Composites with Metal Oxide Nanoparticles

With the surge in research on carbonaceous nanomaterials, the combination of these entities with metal oxide nanoparticles is enticing as the electronic properties of materials such as graphene may influence particle characteristics. For this reason, the interaction of titania nanoparticles with B- and N-doped graphene has been investigated recently in order to study the photodegradation of dye molecules by these composites. Anatase Ti02... 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

Composite materials can be formed by numerous methods. Two modes in which incorporation of the inorganic material in the template can be achieved will be discussed sol-gel processes or nanoparticle infiltration. They are both solution methods that can be processed at low temperatures, hence allowing the use of polymeric templates. In the first method the sol-gel chemistry is performed after the incorporation of a metal oxide precursor in the polymer matrix or around the template entities. The second method makes use of preformed metal oxide nanoparticles, which are infiltrated into the organic scaffold or suspended in solution with the individual structures for controlled adhesion. 

The flux of metal atoms in vacuum (Pd, Sn, Al, Ti, Zn), evaporated from a bulk sample condenses onto a cooled substrate together with the monomer. The condensate consists of nanoparticles of the metal and the monomer (Fig. 1). Upon heating the substrate to ambient temperature the monomer polymerises to PPX. The structure thus obtained is a porous matrix with dispersed nanoparticles in it. The properties of these nanocomposites containing metal and/or metal-oxide nanoparticles in the polymeric matrix are presented. Manipulation of the synthesis conditions, i.e., the distance between the vapour source and the substrate, the tilt angle of the beam, and the deposition time allowed for optimising the deposition regime. Measuring the electrical resistance of the condensate and composite permitted the control of the film formation in relation to the oxidation behaviour. 

Using a dynamic collection of droplets in a gas medium is also a well-known procedure to restrict the reaction volumes and obtain nanoparticulate materials by a thermally promoted reaction (spray pyrolysis]. Chidembo et al. [112] used an in situ spray pyrolysis approach to fabricate metal oxide-graphene composites with highly porous morphologies. The materials exhibited unique globular structures comprising metal oxide nanoparticles intercalated between graphene sheets. 

Since every preparation route has specific advantages and drawbacks, it is not possible to define a general strategy for obtaining nanocomposite aerogels. Nevertheless, metal and metal oxide nanoparticles supported on aerogel matrices with given compositional and microstructural features have been effectively prepared. 

Composite electrode material of Mn and Fe metal oxides with ratio ranging from 3.5 1 to 4.5 1 for use in ESs. Metal oxides react to form gel and are subsequently dried in supercritical CO2 to form powder. An electrode with a weight composition of 15 to 60% carbon and 40 to 80% metal oxide nanoparticles achieved nunimum specific capacitance of 500 F/g at 1 mV/s in IM KOH electrolyte. 

The synthetic developments made in the past few years in the field of metal oxide nanoparticles are allowing us to prepare materials which not only have exotic structures, but we now also have much greater control over resulting particle shape and size. While this review is not an exhaustive list of the achievements made in this field, it demonstrates the huge advancements made in innovative approaches and low-temperature synthetic design. These apply not only to binary metal oxides, but also to more complex nanoparticle systems and composites. Hand-in-hand with synthetic developments has been advances in characterisation of nanostmetures, which are providing us with mueh-needed insight into the stmeture-property-function relationships in these materials. 

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