Showing papers on "Oxide published in 2011"
TL;DR: The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER.
Abstract: Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable-energy technologies including fuel cells and water splitting. Despite tremendous efforts, developing oxygen electrode catalysts with high activity at low cost remains a great challenge. Here, we report a hybrid material consisting of Co₃O₄ nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Although Co₃O₄ or graphene oxide alone has little catalytic activity, their hybrid exhibits an unexpected, surprisingly high ORR activity that is further enhanced by nitrogen doping of graphene. The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions. The same hybrid is also highly active for OER, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER. The unusual catalytic activity arises from synergetic chemical coupling effects between Co₃O₄ and graphene.
4,898 citations
TL;DR: The high activity of BSCF was predicted from a design principle established by systematic examination of more than 10 transition metal oxides, which showed that the intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an eg symmetry of surface transition metal cations in an oxide.
Abstract: The efficiency of many energy storage technologies, such as rechargeable metal-air batteries and hydrogen production from water splitting, is limited by the slow kinetics of the oxygen evolution reaction (OER). We found that Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3–δ (BSCF) catalyzes the OER with intrinsic activity that is at least an order of magnitude higher than that of the state-of-the-art iridium oxide catalyst in alkaline media. The high activity of BSCF was predicted from a design principle established by systematic examination of more than 10 transition metal oxides, which showed that the intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an e g symmetry of surface transition metal cations in an oxide. The peak OER activity was predicted to be at an e g occupancy close to unity, with high covalency of transition metal–oxygen bonds.
3,876 citations
TL;DR: In this article, a large database of HO* and HOO* adsorption energies on oxide surfaces was used to analyze the reaction free energy diagrams of all the oxides in a general way.
Abstract: Trends in electrocatalytic activity of the oxygen evolution reaction (OER) are investigated on the basis of a large database of HO* and HOO* adsorption energies on oxide surfaces. The theoretical overpotential was calculated by applying standard density functional theory in combination with the computational standard hydrogen electrode (SHE) model. We showed that by the discovery of a universal scaling relation between the adsorption energies of HOO* vs HO*, it is possible to analyze the reaction free energy diagrams of all the oxides in a general way. This gave rise to an activity volcano that was the same for a wide variety of oxide catalyst materials and a universal descriptor for the oxygen evolution activity, which suggests a fundamental limitation on the maximum oxygen evolution activity of planar oxide catalysts.
2,923 citations
TL;DR: In this paper, the antibacterial activity of four types of graphene-based materials (graphite (Gt), graphite oxide (GtoO), reduced graphene oxide (rGO), and reduced GtO) toward a bacterial model (Escherichia coli) was investigated.
Abstract: Health and environmental impacts of graphene-based materials need to be thoroughly evaluated before their potential applications. Graphene has strong cytotoxicity toward bacteria. To better understand its antimicrobial mechanism, we compared the antibacterial activity of four types of graphene-based materials (graphite (Gt), graphite oxide (GtO), graphene oxide (GO), and reduced graphene oxide (rGO)) toward a bacterial model—Escherichia coli. Under similar concentration and incubation conditions, GO dispersion shows the highest antibacterial activity, sequentially followed by rGO, Gt, and GtO. Scanning electron microscope (SEM) and dynamic light scattering analyses show that GO aggregates have the smallest average size among the four types of materials. SEM images display that the direct contacts with graphene nanosheets disrupt cell membrane. No superoxide anion (O2•–) induced reactive oxygen species (ROS) production is detected. However, the four types of materials can oxidize glutathione, which serves ...
2,279 citations
TL;DR: These findings reflect the critical influences of the σ orbital and metal-oxygen covalency on the competition between O(2)(2-)/OH(-) displacement and OH(-) regeneration on surface transition-metal ions as the rate-limiting steps of the ORR, and highlight the importance of electronic structure in controlling oxide catalytic activity.
Abstract: The prohibitive cost and scarcity of the noble-metal catalysts needed for catalysing the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries limit the commercialization of these clean-energy technologies. Identifying a catalyst design principle that links material properties to the catalytic activity can accelerate the search for highly active and abundant transition-metal-oxide catalysts to replace platinum. Here, we demonstrate that the ORR activity for oxide catalysts primarily correlates to σ-orbital (e(g)) occupation and the extent of B-site transition-metal-oxygen covalency, which serves as a secondary activity descriptor. Our findings reflect the critical influences of the σ orbital and metal-oxygen covalency on the competition between O(2)(2-)/OH(-) displacement and OH(-) regeneration on surface transition-metal ions as the rate-limiting steps of the ORR, and thus highlight the importance of electronic structure in controlling oxide catalytic activity.
2,241 citations
TL;DR: A highly active photocathode for solar H(2) production is presented, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electroDeposited Pt nanoparticles.
Abstract: A clean and efficient way to overcome the limited supply of fossil fuels and the greenhouse effect is the production of hydrogen fuel from sunlight and water through the semiconductor/water junction of a photoelectrochemical cell, where energy collection and water electrolysis are combined into a single semiconductor electrode. We present a highly active photocathode for solar H(2) production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electrodeposited Pt nanoparticles. The roles of the different surface protection components were investigated, and in the best case electrodes showed photocurrents of up to -7.6 mA cm(-2) at a potential of 0 V versus the reversible hydrogen electrode at mild pH. The electrodes remained active after 1 h of testing, cuprous oxide was found to be stable during the water reduction reaction and the Faradaic efficiency was estimated to be close to 100%.
1,856 citations
TL;DR: The graphene oxide nanosheets may be suitable materials in heavy metal ion pollution cleanup if they are synthesized in large scale and at low price in near future.
Abstract: Graphene has attracted multidisciplinary study because of its unique physicochemical properties. Herein, few-layered graphene oxide nanosheets were synthesized from graphite using the modified Hummers method, and were used as sorbents for the removal of Cd(II) and Co(II) ions from large volumes of aqueous solutions. The effects of pH, ionic strength, and humic acid on Cd(II) and Co(II) sorption were investigated. The results indicated that Cd(II) and Co(II) sorption on graphene oxide nanosheets was strongly dependent on pH and weakly dependent on ionic strength. The abundant oxygen-containing functional groups on the surfaces of graphene oxide nanosheets played an important role on Cd(II) and Co(II) sorption. The presence of humic acid reduced Cd(II) and Co(II) sorption on graphene oxide nanosheets at pH < 8. The maximum sorption capacities (Csmax) of Cd(II) and Co(II) on graphene oxide nanosheets at pH 6.0 ± 0.1 and T = 303 K were about 106.3 and 68.2 mg/g, respectively, higher than any currently reporte...
1,557 citations
TL;DR: In this article, the properties of hydrazine-reduced graphite oxide (GO) particles were analyzed by elemental analysis, XPS, TGA, XRD, and SEM.
1,402 citations
TL;DR: This work demonstrates the scalable fabrication of a new type of all-carbon, monolithic supercapacitor by laser reduction and patterning of graphite oxide films, which show good cyclic stability, and energy storage capacities comparable to existing thin-filmsupercapacitors.
Abstract: All-carbon microscale supercapacitors can be simply and scalably fabricated by the laser patterning and reduction of graphene oxide.
1,312 citations
TL;DR: In this article, a combination of high-resolution in situ X-ray photoemission and Xray absorption spectroscopies was used to monitor the deoxygenation process and comprehensively evaluate the electronic structure of graphene oxide thin films at different stages of the thermal reduction process.
Abstract: Despite the recent developments in graphene oxide due to its importance as a host precursor of graphene, the detailed electronic structure and its evolution during the thermal reduction remain largely unknown, hindering its potential applications. We show that a combination of high-resolution in situ X-ray photoemission and X-ray absorption spectroscopies offer a powerful approach to monitor the deoxygenation process and comprehensively evaluate the electronic structure of graphene oxide thin films at different stages of the thermal reduction process. It is established that the edge plane carboxyl groups are highly unstable, whereas carbonyl groups are more difficult to remove. The results consistently support the formation of phenol groups through reaction of basal plane epoxide groups with adjacent hydroxyl groups at moderate degrees of thermal activation (∼400 °C). The phenol groups are predominant over carbonyl groups and survive even at a temperature of 1000 °C. For the first time, a drastic increase...
1,265 citations
TL;DR: The synthesis approach presents a promising route for a large-scale production of RG-O platelet/metal oxide nanoparticle composites as electrode materials for Li-ion batteries.
Abstract: Reduced graphene oxide/Fe2O3 composite was prepared using a facile two-step synthesis by homogeneous precipitation and subsequent reduction of the G-O with hydrazine under microwave irradiation to yield reduced graphene oxide (RG-O) platelets decorated with Fe2O3 nanoparticles. As an anode material for Li-ion batteries, the RG-O/Fe2O3 composite exhibited discharge and charge capacities of 1693 and 1227 mAh/g, respectively, normalized to the mass of Fe2O3 in the composite (and ∼1355 and 982 mAh/g, respectively, based on the total mass of the composite), with good cycling performance and rate capability. Characterization shows that the Fe2O3 nanoparticles are uniformly distributed on the surface of the RG-O platelets in the composite. The total specific capacity of RG-O/Fe2O3 is higher than the sum of pure RG-O and nanoparticle Fe2O3, indicating a positive synergistic effect of RG-O and Fe2O3 on the improvement of electrochemical performance. The synthesis approach presents a promising route for a large-sca...
TL;DR: Graphene films grown by chemical vapor deposition are demonstrated for the first time to protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy from air oxidation, allowing pure metal surfaces only one atom away from reactive environments.
Abstract: The ability to protect refined metals from reactive environments is vital to many industrial and academic applications. Current solutions, however, typically introduce several negative effects, including increased thickness and changes in the metal physical properties. In this paper, we demonstrate for the first time the ability of graphene films grown by chemical vapor deposition to protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy from air oxidation. In particular, graphene prevents the formation of any oxide on the protected metal surfaces, thus allowing pure metal surfaces only one atom away from reactive environments. SEM, Raman spectroscopy, and XPS studies show that the metal surface is well protected from oxidation even after heating at 200 °C in air for up to 4 h. Our work further shows that graphene provides effective resistance against hydrogen peroxide. This protection method offers significant advantages and can be used on any metal that catalyzes graphene growth.
TL;DR: It is demonstrated that particle size, particulate state, and oxygen content/surface charge of graphene have a strong impact on biological/toxicological responses to red blood cells.
Abstract: Two-dimensional carbon-based nanomaterials, including graphene oxide and graphene, are potential candidates for biomedical applications such as sensors, cell labeling, bacterial inhibition, and drug delivery. Herein, we explore the biocompatibility of graphene-related materials with controlled physical and chemical properties. The size and extent of exfoliation of graphene oxide sheets was varied by sonication intensity and time. Graphene sheets were obtained from graphene oxide by a simple (hydrazine-free) hydrothermal route. The particle size, morphology, exfoliation extent, oxygen content, and surface charge of graphene oxide and graphene were characterized by wide-angle powder X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, dynamic light scattering, and zeta-potential. One method of toxicity assessment was based on measurement of the efflux of hemoglobin from suspended red blood cells. At the smallest size, graphene oxide showed the greatest hemolytic activity, whereas ag...
TL;DR: Combustion processing is now reported as a new low-temperature route for the deposition of diverse metal oxide films, and high-performance transistors are demonstrated using this method as discussed by the authors.
Abstract: Solution-deposited metal oxides show great potential for large-area electronics, but they generally require high annealing temperatures, which are incompatible with flexible polymeric substrates. Combustion processing is now reported as a new low-temperature route for the deposition of diverse metal oxide films, and high-performance transistors are demonstrated using this method.
01 Sep 2011
TL;DR: Exfoliated graphene oxide and reduced graphene oxide have been used for the adsorption of various charged dyes such as methylene blue, methyl violet, rhodamine B, and orange G from aqueous solutions using UV-Visible spectroscopy and FT-IR spectroscopic techniques.
Abstract: In the present study, exfoliated graphene oxide (EGO) and reduced graphene oxide (rGO) have been used for the adsorption of various charged dyes such as methylene blue, methyl violet, rhodamine B, and orange G from aqueous solutions. EGO consists of single layer of graphite decorated with oxygen containing functional groups such as carboxyl, epoxy, ketone, and hydroxyl groups in its basal and edge planes. Consequently, the large negative charge density available in aqueous solutions helps in the effective adsorption of cationic dyes on EGO while the adsorption is negligible for anionic dyes. On the other hand, rGO that has high surface area does not possess as high a negative charge and is found to be very good adsorbent for anionic dyes. The adsorption process is followed using UV-Visible spectroscopy, while the material before and after adsorption has been characterized using physicochemical and spectroscopic techniques. Various isotherms have been used to fit the data, and kinetic parameters were evaluated. Raman and FT-IR spectroscopic data yield information on the interactions of dyes with the adsorbent.
TL;DR: It is shown that a tailor-made, photoreduced graphene oxide can shuttle photogenerated electrons from an O(2)-evolving photocatalyst (BiVO(4) to a H(2)/Ru/SrTiO(3):Rh, tripling the consumption of electron-hole pairs in the water splitting reaction under visible-light irradiation.
Abstract: The effectiveness of reduced graphene oxide as a solid electron mediator for water splitting in the Z-scheme photocatalysis system is demonstrated. We show that a tailor-made, photoreduced graphene oxide can shuttle photogenerated electrons from an O2-evolving photocatalyst (BiVO4) to a H2-evolving photocatalyst (Ru/SrTiO3:Rh), tripling the consumption of electron–hole pairs in the water splitting reaction under visible-light irradiation.
TL;DR: The formation of amorphous metal oxide semiconducting thin-films using a ‘sol–gel on chip’ hydrolysis approach from soluble metal alkoxide precursors affords unprecedented high field-effect mobilities, reproducible and stable turn-on voltages Von≈0 V and high operational stability at maximum process temperatures as low as 230 °C.
Abstract: A low-temperature, solution-based preparation of amorphous, metal oxide semiconducting thin-films is reported. This ‘sol–gel on chip’ hydrolysis approach yields thin-film transistors with high field-effect mobilities, reproducible and stable turn-on voltages and high operational stability.
TL;DR: Ultrafine Pd nanoparticles monodispersed on graphene oxide (GO) surfaces were successfully prepared by the redox reaction between PdCl(4)(2-) and GO, allowing it to express high electrocatalytic ability in formic acid and ethanol oxidation relative to a commercial Pd/C catalyst.
Abstract: Ultrafine Pd nanoparticles monodispersed on graphene oxide (GO) surfaces were successfully prepared by the redox reaction between PdCl42− and GO. The as-made catalyst is very “clean” as a result of the surfactant-free formation process, allowing it to express high electrocatalytic ability in formic acid and ethanol oxidation relative to a commercial Pd/C catalyst. This simple and straightforward method is of significance for the facile preparation metal nanocatalysts with high catalytic activity on proper supporting materials.
TL;DR: In this article, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures, in a review of the literature on carbon monoxide over oxide catalysts.
Abstract: Oxidation into CO2 is a major solution to CO abatement in air depollution treatments. The development of catalytic converters led to an extraordinary high number of publications on metal catalysts during the last fifty years. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest, even if some oxides are known to present remarkable activity since the beginning of the 20th century. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures
TL;DR: In this paper, two types of low-temperature catalysts, the metal oxide catalyst and metal exchanged zeolite catalyst, were reviewed for selective catalytic reduction of NO x with ammonia (NH 3 -SCR) especially at temperature below 200°C.
TL;DR: In this article, the role of trapped water and the evolution of oxygen during annealing of reduced graphene oxide (GO) is investigated, and the interactions between randomly arranged nearby oxygen species are found to affect the spectral response (red and blue shifts) and the overall chemistry during the reduction process.
Abstract: Understanding the thermal reduction of graphene oxide (GO) is important for graphene exfoliation, and chemical and morphological modifications. In this process, the role of trapped water and the evolution of oxygen during annealing are still not well-understood. To unravel the complex mechanisms leading to the removal of oxygen in reduced GO, we have performed in situ transmission infrared absorption spectroscopy measurements of GO films upon thermal annealing at 60–850 °C in vacuum (10–3–10–4 Torr). Using cluster-based first-principles calculations, epoxides, ethers (pyrans and furans), hydroxyls, carboxyls, lactols, and various types of ketones and their possible derivatives have been identified from the spectroscopic data. Furthermore, the interactions between randomly arranged nearby oxygen species are found to affect the spectral response (red and blue shifts) and the overall chemistry during annealing. For instance, the initial composition of oxygen species (relative amounts and types of species, su...
TL;DR: In this article, two MCP Realizer machines were used to process 6061 and AlSi12 alloys, one processing at 50 W and the other at 100 W laser power.
TL;DR: In this paper, the performance data of metal oxide thin-film electrodes have been presented, and the supercapacitors exhibited the specific capacitance values between 50 and 1100 F g-1, which are quite comparable with bulk electrode values.
TL;DR: In this article, a detailed lithium de-intercalation mechanism was proposed for high voltage high energy density cathode materials, which showed clear evidence of a new spinel-like solid phase formed on the surface of the electrode materials after high-voltage cycling.
Abstract: High voltage cathode materials Li-excess layered oxide compounds Li[NixLi1/3−2x/3Mn2/3−x/3]O2 (0 < x < 1/2) are investigated in a joint study combining both computational and experimental methods. The bulk and surface structures of pristine and cycled samples of Li[Ni1/5Li1/5Mn3/5]O2 are characterized by synchrotron X-Ray diffraction together with aberration corrected Scanning Transmission Electron Microscopy (a-S/TEM). Electron Energy Loss Spectroscopy (EELS) is carried out to investigate the surface changes of the samples before/after electrochemical cycling. Combining first principles computational investigation with our experimental observations, a detailed lithium de-intercalation mechanism is proposed for this family of Li-excess layered oxides. The most striking characteristics in these high voltage high energy density cathode materials are 1) formation of tetrahedral lithium ions at voltage less than 4.45 V and 2) the transition metal (TM) ions migration leading to phase transformation on the surface of the materials. We show clear evidence of a new spinel-like solid phase formed on the surface of the electrode materials after high-voltage cycling. It is proposed that such surface phase transformation is one of the factors contributing to the first cycle irreversible capacity and the main reason for the intrinsic poor rate capability of these materials.
TL;DR: The residual defects and groups in chemically reduced graphene oxide cannot not only improve the impedance match characteristic and prompt energy transition from contiguous states to Fermi level, but also introduce defect polarization relaxation and groups' electronic dipole relaxation, which are all in favor of electromagnetic wave penetration and absorption as mentioned in this paper.
Abstract: The residual defects and groups in chemically reduced graphene oxide cannot only improve the impedance match characteristic and prompt energy transition from contiguous states to Fermi level, but also introduce defect polarization relaxation and groups’ electronic dipole relaxation, which are all in favor of electromagnetic wave penetration and absorption The chemically reduced graphene oxide shows enhanced microwave absorption compared with graphite and carbon nanotubes, and can be expected to display better absorption than high quality graphene, exhibiting a promising prospect as microwave absorbing material
TL;DR: In this paper, Na2Ti3O7 is found to reversibly uptake 2 Na ions per formula unit (200 mA h/g) at an average potential of 0.3 V and is hence a very promising negative electrode material for building sodium ion bat...
Abstract: Na2Ti3O7 is found to reversibly uptake 2 Na ions per formula unit (200 mA h/g) at an average potential of 0.3 V and is hence a very promising negative electrode material for building sodium ion bat...
TL;DR: In this article, an asymmetric supercapacitor (ASC) was fabricated using reduced graphene oxide (RGO) sheets modified with ruthenium oxide (RuO2) or polyaniline (PANi) as the anode and cathode, respectively.
Abstract: An asymmetric supercapacitor (ASC) was fabricated using reduced graphene oxide (RGO) sheets modified with ruthenium oxide (RGO–RuO2) or polyaniline (RGO–PANi) as the anode and cathode, respectively. The ASC exhibited a significantly improved capacitive performance in comparison with that of the symmetric supercapacitors fabricated with RGO–RuO2 or RGO–PANi as the electrodes. The improvement was attributed to the broadened potential window in an aqueous electrolyte, leading to an energy density of 26.3 W h kg−1, about two-times higher than that of the symmetrical supercapacitors based on RGO–RuO2 (12.4 W h kg−1) and RGO–PANi (13.9 W h kg−1) electrodes. In addition, a power density of 49.8 kW kg−1 was obtained at an energy density of 6.8 W h kg−1.
TL;DR: The results show that the adsorptions of Cr(III) on the composites is strongly dependent on contact time, agitation speed and pH, in the batch mode; and on flow rate and the bed thickness in the fixed bed mode.
TL;DR: Graphene nanosheets were directly deposited onto a glassy carbon electrode through cyclic voltammetric reduction of a graphene oxide colloidal solution as discussed by the authors, and the resulting electrodes were characterized by electrochemical methods and scanning electron microscopy.
TL;DR: In this article, a nanocomposite anodes for the oxidation of water required to produce renewable fuels is presented. But the anodes are not suitable for large-scale electrochemical energy production with minimal global warming gas emission.
Abstract: A leading approach for large-scale electrochemical energy production with minimal global-warming gas emission is to use a renewable source of electricity, such as solar energy, to oxidize water, providing the abundant source of electrons needed in fuel synthesis. We report corrosion-resistant, nanocomposite anodes for the oxidation of water required to produce renewable fuels. Silicon, an earth-abundant element and an efficient photovoltaic material, is protected by atomic layer deposition (ALD) of a highly uniform, 2 nm thick layer of titanium dioxide (TiO(2)) and then coated with an optically transmitting layer of a known catalyst (3 nm iridium). Photoelectrochemical water oxidation was observed to occur below the reversible potential whereas dark electrochemical water oxidation was found to have low-to-moderate overpotentials at all pH values, resulting in an inferred photovoltage of ~550 mV. Water oxidation is sustained at these anodes for many hours in harsh pH and oxidative environments whereas comparable silicon anodes without the TiO(2) coating quickly fail. The desirable electrochemical efficiency and corrosion resistance of these anodes is made possible by the low electron-tunnelling resistance (<0.006 Ω cm(2) for p(+)-Si) and uniform thickness of atomic-layer deposited TiO(2).