Gc Allen

Photocatalytic oxidation of NOx gases using TiO2: a surface spectroscopic approach

The bandgap of solid-state TiO2 (3.2 eV) enables it to be a useful photocatalyst in the ultraviolet (lambda < 380 nm) region of the spectrum. A clean TiO2 surface in the presence of sunlight therefore enables the removal of harmful NOx gases from the atmosphere by oxidation to nitrates. These properties, in addition to the whiteness, relative cheapness and non-toxicity, make TiO2 ideal for the many de-NOX catalysts that are currently being commercially exploited both in the UK and Japan for concrete paving materials in inner cities. There is need, however, for further academic understanding of the surface reactions involved. Hence, we have used surface specific techniques, including X-ray photoelectron spectroscopy and Raman spectroscopy, to investigate the NOx adsorbate reaction at the TiO2 substrate surface.

Inhibition of hydroxyapatite dissolution by whole casein: the effects of pH, protein concentration, calcium, and ionic strength

Formulating drinks with reduced erosive potential is one approach for reducing dental erosion. In this study, whole casein was added to citric acid solutions representative of soft drinks, and the hydroxyapatite dissolution rate was assessed. Adding 0.02% (w/v) casein to acid solutions significantly reduced the hydroxyapatite dissolution rate by 51 +/- 4% at pH values of 2.80, 3.00, 3.20, 3.40, and 3.60, although the baseline dissolution rates of course varied as a function of pH. The protein concentration [0.002, 0.02, and 0.2% (w/v) casein] had no significant effect on dissolution inhibition. Adding both casein and calcium to citric acid resulted in a further reduction in the dissolution rate at low and intermediate calcium concentrations (5 and 10 mM) but not at higher calcium concentrations (20 and 50 mM). Ionic strength had no significant impact on the efficacy of casein. Casein also significantly reduced the hydroxyapatite dissolution rate when the hydroxyapatite was coated with a salivary pellicle. The reduction in dissolution rate is ascribed to firmly adsorbed casein on the hydroxyapatite surface, which stabilizes the crystal surface and inhibits ion detachment.

Visualisation of human plasma fibrinogen adsorbed on titanium implant surfaces with different roughness

Direct visualisation of adsorbed human plasma proteins on biomaterial surfaces may help the understanding of the performance of implants. The aim of this study was the visualisation of human plasma fibrinogen (HPF) adsorbed on different titanium implants with characterised surface properties. Seven types of titanium samples were used: mechanically polished (P); mechanically ground (G); sand blasted with alumina (B); sand blasted with alumina and etched in HF/HNO3 (BN); sand blasted with alumina and etched in HCl/H2SO4 (SLA); mechanically polished and etched in HCl/H2SO4 (PSLA); and plasma sprayed (TPS). The surface morphology and roughness of these surfaces were analysed by atomic force microscopy (AFM). The surface chemical composition of the implants was analysed by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). The seven processing methods influenced the roughness and produced distinct morphologies of the implant surfaces. The main chemical constituent of all surfaces was TiO2, with some samples showing the presence of other elements. HPF adsorbed on the titanium surfaces was visualised by AFM phase imaging. Visualisation of the adsorbed HPF was successfully achieved on samples P and G, but not on the other five samples. Differences in the dimensions and in the phase contrast of HPF molecules adsorbed on P and G surfaces were observed and discussed. No correlation between surface topography and morphology of the adsorbed proteins was observed on surfaces P and G. Possible reasons for not detecting HPF with AFM on five of the seven sample types were attributed to the surface physico-chemical properties of these samples.

Characterisation of the Spinels MxCo1-xFe2O4(M=Mn,Fe or Ni) using X-ray Photoelectron Spectroscopy

Abstract X-ray photoelectron spectra have been recorded from a series of characterised cobalt-iron containing spinels M x Co 1− x Fe 2 O 4 where M = Mn, Fe or Ni and 0 ≤ x ≤ 1. The binding energies of photoelectron peaks observed for the transition metal ions were recorded. Changes in their relative positions were noted and the measured intensities used to determine the surface composition. The observed surface segregation of certain elements were interpreted in terms of kinetic and thermodynamic factors.

Removal of Uranium(VI), Lead(II) at the Surface of TiO2 Nanotubes Studied by X-Ray Photoelectron Spectroscopy

A thin film of well-ordered anatase TiO2 nanotubes prepared by anodic oxidation of titanium metal were synthesised and used as adsorbent medium for the purification of water from aqueous uranium and lead. The amount of subtracted metal ions was quantified by using X-ray photoelectron spectroscopy at the surface of the reacted TiO2 surface. Batch experiments for the sorption of U and Pb at the surface of the titania substrate were carried out in separated solution equilibrated with air of uranyl acetate and lead nitrate, in the pH range 3–9. For uranium, the experiments were also repeated in anoxic (N2) atmosphere. The amount of metal ions adsorbed onto the titania medium was quantified by measurements of the surface coverage expressed in atomic percent, by recording high-resolution XPS spectra in the Ti2p, U4f and Pb4f photoelectron regions. Adsorption of the uranyl species in air atmosphere as a function of pH showed an adsorption edge near pH 4 with a maximum at pH 7. At higher pH the presence of very stable uranyl–carbonate complexes prevented any further adsorption. Further adsorption increased until pH 8.5 was obtained when the uranyl solution was purged from dissolved CO2. Lead ion showed a sorption edge at pH 6, with a maximum uptake at pH 8. The results showed that the uptake of uranium and lead on the selected titania medium is remarkably sensitive to the solution pH. This study demonstrates the reliability of this type of material for treating water polluted with heavy metals as well as leachates from radioactive nuclear wastes.

Mechanical properties and radiopacity of experimental glass-silica-metal hybrid composites

OBJECTIVES Experimental glass-silica-metal hybrid composites (polycomposites) were developed and tested mechanically and radiographically in this fundamental pilot study. To determine whether mechanical properties of a glass-silica filled two-paste dental composite based on a Bis-GMA/polyglycol dimethacrylate blend could be improved through the incorporation of titanium (Ti) particles (particle size ranging from 1 to 3 microm) or silver-tin-copper (Ag-Sn-Cu) particles (particle size ranging from 1 to 50 microm) we measured the diametral tensile strength, fracture toughness and radiopacity of five composites. METHODS The five materials were: I, the original unmodified composite (control group); II, as group I but containing 5% (wt/wt) of Ti particles; III, as group II but with Ti particles treated with 4-methacryloyloxyethyl trimellitate anhydride (4-META) to promote Ti-resin bonding; IV, as group I but containing 5% (wt/wt) of Ag-Sn-Cu particles; and V, as group IV but with the metal particles treated with 4-META. Ten specimens of each group were tested in a standard diametral tensile strength test and a fracture toughness test using a single-edge notched sample design and five specimens of each group were tested using a radiopacity test. RESULTS The diametral tensile strength increased statistically significantly after incorporation of Ti treated with 4-META, as tested by ANOVA (P=0.004) and Fishers LSD test. A statistically significant increase of fracture toughness was observed between the control group and groups II, III and V as tested by ANOVA (P=0.003) and Fishers LSD test. All other groups showed no statistically significant increase in diametral tensile strength and fracture toughness respectively when compared to their control groups. No statistically significant increase in radiopacity was found between the control group and the Ti filled composite, whereas a statistically significant increase in radiopacity was found between the control group and the Ag-Sn-Cu filled composite as tested by ANOVA (P=0.000) and Fishers LSD procedure. SIGNIFICANCE The introduction of titanium and silver-tin-copper fillers has potential as added components in composites to provide increased mechanical strength and radiopacity, for example for use in core materials.

UD3 formation on uranium: evidence for grain boundary precipitation

The formation of UD3 on the surface of annealed uranium was induced at 320°C and 500 mbar pressure. The location of the deuteride (UD3) precipitates with respect to metal grain boundaries at the sample surface was studied by secondary ion mass spectrometry (SIMS) and focused ion beam (FIB) methods. Ion etching was used to remove the passive oxide layer on the sample surface and reveal the underlying structure of the metal grains. There is strong evidence for the formation of UD3 phases along the metal grain boundaries at the uranium surface.

Mode control in vertical-cavity surface-emitting lasers by post-processing using focused ion-beam etching

Single-mode emission is achieved in previously multimode gain-guided vertical-cavity surface-emitting lasers (VCSELs) by localized modification of the mirror reflectivity using focused ion-beam etching. Reflectivity engineering is also demonstrated to suppress transverse mode emission in an oxide-confined device, reducing the spectral width from 1.2 nm to less than 0.5 nm.

Controls on polyacrylamide adsorption to quartz, kaolinite, and feldspar

Abstract Potentiometric titrations of quartz, kaolinite, feldspar, and partially hydrolysed polyacrylamide (HPAM), and sorption measurements of HPAM on the minerals, allows identification of the general mechanisms of polyacrylamide adsorption to aluminosilicates and quartz. Adsorption was monitored at the mineral solution interface by way of X-ray photoelectron spectroscopy (XPS). XPS spectra of the unreacted minerals show bands in the Ols, Si2p, Al2p, and Cls regions. Additional peaks are observed in the C1s and N1s regions after treatment with polyacrylamide and the latter is used in this study to monitor polymer adsorption. N1s peak intensities increase with polymer concentration to a maximum corresponding to surface site saturation. At a fixed polymer concentration, adsorption varies with pH-dependent surface charge. The adsorption mechanism changes with pH, reflecting variation in the pH-dependent concentrations of ionizable groups on polyacrylamide and at aluminosilicate surfaces, and the extent of hydrogen-bonding between uncharged mineral surface sites and polymer amide groups.

Theoretical development and validation of a Sharp Front model of the dewatering of a slurry by an absorbent substrate

The absorption of water from a slurry into an absorbent substrate is analysed using Sharp Front theory. The analysis describes the relationship between the sorptivity S of the substrate, the desorptivity R of the slurry and the transfer sorptivity A between slurry and substrate, and leads to the relationship 1/A2 = 1/R2 + 1/S2. Experimental data are presented which validate this equation for the practically important case of the absorption of water from soft mortar mixes by fired clay bricks. A unique feature of the experimental work is the measurement of the desorptivity of the mortars at a pressure equal to the wetting front capillary pressure of the clay brick substrate. Analysis of the experimental data also enables, for the first time, the calculation of the capillary potential at the slurry/substrate interface. The analysis has relevance to many aspects of ceramic and mineral processing, industrial filtration and construction engineering.