Sains Malaysiana 37(3)(2008): 277–280
Controlled-Growth of ZnO Nanowires with Different Processing Temperatures
(Pertumbuhan Terkawal Nanodawai ZnO pada Suhu Pemprosesan yang Berbeza) YAP CHIN CHI, MUHAMMAD YAHAYA,
MUHAMAD MAT SALLEH & DEE CHANG FU
ABSTRACT
ZnO nanowires have been synthesized using a catalyst-free carbothermal reduction approach on SiO2-coated Si substrates in a flowing nitrogen atmosphere with a mixture of ZnO and graphite as reactants. The collected ZnO nanowires have been characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and photoluminescence spectroscopy. Controlled growth of the ZnO nanowires was achieved by manipulating the reactants heating temperature from 700ºC to 1000ºC. It was found that the optimum temperature to synthesize high density and long ZnO nanowires was about 800ºC. The possible growth mechanism of ZnO nanowires is also proposed.
Keywords: ZnO; graphite; nanowires; carbothermal; scanning electron microscopy
ABSTRAK
Nanodawai ZnO telah disintesis dengan menggunakan pendekatan penurunan karboterma tanpa mangkin pada substrat Si yang disalut SiO2 di bawah aliran gas nitrogen dengan campuran ZnO dan grafit sebagai reaktan. Nanodawai ZnO yang terkumpul dicirikan dengan menggunakan belauan sinar-X, mikroskopi imbasan elektron, spektroskopi sebaran tenaga dan spektroskopi fotoluminesen. Pertumbuhan terkawal nanodawai ZnO telah dicapai dengan memanipulasi suhu pemanasan reaktan daripada 700ºC hingga 1000ºC. Didapati suhu yang optimum untuk mensintesis nanodawai ZnO yang berketumpatan tinggi dan panjang adalah pada 800ºC. Selain itu, mekanisme pertumbuhan ZnO juga dicadangkan.
Kata kunci: ZnO; grafit; nanodawai; karboterma; mikroskopi imbasan elektron
INTRODUCTION
One-dimensional (1-D) semiconductor nanomaterials have been attracting increasing attention due to their outstanding properties, which are different from bulk materials. For example, ZnO with wide direct band gap (3.37 eV) and large exciton binding energy (60 meV) possesses several advantages for use in electronic and optoelectronics devices (Li et al. 2004; Baxter & Aydil 2006; He et al. 2007). In the past years, the number of ZnO nanowires synthesis techniques has grown exponentially (Yao et al. 2002;
Banerjee et al. 2004; Chen et al. 2004; Wang et al. 2004;
Zhang et al. 2004; Park et al. 2005; Wang et al. 2005; Zeng et al. 2005; Bae et al. 2006; Lim et al. 2006; Sun et al.
2006). The carbothermal reduction approach is one of the most promising and frequently used techniques to grow ZnO nanowires (Yao et al. 2002; Banerjee et al. 2004; Park et al. 2005; Wang et al. 2005; Lim et al. 2006).
The synthesis of ZnO nanowires using catalyst-free carbothermal reduction approach is described. The growth mechanism can be explained by self-catalyzed vapor- liquid-solid (VLS) mechanism. Besides, the effect of reactants heating temperature on the growth of ZnO nanowires is also discussed. It was found that the optimum temperature to grow high density and long ZnO nanowires was about 800ºC.
METHODOLOGY
The silicon wafers were cleaned with acetone and methanol in an ultrasonic bath followed by rinsing with distilled water. The cleaned wafers were dried and transferred into an electron-beam evaporation system, where thin films of SiO2 (150 nm) were deposited. The source material was a mixture of commercial ZnO powders with a purity of 99.9% and graphite with a purity of 99.99% in a mole ratio of 1:1. The ZnO powders and graphite were purchased from Aldrich Chemical Company. They were milled and loaded on a ceramic wafer and placed in the center of a quartz tube heated in an electric tube furnace. The tube was heated at 1000ºC, 900ºC, 800°C and 700ºC for 4 hours to examine the effect of reactants heating temperature on the growth of ZnO nanowires. Nitrogen gas (99.999%) was introduced into the quartz tube throughout the heating process at a flowing rate of 1500 l/m. After cooling down to room temperature, white products were deposited on the substrates as well as the inner wall of the quartz tube.
Scanning electron microscopy (SEM) (1450VP, LEO), energy-dispersive X-ray spectroscopy (EDS) attached to the
SEM and X-ray diffraction (Siemens D-5000) were used to investigate the morphology, composition and crystal structure of the as-grown nanowires. Besides, the photoluminescence properties of nanowires at room
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temperature were also investigated using a Perkin Elmer
LS55 Luminescence Spectrometer.
RESULTS AND DISCUSSION
Figure 1 shows SEM image of as-obtained ZnO nanowires grown at different temperature from 700ºC to 1000ºC. The diameters of ZnO nanowires grown at 700ºC normally range from 50 nm to 80 nm and their lengths are 3–6 μm (Figure 1(a)). With increasing the growth temperature to 800ºC, ZnO nanowires were formed in very high yield as shown in Figure 1(b). The diameters of the nanowires range from 50 to 90 nm and lengths are more than 10 μm.
However, most of the nanowires entangled with one another (Figure 1(b)) that makes it difficult to measure the length accurately. At growth temperature of 900ºC, short and small diameter nanowires of 4–10 μm in length and 50–90 nm in diameter were formed as shown in Figure 1(c). As the growth temperature was increased to 1000°C, short and large diameter nanowires of 2–10 μm in length and 100–250 nm in diameter were formed in very high yield as shown in Figure 1(d). It was found that the optimum temperature to grow high density and long ZnO nanowires for the present experiment was about 800ºC.
The composition of the ZnO nanowire grown at 800ºC was examined by EDS (Figure 2). Si peak originated from
FIGURE 1. SEM images of ZnO nanowires grown at different temperature: a) 700ºC, b) 800ºC, c) 900ºC and d) 1000ºC
(a) (b)
(c) (d)
FIGURE 2. EDS spectrum of the ZnO nanowires on SiO2-coated Si substrate grown at 800ºC
Intensity (a.u.)
Energy (keV)
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the SiO2-coated Si substrates, while Au peak was caused by gold coating (to avoid charging during SEM image recording). Besides, only Zn and O were detected from the sample, confirming the composition of ZnO nanowires.
However, the atomic composition ratio of O to Zn could not be determined since a portion of O originated from the SiO2-coated Si substrates.
The products were examined by X-ray diffraction
(XRD) to investigate the crystal structure. A typical result of ZnO nanowires synthesized at 800ºC is shown in Figure 3. All intense peaks are assigned to the hexagonal wurtzite zinc oxide, with the cell constant of a = 3.249 Å and c = 5.205 Å, indicating that the products are pure ZnO. The strongest (002) peak shows that the preferred growth orientation of nanowires is along the c-axis.
The PL spectrum of ZnO nanowires grown at 800ºC, recorded by using an excitation wavelength of 330 nm at room temperature, shows a characteristic narrow band at 390 nm and a weaker band at about 490 nm (Figure 4).
The 390 nm emission corresponds to the recombination
of free excitons between conductive band and valence band and is called near band-edge emission, while the longer wavelength band can be attributed to the radial recombination of a photo-generated hole with electron that belongs to the singly ionized oxygen vacancy in ZnO (Chen et al. 2004). The strong UV emission in the PL spectra indicates that the ZnO nanowires have a good crystal quality with few oxygen vacancies (Wang et al. 2005).
Since no metal-catalyst was employed, the growth mechanism of ZnO nanowires can be explained using the self-catalyzed vapor-liquid-solid (VLS) model (Zhang et al.
2004). At the temperature range of the location where the source materials were loaded (700 – 1000ºC), graphite (carbon) reduced ZnO into Zn or Zn suboxides (ZnOx, x <
1) with low melting point (419ºC) by the following reactions (Wang et al. 2005):
ZnO(s) + C(s)→ Zn(g) + CO(g). (1) ZnO(s) + CO(g)→ Zn(g) + CO2(g). (2)
FIGURE 3. XRD pattern of the ZnO nanowires grown at 800ºC
FIGURE 4. Room temperature photoluminescence spectrum from ZnO nanowires grown at 800ºC
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The gaseous products would be transferred to the substrates placed at the lower temperature region by the nitrogen carrier gas. The vapors condensed on the substrate and the following reactions may occur:
Zn(g) + CO(g)→ ZnO(s) + C(s). (3) C(s) + CO2(g)→ 2CO(g). (4) The initially formed ZnO could provide an ideal nuclei site for the further growth of nanowires (Yao et al. 2002).
Temperature influences the growth of the ZnO nanowires.
At high temperature (1000ºC), more Zn or Zn suboxides vapors were produced than that at low temperature (Banerjee et al. 2004). The vapors condensed on the substrate and might combine to form bigger nuclei sites for the further growth of nanowires. As a result, high density and large diameters ZnO nanowires were obtained.
At moderate heating temperature (800ºC), the vaporization rate of Zn or Zn suboxides was lower than that at 1000ºC.
This means that the exhaustion of gaseous product was slower at sintering temperature of 800ºC than 1000ºC.
Therefore, longer length ZnO nanowires were obtained at 800ºC as the heating time was kept invariable. In the case of 700ºC, only a few nuclei sites was formed, so the length and the density of the nanowires reduced dramatically.
CONCLUSION
The synthesis of ZnO nanowires using catalyst-free carbothermal reduction approach has been demonstrated.
ZnO nanowires can be grown at different reactants heating temperature from 700ºC to 1000ºC. It was found that long and thin ZnO nanowires could be synthesized in very high density at 800ºC. The growth mechanism of ZnO nanowires can be explained using the self-catalyzed VLS
model.
ACKNOWLEDGEMENT
This work has been carried out with the support of Malaysian Ministry of Science, Technology and Innovation
(MOSTI), under the Science Fund 03-01-02-SF0082.
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Yap Chin Chi & Muhammad Yahaya School of Applied Physics
Faculty of Science and Technology Universiti Kebangsaan Malaysia 43600 Bangi, Selangor D.E.
Malaysia
Muhamad Mat Salleh & Dee Chang Fu
Institute of Microengineering and Nanotechnology (IMEN)
Universiti Kebangsaan Malaysia 43600 Bangi, Selangor D.E.
Malaysia
Received : 12 June 2007 Accepted : 18 November 2007