《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_Structure-directing self-organized, one-dimensional ZnO single-crystal whiskers

Availableonlineatwww.sciencedirect.com SCIENCE DIRECT state ELSEVIER olid State Sciences 7(2005)45-51 Sciences Structure-directing If-organized. one-dimensional Zno single-crystal whiskers Hongwei Hou, Yi Xie*, Qing Li Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026. PR China Received 8 January 2004; received in revised form 7 July 2004; accepted 11 October 2004 Available online 15 December 2004 Abstract One-dimensional (ID)Zno tubular and epitaxial whiskers were synthesized in a structure-directing agent(CTAB) under hydrothermal growth conditions, respectively. FE-SEM, SEM, TEM and SaEd observation reveals that the hexagonal open-ended Zno tubular are with diameters of 4 um and the length of up to 20 um, and zno epitaxial whiskers with diameters of 4 um and the length of 10 structure directing effect of CTAB, the pH value of the system and structure of wurtzite Zno play important roles in the growth of Znd crystalline whiskers @2004 Elsevier SAS. All rights reserved. Keywords: ZnO; Structure-directing: Single-crystal whiskers; Tubular; Epitaxial 1. Introduction whiskers were shaped via oxidation of the Zns powders at 1 100C [12] or formed by pyrolysis of zinc acetylaceto- Recently, many research efforts have been invested in nate at 500C[13]. And dendritic side-branch Zno whiskers the area of wide band-gap semiconductor materials due to on the edge of the hexagonal columnar structure were syn- their potential applications in short wavelength optical de- thesized using a chemical vapor deposition technique [14] vices [1]. ZnO is a ll-VI compound semiconductor with a However, to the best of our knowledge, no epitaxial ZnO wide and direct band gap of 3.3 eV and large exciton bind- single-crystal whiskers have been reported, and also lit ing energy (60 meV)[2]. From this aspect, Zno has been tle work has been done for the tubular ZnO single-crystal investigated as solar cells, gas sensors, a short-wavelength whiskers using mild, low cost and easy controllable metho light-emitting and laser diodes and piezoelectric material Here, for the first time, we report a simple hydrothermal syn- [3, 4]. Because of its rigidity, high aspect ratio and excellent thesis of one-dimensional(ID)tubular ZnO single-crystal chemical stability, ZnO crystal whiskers have been success- whiskers and epitaxial ZnO single-crystal whiskers using fully used as probing tip for atomic force microscopy and structure-directing agent cetyltrimethylammonium bromide scanning tunneling microscopy [5]and reinforced compos- ( CTAB), respectively. It is expected that the novel tubular e materials [6] whiskers may offer exciting opportunities for potential appli Up to now, evaporation growth method [6-8] and hy- cations in catalysis, microsized waveguides or optical fibers drothermal method [9], have been applied to grow ZnO pris- [15] and the novel epitaxial whiskers may offer exciting op- matic whiskers. Recently, polycrystalline tubular ZnO was portunities for homozygote in photoelectronic devices fabricated by a solvothermal route using ethanol as solvent [10] and synthesized by thermal evaporation of the Zn/zno powders at1300°C The single-crystalline tubular Zno 2. Experimental Corresponding author. The synthesis of Zno single-crystal whiskers was car- ried out as follows: Analytical pure zinc(If)acetate dihydrate 1293-2558/S-see front matter 2004 Elsevier SAS. All rights reserved dor: 10. 1016/j. solidstatesciences 2004 10.037

Solid State Sciences 7 (2005) 45–51 www.elsevier.com/locate/ssscie Structure-directing self-organized, one-dimensional ZnO single-crystal whiskers Hongwei Hou, Yi Xie ∗ , Qing Li Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Received 8 January 2004; received in revised form 7 July 2004; accepted 11 October 2004 Available online 15 December 2004 Abstract One-dimensional (1D) ZnO tubular and epitaxial whiskers were synthesized in a structure-directing agent (CTAB) under hydrothermal growth conditions, respectively. FE-SEM, SEM, TEM and SAED observation reveals that the hexagonal open-ended ZnO tubular whiskers are with diameters of 4 µm and the length of up to 20 µm, and ZnO epitaxial whiskers with diameters of 4 µm and the length of 10 µm. The structure directing effect of CTAB, the pH value of the system and structure of wurtzite ZnO play important roles in the growth of ZnO single crystalline whiskers.  2004 Elsevier SAS. All rights reserved. Keywords: ZnO; Structure-directing; Single-crystal whiskers; Tubular; Epitaxial 1. Introduction Recently, many research efforts have been invested in the area of wide band-gap semiconductor materials due to their potential applications in short wavelength optical de￾vices [1]. ZnO is a II–VI compound semiconductor with a wide and direct band gap of 3.3 eV and large exciton bind￾ing energy (60 meV) [2]. From this aspect, ZnO has been investigated as solar cells, gas sensors, a short-wavelength light-emitting and laser diodes and piezoelectric material [3,4]. Because of its rigidity, high aspect ratio and excellent chemical stability, ZnO crystal whiskers have been success￾fully used as probing tip for atomic force microscopy and scanning tunneling microscopy [5] and reinforced compos￾ite materials [6]. Up to now, evaporation growth method [6–8] and hy￾drothermal method [9], have been applied to grow ZnO pris￾matic whiskers. Recently, polycrystalline tubular ZnO was fabricated by a solvothermal route using ethanol as solvent [10] and synthesized by thermal evaporation of the Zn/ZnO powders at 1300 ◦C [11]. The single-crystalline tubular ZnO * Corresponding author. E-mail address: yxielab@ustc.edu.cn (Y. Xie). whiskers were shaped via oxidation of the ZnS powders at 1100 ◦C [12] or formed by pyrolysis of zinc acetylaceto￾nate at 500 ◦C [13]. And dendritic side-branch ZnO whiskers on the edge of the hexagonal columnar structure were syn￾thesized using a chemical vapor deposition technique [14]. However, to the best of our knowledge, no epitaxial ZnO single-crystal whiskers have been reported, and also lit￾tle work has been done for the tubular ZnO single-crystal whiskers using mild, low cost and easy controllable method. Here, for the first time, we report a simple hydrothermal syn￾thesis of one-dimensional (1D) tubular ZnO single-crystal whiskers and epitaxial ZnO single-crystal whiskers using structure-directing agent cetyltrimethylammonium bromide (CTAB), respectively. It is expected that the novel tubular whiskers may offer exciting opportunities for potential appli￾cations in catalysis, microsized waveguides or optical fibers [15] and the novel epitaxial whiskers may offer exciting op￾portunities for homozygote in photoelectronic devices. 2. Experimental The synthesis of ZnO single-crystal whiskers was car￾ried out as follows: Analytical pure zinc(II) acetate dihydrate 1293-2558/$ – see front matter  2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.10.037

H Hou et al. /Solid State Sciences 7(2005)45-51 (0.01 mol) and cetyltrimethylammonium bromide(CTAB) (0.005 mol)were added into a 40 ml Teflon-lined autoclave which was then filled with distilled water up to 75% of the total volume Under constant stirring, the addition of 25% ammonia adjusts the pH of mixed solution to certain value For tubular Zno whiskers, the pH value was adjusted to 10, and then maintained at 140C for 12 h For epitaxial Zno whiskers, the pH value was adjusted to 8, and then was main- ined at 110oC for 12 h. after the autoclave was cooled to room temperature naturally, the white precipitate was col lected. and washed with distilled water for several times Finally, the samples were obtained and dried at 60C in air for 3-4 h The phase identification of the samples was carried out on X-ray powder diffraction(XRD) patterns, using a MAC Science Co. Ltd. MXP 18 AHF X-ray diffractometer with Cu-Ka radiation(= 1.54056 A). The morphology of the products was measured by scanning electron microscopy EM Hitachi X-650) transmission electron microscope (TEM Hitachi H-800)and selected area electron diffrac tion(SAED). Further structural analysis was taken by field 140.C, 12 h) Fig. 2. FE-SEM image of the as-grown Zno tubular whiskers(pH= 10 emission scanning electron microscopy(FE-SEM Hitachi S- 4200). Room temperature photoluminescence(PL)spectra strong peaks also confirmed the products were well crystal were recorded on a Confocal Laser MicroRaman Spectrom- eter (JYLABRAM-HR) using a He-Cd laser with a wave- length of 325 nm as the excitation source 6 In our experiment, the system of pH= 10 at 140C 12 h produce tubular Zno whiskers by using structure- directing agent CTAB. FESEM image shown in Fig. 2 re- veals the panoramic morphology of the as-grown Zno tubu- 3. Results and discussion lar whiskers. The general morphology shows that the sample All obtained Zno samples are of wurtzite structure consists of a large quantity of tubular whiskers. Most of the (hexagonal phase, space group P63mc)and a typical XRD whiskers have fairly uniform diameters of around 4 um and the length of up to 20 um, and the whiskers clearly display a pattern of the ZnO products is shown in Fig. 1. All of the hollow cavity, namely tubular structure. No other morpholo- diffraction peaks can be indexed as those from the known hexagonal ZnO reported in JCPDS card No 36-1451. Com. gles are observed in the product Further morphology characterization of the Zno tubu- pared with the standard diffraction patterns, no characteristic lar whiskers were performed using FE-SEM as shown in peaks from impurities, such as Zn, are detected, indicating Fig 3. The image of Fig. 3(a) shows that an individual tubu- the high purity of the product. In addition, the sharp and lar whisker with an open-ended structure, which has smooth, well-defined crystallographic facets and regular prismatic hexagon with the width of 2 um and angles about 120o respectively. The SAED pattern of Fig. 3(b), taken on the Zno tubular whisker, exhibits that the as-synthesized Zno tubular whiskers are single crystal with the oriented growth direction along c-axis. Different parts of this whisker show exactly the same electron pattern, indicating the single crys- tallinity of the whole single whisker. Dozens of individual whiskers have been examined using this method which con- firmed their single crystallinity. The former XRD results do not show an inconsistent result. The peaks of other crys- tal planes still appear in the XRD pattern(Fig. 1), due to the fact that this XRD pattern was measured on the powder product in which the tubular ZnO single crystal whiskers were randomly mixed Fig 3(c-f) show the tubular ZnO 2theta/deg. prismatic whisker with one open-end and the other close- end. The hollow cavity of tubular Zno whisker has the same Fig 1. XRD patterns of as-prepared zno regular prismatic hexagon with the individual whisker, and

46 H. Hou et al. / Solid State Sciences 7 (2005) 45–51 (0.01 mol) and cetyltrimethylammonium bromide (CTAB) (0.005 mol) were added into a 40 ml Teflon-lined autoclave, which was then filled with distilled water up to 75% of the total volume. Under constant stirring, the addition of 25% ammonia adjusts the pH of mixed solution to certain value. For tubular ZnO whiskers, the pH value was adjusted to 10, and then maintained at 140 ◦C for 12 h. For epitaxial ZnO whiskers, the pH value was adjusted to 8, and then was main￾tained at 110 ◦C for 12 h. After the autoclave was cooled to room temperature naturally, the white precipitate was col￾lected, and washed with distilled water for several times. Finally, the samples were obtained and dried at 60 ◦C in air for 3–4 h. The phase identification of the samples was carried out on X-ray powder diffraction (XRD) patterns, using a MAC Science Co. Ltd. MXP 18 AHF X-ray diffractometer with Cu-Kα radiation (λ = 1.54056 Å). The morphology of the products was measured by scanning electron microscopy (SEM Hitachi X-650) transmission electron microscope (TEM Hitachi H-800) and selected area electron diffrac￾tion (SAED). Further structural analysis was taken by field emission scanning electron microscopy (FE-SEM Hitachi S- 4200). Room temperature photoluminescence (PL) spectra were recorded on a Confocal Laser MicroRaman Spectrom￾eter (JYLABRAM-HR) using a He–Cd laser with a wave￾length of 325 nm as the excitation source. 3. Results and discussion All obtained ZnO samples are of wurtzite structure (hexagonal phase, space group P63mc) and a typical XRD pattern of the ZnO products is shown in Fig. 1. All of the diffraction peaks can be indexed as those from the known hexagonal ZnO reported in JCPDS card No. 36-1451. Com￾pared with the standard diffraction patterns, no characteristic peaks from impurities, such as Zn, are detected, indicating the high purity of the product. In addition, the sharp and Fig. 1. XRD patterns of as-prepared ZnO. Fig. 2. FE-SEM image of the as-grown ZnO tubular whiskers (pH = 10, 140 ◦C, 12 h). strong peaks also confirmed the products were well crystal￾lized. In our experiment, the system of pH = 10 at 140 ◦C for 12 h produce tubular ZnO whiskers by using structure￾directing agent CTAB. FESEM image shown in Fig. 2 re￾veals the panoramic morphology of the as-grown ZnO tubu￾lar whiskers. The general morphology shows that the sample consists of a large quantity of tubular whiskers. Most of the whiskers have fairly uniform diameters of around 4 µm and the length of up to 20 µm, and the whiskers clearly display a hollow cavity, namely tubular structure. No other morpholo￾gies are observed in the product. Further morphology characterization of the ZnO tubu￾lar whiskers were performed using FE-SEM as shown in Fig. 3. The image of Fig. 3(a) shows that an individual tubu￾lar whisker with an open-ended structure, which has smooth, well-defined crystallographic facets and regular prismatic hexagon with the width of 2 µm and angles about 120◦, respectively. The SAED pattern of Fig. 3(b), taken on the ZnO tubular whisker, exhibits that the as-synthesized ZnO tubular whiskers are single crystal with the oriented growth direction along c-axis. Different parts of this whisker show exactly the same electron pattern, indicating the single crys￾tallinity of the whole single whisker. Dozens of individual whiskers have been examined using this method, which con- firmed their single crystallinity. The former XRD results do not show an inconsistent result. The peaks of other crys￾tal planes still appear in the XRD pattern (Fig. 1), due to the fact that this XRD pattern was measured on the powder product in which the tubular ZnO single crystal whiskers were randomly mixed. Fig. 3(c–f) show the tubular ZnO prismatic whisker with one open-end and the other close￾end. The hollow cavity of tubular ZnO whisker has the same regular prismatic hexagon with the individual whisker, and

H. Hou et al. /Solid State Sciences 7(2005)45-51 (0111 (0110 Fig 3. FE-SEM and SAED micrographs of Zno tubular whiskers(pH= 10, Fig 4 SEM of the as-synthesized Zno tubular whiskers (pH= 10 140.C, 12 h).(a) The FE-SEM image on the surface of hollow Z 140C, 12 h).(a) The obliq Wended tubular Zno whisker.(b)The whisker. (b) The SAED pattern of the tubular Zno whisker. (c, d)The rupture tubular Zno whisker.(c)The mid-broken tubular Zno whiskers. FE-SEM image on the open end of the tubular Zno whisker. (e)The (d) The split tubular ZnO whisker. FE-SEM on the close end of an individual tubular Zno whisker (f)The FE-SEM image on the top of the tubular Zno whisker the end facet of tubular Zno prismatic whiskers is hexag- onal, smooth and well-defined crystallographic. The hexag onal concave groove with the width of 300 nm was clearly showed between the top-viewed hexagonal Zno whisker and the hexagonal inset hollow cavity. The inset hexagonal hol low cavity, with the width of I um and angles about 1200, has smooth, well-defined inner crystallographic facets. At the end of the hollow cavity, six identical trapezoid bevels connect the clearly ended hexagon to the uniform inner cav ity facets The oblique open-ended, the ruptur, the middle-broken the split tubular Zno whisker can be found in the as wn product, as showing in the SEM images of Fig. 4, respectively, all confirming their hexagonal hollow nature. Comparatively, in our experiment, the system of pH=8 at 110C for 12 h produce epitaxial Zno whiskers when using structure-directing agent CTAB. Fig. 5 shows the com- prehensive image of ZnO epitaxial whiskers, which demon- strates that the sample consists of a large scale of epi- taxial whiskers with smooth and symmetrically hexagonal shapes and well-knit structure. The yield of the Zno ep taxial whiskers synthesized by this hydrothermal method Further morphology description of the Zno epitaxial Fig. 5. SEM image of the as-synthesized ZnO epitaxial whiskers(pH=8, whiskers was accomplished using FE-SEM as shown l10°C,12h)

H. Hou et al. / Solid State Sciences 7 (2005) 45–51 47 Fig. 3. FE-SEM and SAED micrographs of ZnO tubular whiskers (pH = 10, 140 ◦C, 12 h). (a) The FE-SEM image on the surface of hollow ZnO whisker. (b) The SAED pattern of the tubular ZnO whisker. (c, d) The FE-SEM image on the open end of the tubular ZnO whisker. (e) The FE-SEM image on the close end of an individual tubular ZnO whisker. (f) The FE-SEM image on the top of the tubular ZnO whisker. the end facet of tubular ZnO prismatic whiskers is hexag￾onal, smooth and well-defined crystallographic. The hexag￾onal concave groove with the width of 300 nm was clearly showed between the top-viewed hexagonal ZnO whisker and the hexagonal inset hollow cavity. The inset hexagonal hol￾low cavity, with the width of 1 µm and angles about 120◦, has smooth, well-defined inner crystallographic facets. At the end of the hollow cavity, six identical trapezoid bevels connect the clearly ended hexagon to the uniform inner cav￾ity facets. The oblique open-ended, the ruptur, the middle-broken and the split tubular ZnO whisker can be found in the as grown product, as showing in the SEM images of Fig. 4, respectively, all confirming their hexagonal hollow nature. Comparatively, in our experiment, the system of pH = 8 at 110 ◦C for 12 h produce epitaxial ZnO whiskers when using structure-directing agent CTAB. Fig. 5 shows the com￾prehensive image of ZnO epitaxial whiskers, which demon￾strates that the sample consists of a large scale of epi￾taxial whiskers with smooth and symmetrically hexagonal shapes and well-knit structure. The yield of the ZnO epi￾taxial whiskers synthesized by this hydrothermal method is above 95%. Further morphology description of the ZnO epitaxial whiskers was accomplished using FE-SEM as shown in Fig. 4. SEM images of the as-synthesized ZnO tubular whiskers (pH = 10, 140 ◦C, 12 h). (a) The oblique open-ended tubular ZnO whisker. (b) The rupture tubular ZnO whisker. (c) The mid-broken tubular ZnO whiskers. (d) The split tubular ZnO whisker. Fig. 5. SEM image of the as-synthesized ZnO epitaxial whiskers (pH = 8, 110 ◦C, 12 h)

H Hou et al. /Solid State Sciences 7(2005)45-51 Fig.6. FE-SEM images for the as-synthesized ZnO epitaxial whiskers Fig. 7. TEM and SAED image of Zno epitaxial whiskers(pH=8, 110C, (pH=8, 110C, 12 h).(a) The panoramic morphology of Zno epitaxial 12 h).(a)The TEM image of overview ZnO epitaxial whiskers (b)The whiskers. ( b) The edge image of ZnO epitaxial whiskers. (c)The side face SAED pattem of the epitaxial whiskers. (c)The TEM image of individual image of Zno epitaxial whiskers. symmetrically epitaxial whisker.(d) The TEM image of one special single ZnO epitaxial whisker. Fig. 6. Fig. 6(a), the panoramic morphology of the sample, shows that the sample consists of epitaxial whiskers with straight and symmetrical whiskers. The image of some sep- b: epitaxial whiskers arated independent whiskers suggests that the whisker has nearly smooth, well-defined crystallographic facets and reg- ular prismatic hexagon, with the width of 2 um and angles about 1200, respectively. The edge image of ZnO epitaxial whiskers, as shown in Fig. 6(b), clearly exhibits two hexag - onal prism united together with end facet. The side face image of ZnO epitaxial whiskers(Fig. 6(c), which accords well with the TEM results. showed that the individual Zno epitaxial whisker, which has nearly smooth face, extending outside continuously without curving and clearly show sym- metrical feature Further morphology characterization of the ZnO epitax Wavelength/nm ial whiskers was performed using TEM shown in Fig. 7, Fig 8. Room-temperature PL spectrum of the as-grown ZnO tubular which accords well to the FesEm results. Fig. 7(a) show whiskers and epitaxial whiskers the general morphology of ZnO epitaxial whiskers prepared in which the symmetrically epitaxial whiskers inextricably Room-temperature photoluminescence(PL)of the as- united together by two single whiskers. Most of the epi- grown products were studied and the Pl spectrum the Zno taxial whiskers have fairly uniform diameters of around tubular whiskers and epitaxial whiskers were shown in 4 um and the length of about 10 um. The SAED pattern of Fig 8. Fig 8(a), showing PL spectrum of as-grown ZnO Fig. 7(b), taken on the Zno epitaxial whisker, exhibits that tubular whiskers, consists of a strong shoulder emission the as-synthesized Zno epitaxial whiskers are single crys- band located at 400 and 408 nm, and a weak and broad tal with the oriented growth direction along c-axis. Different emission band centered at 533 nm. The two shoulder nearly parts of this whisker show exactly the same electron pat overlapped violet emission(400, 408 nm), having slight red tern, indicating the single crystallinity of the whole single shift in the near band-edge emission, differ from the band whisker. Dozens of individual whiskers have been examined gap of bulk ZnO(around 380 nm)[16-18], which comes using this method, which confirmed their single crystallinity from the recombination of free exciton [12, 19]. The green Fig. 7(c)confirms the individual ZnO epitaxial whisker, emission at 533 nm is related to the singly ionized oxygen which has nearly smooth face, extending outside continu- vacancy, and this emission results from the recombination of ously without curving and clearly show symmetrical feature. photogenerated hole with a singly ionized charge state of the ig. 7(d) reveals the morphology of one special single Zno specific defect[20, 21]. The photoluminescence spectrum of hisker, whose perfect symmetrical structure clearly shows the as-grown ZnO epitaxial whiskers was shown in Fig. 8(b) the orientation adhesion in which a strong shoulder emission band located at 411 and

48 H. Hou et al. / Solid State Sciences 7 (2005) 45–51 Fig. 6. FE-SEM images for the as-synthesized ZnO epitaxial whiskers (pH = 8, 110 ◦C, 12 h). (a) The panoramic morphology of ZnO epitaxial whiskers. (b) The edge image of ZnO epitaxial whiskers. (c) The side face image of ZnO epitaxial whiskers. Fig. 6. Fig. 6(a), the panoramic morphology of the sample, shows that the sample consists of epitaxial whiskers with straight and symmetrical whiskers. The image of some sep￾arated independent whiskers suggests that the whisker has nearly smooth, well-defined crystallographic facets and reg￾ular prismatic hexagon, with the width of 2 µm and angles about 120◦, respectively. The edge image of ZnO epitaxial whiskers, as shown in Fig. 6(b), clearly exhibits two hexag￾onal prism united together with end facet. The side face image of ZnO epitaxial whiskers (Fig. 6(c)), which accords well with the TEM results, showed that the individual ZnO epitaxial whisker, which has nearly smooth face, extending outside continuously without curving and clearly show sym￾metrical feature. Further morphology characterization of the ZnO epitax￾ial whiskers was performed using TEM shown in Fig. 7, which accords well to the FESEM results. Fig. 7(a) shows the general morphology of ZnO epitaxial whiskers prepared, in which the symmetrically epitaxial whiskers inextricably united together by two single whiskers. Most of the epi￾taxial whiskers have fairly uniform diameters of around 4 µm and the length of about 10 µm. The SAED pattern of Fig. 7(b), taken on the ZnO epitaxial whisker, exhibits that the as-synthesized ZnO epitaxial whiskers are single crys￾tal with the oriented growth direction along c-axis. Different parts of this whisker show exactly the same electron pat￾tern, indicating the single crystallinity of the whole single whisker. Dozens of individual whiskers have been examined using this method, which confirmed their single crystallinity. Fig. 7(c) confirms the individual ZnO epitaxial whisker, which has nearly smooth face, extending outside continu￾ously without curving and clearly show symmetrical feature. Fig. 7(d) reveals the morphology of one special single ZnO whisker, whose perfect symmetrical structure clearly shows the orientation adhesion. Fig. 7. TEM and SAED image of ZnO epitaxial whiskers (pH = 8, 110 ◦C, 12 h). (a) The TEM image of overview ZnO epitaxial whiskers. (b) The SAED pattern of the epitaxial whiskers. (c) The TEM image of individual symmetrically epitaxial whisker. (d) The TEM image of one special single ZnO epitaxial whisker. Fig. 8. Room-temperature PL spectrum of the as-grown ZnO tubular whiskers and epitaxial whiskers. Room-temperature photoluminescence (PL) of the as￾grown products were studied and the PL spectrum the ZnO tubular whiskers and epitaxial whiskers were shown in Fig. 8. Fig. 8(a), showing PL spectrum of as-grown ZnO tubular whiskers, consists of a strong shoulder emission band located at 400 and 408 nm, and a weak and broad emission band centered at 533 nm. The two shoulder nearly￾overlapped violet emission (400, 408 nm), having slight red shift in the near band-edge emission, differ from the band gap of bulk ZnO (around 380 nm) [16–18], which comes from the recombination of free exciton [12,19]. The green emission at 533 nm is related to the singly ionized oxygen vacancy, and this emission results from the recombination of photogenerated hole with a singly ionized charge state of the specific defect [20,21]. The photoluminescence spectrum of the as-grown ZnO epitaxial whiskers was shown in Fig. 8(b), in which a strong shoulder emission band located at 411 and

H. Hou et al. /Solid State Sciences 7(2005)45-51 447nm, was contributed by the near band edge emission of Zn2++20H-- Zn(OH)2, the wide bandgap. The lack of green band in Fig. 5 is mainly indicative of a low surface area to volume ratio [ 22]. Further Zn(OH)2=Zno H20(pH=8 study is in progress to identify the origin of observed peaks At pH=8, the amount of ammonia cannot form the and the changes introduced by different ZnO morphologies. Zn(NH3)42+ but come into being the Zn(OH)2, which can Generally, the growth of the tubular whiskers and epitax be validated by the white turbidity of the initial system ial whiskers are suggested to follow a mechanism combined Therefore, the Zno crystal structure is crucial to synthe- by the effect of CTAB, the pH of the system, the structure size epitaxial whiskers at lower pH value as pH=8, under of ZnO wurtzite under the hydrothermal growth conditions. the effect of structure-directing agent CTAB. The illustration But, careful analysis indicated that the formation mechanism shown in Fig 9(a, b)clearly indicates the Zno crystal struc of ZnO tubular and epitaxial whiskers are totally different ture. As shown in Fig. 9(a), the face of tetrahedron Zn-O4 under the different pH value of the system parallel to positive polar c(0001), and the vertex angle is In the formation of tubular ZnO whiskers at pH= 10, in the face of negative polar (0001)[24]. Fig. 9(b)exhibits he initial tubular ZnO seed crystal come into being from the projection of tetrahedron Zn-O4 in [1120]. The distri- the hydration of Zn(NH3)4-[10, 19]. Under the pH= 10, bution of Zn in c is not symmetrical, which lean to(0001) droxyl OH- and the Zn(NH3)42+ form the initial Zno but apart from(0001) Zn and O are symmetrically distrib- eed crystal according to ute in [1100][25-28. In the same way, appropriate amount NH3+H,O: NH3- H2O= NH4++OH- (1) of CTAB also acts as the structure-directing agent in the for- Zn2++4NH3- Zn(NH3)42+ (2) the 1D epitaxial whiskers cannot shape even if with the ex Zn(NH3 )4++20H" istence of ammonia. Our experiments conform that other ZnO↓+4NH3↑+H2O(pH=10) surfactants, such as sodium dodecyl benzene sulphonate At the beginning of the reaction, because of the lower tem- perature and the production of small amounts of NH,, the ZnO seed crystal is liable to form. At the elevated tempe ature, the production of NH3, the existence of a gas-liquid equilibrium and the structure-directing agent of CTAB in the airtight autoclave make the ZnO seed crystal grow along certain orientation to form hollow structure. without the presence of ammonia, the ID tubular whiskers cannot form even if the presence of structure-directing agent CTAB. Our complement experiments show that when sodium hydrox de is used as a substitute for the ammonia in the pres- ence of CTAB. the tubular whiskers also cannot come forth and only irregular short rods can be obtained[23]. Ap- propriate amount of CTAB acts as the structure-directing agent in the formation of tubular Zno whiskers. While ab- sence of CTAB, the ID tubular whiskers cannot shape even if with the existence of ammonia. Our experiments con- form that other surfactants, such as sodium dodecyl benzene sulphonate(SDBS) and sodium dodecyl sulfate(SDS),can- not come into being the tubular whiskers, indicating that CTAB plays an undeniable and irreplaceable role. It has been found that the optimal molar ratio of zinc(l) acetate dihy drate to CTAB(2: 1)favors the growth of Zno whiskers The ID Zno tubular whiskers can only be formed with the cooperation of structure-directing agent of CTAB and the hydration of Zn(NH3)42+ under pH= 10 in hydrothermal growth conditions In the formation of epitaxial ZnO whiskers at pH=8, the initial epitaxial ZnO seed crystals come into being from the dehydration of Zn(OH)2. Under the pH=8, hydroxyl OH- Fig 9. The schematic diagram of ZnO crystal structure (a) The projec- and the Zn(OH)2 form the initial ZnO seed crystal according tion of ZnO crystal structure in c(0001)face. (b)The projection of Zn-O4 tetrahedron in(1120)face. (c) Intergrowth on the positive and negative po- lar faces. (d)Zn-O4 tetrahedrons connection on positive and negative polar NH3+H2O- NH3-H2O= NH4++OH (4) faces along c-axis

H. Hou et al. / Solid State Sciences 7 (2005) 45–51 49 447 nm, was contributed by the near band edge emission of the wide bandgap. The lack of green band in Fig. 5 is mainly indicative of a low surface area to volume ratio [22]. Further study is in progress to identify the origin of observed peaks and the changes introduced by different ZnO morphologies. Generally, the growth of the tubular whiskers and epitax￾ial whiskers are suggested to follow a mechanism combined by the effect of CTAB, the pH of the system, the structure of ZnO wurtzite under the hydrothermal growth conditions. But, careful analysis indicated that the formation mechanism of ZnO tubular and epitaxial whiskers are totally different under the different pH value of the system. In the formation of tubular ZnO whiskers at pH = 10, the initial tubular ZnO seed crystal come into being from the hydration of Zn(NH3)4 2+ [10,19]. Under the pH = 10, hydroxyl OH− and the Zn(NH3)4 2+ form the initial ZnO seed crystal according to: NH3 + H2O
NH3·H2O
NH4 + + OH−, (1) Zn2+ + 4NH3 → Zn(NH3)4 2+, (2) Zn(NH3)4 2+ + 2OH− → ZnO↓ + 4NH3↑ + H2O (pH = 10). (3) At the beginning of the reaction, because of the lower tem￾perature and the production of small amounts of NH3, the ZnO seed crystal is liable to form. At the elevated temper￾ature, the production of NH3, the existence of a gas–liquid equilibrium and the structure-directing agent of CTAB in the airtight autoclave make the ZnO seed crystal grow along certain orientation to form hollow structure. Without the presence of ammonia, the 1D tubular whiskers cannot form even if the presence of structure-directing agent CTAB. Our complement experiments show that when sodium hydrox￾ide is used as a substitute for the ammonia in the pres￾ence of CTAB, the tubular whiskers also cannot come forth and only irregular short rods can be obtained [23]. Ap￾propriate amount of CTAB acts as the structure-directing agent in the formation of tubular ZnO whiskers. While ab￾sence of CTAB, the 1D tubular whiskers cannot shape even if with the existence of ammonia. Our experiments con￾form that other surfactants, such as sodium dodecyl benzene sulphonate (SDBS) and sodium dodecyl sulfate (SDS), can￾not come into being the tubular whiskers, indicating that CTAB plays an undeniable and irreplaceable role. It has been found that the optimal molar ratio of zinc(II) acetate dihy￾drate to CTAB (2:1) favors the growth of ZnO whiskers. The 1D ZnO tubular whiskers can only be formed with the cooperation of structure-directing agent of CTAB and the hydration of Zn(NH3)4 2+ under pH = 10 in hydrothermal growth conditions. In the formation of epitaxial ZnO whiskers at pH = 8, the initial epitaxial ZnO seed crystals come into being from the dehydration of Zn(OH)2. Under the pH = 8, hydroxyl OH− and the Zn(OH)2 form the initial ZnO seed crystal according to: NH3 + H2O
NH3·H2O
NH4 + + OH−, (4) Zn2+ + 2OH−
Zn(OH)2, (5) Zn(OH)2
ZnO + H2O (pH = 8). (6) At pH = 8, the amount of ammonia cannot form the Zn(NH3)4 2+ but come into being the Zn(OH)2, which can be validated by the white turbidity of the initial system. Therefore, the ZnO crystal structure is crucial to synthe￾size epitaxial whiskers at lower pH value as pH = 8, under the effect of structure-directing agent CTAB. The illustration shown in Fig. 9(a,b) clearly indicates the ZnO crystal struc￾ture. As shown in Fig. 9(a), the face of tetrahedron Zn–O4 parallel to positive polar c (0001), and the vertex angle is in the face of negative polar (0001) [24]. Fig. 9(b) exhibits the projection of tetrahedron Zn–O4 in [1120]. The distri￾bution of Zn in c is not symmetrical, which lean to (0001) but apart from (0001). Zn and O are symmetrically distrib￾ute in [1100] [25–28]. In the same way, appropriate amount of CTAB also acts as the structure-directing agent in the for￾mation of epitaxial ZnO whiskers. While absence of CTAB, the 1D epitaxial whiskers cannot shape even if with the ex￾istence of ammonia. Our experiments conform that other surfactants, such as sodium dodecyl benzene sulphonate Fig. 9. The schematic diagram of ZnO crystal structure. (a) The projec￾tion of ZnO crystal structure in c (0001) face. (b) The projection of Zn–O4 tetrahedron in (1120) face. (c) Intergrowth on the positive and negative po￾lar faces. (d) Zn–O4 tetrahedrons connection on positive and negative polar faces along c-axis

H Hou et al. /Solid State Sciences 7(2005)45-51 (SDBS) and sodium dodecyl sulfate (SDS), cannot come be formed under the synergic effect of structure-directing into being the epitaxial whiskers, indicating that CTAB plays agent of CTAB and the dehydration of Zn(Oh)2 under the an undeniable and irreplaceable role. It has been found that pH=8 in hydrothermal growth conditions. It is expected the optimal molar ratio of zinc(If) acetate dihydrate to CTAb that the novel tubular whiskers may offer exciting oppor- (2: 1)favors the growth of Zno epitaxial whiskers tunities for potential applications in catalysis, microsized Under the effect of structure-directing agent CTAB, the waveguides or optical fibers. The novel epitaxial whiskers direction of stable connection of anionic coordination poly- may offer exciting opportunities for homozygote in photo- hedrons [Zn-O4] is the determinant to epitaxial whiskers electronic devices. In addition, we believe that this simple under the condition of pH=8 in hydrothermal growth con- process can also be applied to synthesize other tubular and ditions. Because of the crystal ZnO characteristic, the link- epitaxial whiskers for functional materials. Further research ing mode between ZnO particles is the same as that of the is still under progress in our laboratory growth unit in the interface, which be connected along the direction of stable connection of the anionic coordination polyhedrons, i.e., along the fastest growth direction of the Acknowledgements corresponding crystal [29, 30]. The connection schematic di- agram of ZnO epitaxial whiskers was shown in Fig 9(c, d) Financial support from the National Natural Science Fig 9(c)exhibits the connection view of connection between Foundation of China and the Chinese Ministry of Educa- the positive c (0001)and negative polar faces c(0001). tion is gratefully acknowledged The symmetrical characteristic of the geometric distribution of the epitaxial whiskers is corresponding to the symmetry of the crystal structure. Zn-O4 tetrahedrons connection onReferences positive and negative polar faces along c-axis, as shown in Fig. 9(d), reveals the connection direction of growth unit [1 C.M. Lieber, Solid State Commun. 107(1998)607. The ID ZnO epitaxial whiskers can only be formed under [2] V Srikant, D.R. Clarke, J Appl. Phys. 83(1998)5447 the synergic effect of structure-directing agent of CTAB and B ]L Guo, Y.L. Ji, H.B. Xu, P. Simon, Z.Y. Wu, J. Amer. Chem. Soc. 124 the direction of stable connection of anionic coordination polyhedrons [Zn-O4]- under the ph=8 in hydrothermal [4J. Zhang, L D. Sun, J. L. Yin, H.L. Sun, C S. Liao, C H. Yan, Chem. Mater.14(2002)4172 growth conditions 5 T. Yoshida, H. Naito, M. Okuda, S Ehara, T. Takagi, O. Kusumoto, n the process of tubular and epitaxial Zno whiskers, sin- H. Kado, K. Yokoyama, T. Tohda, Appl. Phys. Lett. 64(1994) gle whisker's unique growth direction is explained by the favorable energy along a specific growth direction [31, 32] [6]JQ. Hu, Q. Li, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Mater. 14 The hexagonal prismatic morphology and the facet outlook (2002)1216 of the whiskers are caused by the different growth rates of [7 C.X. Xu, X.W. Sun, Jpn J Appl. Phys. 1(42)(2003)4949 [8] H. Najafov, Y. Fukada, S. Ohshio, S. lida, H Saitoh, Jpn. J, Appl the crystalline faces [ 6] hys.1(42)(2003)3490 [9]WJ.Li, E.W. Shi, W.Z. Zhong, w.Z. Yin, J Cryst Growth 203(1999 4. Conclusions 10J. Zhang, L D Sun, C.S. Liao, C.H. Yan, Chem. Commun.(Cam- bridge)(2002)262 [11]YJ, Xing, Z.H. Xi, Z.Q. Xue, X.D. Zhang, J.H. Song, R.M. Wang, In summary, the synthesis of Zno tubular and epitaxial J. Xu, Y Song, S L Zhang, D.P. Yu, Appl. Phys. Lett. 83(2003)1689 whiskers have been described via a simple mild hydrother [12]JQ. Hu, Y Bando, Appl. Phys. Lett. 82(2003)1401 mal method The sample of tubular whiskers has fairly uni- [13]JJ.Wu, S.C. Liu, C.T. Wu, K.H. Chen, L.C. Chen, Appl. Phys form diameters of around 4 um and the length of up to Let.81(2002)1312 20 um. The hollow cavity of tubular Zno whisker has the [14]H. Saitoh, M. Satoh, S. Ohshio, J. Ceram. Soc. Jpn. 110(2002) same regular prismatic hexagon with the individual whisker [15]AJ. Strauss, Phys. Rev. Lett. 16(1966)1193 Its photoluminescence spectrum exhibits a strong shoulder [16]C K. Xu, G.D. Xu, Y.K. Liu, G.H. Wang, Solid State Commun. 122 emission band located at 400 and 408 nm and a weak and (2002)175 large scale of epitaxial whiskers with smooth and syme a [17) U. Koch, A Fojtik, H. Weller, A.Henglein, Chem. Phys.Lett.122 broad emission band centered at 533 nm. The sample of [18]S Monticone, R. Tufen, A.V. Kanaev, J Phys. Chem. B 102(1998) rically hexagonal shapes and well-knit structure has fairly uniform diameters of around 4 um and the length of about [19]B D Yao, H.Z. Shi, H.J. Bi, L D Zhang, J Phys. Condens Mater. 12 10 um. Its photoluminescence spectrum exhibits a strong (2000)6265 shoulder emission band located at 411 and 447 nm. The 20]K. Vanheusden, W.L. Warren, C H. Seager, D.R. Tallant, J.A. Voigt, ID Zno tubular whiskers can only be formed with the co- B.E. Ganade, J. Appl. Phys. 79(1996)7983 21]YLi, G.S. Cheng, L D Zhang, J Mater. Res 15(2000)2305 operation of structure-directing agent of CTAB and the hy- [22]PD Yang, H.Q.Yan,S.Mao,R.Russo,J.Johnson,R.Sayka dration of Zn(NH3)4- under the pH= 10 in hydrothermal N. Morris, J. Pham, R.R. He, H.J. Choi, Adv. Funct Mat. 12(20 growth conditions. The ID ZnO epitaxial whiskers can only

50 H. Hou et al. / Solid State Sciences 7 (2005) 45–51 (SDBS) and sodium dodecyl sulfate (SDS), cannot come into being the epitaxial whiskers, indicating that CTAB plays an undeniable and irreplaceable role. It has been found that the optimal molar ratio of zinc(II) acetate dihydrate to CTAB (2:1) favors the growth of ZnO epitaxial whiskers. Under the effect of structure-directing agent CTAB, the direction of stable connection of anionic coordination poly￾hedrons [Zn–O4] 6− is the determinant to epitaxial whiskers under the condition of pH = 8 in hydrothermal growth con￾ditions. Because of the crystal ZnO characteristic, the link￾ing mode between ZnO particles is the same as that of the growth unit in the interface, which be connected along the direction of stable connection of the anionic coordination polyhedrons, i.e., along the fastest growth direction of the corresponding crystal [29,30]. The connection schematic di￾agram of ZnO epitaxial whiskers was shown in Fig. 9(c,d). Fig. 9(c) exhibits the connection view of connection between the positive c (0001) and negative polar faces c (0001). The symmetrical characteristic of the geometric distribution of the epitaxial whiskers is corresponding to the symmetry of the crystal structure. Zn–O4 tetrahedrons connection on positive and negative polar faces along c-axis, as shown in Fig. 9(d), reveals the connection direction of growth unit. The 1D ZnO epitaxial whiskers can only be formed under the synergic effect of structure-directing agent of CTAB and the direction of stable connection of anionic coordination polyhedrons [Zn–O4] 6− under the pH = 8 in hydrothermal growth conditions. In the process of tubular and epitaxial ZnO whiskers, sin￾gle whisker’s unique growth direction is explained by the favorable energy along a specific growth direction [31,32]. The hexagonal prismatic morphology and the facet outlook of the whiskers are caused by the different growth rates of the crystalline faces [6]. 4. Conclusions In summary, the synthesis of ZnO tubular and epitaxial whiskers have been described via a simple mild hydrother￾mal method. The sample of tubular whiskers has fairly uni￾form diameters of around 4 µm and the length of up to 20 µm. The hollow cavity of tubular ZnO whisker has the same regular prismatic hexagon with the individual whisker. Its photoluminescence spectrum exhibits a strong shoulder emission band located at 400 and 408 nm, and a weak and broad emission band centered at 533 nm. The sample of a large scale of epitaxial whiskers with smooth and symmet￾rically hexagonal shapes and well-knit structure has fairly uniform diameters of around 4 µm and the length of about 10 µm. Its photoluminescence spectrum exhibits a strong shoulder emission band located at 411 and 447 nm. The 1D ZnO tubular whiskers can only be formed with the co￾operation of structure-directing agent of CTAB and the hy￾dration of Zn(NH3)4 2+ under the pH = 10 in hydrothermal growth conditions. The 1D ZnO epitaxial whiskers can only be formed under the synergic effect of structure-directing agent of CTAB and the dehydration of Zn(OH)2 under the pH = 8 in hydrothermal growth conditions. It is expected that the novel tubular whiskers may offer exciting oppor￾tunities for potential applications in catalysis, microsized waveguides or optical fibers. The novel epitaxial whiskers may offer exciting opportunities for homozygote in photo￾electronic devices. In addition, we believe that this simple process can also be applied to synthesize other tubular and epitaxial whiskers for functional materials. Further research is still under progress in our laboratory. Acknowledgements Financial support from the National Natural Science Foundation of China and the Chinese Ministry of Educa￾tion is gratefully acknowledged. References [1] C.M. Lieber, Solid State Commun. 107 (1998) 607. [2] V. Srikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447. [3] L. Guo, Y.L. Ji, H.B. Xu, P. Simon, Z.Y. 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