Transport and thermoelectric properties of the LaAlO3_3/SrTiO3_3 interface

The transport and thermoelectric properties of the interface between SrTiO$_3$ and a 26-monolayer thick LaAlO$_3$-layer grown at high oxygen-pressure have been investigated at temperatures from 4.2 K to 100 K and in magnetic fields up to 18 T. For $T>$ 4.2 K, two different electron-like charge carriers originating from two electron channels which contribute to transport are observed. We probe the contributions of a degenerate and a non-degenerate band to the thermoelectric power and develop a consistent model to describe the temperature dependence of the thermoelectric tensor. Anomalies in the data point to an additional magnetic field dependent scattering.Comment: 7 pages, 4 figure

Photodeposition of platinum nanoparticles on well-defined Tungsten oxide: controlling oxidation state, particle size and geometrical distribution

In this thesis, structure-directed photodeposition of the cocatalyst platinum (Pt) on monoclinic tungsten oxide (WO3) nanoplates is described, both considering fundamental aspects, as well as usefulness for applications in photocatalytic propane oxidation. Before such studies are described, the concepts of crystal facet engineering of WO3 nanoplates and photodeposition of platinum on commercial WO3 are explained. For the crystal facet engineering studies, WO3 with plate-like morphology was synthesized through a hydrothermal synthesis method. A solution containing sodium tungstate, hydrochloric acid and citric acid was treated in a sealed autoclave at elevated temperatures. It is demonstrated that in the synthesis procedure the pH should be sufficiently low and the temperature sufficiently high to obtain monoclinic WO3 nanoplates. In photodeposition studies of Pt on commercial WO3, the influence of the sacrificial agent methanol was investigated in detail. When methanol is used, all platinum in the solution is deposited in a metallic state on the WO3 surface, in the form of large clusters of particles. In the absence of methanol, not all platinum can be deposited on the WO3. Platinum is then found on the surface as highly dispersed nanoparticles in an oxidized state. The results make clear that the dispersion and oxidation state of platinum photodeposited on tungsten oxide can be carefully controlled by the potential usage of methanol or, if needed, hydrogenation.\ud \ud When structure-directed photodeposition studies of Pt on WO3 nanoplates are performed, Pt particles are found on the edges/subordinate facets of the plate-like WO3. This is likely due to sorption effects rather than light-induced charge separation. It is concluded that the anionic Pt precursor [PtCl6]2- must have adsorbed on the positive sites of the WO3 nanoplates before being reduced. The presence of Pt on WO3 nanoplates has negative consequences on the photocatalytic activity of the samples in propane oxidation, especially when it is present as PtII rather than Pt0. The results demonstrate that structure-directed photodeposition of Pt on WO3 nanoplates does not have a beneficial effect on the activity in photocatalytic propane oxidation, in contrast to popular believe that structure-directed photodeposition should result in enhanced photocatalytic activities

Methods, Mechanism, and Applications of Photodeposition in Photocatalysis: A Review

In this review, for a variety of metals and semiconductors, an attempt is made to generalize observations in the literature on the effect of process conditions applied during photodeposition on (i) particle size distributions, (ii) oxidation states of the metals obtained, and (iii) consequences for photocatalytic activities. Process parameters include presence or absence of (organic) sacrificial agents, applied pH, presence or absence of an air/inert atmosphere, metal precursor type and concentration, and temperature. Most intensively reviewed are studies concerning (i) TiO2; (ii) ZnO, focusing on Ag deposition; (iii) WO3, with a strong emphasis on the photodeposition of Pt; and (iv) CdS, again with a focus on deposition of Pt. Furthermore, a detailed overview is given of achievements in structure-directed photodeposition, which could ultimately be employed to obtain highly effective photocatalytic materials. Finally, we provide suggestions for improvements in description of the photodeposition methods applied when included in scientific papers

The Effect of Methanol on the Photodeposition of Pt Nanoparticles on Tungsten Oxide

Formation of platinum nanoparticles on tungsten oxide by photodeposition from acidic [PtCl6]2- solutions in the absence or presence of methanol is studied in detail. Without methanol, [PtCl6]2--ions adsorbed on the WO3 surface are converted to highly dispersed PtO/Pt(OH)2 particles upon illumination, the maximum achievable amount being limited to 10-30% of [PtCl6]2- present in solution. Inclusion of methanol not only promotes adsorption of [PtCl6]2- ions, but upon illumination results in a deposited Pt quantity close to 100% of [PtCl6]2-. The obtained Pt particles are in the metallic state, large, and often clustered. The limited loading and presence of PtO/Pt(OH)2 in the absence of methanol is explained by reduction of WO3-surface sites, accompanied by water oxidation catalyzed by PtO/Pt(OH)2 deposits. The extensive reduction of Pt in methanol solutions is likely induced by methoxy-radicals, oxidatively formed by reaction of methanol with photogenerated holes. This study provides guidelines for optimization of synthesis procedures of Pt/WO3 (photo)catalysts

Room Temperature Oxidation of Ethanol to Acetaldehyde over Pt/WO3

Oxidation of ethanol to acetaldehyde is of relevance to synthetic chemistry and development of sensors for detection of gas phase ethanol. It is demonstrated by gas phase analysis and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) that a Pt/WO3 composite shows significant activity in oxidative dehydrogenation of ethanol to acetaldehyde at room temperature. DRIFTS also demonstrates consecutive surface chemistry occurs, yielding predominantly ethyl acetate. The oxidation state of the most active Pt particles is determined to be Pt0, rather than PtO, by studying variations in catalyst preparation methodology, and characterization of various samples by X-ray photoelectron spectroscopy. Illumination has very little effect on surface reactivity and selectivity, but promotes formation of gas phase acetaldehyde

Selective Electrochemical Oxidation of H2O to H2O2 using Boron-doped diamond: an experimental and techno-economic evaluation

Selective water oxidation to hydrogen peroxide has emerged as an economically attractive replacement for oxygen in electrochemical hydrogen production by water splitting. Here, boron-doped diamond (BDD) is shown to be a promising anode material for anodic H2O2 formation. Faradaic efficiencies of up to 31.7% at 2.90 V versus the reference hydrogen electrode and a current density of 39.8 mA cm–2 were observed, corresponding to a H2O2 production rate of 3.93 μmol min–1 cm–2. A techno-economic evaluation based on the experimentally obtained values demonstrates that the corresponding levelized cost of hydrogen (LCH) is significant ($62.0 kg–1). Particularly, the current market price of BDD limits its implementation as a selective water oxidation anode for H2O2 generation. The sensitivity analysis however suggests that the LCH can be significantly improved by either decreasing the anode cost or increasing the current density. Both approaches are in fact feasible to allow for cost-effective electrochemical H2 production and even competition with H2 obtained from steam methane reforming. This study will guide ongoing research efforts toward BDD development and implementation of selective water oxidation to hydrogen peroxide