Showing papers on "Oxide published in 2006"
01 Mar 2006
TL;DR: In this article, the authors present a solution to Fick's second law for a semi-infinite solid and a rigorous derivation of the kinetics of internal oxidation, as well as the effects of impurities on oxide defect structure.
Abstract: Acknowledgments Preface Introduction 1. Methods of investigation 2. Thermodynamic fundamentals 3. Mechanisms of oxidation 4. Oxidation of pure metals 5. Oxidation of alloys 6. Oxidation by oxidants other than oxygen 7. Reactions of metals in mixed environments 8. Hot corrosion 9. Erosion-corrosion of metals in oxidizing atmospheres 10. Protective coatings 11. Atmosphere control for the protection of metals during production processes Appendix A. Solution to Fick's second law for a semi-infinite solid Appendix B. Rigorous derivation of the kinetics of internal oxidation Appendix C. Effects of impurities on oxide defect structure Index.
2,200 citations
TL;DR: In this article, a number of functionalized graphite oxides were prepared by treatment of graphite oxide (GO) with organic isocyanates, which can then be exfoliated into functionalized graphene oxide nanoplatelets that can form a stable dispersion in polar aprotic solvents.
2,103 citations
TL;DR: Combining virus-templated synthesis at the peptide level and methods for controlling two-dimensional assembly of viruses on polyelectrolyte multilayers provides a systematic platform for integrating these nanomaterials to form thin, flexible lithium ion batteries.
Abstract: The selection and assembly of materials are central issues in the development of smaller, more flexible batteries. Cobalt oxide has shown excellent electrochemical cycling properties and is thus under consideration as an electrode for advanced lithium batteries. We used viruses to synthesize and assemble nanowires of cobalt oxide at room temperature. By incorporating gold-binding peptides into the filament coat, we formed hybrid gold-cobalt oxide wires that improved battery capacity. Combining virus-templated synthesis at the peptide level and methods for controlling two-dimensional assembly of viruses on polyelectrolyte multilayers provides a systematic platform for integrating these nanomaterials to form thin, flexible lithium ion batteries.
1,791 citations
TL;DR: The results suggest that under low-loading conditions, the H(2) adsorption behavior of MOFs can be improved by imparting larger charge gradients on the metal oxide units and adjusting the link metrics to constrict the pore dimensions; however, a large pore volume is still a prerequisite feature.
Abstract: The dihydrogen adsorption isotherms of eight metal-organic frameworks (MOFs), measured at 77 K up to a pressure of 1 atm, have been examined for correlations with their structural features. All materials display approximately Type I isotherms with no hysteresis, and saturation was not reached for any of the materials under these conditions. Among the six isoreticular MOFs (IRMOFs) studied, the catenated materials exhibit the largest capacities on a molar basis, up to 9.8 H(2) per formula unit. The addition of functional groups (-Br, -NH(2), -C(2)H(4)-) to the phenylene links of IRMOF-1 (MOF-5), or their replacement with thieno[3,2-b]thiophene moieties in IRMOF-20, altered the adsorption behavior by a minor amount despite large variations in the pore volumes of the resulting materials. In contrast, replacement of the metal oxide units with those containing coordinatively unsaturated metal sites resulted in greater H(2) uptake. The enhanced affinities of these materials, MOF-74 and HKUST-1, were further demonstrated by calculation of the isosteric heats of adsorption, which were larger across much of the range of coverage examined, compared to those of representative IRMOFs. The results suggest that under low-loading conditions, the H(2) adsorption behavior of MOFs can be improved by imparting larger charge gradients on the metal oxide units and adjusting the link metrics to constrict the pore dimensions; however, a large pore volume is still a prerequisite feature.
1,670 citations
TL;DR: In this article, a review of the development of high-k gate oxides such as hafnium oxide (HFO) and high-K oxides is presented, with the focus on the work function control in metal gate electrodes.
Abstract: The scaling of complementary metal oxide semiconductor transistors has led to the silicon dioxide layer, used as a gate dielectric, being so thin (14?nm) that its leakage current is too large It is necessary to replace the SiO2 with a physically thicker layer of oxides of higher dielectric constant (?) or 'high K' gate oxides such as hafnium oxide and hafnium silicate These oxides had not been extensively studied like SiO2, and they were found to have inferior properties compared with SiO2, such as a tendency to crystallize and a high density of electronic defects Intensive research was needed to develop these oxides as high quality electronic materials This review covers both scientific and technological issues?the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure and reactions, their electronic structure, bonding, band offsets, electronic defects, charge trapping and conduction mechanisms, mobility degradation and flat band voltage shifts The oxygen vacancy is the dominant electron trap It is turning out that the oxides must be implemented in conjunction with metal gate electrodes, the development of which is further behind Issues about work function control in metal gate electrodes are discussed
1,520 citations
TL;DR: A large electric-field response of quasi–two-dimensional electron gases generated at interfaces in epitaxial heterostructures grown from insulating oxides is reported, indicating that the conductivity of the electron gases can be modulated through a quantum phase transition from an insulating to a metallic state.
Abstract: We report on a large electric-field response of quasi-two-dimensional electron gases generated at interfaces in epitaxial heterostructures grown from insulating oxides. These device structures are characterized by doping layers that are spatially separated from high-mobility quasi-two-dimensional electron gases and therefore present an oxide analog to semiconducting high-electron mobility transistors. By applying a gate voltage, the conductivity of the electron gases can be modulated through a quantum phase transition from an insulating to a metallic state.
1,356 citations
TL;DR: In this article, the surface of the oxide films and the interface between the polymer and the oxide was studied with the help of atomic force microscopy, and the effect of the thickness of oxide layer on electrical characteristics of the device was also studied and optimized thickness was achieved to give high power conversion efficiency of 3.3% under simulated AM1.5G illumination of 100mW∕cm2.
Abstract: Polymer-based photovoltaic cells have been fabricated by inserting a thin, transparent, transition metal oxide layer between the transparent anode (indium tin oxide) and the polymer layer. Two different transition metal oxides, namely vanadium oxide and molybdenum oxide, were used and the device performance was compared. The surface of the oxide films and the interface between the polymer and the oxide was studied with the help of atomic force microscopy. The effect of the thickness of the oxide layer on electrical characteristics of the device was also studied and optimized thickness was achieved to give high power conversion efficiency of 3.3% under simulated AM1.5G illumination of 100mW∕cm2.
1,033 citations
Patent•
22 Nov 2006TL;DR: In this article, a transparent conductive film is produced by growing ZnO doped with a group III element oxide on a substrate and has a region with a crystal structure in which a c-axis grows along a plurality of different directions.
Abstract: A ZnO-based transparent conductive film is produced by growing ZnO doped with a group III element oxide on a substrate and has a region with a crystal structure in which a c-axis grows along a plurality of different directions. The transparent conductive film produced by growing ZnO doped with a group III element oxide on a substrate has a ZnO (002) rocking curve full width at half maximum of about 13.5° or more. ZnO is doped with a group III element oxide so that the ratio of the group III element oxide in the transparent conductive film is about 7% to about 40% by weight. The transparent conductive film is formed on the substrate with a SiNx thin film provided therebetween. The transparent conductive film is formed on the substrate by a thin film formation method with a bias voltage applied to the substrate.
1,021 citations
Book•
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TL;DR: In this article, it was shown that supported gold catalysts can affect the oxidation of carbon monoxide at or below ambient temperature, with high activity at sites at the gold support interface, with the support making a vital contribution.
Abstract: Despite occasional references in the older literature to the ability of gold to catalyze certain reactions, the metal has until recently had the reputation of being one of the least catalytically useful. The recent discovery that some supported gold catalysts can affect the oxidation of carbon monoxide at or below ambient temperature has, however, focused attention on the metal's ability in this respect. For oxidation of carbon monoxide at low temperature, catalysts comprising small (<5 nm) gold particles supported preferably on an oxide of the first transition series (e.g., TiO2, α-Fe2O3) are needed. Deposition–precipitation and coprecipitation are better methods than impregnation for this purpose and provide the desired intimacy of contact between metal and support. High activity may well originate at sites at the gold–support interface, with the support making a vital contribution. Stable activity can result by optimizing aging in solution during the preparation, and low calcination temperatures are ge...
984 citations
TL;DR: In this paper, a review of solid electrolytes being developed for solid oxide fuel cells, including zirconia-, ceria-and lanthanum gallate-based materials, are reviewed and compared.
837 citations
TL;DR: In this paper, the effect of interfacial buffer layers (vanadium oxide (V2O5) and cesium carbonate (Cs2CO3) on the performance of polymer solar cells based on regioregular poly-(3-hexylthiophene) and [6,6]-phenyl C60 butyric acid methyl ester blend was investigated.
Abstract: We investigate the effect of interfacial buffer layers—vanadium oxide (V2O5) and cesium carbonate (Cs2CO3)—on the performance of polymer solar cells based on regioregular poly-(3-hexylthiophene) and [6,6]-phenyl C60 butyric acid methyl ester blend. The polarity of solar cells can be controlled by the relative positions of these two interfacial layers. Efficient inverted polymer solar cells were fabricated with the structure of indium tin oxide (ITO)/Cs2CO3/polymer blend/vanadium oxide (V2O5)/aluminum (Al). Short-circuit current of 8.42mA∕cm2, open-circuit voltage of 0.56V, and power conversion efficiency of 2.25% under a AM1.5G 130mW∕cm2 condition were achieved. The interfacial layers were also used to fabricate polymer solar cells using ITO and a thin gold (Au) layer as the transparent electrodes. The thickness of V2O5 layer (10nm) makes it an effective protective layer for the active layer so that ITO can be used for both the electrodes, enabling highly efficient transparent polymer solar cells (i.e., p...
TL;DR: In this article, the magnetic moment was observed in undoped semiconducting or insulating oxides to become ferromagnetic at room temperature on MgO substrates, where the magnetic field was applied parallel to the film plane.
Abstract: Remarkable room-temperature ferromagnetism was observed in undoped $\mathrm{Ti}{\mathrm{O}}_{2}$, $\mathrm{Hf}{\mathrm{O}}_{2}$, and ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ thin films. The magnetic moment is rather modest in the case of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ films on MgO substrates (while on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates, it is negative showing diamagnetism) when the magnetic field was applied parallel to the film plane. In contrast, it is very large in the other two cases (about 20 and $30\phantom{\rule{0.3em}{0ex}}\mathrm{emu}∕{\mathrm{cm}}^{3}$ for $200\text{\ensuremath{-}}\mathrm{nm}$-thick $\mathrm{Ti}{\mathrm{O}}_{2}$ and $\mathrm{Hf}{\mathrm{O}}_{2}$ films, respectively). Since bulk $\mathrm{Ti}{\mathrm{O}}_{2}$, $\mathrm{Hf}{\mathrm{O}}_{2}$, and ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ are clearly diamagnetic, and moreover, there are no contaminations in any substrate, we must assume that the thin film form, which might create necessary defects or oxygen vacancies, would be the reason for undoped semiconducting or insulating oxides to become ferromagnetic at room temperature.
TL;DR: In this paper, a new class of supercapacitors based on nanocrystalline vanadium nitride is reported, which can deliver an impressive specific capacitance of 1340 F g when tested at a scan rate of 2 mV s.
Abstract: Supercapacitors have been known for over thirty years but of late are emerging as attractive electrochemical energy-storage and conversion devices for future electrical vehicle application with complementary electrochemical characteristics to rechargeable batteries and fuel cells. Amongst the numerous materials studied to date, various forms of ruthenium oxides are clearly noteworthy, exhibiting superior electrochemical response. Unfortunately, the expensive nature of ruthenium has limited its technological viability. A new class of supercapacitors based on nanocrystalline vanadium nitride is reported here, which can deliver an impressive specific capacitance of 1340 F g when tested at a scan rate of 2 mV s. An even more impressive capacitance of 554 F g is noted at a higher scan rate of 100 mV s. Such a high capacitance, which exceeds that of RuO2·nH2O, is believed to be caused by a series of reversible redox reactions through hydroxy bonding confined to a few atomic layers of vanadium oxide on the surface of the underlying nitride nanocrystals, which exhibit a metallic electronic conductivity (rbulk = 1.67 × 10 6 X m). Such a modification of the nanocrystal surface chemistry may lead to the development of supercapacitors that exhibit very high and stable power densities. Supercapacitors are generally classified into electrical double layer capacitors (EDLCs), which build up electrical charge at the electrode/electrolyte interface as described by the Gouy–Chapman–Stern–Grahame model, and pseudocapacitors, which utilize a redox reaction at the interface at certain potentials. Both rely on the physicochemical changes that occur at the electrode/electrolyte interface. Hence, understanding the surface properties is crucial for achieving high power and energy densities. High-surface-area carbon-based materials are widely studied for EDLCs. On the other hand, crystalline RuO2 [1–3,7] and amorphous RuO2·nH2O [1,4–6] are well-known pseudocapacitors that exhibit a specific capacitance as high as 350 and 720 F g, respectively, due to the redox activity via proton adsorption in an acidic electrolyte. Despite the expensive nature of ruthenium oxide, it has been the focus of intense research since Ru offers a variety of oxidation states (II–IV) while exhibiting good electronic conductivity (rbulk = 2.8 × 10 6 X m). Vanadium also exhibits numerous oxidation states (II–V) similar to that of ruthenium in V2O5·nH2O, but its poor electronic conductivity (rbulk ≈ 1 ∼ 10 X m) renders the oxide unsuitable for use in high-rate electrochemical devices. However, exploiting the good electronic conductivity of the vanadium nitrides combined with the variety of oxidation states exhibited by V in vanadium oxides could lay the foundation for a new class of high-performance supercapacitors. The synthesis of these nanocrystalline vanadium nitrides with controlled surface oxidation (as in the present study) results in a unique class of supercapacitors without much loss in the overall electrical conductivity. Furthermore, the low cost, high molar density (≈6 g cm), and good chemical resistance of the transition metal nitrides render them excellent candidates for the next generation of supercapacitors. Although no detailed studies have been conducted, in the past Thompson and co-workers have explored transition metal nitrides and carbides for supercapacitors with moderate specific capacitances (< 226 F g). Amorphous V2O5·nH2O has also been tested for its supercapacitor response, but despite mixing a large amount of carbon (25 wt.-%) to improve its poor electronic conductivity (see above), the highest specific capacitance reported to date is 350 F g at a scan rate of 5 mV s. In this study, a low-temperature route based on a two-step ammonolysis reaction of VCl4 in anhydrous chloroform is used to synthesize nanocrystalline VN (see Experimental). The nanometer-sized crystals increase the susceptibility for surface oxidation, while the high surface area of the nitrides provides more redox-reaction sites. Such a VCl4/NH3 reaction, although known, tends to be largely influenced by the type of solvent used. X-ray diffraction (XRD) and highresolution transmission electron microscopy (HRTEM) are used to characterize the as-prepared and heat-treated VN nanocrystals. The as-prepared precursor consists of amorphous V(NH2)3Cl and crystalline NH4Cl, which transforms into the rock salt (Fm3m)-structured VN at 400 °C, which is C O M M U N IC A TI O N S
TL;DR: The gas experiments presented confirm good sensing properties, the possibility to use dopants and catalyser such in thin film gas sensors and the real integration in low power consumption transducers of single crystalline nanobelts prove the feasibility of large scale manufacturing of well-organized sensor arrays based on different nanostructures.
TL;DR: In this article, a buffer film with a thickness of only several nanometers interposed between the electrode and electrolyte materials is proposed to improve the high-rate capability of solid-state lithium batteries.
Abstract: Rechargeable lithium batteries are widely used in portable equipment today. However, there have always been safety issues arising from their combustible organic electrolytes. These issues are becoming more serious with the increasing size of batteries for use in electric vehicle (EV) or load-leveling applications. Nonflammable solid electrolytes would be the ultimate solution to the safety issue. Despite their high safety, the energy densities and power densities of solid systems have been too low for their practical use. We have succeeded in increasing their energy densities to levels comparable to those of liquid ones. However, power densities, or high-rate capabilities, remain poor. In this communication, we report that a buffer film with a thickness of only several nanometers interposed between the electrode and electrolyte materials improves the high-rate capability of solid-state lithium batteries. Low ionic conductivities of solid electrolytes have been the reason for the poor high-rate capability of solid-state lithium batteries. Although the conductivities of recently discovered solid electrolytes (> 10 S cm) are slightly lower than those observed for liquid electrolytes, Li ionic conduction in the solid electrolytes has become as fast as that of liquid electrolytes, by taking into account the fact that the transport number of ions in inorganic electrolytes is unity. However, the high-rate capability of solid systems, including solid electrolytes, remains inferior. This fact strongly suggests that the rate-controlling step is not in the bulk of the solid electrolytes, but at the interface between the electrode and the electrolyte materials. Ionic conduction at interfaces between different kinds of ionic conductors, or heterojunctions, is characterized by the term “nanoionics”; a frontier study was done for a LiI– Al2O3 composite, [7] and a sophisticated example was presented by Sata et al. In the latter, two kinds of F ion conductors, BaF2 and CaF2, were brought into contact with each other. Part of the F ions then transferred from one side to the other to reach an equilibrium, which produced vacancies in the former and interstitial ions in the latter, both of which contributed to ionic conduction at the interface and enhanced the ionic conduction. Similar nanoionic phenomena should take place at the interface between the electrode and the electrolyte materials, forming a space-charge layer. Because the compositions and structures of solid electrolytes have been well tailored to achieve high ionic conductivities, the ionic conductivity of the space-charge layer, where the compositions deviate from the optima, should be lower than that of the bulk, increasing the interfacial resistance. For instance, the Li4GeS4–Li3PS4 (thio-LISICON) system used in the present study has a high ionic conductivity of the order of 10 S cm at its optimum composition. However, variation in the composition reduces it to 10 S cm. The detrimental increase in the interfacial resistance would be prominent in bulk-type or non-thin-film solid-state lithium batteries. Sulfide electrolytes should be used in such batteries, because the ionic conductivities of oxide solid electrolytes, for example, are so low that they are available only in thin-film systems. On the other hand, the cathode should be an oxide, such as LiCoO2, because of its high electrode potential. [10,11]
TL;DR: In this paper, a simple one-pot synthesis via a hydrothermal approach was proposed to synthesize a wide range of metal oxide hollow spheres, including Fe2O3, NiO, Co3O4, CeO2, MgO, and CuO hollow spheres.
Abstract: Hollow spheres of crystalline metal oxides were synthesized in a simple one-pot synthesis via a hydrothermal approach. Various metal salts were dissolved together with carbohydrates in water, and the mixtures were heated to 180 °C in an autoclave. During the hydrothermal treatment, carbon spheres are formed with metal ions incorporated into their hydrophilic shell. The removal of carbon via calcination yields hollow metal oxide spheres. Using this process, we can produce a wide range of metal oxide hollow spheres that are not accessible via sol−gel chemistry. In this paper, we report the synthesis of Fe2O3, NiO, Co3O4, CeO2, MgO, and CuO hollow spheres that are composed of nanoparticles. The surface area and thickness of the shell can be varied or controlled by the carbohydrate:metal salt concentration.
TL;DR: In this paper, the electrochemical behavior of Ti-6Al-4V and Ti-7Al-7Nb alloys, commonly used implant materials, particularly for orthopaedic and osteosynthesis applications, was investigated together with that of Ti−13Nb−13Zr alloy in Hank's solution at 37°C.
TL;DR: In this article, the authors used the scanning tunnelling microscope (STM) to image the reaction of water molecules with bridging-oxygen vacancies on a model oxide surface, rutile TiO2(110).
Abstract: The chemistry of metal oxide surfaces has long been thought to be dominated by reactions involving defects1,2. These are minority sites such as oxygen vacancies. Thus far, it has proved difficult to obtain direct experimental evidence to support this idea, although some progress has been made3,4,5. Here, we use the scanning tunnelling microscope (STM) to image the reaction of water molecules with bridging-oxygen vacancies on a model oxide surface, rutile TiO2(110). In a form of single-molecule chemistry, individual oxygen vacancies are observed being transformed into OH species as a water molecule dissociates in the vacancy. We use the STM tip to selectively desorb individual H atoms, whilst leaving the vacancies intact. This allows us to distinguish between vacancies and OH, which have a similar appearance in STM. In a very clear way, these results validate the view that defects can play a key role in oxide surface reactions.
TL;DR: In this article, a mixture of nanocrystalline zinc oxide nanoparticles (nc-ZnO) and regioregular poly(3-hexylthiophene) (P3HT) processed from solution have been used to construct hybrid polymer-metal oxide bulk-heterojunction solar cells.
Abstract: Blends of nanocrystalline zinc oxide nanoparticles (nc-ZnO) and regioregular poly(3-hexylthiophene) (P3HT) processed from solution have been used to construct hybrid polymer–metal oxide bulk-heterojunction solar cells. Thermal annealing of the spin-cast films significantly improves the solar-energy conversion efficiency of these hybrid solar cells to ∼ 0.9 %. Photoluminescence and photoinduced absorption spectroscopy demonstrate that charge-carrier generation is not quantitative, because a fraction of P3HT appears not to be in contact with or in close proximity to ZnO. The coarse morphology of the films, also identified by tapping-mode atomic force microscopy, likely limits the device performance.
TL;DR: In this article, the authors investigated the sintering inhibition mechanism of Pt in a Pt/ceria-based oxide catalyst under oxidizing condition at high temperature using X-ray absorption analysis.
TL;DR: In this paper, the authors proposed two important criteria for selecting an EC material: the time constant of the ion-intercalation reaction, which is limited both by the diffusion coefficient and by the length of the diffusion path, and the coloration efficiency, the change in optical density (OD) per unit inserted charge, that is, CE= D(OD)/DQ.
Abstract: Electrochromic (EC) materials change their optical properties (darken/lighten) in the presence of a small electric potential difference, and are suitable for application in energy-efficient windows, antiglare automobile rear-view mirrors, sunroofs, displays, and hydrogen sensors. [1–4] The operation of conventional EC devices depends on the reversible electrochemical double injection of positive ions (H + ,L i + ,N a + ) and electrons into the host lattice of multivalent transition metal oxide materials, [5–10] with positive-ion insertion required to satisfy charge neutrality. However, diffusion of positive ions into the oxide layer is often slow, taking minutes to complete. Since the chemical diffusion coefficient of protons (DH+ )i s an order of magnitude larger than that of lithium ions (DLi+), EC systems based on proton electrolytes (e.g., aqueous H2SO4) are mandatory for display applications and preferred for other applications. Unfortunately, proton insertion currently results in rapid degradation of EC films. There are two important criteria for selecting an EC material. The first is the time constant of the ion-intercalation reaction, which is limited both by the diffusion coefficient and by the length of the diffusion path. While the former depends on the chemical structure and crystal structure of the metal oxide, the latter is determined by the material’s microstructure. [11] In the case of a nanoparticle, the smallest dimension is represented by the diffusion path length. Thus, designing a nanostructure with a small radius, while maintaining the proper crystal structure, is key to obtaining a material with fast insertion kinetics, enhanced durability, and superior performance. The second important criterion is coloration efficiency (CE), the change in optical density (OD) per unit inserted charge (Q), that is, CE= D(OD)/DQ. [12] A high CE provides
TL;DR: The methods used to deposit a catalyst on structured surfaces are reviewed in this paper, where physical methods such as PVD and chemical methods (sol-gel, CVD, direct synthesis, etc.) are described.
Abstract: The methods used to deposit a catalyst on structured surfaces are reviewed. Physical methods such as PVD and chemical methods (sol–gel, CVD, direct synthesis, etc.) are described. The coating of catalysts based on oxide, zeolite or carbon support is detailed on various surfaces such as silicon or steel microstructured reactors, cordierite monoliths or foams, fibres, tubes, etc.
TL;DR: This is the first report of Au completely wetting an oxide surface and demonstrates that ultrathin Au films on a oxide surface have exceptionally high catalytic activity, comparable to the activity observed for Au nanoparticles.
Abstract: Ordered gold (Au) mono- and bilayer structures have been synthesized on a highly reduced titania surface. The Au bilayer exhibits a significantly higher catalytic activity for carbon monoxide oxidation than does the Au monolayer structure. This is the first report of Au completely wetting an oxide surface and demonstrates that ultrathin Au films on an oxide surface have exceptionally high catalytic activity, comparable to the activity observed for Au nanoparticles. This discovery is a key to understanding the nature of the active site of supported Au catalysts.
TL;DR: General mechanistic principles for aqueous and nonaqueous sol-gel processes are presented, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents.
Abstract: Nonaqueous-solution routes to metal oxide nanoparticles are a valuable alternative to the known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and ability to control the crystal growth without the use of surfactants. In the first part of the review we give a detailed overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes. In most of these synthesis approaches, the organic solvent plays the role of the reactant that provides the oxygen for the metal oxide, controls the crystal growth, influences particle shape, and, in some cases, also determines the assembly behavior. We have a closer look at the following reaction systems in this order: 1) metal halides in alcohols, 2) metal alkoxides, acetates, and acetylacetonates in alcohols, 3) metal alkoxides in ketones, and 4) metal acetylacetonates in benzylamine. All these systems offer some peculiarities with respect to each other, providing many possibilities to control and tailor the particle size and shape, as well as the surface and assembly properties. In the second part we present general mechanistic principles for aqueous and nonaqueous sol-gel processes, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents. Depending on the system several mechanisms have been postulated: 1) alkyl halide elimination, 2) elimination of organic ethers, 3) ester elimination, 4) C--C bond formation between benzylic alcohols and alkoxides, 5) ketimine and aldol-like condensation reactions, 6) oxidation of metal nanoparticles, and 7) thermal decomposition methods.
TL;DR: This work describes a synthetic methodology that takes advantage of recent advances in the synthesis of metal and oxide nanomaterials and helps to bring together these two classes of materials for catalysis applications and develops efficient gold catalysts for green chemistry processes.
Abstract: Despite recent exciting progress in catalysis by supported gold nanoparticles, there remains the formidable challenge of preparing supported gold catalysts that collectively incorporate precise control over factors such as size and size-distribution of the gold nanoparticles, homogeneous dispersion of the particles on the support, and the ability to utilize a wide range of supports that profoundly affect catalytic performance. Here, we describe a synthetic methodology that achieves these goals. In this strategy, weak interface interactions evenly deposit presynthesized organic-capped metal nanoparticles on oxide supports. The homogeneous dispersion of nanoparticles on oxides is then locked in place, without aggregation, through careful calcination. The approach takes advantage of recent advances in the synthesis of metal and oxide nanomaterials and helps to bring together these two classes of materials for catalysis applications. An important feature is that the strategy allows metal nanoparticles to be well dispersed on a variety of oxides with few restrictions on their physical and chemical properties. Following this synthetic procedure, we have successfully developed efficient gold catalysts for green chemistry processes, such as the production of ethyl acetate from the selective oxidation of ethanol by oxygen at 100 degrees C.
TL;DR: The obtained results suggest a surface initiated reaction involving Msurf2+ (Fe, Co or Mn), producing HO radicals, which can lead to two competitive reactions, i.e. the decomposition of H2O2 or the oxidation of organics present in the aqueous medium.
TL;DR: In a preliminary study on the gas sensitivity of SnO2 hollow spheres, considerably reduced "recovery times" were noted, exemplifying the distinct properties imparted by the hollow structure.
Abstract: A general method for the synthesis of metal oxide hollow spheres has been developed by using carbonaceous polysaccharide microspheres prepared from saccharide solution as templates. Hollow spheres of a series of metal oxides (SnO2, Al2O3, Ga2O3, CoO, NiO, Mn3O4, Cr2O3, La2O3, Y2O3, Lu2O3, CeO2, TiO2, and ZrO2) have been prepared in this way. The method involves the initial absorption of metal ions from solution into the functional surface layer of carbonaceous saccharide microspheres; these are then densified and cross-linked in a subsequent calcination and oxidation procedure to form metal oxide hollow spheres. Metal salts are used as starting materials, which widens the accessible field of metal oxide hollow spheres. The carbonaceous colloids used as templates have integral and uniform surface functional layers, which makes surface modification unnecessary and ensures homogeneity of the shell. Macroporous films or cheese-like nanostructures of oxides can also be prepared by slightly modified procedures. XRD, TEM, HRTEM, and SAED have been used to characterize the structures. In a preliminary study on the gas sensitivity of SnO2 hollow spheres, considerably reduced “recovery times” were noted, exemplifying the distinct properties imparted by the hollow structure. These hollow or porous nanostructures have the potential for diverse applications, such as in gas sensitivity or catalysis, or as advanced ceramic materials.
TL;DR: The structures, the energetics, and the charge distribution of the betaine cation and the bis(trifluoromethylsulfonyl)imide anion, as well as the cation-anion pairs, were studied by density functional theory calculations.
Abstract: Protonated betaine bis(trifluoromethylsulfonyl)imide is an ionic liquid with the ability to dissolve large quantities of metal oxides. This metal-solubilizing power is selective. Soluble are oxides of the trivalent rare earths, uranium(VI) oxide, zinc(II) oxide, cadmium(II) oxide, mercury(II) oxide, nickel(II) oxide, copper(II) oxide, palladium(II) oxide, lead(II) oxide, manganese(II) oxide, and silver(I) oxide. Insoluble or very poorly soluble are iron(III), manganese(IV), and cobalt oxides, as well as aluminum oxide and silicon dioxide. The metals can be stripped from the ionic liquid by treatment of the ionic liquid with an acidic aqueous solution. After transfer of the metal ions to the aqueous phase, the ionic liquid can be recycled for reuse. Betainium bis(trifluoromethylsulfonyl)imide forms one phase with water at high temperatures, whereas phase separation occurs below 55.5 degrees C (temperature switch behavior). The mixtures of the ionic liquid with water also show a pH-dependent phase behavior: two phases occur at low pH, whereas one phase is present under neutral or alkaline conditions. The structures, the energetics, and the charge distribution of the betaine cation and the bis(trifluoromethylsulfonyl)imide anion, as well as the cation-anion pairs, were studied by density functional theory calculations.
TL;DR: Several findings show that the superoxide ion does not undergo transformations into charged atomic oxygen at the surface, and represents a dead-end form of low-temperature oxygen adsorption on reduced metal oxide.
Abstract: Tin dioxide is the most commonly used material in commercial gas sensors based on semiconducting metal oxides. Despite intensive efforts, the mechanism responsible for gas-sensing effects on SnO(2) is not fully understood. The key step is the understanding of the electronic response of SnO(2) in the presence of background oxygen. For a long time, oxygen interaction with SnO(2) has been treated within the framework of the "ionosorption theory". The adsorbed oxygen species have been regarded as free oxygen ions electrostatically stabilized on the surface (with no local chemical bond formation). A contradiction, however, arises when connecting this scenario to spectroscopic findings. Despite trying for a long time, there has not been any convincing spectroscopic evidence for "ionosorbed" oxygen species. Neither superoxide ions O(2)(-), nor charged atomic oxygen O,(-) nor peroxide ions O(2)(2-) have been observed on SnO(2) under the real working conditions of sensors. Moreover, several findings show that the superoxide ion does not undergo transformations into charged atomic oxygen at the surface, and represents a dead-end form of low-temperature oxygen adsorption on reduced metal oxide.