Hydrazine reduction of metal ions to porous submicro-structures of Ag, Pd, Cu, Ni, and Bi

Article abstract of DOI:10.1016/j.jssc.2012.02.059

Porous submicro-structures of Ag, Pd, Cu, Ni, and Bi with high surface area have been prepared by the reduction of hydrazine in glycerol-ethanol solution at room temperature or 120-180 °C. Phase purity, morphology, and specific surface area have been characterized. The reactions probably undergo three different mechanisms: simple reduction for Ag and Pd, coordination-then- reduction for Cu and Ni, and hydrolysis-then-reduction for Bi. The reductant hydrazine also plays an important role to the formation of the porous submicro-structure. The reaction temperature influences the size of the constituent particles and the overall architecture of the submicro-structure so as to influence the surface area value. The as-prepared porous metals have shown the second largest surface area ever reported, which are smaller than those made by the reduction of NaBH₄, but larger than those made by hard or soft template methods.

Full text of DOI:10.1016/j.jssc.2012.02.059

Journal of Solid State Chemistry 191 (2012) 19–26
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Hydrazine reduction of metal ions to porous submicro-structures
of Ag, Pd, Cu, Ni, and Bi
Yue Wang, Yong-Fang Shi, Yu-Biao Chen, Li-Ming Wu n
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Porous submicro-structures of Ag, Pd, Cu, Ni, and Bi with high surface area have been prepared by the
reduction of hydrazine in glycerol–ethanol solution at room temperature or 120–180 1C. Phase purity,
morphology, and specific surface area have been characterized. The reactions probably undergo three
different mechanisms: simple reduction for Ag and Pd, coordination-then-reduction for Cu and Ni, and
hydrolysis-then-reduction for Bi. The reductant hydrazine also plays an important role to the formation
of the porous submicro-structure. The reaction temperature influences the size of the constituent
particles and the overall architecture of the submicro-structure so as to influence the surface area value.
The as-prepared porous metals have shown the second largest surface area ever reported, which
are smaller than those made by the reduction of NaBH4, but larger than those made by hard or soft
template methods.
Received 14 December 2011
Received in revised form
17 February 2012
Accepted 26 February 2012
Available online 6 March 2012
Keywords:
Metal porous submicro-structures
Surface area
Sponge-like submicro-structure
& 2012 Elsevier Inc. All rights reserved.
1. Introduction
and NiAg nanoparticles [22]. Our experimental results suggest
that the formation of the submicro-sized porous structures may
Porous metal show interesting properties, such as high surface
area, low density, high thermal conductivity, which are important
in many fields, for example, catalysis for gas-phase reaction [1a]
or low-temperature fuel cells [1b], heat dissipation [2], electro-
chemistry [3,4], sensors [5,6], actuators [7], and biofiltration
membrane for antibacterial [8]. However, compared with those
of porous metal oxides [9–12], the preparation of porous metal
frameworks with high surface area remains a challenge in the
nano science and technology.
undergo different mechanisms: simple reduction for Ag and Pd,
coordination-then-reduction for Cu and Ni, and hydrolysis-then-
reduction for Bi. The influences of reaction temperature and time
on the morphology are studied. Brunauer–Emmett–Teller (BET)
tests indicate that the as-prepared submicro-structures have
shown the second largest surface area ever reported, which are
smaller than those made by the reduction of NaBH4, but larger
than those made by hard or soft template methods.
Porous metals are usually prepared by hard template methods,
such as thermal reduction of metal-ion-impregnated porous
insoluble supporters (e.g., alumina and pumice) or colloidal
method (e.g., polystyrene latex template) [13–16]. Recently, Ag
sponges could be prepared via soft template methods, such as
dextran [17], poly(ethyleneimine) hydrogel [18], or ionic liquid
N-(2-hydroxyethyl) ammonium formate [19]. A very recent work
reports the preparation of Au, Ag, Pd, and Pt nanosponges with
ligaments of size 5–10 or 20–60 nm by the reduction of NaBH4,
which show extremely high surface area [8].
2. Experimental section
Silver nitrate (AgNO3, A.R.), bismuth nitrate pentahydrate
(Bi(NO3)3 ꢀ 5H2O, A.R.), cupric acetate monohydrate (Cu(CH3COO)2 ꢀ
H2O, A.R.), nickel(II) acetate tetrahydrate (Ni(CH3COO)2 ꢀ 4H2O, A.R.),
palladium(II) acetate (Pd(CH3COO)2, A.R.), glycerol (C3H8O3, A.R.),
ethanol (CH3CH2OH, A.R), and hydrazine hydrate 85% (N2H4 ꢀ H2O,
A.R.) were purchased from Shanghai Chemical Co. All reactants are
used as received.
In this paper, we report a facile hydrazine reduction approach
to produce the porous metal submicro-structures. Hydrazine
(N2H4) is a cheap agent, and has been used to produce metal
nanocrystals, such as Ni nanoflowers [20], Co nanospheres [21],
2.1. Syntheses of metal porous submicro-structures
As a representative, a typical process for synthesis Ag porous
submicro-structure was described here: 0.136 g (0.8 mmol) of AgNO3
was added into a mixture of 4 mL of ethanol and 6 mL (82.3 mmol) of
glycerol in a 25 mL beaker and stirred for 5 min to give a clear
colorless solution, then, 6 mL (105 mmol) of N2H4 ꢀ H2O was added,
n Corresponding author.
E-mail address: liming_wu@fjirsm.ac.cn (L.-M. Wu).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2012.02.059

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Y. Wang et al. / Journal of Solid State Chemistry 191 (2012) 19–26
gray suspended precipitant started to form and the suspension was
stirred for 15 min before transferred into a 25 mL teflon-lined
stainless steel autoclave, which was sealed and heated at 180 1C for
24 h. After the autoclave cooled naturally, the solid product was
collected by centrifugation, then washed with ethanol and water
several times, and dried in air at room temperature. For Pd, Cu, Ni,
and Bi, the similar processes are carried out with corresponding
metallic salt (Pd(CH3COO)2, Cu(CH3COO)2 ꢀ H2O, Ni(CH3COO)2 ꢀ 4H2O,
and Bi(NO3)3 ꢀ 5H2O), respectively.
The parallel reactions were also carried out at room tempera-
ture. All reactants were added following the similar procedure as
described above in air with stirring. After the adding of hydrazine,
the as-produced suspended precipitant (Pd: black, Ag: gray) was
collected at 3 min, 15 min, 2 h, and 24 h by centrifugation, then
washed with ethanol and water several times, and dried in air at
room temperature. In the cases of other metal ions, the color
changes are diverse; the blue Cu2þ-glycerol/ethanol solution
turns brown after the adding of hydrazine, the green Ni2þ
glycerol/ethanol solution turns purple, and the colorless Bi3þ
glycerol/ethanol solution becomes a milky white suspension.
-
-
2.2. Characterizations
The XRD patterns were collected at room temperature with
the aid of a D-MAX-2500 diffractometer and Cu K radiation. The
a
SEM images were obtained using a JSM-6700 F scanning electron
microscopy equipped with a field emission gun. Measurements of
the BET (Brunauer–Emmett–Teller) isotherm using N2 desorption
were collected on a ASAP2020CþM surface area and porosity
analyzer at 77 K after the sample had been degassed in the flow of
N2 at 130 1C for 10 h.
3. Results and discussion
3.1. Silver sponge-like submicro-structure.
The powder X-ray diffraction patterns (XRD) show that pure
phased cubic Ag products (indexed as JCPDS 65–2871) are
obtained via the reduction of silver nitrate by hydrazine, at 180,
150, 120 1C for 24 h or even at room temperature (Fig. 1). The SEM
Fig. 1. The XRD patterns of Ag sponge-like submicro-structures obtained at
different temperatures for 24 h: (a) 180 1C; (b) 150 1C; (c) 120 1C; (d) room
temperature.
Fig. 2. The SEM images of Ag sponge-like submicro-structures obtained at different temperatures for 24 h: (a)–(c) 180 1C; (d)–(f) 150 1C; (g)–(i) 120 1C.

Y. Wang et al. / Journal of Solid State Chemistry 191 (2012) 19–26
21
images shows in Fig. 2 indicate that the porous Ag metal with
sponge-like submicro-structure was obtained at 120, 150, and
180 1C. The silver monoliths can be as large as several hundred
micrometers, which typically consist of aggregated particles with
sizes in the range of 150–500 nm. These particles are not gathered
mechanically, but fused together in a way that fabricates a porous
submicro-structure. (Fig. 2(c), (f), and (i)) The sponge-like sub-
micro-structures are quite robust at elevated temperatures, such
as 120, 150, and 180 1C; (Fig. 2) and can endure intense ultrasonic
treatment during the washing process.
observed yet. (SEM: Fig. 3(b); XRD: Fig. S1(b)) Beautiful porous
framework forms at room temperature for 2 h (SEM: Fig. 3(c), (d);
XRD: Fig. S1(c)), and porous sponge-like submicro-structure
forms at room temperature for 24 h (Fig. 3(e) and (f); XRD:
Fig. 1(d)). These results indicate that the formation of the Ag
porous sponge-like submicro-structures may include nucleation,
growth, and fusion processes.
3.2. Palladium sponge-like submicro-structure
Naturally, as the increase of the reaction temperature, the sizes
of the constituent particles (i.e., building blocks of the sponge-like
Ag submicro-structures) increase. For example, the sizes of the
constituent fused-particles at room temperature are 100–200 nm
(Fig. 3(e), (f)); 120 1C, 150–400 nm (Fig. 2(g)–(i)); 150 1C, 200–
500 nm (Fig. 2(d)–(f)); and 180 1C, 300–700 nm (Fig. 2(a)–(c)).
Such an increase gives rise to the decrease of the specific surface
area as indicated by the BET measurement as discussed
below. The experimental results also indicated that the cubic
silver phase could be formed at room temperature for 3 min
(XRD: Fig. S1a), but the morphology was dispersed particle
with the size range of 100–200 nm (SEM: Fig. 3(a)) instead of
the sponge-like submicro-structure. Similar phenomenon was
also found for the reactions at room temperature for 15 min,
under which the fusion of particles has begun, but the complete
closed circle constructed by the fused-particles has not been
Similarly, porous palladium metal with sponge-like submicro-
structure can be prepared at 120–180 1C and room temperature. The
XRD patterns (Fig. 4) can be well indexed as pure cubic Pd according
to JCPDS 65–2867. The SEM images (Fig. 5) indicate the formation of
sponge-like submicro-structures of Pd at 120, 150, and 180 1C. The
sponges could be larger than 100 mm (Fig. 5(a), (d), (g), (j)), which
are aggregated by irregular particles with size of 60–80 nm
(Fig. 5(c), (f), (i), (l)). But the degree of the inter particle fusion is
not as uniform as that in Ag. These Pd porous structures (Fig. 5(b),
(e), (h), (k)) are more loosely organized in comparison with the silver
sponges, which may be related with the smaller size of the building
block, for instance, the sizes of the aggregated irregular particles for
Pd sponges obtained at room temperature are of about 20 nm
(Fig. 5(l), magnified as Fig. S2), which are much smaller than the
100–200 nm for Ag sponges formed at room temperature (Fig. 3(f)).
Fig. 3. The SEM images of Ag submicro-structures obtained at room temperature for different time: (a) less than 3 min; (b) 15 min; (c), (d) 2 h; (e), (f) 24 h.

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Y. Wang et al. / Journal of Solid State Chemistry 191 (2012) 19–26
3.3. Copper loose sponge-like submicro-structure
Different from silver and palladium, copper cannot be
reduced by hydrazine at room temperature, since the blue
Cu2 þ-glycerol/ethanol solution turns brown with the addition
of hydrazine, which contains no precipitation after a centri-
fugation at a rate of 10,000 rad/min for 30 min. At elevated
temperatures, 120–180 1C, pure phased cubic metallic Cu
(JCPDS 04–0836) can be obtained. (Fig. 6) This difference is
related to the different electromotive force of the correspond-
ing redox reaction and the different reaction mechanism
discussed below. Loosely organized sponge-like porous Cu
made at 120 and 180 1C, which typically consists of fused-
particles with sizes of 250–400 nm (Fig. 7) share similar
morphologies except the different roughness of the constitu-
ent particle surfaces. For example, the particle surfaces at 120 1C
are rougher than those obtained at 180 1C (Fig. 7(d) vs (b)). This is
true because higher temperature benefits the Ostwald ripening of
small particles. The loose organization of the Cu sponge-like
submicro-structure are similar to that of Pd, but the sizes of the
fused building particles are as large as those in Ag, these two
Fig. 4. The XRD patterns of Pd sponge-like submicro-structures obtained at
different temperatures: (a) 180 1C, 24 h; (b) 150 1C, 24 h; (c) 120 1C, 24 h;
(d) room temperature, 2 h.
Fig. 5. The SEM images of Pd sponge-like submicro-structures obtained at different temperatures: (a)–(c) 180 1C, 24 h; (d)–(f) 150 1C, 24 h; (g)–(i) 120 1C, 24 h; (j)–(l) room
temperature, 2 h.

Y. Wang et al. / Journal of Solid State Chemistry 191 (2012) 19–26
23
factors confine its specific surface area as indicated by the BET
measurement described below, which is smaller than those of
both Pd and Ag.
Fig. 2(a)–(c), thus the specific surface area of Ag is lower than
either that of Ni, or Pd as discussed below.
Different from the fluffy Ni porous framework, the as-prepared
Bi products are porous monoliths (Fig. 10) similar as Ag (Fig. 2)
and Pd (Fig. 5). But the inter particle fusion is remarkable, which
is indicated by the size of the irregular particles, for example, at
180 1C, the size range is about 600–3000 nm (Fig. 10(a), (b)), and
120 1C, 150–1000 nm (Fig. 10(c), (d)). Such a fusion leads to the
disappearance of pores so as to result in a low specific surface
area. The as-prepared Bi exhibits the lowest specific surface area
among the studied metals as discussed below. The intrinsic
reason may be related to their different melting points, the lower
the melting point, the easier the inter particle fusion. In principle,
the sizes of the constituent fused-particle seem to follow a
decrease trend in accordance with the increase trend of the
melting point: for example, the submicro-structures produced at
180 1C are constructed by interconnected particles with sizes of
3.4. Nickel and bismuth porous frameworks
Pure phased cubic metallic nickel (JCPDS 04–0850) and rhom-
bohedral bismuth (JCPDS 44–1246) are obtained at 120–180 1C
for 24 h. (Fig. 8) Similar as Cu, Ni or Bi metal cannot be reduced at
room temperature because of the possible coordination reaction
or hydrolysis reaction of Ni2þ or Bi3þ as discussed below. The as-
prepared fluffy porous framework of Ni is aggregated by irregular
particles with sizes of 200–300 nm. (Fig. 9) At 180 1C, Ni porous
submicro-structure appears to be fluffier than that of Cu, even
though their basic building units are in the similar size range
(Fig. 9(a), (c) vs Fig. 7(a), (b)), therefore, the as-prepared Ni shows
higher specific surface area than Cu as discussed below. On the
other hand, the as-prepared Ag sponges contain larger particles
of 300–700 nm and are more closely organized as shown in
Fig. 6. The XRD patterns of Cu loose sponge-like submicro-structures obtained at
different temperatures for 24 h: (a) 180 1C; (b) 120 1C.
Fig. 8. The XRD patterns of the Ni and Bi porous frameworks obtained at different
temperatures for 24 h: (a), (c) 180 1C; (b), (d) 120 1C.
Fig. 7. The SEM images of Cu loose sponge-like submicro-structures obtained at different temperatures for 24 h: (a), (b) 180 1C; (c), (d) 120 1C.

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Y. Wang et al. / Journal of Solid State Chemistry 191 (2012) 19–26
Fig. 9. The SEM images of the Ni porous frameworks obtained at 180 1C for 24 h.
Fig. 10. The SEM images of Bi porous frameworks obtained at different temperatures for 24 h: (a), (b) 180 1C; (c), (d) 120 1C.
(Bi, 600–3000 nm)4(Ag, 300–700 nm)4(Cu, 250–400 nm)4(Ni,
200–300 nm)4(Pd, 60–80 nm) vs melting point trend of the
corresponding bulk metal: Bi, 271.4 1CoAg, 961.9 1CoCu,
1084.9 1CoNi, 1455 1CoPd, 1555 1C [23].
3.5. The role of N2H4 to aid the formation of sponge-like
submicro-structure
Scheme 1. Proposed reaction mechanisms. (M¼Ag, Pd, Ni, Cu, Bi).
The experimental results indicate that the N2H4 is necessary to not
only the reduction but also the formation of the sponge-like sub-
micro-structure under our experimental conditions. Parallel reactions
without hydrazine at room temperature generate no metal at all; at
180 1C for 24 h, generate Ag, Pd, and Cu, but neither Ni nor Bi.
Although cubic Ag, Pd, and Cu phases are formed at 180 1C without
N2H4, the sponge-like morphology cannot be produced; instead, only
dispersed irregular particles with large sizes are obtained (Ag,
3.6. The possible mechanism
Glycerol is known as a good chelate ligand in many metal
complexes, such as Pb(C3H6O3) [25]. Therefore, in our experi-
ments, the first step probably is the formation of the glycerol
complex, noted as M(C3H8ꢁnO3). The subsequent addition of N2H4
cause three different phenomena: (1) in cases of Agþ and Pd2þ
,
Cu:41
m
m; Pd: 4100–200 nm; XRD and SEM: Figs. S3–S5). The
the precipitant of Ag or Pd metal is detected by XRD shortly after
the addition; (2) in the cases of Cu2þ and Ni2þ, the coordination
of hydrazine to the metal ions generating [M(N2H4)2]2þ [20], is
suggested by the color change of the solution (blue to brown
for Cu, and green to purple for Ni); (3) in case of Bi3þ, the
hydrolysis of Bi3þ is indicated by the immediate formation of the
white suspension. For the latter two cases, the coordination
complexes and the hydrolysate BiOþ can be reduced by excess
N2H4 at elevated temperatures. The whole process may be
generated N2 gas [20,24] may be important to the formation of the
sponge-like or fluffy porous framework. According to the previous
work, small metal particles could self-assembled into 1D or 2D-
structures in the present of N2H4 and ethylene glycol (EG) [20].
Similarly, in this work, with the presence of N2H4 and glycerol, 3D-
porous structures have been obtained. Besides, the large viscosity of
glycerol solution may also help to hinder the inter-particle fusion of
the small particles.

Y. Wang et al. / Journal of Solid State Chemistry 191 (2012) 19–26
25
Fig. 11. Nitrogen adsorption–desorption isotherms (at 77 K) of the metal submicro-structures: (a) Ag, made at room temperature; (b) Cu, made at 120 1C, (c) Pd, made at
room temperature; (d) Ni, made at 180 1C; (e) Bi, made at 120 1C.
illustrated in Scheme 1. Therefore, the formation of glycerol
complex, M(C3H8ꢁnO3), may be another important factor in the
generation of metal porous structures.
Ni (made at 180 1C), and Pd (made at room temperature),
respectively. These values are smaller than 16–81 m2/g for Ag
and Pd reduced by NaBH4 which are constructed by smaller
fused-particles with sizes of less than 20 nm;[8] but higher than
the values of 1.0–2.0 m2/g achieved by both hard and soft
templating methods [13,17,19]. The reaction temperature shows
an effect on the BET surface area value. For example, Pd made at
room temperature has a value of 11.43, which decreases to
4.68 m2/g for that made at 150 1C. (Fig. S6) Together with the
morphology studies described above, the surface area values are
correlated to the overall architecture of the porous submicro-
structure as well as the size of the constituent particles. For
example, the fluffier the porous submicro-structure is, the larger
the surface area is; such as Pd vs Ag, or Ni vs Ag. And the smaller
the constituent particle is, the larger surface area is; for example,
Bi made at 120 1C, 150–1000 nm vs Cu at 120 1C, 250–400 nm.
3.7. The reduction tendency
Why can the reduction of Ag and Pd be observed at room
temperature, while those of Cu, Ni, and Bi cannot? As proposed
above, this is caused by the different mechanisms. Meanwhile, this
can also be understood by the different reaction Gibbs energies.
N2H4 is a strong reductant under alkaline condition, indicated by
N2H4þ4 OHꢁ ¼N2þ4H2Oþ4eꢁ, Eꢂ(N2H4/N2)¼ ꢁ1.15 V [24].
The redox reaction of Mnþ and N2H4 can be described as
follows:
ð4=nÞMnþ N2H4 þ4OH2 ¼ ð4=nÞMkþN2mþ4H2O
ðEq:1Þ
The redox electromotive force (emf) of the reaction is calculated
according to Eꢂ¼Eꢂ(N2H4/N2)þEꢂ(Mnþ/M). Under the standard
condition, the Gibbs energies of the redox reactions should follow
the same trend of the emfs as well as the trend of the standard
electrode potentials, that is Eꢂ(Pd2þ/Pd),þ0.987 V4Eꢂ(Agþ/Ag),
þ0.799 V4Eꢂ(Cu2þ/Cu),þ0.337 V4Eꢂ(Ni2þ/Ni),þ0.257 V.
Therefore, the reduction of Pd, and Ag is easier than that of Cu and Ni.
This in principle agrees with the experimental results. For example,
Pd and Ag can be reduced at room temperature. In case of Bi, because
the hydrolysis of Bi3þ to BiOþ (white suspension), and the standard
electrode potential Eꢂ(BiOþ/Bi) is ofþ0.320 V, the reduction cannot
be found at room temperature as well. In cases of Ni, Cu, the
electrode potential of Eꢂ([M(N2H4)2]2þ/M) is not available, we
consider the data may be close to that of Eꢂ(BiOþ/Bi). Based on this
view, Auþ(Eꢂ(Auþ/Au),þ1.68 V), Pt2þ (Eꢂ(Pt2þ/Pt),þ1.2 V), may
also be reduced by the same method.
4. Conclusion
In summary, the reduction of metal ions by hydrazine in
glycerol–ethanol solution from room temperature to 180 1C
undergo three different processes: (1) simple reduction for Ag
and Pd; (2) coordination-then-reduction for Cu and Ni; and
(3) hydrolysis-then-reduction for Bi. The reductant hydrazine (in
cases of Cu and Ni, also as a coordination ligand) also plays an
important role to the formation of the porous submicro-structures.
The reaction temperature is found to influence the size of the
constituent particles and the overall architecture of the porous
submicro-structures, which are intrinsically related to the melting
points of the corresponding bulk metals. The as-prepared porous
metals have shown the second largest surface area ever reported,
which are smaller than those made by the reduction of NaBH4, but
larger than those made by hard or soft template methods.
3.8. The specific surface area
Measurements of the BET (Brunauer–Emmett–Teller) isotherm
using N2 desorption were collected at liquid nitrogen temperature
(77 K). As shown in Fig. 11, the specific surface area data are 2.75,
3.23, 3.67, 6.07, and 11.43 m2/g for as-prepared Bi (made at
120 1C), Cu (made at 120 1C), Ag (made at room temperature),
Acknowledgment
This research was supported by 973 Program (2010CB933501)
and the National Natural Science Foundation of China under
projects (20973175, 20803080).

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Y. Wang et al. / Journal of Solid State Chemistry 191 (2012) 19–26
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