paper by this group, they synthesize polymer-stabilized Pt colloids with small particle sizes and narrow size distributions by a continuous microwave synthesis
[174]. This synthesis method has good reproducibility for synthesizing uniform
metal colloids in bulk amounts.
Komarneni and coworkers [175] conducted a polyol reaction for the preparation
of Pt and Ag nanoparticles. The synthesis of the metal nanoparticles [175] was
conducted in a double-walled digestion vessel which has an inner liner and a cover
made of Teflon PFA and an outer high-strength shell of Ultem polyetherimide.
The starting precursors (chloroplatinic acid or silver nitrate) were mixed with
ethylene glycol and poly(N-vinyl-2-pyrrolidone) of different molecular weights, with
and without NaOH, and reacted at 150
C for 15 min in a microwave system. Large
Pt particles 100–400 and 50–150 nm were obtained with PVP molecular weights
of 8,000 and 10,000, respectively. Non-uniform Pt particles of <10 nm were crystallized from PVP of 40,000 molecular weight. For the Ag particles strings were
obtained. The growth of the Ag metal particles in the form of strings decreased as
the molecular weight of the PVP increased. With a PVP molecular weight of
1,300,000 and without NaOH Ag particles of 100 nm were obtained.
In another attempt [176], silver nanoparticles, yellow in color, were prepared by a
microwave high-pressure synthetic method, using polyacrylamide as the reducing
agent and stabilizer. The maximum absorption peak of colloidal silver is at 421.6
nm, and its strongest resonance scattering peak is at 470 nm. The experimental
results showed that the silver nanoparticles are well dispersed and the colloidal
solution has good stability. The average diameter of silver nanoparticles is 66 nm.
The procedure was rapid and convenient.
Spherical and uniform Pt nanoparticles, 3.5–4.0 nm supported on carbon, were
prepared by microwave irradiation [177]. The products exhibited very high electrocatalytic activity in the room-temperature oxidation of liquid methanol. The preparation method was similar to that of Komarneni [175], namely, a polyol reduction.
A 100 mL beaker, containing chloroplatinic acid dissolved in an ethylene glycol,
KOH solution was added dropwise. Carbon XC-72 was uniformly dispersed in the
mixed solution by ultrasound. The beaker was placed in the center of a microwave
oven (National NN-S327WF, 2450 MHz, 700 W) and heated for 60 s.
Towards the end of 2002, a paper [178] appeared, claiming to have prepared
polychrome silver nanoparticles by a soft solution approach under microwave irradiation from a solution of silver nitrate, AgNO 3 , in the presence of poly (N-vinyl2-pyrrolidone) PVP, without any other reducing agent. Different morphologies of
silver colloids with lovely colors could be obtained using different solvents as the
reaction medium. The influence of the solvent on the morphology of silver was
investigated. In a typical procedure, the reaction solutions were prepared by dissolving PVP and AgNO 3 ethanol in a Pyrex flask to obtain a homogeneous reaction
mixture, which was placed on the turntable of the microwave oven. The mixture
was irradiated discontinuously at a power of 300 W.
6.3.1.2 The Synthesis of Nanoparticles of Metal Oxides by MWH
Similar to sonochemical research, much attention has been devoted to the preparation of nanosized metallic oxides by MWH. When surveying the literature, we
6.3 Microwave Heating 157
[174]. This synthesis method has good reproducibility for synthesizing uniform
metal colloids in bulk amounts.
Komarneni and coworkers [175] conducted a polyol reaction for the preparation
of Pt and Ag nanoparticles. The synthesis of the metal nanoparticles [175] was
conducted in a double-walled digestion vessel which has an inner liner and a cover
made of Teflon PFA and an outer high-strength shell of Ultem polyetherimide.
The starting precursors (chloroplatinic acid or silver nitrate) were mixed with
ethylene glycol and poly(N-vinyl-2-pyrrolidone) of different molecular weights, with
and without NaOH, and reacted at 150
C for 15 min in a microwave system. Large
Pt particles 100–400 and 50–150 nm were obtained with PVP molecular weights
of 8,000 and 10,000, respectively. Non-uniform Pt particles of <10 nm were crystallized from PVP of 40,000 molecular weight. For the Ag particles strings were
obtained. The growth of the Ag metal particles in the form of strings decreased as
the molecular weight of the PVP increased. With a PVP molecular weight of
1,300,000 and without NaOH Ag particles of 100 nm were obtained.
In another attempt [176], silver nanoparticles, yellow in color, were prepared by a
microwave high-pressure synthetic method, using polyacrylamide as the reducing
agent and stabilizer. The maximum absorption peak of colloidal silver is at 421.6
nm, and its strongest resonance scattering peak is at 470 nm. The experimental
results showed that the silver nanoparticles are well dispersed and the colloidal
solution has good stability. The average diameter of silver nanoparticles is 66 nm.
The procedure was rapid and convenient.
Spherical and uniform Pt nanoparticles, 3.5–4.0 nm supported on carbon, were
prepared by microwave irradiation [177]. The products exhibited very high electrocatalytic activity in the room-temperature oxidation of liquid methanol. The preparation method was similar to that of Komarneni [175], namely, a polyol reduction.
A 100 mL beaker, containing chloroplatinic acid dissolved in an ethylene glycol,
KOH solution was added dropwise. Carbon XC-72 was uniformly dispersed in the
mixed solution by ultrasound. The beaker was placed in the center of a microwave
oven (National NN-S327WF, 2450 MHz, 700 W) and heated for 60 s.
Towards the end of 2002, a paper [178] appeared, claiming to have prepared
polychrome silver nanoparticles by a soft solution approach under microwave irradiation from a solution of silver nitrate, AgNO 3 , in the presence of poly (N-vinyl2-pyrrolidone) PVP, without any other reducing agent. Different morphologies of
silver colloids with lovely colors could be obtained using different solvents as the
reaction medium. The influence of the solvent on the morphology of silver was
investigated. In a typical procedure, the reaction solutions were prepared by dissolving PVP and AgNO 3 ethanol in a Pyrex flask to obtain a homogeneous reaction
mixture, which was placed on the turntable of the microwave oven. The mixture
was irradiated discontinuously at a power of 300 W.
6.3.1.2 The Synthesis of Nanoparticles of Metal Oxides by MWH
Similar to sonochemical research, much attention has been devoted to the preparation of nanosized metallic oxides by MWH. When surveying the literature, we
6.3 Microwave Heating 157