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Structural and luminescent properties of ZnO nanorods prepared from aqueous solution
Materials Letters 61 (2007) 1876 – 1880 www.elsevier.com/locate/matlet
Structural and luminescent properties of ZnO nanorods prepared from aqueous solution Fei Li ⁎, Zhen Li, Fu Jiang Jin School of Materials Science and Chemical Engineering, China University of Geosciences, No. 388, Lumo Road, Wuhan 430074, PR China Received 29 May 2006; accepted 23 July 2006 Available online 18 August 2006
Abstract A simple method to fabricate the one-dimensional nanostructure of zinc oxide nanorods from aqueous solution was presented. ZnO nanorods, prepared under different concentrations of precursors, were characterized by techniques such as XRD and SEM. The results indicated that the ZnO nanorods were single crystalline and grown in the direction of [001] with the hexagonal wurtzite structure. Photoluminescence measurement of the ZnO nanorods was carried out and it showed that a strong near-band-gap emission dominated the PL spectra with several weak emission peaks related to the deep level. We also analyzed the effect of the concentration of precursors on the luminescent properties of ZnO nanorods. In addition, the growth mechanism of ZnO nanorods from aqueous solution was preliminarily discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: One-dimensional nanostructure; Zinc oxide; Aqueous solution; Photoluminescence
1. Introduction One-dimensional nanostructures have attracted great interest because of their importance in fundamental research and technological application [1]. Zinc oxide is one of the most important semiconductor due to its wide direct band gap (3.37 eV) at room temperature with a large exciton binding energy (60 meV), which has made ZnO a good candidate for field emission display, solar cell, chemical sensors, short-wavelength light-emitting diodes (LEDs) and laser diodes (LDs) [2– 6]. A recent report of ultraviolet (UV) lasing from well-aligned ZnO nanorod arrays at room temperature [7] has greatly stimulated the study on one-dimensional ZnO nanostructure. So far, ZnO nanorods have been synthesized by various methods such as catalyst-driven vapor–liquid–solid (VLS) growth [7–9], chemical vapor deposition(CVD) [10,11], thermal evaporation [12,13], metalorganic vapor-phase epitaxy
⁎ Corresponding author. E-mail address: [email protected] (F. Li). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.157
[14] and hydrothermal approaches from aqueous solution [15,16]. Compared with other growth methods, hydrothermal approach from aqueous solution has the advantages of low temperature and simple equipment, which is more suitable and economical for a large-scale preparation of ZnO nanorods. In this work, we reported the hydrothermal growth of large area, high quality single crystalline ZnO nanorods on glass substrate under different concentrations of precursors. We investigated the structural and luminescent properties of the ZnO nanorods and preliminarily discussed the growth mechanism of the ZnO nanorods. 2. Experiment section 2.1. Reagent All chemicals (Shanghai Chemicals Co. Ltd.) used in this work were of analytical reagent grade and used as received without further purification. All the aqueous solutions were prepared using double distilled water. Ordinary glass plates were used as substrates and were cleaned by standard procedures prior to use.
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(002) and (101) planes, respectively, I0002 and I0101 are the corresponding values of standard diffraction intensities measured from randomly oriented powder samples. For materials with randomly crystallographic orientation, e.g. powders, the texture coefficient is 0.5. The values of the TC002 in our samples prepared under different concentrations are 0.85(a), 0.82(b) and 0.72(c), respectively, which indicate high c-orientation for samples a and b. For sample c, prepared under the smallest concentration, the degree of preferential c-orientation growth is inferior to that of samples a and b. 3.2. SEM morphology
Fig. 1. X-ray diffraction patters for ZnO nanorods prepared under different concentrations of precursors on glass substrates. Concentration: (a) 0.1 M, (b) 0.01 M, (c) 0.001 M.
2.2. Hydrothermal deposition in aqueous solution The precursor solutions were prepared by mixing Zn (NO3)2·6H2O with methenamine ((CH2)6N4). The volume of the solution was 60 ml and the concentration of both Zn (NO3)2·6H2O and methenamine were kept at 0.1 M, 0.01 M, 0.001 M, respectively. The hydrothermal growth in aqueous solution was carried out at 90 °C for 3 h in a sealed glass beaker placed in a sealed kettle by immersing the substrates in precursor solutions. The ZnO nanostructures were washed by distilled water and then dried in ambient atmosphere at room temperature. 2.3. Characterization Powder X-ray diffraction (XRD) analysis was performed on a Dmax-3β diffractometer with nickel-filtered Cu Kα radiation. The morphology and distribution of the nanorods were characterized by scanning electron microscopy (SEM) (Quanta200). The photoluminescence spectra were measured at room temperature on LS-55 spectrophotometer using Xe lamp with a wave-length of 216 nm as the excitation source. 3. Results and discussion 3.1. Crystalline structure Fig. 1 illustrates X-ray diffraction patterns of the as-prepared ZnO nanorods on glass substrate. All the peaks of the nanorods prepared under different concentrations of the precursors can be well indexed to hexagonal wurtzite ZnO (JCPDS card No. 36-1451, a = 0.325 nm and c = 0.521 nm) with high crystallinity. No diffraction peaks from impurity phase are detected in any of the three samples within the limit of our XRD measurement. The degree of the orientation can be illustrated by the relative texture coefficient [17], which is given by TC002 ¼
0 I002 =I002 ; 0 0 I002 =I002 þ I101 =I101
where TC002 is the relative texture coefficient of diffraction peaks (002) over (101), I002 and I101 are the measured diffraction intensities due to
The SEM images of the ZnO nanorods, prepared under different concentration on glass substrates, are shown in Fig. 2. Using the simple low-temperature hydrothermal method from aqueous solution, ZnO nanorods were successfully obtained. From Fig. 2, we can see that the average diameter of the ZnO nanorods is ∼ 400 nm, ∼ 300 nm, ∼ 300 nm for samples a, b and c, respectively. The average lengths of samples a, b, and c are 2 μm, 6 μm, 3 μm, respectively. In our experiment, the diameter of ZnO nanorods prepared under different concentrations is in the range of the same magnitude, so is the length of ZnO nanorods. This result is quite different from those in Vayssieres [15] and Guo's work [16], where the nanorods were prepared on modified substrates and the nanorod width decreased the same order of magnitude as the decrease of concentration of reactant, and at the same time, the nanorod length increased greatly. It's believed that the higher concentration of the solution, the more easily solute separates. However, the critical sizes of the nuclei are determined by the characteristic of the as-grown crystal itself and almost have nothing to do with the concentration of the solution, together with the isotropic and amorphous glass substrates, therefore, despite the large difference of the concentration of the precursor solutions, there is no remarkable variation of the diameter of the ZnO nanorods. Furthermore, because of the consumption of the solute, the growth of ZnO nanorods is impossible to continue without limit, so that very long nanorods can't be formed. When the concentration of the precursor solution is 0.1 M, ZnO nanorods were very dense and uniform, and the average length was about 2 μm, for concentration of 0.01 M, ZnO nanorods were very sparse and a little long, about 6 μm. For concentration of 0.001 M, the density of ZnO nanorods were a bit larger than that of concentration 0.01 M, together with its small quantity of solute, the average length was shorter than that of 0.01 M, about 3 μm. In Vayssieres [15] and Guo's [16] experiments, the substrates used are polycrystalline ITO glass and nanostructured ZnO thin film; we believe that these polycrystalline and crystalline structures have an effect on the nucleation of the ZnO. The ZnO nanorods or nanowires can grow on the basis of the seeds on the surface of these substrates to get a small diameter and long length. We can also find that the density, uniformity and alignment of ZnO nanorods prepared under the concentration of 0.1 M is much better than those synthesized under the concentration of 0.01 M and 0.001 M. We deem that under a very small concentration of precursors' solution, such as 0.01 M and 0.001 M, the density, uniformity and alignment of the nanorods are difficult to be controlled. The nanorods in Fig. 2A are comparatively regular columns, while the ends of the ZnO nanorods in Fig. 2B and Fig. 2C become sharp, having the trend of forming tips. The enlarged SEM image in Fig. 2A further indicates the shape of the nanorod is hexagonal, which also verifies the analysis of XRD. There is a common structure in all the samples that ZnO nanorods constitute flowerlike ZnO nanostructure, which was also observed by Gao [18] and Xu [19]. In addition, with the decrease of the concentration of the precursors, the number of the ZnO flowerlike structure
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Fig. 2. SEM micrograph of the surface morphology of ZnO nanorods prepared under different concentrations of precursors on glass substrates. (A) 0.1 M, (B) 0.01 M, (C) 0.001 M.
decreases rapidly. In Fig. 2C, we can also see the dendritic structure of ZnO nanorods. From the results of XRD and SEM, we come to a conclusion that dense, well-aligned and uniform ZnO nanorods can be prepared via hydrothermal method from aqueous solution by virtue of controlling the concentration of the precursor properly, such as 0.1 M. Compared with the methods of catalyst-driven vapor–liquid–solid (VLS) growth [7], chemical vapor deposition(CVD) [10], thermal evaporation [12], the degree of well-aligned and uniformity of ZnO nanorods prepared by hydrothermal method is much better, and the cost of the hydrothermal method is much cheaper than the methods mentioned above, so the hydrothermal method has a promising future in the large-scale production of ZnO nanorods. But how to decrease the diameter of ZnO nanorods is still a problem for hydrothermal preparation, which needs further study. 3.3. Growth mechanism of ZnO nanorods Due to the difficulty for divalent metal ions to precipitate in aqueous solution by hydrolysis-condensation in neutral or acidic medium, ZnO
were prepared by aqueous thermal-decomposition of Zn2+ amino complex. The reaction equation can be described as follows: ðCH2 Þ6 N4 þ 10H2 O ¼ 6HCHO þ 4NH3 :H2 O
ð1Þ
Zn2þ 4NH3 :H2 O ¼ ½ZnðNH3 Þ4 2þ þ 4H2 O − ¼ ZnðOHÞ2 ðsÞ þ 4NHþ 4 þ 2OH
ð2Þ
ZnðOHÞ2 ðsÞ ¼ ZnOðsÞ þ H2 O
ð3Þ
In most cases, the homogeneous nucleation of solid phases requires a higher activation energy, and therefore, heteronucleation will be promoted and will be energetically more favorable [20]. Consequently, the whole hydrothermal growth process of ZnO nanorods contains three steps: 1) Formation of ZnO nuclei Heterogenous nucleation takes place on the glass substrate and ZnO nuclei are formed. The sizes of these ZnO nuclei are in the same range of hundreds of nm.
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Fig. 3. Simplified schematic representation for ZnO nanorods prepared under different concentrations of precursors. (A) 0.1 M, (B) 0.01 M, (C) 0.001 M.
2) ZnO nuclei aggregation Too many ZnO nuclei appear simultaneously at the beginning of the reaction, and then a part of these nuclei aggregates into a quasisphere, the residual nuclei are absorbed onto the glass substrate. Moreover, the greater the concentration of the precursors, the more the nuclei, the greater part of the nuclei conglomerating into quasisphere, the larger the quasi-sphere, the more the number of the nuclei occupying the surface of the quasi-sphere. On the contrary, the lower the concentration, the less the number of the nuclei occupying the surface of the quasi-sphere, and the more the number of the dispersed nuclei. 3) Growth of the nuclei into the ZnO nanorods and the formation of the flowerlike structure The velocities of crystal growth in different directions are reported to be [100] N [101] N [001] ≈ [001]. Accordingly, the theoretical and most stable crystal habit is a hexagon elongated along the c-axis and the end has the trend of contraction to get the lowest-energy state [21]. Therefore, the flowerlike structure is formed, and the greater the concentration, the more and denser the flowerlike structure, which can be clearly seen in Fig. 2A. With low concentration, the adjacently dispersed ZnO nanorods will interconnect with each other, then the dendritic structure of ZnO nanorods is generated, which can be seen in Fig. 2C.
The strong UV emission band results from near-band-gap emission, namely the recombination of free excitons through an exciton–exciton collision process [24,25]. It was difficult to observe UV emission from bulk ZnO at room temperature because the intensity of the emission would decrease rapidly with the increase of temperature due to the thermal quenching effect [24]. The weak violet emission peak at about 430 nm, according to Jin [26], may be due to the existence of oxygendepletion interface traps in the ZnOx film. In our work, ZnOx particles may be found in the hydrothermal growth of the ZnO nanorods. In addition, two green light luminescence peaks were observed at about 485 nm and 530 nm. It is generally accepted that the green light emission was referred to a deep-level or trap-state emission. Vanheusden [27] proved that the green transition has been attributed to the singly ionized oxygen vacancy in the ZnO. In detail, according to the energy band of oxygen vacancy [28], the peak at 485 nm results from radiative recombination of a hole occupying the zinc vacancy energy band with an electron occupying the low oxygen vacancy energy band, and the peak at about 530 nm is due to the radiative recombination of a photogenerated hole in the valence band with an
The whole growth process under different concentrations of the precursors can be illustrated in Fig. 3. 3.4. Photoluminescence The luminescent property of ZnO nanorods has been investigated extensively for their potential use as photoelectrical materials [22,23]. To study the influence of the concentration of the precursors on the luminescent property of the ZnO nanorods, PL measurements were conducted at room temperature under the excitation of 216 nm and the results are presented in Fig. 4. The PL spectra of ZnO nanorods prepared under different concentrations show similar PL features, in which four obvious luminescence bands have been observed, including a strong UV, a weak violet, two weak green emission bands centered at about 380 nm, 425 nm, 485 nm and 530 nm, respectively.
Fig. 4. Luminescent spectra of ZnO nanorods prepared under different concentrations of precursors on glass substrates. (a) 0.1 M, (b) 0.001 M, (c) 0.01 M.
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electron occupying the deep oxygen vacancy energy band. The intensity of the peaks is comparatively weak for the reason of the low density of oxygen vacancies in these single crystalline ZnO nanorods, which is different from the result of Yu [29], whose result is that green emission was stronger than UV emission. This proves the high crystalline quality of the ZnO nanorods from another aspect. From Fig. 4, we can also find that the intensity of the spectra is a N b N c. Combining with the previous SEM results, we can conclude that this result is related to the yield density of ZnO nanorods. When the concentration of precursors is 0.1 M, the yield density is largest and the intensity is the strongest. Furthermore, the slight blue shift in UV emission for sample c is possible because of the largest aspect ratio of the ZnO nanorods.
4. Conclusions In conclusion, large quantity and single crystalline ZnO nanorods have been prepared through a simple low temperature hydrothermal method from aqueous solution. XRD and SEM analysis revealed that the ZnO nanorods are hexagonal wurtzite ZnO structure with high crystallinity and preferred growth direction of the c-axis. Photoluminescence spectrum at room temperature shows that ZnO nanorod arrays has strong nearband UV emission and several weak emissions in the blue and green bands, which may suggest a new and good candidate for fabricating optoelectronic nanodevices, such as light-emitting diodes (LEDs) and laser diodes (LDs). It can also be concluded that hydrothermal method from aqueous solution has great potential in large-scale and low-cost preparation of ZnO nanorods. Acknowledgments This work was funded by the Research Foundation for Outstanding Young Teachers, China University of Geosciences (Wuhan, No.CUGQNL0632). The financial support is gratefully appreciated. References [1] J. Hu, T.W. Odom, L.C.M. Ieber, Acc. Chem. Res. 32 (1999) 435–445. [2] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, Appl. Phys. Lett. 81 (2002) 3648–3650.
[3] N. Beermann, L. Vayssieres, S.E. Lindquist, A. Hagfeldt, J. Electrochem. Soc. 147 (2000) 2456–2461. [4] S.C. Minne, S.R. Manalis, C.F. Quate, Appl. Phys. Lett. 67 (1995) 3918–3920. [5] H. Cao, J.Y. Xu, D.Z. Zhang, S.-H. Chang, S.T. Ho, E.W. Seelig, X. Liu, R.P.H. Chang, Phys. Rev. Lett. 84 (2000) 5584–5587. [6] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230–2232. [7] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897–1899. [8] S. Chakrabarti, S. Chaudhuri, Mater. Chem. Phys. 87 (2004) 196–200. [9] Y.W. Wang, L.D. Zhang, G.Z. Wang, X.S. Peng, Z.Q. Chu, C.H. Liang, J. Cryst. Growth 234 (2002) 171–175. [10] J.J. Wu, S.C. Liu, Adv. Mater. 14 (2002) 215–218. [11] H. Saitoh, M. Saitoh, N. Tanaka, Y. Ueda, S. Ohshio, Jpn. J. Appl. Phys. 38 (1999) 6873–6877. [12] Z.W. pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947–1949. [13] J. Lee, K. Park, M. Kang, I. Park, S. Kim, W.K. Cho, H.S. Han, S. Kim, J. Cryst. Growth 254 (2003) 423–431. [14] W.I. Park, D.H. Kim, S.W. Juang, G.C. Yi, Appl. Phys. Lett. 80 (2002) 4232–4234. [15] Lionel Vayssieres, Adv. Mater. 15 (2003) 464–466. [16] M. Guo, P. Diao, X.D. Wang, S.M. Cai, J. Solid State Chem. 178 (2005) 310–3215. [17] Y. Kajikawa, S. Noda, H. Komiyama, Chem. Vap. Depos. 8 (2002) 99–104. [18] X.D. Gao, X.M. Li, W.D. Yu, J. Solid State Chem. 178 (2005) 1138–1144. [19] C.L. Xu, D.H. Qin, H. Li, Y. Guo, T. Xu, H.L. Li, Mater. Lett. 58 (2004) 3976–3979. [20] Lionel Vayssieres, Karin Keis, Sten-Eric Lindquist, Anders Hagfeldt, J. Phys. Chem., B 105 (2001) 3350–3352. [21] W.J. Li, E.W. Shi, W.Z. Zhong, Z.W. Yin, J. Cryst. Growth 203 (1999) 186–196. [22] W.I. Park, D.H. Kim, S.-W. Jung, Gyu-Chul Yi, Appl. Phys. Lett. 80 (2002) 4232–4234. [23] V.A.L. Roy, A.B. Djurisic, W.K. Chan, J. Gao, H.F. Lui, C. Surya, Appl. Phys. Lett. 83 (2003) 141–143. [24] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407–409. [25] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113–116. [26] B.J. Jin, S. Im, S.Y. Lee, Thin Solid Films 366 (2000) 107–110. [27] K. Vanheusden, W.L. Warren, C.H. Ceager, D.R. Tallant, J.A. Voigt, J. Appl. Phys. 79 (1996) 7983–7990. [28] G.L. Song, K.X. Sun, Acta Photonica Sin. 35 (2006) 389–393. [29] K. Yu, Y. Zhang, L. Luo, W. Wang, Z. Zhu, J. Wang, Y. Cui, H. Ma, W. Lu, Appl. Phys., A 79 (2004) 443–446.
Structural and luminescent properties of ZnO nanorods prepared from aqueous solution Fei Li ⁎, Zhen Li, Fu Jiang Jin School of Materials Science and Chemical Engineering, China University of Geosciences, No. 388, Lumo Road, Wuhan 430074, PR China Received 29 May 2006; accepted 23 July 2006 Available online 18 August 2006
Abstract A simple method to fabricate the one-dimensional nanostructure of zinc oxide nanorods from aqueous solution was presented. ZnO nanorods, prepared under different concentrations of precursors, were characterized by techniques such as XRD and SEM. The results indicated that the ZnO nanorods were single crystalline and grown in the direction of [001] with the hexagonal wurtzite structure. Photoluminescence measurement of the ZnO nanorods was carried out and it showed that a strong near-band-gap emission dominated the PL spectra with several weak emission peaks related to the deep level. We also analyzed the effect of the concentration of precursors on the luminescent properties of ZnO nanorods. In addition, the growth mechanism of ZnO nanorods from aqueous solution was preliminarily discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: One-dimensional nanostructure; Zinc oxide; Aqueous solution; Photoluminescence
1. Introduction One-dimensional nanostructures have attracted great interest because of their importance in fundamental research and technological application [1]. Zinc oxide is one of the most important semiconductor due to its wide direct band gap (3.37 eV) at room temperature with a large exciton binding energy (60 meV), which has made ZnO a good candidate for field emission display, solar cell, chemical sensors, short-wavelength light-emitting diodes (LEDs) and laser diodes (LDs) [2– 6]. A recent report of ultraviolet (UV) lasing from well-aligned ZnO nanorod arrays at room temperature [7] has greatly stimulated the study on one-dimensional ZnO nanostructure. So far, ZnO nanorods have been synthesized by various methods such as catalyst-driven vapor–liquid–solid (VLS) growth [7–9], chemical vapor deposition(CVD) [10,11], thermal evaporation [12,13], metalorganic vapor-phase epitaxy
⁎ Corresponding author. E-mail address: [email protected] (F. Li). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.157
[14] and hydrothermal approaches from aqueous solution [15,16]. Compared with other growth methods, hydrothermal approach from aqueous solution has the advantages of low temperature and simple equipment, which is more suitable and economical for a large-scale preparation of ZnO nanorods. In this work, we reported the hydrothermal growth of large area, high quality single crystalline ZnO nanorods on glass substrate under different concentrations of precursors. We investigated the structural and luminescent properties of the ZnO nanorods and preliminarily discussed the growth mechanism of the ZnO nanorods. 2. Experiment section 2.1. Reagent All chemicals (Shanghai Chemicals Co. Ltd.) used in this work were of analytical reagent grade and used as received without further purification. All the aqueous solutions were prepared using double distilled water. Ordinary glass plates were used as substrates and were cleaned by standard procedures prior to use.
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(002) and (101) planes, respectively, I0002 and I0101 are the corresponding values of standard diffraction intensities measured from randomly oriented powder samples. For materials with randomly crystallographic orientation, e.g. powders, the texture coefficient is 0.5. The values of the TC002 in our samples prepared under different concentrations are 0.85(a), 0.82(b) and 0.72(c), respectively, which indicate high c-orientation for samples a and b. For sample c, prepared under the smallest concentration, the degree of preferential c-orientation growth is inferior to that of samples a and b. 3.2. SEM morphology
Fig. 1. X-ray diffraction patters for ZnO nanorods prepared under different concentrations of precursors on glass substrates. Concentration: (a) 0.1 M, (b) 0.01 M, (c) 0.001 M.
2.2. Hydrothermal deposition in aqueous solution The precursor solutions were prepared by mixing Zn (NO3)2·6H2O with methenamine ((CH2)6N4). The volume of the solution was 60 ml and the concentration of both Zn (NO3)2·6H2O and methenamine were kept at 0.1 M, 0.01 M, 0.001 M, respectively. The hydrothermal growth in aqueous solution was carried out at 90 °C for 3 h in a sealed glass beaker placed in a sealed kettle by immersing the substrates in precursor solutions. The ZnO nanostructures were washed by distilled water and then dried in ambient atmosphere at room temperature. 2.3. Characterization Powder X-ray diffraction (XRD) analysis was performed on a Dmax-3β diffractometer with nickel-filtered Cu Kα radiation. The morphology and distribution of the nanorods were characterized by scanning electron microscopy (SEM) (Quanta200). The photoluminescence spectra were measured at room temperature on LS-55 spectrophotometer using Xe lamp with a wave-length of 216 nm as the excitation source. 3. Results and discussion 3.1. Crystalline structure Fig. 1 illustrates X-ray diffraction patterns of the as-prepared ZnO nanorods on glass substrate. All the peaks of the nanorods prepared under different concentrations of the precursors can be well indexed to hexagonal wurtzite ZnO (JCPDS card No. 36-1451, a = 0.325 nm and c = 0.521 nm) with high crystallinity. No diffraction peaks from impurity phase are detected in any of the three samples within the limit of our XRD measurement. The degree of the orientation can be illustrated by the relative texture coefficient [17], which is given by TC002 ¼
0 I002 =I002 ; 0 0 I002 =I002 þ I101 =I101
where TC002 is the relative texture coefficient of diffraction peaks (002) over (101), I002 and I101 are the measured diffraction intensities due to
The SEM images of the ZnO nanorods, prepared under different concentration on glass substrates, are shown in Fig. 2. Using the simple low-temperature hydrothermal method from aqueous solution, ZnO nanorods were successfully obtained. From Fig. 2, we can see that the average diameter of the ZnO nanorods is ∼ 400 nm, ∼ 300 nm, ∼ 300 nm for samples a, b and c, respectively. The average lengths of samples a, b, and c are 2 μm, 6 μm, 3 μm, respectively. In our experiment, the diameter of ZnO nanorods prepared under different concentrations is in the range of the same magnitude, so is the length of ZnO nanorods. This result is quite different from those in Vayssieres [15] and Guo's work [16], where the nanorods were prepared on modified substrates and the nanorod width decreased the same order of magnitude as the decrease of concentration of reactant, and at the same time, the nanorod length increased greatly. It's believed that the higher concentration of the solution, the more easily solute separates. However, the critical sizes of the nuclei are determined by the characteristic of the as-grown crystal itself and almost have nothing to do with the concentration of the solution, together with the isotropic and amorphous glass substrates, therefore, despite the large difference of the concentration of the precursor solutions, there is no remarkable variation of the diameter of the ZnO nanorods. Furthermore, because of the consumption of the solute, the growth of ZnO nanorods is impossible to continue without limit, so that very long nanorods can't be formed. When the concentration of the precursor solution is 0.1 M, ZnO nanorods were very dense and uniform, and the average length was about 2 μm, for concentration of 0.01 M, ZnO nanorods were very sparse and a little long, about 6 μm. For concentration of 0.001 M, the density of ZnO nanorods were a bit larger than that of concentration 0.01 M, together with its small quantity of solute, the average length was shorter than that of 0.01 M, about 3 μm. In Vayssieres [15] and Guo's [16] experiments, the substrates used are polycrystalline ITO glass and nanostructured ZnO thin film; we believe that these polycrystalline and crystalline structures have an effect on the nucleation of the ZnO. The ZnO nanorods or nanowires can grow on the basis of the seeds on the surface of these substrates to get a small diameter and long length. We can also find that the density, uniformity and alignment of ZnO nanorods prepared under the concentration of 0.1 M is much better than those synthesized under the concentration of 0.01 M and 0.001 M. We deem that under a very small concentration of precursors' solution, such as 0.01 M and 0.001 M, the density, uniformity and alignment of the nanorods are difficult to be controlled. The nanorods in Fig. 2A are comparatively regular columns, while the ends of the ZnO nanorods in Fig. 2B and Fig. 2C become sharp, having the trend of forming tips. The enlarged SEM image in Fig. 2A further indicates the shape of the nanorod is hexagonal, which also verifies the analysis of XRD. There is a common structure in all the samples that ZnO nanorods constitute flowerlike ZnO nanostructure, which was also observed by Gao [18] and Xu [19]. In addition, with the decrease of the concentration of the precursors, the number of the ZnO flowerlike structure
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Fig. 2. SEM micrograph of the surface morphology of ZnO nanorods prepared under different concentrations of precursors on glass substrates. (A) 0.1 M, (B) 0.01 M, (C) 0.001 M.
decreases rapidly. In Fig. 2C, we can also see the dendritic structure of ZnO nanorods. From the results of XRD and SEM, we come to a conclusion that dense, well-aligned and uniform ZnO nanorods can be prepared via hydrothermal method from aqueous solution by virtue of controlling the concentration of the precursor properly, such as 0.1 M. Compared with the methods of catalyst-driven vapor–liquid–solid (VLS) growth [7], chemical vapor deposition(CVD) [10], thermal evaporation [12], the degree of well-aligned and uniformity of ZnO nanorods prepared by hydrothermal method is much better, and the cost of the hydrothermal method is much cheaper than the methods mentioned above, so the hydrothermal method has a promising future in the large-scale production of ZnO nanorods. But how to decrease the diameter of ZnO nanorods is still a problem for hydrothermal preparation, which needs further study. 3.3. Growth mechanism of ZnO nanorods Due to the difficulty for divalent metal ions to precipitate in aqueous solution by hydrolysis-condensation in neutral or acidic medium, ZnO
were prepared by aqueous thermal-decomposition of Zn2+ amino complex. The reaction equation can be described as follows: ðCH2 Þ6 N4 þ 10H2 O ¼ 6HCHO þ 4NH3 :H2 O
ð1Þ
Zn2þ 4NH3 :H2 O ¼ ½ZnðNH3 Þ4 2þ þ 4H2 O − ¼ ZnðOHÞ2 ðsÞ þ 4NHþ 4 þ 2OH
ð2Þ
ZnðOHÞ2 ðsÞ ¼ ZnOðsÞ þ H2 O
ð3Þ
In most cases, the homogeneous nucleation of solid phases requires a higher activation energy, and therefore, heteronucleation will be promoted and will be energetically more favorable [20]. Consequently, the whole hydrothermal growth process of ZnO nanorods contains three steps: 1) Formation of ZnO nuclei Heterogenous nucleation takes place on the glass substrate and ZnO nuclei are formed. The sizes of these ZnO nuclei are in the same range of hundreds of nm.
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Fig. 3. Simplified schematic representation for ZnO nanorods prepared under different concentrations of precursors. (A) 0.1 M, (B) 0.01 M, (C) 0.001 M.
2) ZnO nuclei aggregation Too many ZnO nuclei appear simultaneously at the beginning of the reaction, and then a part of these nuclei aggregates into a quasisphere, the residual nuclei are absorbed onto the glass substrate. Moreover, the greater the concentration of the precursors, the more the nuclei, the greater part of the nuclei conglomerating into quasisphere, the larger the quasi-sphere, the more the number of the nuclei occupying the surface of the quasi-sphere. On the contrary, the lower the concentration, the less the number of the nuclei occupying the surface of the quasi-sphere, and the more the number of the dispersed nuclei. 3) Growth of the nuclei into the ZnO nanorods and the formation of the flowerlike structure The velocities of crystal growth in different directions are reported to be [100] N [101] N [001] ≈ [001]. Accordingly, the theoretical and most stable crystal habit is a hexagon elongated along the c-axis and the end has the trend of contraction to get the lowest-energy state [21]. Therefore, the flowerlike structure is formed, and the greater the concentration, the more and denser the flowerlike structure, which can be clearly seen in Fig. 2A. With low concentration, the adjacently dispersed ZnO nanorods will interconnect with each other, then the dendritic structure of ZnO nanorods is generated, which can be seen in Fig. 2C.
The strong UV emission band results from near-band-gap emission, namely the recombination of free excitons through an exciton–exciton collision process [24,25]. It was difficult to observe UV emission from bulk ZnO at room temperature because the intensity of the emission would decrease rapidly with the increase of temperature due to the thermal quenching effect [24]. The weak violet emission peak at about 430 nm, according to Jin [26], may be due to the existence of oxygendepletion interface traps in the ZnOx film. In our work, ZnOx particles may be found in the hydrothermal growth of the ZnO nanorods. In addition, two green light luminescence peaks were observed at about 485 nm and 530 nm. It is generally accepted that the green light emission was referred to a deep-level or trap-state emission. Vanheusden [27] proved that the green transition has been attributed to the singly ionized oxygen vacancy in the ZnO. In detail, according to the energy band of oxygen vacancy [28], the peak at 485 nm results from radiative recombination of a hole occupying the zinc vacancy energy band with an electron occupying the low oxygen vacancy energy band, and the peak at about 530 nm is due to the radiative recombination of a photogenerated hole in the valence band with an
The whole growth process under different concentrations of the precursors can be illustrated in Fig. 3. 3.4. Photoluminescence The luminescent property of ZnO nanorods has been investigated extensively for their potential use as photoelectrical materials [22,23]. To study the influence of the concentration of the precursors on the luminescent property of the ZnO nanorods, PL measurements were conducted at room temperature under the excitation of 216 nm and the results are presented in Fig. 4. The PL spectra of ZnO nanorods prepared under different concentrations show similar PL features, in which four obvious luminescence bands have been observed, including a strong UV, a weak violet, two weak green emission bands centered at about 380 nm, 425 nm, 485 nm and 530 nm, respectively.
Fig. 4. Luminescent spectra of ZnO nanorods prepared under different concentrations of precursors on glass substrates. (a) 0.1 M, (b) 0.001 M, (c) 0.01 M.
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electron occupying the deep oxygen vacancy energy band. The intensity of the peaks is comparatively weak for the reason of the low density of oxygen vacancies in these single crystalline ZnO nanorods, which is different from the result of Yu [29], whose result is that green emission was stronger than UV emission. This proves the high crystalline quality of the ZnO nanorods from another aspect. From Fig. 4, we can also find that the intensity of the spectra is a N b N c. Combining with the previous SEM results, we can conclude that this result is related to the yield density of ZnO nanorods. When the concentration of precursors is 0.1 M, the yield density is largest and the intensity is the strongest. Furthermore, the slight blue shift in UV emission for sample c is possible because of the largest aspect ratio of the ZnO nanorods.
4. Conclusions In conclusion, large quantity and single crystalline ZnO nanorods have been prepared through a simple low temperature hydrothermal method from aqueous solution. XRD and SEM analysis revealed that the ZnO nanorods are hexagonal wurtzite ZnO structure with high crystallinity and preferred growth direction of the c-axis. Photoluminescence spectrum at room temperature shows that ZnO nanorod arrays has strong nearband UV emission and several weak emissions in the blue and green bands, which may suggest a new and good candidate for fabricating optoelectronic nanodevices, such as light-emitting diodes (LEDs) and laser diodes (LDs). It can also be concluded that hydrothermal method from aqueous solution has great potential in large-scale and low-cost preparation of ZnO nanorods. Acknowledgments This work was funded by the Research Foundation for Outstanding Young Teachers, China University of Geosciences (Wuhan, No.CUGQNL0632). The financial support is gratefully appreciated. References [1] J. Hu, T.W. Odom, L.C.M. Ieber, Acc. Chem. Res. 32 (1999) 435–445. [2] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, Appl. Phys. Lett. 81 (2002) 3648–3650.
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