Showing papers on "Oxide published in 2012"

Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction

Abstract: We present two different ways to fabricate nitrogen-doped graphene (N-graphene) and demonstrate its use as a metal-free catalyst to study the catalytic active center for the oxygen reduction reaction (ORR). N-graphene was produced by annealing of graphene oxide (G-O) under ammonia or by annealing of a N-containing polymer/reduced graphene oxide (RG-O) composite (polyaniline/RG-O or polypyrrole/RG-O). The effects of the N precursors and annealing temperature on the performance of the catalyst were investigated. The bonding state of the N atom was found to have a significant effect on the selectivity and catalytic activity for ORR. Annealing of G-O with ammonia preferentially formed graphitic N and pyridinic N centers, while annealing of polyaniline/RG-O and polypyrrole/RG-O tended to generate pyridinic and pyrrolic N moieties, respectively. Most importantly, the electrocatalytic activity of the catalyst was found to be dependent on the graphitic N content which determined the limiting current density, while the pyridinic N content improved the onset potential for ORR. However, the total N content in the graphene-based non-precious metal catalyst does not play an important role in the ORR process.

Metal oxide hollow nanostructures for lithium-ion batteries

Abstract: Metal oxide hollow structures have received great attention because of their many promising applications in a wide range of fields. As electrode materials for lithium-ion batteries (LIBs), metal oxide hollow structures provide high specific capacity, superior rate capability, and improved cycling performance. In this Research News, we summarize the recent research activities in the synthesis of metal oxide hollow nanostructures with controlled shape, size,composition, and structural complexity, as well as their applications in LIBs. By focusing on hollow structures of some binary metal oxides (such as SnO 2 ,TiO 2 , Fe 2 O 3 , Co 3 O 4 ) and complex metal oxides, we seek to provide some rational understanding on the effect of nanostructure engineering on the electrochemical performance of the active materials. It is thus anticipated that this article will shed some light on the development of advanced electrode materials for next-generation LIBs.

Aqueous CO2 reduction at very low overpotential on oxide-derived Au nanoparticles.

Abstract: Carbon dioxide reduction is an essential component of many prospective technologies for the renewable synthesis of carbon-containing fuels. Known catalysts for this reaction generally suffer from low energetic efficiency, poor product selectivity, and rapid deactivation. We show that the reduction of thick Au oxide films results in the formation of Au nanoparticles (“oxide-derived Au”) that exhibit highly selective CO2 reduction to CO in water at overpotentials as low as 140 mV and retain their activity for at least 8 h. Under identical conditions, polycrystalline Au electrodes and several other nanostructured Au electrodes prepared via alternative methods require at least 200 mV of additional overpotential to attain comparable CO2 reduction activity and rapidly lose their activity. Electrokinetic studies indicate that the improved catalysis is linked to dramatically increased stabilization of the CO2•– intermediate on the surfaces of the oxide-derived Au electrodes.

Graphene oxide as a sulfur immobilizer in high performance lithium/sulfur cells

Abstract: The loss of sulfur cathode material as a result of polysulfide dissolution causes significant capacity fading in rechargeable lithium/sulfur cells. Embodiments of the invention use a chemical approach to immobilize sulfur and lithium polysulfides via the reactive functional groups on graphene oxide. This approach obtains a uniform and thin (˜tens of nanometers) sulfur coating on graphene oxide sheets by a chemical reaction-deposition strategy and a subsequent low temperature thermal treatment process. Strong interaction between graphene oxide and sulfur or polysulfides demonstrate lithium/sulfur cells with a high reversible capacity of 950-1400 mAh g −1 , and stable cycling for more than 50 deep cycles at 0.1 C.

Solution-cast metal oxide thin film electrocatalysts for oxygen evolution.

Abstract: Water oxidation is a critical step in water splitting to make hydrogen fuel. We report the solution synthesis, structural/compositional characterization, and oxygen evolution reaction (OER) electrocatalytic properties of ∼2–3 nm thick films of NiOx, CoOx, NiyCo1–yOx, Ni0.9Fe0.1Ox, IrOx, MnOx, and FeOx. The thin-film geometry enables the use of quartz crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the electrocatalytic activity and electrochemical properties of the oxides. Ni0.9Fe0.1Ox was found to be the most active water oxidation catalyst in basic media, passing 10 mA cm–2 at an overpotential of 336 mV with a Tafel slope of 30 mV dec–1 with oxygen evolution reaction (OER) activity roughly an order of magnitude higher than IrOx control films and similar to the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of layered Ni0.9Fe0.1OOH oxyhydroxide species with nearly every Ni atom electrochemically active. In contrast to pr...

Covalent hybrid of spinel manganese-cobalt oxide and graphene as advanced oxygen reduction electrocatalysts.

Abstract: Through direct nanoparticle nucleation and growth on nitrogen doped, reduced graphene oxide sheets and cation substitution of spinel Co3O4 nanoparticles, a manganese–cobalt spinel MnCo2O4/graphene hybrid was developed as a highly efficient electrocatalyst for oxygen reduction reaction (ORR) in alkaline conditions Electrochemical and X-ray near-edge structure (XANES) investigations revealed that the nucleation and growth method for forming inorganic–nanocarbon hybrids results in covalent coupling between spinel oxide nanoparticles and N-doped reduced graphene oxide (N-rmGO) sheets Carbon K-edge and nitrogen K-edge XANES showed strongly perturbed C–O and C–N bonding in the N-rmGO sheet, suggesting the formation of C–O–metal and C–N–metal bonds between N-doped graphene oxide and spinel oxide nanoparticles Co L-edge and Mn L-edge XANES suggested substitution of Co3+ sites by Mn3+, which increased the activity of the catalytic sites in the hybrid materials, further boosting the ORR activity compared with th

Efficient Synthesis of Heteroatom (N or S)‐Doped Graphene Based on Ultrathin Graphene Oxide‐Porous Silica Sheets for Oxygen Reduction Reactions

Abstract: Heteroatom (N or S)-doped graphene with high surface area is successfully synthesized via thermal reaction between graphene oxide and guest gases (NH3 or H2S) on the basis of ultrathin graphene oxide-porous silica sheets at high temperatures. It is found that both N and S-doping can occur at annealing temperatures from 500 to 1000 °C to form the different binding configurations at the edges or on the planes of the graphene, such as pyridinic-N, pyrrolic-N, and graphitic-N for N-doped graphene, thiophene-like S, and oxidized S for S-doped graphene. Moreover, the resulting N and S-doped graphene sheets exhibit good electrocatalytic activity, long durability, and high selectivity when they are employed as metal-free catalysts for oxygen reduction reactions. This approach may provide an efficient platform for the synthesis of a series of heteroatom-doped graphenes for different applications.

Covalent Hybrid of Spinel Manganese-Cobalt Oxide and Gra-phene as Advanced Oxygen Reduction Electrocatalysts

Abstract: Through direct nanoparticle nucleation and growth on nitrogen doped, reduced graphene oxide sheets and cation substitution of spinel Co3O4 nanoparticles, a manganese-cobalt spinel MnCo2O4/graphene hybrid was developed as a highly efficient electrocatalyst for oxygen reduction reaction (ORR) in alkaline conditions. Electrochemical and X-ray near edge structure (XANES) investigations revealed that the nucleation and growth method for forming inorganic-nanocarbon hybrid results in covalent coupling between spinel oxide nanoparticles and N-doped reduced graphene oxide (N-rmGO) sheets. Carbon K-edge and nitrogen K-edge XANES showed strongly perturbed C-O and C-N bonding in the N-rmGO sheet, suggesting the formation of C-O-metal and C-N-metal bonds between N-doped graphene oxide and spinel oxide nanoparticles. Co L-edge and Mn L-edge XANES suggested substitu-tion of Co3+ sites by Mn3+, which increased the activity of the catalytic sites in the hybrid materials, further boosting the ORR activity compared to the pure cobalt oxide hybrid. The covalently bonded hybrid afforded much greater activity and durability than the physi-cal mixture of nanoparticles and carbon materials including N-rmGO. At the same mass loading, the MnCo2O4/N-graphene hybrid can outperform Pt/C in ORR current density at medium overpotentials with superior stability to Pt/C in alkaline solutions.

Nanostructured metal oxide-based materials as advanced anodes for lithium-ion batteries.

Abstract: The search for new electrode materials for lithium-ion batteries (LIBs) has been an important way to satisfy the ever-growing demands for better performance with higher energy/power densities, improved safety and longer cycle life. Nanostructured metal oxides exhibit good electrochemical properties, and they are regarded as promising anode materials for high-performance LIBs. In this feature article, we will focus on three different categories of metal oxides with distinct lithium storage mechanisms: tin dioxide (SnO2), which utilizes alloying/dealloying processes to reversibly store/release lithium ions during charge/discharge; titanium dioxide (TiO2), where lithium ions are inserted/deinserted into/out of the TiO2 crystal framework; and transition metal oxides including iron oxide and cobalt oxide, which react with lithium ions via an unusual conversion reaction. For all three systems, we will emphasize that creating nanomaterials with unique structures could effectively improve the lithium storage properties of these metal oxides. We will also highlight that the lithium storage capability can be further enhanced through designing advanced nanocomposite materials containing metal oxides and other carbonaceous supports. By providing such a rather systematic survey, we aim to stress the importance of proper nanostructuring and advanced compositing that would result in improved physicochemical properties of metal oxides, thus making them promising negative electrodes for next-generation LIBs.

Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review

Abstract: Metal oxide gas sensors are predominant solid-state gas detecting devices for domestic, commercial and industrial applications, which have many advantages such as low cost, easy production, and compact size However, the performance of such sensors is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties Lots of metal oxide nanostructures have been developed to improve the gas sensing properties such as sensitivity, selectivity, response speed, and so on Here, we provide a brief overview of metal oxide nanostructures and their gas sensing properties from the aspects of particle size, morphology and doping When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called "small size effect", yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them In view of those reasons, nanostructures with many kinds of shapes such as porous nanotubes, porous nanospheres and so on have been investigated, that not only possessed large surface area and relatively mass reactive sites, but also formed relatively loose film structures which is an advantage for gas diffusion Besides, doping is also an effective method to decrease particle size and improve gas sensing properties Therefore, the gas sensing properties of metal oxide nanostructures assembled by nanoparticles are reviewed in this article The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given

Highly Conductive and Porous Activated Reduced Graphene Oxide Films for High-Power Supercapacitors

Abstract: We present a novel method to prepare highly conductive, free-standing, and flexible porous carbon thin films by chemical activation of reduced graphene oxide paper. These flexible carbon thin films possess a very high specific surface area of 2400 m2 g–1 with a high in-plane electrical conductivity of 5880 S m–1. This is the highest specific surface area for a free-standing carbon film reported to date. A two-electrode supercapacitor using these carbon films as electrodes demonstrated an excellent high-frequency response, an extremely low equivalent series resistance on the order of 0.1 ohm, and a high-power delivery of about 500 kW kg–1. While higher frequency and power values for graphene materials have been reported, these are the highest values achieved while simultaneously maintaining excellent specific capacitances and energy densities of 120 F g–1 and 26 W h kg–1, respectively. In addition, these free-standing thin films provide a route to simplify the electrode-manufacturing process by eliminating...

Facile Synthesis of Nitrogen-Doped Graphene via Pyrolysis of Graphene Oxide and Urea, and its Electrocatalytic Activity toward the Oxygen-Reduction Reaction

Abstract: excellent thermal conductivity, [ 2 ] and a high optical transparency. [ 3 ] These properties lead to very promising applications of graphene in electronic devices, [ 4 ] transparent electrodes, [ 5 ] and energy-storage devices. [ 6 , 7 ] Recently, it was found that graphene has an extraordinary catalytic activity. [ 8–14 ] For example, N-doped graphene has a high catalytic activity toward the oxygen-reduction reaction (ORR). [ 8 ] Furthermore, graphene oxide (GO), an important derivative of graphene, is an effi cient catalyst for oxidation and hydration reactions of various alcohols, [ 9 ] whereas reduced GO can be used for catalyzing the hydrogenation of nitrobenzene. [ 10 ] Studies have shown that the heteroatoms in graphene derivatives, such as N and O, play a critical role in their catalytic activities. [ 10 , 15 , 16 ] Thus, the introduction of dopants into the graphene lattice has been the focus of much research in order to achieve a high catalytic activity toward target reactions. Among various doped graphenes, nitrogen-doped graphene (NG) has attracted much attention because of its high catalytic activity toward the ORR – the electrode reaction that often limits the performance of the cathode in a fuel cell or a metalair battery. Conventional Pt-based catalysts have a high intrinsic catalytic activity toward the ORR, but suffer from the drawbacks of high cost, poor long-term stability, and susceptibility to the crossover effect, which hinder the commercial viability of Ptloaded fuel cells. [ 17 ] Thus the search for an alternative catalyst, such as NG, is of great importance in replacing these expensive Pt-based catalysts. To prepare NG, chemical-vapor deposition in the presence of N-containing precursors is the most-common method; [ 18 ] arc discharge of graphite electrodes in a H 2 /pyridine or H 2 /NH 3 atmosphere can also produce NG. [ 19 ] However, the extremely low yield and high cost of these methods limit their application only to fundamental studies. Later, it was found that GO can be

Polyaniline-grafted reduced graphene oxide for efficient electrochemical supercapacitors

Abstract: An alternative and effective route to prepare conducting polyaniline-grafted reduced graphene oxide (PANi-g-rGO) composite with highly enhanced properties is reported. In order to prepare PANi-g-rGO, amine-protected 4-aminophenol was initially grafted to graphite oxide (GO) via acyl chemistry where a concomitant partial reduction of GO occurred due to the refluxing and exposure of GO to thionyl chloride vapors and heating. Following the deprotection of amine groups, an in situ chemical oxidative grafting of aniline in the presence of an oxidizing agent was carried out to yield highly conducting PANi-g-rGO. Electron microscopic studies demonstrated that the resultant composite has fibrillar morphology with a room-temperature electrical conductivity as high as 8.66 S/cm and capacitance of 250 F/g with good cycling stability.

Oxygen Reduction Electrocatalyst Based on Strongly Coupled Cobalt Oxide Nanocrystals and Carbon Nanotubes

Abstract: Electrocatalyst for oxygen reduction reaction (ORR) is crucial for a variety of renewable energy applications and energy-intensive industries. The design and synthesis of highly active ORR catalysts with strong durability at low cost is extremely desirable but remains challenging. Here, we used a simple two-step method to synthesize cobalt oxide/carbon nanotube (CNT) strongly coupled hybrid as efficient ORR catalyst by directly growing nanocrystals on oxidized multiwalled CNTs. The mildly oxidized CNTs provided functional groups on the outer walls to nucleate and anchor nanocrystals, while retaining intact inner walls for highly conducting network. Cobalt oxide was in the form of CoO due to a gas-phase annealing step in NH3. The resulting CoO/nitrogen-doped CNT (NCNT) hybrid showed high ORR current density that outperformed Co3O4/graphene hybrid and commercial Pt/C catalyst at medium overpotential, mainly through a 4e reduction pathway. The metal oxide/carbon nanotube hybrid was found to be advantageous o...

Understanding Aqueous Dispersibility of Graphene Oxide and Reduced Graphene Oxide through pKa Measurements

Abstract: The chemistry underlying the aqueous dispersibility of graphene oxide (GO) and reduced graphene oxide (r-GO) is a key consideration in the design of solution processing techniques for the preparation of processable graphene sheets. Here, we use zeta potential measurements, pH titrations, and infrared spectroscopy to establish the chemistry underlying the aqueous dispersibility of GO and r-GO sheets at different values of pH. We show that r-GO sheets have ionizable groups with a single pK value (8.0) while GO sheets have groups that are more acidic (pK = 4.3), in addition to groups with pK values of 6.6 and 9.0. Infrared spectroscopy has been used to follow the sequence of ionization events. In both GO and r-GO sheets, it is ionization of the carboxylic groups that is primarily responsible for the build up of charge, but on GO sheets, the presence of phenolic and hydroxyl groups in close proximity to the carboxylic groups lowers the pKa value by stabilizing the carboxylate anion, resulting in superior wate...

Transition Metal Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies

Abstract: Transition metal oxides are capable of a wide range of work functions. This quality allows them to be used in many applications that involve charge transfer with adsorbed molecules, for example as heterogeneous catalysts, as charge-injection layers in organic electronics, and as electrodes in fuel cells. Chemical and structural factors can alter transition-metal oxide work functions, often making their work functions difficult to control. Little is known about the effects of the cation oxidation state and point defects on the oxide work function. It is necessary to understand how such chemical and structural factors affect work functions in order to controllably tune transition metal oxides for desired applications. Here, a correlation between the oxide work function and cation oxidation state is demonstrated. This correlation is attributed to the change in cation electronegativity with oxidation state. A model is presented that relates the work function to the oxygen deficiency for d0 oxides in the limit of dilute oxygen vacancies. It is proposed that the rapid initial decrease in work function, observed for d0 oxides, is caused by an increase in the density of donor-like defect states. It is also shown that oxides tend to have decreased work functions near a metal/metal-oxide interface as a consequence of the relationship between defects and work function. These insights provide guidelines for tuning transition metal oxide work functions.

Reduced graphene oxide for catalytic oxidation of aqueous organic pollutants.

Abstract: We discovered that chemically reduced graphene oxide, with an ID/IG >1.4 (defective to graphite) can effectively activate peroxymonosulfate (PMS) to produce active sulfate radicals. The produced sulfate radicals (SO4•—) are powerful oxidizing species with a high oxidative potential (2.5–3.1 vs 2.7 V of hydroxyl radicals), and can effectively decompose various aqueous contaminants. Graphene demonstrated a higher activity than several carbon allotropes, such as activated carbon (AC), graphite powder (GP), graphene oxide (GO), and multiwall carbon nanotube (MWCNT). Kinetic study of graphene catalyzed activation of PMS was carried out. It was shown that graphene catalysis is superior to that on transition metal oxide (Co3O4) in degradation of phenol, 2,4-dichlorophenol (DCP) and a dye (methylene blue, MB) in water, therefore providing a novel strategy for environmental remediation.

Tunable photoluminescence from graphene oxide.

Abstract: Graphene oxide (GO) is a graphene sheet modified with oxygen functional groups in the form of epoxy and hydroxy groups on the basal plane and various other types at the edges. It exhibits interesting steady-state photoluminescence (PL) properties. For example, low-energy fluorescence in red to near infrared (NIR) wavelengths (from 600– 1100 nm) has been detected for suspensions and solid thin films of as-synthesized GO. 3] In addition, broad luminescence from 400 to 800 nm from oxygen plasma-treated, mechanically exfoliated, single-layer graphene sheet has been reported. Blue fluorescence with a relatively narrow bandwidth when excited with UV irradiation has also been detected from chemically reduced GO (rGO) and graphene quantum dots. 6] Recently, chemically modified GO or rGO with n-butylamine or Mn has also demonstrated PL emission at a range of energies. 10] A detailed explanation of the origin of such variable energy PL in GO has yet to be elucidated. This is partly because the sample preparation and reduction methods varied, making it difficult to compare the results. Herein, we have prepared GO suspensions that exhibit virtually all of the PL features observed by different groups, through careful and gradual reduction of the GO. The systematic evolution of the electronic structure and comprehensive analysis of steady-state and transient PL along with photoluminescence excitation (PLE) spectroscopy measurements indicate that two different types of electronically excited states are responsible for the observed emission characteristics. GO was synthesized using the modified Hummers method, the details of which have been reported. GO usually contains a large fraction of sp hybridized carbon atoms bound to oxygen functional groups, which makes it an insulator. Reduction can be achieved chemically (e.g. hydrazine exposure) or by thermal annealing in inert environments. Photothermal reduction of GO can be achieved by exposing GO samples to a Xenon flash in ambient conditions. In this study, we prepared aqueous GO solutions and subjected them to steady-state Xe lamp irradiation (500 W) with different exposure times of up to three hours. In contrast to reduction by an instantaneous flash, this method provides a controllable, gradual transformation from GO to rGO, allowing exploration of the PL evolution and emission mechanisms from as-synthesized GO to rGO. The deoxygenation of GO after reduction was confirmed by X-ray photoelectron spectroscopy (XPS), as shown in Figure 1. The C 1s signals of the original GO can be deconvoluted into signals for the C=C bond in aromatic rings (284.6 eV), C O bond (286.1 eV), C=O bond (287.5 eV), and C(=O) OH bond (289.2 eV), in agreement with previous assignments. Increased sp carbon bonding with increased reduction time can be clearly measured, which

Magnetically Driven Floating Foams for the Removal of Oil Contaminants from Water

Abstract: In this study, we present a novel composite material based on commercially available polyurethane foams functionalized with colloidal superparamagnetic iron oxide nanoparticles and submicrometer polytetrafluoroethylene particles, which can efficiently separate oil from water. Untreated foam surfaces are inherently hydrophobic and oleophobic, but they can be rendered water-repellent and oil-absorbing by a solvent-free, electrostatic polytetrafluoroethylene particle deposition technique. It was found that combined functionalization of the polytetrafluoroethylene-treated foam surfaces with colloidal iron oxide nanoparticles significantly increases the speed of oil absorption. Detailed microscopic and wettability studies reveal that the combined effects of the surface morphology and of the chemistry of the functionalized foams greatly affect the oil-absorption dynamics. In particular, nanoparticle capping molecules are found to play a major role in this mechanism. In addition to the water-repellent and oil-ab...

Interfacial Microstructure and Properties of Carbon Fiber Composites Modified with Graphene Oxide

Abstract: The performance of carbon fiber-reinforced composites is dependent to a great extent on the properties of fiber–matrix interface. To improve the interfacial properties in carbon fiber/epoxy composites, we directly introduced graphene oxide (GO) sheets dispersed in the fiber sizing onto the surface of individual carbon fibers. The applied graphite oxide, which could be exfoliated to single-layer GO sheets, was verified by atomic force microscope (AFM). The surface topography of modified carbon fibers and the distribution of GO sheets in the interfacial region of carbon fibers were detected by scanning electron microscopy (SEM). The interfacial properties between carbon fiber and matrix were investigated by microbond test and three-point short beam shear test. The tensile properties of unidirectional (UD) composites were investigated in accordance with ASTM standards. The results of the tests reveal an improved interfacial and tensile properties in GO-modified carbon fiber composites. Furthermore, significa...

Nitrogen-doped graphene and its iron-based composite as efficient electrocatalysts for oxygen reduction reaction.

Abstract: The high cost of platinum-based electrocatalysts for the oxygen reduction reaction (ORR) has hindered the practical application of fuel cells. Thanks to its unique chemical and structural properties, nitrogen-doped graphene (NG) is among the most promising metal-free catalysts for replacing platinum. In this work, we have developed a cost-effective synthesis of NG by using cyanamide as a nitrogen source and graphene oxide as a precursor, which led to high and controllable nitrogen contents (4.0% to 12.0%) after pyrolysis. NG thermally treated at 900 °C shows a stable methanol crossover effect, high current density (6.67 mA cm–2), and durability (∼87% after 10 000 cycles) when catalyzing ORR in alkaline solution. Further, iron (Fe) nanoparticles could be incorporated into NG with the aid of Fe(III) chloride in the synthetic process. This allows one to examine the influence of non-noble metals on the electrocatalytic performance. Remarkably, we found that NG supported with 5 wt % Fe nanoparticles displayed ...

Influence of Oxygen Evolution during Water Oxidation on the Surface of Perovskite Oxide Catalysts

Abstract: Perovskites such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF82) can be highly active for the oxygen evolution reaction (OER) upon water oxidation in alkaline solution. Here we report that BSCF82 can quickly undergo amorphization of its surface at OER potentials, which is accompanied by reduced surface concentrations of Ba2+ and Sr2+ ions as well as increased pseudocapacitive and OER currents. Such quick amorphization during OER was also observed for perovskite catalysts with similar OER activities such as Ba0.5Sr0.5Co0.4Fe0.6O3−δ and SrCo0.8Fe0.2O3−δ. In contrast, perovskite oxides with lower OER activities than BSCF82 did not undergo this transformation when subjected to identical electrochemical conditions. These findings demonstrate that the active chemistry and structure of oxide catalysts during OER can significantly differ from those of the as-synthesized material and that understanding how the oxide surface may change and impact the OER activity is critical to the design of highly active and stable OER catal...

The Origin of Fluorescence from Graphene Oxide

Abstract: Time-resolved fluorescence measurements of graphene oxide in water show multiexponential decay kinetics ranging from 1 ps to 2 ns. Electron-hole recombination from the bottom of the conduction band and nearby localized states to wide-range valance band is suggested as origin of the fluorescence. Excitation wavelength dependence of the fluorescence was caused by relative intensity changes of few emission species. By introducing the molecular orbital concept, the dominant fluorescence was found to originate from the electronic transitions among/between the non-oxidized carbon regions and the boundary of oxidized carbon atom regions, where all three kinds of functionalized groups C-O, C = O and O = C-OH were participating. In the visible spectral range, the ultrafast fluorescence of graphene oxide was observed for the first time.

Room-temperature metastability of multilayer graphene oxide films

Abstract: Graphene oxide potentially has multiple applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is crucial to enable future applications of this material. Here, a combined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene through the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly stable reduced O/C ratio, and exhibits a structure deprived of epoxide groups and enriched in hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favours the reduction of epoxide groups and the formation of water molecules.

Probing the catalytic activity of porous graphene oxide and the origin of this behaviour

Abstract: Graphene oxide has been proposed as an alternative to precious metals for the catalysis of aerobic oxidative reactions; however, high catalyst loadings are needed. Here a simple base and acid treatment is shown to enhance its catalytic activity for the oxidative coupling of amines under ambient conditions.

Synthesis of a metallic mesoporous pyrochlore as a catalyst for lithium–O2 batteries

Abstract: The lithium–O2 ‘semi-fuel’ cell based on the reversible reaction of Li and O2 to form Li2O2 can theoretically provide energy densities that exceed those of Li-ion cells by up to a factor of five. A key limitation that differentiates it from other lithium batteries is that it requires effective catalysts (or ‘promoters’) to enable oxygen reduction and evolution. Here, we report the synthesis of a novel metallic mesoporous oxide using surfactant templating that shows promising catalytic activity and results in a cathode with a high reversible capacity of 10,000 mAh g(−1) (∼1,000 mAh g(−1) with respect to the total electrode weight including the peroxide product). This oxide also has a lower charge potential for oxygen evolution from Li2O2 than pure carbon. The properties are explained by the high fraction of surface defect active sites in the metallic oxide, and its unique morphology and variable oxygen stoichiometry. This strategy for creating porous metallic oxides may pave the way to new cathode architectures for the Li–O2 cell.

Green Synthesis of Biphasic TiO2–Reduced Graphene Oxide Nanocomposites with Highly Enhanced Photocatalytic Activity

Abstract: A series of TiO2–reduced graphene oxide (RGO) nanocomposites were prepared by simple one-step hydrothermal reactions using the titania precursor, TiCl4 and graphene oxide (GO) without reducing agents. Hydrolysis of TiCl4 and mild reduction of GO were simultaneously carried out under hydrothermal conditions. While conventional approaches mostly utilize multistep chemical methods wherein strong reducing agents, such as hydrazine, hydroquinone, and sodium borohydride are employed, our method provides the notable advantages of a single step reaction without employing toxic solvents or reducing agents, thereby providing a novel green synthetic route to produce the nanocomposites of RGO and TiO2. The as-synthesized nanocomposites were characterized by several crystallographic, microscopic, and spectroscopic characterization methods, which enabled confrimation of the robustness of the suggested reaction scheme. Notably, X-ray diffraction and transmission electron micrograph proved that TiO2 contained both anatas...