The objectives of this research are to synthesize and characterize nanomaterials of controlled size and shape so that they can be used in several different applications. Nanophase metal particles have attracted a great deal of interest and have found applications in different fields such as catalysis, optical, microelectronic and magnetic devices and biological diagnostic probes due to their special properties that differ markedly from those of bulk materials. Noble metal nanophases such as Ag, Pt, Pd etc., are the focus of catalytic applications because these nanoparticles can be hybridized with other substrates for enhanced catalytic functions. A one-step microwave-assisted interface reaction using dodecylthiol and ethylene glycol was developed for the direct synthesis of hexagonally arranged spherical silver nanoparticles of about 10 nm. Microwave-assisted solvothermal methods using ethylene glycol, ethanol or methanol as reducing agents were found to be useful for Pt and Pd nanoparticle synthesis at low temperatures. Under conventional hydrothermal conditions, some nanowires such as Te were synthesized with the assistance of biomolecules. Nanowires and nanorods of metals are expected to be useful as interconnects in electronic devices with super-functions. Nanowires of Pt were grown in the nanochannels of mesoporous materials such as SBA-15, which served as a template. Porous alumina membrane was used as a template for the growth of oriented SBA-15 nanorods with the nanoporous channels parallel to the channels of alumina and this hybridized material is expected to find super-function in nanowire fabrication and bio-molecule separations. This review paper shows that conventional and microwave-assisted hydro- or solvothermal methods are eminently suited for the synthesis of nanomaterials of controlled size and shape under environmentally benign conditions.
Keywords

Nanowires, Nanophases, Solvothermal process, Hydrothermal Process, Biomolecules
Introduction

During the last two decades, there has been an enormous interest in nanostructures due to their conspicuous physical-chemical properties that differ markedly from those of bulk materials[1]. Various methods, such as hydrothermal and solvothermal routes[2], surfactant-assisted approach[3], have been utilized for the synthesis of nanomaterials. Most physical and chemical properties of these nanomaterials are sensitively dependent on their size and shape, so materials scientists are still focusing on developing simple and effective methods for the fabrication of nanomaterials of controlled size and morphology[4].

Since metal nanoparticles have various applications, the synthesis of metal nanoparticles has attracted much attention especially in the last decade[5]. A variety of techniques have been developed to synthesize metal nanoparticles, including chemical reduction using a number of chemical reductants including NaBH4, N2H4, NH2OH, ethanol, ethylene glycol and N,N-dimethyformamide (DMF)[6-10], aerosol technique[11], electrochemical or sonochemical deposition[12, 13], photochemical reduction[14], and laser irradiation technique[15]. Because of the size-dependent properties, many physical, chemical and electrochemical methods have been employed to get the metal nanoparticles with uniform size, such as NaBH4-reduction approach resulting in the thiol-capped 1.8-3.5 nm diameter silver nanoparticles and alcohol reduction of fatty acid silver salts under microwave irradiation[16, 17]. The assembly of uniform nanoparticles into well-defined two- and three-dimensional (2D and 3D) superlattices is critically important to chemical, optical, magnetic and electronic nanodevices and would bring possibilities to brand-new properties and applications that result from the spatial orientation and arrangement of the nanocrystals[18]. Therefore, several approaches, such as self-assembly[19], Langmuir-Blodgett (LB) techniques[7], and electrophoretic deposition method[20] have been used in order to obtain self-organized lattices of metal, oxide and chalcogenide nanoparticles including silver[11], gold[21], cobalt[22], indium[23], α-Fe2O3[24], cobalt oxide[25], BaTiO3[26], CdS[27], CdSe[28], and Ag2S[29] nanoparticle arrays.

Besides the uniform and assembled nanoparticles, one-dimensional (1D) nanostructures, such as nanorods and nanowires, are also of particular interest not only because of their great potential for testing and understanding fundamental concepts but also because of their wide applications as interconnects in electronic devices with super-functions[30]. The synthesis of 1D nanostructures and guiding these nanometer-scaled building blocks to ordered superstructures would offer great opportunities to investigate the size- and dimensionality-dependent properties of these materials and could lead to the construction of nanoscale devices[31]. Until now, great progress has been made in the shape control of nanomaterials and a range of different 1D nanostructures have been fabricated by various techniques, such as Vapor-Liquid-Solid (VLS) growth mechanism[32], micro-emulsion method[3], hydrothermal (or solvothermal) technique[2] and template methods[33]. Among the various methods, hard template method is an effective method to obtain the nanostructures with low dimensionality. Porous alumina membrane and mesoporous materials such as SBA-15 are two of the most used templates. Nanowires of Ag, Pt, and Au were grown in the nanochannels of SBA-15[34], and many other nanorods arrays have been obtained by porous alumina membrane[35]. However, the pore size of the alumina membrane is from dozens of nanometers to several hundred nanometers and the SBA-15 is usually in powder form or as membranes with its channels parallel to plane of the substrate, which limits their applications in nanodevice fabrication[36]. Combining these two templates by introducing SBA-15 into alumina membrane channels is expected to find super-function in nanowire fabrication and bio-molecule separations.

This review paper shows that conventional and microwave-assisted hydro- or solvothermal methods are highly suited for the synthesis of nanomaterials of controlled size and shape under environmentally benign conditions for several different applications.