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Single crystalline ZnO nanorods grown by a simple hydrothermal process
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 0 6 3–1 0 6 7
Short communication
Single crystalline ZnO nanorods grown by a simple hydrothermal process L.Z. Pei a,⁎, H.S. Zhao a , W. Tan b , H.Y. Yu a , Y.W. Chen c , Qian-Feng Zhang a a
School of Materials Science and Engineering, Institute of Molecular Engineering and Applied Chemistry, Key Lab of Materials Science and Processing of Anhui Province, Anhui University of Technology, Ma'anshan, Anhui 243002, PR China b Henkel Huawei Electronics Co. Ltd., Lian'yungang, Jiangsu 222006, PR China c Department of Materials Science, Fudan University, Shanghai 200433, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
Single crystalline ZnO nanorods with wurtzite structure have been prepared by a simple
Received 17 February 2009
hydrothermal process. The microstructure and composition of the products were studied by
Accepted 10 March 2009
X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM, energy dispersive X-ray spectrum (EDS) and
Keywords:
Raman spectrum. The nanorods have diameters ranging from 100 nm to 800 nm and length
ZnO nanorods
of longer than 10 µm. Raman peak at 437.8 cm− 1 displays the characteristic peak of wurtzite
Semiconductors
ZnO. Photoluminescence (PL) spectrum shows a blue light emission at 441 nm, which is
Nanomaterials
related to radiative recombination of photo-generated holes with singularly ionized oxygen
Electron microscopy
vacancies.
Crystal growth
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 devices [1]. As a wide direct band-gap (3.37 eV) semiconductor, ZnO emits blue light at room temperature. The one-dimensional (1D) ZnO nanomaterials, such as nanowires or nanorods, are especially attractive due to their tunable electronic and opto-electronic properties, and the potential applications in the nanoscale electronic and opto-electronic devices [2], self-powdered nanosystems [3,4] and sensors [5]. In recent years, strong efforts have been made to fabricated ZnO nanorods by a vapor-phase transport process [6,7], thermal evaporation [8], anodic alumina membrane templates [9], chemical vapor deposition [10] and metal organic chemical vapor deposition [11] etc. Generally, these preparation methods involve complex procedures, sophisticated equipment and rigorous experimental conditions.
© 2009 Elsevier Inc. All rights reserved.
Therefore, the development of simple preparation route to ZnO nanorods is of great significance. Very recently, hydrothermal methods were applied to the preparation of ZnO nanorods resulting in excellent crystalline quality, chemical and thermal stability [1,12–14]. Hydrothermal preparation is becoming popular for environmental reason because water is used as the reaction solvent than organics. Typically, hydrothermal preparation uses an autoclave with the starting solution being heated gradually and aged for hours or even days. During the heat-up and cool-down time, the hydrothermal reaction takes place to produce nuclei and subsequent crystal growth [15]. So the hydrothermal method has been recently reported as a simple way to grow one-dimensional nanostructures because of its easy and low-cost procedure, low growth temperature, great control over experimental parameters and convenience for synthesis in high quality [16–19]. The formation of the ZnO nanorods by the hydrothermal method was mostly attributed to the presence of a surfactant or organic solvent (such as dodecyl
⁎ Corresponding author. Tel./fax: +86 5552311570. E-mail addresses: [email protected], [email protected] (L.Z. Pei). 1044-5803/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2009.03.002
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 0 6 3–1 0 6 7
microscopy samples were prepared by putting several drops of solution with ZnO nanorods onto a standard copper grid with a porous carbon film after the nanorod samples were dispersed into distilled water and treated for about 10 min using supersonic wave apparatus. Transmission electron microscopy and high-resolution transmission electron microscopy observations were performed using JEOL JEM-2100 transmission electron microscope operating with 1.9 Å point-to-point resolution operating with a 200-kV accelerating voltage with a GATAN digital photography system. Raman measurements were conducted using a Renishaw inVia Raman Microscope. The light source is He–Ne laser with the wavelength of 514.5 nm. photoluminescence measurement was carried out at room temperature using 380 nm as the excitation wavelength with a Fig. 1 – X-ray diffraction pattern of the ZnO nanorods.
benzene sulfonic acid sodium salt DBS, cetyltrimethylammonium bromide CTAB, poly-(oxyethylene) nonyl phenolether NP, poly-(vinyl pyrrolidone) PVP, hexomethylenetetramine HMT, etc.) and different mechanisms were proposed. Here we report a simple approach to the preparation of ZnO nanorods using zinc substrate under hydrothermal conditions without any other surfactants and organic solvent. The structure, morphology and optical property of the ZnO nanorods are characterized and the formation process is discussed.
2.
Experimental
The ZnO nanorods were prepared through a hydrothermal route in an autoclave. A pure zinc foil was used as the substrate for preparing ZnO nanorods. 48 ml distilled water was put into the autoclave. The zinc substrate with the size of 6 × 2 cm was put into distilled water cleaning 10 min using supersonic wave apparatus in order to insure the cleanness of the surface. Then, the latter was fixed in the stainless steel bracket in the center of the autoclave. After the autoclave was sealed safely, it was heated to 200 °C of temperature, 1.35 MPa of pressure, 100 rpm of the rotating speed for the stirrer. The temperature and pressure were maintained for 1 h. Subsequently the autoclave was cooled in air. Finally, the substrate with bulk white deposit was obtained after the experiment. The obtained products were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrum, transmission electron microscopy, selected area electron diffraction, high-resolution transmission electron microscopy, Raman and photoluminescence spectra. X-ray diffraction pattern was carried out on a Siemens D5000 X-ray diffractometer equipped with a graphite monochromatized Cu Kα radiation (λ = 1.5406 Å). The samples were scanned at a scanning rate of 0.02°/s in the 2θ range of 10–80°. Scanning electron microscopy observation was performed using JEOL JSM5410 SEM with a 15-kV accelerating voltage. Chemical analysis was performed with an energy dispersive spectrum attached to the scanning electron microscopy. The product was mechanically scrapped from the zinc substrate. Transmission electron microscopy and high-resolution transmission electron
Fig. 2 – (a) General SEM image of the ZnO nanorods. (b) SEM image at the top of the ZnO nanorods. (a) ×3500, (b) ×7500. (c) The corresponding EDS spectrum of the ZnO nanorods.
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 0 6 3–1 0 6 7
luminescence spectrometer (Edinburgh Instruments, FLS920) in the range of 400–600 nm.
3.
Results and Discussion
Fig. 1 shows the X-ray diffraction (XRD) pattern of the product. All of the diffraction peaks can be indexed within experimental error as hexagonal ZnO phase (wurtzite structure) with lattice constants of a = 3.2508 Å, and c = 5.2069 Å by comparison with the data from the Joint Committee on Powder Diffraction Standards (JCPDS) card for ZnO (JCPDS 36-1451) which is constant with the results of literatures [6–14,20–22]. The strong and narrow diffraction peaks indicate that the material has a good crystallinity and size. No characteristic peaks from other impurities are detected. The strong diffraction peaks of (101) and (002) suggest that the <101> and <002> are the main preferred growth orientations of the ZnO nanorods. The typical scanning electron microscopy (SEM) images of the as-prepared product are shown in Fig. 2(a) and (b). An abundant number of free-standing nanorods are observed from the surface of the substrate. ZnO nanorods with diameters ranging from 100 nm to 800 nm have been prepared conveniently by this method. The length of the ZnO nanorods is about several micrometers, even longer than 10 µm. Fig. 2 (b) is the SEM image at the top of the ZnO nanorods demonstrating the hexagonal shape. The nanorods are composed of Zn and O according to the typical EDS spectrum
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(Fig. 2 (c)) collected from the nanorods. The observed peaks correspond to Zn and O showing that the nanorods are composed of Zn and O. Fig. 3(a) is the typical transmission electron microscopy (TEM) image of ZnO nanorods showing the similar morphology as that of the SEM observations. The ZnO nanorods are straight and smooth. However, different from the SEM images, ZnO nanorods with fracture surface can be observed (shown by white arrow in Fig. 3 (a)) which may be caused owing to the mechanical fracture of ZnO nanorods during the separation from the zinc substrates. The corresponding selected area electron diffraction (SAED) (inset in the right-upper part of Fig. 3 (a)) pattern indicates that the nanorods are wurtzite single crystalline in nature. The as-prepared ZnO nanorods were further confirmed to be single crystalline structure using HRTEM combined with a fast Fourier transform (FFT) analysis. Fig. 3 (b) and (c) are the typical high-resolution transmission electron microscopy (HRTEM) images at the middle part and the edge of the nanorod, respectively showing the perfect single crystalline structure of the ZnO nanorods. The HRTEM images also further verify the SAED result. The insets in the right-upper parts of Fig. 3 (b) and (c) correspond to the FFT of the HRTEM images, which are consistent with the experimental electron diffraction pattern given in the inset of Fig. 3 (a). Therefore, the ZnO nanorods are composed of single crystalline wurtzite ZnO structure without dislocations and stacking faults. Another effective approach to investigate the phase and purity of the nanostructures is Raman scattering. ZnO has a
Fig. 3 – (a) General TEM image of the ZnO nanorods, the inset in the right-upper part is the corresponding SAED pattern. ×20,000 (b) and (c) are the typical HRTEM images at the middle part and edge of the nanorod, respectively which show the single crystalline structure. Insets in the right-upper parts are the corresponding diffraction patterns obtained by FFT. ×5,000,000 (d) Raman spectrum of the ZnO nanorods.
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Fig. 4 – PL spectrum of the ZnO nanorods.
wurtzite structure and belongs to the C46V space group or 6 mm symmetry [23,24]. The different symmetries involved govern whether vibrations are Raman active and appear in the spectra, while changes in lattice spacing and chemical environment may shift the vibrational frequencies. Representative Raman spectrum of the ZnO nanorods is shown in Fig. 3 (d). When considering the wurtzite type ZnO (space group P63mc), phonon modes E2 (low and high frequency), A1 (TO— transversal acoustic mode and LO—longitudinal optical components) and E1 (TO and LO) are expected. The high frequency E2 mode is clearly visible at 437.8 cm− 1 with a width of about 50 cm− 1, indicating the good crystal quality [23,25,26]. Due to size effect, the Raman bands become much broader [27]. The weak Raman peak at 379.4 cm− 1 can be assigned to the A1 mode of ZnO. The strong Raman peak at 437.8 cm− 1 is one of the characteristic peaks of wurtzite ZnO attributed to the E2 mode [23,25,26]. The Raman spectrum of the investigated ZnO nanorods depends on the collecting configuration and the crystal face. The photoluminescence (PL) property of the ZnO nanorods is one of the most interesting and important properties that has been intensively investigated recently. The PL spectrum of the as-prepared ZnO nanorods is given in Fig. 4. Different from the UV emission peaks of ZnO nanorods at 385 nm and 386 nm observed respectively by Guo et al. and Cheng et al. [28,29], it is clear from the spectrum that the spectrum consists of a widened shoulder peak from 400 nm to 430 nm and a sharp one centered at 441 nm showing the strong blue light emission ability. The above peak positions are very close to the recent results obtained by Gao, Ni and co-workers [20,30]. Usually, the UV emission is considered to be attributed to the near band edge emission of the wide band-gap of ZnO due to the annihilation of excitons [28,31]. However, the blue luminescence is considered to be the result of radiative recombination of photo-generated holes with singularly ionized oxygen vacancies [23,32,33]. In addition, the stronger blue light emission should be attributed to much more defective of the nanostructures prepared at lower temperature than those prepared at much higher temperatures, at which the UV emission is stronger [23,34]. According to previous research results on the formation mechanism of ZnO nanorods [1,12–23,28–30] the formation of
the ZnO nanorods was mostly attributed to the presence of a surfactant, organic solvent or solution concentration. However, no surfactants or organic solvent were used in our experiments. The starting material is only zinc substrate and the solvent is water. The water supplies the pressure of 1.35 MPa in the autoclave under the hydrothermal condition of 200 °C. An experiment under ordinary atmosphere condition without hydrothermal pressure was carried out. But no ZnO nanorods were found. Therefore, the hydrothermal pressure and oxygen in the hydrothermal environment are considered to play the important roles on the formation and growth of the ZnO nanorods. ZnO is believed to originate from the surface of the zinc substrate owing to the oxidation effect. Zn and ZnO nanoparticles on the surface of zinc substrate form owing to the oxidization effect under the hydrothermal pressure. Zn on the surface of the zinc substrate is further oxidized from the oxygen originating from the hydrothermal environment to form ZnO nanoclusters. The ZnO nanoclusters absorb oxygen, Zn continuously and begin to form ZnO nanorods in the c-axis direction by the intrinsic crystallographic structure [21]. The growth of ZnO nanorods stops gradually due to the continuously consumption of Zn nanoparticles on the surface of the Zn substrate and no enough ZnO supplying for the growth of the ZnO nanorods [35].
4.
Conclusions
In summary, we have developed a simple hydrothermal method to prepare ZnO nanorods with wurtzite structure in the diameter regime of 100 nm to 800 nm. The length is several micrometers, even up to 10 µm. The strong Raman peak at 437.8 cm− 1 is one of the characteristic peaks of wurtzite ZnO. The blue light emission PL peak of the ZnO nanorods is considered to be caused by radiative recombination of photogenerated holes with singularly ionized oxygen vacancies. The main advantage of the proposed method is its simplicity. This simple approach should promise us a future large-scale preparation of other nanomaterials for many important applications in nanotechnology.
Acknowledgments This work was supported by the National Basic Research Program of China (973 Program, 2008CB617605), the Program for New Century Excellent Talents in University of China (NCET-06-0556) and Natural Science Foundation of Anhui Provincial Education Department of China (KJ2007B077). The financial support is gratefully appreciated.
REFERENCES [1] Sun XM, Chen X, Deng ZX, Li YD. A CTAB-assisted hydrothermal orientation growth of ZnO nanorods. Mater Chem Phys 2002;78:99–104. [2] Huang MH, Wu Y, Feick H, Tran N, Weber E, Yang P. Catalytic growth of zinc oxide nanowires by vapor transport. Adv Mater 2001;13:113–6.
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[3] Wang ZL. Towards self-powered nanosystems: from nanogenerators to nanopiezotronics. Adv Funct Mater 2008;18:1–15. [4] Zhou J, Gu YD, Fei P, Mai WJ, Gao YF, Yang RS, et al. Flexible piezotronic strain sensor. Nano Lett 2008;8:3035–40. [5] Wang CH, Chu XF, Wu MM. Detection of H2S down to ppb levels at room temperature using sensors based on ZnO nanorods. Sens Actuators, B 2006;113:320–3. [6] Vayssieres L. Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions. Adv Mater 2003;15:464–6. [7] Zhang J, Yu WY, Zhang LD. Fabrication of semiconducting ZnO nanobelts using a halide source and their photoluminescence properties. Phys Lett, A 2002;299:276–81. [8] Li F, Li Z, Jin FJ. Fabrication and characterization of ZnO micro and nanostructures prepared by thermal evaporation. Physica, B 2008;403:664–9. [9] Zheng MJ, Zhang LD, Li GH, Shen WZ. Fabrication and optical properties of large-scale uniform zinc oxide nanowire arrays by one-step electrochemical deposition technique. Chem Phys Lett 2002;363:123–8. [10] Wu JJ, Liu SC. Low-temperature growth of well-aligned ZnO nanorods by chemical vapor deposition. Adv Mater 2002;14:215–8. [11] Park WI, yi GC, Kim JW, Park SM. Schottky nanocontacts on ZnO nanorod arrays. Appl Phys Lett 2003;82:4358. [12] Wei HY, Wu YS, Lun N, Hu CX. Hydrothermal synthesis and characterization of ZnO nanorods. Mater Sci Eng, A 2005;393:80–2. [13] Xu JQ, Chen YP, Li YD, Shen JN. Gas sensing properties of ZnO nanorods prepared by hydrothermal method. J Mater Sci 2005;40:2919–21. [14] Guo M, Diao P, Cai SM. Hydrothermal growth of well-aligned ZnO nanorod arrays: Dependence of morphology and alignment ordering upon preparing conditions. J Solid State Chem 2005;178:1864–73. [15] Sue K, Kimura K, Yamamoto M, Arai. Rapid hydrothermal synthesis of ZnO nanorods without organics. Mater Lett 2004;58:3350–2. [16] Tam KH, Cheung CK, Leung YH, Djurišic AB, Ling CC, Beling CD, et al. Defects in ZnO nanorods prepared by a hydrothermal method. J Phys Chem, B 2006;110:20865–71. [17] Greene LE, Law M, Tan DH, Montano M, Goldberger J, Somorjai G, et al. General route to vertical ZnO nanowire. Nano Lett 2005;5:1231–8. [18] Law M, Greene LE, Johnson JC, Saykally R, Yang P. Nanowire dye-sensitized solar cells. Nat Mater 2005;4:455–9. [19] Greene LE, Law M, Goldberger J, Kim F, Johnson JC, Zhang Y, et al. Low-temperature wafer-scale production of ZnO nanowire arrays. Angew Chem, Int Ed 2003;42:3031–4.
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[20] Ni YH, Wei XW, Hong JM, Ye Y. Hydrothermal preparation and optical properties of ZnO nanorods. Mater Sci Eng, B 2005;121:42–7. [21] Li ZK, Huang XT, Liu JP, Ai HH. Single crystalline ZnO nanowires on zinc substrate by a simple hydrothermal synthesis method. Mater Lett 2008;62:2507–11. [22] Wei HY, Wu YS, Lun N, Hu CX. Hydrothermal synthesis and characterization of ZnO nanorods. Mater Sci Eng, A 2005;393:80–2. [23] Lupan O, Chow L, Chai GY, Roldan B, Naitabdi A, Schulte A, et al. Nanofabrication and characterization of ZnO nanorod arrays and branched microrods by aqueous solution route. Mater Sci Eng, B 2007;145:57–66. [24] Calleja JM, Cardona M. Resonant Raman scattering in ZnO. Phys Rev, B 1977;16:3753–61. [25] Damen TC, Porto SPS, Tell B. Raman effect in zinc oxide. Phys Rev 1966;142:570–4. [26] Li Z, Xiong Y, Xie Y. Selective growth of ZnO nanostructures with coordination polymers. Nanotechnology 2005;16:2303–8. [27] Xie J, Ji TH, Ou-Yang XH, Xiao ZY, Shi HJ. Preparation of SrTiO3 nanomaterial from layered titanate nanotubes or nanowires. Solid State Commun 2008;147:226–9. [28] Guo L, Ji YL, Xu HB, Simon P, Wu ZY. Regularly shaped, single-crystalline ZnO nanorods with wurtzite structure. J Am Chem Soc 2002;124:14864–5. [29] Cheng CW, Xu GY, Zhang HQ, Luo Y, Li YY. Solution synthesis, optical and magnetic properties of Zn1 − xCoxO nanowires. Mater Lett 2008;62:3733–5. [30] Wang JM, Gao L. Synthesis of uniform rod-like, multi-pod-like ZnO whiskers and their photoluminescence properties. J Cryst Growth 2004;262:290–4. [31] Monticone S, Tufeu R, Kanaev AV. Complex nature of the UV and visible fluorescence of colloidal ZnO nanoparticles. J Phys Chem, B 1998;102:2854–62. [32] Hsu CL, Chang SJ, Hung HC, Lin YR, Huang CJ, Tseng YK, et al. Vertical single-crystal ZnO nanowires grown on ZnO:Ga/glass templates. IEEE Trans Nanotechnol 2005;4:649–54. [33] Li Y, Meng GW, Zhang LD, Phillipp F. Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties. Appl Phys Lett 2000;76:2011. [34] Park SH, Kim SH, Han SW. Growth of homoepitaxial ZnO film on ZnO nanorods and light emitting diode applications. Nanotechnology 2007;18:055608. [35] Liu JP, Huang XT, Li YY, Sulieman KM, He X, Sun FL. Self-assembled CuO monocrystalline nanoarchitectures with controlled dimensionality and morphology. Cryst Growth Des 2006;6:1690–6.
Short communication
Single crystalline ZnO nanorods grown by a simple hydrothermal process L.Z. Pei a,⁎, H.S. Zhao a , W. Tan b , H.Y. Yu a , Y.W. Chen c , Qian-Feng Zhang a a
School of Materials Science and Engineering, Institute of Molecular Engineering and Applied Chemistry, Key Lab of Materials Science and Processing of Anhui Province, Anhui University of Technology, Ma'anshan, Anhui 243002, PR China b Henkel Huawei Electronics Co. Ltd., Lian'yungang, Jiangsu 222006, PR China c Department of Materials Science, Fudan University, Shanghai 200433, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
Single crystalline ZnO nanorods with wurtzite structure have been prepared by a simple
Received 17 February 2009
hydrothermal process. The microstructure and composition of the products were studied by
Accepted 10 March 2009
X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM, energy dispersive X-ray spectrum (EDS) and
Keywords:
Raman spectrum. The nanorods have diameters ranging from 100 nm to 800 nm and length
ZnO nanorods
of longer than 10 µm. Raman peak at 437.8 cm− 1 displays the characteristic peak of wurtzite
Semiconductors
ZnO. Photoluminescence (PL) spectrum shows a blue light emission at 441 nm, which is
Nanomaterials
related to radiative recombination of photo-generated holes with singularly ionized oxygen
Electron microscopy
vacancies.
Crystal growth
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 devices [1]. As a wide direct band-gap (3.37 eV) semiconductor, ZnO emits blue light at room temperature. The one-dimensional (1D) ZnO nanomaterials, such as nanowires or nanorods, are especially attractive due to their tunable electronic and opto-electronic properties, and the potential applications in the nanoscale electronic and opto-electronic devices [2], self-powdered nanosystems [3,4] and sensors [5]. In recent years, strong efforts have been made to fabricated ZnO nanorods by a vapor-phase transport process [6,7], thermal evaporation [8], anodic alumina membrane templates [9], chemical vapor deposition [10] and metal organic chemical vapor deposition [11] etc. Generally, these preparation methods involve complex procedures, sophisticated equipment and rigorous experimental conditions.
© 2009 Elsevier Inc. All rights reserved.
Therefore, the development of simple preparation route to ZnO nanorods is of great significance. Very recently, hydrothermal methods were applied to the preparation of ZnO nanorods resulting in excellent crystalline quality, chemical and thermal stability [1,12–14]. Hydrothermal preparation is becoming popular for environmental reason because water is used as the reaction solvent than organics. Typically, hydrothermal preparation uses an autoclave with the starting solution being heated gradually and aged for hours or even days. During the heat-up and cool-down time, the hydrothermal reaction takes place to produce nuclei and subsequent crystal growth [15]. So the hydrothermal method has been recently reported as a simple way to grow one-dimensional nanostructures because of its easy and low-cost procedure, low growth temperature, great control over experimental parameters and convenience for synthesis in high quality [16–19]. The formation of the ZnO nanorods by the hydrothermal method was mostly attributed to the presence of a surfactant or organic solvent (such as dodecyl
⁎ Corresponding author. Tel./fax: +86 5552311570. E-mail addresses: [email protected], [email protected] (L.Z. Pei). 1044-5803/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2009.03.002
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 0 6 3–1 0 6 7
microscopy samples were prepared by putting several drops of solution with ZnO nanorods onto a standard copper grid with a porous carbon film after the nanorod samples were dispersed into distilled water and treated for about 10 min using supersonic wave apparatus. Transmission electron microscopy and high-resolution transmission electron microscopy observations were performed using JEOL JEM-2100 transmission electron microscope operating with 1.9 Å point-to-point resolution operating with a 200-kV accelerating voltage with a GATAN digital photography system. Raman measurements were conducted using a Renishaw inVia Raman Microscope. The light source is He–Ne laser with the wavelength of 514.5 nm. photoluminescence measurement was carried out at room temperature using 380 nm as the excitation wavelength with a Fig. 1 – X-ray diffraction pattern of the ZnO nanorods.
benzene sulfonic acid sodium salt DBS, cetyltrimethylammonium bromide CTAB, poly-(oxyethylene) nonyl phenolether NP, poly-(vinyl pyrrolidone) PVP, hexomethylenetetramine HMT, etc.) and different mechanisms were proposed. Here we report a simple approach to the preparation of ZnO nanorods using zinc substrate under hydrothermal conditions without any other surfactants and organic solvent. The structure, morphology and optical property of the ZnO nanorods are characterized and the formation process is discussed.
2.
Experimental
The ZnO nanorods were prepared through a hydrothermal route in an autoclave. A pure zinc foil was used as the substrate for preparing ZnO nanorods. 48 ml distilled water was put into the autoclave. The zinc substrate with the size of 6 × 2 cm was put into distilled water cleaning 10 min using supersonic wave apparatus in order to insure the cleanness of the surface. Then, the latter was fixed in the stainless steel bracket in the center of the autoclave. After the autoclave was sealed safely, it was heated to 200 °C of temperature, 1.35 MPa of pressure, 100 rpm of the rotating speed for the stirrer. The temperature and pressure were maintained for 1 h. Subsequently the autoclave was cooled in air. Finally, the substrate with bulk white deposit was obtained after the experiment. The obtained products were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrum, transmission electron microscopy, selected area electron diffraction, high-resolution transmission electron microscopy, Raman and photoluminescence spectra. X-ray diffraction pattern was carried out on a Siemens D5000 X-ray diffractometer equipped with a graphite monochromatized Cu Kα radiation (λ = 1.5406 Å). The samples were scanned at a scanning rate of 0.02°/s in the 2θ range of 10–80°. Scanning electron microscopy observation was performed using JEOL JSM5410 SEM with a 15-kV accelerating voltage. Chemical analysis was performed with an energy dispersive spectrum attached to the scanning electron microscopy. The product was mechanically scrapped from the zinc substrate. Transmission electron microscopy and high-resolution transmission electron
Fig. 2 – (a) General SEM image of the ZnO nanorods. (b) SEM image at the top of the ZnO nanorods. (a) ×3500, (b) ×7500. (c) The corresponding EDS spectrum of the ZnO nanorods.
M A TE RI A L S C H A RAC TE RI ZA T ION 6 0 ( 2 00 9 ) 1 0 6 3–1 0 6 7
luminescence spectrometer (Edinburgh Instruments, FLS920) in the range of 400–600 nm.
3.
Results and Discussion
Fig. 1 shows the X-ray diffraction (XRD) pattern of the product. All of the diffraction peaks can be indexed within experimental error as hexagonal ZnO phase (wurtzite structure) with lattice constants of a = 3.2508 Å, and c = 5.2069 Å by comparison with the data from the Joint Committee on Powder Diffraction Standards (JCPDS) card for ZnO (JCPDS 36-1451) which is constant with the results of literatures [6–14,20–22]. The strong and narrow diffraction peaks indicate that the material has a good crystallinity and size. No characteristic peaks from other impurities are detected. The strong diffraction peaks of (101) and (002) suggest that the <101> and <002> are the main preferred growth orientations of the ZnO nanorods. The typical scanning electron microscopy (SEM) images of the as-prepared product are shown in Fig. 2(a) and (b). An abundant number of free-standing nanorods are observed from the surface of the substrate. ZnO nanorods with diameters ranging from 100 nm to 800 nm have been prepared conveniently by this method. The length of the ZnO nanorods is about several micrometers, even longer than 10 µm. Fig. 2 (b) is the SEM image at the top of the ZnO nanorods demonstrating the hexagonal shape. The nanorods are composed of Zn and O according to the typical EDS spectrum
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(Fig. 2 (c)) collected from the nanorods. The observed peaks correspond to Zn and O showing that the nanorods are composed of Zn and O. Fig. 3(a) is the typical transmission electron microscopy (TEM) image of ZnO nanorods showing the similar morphology as that of the SEM observations. The ZnO nanorods are straight and smooth. However, different from the SEM images, ZnO nanorods with fracture surface can be observed (shown by white arrow in Fig. 3 (a)) which may be caused owing to the mechanical fracture of ZnO nanorods during the separation from the zinc substrates. The corresponding selected area electron diffraction (SAED) (inset in the right-upper part of Fig. 3 (a)) pattern indicates that the nanorods are wurtzite single crystalline in nature. The as-prepared ZnO nanorods were further confirmed to be single crystalline structure using HRTEM combined with a fast Fourier transform (FFT) analysis. Fig. 3 (b) and (c) are the typical high-resolution transmission electron microscopy (HRTEM) images at the middle part and the edge of the nanorod, respectively showing the perfect single crystalline structure of the ZnO nanorods. The HRTEM images also further verify the SAED result. The insets in the right-upper parts of Fig. 3 (b) and (c) correspond to the FFT of the HRTEM images, which are consistent with the experimental electron diffraction pattern given in the inset of Fig. 3 (a). Therefore, the ZnO nanorods are composed of single crystalline wurtzite ZnO structure without dislocations and stacking faults. Another effective approach to investigate the phase and purity of the nanostructures is Raman scattering. ZnO has a
Fig. 3 – (a) General TEM image of the ZnO nanorods, the inset in the right-upper part is the corresponding SAED pattern. ×20,000 (b) and (c) are the typical HRTEM images at the middle part and edge of the nanorod, respectively which show the single crystalline structure. Insets in the right-upper parts are the corresponding diffraction patterns obtained by FFT. ×5,000,000 (d) Raman spectrum of the ZnO nanorods.
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Fig. 4 – PL spectrum of the ZnO nanorods.
wurtzite structure and belongs to the C46V space group or 6 mm symmetry [23,24]. The different symmetries involved govern whether vibrations are Raman active and appear in the spectra, while changes in lattice spacing and chemical environment may shift the vibrational frequencies. Representative Raman spectrum of the ZnO nanorods is shown in Fig. 3 (d). When considering the wurtzite type ZnO (space group P63mc), phonon modes E2 (low and high frequency), A1 (TO— transversal acoustic mode and LO—longitudinal optical components) and E1 (TO and LO) are expected. The high frequency E2 mode is clearly visible at 437.8 cm− 1 with a width of about 50 cm− 1, indicating the good crystal quality [23,25,26]. Due to size effect, the Raman bands become much broader [27]. The weak Raman peak at 379.4 cm− 1 can be assigned to the A1 mode of ZnO. The strong Raman peak at 437.8 cm− 1 is one of the characteristic peaks of wurtzite ZnO attributed to the E2 mode [23,25,26]. The Raman spectrum of the investigated ZnO nanorods depends on the collecting configuration and the crystal face. The photoluminescence (PL) property of the ZnO nanorods is one of the most interesting and important properties that has been intensively investigated recently. The PL spectrum of the as-prepared ZnO nanorods is given in Fig. 4. Different from the UV emission peaks of ZnO nanorods at 385 nm and 386 nm observed respectively by Guo et al. and Cheng et al. [28,29], it is clear from the spectrum that the spectrum consists of a widened shoulder peak from 400 nm to 430 nm and a sharp one centered at 441 nm showing the strong blue light emission ability. The above peak positions are very close to the recent results obtained by Gao, Ni and co-workers [20,30]. Usually, the UV emission is considered to be attributed to the near band edge emission of the wide band-gap of ZnO due to the annihilation of excitons [28,31]. However, the blue luminescence is considered to be the result of radiative recombination of photo-generated holes with singularly ionized oxygen vacancies [23,32,33]. In addition, the stronger blue light emission should be attributed to much more defective of the nanostructures prepared at lower temperature than those prepared at much higher temperatures, at which the UV emission is stronger [23,34]. According to previous research results on the formation mechanism of ZnO nanorods [1,12–23,28–30] the formation of
the ZnO nanorods was mostly attributed to the presence of a surfactant, organic solvent or solution concentration. However, no surfactants or organic solvent were used in our experiments. The starting material is only zinc substrate and the solvent is water. The water supplies the pressure of 1.35 MPa in the autoclave under the hydrothermal condition of 200 °C. An experiment under ordinary atmosphere condition without hydrothermal pressure was carried out. But no ZnO nanorods were found. Therefore, the hydrothermal pressure and oxygen in the hydrothermal environment are considered to play the important roles on the formation and growth of the ZnO nanorods. ZnO is believed to originate from the surface of the zinc substrate owing to the oxidation effect. Zn and ZnO nanoparticles on the surface of zinc substrate form owing to the oxidization effect under the hydrothermal pressure. Zn on the surface of the zinc substrate is further oxidized from the oxygen originating from the hydrothermal environment to form ZnO nanoclusters. The ZnO nanoclusters absorb oxygen, Zn continuously and begin to form ZnO nanorods in the c-axis direction by the intrinsic crystallographic structure [21]. The growth of ZnO nanorods stops gradually due to the continuously consumption of Zn nanoparticles on the surface of the Zn substrate and no enough ZnO supplying for the growth of the ZnO nanorods [35].
4.
Conclusions
In summary, we have developed a simple hydrothermal method to prepare ZnO nanorods with wurtzite structure in the diameter regime of 100 nm to 800 nm. The length is several micrometers, even up to 10 µm. The strong Raman peak at 437.8 cm− 1 is one of the characteristic peaks of wurtzite ZnO. The blue light emission PL peak of the ZnO nanorods is considered to be caused by radiative recombination of photogenerated holes with singularly ionized oxygen vacancies. The main advantage of the proposed method is its simplicity. This simple approach should promise us a future large-scale preparation of other nanomaterials for many important applications in nanotechnology.
Acknowledgments This work was supported by the National Basic Research Program of China (973 Program, 2008CB617605), the Program for New Century Excellent Talents in University of China (NCET-06-0556) and Natural Science Foundation of Anhui Provincial Education Department of China (KJ2007B077). The financial support is gratefully appreciated.
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