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distribution and shape of the particles from the micrometer scale to the nanometer scale. Over the past two decades, the
polyol process has been studied much and adapted for the preparation of ferromagnetic metal powders [8,9],
nanometer noble metal powders [10,11], metallic nanorods and nanowires [12,13], and nanoscale oxide particles [14].
Even though the polyol process has been used widely for the preparation of various metal powders, the polyol reaction
mechanisms have been studied little. Blin et al. [15] have studied the production of EG through oxidation, and the
diacetyl was detected in the EG–Ni(OH)2–H2O system in 1989. However, the redox reaction in solution was related to
the reduction potential, which varied with the pH value when H+ or OHꢁ participated in the formation of metal
powders. Consequently, studying the thermodynamics of the preparation of nanometer nickel powders from EG can
give rise to a better understanding of the reduction mechanism.
In this work, a process for preparing nanometer nickel powders by the polyol method was described. The E–pH
diagram of the Ni–EG–H2O system was studied according to the thermodynamics of the potential reaction between
Ni(OH)2 and EG. The oxidation reaction direction and the oxidation products of EG were investigated and identified
through the IR spectrum.
2. Experimental
The reagents used in the experiments, Ni(NO3)2ꢀ6H2O, NaOH, ethanol, and EG, were all analytical grade reagents.
Typically, a given amount of Ni(NO3)2ꢀ6H2O with 1% polyvinylpyrrolidone (PVP, molecular weight 10,000) was
dissolved in distilled water, and 1 M NaOH solution was added to adjust the pH to 11. The mixed solution was stirred
at 60 8C for 2 h. Then, the Ni(OH)2 slurry was filtered using a Buchner funnel. The water content of the Ni(OH)2 was
77.0%.
The obtained Ni(OH)2, 3.0 g NaOH, 0.4 g PVP, and 100 mL EG were added to a three-necked flask. The mixture of
Ni(OH)2, NaOH, PVP, and EG was stirred to form Ni(OH)2 slurry, and then refluxed at 180 8C for 4 h, filtered using a
Buchner funnel, washed thrice with distilled water and twice with ethanol, and dried at 45 8C for 24 h. Gray–black
nickel powders were obtained. Twenty milliliters CaCl2 solution was added into filtrate, and the sediments were
filtered, dried and then characterized by X-ray diffraction (XRD).
The filtrate was examined using a Nexus 670 IR (Thermo Electron) with a KBr coating, and the nickel powders
were characterized by their X-ray diffraction patterns (Rigaku, Dmax/2550) and using a field-emission scanning
electron micrograph (FE-SEM) (Sirion200, EDAX).
3. Results and discussion
3.1. Thermodynamics of Ni–EG–H2O system
A redox reaction in solution depends not only on the concentration of the ion but also on the pH of the solution
according to the Nernst equation. The free energy of formation of the main substance in the Ni–EG–H2O system
is shown in Table 1. The value of the reduction potential of Ni(OH)2 in EG is not available in the literature. The
order of the potentials of a given electrode reaction as a rule does not significantly differ for different media.
Moreover, the half-cell potentials of metal ions in EG are also in agreement with their active order in an aqueous
solution [16].
Table 1
The free energy of formation of the major substances in the Ni–EG–H2O system (T = 298 K)
DGuf ðkJ=molÞ
Species
H2O
Ni2+
ꢁ238.09
ꢁ192.28
ꢁ448.98
0
Ni(OH)2
Ni
EG
ꢁ324.62
ꢁ277.98
ꢁ529.91
ꢁ157.89
Diacetyl
2ꢁ
CO3
OHꢁ