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Synthesis of nanowires, nanorods and nanoparticles of ZnO through modulating the ratio of water to methanol by using a mild and simple solution method
Materials Chemistry and Physics 89 (2005) 326–331
Synthesis of nanowires, nanorods and nanoparticles of ZnO through modulating the ratio of water to methanol by using a mild and simple solution method Hualan Zhou, Zhuang Li∗ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, Jinlin 130022, China Received 8 April 2004; received in revised form 2 September 2004; accepted 9 September 2004
Abstract ZnO nanowires, nanorods and nanoparticles through modulating the ratio of water to methanol have been synthesized by using a mild and simple solution method. The as-prepared ZnO nanostructures have been characterized by atomic force microscopy and X-ray photoelectron spectroscopy. With the increase of the ratio of water to methanol, the morphology of ZnO nanostructures varied form denser nanowires, to sparse nanowires, to nanorods, and then to nanoparticles. The ratio of water to methanol is supposed to play an important role in the formation of ZnO nanostructures. The mechanism of formation is related to the chemical potential, which is simply proportional to their surface ratio. © 2004 Elsevier B.V. All rights reserved. Keywords: ZnO nanostructures; Chemical synthesis; Atomic force microscopy; X-ray photoelectron spectroscopy; Chemical potential
1. Introduction Recently, one-dimensional (1D) nanostructures such as wires, rods and tubes have become the focus of intensive research owing to their unique applications in mesoscale physics and fabrication of nanoscale devices [1–6]. The ability to precisely control or tune their dimensions, chemical composition, surface properties, the phase purity and crystal structure have become more and more important for the ulitilization of 1D nanostructures in a broad range of areas [7]. The growth of semiconductor nanostructures has attracted much more interest due to their ability to provide low-dimensional electron confinement. Semiconductor nanowires have exhibited stronger size-dependent properties in electronics and optoelectronics and their 1D nanostructures are liable to be integrated in functional nanodevices [8–11]. Zinc oxide, which is a wide band-gap (3.37 eV at 298 K) semiconductor with a large exciton binding energy (60 meV), has been widely investigated as a short-wavelength ∗
Corresponding author. Tel.: +86 431 526 2057; fax: +86 431 526 2057. E-mail addresses: [email protected], [email protected] (Z. Li).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.09.006
light-emitting transport conductor and piezoelectric material [12–14]. Due to the potential application of ZnO nanostructures in nanoscale optoelectronic devices, it is significant to synthesize these nanostructures. Various chemical, electrochemical and physical deposition techniques have been reported to prepare an oriented structure of ZnO nanorods and nanowires with average diameters typically ranging over an order of magnitude from 20 to 200 nm, and with lengths from several micrometers up to 10 m. For instance, catalytic growth via the vapor–liquid–solid epitaxial mechanism [15], metal–organic chemical vapor deposition [16], pulsed laser deposition [17] and templating against anodic alumina membrane [18] have been successful in creating highly oriented arrays of anisotropic nanorods of ZnO. In this paper, we describe the synthesis of ZnO nanowires, nanorods and nanoparticles through modulating the ratio of water to methanol by using a mild and simple route. The obtained ZnO nanostructures have been characterized by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The influence of the water-to-methanol ratio on the formation of different ZnO nanostructures is discussed and the possible mechanism of formation is also given.
H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
327
2. Experimental details 2.1. Sample preparation All chemical reagents used in our experiments were purchased from Beijing Chemical Engineering Factory. NaOH, Zn(NO3 )2 and methanol were of analytical grade purity. The water used in our work was deionized and distilled. The preparation was carried out by using a solution consisting of Zn(NO3 )2 and NaOH in the presence of methanol. A solution (20 ml) containing 0.1 M Zn(NO3 )2 in methanol was added dropwise and under stirring to 100 ml of 0.1 M NaOH in methanol. The solution was then diluted by adding methanol of 120 ml. Next, different mixtures of methanol and water were added to a certain amount of the above solution, resulting in water-to-methanol ratios of 0.005, 0.006, 0.008 and 0.04 (samples I–IV). Altogether, four kinds of solution were obtained and all stored in refrigerator for 2 h. The stock solution was ready for use.
Fig. 1. AFM image of the as-prepared denser ZnO nanowires.
2.2. Characterization Samples for AFM analysis were prepared by dropping 15 l solution containing the product onto a new piece of cleaved mica and drying in air at room temperature. A Nanoscope IIIa Multimode microscope (Digital Instruments, Santa Barbara, CA) was used to show the morphology of the ZnO nanostructures on a piece of cleaved mica. XPS measurements were made in a Vacuum Generators ESCALab MK II instrument (VG Co., UK), which consists of a dual anode X-ray source, a hemispherical analyzer, and a sample stage with multi-axial adjustability. 300 W Mg X-rays (hυ = 1253.6 eV) were used to excite photoelectrons from the Zn 2p and O 1s core levels.
3. Results and discussion
Fig. 2. AFM image of the as-prepared sparser ZnO nanowires.
were formed, where the ratio of water to methanol was 0.008. However, when the ratio of water to methanol increased to 0.04, ZnO nanoparticles were well distributed on the mica.
3.1. The influence of water-to-methanol ratio on the morphology of the fabricated ZnO The morphology of the as-prepared interconnected twodimensional (2D) nanowire network, sparse nanowires, nanorods and nanoparticles was examined by AFM. AFM images of samples I–IV are shown in Figs. 1–4, respectively. In sample I (Fig. 1), denser nanowires were formed when the ratio of water to methanol was 0.005 and these nanowires finally grew into an interconnected 2D nanowire network. From a section analysis of the AFM image, we know that the diameter of these nanowires is from 9 to 31 nm. In sample II (Fig. 2), the wire density in the network is strongly reduced and sparse nanowires with a uniform diameter of 21 nm and a length up to 2 m were successfully achieved, when the ratio of water to methanol was 0.006. In sample III (Fig. 3), nanorods with a diameter of 33 nm and a length of 766 nm
Fig. 3. AFM image of the as-prepared ZnO nanorods.
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H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
3.2. XPS characterization of the morphology of ZnO
Fig. 4. AFM image of the as-prepared ZnO nanoparticles.
To identify the successful conversion of Zn(NO3 )2 to ZnO, we carried out XPS experiments. Fig. 5 shows the Zn 2p and O 1s core level spectra of Zn(NO3 )2 . The binding energy of the Zn 2p3 component is recorded to be 1023.2 eV. The binding energy of O 1s is 531.8 eV. Fig. 6 shows the Zn 2p and O 1s core level spectra of the as-prepared sample I. From Fig. 6, we know that the binding energy of the Zn 2p3 component is recorded to be 1021.8 eV and the O 1s is 531.8 eV. The shift of Zn 2p3 from 1023.2 to 1021.8 ev demonstrates that the conversion of Zn(NO3 )2 to ZnO was successfully achieved by this simple route. At the same time, Figs. 7–9 show the XPS spectra of samples II–IV, respectively. From the figures, we know that the binding energy of the Zn 2p3 component in samples II–IV is accordingly recorded to be 1021.8, 1021.9 and 1022.6 eV. The conversion from Zn(NO3 )2 to ZnO was
Fig. 5. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from Zn(NO3 )2 .
Fig. 6. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from the as-prepared denser ZnO nanowires.
H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
Fig. 7. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from the as-prepared sparser ZnO nanowires.
Fig. 8. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from the as-prepared ZnO nanorods.
Fig. 9. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from the as-prepared ZnO nanoparticles.
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330
H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
confirmed. From the figures, we can see that binding energy of the Zn 2p component is slightly different due to the different morphology of the fabricated ZnO nanostructures. From the experiment, we know that the ratio of water to methanol plays an important role in the formation of ZnO nanostructures. With the increase of the ratio of water to methanol, the morphology of ZnO nanostructures fabricated varied from an interconnected 2D nanowire network, to sparse nanowires, to nanorods and then to nanoparticle, as shown in Fig. 10. 3.3. The possible mechanism of the formation The possible mechanism is given below. First, we begin to study from Gibbs–Thompson equation [Eq. (1)]. Gibbs–Thompson equation is the basis of classic crystallization theory. Although it may not be accurate enough for a quantitative description of a crystallization process in the nanometer regime, it should provide us with a starting point. 2σVm Sr = Sb exp (1) rRT where r is the radius of the crystal, σ the specific surface energy, Vm the molar volume of the material, Sb and Sr are the solubilities of bulk crystals and crystals with a radius r, respectively, R is the gas constant, and T the absolute temperature. However, in the nanometer regime it is impossible to reach an ultimate chemical equilibrium between the nanocrystal and the monomers at the concentration Sr determined by Eq. (1). A simple mathematical treatment can change Eq. (1) to Eq. (2): 2σVm (2) rRT If µr and µb respresent the chemical potentials of the crystals with a radius r and with an infinite size, Eq. (2) can be converted into Eq. (3): RT ln Sr = RT ln Sb +
2σVm (3) rRT For spherical crystals, the number of the surface atoms and the total atoms should be proportional to the surface area and the volume, respectively. If we define the surface ratio as δ, then
µ r = µb +
δ=
k1 4πr 2 k3 = r (k1 4πr 3 )/3
(4)
where k1 , k2 and k3 are proportional constants. Combining Eqs. (3) and (4), we can get µr ∝ δ
(5)
Eq. (5) indicates that the relative chemical potential of crystals is simply proportional to their surface ratio. This equation should be applicable to crystals with different shapes. The driving force of minimizing the surface energy of a
Fig. 10. Schematic illustration showing the formation of different morphologies of ZnO nanostructrues in our experiment.
given nanocrystal removed those atoms from the relatively higher surface energy position to the lower ones and made the nanocrystal with a relatively smooth surface. When a crystallization system starts by quickly mixing two precursors together, the classical nucleation theory determines the minimum stable size of the nuclei to be formed in the solution using the Gibbs–Thompson equation [Eq. (1)]. In principle, the higher the monomer concentration is, the smaller the critical nuclei can be, the easier the nucleation process can take place. The inevitable formation of critically sized nanoclusters in the nucleation stage for the growth of anisotropic shapes likely plays a key role in determining the nature of all of the following events, and the size/shape of the resulting nanocrystals [19]. The critically sized nuclei deeply impact the following growth process of the elongated nanostructures. A relatively high monomer concentration will promote to grow longer in the following 1D growth. Conversely, a relatively low monomer concentration will promote 2D growth, so bigger nanoparticle is formed [20]. Additionally, water also accelerates the growth of colloidal ZnO particles [21]. Altogether, with an increase of the ratio of water to methanol, namely with the decrease of monomer concentration, it is more liable to form nanoparticles due to the relatively low chemical potential. By simply increasing the precursor concentration, the morphology of ZnO nanocrystals varied from short rods to long rods, and then to nanowires. However, when the rods grew to a certain length in the 1D growth stage, they always turned not symmetric along the c-axis, but with a long-skimmy tail. As shown in Fig. 2, the nanowire is not very uniform, but has a long skimmy tail.
4. Conclusions In summary, we have demonstrated that a simple solution process can be employed to achieve the synthesis of ZnO nanowires, nanorods and nanoparticles through modu-
H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
lating the ratio of water to methanol. AFM and XPS studies have been used to characterize the as-prepared ZnO nanostructures. With an increase of the ratio of water to methanol, namely with the decrease of monomer concentration, the morphology of ZnO nanostructures varied form denser nanowires, to sparse nanowires, to nanorods, and then to nanoparticles. The ratio of water to methanol is supposed to have an important role in the formation of ZnO nanostructures. A relatively high monomer concentration promotes the 1D growth. A relatively low monomer concentration will promote the 2D growth. The mechanism of formation is related to the chemical potential, which is simply proportional to their surface ratio.
Acknowledgement This project was financially supported by the National Natural Science Foundation of China (Grant No. 30070417).
References [1] R.S. Ruoff, Nature 372 (1994) 732. [2] P.M. Ajayan, O. Stephan, P. Redlich, C. Colliex, Nature 375 (1995) 564. [3] Y.K. Chen, M.L.H. Green, S.C. Tsang, Chem. Commun. 21 (1996) 2489.
331
[4] C.R. Martin, Science 266 (1994) 1961. [5] C. Huber, M. Sadoqi, T. Huber, D. Chacko, Adv. Mater. 7 (1995) 316. [6] D. Routkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. B 100 (1996) 14037. [7] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353. [8] J. Yu, S. Chung, J.R. Heath, J. Phys. Chem. B 104 (2000) 11864. [9] J.F. Wang, M.S. Gudiksen, X.F. Duan, Y. Cui, C.M. Lieber, Science 293 (2001) 1455. [10] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [11] J.C. Johnson, H.Q. Yan, R.D. Schaller, P.B. Petersen, P.D. Yang, R.J. Saykally, Nanoletters 2 (2002) 279. [12] 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. [13] H. Cao, J.Y. Xu, E.W. Seelig, R.P.H. Chang, Appl. Phys. Lett. 84 (2000) 2997. [14] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230. [15] M. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [16] S. Liu, J.J. Wu, Mater. Res. Soc. Symp. Proc. 703 (2002) 241. [17] J.H. Choi, H. Tabata, T. Kawai, J. Cryst. Growth 226 (2001) 493. [18] Y. Li, G.W. Meng, L.D. Zhang, F. Phillip, Appl. Phys. Lett. 76 (2000) 2011. [19] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 124 (2002) 3343. [20] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 123 (2001) 1389. [21] L. Spanhel, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 6632.
Synthesis of nanowires, nanorods and nanoparticles of ZnO through modulating the ratio of water to methanol by using a mild and simple solution method Hualan Zhou, Zhuang Li∗ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, Jinlin 130022, China Received 8 April 2004; received in revised form 2 September 2004; accepted 9 September 2004
Abstract ZnO nanowires, nanorods and nanoparticles through modulating the ratio of water to methanol have been synthesized by using a mild and simple solution method. The as-prepared ZnO nanostructures have been characterized by atomic force microscopy and X-ray photoelectron spectroscopy. With the increase of the ratio of water to methanol, the morphology of ZnO nanostructures varied form denser nanowires, to sparse nanowires, to nanorods, and then to nanoparticles. The ratio of water to methanol is supposed to play an important role in the formation of ZnO nanostructures. The mechanism of formation is related to the chemical potential, which is simply proportional to their surface ratio. © 2004 Elsevier B.V. All rights reserved. Keywords: ZnO nanostructures; Chemical synthesis; Atomic force microscopy; X-ray photoelectron spectroscopy; Chemical potential
1. Introduction Recently, one-dimensional (1D) nanostructures such as wires, rods and tubes have become the focus of intensive research owing to their unique applications in mesoscale physics and fabrication of nanoscale devices [1–6]. The ability to precisely control or tune their dimensions, chemical composition, surface properties, the phase purity and crystal structure have become more and more important for the ulitilization of 1D nanostructures in a broad range of areas [7]. The growth of semiconductor nanostructures has attracted much more interest due to their ability to provide low-dimensional electron confinement. Semiconductor nanowires have exhibited stronger size-dependent properties in electronics and optoelectronics and their 1D nanostructures are liable to be integrated in functional nanodevices [8–11]. Zinc oxide, which is a wide band-gap (3.37 eV at 298 K) semiconductor with a large exciton binding energy (60 meV), has been widely investigated as a short-wavelength ∗
Corresponding author. Tel.: +86 431 526 2057; fax: +86 431 526 2057. E-mail addresses: [email protected], [email protected] (Z. Li).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.09.006
light-emitting transport conductor and piezoelectric material [12–14]. Due to the potential application of ZnO nanostructures in nanoscale optoelectronic devices, it is significant to synthesize these nanostructures. Various chemical, electrochemical and physical deposition techniques have been reported to prepare an oriented structure of ZnO nanorods and nanowires with average diameters typically ranging over an order of magnitude from 20 to 200 nm, and with lengths from several micrometers up to 10 m. For instance, catalytic growth via the vapor–liquid–solid epitaxial mechanism [15], metal–organic chemical vapor deposition [16], pulsed laser deposition [17] and templating against anodic alumina membrane [18] have been successful in creating highly oriented arrays of anisotropic nanorods of ZnO. In this paper, we describe the synthesis of ZnO nanowires, nanorods and nanoparticles through modulating the ratio of water to methanol by using a mild and simple route. The obtained ZnO nanostructures have been characterized by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The influence of the water-to-methanol ratio on the formation of different ZnO nanostructures is discussed and the possible mechanism of formation is also given.
H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
327
2. Experimental details 2.1. Sample preparation All chemical reagents used in our experiments were purchased from Beijing Chemical Engineering Factory. NaOH, Zn(NO3 )2 and methanol were of analytical grade purity. The water used in our work was deionized and distilled. The preparation was carried out by using a solution consisting of Zn(NO3 )2 and NaOH in the presence of methanol. A solution (20 ml) containing 0.1 M Zn(NO3 )2 in methanol was added dropwise and under stirring to 100 ml of 0.1 M NaOH in methanol. The solution was then diluted by adding methanol of 120 ml. Next, different mixtures of methanol and water were added to a certain amount of the above solution, resulting in water-to-methanol ratios of 0.005, 0.006, 0.008 and 0.04 (samples I–IV). Altogether, four kinds of solution were obtained and all stored in refrigerator for 2 h. The stock solution was ready for use.
Fig. 1. AFM image of the as-prepared denser ZnO nanowires.
2.2. Characterization Samples for AFM analysis were prepared by dropping 15 l solution containing the product onto a new piece of cleaved mica and drying in air at room temperature. A Nanoscope IIIa Multimode microscope (Digital Instruments, Santa Barbara, CA) was used to show the morphology of the ZnO nanostructures on a piece of cleaved mica. XPS measurements were made in a Vacuum Generators ESCALab MK II instrument (VG Co., UK), which consists of a dual anode X-ray source, a hemispherical analyzer, and a sample stage with multi-axial adjustability. 300 W Mg X-rays (hυ = 1253.6 eV) were used to excite photoelectrons from the Zn 2p and O 1s core levels.
3. Results and discussion
Fig. 2. AFM image of the as-prepared sparser ZnO nanowires.
were formed, where the ratio of water to methanol was 0.008. However, when the ratio of water to methanol increased to 0.04, ZnO nanoparticles were well distributed on the mica.
3.1. The influence of water-to-methanol ratio on the morphology of the fabricated ZnO The morphology of the as-prepared interconnected twodimensional (2D) nanowire network, sparse nanowires, nanorods and nanoparticles was examined by AFM. AFM images of samples I–IV are shown in Figs. 1–4, respectively. In sample I (Fig. 1), denser nanowires were formed when the ratio of water to methanol was 0.005 and these nanowires finally grew into an interconnected 2D nanowire network. From a section analysis of the AFM image, we know that the diameter of these nanowires is from 9 to 31 nm. In sample II (Fig. 2), the wire density in the network is strongly reduced and sparse nanowires with a uniform diameter of 21 nm and a length up to 2 m were successfully achieved, when the ratio of water to methanol was 0.006. In sample III (Fig. 3), nanorods with a diameter of 33 nm and a length of 766 nm
Fig. 3. AFM image of the as-prepared ZnO nanorods.
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H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
3.2. XPS characterization of the morphology of ZnO
Fig. 4. AFM image of the as-prepared ZnO nanoparticles.
To identify the successful conversion of Zn(NO3 )2 to ZnO, we carried out XPS experiments. Fig. 5 shows the Zn 2p and O 1s core level spectra of Zn(NO3 )2 . The binding energy of the Zn 2p3 component is recorded to be 1023.2 eV. The binding energy of O 1s is 531.8 eV. Fig. 6 shows the Zn 2p and O 1s core level spectra of the as-prepared sample I. From Fig. 6, we know that the binding energy of the Zn 2p3 component is recorded to be 1021.8 eV and the O 1s is 531.8 eV. The shift of Zn 2p3 from 1023.2 to 1021.8 ev demonstrates that the conversion of Zn(NO3 )2 to ZnO was successfully achieved by this simple route. At the same time, Figs. 7–9 show the XPS spectra of samples II–IV, respectively. From the figures, we know that the binding energy of the Zn 2p3 component in samples II–IV is accordingly recorded to be 1021.8, 1021.9 and 1022.6 eV. The conversion from Zn(NO3 )2 to ZnO was
Fig. 5. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from Zn(NO3 )2 .
Fig. 6. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from the as-prepared denser ZnO nanowires.
H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
Fig. 7. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from the as-prepared sparser ZnO nanowires.
Fig. 8. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from the as-prepared ZnO nanorods.
Fig. 9. XPS spectra of the Zn 2p (a) and O 1s (b) core levels recorded from the as-prepared ZnO nanoparticles.
329
330
H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
confirmed. From the figures, we can see that binding energy of the Zn 2p component is slightly different due to the different morphology of the fabricated ZnO nanostructures. From the experiment, we know that the ratio of water to methanol plays an important role in the formation of ZnO nanostructures. With the increase of the ratio of water to methanol, the morphology of ZnO nanostructures fabricated varied from an interconnected 2D nanowire network, to sparse nanowires, to nanorods and then to nanoparticle, as shown in Fig. 10. 3.3. The possible mechanism of the formation The possible mechanism is given below. First, we begin to study from Gibbs–Thompson equation [Eq. (1)]. Gibbs–Thompson equation is the basis of classic crystallization theory. Although it may not be accurate enough for a quantitative description of a crystallization process in the nanometer regime, it should provide us with a starting point. 2σVm Sr = Sb exp (1) rRT where r is the radius of the crystal, σ the specific surface energy, Vm the molar volume of the material, Sb and Sr are the solubilities of bulk crystals and crystals with a radius r, respectively, R is the gas constant, and T the absolute temperature. However, in the nanometer regime it is impossible to reach an ultimate chemical equilibrium between the nanocrystal and the monomers at the concentration Sr determined by Eq. (1). A simple mathematical treatment can change Eq. (1) to Eq. (2): 2σVm (2) rRT If µr and µb respresent the chemical potentials of the crystals with a radius r and with an infinite size, Eq. (2) can be converted into Eq. (3): RT ln Sr = RT ln Sb +
2σVm (3) rRT For spherical crystals, the number of the surface atoms and the total atoms should be proportional to the surface area and the volume, respectively. If we define the surface ratio as δ, then
µ r = µb +
δ=
k1 4πr 2 k3 = r (k1 4πr 3 )/3
(4)
where k1 , k2 and k3 are proportional constants. Combining Eqs. (3) and (4), we can get µr ∝ δ
(5)
Eq. (5) indicates that the relative chemical potential of crystals is simply proportional to their surface ratio. This equation should be applicable to crystals with different shapes. The driving force of minimizing the surface energy of a
Fig. 10. Schematic illustration showing the formation of different morphologies of ZnO nanostructrues in our experiment.
given nanocrystal removed those atoms from the relatively higher surface energy position to the lower ones and made the nanocrystal with a relatively smooth surface. When a crystallization system starts by quickly mixing two precursors together, the classical nucleation theory determines the minimum stable size of the nuclei to be formed in the solution using the Gibbs–Thompson equation [Eq. (1)]. In principle, the higher the monomer concentration is, the smaller the critical nuclei can be, the easier the nucleation process can take place. The inevitable formation of critically sized nanoclusters in the nucleation stage for the growth of anisotropic shapes likely plays a key role in determining the nature of all of the following events, and the size/shape of the resulting nanocrystals [19]. The critically sized nuclei deeply impact the following growth process of the elongated nanostructures. A relatively high monomer concentration will promote to grow longer in the following 1D growth. Conversely, a relatively low monomer concentration will promote 2D growth, so bigger nanoparticle is formed [20]. Additionally, water also accelerates the growth of colloidal ZnO particles [21]. Altogether, with an increase of the ratio of water to methanol, namely with the decrease of monomer concentration, it is more liable to form nanoparticles due to the relatively low chemical potential. By simply increasing the precursor concentration, the morphology of ZnO nanocrystals varied from short rods to long rods, and then to nanowires. However, when the rods grew to a certain length in the 1D growth stage, they always turned not symmetric along the c-axis, but with a long-skimmy tail. As shown in Fig. 2, the nanowire is not very uniform, but has a long skimmy tail.
4. Conclusions In summary, we have demonstrated that a simple solution process can be employed to achieve the synthesis of ZnO nanowires, nanorods and nanoparticles through modu-
H. Zhou, Z. Li / Materials Chemistry and Physics 89 (2005) 326–331
lating the ratio of water to methanol. AFM and XPS studies have been used to characterize the as-prepared ZnO nanostructures. With an increase of the ratio of water to methanol, namely with the decrease of monomer concentration, the morphology of ZnO nanostructures varied form denser nanowires, to sparse nanowires, to nanorods, and then to nanoparticles. The ratio of water to methanol is supposed to have an important role in the formation of ZnO nanostructures. A relatively high monomer concentration promotes the 1D growth. A relatively low monomer concentration will promote the 2D growth. The mechanism of formation is related to the chemical potential, which is simply proportional to their surface ratio.
Acknowledgement This project was financially supported by the National Natural Science Foundation of China (Grant No. 30070417).
References [1] R.S. Ruoff, Nature 372 (1994) 732. [2] P.M. Ajayan, O. Stephan, P. Redlich, C. Colliex, Nature 375 (1995) 564. [3] Y.K. Chen, M.L.H. Green, S.C. Tsang, Chem. Commun. 21 (1996) 2489.
331
[4] C.R. Martin, Science 266 (1994) 1961. [5] C. Huber, M. Sadoqi, T. Huber, D. Chacko, Adv. Mater. 7 (1995) 316. [6] D. Routkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. B 100 (1996) 14037. [7] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353. [8] J. Yu, S. Chung, J.R. Heath, J. Phys. Chem. B 104 (2000) 11864. [9] J.F. Wang, M.S. Gudiksen, X.F. Duan, Y. Cui, C.M. Lieber, Science 293 (2001) 1455. [10] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [11] J.C. Johnson, H.Q. Yan, R.D. Schaller, P.B. Petersen, P.D. Yang, R.J. Saykally, Nanoletters 2 (2002) 279. [12] 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. [13] H. Cao, J.Y. Xu, E.W. Seelig, R.P.H. Chang, Appl. Phys. Lett. 84 (2000) 2997. [14] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230. [15] M. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [16] S. Liu, J.J. Wu, Mater. Res. Soc. Symp. Proc. 703 (2002) 241. [17] J.H. Choi, H. Tabata, T. Kawai, J. Cryst. Growth 226 (2001) 493. [18] Y. Li, G.W. Meng, L.D. Zhang, F. Phillip, Appl. Phys. Lett. 76 (2000) 2011. [19] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 124 (2002) 3343. [20] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 123 (2001) 1389. [21] L. Spanhel, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 6632.