Synthesis and stabilization of Ni nanoparticles in a pure aqueous CTAB solution

Article abstract of DOI:10.1246/cl.2004.406

Ni nanoparticles (7-20nm) were synthesized by hydrazine reduction in a pure aqueous solution at 25°C using CTAB as a capping agent and an appropriate amount of bases as catalysts. No organic solvents and extra inert gases were necessary. It was suggested that Ni nanoparticles were stabilized by a bilayer structure of CTAB.

Full text of DOI:10.1246/cl.2004.406

406
Chemistry Letters Vol.33, No.4 (2004)
Synthesis and Stabilization of Ni Nanoparticles in a Pure Aqueous CTAB Solution
Szu-Han Wu and Dong-Hwang Chenꢀ
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 701
(Received January 5, 2004; CL-040008)
Ni nanoparticles (7–20nm) were synthesized by hydrazine
cluded that the nanoparticles prepared in this work were pure Ni
of fcc structure.
reduction in a pure aqueous solution at 25 ꢁC using CTAB as a
capping agent and an appropriate amount of bases as catalysts.
No organic solvents and extra inert gases were necessary. It
was suggested that Ni nanoparticles were stabilized by a bilayer
structure of CTAB.
In the past two decades, considerable attention has been de-
voted to the synthesis of metal nanoparticles because of their un-
usual properties and potential applications in optical, electronic,
catalytic, magnetic materials, and so on. Among various kinds of
metal nanoparticles, the preparations of some metal nanoparti-
cles such as Ni, Cu, and Fe are relatively difficult because they
are easily oxidized. Ni nanoparticles have attracted much atten-
tion because of their applications as catalysts and conducting and
magnetic materials. Until now, only few works on the prepara-
tion of Ni nanoparticles have been reported and they usually
were performed in organic media to avoid the formation of nick-
el oxide or hydroxide.1–3
The synthesis of Ni nanoparticles in pure aqueous solutions
is attractive because of the absence of organic pollution. Recent-
ly, we have reported the preparation of Ni nanoparticles in aque-
ous solution of cationic surfactants cetyltrimethylamonium bro-
mide/tetradodecylammonium bromide (CTAB/TC12AB),4
which was usually used for the syntheses of rod-like or fiber-like
Au, Ag, CdS, and CdSe nanoparticles.5–10 However, an elevated
reaction temperature and the addition of acetone were necessary.
In this work, we prepared Ni nanoparticles in a pure aqueous
CTAB solution at 25 ꢁC. The role of CTAB was also examined.
In general, for the synthesis of Ni nanoparticle, an aqueous
solution of 10 mL was first prepared by dissolving CTAB
(0.01 M) and nickel chloride (0.005–0.05 M) in water. Then, hy-
drazine (0.25–2 M) and NaOH solutions (0.02 M) were added in
sequence. At 25 ꢁC, Ni nanoparticles were formed after about
20 h in a capped bottle.
Figure 1 shows the typical transmission electron microscope
(TEM), electron-diffraction pattern and high-resolution TEM
(HRTEM) images of Ni nanoparticles. The TEM image indicat-
ed that these particles essentially were very fine and monodis-
persed with a mean diameter of 15.4 nm and a standard deviation
of 1.9 nm. This revealed that the presence of CTAB indeed re-
stricted the growth of Ni nanoparticles efficiently. The elec-
tron-diffraction pattern showed three fringe patterns with plane
ꢀ
distances of 2.03, 1.76, 1.02 A. They related to the (111),
(200), (222) planes of pure face-centered cubic (fcc) Ni. From
ꢀ
HRTEM, the interreticular distance was found to be 2.06 A. It re-
lated to the (111) plane of pure fcc Ni. In addition, the X-ray dif-
fraction (XRD) analysis also indicated three characteristic peaks
for Ni (2ꢀ ¼ 44:5, 51.8, and 76.4), corresponding to their indices
((111), (200), and (222)). No oxides or hydroxide such as NiO,
Ni2O3, and Ni(OH)2 were formed. Accordingly, it could be con-
Figure 1. TEM, electron-diffraction pattern, and HRTEM im-
ages of Ni nanoparticles. [NiCl2] ¼ 25 mM; [N2H5OH] ¼
0:25 M; [CTAB] ¼ 0:01 M; 25 ꢁC.
Similar to our previous work,3,4 it was notable that the addi-
tion of NaOH was necessary for the formation of Ni nanoparti-
cles. In this work, it was found that other bases such as KOH,
Ba(OH)2, or tetraethylammonium hydroxide also led to the same
result. The additions of these bases resulted in the increase of so-
lution pH from 10.2 to 10.6. However, no Ni nanoparticles were
formed if much more hydrazine was added to raise the solution
pH up to 10.6. This reveals that theses bases probably acted as
catalysts instead of a simple role to adjust the solution pH. Fur-
ther investigation is necessary.
Furthermore, no matter whether the synthesis reaction was
performed in an inert atmosphere (N2 gas) or not, only metallic
Ni nanoparticles were obtained. Since N2 gas might be generated
to create an inert atmosphere automatically via the formation re-
action and the catalytic decomposition of excess hydrazine by
the resultant Ni nanoparticles,3 the input of extra N2 gas was
not necessary.
By
a superconducting quantum interference device
(SQUID) magnetometer, the saturation magnetization (Ms), re-
manent magnetization (Mr), and coercivity (Hc) of the resultant
Ni nanoparticles at 25 ꢁC were determined to be 31.8 emu/g,
1.86 emu/g, and 26.8 Oe, respectively. In addition, at an applied
field of 10 kOe, the magnetization of Ni nanoparticles increased
with the decrease of temperature at 5–400 K owing to the de-
crease in thermal energy. All these magnetic properties reflected
the nature of nanoparticles.2–4
The size of Ni nanoparticles decreased with decreasing nick-
el chloride concentration or increasing hydrazine concentration.
When [N2H5OH] ¼ 0:4 M, the mean diameters of Ni nanoparti-
cles (20–9 nm) decreased with the decrease of nickel chloride
concentration (0.05–0.01 M) and remained unchanged at
[NiCl2] < 0:01 M. At [NiCl2] ¼ 0:025 M, the mean diameters
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.4 (2004)
407
of Ni nanoparticles (15–10 nm) decreased with the increase of
hydrazine concentration (0.25–0.5 M), and remained at 10 nm
when [hydrazine] > 1:0 M. Both the concentration effects re-
vealed that the size of Ni nanoparticles was not affected by hy-
drazine concentration when [N2H5OH]=[NiCl2] > 40. These
phenomena were similar to those observed in other media.
Since CTAB is a cationic surfactant, nickel ions would not
be adsorbed on the micelles. Hence, it was suggested that Ni
nanoparticles were capped by CTAB molecules after they were
formed. First, the Fourier transform infrared (FTIR) spectrum of
the resultant Ni nanoparticles and free CTAB were studied. The
resultant Ni nanoparticles were washed several times with alco-
hol and water to remove free CTAB before the measurement. As
shown in Figure 2, the resultant Ni nanoparticles and free CTAB
had the similar absorption bands. Since Ni has no characteristic
absorption bands in the examined wavenumber range, this re-
vealed that CTAB was indeed capped on the surface of Ni nano-
particles. In addition, the resultant Ni nanoparticles did not ex-
hibit any absorption bands different from those of free CTAB.
This implied that CTAB might be just adsorbed on the surface
of Ni nanoparticles, not decomposed catalytically by Ni nano-
particles as observed in the case of ethylene glycol.3 For the syn-
thesis of metal nanoparticles in ethylene glycol, extra protective
polymers were usually necessary because ethylene glycol was
not an efficient surfactant or capping agent and could be used just
as a solvent or a reducing agent. In the case of Ni nanoparticles,
the adsorbed ethylene glycol was decomposed catalytically by
Ni nanoparticles and self-created a protective layer on the sur-
face.3,11 Thus, compared to ethylene glycol, the surfactant
CTAB could be used directly as an efficient capping agent for
the synthesis of metal nanoparticles.
120
80
40
0
(a)
(b)
0
200 400 600 800 1000
Teamperature / oC
Figure 3. TGA curves for CTAB (b) and CTAB-capped
Ni nanoparticles (a) obtained at [NiCl2] ¼ 0:025 M,
[N2H5OH] ¼ 0:25 M, and [CTAB] ¼ 0:01 M.
to the outer layer, whose headgroups were in the aqueous solu-
tion, through hydrophobic interaction. On the basis of the sug-
gestion, the first and second weight-loss steps in the TGA curve
might be attributed to the releases of the outer and inner layers,
respectively. Similar phenomenon and suggestion were also re-
ported in the assembly of cationic surfactants on the surface of
Au nanorods.12
In conclusion, Ni nanoparticles were synthesized in a pure
aqueous solution at 25 ꢁC. No extra inert gases were necessary
and the formation of a bilayer structure of CTAB was suggested.
Compared to our previous works,2–4 both elevated temperature
and organic solvent were unnecessary. Also, CTAB was used
directly as an efficient capping agent, not to form micelles or
reverse micelles for the stabilization of Ni nanoparticles. The
simple method will be helpful for the production of Ni nanopar-
ticles.
(a)
We are grateful to the National Science Council of the
Republic of China for the support of this research (Contract
No. NSC 91-2214-E006-019)
(b)
References
1
L. K. Kurihara, G. M. Chow, and P. E. Schoen, Nanostruct.
Mater., 5, 607 (1995).
2
3
D. H. Chen and S. H. Wu, Chem. Mater., 12, 1354 (2000).
S. H. Wu and D. H. Chen, J. Colloid Interface Sci., 259, 282
(2003).
4000
3000
2000
1000
0
Wavenumber/cm-1
4
D. H. Chen and C. H. Hsieh, J. Mater. Chem., 12, 2412
(2002).
Figure 2. FTIR spectra of the resultant Ni nanoparticles (a) and
free CTAB (b).
5
6
T. Pal, T. K. Sau, and N. R. Jana, Langmuir, 13, 1481 (1997).
Y. Y. Yu, S. S. Chang, C. L. Lee, and C. R. C. Wang, J. Phys.
Chem. B, 101, 6661 (1997).
C. C. Chen, C. Y. Chao, and Z. H. Lang, Chem. Mater., 12,
1516 (2000).
N. R. Jana, L. Gearheart, and C. J. Murphy, Adv. Mater., 13,
1389 (2001).
B. Nikoobakht and M. A. El-Sayed, Langmuir, 17, 6368
(2001).
To further investigate the stabilization mechanism of Ni
nanoparticles by CTAB, thermogravimetric analysis (TGA)
was conducted. The TGA curves for CTAB and CTAB-capped
Ni nanoparticles are indicated in Figure 3. In the case of CTAB,
the weight loss occurred sharply from 250 to 350 ꢁC, revealing
CTAB molecules were decomposed thermally within this tem-
perature range. However, in the case of CTAB-capped Ni nano-
particles, the TGA curve shows two weight loss steps. The first
weight-loss step occurred sharply within 250–400 ꢁC and the
second one appeared gradually from 400 to 500 ꢁC. This implied
that CTAB molecules were present on the surface of Ni nanopar-
ticles in two different forms. Thus, it could be suggested that a
bilayer structure of CTAB was formed: the inner layer was
bound to the Ni surface via the headgroups and was connected
7
8
9
10 A. Kameo, A. Suzuki, K. Torigoe, and K. Esumi, J. Colloid
Interface Sci., 241, 289 (2001).
11 D. H. Chen, J. P. Lin, S. H. Wu, and C. T. Wang, Chem. Lett.,
32, 662 (2003).
12 B. Nikoobakht and M. A. El-Sayed, Langmuir, 17, 6368
(2001).
Published on the web (Advance View) March 6, 2004; DOI 10.1246/cl.2004.406