Facile synthesis of a metastable nanocrystalline Ni3N from nickel nanoparticle

Article abstract of DOI:10.1016/j.jallcom.2007.11.031

Nanoparticle has been a hot topic because of its excellent properties comparing to the corresponding bulk material in many aspects. Here we described a successful use of nickel nanoparticle prepared by a reduction of Ni²⁺ with hydrazine to synthesize metastable nanocrystalline Ni₃N at 513 K in NH₃ flow. In comparison, a nickel precursor prepared by a reduction of NiO in hydrogen was not nitrided when it was treated with NH₃ at 533 K. The samples were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). The XRD patterns showed that the nickel particles prepared by hydrazine reduction had a smaller crystalline size (18.3 nm) than that of nickel particles from hydrogen reduction (49.5 nm), resulting in a low nitriding temperature. As-synthesized Ni₃N had spherical morphology and average particle size of ～30 nm from TEM observation. The temperature-programmed decomposition of Ni₃N showed that it began to decompose at 540 K and was finally transformed into metallic nickel.

Full text of DOI:10.1016/j.jallcom.2007.11.031

Journal of Alloys and Compounds 466 (2008) 352–355
Facile synthesis of a metastable nanocrystalline Ni3N from nickel
nanoparticle
Zhiqiang Wang, Wenjing Yu, Juan Chen, Minghui Zhang∗, Wei Li, Keyi Tao
Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, PR China
Received 29 August 2007; received in revised form 4 November 2007; accepted 11 November 2007
Available online 19 November 2007
Abstract
Nanoparticle has been a hot topic because of its excellent properties comparing to the corresponding bulk material in many aspects. Here we
described a successful use of nickel nanoparticle prepared by a reduction of Ni2+ with hydrazine to synthesize metastable nanocrystalline Ni3N at
513 K in NH3 flow. In comparison, a nickel precursor prepared by a reduction of NiO in hydrogen was not nitrided when it was treated with NH3 at
533 K. The samples were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). The XRD patterns showed that
the nickel particles prepared by hydrazine reduction had a smaller crystalline size (18.3 nm) than that of nickel particles from hydrogen reduction
(49.5 nm), resulting in a low nitriding temperature. As-synthesized Ni3N had spherical morphology and average particle size of ∼30 nm from TEM
observation. The temperature-programmed decomposition of Ni3N showed that it began to decompose at 540 K and was finally transformed into
metallic nickel.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Nitride materials; Transmission electron microscopy; X-ray diffraction; Amorphous materials; Thermal analysis
1. Introduction
electrode materials and superconductor materials [6–9]. Ni3N
is a typical metastable nitride among them. Up to date, several
preparation methods have been reported, including rapid and
exothermic solid-state metathesis (SSM) reactions, supercritical
synthesis and laser-aided synthesis, etc. [10–14]. For example,
SSM and supercritical synthesis do not need higher temperature
than 500 K, but always introduce the by-product as the impuri-
ties and need further treatment. Nickel nitrides have also been
prepared by sputtering of nickel films in Ar/N2 mixed gas, as
well as through ion bombardment of nickel films using N+ ions
[15,16]. These methods mentioned above either need rigorous
synthesisconditionsorinevitablybringimpuritiesfromthestart-
ing materials or substrates. The Ni3N can be formed when nickel
metal is heated in a NH3 flow at 781 K [17], but a phase-pure
Ni3N cannot be obtained. So, it is necessary to develop a facile
method to synthesize metastable nitrides at low temperature.
In this work, we described a successful use of nanoparticle
nickel prepared by the reduction of Ni2+ with hydrazine to syn-
thesize nanocrystalline Ni3N at 513 K. The further study showed
that when large crystalline size of Ni precursor prepared by the
reduction of NiO in a H2 flow was performed, nickel nitride
could not be formed even at 533 K. While an amorphous Ni(B)
prepared by the reduction of Ni2+ by KBH4 was nitrided in a
Recently, metal nitrides have been extensively studied for
their particular properties and wide applications in catalysts and
optoelectronic materials [1–4]. The researches mainly focused
on the materials with high melting point, for example GaN, TiN,
Mo2N, WN VN, etc., while the thermal unstable metal nitrides
(e.g., Ni, Co, Cu, Ru Pt, etc.) and their synthesis routes were less
reported because those nitrides mostly decomposed to pure met-
als at a quite low temperature. So, phase-pure metastable nitride
could not be prepared through nitriding metal powders in NH3
flow because of their thermal instability. Those metastable mate-
rials have many properties that manifest themselves in variety
of ways, including, magnetic interactions, small band gap semi-
conductor behavior and reactive catalytic surface structures [5].
They are the potential optical, electrode, magnetic and catalytic
materials, meanwhile some of them are amenable to reductive
lithiation or intercalation reactions forming compounds such
as LixM3N (M: Ni, Co and Cu) that have potential as battery
∗
Corresponding author. Tel.: +86 22 23507730; fax: +86 22 23507730.
E-mail address: zhangmh@nankai.edu.cn (M. Zhang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.11.031

Z. Wang et al. / Journal of Alloys and Compounds 466 (2008) 352–355
353
NH3 flow, it resulted in Ni3N and Ni3B as impurity. The profile
of temperature-programmed decomposition showed that the as-
synthesized metastable Ni3N began to decompose and release
N2 at ∼540 K, and the metallic nickel was finally formed at
a higher temperature. This showed that the crystalline size of
nickel precursor had an important effect on the nitriding process
and composition of final product. The research provided a facile
route to prepare phase-pure metastable Ni3N at mild condition
and relatively low temperature.
2. Experimental
All reagents such as hydrazine (80 wt.%), Ni(NO3)2·6H2O and KBH4 were
used as received without further purification. The preparation of nickel nanopar-
ticles by the reduction of Ni2+ with hydrazine was similar to Yu’s [18] and
Boudjahem’s [19]. In a typical experiment, 6.88 g Ni(NO3)2·6H2O was dis-
solved in 150 mL water under stirring. When the solution was heated to 333 K,
15 mL hydrazine was introduced to the solution. Immediately, 2.3 g sodium
hydroxide was added to the resulting solution. After 15 min, a black precipi-
tation generated and a mass of gas bubbles were observed. Until no obvious
bubbles were observed, the final precipitate was collected, and washed by water
then by ethanol several times and stored in ethanol. This nickel precursor was
denoted as Ni(N2H4). Amorphous Ni(B) precursor was synthesized by the way
as follows: 6.88 g Ni(NO3)2·6H2O was dissolved in 150 mL water under stirring,
then 10 mL 2 M KBH4 was added to the solution by stepwise dropping at room
temperature. When KBH4 was introduced, a precipitation occurred at once. The
precipitation was washed and stored as described above. Ni(H2) precursor was
prepared by the reduction of nickel oxide in H2 at 873 K for 2 h. Nickel oxide
was gained by calcinations of Ni(NO3)2 in the air at 773 K for 5 h. In the typical
nitriding process, Ni(N2H4), Ni(B) and Ni(H2) samples were heated in a reactor
under a flow of NH3 at a rate of 30 mL/min. The temperature was increased
linearly at a rate of 10 K/min from room temperature to 533 K and then kept at
533 K for 5 h. All the products were naturally cooled to room temperature under
NH3 and passivated for 10 h in a flow of 1% (v/v) O2/N2 mixed gas.
Powder XRD patterns were obtained using a D/max-2500 powder X-ray
diffractometer (Cu K␣ radiation). Data acquisition, peak identification and data
plotting were accomplished using a Data General Computer System. Transmis-
sion electron microscope (TEM) images were acquired using a Tecnai G2 T20
operating at an accelerating voltage of 200 kV. The temperature-programmed
decomposition of Ni3N was measured as follows: the sample was placed in
a quartz tube and vacuumed at room temperature for 3 h, then switched to
argon flow with 20 mL/min for 30 min. The system was heated linearly at a
rate of 10 K min−1, and the decomposition product was detected by thermal
conductivity detector (TCD).
Fig. 1. Unit cell structure of Ni3N. Ni atoms are shown as grey spheres and N
atoms are shown as black spheres.
fcc Ni (1 1 1), (2 0 0) and (2 2 0) crystallographic planes accord-
ing to JCPDS 04-0850. When nitriding at 473 and 493 K, the full
widths at half maximum (FWHM) of the peak at 2θ = 44.4◦ are
little smaller than that of nanocrystalline Ni(N2H4) (Fig. 2b and
c). It indicates that the crystalline size of Ni(N2H4) increases
at 473 and 493 K, but the nanocrystalline Ni(N2H4) still can
not be nitrided by NH3. Six diffraction peaks locating at about
2θ = 38.8◦, 41.8◦, 44.3◦, 58.5◦, 70.4◦ and 78.3◦ are observed
at the nitriding temperature of 513 K (Fig. 2d), which could be
identified to the (1 1 0), (0 0 2), (1 1 1), (1 1 2), (3 0 0) and (1 1 3)
crystallographic planes of hexagonal Ni3N, respectively. Mean-
while the corresponding peak intensity of nickel at 2θ = 44.4◦
was weaker than that of Ni(N2H4) (Fig. 2a). When the nitrid-
ing temperature was increased to 533 K (Fig. 2e), the diffraction
peaks of Ni disappeared and a single phase Ni3N was observed
[6,13]. This suggests that a phase-pure Ni3N can be successfully
synthesized through nitriding nanocrystalline nickel at 533 K
under atmospheric pressure. The as-synthesized Ni3N has lattice
3. Results and discussion
The Ni3N has an inverse-ReO3 hexagonal structure with one
Ni3N per unit cell and is less dense than the elemental nickel
face-centeredcubic(fcc)structurebecausetheNatomsareintro-
duced into the cell (Fig. 1). The bond of Ni N is close to the
covalent bond rather than ionic bond in the nitride. The thermal
instability of Ni3N is also relative to the weak covalent bond of
Ni N [20].
The reaction between Ni and NH3 can be described as fol-
lows:
3Ni + NH3 = Ni3N + 3/2H2
The powder X-ray diffraction patterns of nanocrystalline
Ni(N2H4) and its nitriding products prepared at different tem-
peratures are shown in Fig. 2. Three diffraction peaks of
nanocrystalline Ni(N2H4) (Fig. 2a) are corresponding to typical
Fig. 2. Powder X-ray diffraction patterns of as-synthesized Ni(N2H4) (a) and
nitriding products at 473 K (b), 493 K (c), 513 K (d), 533 K (e) for 5 h.

354
Z. Wang et al. / Journal of Alloys and Compounds 466 (2008) 352–355
Fig. 3. Powder X-ray diffraction patterns of amorphous Ni(B) (a) and Ni(B)
nitrided in the NH3 flow at 533 K for 5 h (b), where ‘*’ stands for impurity-Ni3B.
Fig. 4. The powder X-ray diffraction patterns of Ni(N2H4) (a), Ni(H2) (b),
nitriding product of Ni(N2H4) (c) and nitriding product of Ni(H2) (d) in the
NH3 flow at 533 K for 5 h.
˚
˚
parameters of a = 4.628 A and c = 4.309 A. Those parameters are
calculated from XRD patterns and well match with the reported
˚
˚
values of a = 4.621 A and c = 4.304 A (JCPDS 10-0280).
The amorphous Ni(B) was also used as another nitrid-
ing precursor, which was synthesized by reducing Ni2+ with
KBH4 at room temperature. The amorphous Ni(B) belongs
to a class of material with long-range disordered but short-
range ordered structure, and has a high concentration of
highly unsaturated coordinative sites. As shown in Fig. 3a,
the Ni(B) is of amorphous structure as a broad diffraction
peak around 45◦ is observed [21]. When the amorphous
Ni(B) was nitrided in a NH3 flow at 533 K, the Ni3N with
Ni3B as an impurity was observed (Fig. 3b). Deng and
coworkers [22] had confirmed that an amorphous structure
of Ni(B) could be maintained even heating at 576 K. In our
experiment, the nitriding temperature is far below the crys-
tallining temperature of Ni(B). However, a phase-pure Ni3N
could not be obtained because of the existence of boron
element, which resulted in Ni3B phase separating out at
573 K.
lated by Scherrer’s formula from the line broadening, and the
results are listed in Table 1. The crystalline size of Ni(N2H4)
(18.3 nm) is smaller than that of Ni(H2) (49.5 nm). Lu and
coworkers demonstrated that iron plate, the surface of which
was refined at the nanometer scale by surface mechanical attri-
tion treatment, could be nitrided at 573 K [23]. It was pointed
out that, nanostructures have high density of non-equilibrium
defects and store an excess Gibbs free energy in the grain
boundaries to constitute a thermodynamic extra driving force
for a nitride formation. In terms of the nitridation of nickel,
Ni(N2H4) precursor with the smaller crystalline size will store
higher excess free energy which can promote the formation
of metastable Ni3N at a lower temperature below its decom-
position temperature, and Ni(H2) with a larger crystalline size
cannot be nitrided at the same temperature. For the amorphous
Ni(B), a disordering structure and a large number of defects
may also store abundant excess free energy. Using amorphous
Ni(B) as the nitriding precursor, the extra driving force stored in
the amorphous material enables the formation of Ni3N at a low
temperature.
For comparison, a Ni(H2) prepared by the reduction of NiO
in a H2 flow was used as nitriding precursor. The XRD patterns
of Ni(H2) and its nitriding product are shown in Fig. 4. It can
be seen that the Ni(H2) precursor possesses the fcc structure,
and is not nitrided even at 533 K for 5 h. The XRD patterns of
Ni(N2H4) and its final nitrided product at the same nitriding
conditions are also presented in Fig. 4. The lattice parameters
calculated from XRD patterns are a = 3.525 A for Ni(N2H4)
and a = 3.524 A for Ni(H2), respectively. They are in agree-
ment with the expected values a = 3.525 A (JCPDS 701849).
Fig. 5 shows the TEM micrograph of Ni3N particles nitrided
from Ni(N2H4) at 533 K. These particles are spherical and
uniform in distribution with the average size of 30 nm. The
aggregation of Ni3N particles is observed, which can be
ascribed to the strong interaction among the magnetic dipoles of
particles.
˚
˚
˚
Table 1
The Ni(N2H4) can be nitrided to Ni3N, while the Ni(H2) can-
not be nitrided at the same temperature. Both precursors are
all fcc structures which can be confirmed by the powder X-ray
diffraction patterns, while the products are different. This dif-
ference is caused by the crystalline sizes of precursors, which
can be seen from Fig. 4. The crystalline size of Ni(N2H4) is
smaller than that of the Ni(H2) because it has larger FWHM
of XRD peaks. The crystalline sizes of the samples are calcu-
The properties of precursors and final products
The precursor
The crystalline size of
precursor calculated from
XRD patterns
The composition
of products at
533 K
Ni(N2H4)
Ni(B)
Ni(H2)
18.3 nm
Amorphous
49.5 nm
Ni3N
Ni3N + Ni3B
Ni

Z. Wang et al. / Journal of Alloys and Compounds 466 (2008) 352–355
355
4. Conclusions
A facile route to synthesize Ni3N at a low temperature of
533 K is developed. In comparison with Ni(H2) and amorphous
Ni(B), a phase-pure Ni3N could be formed from the Ni(N2H4)
by nitriding with NH3 under a mild condition. TEM micro-
graph shows that the as-synthesized Ni3N has an average particle
size of ∼30 nm. Temperature-programmed decomposition study
shows that Ni3N is a metastable material, and it begins to decom-
pose at 540 K and finally converts to metallic Ni. It can be
predicted that such a nitriding method based on nanoparticle
metal with the small crystalline size can be expanded to synthe-
size other metastable nitrides at a reducing temperature below
its decomposition temperature.
Acknowledgements
The authors thank support from the National Natural Science
Foundation of China (Grant 20403009) and the Key Project of
Chinese Ministry of Education (Grant 105045).
Fig. 5. TEM micrograph of Ni3N nanoparticles obtained at 533 K in NH3 flow.
References
[1] P. Kroll, Phys. Rev. Lett. 90 (2003) 125501–125504.
[2] S.H. Jhi, J. Ihm, S.G. Louie, M.L. Cohen, Nature 399 (1999) 132–134.
[3] W. Li, Z.Q. Wang, M.H. Zhang, K.Y. Tao, Catal. Commun. 6 (2005)
656–660.
[4] E. Gregoryanz, C. Sanloup, M. Somayazulu, J. Badro, G. Fiquet, H.K. Mao,
R.J. Hemley, Nat. Mater. 3 (2004) 294–297.
[5] J.C. Crowhurst, A.F. Goncharov, B. Sadigh, C.L. Evans, P.G. Morrall, J.L.
Ferreira, A.J. Nelson, Science 311 (2006) 1275–1278.
[6] F. Gulo, A. Simon, J. Kohler, R.K. Kremer, Angew. Chem. Int. Ed. 43
(2004) 2032–2034.
[7] M. Nishijima, T. Kagohashi, Y. Takeda, M. Imanishi, O. Yamamoto, J.
Power Sources 68 (1997) 510–514.
[8] T. Shodai, S. Okada, S. Tobishima, J. Yamaki, J. Power Sources 68 (1997)
515–518.
[9] Y. Wang, Z.W. Fu, X.L. Yue, Q.Z. Qin, J. Electrochem. Soc. 151 (2004)
E162–E167.
[10] K. Peter, J. Solid State Chem. 176 (2003) 530–537.
[11] D. Sophie, A. Cyril, L. Anne, W. Franc¸ois, G. Ste´phane, E. Jean, C.
Franc¸ois, J. Mater. Chem. 14 (2004) 228–232.
Fig. 6. The profile of Ni3N temperature-programmed decomposed in Ar flow.
[12] A. Leineweber, H. Jacobs, S. Hull, Inorg. Chem. 40 (2001) 5818–5822.
[13] N.S. Gajbhiye, R.S. Ningthoujarn, J. Weissmuller, Phys. Stat. Sol. A 189
(2002) 691–695.
[14] A. Leinenweber, H. Jacobs, W. Kockelmann, S. Hull, D. Hinz-Hubner, J.
Alloy Compd. 384 (2004) 1–2.
Ni3N is a thermal unstable material and is decomposed to
metallic nickel and nitrogen gas, when it is heated above its
decomposition temperature. The reaction can be described as
follows:
[15] I.M. Neklyudov, A.N. Morozov, Physica B 350 (2004) 325–337.
[16] M. Kawamura, Y. Abe, K. Sasaki, Vacuum 59 (2000) 721–726.
[17] R. Juza, W. Sachsze, J. Anorg. Allg. Chem. 251 (1943) 201–212.
[18] K. Yu, D.J. Kim, H.S. Chung, H. Liang, Mater. Lett. 57 (2003) 3992–3997.
[19] A.G. Boudjahem, S. Monteverdi, M. Mercy, M.M. Bettahar, J. Catal. 221
(2004) 325–334.
[20] C. Guillaume, J.P. Morniroli, D.J. Frost, G. Serghiou, J. Phys.: Condens.
Matter 18 (2006) 8651–8660.
[21] R.B. Zhang, F.Y. Li, N. Zhang, Q.J. Shi, Appl. Catal. A: Gen. 239 (2003)
17–23.
2Ni3N = 6Ni + N2
Temperature-programmed decomposition was measured to
study its thermal stability. As shown in Fig. 6, the Ni3N begins
todecomposeat540 K. Thisindicatesthatthesynthesistempera-
tureofNi3Nshouldnotbehigherthan540 K. Thedecomposition
rate achieves the maximum at 827 K. The final solid is fcc struc-
ture Ni, which can be confirmed from the inserted XRD pattern
in Fig. 5. The metastability of Ni3N is attributed to the low max-
imum solubility of nitrogen in Ni lattice and weak covalent bond
of Ni N [13,20].
[22] D. Xue, H. Chen, G.H. Wu, J.F. Deng, Appl. Catal. A: Gen. 214 (2001)
87–94.
[23] W.P. Tong, N.R. Tao, Z.B. Wang, J. Lu, K. Lu, Science 299 (2003) 686–689.