Bonding of gold nanoclusters to oxygen vacancies on rutile TiO2(110)

Through an interplay between scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that bridging oxygen vacancies are the active nucleation sites for Au clusters on the rutile TiO2(110) surface. We find that a direct correlation exists between a decrease in density of vacancies and the amount of Au deposited. From the DFT calculations we find that the oxygen vacancy is indeed the strongest Au binding site. We show both experimentally and theoretically that a single oxygen vacancy can bind 3 Au atoms on average. In view of the presented results, a new growth model for the TiO2(110) system involving vacancy-cluster complex diffusion is presented

Small Polarons in Transition Metal Oxides

The formation of polarons is a pervasive phenomenon in transition metal oxide compounds, with a strong impact on the physical properties and functionalities of the hosting materials. In its original formulation the polaron problem considers a single charge carrier in a polar crystal interacting with its surrounding lattice. Depending on the spatial extension of the polaron quasiparticle, originating from the coupling between the excess charge and the phonon field, one speaks of small or large polarons. This chapter discusses the modeling of small polarons in real materials, with a particular focus on the archetypal polaron material TiO2. After an introductory part, surveying the fundamental theoretical and experimental aspects of the physics of polarons, the chapter examines how to model small polarons using first principles schemes in order to predict, understand and interpret a variety of polaron properties in bulk phases and surfaces. Following the spirit of this handbook, different types of computational procedures and prescriptions are presented with specific instructions on the setup required to model polaron effects.Comment: 36 pages, 12 figure

Enhanced Photocatalytic H2 Production in Core–Shell Engineered Rutile TiO2

The authors thank the Major Basic Research Program, Ministry of Science and Technology of China (2014CB239401), NSFC (Nos. 51422210, 51572266, 51561130157, 51172243, 51521091). GL thanks Newton Advanced FellowshipA rationally designed crystalline Ti3+ core/amorphous Ti4+ shell configuration can reverse the population disparity between holes and electrons reaching the surface of microsized rutile TiO2 photocatalyst, thus significantly enhancing its photocatalytic activity by two orders of magnitude in terms of the hydrogen production rate under the irradiation of UV–vis light.PostprintPeer reviewe

Highly efficient photoanodes based on cascade structural semiconductors of Cu2Se/CdSe/TiO2: a multifaceted approach to achieving microstructural and compositional control

Hydrogen produced by splitting water is receiving significant attention due to the rising global energy demand and growing climate concerns. The photocatalytic decomposition of water converts solar energy into clean hydrogen, and may help mitigate the crisis of fossil fuel depletion. However, the photocatalytic hydrogen production remains challenging to obtain high and stable photoconversion efficiency. Here, we report a highly efficient photoanode based on the coaxial heterogeneous cascade structure of Cu2Se/CdSe/TiO2 synthesized via a simple room-temperature and low-cost electrochemical deposition method. The microstructure and composition of the Cu2Se top layer are regulated and controlled by doping Cu with various amounts in different zones of the CdSe/TiO2 coaxial heterojunction and then using a simple integral annealing process. Surprisingly, a little effort made to achieve the Cu2Se top layer utilizing such doped CdSe/TiO2 exhibits a significant enhancement in photocatalytic activity. The maximum stable photocurrent density of the sample with the optimal copper zone and doping concentration has reached up to 28 mA cm−2, which can be attributed to the success in the uniform dispersion of the three-layer heterogeneous nanojunctions among the anatase nanotube wall from the top to the bottom. This results in a stepwise structure of band-edge levels in the Cu2Se/CdSe/TiO2 photoelectrode that is conducive to enhancing effectively the separation of the photogenerated electron–hole pairs

Water-Gated Charge Doping of Graphene Induced by Mica Substrates

We report on the existence of water-gated charge doping of graphene deposited on atomically flat mica substrates. Molecular films of water in units of ~0.4 nm-thick bilayers were found to be present in regions of the interface of graphene/mica hetero-stacks prepared by micromechanical exfoliation of kish graphite. The spectral variation of the G and 2D bands, as visualized by Raman mapping, shows that mica substrates induce strong p-type doping in graphene, with hole densities of $(9 \pm 2) \times 1012 cm${-2}$. The ultrathin water films, however, effectively block interfacial charge transfer, rendering graphene significantly less hole-doped. Scanning Kelvin probe microscopy independently confirmed a water-gated modulation of the Fermi level by 0.35 eV, in agreement with the optically determined hole density. The manipulation of the electronic properties of graphene demonstrated in this study should serve as a useful tool in realizing future graphene applications.Comment: 15 pages, 4 figures; Nano Letters, accepted (2012

Ion-exchangeable semiconductor materials for visible light-induced photocatalysis

The use of semiconductor materials for solar fuel production and environmental remediation has attracted increasing attention in the past decades due to their potential to address important energy and environmental problems. Ion-exchangeable semiconductor materials represent one family of promising materials due to their unique crystal structures and structure-related photocatalytic activity. However, most of the ion-exchangeable semiconductor materials can only absorb UV light due to their wide band-gap. To efficiently utilize solar energy, it is indispensable to develop visible light-responsive semiconductor materials which can efficiently absorb solar electromagnetic radiation reaching the Earth's surface. In this review article, we summarize the recent advances on ion-exchangeable semiconductor materials as visible light-responsive photocatalysts with particular focus on the band-gap engineering strategies and their photocatalytic applications

Effects of plasmon excitation on photocatalytic activity of Ag/TiO 2 and Au/TiO2 nanocomposites

Model nanocomposite photocatalysts consisting of undoped TiO2 films with optically active Ag or Au nanoparticles (NPs) were designed, fabricated, and examined to address the role of plasmon excitations in their performance. Different composition configurations were tested in which the NPs were either facing the reaction environment or not, and in direct contact or not with TiO2. We found, as measured for the reactions of methanol and ethylene oxidation in two different photoreactors, that composites always show enhanced activity (up to x 100 for some configurations) compared to bare TiO2. We deduced from in situ localized surface plasmon resonance spectroscopy measurements that the interfacial charge transfer from TiO2 to NPs plays a major role in the activity enhancement for composite configurations where particles are in direct contact with TiO2. Plasmonic near- and far-field effects were only observed when the plasmon resonance energy overlaps with the bandgap energy of undoped TiO2. (C) 2013 Elsevier Inc. All rights reserved

Suspension high velocity oxy-fuel spraying of TiO2: a quantitative approach to phase composition

A range of coatings from a water based suspension of anatase has been prepared by suspension high velocity oxy-fuel spraying with the aim to study effects of heat power of the flame on phase composition, microstructure and surface topography. Three most commonly used approaches of quantitative phase analysis have been scrutinized with respect to their applicability and as some of the coatings showed presence of preferred orientation and it was argued that quantitative Rietveld refinement is the most accurate method for phase composition determination. Coatings had a layered duplex anatase/rutile microstructure with fraction of rutile increasing exponentially with heat power. Spraying at the lower heat power led to a lower surface roughness and higher power resulted in surfaces with pronounced humps, which were distributed homogeneously on the surface. The emergence of humps is related to an increase in macroscopic surface area of up to 30% with respect to the flat coating

Doped Framework Iron Hydroxyl Phosphate as Photocatalyst for Hydrogen Production from Water/Methanol Mixtures

[EN] In the search for novel photocatalysts for hydrogen production and with the alpha-Fe2O3 photoelectrocatalyst as a recent precedent, we report herein the preparation, semiconductor properties and photocatalytic activity of metal-doped (0.1-5 wt.-% loading) iron hydroxyl phosphate (FeP). X-ray diffraction analyses of FeP samples subjected to extended photocatalytic irradiation showed the stability of this framework phosphate under photocatalytic conditions. Doping increased the photocatalytic efficiency of FeP for all dopants, with the optimal doping level between 0.1 and 1%. Under the optimized conditions (Cr at 1% doping), the photocatalytic activity of FeP reached a hydrogen production rate of 35.82 mu molg(Fe)(-1) in the absence of platinum as co-catalyst. The conduction flat band potential was estimated by photocurrent measurements or impedance spectroscopy to be 0.1 eV versus NHE and the charge carrier density 2.6 x 10(20) carriers cm(-3). Transient absorption spectroscopy revealed a transient species decaying on the microsecond time-scale characterized by a broad band spanning 300-750 nm. This transient was attributed to the charge-separated state. These results are promising for the development of novel photocatalytic materials based on framework metal phosphate.Financial support by the Spanish Ministry of Economy and Competitiveness (MEC) (Severo Ochoa and CTQ20212-32315) and the Generalidad Valenciana (Prometeo 2012/014) is gratefully acknowledged. M. S. thanks the Spanish Consejo Superior de Investigaciones Cientificas (CSIC) and Technical University of Valencia for a postgraduate scholarship.Serra, M.; García Baldoví, H.; Alvaro Rodríguez, MM.; García Gómez, H. (2015). Doped Framework Iron Hydroxyl Phosphate as Photocatalyst for Hydrogen Production from Water/Methanol Mixtures. European Journal of Inorganic Chemistry. 2015(25):4237-4243. https://doi.org/10.1002/ejic.201500629S42374243201525Amao, Y. (2011). 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