- Email: [email protected]
Controlled growth of ZnO nanomaterials via doping Sb
ARTICLE IN PRESS
Journal of Crystal Growth 266 (2004) 511–518
Controlled growth of ZnO nanomaterials via doping Sb D.W. Zenga,*, C.S. Xiea, B.L. Zhua, R. Jianga, X. Chena, W.L. Songa, J.B. Wangb, J. Shib a
The State Key Laboratory of Plastic Forming Simulation and Mould Technology, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b Electronic Microscope Center, Department of Physics, Wuhan University, Wuhan, China Received 22 December 2003; accepted 3 March 2004 Communicated by K. Nakajima
Abstract ZnO nanomaterials with wurtzite structure were synthesized by simple thermal evaporation method from pure Zn or Zn–Sb alloys with three Sb/Zn mole ratios (1:50, 1:15, 1:6) at the same eperimental conditions. XRD patterns, Raman spectra and XPS analysis clearly indicate that Sb element has been doped into the ZnO lattice. As the Sb/Zn mole ratio of the evaporated materials increases, the Sb content doped into the ZnO lattice increases, and the shape of ZnO nanomaterials varies from tetrapod-shaped whisker, nanorod (aspect ratio 5:1 or 2:1) to hexagon nanoplate, which implies that the shape of the ZnO nanomaterials could be controlled via doping Sb. Finally, the controlled growth mechanism of ZnO nanomaterials was discussed in detail. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.h Keywords: Zinc oxide (ZnO) nanomaterials; Controlled growth; Doping; Antimony (Sb)
1. Introduction Zinc oxide (ZnO), a wide bandgap (3.37 eV) semiconductor with large exciton binding energy (60 meV), has been investigated as a short-wavelength light-emitting, transparent conducting and piezoelectric materials for fabricating solar cells, varistors, transducers, transparent conducting electrodes, sensors, and catalysts [1–3]. ZnO with a wurtzite structure is an n-type semiconductor *Corresponding author. Tel.: +86-87541490. E-mail address: [email protected] (D.W. Zeng).
material with both good electrical and optical properties because of a deviation from the stoichiometry due to the existence of intrinsic defects such as O vacancies and Zn interstitials [4]. However, the properties of the ZnO are unstable and cannot meet the increasing needs for the applications. To enhance the electrical/optical properties, ZnO was usually doped with some dopants such as Al, Si, In, and Ga [5–8]. Al-doped ZnO increases its conductivity without impairing the optical transmission and is regarded as a potential alternative candidate for ITO materials [9,10].
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.03.014
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Doping In into the ZnO lattice can significantly enhance the gas sensitivity to the volatile organic compounds [11]. Sb-doped ZnO nanoparticles have higher resistance and diffuse reflectivity than the undoped ZnO [12]. On the other hand, doping Sb can alter the morphology of the ZnO nanomaterials, including crystal size, orientation and aspect ratio, which also have an important effect on their electrical/optical, gas sensitivity and catalytic properties. However, to our knowledge, the controlled growth of the ZnO nanomaterials via doping has not been reported. The vapor condensation method is extensively applied to produce the doped or pure ZnO nanomaterials due to its robust and reliable control of the shape and size of the nanoparticles without requiring the expensive and complex equipments. In this paper, Sb-doped or pure ZnO nanomaterials at the atmosphere of a flowing Ar gas-mixed O2 were produced by the vapor condensation method. An attempt to control the growth of the ZnO nanomaterials was made via doping Sb, which was evaporated from the Zn–Sb alloys with three Sb/Zn mole ratios of 1:50, 1:15 and 1:6, and the controlled growth mechanism was discussed in detail.
evaporate the Zn or Zn–Sb alloys. The pure or Sb-doped ZnO nanomaterials were synthesized via the oxidation reaction with O2, and transported towards the collection chamber for further examination. A JEM-2010 transmission electron microscope (TEM) and a JEM-2010 FEFM high-resolution TEM (HRTEM) were employed to observe the morphology of the Sb-doped ZnO nanomaterials and determine their structure and size. An energy dispersion X-ray (EDX) spectrometer as an attachment of HRTEM was used to analyze the chemical composition of the ZnO nanomaterials. Their phase structure and chemical state were identified by an X-ray diffractometry (XRD, Rigaku R/Max-3B) with Cu Ka1 incident radiation and an X-ray photoelectron spectrometer (XPS, KRATOS XSM800) with nonmonochromatic Mg Ka X-ray radiation, respectively. The C 1s signal was adjusted to a position of 284.8 eV. Raman spectrum measurement was taken on a LABRAM-HR laser Raman spectrometer at the room temperature. An Ar-ion laser with a wavelength of 514.5 nm in a back-scattering configuration was used to excite the sample. Samples for Raman spectra measurement were prepared by pressing the ZnO nanomaterials into a disk-shaped compact.
2. Experimental procedures The schematic diagram of basic setup for the vapor condensation was shown in Ref. [13]. In this case, only a high-frequency inductive resource was used as a heating resource. The Zn–Sb alloys with three Sb/Zn mole ratios of 1:50, 1:15, 1:6 and pure Zn were casted into a small ingot in a vacuum chamber using a high-frequency induction furnace. A graphite crucible containing Zn or Zn–Sb alloys was placed in a chamber. The chamber was pumped to a pressure of B10 Pa using a mechanical rotary pump and sequentially filled with an Ar gas-mixed O2 to 1.0 104 Pa, establishing a convective flow balance by controlling the pumping speed and the mixed gas injection. Meanwhile, the oxygen partial pressure of 2000 Pa was obtained by tuning the oxygen flux. After the oxygen partial pressure was kept constant for 20 min, the inductive resource was applied to heat and
3. Results and discussion XRD patterns of the as-synthesized products are shown in Fig. 1. All of the diffraction peaks are indexed to be those of pure hexagonal wurtzite ZnO. No signals of the metallic Sb and its oxides as well as the metallic Zn are detected in Fig. 1. This suggests that the Sb has been doped into the ZnO lattice and the Sb-doped ZnO is synthesized from the Zn–Sb alloys. Fig. 2 shows the morphology of the ZnO nanomaterials made from pure Zn and Zn–Sb alloys with three mole ratios. Without Sb, the undoped ZnO nanomaterials are tetrapod-like whiskers with white color, and their average diameter and length of the needles are about 10 and 150 nm, respectively. The aspect ratio (length to width) is about 15:1. It should be noted that the
ARTICLE IN PRESS D.W. Zeng et al. / Journal of Crystal Growth 266 (2004) 511–518
Sb:Zn=1:6
(200) (112)
(103)
(110)
(101)
(102)
Intensity (a.u.)
(100) (002)
portion of the needles has a saltation on their diameter to form a bottle neck, thus differing from the tetrapod-like ZnO nanowhisker synthesized by hybrid induction and laser-heating method [14,15],
Sb:Zn=1:15 Sb:Zn=1:50 pure Zn
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30
40
50
60
70
2θ Fig. 1. XRD patterns of ZnO nanomaterials produced from Zn or Zn–Sb alloys with three Sb/Zn mole ratios of 1:50, 1:15 and 1:6.
of which the diameter becomes small gradually along the entire length. When the ZnO nanomaterials are synthesized from the Zn–Sb alloys, their needles of the ZnO nanomaterials disappear and the color becomes salmon pink. In addition, the higher the Sb/Zn mole ratio, the deeper the color. The changes in the color imply that the Sb has been doped into the ZnO lattice and the optical property of the ZnO nanomaterials could be modified via doping Sb. Moreover, as the Sb/Zn mole ratio of the evaporated materials increases from 1:50 to 1:6, the shape of the Sb-doped ZnO nanomaterials varies from the nanorod to the regular hexagon nanoparticle (see Fig. 2). At the small Sb/Zn mole ratio of 1:50, the width and length of the Sb-doped ZnO nanorods are 50 and 250 nm, respectively. The ZnO nanorods became fatter and the aspect ratio was markedly decreased to 1:5. Further, increasing the Sb/Zn mole ratio to 1:15 caused the formation of short nanorods (aspect ratio 1:2). At 1:6, the ZnO crystals became regular hexagon nanoparticles. They are in a fine dispersion and contact with each other in an
100 nm (a)
100 nm (b)
100 nm (c)
513
100 nm (d)
Fig. 2. TEM images of the ZnO nanomaterials: (a) pure ZnO; (b) Sb:Zn=1:50; (c) Sb:Zn=1:15; (d) Sb:Zn=1:6.
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3 2
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437
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600
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Fig. 4. Raman spectra of the ZnO nanomaterials.
Sb 3d5/2 O1s
1 Sb:Zn=1:50 2 Sb:Zn=1:6 Intensity (a.u.)
1 Sb:Zn=1:50 2 Sb:Zn=1:6 Intensity (a.u.)
2p3/2 peak area at 1:50 and 1:6 are 0.4937 and 2.217, respectively. This indicates that the doped Sb content increases with an increase in the Sb/Zn mole ratio of the evaporated materials. The Raman spectra were recorded from a Raman shift of 50 to 650 cm1. To observe the shifting of Raman peak more clearly, Fig. 4 shows the Raman spectra between 350 and 650 cm1 of the pure ZnO nanowhisker, Sb-doped ZnO nanorod at 1:50 and hexagonal ZnO nanoparticles at 1:6. It was found that the red shift of the Raman peaks at both about 437 cm1 (E2(high)) and 581 cm1 (E1(LO)) increase as the Sb/Zn mole ratio increases. Compared with pure ZnO nanowhisker, the peaks at about 437 and 581 cm1 of the Sb-doped nanomaterials at 1:6 are red shifted by about 4 and 10 cm1, respectively. Moreover, the intensity ratio of a new Raman peak at
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unagglomerate pattern. Their average diameter is about 60 nm in a narrow distribution. These results clearly indicate that the morphology and aspect ratio of the ZnO nanomaterials are controlled via doping the Sb element. Owing to the different evaporation pressures for the Sb and Zn metals, selective evaporation phenomenon may occur during the synthesizing process of the ZnO nanomaterials. Compared with the metal Sb, the metal Zn can be evaporated much easier. Therefore, the Sb doped into the ZnO nanomaterials is much smaller than that of the evaporated Zn–Sb alloys. Similar to the XRD analysis results, no signals of the Sb element are detected by EDX analysis, even though inside the hexagonal nanoparticles synthesized at 1:6 (the original Sb percent 16.7 at.%). These results indicate that a little Sb element may be doped into the ZnO lattice. To further confirm the conclusion mentioned above, the chemical state and Raman spectra of the ZnO nanomaterials were measured. Fig. 3 shows the Sb 3d and Zn 2p3/2 spectra recorded from the Sb-doped ZnO nanomtaerials at 1:50 and 1:6. The characteristic peaks of antimony (Sb 3d3/2 539.6 eV, Sb 3d5/2 530.4 eV including the O 1s peak) and zinc (Zn 2p3/2 1021 eV) are detected, clearly indicating the existence of the Sb element in the ZnO nanomaterials. The XPS analysis also shows that Sb and Zn are all in the oxide state. Due to superimposition of the O 1s and Sb 3d5/2 at about 530 eV, the Sb atomic percent cannot be estimated directly from the area of the Sb 3d peak, but ratios calculated between the Sb 3d3/2 and Zn
Intensity (a.u.)
514
Sb 3d3/2
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2 2
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540
(a)
535 530 Binding energy (eV)
1026
525
(b)
1024
1022
1020
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1016
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Fig. 3. XPS spectra obtained from the ZnO nanomaterials at 1:50 and 1:6. (a) Sb3d and O 1s, (b) Zn 2p3/2.
650
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525 cm1 to peak at about 437 cm1 increases gradually as well as that of the peak at 581 cm1 with an increase in the Sb/Zn mole ratio. The peak at 581 cm1 is broadened by 20 cm1 after doping Sb. These position and shape variations of the Raman spectra peaks at about 437 and 581 cm1 are in good agreement with those of the Sb-doped ZnO nanoparticles with an increase in the doped Sb content [16], which is due to the finite size effects induced by the existence of Sb inside the ZnO nanomaterials. Therefore, Raman and XPS analysis data for the undoped and Sb-doped ZnO nanomaterials further confirm the conclusion that the Sb element has been doped into the ZnO lattice, and the doped Sb content of the ZnO nanomaterials increases as the Sb/Zn mole ratio increases, which may be responsible for the morphology evolution from the nanowhisker, nanorod to the hexagonal nanoparticle. TEM morphology of the needles with a neck and its selected area electron diffraction pattern (SAED) are shown in Fig. 5(a). The SAED pattern recorded from ½2 1% 1% 0 axis zone reveals that the tetrapod-like nanowhiskers without Sb have the wurtzite structure, in agreement with the XRD analysis, and the needle grows along [0 0 0 1] direction as reported by Suyama et al. [17]. The crystal fringes of the needle with the interplanar spacing d0 0 0 1=0.52 nm (see Fig. 5b) also confirms the above conclusion. It is worth noting that stacking faults are the most commonly observed defect near the neck, which may influence the optical properties of the ZnO nanowhiskers.
Although the synthesis reaction was rather simple, the formation of the ZnO nanowhiskers would involve a very complicated process. When the pure Zn ingot is heated beyond its evaporating point, Zn vapor zone is generated above the metal liquid. According to the octahedral multiple twin (octa-twin) embryos model [18–20], there exists a large amount of the octa-twin embryos inside the vapor zone. The embryos are usually relaxed by cracking along the twin boundary and ZnO protrudes from the cracks via oxidation reaction. Meanwhile, the oxidation reaction releases energy that accelerates the growth of the ZnO protrusion. Since the (0 0 0 1) plane of ZnO with wurtzite structure is the closest packed plane in the crystal and has the lowest energy, the [0 0 0 1] direction is the preferred growth orientation [21,22] and then the protrusions grow along the [0 0 0 1] orientation by absorbing O and Zn atoms, which leads to the formation of the needles. Finally, the tetrapodshaped ZnO is produced when the needles stop their growth. The formation of the neck in the needles caused by the decreased Zn vapor supply during the growth process. At the onset of the evaporation, vapor Zn atoms were evaporated from a larger melted Zn surface. Owing to the rapid evaporation, the evaporated liquid surface shrank and became smaller rapidly, resulting in a lessened supply of Zn atoms compared with their state at the beginning. Thus, the diameter of the needles formed later was much smaller. After doping the small Sb into the ZnO lattice, the growth orientation of the nanorods is still
[0001] [0001]
0110
0.52 nm
0110
0001
50 nm (a)
[2110]
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(b) Fig. 5. (a) SAED pattern and (b) HRTEM image of the tetrapod-like whisker.
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[0001]
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(f)
Fig. 6. (a, c, e) SAED patterns and (b, d, f) HRTEM images of Sb-doped ZnO nanomaterials from the alloys with Sb:Zn=1:50, Sb:Zn=1:15 and Sb:Zn=1:6, respectively.
along the (0 0 0 1) direction (see the SAED patterns and HRTEM images in Fig. 6a–d) like that of the needles of the tetrapod ZnO nanowhiskers, but the aspect ratio of the nanorods is much smaller than that of the needles, which implies that the crystal growth of the nanorods along the [0 0 0 1] orientation is slowed down. Furthermore, much higher doped Sb content can produce hexagonal platelike ZnO nanoparticles with the sharper edges and corners (Fig. 6e), about 90 nm in width and 10 nm
in thickness, rather than the nanorods. The SAED pattern in the inset and the HRTEM image show that the thickness direction is the [0 0 0 1] orientation, suggesting that the crystal growth along the [0 0 0 1] direction was completely suppressed at the high doped Sb content, but the crystals were still able to grow sideways in the form of the thin platelets. The hexagonal nanoplate has a perfect single crystal without any defects such as the dislocations or twins (see Fig. 6f). In addition,
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there exists an increase of the interplanar spacings of the Sb-doped ZnO nanomaterials at the Sb\Zn mole ratios of 1:15 and 1:6, which is another proof that the Sb element has been doped into the ZnO lattice. XPS and Raman analysis data show that the doped Sb content increases with an increase of the Sb/Zn mole ratio increases. Fig. 7 shows that the aspect ratio is rapidly decreased as the Sb/Zn mole ratio increases. Those correlations suggest that the doped Sb content slowed down or suppressed the crystal growth along the (0 0 0 1) orientation and therefore provided a simple approach to controlling the morphology and aspect ratio of the ZnO nanomaterials. The effects of the doped Sb content on the morphology of the ZnO nanomaterials are highly consistent with those of the citrate ion concentration [23]. When the Zn–Sb alloys are used as the evaporated sources, the tetrapod-like feature completely disappears, which suggests that the octa-twin embryos model is not suitable for the nucleation and growth of the Sb-doped ZnO nanomaterials. We speculate that the embryos have a wurtzite structure due to the Sb existence and then reacts with O2 to produce wurtzite structural ZnO, which further serves as some seeds for the growth of Sb-doped ZnO nanomaterials. Recently, Tian et al. found that the citrate anions concentration can control the morphology of the
Aspect ratio
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10
5
0 0.00
0.05
0.10
0.15
Sb/Zn mole ratio Fig. 7. Aspect ratios (length to width) as a function of the Sb/ Zn mole ratio.
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complex and oriented ZnO nanostructures by absorbing preferentially on the (0 0 0 1) plane and modifying this surface chemistry properties [23]. As the added citrate ions increased, the ZnO nanostructure growth along the [0 0 0 1] orientation was slowed down or inhibited; the ZnO nanorods became fatter and shorter, and the thin hexagonal nanoplates produced finally. In this case, we speculate that the doped Sb into the lattice ZnO modifies the surface energy of the crystal facets. When a very small quantity of the Sb was doped, the difference betweens the facets became small but the surface energy of the (0 0 0 1) plane was still the lowest, and the nanorod growth along [0 0 0 1] direction was slowed down. Moreover, much higher doped Sb content could lead to the {1 1 0 0} planes having the lowest surface energy and becoming energetically favorable, resulting in the crystal growth along [0 0 0 1] direction being completely suppressed but sideways growth occuring, resulting in the formation of the thin nanoplates. Therefore, the role of the doped Sb like the citrate ions could be slowed down or could prohibit the ZnO nanomaterial growth along the [0 0 0 1] direction by tailoring the surface properties.
4. Conclusions We report large-scale synthesis of the pure or Sb-doped ZnO nanomaterials with the wurtzite structure by a simple thermal evaporation method. It is found that the shape of ZnO nanomaterials varies from tetrapod-shaped whisker, nanorod (aspect ratio 5:1 or 2:1) to the hexagon nanoplate and the aspect ratio is markedly decreased, as the Sb content doped into the ZnO lattice increases. The doped Sb content can readily control the nucleation and growth and predict the morphology alteration by modifying the surface energy of the crystalline facets. For the smaller doped Sb content, the [0 0 0 1] orientation is still the growth direction, but the crystal growth of the ZnO nanorods is slowed down. Much higher doped Sb completely inhibits the crystal growth along the [0 0 0 1] direction, resulting in the formation of the thin nanoplates. To a better understanding of how
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the doped Sb affects the ZnO nanocrystal growth, a cluster modeling technique needs to be established to estimate the surface energy in future, which is very important in nanomaterials sciences.
Acknowledgements The authors gratefully acknowledge the financial support by National Natural Science Foundation of China (NFSC, Grant No. 50271029 and 50041024), the Key Project for Science and Technology Research of Ministry of Education (Grant No. 00084), Science and Technology Planning Project of Wuhan (Grant No. 20011007088-5), and Open Foundation of the State Key Laboratory of Plastic Forming Simulation and Mould Technology.
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[7] G.A. Hirata, J. Mckittrik, T. Cheeks, J.M. Siqueiros, J.A. Diaz, O. Contreras, O.A. Lopez, Thin Solid Films 288 (1996) 29. [8] P. Nunes, B. Fernandes, E. Fortunan, P. Vilarinlo, R. Martins, Thin Solid Films 337 (1999) 176. [9] K.T.R. Reddy, R.W. Miles, J. Mater. Sci. Lett. 17 (1998) 279. [10] Z.C. Jin, I. Hamberg, C.G. Granqvist, J. Appl. Phys. 64 (1988) 5117. [11] B.L. Zhu, D.W. Zeng, C.S. Xie, J. Wu, W.L. Song, J. Mater. Sci. Mater. Electron. 14 (2003) 521. [12] D.W. Zeng, C.S. Xie, B.L. Zhu, W.L. Song, A.H. Wang, Mater. Sci. Eng. B 104 (2003) 68. [13] D.W. Zeng, C.S. Xie, B.L. Zhu, W.L. Song, A.H. Wang, Mater. Sci. Eng. A 336 (2004) 332. [14] W.T. Nichols, J.W. Keto, D.E. Henneke, J.R. Brock, G. Malyavanatham, M.F. Becker, H.D. Glicksman, Appl. Phys. Lett. 78 (2001) 1128. [15] R. Wu, C.S. Xie, J.H. Hu, H. Xia, A.H. Wang, J. Crystal Growth 217 (2000) 274. [16] J. Zuo, C.Y. Xu, L.H. Zhang, B.K. Xu, R. Wu, J. Raman Spectrosc. 32 (2001) 979. [17] Y. Suyama, Y. Tomokiyo, T. Manabe, E. Tanaka, J. Am. Ceram. Soc. 71 (1988) 391. [18] M. Fujii, H. Iwanaga, M. Ichihara, S. Takeuchi, J. Crystal Growth 128 (1993) 1095. [19] H. Iwananga, M. Fujii, M. Ichihara, S. Takeuchi, J. Crystal Growth 134 (1993) 275. [20] H. Iwananga, M. Fujii, S. Takeuchi, J. Crystal Growth 183 (1998) 190. [21] D. Lee, S. Yang, M. Choi, Appl. Phys. Lett. 79 (2001) 2459. [22] J.Q. Hu, N.B. Wong, C.S. Quan Li, S.T. Lee, Lee, Chem. Mater. 14 (2002) 1216. [23] Z.R.R. Tian, J. Voigt, J. Liu, B. Mckenzie, M.J. Mcdermott, M.A. Rodriguez, H. Konishi, H.F. Xu, Nat. Mater. 2 (2003) 821.
Journal of Crystal Growth 266 (2004) 511–518
Controlled growth of ZnO nanomaterials via doping Sb D.W. Zenga,*, C.S. Xiea, B.L. Zhua, R. Jianga, X. Chena, W.L. Songa, J.B. Wangb, J. Shib a
The State Key Laboratory of Plastic Forming Simulation and Mould Technology, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b Electronic Microscope Center, Department of Physics, Wuhan University, Wuhan, China Received 22 December 2003; accepted 3 March 2004 Communicated by K. Nakajima
Abstract ZnO nanomaterials with wurtzite structure were synthesized by simple thermal evaporation method from pure Zn or Zn–Sb alloys with three Sb/Zn mole ratios (1:50, 1:15, 1:6) at the same eperimental conditions. XRD patterns, Raman spectra and XPS analysis clearly indicate that Sb element has been doped into the ZnO lattice. As the Sb/Zn mole ratio of the evaporated materials increases, the Sb content doped into the ZnO lattice increases, and the shape of ZnO nanomaterials varies from tetrapod-shaped whisker, nanorod (aspect ratio 5:1 or 2:1) to hexagon nanoplate, which implies that the shape of the ZnO nanomaterials could be controlled via doping Sb. Finally, the controlled growth mechanism of ZnO nanomaterials was discussed in detail. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.h Keywords: Zinc oxide (ZnO) nanomaterials; Controlled growth; Doping; Antimony (Sb)
1. Introduction Zinc oxide (ZnO), a wide bandgap (3.37 eV) semiconductor with large exciton binding energy (60 meV), has been investigated as a short-wavelength light-emitting, transparent conducting and piezoelectric materials for fabricating solar cells, varistors, transducers, transparent conducting electrodes, sensors, and catalysts [1–3]. ZnO with a wurtzite structure is an n-type semiconductor *Corresponding author. Tel.: +86-87541490. E-mail address: [email protected] (D.W. Zeng).
material with both good electrical and optical properties because of a deviation from the stoichiometry due to the existence of intrinsic defects such as O vacancies and Zn interstitials [4]. However, the properties of the ZnO are unstable and cannot meet the increasing needs for the applications. To enhance the electrical/optical properties, ZnO was usually doped with some dopants such as Al, Si, In, and Ga [5–8]. Al-doped ZnO increases its conductivity without impairing the optical transmission and is regarded as a potential alternative candidate for ITO materials [9,10].
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.03.014
ARTICLE IN PRESS 512
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Doping In into the ZnO lattice can significantly enhance the gas sensitivity to the volatile organic compounds [11]. Sb-doped ZnO nanoparticles have higher resistance and diffuse reflectivity than the undoped ZnO [12]. On the other hand, doping Sb can alter the morphology of the ZnO nanomaterials, including crystal size, orientation and aspect ratio, which also have an important effect on their electrical/optical, gas sensitivity and catalytic properties. However, to our knowledge, the controlled growth of the ZnO nanomaterials via doping has not been reported. The vapor condensation method is extensively applied to produce the doped or pure ZnO nanomaterials due to its robust and reliable control of the shape and size of the nanoparticles without requiring the expensive and complex equipments. In this paper, Sb-doped or pure ZnO nanomaterials at the atmosphere of a flowing Ar gas-mixed O2 were produced by the vapor condensation method. An attempt to control the growth of the ZnO nanomaterials was made via doping Sb, which was evaporated from the Zn–Sb alloys with three Sb/Zn mole ratios of 1:50, 1:15 and 1:6, and the controlled growth mechanism was discussed in detail.
evaporate the Zn or Zn–Sb alloys. The pure or Sb-doped ZnO nanomaterials were synthesized via the oxidation reaction with O2, and transported towards the collection chamber for further examination. A JEM-2010 transmission electron microscope (TEM) and a JEM-2010 FEFM high-resolution TEM (HRTEM) were employed to observe the morphology of the Sb-doped ZnO nanomaterials and determine their structure and size. An energy dispersion X-ray (EDX) spectrometer as an attachment of HRTEM was used to analyze the chemical composition of the ZnO nanomaterials. Their phase structure and chemical state were identified by an X-ray diffractometry (XRD, Rigaku R/Max-3B) with Cu Ka1 incident radiation and an X-ray photoelectron spectrometer (XPS, KRATOS XSM800) with nonmonochromatic Mg Ka X-ray radiation, respectively. The C 1s signal was adjusted to a position of 284.8 eV. Raman spectrum measurement was taken on a LABRAM-HR laser Raman spectrometer at the room temperature. An Ar-ion laser with a wavelength of 514.5 nm in a back-scattering configuration was used to excite the sample. Samples for Raman spectra measurement were prepared by pressing the ZnO nanomaterials into a disk-shaped compact.
2. Experimental procedures The schematic diagram of basic setup for the vapor condensation was shown in Ref. [13]. In this case, only a high-frequency inductive resource was used as a heating resource. The Zn–Sb alloys with three Sb/Zn mole ratios of 1:50, 1:15, 1:6 and pure Zn were casted into a small ingot in a vacuum chamber using a high-frequency induction furnace. A graphite crucible containing Zn or Zn–Sb alloys was placed in a chamber. The chamber was pumped to a pressure of B10 Pa using a mechanical rotary pump and sequentially filled with an Ar gas-mixed O2 to 1.0 104 Pa, establishing a convective flow balance by controlling the pumping speed and the mixed gas injection. Meanwhile, the oxygen partial pressure of 2000 Pa was obtained by tuning the oxygen flux. After the oxygen partial pressure was kept constant for 20 min, the inductive resource was applied to heat and
3. Results and discussion XRD patterns of the as-synthesized products are shown in Fig. 1. All of the diffraction peaks are indexed to be those of pure hexagonal wurtzite ZnO. No signals of the metallic Sb and its oxides as well as the metallic Zn are detected in Fig. 1. This suggests that the Sb has been doped into the ZnO lattice and the Sb-doped ZnO is synthesized from the Zn–Sb alloys. Fig. 2 shows the morphology of the ZnO nanomaterials made from pure Zn and Zn–Sb alloys with three mole ratios. Without Sb, the undoped ZnO nanomaterials are tetrapod-like whiskers with white color, and their average diameter and length of the needles are about 10 and 150 nm, respectively. The aspect ratio (length to width) is about 15:1. It should be noted that the
ARTICLE IN PRESS D.W. Zeng et al. / Journal of Crystal Growth 266 (2004) 511–518
Sb:Zn=1:6
(200) (112)
(103)
(110)
(101)
(102)
Intensity (a.u.)
(100) (002)
portion of the needles has a saltation on their diameter to form a bottle neck, thus differing from the tetrapod-like ZnO nanowhisker synthesized by hybrid induction and laser-heating method [14,15],
Sb:Zn=1:15 Sb:Zn=1:50 pure Zn
20
30
40
50
60
70
2θ Fig. 1. XRD patterns of ZnO nanomaterials produced from Zn or Zn–Sb alloys with three Sb/Zn mole ratios of 1:50, 1:15 and 1:6.
of which the diameter becomes small gradually along the entire length. When the ZnO nanomaterials are synthesized from the Zn–Sb alloys, their needles of the ZnO nanomaterials disappear and the color becomes salmon pink. In addition, the higher the Sb/Zn mole ratio, the deeper the color. The changes in the color imply that the Sb has been doped into the ZnO lattice and the optical property of the ZnO nanomaterials could be modified via doping Sb. Moreover, as the Sb/Zn mole ratio of the evaporated materials increases from 1:50 to 1:6, the shape of the Sb-doped ZnO nanomaterials varies from the nanorod to the regular hexagon nanoparticle (see Fig. 2). At the small Sb/Zn mole ratio of 1:50, the width and length of the Sb-doped ZnO nanorods are 50 and 250 nm, respectively. The ZnO nanorods became fatter and the aspect ratio was markedly decreased to 1:5. Further, increasing the Sb/Zn mole ratio to 1:15 caused the formation of short nanorods (aspect ratio 1:2). At 1:6, the ZnO crystals became regular hexagon nanoparticles. They are in a fine dispersion and contact with each other in an
100 nm (a)
100 nm (b)
100 nm (c)
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Fig. 2. TEM images of the ZnO nanomaterials: (a) pure ZnO; (b) Sb:Zn=1:50; (c) Sb:Zn=1:15; (d) Sb:Zn=1:6.
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Fig. 4. Raman spectra of the ZnO nanomaterials.
Sb 3d5/2 O1s
1 Sb:Zn=1:50 2 Sb:Zn=1:6 Intensity (a.u.)
1 Sb:Zn=1:50 2 Sb:Zn=1:6 Intensity (a.u.)
2p3/2 peak area at 1:50 and 1:6 are 0.4937 and 2.217, respectively. This indicates that the doped Sb content increases with an increase in the Sb/Zn mole ratio of the evaporated materials. The Raman spectra were recorded from a Raman shift of 50 to 650 cm1. To observe the shifting of Raman peak more clearly, Fig. 4 shows the Raman spectra between 350 and 650 cm1 of the pure ZnO nanowhisker, Sb-doped ZnO nanorod at 1:50 and hexagonal ZnO nanoparticles at 1:6. It was found that the red shift of the Raman peaks at both about 437 cm1 (E2(high)) and 581 cm1 (E1(LO)) increase as the Sb/Zn mole ratio increases. Compared with pure ZnO nanowhisker, the peaks at about 437 and 581 cm1 of the Sb-doped nanomaterials at 1:6 are red shifted by about 4 and 10 cm1, respectively. Moreover, the intensity ratio of a new Raman peak at
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unagglomerate pattern. Their average diameter is about 60 nm in a narrow distribution. These results clearly indicate that the morphology and aspect ratio of the ZnO nanomaterials are controlled via doping the Sb element. Owing to the different evaporation pressures for the Sb and Zn metals, selective evaporation phenomenon may occur during the synthesizing process of the ZnO nanomaterials. Compared with the metal Sb, the metal Zn can be evaporated much easier. Therefore, the Sb doped into the ZnO nanomaterials is much smaller than that of the evaporated Zn–Sb alloys. Similar to the XRD analysis results, no signals of the Sb element are detected by EDX analysis, even though inside the hexagonal nanoparticles synthesized at 1:6 (the original Sb percent 16.7 at.%). These results indicate that a little Sb element may be doped into the ZnO lattice. To further confirm the conclusion mentioned above, the chemical state and Raman spectra of the ZnO nanomaterials were measured. Fig. 3 shows the Sb 3d and Zn 2p3/2 spectra recorded from the Sb-doped ZnO nanomtaerials at 1:50 and 1:6. The characteristic peaks of antimony (Sb 3d3/2 539.6 eV, Sb 3d5/2 530.4 eV including the O 1s peak) and zinc (Zn 2p3/2 1021 eV) are detected, clearly indicating the existence of the Sb element in the ZnO nanomaterials. The XPS analysis also shows that Sb and Zn are all in the oxide state. Due to superimposition of the O 1s and Sb 3d5/2 at about 530 eV, the Sb atomic percent cannot be estimated directly from the area of the Sb 3d peak, but ratios calculated between the Sb 3d3/2 and Zn
Intensity (a.u.)
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2 2
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540
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535 530 Binding energy (eV)
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Fig. 3. XPS spectra obtained from the ZnO nanomaterials at 1:50 and 1:6. (a) Sb3d and O 1s, (b) Zn 2p3/2.
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525 cm1 to peak at about 437 cm1 increases gradually as well as that of the peak at 581 cm1 with an increase in the Sb/Zn mole ratio. The peak at 581 cm1 is broadened by 20 cm1 after doping Sb. These position and shape variations of the Raman spectra peaks at about 437 and 581 cm1 are in good agreement with those of the Sb-doped ZnO nanoparticles with an increase in the doped Sb content [16], which is due to the finite size effects induced by the existence of Sb inside the ZnO nanomaterials. Therefore, Raman and XPS analysis data for the undoped and Sb-doped ZnO nanomaterials further confirm the conclusion that the Sb element has been doped into the ZnO lattice, and the doped Sb content of the ZnO nanomaterials increases as the Sb/Zn mole ratio increases, which may be responsible for the morphology evolution from the nanowhisker, nanorod to the hexagonal nanoparticle. TEM morphology of the needles with a neck and its selected area electron diffraction pattern (SAED) are shown in Fig. 5(a). The SAED pattern recorded from ½2 1% 1% 0 axis zone reveals that the tetrapod-like nanowhiskers without Sb have the wurtzite structure, in agreement with the XRD analysis, and the needle grows along [0 0 0 1] direction as reported by Suyama et al. [17]. The crystal fringes of the needle with the interplanar spacing d0 0 0 1=0.52 nm (see Fig. 5b) also confirms the above conclusion. It is worth noting that stacking faults are the most commonly observed defect near the neck, which may influence the optical properties of the ZnO nanowhiskers.
Although the synthesis reaction was rather simple, the formation of the ZnO nanowhiskers would involve a very complicated process. When the pure Zn ingot is heated beyond its evaporating point, Zn vapor zone is generated above the metal liquid. According to the octahedral multiple twin (octa-twin) embryos model [18–20], there exists a large amount of the octa-twin embryos inside the vapor zone. The embryos are usually relaxed by cracking along the twin boundary and ZnO protrudes from the cracks via oxidation reaction. Meanwhile, the oxidation reaction releases energy that accelerates the growth of the ZnO protrusion. Since the (0 0 0 1) plane of ZnO with wurtzite structure is the closest packed plane in the crystal and has the lowest energy, the [0 0 0 1] direction is the preferred growth orientation [21,22] and then the protrusions grow along the [0 0 0 1] orientation by absorbing O and Zn atoms, which leads to the formation of the needles. Finally, the tetrapodshaped ZnO is produced when the needles stop their growth. The formation of the neck in the needles caused by the decreased Zn vapor supply during the growth process. At the onset of the evaporation, vapor Zn atoms were evaporated from a larger melted Zn surface. Owing to the rapid evaporation, the evaporated liquid surface shrank and became smaller rapidly, resulting in a lessened supply of Zn atoms compared with their state at the beginning. Thus, the diameter of the needles formed later was much smaller. After doping the small Sb into the ZnO lattice, the growth orientation of the nanorods is still
[0001] [0001]
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0001
50 nm (a)
[2110]
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(b) Fig. 5. (a) SAED pattern and (b) HRTEM image of the tetrapod-like whisker.
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[0001]
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Fig. 6. (a, c, e) SAED patterns and (b, d, f) HRTEM images of Sb-doped ZnO nanomaterials from the alloys with Sb:Zn=1:50, Sb:Zn=1:15 and Sb:Zn=1:6, respectively.
along the (0 0 0 1) direction (see the SAED patterns and HRTEM images in Fig. 6a–d) like that of the needles of the tetrapod ZnO nanowhiskers, but the aspect ratio of the nanorods is much smaller than that of the needles, which implies that the crystal growth of the nanorods along the [0 0 0 1] orientation is slowed down. Furthermore, much higher doped Sb content can produce hexagonal platelike ZnO nanoparticles with the sharper edges and corners (Fig. 6e), about 90 nm in width and 10 nm
in thickness, rather than the nanorods. The SAED pattern in the inset and the HRTEM image show that the thickness direction is the [0 0 0 1] orientation, suggesting that the crystal growth along the [0 0 0 1] direction was completely suppressed at the high doped Sb content, but the crystals were still able to grow sideways in the form of the thin platelets. The hexagonal nanoplate has a perfect single crystal without any defects such as the dislocations or twins (see Fig. 6f). In addition,
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there exists an increase of the interplanar spacings of the Sb-doped ZnO nanomaterials at the Sb\Zn mole ratios of 1:15 and 1:6, which is another proof that the Sb element has been doped into the ZnO lattice. XPS and Raman analysis data show that the doped Sb content increases with an increase of the Sb/Zn mole ratio increases. Fig. 7 shows that the aspect ratio is rapidly decreased as the Sb/Zn mole ratio increases. Those correlations suggest that the doped Sb content slowed down or suppressed the crystal growth along the (0 0 0 1) orientation and therefore provided a simple approach to controlling the morphology and aspect ratio of the ZnO nanomaterials. The effects of the doped Sb content on the morphology of the ZnO nanomaterials are highly consistent with those of the citrate ion concentration [23]. When the Zn–Sb alloys are used as the evaporated sources, the tetrapod-like feature completely disappears, which suggests that the octa-twin embryos model is not suitable for the nucleation and growth of the Sb-doped ZnO nanomaterials. We speculate that the embryos have a wurtzite structure due to the Sb existence and then reacts with O2 to produce wurtzite structural ZnO, which further serves as some seeds for the growth of Sb-doped ZnO nanomaterials. Recently, Tian et al. found that the citrate anions concentration can control the morphology of the
Aspect ratio
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5
0 0.00
0.05
0.10
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Sb/Zn mole ratio Fig. 7. Aspect ratios (length to width) as a function of the Sb/ Zn mole ratio.
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complex and oriented ZnO nanostructures by absorbing preferentially on the (0 0 0 1) plane and modifying this surface chemistry properties [23]. As the added citrate ions increased, the ZnO nanostructure growth along the [0 0 0 1] orientation was slowed down or inhibited; the ZnO nanorods became fatter and shorter, and the thin hexagonal nanoplates produced finally. In this case, we speculate that the doped Sb into the lattice ZnO modifies the surface energy of the crystal facets. When a very small quantity of the Sb was doped, the difference betweens the facets became small but the surface energy of the (0 0 0 1) plane was still the lowest, and the nanorod growth along [0 0 0 1] direction was slowed down. Moreover, much higher doped Sb content could lead to the {1 1 0 0} planes having the lowest surface energy and becoming energetically favorable, resulting in the crystal growth along [0 0 0 1] direction being completely suppressed but sideways growth occuring, resulting in the formation of the thin nanoplates. Therefore, the role of the doped Sb like the citrate ions could be slowed down or could prohibit the ZnO nanomaterial growth along the [0 0 0 1] direction by tailoring the surface properties.
4. Conclusions We report large-scale synthesis of the pure or Sb-doped ZnO nanomaterials with the wurtzite structure by a simple thermal evaporation method. It is found that the shape of ZnO nanomaterials varies from tetrapod-shaped whisker, nanorod (aspect ratio 5:1 or 2:1) to the hexagon nanoplate and the aspect ratio is markedly decreased, as the Sb content doped into the ZnO lattice increases. The doped Sb content can readily control the nucleation and growth and predict the morphology alteration by modifying the surface energy of the crystalline facets. For the smaller doped Sb content, the [0 0 0 1] orientation is still the growth direction, but the crystal growth of the ZnO nanorods is slowed down. Much higher doped Sb completely inhibits the crystal growth along the [0 0 0 1] direction, resulting in the formation of the thin nanoplates. To a better understanding of how
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the doped Sb affects the ZnO nanocrystal growth, a cluster modeling technique needs to be established to estimate the surface energy in future, which is very important in nanomaterials sciences.
Acknowledgements The authors gratefully acknowledge the financial support by National Natural Science Foundation of China (NFSC, Grant No. 50271029 and 50041024), the Key Project for Science and Technology Research of Ministry of Education (Grant No. 00084), Science and Technology Planning Project of Wuhan (Grant No. 20011007088-5), and Open Foundation of the State Key Laboratory of Plastic Forming Simulation and Mould Technology.
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