Solvothermal preparation of silicon nanocrystals

Article abstract of DOI:10.1246/cl.2008.644

Silicon nanocrystals have been produced successfully by using silicon monoxide as the starting material via a simple solvothermal preparation method. The temperature needed in the solvothermal preparation is found to be as low as 220°C when ethylenediamine is used as the solvent. The as-prepared silicon samples are of single-crystal nature with diameters around 5 nanometers and dispersed in an amorphous silica. The silicon nanocrystals became a little larger in size after removing the amorphous silica, according to the results of transmission electron microscope (TEM) characterization. Copyright

Full text of DOI:10.1246/cl.2008.644

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Chemistry Letters Vol.37, No.6 (2008)
Solvothermal Preparation of Silicon Nanocrystals
ꢀ
Jun Qiu, Wanqing Shen, Ruijie Yu, and Baodian Yao
Department of Chemistry, Fudan University, Shanghai 200433, P. R. China
(Received March 5, 2008; CL-080248; E-mail: yaobd@fudan.edu.cn)
Silicon nanocrystals have been produced successfully by
using silicon monoxide as the starting material via a simple
solvothermal preparation method. The temperature needed in
at 40 kV and 40 mA. TEM images were obtained on a JEM-
2010 transmission electron microscope in combination with an
energy dispersive X-ray spectroscope (EDX) operating at an ac-
celeration voltage of 200 kV. Sample grids were prepared by
sonicating the powdered samples in absolute ethanol for
several minutes and evaporating one drop of the suspension on
carbon film supported on copper grids.
ꢁ
the solvothermal preparation is found to be as low as 220 C
when ethylenediamine is used as the solvent. The as-prepared
silicon samples are of single-crystal nature with diameters
around 5 nanometers and dispersed in an amorphous silica.
The silicon nanocrystals became a little larger in size after re-
moving the amorphous silica, according to the results of trans-
mission electron microscope (TEM) characterization.
Figure 1 showed the XRD patterns of SiO source material,
ꢁ
and two solvothermal products obtained at 200 and 220 C, re-
spectively. The starting material of SiO powders are amorphous
and no crystal Si could be detected (Figure 1a). For samples
ꢁ
solvothermal treated at temperature lower than 200 C, the
XRD result (Figure 1b) showed no Si crystal, too. When the sol-
The first observation of room-temperature luminescence
1
ꢁ
vothermal temperature increased to 220 C, diamond-cubic sili-
con (Joint committee on Powder Diffraction Standard, JCPDS
more than a decade ago in silicon-implanted SiO2 or in porous
silicon2 has triggered a strong interest in the fabrication of
˚
card: 27-1402, a ¼ 5:43088 A) XRD peaks can be clearly seen
silicon nanocrystals (Si-NCs) and their properties. Some of this
interest arises from scientific curiosity concerning size-depend-
ent optical absorption and photoluminescent response and from
the potential applications offered by the seemingly forbidden
band gap transition that leads to photoluminescence not mani-
fested in bulk Si. To date, controlled preparation of Si-NCs is
still the focus of intense research, and many advances have been
achieved in this regard. For Si-NCs confined in matrix, the
known methodologies include the formation of Si-NCs by ion
(Figure 1c), which indicated that the disproportionation reaction
of SiO had been initiated:
2
SiO ! Si þ SiO2
ð1Þ
Therefore, this solvothermal process endows reaction 1 at
very low initiating temperature which is usually higher than
ꢁ
8
8
00 C in the case of vapor decomposition processes. The broad
XRD peaks in Figure 1c also indicated the crystallite size of Si
products in nanoscale, which was around 5 nm calculated from
line-broading of (111) peak by the Scherrer formula.
1,3
implantation, by thermal crystallization of amorphous Si/SiO2
4
superlattice, and by phase separation of thermal annealed SiO/
5
Deionized water is also adopted as the solvent to produce
Si-NCs, however, no Si crystal was detected in samples obtained
by a similar procedure at temperatures ranging from 150 to
SiO2 superlattice. For freestanding Si-NCs, the methods include
6
solution-based silicon precursor reduction, silicon precursor
7
thermolysis, and pyrolysis by using usually hazardous, expen-
ꢁ
ꢁ
2
2
50 C. In cases of hydrothermal temperature higher than
20 C, gas release can be observed from the reacted autoclave
sive silicon precursors. It might be imaged that it is essential
to tightly control Si-NCs over the size, shape, crystallographic
orientation, and surface chemistry. Thus, it is still a challenging
work to develop new process and technology, especially by
using cheap and non-hazardous silicon precursor, to effectively
address all of these issues.
after cooling, which implied that certain gaseous products
were formed in the autoclave. The following two reactions are
1400
In this contribution, we report one simple solvothermal
method for the preparation of silicon nanocrystals with several
nanometers in size by using readily available, nonhazardous sil-
icon monoxide (SiO) as precursors. Samples are characterized
by X-ray diffraction (XRD) and TEM.
1
1
200
000
From up to bottom:
a: SiO raw materials
b: SiO-200
c: SiO-220
800
The starting chemicals, 99.9% SiO powders and A. R.
reagent ethylenediamine, purchased from Beijing Chemical
Factory were used without further purification. In short, 0.2 g
of SiO powders and 15 mL of pure ethylenediamine mixture
was put into a 20-mL Teflon-lined stainless steel autoclave,
600
a
4
2
00
00
0
b
c
which was then thermally treated in a preheated oven at
ꢁ
2
20 C for 24 h. The as-prepared samples were filtered, washed
ꢁ
with deioned water and dried at 90 C. Part of the products
was treated with concentrated HF to remove SiO2 and unreacted
SiO for transmission electron microscope characterization.
The powder XRD data were recorded on a German Bruker
D4 X-ray diffractometer with Ni-filtered Cu Kꢀ X-ray source
20
30
40
50
60
70
2
θ
Figure 1. XRD patterns of SiO raw materials untreated (a),
ꢁ
solvothermal treated at 200 (b) and at 220 C (c).
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.6 (2008)
645
In another attempt to obtain Si products by using phenol
as an acidic organic solvent, Si crystal was also not obtained
in samples prepared at temperatures ranging from 150 to
ꢁ
2
50 C. Therefore, it seems that the basic organic reagent ethyl-
enediamine is likely helpful to catalyze the initiation of
reaction 1 at considerable low temperatures.
Figure 2 shows the TEM images of as-prepared and
HF-treated Si-NCs. It can be clearly seen that the as-prepared
Si-NCs were dispersed in an amorphous matrix (Figure 2a)
and seemingly became more regular in shape after HF treatment
(
Figure 2b). The diameters of Si-NCs were all below 20 nm
in sizes and most of them around 8 nm (Figure 2b), which was
larger than that calculated from XRD results based on the
Scherrer formula. The elemental analysis showed only Si exis-
tence in HF-treated samples according to the EDX spectrum
(
not shown here). The selected area electron diffraction (SAED)
pattern (inset of Figure 2b) can be well indexed with diamond-
cubic silicon, which further confirmed the crystalline nature of
as-prepared Si products.
b
In summary, we demonstrated for the first time the low-tem-
perature solvothermal formation of Si-NCs using readily availa-
ble, nonhazardous SiO as Si raw material. Compared to hydro-
thermal process, silicon nanoproducts could be expected to be
mass-produced by solvothermal process. Further work is ongo-
ing to explore the optical properties of as-prepared Si-NCs, as
well as the possiblity of solvothermal preparation of one-dimen-
sional silicon nanomaterials, such as nanowires and nanotubes.
This work was supported by NSFC (no. 20503006),
Shanghai Sci. & Tech. Committee (no. 05ZR14014) and
Shanghai Leading Academic Discipline Project, Project
Number: B108.
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6
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Though further investigations are needed to confirm the ac-
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8
9
9
,10
even when Si products can be obtained.
In this regard, the
present solvothermal preparation of Si-NCs using ethylene-
diamine as solvents is helpful to mass production of Si-NCs
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Published on the web (Advance View) May 17, 2008; doi:10.1246/cl.2008.644