Showing papers on "Oxide published in 2008"
TL;DR: In this paper, the peak positions and peak shape parameters of Fe 3p for Fe2+ and Fe3+ were derived from the XPS spectra of the standard samples of 2FeO·SiO2 and Fe2O3, respectively.
4,434 citations
TL;DR: These experiments demonstrate solution-processed GO films have potential as transparent electrodes and sheet resistance and optical transparency using different reduction treatments.
Abstract: Processable, single-layered graphene oxide (GO) is an intriguing nanomaterial with tremendous potential for electronic applications. We spin-coated GO thin-films on quartz and characterized their sheet resistance and optical transparency using different reduction treatments. A thermal graphitization procedure was most effective, producing films with sheet resistances as low as 102 −103 Ω/square with 80% transmittance for 550 nm light. Our experiments demonstrate solution-processed GO films have potential as transparent electrodes.
3,011 citations
TL;DR: Experimental evidence is provided to support this general model of memristive electrical switching in oxide systems, and micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching are built.
Abstract: Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the ‘memristor’ (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron–ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance. Nanoscale metal/oxide/metal devices that are capable of fast non-volatile switching have been built from platinum and titanium dioxide. The devices could have applications in ultrahigh density memory cells and novel forms of computing.
2,744 citations
TL;DR: In all of these solvents, full exfoliation of the graphite oxide material into individual, single-layer graphene oxide sheets was achieved by sonication, and graphene oxide dispersions exhibited long-term stability and were made of sheets between a few hundred nanometers and a few micrometers large.
Abstract: The dispersion behavior of graphene oxide in different organic solvents has been investigated. As-prepared graphite oxide could be dispersed in N,N-dimethylformamide, N-methyl-2-pyrrolidone, tetrahydrofuran, and ethylene glycol. In all of these solvents, full exfoliation of the graphite oxide material into individual, single-layer graphene oxide sheets was achieved by sonication. The graphene oxide dispersions exhibited long-term stability and were made of sheets between a few hundred nanometers and a few micrometers large, similar to the case of graphene oxide dispersions in water. These results should facilitate the manipulation and processing of graphene-based materials for different applications.
2,514 citations
TL;DR: In this article, the effect of oxidation on the structural integrity of multiwalled carbon nanotubes through acidic (nitric acid and a mixture of sulfuric acid and hydrogen peroxide) and basic (ammonium hydroxide/hydrogen peroxide), agents has been studied.
2,454 citations
TL;DR: The photocatalytic methodology not only provides an on-demand UV-assisted reduction technique but also opens up new ways to obtain photoactive graphene-semiconductor composites.
Abstract: Graphene oxide suspended in ethanol undergoes reduction as it accepts electrons from UV-irradiated TiO2 suspensions. The reduction is accompanied by changes in the absorption of the graphene oxide, as the color of the suspension shifts from brown to black. The direct interaction between TiO2 particles and graphene sheets hinders the collapse of exfoliated sheets of graphene. Solid films cast on a borosilicate glass gap separated by gold-sputtered terminations show an order of magnitude decrease in lateral resistance following reduction with the TiO2 photocatalyst. The photocatalytic methodology not only provides an on-demand UV-assisted reduction technique but also opens up new ways to obtain photoactive graphene-semiconductor composites.
2,397 citations
TL;DR: Significant enhancement in mechanical stiffness and fracture strength of graphene oxide paper, a novel paperlike material made from individual graphene oxide sheets, can be achieved upon modification with a small amount of Mg(2+) and Ca(2+).
Abstract: Significant enhancement in mechanical stiffness (10–200%) and fracture strength (∼50%) of graphene oxide paper, a novel paperlike material made from individual graphene oxide sheets, can be achieved upon modification with a small amount (less than 1 wt %) of Mg2+ and Ca2+. These results can be readily rationalized in terms of the chemical interactions between the functional groups of the graphene oxide sheets and the divalent metals ions. While oxygen functional groups on the basal planes of the sheets and the carboxylate groups on the edges can both bond to Mg2+ and Ca2+, the main contribution to mechanical enhancement of the paper comes from the latter.
1,630 citations
TL;DR: Reduced graphene oxide is demonstrated as the active material for high-performance molecular sensors fabricated from exfoliated graphene oxide platelets that are deposited to form an ultrathin continuous network.
Abstract: We demonstrate reduced graphene oxide as the active material for high-performance molecular sensors. Sensors are fabricated from exfoliated graphene oxide platelets that are deposited to form an ultrathin continuous network. These graphene oxide networks are tunably reduced toward graphene by varying the exposure time to a hydrazine hydrate vapor. The conductance change of the networks upon exposure to trace levels of vapor is measured as a function of the chemical reduction. The level of reduction affects both the sensitivity and the level of 1/f noise. The sensors are capable of detecting 10 s exposures to simulants of the three main classes of chemical-warfare agents and an explosive at parts-per-billion concentrations.
1,629 citations
TL;DR: In this article, a general approach for the preparation of graphene−metal particle nanocomposites in a water−ethylene glycol system using graphene oxide as a precursor and metal nanoparticles (Au, Pt and Pd) as building blocks.
Abstract: Graphene sheets, which possess unique nanostructure and a variety of fascinating properties, can be considered as promising nanoscale building blocks of new composites, for example, a support material for the dispersion of nanoparticles. Here, we present a general approach for the preparation of graphene−metal particle nanocomposites in a water−ethylene glycol system using graphene oxide as a precursor and metal nanoparticles (Au, Pt and Pd) as building blocks. These metal nanoparticles are adsorbed on graphene oxide sheets and play a pivotal role in catalytic reduction of graphene oxide with ethylene glycol, leading to the formation of graphene−metal particle nanocomposites. The typical methanol oxidation of graphene−Pt composites in cyclic voltammograms analyses indicated its potential application in direct methanol fuel cells, bringing graphene−particle nanocomposites close to real technological applications.
1,478 citations
TL;DR: It is reported that increasing the pressure causes a steep increase in the onset Tc of F-doped LaOFeAs, to a maximum of ∼43 K at ∼4 GPa, which is the highest Tc reported to date.
Abstract: The hunt for new materials exhibiting high-temperature superconductivity is on again. A complex iron-based oxide, containing lanthanum and arsenic, was recently found to exhibit a transition temperature (Tc) of about 26 K when doped with fluoride ions. That's respectable, but far from the heights achieved in copper oxide superconductors. Now Takahashi et al. show that the application of around 40,000 atmospheres of pressure can raise the Tc of this material substantially, to about 43 K. This is the highest tc yet reported for a non-copper-based material. What is more, this record is unlikely to last for long: the complexity of 'iron oxypnictides' of this type offers considerable flexibility for chemical modification, and we can expect to hear of yet higher transition temperatures. This paper — and the prospect of a new wave of superconductor fever — is the subject of an Editorial in the 24 April issue of Nature (452, 914; 2008). The application of pressure can raise the superconducting transition temperature of oxypnictide (a pnicogen being a group V element) substantially, to a maximum value of about 43 K. This is the highest transition temperature yet reported for a non-copper-based material, but this record is unlikely to last for long: the material system offers considerable flexibility for chemical modification, and we can reasonably anticipate that this record will soon be superseded. The iron- and nickel-based layered compounds LaOFeP (refs 1, 2) and LaONiP (ref. 3) have recently been reported to exhibit low-temperature superconducting phases with transition temperatures Tc of 3 and 5 K, respectively. Furthermore, a large increase in the midpoint Tc of up to ∼26 K has been realized4 in the isocrystalline compound LaOFeAs on doping of fluoride ions at the O2- sites (LaO1-xFxFeAs). Experimental observations5,6 and theoretical studies7,8,9 suggest that these transitions are related to a magnetic instability, as is the case for most superconductors based on transition metals. In the copper-based high-temperature superconductors, as well as in LaOFeAs, an increase in Tc is often observed as a result of carrier doping in the two-dimensional electronic structure through ion substitution in the surrounding insulating layers, suggesting that the application of external pressure should further increase Tc by enhancing charge transfer between the insulating and conducting layers. The effects of pressure on these iron oxypnictide superconductors may be more prominent than those in the copper-based systems, because the As ion has a greater electronic polarizability, owing to the covalency of the Fe–As chemical bond, and, thus, is more compressible than the divalent O2- ion. Here we report that increasing the pressure causes a steep increase in the onset Tc of F-doped LaOFeAs, to a maximum of ∼43 K at ∼4 GPa. With the exception of the copper-based high-Tc superconductors, this is the highest Tc reported to date. The present result, together with the great freedom available in selecting the constituents of isocrystalline materials with the general formula LnOTMPn (Ln, Y or rare-earth metal; TM, transition metal; Pn, group-V, ‘pnicogen’, element), indicates that the layered iron oxypnictides are promising as a new material platform for further exploration of high-temperature superconductivity.
1,084 citations
TL;DR: It is found that O 2 etching kinetics vary strongly with the number of graphene layers in the sample, and three-layer-thick samples show etching similar to bulk natural graphite.
Abstract: Patterned graphene shows substantial potential for applications in future molecular-scale integrated electronics. Environmental effects are a critical issue in a single-layer material where every atom is on the surface. Especially intriguing is the variety of rich chemical interactions shown by molecular oxygen with aromatic molecules. We find that O2 etching kinetics vary strongly with the number of graphene layers in the sample. Three-layer-thick samples show etching similar to bulk natural graphite. Single-layer graphene reacts faster and shows random etch pits in contrast to natural graphite where nucleation occurs at point defects. In addition, basal plane oxygen species strongly hole dope graphene, with a Fermi level shift of ∼0.5 eV. These oxygen species desorb partially in an Ar gas flow, or under irradiation by far UV light, and readsorb again in an O2 atmosphere at room temperature. This strongly doped graphene is very different from “graphene oxide” made by mineral acid attack.
TL;DR: In this article, an XPS study of Ce 3D emission spectra dominated by atomic multiplet effects in core level spectroscopy of rare earth compounds (Ce oxides) was presented.
Abstract: This article presents an XPS study of Ce 3d emission spectra dominated by atomic multiplet effects in core level spectroscopy of rare earth compounds (Ce oxides). Core level spectroscopy has been used to study the electronic states of Ce 3d5/2 and Ce 3d3/2 levels in Ce4+ and Ce3+ states. The well-resolved components of Ce 3d5/2 and Ce 3d3/2 spin-orbit components, due to various final states (4f0, 4f1, 4f2 configurations), were determined on 3d XPS spectra from commercial powders (CeO2, CePO4).
These results were used to study the 3d spin-orbit component of mixed cerium-titanium oxide.
This compound was prepared by co-melting commercial powders of CeO2 and TiO2 at 1800 K under air using a solar furnace with a flux density of 16 MW.m−2 at the focal point of the parabolic concentrator. The mixed oxide Ce2Ti2O7 was produced and contained Ce(III) species which may be reactive with water to give back the initial metal oxides and generate hydrogen, a valuable product considered as a promising energy carrier in the future in replacement of oil.
The 3d photoemission spectra revealed the presence of mixed components attributed to mainly Ce(III) and Ce(IV) species. Copyright © 2008 John Wiley & Sons, Ltd.
TL;DR: Step-by-step controllable thermal reduction of individual graphene oxide sheets, incorporated into multiterminal field effect devices, was carried out at low temperatures with simultaneous electrical measurements.
Abstract: Step-by-step controllable thermal reduction of individual graphene oxide sheets, incorporated into multiterminal field effect devices, was carried out at low temperatures (125−240 °C) with simultaneous electrical measurements. Symmetric hysteresis-free ambipolar (electron- and hole-type) gate dependences were observed as soon as the first measurable resistance was reached. The conductivity of each of the fabricated devices depended on the level of reduction (was increased more than 106 times as reduction progressed), strength of the external electrical field, density of the transport current, and temperature.
TL;DR: It is demonstrated here how this can affect the catalytic reactivity of iron catalysts in Fischer-Tropsch synthesis (FTS), using in situ XRD under conditions close to the reaction conditions, that the distribution of iron carbide and oxide phases is modulated in the CNT-confined system.
Abstract: Following our previous findings that confinement within carbon nanotubes (CNTs) can modify the redox properties of encapsulated iron oxides, we demonstrate here how this can affect the catalytic reactivity of iron catalysts in Fischer−Tropsch synthesis (FTS). The investigation, using in situ XRD under conditions close to the reaction conditions, reveals that the distribution of iron carbide and oxide phases is modulated in the CNT-confined system. The iron species encapsulated inside CNTs prefer to exist in a more reduced state, tending to form more iron carbides under the reaction conditions, which have been recognized to be essential to obtain high FTS activity. The relative ratio of the integral XRD peaks of iron carbide (FexCy) to oxide (FeO) is about 4.7 for the encapsulated iron catalyst in comparison to 2.4 for the iron catalyst dispersed on the outer walls of CNTs under the same conditions. This causes a remarkable modification of the catalytic performance. The yield of C5+ hydrocarbons over the e...
TL;DR: In this paper, X-ray photo-electron spectroscopy (XPS) measurements were used to study the growth of polycrystalline copper oxide layers in the presence of ambient air conditions for long periods.
Abstract: Qualitative and quantitative studies of the oxidation of polycrystalline copper (Cu) thin films upon exposure to ambient air conditions for long periods (on the order of several months) are reported in this work. Thin films of Cu, prepared by thermal evaporation, were analyzed by means of X-ray photoelectron spectroscopy (XPS) to gain an understanding on the growth mechanism of the surface oxide layer. Analysis of high-resolution Cu LMM, Cu2p3/2, and O1s spectra was used to follow the time dependence of individual oxide overlayer thicknesses as well as the overall oxide composite thickness. Transmission electron microscopy (TEM) and spectroscopic ellipsometry (SE) were used to confirm the results obtained from XPS measurements. Three main stages of copper oxide growth were observed: (a) the formation of a Cu2O layer, most likely due to Cu metal ionic transport toward the oxide−oxygen interface, (b) the formation of a Cu(OH)2 metastable overlayer, due to the interactions of Cu ions with hydroxyl groups pr...
TL;DR: In this paper, the authors reported that tin monoxide (SnO) has a high hole mobility and produces good p-type oxide thin-film transistors (TFTs).
Abstract: This paper reports that among known p-type oxide semiconductors, tin monoxide (SnO) has a high hole mobility and produces good p-type oxide thin-film transistors (TFTs) Device-quality SnO films were grown epitaxially on (001) yttria-stabilized zirconia substrates at 575°C by pulsed laser deposition These exhibited a Hall mobility of 24cm2V−1s−1 at room temperature Top-gated TFTs, using epitaxial SnO channels, exhibited field-effect mobilities of 13cm2V−1s−1, on/off current ratios of ∼102, and threshold voltages of 48V
TL;DR: In this article, the authors investigated the surface reactivity of cobalt oxides in low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy.
Abstract: Cobalt oxides comprise two readily accessible cation oxidation states: Co 2+ and Co 3+ , which are thermodynamically competitive under common ambient conditions, and redox mechanisms connecting the two states are largely responsible for their success in partial oxidation catalysis. In our studies, CoO(1 0 0), Co 3 O 4 (1 1 0), and Co 3 O 4 (1 1 1) single crystal substrates have been investigated with X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and low energy electron diffraction (LEED) for their surface reactivity toward O 2 and H 2 O and for their stability under reducing UHV conditions. There is facile inter-conversion between CoO and Co 3 O 4 stoichiometry at the oxide surface which, despite the compositional variability, remains well ordered in long-range structure. Surface impurities, however, can pin the surface at either CoO or Co 3 O 4 compositional extremes. Contrary to reports of a pressure gap that creates difficulty in oxide hydroxylation under UHV, it is pos sible to hydroxylate both cobalt monoxide and spinel oxide substrates with H 2 O, provided sufficient activation is available to dis sociate the water molecule.
TL;DR: In this paper, the effect of water vapour on the formation of a more porous scale is discussed. But, while there is experimental evidence for altered mechanical behaviour, there is very little data on relevant mechanical properties.
TL;DR: A critical review describes some illustrative examples, emphasizes the role of the interplay between theory and experiment, and relates some recent findings related to the possibility to control the charge state of a supported nanoparticle on an ultrathin oxide film.
Abstract: Ultrathin oxide films on metals offer new opportunities for the design of supported nanoclusters with potential use in catalysis. This requires a characterization at the atomistic level of the structure and composition of the thin film, of its morphology and defect structure. A proper selection of metal/oxide interface, film thickness, lattice mismatch, etc. makes it possible to prepare collections of supported metal particles with novel properties. This critical review describes some illustrative examples, emphasizes the role of the interplay between theory and experiment, and relates some recent findings related to the possibility to control the charge state of a supported nanoparticle on an ultrathin oxide film (211 references).
TL;DR: In this paper, the authors describe the anodic growth of self-organized TiO 2 nanotube layers in glycerol/water/ammonium fluoride electrolytes, and show that there is a significant effect of even small additions of water (0.67vol%) into the resulting geometry, and that during anodization permanently a high-field oxide layer is present on the tube bottom.
TL;DR: In this article, a stable, cation-ordered oxides with layered lattice structures were obtained with medium-sized Ln 3+ ions over a wide range of oxygen partial pressures, a property essential for applications as oxygen separation membranes and solid oxide fuel cell cathodes.
TL;DR: In this article, a model for the thermodynamic properties of rhombohedral oxide solid solutions in the system Fe2O3-FeTiO3 -MgTiO 3-MnTiO4 (containing minor amounts of Al 2O3) is presented.
Abstract: A model for the thermodynamic properties of rhombohedral oxide solid solutions in the system Fe2O3-FeTiO3-MgTiO3-MnTiO3 (containing minor amounts of Al2O3) is presented. The model accounts for temperature and compositionally dependent long-range cation-order and the related high to low symmetry structural phase transition. The model is calibrated from the cation-ordering data of Harrison and others (2000; Harrison and Redfern, 2001) and experimental data on Fe+2Ti ⇔ (Fe+3)2 exchange between rhombohedral oxide and spinel from Lattard and others (2005) and Evans and others (2006). Successful calibration require introduction of an energetic contribution attributed to short-range cation-order, which reduces the configurational entropy of the solid solution. The resultant thermodynamic model for the rhombohedral oxides is internally consistent with the model for spinel solid solutions of Sack and Ghiorso (1991a, 1991b) and with the endmember thermodynamic properties database of Berman (1988); a new model equation for the isobaric heat capacity of ulvospinel (cubic Fe2TiO4) is proposed and values of the enthalpy of formation, −1490.417 kJ/mol, and third law entropy, 184.199 J/K-mol, at 298.15 K and 105 Pa are recommended. The new model forms the basis of a revised FeTi-oxide geothermometer/oxygen barometer, which is applied to a newly compiled dataset of natural two oxide pairs from silicic volcanic rocks. Results are compared to previous formulations with the general conclusion that the new model gives a better estimate of oxidation state for magmas that equilibrated under conditions more oxidizing than the nickel-nickel oxide buffer. Estimates of oxygen fugacity are fairly insensitive to analytical uncertainties in oxide compositions. By contrast, temperature estimates are especially sensitive to analytical error and to the abundances of “minor” constituents. Application of the geothermometer to oxide pairs that grew under conditions where the rhombohedral phase was cation disordered (that is high temperature or at oxygen fugacities greater by about one log10 unit than the nickel-nickel oxide buffer) results in an uncertainty due solely to analytical error of at least 50°C and sometimes as high as 100 °C. Temperature estimates from the new geothermometer can be made using either the Fe+2Ti ⇔ (Fe+3)2 exchange or Fe+2 ⇔ Mg exchange between the two oxides. Comparison of the two temperature estimates provides a means of evaluating the internal consistency of coexisting oxide compositions and assessing the extent of disequilibrium. Temperatures calculated from the new model are found to be consistent with experimental phase relations for the stability of cummingtonite in silicic volcanics. Other petrologic constraints on derived temperatures are examined including limits on the width of the miscibility gap and the development of self-reversed remanent magnetization in the rhombohedral series. Software that implements the new thermodynamic model and the two-oxide geothermometer/oxygen barometer is available from http://www.ofm-research.org/.
TL;DR: In this article, a comparative analysis of electrochemical and transport properties in the major families of cathode and anode compositions for intermediate-temperature solid oxide fuel cells (SOFCs) and materials science-related factors affecting electrode performance is presented.
Abstract: This work is focused on the comparative analysis of electrochemical and transport properties in the major families of cathode and anode compositions for intermediate-temperature solid oxide fuel cells (SOFCs) and materials science-related factors affecting electrode performance. The first part presents a brief overview of the electrochemical and chemical reactions in SOFCs, specific rate-determining steps of the electrode processes, solid oxide electrolyte ceramics, and effects of partial oxygen ionic and electronic conductivities in the SOFC components. The aspects associated with materials compatibility, thermal expansion, stability, and electrocatalytic behavior are also briefly discussed. Primary attention is centered on the experimental data and approaches reported during the last 10–15 years, reflecting the main challenges in the field of materials development for the ceramic fuel cells.
TL;DR: Surfactant-free processes, which have become a valuable alternative to surfactant-assisted as well as to traditional aqueous sol-gel chemistry routes, are discussed.
Abstract: Surfactant-free nonaqueous (and/or nonhydrolytic) sol-gel routes constitute one of the most versatile and powerful synthesis methodologies for nanocrystalline metal oxides with high compositional homogeneity and purity. Although the synthesis protocols are particularly simple, involving only metal oxide precursors and common organic solvents, the obtained uniform nanocrystals exhibit an immense variety of sizes and shapes. The small number of reactants in these routes enables the study of the chemical mechanisms involved in metal oxide formation. Nonhydrolytic routes to inorganic nanomaterials that used surfactants as size- and shape-controlling agents have been discussed recently. This Minireview supplements this topic by discussing surfactant-free processes, which have become a valuable alternative to surfactant-assisted as well as to traditional aqueous sol-gel chemistry routes.
TL;DR: In this article, a soft chemistry method was used to synthesize BaCe 0.3 Zr 0.5 Y 0.2 O 3 −−−− δ (BCZY, with 0.8) proton conductors to combine the high propton conductivity of barium cerate and good chemical stability of Barium zirconate.
TL;DR: To modify oxide structure and introduce a thin conductive film on Li4Ti5O12, thermal nitridation was adopted for the first time and shows great electrochemical properties at high current densities.
Abstract: To modify oxide structure and introduce a thin conductive film on Li4Ti5O12, thermal nitridation was adopted for the first time. NH3 decomposes surface Li4Ti5O12 to conductive TiN at high temperature, and surprisingly, it also modifies the surface structure in a way to accommodate the single phase Li insertion and extraction. The electrochemically induced Li4+δTi5O12 with a TiN coating layer shows great electrochemical properties at high current densities.
TL;DR: In this paper, the authors describe the deposition and optoelectronic properties of reduced graphene oxide thin films with thicknesses ranging from 1-10nm by the vacuum filtration method.
Abstract: The deposition and optoelectronic properties of reduced graphene oxide thin films are described. Thin films with thicknesses ranging from 1–10nm have been deposited by the vacuum filtration method. The conductivity of the thin films can be varied over six orders of magnitude by varying the filtration volume of the graphene oxide aqueous suspension while maintaining the transmittance between 60%–95%. In addition, enhancement in the conductance through Cl doping is demonstrated. The combination of the reduction and Cl treatments make the reduced graphene oxide thin films sufficiently conducting to incorporate them as the hole collecting electrode in proof of concept organic photovoltaic devices.
TL;DR: An overview of the current understanding of electro-oxidation of Pd in aqueous acidic and basic electrolytes is presented in this paper, where various experimental procedures commonly used to determine the real surface area of Palladium electrodes are outlined and compared.
TL;DR: Gold nanoparticles supported on metal oxides are efficient catalysts for important oxidation process, including selective oxidation of hydrocarbons and oxidation of various volatile organic compounds (VOCs), such as CO, CH3OH, and HCHO at moderately elevated temperatures.
Abstract: One of the great challenges in catalysis is to devise new catalysts that have high activity when illuminated by visible light. Solving this challenge will allow us to use sunlight, an abundant and clean low-cost energy source, to drive chemical reactions. Visible light (wavelength l> 400 nm) constitutes around 43% of solar energy, and the energy of sunlight to the Earth is about 10000 times more than the current energy consumption of the world. Many approaches have been proposed to develop visible light photocatalysts, including doping TiO2 with metal ions or metal atom clusters, [4,5] incorporating nitrogen and carbon into TiO2, and employing other metal oxides or polymetallates as catalyst materials. 5,8] Research has been mainly concentrated on semiconductor oxides. Sulfides have also been studied, but they are not suitable catalysts because of their poor chemical stability. However, searching for catalysts that can work under visible light should not be limited to semiconductor materials with band-gap structure, but can be extended to other materials, such as gold nanoparticles. It can be said that glaziers in medieval forges were the first nanotechnologists who produced colors with gold nanoparticles of different sizes, although they had little understanding of the modern day principles which have become a hot topic in the last two decades. In recent years there have been numerous studies on the optical properties of gold nanoparticles. Gold nanoparticles absorb visible light intensely because of the surface plasmon resonance (SPR) effect. The electromagnetic field of incident light couples with the oscillations of conduction electrons in gold particles, resulting in strong-field enhancement of the local electromagnetic fields near the rough surface of gold nanoparticles. The enhanced local field strength can be over 500 times larger than the applied field for structures with sharp apices, edges, or concave curvature (e.g. nanowires, cubes, triangular plates, and nanoparticle junctions). The SPR absorption may cause rapid heating of the nanoparticles. Gold nanoparticles supported on metal oxides are efficient catalysts for important oxidation process, including selective oxidation of hydrocarbons and oxidation of various volatile organic compounds (VOCs), such as CO, CH3OH, and HCHO at moderately elevated temperatures. Therefore, the combination of the SPR absorption and the catalytic activity of gold nanoparticles presents an important opportunity: if the heated gold nanoparticles could activate the organic molecules on them to induce oxidation of the organic compounds, then oxidation on gold catalysts can be driven by visible light at ambient temperature. Moreover, the SPR is a local effect, limited to the noble metal particles, so that the light only heats gold nanoparticles, which generally account for a few percent of the overall catalyst mass. This leads to significant saving in energy consumption for catalyzing organic compound oxidation. To verify the possibility of driving the VOC oxidation with visible light at room temperature, we prepared gold particles supported on various oxide powders. ZrO2 and SiO2 powders were first chosen as supports, because their band gaps are circa 5.0 eV and circa 9.0 eV, respectively, which are much larger than the energies of the photons of visible light (less than 3.0 eV). Thus, the light cannot excite electrons from the valence band to the conduction band. It is also impossible for the gold nanoparticles on ZrO2 to reduce the band gaps of ZrO2 enough for visible light photons to be absorbed and excite electrons in ZrO2. Thus, the catalytic activity is not caused by the same mechanism as occurs in semiconductor photocatalysts, but is due to the SPR effect of gold nanoparticles. The changes in the concentrations of the reactant (HCHO, 100 ppm) and product (CO2), when gold supported on ZrO2 was used as the catalyst, are depicted in Figure 1a. The initial concentration of HCHO in the glass vessel was 100 ppm. HCHO content decreased by 64% in two hours under the irradiation of six light tubes of blue light (with wavelength between 400 and 500 nm and the irradiation energy determined to be 0.17 Wcm 2 at the position of glass slides coated with the gold catalysts), and the CO2 content in the vessel increased accordingly. These results confirm that the oxidation of formaldehyde to carbon dioxide proceeds to a large extent at ambient temperature. The turnover frequency was calculated as being about 1.2 = 10 3 molecules of [*] Dr. X. Chen, Prof. H.-Y. Zhu, Z.-F. Zheng School of Physical and Chemical Sciences Queensland University of Technology Brisbane, Qld 4001 (Australia) Fax: (+61)7-3864-1804 E-mail: hy.zhu@qut.edu.au