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Effect of carbon source on the carbothermal reduction for the fabrication of ZnO nanostructure
Applied Surface Science 253 (2006) 1601–1605 www.elsevier.com/locate/apsusc
Effect of carbon source on the carbothermal reduction for the fabrication of ZnO nanostructure Y.S. Lim *, J.W. Park, M.S. Kim, J. Kim Corporate R&D, LG Chem/Research Park, 104-1 Moonji-dong, Yuseong-gu, 305-380 Daejeon, Republic of Korea Received 7 January 2006; received in revised form 16 February 2006; accepted 21 February 2006 Available online 31 March 2006
Abstract Surface area effect of carbon source on the carbothermal reduction for the fabrication of ZnO nanostructure was investigated. For a systematic comparison, graphite and three kinds of carbon black powder were used as source materials for the carbothermal reduction. Depending on the surface area, the carbothermal reduction at 800 8C for 30 min resulted in Zn-silicate island or ZnO nanorod at the same experimental condition. These structures were characterized with a scanning electron microscopy, a transmission electron microscopy, an energy dispersive spectroscopy and an X-ray photoelectron spectroscopy. The results show that the reducing power of ZnO(s) source into Zn(g) vapor is strongly dependent on the surface area of carbon source, and that the fabrication of ZnO nanostructure can be performed more efficiently by using carbon source with large surface area. # 2006 Elsevier B.V. All rights reserved. Keywords: ZnO; Carbothermal reduction; Nanorod; Transmission electron microscopy; X-ray photoelectron spectroscopy
1. Introduction Among the various methods for fabricating ZnO nanostructure, carbothermal reduction is known to be a relatively simple, cheap and suitable process for large scale production [1]. By heating the mixture of ZnO and carbon powder up to a reaction temperature, ZnO(s) powder can be reduced into Zn(g) by C(s) through solid–solid reaction [1,2]. The vaporized Zn(g) can be deposited on a substrate, resulting in ZnO nanostructure. As the carbon source for the carbothermal reduction process, graphite carbon has been generally used [1–3]. However, due to its small surface area (<10 m2/g), the reaction between source materials of graphite and ZnO(s) powders was not strong. Therefore, for the fabrication of ZnO nanostructures with graphite carbon source, high temperature and/or precise gas control system had been required [1–5]. Recently, there were challenges to enhance the carbothermal reduction by using other carbon source rather than graphite [6–9]. By using various carbon sources, such as graphite, nanotube, carbon black and carbon nanoparticle, Rao et al. successfully synthesized a variety of oxide, nitride and carbide
* Corresponding author. E-mail address: [email protected] (Y.S. Lim). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.051
nanowires [6,7]. Leung et al. showed that the carbothermal reduction for ZnO nanostructure fabrication could be significantly enhanced by using single-walled carbon nanotube [8,9]. Moreover, with the mixture of ZnO nanoparticle and conventional graphite powder, the enhanced reduction was also achieved [8,9]. Because the reduction of ZnO(s) into vaporized Zn(g) is achieved by the solid–solid reaction between C(s) and ZnO(s), these results imply that the surface area of starting materials is very important factor for the carbothermal reduction process. Here, we report the effect of surface area of carbon source to enhance the solid–solid reaction between ZnO(s) and C(s) during carbothermal reduction for ZnO nanostructure fabrication. Although there are some reports on the growth of ZnO nanostructure by using the carbon source with large surface area [6–9], but a systematic comparison of the effect of the surface area has not yet been reported. For the comparison, graphite and three kinds of carbon black powder were used as the carbon source for the fabrication of ZnO nanostructure in this experiment. Carbon black is a cheaper and more abundant material than single-walled carbon nanotube, and it has a larger surface area (As = 30–1600 m2/g) than graphite (<10 m2/g). Moreover, one can easily choose desirable surface area of carbon black powder within the range of 30–1600 m2/g for specific purpose of experiment.
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In this experiment, different kinds of Zn-related structure were synthesized after carbothermal reduction at 800 8C for 30 min, depending on the surface area of carbon source. The structure was characterized with a scanning electron microscopy (SEM), a transmission electron microscopy (TEM), an energy dispersive spectroscopy (EDS) and an X-ray photoelectron spectroscopy (XPS). These results show that the reducing power of ZnO(s) source into Zn(g) vapor is strongly dependent on the surface area of carbon source, and that the fabrication of ZnO nanostructure can be performed more efficiently by using carbon source with large surface area. This result is not only useful for the fabrication of ZnO nanostructure, but also applicable to the nanostructure fabrication of various kinds of other oxides. 2. Experimental details Source powder for the carbothermal reduction was prepared with a fixed amount of the mixture of high purity ZnO powder (0.05 g) and carbon powder (0.05 g). In this experiment, graphite (As = 3.5 m2/g) and three kinds of carbon black powder (As = 70, 236 and 1440 m2/g, labeled as C1–C3, respectively) were used as carbon source. The surface area was measured by Brunauer– Emmett–Teller (BET) method with Micromeritics ASAP 2010 volumetric sorption analyzer at 77.35 K using high-purity nitrogen in the range of relative pressures from 106 to 1. Before the measurements, all samples were degassed at 200 8C for 2 h. The carbon powder was mixed with ZnO powder, ground by mortar and pestle, and loaded on a platinum boat. The boat was covered with Au-catalyzed Si substrate (p-type, 1– 10 V cm), and inserted into a horizontal tube furnace. The platinum boat was heated up to 800 8C for 30 min in air. After the carbothermal reduction process, the resulting structures were characterized with a SEM, a TEM, an EDS and an XPS. SEM analysis was performed with a Hitachi S-4800 Scanning electron microscope, operated at 15 keV for secondary electron imaging. TEM characterization was performed with a
JEOL JEM-ARM1300S high-voltage transmission electron microscope (HVEM), operated at 1250 keV. For the compositional analysis, energy dispersive spectroscopy (EDS) elemental mapping was achieved with JEOL JEM-2100F, operated at 200 keV. The XPS spectra were obtained on a VG ESCALAB 250 system with monochromatic Al Ka (1486.6 eV) as the excitation source. 3. Results and discussion Fig. 1 shows SEM micrographs of the surface of Aucatalyzed Si substrate after 30 min carbothermal reduction of the mixture of ZnO(s) and C(s) source materials, and all the structures shown in Fig. 1 were obtained at the same experimental condition except for carbon source. When graphite carbon (As = 3.5 m2/g) was used as a starting material, there was no Zn-related structure, but uniformly distributed Au nanoparticles (size = 5–10 nm) could be observed on the substrate. Although there is a region which looks like an island (marked by arrows), but it is not clear. However, with a C1 carbon black source (As = 70 m2/g), a certain island structure was clearly observed on the Si substrate at the same experimental condition, as shown in Fig. 1(b). The island was proven to be Zn-silicate by EDS mapping image, and the result is discussed later. Au nanoparticles of which size is 10–20 nm were located along the boundary of the Zn-silicate island, and this result shows that the Zn-silicate islands are preferentially nucleated at the corner between Au nanoparticles and substrate. The substrate was not fully covered with the islands, and large amount of Au nanoparticles (size = 5–10 nm) could also be observed on the substrate in the absence of the islands. The size of Au nanoparticle size shows that the particles were merged during the formation of Zn-silicate island. When we used a C2 carbon black source (As = 236 m2/g), the substrate was completely covered with Zn-silicate islands, as shown in Fig. 1(c). This result means that the supply of the reacting element for the formation of Zn-silicate, vaporized
Fig. 1. SEM micrographs of samples fabricated by using (a) graphite, (b) C1, (c) C2 and (d) C3 carbon source. The inset shows a well-faceted ZnO nanorod.
Y.S. Lim et al. / Applied Surface Science 253 (2006) 1601–1605
Zn(g), was enhanced by using C2 carbon source rather than graphite and C1. Therefore, it is found that carbon source with large surface area more efficiently provides Zn(g) by the enhanced solid–solid reaction between C(s) and ZnO(s) source materials during the carbothermal reduction [8,9]. As well as the formation of Zn-silicate, the carbothermal reduction with carbon black source resulted in ZnO nanostructure. When we used a C3 carbon black source (As = 1440 m2/ g), the carbothermal reduction resulted in ZnO nanorod, as shown in Fig. 1(d). The average length and diameter of the nanorod were 1 mm and 100 nm, respectively. The tip of nanorod (in the inset) has a fine hexagonal shape, so that the growth direction of the nanorod is expected as h0 0 0 1i. Comparing to the formation of Zn-silicate island, more Zn(g) should be provided for the formation and the growth of ZnO nanorod shown in Fig. 1(d). In this experiment, it was simply achieved by increasing the surface area of the carbon source without any change of experimental parameters. It is induced by the increase of the interaction area between ZnO(s) and C(s) source materials for the reduction into Zn(g), and this result confirms that the reduction of ZnO(s) into vaporized Zn(g) is enhanced by increasing the surface area of carbon source. Fig. 2(a–d) show cross-sectional TEM micrographs, corresponding to Fig. 1(a–d), respectively. When graphite was used as a carbon source, there was no Zn-related structure on the Si substrate. As shown in Fig. 2(a), thermal oxidation of Si substrate occurred below Au nanoparticles due to the high temperature process at 800 8C in air. The SiO2/Si interface was atomically flat and the oxide thickness was 6 nm. With carbon black sources of C1 and C2, island structure was produced on the thermally grown SiO2 layer, as shown in Fig. 2(b and c). The island has an elliptical shape along the
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cross-sectional view, and the degree of the undulation of SiO2/ Si interface is very consistent to the shape of the island. The oxide thicknesses in Fig. 2(b and c) were around 4–4.5 nm. When we used a C3 carbon source, the Zn-silicate islands were merged in Zn-silicate layer on 2.5 nm-thick SiO2. As shown in Fig. 2(d), the SiO2/Si interface seems to be quite flat, but not atomically. This result indicates that the interface undulation also merged in a flat interface as well as the Znsilicate islands. The carbothermal reduction produced a nanorod on the silicate layer, consistent with Fig. 1(d). Because the SiO2 thickness continuously decreases with increasing the surface area of carbon source, it can be proposed that there was a consumption of SiO2 during the carbothermal reduction. Therefore, the island structure, which produced by the reaction between SiO2 and Zn(g) during the carbothermal reduction process, is supposed to be Zn-silicate [10,11]. To find the composition of the island structure, elemental mapping of the structure was performed with EDS. Fig. 3(a–c) shows the EDS elemental mapping images of Zn, Si and O, respectively. As shown in Fig. 3, the island was composed of Zn, Si and O. Therefore, the island consists of Zn-silicate and it is very consistent with our assumption from the thickness of thermal oxide. With these results, it is confirmed that the Zn-silicate island is produced by the reaction between vaporized Zn(g) and thermally grown oxide during the carbothermal reduction. The Zn-silicate island could grow and cover the substrate completely by using a carbon source with larger surface area. After merging in a Zn-silicate layer of the island, ZnO nanorod could be grown on the Zn-silicate layer as shown in Fig. 2(d). Therefore, these results show that Zn-silicate island structure is produced at the initial stage of the reaction with C3 carbon black source, and that the ZnO nanorod is grown directly on
Fig. 2. Cross-sectional TEM micrographs of samples fabricated by using (a) graphite, (b) C1, (c) C2 and (d) C3 carbon source.
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Fig. 3. EDS mapping images of a Zn-silicate island with the element of (a) Zn, (b) Si and (c) O.
the silicate layer in this experiment. The crystal structure of the nanorod is investigated with high-resolution TEM (HRTEM). Fig. 4 is an HRTEM micrograph, showing an interfacial region between ZnO nanorod and Si substrate. Because the zone axis was oriented along 1 1 0 0 direction of ZnO, the lattice fringe of Si is not clearly identified. For the same reason, the interface between Zn-silicate and SiO2 layer is not so much clear as in Fig. 2(d). This result shows that the nanorod is composed of hexagonal wurtzite ZnO, and that the growth direction of the ZnO nanorod is h0 0 0 1i, as expected in the inset of Fig. 1(d). XPS spectra of those samples are shown in Fig. 5, and the result is summarized in Table 1. In Fig. 5(a), there is no Zn 2p3/2 peak in the sample produced with graphite carbon source. It means that graphite carbon source could not reduce ZnO(s) powder enough to produce any Zn-related structure in this experimental condition, and this result is very consistent with that in Figs. 1(a) and 2(a). However, in the structures formed by using C1 and C2 carbon black sources, Zn 2p3/2 peaks are clearly identified at the binding energy of 1023.4–1023.6 eV. Considering the results in Figs. 1 and 2, the binding energy is corresponding to the Zn-silicate islands. As shown in Table 1, the atomic percent of Zn increases with increasing surface area of carbon source, and it is directly related with the coverage of Zn-silicate islands on the substrate. When we used a C3 carbon
Fig. 4. A high-resolution TEM micrograph
of the interfacial area between ZnO nanorod and Si substrate along 1 1 0 0 zone axis of ZnO.
Fig. 5. XPS spectra of (a) Zn 2p3/2 and (b) O 1s obtained from the samples fabricated by using graphite and carbon black sources.
Y.S. Lim et al. / Applied Surface Science 253 (2006) 1601–1605
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Table 1 Binding energies and atomic percents of the elements in the samples depending on the carbon source Carbon source
Graphite C1 C2 C3
Binding energy (eV)
Atomic percent (%)
Zn 2p3
O 1s
Au 4d5
Si 2s
Zn 2p3
O 1s
Au 4d5
Si 2s
– 1023.6 1023.4 1022.0
532.6 532.8 532.2 530.6
334.8 335.2 335.4 335.0
102.8 103.2 102.8 103.0
0.32 9.39 30.67 53.19
57.32 55.38 49.74 42.94
4.30 4.08 4.44 1.96
38.06 31.15 15.15 1.91
black source, the Zn peak, which originates from wurtzite ZnO, is shown at the binding energy of 1022.0 eV, and this result is very consistent with SEM and TEM results. O 1s spectra also show the same tendency, as shown in Fig. 5(b). In the case of graphite carbon source, oxygen peak is originating from SiO2 at the binding energy of 532.6 eV. In the samples produced by C1 and C2, the peaks are located at 532.2–532.8 eV. Because this value is very close to the oxygen binding energy of SiO2, Zn-silicate could not be distinguished from SiO2. However, in the case of C3, the oxygen peak originating from the ZnO nanorod was located at 530.6 eV. Therefore, it was proven that the carbothermal reduction process for ZnO nanofabrication is strongly dependent on the surface area of carbon. Because all the experiments were performed at the same experimental condition, the formation of the Zn-silicate island and ZnO nanorod is closely related with the efficiency of Zn(g) vaporization [8,9]. From the coverage of Zn-silicate islands on the substrate in Fig. 1, it was found that carbon source with larger surface area more efficiently supplies Zn(g) for the formation of Zn-silicate. After merging in a layer of Zn-silicate islands, we could grow ZnO nanorod on the Zn-silicate layer by using the carbon source with the largest surface area. With this result, it is concluded that the carbothermal reduction of ZnO(s) is strongly dependent on the interaction area between ZnO(s) and C(s) source materials, and that the supply of Zn(g) for the formation of Zn-related structure could be enhanced by increasing the surface area of carbon source. 4. Conclusions In summary, we investigated the effect of the surface area of carbon source on carbothermal reduction. By controlling the surface area, Zn-silicate and ZnO nanorod were grown
on Si substrate at 800 8C for 30 min in air. With SEM, TEM, EDS and XPS results, it is concluded that the reducing power of ZnO(s) source into Zn(g) vapor is strongly dependent on the surface area of carbon source, and that the fabrication of ZnO nanostructure can be performed more efficiently by using carbon source with large surface area. We believe that this result is not only useful for the formation of ZnO nanostructure, but also applicable to the nanofabrication of various kinds of other oxides. Acknowledgement The authors would like to thank Y.M. Kim and Y.J. Kim (Korea Basic Science Institute, Daejeon, Korea) for highvoltage transmission electron microscopy measurement. References [1] S.Y. Li, P. Lin, C.Y. Lee, T.Y. Tseng, J. Appl. Phys. 95 (2004) 3711. [2] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002) 757. [3] D. Banerjee, J.Y. Lao, D.Z. Wang, J.Y. Huang, Z.F. Ren, D. Steeves, B. Kimball, M. Sennett, Appl. Phys. Lett. 83 (2003) 2061. [4] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [5] C.X. Xu, X.W. Sun, B.J. Chen, Appl. Phys. Lett. 84 (2004) 1540. [6] C.N.R. Rao, G. Gundiah, F.L. Deepak, A. Govindaraj, A.K. Cheetham, J. Mater. Chem. 14 (2004) 440. [7] G. Gundiah, F.L. Deepak, A. Govindaraj, C.N.R. Rao, Top. Catal. 24 (2003) 137. [8] Y.H. Leung, A.B. Djurisˇic´, J. Gao, M.H. Xie, Z.F. Wei, S.J. Xu, W.K. Chan, Chem. Phys. Lett. 394 (2004) 452. [9] Y.H. Leung, A.B. Djurisˇic´, J. Gao, M.H. Xie, Z.F. Wei, W.K. Chan, Chem. Phys. Lett. 385 (2004) 155. [10] X. Xu, P. Wang, Z. Qi, H. Ming, J. Xu, H. Liu, C. Shi, G. Lu, W. Ge, J. Phys.: Condens. Matter 15 (2003) L607. [11] X. Xu, C. Guo, Z. Qi, H. Liu, J. Xu, C. Shi, C. Chong, W. Huang, Y. Zhou, C. Xu, Chem. Phys. Lett. 364 (2002) 57.
Effect of carbon source on the carbothermal reduction for the fabrication of ZnO nanostructure Y.S. Lim *, J.W. Park, M.S. Kim, J. Kim Corporate R&D, LG Chem/Research Park, 104-1 Moonji-dong, Yuseong-gu, 305-380 Daejeon, Republic of Korea Received 7 January 2006; received in revised form 16 February 2006; accepted 21 February 2006 Available online 31 March 2006
Abstract Surface area effect of carbon source on the carbothermal reduction for the fabrication of ZnO nanostructure was investigated. For a systematic comparison, graphite and three kinds of carbon black powder were used as source materials for the carbothermal reduction. Depending on the surface area, the carbothermal reduction at 800 8C for 30 min resulted in Zn-silicate island or ZnO nanorod at the same experimental condition. These structures were characterized with a scanning electron microscopy, a transmission electron microscopy, an energy dispersive spectroscopy and an X-ray photoelectron spectroscopy. The results show that the reducing power of ZnO(s) source into Zn(g) vapor is strongly dependent on the surface area of carbon source, and that the fabrication of ZnO nanostructure can be performed more efficiently by using carbon source with large surface area. # 2006 Elsevier B.V. All rights reserved. Keywords: ZnO; Carbothermal reduction; Nanorod; Transmission electron microscopy; X-ray photoelectron spectroscopy
1. Introduction Among the various methods for fabricating ZnO nanostructure, carbothermal reduction is known to be a relatively simple, cheap and suitable process for large scale production [1]. By heating the mixture of ZnO and carbon powder up to a reaction temperature, ZnO(s) powder can be reduced into Zn(g) by C(s) through solid–solid reaction [1,2]. The vaporized Zn(g) can be deposited on a substrate, resulting in ZnO nanostructure. As the carbon source for the carbothermal reduction process, graphite carbon has been generally used [1–3]. However, due to its small surface area (<10 m2/g), the reaction between source materials of graphite and ZnO(s) powders was not strong. Therefore, for the fabrication of ZnO nanostructures with graphite carbon source, high temperature and/or precise gas control system had been required [1–5]. Recently, there were challenges to enhance the carbothermal reduction by using other carbon source rather than graphite [6–9]. By using various carbon sources, such as graphite, nanotube, carbon black and carbon nanoparticle, Rao et al. successfully synthesized a variety of oxide, nitride and carbide
* Corresponding author. E-mail address: [email protected] (Y.S. Lim). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.051
nanowires [6,7]. Leung et al. showed that the carbothermal reduction for ZnO nanostructure fabrication could be significantly enhanced by using single-walled carbon nanotube [8,9]. Moreover, with the mixture of ZnO nanoparticle and conventional graphite powder, the enhanced reduction was also achieved [8,9]. Because the reduction of ZnO(s) into vaporized Zn(g) is achieved by the solid–solid reaction between C(s) and ZnO(s), these results imply that the surface area of starting materials is very important factor for the carbothermal reduction process. Here, we report the effect of surface area of carbon source to enhance the solid–solid reaction between ZnO(s) and C(s) during carbothermal reduction for ZnO nanostructure fabrication. Although there are some reports on the growth of ZnO nanostructure by using the carbon source with large surface area [6–9], but a systematic comparison of the effect of the surface area has not yet been reported. For the comparison, graphite and three kinds of carbon black powder were used as the carbon source for the fabrication of ZnO nanostructure in this experiment. Carbon black is a cheaper and more abundant material than single-walled carbon nanotube, and it has a larger surface area (As = 30–1600 m2/g) than graphite (<10 m2/g). Moreover, one can easily choose desirable surface area of carbon black powder within the range of 30–1600 m2/g for specific purpose of experiment.
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In this experiment, different kinds of Zn-related structure were synthesized after carbothermal reduction at 800 8C for 30 min, depending on the surface area of carbon source. The structure was characterized with a scanning electron microscopy (SEM), a transmission electron microscopy (TEM), an energy dispersive spectroscopy (EDS) and an X-ray photoelectron spectroscopy (XPS). These results show that the reducing power of ZnO(s) source into Zn(g) vapor is strongly dependent on the surface area of carbon source, and that the fabrication of ZnO nanostructure can be performed more efficiently by using carbon source with large surface area. This result is not only useful for the fabrication of ZnO nanostructure, but also applicable to the nanostructure fabrication of various kinds of other oxides. 2. Experimental details Source powder for the carbothermal reduction was prepared with a fixed amount of the mixture of high purity ZnO powder (0.05 g) and carbon powder (0.05 g). In this experiment, graphite (As = 3.5 m2/g) and three kinds of carbon black powder (As = 70, 236 and 1440 m2/g, labeled as C1–C3, respectively) were used as carbon source. The surface area was measured by Brunauer– Emmett–Teller (BET) method with Micromeritics ASAP 2010 volumetric sorption analyzer at 77.35 K using high-purity nitrogen in the range of relative pressures from 106 to 1. Before the measurements, all samples were degassed at 200 8C for 2 h. The carbon powder was mixed with ZnO powder, ground by mortar and pestle, and loaded on a platinum boat. The boat was covered with Au-catalyzed Si substrate (p-type, 1– 10 V cm), and inserted into a horizontal tube furnace. The platinum boat was heated up to 800 8C for 30 min in air. After the carbothermal reduction process, the resulting structures were characterized with a SEM, a TEM, an EDS and an XPS. SEM analysis was performed with a Hitachi S-4800 Scanning electron microscope, operated at 15 keV for secondary electron imaging. TEM characterization was performed with a
JEOL JEM-ARM1300S high-voltage transmission electron microscope (HVEM), operated at 1250 keV. For the compositional analysis, energy dispersive spectroscopy (EDS) elemental mapping was achieved with JEOL JEM-2100F, operated at 200 keV. The XPS spectra were obtained on a VG ESCALAB 250 system with monochromatic Al Ka (1486.6 eV) as the excitation source. 3. Results and discussion Fig. 1 shows SEM micrographs of the surface of Aucatalyzed Si substrate after 30 min carbothermal reduction of the mixture of ZnO(s) and C(s) source materials, and all the structures shown in Fig. 1 were obtained at the same experimental condition except for carbon source. When graphite carbon (As = 3.5 m2/g) was used as a starting material, there was no Zn-related structure, but uniformly distributed Au nanoparticles (size = 5–10 nm) could be observed on the substrate. Although there is a region which looks like an island (marked by arrows), but it is not clear. However, with a C1 carbon black source (As = 70 m2/g), a certain island structure was clearly observed on the Si substrate at the same experimental condition, as shown in Fig. 1(b). The island was proven to be Zn-silicate by EDS mapping image, and the result is discussed later. Au nanoparticles of which size is 10–20 nm were located along the boundary of the Zn-silicate island, and this result shows that the Zn-silicate islands are preferentially nucleated at the corner between Au nanoparticles and substrate. The substrate was not fully covered with the islands, and large amount of Au nanoparticles (size = 5–10 nm) could also be observed on the substrate in the absence of the islands. The size of Au nanoparticle size shows that the particles were merged during the formation of Zn-silicate island. When we used a C2 carbon black source (As = 236 m2/g), the substrate was completely covered with Zn-silicate islands, as shown in Fig. 1(c). This result means that the supply of the reacting element for the formation of Zn-silicate, vaporized
Fig. 1. SEM micrographs of samples fabricated by using (a) graphite, (b) C1, (c) C2 and (d) C3 carbon source. The inset shows a well-faceted ZnO nanorod.
Y.S. Lim et al. / Applied Surface Science 253 (2006) 1601–1605
Zn(g), was enhanced by using C2 carbon source rather than graphite and C1. Therefore, it is found that carbon source with large surface area more efficiently provides Zn(g) by the enhanced solid–solid reaction between C(s) and ZnO(s) source materials during the carbothermal reduction [8,9]. As well as the formation of Zn-silicate, the carbothermal reduction with carbon black source resulted in ZnO nanostructure. When we used a C3 carbon black source (As = 1440 m2/ g), the carbothermal reduction resulted in ZnO nanorod, as shown in Fig. 1(d). The average length and diameter of the nanorod were 1 mm and 100 nm, respectively. The tip of nanorod (in the inset) has a fine hexagonal shape, so that the growth direction of the nanorod is expected as h0 0 0 1i. Comparing to the formation of Zn-silicate island, more Zn(g) should be provided for the formation and the growth of ZnO nanorod shown in Fig. 1(d). In this experiment, it was simply achieved by increasing the surface area of the carbon source without any change of experimental parameters. It is induced by the increase of the interaction area between ZnO(s) and C(s) source materials for the reduction into Zn(g), and this result confirms that the reduction of ZnO(s) into vaporized Zn(g) is enhanced by increasing the surface area of carbon source. Fig. 2(a–d) show cross-sectional TEM micrographs, corresponding to Fig. 1(a–d), respectively. When graphite was used as a carbon source, there was no Zn-related structure on the Si substrate. As shown in Fig. 2(a), thermal oxidation of Si substrate occurred below Au nanoparticles due to the high temperature process at 800 8C in air. The SiO2/Si interface was atomically flat and the oxide thickness was 6 nm. With carbon black sources of C1 and C2, island structure was produced on the thermally grown SiO2 layer, as shown in Fig. 2(b and c). The island has an elliptical shape along the
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cross-sectional view, and the degree of the undulation of SiO2/ Si interface is very consistent to the shape of the island. The oxide thicknesses in Fig. 2(b and c) were around 4–4.5 nm. When we used a C3 carbon source, the Zn-silicate islands were merged in Zn-silicate layer on 2.5 nm-thick SiO2. As shown in Fig. 2(d), the SiO2/Si interface seems to be quite flat, but not atomically. This result indicates that the interface undulation also merged in a flat interface as well as the Znsilicate islands. The carbothermal reduction produced a nanorod on the silicate layer, consistent with Fig. 1(d). Because the SiO2 thickness continuously decreases with increasing the surface area of carbon source, it can be proposed that there was a consumption of SiO2 during the carbothermal reduction. Therefore, the island structure, which produced by the reaction between SiO2 and Zn(g) during the carbothermal reduction process, is supposed to be Zn-silicate [10,11]. To find the composition of the island structure, elemental mapping of the structure was performed with EDS. Fig. 3(a–c) shows the EDS elemental mapping images of Zn, Si and O, respectively. As shown in Fig. 3, the island was composed of Zn, Si and O. Therefore, the island consists of Zn-silicate and it is very consistent with our assumption from the thickness of thermal oxide. With these results, it is confirmed that the Zn-silicate island is produced by the reaction between vaporized Zn(g) and thermally grown oxide during the carbothermal reduction. The Zn-silicate island could grow and cover the substrate completely by using a carbon source with larger surface area. After merging in a Zn-silicate layer of the island, ZnO nanorod could be grown on the Zn-silicate layer as shown in Fig. 2(d). Therefore, these results show that Zn-silicate island structure is produced at the initial stage of the reaction with C3 carbon black source, and that the ZnO nanorod is grown directly on
Fig. 2. Cross-sectional TEM micrographs of samples fabricated by using (a) graphite, (b) C1, (c) C2 and (d) C3 carbon source.
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Fig. 3. EDS mapping images of a Zn-silicate island with the element of (a) Zn, (b) Si and (c) O.
the silicate layer in this experiment. The crystal structure of the nanorod is investigated with high-resolution TEM (HRTEM). Fig. 4 is an HRTEM micrograph, showing an interfacial region between ZnO nanorod and Si substrate. Because the zone axis was oriented along 1 1 0 0 direction of ZnO, the lattice fringe of Si is not clearly identified. For the same reason, the interface between Zn-silicate and SiO2 layer is not so much clear as in Fig. 2(d). This result shows that the nanorod is composed of hexagonal wurtzite ZnO, and that the growth direction of the ZnO nanorod is h0 0 0 1i, as expected in the inset of Fig. 1(d). XPS spectra of those samples are shown in Fig. 5, and the result is summarized in Table 1. In Fig. 5(a), there is no Zn 2p3/2 peak in the sample produced with graphite carbon source. It means that graphite carbon source could not reduce ZnO(s) powder enough to produce any Zn-related structure in this experimental condition, and this result is very consistent with that in Figs. 1(a) and 2(a). However, in the structures formed by using C1 and C2 carbon black sources, Zn 2p3/2 peaks are clearly identified at the binding energy of 1023.4–1023.6 eV. Considering the results in Figs. 1 and 2, the binding energy is corresponding to the Zn-silicate islands. As shown in Table 1, the atomic percent of Zn increases with increasing surface area of carbon source, and it is directly related with the coverage of Zn-silicate islands on the substrate. When we used a C3 carbon
Fig. 4. A high-resolution TEM micrograph
of the interfacial area between ZnO nanorod and Si substrate along 1 1 0 0 zone axis of ZnO.
Fig. 5. XPS spectra of (a) Zn 2p3/2 and (b) O 1s obtained from the samples fabricated by using graphite and carbon black sources.
Y.S. Lim et al. / Applied Surface Science 253 (2006) 1601–1605
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Table 1 Binding energies and atomic percents of the elements in the samples depending on the carbon source Carbon source
Graphite C1 C2 C3
Binding energy (eV)
Atomic percent (%)
Zn 2p3
O 1s
Au 4d5
Si 2s
Zn 2p3
O 1s
Au 4d5
Si 2s
– 1023.6 1023.4 1022.0
532.6 532.8 532.2 530.6
334.8 335.2 335.4 335.0
102.8 103.2 102.8 103.0
0.32 9.39 30.67 53.19
57.32 55.38 49.74 42.94
4.30 4.08 4.44 1.96
38.06 31.15 15.15 1.91
black source, the Zn peak, which originates from wurtzite ZnO, is shown at the binding energy of 1022.0 eV, and this result is very consistent with SEM and TEM results. O 1s spectra also show the same tendency, as shown in Fig. 5(b). In the case of graphite carbon source, oxygen peak is originating from SiO2 at the binding energy of 532.6 eV. In the samples produced by C1 and C2, the peaks are located at 532.2–532.8 eV. Because this value is very close to the oxygen binding energy of SiO2, Zn-silicate could not be distinguished from SiO2. However, in the case of C3, the oxygen peak originating from the ZnO nanorod was located at 530.6 eV. Therefore, it was proven that the carbothermal reduction process for ZnO nanofabrication is strongly dependent on the surface area of carbon. Because all the experiments were performed at the same experimental condition, the formation of the Zn-silicate island and ZnO nanorod is closely related with the efficiency of Zn(g) vaporization [8,9]. From the coverage of Zn-silicate islands on the substrate in Fig. 1, it was found that carbon source with larger surface area more efficiently supplies Zn(g) for the formation of Zn-silicate. After merging in a layer of Zn-silicate islands, we could grow ZnO nanorod on the Zn-silicate layer by using the carbon source with the largest surface area. With this result, it is concluded that the carbothermal reduction of ZnO(s) is strongly dependent on the interaction area between ZnO(s) and C(s) source materials, and that the supply of Zn(g) for the formation of Zn-related structure could be enhanced by increasing the surface area of carbon source. 4. Conclusions In summary, we investigated the effect of the surface area of carbon source on carbothermal reduction. By controlling the surface area, Zn-silicate and ZnO nanorod were grown
on Si substrate at 800 8C for 30 min in air. With SEM, TEM, EDS and XPS results, it is concluded that the reducing power of ZnO(s) source into Zn(g) vapor is strongly dependent on the surface area of carbon source, and that the fabrication of ZnO nanostructure can be performed more efficiently by using carbon source with large surface area. We believe that this result is not only useful for the formation of ZnO nanostructure, but also applicable to the nanofabrication of various kinds of other oxides. Acknowledgement The authors would like to thank Y.M. Kim and Y.J. Kim (Korea Basic Science Institute, Daejeon, Korea) for highvoltage transmission electron microscopy measurement. References [1] S.Y. Li, P. Lin, C.Y. Lee, T.Y. Tseng, J. Appl. Phys. 95 (2004) 3711. [2] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002) 757. [3] D. Banerjee, J.Y. Lao, D.Z. Wang, J.Y. Huang, Z.F. Ren, D. Steeves, B. Kimball, M. Sennett, Appl. Phys. Lett. 83 (2003) 2061. [4] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [5] C.X. Xu, X.W. Sun, B.J. Chen, Appl. Phys. Lett. 84 (2004) 1540. [6] C.N.R. Rao, G. Gundiah, F.L. Deepak, A. Govindaraj, A.K. Cheetham, J. Mater. Chem. 14 (2004) 440. [7] G. Gundiah, F.L. Deepak, A. Govindaraj, C.N.R. Rao, Top. Catal. 24 (2003) 137. [8] Y.H. Leung, A.B. Djurisˇic´, J. Gao, M.H. Xie, Z.F. Wei, S.J. Xu, W.K. Chan, Chem. Phys. Lett. 394 (2004) 452. [9] Y.H. Leung, A.B. Djurisˇic´, J. Gao, M.H. Xie, Z.F. Wei, W.K. Chan, Chem. Phys. Lett. 385 (2004) 155. [10] X. Xu, P. Wang, Z. Qi, H. Ming, J. Xu, H. Liu, C. Shi, G. Lu, W. Ge, J. Phys.: Condens. Matter 15 (2003) L607. [11] X. Xu, C. Guo, Z. Qi, H. Liu, J. Xu, C. Shi, C. Chong, W. Huang, Y. Zhou, C. Xu, Chem. Phys. Lett. 364 (2002) 57.