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.
Experimental
Microwave-Assisted Solvothermal Synthesis of Metal Nanoparticles

For the hexagonally arranged spherical silver nanoparticles, 0.15 g AgNO3 was added in a Teflon vessel of a double-walled digestion vessel used in MARS-5 system. Then 10 ml toluene, 1 ml dodecylthiol and 4 ml ethylene glycol were added into the vessel in order. After sealing, the vessel was treated at 160°C for 3 hours using a microwave digestion system, MARS-5 (CEM Corp.). After cooling to room temperature, the product was collected and an interface between two layers was found to be full of black product.

Pt and Pd nanoparticles were synthesized by microwave-assisted solvothermal method. PVP with an average molecular weight of 40K was used as a capping agent in all the experiments. Dihydrogen hexachloroplatinate (IV), and palladium (II) 2,4-pentanedionate were used as metal precursors. PVP was dissolved in methanol or ethanol and then the metal salts were added. The reactants were heated for 60 min at 90°C when methanol was used as a reducing agent and at 120°C when ethanol was used as a reducing agent for 60 min under microwave irradiation.
Biomolecule-Assisted Hydrothermal Synthesis For Te Nanowires[37]

For the elemental tellurium nanowires, 0.15g H2TeO4·2H2O was mixed with 0.075 g alginic acid in 10 ml distilled water in a Telfon-lined stainless autoclave. After sealing, the autoclave was heated to 150°C and kept for 15 hours. After cooling to room temperature, the solid product was collected by centrifugation at 2000 rpm for ~10 min and washed with distilled water and alcohol several times, followed by drying in air at room temperature.
Sol-Gel Method for The Growth of SBA-15 Nanorods Array Inside Porous Alumina Membrane[38]

In the synthesis of SBA-15 nanowire arrays inside porous alumina membrane, a sol solution was made by dissolving 1 g Pluronic P123 (PEO20PO70EO20, Mav=5800, Aldrich) in 5 g ethanol and 0.2 g 2 M HCl solution and mixing with 2.08 g tetraethyl orthosilicate (TEOS, 98%, Aldrich). Then a simple piece of porous alumina membrane was put into the sol solution. After the sol solution was left at room temperature (about 25°C) for 20 h to make sol change to gel, some amount of liquid paraffin wax with thickness of 1 mm was poured onto the gel and then it was kept at 60°C for 20 h. Then, liquid paraffin was removed and the sample was calcined in the alumina membrane at 540°C for 6 h.
Nanowires of Pt Inside SBA-15

First, SBA-15 was prepared and treated with H2PtCl6 followed by H2 reduction at 400°C in order to prepare nanowires of Pt inside SBA-15. The SBA-15 was then dissolved in dilute HF solution to recover Pt nanowries.
Characterization

The morphology, crystallinity, and size of products have been determined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Selected area electron diffraction (SAED) was used to identify the crystalline phases. TEM was carried out with a Philips 420 transmission electron microscope operated at 120 kV and SEM was carried out with a Hitachi S-3500N scanning electron microscope.
Results and Discussion
Microwave-Assisted Solvothermal Synthesis of Metal Nanoparticles

Due to the spatial orientation and arrangement of the nanocrystals, 2D and/or 3D nanoparticles superlattices would bring possibilities to brand-new properties and applications, which make their syntheses be a focusing area in the current research field [18-20]. For the assembly of uniform Ag nanocrystals, the presynthesis of uniform nanoparticles or precursors is usually required followed by the organization process by surfactants or ligands. The development of a simple and direct method for the fabrication of such crystals is a major challenge for future research. Herein we report a general and one-step microwave-assisted interface-reaction for the synthesis and assembly of monodispersed silver nanoparticles. By using dodecylthiol as directing reagent and ethylene glycol as reducing agent, hexagonally arranged spherical silver nanoparticles can be obtained by a one-step interface-reaction under microwave-assisted solvothermal conditions without the requirement of the pre-synthesis of uniform silver nanoparticles or special precursors and the technique of size-selective precipitation. In the synthetic system, ethylene glycol and toluene form two layers with an interface where the thiol group of dodecylthiol might react with silver ions to form an inorganic-organic complex, which is reduced to elemental silver by ethylene glycol under microwave-solvothermal conditions. After the reaction, a black thin layer of silver nanoparticles is found at the interface and the formed silver nanoparticles automatically compact-pack to form ordered superstructures. Figure 1 shows the TEM images of the as-prepared sample, from which it can be clearly seen that the sample consists of a hexagonal-like ordered superstructure of monodispersed silver nanoparticles. Figure 1a displays a TEM image with low magnification, clearly showing that the two-dimensional (2D) hexagonal superlattice is the typical structure of the as-prepared silver sample. A TEM image of the silver sample with high magnification (Figure 1b) displays clearly that these nanoparticles are monodispersed with an average diameter of ~ 10 nm and the inter-particle spacings are calculated to be about 2 nm. Figure 1c shows its Fourier transform power spectrum. It displays ordered hexagonal-like spot arrays, which confirms the formation of the hexagonally arranged silver superlattice. The SAED pattern of the sample, showed in Figure 1d, exhibits polycrystalline diffraction rings, which can be indexed as cubic-phase metal silver, indicating that these nanoparticles are crystalline metal silver.