Synthesis and characterization of reduced transition metal oxides and nanophase metals with hydrazine in aqueous solution

Article abstract of DOI:10.1016/S0025-5408(02)00983-2

Reduction of the first row (3d) transition metal complexes in aqueous solution using hydrazine as the reducing agent has been investigated systematically. Nanoscale reduced metal oxides such as VO₂(B), Cr₂O₃ were obtained through hydrazine reduction followed by proper thermal treatments. Nanocrystalline γ-Mn₂O₃ was synthesized directly through aqueous phase reduction at room temperature. Nanoclusters of cobalt, copper and the one-dimensional single crystal nickel nanowhiskers were synthesized by direct reduction without post-annealing process. All the products were characterized on their structure and micro-morphology. The reaction details and features were described and discussed.

Full text of DOI:10.1016/S0025-5408(02)00983-2

Materials Research Bulletin 38 (2003) 169±176
Synthesis and characterization of reduced transition metal oxides
and nanophase metals with hydrazine in aqueous solution
a,*
Zhou Gui , Rong Fan , Weiqin Mo , Xianhui Chen , Ling Yang , Yuan Hu
b
b
b,c
a
a
a
State Key Lab of Fire Science, University of Science and Technology of China, Hefei 230027, PRChina
b
Department of Physics, University of Science and Technology of China, Hefei 230027, PRChina
Structure Research Laboratory, University of Science and Technology of China, Hefei 230027, PR China
c
Received 22 February 2002; received in revised form 17 April 2002; accepted 26 September 2002
Abstract
Reduction of the ®rst row (3d) transition metal complexes in aqueous solution using hydrazine as the reducing
agent has been investigated systematically. Nanoscale reduced metal oxides such as VO (B), Cr O were
2
2 3
obtained through hydrazine reduction followed by proper thermal treatments. Nanocrystalline g-Mn O was
2
3
synthesized directly through aqueous phase reduction at room temperature. Nanoclusters of cobalt, copper and
the one-dimensional single crystal nickel nanowhiskers were synthesized by direct reduction without post-
annealing process. All the products were characterized on their structure and micro-morphology. The reaction
details and features were described and discussed.
#
2002 Elsevier Science Ltd. All rights reserved.
Keywords: Inorganic compounds; Nanostructure; Chemical synthesis
1
. Introduction
Low temperature soft chemistry preparative techniques recently attract increasing interests in the
synthesis of technologically important transition metal oxides and nanophase metals due to their unique
advantages over the conventional high temperature solid state reactions in approaching novel
metastable phases and controlling homogeneity, grain size and micro-morphology of products [1,2]. It
is of great signi®cance to turn down the reaction temperature, which creates new chances in synthetic
chemistry techniques [3], including sol±gel method [4], hydrothermal process [5], solverthermal
process [6]and molten salts process [7]. Recently, a great deal of work has been done on using new
reducing agents for preparing reduced metal oxides. These oxides are important for many applications
such as catalysts [8], high-energy-density battery cathodes [9], magnetic materials [10] and so on.
*
Corresponding author. Tel.: ‡86-551-3601288; fax: ‡86-551-3601669.
E-mail address: zgui.@ustc.edu.cn (Z. Gui).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0025-5408(02)00983-2

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Z. Gui et al. / Materials Research Bulletin 38 (2003) 169±176
Manthiram and coworkers reported that alkali borohydrides can be used as the reducing agent to
generate binary oxides VO , M oO, M nO , Cr O and some ternary oxides like the tungsten bronze
Na WO [11±15]. Goodenough and coworkers made use of the hydroxyamine hydrocloride
2
2
2Àd
2 3
x
3
(
NH OHÁHCl) to produce reduced transition metal oxides such as Fe O , VO , and MoO at relatively
2
3
4
2
2
low temperatures [16,17]. Ultra®ne particles of the metal Fe, Ni, Co and their alloys were prepared by
the reduction of Fe , Co and Ni in the aqueous solutions using alkali borohydrides [18].
Hydrazine is an important reducing agent in organic synthesis. Recently, it was demonstrated useful
2‡
2‡
2‡
for preparing II±VI group semiconductor nanorods [19], and layered MoE (E ˆ S, Se) [20]. Gibson
2
2‡
and Putzer reported that anisometric cobalt nanoclusters were obtained by sonication of aqueous Co
and hydrazine [21]. Ultra®ne nickel particles were prepared by Li et al. [22] through the hydrothermal
reduction of aqueous Ni salts with hydrazine assisted by a selected surfactant.
In the present work, we systematically studied the reduction of the ®rst row (3d) transition metal
complexes in aqueous solutions at low temperature with hydrazine as the reducing agent. Nanoscale
oxides of VO , Cr O , g-Mn O , nanoparticles of Co, Cu and one-dimensional single crystal Ni
2
2
3
2 3
nanowhiskers were obtained.
2
. Experimental
A given amount of analytically pure transition metal source compounds (e.g. NH VO , K Cr O , and
KMnO ) were dissolved in distilled water and then more distilled water was added until its total volume
4
3
2
2 7
4
reached up to 50 ml. After that, the hydrazine (N H ÁH O) solution (85 vol.%) was slowly dripped into
2
4
2
the solution under constant stirring until precipitates formed. In the case of vanadium oxides,
hydrochloric acid should be pre-added to adjust the pH value of the solution to the weak acidic
condition. The precipitates were allowed to settle overnight and collected by washing and ®ltration.
Thermal treatment of the chromium and vanadium products, thermal treatment at elevated temperatures
is leads to crystallization.
For the synthesis of Ni nanowhiskers, 50 ml of 0.1 MNi(CH COO) aqueous solution and 5 ml of
3
2
2‡
ammonia solution (28%) were added into a beaker to give a dark blue [Ni(NH ) ] solution. Then 5 ml
4
3
of 17 Mhydrazine solution was dripped into this above solution slowly. This solution is pretty clear and
no solid formed at this stage. Here pH value of the solution was around 10.5. When this solution was
heated up to 100 8C, it can be observed that some black solid formed. The hot plate was removed
immediately after the initiation of the reaction. This reaction generated a great deal of heat, which
drove the reaction to proceed ahead and therefore, is shown to be self-sustained. After 30 min of
reacting the resulting solids, many of which were ¯oating on the surface of the solution, were collected
by ®ltration and washing.
For the synthesis of Co nanoclusters, 0.01 mol of analytically pure Co(CH COO) Á6H O was
3
2
2
dissolved in 5 ml of distilled water and directly dripped into 50 ml of 17 M(85%) hydrazine
(
N H ÁH O) solution under heating and constant stirring. The pH value of the resulting solution is 12.
2
4
2
Gray precipitate formed slowly after the solution was heated to boiling temperature and held for
4
0 min.
For the synthesis of nanometer Cu metal, the surfactant of sodium dodecyl sulphonate (SDS) is
needed to con®ne the product size. Hydrazine was dripped into the solution of CuSO /SDS to produce a
4
black precipitate at room temperature.

Z. Gui et al. / Materials Research Bulletin 38 (2003) 169±176
171
All products were collected after ®ltration, washed with water and absolute alcohol, then dried in
vacuum at 80 8C for 5 h and stored in a desiccator. X-ray powder diffraction (XRD) analysis was
conducted on a Rigaku D/Max X-ray diffractometer with graphite monochromated Cu Ka radiation
Ê
(
l ˆ 1:5418 A) to verify the formation of products. Transmission electron microscopy (TEM) images
of the samples were collected on Hitachi H-800 electron microscope operated at 200 keV. High-
resolution electron microscopy (HREM) images and X-ray energy dispersion spectrum (EDS) analysis
were recorded on a JEOL 2010 operated at 200 keV.
3
. Results and discussion
The optimum reaction conditions and the products obtained are listed in Table 1. The as-prepared
VO directly from room temperature reduction shows no Bragg re¯ections in the X-ray diffraction
2
pattern. For further characterization of phase transformation of the as-prepared VO , it was heated in a
2
¯owing N atmosphere at various temperatures for 24±48 h and then analyzed by X-ray diffraction. The
results as shown in Fig. 1(a±d) reveal that only the metastable VO (B) is present in the product obtained
2
2
below 300 8C, while the monoclinic rutile VO (R) begins to form at around 350 8C. This conversion
2
completes at around 650 8C that is much higher than the phase transition temperature of VO (B) to
2
VO (R) reported in the previous literature [23]. When the as-prepared VO was exposed in air
atmosphere for 2 weeks, and then heated in a ¯owing N atmosphere at 650 8C, only V O was
2
2
2
2 5
obtained. This suggests that the as-prepared VO is easily oxidized by oxygen in the air at room
2
temperature. The nanocrystalline VO (B) we prepared has the potential application as a cathode for
2
lithium cells. The chromium oxide obtained from room temperature reduction shows no discernible
Bragg re¯ections in the XRD pattern; when it was annealed at 500 8C in a ¯owing N atmosphere for
2
2
4 h, X-ray powder diffraction pattern shows that the product is hexagonal Cr O after annealing at
2 3
5
show that they are quasi-spherical particles with the average diameter of 35 nm for VO (B) and 30 nm
00 8C, in (Fig. 2a). TEMimages ( Fig. 3a and b) for the samples of VO (B) and Cr O after annealing
2
2 3
2
for Cr O , respectively.
2
3
It is interesting that the sample of manganese oxides directly from room temperature reduction is
found to be crystalline g-Mn O as shown in Fig. 2b. To our knowledge, this is the ®rst report that the
2
3
Table 1
The starting materials, reaction conditions and the products obtained
Metal sources
Concentration
Hydrazine (N
2
H
4
ÁH
Volume
(ml)
2
O)
pH
Additament
Crystal
temperature (8C)
Final
products
Volume
ml)
Concentration
(M)
(
0
0
0
0
0
2
.2 MNH VO3
4
50
50
50
50
50
5
3.4
17
1.5
5
3.5
300
500
VO (B)
2
.2 MK
.2 MKMnO
2
CrO
7
4
2 3
Cr O
17
17
5
g-Mn
Cu
Ni
2 3
O
a
.2 MCuSO 4
.1 MNiAc
MCoAc
2
SDS
NH
2
17
17
3
50
10.5
12
3
H
2
O
b
2
Co
a
SDS: sodium dodecyl sulfonate.
During this experiment, aqueous CoAc was dripped into 50 ml of 85% hydrazine solution under stirring.
b
2

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72
Z. Gui et al. / Materials Research Bulletin 38 (2003) 169±176
Fig. 1. XRD patterns of VO upon heating at (a) 240, (b) 350, (c) 600 and (d) 650 8C for 36 h.
2
crystalline g-Mn O can be directly synthesized through a simple redox method at room temperature.
2
3
Manthiram et al. [13] obtained amorphous Mn O by reduction of KMnO with KBH in aqueous
solutions, but a high temperature thermal treatment was needed to obtain a crystalline product. The
reducing agent N H may play a complex role in the formation of the crystalline product and the
2
3
4
4
2
4
con®nement of crystal size. From the TEMimage as shown in Fig. 3c, the g-Mn O obtained comprises
2
3
homogenous nanoparticles with an average diameter of 8 nm. The surface self-organization of these
particles results in the monolayer ®lm of nanocrystallites [25].
2‡
The reduction of [Ni(NH ) ] by hydrazine is found to be an interesting self-sustained reaction
3
4
initiated by short time heating. Signi®cant exotherm effect drives the reaction to proceed forward
completely. A black sponge-like solid formed and ¯oated on the solution surface. XRD pattern as
shown in Fig. 2c indicates it is the face-centered cubic (fcc) structure of nickel. SEMimages ( Fig. 3d)
show that the as-prepared black Ni comprised of an intercrossed framework, which is highly porous and
is of low density, just like sponge. After ground and dispersed by ultrasound, this sample is observed to
consist of crystal whiskers typically with a width of 10±20 nm and a length 200±300 nm indicated by
TEMimage in Fig. 3e. Fig. 3f is the high-resolution election microscopy (HERM) image for Ni
nanowhiskers. The unambiguous continuous fringes continuously in the individual whiskers indicate
that each Ni whisker is a single crystal. Compositional analysis of the individual Ni nanowhiskers
carried out by X-ray electron dispersive spectroscopy shows that Ni is pure. The growth of one-
dimensional nanowhiskers of nickel may attribute to the coordination effect of NH3 ligand.
NH CH CH NH (en) has been reported to be the solvent molecular coordination template agent
2
2
2
2
2
‡
assisting the growth of CdS nanorods due to the assembly of square-planar coordinated [Cd(en) ]
2
2
‡
along the perpendicular direction [24]. In our work [Ni(NH ) ]
3
also adopts square-planar
4

Z. Gui et al. / Materials Research Bulletin 38 (2003) 169±176
173
Fig. 2. XRD patterns for the products of (a) Cr O , (b) Mn O , (c) Ni, (d) Co, and (e) Cu.
2
3
2 3
coordination and is structurally anisotropic. Therefore, the crystal growth used as a building block may
lead to the formation of one-dimensional nanostructure. To our knowledge, this is the ®rst simple
synthesis of single crystal Ni nanowhiskers through a chemical reduction.
For cobalt, only one peak can be observed in the XRD pattern (Fig. 2d); it could be indexed either to
1 1 1) in cubic Co (fcc) or as (0 0 2) in hexagonal Co (hcp). However, electron diffraction in Fig. 3h
(
reveals the coexistence of fcc and hcp cobalt. The disappearance of corresponding peaks in XRD
pattern may result from the formation of stacking faults, which lead to the collapse of periodicity along
some directions. The sample of cobalt comprises homogenous nanoclusters with an average diameter of
2
cobalt cannot be prepared by direct reduction of Co with hydrazine, while a sonication process is
0 nm, as shown in the TEMimage of Fig. 3g. Gibson and Putzer pointed out that the nanoclusters of
2‡
2
‡
necessary for obtaining Co nanoclusters [21]. We also found that dripping hydration to Co solution
did not generate cobalt metal, instead some kind of cobalt hydroxides were formed. But, gray

1
74
Z. Gui et al. / Materials Research Bulletin 38 (2003) 169±176
2 2 3 2 3
Fig. 3. TEMimages for the products of (a) VO , (b) Cr O , (c) Mn O , (e) Ni, (g) Co, and (i) Cu; (d) SEMimage for sponge
Ni; (h) ED pattern of Co nanocluster; (f) HREMimage for Ni nanowhiskers.

Z. Gui et al. / Materials Research Bulletin 38 (2003) 169±176
175
2‡
precipitate forms slowly when Co solution is directly dripped into 85% hydrazine under heating near
boiling. In addition when the ligand N H ÁH O was also pre-added to CoAc solution, one-dimensional
2
4
2
2
Co nanowhisker cannot be obtained. This is different from the synthesis of Ni nanowhiskers. The
preparation of Co nanoclusters is a reduction reaction in high concentration hydrazine solution, while
the formation of Ni nanowhiskers is a slow self-sustained process, which allows the possibility of the
gradual growth of one-dimensional whiskers.
2
‡
The reduction of Cu solution with hydrazine produced golden colored Cu particles without the
assistance of any surfactant. By surfactant assisted (SDS) reduction, black nanophase copper particles
were obtained. The XRD pattern for the as-prepared Cu nanoparticles as shown in Fig. 2d can be
indexed to face-centered cubic structure. TEMimage of Fig. 3i indicates that the obtained Cu sample is
comprised of nanoparticles with an average diameter of 35 nm. It is found that the choice of starting
materials is important: if CuCl is used instead of CuSO as the starting agent, keeping other
2
4
experimental variables unchanged, the product is a mixture of Cu and CuCl. That is, because of the
formation of undissolvable CuCl, which prohibits the further reduction of Cu(I) to Cu(0).
4
. Conclusion
We have systematically studied the low temperature chemical reduction of 3d transition metal cations
or metalates using hydrazine as the reductant in aqueous solution. Nanoscale reduced oxides VO (B),
2
Cr O and g-Mn O and metals (Co, Ni, and Cu) have been obtained. The structure and micro-
2
3
2 3
morphology of the products were well characterized. It is of special interest that the nanocrystalline g-
Mn O could be prepared by a direct reduction at room temperature without post-thermal treatment for
crystallization, and one-dimensional single crystal Ni nanowhiskers have been obtained by a simple
direct chemical reduction with the aid of NH ligand. The nanophase reduced transition metal oxides
2
3
3
are technologically important for catalysts. The magnetic metal (Co, Ni) nanoclusters or whiskers have
great signi®cance in fabricating single-domain magnetic devices.
Acknowledgements
This work was supported by a grant of Natural Science Foundation of China, No. 50003008 and
supported by the China NKBRSF Project, No. 2001CB409600.
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