Study on thermodynamics and oxidation mechanism of ethylene glycol in the preparation of nanometer nickel powders

Article abstract of DOI:10.1016/j.materresbull.2006.03.028

Nanometer nickel powders have been prepared using the polyol method with NaOH, Ni(NO₃)₂·6H₂O, ethylene glycol (EG), and polyvinylpyrrolidone (PVP) as raw materials. The thermodynamics of the reaction system was studied, and the E-pH diagram of Ni-EG-H₂O was plotted. The oxidation products of EG were predicted from the E-pH diagram, and CO₃^2- in alkaline solutions was identified as the product through the IR spectrum and CaCO₃ sediment. Field-emission scanning electron micrograph (FE-SEM) showed that spherical nanometer nickel powders were obtained.

Full text of DOI:10.1016/j.materresbull.2006.03.028

Materials Research Bulletin 41 (2006) 2130–2136
www.elsevier.com/locate/matresbu
Study on thermodynamics and oxidation mechanism
of ethylene glycol in the preparation of nanometer
nickel powders
*
Jin Shengming , Yuan Liangsheng, Zhou Ying,
*
Qiu Guanzhou , Wan Cuifeng
Department of Inorganic Materials, School of Resources Processing and Bioengineering,
Peace Building, Central South University, Changsha, Hunan 410083, People’s Republic of China
Received 20 October 2005; received in revised form 21 March 2006; accepted 29 March 2006
Available online 21 April 2006
Abstract
Nanometer nickel powders have been prepared using the polyol method with NaOH, Ni(NO3)2ꢀ6H2O, ethylene glycol (EG), and
polyvinylpyrrolidone (PVP) as raw materials. The thermodynamics of the reaction system was studied, and the E–pH diagram of
Ni–EG–H2O was plotted. The oxidation products of EG were predicted from the E–pH diagram, and CO32ꢁ in alkaline solutions
was identified as the product through the IR spectrum and CaCO3 sediment. Field-emission scanning electron micrograph (FE-
SEM) showed that spherical nanometer nickel powders were obtained.
# 2006 Elsevier Ltd. All rights reserved.
Keywords: A. Metals; A. Nanostructures; B. Chemical synthesis
1. Introduction
Recently, attention has been increasingly focused on uniform-shape and high-purity nanometer metal powders
with mean particle size ranging from several nanometers to 100 nm. This attention has arisen from their numerous
applications in batteries, hard alloys, and catalysts, and in electrical applications [1]. Among the various kinds of
nanometer metal powder, those containing Ni, Co, Fe, and Cu are relatively difficult to prepare because they
oxidize easily. Many methods of production of such metal powders of various sizes and shapes have been reported
such as the sonochemical method [2], the microemulsion technique [3], and the microwave irradiation method [4].
However, these methods are not appropriate for large-scale production of powders because of the difficulty of the
technique and the expensive equipment involved. Usually, chemical reduction in solution is suited for
the preparation of nanometer powders. Controlling the size and shape of powders presents a challenge to this
field [5,6].
´
A new method of preparation of metal powders, the polyol process, was considered by Fievet et al. [7] for
deoxidizing Co(OH)2 or Ni(OH)2 with ethylene glycol (EG). The polyol process enables an accurate control of the size
* Corresponding authors. Tel.: +86 731 88377204; fax: +86 731 8710804.
E-mail address: shmjin@mail.csu.edu.cn (J. Shengming).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.03.028

J. Shengming et al. / Materials Research Bulletin 41 (2006) 2130–2136
2131
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

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J. Shengming et al. / Materials Research Bulletin 41 (2006) 2130–2136
In an aqueous system, to consider any possible M-bearing species MiHjOqk, the following general reaction can be
used [17]:
M þ wH2O þ mHþ þ ne ¼ rMiHjOqk;
where i ¼ 0, r = 1/i, w = k/i, m = (j ꢁ 2k)/i, n = (j ꢁ 2k ꢁ q)/i.
With aH2O ¼ 1, the Nernst formula can be expressed as,
DGꢂ RT
2:303mRT
r
E ¼ ꢁ
ꢁ
lnðaM H O Þ ꢁ
pH
i
j
k
nF
nF
nF
The component activity term is then
lgðaM H O Þ ¼ ApH þ BE þ C
i
j
k
where A = ꢁm, B = (ꢁnF/2.303RT), C = (ꢁDG8/2.303RT).
The above formula can be applied to any redox reaction. The potential values in an aqueous solution were used for
estimating those in the Ni–EG–H2O system. The standard potential values could be calculated from the free energies
of formation of the substances listed in Table 1. Therefore, the E–pH diagram can be plotted at 298 K when the
activities of the related compound, ion and gas fugacity are one during calculation.
In the reaction system of EG and Ni(OH)2, the potential equilibrium conditions are as follows:
(1) Reactions corresponding to Ni:
Ni2þðaqÞ þ 2e Ð NiðsÞ
’1 ¼ ꢁ0:23V
(1)
(2)
(3)
NiðOHÞ2ðsÞ þ 2HþðaqÞ Ð Ni2þðaqÞ þ 2H2OðlÞ
pH ¼ 6:40
NiðOHÞ2ðsÞ þ 2e Ð NiðsÞ þ 2OHꢁðaqÞ
’2 ¼ ð0:17 ꢁ 0:059pHÞV
(2) Reactions corresponding to EG:
C4H6O2ðaqÞ þ 4H2OðlÞ þ 2e Ð 2C2H6O2ðlÞ þ 2OHꢁðaqÞ
’3 ¼ ðꢁ0:95 ꢁ 0:059pHÞV
(4)
(5)
(6)
(7)
4CO3 ðaqÞ þ 16H2OðlÞ þ 18e Ð C4H6O2ðaqÞ þ 26OHꢁðaqÞ
ꢁ2
’4 ¼ ðꢁ0:89 ꢁ 0:085pHÞV
H2CO3 Ð HCO3ꢁ þ Hþ
pH ¼ 6:38
HCO3 Ð CO3 þ Hþ
pH ¼ 10:25
ꢁ
ꢁ2
(3) Water stability:
O2 þ 4HþðaqÞ þ 4e Ð H2OðlÞ
’
¼ ð1:229 ꢁ 0:059pHÞV
(a)
(b)
o2
2HþðaqÞ þ 2e Ð H2
’
¼ ðꢁ0:059pHÞV
H2
Fig. 1 shows the E–pH diagram (Pourbaix diagram) of the Ni–EG–H2O system obtained from the above equilibria.
In Fig. 1, the dotted lines labelled a and b represent, respectively, the equilibrium reaction of the reduction of water to

J. Shengming et al. / Materials Research Bulletin 41 (2006) 2130–2136
2133
Fig. 1. E–pH diagram of Ni–EG–H2O system (T = 298 K).
gaseous H2 and of the oxidation of water to O2 when the partial fugacity of H2 or O2 is 1. The lines labelled 4 and 5
2ꢁ
denote, respectively, the equilibrium reaction of the oxidation of EG to diacetyl and the oxidation of diacetyl to CO3
.
Lines 6 and 7 represent the dissociation of H2CO3 and HCO3ꢁ, respectively. The lines labeled 1, 2, and 3 correspond to
the equilibrium between Ni, Ni(OH)2, and Ni2+. The E–pH diagram is divided into the regions of stability of diacetyl,
2ꢁ
EG, H2CO3, HCO3ꢁ, and CO3 by lines 4, 5, 6, and 7.
The E–pH diagram of the Ni–EG–H2O system can be used to theoretically understand the accumulated sum of
experimental findings and predict the oxidation products of EG at different pH values. According to Fig. 1, lines 4 and
5 intersect at pH 2.3, which indicates the variation of the stability of diacetyl and EG. When the pH value is greater than
2.3, the region of diacetyl is below line 4 and thus diacetyl is not thermodynamically stable in the reaction system.
Diacetyl tends to be oxidized completely to H2CO3, HCO3ꢁ, and CO32ꢁ at different pH values by Ni(OH)2. In contrast,
when the pH value is less than 2.3, the diacetyl region is above line 4, and the thermodynamic stability of diacetyl is
greater than that of EG in the reaction system, and an intermediate of diacetyl may form and then be oxidized to form
H2CO3. It is obvious that through thermodynamic considerations, H2CO3 is the product of total oxidation of EG. In
addition, when the pH value is less than 2.3, diacetyl can be obtained. Diacetyl has been detected as the main product
of EG in acidic solutions in the literature [7,15]. It must be pointed out that Fig. 1 may be invalid practically because of
the fact that the reaction temperature is not 298 K but 453 K.
3.2. Characterization of nickel powder
XRD patterns of nickel powder prepared using the polyol process are shown in Fig. 2. Three peaks at 2u = 44.58,
51.88, and 76.48 could be indexed to a face-centered cubic system, and the corresponding Miller indices were marked
for each diffraction peak. No peaks of nickel hydroxides or other impurities were detected, indicating the complete
reduction of nickel hydroxides. The strong and sharp peaks revealed that the nickel particles were well crystalline.
Fig. 3 shows field-emission scanning electron micrographs of the same sample. The micrographs show that spherical
nanometer nickel powders were obtained.
3.3. Analysis of the oxidation product of EG
The IR spectrum of the filtrate is shown in Fig. 4. The peaks may be assigned to three kinds of substance: the
vibration peaks of all kinds of functional groups of EG, the solvent and reactants, the vibration peaks of C O of PVP,
surfactant, and the vibration peaks of the resultant carbonate. The main vibration peaks are assigned as follows: (1) the
experimental bands shown in Fig. 4 have the greatest similarity to the standard IR spectrum of EG; (2) the band at
1641 cmꢁ1 corresponds to the stretching –C O vibration of PVP and the other bands of PVP are overlapped by that of
EG; (3) typical vibration peaks assigned to CO32ꢁ with the structure of a symmetric triangle in a plane (D3h) at 1413

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J. Shengming et al. / Materials Research Bulletin 41 (2006) 2130–2136
Fig. 2. XRD patterns of nickel powders prepared using the polyol process.
Fig. 3. FE-SEM micrographs of the samples in Fig. 2.
Fig. 4. The IR spectrum of the filtrate.

J. Shengming et al. / Materials Research Bulletin 41 (2006) 2130–2136
2135
Fig. 5. XRD patterns of sediment from filtrate.
and 687 cmꢁ1 correspond to asymmetric stretching and in-plane bending of CO32ꢁ, respectively, and the out-of-plane
bend vibration peak at 879 cmꢁ1 is overlapped by the corresponding peak of EG.
However, the band at 1413 and 879 cmꢁ1 could be connected with vinyl-group of PVP and band at 879 cmꢁ1
with valence vibration of C–N. The characterization of sediment from filtrate was carried out by XRD (shown
in Fig. 5).
The results show that the characteristic peaks for CaCO3 are observed, indicating that the CaCO3 sediment come
into being. The reaction formula was as follows when the CaCl2 solution was added into filtrate:
CaCl2 þ NaCO3 ¼ CaCO3 # þ 2NaCl
It is obvious that the complete oxidation of EG in an alkaline solution yielded CO32ꢁ because the typical peaks of
CaCO3 shown in Fig. 5 are in accordance with that predicted from Fig. 1.
4. Conclusions
Nanometer nickel powders have been successfully prepared using the polyol method. The thermodynamics of the
preparation of nanometer nickel powders using the polyol method have been investigated, and the E–pH diagram of
the Ni–EG–H2O system shows that the pH value of 2.3 is the intersection of the oxidation equilibria of EG and
diacetyl. When the pH value is greater than 2.3, diacetyl is not stable thermodynamically in the reaction system, and
complete oxidation of EG occurs directly with Ni(OH)2; when the pH value is less than 2.3, complete oxidation of EG
occurs through an intermediate diacetyl. The typical peaks of CaCO3 shown in the XRD patterns of the sediment from
the filtrate confirm the complete oxidation of EG in an alkaline solution, in accordance with the results obtained from
the E–pH diagram.
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