Synthesis and characterization of Cu3P hollow spheres by a facile soft-template process

Article abstract of DOI:10.1016/j.jallcom.2008.06.052

Hollow Cu₃P microspheres have been successfully synthesized by a facile ethylenediamine tetraacetic acid (EDTA) mediated solvothermal route using CuSO₄·5H₂O and yellow phosphorus as starting materials in a mixture solution

Full text of DOI:10.1016/j.jallcom.2008.06.052

Journal of Alloys and Compounds 474 (2009) 233–236
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of Cu3P hollow spheres
by a facile soft-template process
Xinjun Wang∗, Fuquan Wan, Juan Liu, Youjun Gao, Kai Jiang∗
College of Chemistry and Environmental Science, Henan Normal University, 46# East of Construction Road, Xinxiang, Henan 453007, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Hollow Cu3P microspheres have been successfully synthesized by a facile ethylenediamine tetraacetic
acid (EDTA) mediated solvothermal route using CuSO4·5H2O and yellow phosphorus as starting materials
in a mixture solution of ethylene glycol (EG), ethanol and water. The formation of these hollow spheres is
attributed to the oriented aggregation of Cu3P nanocrystals around the gas–liquid interface between PH3
and the mixture solution. The possible growth mechanism is proposed.
Received 28 December 2007
Received in revised form 8 June 2008
Accepted 15 June 2008
Available online 23 July 2008
© 2008 Elsevier B.V. All rights reserved.
Keywords:
Metals
Nanostructures
Scanning and transmission electron
microscopy
1. Introduction
challenge to develop simple methods for the fabrication of hol-
low nano- and microspheres of solid material. Recently, a lot
The morphology of inorganic solid materials is an important
factor to their properties, thus, the preparation of inorganic com-
pounds with special morphologies has attracted a great deal of
interest [1]. Up to now, a number of novel nano/microstructures
have been prepared with different morphologies. Among these
novel nano/microstructures, inorganic hollow sphere have received
considerable attention because of their diverse applications, such
as in drug delivery [2], heterogeneous catalysis [3], nanostructured
composites [4], and the protection of enzymes and proteins [5],
bioencapsulation [6]. Various methods have been developed for
preparing hollow spheres. For example, hollow polymer, oxide,
and glass composite microspheres with diameters generally in
the micrometer-size range can be produced by spray drying tech-
niques, which use nozzle systems to dispense individual liquid
droplets of uniform size [7]. Another method for obtaining hollow
spheres is involved with the direct synthesis of intact inorganic
shells around various sacrificial templates, such as polystyrene
latex spheres [8,9], vesicles [10], liquid droplets [11], latex templates
[12], microemulsion droplets [13], silica spheres [14]. However,
in most cases, the pure product was obtained only after the
complete removal of the templates, which makes the experi-
ments become more complicated. However, it still remains a
of inorganic hollow spheres have been prepared via a bubble
template route. The use of gas bubbles produced during the
reaction to provide aggregation centers is a novel and effective
method to fabricate hollow microspheres. Compared to the other
template-synthetic methods, this soft-template method is very
simple, convenient and avoids the introduction of impurities, and
is therefore suitable for modern chemical synthesis. For exam-
ple, Li and co-workers [15] prepared ZnSe hollow sphere by using
N2 bubbles as soft template. Lu and co-workers [16] synthesized
ZnS hollow sphere by taking H2S bubbles as the aggregation
centers. Han et al. [17] obtained CaCO3 hollow spheres by the
aggregation of nano-sized spherical particles on CO2/N2 bubble
surface.
Due to their excellent properties and potential application, tran-
sition metal phosphides have attracted more and more attention.
Among these metal phosphides, Cu3P is widely used as a potential
electrode material in lithium batteries [18,19], a kind of fine solder
and important alloying addition [20], a reinforcing agent in high
speed steel (HSS) composite materials [21] and it can enhance the
sintering behavior of 316L stainless steel [22].
In this paper, we report a facile one-pot soft-template method
for the synthesis of hollow Cu3P microspheres. In our experiment,
copper sulfate (CuSO4 · 5H2O) and yellow phosphorus were used as
Cu source and P source, respectively, and the desired samples were
obtained in a mixed solvent (EG, ethanol and water) at relatively
low temperature (200 ◦C). EDTA is used to regulate the pH values
∗
Corresponding authors. Tel.: +86 373 3326336; fax: +86 373 3326336.
E-mail address: wxjtg2006@126.com (X. Wang).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.06.052

234
X. Wang et al. / Journal of Alloys and Compounds 474 (2009) 233–236
ues (JPCDS Card No. 71-2261). No peaks of impurities were detected,
indicating the high purity of the products.
Fig. 2 shows the scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images of the products
obtained at 200 ◦C for 17 h in presence of EDTA in the mixture solu-
tion of water, EG and ethanol. Fig. 2a is a representative TEM image
of the Cu3P hollow spheres. The contrast between the dark edge
and pale inner part provides a direct proof for its hollow nature.
The size and wall thickness of these hollow spheres are calcu-
lated to be approximately 0.8–1.0 ␮m and 50–80 nm, respectively.
Fig. 2b shows a SEM image at low magnification of the as-prepared
Cu3P microspheres. It was clearly demonstrated that the major-
ity of the products exhibited spherical morphology. The average
diameter of Cu3P spheres is about 0.8–1.0 ␮m. The broken shells
observed on some of the spheres indicate the hollow structure of
them, and a magnified image of one hollow sphere with the bro-
ken part is shown in Fig. 2c, confirming the hollow structure of
these Cu3P microspheres. From Fig. 2c, the wall thickness of the
microspheres is estimated to be approximately 50–80 nm, which
is consistent with the TEM observation. The agglomerates struc-
ture maybe result from the high surface energies of Cu3P hollow
spheres.
Fig. 1. Typical XRD pattern of the obtained sample.
and the rate of releasing of Cu2+ ions for the formation of Cu3P
hollow spheres.
2. Experimental
In a typical experiment, the desired amount of copper sulfate (CuSO4·5H2O,
0.517 g) and 0.748 g EDTA were dissolved in 20 ml deionized water under stirring.
After the solution became transparent, a mixture of 10 ml EG and 10 ml ethanol
was added to the above solution. Several minutes later, the mixed solution was
transferred to a 50 ml Telfon-lined stainless steel autoclave and appropriate amount
of yellow phosphorus (0.45 g) was added to the above system. Then the autoclave
was sealed and maintained at 200 ◦C for 17 h and then cooled to room tempera-
ture naturally. The resulting black precipitate was separated by centrifugation and
washed respectively with distilled water, carbon disulfide (CS2) and absolute ethanol
to remove the residual reactants, excess yellow phosphorous and by-products. After
that, the obtained sample was dried in vacuum at 60 ◦C for 6 h.
In our prior works [23], our group have successfully fabricated
Cu3P hollow sphere by a two-step solvent-assisted coordination
and reduction process, in which EG serves not only as a reduc-
ing reagent, but also as a complexing solvent. This process can be
described as follows:
P
EG
4
Cu(OH)2 −→ [Cu(EG)x]2+
−→ Cu −→ Cu3P
(2)
3. Results and discussion
coordination
(1)
reduction redox
(3)
Fig. 1 shows the XRD pattern of the obtained sample synthe-
sized at 200 ◦C for 17 h and 6 h. All the diffraction peaks can be
readily indexed as the hexagonal phase Cu3P with lattice constants
However, we have not exactly understood how on earth to form the
hollow spherical structure so far. The possible formation process
may be due to the Kirkendall effect.
´
´
˚
˚
of a = 6.9595 A and c = 7.1467 A, which are close to the literature val-
Fig. 2. TEM and SEM images of the as-obtained sample at 200 ◦C for 17 h.

X. Wang et al. / Journal of Alloys and Compounds 474 (2009) 233–236
235
Fig. 3. The TEM images of the obtained sample synthesized at 200 ◦C for (a) 12 h and (b) 23 h, respectively. (c) TEM image of the sample obtained without using EDTA.
In the present paper, the solvent and Cu source is different from
the prior report and the system is acid environment, which is also
different from the prior report. Based on the experiment results,
the possible growth mechanism of these hollow spheres may be
regarded as an oriented aggregation process. The probable reaction
process for the formation of Cu3P microspheres can be summarized
as follows:
studied here may be mainly oriented attachment mechanism. The
process is similar to the formation of ZnO nanorod reported by
Ge [24]. However, the growth mechanism is still not completely
understood and further studies are on the way.
EDTA is well known to form stable chelates with both transition-
metal ions and main group ions at different pH values in solution
[25]. Copper ions can chelate with EDTA and form stable complexes
at low pH. In the present synthesis, the pH value is found to be
∼2 at the initial stage without additional adjustments. With the
increment of the reaction temperature, the stability of this com-
plex weakens gradually and slowly releases the free Cu2+ ions. Due
to the lower pH value in the reaction system, the phosphorus under-
goes the reaction shown in Eqs. (2)–(4). It is similar to the reaction
reported by Liu [26]. The by-products H3PO3 and H3PO4 undergo
a circular reaction shown in Eqs. (2) and (3), which increase the
amount of PH3 and accelerate the reaction of PH3 with the copper
ions until one of the raw materials runs out. As there is no hard
template used in the reaction system, the formation of the hollow
microspheres may be attributed to the formation of PH3 gas bubbles
that evolved during the reaction. As shown in Eqs. (1)–(3), the reac-
tion can form PH3 gas bubbles, which may act as the temporary soft
templates. Driven by the minimization of interfacial energy, small
Cu3P nanoparticles may aggregate around the gas–liquid interface
between PH3 and solution, and finally Cu3P hollow spheres were
formed.
When experiments were carried out without EDTA, no such
hollow sphere morphology was produced, only irregular solid
spheres were obtained (as shown in Fig. 3c). The reason may
lie in the fact that the rate of releasing copper ions was slower
than the rate of forming PH3 in presence of EDTA. So the excess
PH3 could form PH3 bubbles, which favors the formation of
the hollow structure. When EDTA was absent, the PH3 rapidly
reacted with free Cu2+ ions to give rise to small Cu3P parti-
cles and no temporary soft templates could help form hollow
structure.
Na2EDTA + Cu2+ → Cu-EDTA + 2Na+
2P4 + 12H2O → 3H3PO4 + 5PH3
P4 + 6H3PO4 + 6H2O → 10H3PO3
4H3PO3 → PH3 + 3H3PO4
(1)
(2)
(3)
(4)
(5)
Cu-EDTA → Cu2+ + EDTA (therateisslow)
Ph
Ph
3
3
Cu2+−→ Cu22+−→ Cu3P
(6)
In order to investigate the reaction process, the reaction is con-
ducted at 200 ◦C for 6 h, 12 h, 17 h and 23 h, respectively. When the
reaction time was 6 h, the obtained product was still hexagonal
phase Cu3P, and no other impurities (such as metal copper) were
detected. The XRD pattern of the sample is shown inset in Fig. 1.
Therefore, we infer that copper should not be an intermediate. This
result is different from our prior report [23]. When the reaction time
was 12 h, the hollow structure was irregular and it was undergoing
a process of oriented aggregation of the primary Cu3P nanoparticles
(as shown in Fig. 3a). It should have resulted from the slower libera-
tion of Cu2+ that led to the formation of incompact hollow structure.
When the reaction time was prolonged to 17 h, a large quantity of
nearly uniform hollow spheres of Cu3P was formed, as shown by
the images in Fig. 2. However, when the reaction time was extended
further to 23 h, the hollow spherical structure gradually grew to
rod-like shapes (as shown in Fig. 3b). A possible growth process
for Cu3P nanorods involves spontaneous self-organization of adja-
cent particles and dominant growth mechanism of Cu3P nanorods

236
X. Wang et al. / Journal of Alloys and Compounds 474 (2009) 233–236
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novel one-step solvothermal method. We found that reaction time
played important role in the synthesis of hollow spheres of Cu3P in
our experiment. Based on the results of the experiments, the growth
process of Cu3P hollow sphere was also proposed. Compared with
previous methods of preparing hollow spheres, this soft-template
reaction route provides a convenient path for the synthesis of high
quality Cu3P hollow spheres and special growth process of Cu3P
may provide a wider space for further studying other metal phos-
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The National Natural Science Foundation of PR China (No.
20571025), the Natural Science Foundation of Henan Province
(No. 0611020300) and Henan Innovation of University Promi-
nent Research Talents (No. 2005 KYCX05) are greatly appre-
ciated.
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