Hilde Poelman

Effect of microstructure and crystallinity on the photocatalytic activity of TiO2 thin films deposited by dc magnetron sputtering

TiO2 thin films were deposited by dc magnetron sputtering from sub-stoichiometric TiO2−x targets. The as deposited films were XRD amorphous and their microstructure was varied by changing the Ar-pressure and the applied dc power during sputtering. An anneal treatment at 673 K/1 h in air resulted in the crystallization of a preferentially oriented anatase phase, the degree of which depended on the deposition conditions. The microstructure, optical and photocatalytic properties of the thin films were investigated using SEM, UV–VIS spectroscopy and a custom made photocatalytic test reactor, respectively. The thin film density was estimated from their refractive index. Density and crystallinity were the most important factors determining the photocatalytic activity of the thin films.

Super-dry reforming of methane intensifies CO2 utilization via Le Chatelier’s principle

Upgrading CO2 with methane The use of carbon dioxide as a reactant could help to mitigate its impact on climate, but it is difficult to activate as an oxidant. Buelens et al. combined methane in a high-temperature “super-dry” reforming process that generates reactive carbon monoxide. Both molecules were fed into a reactor containing a nickel methane-reforming catalyst, an iron oxide solid oxygen carrier, and calcium oxide as a CO2 sorbent. The adsorbed CO2 was treated with an inert gas purge that shifted the equilibrium, releasing mainly CO. This isothermal process avoids carbon buildup and can be used with biogas methane that contains substantial levels of CO2. Science, this issue p. 449 A two-step isothermal process converts carbon dioxide and methane into carbon monoxide for chemical and fuel production Efficient CO2 transformation from a waste product to a carbon source for chemicals and fuels will require reaction conditions that effect its reduction. We developed a “super-dry” CH4 reforming reaction for enhanced CO production from CH4 and CO2. We used Ni/MgAl2O4 as a CH4-reforming catalyst, Fe2O3/MgAl2O4 as a solid oxygen carrier, and CaO/Al2O3 as a CO2 sorbent. The isothermal coupling of these three different processes resulted in higher CO production as compared with that of conventional dry reforming, by avoiding back reactions with water. The reduction of iron oxide was intensified through CH4 conversion to syngas over Ni and CO2 extraction and storage as CaCO3. CO2 is then used for iron reoxidation and CO production, exploiting equilibrium shifts effected with inert gas sweeping (Le Chatelier’s principle). Super-dry reforming uses up to three CO2 molecules per CH4 and offers a high CO space-time yield of 7.5 millimole CO per second per kilogram of iron at 1023 kelvin.

Comparison of Thermal and Plasma-Enhanced ALD/CVD of Vanadium Pentoxide

Vanadium pentoxide was deposited by atomic layer deposition (ALD) from vanadyl-tri-isopropoxide (VTIP). Water or oxygen was used as a reactive gas in thermal and plasma-enhanced (PE) processes. For PE ALD, there was a wide ALD temperature window from 50 to 200°C. Above 200°C, VTIP decomposed thermally, resulting in the chemical vapor deposition (CVD) of vanadium pentoxide. The PE ALD reactions saturated much faster than during thermal ALD, leading to a growth rate of approximately 0.7 A/cycle during PE ALD using H 2 O or O 2 . Optical emission spectroscopy showed combustion-like reactions during the plasma step. X-ray diffraction was performed to determine the crystallinity of the films after deposition and after postannealing under He or O 2 atmosphere. Films grown with CVD at 300°C and PE O 2 ALD at 150°C were (001)-oriented V 2 O 5 as deposited, while thermal and PE H 2 O ALD films grown at 150°C were amorphous as deposited. The crystallinity of the PE O 2 ALD could be correlated to its high purity, while the other films had significant carbon contamination, as shown by X-ray photoelectron spectroscopy. Annealing under He led to oxygen-deficient films, while all samples eventually crystallized into V 2 O 5 under O 2 .

TiO2 films prepared by DC magnetron sputtering from ceramic targets

Abstract The deposition of stoichiometric TiO 2 films for V 2 O 5 /TiO 2 anatase catalysts was investigated. DC magnetron sputtering from ceramic oxide targets in an argon/oxygen atmosphere was chosen as deposition technique. The target behaviour upon sputtering was followed by means of the target voltage. The layers were prepared with different oxygen concentrations in the plasma and the influence upon electronic and optical properties of the deposited layers was investigated. Surface XPS measurements showed that stoichiometry is obtained for low oxygen mole fractions, while for higher oxygen contributions, Ti 3+ species are increasingly present. This departure from stoichiometry is ascribed to active resputtering of the deposited layer by O − ions. Due to the higher sputter yield of oxygen as compared to titanium, this resputtering resulted in the observed reduction. From optical transmission measurements, the refractive index was found to decrease with increasing oxygen mole fraction, while the band gap increased considerably. The variation of the optical absorption with the oxygen concentration in the plasma was found to confirm the XPS analysis regarding the stoichiometry of the layers. Polycrystalline anatase layers were obtained at elevated substrate temperatures.

Electronic and optical characterisation of TiO2 films deposited from ceramic targets

Abstract The deposition of stoichiometric TiO2 films for abrasion resistant V2O5/TiO2 anatase catalysts was investigated. DC magnetron sputtering from ceramic oxide targets in an argon/oxygen atmosphere was chosen as deposition technique. Smaller amounts of oxygen gas proved necessary to achieve stoichiometry in the deposited layers, than during deposition processes involving metal targets. TiO2 layers were prepared with different oxygen mole fractions and the influence upon electronic and optical properties was investigated. Surface XPS measurements showed that stoichiometry is obtained for low oxygen mole fractions, while for higher oxygen contributions, Ti3+ species are increasingly present. This departure from stoichiometry was ascribed to active resputtering of the deposited layer by O− ions. Due to the higher sputter yield of oxygen as compared to titanium, this resputtering resulted in the observed reduction. From optical transmission measurements, the refractive index was found to decrease with increasing oxygen mole fraction, while the band gap increased considerably. Post-deposition annealing in oxygen improved the crystallinity of the layers. The refractive index and optical band gap were both reduced by this treatment and the oxygen deficiency of the layers was in general seen to decrease.

Observation of the V2O5(001) surface using ambient atomic force microscopy.

Constant force images of the V2O5(001) surface were recorded in ambient conditions with atomic force microscopy. All images exhibit the 11.5 A × 3.5 A. periodicity expected for a bulk terminated surface. However, images reveal differences from the ideal structure. The experimental results are interpreted in terms of preferential adsorption sites for water molecules. Because these sites are thought to influence the catalytic properties of the surface, their characterization is an important step towards understanding how the atomic-scale structure of a surface influences its properties.

Effect of Substrate Sodium Content on Crystallization and Photocatalytic Activity of TiO2 Films Prepared by DC Magnetron Sputtering

The effect of sodium content of the glass support on the crystallinity of sputtered TiO2 films and photocatalytic breakdown of ethanol has been studied. It was found that the activity of the as-deposited (amorphous) films does not depend on the type of support used. The chemical composition of the glass support does influence the activity of annealed films. When using soda-lime glass support sodium diffuses into the film upon annealing, suppressing anatase crystallization and decreasing its photocatalytic activity. To decrease the influence of sodium, soda-lime glass coated with an e-beam evaporated SiO2 barrier layer was used with good result. A reduced sodium concentration in the film leads to well crystallized anatase after annealing. An increased photocatalytic activity was observed for these films.

Observation of surface phonons on the (001) and (100) surfaces of anatase minerals

Abstract The surface properties of TiO2-anatase are of great importance in chemical technology because this material is frequently used as a support for oxide catalysts. In the present work, surface phonons were observed on the (001) and (100) surfaces of anatase minerals by means of HREELS. After comparison with theoretical values and infrared data a preliminary assignment of the observed modes was achieved.

Mg–Fe–Al–O for advanced CO2 to CO conversion: carbon monoxide yield vs. oxygen storage capacity

A detailed study of new oxygen carrier materials, Mg–Fe–Al–O, with various loadings of iron oxide (10–100 wt% Fe2O3) is carried out in order to investigate the relationship between material transformation, stability and CO yield from CO2 conversion. In situ XRD during H2-TPR, CO2-TPO and isothermal chemical looping cycles as well as Mossbauer spectroscopy are employed. All samples show the formation of a spinel phase, MgFeAlOx. High loadings of iron oxide (50–90 wt%) lead to both spinel and Fe2O3 phases and show deactivation in cycling as a result of Fe2O3 particle sintering. During the reduction, reoxidation and cycling of the spinel MgFeAlOx phase, only limited sintering occurs. This is evidenced by the stable spinel crystallite sizes (∼15–20 nm) during isothermal cycling. The reduction of MgFe3+AlOx starts at 400 °C and proceeds via partial reduction to MgFe2+AlOx. Prolonged cycling and higher temperatures (>750 °C) lead to deeper reduction and segregation of Fe from the spinel structure. Very high stability and CO yield from CO2 conversion are found in Mg–Fe–Al–O materials with 10 wt% Fe2O3, i.e. the lowest oxygen storage capacity among the tested samples. Compared to 10 wt% Fe2O3 supported on Al2O3 or MgO, the CO yield of the 10 wt% Fe2O3–MgFeAlOx spinel is ten times higher.

A HREELS study of the V2O5 (001) surface phonon spectrum

Abstract In this work, HREELS is used to investigate the surface vibrational properties of V 2 O 5 , a multi-phonon-mode, anisotropic, layered semiconductor. An adequate description of the complex loss spectrum is provided by the dielectric theory, extended to anisotropic materials, which together with infrared data allows to generate theoretical spectra. These simulations accurately reproduce the peak positions. The relative loss intensities are also in fair agreement with the experimental data. On comparing the HREELS spectra with infrared reflection data, the assignment of the single loss frequencies is straightforward.