15-06-2013, 12:17 PM
THE WORLD OF OXIDE NANOMATERIALS
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INTRODUCTION
Metal oxides play a very important role in many areas of chemistry, physics and materials science .The metal elements are able to form a large number of oxide compounds. They can adopt a number of structural geometries with an electronic structure that can exhibit metallic, semiconductor or insulator character. Metal oxides are used in the fabrication of microelectronic circuits, sensors, piezoelectric devices, fuel cells, coatings for the passivation of surfaces against corrosion and as catalysts .
Nanotechnology is an emerging field of technology with a specific goal, to make nanostructures or nanoarrays with special properties with respect to those of bulk or single particle species .Oxide nanoparticles can exhibit unique physical and chemical properties due to their limited size and a high density of comer or edge surface sites. In any material particle size is expected to influence two important groups of basic properties.
The first one comprises the structural characteristics, namely the lattice symmetry and cell parameters Bulk oxides are usually robust and stable systems with well-defined crystallographic structures. However, the growing importance of surface free energy and stress with decreasing particle size must be considered: changes in thermodynamic stability associate with size can induce modification of cell parameters and/or structural transformations and inExtreme cases the nanoparticles can disappear due to interactions with its surrounding environment and a high surface free energy .
In order to display mechanical or structural stability, a nanoparticJe must have a low surface free energy. As a consequence of this requirement, phases that have a low stability in bulk materials can become very stable in nanostructures.
The second important effect of size is related to the electronic properties of the oxide. in any material, then nanostructure produces the so-called quantum size or confinement effects which essentially arise from the presence of discrete, atom-like electronic states. From a solid-state point of view, these states can be considered as being a superposition of bulk-like states with a concomitant increase in oscillator strength. Additional general electronic effects of quantum confinement experimentally probed on oxides are related to the energy shift of excit on levels and optical band gap .An important factor to consider when dealing with the electronic properties of a bulk oxide surface are the long-range effects of the Madelung field, which are not present or limited in a nanostructured oxide. Theoretical studies for oxides show a redistribution of charge when going from large periodic structures to small clusters or aggregates which must be roughly considered to be relatively small for ionic solids while significantly larger for covalent ones
Properties of nanoparticulated oxides
The current knowledge on oxide materials allows to affirm that most of their physico¬-chemical properties display an acute size dependence. Physico-chemical properties of special relevance in Chemistry are mostly related to the industrial use of oxides as sensors, ceramics, absorbents and catalysts. A bunch of novel application with in these fields rely on the size¬ dependence of the optical,(electronic and/or ionic) transport, mechanical and, obviously, surface/chemical (redox, acid/base) properties of oxide nanomaterials.
Synthesis of nanoparticulated oxides
The first requirement of any novel study of nanoparticulated oxides is the synthesis of the material. The development of systematic studies for the synthesis of oxide nanoparticles is a current challenge and essentially,the corresponding preparation methods may be grouped in two main streams based upon the liquid-solid and gas-solid nature of the transformations.
Liquid-solid transformations are possibly the most broadly used in order to control morphological characterstics with certain "chemical" versatility and usually follow a "bottom-up" approach. A number of specific methods shave been developed, among which those broadly in use are:
Sol-gel processing
The sol-gel process is a wet-chemical technique widely used in the fields of materials science and ceramic engineering. Such methods are used primarily for the fabrication of materials (typically metal oxides) starting from a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers Typical precursor are metal alkoxides and metal salts (such as chloride nitrates and acetates), which undergo various forms of hydrolysis an polycondensation reactions.
The method prepares metal oxides via hydrolysis of precursors .usually alkoxides in alcoholic solution, resulting in the corresponding oxo-hydroxide. Condensation of molecules by giving off water leads to the formation of a network of the metal hydroxide Hydroxyl-¬species undergo polymerization by condensation and form a dense porous gel. Appropriatedrying and calcinations lead to ultrafine porous oxides
Microwave assisted synthesis
Microwave assisted synthesis technique offers a new route for synthesis where the reaction can be carried out in few- minutes. Microwave synthesis has the potential to be a useful tool for growing needs of nanotechnology Advantages of microwave synthesis of nanoparticles include uniform temperatures, less time for synthesis, product uniformity and much higher yields. Microwaves are electromagnetic waves with frequencies lying in the range of 300 MHz to 300 MHz. Microwaves cause maximum reorientation of the molecules at their natural frequency resulting into maximum heating of the materials. In conventional, kitchen microwave oven, molecules of the waters are excited directly by electromagnetic field at frequency 2.45 GHz, which is the resonance frequency of water molecules. However some inorganic solvents like ethanol, methanol etc. can also be excited with the same frequency. Even the semiconducting materials with high dielectric constant also absorb this electromagnetic energy resulting in increase of its temperature violently In conventional synthesis of ZnO nanoparticles (Spanhel method) 3 hr of processing is required because of flow of heat due to conduction. But microwave radiation cause rotation of molecules at their natural frequency and hence lead to reduction in synthesis time and uniform heating within the sample. The essential requirement for microwave synthesis of the metal oxide is that the precursor has to have a high dielectric constant. Therefore, when mixture of two different materials having different dielectric constant is exposed to microwave adiation, material with higher value of Dielectric constant absorbs more energy and gets heated rapidly compared to the other. The homogeneous mixing of precursor into solvent may be advantageous provided the precursor has more dielectric constant than the solvent. Ethanol is a very good solvent in which metal acetates are soluble and ethanol even does not precipitate. However the disadvantage of this technique is 2-3 hrs processing time in refluxing or distillation under continuous stirring.
Vapour condensation
Vapour condensation synthesis technique is used to make metallic an metal oxide ceramic nanoparticles It involves evaporation of a solid metal followed by rapid condensation to form nanosized clusters that settle in the form of a powder. Various approaches to vaporizing the metal can be used and variation of the medium into which the vapour is released affects the nature and size of the particles. Inert gases are used to avoid oxidation when creating metal nanoparticles, whereas a reactive oxygen atmosphere is used to produce metal oxide ceramic nanoparticles. A technique that is arguably not really vapour condensation is the "exploding wire" technique, developed originally in Russia-and currently used by Argonide An electrical arc is created at the surface of a metal wire with sufficient energy 10 explode or vaporize Clusters of atoms (similar to blowing a filament in a light bulb). These clusters then condense within an inert gas into nanoscale particles.
[attachment=55459]
INTRODUCTION
Metal oxides play a very important role in many areas of chemistry, physics and materials science .The metal elements are able to form a large number of oxide compounds. They can adopt a number of structural geometries with an electronic structure that can exhibit metallic, semiconductor or insulator character. Metal oxides are used in the fabrication of microelectronic circuits, sensors, piezoelectric devices, fuel cells, coatings for the passivation of surfaces against corrosion and as catalysts .
Nanotechnology is an emerging field of technology with a specific goal, to make nanostructures or nanoarrays with special properties with respect to those of bulk or single particle species .Oxide nanoparticles can exhibit unique physical and chemical properties due to their limited size and a high density of comer or edge surface sites. In any material particle size is expected to influence two important groups of basic properties.
The first one comprises the structural characteristics, namely the lattice symmetry and cell parameters Bulk oxides are usually robust and stable systems with well-defined crystallographic structures. However, the growing importance of surface free energy and stress with decreasing particle size must be considered: changes in thermodynamic stability associate with size can induce modification of cell parameters and/or structural transformations and inExtreme cases the nanoparticles can disappear due to interactions with its surrounding environment and a high surface free energy .
In order to display mechanical or structural stability, a nanoparticJe must have a low surface free energy. As a consequence of this requirement, phases that have a low stability in bulk materials can become very stable in nanostructures.
The second important effect of size is related to the electronic properties of the oxide. in any material, then nanostructure produces the so-called quantum size or confinement effects which essentially arise from the presence of discrete, atom-like electronic states. From a solid-state point of view, these states can be considered as being a superposition of bulk-like states with a concomitant increase in oscillator strength. Additional general electronic effects of quantum confinement experimentally probed on oxides are related to the energy shift of excit on levels and optical band gap .An important factor to consider when dealing with the electronic properties of a bulk oxide surface are the long-range effects of the Madelung field, which are not present or limited in a nanostructured oxide. Theoretical studies for oxides show a redistribution of charge when going from large periodic structures to small clusters or aggregates which must be roughly considered to be relatively small for ionic solids while significantly larger for covalent ones
Properties of nanoparticulated oxides
The current knowledge on oxide materials allows to affirm that most of their physico¬-chemical properties display an acute size dependence. Physico-chemical properties of special relevance in Chemistry are mostly related to the industrial use of oxides as sensors, ceramics, absorbents and catalysts. A bunch of novel application with in these fields rely on the size¬ dependence of the optical,(electronic and/or ionic) transport, mechanical and, obviously, surface/chemical (redox, acid/base) properties of oxide nanomaterials.
Synthesis of nanoparticulated oxides
The first requirement of any novel study of nanoparticulated oxides is the synthesis of the material. The development of systematic studies for the synthesis of oxide nanoparticles is a current challenge and essentially,the corresponding preparation methods may be grouped in two main streams based upon the liquid-solid and gas-solid nature of the transformations.
Liquid-solid transformations are possibly the most broadly used in order to control morphological characterstics with certain "chemical" versatility and usually follow a "bottom-up" approach. A number of specific methods shave been developed, among which those broadly in use are:
Sol-gel processing
The sol-gel process is a wet-chemical technique widely used in the fields of materials science and ceramic engineering. Such methods are used primarily for the fabrication of materials (typically metal oxides) starting from a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers Typical precursor are metal alkoxides and metal salts (such as chloride nitrates and acetates), which undergo various forms of hydrolysis an polycondensation reactions.
The method prepares metal oxides via hydrolysis of precursors .usually alkoxides in alcoholic solution, resulting in the corresponding oxo-hydroxide. Condensation of molecules by giving off water leads to the formation of a network of the metal hydroxide Hydroxyl-¬species undergo polymerization by condensation and form a dense porous gel. Appropriatedrying and calcinations lead to ultrafine porous oxides
Microwave assisted synthesis
Microwave assisted synthesis technique offers a new route for synthesis where the reaction can be carried out in few- minutes. Microwave synthesis has the potential to be a useful tool for growing needs of nanotechnology Advantages of microwave synthesis of nanoparticles include uniform temperatures, less time for synthesis, product uniformity and much higher yields. Microwaves are electromagnetic waves with frequencies lying in the range of 300 MHz to 300 MHz. Microwaves cause maximum reorientation of the molecules at their natural frequency resulting into maximum heating of the materials. In conventional, kitchen microwave oven, molecules of the waters are excited directly by electromagnetic field at frequency 2.45 GHz, which is the resonance frequency of water molecules. However some inorganic solvents like ethanol, methanol etc. can also be excited with the same frequency. Even the semiconducting materials with high dielectric constant also absorb this electromagnetic energy resulting in increase of its temperature violently In conventional synthesis of ZnO nanoparticles (Spanhel method) 3 hr of processing is required because of flow of heat due to conduction. But microwave radiation cause rotation of molecules at their natural frequency and hence lead to reduction in synthesis time and uniform heating within the sample. The essential requirement for microwave synthesis of the metal oxide is that the precursor has to have a high dielectric constant. Therefore, when mixture of two different materials having different dielectric constant is exposed to microwave adiation, material with higher value of Dielectric constant absorbs more energy and gets heated rapidly compared to the other. The homogeneous mixing of precursor into solvent may be advantageous provided the precursor has more dielectric constant than the solvent. Ethanol is a very good solvent in which metal acetates are soluble and ethanol even does not precipitate. However the disadvantage of this technique is 2-3 hrs processing time in refluxing or distillation under continuous stirring.
Vapour condensation
Vapour condensation synthesis technique is used to make metallic an metal oxide ceramic nanoparticles It involves evaporation of a solid metal followed by rapid condensation to form nanosized clusters that settle in the form of a powder. Various approaches to vaporizing the metal can be used and variation of the medium into which the vapour is released affects the nature and size of the particles. Inert gases are used to avoid oxidation when creating metal nanoparticles, whereas a reactive oxygen atmosphere is used to produce metal oxide ceramic nanoparticles. A technique that is arguably not really vapour condensation is the "exploding wire" technique, developed originally in Russia-and currently used by Argonide An electrical arc is created at the surface of a metal wire with sufficient energy 10 explode or vaporize Clusters of atoms (similar to blowing a filament in a light bulb). These clusters then condense within an inert gas into nanoscale particles.