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Preparation and characterization of nano-zinc oxide
Journal of Materials Processing Technology 189 (2007) 379–383
Preparation and characterization of nano-zinc oxide Yang Liu a,b,∗ , Jian-er Zhou a , Andre Larbot b , Michel Persin b a
b
Material School, Jingdezhen Ceramic Institute, Jiangxi 333001, PR China I.E.M.-Campus CNRS 1919, Route de Mende 34296 Montpellier Cedex 5, France
Received 24 June 2006; received in revised form 23 December 2006; accepted 3 February 2007
Abstract Zinc oxide nanocrystals were prepared by homogeneous precipitation method using urea and zinc nitrate as raw materials. The orientation adhesion of nanocrystallites formed by connection of crystallites was discussed by the growth unit model of anionic coordination polyhedrons. It was shown that zinc oxide nanocrystal growth was more easily to occur along c-axis, which was primarily formed by the connection of positive ¯ secondly by the positive polar faces c(0 0 0 1) and negative polar hexagonal cone faces p(1 0 1¯ 1) and negative hexagonal cone faces p (1 0 1¯ 1), ¯ faces −c(0 0 0 1). © 2007 Elsevier B.V. All rights reserved. Keywords: Zinc oxide nanocrystals; Homogeneous precipitation; Orientation adhesion; Anionic coordination polyhedrons; Growth unit
1. Introduction
2. Experimental process
Zinc oxide is a new and important semiconductor material, which is useful in various applications such as photo-electric devices [1], electronic devices [2], surface acoustic wave devices [3], field emitters [4], sensors [5], ultraviolet lasers [6], solar cells [7], etc. Till now, many methods have been developed to synthesize zinc oxide nanocrystals including vapor phase growth [8], vapor–liquid–solid process [9], soft chemical method [10], electrophoretic deposition [11], sol–gel process [12], etc. Depending on the adopted synthesis method, zinc oxide nanocrystals would show various morphologies under different formation mechanisms. In this paper, zinc oxide nanocrystals were prepared by homogeneous precipitation method using urea and zinc nitrate as raw materials, and characterized by means of powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The orientation adhesion mechanism was discussed by the growth unit model of anionic coordination polyhedrons from the viewpoint of crystal chemistry.
Nano-zinc oxide nanocrystals were prepared with zinc nitrate (analyticalgrade product) and urea (analytical-grade product) as raw materials. Solution concentrations of zinc nitrate were 0.1, 0.3 and 1.0 M, respectively. Urea was used as the precipitant and the molar ratio of zinc nitrate and urea was 1:2. Preparation process was as the following: first preparing solutions at above concentrations, put the solutions at 95 ◦ C for 4 h under saturated vapor pressure, and then obtain precipitates, drying the precipitates at room temperature (25 ◦ C) for 24 h, and then thermal treatment at 500 ◦ C for 10 min and finally get nanometer zinc oxide powders. Zinc oxide nanocrystals were characterized with XRD (D8 advance diffractometer) and SEM (Hitachi-S-4600).
∗
Corresponding author at: Material School, Jingdezhen Ceramic Institute, Jiangxi 333001, PR China. Tel.: +86 798 8493916; fax: +86 798 8493916. E-mail address: [email protected] (Y. Liu). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.02.007
3. Results and discussions 3.1. XRD analysis Fig. 1(a) shows XRD pattern of as-synthesized sample prepared from 0.1 M Zn2+ solution, which is consistent with that of JCPDS card No. 003-787. It proves that chemical composition of the precipitate is Zn4 CO3 (OH)6 ·H2 O. Fig. 1(b) shows XRD pattern of zinc oxide nanocrystals sample calcined at 500 ◦ C, which indicates that the synthesized zinc oxide sample is well-crystallized and the diffraction peaks could be indexed to the hexagonal structure (space group p63mc, JCPDS card No. 005-0664). Compared with the standard diffraction patterns, no characteristic peaks from other phases were detected.
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3.3.1. Zinc oxide polar growth habit Under hydrothermal synthesis conditions, many parameters such as species concentration, synthesis temperature and solution basicity have great impacts on the size and the morphology of the resulted crystals [14]. With the increase of the solution’s basicity, zinc oxide crystals morphology would change from long prismatic shape to short prismatic shape and then to grainy, which displays polar growth habit. The difference of crystal growth speed between positive and negative cone faces decreases gradually with the increase of solution basicity [15] (Fig. 4).
Fig. 1. XRD patterns of samples prepared with 0.1 M Zn2+ solution. (a) Uncalcined precipitate, (b) precipitate calcined at 500 ◦ C for 10 min.
3.3.2. Orientation adhesion mechanism of nano-zinc oxide Nano-zinc oxide crystals, prepared by homogeneous precipitation method using urea and zinc nitrate as raw materials with 0.1 M Zn2+ (0.2 M urea) concentration, show structures with orientation adhesion. However, nano-zinc oxide crystals prepared by hydrothermal method with good dispersion properties are less orientation adhesion. This is because crystal grain grows freely under hydrothermal condition. Samples with perfect crystal form, good dispersion and without agglomeration would be obtained. When prepared by homogeneous precipitation method, zinc oxide nanocrystals are formed from intermediate precipitate product zinc carbonate hydroxide hydrate and thus zinc oxide nanocrystals would crystallize under forced state. It could be analyzed by following process: Urea decomposes when temperature rises: CO(NH2 )2 + 3H2 O → CO2 ↑ + 2NH3 H2 O
3.2. SEM analysis As shown in Fig. 2(a), when Zn2+ concentration was 0.1 M (while urea concentration was 0.2 M), it is found that zinc oxide nanocrystals are intergrowth and most of them are structured with orientation adhesion and dendritic crystals. This orientation adhesion occurs along c-axis as shown in Fig. 2(b). On the other hand, with the increase of monomer concentration, the synthesized zinc oxide nanocrystals become grainy, as shown in Fig. 2(c and d). 3.3. Structure and morphology of zinc oxide nanocrystals Zinc oxide’s structure belongs to hexagonal system with 4 = P63mc [13]. Zn atom is arranged in hexagspace group C6v onal close packing and each Zn2+ ion is surrounded by four oxygen atoms to form [Zn-O4 ]6− tetrahedron. Every tetrahedron is connected through the corners to form the 3-D structure. One face of the tetrahedron is paralleled to face +c(0 0 0 1), while ¯ as one corner of the tetrahedron is directed to face −c(0 0 0 1), schematically illustrated in Fig. 3(a). As shown in Fig. 3(b), distribution of Zn2+ along c-axis is not in symmetry and Zn2+ is not located in the middle of two oxygen atom layers, but near to face + c direction, which is the intrinsic factor resulting in zinc oxide with polar character. Theoretical polar growth morphology of zinc oxide crystal is hexagonal single cone with symmetrical form of L6 P. L6 is z-axis. The exposured crystal ¯ hexagoplanes are hexagonal single cone p{1 0 1¯ 1}, p {1 0 1¯ 1}, nal prism m{1 0 1 0} and single-sided face c{1 0 0 1}(Fig. 3(c)).
Hydrolyzed product reacts with zinc nitrate to form zinc carbonate hydroxide hydrate: 4Zn2+ + CO3 2− + 6OH− + H2 O → Zn4 CO3 (OH)6 ·H2 O ↓ Zinc carbonate hydroxide hydrate calcined at 500 ◦ C for 10 min would form zinc oxide: Zn4 CO3 (OH)6 ·H2 O → 4ZnO + 4H2 O + CO2 ↑ It could be seen from the above discussed reactions that there are two growth units. One is (ZnCO3 )4 6− tetrahedron and the other is Zn(OH)4 2− tetrahedron. Zinc carbonate is formed on combination of (ZnCO3 )4 6− tetrahedrons joined with C-O3 triangles, when heated zinc oxide was obtained. Zn(OH)4 2− tetrahedrons are combined each other at the corners. Two Zn2+ atoms are connected in above reactions to form zinc oxide. For rapid reaction, the resulted crystal size is very small, which makes some unstable index crystal faces exposured when the reaction stopped. Small crystal grains with large surface area, so the face and corner of [Zn-O4 ]6− tetrahedron were fully exposured. Since crystal grains possess polar growth habit, each crystal grain is equivalent to a large polar molecule, whose positive and negative polar faces are connected to form the crystals with orientation adhesion and the connected direction is pole axis (c-axis). It could be seen from the orientation adhesion that, one was positive and negative hexagonal pyramidal faces connected each other, this was because [Zn-O4 ]6− tetrahedrons with different exposured state on positive and negative hexagonal pyramidal faces, [ZnO4 ]6− tetrahedron had one polar face and one corner exposured on p{1 0 1¯ 1} cone face, while on negative polar face [Zn-O4 ]6−
Y. Liu et al. / Journal of Materials Processing Technology 189 (2007) 379–383
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Fig. 2. SEM images of sample zinc oxide nanocrystals prepared under different conditions. (a and b) Prepared with 0.1 M Zn2+ , (c) prepared with 0.3 M Zn2+ , (d) prepared with 1.0 M Zn2+ .
tetrahedron had one edge or face exposured, seen in Fig. 5. Positive and negative hexagonal pyramidal faces connect each other to match its structure, which is the prerequisite for the formation of orientation adhesion, therefore, c-axes of orientation adhesion crystal grains are paralleled to each other, as seen in Fig. 6. The other orientation adhesion is such intergrowth that its positive and negative hexagonal pyramidal faces {0 0 0 1} and ¯ are parallel to c-axis, as seen in Fig. 7. No matter inter{0 0 0 1} growth along pyramidal faces or along positive and negative polar faces, orientation adhesion crystals distribute along c-axis and obviously possess the property of polar growth habit.
Zinc oxide crystal grains present the morphology of hexagon by homogeneous precipitation method with urea concentration up to 2.0 M, as seen in Fig. 2(d). Urea molecules possess dipole property with O side as negative and NH2 side as positive. Thus, urea molecules could be easily enriched on the positive and negative hexagonal cone faces of zinc oxide crystals. The growth of zinc oxide crystals with positive and negative hexagonal cone faces is forbidden and effected its normal growth along c-axis. At this moment, crystals grow along cone face m{1 0 1¯ 0} and take the morphology of hexagon.
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Fig. 5. [Zn-O4 ]6− tetrahedrons on positive and negative hexagonal cone faces.
Fig. 6. Orientation adhesion of zinc oxide. (a) Intergrowth on positive and negative hexagonal cone faces, (b) the accordant connection of [Zn-O4 ]6− tetrahedrons on positive and negative hexagonal cone faces.
Fig. 3. The structure of zinc oxide crystal. (a) Orientation relation between faces c, p, p and [Zn-O4 ]6− tetrahedrons, (b) Zn2+ orientation of the [Zn-O4 ]6− tetrahedrons at {1 0 1 0} (Zn, O atoms, are not in symmetry along c-axis), (c) Theoretical polar growth morphology of zinc oxide crystal.
Fig. 4. Relation of zinc oxide crystal habit with basicity solution.
Fig. 7. Orientation adhesion of zinc oxide: (a) Intergrowth on positive and negative hexagonal cone faces, (b) [Zn-O4 ]6− tetrahedrons on positive and negative hexagonal cone faces.
Y. Liu et al. / Journal of Materials Processing Technology 189 (2007) 379–383
4. Conclusions The synthesis and characterization of nanosized zinc oxide crystals has been studied in this paper with different crystal habits and morphologies. The result shows that nano-zinc oxide crystals typically grow with the character of polar crystal’s growth habit. On the other hand, the change of reaction parameters would lead to the synthesized nanosized zinc oxide crystals with different morphology. Relations between reaction parameters and sample morphologies of nano-zinc crystals are discussed from the viewpoint of crystal chemistry. Formation mechanism of this crystal habit is well described, especially for the character of polar growth of the crystals, which could be used for the preparation of nano-zinc oxide crystals. Acknowledgements This work is financially supported by the National project of advanced research of China (Grant No. 2004CCAO7500) and AFCRST (Association Franco-Chinoise pour la Recherche Scientifique et Technique). References [1] M. Purica, E. Budianu, E. Rusu, ZnO thin films on semiconductors substrate for large area photo-detector applications, Thin Solid Films 383 (1–2) (2001) 284.
383
[2] T. Aoki, Y. Hatannaka, D.C. Look, ZnO diode fabricated by excimer-laser doping, Appl. Phys. Lett. 76 (2000) 3257. [3] C.R. Gorla, N.W. Emanetoglu, S. Liang, et al., Structural, optical and surface acoustic wave properties of epitaxial ZnO films grown on (0 1 1 over-bar 2) sapphire by metalorganic chemical vapor deposition, J. Appl. Phys. 85 (5) (1999) 2595. [4] S.H. Jo, J.Y. Lao, Z.F. Ren, et al., Field-emission studies on thin films of zinc oxides nanowires, Appl. Phys. Lett. 83 (23) (2003) 4821. [5] M.S. Arnold, P. Avouris, Z.W. Pan, et al., Field-effect transistors based on single semiconducting oxide nanobelts, J. Phys. Chem. B 107 (3) (2003) 659. [6] H.Q. Yan, R.R. He, J. Johnson, et al., Dendritic nanowire ultraviolet laser array, J. Am. Chem. Soc. 125 (2003) 4728. [7] N. Senoussaoui, M. Krause, J. Muller, et al., Thin-film solar cells with periodic grating coupler, Thin Solid Films 451–452 (2004) 397. [8] X.C. Sun, H.Z. Zhang, J. Xu, et al., Shape controllable synthesis of ZnO nano-rod arrays via vapor phase growth, Solid State Commun. 129 (12) (2004) 803. [9] P.X. Gao, Z.L. Wang, Nanopropeller arrays of zinc oxide, Appl. Phys. Lett. 84 (15) (2004) 2883. [10] L. Vayssieres, Growth of arrayed nano-rods and nanowires of ZnO from aqueous solution, Adv. Mater. 15 (5) (2003) 464. [11] C.H. Liu, J.A. Zapien, Y. Yao, et al., High-density ordered ultraviolet lightemitting ZnO nanowire arrays, Adv. Mater. 15 (10) (2003) 838. [12] H. Zhang, X.Y. Ma, J. Xu, et al., Arrays of ZnO nanowires fabricated by a simple chemical solution route, Nanotechnology 14 (4) (2003) 423. [13] C. Liu, Y. Zhen, W. Zhong, Z. Wang, X. Hu, Epitaxy of ZnO nanocrystal, J. Synth. Cryst. 28 (3) (1999) 244. [14] B. Wang, E. Shi, W. Zhong, C. Xia, W. Li, Z. Yin, Growth habits and mechanism of ZnO, Micro-Cryst. Hydrotherm. Cond. 25 (2) (1997) 223. [15] W. Zhong, S. Hua, Growth units of anion coordination polyhedra and habits, J. Chin. Ceram. Soc. 23 (4) (1995) 464.
Preparation and characterization of nano-zinc oxide Yang Liu a,b,∗ , Jian-er Zhou a , Andre Larbot b , Michel Persin b a
b
Material School, Jingdezhen Ceramic Institute, Jiangxi 333001, PR China I.E.M.-Campus CNRS 1919, Route de Mende 34296 Montpellier Cedex 5, France
Received 24 June 2006; received in revised form 23 December 2006; accepted 3 February 2007
Abstract Zinc oxide nanocrystals were prepared by homogeneous precipitation method using urea and zinc nitrate as raw materials. The orientation adhesion of nanocrystallites formed by connection of crystallites was discussed by the growth unit model of anionic coordination polyhedrons. It was shown that zinc oxide nanocrystal growth was more easily to occur along c-axis, which was primarily formed by the connection of positive ¯ secondly by the positive polar faces c(0 0 0 1) and negative polar hexagonal cone faces p(1 0 1¯ 1) and negative hexagonal cone faces p (1 0 1¯ 1), ¯ faces −c(0 0 0 1). © 2007 Elsevier B.V. All rights reserved. Keywords: Zinc oxide nanocrystals; Homogeneous precipitation; Orientation adhesion; Anionic coordination polyhedrons; Growth unit
1. Introduction
2. Experimental process
Zinc oxide is a new and important semiconductor material, which is useful in various applications such as photo-electric devices [1], electronic devices [2], surface acoustic wave devices [3], field emitters [4], sensors [5], ultraviolet lasers [6], solar cells [7], etc. Till now, many methods have been developed to synthesize zinc oxide nanocrystals including vapor phase growth [8], vapor–liquid–solid process [9], soft chemical method [10], electrophoretic deposition [11], sol–gel process [12], etc. Depending on the adopted synthesis method, zinc oxide nanocrystals would show various morphologies under different formation mechanisms. In this paper, zinc oxide nanocrystals were prepared by homogeneous precipitation method using urea and zinc nitrate as raw materials, and characterized by means of powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The orientation adhesion mechanism was discussed by the growth unit model of anionic coordination polyhedrons from the viewpoint of crystal chemistry.
Nano-zinc oxide nanocrystals were prepared with zinc nitrate (analyticalgrade product) and urea (analytical-grade product) as raw materials. Solution concentrations of zinc nitrate were 0.1, 0.3 and 1.0 M, respectively. Urea was used as the precipitant and the molar ratio of zinc nitrate and urea was 1:2. Preparation process was as the following: first preparing solutions at above concentrations, put the solutions at 95 ◦ C for 4 h under saturated vapor pressure, and then obtain precipitates, drying the precipitates at room temperature (25 ◦ C) for 24 h, and then thermal treatment at 500 ◦ C for 10 min and finally get nanometer zinc oxide powders. Zinc oxide nanocrystals were characterized with XRD (D8 advance diffractometer) and SEM (Hitachi-S-4600).
∗
Corresponding author at: Material School, Jingdezhen Ceramic Institute, Jiangxi 333001, PR China. Tel.: +86 798 8493916; fax: +86 798 8493916. E-mail address: [email protected] (Y. Liu). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.02.007
3. Results and discussions 3.1. XRD analysis Fig. 1(a) shows XRD pattern of as-synthesized sample prepared from 0.1 M Zn2+ solution, which is consistent with that of JCPDS card No. 003-787. It proves that chemical composition of the precipitate is Zn4 CO3 (OH)6 ·H2 O. Fig. 1(b) shows XRD pattern of zinc oxide nanocrystals sample calcined at 500 ◦ C, which indicates that the synthesized zinc oxide sample is well-crystallized and the diffraction peaks could be indexed to the hexagonal structure (space group p63mc, JCPDS card No. 005-0664). Compared with the standard diffraction patterns, no characteristic peaks from other phases were detected.
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3.3.1. Zinc oxide polar growth habit Under hydrothermal synthesis conditions, many parameters such as species concentration, synthesis temperature and solution basicity have great impacts on the size and the morphology of the resulted crystals [14]. With the increase of the solution’s basicity, zinc oxide crystals morphology would change from long prismatic shape to short prismatic shape and then to grainy, which displays polar growth habit. The difference of crystal growth speed between positive and negative cone faces decreases gradually with the increase of solution basicity [15] (Fig. 4).
Fig. 1. XRD patterns of samples prepared with 0.1 M Zn2+ solution. (a) Uncalcined precipitate, (b) precipitate calcined at 500 ◦ C for 10 min.
3.3.2. Orientation adhesion mechanism of nano-zinc oxide Nano-zinc oxide crystals, prepared by homogeneous precipitation method using urea and zinc nitrate as raw materials with 0.1 M Zn2+ (0.2 M urea) concentration, show structures with orientation adhesion. However, nano-zinc oxide crystals prepared by hydrothermal method with good dispersion properties are less orientation adhesion. This is because crystal grain grows freely under hydrothermal condition. Samples with perfect crystal form, good dispersion and without agglomeration would be obtained. When prepared by homogeneous precipitation method, zinc oxide nanocrystals are formed from intermediate precipitate product zinc carbonate hydroxide hydrate and thus zinc oxide nanocrystals would crystallize under forced state. It could be analyzed by following process: Urea decomposes when temperature rises: CO(NH2 )2 + 3H2 O → CO2 ↑ + 2NH3 H2 O
3.2. SEM analysis As shown in Fig. 2(a), when Zn2+ concentration was 0.1 M (while urea concentration was 0.2 M), it is found that zinc oxide nanocrystals are intergrowth and most of them are structured with orientation adhesion and dendritic crystals. This orientation adhesion occurs along c-axis as shown in Fig. 2(b). On the other hand, with the increase of monomer concentration, the synthesized zinc oxide nanocrystals become grainy, as shown in Fig. 2(c and d). 3.3. Structure and morphology of zinc oxide nanocrystals Zinc oxide’s structure belongs to hexagonal system with 4 = P63mc [13]. Zn atom is arranged in hexagspace group C6v onal close packing and each Zn2+ ion is surrounded by four oxygen atoms to form [Zn-O4 ]6− tetrahedron. Every tetrahedron is connected through the corners to form the 3-D structure. One face of the tetrahedron is paralleled to face +c(0 0 0 1), while ¯ as one corner of the tetrahedron is directed to face −c(0 0 0 1), schematically illustrated in Fig. 3(a). As shown in Fig. 3(b), distribution of Zn2+ along c-axis is not in symmetry and Zn2+ is not located in the middle of two oxygen atom layers, but near to face + c direction, which is the intrinsic factor resulting in zinc oxide with polar character. Theoretical polar growth morphology of zinc oxide crystal is hexagonal single cone with symmetrical form of L6 P. L6 is z-axis. The exposured crystal ¯ hexagoplanes are hexagonal single cone p{1 0 1¯ 1}, p {1 0 1¯ 1}, nal prism m{1 0 1 0} and single-sided face c{1 0 0 1}(Fig. 3(c)).
Hydrolyzed product reacts with zinc nitrate to form zinc carbonate hydroxide hydrate: 4Zn2+ + CO3 2− + 6OH− + H2 O → Zn4 CO3 (OH)6 ·H2 O ↓ Zinc carbonate hydroxide hydrate calcined at 500 ◦ C for 10 min would form zinc oxide: Zn4 CO3 (OH)6 ·H2 O → 4ZnO + 4H2 O + CO2 ↑ It could be seen from the above discussed reactions that there are two growth units. One is (ZnCO3 )4 6− tetrahedron and the other is Zn(OH)4 2− tetrahedron. Zinc carbonate is formed on combination of (ZnCO3 )4 6− tetrahedrons joined with C-O3 triangles, when heated zinc oxide was obtained. Zn(OH)4 2− tetrahedrons are combined each other at the corners. Two Zn2+ atoms are connected in above reactions to form zinc oxide. For rapid reaction, the resulted crystal size is very small, which makes some unstable index crystal faces exposured when the reaction stopped. Small crystal grains with large surface area, so the face and corner of [Zn-O4 ]6− tetrahedron were fully exposured. Since crystal grains possess polar growth habit, each crystal grain is equivalent to a large polar molecule, whose positive and negative polar faces are connected to form the crystals with orientation adhesion and the connected direction is pole axis (c-axis). It could be seen from the orientation adhesion that, one was positive and negative hexagonal pyramidal faces connected each other, this was because [Zn-O4 ]6− tetrahedrons with different exposured state on positive and negative hexagonal pyramidal faces, [ZnO4 ]6− tetrahedron had one polar face and one corner exposured on p{1 0 1¯ 1} cone face, while on negative polar face [Zn-O4 ]6−
Y. Liu et al. / Journal of Materials Processing Technology 189 (2007) 379–383
381
Fig. 2. SEM images of sample zinc oxide nanocrystals prepared under different conditions. (a and b) Prepared with 0.1 M Zn2+ , (c) prepared with 0.3 M Zn2+ , (d) prepared with 1.0 M Zn2+ .
tetrahedron had one edge or face exposured, seen in Fig. 5. Positive and negative hexagonal pyramidal faces connect each other to match its structure, which is the prerequisite for the formation of orientation adhesion, therefore, c-axes of orientation adhesion crystal grains are paralleled to each other, as seen in Fig. 6. The other orientation adhesion is such intergrowth that its positive and negative hexagonal pyramidal faces {0 0 0 1} and ¯ are parallel to c-axis, as seen in Fig. 7. No matter inter{0 0 0 1} growth along pyramidal faces or along positive and negative polar faces, orientation adhesion crystals distribute along c-axis and obviously possess the property of polar growth habit.
Zinc oxide crystal grains present the morphology of hexagon by homogeneous precipitation method with urea concentration up to 2.0 M, as seen in Fig. 2(d). Urea molecules possess dipole property with O side as negative and NH2 side as positive. Thus, urea molecules could be easily enriched on the positive and negative hexagonal cone faces of zinc oxide crystals. The growth of zinc oxide crystals with positive and negative hexagonal cone faces is forbidden and effected its normal growth along c-axis. At this moment, crystals grow along cone face m{1 0 1¯ 0} and take the morphology of hexagon.
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Fig. 5. [Zn-O4 ]6− tetrahedrons on positive and negative hexagonal cone faces.
Fig. 6. Orientation adhesion of zinc oxide. (a) Intergrowth on positive and negative hexagonal cone faces, (b) the accordant connection of [Zn-O4 ]6− tetrahedrons on positive and negative hexagonal cone faces.
Fig. 3. The structure of zinc oxide crystal. (a) Orientation relation between faces c, p, p and [Zn-O4 ]6− tetrahedrons, (b) Zn2+ orientation of the [Zn-O4 ]6− tetrahedrons at {1 0 1 0} (Zn, O atoms, are not in symmetry along c-axis), (c) Theoretical polar growth morphology of zinc oxide crystal.
Fig. 4. Relation of zinc oxide crystal habit with basicity solution.
Fig. 7. Orientation adhesion of zinc oxide: (a) Intergrowth on positive and negative hexagonal cone faces, (b) [Zn-O4 ]6− tetrahedrons on positive and negative hexagonal cone faces.
Y. Liu et al. / Journal of Materials Processing Technology 189 (2007) 379–383
4. Conclusions The synthesis and characterization of nanosized zinc oxide crystals has been studied in this paper with different crystal habits and morphologies. The result shows that nano-zinc oxide crystals typically grow with the character of polar crystal’s growth habit. On the other hand, the change of reaction parameters would lead to the synthesized nanosized zinc oxide crystals with different morphology. Relations between reaction parameters and sample morphologies of nano-zinc crystals are discussed from the viewpoint of crystal chemistry. Formation mechanism of this crystal habit is well described, especially for the character of polar growth of the crystals, which could be used for the preparation of nano-zinc oxide crystals. Acknowledgements This work is financially supported by the National project of advanced research of China (Grant No. 2004CCAO7500) and AFCRST (Association Franco-Chinoise pour la Recherche Scientifique et Technique). References [1] M. Purica, E. Budianu, E. Rusu, ZnO thin films on semiconductors substrate for large area photo-detector applications, Thin Solid Films 383 (1–2) (2001) 284.
383
[2] T. Aoki, Y. Hatannaka, D.C. Look, ZnO diode fabricated by excimer-laser doping, Appl. Phys. Lett. 76 (2000) 3257. [3] C.R. Gorla, N.W. Emanetoglu, S. Liang, et al., Structural, optical and surface acoustic wave properties of epitaxial ZnO films grown on (0 1 1 over-bar 2) sapphire by metalorganic chemical vapor deposition, J. Appl. Phys. 85 (5) (1999) 2595. [4] S.H. Jo, J.Y. Lao, Z.F. Ren, et al., Field-emission studies on thin films of zinc oxides nanowires, Appl. Phys. Lett. 83 (23) (2003) 4821. [5] M.S. Arnold, P. Avouris, Z.W. Pan, et al., Field-effect transistors based on single semiconducting oxide nanobelts, J. Phys. Chem. B 107 (3) (2003) 659. [6] H.Q. Yan, R.R. He, J. Johnson, et al., Dendritic nanowire ultraviolet laser array, J. Am. Chem. Soc. 125 (2003) 4728. [7] N. Senoussaoui, M. Krause, J. Muller, et al., Thin-film solar cells with periodic grating coupler, Thin Solid Films 451–452 (2004) 397. [8] X.C. Sun, H.Z. Zhang, J. Xu, et al., Shape controllable synthesis of ZnO nano-rod arrays via vapor phase growth, Solid State Commun. 129 (12) (2004) 803. [9] P.X. Gao, Z.L. Wang, Nanopropeller arrays of zinc oxide, Appl. Phys. Lett. 84 (15) (2004) 2883. [10] L. Vayssieres, Growth of arrayed nano-rods and nanowires of ZnO from aqueous solution, Adv. Mater. 15 (5) (2003) 464. [11] C.H. Liu, J.A. Zapien, Y. Yao, et al., High-density ordered ultraviolet lightemitting ZnO nanowire arrays, Adv. Mater. 15 (10) (2003) 838. [12] H. Zhang, X.Y. Ma, J. Xu, et al., Arrays of ZnO nanowires fabricated by a simple chemical solution route, Nanotechnology 14 (4) (2003) 423. [13] C. Liu, Y. Zhen, W. Zhong, Z. Wang, X. Hu, Epitaxy of ZnO nanocrystal, J. Synth. Cryst. 28 (3) (1999) 244. [14] B. Wang, E. Shi, W. Zhong, C. Xia, W. Li, Z. Yin, Growth habits and mechanism of ZnO, Micro-Cryst. Hydrotherm. Cond. 25 (2) (1997) 223. [15] W. Zhong, S. Hua, Growth units of anion coordination polyhedra and habits, J. Chin. Ceram. Soc. 23 (4) (1995) 464.