Abstract

Metallic nanoparticles become of current interests because they exhibit unique properties compared with those of metal atoms or bulk metal due to the quantum size effect and their large surface area, which make them attractive for applications in optics, electronics, catalysis biology and medicine. TiO2 has been used for environmental remediation purposes such as in the purification of water and air and also solar-to chemical energy conversion via water splitting (H2 generation) and CO2 reduction into light hydrocarbons. Nevertheless, due to large band gap of pure TiO2 (from 3.0 to 3.2 eV depending on the crystal structure) it could be activated only by UV irradiation, limiting the utilization of sunlight as an irradiation source in photocatalytic reactions. Noble metallic nanomaterials are of particular interest because they revealed catalytic activity and can be used to enhance the photocatalytic activity of titanium (IV) oxide in visible light. Noble metal nanoparticles possess ability to absorb visible light, due to localized surface plasmon resonance (LSPR). The role of loaded metal is trapping and subsequent transfer of photoexcited electrons on TiO2 surface. The activity of the photocatalyst will generally depend on the size of the metal particles, where a catalyst with small particles will give high activity due to the large number of atoms available on the metal particles' surfaces. In addition silver nanoparticles have strong inhibitory and bactericidal effects in a broad spectrum of antimicrobial activities. The activity of the catalyst will generally depend on the size of the metal particles, where a catalyst with small particles will give high activity due to the large number of atoms available on the metal particles' surfaces. For some types of catalytic reactions, the selectivity will depend on the nature of the catalytic sites (assemblies of few atoms) on the surface of the metal particle. Depending on the particle size, different types of sites are present. There are also socalled structure sensitive reactions for which the activity per active site (turnover frequency) varies with the metal particle size.During the last few years, many methods have been employed to prepare metallic nanoparticles. such as chemical reduction of metal ions [1,2], thermal decomposition in organic solvents [3,4], chemical and photochemical reduction in reverse micelles [5,6], sonochemical [7] deposition and reduction by γ-radiation [8]. Both physical and chemical methods had been reported in the literature. For chemical methods, the choice of preparation methods and reaction conditions such as reducing agent or stabilizers, is an important task. The methods of chemical reduction based on reduction of metal salt in the presence of a suitable stabilizer play an important role in protecting nanoparticles from agglomeration. . In these methods, reverse microemulsion method has received considerable attentions. Microemulsion is an isotropic and thermodynamically stable system which consist of three components: water, organic phase and amphiphillic molecule, the surfactant. The particle sizes generated can be controlled by the nanodroplet size of the inner phase of the microemulsion. In W/O microemulsions the water droplets surrounded by surfactant molecules act as chemical microreactors, which can be used in particular to obtain ultrafine and monodisperse metal nanoparticles. The interchange of the reactants takes place during the collisions of the water droplets in the microemulsions. In this review, besides an overview on the general methods for the preparation of noble metallic, bimetallic and semiconductor nanoparticles, the main content aims to describe the preparation of bimetallic nanoparticles with various morphologies in microemulsion systems, the physical properties of obtained nanoparticles and the effects of the concerned parameters on the particle formation in microemulsion.Many kinds of nanoparticles have been prepared in w/o microemulsions including metals, metal oxides, organic polymers and bimetallic nanoparticles. Nanobimetallic particles, in which two kinds of metals are contained in one particle, have unique catalytic, electronic and optical properties distinct from those of the corresponding monometallic particles [28-30]. However, until recently when we prepared Ag/Au-TiO2 and Ag/Cu-TiO2 bimetallic nanoparticles the preparation of bimetallic nanoparticles in w/o microemulsions had not been attempted except for metallic nanoparticles Pd/Pt, Cu-Au, Au-Pt, Au-Pd, Ag/Au, and Pd-Pt.Effects of stabilizer type, continuous phase, reducing agent and reaction conditions on the particle size and polydispersity are summarized and evaluated. The influence of several other parameters such as temperature, the incident light, the nature of metal salts and addition of electrolyte are also reviewed. The effect of crystalline form of TiO2 and gold/silver or copper/silver nanoparticles size on photocatalytic activity in visible light were investigated [9,10]. These results indicate that the Ag/Au-TiO2 and Ag/Cu-TiO2 nanocomposites have shown more significant visible light photoactivity than both Ag-TiO2 and Au-TiO2 photocatalysts and represent a remarkable step forward in visible light photocatalysis.

Authors (4)