Thorn-like ZnO/CNT composites via the hydrothermal method with different seed layer Hsi-Chao Chen,1,2,* Ssu-Fan Lin,1 and Kuo-Ting Huang2 1
Graduate School of Electronic and Optoelectronic Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan 2
Graduate School of Science and Technology, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan *Corresponding author: [email protected] Received 29 August 2013; revised 14 November 2013; accepted 14 November 2013; posted 22 November 2013 (Doc. ID 196688); published 10 January 2014
This paper studies the preparation of a zinc oxide (ZnO) seed layer deposited onto carbon nanotubes (CNTs) via RF sputtering, and the growth of ZnO/CNT composite via the hydrothermal method. The ZnO “thorns” have been successfully deposited on CNT “stems.” The research focuses on the ZnO seed layer with different sputtering times and annealing processes. A scanning electron microscopy (SEM) investigation showed that the length and amount of ZnO thorns decreased with increased sputtering time of the seed layer. The x-ray diffraction (XRD) results showed that the ZnO crystalline structures improved after the seed layer, annealing at peaks of (100), (002), and (101). The results of Raman spectra showed that the ZnO seed layer deposited onto the CNTs, and the annealing, caused damage to the CNTs, reducing the IG ∕ID ratio from 0.9 to 0.5. Furthermore, the highest UV emission of thorn-like ZnO/CNTs occurred at a peak of 380 nm with seed layer deposition time of 2 min, after annealing. © 2013 Optical Society of America OCIS codes: (160.0160) Materials; (160.4236) Nanomaterials; (160.4670) Optical materials; (160.4760) Optical properties. http://dx.doi.org/10.1364/AO.53.00A242
1. Introduction
Carbon nanotubes (CNTs) have attracted a great deal of attention because of their good mechanical properties, electrical conductivity, and chemical properties [1–3]. Therefore, in recent years, there has been an increasing interest in the composite materials of CNTs [4–6]. Furthermore, zinc oxide (ZnO) has potential applications in optoelectronic devices, such as solar cells [7,8], sensors [9,10], displays [11,12], and so on, due to its wide direct bandgap (Eg 3.37 eV) and high exciting binding energy (Ex 60 meV) [13,14]. Hence, the growth of ZnO on CNTs leads to heterojunctions of semiconducting ZnO and metallic CNT, which might be very useful in optoelectronic device applications. In 2002, ZnO nanowires were first grown on the 1559-128X/14/04A242-06$15.00/0 © 2014 Optical Society of America A242
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surfaces of CNTs without the presence of a catalyst [15]. In 2006, ZnO nanowires were successfully grown on modified well-aligned CNT arrays, via a hydrothermal process [16]. More recently, mesoporous ZnO/CNT films were fabricated into the working electrodes of dye-sensitized solar cells (DSSCs); the acid-treated CNTs increased the power conversion efficiency of DSSC that had been achieved [17]. There are many applications for ZnO/CNT composites; however, some problematic issues appear in the production process, such as the reconstruction, stability, lifetime, and large area, all affecting the cost. Subsequently, a simple process, using radio frequency (RF) sputtering, deposited a ZnO seed layer onto CNTs, which were grown into thorn-like ZnO structures, via a hydrothermal method. The effect of the annealing on the seed layer was also studied. The results showed that the growth ratio of the ZnO tips decreases with the thickening seed
layer and that the ZnO crystalline structure improved after seed layer annealing by x-ray diffraction (XRD). The ZnO seed layer deposited onto the CNTs, and the annealing, damaged the CNTs, reducing the IG ∕ID ratio from 0.9 to 0.5 by Raman spectra. Finally, the highest UV emission of thorn-like ZnO/CNTs occurred at a peak of 380 nm with a seed layer deposition time of 2 min, after annealing. 2. Experimental Procedure
The preparation of composite thorn-like ZnO/CNT is divided into four steps, as shown in Fig. 1. First, an Fe catalyst was deposited by direct current (DC) sputtering on a silicon wafer. Second, CNTs were grown by thermal chemical vapor deposition (CVD) at 700°C for 3 min. In the third step, ZnO was deposited onto the CNT surfaces as a seed layer, by RF sputtering. The base pressure, below 1 × 10−6 Torr, was maintained prior to the deposition. High purity Ar and O2 gases filled the chamber at the flow rate of 18 and 2 sccm, respectively, and the working pressure was 1.1 × 10−2 Torr. RF power of 100 W was supplied for the sputtering of the ZnO seed layer, and the deposition rate was about 10 nm∕min. At room temperature, the deposition times were 2, 5, and 10 min. After deposition of the seed layer, the samples were annealed at 500°C in ambient air for 1 h that was an optional step. Finally, the thorn-like ZnO was grown on CNT stems via a hydrothermal method. The deposited condition of the hydrothermal process was zinc acetate and hexamethylenetetramines (HMTA) dissolved in 80 ml of de-ionized water, with a concentration of 0.0125 M. The solution and ZnO seed layer–CNT sample were transferred into a Teflon-lined stainless autoclave. The autoclave was sealed and maintained at 90°C for 1 h. The morphology of ZnO/CNT composites was analyzed with field emission scanning electron microscopy (FESEM, JSM-6701F). The crystalline structure of the ZnO/CNT composites was characterized by (XRD, Bruker D8). Cobalt K α radiation (λk 0.178892 nm) from an x ray tube with a normal focus was employed
at a grazing incidence angle of 1°. Raman spectra measurements were taken with a TRIAX 550 (Jobin Yvon HORIBA S. A. S., France) using a 532 nm laser as the excitation source. Photoluminescence (PL) measurements were performed with a He–Cd laser (325 nm) as the excitation source at room temperature. 3. Results and Discussion A. Measurement of Morphology
Figure 2 shows the SEM morphology of the ZnO seed layer on the CNT surface via RF sputtering, in which (a), (b), and (c) involved deposition times of 2, 5, and 10 min without the annealing process, respectively. Corresponding to the three previous samples, (d), (e), and (f) were passed through the annealing process after sputtering. As shown in Fig. 2, the amount of ZnO particles adhering to the CNTs increases the diameter of the tube with the increased sputtering time. The thickness did not change after the annealing. By comparing the seed layer without and with annealing, we found that there was no difference between them with regard to the surface morphology of the ZnO seed layer; however, the annealing process could improve the crystallization of ZnO with annealing. Figure 3 shows the SEM images of thorn-like ZnO/CNT surface morphology via a hydrothermal method, at different seed layer sputtering times, in which (a), (b), and (c) are the ZnO seed layers without annealing, and (d), (e), and (f) are with annealing. The length and amount of ZnO thorns decreased with the increased sputtering time of the seed layer; however, the diameter of the ZnO thorn increased with the decreasing ZnO thorn numbers, due to lower nucleation density, which means there was nothing to effectively inhibit the growth of the laterals. Figures 3(d)–3(f) show the ZnO/CNTs surface morphology with the annealing process, and that the growth trend of ZnO thorns was the same for them, without the annealing process.
Fig. 1. Schematic diagram of the growth process of the thorn-like ZnO/CNT composites. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 2. SEM images of ZnO-seed-layer/CNT surface morphology at different sputtering times: (a) 2 min, (b) 5 min, and (c) 10 min, without annealing, and (d) 2 min, (e) 5 min, (f) 10 min, with the annealing process.
Figure 4 shows the length of ZnO thorns with different sputtering times and annealing of the ZnO seed layer. The length of the ZnO thorns changed from 320 to 260 nm without annealing and 220 to 140 nm with annealing. It is clear to see that the length of the ZnO thorns with annealing become smaller than without annealing, as the annealing process increases the grain size of the seed layer. This phenomenon was also studied in the XRD patterns. B.
Measurement of XRD
Figure 5 shows the XRD patterns of ZnO seed layers deposited onto CNT surfaces with and without annealing. The three main diffraction peaks (100), (002), and (101) correspond to ZnO. The grain size of the crystallites was calculated using a Scherrer’s formula, d
0.9λ ; β cos θ
(1)
where λ, θ, and β are the x ray wavelength, Bragg diffraction angle, and full width at half-maximum (FWHM) after correcting the instrument’s peak broadening. In Fig. 5, the diffraction peaks (100), (002), (101), and (110) of the ZnO seed layer clearly appear in the seed layers without annealing. After the annealing process, the FWHM of the four peaks became narrower and also revealed the other peaks of (102), (103), (112), and (201). According to Scherrer’s equation, the FWHM of the XRD peaks was narrowing due to the increased grain size after the annealing process [18]. Moreover, peak intensity increases with the deposition time of the ZnO seed layer, up to 2–10 min. Figure 6 shows XRD patterns of thornlike ZnO/CNTs that were grown via hydrothermal process without and with annealing. The intensity of the (002) peak was higher than the (100) peak after the annealing process, which could be attributed to the ZnO thorns’ growth along the c-axis. In other
Fig. 3. SEM images of thorn-like ZnO/CNT surface morphologies created using a hydrothermal process with different seed layer deposition times: (a) 2 min, (b) 5 min, and (c) 10 min without seed layer annealing, and (d) 2 min, (e) 5 min, and (f) 10 min, with seed layer annealing. A244
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Fig. 4. Average length of the ZnO thorns at the different sputtering times of the ZnO seed layer.
words, the ZnO crystallization improved by using the annealing process. However, the peak intensity decreased after the annealing process as the length of the ZnO thorn became shorter (see Fig. 4). The thorn-like ZnO/CNT composite structures, grown via a hydrothermal method, had crystallization in the (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes. The peak positions show the JCPDS standard (No. 79-2205) for ZnO with a hexagonal wurtzite structure. In addition, the peak at 2θ 26° originates from the graphite layers of the CNT’s film. C.
Measurement of Raman Spectra
Figure 7 shows the Raman spectra of the ZnO seed layer deposited on the CNTs with different sputtering times, with and without annealing. The D band peak was the defect mode that existed at about 1325 cm−1 , and the G band peak was the graphite C–C tangential stretching mode that existed at about 1580 cm−1 [19]. As shown in Fig. 7, when the ZnO seed layer was deposited on the CNTs, the D band
Fig. 5. XRD patterns of ZnO seed layer/CNTs with different deposition times, with (a), (b), and (c) without annealing, and (d), (e), and (f) with annealing.
Fig. 6. XRD patterns of thorn-like ZnO/CNTs with different seed layer sputtering times, with (a), (b), and (c) without annealing, and (d), (e), and (f) with annealing.
strength became higher than the G band, and inverted to the pure CNT model. The IG ∕ID would get by using a Lorentz fitting to calculate the integral area ratio of the G and D bands, which could evaluate the perfect quality of the CNT crystalline structure. When the IG ∕ID ratio of CNTs with deposited ZnO seed layer was lower than 0.96 of the pure CNTs, it means that sputtering ZnO onto the CNTs has destroyed the graphite structure and deteriorated the quality of the CNTs. For all of the ZnO seed layer/CNT films, there was a clear shoulder peak at 1620 cm−1 after annealing; the peak was the D’ band, which means that there was a broken key among these carbon atoms, or the loss of an atom from the special position of the periodic lattices, which generated vacancy defects. During the annealing process of the ZnO seed layer, re-crystallization action could also damage the crystallization of CNTs. The ratio of IG ∕ID shows no clear trends for the ZnO/CNTs after hydrothermal growth. Figure 8 shows the Raman spectra of thorn-like ZnO/CNT
Fig. 7. Raman spectra of ZnO seed layers deposited on CNT surfaces using different sputtering times, where (a) is pure CNT (no seed layer), and (b), (c), and (d) without annealing, and (e), (f), and (g) with annealing. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 8. Raman spectra of thorn-like ZnO/CNTs with different seed layer sputtering times, with (a), (b), and (c) without annealing, and (d), (e), and (f) with annealing.
composites with different deposition times of the ZnO seed layer, with and without annealing. There was only a small difference between the ratios of IG ∕ID . The shoulder peak also existed at 1620 cm−1 for the ZnO seed layer/CNTs with the annealing process. Therefore, sputtering and annealing the ZnO seed layer were the main factors affecting the quality of CNT performance. D.
Measurement of PL Spectra
Figure 9 shows the photoluminescence (PL) spectra of the ZnO seed layer deposited on the CNTs with different sputtering times. The PL spectrum of ZnO is composed of two parts, an excitonic-related near band edge (NBE) emission peak in the ultraviolet region (380 nm) and a broad deep level emission band at around the green ray region (570 nm). The broad deep level emission band is associated with electron-hole recombination at a deep level, presumably caused by oxygen vacancy or zinc interstitial defect [20]. In Fig. 9, ZnO seed layer/CNTs with 10 min sputtering time and annealing had the maximum NBE emission peak, which could be attributed to
Fig. 10. PL spectra of thorn-like ZnO/CNTs with different seed layer sputtering times, in which (a), (b), and (c) were without the annealing process, and (d), (e), and (f) were with the annealing process.
the higher crystal quality of ZnO seed layer [21]. The UV emission intensity increased with the thickness of the ZnO. Hence, the variation of NBE emission is consistent with the quality evaluated from the XRD spectra in Fig. 5. Figure 10 shows the PL spectra of thorn-like ZnO/ CNTs with different sputtering times of ZnO seed layer. According to the SEM results, NBE emission intensity increases with the higher density of ZnO thorns. The crystallization of ZnO was improved by the annealing process and the highest intensity of UV emission appears at the sputtering time of 2 min, with annealing. Hence, the strong UV emission in thorn-like ZnO/CNTs may be attributed to the higher density of ZnO thorns and the better crystallization of ZnO. Furthermore, the PL spectrum of the ZnO presents a redshift of the NBE emission peak of approximately 70 meV that could be observed with the increased thickness of the ZnO seed layer after the annealing process (see Figs. 9 and 10). However, the thickness of ZnO seed layers were larger than the exciton Bohr radius of ZnO, which is about 2 nm; thus, the quantum confinement effect is not the main cause of the redshift in the NBE emission peak. This could be attributed to band tail, or defect-related states caused by the relaxation of the compressive strain [22]. During annealing, the ZnO seed was re-crystallized and grain growth may have taken place, thereby, changing the bandgap of ZnO and causing a corresponding redshift in the PL spectrum. 4. Conclusions
Fig. 9. Photoluminescence spectra of ZnO seed layers deposited on CNTs’ surface by different sputtering times, with (a), (b), and (c) without annealing, and (d), (e), and (f) with annealing. A246
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A novel composite material composed of ZnO “thorns” on CNT “stems,” is prepared for the seed layer by RF sputtering and the growth of ZnO on the CNT surfaces via a hydrothermal process. A comparison was made between ZnO seed layers deposited with different times of 2, 5, and 10 min, and with and without the annealing process. The SEM images showed that the ZnO thorn length decreased
with increased sputtering time of the seed layer, and that the diameter of the ZnO thorn increased with decreasing ZnO length. The XRD images showed that ZnO is a hexagonal wurtzite structure, with clear peaks at (100), (002), and (101), and that the annealing process could improve the crystallization of ZnO. Raman spectra showed the D band and G band of CNTs at 1325 cm−1 and 1580 cm−1 , respectively. The ZnO seed layer deposited on the CNTs, and the annealing process, damaged the CNTs, reducing the IG ∕ID ratio. The intrinsic emission of ZnO was at 380 nm; however, the annealing of the ZnO seed layer had a redshift of UV emission. The largest UV emission of thorn-like ZnO/CNTs occurred with a ZnO seed layer sputtering time of 2 min, with annealing. In this research, the thornlike ZnO/CNT composite material could be fabricated successfully via a hydrothermal method. The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under contract No. NSC 102-2220-E-224-002- and NSC 102 - 2622 E - 224 - 003 - CC3. References 1. S. B. Yang, B. S. Kong, D. H. Jung, Y. K. Baek, C. S. Han, S. K. Oh, and H. T. Jung, “Recent advances in hybrids of carbon nanotube network films and nanomaterials for their potential applications as transparent conducting films,” Nanoscale 3, 1361–1373 (2011). 2. D. S. Hecht, L. Hu, and G. Irvin, “Emerging transparent electrodes based on thin films of carbon nanotubes, graphene, and metallic nanostructures,” Adv. Mater. 23, 1482–1513 (2011). 3. E. T. Thostenson, Z. F. Ren, and T. W. Chou, “Advances in the science and technology of carbon nanotubes and their composites: a review,” Compos. Sci. Technol. 61, 1899–1912 (2001). 4. X. Zhang, J. S. Lee, G. S. Lee, D. K. Cha, M. J. Kim, D. J. Yang, and S. K. Manohar, “Chemical synthesis of PEDOT nanotubes,” Macromolecules 39, 470–472 (2006). 5. A. O’Connor, S. Deb, J. N. Coleman, and Y. K. Gun’koa, “Development of transparent, conducting composites by surface infiltration of nanotubes into commercial polymer films,” Carbon 47, 1983–1988 (2009). 6. H. S. Park, J. S. Kim, B. G. Choi, S. M. Jo, D. Y. Kim, W. H. Hong, and S. Y. Janga, “Sonochemical hybridization of carbon nanotubes with gold nanoparticles for the production of flexible transparent conducing films,” Carbon 48, 1325–1330 (2010).
7. Q. F. Zhang, C. S. Dandeneau, X. Y. Zhou, and G. Z. Cao, “ZnO nanostructures for dye-sensitized solar cells,” Adv. Mater. 21, 4087–4108 (2009). 8. Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, “ZnO nanostructures as efficient antireflection layers in solar cells,” Nano Lett. 8, 1501–1505 (2008). 9. Q. Wan, Q. H. Li, Y. J. Chen, T. H. Wang, X. L. He, J. P. Li, and C. L. Lin, “Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors,” Appl. Phys. Lett. 84, 3654–3656 (2004). 10. J. Q. Xu, Q. Y. Pan, Y. A. Shun, and Z. Z. Tian, “Grain size control and gas sensing properties of ZnO gas sensor,” Sensor. Actuat. B 66, 277–279 (2000). 11. S. H. K. Park, C. S. Hwang, M. Ryu, S. Yang, C. Byun, J. Shin, J. I. Lee, K. Lee, M. S. Oh, and S. Im, “Transparent and photo-stable ZnO thin-film transistors to drive an active matrix organic-light-emitting-diode display panel,” Adv. Mater. 21, 678–682 (2009). 12. B. Y. Oh, M. C. Jeong, T. H. Moon, W. Lee, J. M. Myoung, J. Y. Hwang, and D. S. Seo, “Transparent conductive Al-doped ZnO films for liquid crystal displays,” J. Appl. Phys. 99, 124505 (2006). 13. Z. L. Wang, “Zinc oxide nanostructures: growth, properties and applications,” J. Phys. 16, R829–R858 (2004). 14. U. Ozgur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho, and H. Morkoc, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98, 041301 (2005). 15. H. Kim and W. Sigmund, “Zinc oxide nanowires on carbon nanotubes,” Appl. Phys. Lett. 81, 2085–2087 (2002). 16. W. D. Zhang, “Growth of ZnO nanowires on modified well-aligned carbon nanotube arrays,” Nanotechnology 17, 1036–1040 (2006). 17. W. C. Chang, Y. Y. Cheng, W. C. Yu, Y. C. Yao, C. H. Lee, and H. H. Ko, “Enhancing performance of ZnO dye-sensitized solar cells by incorporation of multiwalled carbon nanotubes,” Nanoscale Res. Lett. 7, 166 (2012). 18. L. Patterson, “The Scherrer formula for x-ray particle size determination,” Phys. Rev. 56, 978–982 (1939). 19. F. M. Huang, K. T. Yue, P. H. Tan, S. L. Zhang, Z. J. Shi, X. H. Zhou, and Z. N. Gu, “Temperature dependence of the Raman spectra of carbon nanotubes,” J. Appl. Phys. 84, 4022–4024 (1998). 20. Y. G. Wang, S. P. Lau, H. W. Lee, S. F. Yu, B. K. Tay, X. H. Zhang, K. Y. Tse, and H. H. Hng, “Comprehensive study of ZnO films prepared by filtered cathodic vacuum arc at room temperature,” J. Appl. Phys. 94, 1597–1604 (2003). 21. N. Ouldhamadouche, A. Achour, I. Musa, K. Ait Aissa, F. Massuyeau, P. Y. Jouan, M. Kechouane, L. Le Brizoual, E. Faulques, N. Barreau, and M. A. Djouadi, “Structural and photoluminescence characterization of vertically aligned multiwalled carbon nanotubes coated with ZnO by magnetron sputtering,” Thin Solid Films 520, 4816–4819 (2012). 22. R. Chakraborty, S. Dhara, and P. K. Giri, “Effect of rapid thermal annealing on microstructure and optical properties of ZnO nanorods,” Int. J. Nanosci. 10, 65–68 (2011).
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Graduate School of Electronic and Optoelectronic Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan 2
Graduate School of Science and Technology, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan *Corresponding author: [email protected] Received 29 August 2013; revised 14 November 2013; accepted 14 November 2013; posted 22 November 2013 (Doc. ID 196688); published 10 January 2014
This paper studies the preparation of a zinc oxide (ZnO) seed layer deposited onto carbon nanotubes (CNTs) via RF sputtering, and the growth of ZnO/CNT composite via the hydrothermal method. The ZnO “thorns” have been successfully deposited on CNT “stems.” The research focuses on the ZnO seed layer with different sputtering times and annealing processes. A scanning electron microscopy (SEM) investigation showed that the length and amount of ZnO thorns decreased with increased sputtering time of the seed layer. The x-ray diffraction (XRD) results showed that the ZnO crystalline structures improved after the seed layer, annealing at peaks of (100), (002), and (101). The results of Raman spectra showed that the ZnO seed layer deposited onto the CNTs, and the annealing, caused damage to the CNTs, reducing the IG ∕ID ratio from 0.9 to 0.5. Furthermore, the highest UV emission of thorn-like ZnO/CNTs occurred at a peak of 380 nm with seed layer deposition time of 2 min, after annealing. © 2013 Optical Society of America OCIS codes: (160.0160) Materials; (160.4236) Nanomaterials; (160.4670) Optical materials; (160.4760) Optical properties. http://dx.doi.org/10.1364/AO.53.00A242
1. Introduction
Carbon nanotubes (CNTs) have attracted a great deal of attention because of their good mechanical properties, electrical conductivity, and chemical properties [1–3]. Therefore, in recent years, there has been an increasing interest in the composite materials of CNTs [4–6]. Furthermore, zinc oxide (ZnO) has potential applications in optoelectronic devices, such as solar cells [7,8], sensors [9,10], displays [11,12], and so on, due to its wide direct bandgap (Eg 3.37 eV) and high exciting binding energy (Ex 60 meV) [13,14]. Hence, the growth of ZnO on CNTs leads to heterojunctions of semiconducting ZnO and metallic CNT, which might be very useful in optoelectronic device applications. In 2002, ZnO nanowires were first grown on the 1559-128X/14/04A242-06$15.00/0 © 2014 Optical Society of America A242
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surfaces of CNTs without the presence of a catalyst [15]. In 2006, ZnO nanowires were successfully grown on modified well-aligned CNT arrays, via a hydrothermal process [16]. More recently, mesoporous ZnO/CNT films were fabricated into the working electrodes of dye-sensitized solar cells (DSSCs); the acid-treated CNTs increased the power conversion efficiency of DSSC that had been achieved [17]. There are many applications for ZnO/CNT composites; however, some problematic issues appear in the production process, such as the reconstruction, stability, lifetime, and large area, all affecting the cost. Subsequently, a simple process, using radio frequency (RF) sputtering, deposited a ZnO seed layer onto CNTs, which were grown into thorn-like ZnO structures, via a hydrothermal method. The effect of the annealing on the seed layer was also studied. The results showed that the growth ratio of the ZnO tips decreases with the thickening seed
layer and that the ZnO crystalline structure improved after seed layer annealing by x-ray diffraction (XRD). The ZnO seed layer deposited onto the CNTs, and the annealing, damaged the CNTs, reducing the IG ∕ID ratio from 0.9 to 0.5 by Raman spectra. Finally, the highest UV emission of thorn-like ZnO/CNTs occurred at a peak of 380 nm with a seed layer deposition time of 2 min, after annealing. 2. Experimental Procedure
The preparation of composite thorn-like ZnO/CNT is divided into four steps, as shown in Fig. 1. First, an Fe catalyst was deposited by direct current (DC) sputtering on a silicon wafer. Second, CNTs were grown by thermal chemical vapor deposition (CVD) at 700°C for 3 min. In the third step, ZnO was deposited onto the CNT surfaces as a seed layer, by RF sputtering. The base pressure, below 1 × 10−6 Torr, was maintained prior to the deposition. High purity Ar and O2 gases filled the chamber at the flow rate of 18 and 2 sccm, respectively, and the working pressure was 1.1 × 10−2 Torr. RF power of 100 W was supplied for the sputtering of the ZnO seed layer, and the deposition rate was about 10 nm∕min. At room temperature, the deposition times were 2, 5, and 10 min. After deposition of the seed layer, the samples were annealed at 500°C in ambient air for 1 h that was an optional step. Finally, the thorn-like ZnO was grown on CNT stems via a hydrothermal method. The deposited condition of the hydrothermal process was zinc acetate and hexamethylenetetramines (HMTA) dissolved in 80 ml of de-ionized water, with a concentration of 0.0125 M. The solution and ZnO seed layer–CNT sample were transferred into a Teflon-lined stainless autoclave. The autoclave was sealed and maintained at 90°C for 1 h. The morphology of ZnO/CNT composites was analyzed with field emission scanning electron microscopy (FESEM, JSM-6701F). The crystalline structure of the ZnO/CNT composites was characterized by (XRD, Bruker D8). Cobalt K α radiation (λk 0.178892 nm) from an x ray tube with a normal focus was employed
at a grazing incidence angle of 1°. Raman spectra measurements were taken with a TRIAX 550 (Jobin Yvon HORIBA S. A. S., France) using a 532 nm laser as the excitation source. Photoluminescence (PL) measurements were performed with a He–Cd laser (325 nm) as the excitation source at room temperature. 3. Results and Discussion A. Measurement of Morphology
Figure 2 shows the SEM morphology of the ZnO seed layer on the CNT surface via RF sputtering, in which (a), (b), and (c) involved deposition times of 2, 5, and 10 min without the annealing process, respectively. Corresponding to the three previous samples, (d), (e), and (f) were passed through the annealing process after sputtering. As shown in Fig. 2, the amount of ZnO particles adhering to the CNTs increases the diameter of the tube with the increased sputtering time. The thickness did not change after the annealing. By comparing the seed layer without and with annealing, we found that there was no difference between them with regard to the surface morphology of the ZnO seed layer; however, the annealing process could improve the crystallization of ZnO with annealing. Figure 3 shows the SEM images of thorn-like ZnO/CNT surface morphology via a hydrothermal method, at different seed layer sputtering times, in which (a), (b), and (c) are the ZnO seed layers without annealing, and (d), (e), and (f) are with annealing. The length and amount of ZnO thorns decreased with the increased sputtering time of the seed layer; however, the diameter of the ZnO thorn increased with the decreasing ZnO thorn numbers, due to lower nucleation density, which means there was nothing to effectively inhibit the growth of the laterals. Figures 3(d)–3(f) show the ZnO/CNTs surface morphology with the annealing process, and that the growth trend of ZnO thorns was the same for them, without the annealing process.
Fig. 1. Schematic diagram of the growth process of the thorn-like ZnO/CNT composites. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 2. SEM images of ZnO-seed-layer/CNT surface morphology at different sputtering times: (a) 2 min, (b) 5 min, and (c) 10 min, without annealing, and (d) 2 min, (e) 5 min, (f) 10 min, with the annealing process.
Figure 4 shows the length of ZnO thorns with different sputtering times and annealing of the ZnO seed layer. The length of the ZnO thorns changed from 320 to 260 nm without annealing and 220 to 140 nm with annealing. It is clear to see that the length of the ZnO thorns with annealing become smaller than without annealing, as the annealing process increases the grain size of the seed layer. This phenomenon was also studied in the XRD patterns. B.
Measurement of XRD
Figure 5 shows the XRD patterns of ZnO seed layers deposited onto CNT surfaces with and without annealing. The three main diffraction peaks (100), (002), and (101) correspond to ZnO. The grain size of the crystallites was calculated using a Scherrer’s formula, d
0.9λ ; β cos θ
(1)
where λ, θ, and β are the x ray wavelength, Bragg diffraction angle, and full width at half-maximum (FWHM) after correcting the instrument’s peak broadening. In Fig. 5, the diffraction peaks (100), (002), (101), and (110) of the ZnO seed layer clearly appear in the seed layers without annealing. After the annealing process, the FWHM of the four peaks became narrower and also revealed the other peaks of (102), (103), (112), and (201). According to Scherrer’s equation, the FWHM of the XRD peaks was narrowing due to the increased grain size after the annealing process [18]. Moreover, peak intensity increases with the deposition time of the ZnO seed layer, up to 2–10 min. Figure 6 shows XRD patterns of thornlike ZnO/CNTs that were grown via hydrothermal process without and with annealing. The intensity of the (002) peak was higher than the (100) peak after the annealing process, which could be attributed to the ZnO thorns’ growth along the c-axis. In other
Fig. 3. SEM images of thorn-like ZnO/CNT surface morphologies created using a hydrothermal process with different seed layer deposition times: (a) 2 min, (b) 5 min, and (c) 10 min without seed layer annealing, and (d) 2 min, (e) 5 min, and (f) 10 min, with seed layer annealing. A244
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Fig. 4. Average length of the ZnO thorns at the different sputtering times of the ZnO seed layer.
words, the ZnO crystallization improved by using the annealing process. However, the peak intensity decreased after the annealing process as the length of the ZnO thorn became shorter (see Fig. 4). The thorn-like ZnO/CNT composite structures, grown via a hydrothermal method, had crystallization in the (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes. The peak positions show the JCPDS standard (No. 79-2205) for ZnO with a hexagonal wurtzite structure. In addition, the peak at 2θ 26° originates from the graphite layers of the CNT’s film. C.
Measurement of Raman Spectra
Figure 7 shows the Raman spectra of the ZnO seed layer deposited on the CNTs with different sputtering times, with and without annealing. The D band peak was the defect mode that existed at about 1325 cm−1 , and the G band peak was the graphite C–C tangential stretching mode that existed at about 1580 cm−1 [19]. As shown in Fig. 7, when the ZnO seed layer was deposited on the CNTs, the D band
Fig. 5. XRD patterns of ZnO seed layer/CNTs with different deposition times, with (a), (b), and (c) without annealing, and (d), (e), and (f) with annealing.
Fig. 6. XRD patterns of thorn-like ZnO/CNTs with different seed layer sputtering times, with (a), (b), and (c) without annealing, and (d), (e), and (f) with annealing.
strength became higher than the G band, and inverted to the pure CNT model. The IG ∕ID would get by using a Lorentz fitting to calculate the integral area ratio of the G and D bands, which could evaluate the perfect quality of the CNT crystalline structure. When the IG ∕ID ratio of CNTs with deposited ZnO seed layer was lower than 0.96 of the pure CNTs, it means that sputtering ZnO onto the CNTs has destroyed the graphite structure and deteriorated the quality of the CNTs. For all of the ZnO seed layer/CNT films, there was a clear shoulder peak at 1620 cm−1 after annealing; the peak was the D’ band, which means that there was a broken key among these carbon atoms, or the loss of an atom from the special position of the periodic lattices, which generated vacancy defects. During the annealing process of the ZnO seed layer, re-crystallization action could also damage the crystallization of CNTs. The ratio of IG ∕ID shows no clear trends for the ZnO/CNTs after hydrothermal growth. Figure 8 shows the Raman spectra of thorn-like ZnO/CNT
Fig. 7. Raman spectra of ZnO seed layers deposited on CNT surfaces using different sputtering times, where (a) is pure CNT (no seed layer), and (b), (c), and (d) without annealing, and (e), (f), and (g) with annealing. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 8. Raman spectra of thorn-like ZnO/CNTs with different seed layer sputtering times, with (a), (b), and (c) without annealing, and (d), (e), and (f) with annealing.
composites with different deposition times of the ZnO seed layer, with and without annealing. There was only a small difference between the ratios of IG ∕ID . The shoulder peak also existed at 1620 cm−1 for the ZnO seed layer/CNTs with the annealing process. Therefore, sputtering and annealing the ZnO seed layer were the main factors affecting the quality of CNT performance. D.
Measurement of PL Spectra
Figure 9 shows the photoluminescence (PL) spectra of the ZnO seed layer deposited on the CNTs with different sputtering times. The PL spectrum of ZnO is composed of two parts, an excitonic-related near band edge (NBE) emission peak in the ultraviolet region (380 nm) and a broad deep level emission band at around the green ray region (570 nm). The broad deep level emission band is associated with electron-hole recombination at a deep level, presumably caused by oxygen vacancy or zinc interstitial defect [20]. In Fig. 9, ZnO seed layer/CNTs with 10 min sputtering time and annealing had the maximum NBE emission peak, which could be attributed to
Fig. 10. PL spectra of thorn-like ZnO/CNTs with different seed layer sputtering times, in which (a), (b), and (c) were without the annealing process, and (d), (e), and (f) were with the annealing process.
the higher crystal quality of ZnO seed layer [21]. The UV emission intensity increased with the thickness of the ZnO. Hence, the variation of NBE emission is consistent with the quality evaluated from the XRD spectra in Fig. 5. Figure 10 shows the PL spectra of thorn-like ZnO/ CNTs with different sputtering times of ZnO seed layer. According to the SEM results, NBE emission intensity increases with the higher density of ZnO thorns. The crystallization of ZnO was improved by the annealing process and the highest intensity of UV emission appears at the sputtering time of 2 min, with annealing. Hence, the strong UV emission in thorn-like ZnO/CNTs may be attributed to the higher density of ZnO thorns and the better crystallization of ZnO. Furthermore, the PL spectrum of the ZnO presents a redshift of the NBE emission peak of approximately 70 meV that could be observed with the increased thickness of the ZnO seed layer after the annealing process (see Figs. 9 and 10). However, the thickness of ZnO seed layers were larger than the exciton Bohr radius of ZnO, which is about 2 nm; thus, the quantum confinement effect is not the main cause of the redshift in the NBE emission peak. This could be attributed to band tail, or defect-related states caused by the relaxation of the compressive strain [22]. During annealing, the ZnO seed was re-crystallized and grain growth may have taken place, thereby, changing the bandgap of ZnO and causing a corresponding redshift in the PL spectrum. 4. Conclusions
Fig. 9. Photoluminescence spectra of ZnO seed layers deposited on CNTs’ surface by different sputtering times, with (a), (b), and (c) without annealing, and (d), (e), and (f) with annealing. A246
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A novel composite material composed of ZnO “thorns” on CNT “stems,” is prepared for the seed layer by RF sputtering and the growth of ZnO on the CNT surfaces via a hydrothermal process. A comparison was made between ZnO seed layers deposited with different times of 2, 5, and 10 min, and with and without the annealing process. The SEM images showed that the ZnO thorn length decreased
with increased sputtering time of the seed layer, and that the diameter of the ZnO thorn increased with decreasing ZnO length. The XRD images showed that ZnO is a hexagonal wurtzite structure, with clear peaks at (100), (002), and (101), and that the annealing process could improve the crystallization of ZnO. Raman spectra showed the D band and G band of CNTs at 1325 cm−1 and 1580 cm−1 , respectively. The ZnO seed layer deposited on the CNTs, and the annealing process, damaged the CNTs, reducing the IG ∕ID ratio. The intrinsic emission of ZnO was at 380 nm; however, the annealing of the ZnO seed layer had a redshift of UV emission. The largest UV emission of thorn-like ZnO/CNTs occurred with a ZnO seed layer sputtering time of 2 min, with annealing. In this research, the thornlike ZnO/CNT composite material could be fabricated successfully via a hydrothermal method. The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under contract No. NSC 102-2220-E-224-002- and NSC 102 - 2622 E - 224 - 003 - CC3. References 1. S. B. Yang, B. S. Kong, D. H. Jung, Y. K. Baek, C. S. Han, S. K. Oh, and H. T. Jung, “Recent advances in hybrids of carbon nanotube network films and nanomaterials for their potential applications as transparent conducting films,” Nanoscale 3, 1361–1373 (2011). 2. D. S. Hecht, L. Hu, and G. Irvin, “Emerging transparent electrodes based on thin films of carbon nanotubes, graphene, and metallic nanostructures,” Adv. Mater. 23, 1482–1513 (2011). 3. E. T. Thostenson, Z. F. Ren, and T. W. Chou, “Advances in the science and technology of carbon nanotubes and their composites: a review,” Compos. Sci. Technol. 61, 1899–1912 (2001). 4. X. Zhang, J. S. Lee, G. S. Lee, D. K. Cha, M. J. Kim, D. J. Yang, and S. K. Manohar, “Chemical synthesis of PEDOT nanotubes,” Macromolecules 39, 470–472 (2006). 5. A. O’Connor, S. Deb, J. N. Coleman, and Y. K. Gun’koa, “Development of transparent, conducting composites by surface infiltration of nanotubes into commercial polymer films,” Carbon 47, 1983–1988 (2009). 6. H. S. Park, J. S. Kim, B. G. Choi, S. M. Jo, D. Y. Kim, W. H. Hong, and S. Y. Janga, “Sonochemical hybridization of carbon nanotubes with gold nanoparticles for the production of flexible transparent conducing films,” Carbon 48, 1325–1330 (2010).
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