Smart Solution Chemistry to Sn-Containing Intermetallic Compounds through a Self-Disproportionation Process

Article abstract of DOI:10.1002/chem.201601681

Developing new methods to synthesize intermetallics is one of the most critical issues for the discovery and application of multifunctional metal materials; however, the synthesis of Sn-containing intermetallics is challenging. In this work, we demonstrated for the first time that a self-disproportionation-induced in situ process produces cavernous Sn?Cu intermetallics (Cu₃Sn and Cu₆Sn₅). The successful synthesis is realized by introducing inorganic metal salts (SnCl₂?2 H₂O) to NaOH aqueous solution to form an intermediate product of reductant (Na₂SnO₂) and by employing steam pressures that enhance the reduction ability. Distinct from the traditional in situ reduction, the current reduction process avoided the uncontrolled phase composition and excessive use of organic regents. An insight into the mechanism was revealed for the Sn?Cu case. Moreover, this method could be extended to other Sn-containing materials (Sn?Co, Sn?Ni). All these intermetallics were attempted in the catalytic effect on thermal decompositions of ammonium perchlorate. It is demonstrated that Cu₃Sn showed an outstanding catalytic performance. The superior property might be primarily originated from the intrinsic chemical compositions and cavernous morphology as well. We supposed that this smart solution reduction methodology reported here would provide a new recognition for the reduction reaction, and its modified strategy may be applied to the synthesis of other metals, intermetallics as well as some unknown materials.

Full text of DOI:10.1002/chem.201601681

DOI: 10.1002/chem.201601681
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&
Metal–Metal Interactions |Hot Paper|
Smart Solution Chemistry to Sn-Containing Intermetallic
Compounds through a Self-Disproportionation Process
Yuelan Zhang,[a, b] Liping Li,*[b] Qi Li,[b] Jianming Fan,[b] Jing Zheng,[b] and Guangshe Li*[a]
Chem. Eur. J. 2016, 22, 1 – 10
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Abstract: Developing new methods to synthesize interme-
tallics is one of the most critical issues for the discovery and
application of multifunctional metal materials; however, the
synthesis of Sn-containing intermetallics is challenging. In
this work, we demonstrated for the first time that a self-dis-
proportionation-induced in situ process produces cavernous
SnꢀCu intermetallics (Cu3Sn and Cu6Sn5). The successful syn-
thesis is realized by introducing inorganic metal salts
(SnCl2·2H2O) to NaOH aqueous solution to form an inter-
mediate product of reductant (Na2SnO2) and by employing
steam pressures that enhance the reduction ability. Distinct
from the traditional in situ reduction, the current reduction
process avoided the uncontrolled phase composition and
excessive use of organic regents. An insight into the mecha-
nism was revealed for the SnꢀCu case. Moreover, this
method could be extended to other Sn-containing materials
(SnꢀCo, SnꢀNi). All these intermetallics were attempted in
the catalytic effect on thermal decompositions of ammoni-
um perchlorate. It is demonstrated that Cu3Sn showed an
outstanding catalytic performance. The superior property
might be primarily originated from the intrinsic chemical
compositions and cavernous morphology as well. We sup-
posed that this smart solution reduction methodology re-
ported here would provide a new recognition for the reduc-
tion reaction, and its modified strategy may be applied to
the synthesis of other metals, intermetallics as well as some
unknown materials.
Introduction
affect the electronic environment and surface active sites in
relevant reactions. Because there is a strong relationship be-
tween the formation of metallic bonds and synthetic strategy,
numerous efforts have been explored recently for the prepara-
tion of Sn-containing intermetallic compounds. For example,
many synthetic routes have been reported for the Sn-contain-
ing inermetallics, such as physical (ball-milling),[9] electrochemi-
cal (electrodeposition and electroplating),[10] and chemical re-
duction (reductants like N2H4, NaBH4, polyalcohol, oleylamine,
etc.) processes.[11] With these synthetic routes, well-defined
monodisperse SnꢀCu,[11c,d] Pt3Sn,[12] and SnꢀSb, etc.,[11e] nano-
intermetallic compounds have been synthesized successfully.
Unfortunately, an important restriction on the fabrication of
monodisperse particles lies in the strict condition and utiliza-
tion of toxic organic capping agents, solvents, reducing
agents, and organo-metals. Obviously, using these organic re-
agents or organo-metal salts inevitably leads to a high-cost of
fabrication method and a striking negative effect from environ-
mental concerns,[13] which further restricts the applications of
these compounds due to the long time to commercialization.
On the other hand, traditional inorganic synthesis methods
have been demonstrated to possess many advantages in ob-
taining products in large quantities except for the difficulty in
controlling the phase compositions. Therefore, an improved
direct preparation of organic-free Sn-containing intermetallic
method is necessary and urgent.
As a special class of metal materials, intermetallic compounds
exhibit optical, electrical, magnetic, and catalytic properties dif-
ferent from single components, metal mixtures, or disordered
alloys.[1] In general, the newly formed metal bonding and rear-
ranged atomic sequence endow intermetallic compounds with
improved performance such as excellent high-temperature
structure stability,[2] enhanced cycling stability in lithium-ion
batteries,[3] superior oxygen-reduction-reaction activity, and
highly efficient catalytic performance.[4] Intermetallic com-
pounds may be the ideal candidates of low-cost catalysts with
desired performance.[5]
Among varied transition metal intermetallic materials, Sn-
containing intermetallic compounds are promising, because
1) tin is a nontoxic, inexpensive, naturally highly abundant ele-
ment, and 2) compared with Sn metal, Sn-containing interme-
tallic compounds show stable materials properties (like superi-
or lithium-ion-battery properties)[6] and potential applications
like those in solar cells[7] and catalysis.[8] These advanced prop-
erties are directly associated with the newly formed chemical
bonds between diverse metal atoms, which subsequently
[a] Y. Zhang, Dr. G. Li
States Key Laboratory of Inorganic Synthesis and Preparative Chemistry
College of Chemistry, Jilin University, Changchun 130012 (P.R. China)
E-mail: guangshe@jlu.edu.cn
To develop an organic-free method for homogeneous Sn-
containing intermetallic compounds, two main factors have to
be considered: one is associated with the reductants and an-
other is related to the reaction conditions in which atoms rear-
range to form the metallic bond. It is well known that in inor-
ganic reaction system, metal ions would be reduced firstly at
the presence of reductants. Subsequently, the final products
would be formed by the atoms diffusion. The nearly independ-
ently discrete procedures often make the phase compositions
uncontrolled. By virtue of these considerations, an in situ syn-
thetic strategy inspires us. That is, the reduction of metal ions
and the formation of metallic bonding might occur simultane-
ously. To achieve this vision, we subtly designed a self-dispro-
[b] Y. Zhang, Dr. L. Li, Q. Li, J. Fan, J. Zheng
Key Laboratory of Design and Assembly of Functional Nanostructures
Fujian Institute of Research on the Structure of Matter
Fuzhou 350002 (P.R. China)
E-mail: lipingli@fjirsm.ac.cn
Supporting information for this article (containing tables listing products
obtained under different experiment conditions, XRD patterns of all the
products using C4H6MnO4, Fe(NO3)3·9H2O, C4H6CoO4·4H2O, C4H6NiO4·4H2O
and CuCl2·2H2O as the source of metal salts, Rietveld refinement results of
the sample SnꢀCo, SnꢀNi, Cu6Sn5 and Cu3Sn, SEM and EDS data of SnꢀCo
prepared at 2208C, SEM and EDS data of SnꢀNi prepared at 1208C, strain
data of SnꢀCo, Cu6Sn5 and Cu3Sn, XRD patterns of SnO2 (SA1), SnO (SA3)
and Sn (SA5) in 2 theta range of 20 to 808, and the related table of specific
surface area) is available on the WWW under
http://dx.doi.org/10.1002/chem.201601681.
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portionation-induced process to synthesize Sn-containing in-
termetallic compounds in situ utilizing the valence state varia-
bility and tunability of tin element. In this process, stannous
chloride hydrate (SnCl2·2H2O) is designed as both the reagent
and Sn source. Moreover, this self-disproportionation-induced
in situ process is totally different from the traditional in situ re-
duction of intermetallic compounds. For the latter case, extra
reductants are necessary, and the reductant is generally irrele-
vant to the final products.[11d,14] The synthetic mechanism of
the organic-free method was experimentally determined. Fur-
ther, we demonstrate that the obtained SnꢀCu intermetallics
are promising candidates as additives in solid fuels to efficient-
ly decrease the thermal decomposition temperature of ammo-
nium perchlorate (AP). We expect that this new synthetic route
may be modified and applied toward other intermetallic mate-
rials as well as metals and oxides.
Table 1. Experimental conditions and products in the organic-free hydro-
thermal reaction for preparing CuꢀSn intermetallics.
Sample
t
[h]
CNaOH
[m]
CuCl2·2H2O
[mmol]
Products
(XRD analysis)
SA1
SA2
SA3
SA4
SA5
SA6
SA7
SA8
6
6
6
6
6
6
6
6
6
6
0
0.5
1
2
4
0
0.1
1
3
6
6
6
6
6
6
6
6
6
6
6
0
0
0
0
0
2
4
6
8
10
8
8
8
8
8
SnO2
SnO2
SnO+SnO2
SnO+Sn
Sn
Cu6Sn5 +Sn
Cu6Sn5 +Sn[a]
Cu6Sn5 +Cu3Sn
Cu3Sn+Cu[a]
Cu3Sn+Cu
Cu2O
Cu2O+Cu
Cu3Sn+Cu
Cu3Sn+Cu
Cu3Sn+Cu
[a]
SA9
SA10
SA11
SA12
SA13
SA14
SA15
[a] Represents the phases in low amounts.
Results and Discussion
Experimental design and general research
We demonstrated that different Sn-containing intermetallic
compounds have been synthesized by a self-disproportiona-
tion-induced in situ process. The synthetic procedure was de-
scribed as follows (Figure 1). Inexpensive metal salts are used
Figure 1. Outline of the self-disproportionation-induced in situ synthesis
strategy for Sn-containing intermetallic compounds.
Figure 2. XRD patterns and SEM images of the samples: (a,b) Sn (sample
SA5) obtained without addition of Cu species, (c,d) Cu6Sn5 (sample SA7), and
(e,f) Cu3Sn (sample SA9) obtained with different amounts of Cu species in-
volved (insets are the structure diagrams).
as the metal precursors, and stannous chloride alkali solution
serves as both the reductant and tin source. According to the
basic theory of inorganic chemistry, the synthetic process
should consist of three major steps: 1) generation of Na2SnO2
solution when SnCl2·2H2O dissolves in NaOH aqueous solution;
2) hydroxides or oxides form when Mn+ (M=Mn, Fe, Co, Ni,
Cu) is imported into the solution to react with OHꢀ. In this
step, Mn+ ions may be reduced to the lower valence state
ions; 3) during the following hydrothermal process, metallic
clusters may develop gradually and intermetallics finally occur
because the reduction ability of Na2SnO2 is promoted through
the assistance of heat and pressure of the hydrothermal reac-
tions.
termetallics by the combination of XRD and TEM analyses.
Moreover, most XRD diffractions of SA7 in Figure 2c could be
indexed in terms of Cu6Sn5 (JCPDS, No. 45-1488), except for
a weak peak assigned to Sn. Figure 3b presents HRTEM image
of sample SA7. The clear interplane distance of 0.297 nm is
closer to the d-value of (22-1) diffraction of monoclinic Cu6Sn5,
a main constitution of sample SA7. With the same method,
sample SA9 was determined to be Cu3Sn intermetallic (Fig-
ure 2e), and clear interplane distance of 0.216 nm in HRTEM
image (Figure 4c) also agrees with the d-value of (002) diffrac-
tions of Cu3Sn (JCPDS, No. 01-1240).
XRD patterns and SEM images of three typical samples (SA5,
SA7 and SA9 as defined in Table 1) are presented in Figure 2.
Sample SA5 obtained with no introduction of Cu species in the
synthetic procedures shows blocky particles with micron size.
Comparatively, samples SA7 and SA9, obtained with moderate
addition of Cu species, showed cavernous particles in nano-
scale with a large number of holes. X-ray diffraction lines of
sample SA5 in Figure 2a matched the standard data of tetrag-
onal Sn (JCPDS, No. 04-0673) well, suggesting that sample SA5
with a blocky shape was pure metallic Sn. The other two sam-
ples with cavernous morphologies were identified as CuꢀSn in-
During the formation of SnꢀCu intermetallics under hydro-
thermal conditions, the content of Cu species involved greatly
altered the morphology, size distribution as well as the phase
structure of the as-synthesized samples. From the TEM images
of Cu6Sn5 and Cu3Sn in (Figure 3a and 4a), one can see distinct
holes with 100–200 nm in average diameter. Thus, CuꢀSn inter-
metallic compounds with a cavernous morphology and differ-
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cessfully synthesized using a self-disproportionation-induced in
situ reduction reaction without any organic species.
Our synthetic method could be easily applied to the prepa-
ration of Sn/transition intermetallic materials such as SnꢀCo
and SnꢀNi intermetallic compounds (the Supporting Informa-
tion, Table S1). Moreover, all intermetallic compounds are pre-
pared under hydrothermal conditions. The presence of high-
temperature and pressure has a significant effect on the forma-
tion of Sn-intermetallic compounds. Surprisingly, XRD pattern
for the sample obtained at 1208C (Figure 5c) by importing
Figure 3. a) TEM and b) HR-TEM image of Cu6Sn5 (sample SA7); c) TEM-EDS
spectrum of Cu6Sn5 with molar ratio of Cu to Sn at 29.87:29.53; d) dark-field
TEM spectrum of Cu6Sn5 (sample SA7) and EDS elemental mapping images
of e) Cu and f) Sn.
Figure 5. SEM images of the as-synthesized samples: a) SnꢀCo intermetallic
compounds with the addition of Co2+ at 1208C, and b) SnꢀNi intermetallic
compounds with the addition of Ni2+ at 2208C; c) and d) the relevant XRD
patterns of SnꢀCo and SnꢀNi intermetallic compounds (blue and red lines
at the bottom of each Figure represent the standard diffraction lines of
Co3Sn2 (JCPDS, No. 27-1124) and Ni3Sn4 (JCPDS, No. 65-4310), respectively);
e) and f) related EDS images of SnꢀCo and SnꢀNi intermetallic compounds.
Figure 4. a) TEM image, b) TEM-EDS spectrum and c) HR-TEM image of
Cu3Sn (sample SA9); d) dark-field TEM of Cu3Sn and EDS elemental mapping
images of e) Cu and f) Sn.
C4H6CoO4 does not match any diffraction data existed in the
standard diffraction cards because of the disappearance of ad-
ditional peaks, such as that at two theta of 448. Careful data
analysis indicates that the sample obtained at 1208C might be
hexagonal SnꢀCo with a group space of P63/mmc as confirmed
by Rietveld refinements in Figure S4 (the Supporting Informa-
tion). The atomic ratio of Co to Sn at 1.06:1 was further con-
firmed by EDS (Figure 5e). However, higher temperature would
lead to a higher atomic ratio of Co/Sn at 2.56:1, and appear-
ance of impurities like CoSn(OH)6 and other unknown phases
(the Supporting Information, Figure S5b, S3b). Except for the
changed phases, the morphologies of samples were also al-
tered by reaction temperature. From SEM images in Figure 5a,
one can see that SnꢀCo intermetallic prepared at 1208C
showed regular polygon, distinct from the random packed
ent phase compositions can be synthesized by varying the re-
action conditions.
To confirm the element constituent of the as-synthesized
CuꢀSn intermetallics, we examined their EDS spectra and ele-
mental mapping images. Clear Cu and Sn signals were ob-
served, along with the presence of few C or O elements that
may come from non-dried ethanol dispersion liquid and the
absorbance of H2O (Figure 3c and 4b). EDS mapping images
(Figures 3e, f and 4e, f) show the uniform distribution of Sn
and Cu species, which confirms the formation of homogene-
ous CuꢀSn intermetallic particles. In other words, homogene-
ous Cu6Sn5 and Cu3Sn intermetallic compounds have been suc-
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products obtained at 2208C (the Supporting Information, Fig-
ure S5a). The existence of impurities and the higher tempera-
ture might contribute to the cracking substances and the co-
hered bulk morphology of the products obtained at 2208C.
More importantly, well-shaped regular polygon SnꢀCo interme-
tallic compound is obtained for the first time, which is different
from the random stacked particles[6a] and nanowire of SnꢀCo
intermetallic[10d] reported previously. Just like SnꢀCo intermetal-
lic compounds, SnꢀNi was synthesized by the same strategy
(Figure 5d and the Supporting Information, Figure S6b). Differ-
ent from SnꢀCo compounds, SnꢀNi product obtained at
higher temperature of 2208C showed a relatively pure mono-
clinic phase with space group of C2/m (the Supporting Infor-
mation, Figure S7), and the product obtained at lower temper-
ature of 1208C contained excessive Sn (Figure S6c). EDS analy-
ses was further used to detect the atomic ratio of Ni to Sn.
Sample prepared at 1208C (the Supporting Information, Fig-
ure S8b, Ni/Sn=1.07) showed a lower Ni/Sn ratio than that at
2208C (Figure 5 f, Ni/Sn=1.82). From SEM images of SnꢀNi in-
termetallic in Figure 5b, it can be seen that popcorn-like SnꢀNi
intermetallic was synthesized at 2208C, obviously distinct from
those of irregular morphology and porous SnꢀNi intermetallic
microcages prepared by other methods.[15] Comparatively, the
products obtained at lower temperature of 1208C exhibited
a shape of nanoflower and self-assembled nanospheres (the
Supporting Information, Figure S8a). The formation of nonuni-
formed shape may be due to the incomplete reaction at
1208C: as indicated by XRD, there is little foreign matter of Sn
mixed in the final product (the Supporting Information, Fig-
ure S6c). The formation of regular polygon SnꢀCo and pop-
corn-like SnꢀNi intermetallic has indicated the unique feature
of the current method: that is, it could regulate the develop-
ment of metallic clusters and crystal nucleus of intermetallic by
slow reduction process of Na2SnO2. Moreover, the reductant
Na2SnO2 was introduced to the products subtly through a dis-
proportionation reaction. We expect that the changeable and
special morphology of Sn-containing intermetallic compounds
might offer potential application somewhere.
occur as initially intended, which could further demonstrate
that the temperature and pressure are the driving force for the
reduction effect of Na2SnO2. In the preparation of SnꢀCu inter-
metallic, there is also a high-temperature phase transition pro-
cess. To understand the synthesis mechanism, a case study in
SnꢀCu system was conducted in details.
Growth mechanism: A case study for the formation of Snꢀ
Cu intermetallic compounds
To further understand the evolution process and uncover the
growth mechanism of this self-disproportionation-induced in
situ synthesis method of Sn-containing intermetallic, we con-
ducted a series of experiments in a SnꢀCu case by varying the
experimental conditions including the concentration of base
(CNaOH), amount of Cu species (nCuCl2·2H2O), and reaction
time (t). Therefore, fifteen samples were thus obtained, as
listed in Table 1. Surprisingly, the oxidation state of metal ele-
ments in the products decreases serially as NaOH concentra-
tion increases (Figure 6). We found that SnO2 could be ob-
Figure 6. XRD patterns of the samples: a) SA1 (SnO2), b) SA3 (SnO with
a little SnO2), and c) SA5 (Sn) synthesized by adjusting the base concentra-
tion from 0m to 6m. The pink line, red line and green line at the bottom of
each figure represent the standard diffraction lines of SnO2 (JCPDS, No. 41-
1445), SnO (JCPDS, No. 06-0395) and Sn (JCPDS, No. 04-0673), respectively.
The related crystal structures with the main lattice planes marking were
placed in the right panel. The gray, blue and purple planes represent (101),
(110) and (200) planes, respectively.
The reduction reaction induced by high temperature and
pressure of hydrothermal systems may enhance the reduction
ability and hence promote the formation of intermetallic com-
pounds. On the contrary, only oxides and hydroxides were ob-
tained (the Supporting Information, Table S1) under mild con-
ditions (atmosphere, room temperature). In addition to SnꢀCo,
SnꢀNi, and SnꢀCu intermetallic compounds, only oxide (g-
Fe2O3, a-Fe2O3) and hydroxide (MnSn(OH)6) were produced
when adding Fe(NO3)3·9H2O and C4H6MnO4 in the hydrother-
mal reaction (the Supporting Information, Figures S1 and S2).
This observation suggests that the reduction potential plays
a critical role in this designed reaction. Certainly, the metal in-
trinsic properties also influenced the phase compositions of
product because the formation of intermetallic compounds
mainly underwent two processes: the reduction of metal ions
and the combination of metal atoms with metallic bond. It
should be emphasized that room temperature and absence of
pressure could not stimulate the reduction reaction of metal
ions. Only under hydrothermal conditions, the reaction could
tained without or with a small amount of NaOH addition
(CNaOH=0.1 molLꢀ1), and also SnO began to appear along
with SnO2 at a relatively high base concentration (CNaOH=
1 molLꢀ1). Metallic Sn appeared simultaneously with SnO at
a much higher base concentration (CNaOH=3 molLꢀ1), where-
as pure Sn was obtained when the base concentration reached
6 molLꢀ1. The base concentration indeed affects the structure
and morphology of tin oxides, as reported elsewhere.[16] How-
ever, only controlled synthesis of SnO2 and SnO was successful-
ly done when using NH3·H2O as the alkali source,[16a] and the
formation of Sn has not been found ever before. Thus, a series
of oxidation state for tin compounds are synthesized, and the
modulation for the synthesis becomes easily controlled by
base concentration. Under lower base concentration condition,
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the addition of SnCl2·2H2O would form a milky solution due to
the presence of the hydrolysis product Sn(OH)2, which would
be easily oxidized and dehydrated to form SnO2 eventually. In
contrast, with higher base concentration, SnCl2·2H2O would
dissolve in NaOH solution to form Na2SnO2. Na2SnO2 is water
soluble and would undergo a disproportionation reaction as
Equation (1) during the hydrothermal process, resulting in
metal Sn. Similarly, when the base concentration was tuned in
a median range such as adding 1–3m NaOH, the solution con-
tained Na2SnO2 and Sn(OH)2 before hydrothermal reaction. In-
sufficient Na2SnO2 would restrain the oxidation reaction to
some extent, resulting in SnO2 and SnO mixtures after hydro-
thermal reaction.
Cu to Sn also plays an important role besides the temperature.
Cu6Sn5 and Cu3Sn could be synthesized with 4 mmol and
8 mmol of CuCl2·2H2O added, respectively.
To get insight into the growth mechanism of intermetallic
particles, time-dependent preparations were conducted for
Cu3Sn case. In the experimental process, when CuCl2·2H2O was
added into the mixed solution of NaOH and SnCl2, the color of
the suspension changed from green to red gradually. XRD pat-
terns of the red precipitate in Figure 7 (line g, t=0 h) indicates
the formation of Cu2O. In terms of Equation (2), the occurrence
of Cu2O is due to the reduction of Cu species by Na2SnO2. As
the reaction time is prolonged from 0.5 to 1 h (line h, i), the
diffraction peaks of Cu2O vanished gradually, while the peak in-
tensities of Cu3Sn and Cu keep increasing. Then, Cu disap-
peared gradually (line j, k, t=2, 4 h), and relatively pure Cu3Sn
formed eventually (line l, t=6 h). Surprisingly, in this time-de-
pendent evolution process, there is no evidence for the ap-
pearance of Sn, likely due to the direct in situ formation of Sn
in Cu3Sn.
D
2 ½SnO2ꢁ2ꢀ þ H2O
½SnO3ꢁ2ꢀ þ Sn þ 2 OHꢀ
ð1Þ
!
By using the same reaction method, we also synthesized
SnꢀCu intermetallic compounds with phase composition vary-
ing from Cu6Sn5 to Cu3Sn by adjusting the amount of
CuCl2·2H2O. As indicated by XRD patterns in Figure 7, Cu6Sn5
Taking the formation of Cu3Sn for example, according to the
time-dependent evolution process (Figure 7), the overall syn-
thesis is graphically described in Figure 8. Three main paths
Figure 7. a–f) XRD patterns of the samples prepared by adjusting the addi-
tion amount of CuCl2·2H2O at 0, 2, 4, 6, 8, 10 mmol respectively; g–l) XRD
patterns that illustrate the time-dependent evolution process for the forma-
tion of Cu3Sn (sample SA9) at the reaction time of 0, 0.5, 1, 2, 4, 6 h, respec-
tively.
Figure 8. Depiction of the synthesis of SnꢀCu intermetallic compounds
through a simple in situ strategy. Inset in the upper-right corner illustrates
the possible specific formation process of Cu3Sn.
began to appear accompanying with a decrease in diffraction
intensity of Sn at lower concentration of Cu species (line b,
nCuCl2·2H2O=2 mmol). When nCuCl2·2H2O increases to
4 mmol (line c), the phase of Sn disappeared almost. As
nCuCl2·2H2O reaches 6 mmol (line d), a mixture of Cu6Sn5 and
Cu3Sn was obtained. The phase of Cu6Sn5 vanished when
nCuCl2·2H2O is set at 8 mmol (line e), yielding Cu3Sn phase
with a little Cu in the final product (the Supporting Informa-
tion, Figure S11). If continuously increasing nCuCl2·2H2O to
10 mmol (line f), impurity of metallic Cu increased. Previous
study on the evolution of Cu6Sn5 to Cu3Sn is basically depen-
dent on the control of temperature or aging time: lower tem-
perature tends to form Cu6Sn5, whereas a higher temperature
is more likely to produce Cu3Sn.[11a,c] If the temperature remains
unchanged, Cu6Sn5 is initially formed and then comes to
Cu3Sn, because Cu6Sn5 grows much faster than Cu3Sn.[17] How-
ever, in the present hydrothermal reaction, the proportion of
are depicted as follows: 1) before heating, Cu2+ is reduced to
Cu2O by the pre-formed Na2SnO2 solution with SnCl2·2H2O dis-
solved in NaOH solution [Eq. (2)]; 2) in the initial hydrothermal
reaction, that is, heating the solution in Teflon-lined stainless
steel autoclave, as the temperature and pressure increased in
the hydrothermal process, the reduction ability of Na2SnO2 en-
hanced and subsequently furthers the reduction of Cu2O to
metallic Cu [Eq. (3)] because the redox potential of Cu2O/Cu
(ꢀ0.360 V vs. SHE, SHE=standard hydrogen electrode) is
higher than [SnO2]2ꢀ/Sn (ꢀ0.909 V vs. SHE); 3) at last, yet the
most critical step, [SnO2]2ꢀ ions around Cu particles are dispro-
portionated to Sn, which in situ bonds with Cu to form Cu3Sn,
easily removing [SnO3]2ꢀ from the surrounding solution. The
2ꢀ
disproportionation reaction of SnO2 and the formation pro-
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cess of Cu3Sn might occur almost at the same time as in Equa-
tion (4) because no diffraction peak of Sn was identified in the
products of intermediate procedures. The possible critical step
of Cu3Sn was depicted as the upper-right corner, inset in
Figure 8, in which Sn rapidly bonded with Cu. We could also
propose that Sn atoms formed directly in the lattice position
of Cu3Sn in this specific in situ method.
cess of AP. It is well known that[20] the LTD process involves
a heterogeneous process that includes 1) proton transfer from
+
ꢀ
NH4 to ClO4 accompanied with hydrogen transfer over the
lattice to yield NH3 and HClO4, and followed by 2) the decom-
position of HClO4 and reaction with NH3. Compared with pure
AP, all as-prepared Sn-containing intermetallic samples have
shown a promoted effect on the decomposition of AP, espe-
cially SnꢀCu intermetallic compounds. The low/high tempera-
ture decomposition (LTD/HTD) peak was reduced from 326/
4238C for AP to 317/3848C for Cu6Sn5 and further to 310/
3348C for Cu3Sn, reducing near 1008C in HTD process.
½SnO2ꢁ2ꢀ þ 2 Cu2þ þ 4OHꢀ ! Cu2O þ ½SnO3ꢁ2ꢀ þ 2 H2O
ð2Þ
ð3Þ
ð4Þ
2ꢀ
½SnO2ꢁ2ꢀ þ Cu2O ! 2 Cu þ ½SnO3ꢁ
We examined four possible factors that could be responsible
for the improved catalytic performance:
3 Cu þ 2½SnO2ꢁ2ꢀ þ H2O ! Cu3Sn þ ½SnO3ꢁ2ꢀ þ 2OHꢀ
1) Strain. A recent work[21] concludes that strain can increase
the catalytic activities. We calculated the effective strain of
Sn-containing intermetallic compounds. Thus, effective
strain was calculated using the Williams and Hall theorem
(the Supporting Information, Figure S12).[22] Except for Snꢀ
Ni with lots of overlapping peaks, which is difficult to calcu-
late, all the other samples are found to show much smaller
strain (hCu3Sn =0.49%, hCu6Sn5 =0.13%, hSnꢀCo =1.40%), illus-
trating that the strain might has little effect in this system.
2) Specific surface area. For almost all porous materials, sur-
face area is often considered important for catalysis be-
cause of its relevance to absorption and desorption. BET
data (the Supporting Information, Table S2) of all as-pre-
pared intermetallics are extremely low (<4 m2gꢀ1), which
could help us to get rid of the influence of contact area on
the catalytic performance.
Promoted effect in the thermal decomposition of AP
As a typical inorganic energy material, AP is one of the main
oxidizing agents that have been used in various propellants,
and the burning ability of propellants is highly relevant to the
thermal decomposition of AP.[18] Typically, metal, alloy powders,
and metal oxides have advantage in the decomposition behav-
ior of AP.[18a,b,19] Previous investigations have shown that Ni, Co,
Cu, AgNi, and so on[18a,b] have a significant catalytic effect in re-
ducing the decomposition temperature. Sn-containing inter-
metallics may show good activity in this regard. To investigate
the catalytic performance in AP thermal decomposition, varied
Sn-containing intermetallic materials as-synthesized were ini-
tially mixed with AP at a ratio of 2% (SnxMy/AP). Figure 9a
shows the raw TG data and their first derivative curves for
pure AP and AP mixed with 2% as-prepared Sn-containing in-
termetallic additives by mass. For pure AP, two peaks were ob-
served in the first derivative curve of TG data, indicating two
mass loss processes. The first mass loss peak was located at
about 3268C is associated with the low-temperature decompo-
sition (LTD) process of AP and the second peak at 4238C is as-
sociated with the high-temperature decomposition (HTD) pro-
3) Compositions. As mentioned above, lower surface area
with a superior performance means plenty of high activity
sites exist. Thus, we suppose that the composition plays
a leading role in improving the catalytic properties, since as
reported elsewhere, the conductivity of Cu is the best
among non-noble metals and Cu3Sn showed similar con-
ductivity with Cu.[23] Thus, the accelerated electron transfer
process would promote the catalytic activities.
4) Cavernous morphology. Cavernous morphology with large
amounts of holes would provide better dispersion effect
for catalytic reaction.[24]
Representative DSC curves for AP carried out in different
ramping rate are shown in Figure 9(b,c) to illustrate the heat
release during AP decomposition process. For pure AP and AP/
Cu3Sn mixture, three peaks were observed in their respective
DSC curves. The first endothermic peak at about 2408C, with
no associated weight loss, is due to the phase transformation
of AP from orthorhombic to cubic, and the transformation
peak slowly moved to higher temperature as the ramping rate
increased. The second exothermic peak with broad shape lo-
cated in ꢂ3008C corresponds to the LTD process, and the
third peak in high temperature belongs to the HTD process.
When the ramping rate was controlled at 5 Kminꢀ1, 10 Kminꢀ1
or 15 Kminꢀ1, the HTD peak for AP changed to endothermic
(Figure 9b), which is the same as pure AP decomposition with-
out lids reported before.[18a] This endothermic peak can be al-
Figure 9. a) TG curves (black line) for the decomposition of AP and mixtures
of AP with 2 wt% as-prepared sample (SnꢀCo synthesized at 1208C, SnꢀNi
synthesized at 2208C, Cu6Sn5, Cu3Sn) at ramping rate of 15 Kminꢀ1 measured
using aluminum pans; red lines are the first derivative curves of the raw TG
data. DSC curves of b) AP without and c) with additives of 2% Cu3Sn at dif-
ferent ramping rates.
Chem. Eur. J. 2016, 22, 1 – 10
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(180 mmol) were dissolved in deionized water (30 mL), then
SnCl2·H2O (15.8 mmol) was added after the dissolution induced
warming of base solution cooling down to room temperature. The
metal compound (C4H6MnO4, Fe(NO3)3·9H2O, C4H6CoO4·4H2O,
C4H6NiO4·4H2O, CuCl2·2H2O) (4 mmol) was added at the end of the
stirring process for a few minutes before hydrothermal treatment.
The obtained solution was transferred to a 100 mL Teflon-lined
stainless steel autoclave. Different reaction conditions, including
room temperature reaction, hydrothermal at 1208C and 2208C for
6 h, were explored to investigate their influence on the products,
as illustrated in Table S1 (the Supporting Information).
tered to exothermic peak when the as-prepared Cu3Sn was
added in AP (Figure 9c). For the decomposition behavior of
AP/Cu3Sn, both LTD and HTD peaks remained exothermic
peak. However, the decomposition temperature is dependent
on the ramping rate and the location of these peaks shifted to
higher temperatures as the ramping rate increased, which has
been described by the Kissinger correlation.[25]
Conclusion
We have developed a new, self-disproportionation-induced in
situ synthesis approach to synthesize cavernous Cu6Sn5 and
Cu3Sn intermetallic compounds. This synthetic method has
been directly applied to other Sn-containing intermetallic ma-
terials such as SnꢀCo and SnꢀNi. Owing to the high tempera-
ture and pressure during hydrothermal procedure, the reduc-
tion ability of intermediate Na2SnO2 increased gradually, and
hence an in situ reduction reaction occurred, that is to say the
disproportionation of Na2SnO2 and the formation of SnxMy oc-
curred simultaneously. The special growth environment also re-
sulted in the well-shaped samples (popcorn-like SnꢀNi, regular
polygon SnꢀCo). All these syntheses utilize only inexpensive
inorganic commercial precursors and the insight mechanism
was confirmed through a series of experiments and several
combined measurements. Additionally, we showed that SnꢀCu
intermetallic compounds are promising candidates as catalytic
additives in promoting the thermal decomposition of ammoni-
um perchlorate (AP). This method will enable access to previ-
ously unknown materials. Further, the new inorganic self-dis-
proportionation-induced in situ synthesis strategy reported
here would afford a new recognition for reduction reaction
and open up horizons for the synthesis toward other impor-
tant materials.
Controlled synthesis of CuꢀSn intermetallic compounds
CuꢀSn intermetallic compounds were synthesized similar to the
preparation conditions of Sn-containing intermetallic compounds
as described above except for the varied amounts of CuCl2·2H2O
(2, 4, 5, 6, 8 and 10 mmol) and reaction time (0, 0.5, 1, 2, 4 and 6 h)
in the synthetic procedure. The as-obtained samples are labeled as
SA1 to SA10 as illustrated in Table 1.
Characterization
The structures of the samples were characterized by X-ray diffrac-
tion (XRD) on a Rigaku Mini FlexII bench top X-ray diffractometer
using a copper target. The lattice parameters of the samples were
measured by a least-squares method using Rietica Rietveld soft-
ware. Size, morphology, and elemental distribution of the products
were investigated by transmission electron microscope (TEM) using
JEM-2010 apparatus with an acceleration voltage of 200 kV. Sam-
ples for the measurements of TEM, high resolution transmission
electron microscope (HRTEM), and energy disperse spectroscopy
(EDS) mapping images were prepared by dispersing the powder in
ethanol and leaving a droplet of the suspension on a molybdenum
microscope grid covered with perforated carbon. Particle morphol-
ogies of the prepared samples were studied by field-emission scan-
ning electron microscopy (SEM) (JEOL, model JSM-6700). The cata-
lytic roles of Sn and Sn-containing intermetallic compounds in the
thermal decomposition of ammonium perchlorate (AP) were de-
tected by thermogravimetric analysis (TG) and differential scanning
calorimeter (DSC) in N2 atmosphere over the temperature range of
100–5008C (Netzsch STA 449F3 thermal analyzer). To investigate
the influence of ramping rates in the thermal decomposition, the
ramping rates were varied from 5 Kminꢀ1 to 20 Kminꢀ1 with
a fixed mass ratio (sample/AP=2%). Nitrogen adsorption isotherm
was performed in an apparatus of Micromeritics ASAP 2020. Specif-
ic surface area was calculated by using the BET equation.
Experimental Section
Synthesis of tin and tin oxide
All chemicals were used as received analytical reagent (AR) without
further purification. SnO2, SnO, and Sn was synthesized by a hydro-
thermal process using stannous chloride (SnCl2·2H2O) and sodium
hydroxide (NaOH) as the starting materials. Details of the prepara-
tion are described as follows: A certain amount of NaOH (0, 3, 30,
90, and 180 mmol) was dissolved in deionized water (30 mL), then
SnCl2·H2O (15.8 mmol) was added and after the base solution cool-
ing to room temperature. The obtained solution was transferred to
a 100 mL Teflon-lined stainless steel autoclave, which was allowed
to react at 1208C for 6 h. When the reaction was ended, Teflon au-
toclave cooled naturally to room temperature. The resulting sus-
pensions were collected by centrifugation at the speed of
8000 rmp for 5 min, and the precipitates were washed four to five
times to remove the Clꢀ residue solution. Finally, the products
were dried at 608C in air for 6 h.
Acknowledgements
This work was financially supported by NSFC (Grants
21025104, 21271171, and 91022018)
Keywords: metal–metal interactions · reduction · synthetic
methods · tin · transition metals
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Published online on && &&, 0000
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FULL PAPER
Smart solution: A new self-dispropor-
tionation-induced in situ reduction
strategy for the synthesis of Sn-contain-
ing intermetallic compounds was pro-
posed for the first time. Using this strat-
egy, polyhedron SnꢀCo, popcorn-like
SnꢀNi and cavernous CuꢀSn intermetal-
lic compounds were synthesized (see
figure).
&
Metal–Metal Interactions
Y. Zhang, L. Li,* Q. Li, J. Fan, J. Zheng,
G. Li*
&& – &&
Smart Solution Chemistry to Sn-
Containing Intermetallic Compounds
through a Self-Disproportionation
Process
Metal–Metal Interactions
Sn-containing intermetallics were synthesized by an in-situ
self-disproportionation-induced process. [SnO2]2ꢀ, acting
a dual role of tin source and reductant, could produce bulk
element Sn and soluble [SnO3]2ꢀ under hydrothermal
condition (pictured as orange tree trunk). [SnO2]2ꢀ would
also react with transition-metal cations, once introduced in
this reaction system, which led to intermetallics. The results
reported herein help to find a new strategy of synthesizing
functional intermetallics at ambient conditions. More
information can be found in the Full Paper by Y. Zhang et al.
on page ff. &&.
&
&
Chem. Eur. J. 2016, 22, 1 – 10
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!