Alan V. Chadwick

Titania and silver–titania composite films on glass—potent antimicrobial coatings

Titania (anatase) and Ag-doped titania (anatase) coatings were prepared on glass microscope slides by a sol - gel dip-coating method. The resultant coatings were characterised by X-ray diffraction, X-ray absorption near edge structure (XANES), Raman, scanning electron microscopy (SEM), wavelength dispersive X-ray (WDX) analysis, X-ray photoelectron spectroscopy (XPS) and UV-vis techniques and shown to consist of anatase with ca. 0.2 - 1 atom% Ag2O. Photocatalytic activity of the coatings was determined by photomineralisation of stearic acid, monitored by FT-IR spectroscopy. Photocatalytically-active coatings were screened for their antibacterial efficacy against Staphylococcus aureus (NCTC 6571), Escherichia coli ( NCTC 10418) and Bacillus cereus (CH70-2). Ag-doped titania coatings were found to be significantly more photocatalytically and antimicrobially active than a titania coating. No antimicrobial activity was observed in the dark - indicating that silver ion diffusion was not the mechanism for antimicrobial action. The mode of action was explained in terms of a charge separation model. The coatings also demonstrated significantly higher activity against the Gram-positive organisms than against the Gram-negative. The Ag2O - TiO2 coating is a potentially useful coating for hard surfaces in a hospital environment due to its robustness, stability to cleaning and reuse, and its excellent antimicrobial response.

Synthesis of ordered mesoporous NiO with crystalline walls and a bimodal pore size distribution

A mesoporous solid with crystalline walls and an ordered pore structure exhibiting a bimodal pore size distribution (3.3 and 11 nm diameter pores) has been synthesized. Previous attempts to synthesize solids with large ordered mesopores by hard templating focused on the preparation of templates with thick walls (the thick walls become the pores in the target materials), something that has proved difficult to achieve. Here the large pores (11 nm) do not depend on the synthesis of a template with thick walls but instead on controlling the microporous bridging between the two sets of mesopores in the KIT-6 template. Such control determines the relative proportion of the two pore sizes. The wall thickness of the 3D cubic NiO mesopore has also been varied. Preliminary magnetic characterization indicates the freezing of uncompensated moments or blocking of superparamagnetism.

Definitions of terms relating to the structure and processing of sols, gels, networks, and inorganic-organic hybrid materials (IUPAC Recommendations 2007)

This document defines terms related to the structure and processing of inorganic, polymeric, and inorganic-organic hybrid materials from precursors, through gels to solid products. It is divided into four sections - precursors, gels, solids, and processes - and the terms have been restricted to those most commonly encountered. For the sake of completeness and where they are already satisfactorily defined for the scope of this document, terms from other IUPAC publications have been used. Otherwise, the terms and their definitions have been assembled in consultation with experts in the relevant fields. The definitions are intended to assist the reader who is unfamiliar with sol-gel processing, ceramization, and related technologies and materials, and to serve as a guide to the use of standard terminology by those researching in these areas.

On the behavior of the LixNiO2 system: an electrochemical and structural overview

Lithium nickel oxide exhibits a departure from stoichiometry (Li1 − zNi1 + zO2) consisting in the presence of extra-nickel ions within the lithium sites. Using optimized experimental synthesis conditions, compositions very close to the ideal stoichiometry (z = 0.02) can be obtained. By using the sensitivity of the lithium site isotropic temperature factor to the stoichiometry, the amount of extra-nickel ions can be determined in a very precise way. The loss of reversibility at the first cycle is mainly related to the change in the oxidation state of the extra-nickel ions, which induces a local collapse of the structure and makes difficult the lithium re-intercalation. A systematic structural study of LixNiO2 phases has been performed by extended X-ray absorption fine structure (EXAFS) as well as X-ray and electron diffraction. In the case of the starting Li0.98Ni1.02O2 phase, a local distortion of the NiO6 octahedra, resulting from a dynamic Jahn-Teller effect of low spin trivalent nickel ions has been evidenced from the EXAFS study. For the partially de-intercalated materials (0.50 < x < 0.75) which crystallize in the monoclinic system, the EXAFS study shows that the NiO6 octahedra are only slightly distorted due to the occurrence of a hopping phenomenon between NiIV and NiIII. Electron diffraction experiments show the existence of a superstructure due to a peculiar lithium-ion ordering. Systematic electrochemical studies have shown that this ordering is strongly sensititve to the presence of extra-nickel ions.

Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen

During the charging and discharging of lithium-ion-battery cathodes through the de- and reintercalation of lithium ions, electroneutrality is maintained by transition-metal redox chemistry, which limits the charge that can be stored. However, for some transition-metal oxides this limit can be broken and oxygen loss and/or oxygen redox reactions have been proposed to explain the phenomenon. We present operando mass spectrometry of (18)O-labelled Li1.2[Ni0.13(2+)Co0.13(3+)Mn0.54(4+)]O2, which demonstrates that oxygen is extracted from the lattice on charging a Li1.2[Ni0.13(2+)Co0.13(3+)Mn0.54(4+)]O2 cathode, although we detected no O2 evolution. Combined soft X-ray absorption spectroscopy, resonant inelastic X-ray scattering spectroscopy, X-ray absorption near edge structure spectroscopy and Raman spectroscopy demonstrates that, in addition to oxygen loss, Li(+) removal is charge compensated by the formation of localized electron holes on O atoms coordinated by Mn(4+) and Li(+) ions, which serve to promote the localization, and not the formation, of true O2(2-) (peroxide, O-O ~1.45 Å) species. The quantity of charge compensated by oxygen removal and by the formation of electron holes on the O atoms is estimated, and for the case described here the latter dominates.

Non-cooperative Jahn-Teller effect in LiNiO2 : an EXAFS study

Lithium nickel oxide, used as positive electrode in rechargeable lithium batteries, exhibits a layered structure made of NiO2 slabs between which Li+ ions are inserted in an octahedral environment. In fact, stoichiometric LiNiO2 has never been reported, the true formula is Li1−zNi1+zO2 (0.0<z≤0.20). z depending on the experimental conditions. Special attention devoted to the synthesis conditions, has allowed us to obtain quasi-2D LiNiO2. EXAFS spectroscopy shows clearly the existence of two different NiO bond lengths: at 1.91 A (for four of them) and 2.09 A for the two last ones. This local distortion of the NiO6 octahedra results from the Jahn-Teller effect of the trivalent nickel ion in the low spin state t26e1.

Kinetic factors in the response of organometallic semiconductor gas sensors

Abstract The influences of thermal-treatment and gas-exposure history on the kinetics of the electrical conductivity changes occurring in sublimed films of lead phthalocyanine, chloroaluminium phthalocyanine, fluoroaluminium phthalocyanine and tetraazadibenzo[14]-annulene and its copper complex on exposure to sub-parts per million (ppm) concentrations of nitrogen dioxide (NO 2 ) in air have been studied. Freshly sublimed films are sensitized to nitrogen dioxide on heating in air. Thermally-induced structural changes and the creation of traces of strongly-bound nitrogen dioxide species that exercise a catalytic role in facilitating displacement of oxygen from surface sites are shown to be responsible for these changes. Their significance in determining the kinetics of the response of organic semiconductor gas sensors is discussed, together with implications for the optimization of the operating modes of such sensors.

A structural basis for ionic diffusion in oxide glasses

Abstract The relationships between the environments of cations in alkali silicates measured by X-ray absorption fine structure (XAFS) and magic angle spinning nuclear magnetic resonance (MASNMR) are considered. Both are consistent with the modified random network for glass structure in which modifiers form channels percolating through the network. It is proposed that the mechanisms determining the distribution of bridging and non-bridging oxygen atoms at the glass transition are the same as those that promote ionic transport at lower temperatures in the glass. In particular the results of XAFS and MASNMR can be used to predict the activation energy for ionic transport and the magnitude of the electrical conductivity. Values of these parameters for alkali disilicates are in good agreement with those measured directly from transport properties.

Electron donor–acceptor interactions and surface semiconductivity in molecular crystals as a function of ambient gas

Quantitative studies of the magnitude, rate and reversibility of changes in surface semiconductivity of single crystals of phthalocyanines, perylene, tetracyanoquinodimethane (TCNQ) and molecular complexes as a function of ambient gas are reported. NO2+ N2O4 increases the surface conductivity of phthalocyanines, by factors of up to 108. At low pressures, the magnitude of the increase follows the Freundlich adsorption isotherm, while the rate obeys the Elovich equation. The saturation conductivity at high pressures is similar for all the phthalocyanines and corresponds to complete surface coverage, each adsorbed molecule producing one ionised state. Reversibility on heating in vacuo depends on the metal : (metal free, Ni, Cu, Zn) > (Co, Mn) > Pb. In all cases, treatment with low pressures of NH3 gives rapid reversal of the effects. BF3 gives smaller, irreversible enhancements. The surface conductivity of perylene is enhanced by a factor up to 108 in BF3 and the effect is reversed on treatment with NH3. NO2+ N2O4 produces smaller effects (104), easily reversible. Perylene—TCNQ shows small conductivity increases (102) with both NO2+ N2O4 and NH3, while TCNQ shows similar effects only with NH3. Semiconduction activation energies in the presence of gases enhancing conductivity are reduced to values comparable with those obtained from temperature dependence of photoconduction (0.1–0.2 eV). Conductivity changes are interpreted in terms of production of ionised states following weak chemisorption involving donor–acceptor interactions. The magnitude and reversibility of the changes depends on the nature of the orbitals involved in these interactions, and provides scope for selective gas detection.

High-temperature transport in fluorites

Abstract The high-temperature fluorites are amongst the simplest materials to show superionic conductivity. In this paper the recent experimental and theoretical studies of the nature of disorder and transport in the halide fluorites is reviewed. The majority of the evidence is in favour of anion motion by discrete jumps and not by a cooperative liquid-like diffusion. It is also clear that the anions do not appreciably occupy the cube-centre interstitial sites of the lattice. The exact nature and extent of the disorder has still to be resolved, although the number of mobile defects appears to be relatively small.