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Multi-capping agents in size confinement of ZnO nanostructured particles
Optical Materials 31 (2009) 1570–1574
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Multi-capping agents in size confinement of ZnO nanostructured particles N. Rajeswari Yogamalar, R. Srinivasan, A. Chandra Bose * Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620015, India
a r t i c l e
i n f o
Article history: Received 20 September 2008 Received in revised form 4 March 2009 Accepted 4 March 2009 Available online 8 April 2009 PACS: 78.66.J 61.46 81.05.Y Keywords: ZnO nanoparticles Capping agents PVP CA
a b s t r a c t In the present paper, we report a new approach to synthesize crystalline zinc oxide (ZnO) nanoparticles in the presence of multi-capping agents namely poly-vinylpyrrolidone (PVP) and citric acid (CA), with zinc acetate dihydrate and sodium hydroxide (NaOH as pellets) as a source material and their characteristic studies. The ZnO nanoparticles grown under this simple chemical process involve a heterogeneous chemical reaction in the presence of water as a solvent medium and reaction temperature of 100 °C for 48 h in a closed environment. The structural, optical and chemical features of ZnO nanoparticles were systematically studied by X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and Ultra Violet–visible (UV–vis) absorption spectroscopy. The Williamson–Hall (W–H) plot was also performed to distinguish the effect of crystalline size-induced broadening and strain-induced broadening at Full Width Half Maximum (FWHM) of the XRD profile. The growth mechanism of ZnO nanoparticles and the effective capping mechanism shown by each capping agent are briefly discussed. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction ZnO has received much attention over the past few years because it has a wide range of properties that depends on doping, including a range of conductivity from metallic to insulating (including n-type and p-type conductivity), high transparency, piezoelectricity, room-temperature ferromagnetism, enhanced magneto-optic and chemical-sensing effects. One of the prominent materials in the metal-oxide family, nanostructured ZnO has been intensely studied for its versatile physical properties and promising potential for electronics in particular optoelectronics and piezoelectric applications [1,2]. ZnO is a wide band gap (Eg = 3.37 eV) II–VI semiconductor compound which has a non-centrosymmetric wurtzite structure with polar surfaces. ZnO nanoparticles are frequently studied because of their interest in fundamental study and their applied aspects in solar energy conversion, photocatalysis, light-emitting materials, transparent UV protection films and chemical sensors [3]. As a result, searching for new methodology to synthesize uniform ZnO nanoparticles is still of great importance for both fundamental study and application. Various chemical and physical methods have been developed for the synthesis of ZnO nanoparticles [4,5]. Here, we employ a simple, cost effective, low temperature 100 °C chemical process with water as a solvent. To promote a con* Corresponding author. Tel.: +91 9444065746. E-mail address: [email protected] (A.C. Bose). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.03.002
fined and stable growth of ZnO nanoparticles, the capping molecules PVP and CA are used in the reaction process. It is indicated that the addition of capping molecule could greatly influence the size confinement of ZnO nanostructures, which in turn affects the physical properties. A significant variation in the size of the particle is noted with the addition of capping molecules. However, the other parameters such as reaction temperature, pH value and time also have an influence on the growth of the ZnO nanoparticles. Finally, the possible growth and capping mechanism are discussed briefly [6,7]. 2. Experimental procedure 2.1. Chemical synthesis method The following reagents were used: Zinc acetate dihydrate Zn(CH3COO)2 2H2O, sodium hydroxide NaOH pellets, poly-vinylpyrrolidone (PVP) and citric acid (CA). Throughout, the experiment double distilled water was used for aqueous solution preparation. A stoichiometric ratio of PVP is dissolved in 100 ml distilled water with the slow addition of zinc acetate to the solution. The resulting reaction mixture was constantly stirred for several minutes followed by the dropwise addition of NaOH pellets in the molar ratio of Zn2+/OH = 0.09 and a turbid white precipitate were formed about 10 min later. The mixture is then transferred immediately to a beaker sealed with an aluminium foil for chemical treatment to a temperature of 100 °C for 48 h in a hot air oven.
N.R. Yogamalar et al. / Optical Materials 31 (2009) 1570–1574
After the reaction process, the beaker containing the sample is allowed to cool down to room temperature naturally. The white precipitate deposited at the bottom of the beaker is collected and washed with distilled water and ethanol several times in order to remove the impurity ions as well as the capping molecules (PVP and CA). Finally, ZnO samples are obtained by centrifugation and dehydration of the precipitate at 60 °C. The above said entire process is repeated for the synthesis of ZnO nanoparticles with multicapping agents involving the mixture of two capping molecules namely PVP and CA with molar ratio of CA/PVP = 1:2 in weight. 2.2. Analysis and measurement XRD analyses were performed by a Rigaku diffractometer using Cu Ka1 radiation (1.5406 Å) with 2h ranging from 25° to 65°. The particle morphology was characterized by SEM operated at an accelerating voltage of 20 kV. UV–vis absorption spectra of ZnO nanoparticles dispersed in 2-propanol solutions were obtained by Shimadzu UV-1700 Pharma Spectro Photometer. FT-IR spectrum of the powders (as pellets in KBr) was recorded using Perkin–Elmer Spectrum One FT-IR spectrometer in the range of 4000–400 cm1. 3. Results and discussion This synthesis method is based on a simple chemical treatment with and without the presence of capping agents (CA and PVP) and with water as a reaction solvent. 3.1. Growth mechanism The main zinc species in the solution are ZnO OH, ZnðOHÞ2 4 , ZnO 2 and their concentrations depend on the OH concentration
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(pH value) and temperature. The major quantity of OH concentration derives from the basic mineralizers and also from autoprotolysis of water.
2H2 O ! H3 Oþ þ OH The driving force required to enhance the growth process comes in the solution which from the high concentration of ZnðOHÞ2 4 builds up the super saturation necessary to trigger nucleation on the ZnO seed crystal [8–10]. Possible reaction leading to the growth of ZnO comprises of ZnðOHÞ2 4 ! ZnðOHÞ2 þ 2OH
ZnðOHÞ2 þ 2OH ! ZnO2 2 þ 2H2 O ZnO2 2 þ H2 O ! ZnO þ 2OH
washing with ethanol and drying at low temperature were very important in converting Zn(OH)2 into ZnO as well as for the removal of organic impurities [11]. 3.2. Particle morphology and capping mechanism Fig. 1a–c shows the SEM micrographs capped ZnO nanoparticles grown under simple chemical treatment. It was found that the individually capped ZnO nanoparticles have a better morphology than the multi-capped one. The amount of capping molecules (PVP and CA) used in the solution is an important factor for determining the morphology [12]. PVP is a large polymer molecule (molecular weight = 40,000) although, it could form a sort of capping on the surface of ZnO nanoparticles, they formed a large particle growth as seen in Fig. 1a. All these particles are resulted from
Fig. 1. SEM micrographs of the products prepared by capping ZnO with (a) PVP, (b) CA and (c) PVP-CA.
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the effect of stacking and agglomeration of initial single crystal particles based on the growth conditions and parameters. When polymeric PVP capping molecule is added to the reaction mixture, they simply attaches to the surface of growing particles and by either steric or electrostatic repulsion prevents the further growth of the particles. For the role of PVP, the surface regulating polymer is believed to play a key role in preventing the flocculation of particles, controlling the particle size and its morphology [13,14]. PVP that contains nitrogen (N) and oxygen (O) easily attaches to the surface of ZnO and slows the growth speed of the crystal facets by reducing the surface free energy. As a kinetic controller, PVP adsorbed on specific crystalline surfaces could significantly decrease their growth rate and lead to nanosized particles. ZnO nanoparticles synthesized with CA as a capping molecule (Fig. 1b) shows a plate like structure with varying size and distribution. The CA is an anionic surfactant agent and it is used as a reducing agent in this chemical synthesis. When the CA is added the long chain of CA entangles with each other and hider the carboxylic group (ACOOH) of the CA to react with the hydroxide group on the surface of ZnO. Due to the acidity of CA more NaOH solution is added until a required pH is attained which forms the basis for the synthesis of monodispersed particles. In Fig. 1c the ZnO nanoparticles synthesized in the presence of multi-capping agent is shown. The addition of CA along with PVP to the solution of growing particles prevents further growth by covalently binding to the particle surface. PVP are commonly used to control the size by controlling the growth process and by restricting the reaction space and CA are used for obtaining narrow size distribution of ZnO nanoparticles. But, when both the capping molecules are added simultaneously, some form of reaction hindrance arise resulting in anisotropic shaped particles with less particle size.
lattice spacing for the hexagonal structure at (1 0 1) plane is 1 4 1 1 ¼ where, davg = average crystalline size, k = wave2 2 þ c2 3 a d 101
length of incident X-ray beam (1.5406 Å), h = scattering angle in degree, and b = FWHM in radians. Fig. 3 shows the variation of the lattice parameter (‘a’ and ‘c’) and unit cell volume V with the addition of capping molecules. There is a broadening observed at FWHM of the XRD pattern with the addition of capping agents. This broadening may be the result of the crystalline size-induced broadening or strain-induced broadening. This was distinguished from W–H plot as shown in Fig. 4. The crystalline size and strain can be obtained from the intercept
3.3. Crystal structure and size White powders were obtained from the simple chemical-route synthesis and analyzed using XRD. Fig. 2 shows the XRD pattern for uncapped, PVP capped, CA capped and multi-capped PVP-CA ZnO nanoparticles. All the diffraction peaks can be well indexed to a hexagonal phase ZnO (a = 3.253 Å; c = 5.213 Å). The obtained ZnO samples are of wurtzite structure [15]. The results indicate that the products consist of pure phase and there is no impurity reflection peaks. The sharp peaks indicate that the products were well crystallized and oriented. The average size of the ZnO particle and the crystal is measured by Debye–Scherer formula davg ¼ b0:9k cos h Fig. 3. (a) Variation of lattice parameter ‘a’ and ‘c’ with the crystalline size of ZnO. (b) Plot of unit cell volume V vs. crystalline size of ZnO particles.
Fig. 2. XRD patterns of wurtzite ZnO nanoparticles with and without capping molecules.
Fig. 4. Williamson–Hall plots for uncapped and capped ZnO nanoparticles.
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N.R. Yogamalar et al. / Optical Materials 31 (2009) 1570–1574 Table 1 Various parameter measurement of uncapped and capped ZnO nanopowders from XRD profile. Sample
a (Å)
c (Å)
c/a
V (Å)3
2h (°)
b (FWHM)
Measured d1 0 1 (Å)
Observed d1 0 1 (Å)
davg (nm)
W–H plot (nm)
Strain (no unit)
Uncapped ZnO CA PVP PVP-CA
3.2481 3.2412 3.2446 3.2498
5.2045 5.1798 5.1862 5.196
1.6023 1.598 1.5984 1.5989
47.552 47.123 47.282 47.525
36.266 36.425 36.382 36.301
0.33 0.339 0.379 0.496
2.4746 2.4639 2.4705 2.4747
2.4751 2.4646 2.4675 2.4727
25 24 22 17
47 43 37 34
0.00145 0.00113 0.00076 0.00072
þ ð4 sin hÞ at the y-axis and slope, respectively [16]. b cos h ¼ 0:9k davg where, b is FWHM in radians, k is the X-ray wavelength in Å (1.5406 Å) and e is the strain-induced on the particle [17]. The grain size and strain of the as-grown ZnO nanoparticles are found to vary from 34 nm to 47 nm and 0.00072 to 0.00145, respectively. It has been referred that the addition of high molecular weight capping agent to the growing sample will contribute some strain as they cap over the ZnO nuclei. However, as seen from the W–H plot, the strain values are very small and thus, their effect on broadening is negligible. The estimated average size of the particle and the measured properties from XRD are tabulated in Table 1. From the table it is clear that the addition of capping molecule confines the particle size to a greater extent. The crystal size obtained from uncapped ZnO nanopowder is bigger than the capped crystals. Therefore, capping agent is an important one to control the crystal growth. 3.4. Functional group analysis
from 295 nm to 272 nm as the particle size decreases. An optical energy band gap was determined from the energy dependence of the absorption coefficient a(m). The absorbance A of the sample is given by the formula A ¼ 2:303log10 II0 where, I0 is the transmitted light intensities, I is the incident light intensities, respectively at the particular wavelength. The absorption coefficient a(m) was calculated from the absorbance A. a(m) is proportional to (2.303 103 Aq)/CL, where A is the absorbance of the sample, q is the density of bulk ZnO, C is the concentration of the particles and L is the optical path length in cm [21]. The band gap relation is given by a(m)hm = C(hm Eg)n where, h(m) is the photon energy, Eg is the optical energy band gap, C is a constant and n is an index which takes the value depending on the type of transition responsible for the absorption. For direct transition, n = 1/2 or 2/3 and the former value was found to be more suitable for ZnO. In Fig. 6 the relationship between (ahm)2 and hm is plotted. The value of the optical band gap Eg is obtained graphically by the intersection of the extrapolated line from the
The presence of various chemical functional groups and the formation of ZnO nanoparticles are supported by the FT-IR spectra as shown in Fig. 5. The broad absorption bands at 3420 and 1630 cm1 encompass the OAH stretching vibrations of absorbed water on the ZnO surface. The strong absorption band between 500 and 450 cm1 can be attributed to the stretching modes of ZnAO [18]. Moreover, no absorption bands of PVP (1677 cm1) and CA (carboxylate group at 1580 cm1) can be detected as they are completely washed out at the time of synthesis. But, a small stretching vibration of C@O corresponding to acetate group is found around 1500 to 1600 cm1. The FT-IR spectra obtained for the uncapped and capped ZnO particles are more or less similar (all the spectra are not shown). 3.5. Optical absorption Fig. 6 shows the absorption spectra for the uncapped and capped ZnO nanoparticles [19,20]. Here, the absorption peak shifts
Fig. 5. FT-IR spectra for multi-capped PVP–CA ZnO nanoparticles.
Fig. 6. (a) UV–vis absorption spectra at room temperature of ZnO nanoparticles. (b) Plot of linear portion of (ahm)2 vs. Eg for ZnO nanoparticles and inset shows the variation of energy band gap with crystalline size.
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Table 2 Estimated optical bang gap energies (Eg) of ZnO nanoparticles as a function of particle size obtained at 100 °C for 48 h. Sample
Crystalline size obtained from XRD (nm)
Estimated optical energy band gap Eg (eV)
Uncapped ZnO Capped with PVP Capped with PVP-CA
25 22 17
3.865 3.895 4.001
Acknowledgements The authors would like to thank Dr. M. Chidambaram, Director, NIT, Tiruchirappalli for his valuable support and constant encouragement. The authors thank Dr. M. Ashok, NIT, Tiruchirappalli for UV–vis absorption study. The research was supported by Seed Money for new faculties, from NIT, Tiruchirappalli and DST project (SR/FTP/ETA – 31/07). References
linear part of the curves on the photon energy axis. The optical energy band gap values obtained are summarized in Table 2. As the particle size reduces from 25 to 16 nm, the optical energy band gap increased from 3.86 to 4.00 eV. 4. Conclusion The synthesis and characterization including structural and optical study of nanosized ZnO has been performed. The results show that there is a size confinement with a change in concentration ratio of Zn2+/PVP/CA. PVP and CA are used to cap and confine the particle size. XRD revealed the particle sizes were in the range of 25 nm for uncapped, 22 nm for the PVP capped, nm for CA capped and 16 nm for multi-capped PVP-CA ZnO nanoparticles. FT-IR shows the absence of absorption bands concerned with PVP and CA. Growth mechanism of ZnO nanostructures is mainly due to the driving force of high concentration of ZnðOHÞ2 4 is reported. The results demonstrate that the UV–vis absorption spectra obtained at shorter wavelength indicates a much smaller particle size and strong particle size confinement.
[1] W.K. Hong, J.I. Sohn, D.K. Hwang, S.S. Kwon, G. Jo, S. Song, S.M. Kim, H.J. Ko, S.J. Park, T. Lee, Nano Lett. 8 (2008) 950. [2] T. Busgen, M. Hilgendorff, S. Irsen, F. Wilhelm, A. Rogalev, D. Goll, M. Giersig, J. Phys. Chem. C 112 (2008) 2412. [3] P.K. Shin, Y. Aya, T. Ikegami, K. Ebihara, Thin Solid Films 516 (2008) 3767. [4] J.G. Lu, P. Chang, Z. Fan, Mater. Sci. Eng. R 52 (2006) 49. [5] S. Somiya, R. Roy, Bull. Mater. Sci. 23 (2000) 453. [6] F. Bai, P. He, Z. Jia, X. Huang, Mater. Lett. 59 (2005) 1687. [7] M. Guo, P. Diao, S. Cai, J. Solid State Chem. 178 (2005) 1864. [8] Y. Tong, Y. Liu, L. Dong, D. Zhao, J. Phys. Chem. B 110 (2006) 20263. [9] D. Wang, C. Song, J. Phys. Chem. B 109 (2005) 12697. [10] H.Y. Xu, H.W. YongCai, W.L. He, Ceram. Int. 30 (2004) 93. [11] G. Kwak, K. Yong, J. Phys. Chem. C 112 (2008) 3036. [12] J.H. Lee, K. Kamada, J. Colloid Interface Sci. 316 (2007) 87. [13] Z. Ajji, Nucl. Instrum. Methods Phys. Res. B 265 (2007) 179. [14] C. Wang, E. Shen, E. Wang, L. Gao, Mater. Lett. 59 (2005) 2867. [15] N. Leopt, M.K. Bael, H. Van, Mater. Lett. 61 (2007) 2624. [16] S.T. Tan, B.J. Chen, X.W. Sun, H.S. Kwok, X.H. Zhang, S.J. Chua, W.J. Fan, J. Appl. Phys. 98 (2005) 013505. [17] M.F. Ogawa, Y. Natsume, J. Mater. Sci. Lett. 9 (1990) 1354. [18] D.C. Agarwal, Nucl. Instrum. Methods Phys. Res. B 244 (2006) 136. [19] Y.H. Ni, X.W. Wei, J. Hong, Y. Ye, Mater. Sci. Eng. B 121 (2005) 42. [20] L. Guo, J.X. Cheng, X.Y. Li, Y.J. Yan, S.H. Yang, Mater. Sci. Eng. C 16 (2001) 123. [21] F. Gu, S.F. Wang, M.K. Lu, G.J. Zhou, D. Xu, D.R. Yuan, J. Phys. Chem. B 108 (2004) 8119.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Multi-capping agents in size confinement of ZnO nanostructured particles N. Rajeswari Yogamalar, R. Srinivasan, A. Chandra Bose * Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620015, India
a r t i c l e
i n f o
Article history: Received 20 September 2008 Received in revised form 4 March 2009 Accepted 4 March 2009 Available online 8 April 2009 PACS: 78.66.J 61.46 81.05.Y Keywords: ZnO nanoparticles Capping agents PVP CA
a b s t r a c t In the present paper, we report a new approach to synthesize crystalline zinc oxide (ZnO) nanoparticles in the presence of multi-capping agents namely poly-vinylpyrrolidone (PVP) and citric acid (CA), with zinc acetate dihydrate and sodium hydroxide (NaOH as pellets) as a source material and their characteristic studies. The ZnO nanoparticles grown under this simple chemical process involve a heterogeneous chemical reaction in the presence of water as a solvent medium and reaction temperature of 100 °C for 48 h in a closed environment. The structural, optical and chemical features of ZnO nanoparticles were systematically studied by X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and Ultra Violet–visible (UV–vis) absorption spectroscopy. The Williamson–Hall (W–H) plot was also performed to distinguish the effect of crystalline size-induced broadening and strain-induced broadening at Full Width Half Maximum (FWHM) of the XRD profile. The growth mechanism of ZnO nanoparticles and the effective capping mechanism shown by each capping agent are briefly discussed. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction ZnO has received much attention over the past few years because it has a wide range of properties that depends on doping, including a range of conductivity from metallic to insulating (including n-type and p-type conductivity), high transparency, piezoelectricity, room-temperature ferromagnetism, enhanced magneto-optic and chemical-sensing effects. One of the prominent materials in the metal-oxide family, nanostructured ZnO has been intensely studied for its versatile physical properties and promising potential for electronics in particular optoelectronics and piezoelectric applications [1,2]. ZnO is a wide band gap (Eg = 3.37 eV) II–VI semiconductor compound which has a non-centrosymmetric wurtzite structure with polar surfaces. ZnO nanoparticles are frequently studied because of their interest in fundamental study and their applied aspects in solar energy conversion, photocatalysis, light-emitting materials, transparent UV protection films and chemical sensors [3]. As a result, searching for new methodology to synthesize uniform ZnO nanoparticles is still of great importance for both fundamental study and application. Various chemical and physical methods have been developed for the synthesis of ZnO nanoparticles [4,5]. Here, we employ a simple, cost effective, low temperature 100 °C chemical process with water as a solvent. To promote a con* Corresponding author. Tel.: +91 9444065746. E-mail address: [email protected] (A.C. Bose). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.03.002
fined and stable growth of ZnO nanoparticles, the capping molecules PVP and CA are used in the reaction process. It is indicated that the addition of capping molecule could greatly influence the size confinement of ZnO nanostructures, which in turn affects the physical properties. A significant variation in the size of the particle is noted with the addition of capping molecules. However, the other parameters such as reaction temperature, pH value and time also have an influence on the growth of the ZnO nanoparticles. Finally, the possible growth and capping mechanism are discussed briefly [6,7]. 2. Experimental procedure 2.1. Chemical synthesis method The following reagents were used: Zinc acetate dihydrate Zn(CH3COO)2 2H2O, sodium hydroxide NaOH pellets, poly-vinylpyrrolidone (PVP) and citric acid (CA). Throughout, the experiment double distilled water was used for aqueous solution preparation. A stoichiometric ratio of PVP is dissolved in 100 ml distilled water with the slow addition of zinc acetate to the solution. The resulting reaction mixture was constantly stirred for several minutes followed by the dropwise addition of NaOH pellets in the molar ratio of Zn2+/OH = 0.09 and a turbid white precipitate were formed about 10 min later. The mixture is then transferred immediately to a beaker sealed with an aluminium foil for chemical treatment to a temperature of 100 °C for 48 h in a hot air oven.
N.R. Yogamalar et al. / Optical Materials 31 (2009) 1570–1574
After the reaction process, the beaker containing the sample is allowed to cool down to room temperature naturally. The white precipitate deposited at the bottom of the beaker is collected and washed with distilled water and ethanol several times in order to remove the impurity ions as well as the capping molecules (PVP and CA). Finally, ZnO samples are obtained by centrifugation and dehydration of the precipitate at 60 °C. The above said entire process is repeated for the synthesis of ZnO nanoparticles with multicapping agents involving the mixture of two capping molecules namely PVP and CA with molar ratio of CA/PVP = 1:2 in weight. 2.2. Analysis and measurement XRD analyses were performed by a Rigaku diffractometer using Cu Ka1 radiation (1.5406 Å) with 2h ranging from 25° to 65°. The particle morphology was characterized by SEM operated at an accelerating voltage of 20 kV. UV–vis absorption spectra of ZnO nanoparticles dispersed in 2-propanol solutions were obtained by Shimadzu UV-1700 Pharma Spectro Photometer. FT-IR spectrum of the powders (as pellets in KBr) was recorded using Perkin–Elmer Spectrum One FT-IR spectrometer in the range of 4000–400 cm1. 3. Results and discussion This synthesis method is based on a simple chemical treatment with and without the presence of capping agents (CA and PVP) and with water as a reaction solvent. 3.1. Growth mechanism The main zinc species in the solution are ZnO OH, ZnðOHÞ2 4 , ZnO 2 and their concentrations depend on the OH concentration
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(pH value) and temperature. The major quantity of OH concentration derives from the basic mineralizers and also from autoprotolysis of water.
2H2 O ! H3 Oþ þ OH The driving force required to enhance the growth process comes in the solution which from the high concentration of ZnðOHÞ2 4 builds up the super saturation necessary to trigger nucleation on the ZnO seed crystal [8–10]. Possible reaction leading to the growth of ZnO comprises of ZnðOHÞ2 4 ! ZnðOHÞ2 þ 2OH
ZnðOHÞ2 þ 2OH ! ZnO2 2 þ 2H2 O ZnO2 2 þ H2 O ! ZnO þ 2OH
washing with ethanol and drying at low temperature were very important in converting Zn(OH)2 into ZnO as well as for the removal of organic impurities [11]. 3.2. Particle morphology and capping mechanism Fig. 1a–c shows the SEM micrographs capped ZnO nanoparticles grown under simple chemical treatment. It was found that the individually capped ZnO nanoparticles have a better morphology than the multi-capped one. The amount of capping molecules (PVP and CA) used in the solution is an important factor for determining the morphology [12]. PVP is a large polymer molecule (molecular weight = 40,000) although, it could form a sort of capping on the surface of ZnO nanoparticles, they formed a large particle growth as seen in Fig. 1a. All these particles are resulted from
Fig. 1. SEM micrographs of the products prepared by capping ZnO with (a) PVP, (b) CA and (c) PVP-CA.
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the effect of stacking and agglomeration of initial single crystal particles based on the growth conditions and parameters. When polymeric PVP capping molecule is added to the reaction mixture, they simply attaches to the surface of growing particles and by either steric or electrostatic repulsion prevents the further growth of the particles. For the role of PVP, the surface regulating polymer is believed to play a key role in preventing the flocculation of particles, controlling the particle size and its morphology [13,14]. PVP that contains nitrogen (N) and oxygen (O) easily attaches to the surface of ZnO and slows the growth speed of the crystal facets by reducing the surface free energy. As a kinetic controller, PVP adsorbed on specific crystalline surfaces could significantly decrease their growth rate and lead to nanosized particles. ZnO nanoparticles synthesized with CA as a capping molecule (Fig. 1b) shows a plate like structure with varying size and distribution. The CA is an anionic surfactant agent and it is used as a reducing agent in this chemical synthesis. When the CA is added the long chain of CA entangles with each other and hider the carboxylic group (ACOOH) of the CA to react with the hydroxide group on the surface of ZnO. Due to the acidity of CA more NaOH solution is added until a required pH is attained which forms the basis for the synthesis of monodispersed particles. In Fig. 1c the ZnO nanoparticles synthesized in the presence of multi-capping agent is shown. The addition of CA along with PVP to the solution of growing particles prevents further growth by covalently binding to the particle surface. PVP are commonly used to control the size by controlling the growth process and by restricting the reaction space and CA are used for obtaining narrow size distribution of ZnO nanoparticles. But, when both the capping molecules are added simultaneously, some form of reaction hindrance arise resulting in anisotropic shaped particles with less particle size.
lattice spacing for the hexagonal structure at (1 0 1) plane is 1 4 1 1 ¼ where, davg = average crystalline size, k = wave2 2 þ c2 3 a d 101
length of incident X-ray beam (1.5406 Å), h = scattering angle in degree, and b = FWHM in radians. Fig. 3 shows the variation of the lattice parameter (‘a’ and ‘c’) and unit cell volume V with the addition of capping molecules. There is a broadening observed at FWHM of the XRD pattern with the addition of capping agents. This broadening may be the result of the crystalline size-induced broadening or strain-induced broadening. This was distinguished from W–H plot as shown in Fig. 4. The crystalline size and strain can be obtained from the intercept
3.3. Crystal structure and size White powders were obtained from the simple chemical-route synthesis and analyzed using XRD. Fig. 2 shows the XRD pattern for uncapped, PVP capped, CA capped and multi-capped PVP-CA ZnO nanoparticles. All the diffraction peaks can be well indexed to a hexagonal phase ZnO (a = 3.253 Å; c = 5.213 Å). The obtained ZnO samples are of wurtzite structure [15]. The results indicate that the products consist of pure phase and there is no impurity reflection peaks. The sharp peaks indicate that the products were well crystallized and oriented. The average size of the ZnO particle and the crystal is measured by Debye–Scherer formula davg ¼ b0:9k cos h Fig. 3. (a) Variation of lattice parameter ‘a’ and ‘c’ with the crystalline size of ZnO. (b) Plot of unit cell volume V vs. crystalline size of ZnO particles.
Fig. 2. XRD patterns of wurtzite ZnO nanoparticles with and without capping molecules.
Fig. 4. Williamson–Hall plots for uncapped and capped ZnO nanoparticles.
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N.R. Yogamalar et al. / Optical Materials 31 (2009) 1570–1574 Table 1 Various parameter measurement of uncapped and capped ZnO nanopowders from XRD profile. Sample
a (Å)
c (Å)
c/a
V (Å)3
2h (°)
b (FWHM)
Measured d1 0 1 (Å)
Observed d1 0 1 (Å)
davg (nm)
W–H plot (nm)
Strain (no unit)
Uncapped ZnO CA PVP PVP-CA
3.2481 3.2412 3.2446 3.2498
5.2045 5.1798 5.1862 5.196
1.6023 1.598 1.5984 1.5989
47.552 47.123 47.282 47.525
36.266 36.425 36.382 36.301
0.33 0.339 0.379 0.496
2.4746 2.4639 2.4705 2.4747
2.4751 2.4646 2.4675 2.4727
25 24 22 17
47 43 37 34
0.00145 0.00113 0.00076 0.00072
þ ð4 sin hÞ at the y-axis and slope, respectively [16]. b cos h ¼ 0:9k davg where, b is FWHM in radians, k is the X-ray wavelength in Å (1.5406 Å) and e is the strain-induced on the particle [17]. The grain size and strain of the as-grown ZnO nanoparticles are found to vary from 34 nm to 47 nm and 0.00072 to 0.00145, respectively. It has been referred that the addition of high molecular weight capping agent to the growing sample will contribute some strain as they cap over the ZnO nuclei. However, as seen from the W–H plot, the strain values are very small and thus, their effect on broadening is negligible. The estimated average size of the particle and the measured properties from XRD are tabulated in Table 1. From the table it is clear that the addition of capping molecule confines the particle size to a greater extent. The crystal size obtained from uncapped ZnO nanopowder is bigger than the capped crystals. Therefore, capping agent is an important one to control the crystal growth. 3.4. Functional group analysis
from 295 nm to 272 nm as the particle size decreases. An optical energy band gap was determined from the energy dependence of the absorption coefficient a(m). The absorbance A of the sample is given by the formula A ¼ 2:303log10 II0 where, I0 is the transmitted light intensities, I is the incident light intensities, respectively at the particular wavelength. The absorption coefficient a(m) was calculated from the absorbance A. a(m) is proportional to (2.303 103 Aq)/CL, where A is the absorbance of the sample, q is the density of bulk ZnO, C is the concentration of the particles and L is the optical path length in cm [21]. The band gap relation is given by a(m)hm = C(hm Eg)n where, h(m) is the photon energy, Eg is the optical energy band gap, C is a constant and n is an index which takes the value depending on the type of transition responsible for the absorption. For direct transition, n = 1/2 or 2/3 and the former value was found to be more suitable for ZnO. In Fig. 6 the relationship between (ahm)2 and hm is plotted. The value of the optical band gap Eg is obtained graphically by the intersection of the extrapolated line from the
The presence of various chemical functional groups and the formation of ZnO nanoparticles are supported by the FT-IR spectra as shown in Fig. 5. The broad absorption bands at 3420 and 1630 cm1 encompass the OAH stretching vibrations of absorbed water on the ZnO surface. The strong absorption band between 500 and 450 cm1 can be attributed to the stretching modes of ZnAO [18]. Moreover, no absorption bands of PVP (1677 cm1) and CA (carboxylate group at 1580 cm1) can be detected as they are completely washed out at the time of synthesis. But, a small stretching vibration of C@O corresponding to acetate group is found around 1500 to 1600 cm1. The FT-IR spectra obtained for the uncapped and capped ZnO particles are more or less similar (all the spectra are not shown). 3.5. Optical absorption Fig. 6 shows the absorption spectra for the uncapped and capped ZnO nanoparticles [19,20]. Here, the absorption peak shifts
Fig. 5. FT-IR spectra for multi-capped PVP–CA ZnO nanoparticles.
Fig. 6. (a) UV–vis absorption spectra at room temperature of ZnO nanoparticles. (b) Plot of linear portion of (ahm)2 vs. Eg for ZnO nanoparticles and inset shows the variation of energy band gap with crystalline size.
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Table 2 Estimated optical bang gap energies (Eg) of ZnO nanoparticles as a function of particle size obtained at 100 °C for 48 h. Sample
Crystalline size obtained from XRD (nm)
Estimated optical energy band gap Eg (eV)
Uncapped ZnO Capped with PVP Capped with PVP-CA
25 22 17
3.865 3.895 4.001
Acknowledgements The authors would like to thank Dr. M. Chidambaram, Director, NIT, Tiruchirappalli for his valuable support and constant encouragement. The authors thank Dr. M. Ashok, NIT, Tiruchirappalli for UV–vis absorption study. The research was supported by Seed Money for new faculties, from NIT, Tiruchirappalli and DST project (SR/FTP/ETA – 31/07). References
linear part of the curves on the photon energy axis. The optical energy band gap values obtained are summarized in Table 2. As the particle size reduces from 25 to 16 nm, the optical energy band gap increased from 3.86 to 4.00 eV. 4. Conclusion The synthesis and characterization including structural and optical study of nanosized ZnO has been performed. The results show that there is a size confinement with a change in concentration ratio of Zn2+/PVP/CA. PVP and CA are used to cap and confine the particle size. XRD revealed the particle sizes were in the range of 25 nm for uncapped, 22 nm for the PVP capped, nm for CA capped and 16 nm for multi-capped PVP-CA ZnO nanoparticles. FT-IR shows the absence of absorption bands concerned with PVP and CA. Growth mechanism of ZnO nanostructures is mainly due to the driving force of high concentration of ZnðOHÞ2 4 is reported. The results demonstrate that the UV–vis absorption spectra obtained at shorter wavelength indicates a much smaller particle size and strong particle size confinement.
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