Sol-gel autocombustion synthesis of metals and metal alloys

Article abstract of DOI:10.1002/anie.200903444

Go for the burn! Sol-gel autocombustion is an energy-efficient approach to synthesizing metals, such as Co, Ni, Cu, Ag, Bi, and Co-Ni alloy. Based on differential thermal analysis (DTA) and mass spectrometry measurements (see picture for Ni gel), a reacti

Full text of DOI:10.1002/anie.200903444

Angewandte
Chemie
DOI: 10.1002/anie.200903444
Metal Synthesis
Sol–Gel Autocombustion Synthesis of Metals and Metal Alloys**
Yuwen Jiang, Shaoguang Yang,* Zhenghe Hua, and Hongbo Huang
The development of technology for the synthesis of metals
and alloys continues to be one of the most important
challenges in materials science. A general method for the
synthesis of metals is to reduce metal oxides with hydrogen or
carbon at high temperature. Pure metals are then used as
starting materials to synthesize alloys by a melting method.
There are some existing methods used to synthesize metals
and alloys that have improvement of their capabilities.[1–3]
However, the reduction of energy costs in the synthesis
process is one of the critical factors for making the overall
products cost-effective. Autocombustion is a self-sustaining
method for heat generation from its own exothermic reac-
Figure 1. XRD patterns of the products from sol–gel autocombustion.
All the peaks can be indexed to the corresponding metals and alloy
with no impurities detected.
tion.[4,5] In addition, the sol–gel technique has been widely
used for the synthesis of a variety of metal oxides by mixing of
different elements at the atomic level.[6–13]
Herein, we describe a versatile new strategy for producing
pure metals and alloys by combining the sol–gel method and
autocombustion; similar methods have been successfully used
for the synthesis of metal oxides.[14,15] We used metal nitrate,
citric acid, and ammonia as starting materials; the dried gels
were prepared through a routine sol–gel approach. The
autocombustion was activated at 3008C in a tube furnace. The
resulting materials were characterized by X-ray diffraction
(XRD) with CuKa radiation. This method has the following
features: 1) simple apparatus with inexpensive raw materials;
2) a relatively simple synthesis process with fine and highly
homogeneous powder products; and 3) a very low temper-
ature is required to activate the reaction, and importantly the
combustion can then continue without extra energy supply.
Pure metals are important materials that are extensively
used for their unique properties (such as magnetic, catalytic,
superconducting). Furthermore, atoms of pure metals can be
used as building blocks in the construction of different kinds
of alloys. As shown in Figure 1, pure Co, Ni, Cu, Ag, and Bi
were identified. The morphologies of the synthesized metals
were characterized by transmission electron microscopy
(TEM), which showed that the metals and alloys were
composed of nanoparticles of size from several nanometers
to 100 nm.
Figure 2 shows a typical TEM image of the metallic nickel.
The inset of Figure 2 is a high-resolution TEM image of one
grain. The distance between two fringes is 2.03 ꢀ, which fits
Figure 2. TEM image of the prepared nickel metal. Inset: high-resolu-
tion TEM image of one grain. The particle size is estimated as between
10 and 25 nm. The grain size of the nickel is about 10 nm.
[*] Dr. Y. W. Jiang, Prof. S. G. Yang, Dr. Z. H. Hua, Prof. H. B. Huang
Nanjing National Laboratory of Microstructures
Nanjing University (China)
very well to the d spacing of the (111) plane in nickel. From
this image it is estimated that the grain size of nickel is about
10 nm. TEM images of the metals Co, Cu, Ag, and Bi and
Co0.5Ni0.5 alloy can be found in the Supporting Information.
In addition to the synthesis of pure metals, we have
demonstrated the successful synthesis of a series of CoxNi1Àx
(x = 0.2, 0.4, 0.5, 0.6) alloys. The XRD pattern of Co0.5Ni0.5 is
shown in Figure 1 (see the Supporting Information for other
XRD patterns). The (111) peak shifts to a higher angle with
an increase of the ratio of the Ni atom. As the crystalline unit
Fax: (+86)25-8359-5535
E-mail: sgyang@nju.edu.cn
[**] This work was supported by the National Natural Science
Foundation of China, Natural Science Foundation of Jiangsu
Province (BK2009245), New Century Excellent Talents in University
(07-0430), and the “973” Project (2009CB929501).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903444.
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cell parameter of Ni metal is a little smaller, this result
indicates the formation of Co–Ni alloys.
To dissociate and analyze the resulting molecules while
heating the gel, we used mass spectrometry to record the ion
currents released from the gel. The results show that H2, H2O,
CH4, NO, CO2, NH3, and NO2 species were identified near the
combustion temperature. H2 and CH4 are reducing agents
that can be used in the redox reaction for synthesizing metals
from oxides. The mass spectrometric data for H2 and CH4
near the combustion temperature (Figure 3b) illustrate the
release of reducing gas during combustion.
To obtain a good understanding of the sol–gel autocom-
bustion process in the synthesis of metals and alloys, we
characterized the gel for metallic Ni by using thermogravim-
etry (TG) and differential thermal analysis (DTA). The
experiments were carried out at up to 6008C at a heating rate
of 4 KminÀ1 under an argon atmosphere. Figure 3a shows
Based on the data obtained from the TG–DTA and mass
spectrometry analysis, we speculated that there might be five
reactions occurring during burning of the gel at high temper-
ature: 1) exothermic reaction between fuel and oxidant;
2) the formation of metal oxide(s) through decomposition of
the nitrate(s); 3) generation of CH4 and H2 by the decom-
position of CHx-containing groups of citric acid; 4) exother-
mic reaction between CH4/H2 and oxidant; and 5) reduction
of metals from their corresponding metal oxides by CH4 and
H2 in nascent form.
As others have shown, many oxides were synthesized by
the autocombustion method with high temperature (even
more than 10008C) from the exothermic reaction between
fuel and oxidant during the combustion process.[15,16] In our
work, citric acid is used as the fuel and the nitrate ion is used
as an oxidant. Various ratios between fuel and oxidant led to a
variety of maximum temperatures of the reaction, which
would affect the composition of the gas and thus play an
important role in forming the product. Deshpande et al.[16]
reported that the temperature in an inert atmosphere would
be high when the ratio between fuel and oxidant was low.
Although a high temperature would be helpful for reduction
of metal oxides, superfluous oxidant would constrict this
reduction. However, when the ratio was too high, the
temperature would be decreased and thus not be favorable
for the reduction of metal oxides.
The appropriate ratio between fuel and oxidant is critical
for the synthesis of metals by sol–gel autocombustion. We
optimized the reaction by varying the ratio between fuel and
oxidant; the optimization of nickel synthesis is demonstrated
here. Six samples were synthesized with a C6H8O7/Ni(NO3)2
stoichiometric ratio of 0.3:1, 0.5:1, 0.8:1, 1:1, 1.2:1, and 1.5:1.
The XRD patterns of the samples are shown in Figure 4.
Figure 3. a) TG and DTA curves of Ni gel. The abrupt drop in the TG
curve represents the violent combustion of the gel. The three endo-
thermic peaks (marked with arrows A–C) indicate the evaporation of
the remaining water, the desorption of chemically absorbed water, and
the decomposition of NH4NO3 in the gel, respectively. The exothermic
peak at about 2448C results from the combustion of the gel. b) Mass
spectrometry of the Ni gel, from which H2 and CH4 can be detected
near the combustion temperature.
that the weight loss started around 508C, and the correspond-
ing endothermic peak (arrow A) can be assigned to the
evaporation of the remaining water in the gel. The weak
weight loss at 1308C (arrow B) arises from desorption of
chemically absorbed water. Another endothermic peak at
1708C (arrow C) may come from the decomposition of
NH4NO3 formed during the pH adjustment of the solution
mixture. The TG curve shows an abrupt weight loss from 220
to 2458C, which indicates the combustion of the gel. The
strongest exothermic peak is at about 2448C, which is related
to burning. No more weight loss was seen after combustion.
The TG–DTA studies showed that the sol–gel combustion
reaction was finished at about 2448C.
Figure 4. XRD patterns of the products from sol–gel autocombustion
with different fuel/oxidant ratios as indicated. The NiO peaks are
marked with * and the peaks belonging to Ni metal are marked with
*
. The optimized fuel/oxidant ratio is around 1:1.
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obtained in the remaining loose powder. The activation temperature
for sol–gel autocombustion in a tube furnace was 3008C.
There was a very strong XRD signal from NiO in the sample
with the ratio of 0.3:1. By carefully analyzing the XRD data,
we concluded that when the ratio is higher than 1.5:1 or lower
than 0.5:1, nickel oxide is formed in the products. For the
synthesis of nickel metal, the optimized ratio is from 0.8:1 to
1.2:1. The fuel/oxidant ratio was 1:1 in the synthesis of the
metals and metal alloys.
We also noticed from the XRD patterns that the peak
becomes sharper with increasing oxidant content, which
suggests an increase of the reaction temperature. This result
fits well with the analysis of the influence of fuel/oxidant ratio
on the reaction.
Received: June 25, 2009
Revised: August 5, 2009
Published online: October 1, 2009
Keywords: alloys · autocombustion · metals · sol–gel processes ·
.
thermochemistry
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Experimental Section
Metal nitrates and citric acid (C6H8O7·H2O) were used as starting
materials. The gel was prepared by a routine sol–gel method. In the
cases of Co, Ni, and Cu, the pH value of the aqueous solution was
adjusted to 7 by ammonia. In the cases of Ag and Bi, the metal
nitrates were first dissolved in dilute nitric acid, then an appropriate
amount of citric acid was added, and finally the pH value was adjusted
to around 3 with ammonia. In the case of CoxNi1Àx (x = 0.2, 0.4, 0.5,
0.6) alloy, cobalt nitrate and nickel nitrate with stoichiometric ratio
x/(1Àx) were dissolved in aqueous solution, then the pH value was
adjusted to 7 by ammonia. The resultant solution was poured into a
beaker and heated at 958C to develop a dried gel. Under the
protection of inert gas (such as argon or nitrogen), these dried gels
were heated to the preset temperature to activate combustion. Then,
the gel was combusted violently and a large amount of white gas was
released. After cooling to room temperature, the metals were
[16] K. Deshpande, A. Mukasyan, A. Varma, Chem. Mater. 2004, 16,
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