Rutile Single Crystal Surfaces Research Articles

Irreversible surface changes upon n-type doping – A photoelectrochemical study on rutile single crystals

Single crystal wafers need to provide sufficient electrical conductivity to be employed in photoelectrochemical investigations. Usually, their defect concentrations and donor densities are too low to allow electrochemical measurements. Accordingly, TiO2 rutile single crystal surfaces have to be n-type doped before their electrochemical activity can be evaluated. The doping itself, however, leads to irreversible changes in the surface morphology of the initially smooth crystalline surfaces. In this study, the effects of n-type doping and photoetching on TiO2 rutile single crystal surfaces have been investigated. The photocatalytic and photoelectrochemical activities of the rutile single crystal wafers have been quantified by methanol photooxidation. The results indicate that n-type doping has different impacts on the employed rutile (100) and (110) surfaces. Subsequent photoetching is necessary to achieve comparable donor densities for both single crystal electrodes. Moreover, the rutile (100) surface is producing different product ratios depending on the applied external bias as compared with the rutile (110) surface for methanol and water oxidation.

Microstructure evolution during BaTiO 3 formation by solid-state reactions on rutile single crystal surfaces

To study the modified phase formation sequence observed during sintering of TiO 2–BaCO 3 core-shell powders, model experiments on phase formation and microstructure evolution during BaTiO 3 genesis were performed. Reaction products between BaO vapour and TiO 2 (rutile) single crystals with different orientations, obtained at different temperatures, were investigated by XRD texture analysis, HRTEM and EELS. BaTiO 3 was found in reactions performed at temperatures between 700 and 900 °C. Using Pt markers, in-diffusion of Ba ions was detected at 800 and 850 °C. Ti-rich phases were found after the reaction at 850 °C. At 900 °C, BaTiO 3 forms at the surface of the reaction layer with a well-defined orientation to the rutile substrate. In addition, pores and Ti-rich phases were identified at the BaTiO 3–TiO 2 interface. The pore formation is explained by an out-diffusion of titanium and oxygen ions from the BaTiO 3–TiO 2 interface through the reaction layer to the surface.

Titania Nanoparticles Prepared with Pulsed Laser Ablation of Rutile Single Crystals in Water

The pulsed laser ablation (PLA) experiments of rutile single crystal surfaces were carried out in water solution to prepare nanosized titania particles. The solvated PLA species were transparent as produced and changed to a lighter blue solution with some precursors for filamentous species (in several days), then finally changed to white enlarged filamentous species (in 2−4 weeks). The solvated PLA species were measured as uniform nanoparticles with a size below 10 nm by TEM measurements and showed the absorption−photon energy relation for the direct transition with a band gap of 5.3−5.5 eV. The band-gap values were elucidated with quantum confinement size effects. It was assumed that the primary solvated species should have a size of about 1 nm and they were agglomerated to be the secondary species. The Mie scattering is responsible for the “blue” color, which proves that the size enlargement process exists on the PLA species in water. The filamentous species are composed of mainly the anatase form, which was analyzed with Raman spectroscopy. The XPS results indicate that the Ti species are tetravalent and bonded to oxygen atoms with O 1s binding energy at around 530 eV as TiO2. They were found to be composed to about 1 μm diameter with gathering nanosized particles on SEM pictures. The thin films composed of the PLA filamentous species showed no band gap increase. The process for PLA of rutile in water was studied with different temperatures (T in K) to elucidate the effect of viscosity (η). The yield of PLA species in water decreases nearly linearly with the ratio of T/η representing the diffusion coefficient of the solvated species. The result implies that the PLA species are confined in a media not diffused into water solution.

Hepatocyte performance on different crystallographic faces of rutile

The influence of crystallographic orientation of polished rutile single crystal surfaces of the (1 0 0), (1 1 0) and (1 1 1) orientation on hepatocyte performance was tested in cell culture over 3 days. Cell adhesion was observed on the titanium dioxide surfaces and their performance was measured by means of cell number attached (protein mass), cell viability (neutral red assays) and metabolic activity (thiazolyl tetrazolium bromide assay). Titanium dioxide displays no cytotoxic effects on hepatocytes, and shows a performance in the range of standard collagen-coated tissue culture polystyrene dish. The number of hepatocytes adhered on the different rutile surfaces were not significantly different to those on dense rutile polycrystalline ceramic. These findings suggest that hepatocytes do not recognize the specific differences of differently orientated rutile crystal surfaces.

Towards a structure–activity relationship for oxide supported metals

The chemistry of [RhCl(CO) 2] 2 on rutile single crystal surfaces and high area oxides is described. Dissociative chemisorption leads to RhCl(CO) 2/oxide in most cases, but when the favoured site is removed, as in rutile(1 1 0)(1×2), reductive decomposition to the metal is rapid. In that favoured site, the surface provides a Lewis base site (O) for coordination to Rh and a Lewis acid site (Ti) for Cl. Metal particles formed by the thermolysis can attain SMSI effects. Using energy dispersive EXAFS (EDE) in conjunction with mass spectrometry, the reactions of Rh(CO) 2Cl/alumina and Rh/alumina with NO have been studied in detail using the Rh K-edge. Both react rapidly with NO under mild conditions. For the dicarbonyl, the kinetics of the changes in the Rh structure and the gas phase compositions reveal two different stages in the reaction towards a bent nitrosyl complex, formulated as RhCl(NO)/alumina. For Rh/alumina, there is rapid disruption of the metal particles; O 2 similarly rapidly disrupts supported rhodium. This fluidity of the metal structures strengthens the importance of synchronous structure/reactivity observations. It has long been the aim to derive a molecular level understanding of the heterogeneous catalysts. Such descriptions are often thwarted by the structural complexity and also by the absence of techniques that can be suitably applied to such materials under reaction conditions. We have sought to tackle this problem by a multi-facetted approach stretching from detailed studies of the chemisorption of organo-rhodium compounds onto the surfaces of single-crystal surfaces through the interaction of organometallics onto the surfaces of high area supports to the improved characterisation of oxide-supported metal catalysts. In this report, we review progress towards the elusive structure–reactivity relationship.

Comparative second harmonic generation and X-ray photoelectron spectroscopy studies of the UV creation and O 2 healing of Ti 3+ defects on (110) rutile TiO 2 surfaces

SHG studies on polished and etched (110) rutile TiO 2 single-crystal surfaces suggest that above band gap, low-energy (4.7 eV) ultraviolet photons create stable Ti 3+ surface defects in UHV. Such defects can be healed by subsequently exposing the surface to O 2 gas. These results are similar to recently reported measurements on polished and etched (001) rutile single-crystal surfaces. Observations on the (001) surfaces were interpreted as the photodesorption and re-adsorption of molecular oxygen bound loosely to Ti 3+ defects on the surface as a Ti 4+:O − 2 complex. For the current (110) study, XPS was used to confirm the defect type and to quantify the density of Ti 3+ defects created. Defects on the surfaces studied using XPS were generated using photons of even lower energy (3.4 eV), indicating that the oxygen species removed was very loosely bound. The same defect creation and healing processes were also observed on a nearly defect-free thermally annealed single-crystal surface using XPS. Whether bridging oxygen ions on the (110) surface or O − 2 ions are the photo-labile species has yet to be determined. Transient photodesorption of O 2 at high oxygen pressure also was observed using SHG. SHG has proven to be a sensitive probe of surface defects, consistent with XPS results, under conditions ranging from UHV to atmospheric.