Microwave Chemistry

Considerable knowledge of microwave radiation was obtained during the development of radar before and during the second world war. In the late 1960s it was used as a heating mode for temperature-jump experiments [78]. The first application in chemical research was reported in the early 1970s, when gas-phase discharge was applied to realize decomposition of simple organic compounds [79]. By the early 1980s, two patents concerning polymer chemistry appeared and one was related to starch derivatisation. However, when significant rate accelerations for reactions carried out in a conventional microwave oven were observed in 1986, considerable attention on reactions was laid upon dielectric heating [80,81]. In addition, more advanced microwave ovens were designed. Moreover, ensued are discussions on the causes of microwave reaction rate enhancements, apprehensions about temperature monitoring and control as well as trials of large-scale reactions in microwave ovens.

1.3.1 Microwave-assisted synthesis of metallic nanostructures

Microwave (MW) rapid heating has received considerable attention as a new promising method for the one-pot synthesis of metallic nanostructures in solutions. A variety of metallic nanostructures, including spherical particles, sheets, plates, rods,

wires, tubes and dendrites have generated significant scientific and technological interests because of their unique optical as well as novel chemical and catalytic properties. These nanostructures have been synthesized by various techniques, including chemical reduction of metallic ions in aqueous or organic solvents [82-84].

In general, chemical reduction has been carried out by heating reagent solutions at 65–200°C in an oil bath. In the oil-bath heating, the solvent is heated by conduction and convection, so that there is a large temperature distribution within the solvent.

Recently, microwave dielectric heating has been applied to the rapid synthesis of metallic nanostructures [85-112]. MWs are a portion of the electromagnetic spectrum with frequencies in the range from 300 MHz to 300 GHz. The commonly used frequency is 2.45 GHz. In the microwave frequency range, polar molecules such as H2O undertake to orientate with the electric field. When dipolar molecules attempt to re-orientate with respect to an alternating electric field, they lose energy in the form of heat by molecular friction. The MW power dissipation per unit volume in a material (P) is given by equation:

where c is a constant, E is an electric field in the material, f is the radiation frequency, and ε' and ε'' are the dielectric and dielectric loss constants, respectively. ε' represents the relative permittivity, which is a measure of the ability of a molecule to be polarized by an electric field and tanδ=ε''/ε' is the energy dissipation factor or loss tangent. Equation indicates that ε'' is the most important physical parameter that describes the ability of a material to heat in the MW field. The physical parameters of typical solvents used in MW heating for synthesis of metallic nanostructures are listed in Table 1-4.

Table 1-4 Physical parameters of typical solvents used for microwave heating

Water, alcohols, DMF and ethyleneglycol (EG) have high dielectric losses and high reduction abilities and are ideal solvents for MW rapid heating. The MW heating in these solvents in the presence of surfactants has been used to synthesize nanoparticles of various metals (Ni, Ru, Rh, Pd, Ag, Ir, Pt, Au,), [85-104] metallic compounds (PtRu, TiO2, CdS, CdSe, MoSe2, PbS, HgS, CuInTe2, CuInSe2) and Au/Pd core-shell structures [105-112].

1.3.2 Possible effects of MW heating

There are two effects of MW dielectric heating, thermal and non-thermal [114].

Thermal effects arise from different temperature regimes under MW heating, whereas non-thermal effects result from effects inherent to the MWs. These effects lead to different morphologies and sizes of metallic nanostructures under MW heating from those in the conventional oil-bath heating.

1.3.2.1 Thermal effects (effects of rapid and uniform heating)

MW provides rapid and uniform heating of reagents, solvents, intermediates and products. Fast heating accelerates the reduction of metal precursors and the nucleation of the metal cluster, and results in mono-dispersed small nanostructures. When MWs are incident perpendicular to the solvent surface, their intensity is attenuated in the direction of incidence. However, for most materials, the distance is quite long in the direction of penetration at which the incident power is reduced to half of its initial

value. Therefore, the power dissipation is fairly uniform throughout the solvent. This homogeneous MW heating also provides uniform nucleation and growth conditions, and leads to uniform nanomaterials of small size. Due to the rapid and homogeneous MW heating, a better crystallinity can be obtained. Therefore, such single-crystalline nanostructures as polygonal plates, rods and wires could be synthesized efficiently in many cases.

1.3.2.2 Effects of hot spots and hot surfaces

When solids heated by MW are involved in the reaction system, hot spots are created on the solid–liquid surface. The uniform formation of hot spots and hot surfaces also accelerates the reduction of metal precursors and the nucleation of the metal cluster, and leads to uniform nanostructures of small size.

1.3.2.3 Superheating

Superheating of solvents over boiling points of solvents often occurs as a consequence of the MW dissipation over the whole liquid volume [113]. This effect is especially significant in the presence of a large amount of ions.

1.3.2.4 Non-thermal effects

Non-thermal effects are defined as those that occur under the same temperature profiles of solvents between MW and oil-bath heatings during the reaction. Formation of hot spots and hot surfaces are typical non-thermal effects for the preparation of metallic nanostructures. MW heating induces various thermal and non-thermal effects described above.