Nanocrystalline Metal Oxides Research Articles

A novel approach to metal and metal oxide nanoparticle synthesis: the oil-in-water microemulsion reaction method

A novel and straightforward approach, based on oil-in-water (o/w) microemulsions, was developed for the synthesis of inorganic nanoparticles at ambient conditions. It implies the use of organometallic precursors dissolved in nanometre-scale oil droplets of o/w microemulsions. Addition of reducing or oxidizing/precipitating agents results in the formation of metallic or metal oxide nanoparticles, respectively. Nonionic o/w microemulsion systems were chosen, and several key compositions were selected for nanoparticle synthesis at 25 °C. High Resolution Electron Microscopy revealed that small nanoparticles of metals (Pt, Pd and Rh) and nanocrystalline metal oxide (cerium (IV) oxide with cubic type crystalline structure confirmed by XRD), of less than 7 nm can be obtained in mild conditions.

The application of ultrasound radiation to the synthesis of nanocrystalline metal oxide in a non-aqueous solvent

Highly crystalline metal oxide nanoparticles of TiO 2, WO 3, and V 2O 5 were synthesized in just a few minutes by reacting transition metal chloride with benzyl alcohol using ultrasonic irradiation under argon atmosphere in a non-aqueous solvent. The sonochemical process was conducted at a relatively low temperature, 363 K. A unique crystallization process of these nanoparticles has been observed and characterized by powder X-ray diffraction (PXRD), high resolution scanning electron microscopy (HRSEM), and BET. The particles’ size and shape measured from HRSEM reveal “quasi” zero-dimensional, spherical TiO 2 particles in the range of 3–7 nm. The V 2O 5 particles have a “quasi” one-dimensional ellipsoidal morphology, with lengths in the range of 150–200 nm and widths varying between 40 and 60 nm. The WO 3 particles were obtained as “quasi” two-dimensional platelets with square shapes having facets ranging from 30 to 50 nm. The thickness of these platelets was between 2 and 7 nm. The mechanism of the reactions leading to these three metal oxide nanoparticles in a non-aqueous system is substantiated by Nuclear Magnetic Resonance (NMR), and Electron Spin Resonance (ESR).

Structural characterization of Nd-doped in silica host matrix prepared by wet chemical process

Silica host matrix containing neodymium which is potentially important for the formation of nanocrystalline metal oxides was prepared by solgel method, using tetra-ethoxysilane and Nd(NO 3) 3 as precursor materials. The prepared samples were changed from amorphous to nanocrystallites phase at sintered temperature 550 °C (4 h), 750 °C (8 h) and 950 °C (12 h). The thermally treated sample microstructures were investigated using X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). While a further increase of the temperature at 750 °C and annealing time (8 h) resulted in the formation of cubic and hexagonal Nd 2O 3 nanocrystallites. At constant sintering temperature 950 °C for 12 h, the samples showed sharper and intense peaks. The sizes of Nd 2O 3 nanocrystallites were characterized by XRD with average size ∼46 nm.

Nanocrystalline Metal Oxides for Methane Sensors: Role of Noble Metals

Methane is an important gas for domestic and industrial applications and its source is mainly coalmines. Since methane is extremely inflammable in the coalmine atmosphere, it is essential to develop a reliable and relatively inexpensive chemical gas sensor to detect this inflammable gas below its explosion amount in air. The metal oxides have been proved to be potential materials for the development of commercial gas sensors. The functional properties of the metal oxide‐based gas sensors can be improved not only by tailoring the crystal size of metal oxides but also by incorporating the noble metal catalyst on nanocrystalline metal oxide matrix. It was observed that the surface modification of nanocrystalline metal oxide thin films by noble metal sensitizers and the use of a noble metal catalytic contact as electrode reduce the operating temperatures appreciably and improve the sensing properties. This review article concentrates on the nanocrystalline metal oxide methane sensors and the role of noble metals on the sensing properties.

Interfacial Charge Recombination Between e−−TiO2 and the I−/I3− Electrolyte in Ruthenium Heteroleptic Complexes: Dye Molecular Structure−Open Circuit Voltage Relationship

A series of heteroleptic ruthenium(II) polypyridyl complexes containing phenanthroline ligands have been designed, synthesized, and characterized. The spectroscopic and electrochemical properties of the complexes have been studied in solution and adsorbed onto semiconductor nanocrystalline metal oxide particles. The results show that for two of the ruthenium complexes, bearing electron-donating (-NH2) or electron-withdrawing (-NO2) groups, the presence of the redox-active I(-)/I3(-) electrolyte produces important changes in the interfacial charge transfer processes that limit the device performance. For example, those dyes enhanced the electron recombination reaction between the photoinjected electrons at TiO2 and the oxidized redox electrolyte. In an effort to understand the details of such striking observations, we have monitored the charge transfer reactions taking place at the different interfaces of the devices using time-resolved single photon counting, laser transient spectroscopy, and light-induced photovoltage measurements.

Surfactant‐Free Nonaqueous Synthesis of Metal Oxide Nanostructures

Surfactant-free nonaqueous (and/or nonhydrolytic) sol-gel routes constitute one of the most versatile and powerful synthesis methodologies for nanocrystalline metal oxides with high compositional homogeneity and purity. Although the synthesis protocols are particularly simple, involving only metal oxide precursors and common organic solvents, the obtained uniform nanocrystals exhibit an immense variety of sizes and shapes. The small number of reactants in these routes enables the study of the chemical mechanisms involved in metal oxide formation. Nonhydrolytic routes to inorganic nanomaterials that used surfactants as size- and shape-controlling agents have been discussed recently. This Minireview supplements this topic by discussing surfactant-free processes, which have become a valuable alternative to surfactant-assisted as well as to traditional aqueous sol-gel chemistry routes.

Spectroelectrochemical Characterization of a Pentaheme Cytochrome in Solution and as Electrocatalytically Active Films on Nanocrystalline Metal-Oxide Electrodes

The pentaheme containing cytochrome, NrfA, from Escherichia coli catalyzes the six-electron reduction of nitrite and the five-electron reduction of nitric oxide. Crystallographic and spectroscopic studies have provided a structural framework for these mechanisms. The active site includes a high-spin heme, and four low-spin, bis-his coordinated hemes are positioned to facilitate intra- and intermolecular electron exchange. However, despite the use of protein film voltammetry to provide kinetic descriptions of NrfA catalysis at graphite and gold electrodes, the thermodynamic descriptions of heme redox activity remain incomplete. Here we rectify this situation with the observation of nonturnover signals from NrfA adsorbed on mesoporous SnO2 electrodes. Simultaneous cyclic voltammetry and electronic absorption spectroscopy define reduction potentials for the high- and low-spin hemes. These reduction potentials are shown to be similar to those exhibited by the enzyme in solution and defined by electrodic reduction monitored by magnetic circular dichroism. Thus, NrfA is shown to undergo minimal perturbation of its electronic and thermodynamic properties on adsorption giving confidence to correlations of properties deduced from various methods and in approaches that may well facilitate studies of other oxidoreductases where catalytic protein film voltammetry is well-defined but nonturnover signals elusive.

Chemical modification of nanocrystalline metal oxides: effect of the real structure and surface chemistry on the sensor properties

The relationships between the composition, structure, chemistry of the surface, and sensor properties of nanocomposites SnO2-M n O m (M n O m = Fe2O3, MoO3, V2O5) obtained by chemical precipitation from solutions were analyzed. The relationships between the elemental and phase composition of the nanocomposites and the effect of the composition on the nanostructure and the acidic and oxidation properties of the nanocomposite surface were considered. The modification of the SnO2 surface by other oxides makes it possible to control the type and density of the acid sites and the oxidation properties of the surface and to enhance the selectivity of the materials in the detection of various gases.

Optical and electrochemical properties of polyether derivatives of perylenediimides adsorbed on nanocrystalline metal oxide films

We report optical and electrochemical properties of polyether derivatives of perylenediimides (PDIs) thin films formed in various materials (semiconductor, insulator, amorphous and self-assembly). Perylenediimides adsorbed on nanocrystalline TiO 2 (NT) nanocrystalline alumina (NA), amorphous silicon (PS) and neat self-assemblied (SA) films were prepared and characterized based on spectroscopic, electrochemical, spectro-electrochemical techniques. The absorption and fluorescence spectra of PDIs in chloroform exhibit vibronic features. The fluorescence quantum yields ( Φ f) of PDIs with end amino substituents in chloroform solutions are over 0.95, while the quantum yield of triethoxyphenyl substituted PDI Φ f value is 0.024 in solution. Optical spectroscopy proves that PDIs in metal oxide thin films form aggregated type complexes. An electrochromism, a color change from red to blue/violet, is observed on metal oxide films, that indicates existence of mono and dianion forms of PDIs. Reversibility of electrochemical reductions in NT film depends on the scanning rate. However, electrochromism in NA films is stable and reversibility is independent from scanning rate. Stable mono and diaionic species are formed on NA films. SA films show broad absorption peaks during the voltammetric scan. On the other hand, the first reduction onset potentials of PDIs are almost equal to the onset potential of capacitive current of TiO 2 which lead to low efficiency in dye-sensitized solar cells.

Functionalized SBA-15 and its Catalytic Applications in Selective Organic Transformations

Ordered, mesoporous SBA-15 functionalized with organic and inorganic moieties exhibits efficient catalytic activity in a variety of organic transformations. In this account, reviewing our own work, three-sets of surface-modified SBA-15 materials have been investigated. The first-set of materials consists of SBA-15 modified with organo-acidic (propyl thiol and propyl sulfonic acid) and basic (propyl amine and propyl adenine) moieties. The second-set of materials was prepared by grafting Mn complexes to the organo-functionalized SBA-15. The third-set composes of nanocrystalline metal oxides supported on SBA-15. All these catalysts have been characterized by structural and spectroscopic techniques. Catalytic activities of the first-set of solid materials have been investigated in acid/base-catalyzed reactions viz., ring-opening of epoxides with amines (producing β-amino alcohols), esterification, three-component-Mannich reactions and cycloaddition of CO2 to epoxides. The Mn complexes grafted on organofunctionalized SBA-15 are efficient catalysts for the chemo-, regio- and stereoselective aerial oxidation of monoterpenes at ambient conditions. TiOx, VOx, MoOx and WOx supported on SBA-15 catalyzed biomimetic oxyhalogenation of aromatic compounds. In all these reactions, the functionalized SBA-15 showed high selectivity.

Generic synthesis of a variety of nanocrystalline metal oxides at room temperature

Generic synthesis of nanocrystals of TiO2, ZrO2 and V2O5 under ambient conditions at room temperature is presented. For the first time, it employs a water–octyl-β-D-glucopyranoside–ethylene glycol diethyl ether based microemulsion and a unique set of glycolate precursors.

A room temperature etching route to tungsten oxide hydrate nanoplates with expanded surface area

Tungsten oxide hydrate nanoplates with expanded surface area have been synthesized through the acid treatment of tungstate−aluminum keggin nanocomposite. According to powder X-ray diffraction and W L III-edge X-ray absorption spectroscopic analyses, X-ray amorphous nanocomposite is changed into crystalline WO 3·2H 2O upon the acidic etching of alumina component. Nitrogen adsorption−desorption isotherm and field emission-scanning electron microscopy measurements clearly demonstrated that the surface area of the nanocomposite is increased by the selective etching of alumina component, a result of the formation of the porous stacking assembly of nanoplates. From optoelectrochemical analysis, the etched derivative is found to be applicable as an electrode material for electrochromic devices. Based on the experimental findings, we are able to conclude that the present selective etching method can provide a soft-chemical synthetic route to nanocrystalline metal oxides with expanded surface area.

Saccharide Blocking Layers in Solid State Dye Sensitized Solar Cells

AbstractThe adsorption of saccharides on dye sensitized, nanocrystalline metal oxide films is shown to improve the efficiency of solid state dye sensitized solar cells. The function of the saccharide treatment is evaluated by transient optical studies, and correlated with device photovoltaic performance. A range of saccharides, including cyclodextrins and their linear analogue amylose, are investigated. The saccharide blocking layer is shown to retard interfacial charge recombination losses, resulting in increased device open circuit voltage. Highest device performance is achieved with linear saccharide amylose, resulting in a 60 % improvement in device efficiency relative to the non‐treated control, with a device open circuit voltage of 1 V.

Chemical self-organization length scales in non- and nano-crystalline thin films

This paper identifies different length scales λ s for strain-reducing chemical bonding self-organizations (CBSO) in non-crystalline and nano-crystalline thin films. CBSOs are differentiated spectroscopically, and explained by semi-empirical bond-constraint theory (SE-BCT). Non-crystalline thin film CBSOs are characterized by molecular scale, strain-reducing chemical ordering with λ s > 0.6 nm, and extending to at most 1 nm. The non-random bonding results in reduced defect densities that are enabling for device applications. Nano-crystalline transition metal oxide thin films display qualitatively different properties in two distinct nano-scale regimes in which the length scale metric is defined by π-bond coupling between atoms in strings of neighboring primitive unit cells (PUCs): (i) type I with nano-grain dimensions and λ s ∼ 2 nm (or <4 PUCs), and (ii) type 2 with nano-grain sizes and λ s > 3–4 nm (>6 PUCs). There are also diphasic nano-crystalline/non-crystalline technologically important thin films in which strain percolation is also minimized by CBSOs that combine molecular and PUC scales of order. Representative non-crystalline, nano-crystalline and diphasic nano-crystalline/non-crystalline thin film materials with qualitatively different behaviors and degrees of phase stability/metastability are addressed.

Effect of calcination temperature on morphology, crystallinity and electrochemical properties of nano-crystalline metal oxides (Co 3O 4, CuO, and NiO) prepared via ultrasonic spray pyrolysis

Nano-crystalline metal oxides (Co 3O 4, CuO, and NiO) are synthesized as anode materials for lithium-ion batteries by an ultrasonic spray pyrolysis method. The effects of calcination temperature on the morphology, crystallite size and electrochemical properties of the metal oxides are investigated. X-ray diffraction (XRD) studies show that the crystallite size varies with the final calcination temperature. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations reveal that the calcination temperature strongly influences the morphology of the prepared metal oxides and this results in different electrochemical performance. The existence of a nano-scale microstructure for the prepared metal oxides has a strong relationship with irreversible capacity and capacity retention.

Nanocrystalline Metal Oxides Dispersed Multiwalled Carbon Nanotubes as Supercapacitor Electrodes

RuO2/MWNT, TiO2/MWNT, and SnO2/MWNT nanocrystalline composites for supercapacitor electrodes have been synthesized by chemical reduction method using functionalized MWNT and respective salts. MWNT have been synthesized by thermal catalytic chemical vapor deposition (CCVD) over hydrogen decrepitated Mischmetal (Mm)-based AB3 alloy hydride catalysts. Structural and morphological characterizations of metal oxide dispersed MWNT have been carried out using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM and HRTEM), energy dispersive X-ray analysis (EDAX), and Raman spectroscopy. Electrochemical performance of these electrodes has been investigated using cyclic voltammetry, galvanostatic charge−discharge, and electrochemical impedance spectroscopy. Specific capacitance of RuO2, TiO2, and SnO2 dispersed MWNT electrodes increases compared to that pure MWNT electrode due to the pseudo capacitance of the nanocrystalline metal oxides dispersed on the functionalized MWNT. The present study shows the potential of cost-effective TiO2/MWNT novel nanocrystalline composite material for electrochemical double layer capacitor.

A Multilayered Polymer Light‐Emitting Diode Using a Nanocrystalline Metal‐Oxide Film as a Charge‐Injection Electrode

A Multilayered Polymer Light‐Emitting Diode Using a Nanocrystalline Metal‐Oxide Film as a Charge‐Injection Electrode

Near Infrared Reflectance Properties of Metal Oxide Nanoparticles

Powders of nanocrystalline metal oxides were compared with common macrocrystalline powders and minerals for their ability to reflect near-infrared light (NIR) (750−2500 nm). The nanocrystalline metal oxides were found to possess higher NIR reflectance values (∼15−20%) and this can be attributed to their smaller crystallite sizes coupled with smaller mean aggregate sizes in accordance with the Kubelka−Munk theory.

Hybrid polymer–metal oxide thin films for photovoltaic applications

We review progress in the development of organic–inorganic hybrid photovoltaic materials consisting of a conjugated polymer as an electron donor and a nanocrystalline metal oxide as the electron acceptor. We distinguish two main approaches: (1) where a rigid porous metal oxide structure is filled with polymer and (2) where metal oxide nanoparticles and polymer are co-deposited from solution to form a blend film. In the case of porous structures, performance is limited by the infiltration of polymer into the pores of the metal oxide and control of the nanostructure dimensions. In the case of blends, control of the blend morphology and transport between nanoparticles are limitations. In both cases, further improvements are possible by modifying the metal oxide organic interface to optimise charge transfer, by improving both inter- and intra-particle transport within the metal oxide phase, for example by the use of single crystalline nanorods, and by optimising the choice of electrode materials. Though unlikely to achieve the highest photocurrents, the polymer–metal oxide composites provide a model system to study the effects of interface properties and film morphology on the performance of bulk heterojunction photovoltaic devices.

Nanocrystalline TiO2 Solar Cells Sensitized with InAs Quantum Dots

We report nanocrystalline TiO2 solar cells sensitized with InAs quantum dots. InAs quantum dots of different sizes were synthesized and incorporated in solar cell devices. Efficient charge transfer from InAs quantum dots to TiO2 particles was achieved without deliberate modification of the quantum dot capping layer. A power conversion efficiency of about 1.7% under 5 mW/cm2 was achieved; this is relatively high for a nanocrystalline metal oxide solar cell sensitized with presynthesized quantum dots, but this efficiency could only be achieved at low light intensity. At one sun, the efficiency decreased to 0.3%. The devices are stable for at least weeks under room light in air.