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ZnO hierarchical nanostructures by two-step vapor phase method
Materials Research Bulletin 44 (2009) 1003–1008
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Fabrication of ZnS/ZnO hierarchical nanostructures by two-step vapor phase method Jianwei Zhao a,b,*, Lirong Qin a, Lide Zhang b a b
School of Physical Science and Technology, MOE Key Laboratory on Luminescence and Real-Time Analysis, Southwest University, Chongqing 400715, PR China Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 June 2008 Received in revised form 18 October 2008 Accepted 8 November 2008 Available online 13 November 2008
A simple two-step vapor phase method is presented to fabricate ZnS/ZnO hierarchical nanostructures in bulk quantities. That is ZnS nanobelts were first synthesized and then used as substrate for growth of ZnO nanorod arrays. Investigation results demonstrate that the polar surfaces of ZnS nanobelts could induce a preferred asymmetric growth of ZnO nanorods on the side surfaces. But it is believed that if the local concentration of ZnO was high enough, ZnO nanorods could also grow symmetrically on the top/ bottom surface of the ZnS nanobelts. The optical property of the products was also recorded by means of photoluminescence (PL) spectroscopy. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Vapor deposition D. Optical properties
1. Introduction Over the past decade, synthesis and characterization of onedimensional (1D) nanostructures have attracted great interest due to their novel physical properties and wide potential applications [1–3]. Various 1D nanostructures such as nanowires, nanotubes, and nanobelts have been synthesized and investigated [4–6]. Recently, many efforts have been focused on the integration of these building blocks into two- and three-dimensional ordered superstructures or complex functional architectures, which is a crucial step toward the realization of functional nanosystems [7]. However, the small size of the nanoscale building blocks makes the fabrication process always costly and complex. Self-assembly nanostructures of multiple dimensionality and/or hierarchy are highly desirable and provide an attractive alternative method in terms of realizing mesoscopic assembly of nanodevices. They can be prepared directly by growing nanoscale structures of materials with different dimensions (nanoscale to microscale). In this regard, various methods have been applied to assemble the architectures and some exciting hierarchically structured materials have achieved such as SiO2/Si [8], ZnO/Ni [9] and SnO2/Fe2O3 [10]. However, the development of new hierarchical structures and
* Corresponding author at: School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China. Tel.: +86 2368367930; fax: +86 2368254608. E-mail address: [email protected] (J. Zhao). 0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.11.006
thorough understanding of the growth mechanisms still remain a big challenging subject to chemists and material scientists. Both ZnO and ZnS, two important II–IV group semiconductor compounds with direct wide band gap energy, have prominent application in many fields including electronics, photo-electronics. And many kinds of ZnO [11,12] and ZnS [13,14] nanostructures have been widely investigated. More recently, some fascinating architectures assembled from low-dimensional ZnO and ZnS structures have been reported. Li and Sulieman synthesized respectively the ZnO/ZnS core-shell nanowires [15,16]. Lin et al reported the synthesis of the ZnS/ZnO core-shell nanorod arrays [17]. Yan and Xue reported the preparation of the ZnO/ZnS coreshell spheres [18]. Shen et al. synthesized hierarchical saw-like ZnO nanobelt/ZnS nanowire heterostructures [19]. In this paper, we reported the synthesis of a new ZnO/ZnS hierarchical nanostructure in bulk quantities by a simple two-step vapor phase method. Their growth mechanism and photoluminescence properties have also been discussed. 2. Experiment First, ZnS nanobelts were fabricated in a conventional horizontal tube furnace with a 2-cm inner-diameter alumina tube mounted inside. ZnS powders (99.9%, about 4 g) were placed in a ceramic boat and then put into the center of the alumina tube. Another boat that covered with a Si substrate to act as the deposition substrate (distance from the source: 12 cm) was placed orderly downstream in the alumina tube. The tube furnace was purged with high-purity argon for 2 h prior to heating to eliminate oxygen in the furnace.
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Fig. 1. The morphology and structure of the ZnS nanobelts: (a) low-magnification SEM image, (b) high-magnification SEM image, (c) XRD pattern, and (d) a TEM image (inset is the corresponding SAED pattern).
Under a constant flow of Ar (20 sccm), the furnace was rapidly heated to 1150 8C in 10 min and kept at this temperature for 60 min. After the system was cooled down to room temperature, a white wool-like product was observed on the Si wafer. Second, some pure Zn powders (99.9%, about 0.4 g) were placed in one side of an alumina boat and the Si wafer grown with ZnS nanobelts was placed downstream in the tube (1 cm away from the Zn power). The boat was put into the center of the alumina tube. The tube furnace was purged with high-purity argon for 30 min prior to heating to reduce oxygen, and more oxygen presented in the furnace would make the pre-growing ZnS nanobelts begin to be oxidized [20]. Under the ambient pressure and a constant flow of Ar (20 sccm), the furnace was rapidly heated to 650 8C in 6 min and kept at this temperature for 5 min. After the system was cooled down to room temperature, the resulting product was obtained on the Si wafer. The as-synthesized products were characterized and analyzed by scanning electron microscopy [(SEM) Sirion 200 FEG] equipped with an energy dispersive X-ray spectrometer (EDS), X-ray diffraction [(XRD) PW1710 instrument with Cu Ka radiation], and transmission electron microscopy [(TEM) JEOL 2010, operated at 200 kV] equipped with selected area electron diffraction (SEAD) patterns. Photoluminescence (PL) spectra were obtained using an Edinburgh FLS 920 fluorescence spectrophotometer (Xe 900 lamp with output power of 450 W) at room temperature. 3. Results and discussion The morphology of the ZnS product prepared at the first step was investigated first using scanning electron microscope (SEM).
Fig. 1a is a low-magnification SEM image and it shows that the asprepared product consists of a large quantity of belt-shaped structures with typical lengths in the range of several tens to several hundreds of micrometers. A magnification SEM image (Fig. 1b) of single nanobelt reveals that its surface is smooth. The width of the nanobelt is about several micrometers and the average thickness is about 80 nm. It is worthy to be noted that some ZnS nanosaws had been found in the SEM observation as shown in the inset of Fig. 1b. XRD pattern has been measured for assessing the overall structure and phase purity (Fig. 1c). The major diffraction peaks are all corresponding to ZnS crystal faces. Analyzed from the XRD result, the ZnS crystal is wurtzite (hexagonal) structure with lattice constants of a = 3.809 A˚ and c = 6.252 A˚, which is consistent with standard value of bulk ZnS (JCPDS card no. 36-1450). The microstructures of ZnS nanobelts were further characterized by TEM. A representative TEM image shown in Fig. 1d reveals that the produced ZnS nanobelt is straight and relatively uniform with width of about 2 mm. Dark lines on the belt are due to bending contours. These bending contours are frequently observed in thin TEM samples. The selected area electron diffraction (SAED) pattern (inset of Fig. 1d) indicates that the nanobelt is single crystalline and grows along [1 0 1¯ 0] with side surface of (0 0 0 1). After two-step growth, the resulting product was investigated using SEM. Fig. 2a is a low-magnification SEM image and it shows that the product also consists of a large quantity of belt-shaped structures. Compared with the morphology of above pre-growing ZnS nanobelts, the surfaces of the belts here are rough. Moreover, there are many little nanorods growing on the side surfaces of these belts. With further observations, it can be found that
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Fig. 2. The morphology and structure of the ZnS/ZnO hierarchical nanostructure: (a) low-magnification SEM image, (b)–(d) high-magnification SEM image, (e) EDS spectrum of nanorod part, and (f) XRD pattern.
different from the usual comb-like structure with double-sided teeth [21], there are still some nanorods extending outward from the top/bottom surfaces of the belt in our case (shown in Fig. 2b). It also can be seen that the rough top surface could be attributed to many little grains covering on it. Fig. 2c and d are the further magnification SEM image showing the side part of the belt. They indicate that these nanorod arrays grow vertically on the side surface with high density. The nanorods have almost uniform length of about one micrometer. And every nanorod has a broad head and a uniform diametrical stem. Careful examination of many rods reveals that the diameters of the stem are mostly in a range of 40–90 nm. The chemical composition of these nanorods had been analyzed by EDS (Fig. 2e) and the result indicates that the nanorods contain only Zn and O without the presence of any impurities. Fig. 2f shows the XRD spectrum of the resulting product. It
indicates that this product consists of two compounds. Those strong diffraction peaks are all corresponding to ZnS crystal faces, which should come from the ZnS nanobelts obtained in the first step. The weak ZnO peaks occurred in the pattern match well to those listed in the JCPDS (card no. 36-1451) and should be related to the ZnO nanorods grown on the surface of nanobelts. The typical bright-field TEM images of the product are shown in Fig. 3a. It can be seen that many nanocrystallites cover on the surface of the nanobelt and an array of nanorods grow on the side of the nanobelt, consistent with the above SEM observation. A magnification TEM image (Fig. 3b) shows that the nanorod has a broad head and a uniform diametrical stem. Fig. 3c is the corresponding SAED pattern, which indicates that the nanorods are single crystalline and hexagonal ZnO structure with preferred orientation in the (0 0 0 1) direction. On the basis of the above
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Fig. 3. (a) TEM image of the hierarchical structure, (b) TEM image of the nanorods, and (c) SAED pattern of nanorod part.
analysis, it can be determined that the resulting product we synthesized is a hierarchical structure composed of ZnS nanobelts and ZnO nanorods. From the systemic research on the resulting product obtained at the different region of the Si wafer, it is interesting to see that the density of the ZnO nanorods grown on the top/bottom surface of trunk belt gradually changes from high to low along the carrier gas flow direction, as shown in Fig. 4a and b. Even some hierarchical structures without any nanorods grown on the top/bottom surface have been observed at the end of the Si wafer (Fig. 4c). Additionally, through comparing two sides of individual hierarchical structure (Fig. 4a or b), it also can be found that the density of ZnO nanorods grown on saw-teeth side of the trunk belt are higher than that grown on the opposite side. While, seen from Fig. 4d, the density of nanorods grown on top/bottom surface does not change obviously. We believed these phenomena should be related to the crystal
structure of ZnS nanobelts and the local concentration of ZnO vapor. The simple growth process can be described as follows. For the formation of ZnS nanobelts in the first stage, the only source material used in our synthesis is pure ZnS powder and the assynthesized products are identical to be ZnS nanobelts. So it is likely that the growth is governed by a vapor–solid (VS) process similar to the growth of ZnS nanosheets reported by Fang et al. [22]. In the second stage, Zn powder is heated to generate Zn vapor, which is oxidized into ZnO by the remnant of oxygen. In the mean time, these ZnO vapor would be transferred to the lowtemperature region by Ar gas and nucleate on the pre-growing ZnS nanobelts. As a result, ZnO nanorods sprout out from the surface of the nanobelt and finally ZnO/ZnS hierarchical structure is formed. The schematic diagram for the growth process is illustrated in Fig. 5. Here, a question should be discussed that what
Fig. 4. The SEM images showing the different morphology: hierarchical structure with high density (a) and low density (b) of ZnO nanorods grown on the top/bottom surface, (c) hierarchical structure without ZnO nanorods grown on the top/bottom surface, (d) the side-view SEM image of hierarchical structure.
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Fig. 5. The schematic diagram for the growth process of ZnS/ZnO hierarchical nanostructure.
induced the different growth behavior of ZnO nanorods on the side surface and top/bottom surface of ZnS nanobelts. It is well known that both ZnS and ZnO are wurtzite structure and have two important structural characteristics: noncentral symmetry and polar surfaces. That is, the structure of ZnS (or ZnO) can be described as a number of alternating planes composed of tetrahedrally coordinated S2 (or O2) and Zn2+ ions, stacked alternately along the c-axis. The oppositely charged ions produce positively charged Zn-(0 0 0 1) and negatively charged S (or O)¯ polar surfaces [14,21]. The cation-terminated polar surface ð0 0 0 1Þ is chemically active and it can be the self-catalyst for growing saw¯ tooth structures [23]. While the (0 0 0 1)-S (or O) surface is chemically inactive and typically does not initiate any growth, but controlling experimental conditions could lead to the growth from ¯ surface and the (2¯ 1 1 0) planes [21]. the edges between the (0 0 0 1) In recent years, a series of unique and novel hierarchical heterostructures based on the polar surface have been synthesized and investigated. Shen et al. have reported that the polar surface of ZnO could initiate the growth of ZnS nanowires to form a hierarchical saw-like ZnO nanobelt/ZnS nanowire heterostructures [19]. Wu et al. synthesized ZnO/ZnS heterostructured rings and further proved that ZnO teeth could nucleated and grown on the formed ZnS overlayer [24]. In one of our recent work, a novel nanostructure of side-to-side SnO2/ZnO/SnO2 triaxial nanobelt have been achieved and demonstrated that the polar surfaces of ZnO (0 0 0 1) were all able to induce the heterogenous epitaxial growth [25]. Here, the side surfaces of our ZnS nanobelts produced in the first step just belonged to polar surface, having active (0 0 0 1) ¯ face. They could induce a preferred face and inactive (0 0 0 1) asymmetric nucleation and growth of ZnO nanorods in the second step, resulting in the different density of ZnO nanorods grown on two sides of ZnS nanobelts. Whereas, the growth of ZnO nanorods was difficult on the nonpolar top/bottom surfaces of ZnS nanobelts. Those ZnO clusters adsorbed on the top/bottom surfaces of ZnS nanobelts were apt to form many nanocrystallites finally as shown in Fig. 4c. However, the structures of our product demonstrated that ZnO nanorods could also grow symmetrically on the top/bottom surface of the ZnS nanobelts especially near the source region, just as shown in Fig. 4. In our system, the concentration of ZnO vapor produced in the second step was decreased along the carrier gas flow direction. So it could be deduced that the local concentration of ZnO vapor decided the density of the ZnO nanorods grown on the top/bottom surfaces. In a word, it should be the polar surface and ZnO concentration decided the formation of our ZnS/ZnO hierarchical nanostructures. Certainly, this mechanism proposed needs to be confirmed by more studies. For the building blocks to incorporate into functional nanodevices, the controlled synthesis of the nanostructures for desired size, shape, and orientation should be fulfilled first. However, it has
Fig. 6. The PL spectrum of the ZnS/ZnO hierarchical nanostructure. For comparing, the PL spectra of the pure ZnO nanorods and ZnS nanobelts were also provided.
made less progress in the past years [26]. Especially, there are few reports on the controlled synthesis of hierarchical nanostructures, which is a rather challenging issue. Here, on the base of our experiment results, it is believed that selective growth induced by polar surface would be helpful in designing and preparing hierarchical nanostructures. Furthermore, other experimental parameters such as the local vapor concentration also have an effect on the morphology evolution. This finding may provide some insight into the control growth of hierarchical nanostructures. The room temperature PL spectra of the ZnS nanobelts and ZnS/ ZnO hierarchical nanostructure are shown in Fig. 6 and the excitation wavelength is 325 nm. It can be seen that the ZnS nanobelts have a weak and broad emission band located mainly in the blue region. Kar and Chaudhuri have reported similar emission spectrum from ZnS nanobelts and attributed to the sulfur vacancy and surface states [27]. For pure ZnO nanorods, two luminescence bands have been observed at 382 and 500 nm. It was deemed traditionally that the UV emission corresponded to the near band edge (NBE) peak, the green emission originated from the recombination of the holes with electrons occupying the singly ionized O vacancy, and other structure defects or impurities also have an important effect on the luminescence of ZnO nanostructure [25,28]. Compared with the emission of the ZnS nanobelts and ZnO nanorods, the ZnS/ZnO hierarchical nanostructure shows only a much stronger green emission band centered at 500 nm. It can be deduced that this green emission band resulted from the green emission of ZnO nanorods within the ZnS/ZnO hierarchical nanostructures. In our experiment, the condition of lacking oxygen would bring more O vacancies to the ZnO nanorods grown in the second stage. Moreover, the mismatch at two interfacial regions could result in many defects and stacking faults. Both of the above causes would weaken the UV emission and enhance the green emission greatly, resulting in the large difference in the PL from the pure ZnO nanorods, similar with Shen’s report [19]. As for the disappearance of the blue emission band corresponding to the ZnS nanobelts, it may be related to the anneal process for ZnS in the second stage. More detailed mechanism of the origination of the emission in the as-synthesized ZnS/ZnO hierarchical nanostructures is required and is still under investigation. 4. Conclusion A new ZnS/ZnO hierarchical nanostructure was synthesized in bulk quantities by a two-step chemical vapor deposition method.
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That is the pre-growing ZnS nanobelts were used as substrate to grow the ZnO nanorod arrays. The special composite structure accompanied with its optical properties may endow the products presented here some potential applications in photoelectricity and photocatalyst. Acknowledgments This work was financially supported by the National Major Project of Fundamental Research: Nanomaterials and Nanostructures and Doctoral Fund of Southwest University (Grant Nos. SWUB2007027 and SWUB2007053). References [1] X.F. Duan, Y. Huang, Y. Cui, J.F. Wang, C.M. Liber, Nature 409 (2001) 66. [2] Y.W. Wang, L.D. Zhang, G.W. Meng, C.H. Liang, G.Z. Wang, S.H. Sun, Chem. Commun. 24 (2001) 2632. [3] X.S. Fang, C.H. Ye, L.D. Zhang, T. Xie, Adv. Mater. 17 (2005) 1661. [4] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [5] C.H. Ye, G.W. Meng, Z. Jiang, Y.H. Wang, G.Z. Wang, Z.L. Zhang, J. Am. Chem. Soc. 124 (2002) 15180. [6] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 2947.
[7] S. Lee, Y. Jun, S. Cho, J. Cheon, J. Am. Chem. Soc. 124 (2002) 11244. [8] J.Q. Hu, Y. Bando, J.H. Zhan, X.L. Yuan, T. Sekiguchi, D. Golgerg, Adv. Mater. 17 (2005) 971. [9] S.W. Jung, W.I. Park, G. Yi, M. Kim, Adv. Mater. 15 (2003) 1358. [10] D.F. Zhang, L.D. Sun, C.J. Jia, Z.G. Yan, L.P. You, C.H. Yan, J. Am. Chem. Soc. 127 (2005) 13492. [11] Y. Li, G.W. Meng, L.D. Zhang, F. Phillipp, Appl. Phys. Lett. 76 (2000) 2011. [12] J.Q. Hu, Y. Bando, Appl. Phys. Lett. 82 (2003) 1401. [13] Y.W. Wang, L.D. Zhang, C.H. Liang, G.Z. Wang, X.S. Peng, Chem. Phys. Lett. 357 (2002) 314. [14] D. Moore, C. Ronning, C. Ma, Z.L. Wang, Chem. Phys. Lett. 385 (2004) 8. [15] J. Li, D. Zhao, X. Meng, Z. Zhang, J. Zhang, D. Shen, Y. Lu, X. Fan, J. Phys. Chem. B 110 (2006) 14685. [16] K.M. Sulieman, X.T. Huang, J.P. Liu, M. Tang, Smart Mater. Struct. 16 (2007) 89. [17] Y.F. Lin, Y.J. Hsu, S.Y. Lu, W.S. Chiang, Nanotechnology 17 (2006) 4773. [18] C.L. Yan, D.F. Xue, J. Phys. Chem. B 110 (2006) 7102. [19] G.Z. Shen, D. Chen, C.J. Lee, J. Phys. Chem. B 110 (2006) 15689. [20] Y. Li, L. You, R. Duan, P. Shi, G. Qin, Solid State Commun. 129 (2004) 233. [21] Z.L. Wang, X.Y. Kong, J.M. Zuo, Phys. Rev. Lett. 91 (2003) 185502. [22] X.S. Fang, C.H. Ye, X.S. Peng, Y.H. Wang, Y.C. Wu, L.D. Zhang, J. Crystal Growth 263 (2004) 263. [23] Z.L. Wang, J. Mater. Chem. 15 (2005) 1021. [24] X. Wu, P. Jiang, Y. Ding, W. Cai, S. Xie, Z.L. Wang, Adv. Mater. 19 (2007) 2319. [25] J.W. Zhao, C.H. Ye, X.S. Fang, L.R. Qin, L.D. Zhang, Cryst. Growth Des. 6 (2006) 2643. [26] X.S. Fang, L.D. Zhang, J. Mater. Sci. Technol. 22 (2006) 1. [27] S. Kar, S. Chaudhuri, J. Phys. Chem. B 109 (2005) 3298. [28] J.Q. Hu, Y. Bando, J.H. Zhan, Y.B. Li, T. Sekiguchi, Appl. Phys. Lett. 83 (2003) 4414.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Fabrication of ZnS/ZnO hierarchical nanostructures by two-step vapor phase method Jianwei Zhao a,b,*, Lirong Qin a, Lide Zhang b a b
School of Physical Science and Technology, MOE Key Laboratory on Luminescence and Real-Time Analysis, Southwest University, Chongqing 400715, PR China Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 June 2008 Received in revised form 18 October 2008 Accepted 8 November 2008 Available online 13 November 2008
A simple two-step vapor phase method is presented to fabricate ZnS/ZnO hierarchical nanostructures in bulk quantities. That is ZnS nanobelts were first synthesized and then used as substrate for growth of ZnO nanorod arrays. Investigation results demonstrate that the polar surfaces of ZnS nanobelts could induce a preferred asymmetric growth of ZnO nanorods on the side surfaces. But it is believed that if the local concentration of ZnO was high enough, ZnO nanorods could also grow symmetrically on the top/ bottom surface of the ZnS nanobelts. The optical property of the products was also recorded by means of photoluminescence (PL) spectroscopy. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Vapor deposition D. Optical properties
1. Introduction Over the past decade, synthesis and characterization of onedimensional (1D) nanostructures have attracted great interest due to their novel physical properties and wide potential applications [1–3]. Various 1D nanostructures such as nanowires, nanotubes, and nanobelts have been synthesized and investigated [4–6]. Recently, many efforts have been focused on the integration of these building blocks into two- and three-dimensional ordered superstructures or complex functional architectures, which is a crucial step toward the realization of functional nanosystems [7]. However, the small size of the nanoscale building blocks makes the fabrication process always costly and complex. Self-assembly nanostructures of multiple dimensionality and/or hierarchy are highly desirable and provide an attractive alternative method in terms of realizing mesoscopic assembly of nanodevices. They can be prepared directly by growing nanoscale structures of materials with different dimensions (nanoscale to microscale). In this regard, various methods have been applied to assemble the architectures and some exciting hierarchically structured materials have achieved such as SiO2/Si [8], ZnO/Ni [9] and SnO2/Fe2O3 [10]. However, the development of new hierarchical structures and
* Corresponding author at: School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China. Tel.: +86 2368367930; fax: +86 2368254608. E-mail address: [email protected] (J. Zhao). 0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.11.006
thorough understanding of the growth mechanisms still remain a big challenging subject to chemists and material scientists. Both ZnO and ZnS, two important II–IV group semiconductor compounds with direct wide band gap energy, have prominent application in many fields including electronics, photo-electronics. And many kinds of ZnO [11,12] and ZnS [13,14] nanostructures have been widely investigated. More recently, some fascinating architectures assembled from low-dimensional ZnO and ZnS structures have been reported. Li and Sulieman synthesized respectively the ZnO/ZnS core-shell nanowires [15,16]. Lin et al reported the synthesis of the ZnS/ZnO core-shell nanorod arrays [17]. Yan and Xue reported the preparation of the ZnO/ZnS coreshell spheres [18]. Shen et al. synthesized hierarchical saw-like ZnO nanobelt/ZnS nanowire heterostructures [19]. In this paper, we reported the synthesis of a new ZnO/ZnS hierarchical nanostructure in bulk quantities by a simple two-step vapor phase method. Their growth mechanism and photoluminescence properties have also been discussed. 2. Experiment First, ZnS nanobelts were fabricated in a conventional horizontal tube furnace with a 2-cm inner-diameter alumina tube mounted inside. ZnS powders (99.9%, about 4 g) were placed in a ceramic boat and then put into the center of the alumina tube. Another boat that covered with a Si substrate to act as the deposition substrate (distance from the source: 12 cm) was placed orderly downstream in the alumina tube. The tube furnace was purged with high-purity argon for 2 h prior to heating to eliminate oxygen in the furnace.
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Fig. 1. The morphology and structure of the ZnS nanobelts: (a) low-magnification SEM image, (b) high-magnification SEM image, (c) XRD pattern, and (d) a TEM image (inset is the corresponding SAED pattern).
Under a constant flow of Ar (20 sccm), the furnace was rapidly heated to 1150 8C in 10 min and kept at this temperature for 60 min. After the system was cooled down to room temperature, a white wool-like product was observed on the Si wafer. Second, some pure Zn powders (99.9%, about 0.4 g) were placed in one side of an alumina boat and the Si wafer grown with ZnS nanobelts was placed downstream in the tube (1 cm away from the Zn power). The boat was put into the center of the alumina tube. The tube furnace was purged with high-purity argon for 30 min prior to heating to reduce oxygen, and more oxygen presented in the furnace would make the pre-growing ZnS nanobelts begin to be oxidized [20]. Under the ambient pressure and a constant flow of Ar (20 sccm), the furnace was rapidly heated to 650 8C in 6 min and kept at this temperature for 5 min. After the system was cooled down to room temperature, the resulting product was obtained on the Si wafer. The as-synthesized products were characterized and analyzed by scanning electron microscopy [(SEM) Sirion 200 FEG] equipped with an energy dispersive X-ray spectrometer (EDS), X-ray diffraction [(XRD) PW1710 instrument with Cu Ka radiation], and transmission electron microscopy [(TEM) JEOL 2010, operated at 200 kV] equipped with selected area electron diffraction (SEAD) patterns. Photoluminescence (PL) spectra were obtained using an Edinburgh FLS 920 fluorescence spectrophotometer (Xe 900 lamp with output power of 450 W) at room temperature. 3. Results and discussion The morphology of the ZnS product prepared at the first step was investigated first using scanning electron microscope (SEM).
Fig. 1a is a low-magnification SEM image and it shows that the asprepared product consists of a large quantity of belt-shaped structures with typical lengths in the range of several tens to several hundreds of micrometers. A magnification SEM image (Fig. 1b) of single nanobelt reveals that its surface is smooth. The width of the nanobelt is about several micrometers and the average thickness is about 80 nm. It is worthy to be noted that some ZnS nanosaws had been found in the SEM observation as shown in the inset of Fig. 1b. XRD pattern has been measured for assessing the overall structure and phase purity (Fig. 1c). The major diffraction peaks are all corresponding to ZnS crystal faces. Analyzed from the XRD result, the ZnS crystal is wurtzite (hexagonal) structure with lattice constants of a = 3.809 A˚ and c = 6.252 A˚, which is consistent with standard value of bulk ZnS (JCPDS card no. 36-1450). The microstructures of ZnS nanobelts were further characterized by TEM. A representative TEM image shown in Fig. 1d reveals that the produced ZnS nanobelt is straight and relatively uniform with width of about 2 mm. Dark lines on the belt are due to bending contours. These bending contours are frequently observed in thin TEM samples. The selected area electron diffraction (SAED) pattern (inset of Fig. 1d) indicates that the nanobelt is single crystalline and grows along [1 0 1¯ 0] with side surface of (0 0 0 1). After two-step growth, the resulting product was investigated using SEM. Fig. 2a is a low-magnification SEM image and it shows that the product also consists of a large quantity of belt-shaped structures. Compared with the morphology of above pre-growing ZnS nanobelts, the surfaces of the belts here are rough. Moreover, there are many little nanorods growing on the side surfaces of these belts. With further observations, it can be found that
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Fig. 2. The morphology and structure of the ZnS/ZnO hierarchical nanostructure: (a) low-magnification SEM image, (b)–(d) high-magnification SEM image, (e) EDS spectrum of nanorod part, and (f) XRD pattern.
different from the usual comb-like structure with double-sided teeth [21], there are still some nanorods extending outward from the top/bottom surfaces of the belt in our case (shown in Fig. 2b). It also can be seen that the rough top surface could be attributed to many little grains covering on it. Fig. 2c and d are the further magnification SEM image showing the side part of the belt. They indicate that these nanorod arrays grow vertically on the side surface with high density. The nanorods have almost uniform length of about one micrometer. And every nanorod has a broad head and a uniform diametrical stem. Careful examination of many rods reveals that the diameters of the stem are mostly in a range of 40–90 nm. The chemical composition of these nanorods had been analyzed by EDS (Fig. 2e) and the result indicates that the nanorods contain only Zn and O without the presence of any impurities. Fig. 2f shows the XRD spectrum of the resulting product. It
indicates that this product consists of two compounds. Those strong diffraction peaks are all corresponding to ZnS crystal faces, which should come from the ZnS nanobelts obtained in the first step. The weak ZnO peaks occurred in the pattern match well to those listed in the JCPDS (card no. 36-1451) and should be related to the ZnO nanorods grown on the surface of nanobelts. The typical bright-field TEM images of the product are shown in Fig. 3a. It can be seen that many nanocrystallites cover on the surface of the nanobelt and an array of nanorods grow on the side of the nanobelt, consistent with the above SEM observation. A magnification TEM image (Fig. 3b) shows that the nanorod has a broad head and a uniform diametrical stem. Fig. 3c is the corresponding SAED pattern, which indicates that the nanorods are single crystalline and hexagonal ZnO structure with preferred orientation in the (0 0 0 1) direction. On the basis of the above
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Fig. 3. (a) TEM image of the hierarchical structure, (b) TEM image of the nanorods, and (c) SAED pattern of nanorod part.
analysis, it can be determined that the resulting product we synthesized is a hierarchical structure composed of ZnS nanobelts and ZnO nanorods. From the systemic research on the resulting product obtained at the different region of the Si wafer, it is interesting to see that the density of the ZnO nanorods grown on the top/bottom surface of trunk belt gradually changes from high to low along the carrier gas flow direction, as shown in Fig. 4a and b. Even some hierarchical structures without any nanorods grown on the top/bottom surface have been observed at the end of the Si wafer (Fig. 4c). Additionally, through comparing two sides of individual hierarchical structure (Fig. 4a or b), it also can be found that the density of ZnO nanorods grown on saw-teeth side of the trunk belt are higher than that grown on the opposite side. While, seen from Fig. 4d, the density of nanorods grown on top/bottom surface does not change obviously. We believed these phenomena should be related to the crystal
structure of ZnS nanobelts and the local concentration of ZnO vapor. The simple growth process can be described as follows. For the formation of ZnS nanobelts in the first stage, the only source material used in our synthesis is pure ZnS powder and the assynthesized products are identical to be ZnS nanobelts. So it is likely that the growth is governed by a vapor–solid (VS) process similar to the growth of ZnS nanosheets reported by Fang et al. [22]. In the second stage, Zn powder is heated to generate Zn vapor, which is oxidized into ZnO by the remnant of oxygen. In the mean time, these ZnO vapor would be transferred to the lowtemperature region by Ar gas and nucleate on the pre-growing ZnS nanobelts. As a result, ZnO nanorods sprout out from the surface of the nanobelt and finally ZnO/ZnS hierarchical structure is formed. The schematic diagram for the growth process is illustrated in Fig. 5. Here, a question should be discussed that what
Fig. 4. The SEM images showing the different morphology: hierarchical structure with high density (a) and low density (b) of ZnO nanorods grown on the top/bottom surface, (c) hierarchical structure without ZnO nanorods grown on the top/bottom surface, (d) the side-view SEM image of hierarchical structure.
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Fig. 5. The schematic diagram for the growth process of ZnS/ZnO hierarchical nanostructure.
induced the different growth behavior of ZnO nanorods on the side surface and top/bottom surface of ZnS nanobelts. It is well known that both ZnS and ZnO are wurtzite structure and have two important structural characteristics: noncentral symmetry and polar surfaces. That is, the structure of ZnS (or ZnO) can be described as a number of alternating planes composed of tetrahedrally coordinated S2 (or O2) and Zn2+ ions, stacked alternately along the c-axis. The oppositely charged ions produce positively charged Zn-(0 0 0 1) and negatively charged S (or O)¯ polar surfaces [14,21]. The cation-terminated polar surface ð0 0 0 1Þ is chemically active and it can be the self-catalyst for growing saw¯ tooth structures [23]. While the (0 0 0 1)-S (or O) surface is chemically inactive and typically does not initiate any growth, but controlling experimental conditions could lead to the growth from ¯ surface and the (2¯ 1 1 0) planes [21]. the edges between the (0 0 0 1) In recent years, a series of unique and novel hierarchical heterostructures based on the polar surface have been synthesized and investigated. Shen et al. have reported that the polar surface of ZnO could initiate the growth of ZnS nanowires to form a hierarchical saw-like ZnO nanobelt/ZnS nanowire heterostructures [19]. Wu et al. synthesized ZnO/ZnS heterostructured rings and further proved that ZnO teeth could nucleated and grown on the formed ZnS overlayer [24]. In one of our recent work, a novel nanostructure of side-to-side SnO2/ZnO/SnO2 triaxial nanobelt have been achieved and demonstrated that the polar surfaces of ZnO (0 0 0 1) were all able to induce the heterogenous epitaxial growth [25]. Here, the side surfaces of our ZnS nanobelts produced in the first step just belonged to polar surface, having active (0 0 0 1) ¯ face. They could induce a preferred face and inactive (0 0 0 1) asymmetric nucleation and growth of ZnO nanorods in the second step, resulting in the different density of ZnO nanorods grown on two sides of ZnS nanobelts. Whereas, the growth of ZnO nanorods was difficult on the nonpolar top/bottom surfaces of ZnS nanobelts. Those ZnO clusters adsorbed on the top/bottom surfaces of ZnS nanobelts were apt to form many nanocrystallites finally as shown in Fig. 4c. However, the structures of our product demonstrated that ZnO nanorods could also grow symmetrically on the top/bottom surface of the ZnS nanobelts especially near the source region, just as shown in Fig. 4. In our system, the concentration of ZnO vapor produced in the second step was decreased along the carrier gas flow direction. So it could be deduced that the local concentration of ZnO vapor decided the density of the ZnO nanorods grown on the top/bottom surfaces. In a word, it should be the polar surface and ZnO concentration decided the formation of our ZnS/ZnO hierarchical nanostructures. Certainly, this mechanism proposed needs to be confirmed by more studies. For the building blocks to incorporate into functional nanodevices, the controlled synthesis of the nanostructures for desired size, shape, and orientation should be fulfilled first. However, it has
Fig. 6. The PL spectrum of the ZnS/ZnO hierarchical nanostructure. For comparing, the PL spectra of the pure ZnO nanorods and ZnS nanobelts were also provided.
made less progress in the past years [26]. Especially, there are few reports on the controlled synthesis of hierarchical nanostructures, which is a rather challenging issue. Here, on the base of our experiment results, it is believed that selective growth induced by polar surface would be helpful in designing and preparing hierarchical nanostructures. Furthermore, other experimental parameters such as the local vapor concentration also have an effect on the morphology evolution. This finding may provide some insight into the control growth of hierarchical nanostructures. The room temperature PL spectra of the ZnS nanobelts and ZnS/ ZnO hierarchical nanostructure are shown in Fig. 6 and the excitation wavelength is 325 nm. It can be seen that the ZnS nanobelts have a weak and broad emission band located mainly in the blue region. Kar and Chaudhuri have reported similar emission spectrum from ZnS nanobelts and attributed to the sulfur vacancy and surface states [27]. For pure ZnO nanorods, two luminescence bands have been observed at 382 and 500 nm. It was deemed traditionally that the UV emission corresponded to the near band edge (NBE) peak, the green emission originated from the recombination of the holes with electrons occupying the singly ionized O vacancy, and other structure defects or impurities also have an important effect on the luminescence of ZnO nanostructure [25,28]. Compared with the emission of the ZnS nanobelts and ZnO nanorods, the ZnS/ZnO hierarchical nanostructure shows only a much stronger green emission band centered at 500 nm. It can be deduced that this green emission band resulted from the green emission of ZnO nanorods within the ZnS/ZnO hierarchical nanostructures. In our experiment, the condition of lacking oxygen would bring more O vacancies to the ZnO nanorods grown in the second stage. Moreover, the mismatch at two interfacial regions could result in many defects and stacking faults. Both of the above causes would weaken the UV emission and enhance the green emission greatly, resulting in the large difference in the PL from the pure ZnO nanorods, similar with Shen’s report [19]. As for the disappearance of the blue emission band corresponding to the ZnS nanobelts, it may be related to the anneal process for ZnS in the second stage. More detailed mechanism of the origination of the emission in the as-synthesized ZnS/ZnO hierarchical nanostructures is required and is still under investigation. 4. Conclusion A new ZnS/ZnO hierarchical nanostructure was synthesized in bulk quantities by a two-step chemical vapor deposition method.
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