Ralf Bechstein

Promotion of phenol photodecomposition over TiO2 using Au, Pd, and Au-Pd nanoparticles

Noble metal nanoparticles (Au, Pd, Au-Pd alloys) with a narrow size distribution supported on nanocrystalline TiO(2) (M/TiO(2)) have been synthesized via a sol-immobilization route. The effect of metal identity and size on the photocatalytic performance of M/TiO(2) has been systematically investigated using phenol as a probe molecule. A different phenol degradation pathway was observed when using M/TiO(2) catalysts as compared to pristine TiO(2). We propose a mechanism to illustrate how the noble metal nanoparticles enhance the efficiency of phenol decomposition based on photoreduction of p-benzoquinone under anaerobic conditions. Our results suggest that the metal nanoparticles not only play a role in capturing photogenerated electrons, but are strongly involved in the photocatalytic reaction mechanism. The analysis of the reaction intermediates allows us to conclude that on M/TiO(2) undesired redox reactions that consume photogenerated radicals are effectively suppressed. The analysis of the final products shows that the reusability performance of the catalyst is largely dependent on the pretreatment of the catalyst and the identity of the metal nanoparticle. Interestingly, the as-prepared Pd and Au-Pd decorated TiO(2) materials exhibit excellent long-term photoactivity, in which ~90% of the phenol can be fully decomposed to CO(2) in each cycle.

Designer titania-supported Au-Pd nanoparticles for efficient photocatalytic hydrogen production

Photocatalytic hydrogen evolution may provide one of the solutions to the shift to a sustainable energy society, but the quantum efficiency of the process still needs to be improved. Precise control of the composition and structure of the metal nanoparticle cocatalysts is essential, and we show that fine-tuning the Au-Pd nanoparticle structure modifies the electronic properties of the cocatalyst significantly. Specifically, Pd(shell)-Au(core) nanoparticles immobilized on TiO2 exhibit extremely high quantum efficiencies for H2 production using a wide range of alcohols, implying that chemical byproducts from the biorefinery industry can be used as feedstocks. In addition, the excellent recyclability of our photocatalyst material indicates a high potential in industrial applications. We demonstrate that this particular elemental segregation provides optimal positioning of the unoccupied d-orbital states, which results in an enhanced utilization of the photoexcited electrons in redox reactions. We consider that the enhanced activity observed on TiO2 is generic in nature and can be transferred to other narrow band gap semiconductor supports for visible light photocatalysis.

The importance of bulk Ti3+ defects in the oxygen chemistry on titania surfaces.

The role of bulk defects in the oxygen chemistry on reduced rutile TiO(2)(110)-(1 × 1) has been studied by means of temperature-programmed desorption spectroscopy and scanning tunneling microscopy measurements. Following O(2) adsorption at 130 K, the amount of O(2) desorbing at ∼410 K initially increased with increasing density of surface oxygen vacancies but decreased after further reduction of the TiO(2)(110) crystal. We explain these results by withdrawal of excess charge (Ti(3+)) from the TiO(2)(110) lattice to oxygen species on the surface and by a reaction of Ti interstitials with O adatoms upon heating. Important consequences for the understanding of the O(2)-TiO(2) interaction are discussed.

Water-mediated proton hopping on an iron oxide surface

Water-Assisted Proton Diffusion Proton diffusion on metal oxide surfaces can play an important role in many catalytic processes. The presence of water is thought to accelerate proton diffusion. Merte et al. (p. 889) used high-speed, high-resolution scanning tunneling microscopy to study proton diffusion on an iron oxide. On oxygen-terminated FeO monolayer films formed on Pt, molecular water accelerated proton diffusion. Density function theory calculations implicated a H3O+ transition state in the diffusion process. The presence of adsorbed water enhances proton diffusion, likely through a hydronium ion transition state. The diffusion of hydrogen atoms across solid oxide surfaces is often assumed to be accelerated by the presence of water molecules. Here we present a high-resolution, high-speed scanning tunneling microscopy (STM) study of the diffusion of H atoms on an FeO thin film. STM movies directly reveal a water-mediated hydrogen diffusion mechanism on the oxide surface at temperatures between 100 and 300 kelvin. Density functional theory calculations and isotope-exchange experiments confirm the STM observations, and a proton-transfer mechanism that proceeds via an H3O+-like transition state is revealed. This mechanism differs from that observed previously for rutile TiO2(110), where water dissociation is a key step in proton diffusion.

Stabilization Principles for Polar Surfaces of ZnO

ZnO is a wide band gap metal oxide with a very interesting combination of semiconducting, transparent optical and catalytic properties. Recently, an amplified interest in ZnO has appeared due to the impressive progress made in nanofabrication of tailored ZnO nanostructures and functional surfaces. However, the fundamental principles governing the structure of even the clean low-index ZnO surfaces have not been adequately explained. From an interplay of high-resolution scanning probe microscopy (SPM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments, and density functional theory (DFT) calculations, we identify here a group of hitherto unresolved surface structures which stabilize the clean polar O-terminated ZnO(0001) surface. The found honeycomb structures are truly remarkable since their existence deviates from expectations using a conventional electrostatic model which applies to the opposite Zn-terminated (0001) surface. As a common principle, the differences for the clean polar ZnO surfaces are explained by a higher bonding flexibility of the exposed 3-fold coordinated surface Zn atoms as compared to O atoms.

High-quality Fe-doped TiO2 films with superior visible-light performance

We report on high-quality polycrystalline Fe-doped TiO2 (Fe–TiO2) porous films synthesized via one-step electrochemical oxidation. We demonstrate that delicate properties such as the impurity concentration and the microstructure that strongly influence the performance of the material for photovoltaic and photocatalysis applications can be controlled by adjusting the electrolyte composition. Compared to Fe-doped TiO2 films prepared with traditional phosphate- or silicate-based electrolytes, our newly synthesised Fe–TiO2 films contain solely Fe dopants, which results in excellent photocatalytic and photovoltaic performance under visible light irradiation.

‘All-inclusive’ imaging of the rutile TiO2(110) surface using NC-AFM

Non-contact atomic force microscopy (NC-AFM) at true atomic resolution is used to investigate the (110) surface of rutile TiO(2). We are able to simultaneously resolve both bridging oxygen and titanium atoms of this prototypical oxide surface. Furthermore, the characteristic defect species, i.e. bridging oxygen vacancies, single and double hydroxyls as well as subsurface defects, are identified in the very same frame. We employ density functional theory (DFT) calculations to obtain a comprehensive understanding of the relation between the tip apex structure and the observed image contrast. Our results provide insight into the physical mechanisms behind atomic-scale contrast, indicating that electrostatic interaction can lead to a far more complex contrast than commonly assumed.

Vertical and lateral drift corrections of scanning probe microscopy images

A procedure is presented for image correction of scanning probe microscopy data that is distorted by linear thermal drift. The procedure is based on common ideas for drift correction, which the authors combine to a comprehensive step-by-step description of how to measure drift velocities in all three dimensions and how to correct the images using these velocities. The presented method does not require any knowledge about size or shape of the imaged structures. Thus, it is applicable to any type of scanning probe microscopy image, including images lacking periodic structures. Besides providing a simple, ready-to-use description of lateral and vertical drift correction, they derive all formulas needed from the model of linear drift.

Water clustering on nanostructured iron oxide films

The adhesion of water to solid surfaces is characterized by the tendency to balance competing molecule-molecule and molecule-surface interactions. Hydroxyl groups form strong hydrogen bonds to water molecules and are known to substantially influence the wetting behaviour of oxide surfaces, but it is not well-understood how these hydroxyl groups and their distribution on a surface affect the molecular-scale structure at the interface. Here we report a study of water clustering on a moiré-structured iron oxide thin film with a controlled density of hydroxyl groups. While large amorphous monolayer islands form on the bare film, the hydroxylated iron oxide film acts as a hydrophilic nanotemplate, causing the formation of a regular array of ice-like hexameric nanoclusters. The formation of this ordered phase is localized at the nanometre scale; with increasing water coverage, ordered and amorphous water are found to coexist at adjacent hydroxylated and hydroxyl-free domains of the moiré structure.

Growth of ordered C60 islands on TiO2(110)

Non-contact atomic force microscopy is used to study C(60) molecules deposited on the rutile TiO(2)(110) surface in situ at room temperature. At submonolayer coverages, molecules adsorb preferentially at substrate step edges. Upon increasing coverage, ordered islands grow from the decorated step edges onto the lower terraces. Simultaneous imaging of bridging oxygen rows of the substrate and the C(60) island structure reveals that the C(60) molecules arrange themselves in a centered rectangular superstructure, with the molecules lying centered in the troughs formed by the bridging oxygen rows. Although the TiO(2)(110) surface exhibits a high density of surface defects, the observed C(60) islands are of high order. This indicates that the C(60) intermolecular interaction dominates over the molecule-substrate interactions that may cause structural perturbations on a defective surface. Slightly protruding C(60) strands on the islands are attributed to anti-phase boundaries due to stacking faults resulting from two islands growing together.