some metal hydrides have been investigated for their potential environmentally friendly application as nuclear materials to be used in FR cores. Two types of utilisation of metal hydrides in FR cores are discussed in this paper. One is the application of hafnium hydride as neutron absorber in FR cores. The core design has been carried out to examine its characteristics as well as to evaluate the cost reduction effect. Demonstration of the fabrication of hydride pins has been performed using hydride pellets and stainless steel claddings. The coating technique of the inner cladding surface has also been developed to reduce the permeation of hydrogen through the stainless steel cladding. The physical and chemical properties of the pellet have been measured for the purpose of designing a hafnium hydride pin. Irradiation test of the hydride pins has been performed in the experimental FR, JOYO, Japan Atomic Energy Agency. The other application is the utilisation as a transmutation target of long lived nuclear wastes. Hydride fuel containing ²³⁷Np, ²⁴¹Am and ²⁴³Am has been studied for a candidate transmutation target to be used to reduce the radioactivity of long lived nuclides contained in the nuclear wastes, which are obtained after reprocessing spent fuels.