Large-Scale Synthesis of Uniform Nanotubes of a Nickel Complex by a Solution Chemical Route

Article abstract of DOI:10.1021/ja039381h

Novel layer-rolled nanotubes of a nickel complex have been successfully synthesized by a simple wet chemical method. The nanotubes are assembled by rolling the (111) sheets of [Ni(NH₃)₆]Cl₂ with the assistance of a polymer. The remarkable uniformity and high yields of the nickel complex nanotubes point to future applications in various fields of nanotechnology. Copyright

Full text of DOI:10.1021/ja039381h

Published on Web 03/20/2004
Large-Scale Synthesis of Uniform Nanotubes of a Nickel Complex by a
Solution Chemical Route
Lin Guo,*,† Chenmin Liu,† Rongming Wang,‡ Huibin Xu,*,† Ziyu Wu,§ and Shihe Yang*,|
School of Materials Science and Engineering, Beijing UniVersity of Aeronautics and Astronautics, 100083, China,
Electron Microscopy Laboratory and State Key Laboratory for Mesoscopic Physics, School of Physics,
Peking UniVersity, Beijing 100871, China, Beijing Synchrotron Radiation Facility, Chinese Academy of Sciences,
100039, China, and Department of Chemistry, The Hong Kong UniVersity of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong, China
Received November 2, 2003; E-mail: guolin@buaa.edu.cn; chsyang@ust.hk
In the past few years, tubular nanostructures have aroused
considerable interest due to their exceptional properties and potential
applications in catalysis, composite materials, and optoelectronics.1-4
The discovery of carbon nanotubes (CNT) in 1991 has sprawled
the belief that other layered materials may also roll up into tubes
just as the graphite sheets do.5 This is reasonable because the layered
structures tend to bend to eliminate dangling bonds by the intralayer
linkage. As a result, the syntheses of many inorganic tubular
structures such as MoS2, WS2, BN, and V2O5 nanotubes have been
achieved.6-11 However, these nanotube synthesis methods require
either extreme conditions such as high temperatures or preformed
templates. Therefore, it remains a challenge to find simple and mild
Figure 1. XRD pattern of as-synthesized Ni complex nanotubes.
routes to synthesize tubular nanostructures of inorganic materials.
Recently, relatively mild conditions of chemical precipitation were
adopted for the synthesis of nanotube-like structures of NiS,12 Cu-
(OH)2,13 and titanate.14 Here, we demonstrate a simple solution-
phase approach for the synthesis of novel nanotubes of a nickel
complex [Ni(NH3)6]Cl2. [Ni(NH2NH2)2]Cl2 is found to be the
precursor, which is decomposed to form the nanotubes. The
nanotubes are very stable in ambience after surface modification
using PVP, a very important feat for future applications.
The layer-rolled [Ni(NH3)6]Cl2 nanotubes were obtained by the
reduction of NiCl2‚6H2O with hydrazine monohydrate in the
presence of poly(vinylpyrrolidone) (PVP, Mw 40 000) (see S-1 in
Supporting Information). First, a solution was prepared by dissolv-
ing NiCl2‚6H2O and PVP in ethylene glycol (EG). This was
followed by a dropwise addition of hydrazine monohydrate at room
temperature with simultaneous vigorous agitation. Second, the
homogeneous mixture was heated to the boiling point of EG for
refluxing (∼197 °C). After the mixture was refluxed for 5 h, a
light violet precipitate was obtained, which was separated by
centrifugation and washed with absolute ethanol. The composition
of the as-prepared product was determined by XRD (Figure 1).
The diffraction peaks, except those marked with stars and diamonds
arising respectively from the undecomposed [Ni(N2H4)2]Cl2 (JCPDS
No. 28-0695) and nested nickel particles (JCPDS No. 04-0850),
conform to the {111} facet group, including (111), (222), (333),
and (444), of the fcc structure of [Ni(NH3)6]Cl2 (JCPDS 76-1842).
The disappearance of the diffraction peaks of (220), (311), (400),
(422), etc., when compared to the standard powder XRD pattern,
indicates the rolling of the (111) crystal plane.
Figure 2. (a) TEM image of the as-prepared Ni complex nanotubes at a
low magnification. (b) TEM image of the same samples as in (a) but with
a higher magnification.
ranging from 20 to 40 nm and lengths of ∼2 µm. More importantly,
the nanotube yield is rather high and the purity is outstanding as is
evident from the images. The TEM image at a higher magnification
(Figure 2b) reveals the tubular structure of the product with a strong
contrast between the dark edges and the pale center. However, close
inspection of Figure 2b reveals that the center of the nanotube is
not completely hollow. Instead, the nanotubes are often filled with
amorphous substances, which are decomposed rapidly under the
electron beam, leaving behind a few particles nested in the tubes.
Further information on the tubular structure was acquired using
the HRTEM technique. Figure 3a and 3b shows HRTEM images
of a tube from the portions of the stem and the head, respectively.
The corresponding fast Fourier transformation (FFT) pattern is given
in the inset of Figure 3b. It can be seen that the nanotube has closed
but amorphous tips and the thickness of the nanotube wall is about
3-5 nm. The spacing between the rolling layers in the perpendicular
direction to the tube axis is 0.584 nm, which is almost the same as
the d111 spacing of [Ni(NH3)6]Cl2. This is in excellent agreement
with the XRD result presented above. Moreover, the HRTEM
images show clearly that the upper portion of the tube has one
more layer than the lower portion; there are, respectively, 5 and 4
The low-magnification TEM (Figure 2a) images show that the
product was formed in a rodlike morphology with a remarkably
uniform shape and size. The Ni complex nanotubes have diameters
† Beijing University of Aeronautics and Astronautics.
‡ Peking University.
§ Chinese Academy of Sciences.
| The Hong Kong University of Science and Technology.
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10.1021/ja039381h CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
The posited reactions are as follows:
NiCl2 + 2N2H4 f [Ni(N2H4)2]Cl2
2[Ni(N2H4)2]Cl2 + 5N2H4 f 2[Ni(NH3)6]Cl2V + 3N2v
Yet how are the layer-rolled nanotubes formed during the decom-
position of [Ni(N2H4)2]Cl2? It seems that the presence of hydrated
H2O in NiCl2‚6H2O and N2H4‚H2O plays an important role simply
from the experimental results (see S-3 in Supporting Information).
Conceivably, the decomposition product [Ni(NH3)6]Cl2 may self-
organize into a layer structure induced by the presence of a small
amount of water as proposed by Jiang et al.12 Here, the water
molecules help to passivate the surfaces of the [Ni(NH3)6]2+ layers
by forming the NH‚‚‚OH2 hydrogen bonds. Such a layer-structured
precursor tends to bend under stress and roll into a tubular form
accompanied by the elimination of water. During the rolling process,
some undecomposed [Ni(N2H4)2]Cl2 may be nested in the nano-
tubes. This explains why the decomposition of [Ni(N2H4)2]Cl2 is
very slow. Further studies on the mechanistic details are in progress.
In summary, a novel tubular nanostructure of [Ni(NH3)6]Cl2 has
been synthesized by a simple wet chemical method. The [Ni(NH3)6]-
Cl2 nanotubes are 30-40 nm in diameter and ∼2 µm in length.
The walls of the nanotubes are about 3-5 nm thick. All of our
data indicate that the nanotubes are assembled by rolling the (111)
sheets of the Ni complexes. The advantages of our method for
synthesizing the Ni complex nanotubes lie in its simplicity, high
yield, and mild reaction conditions. Therefore, it offers an attractive
and convenient path to large-scale production. In ongoing experi-
ments, we hope to effectively control the amount of encapsulated
materials in the nanotubes, extend our method to the synthesis of
nanotubes of other transition metal complexes, and investigate the
properties of the new nanotube materials.
Figure 3. (a) Lattice-resolved HRTEM image taken at the stem of a single
nanotube. (b) Lattice-resolved HRTEM image taken at the head of a single
nanotube. The inset shows the corresponding fast Fourier transformation
(FFT) pattern from the digitalized 1024 × 1024 HRTEM image. (c) and
(d): Lattice-resolved HRTEM images of nested Ni nanoparticles, which
are obtained from the decomposition of the amorphous substances in the
nanotube.
layers in Figure 3a and 8 and 7 layers in Figure 3b. This strongly
supports that the nanotubes are formed by the rolling mechanism.
More specifically, the closest-packed (111) sheets of the fcc Ni
complex are continuously rolled into nanotubes. More evidence for
the formation of the [Ni(NH3)6]Cl2 nanotubes can be found in the
Supporting Information (S-2).
Recalling that there are some amorphous substances left inside
the nanotube, which are decomposed rapidly into nested particles
under the electron beam, Figure 3c shows the HRTEM image of a
remnant nanocrystal nested in a nanotube with an interplane spacing
of 0.203 nm. Another interplane spacing of 0.180 nm has also been
found (Figure 3d). These interplane spacings match well the
corresponding literature values of (111) and (200) planes of a pure
nickel crystal (fcc), respectively (JCPDS No. 04-0850). As men-
tioned above, the existence of the fcc Ni phase is also manifest in
the XRD pattern in Figure 1.
In the nanotube synthesis, after the hydrazine monohydrate was
added to the reaction system, the solution turned turbid and sky-
blue, indicating the formation of a stable complex between Ni2+
and N2H4.15 This complex is [Ni(N2H4)2]Cl2, as identified by the
XRD analysis. We believe that the nanotubes are produced from
the decomposition of the complex [Ni(N2H4)2]Cl2. Note that the
decomposition is slow according to our XRD investigation (see
S-5 in Supporting Information). Such a slow decomposition may
be essential for the nanotube formation. The amorphous materials
observed are then from the remnants of [Ni(N2H4)2]Cl2, which is
very sensitive to the electron beam illumination and rapidly
decomposed to fine Ni particles as shown in Figures 2 and 3.
The reactions that account for the growth of the [Ni(NH3)6]Cl2
nanotubes are essentially composed of complexation, reduction, and
decomposition. In the beginning, Ni2+ reacts with hydrazine to form
a sky-blue complex, [Ni(N2H4)2]Cl2, which is very stable in
ambience.15 When the temperature is raised to the boiling point of
ethylene glycol, the [Ni(N2H4)2]Cl2 complex begins to decompose
accompanied by the color change from sky-blue to orange and the
brisk bubbling in the solution. Finally, light violet precipitates are
formed, which have been testified to be [Ni(NH3)6]Cl2 nanotubes.
Acknowledgment. This project was supported by NSFC (No.
20373004 and 10334060) and the Outstanding Young Teacher Fund
of BUAA. S.Y. acknowledges the support from the Research Grant
Council of Hong Kong.
Note Added after ASAP: In the version published on the Web
3/20/2004, the reference to Figure 3d in the first paragraph on this
page is incorrect. The final Web version published 3/26/2004 and
the print version are correct.
Supporting Information Available: Experimental details; ad-
ditional spectroscopic data; data on the effects of hydrated water and
PVP; data on the decomposition of [Ni(N2H4)2]Cl2 (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
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