Preparation of nanocrystalline nickel powders through hydrothermal-reduction method

Article abstract of DOI:10.1016/S0025-5408(98)00180-9

Nanocrystalline nickel powders were prepared in aqueous solution through a thermal-reduction process. The reductive atmosphere was produced from the disproportionation of white phosphorus in basic solution at 100°C. X-ray diffraction (XRD), transmission electron microscopy (TEM), and chemical analysis were employed to characterize the products. XRD revealed that the nickel was single phase. TEM showed that the average particle size was about 26 nm. The purity of the as-prepared nickel was 99.6%, as determined from the spectrophotoscopy. The possible formation mechanism of nanocrystalline nickel is a reduction of nanocrystalline Ni(OH)₂.

Full text of DOI:10.1016/S0025-5408(98)00180-9

Materials Research Bulletin, Vol. 33, No. 12, pp. 1747–1751, 1998
Copyright © 1999 Elsevier Science Ltd
Printed in the USA. All rights reserved
0025-5408/98 $19.00 ϩ .00
PII S0025-5408(98)00180-9
PREPARATION OF NANOCRYSTALLINE NICKEL POWDERS THROUGH
HYDROTHERMAL-REDUCTION METHOD
C. Wang, X.M. Zhang, X.F. Qian, Y. Xie, W.Z. Wang, and Y.T. Qian*
Department of Chemistry and Structure Research Laboratory, University of Science and
Technology of China, Hefei, Anhui 230026, P.R. China
(Refereed)
(Received February 18, 1998; Accepted April 22, 1998)
ABSTRACT
Nanocrystalline nickel powders were prepared in aqueous solution through a
thermal-reduction process. The reductive atmosphere was produced from the
disproportionation of white phosphorus in basic solution at 100°C. X-ray
diffraction (XRD), transmission electron microscopy (TEM), and chemical
analysis were employed to characterize the products. XRD revealed that the
nickel was single phase. TEM showed that the average particle size was about
26 nm. The purity of the as-prepared nickel was 99.6%, as determined from
the spectrophotoscopy. The possible formation mechanism of nanocrystalline
nickel is a reduction of nanocrystalline Ni(OH)2. © 1999 Elsevier Science Ltd
KEYWORDS: A. metals, B. chemical synthesis, C. X-ray diffraction
INTRODUCTION
Hydrothermal synthesis is an important method of preparing nanocrystalline materials,
especially for advanced ceramic powders [1]. Fine metal particles of uniform size and shape
of high purity are of technological importance in the electronics industry and are used in
conducting inks, paints, and adhesives [2]. For the preparation of nanosized metal particles,
ball milling [3], pulsed electrodeposition (PED) [4,5], thermal plasma [5,6], polyol process
[7–9], gas deposition method (GDM) [10], ␥-radiation [11], and many other methods have
been utilized. Although nanosized nickel powder has been prepared by some of these
methods, special apparatus [11] or complicated treatment is required.
*To whom correspondence should be addressed.
1747

1748
C. WANG et al.
Vol. 33, No. 12
In this paper, we report a hydrothermal-reduction method for the preparation of nanoc-
rystalline nickel powder. This synthesis method involves the Ni(OH)2 reacting with the
reductive atmosphere produced from the disproportionation of white phosphorus in basic
solution.
EXPERIMENTAL
For preparing nickel, about 4.0 g NaOH, 1.4 g NiCl2⅐6H2O, and 0.9 g white phosphorus were
added to 80 mL distilled water in a stainless autoclave with a Teflon liner of 100-mL
capacity. The autoclave was heated in an oven at the temperature of 100°C for 24 h. The
product was washed with hydrochloric acid in the pH range of 2–4 and distilled water, then
dried at 60°C in a vacuum drier. For Ni(OH)2, a similar experiment was conducted, without
the addition of white phosphorus and washing with hydrochloric acid.
The final products were characterized by XRD, transmission electron microscopy (TEM),
and chemical analysis. The XRD patterns were recorded on a Rigaku Dmax ␥-A diffrac-
tometer with Cu K␣ radiation (␭ ϭ 1.54178 Å). TEM images of the nanocrystalline powders
were taken with a Hitachi H-800 transmission electron microscope. The purity of nanocrys-
talline nickel powder was determined by diacetyldioxine spectrophotometry at a wavelength
of 530 nm in a 721 Spectrophotometer (Shanghai).
RESULTS AND DISCUSSION
Black nickel powders obtained in the experiment were prepared from a mixture of Ni(OH)2
and Ni by dissolving Ni(OH)2 in hydrochloric acid with pH in the range of 2–4. The XRD
pattern of the mixture is shown in Figure 1a. The XRD pattern of the nanocrystalline nickel
powders is shown in Figure 1b. The broadening in diffraction peaks shows that the products
are very small particles. An average particle size of 20 nm was calculated from the half-width
of the peaks of the XRD pattern, using the Scherrer formula [13]. Pure phase (Fig. 1c) of
Ni(OH)2 was also obtained in the experiment without addition of white phosphorus as a
reductive agent. In Figures 1a and c, one can see that the peak widths of the (100) and (110)
reflections of Ni(OH)2 are not as broad as the others. This is due to the anisotropic shape of
the crystallites, which are smaller in the c direction relative to the a and b directions.
A TEM image of the nickel powder is shown in Figure 2a. This image shows spherical
crystallites with an average size of about 26 nm, which is in agreement with that calculated
with the Scherrer formula. The ED image in Figure 2b shows a cubic phase of pure nickel
powder with a ϭ 9.5235, which is close to that reported by JCPDS 4-850. A TEM image of
the Ni(OH)2 powder is shown in Figure 2c. Square shapes of about 50 nm ϫ 50 nm can be
seen in this TEM image. The purity of the nickel, as determined by diacetyldioxine spec-
trophotometry at a wavelength of 530 nm [12], is 99.6%.
For the full disproportionation of white phosphorus, the mole amount of NaOH should be
in excess of triple that of white phosphorus. Otherwise, NiP2 would form and could not be
removed from the final product. When all the reagents were added to the distilled water in
the autoclave, the following chemical process occurred and amorphous Ni(OH)2 formed
NiCl2 ϩ 2NaOH 3 Ni(OH)2 ϩ 2NaCl
(1)

Vol. 33, No. 12
NANOCRYSTALLINE NICKEL
1749
FIG. 1
The XRD patterns of the products: (a) XRD pattern of a mixture of Ni and Ni(OH)2; (b) XRD
pattern of nanocrystalline nickel powder; and (c) XRD pattern of nanocrystalline Ni(OH)2.
Asterisk (*) denotes peaks of nickel.

1750
C. WANG et al.
Vol. 33, No. 12
FIG. 2
TEM and ED images of the products: (a) TEM image of nanocrystalline nickel powder; (b)
ED image of nickel powder; and (c) TEM image of nanocrystalline Ni(OH)2.
With the elevation of temperature, the crystallization of Ni(OH)2 and disproportionation of
white phosphorus occurred. After heating in the oven at 100°C for 24 h, a mixture of Ni(OH)2
and Ni was produced. Hydrochloric acid was used to dissolve the unreacted Ni(OH)2. (High
concentration acid should be avoided, since it can dissolve the nickel. The favorable range
of the pH of the acid is 2 to 4.) The products were then washed with distilled water and dried
at 60°C in a vacuum drier. The overall formation process of nickel in this system can be
described by the following equations [2,3]:
P4 ϩ (8n Ϫ 4)OHϪ 3 4HPOn2Ϫ ϩ (4n Ϫ 4)H2O ϩ (8n Ϫ 12)e (n ϭ 3, 4)
(2)
Ni(OH)2 ϩ 2e 3 Ni ϩ 2OHϪ
(3)
The formation of nickel can also be the reduction of Ni2ϩ ionized from Ni(OH)2 in the
solution. We prefer this explanation. From Figure 1c, we can see that the half-widths of peaks
of the nickel metal are very close to those of Ni(OH)2, with the exception of the (100) and
(110) peaks. The same phenomenon can be seen between 100 and 180°C for this system. The
explanation for this might be that the redox reaction tends to occur on the 001 faces of
nanocrystalline Ni(OH)2. With the elevation of temperature, the sizes of Ni(OH)2 and Ni
become larger. The high temperature and the 24-h reaction time ensure the entire dispro-
portionation of white phosphorus.
CONCLUSION
A new hydrothermal-reduction method to prepare nanocrystalline nickel with an average size
of 26 nm at 100°C was introduced. The reductive agents were produced from dispropor-
tionation of white phosphorus in basic solution.

Vol. 33, No. 12
NANOCRYSTALLINE NICKEL
1751
ACKNOWLEDGMENTS
This work was supported by the Climbing Program Research–National Foundation Project
and the Chinese National Foundation of Natural Science Research.
REFERENCES
1. W.J. Dawson, Ceram. Bull. 67, 673 (1988).
2. C. Ducamp-Sanquesa, R. Herrara-Urbina, and M. Figlarz, J. Solid State Chem. 100, 272 (1992).
3. C. Dolitis and W.L. Johnson, J. Appl. Phys. 60, 1147 (1986).
4. H. Natter and R. Hempelmann, J. Phys. Chem. 100, 19525 (1996).
5. A. Kato, Ceramics 19, 478 (1984).
6. Y. Morrysohi, M. Futaki, S. Komatsu, and T. Ishigaki, J. Mater. Sci. Lett. 16, 347 (1997).
7. R. Seshadri and C.N.R. Rao, Mater. Res. Bull. 29, 795 (1994).
8. S. Puvvada, S. Bural, G.M. Chow, S.B. Qudri, and B.R. Ratna, J. Am. Chem. Soc. 116, 2135
(1994).
9. S. Ayyappam, R. Srinivasa Gopalan, G.N. Subbana, and C.N.R. Rao, J. Mater. Res. 12, 398
(1997).
10. M. Ichiki, J. Akedo, K. Mori, and Y. Ishikawa, J. Mater. Sci. Lett. 16, 531 (1997).
11. Y.J. Zhu, Y.T. Qian, M.W. Zhang, and Z.Y. Chen, J. Mater. Sci. Lett. 13, 1243 (1994).
12. L.Q. Li, C. Wang, and Y.D. Li, J. Membr. Sci. 135, 173 (1997).
13. C.W.J. Wagner and E.N. Aqua, Adv. X-ray Anal. 7, 46 (1964).