Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Trang 1Hybrid nanocomposite of Fe nanoparticles on SiO 2
nanowires by sublimation route
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People's Republic of China
Received 15 October 2006; accepted 16 December 2006 Available online 23 December 2006
Abstract
A simple route was developed to synthesize the hybrid nanocomposite with Fe nanoparticles (NPs) dispersed on the surface of SiO2nanowires (NWs), where SiO2NWs with the diameter of 20–40 nm were produced by heating single-crystal silicon wafer, and Fe NPs in the size range of 3–
20 nm were generated by heating Fe powders The nucleation and growth of Fe NPs follows the solid–vapor–solid (S–V–S) mechanism, namely,
Fe powders firstly sublime and then Fe atoms deposit on SiO2NWs to form Fe NPs
© 2007 Elsevier B.V All rights reserved
Keywords: Nanocomposites; Electron microscopy; Nucleation; Growth
1 Introduction
Hybrid nanocomposites have attracted intensive attention
recently By attaching nanoparticles on one-dimensional
nanomaterials, such as nanotubes and nanowires, many kinds
of nanocomposites have been obtained For example, carbon
nanotubes have been exploited as the support for different
nanoparticles made by the chemical solution method [1–3],
direct sputtering[4], centrifugation and redispersion[5]; Ni and
Pt NPs were dispersed on the surface of Si NWs by PECVD[6];
Au nanoclusters were directly organized on Ag NWs[7]; Ag,
Cu, Pd and Rh NPs were attached on the sidewall of Si NWs by
the reaction of Si NWs with relevant aqueous solution[8–13]
Nanoparticles-on-nanowires nanocomposites have been
attempted for many applications For example, Al clusters on
carbon nanotubes provide a“proof-of-principle” for the
devel-opment of molecular sensors[14], silver/dendrimer
nanocom-posite exhibits potential application as cell biomarkers[15], and
highly dispersed Pt particles on carbon nanofibers show
effi-cient usage for catalysts[3]
Fe NPs are well-known as catalysts and have magnetism
properties In this work, an efficient and simple route was
developed to prepare a hybrid nanocomposite with Fe NPs on
the surface of SiO2NWs Based on a detailed investigation on the morphology and distribution of Fe NPs using transmission electron microscope (TEM), an S–V–S mechanism was pro-posed to explain the formation of the nanocomposite
2 Experimental p-type b100N orientated silicon wafer with a thickness of
650 ± 25μm was used as silicon source to synthesize SiO2NWs
in a GSL 1600× tubular furnace The wafer was cleaned using 5% HF solution and acetone, placed in an Al2O3boat, and then put in the middle of the tubular furnace The side-view on the heating system is schematically shown inFig 1a The furnace tube was evacuated for 5 min to reach the ultimate vacuum (2 Pa) before the heating treatment, then high-pure Ar (99.99%) was introduced into the tube at a flow rate of 40 sccm and kept flowing, which maintained the furnace pressure at the normal atmosphere pressure The furnace temperature was then raised
to 1300 °C at a heating rate of 10 °C/min and held for 3 h Afterwards, the samples were cooled to room temperature at a cooling rate of 10 °C/min in furnace with the protection of Ar
In the next step, the Si wafer was taken out from Al2O3boat, and Fe powders with the biggest size of 28μm were put at the upstream part of the Al2O3boat, then the Al2O3boat was placed
in the middle of the tubular furnace, as shown inFig 1b The tube was evacuated, filled with high-pure Ar and heated as
⁎ Corresponding author Tel.: +86 22 81523700; fax: +86 22 27405874.
E-mail address: xwdu@tju.edu.cn (X.W Du).
0167-577X/$ - see front matter © 2007 Elsevier B.V All rights reserved.
doi: 10.1016/j.matlet.2006.12.033
Trang 2described above until the furnace temperature rose to 750 °C,
then H2was filled at a flow rate of 10 sccm along with Ar gas
Whereafter, the furnace temperature was raised to 1400 °C at a
heating rate of 10 °C/min and held for 3 h Finally, the samples
were cooled at 10 °C/min in furnace with the protection of Ar and H2to room temperature
The morphology and structure were determined by FEI Technai G2 F20 TEM, and the composition was analyzed by Oxford INCA energy-dispersive X-ray spectroscopy (EDS) attached to the TEM TEM samples were prepared by scratching NWs from the wall of Al2O3boat and transferring onto copper grids with carbon film
3 Results and discussion After the Si wafer was heated in Ar gas, a layer of megascopic wool-like nanowires was found on Si wafer and the wall of the Al2O3boat Fig 2a shows the TEM images of the generated nanowires The nanowires were 20–40 nm in diameter and several tens of microns in length The selected-area-electron-diffraction (SAED) pattern (inset in Fig 2a) on the nanowires reveals only diffusive rings, and no diffraction spot was found, which indicates the amorphous nature of the nanowires.Fig 2b displays the EDS spectrum on the nanowires, which contains the signals of Si, O, C and Cu elements Considering the nanowires were supported by a copper grid with carbon film in TEM samples, C and Cu signals in EDS spectrum are not the intrinsic information and can be neglected, therefore the product should be silica NWs
Fig 1 Schematic diagram of the side-view on the system for synthesizing SiO 2
NWs (a) and that for synthesizing Fe NPs on SiO 2 NWs (b), respectively.
Fig 2 (a) TEM image of the generated NWs and SAED pattern (inset); (b) EDS
spectrum of NWs.
Fig 3 (a) High magnification TEM image of Fe NPs on SiO 2 NWs near the initial Fe powders and SAED pattern (inset) of Fe NPs; (b) EDS spectrum on the circle-marked area in (a).
Trang 3Under the temperature 1300 °C, it is difficult for the silicon atoms in
wafer to escape by diffusion or sublimation, however, the tube furnace
was evacuated before the filling of highly pure Ar gas, and the partial
pressure of oxygen should be very low, and decrease with the reaction
time As O2partial pressure is low enough (less than 10 Pa at 1300 °C),
atoms in bulk silicon could react with O2to form SiO gas[16],
The SiO gas may condense anywhere in the tube furnace by the
following reaction,
As a result, Si/SiO2 nanowires appear, and further oxidation of
nanowires could lead to the formation of pure SiO2nanowires
After the Al2O3boat with SiO2NWs and Fe powders were heated in
H2and Ar mixture, many particles with the size of 3–20 nm appeared
on the surface of SiO2NWs, as shown inFig 3a The inserted SAED
pattern exhibits clear rings corresponding to Fe {111}, {200}, {220},
{311}… planes The EDS spectrum on the circle-marked area inFig 3a
(including Fe NPs and a silicon NW) is shown inFig 3b, and the result
suggests that there exist Fe, O, and Si elements Compared with the
EDS spectrum on SiO2NWs shown inFig 2b, we can deduce that the
nanoparticles attaching on SiO2NWs are composed of Fe atoms
To investigate the formation mechanism of Fe NPs, the distribution
and morphology of Fe NPs were analyzed in detail InFig 4a, the dark
part in the lower-left corner shows an initial Fe particle with a size of
around 4μm, and almost all the generated Fe NPs are on the side of
SiO2NWs facing to the initial Fe particle Especially inFig 4c, Fe NPs
distribute on the surface of SiO2nanowire uniformly, and the size of Fe
NPs is close to the diameter of the nanowire, which approximates to
20 nm
On the other hand, Fe NPs were only found on the SiO2NWs near
the initial Fe powders, as shown inFigs 3a and 4, the density of Fe
nanoparticles decreases with the increase in the distance away from the
initial Fe powders, and finally Fe NPs disappear completely, as shown
inFig 2a
The possible mechanisms on the formation of Fe NPs include
solid–liquid–solid (S–L–S) and S–V–S route In the S–L–S route, the
solid massive Fe powders firstly melt and SiO2 NWs are soaked in
liquid Fe; as temperature decreases, the layer of liquid Fe solidifies and
congregates into Fe NPs In this case, the Fe NPs should distribute
uniformly on the surface of SiO2 NWs; however, the experimental
result conflicts with this mechanism in two aspects: firstly, the massive
Fe powders should not melt at 1400 °C, and secondly, the Fe NPs do
not distribute uniformly and only appear on the front side facing to the initial Fe powders
As for the S–V–S route, when the temperature approaches 1400 °C, the equilibrium vapor pressure between the solid phase and vapor phase of Fe reaches to 0.437 Pa, calculated from the vapor–temperature curve of Fe[17] Therefore, the initial Fe powders sublimate into Fe vapor in a rather high speed, and then Fe vapor is transported to the surface of SiO2NWs by the flowing mixture of Ar and H2 Fe nuclei form by the deposition and segregation of Fe atoms, and further growth
of Fe NPs proceeds with the continuous supply of Fe atoms during the temperature-holding stage Because the direction of the gas flow is from the initial Fe powders to SiO2NWs, Fe atoms and Fe NPs only attach on the front side of SiO2NWs, while no Fe NPs were found on the back side of SiO2NWs
4 Conclusion
Fe NPs dispersed on the surface of SiO2 NWs can be prepared simply by heating silicon wafer and Fe powders The nucleation and growth of Fe NPs follows the S–V–S mechanism, where Fe powders firstly sublime and then the Fe atoms deposit on SiO2nanowires to form Fe NPs It is possible
to obtain various NWs decorated with NPs by changing the starting materials Our method is simple and efficient, thus showing high potential on industrial application
Acknowledgments This work is financially supported by the Natural Science Foundation of China (No 50402010 and No 50672065), Natural Science Foundation of Tianjin City (No 043800711) and 985 Project of Tianjin University
References
[1] G.L Che, B.B Lakshmi, E.R Fisher, C.R Martin, Nature 393 (1998) 346.
[2] K.Y Jiang, A Eitan, L.S Schadler, P.M Ajayan, R.W Siegel, Nano Lett 3 (2003) 275.
[3] M Endo, Y.A Kim, M Ezaka, K Osada, T Yanagisawa, T Hayashi, M Terrones, M.S Dresselhaus, Nano Lett 3 (2003) 723.
[4] C.L Sun, L.C Chen, M.C Su, L.S Hong, O Chyan, C.Y Hsu, K.H Chen, T.F Chang, L Chang, Chem Mater 17 (2005) 3749.
Fig 4 TEM image of Fe NPs on SiO 2 NWs; (a) the area near an initial Fe particle; (b) the area apart from initial Fe powders; (c) a SiO 2 nanowire with separated Fe NPs.
Trang 4[5] M Olek, T Büsgen, M Hilgendorff, M Giersig, J Phys Chem., B 110
(2006) 12901.
[6] A.D LaLonde, M.G Norton, D Zhang, D Gangadean, A Alkhateeb, R.
Padmanabhan, D.N McIlroy, J Nanopart Res 8 (2006) 99.
[7] J Sharma, J.P Vivek, K.P Vijayamohanan, P Singh, C.V Dharmadhikari,
Appl Phys Lett 88 (2006) 193103.
[8] A.A Yasseri, S Sharma, T.I Kamins, Z Li, R.S Williams, Appl Phys., A
Mater Sci Process 82 (2006) 659.
[9] X.H Sun, N.B Wong, C.P Li, S.T Lee, P.S.G Kim, T.K Sham, Chem.
Mater 16 (2004) 1143.
[10] Y.Q Qu, R Porter, F Shan, J.D Carter, T Guo, Langmuir 22 (2006) 6367.
[11] X.H Sun, H.Y Peng, Y.H Tang, W.S Shi, N.B Wong, C.S Lee, S.T Lee,
T.K Sham, J Appl Phys 89 (2001) 6396.
[12] X.H Sun, R Sammynaiken, S.J Naftel, Y.H Tang, P Zhang, P.S Kim, T.K Sham, X.H Fan, Y.F Zhang, C.S Lee, S.T Lee, N.B Wong, Y.F.
Hu, K.H Tan, Chem Mater 14 (2002) 2519.
[13] X.H Sun, C.P Li, N.B Wong, C.S Lee, S.T Lee, B.K Teo, Inorg Chem.
41 (2002) 4331.
[14] Q Zhao, M.B Nardelli, W Lu, J Bernholc, Nano Lett 5 (2005) 847 [15] W Lesniak, A.U Bielinska, K Sun, K.W Janczak, X.Y Shi, J.R Baker Jr., L.P Balogh, Nano Lett 5 (2005) 2123.
[16] X.W Du, X Zhao, S.L Jia, J.J Li, N.Q Zhao, Mater Sci Eng., B, Solid-State Mater Adv Technol 136 (2006) 72.
[17] R.E Honig, D.A Kramer, RCA Rev 30 (1969) 285.