Nanocrystalline Metal Oxides Research Articles

Nanocrystalline Metal Oxides from the Injection of Metal Oxide Sols in Coordinating Solutions: Synthesis, Characterization, Thermal Stabilization, Device Processing, and Gas‐Sensing Properties

AbstractMetal oxide (SnO2, TiO2, In2O3, ZnO) sols are prepared by various sol–gel processes in such a way as to hinder the condensation reactions. The obtained sols are injected at 160 °C into a solution of tetradecene and dodecylamine, and kept under heating for different periods of time. Depending on the starting sol, variously crystallized oxide nanoparticles are obtained, whose phase compositions and chemical structure have been studied by X‐ray diffraction (XRD) and Fourier transform IR spectroscopy. The elimination of the organic residuals has been carried out by thermal treatment, and the thermal evolution of the nanoparticles has been studied by thermal analyses and Raman spectroscopy. High‐resolution transmission electron microscopy studies coupled with XRD measurements show that the thermal treatment does not markedly affect the particle size, which remains in the nanometer‐sized regime (from 3.5 to 8.5 nm, depending on the system), except in the case of ZnO. The thermally purified and stabilized powders, drop‐coated onto alumina substrates with pre‐deposited electrical contacts, have been tested as gas‐sensing devices, displaying outstanding sensing properties even at room temperature.

Triplet State Photosensitization of Nanocrystalline Metal Oxide Electrodes by Zinc-Substituted Cytochrome c: Application to Hydrogen Evolution

The interfacing of nanostructured semiconductor photoelectrodes with redox proteins is an innovative approach to the development of artificial photosynthetic systems. In this paper, we have investigated the photoinduced electron-transfer reactions of zinc-substituted cytochrome c, ZnCyt-c, immobilized on mesoporous, nanocrystalline metal oxide electrodes. Efficient electron injection from the triplet state of ZnCyt-c is observed into TiO(2) electrodes (t(50%) approximately 100 micros) resulting in a long-lived charge-separated state (lifetime of up to 0.4 s). Further studies were undertaken as a function of electrolyte pH and metal oxide employed. Optimum yield of a long-lived charge-separated state was observed employing TiO(2) electrodes at pH 5, consistent with our previous studies of analogous dye-sensitized metal oxide electrodes. The addition of EDTA as a sacrificial electron donor to the electrolyte resulted in efficient photogeneration of molecular hydrogen, with a quantum yield per one absorbed photon of 10 +/- 5%.

Immobilization and Electrochemistry of Negatively Charged Proteins on Modified Nanocrystalline Metal Oxide Electrodes

The immobilization of two acidic, low isoelectric point proteins, green fluorescence protein and ferredoxin (FRD) is investigated on nanocrystalline, mesoporous TiO2 and SnO2 electrodes. Modification of these electrodes with a cationic polypeptide (poly-L-lysine) or an aminosilane prior to protein immobilization is found to enhance protein binding at least ten fold, attributed to more favorable protein/electrode electrostatic interactions. Cyclic voltammetry studies of FRD-modified SnO2 electrodes indicate reversible protein electrochemistry with a midpoint potential of −0.59 V (vs. Ag/AgCl) and an interfacial electron transfer rate constant of 0.45 s−1.

Inorganic Structures as Materials for Gas Sensors

AbstractFor Abstract see ChemInform Abstract in Full Text.

Charge Separation versus Recombination in Dye-Sensitized Nanocrystalline Solar Cells: the Minimization of Kinetic Redundancy

In this paper we focus upon the electron injection dynamics in complete dye-sensitized nanocrystalline metal oxide solar cells (DSSCs). Electron injection dynamics are studied by transient absorption and emission studies of DSSCs and correlated with device photovoltaic performance and charge recombination dynamics. We find that the electron injection dynamics are dependent upon the composition of the redox electrolyte employed in the device. In a device with an electrolyte composition yielding optimum photovoltaic device efficiency, electron injection kinetics exhibit a half time of 150 ps. This half time is 20 times slower than that for control dye-sensitized films covered in inert organic liquids. This retardation is shown to result from the influence of the electrolyte upon the conduction band energetics of the TiO2 electrode. We conclude that optimum DSSC device performance is obtained when the charge separation kinetics are just fast enough to compete successfully with the dye excited-state decay. These conditions allow a high injection yield while minimizing interfacial charge recombination losses, thereby minimizing "kinetic redundancy" in the device. We show furthermore that the nonexponential nature of the injection dynamics can be simulated by a simple inhomogeneous disorder model and discuss the relevance of our findings to the optimization of both dye-sensitized and polymer based photovoltaic devices.

Fabrication and Characterization of Molybdenum Oxide Nanofibers by Electrospinning

ABSTRACTMolybdenum oxide/ Poly (ethylene oxide) composite nanofibers were prepared by combining the sol-gel process and electrspinning technique. An ethanol solution of Poly(ethylene oxide) (PEO) was mixed with molybdenum isopropoxide to form a precursor solution. Composite nanofibers were obtained by electrspinning this viscous solution. By calcination of the composite fibers, pure molybdenum oxide nanofibers and nano-rods were obtained with diameters of 100-nanometer scale. Morphology of the fibers has been characterized by scanning electric microscopy. Components and structures of the final products have been identified by EDAX and grazing incidence XRD. Calcination process has been studied by DSC and TG analysis. The real time dynamics of the structural evolution from composite fibers to nanocrystalline metal oxide fibers has been investigated by synchrotron-based in-situ x-ray diffraction.

Hybrid molecular materials for optoelectronic devices

Hybrid molecular materials based on semiconductor nanocrystalline metal oxides are a subject of intense research for the development of optoelectronic devices. Such devices follow the strategy of combining high surface area mesoporous materials with optically and electrochemically active molecules. Photovoltaic devices, molecular sensors and biosensors are some of the examples discussed in this paper.

Performance of Manganese Oxide/CNTs Composites as Electrode Materials for Electrochemical Capacitors

Nanocrystalline metal oxides can be prepared with large surface area, electrochemical stability, and pseudocapacitive behavior, being able to be used as supercapacitor electrodes. Among the various metal oxides studied, amorphous and hydrated manganese oxide is the most promising for supercapacitor electrodes due to the low cost of the raw material. In the present work, amorphous manganese dioxide is prepared by chemical co-precipitation of Mn(VII) and Mn(II) in water medium, giving small particles of relatively high surface area. Carbon nanotubes (CNTs) are proposed as an alternative additive of carbon black for improving the electrical conductivity of the manganese oxide electrodes used to build capacitors. The results demonstrate that CNTs are effective for increasing the capacitance and improving the electrochemical properties of the electrodes which show a better capacitive behavior than with carbon black. This enhancement is due to the high entanglement of CNTs which form a network of open mesopores, allowing the bulk of to be easily reached by the ions. The performance optimization requires a careful control of the electrolyte pH in order to avoid the irreversible reactions Mn(IV) to Mn(II) at the negative electrode and Mn(IV) to Mn(VII) at the positive one. © 2004 The Electrochemical Society. All rights reserved.

Adsorption of thiophenes out of hydrocarbons using metal impregnated nanocrystalline aluminum oxide

Adsorption of thiophene and its derivatives was carried out using silver impregnated nanocrystalline metal oxides, with special emphasis on aluminum oxide. A modified nanocrystalline aluminum oxide was prepared by an impregnation method and characterized. The adsorption experiments were carried out for thiophene, benzothiophene, dibenzothiophene and 4,6-dimethyl dibenzothiophene. Incorporation of soft Lewis acid sites such as silver ion is necessary for the adsorption of thiophene related molecules. The modified adsorbent was found to retain its adsorption characteristics even after compaction and it can be used for adsorption again after regeneration by heating at modest temperatures. The adsorption properties of the silver impregnated aluminum oxide (prepared by aerogel procedure) (Ag-AP-Al 2O 3) were compared with several other high surface area materials (>200 m 2/g). Detailed characterization studies showed that the active site in Ag-AP-Al 2O 3 is the Ag + ion coordinated to carbonate. Metallic silver nanoparticles do not appear to be effective in thiophene adsorption.

Bacterially Produced Manganese Oxide and Todorokite: UV Raman Spectroscopic Comparison

UV Raman spectroscopic data measured at two distinct conditions, low and high laser power, establish that the manganese oxide (SP6-MnOx) produced by the freshwater bacterium (Leptothrix discophora SP-6) closely resembles the 3 × 3-tunnel todorokite among the MnO2 materials studied. Under the two conditions, the effect of hydration/cation and the phase transition of todorokite or SP6-MnOx to Mn3O4 or birnessite will be described. A higher concentration of Mg2+ incorporated in the framework of the SP6-MnOx than in todorokite is probably responsible for the formation of a new Raman band obtained at high laser powers, matching the most intense UV Raman band of synthetic birnessite. Also, we present the assignment of Raman bands for todorokite mineral and discuss the mutual exclusion principle that should hold for all the MnO2 materials studied. These experiments provide characterization of hydrous, poorly crystalline, or nanocrystalline metal oxides, which are frequently difficult to identify.

Percolation and gas sensitivity in nanocrystalline metal oxide films

We use Monte Carlo simulations to study the influence of gas adsorption on the conductance of nanocrystalline metal oxide films. The films are modeled by a network of intergranular contacts with conductances that depend on the amount of adsorbed gas molecules, and take into account a broad distribution of grain sizes and the possibility that ultrasmall nanograins can be insulating. Using percolation theory, we show that below a critical gas concentration (detection limit), the film can be insulating due to the absence of a percolating cluster of conducting grains. Above this detection limit, the conductance of the film increases rapidly. The detection limit can be tuned by the grain size and the mean coordination number in the film.

Friction layers and friction films on PMC brake pads

A rather new tool, the focused ion beam (FIB) technique, was used to characterise superficial layers at micro-contact areas of a commercial brake pad. The friction material was a polymer matrix composite (PMC) with approximately 50% metal content (semi-metallic) and the counter part was a cast iron rotor. Though the contact areas were not visible as topographic features, they could be identified with the aid of their increased secondary electron emission during FIB-scanning of the surface after tribological activation. Target preparation of micron-sized cross-sections with the FIB enabled the study of superficial layers at predetermined sites at high magnification. Depending on the constituent of the pad, one, two or three layers were identified. The three layer structure comprised: (i) a 100 nm thick friction film containing nanocrystalline metal oxides and an amorphous phase which was enriched with sulphur, (ii) a nanocrystalline friction layer of compacted wear debris accommodating surface roughness and (iii) a severely deformed layer if the supporting constituent was a metal particle. Though the majority of loose wear particles was iron oxide, the friction film which adhered tightly to the pad surface contained a large amount of copper and sulphur, whereas zinc was transferred to the cast iron rotor.

New Synthetic Method of Magnetite Nanocrystallites Using ?-Irradiation

A new method,γ-irradiation, has been developed to prepare nanocrystalline metal, alloys, metal oxide, and composites. The reactions ofγ-irradiation on a rod type FeOOH in the presence of isopropanol and water have been investigated. In the initial stage of theγ-irradiation, FeOOH was turned into magnetite. With the continuedγ-irradiation(over 50000 Gy), FeOOH disappeared and the product was a single phase of magnetite. XRD, SAD, LRTEM and HRTEM were used to characterize the structure and to determine their morphology, respectively. The VSM spectrum was obtained to study magnetic properties of the products.

Energetics of a nanophase zeolite independent of particle size

Silicalite-1 nanocrystals were synthesized with different sizes (40, 95 and 180 nm) from clear solutions and the organic structure directing agent removed by calcination. X-ray diffraction, scanning electron microscopy, nitrogen adsorption, Fourier transform infrared spectroscopy and thermogravimetric analysis were used for characterization. High-temperature solution calorimetry using lead borate (2PbO · B 2O 3) solvent at 974 K measured the enthalpies (Δ H tran) relative to quartz at 298 K. The Δ H tran values are 8.2 ± 0.4 kJ/mol (40 nm), 8.3 ± 0.3 kJ/mol (95 nm) and 8.0 ± 0.5 kJ/mol (180 nm). These values are essentially the same as Δ H tran for microsized MFI crystals, 8.0 ± 0.8 kJ/mol [J. Phys. Chem. B 104 (2000) 10001]. Thus the particle size has no effect on the framework energetics of silicalite. This unusual phenomenon, which is different from that of condensed nanocrystalline metal oxides, is discussed in terms of internal surface area and surface energy of zeolites.

Synthesis and characterization of silica MCM-48 as carrier of size-confined nanocrystalline metal oxides particles inside the pore system

The interpenetrating 3-dimensional channel system of silica MCM-48 has been selected for the deposition of Cu/Zn/O mixed metal oxide particles. With the wet impregnation technique aqueous solutions of metal acetates have been used to load the calcined form of the mesoporous silica. Successive impregnation yielded metal contents of ca. 9 wt.%. Calcination of the composite transformed the acetates to the metal oxides. X-ray powder diffraction and solid-state MAS NMR showed the uptake of the metal salt inside the pore system. N 2-adsorption, X-ray diffraction and TEM confirmed the mesoporous structure. XPS measurements and EXAFS analysis (Cu K- and Zn K-edges) confirmed the metal uptake. Whereas nano-disperse CuO particles have been obtained ZnO shows no regular structure and seems to have reacted with the silicate channel surface by coating the channel wall.

Surface Restructuring of Nanoparticles: An Efficient Route for Ligand−Metal Oxide Crosstalk

Surface modification of nanocrystalline metal oxide particles with enediol ligands was found to result in altered optical properties of nanoparticles. The surface modification results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites. The optical shift is correlated to the dipole moment of the Ti−ligand complexes at the particle surface and decreases with the ligand size. The binding was found to be exclusively characteristic of colloids in the nanocrystalline domain(<20 nm). X-ray near-edge structure measurements at Ti K edge indicate that in this size domain the surface Ti atoms adjust their coordination environment to form undercoordinated sites. These five-coordinated defect sites are the source of novel enhanced and selective reactivity of the nanoparticle toward bidentate ligand binding as observed using IR spectroscopy. Enediol ligands have the optimal geometry for chelating surface Ti atoms, resulting in a five-membered ring coordination complex and restored si...

Preparation of nanocrystalline metal oxide powders with the surfactant-mediated method

Nanocrystalline metal oxides (ZnO, NiO, and SnO2) powders with an average particle diameter of 18–55 nm have been successfully prepared with the surfactant-mediated method. The cationic surfactant (cetyltrimethylammonium bromide, CTAB) and the hydrous metal chlorides (ZnCl2·2H2O, NiCl2·6H2O, and SnCl4·5H2O) appear to be the good candidates for obtaining a high yield of nanoparticles. The resultant products have been characterized by thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), and transmission electron microscopy (TEM). The resulting powders are highly crystalline and largely monodisperse oxide particles. The surfactant-mediated method turned out to be suited for the preparation of the nanocrystalline oxide powders. Through this method, it is possible to obtain nanocrystalline metal oxide powders.

Nanocrystalline metal oxides as destructive adsorbents for organophosphorus compounds at ambient temperatures.

Nanocrystals of magnesium oxide react with organophosphorus compounds at room temperature by dissociative chemisorption, which we term "destructive adsorption". This process involves cleavage of P-O and P-F bonds (but not P-C bonds) and immobilization of the resultant molecular fragments. These ultrafine powders have unusual crystalline shapes and possess high surface concentrations of reactive edge/corner and defect sites, and thereby display higher surface reactivity, normalized for surface area, than typical polycrystalline material. This high surface reactivity coupled with high surface area allows their use for effective decontamination of chemical warfare agents and related toxic substances. Herein data is presented for paraoxon, diisopropylfluorophosphate (DFP), and (CH3CH2O)2P(O)CH2-SC6H5 (DEPTMP). Solid-state NMR and IR spectroscopy indicate that all OR and F groups dissociate; this leaves bound -PO4, -F, and -OR groups for paraoxon, DFP, and DEPTMP, respectively. For paraoxon, it was shown that one monolayer reacts. For DEPTMP, the OR groups dissociate, but not the P-CH2SC6H5 group. The nanocrystalline MgO reacts much faster and in higher capacity than typical activated carbon samples, which physisorb but do not destructively adsorb these phosphorous compounds.

High sensitivity NO 2 sensors for environmental monitoring produced using laser ablated nanocrystalline metal oxides

This paper presents results from a study of gas sensors produced using laser ablated nanocrystalline metal oxides. The devices have been tested for their potential use as NO 2 detectors with particular emphasis on their potential cross sensitivity to NO. Subsequently, these SnO 2 devices are compared with the gas sensing capabilities of other sensors made from In 2O 3 and WO 3 prepared via a similar route. The investigation focussed on studying these sensors at low concentration levels with a view to environmental monitoring applications. Very high sensitivities to low NO 2 levels together with fast response times have been observed for both SnO 2 and In 2O 3 materials.

Lateral Electron Transport inside a Monolayer of Derivatized Fullerenes Anchored on Nanocrystalline Metal Oxide Films

Carboxylated fullerene derivatives adsorbed on nanocrystalline ZrO2 films deposited on conducting glass, display reversible electrochemical behavior with currents being 200 times higher than measured on a monolayer of the redox active fullerene molecule adsorbed on the conducting support. Despite the insulating oxide layer on which the fullerene is adsorbed, cross-surface charge transfer results in the participation of the internal surface of the oxide in the redox process. A mechanism of charge transport involving electron injection from the conducting support, followed by lateral electron hopping within the fullerene monolayer on the oxide, is proposed. Apparent diffusion coefficients as high as 1.5 × 10-8 cm2 s-1 were measured for the electron hopping process. A percolation threshold for electronic conductivity was found at a surface coverage between 40 and 60% of a full monolayer. Co-adsorbed spacer molecules, however, were seen to modify the percolation limit, as no threshold was observed up to 60% fullerene coverage. Significant implications are envisaged with regard to prospective applications in e.g. nano-optoelectronics, electrochemical devices, sensors, solar cells, and redox targeting of adsorbed biomolecules.