Nanomaterials separation by an ultrasonic-assisted phase transfer method

Article abstract of DOI:10.1016/j.cplett.2008.02.090

Nanomaterials in different sizes can be separated by an ultrasonic-assisted phase transfer method. A two-phase polar-non-polar liquid system composed of alcohol (polar, C₁-C₃ or their mixture), alkane (non-polar, C₅-C₁₀ or their mixture) and DI water is used in this method. The separation is achieved via the surface tension of the interface and the dynamic equilibrium established between nanomaterials of different weights. Different nanostructures can be separated to exist in layers of different polarity or in the interface throughout the sonication process, with the small-sized nanostructures staying in the upper layer and the large-sized one in the lower layer and interface.

Full text of DOI:10.1016/j.cplett.2008.02.090

Chemical Physics Letters 455 (2008) 252–255
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Nanomaterials separation by an ultrasonic-assisted phase transfer method
Zhenhui Kang a,b,c, Chi Him Alpha Tsang a,b, Dorothy Duo Duo Ma a,d, Ning-Bew Wong a,b,*,
a,d,e,
*
Shuit-Tong Lee
a
Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong
Department of Biology and Chemistry, City University of Hong Kong, Hong Kong
Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, People’s Republic of China
Department of Physics and Materials Science, City University of Hong Kong, Hong Kong
b
c
d
e
Nano-organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanomaterials in different sizes can be separated by an ultrasonic-assisted phase transfer method. A two-
Received 11 December 2007
In final form 26 February 2008
Available online 2 March 2008
1 3
phase polar–non-polar liquid system composed of alcohol (polar, C –C or their mixture), alkane (non-
polar, C –C10 or their mixture) and DI water is used in this method. The separation is achieved via the
5
surface tension of the interface and the dynamic equilibrium established between nanomaterials of dif-
ferent weights. Different nanostructures can be separated to exist in layers of different polarity or in the
interface throughout the sonication process, with the small-sized nanostructures staying in the upper
layer and the large-sized one in the lower layer and interface.
Ó 2008 Elsevier B.V. All rights reserved.
1
. Introduction
Nanoscale materials in the nanometer scale attracts intense
(SiO) without catalysis [4], and their properties extensively inves-
tigated. Particularly, it has been demonstrated from scanning tun-
neling microscopy (STM) and scanning tunneling spectroscopy
(STS) measurements [4] that small-diameter SiNWs (1–7 nm) have
a band gap energy increasing with decreasing wire diameter, and
also higher oxidation resistance towards air [5]. These findings
suggest promising potential of silicon-based nano-optoelectronic
devices. The oxide-assisted growth (OAG) method is an effective
method for bulk-quantity production of SiNWs which has been de-
scribed in our previous work [4–6,13–16]. However, bulk SiNWs
samples are composed of wires of a distribution of diameters,
and sometimes different morphologies. Therefore, one of the
challenging issues in the field of nanomaterials is the sorting of
high-quality, high-purity quantum-sized SiNWs into different
dimensions and morphologies. In this study we demonstrate the
separation of nanomaterials by an ultrasonic-assisted phase trans-
fer process: the quantum-sized SiNWs (diameter 6 5 nm) can be
separated from the bulk SiNWs product, and additionally SiQDs,
Si nanoparticles (SiNPs), Si nanoribbon (SiNRs) and Si nanocones
(SiNCs) also can be separated from their raw products. Similarly,
silica NWs or silicon nanocores also can be separated from the sil-
ica NWs/silicon nanocore hierarchical structures.
interest, because they exhibit different properties from the corre-
sponding macromaterials [1–3]. Nanomaterials have a larger sur-
face-to-volume ratio, which leads to greater reactivity, different
mechanical strength and electrical properties. As the dimension
further reduces, quantum-size effect emerges and dominates the
behaviour of materials at nanoscale, and drastically changes the
optical, electrical and magnetic behaviours, etc [4–12]. Nanotech-
nology deals with design, characterisation, production, and appli-
cation of nanostructures, devices and systems by controlling
shape and size at the nanometer scale. For nanotechnology, obtain-
ing the size- and shape-controllable nanoscaled building blocks is
an essential task for its further development. There are two ap-
proaches in this issue: The first one is to achieve direct controlled
synthesis of monodisperse distribution nanomaterials, while the
other is to develop a general method for separation of nanomateri-
als according to sizes and morphologies. In comparison, the first
one is more difficult to achieve, while the second one is more
practical.
Silicon (Si) is of great interest to many areas of science and tech-
nology. Recently bulk silicon nanowires (SiNWs) have been synthe-
sized successfully by thermal evaporation of silicon monoxide
2
. Experimental
In our experiments, all the chemicals were purchased from Sig-
ma-Aldrich. SiNWs were prepared by the OAG method via direct
thermal vaporization of pure SiO powder at 1350 °C and a heating
rate of 50 °C/min [14]. The oxide layers of the as-grown SiNWs
were removed by 5% hydrogen fluoride (HF) solution to generate
*
Corresponding authors. Address: Department of Physics and Materials Science,
City University of Hong Kong, Hong Kong. Fax: +852 2784 4696 (S.-T. Lee).
E-mail addresses: bhnbwong@cityu.edu.hk (N.-B. Wong), apannale@cityu.edu.hk
(
S.-T. Lee).
0
009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.02.090

Z. Kang et al. / Chemical Physics Letters 455 (2008) 252–255
253
H–SiNW surfaces. SiQDs and SiNPs were prepared by electrochem-
ical etching of Si wafer [15]. The mixture of SiNRs and SiNCs/SiN-
Ws, and silica nanowires (NWs)/silicon nanocore hierarchical
structures were both prepared by thermal evaporation method
centrifuge tube with a diameter of 3 cm was used as the separation
chamber, which could deal with ꢀ0.1 g SiNWs or ꢀ0.5 g ZnO sam-
ple. Counting the nanoparticles on randomly selected grids in the
transmission electron microscope (TEM), the present separation
is effective and complete with over 95% target sample separated
from the raw materials.
TEM images of SiQDs and SiNWs were obtained by a FEI/Philips
Techal 12 BioTWIN TEM, while the high-resolution (HR)-TEM
images were obtained by a CM200 FEG TEM.
[
16]. The ZnO sample of nanorods and nanoparticles (ZnO NPs)
with different diameter was prepared by plasma-enhanced chem-
ical vapour deposition (PECVD) method. The CdS sample with dif-
ferent diameter nanowires (CdS NWs) and nanoribbons (CdS NRs)
3
was prepared by thermal evaporation method. Typically, 50 cm
3
. Results and discussion
n-hexane/Ethanol/
O/ H-SiNWs
a
b
The separation was carried out in a two-phase liquid system un-
der ultrasonic condition, in which phase transfer took place from
the alcohol/H O to alcohol/alkane phase. The typical two-phase li-
H
2
2
quid system was composed of two organic solvents of alcohol and
alkane together with deionized (DI) water. The alkane used was
Shaking
Interface
H-SiNWs
any alkane from C
to those having less than C
5
to C10 or their mixture. Alcohol was limited
or their mixture. In practice, the raw
n-hexane/ethanol
O/ethanol
Ultrasonic bath
3
H–SiNWs were put into ethanol for ultrasonic dispersion, followed
by adding a suitable amount of n-hexane and DI water to disperse
the liquid for nanomaterial separation with shaking until interface
appeared. After ultrasonic treatment, a small-sized SiNWs, found
in the n-hexane/ethanol (upper) layer, was partially transparent
in color. Fig. 1a shows the working flowchart of the separation pro-
cess. Fig. 1b shows the photograph of H–SiNWs segregated at the
two-phase interface. The bulk sample was a mixture of H–SiNWs
of different diameters (average diameter is about 15 nm), which
is shown in Fig. 2a. The diameter of the separated H–SiNWs at
H
2
Up layer
<
5 nm
Interface
10 nm
n-hexane/ethanol
O/ethanol
≈
H
2
Fig. 1. (a) Flowchart of SiNWs separation, (b) optical photograph showing the
interface in the two-phase system after separation.
Fig. 2. TEM image of (a) bulk H–SiNWs, and (b) H–SiNWs in the interface after separation. TEM images of (c) and (d) small-diameter H–SiNWs in n-hexane/ethanol layer
upper layer) after separation, and (e) HR-TEM of a small-diameter H–SiNW in n-hexane/ethanol layer after separation.
(

2
54
Z. Kang et al. / Chemical Physics Letters 455 (2008) 252–255
the interface was about 10 nm (Fig. 2b), whereas that of the sepa-
rated H–SiNWs in the upper layer was about 2–3 nm (Figs. 2c and
d). Fig. 2e shows the HR-TEM image of the separated H–SiNWs
with a diameter of 2 nm, where the lattice fringe spacing of
In the present ultrasonic-assisted phase transfer method, the
two-phase liquid system plays an important role in nanomaterials
separation. Liquid alkanes (C5–C10) are preferably used as one
phase, while alcohols with 1–3 carbons, especially ethanol, are
preferably used as the second phase. The separation was achieved
by the surface tension of the interface and the dynamic equilibrium
established by the weight of nanomaterials under ultrasonic condi-
tion. For a sample mixture of H–SiNWs of different diameters, the
migration of smaller diameter H–SiNWs (lower mass) towards the
non-polar upper layer is faster. On the contrary, the larger diame-
ter species are trapped in the interface or migrate towards the low-
er layer instead. This differential tendency of segregation can be
understood as follows. Since H–SiNWs used are all hydrogen ter-
minated and the SiH bonds are hydrophobic in nature, the non-po-
lar n-hexane would favor the migration of smaller-sized species
due to their higher non-polar surface character and lower mass,
whereas the larger-sized nanomaterials would preferentially re-
main at the interface. As a result, these factors would favor the
migration of the smaller-sized H–SiNWs to the upper layer, while
the larger-sized ones would be trapped at the interface, or migrate
to the lower layer during separation. Similar phenomena would
also exist for other silicon-based nanostructures, such as SiQDs,
SiNPs, SiNRs, and SiNCs, when the present separation method
was applied to them. Further separation of nanomaterials can be
achieved by regulating the power of ultrasonic bath. In addition,
the same method can also be applied for the separation of other
inorganic semi-conductor nanomaterials (see supporting informa-
tion for separation of ZnO, CdS, etc). Using the same volume of sol-
vent, the separation ability increases with increasing diameter of
the separation apparatus. About 0.1 g SiNWs can be separated
0
.31–0.32 nm agrees well with the h111i plane spacing of Si single
crystal.
Besides SiNWs, the same method was also applied to separate
SiQDs and SiNPs. SiQDs (1–10 nm) were produced by the electro-
chemical method, but small fragments of silicon wafer were also
present in the product as shown in Fig. 3a. After separation, pure
SiQDs were segregated and dispersed in the upper layer as shown
in Fig. 3b, where the TEM image reveals no other small silicon frag-
ments after separation. Separation of SiNPs was also achieved by
the same method from the silicon wafer fragments in the product
as shown in Fig. 3c. Pure SiNPs with a diameter of 30–50 nm were
obtained at the interface after separation as shown in Fig. 3d. Sim-
ilar process also can be used for the separation of other large-sized
and complex Si nanostructures. Fig. 4a shows the TEM image of a
bulk sample consisting of SiNRs, SiNWs and SiNCs. After separation
the TEM image in Fig. 4b shows only SiNWs and SiNCs (30–50 nm
in diameter) in the upper layer. Fig. 4c shows the TEM image of
SiNRs (about 50–100 nm in width and several microns in length)
after separation. Moreover, the method also can be used for the
separation of silica and Si complex structure. Fig. 4d shows the
TEM images of bulk silica NWs and Si hierarchical nanostructure
consisting of high-density amorphous silica nanowires radial
standing on a single-crystal silicon core. After separation, the silica
nanowires can be separated from the silicon core. Fig. 4e shows the
TEM image of silica NWs in the upper layer and interface after sep-
aration, while Fig. 4f shows the Si nanocores in the lower ethanol/
3
2
H O layer.
out using a 50 cm centrifuge tube with a diameter of 3 cm, while
Fig. 3. TEM images of (a) bulk SiQDs sample and (b) small-diameter SiQDs in the upper layer of n-hexane/ethanol after separation. TEM images of (c) bulk SiNPs sample and
d) small-diameter SiNPs in the upper layer of n-hexane/ethanol after separation.
(

Z. Kang et al. / Chemical Physics Letters 455 (2008) 252–255
255
Fig. 4. TEM image of (a) bulk SiNRs, SiNWs and SiNCs sample, (b) SiNWs and SiNCs in n-hexane/ethanol layer after separation, and (c) SiNRs after separation. TEM image of
d) bulk silica NWs and Si nanocore samples, (e) pure silica NWs in the upper layer or interface of n-hexane/ethanol after separation, and (f) pure Si nanocores in the lower
layer.
(
ꢀ
0.15 g can be separated out using a centrifuge tube with a
Appendix A. Supplementary data
diameter of 5 cm. On the other hand, 0.3–0.5 g SiNWs can be
3
separated out by using a 100 cm centrifuge tube with a diameter
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2008.02.090.
of ꢀ5 cm. Significantly, this method can guarantee large-scale
separation of small-sized nanomaterials from the bulk sample
mixture.
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(