- Email: [email protected]
Preparation and characterization of ZnO nanoparticles by a novel sol–gel route
Materials Letters 61 (2007) 3265 – 3268 www.elsevier.com/locate/matlet
Preparation and characterization of ZnO nanoparticles by a novel sol–gel route M. Vafaee a , M. Sasani Ghamsari b,⁎ a
Department of Material Science and Engineering, Sharif University of Technology, 11365-9898, Tehran, Iran b Solid State Laser Division, Laser Research Center, 11365-8486, Tehran, Iran Received 27 September 2006; accepted 9 November 2006 Available online 5 December 2006
Abstract In this study, ZnO nanoparticles with 3 to 4 nm size and spherical shape have been prepared. For the first time, TEA (triethanolamine) as a surfactant has been used for the preparation of ZnO nanoparticles. The best concentration of each component was adjusted through comparison between different sol absorption spectra. The best sol regarding its optical property was subjected to analysis by photoluminescence spectroscopy. TEM micrograph and electron diffraction pattern of these particles were obtained to represent the morphology and crystalline phase of the particles, respectively. Experimental results have shown that the prepared zinc oxide nanoparticles by this method have higher photoluminescence spectra as compared to other methods. © 2006 Elsevier B.V. All rights reserved. Keywords: ZnO nanoparticles; TEA; Surfactant; Photoluminescence spectra
1. Introduction Zinc oxide is a well-known semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy of 60 meV at room temperature [1]. It is used in many applications such as surface acoustic wave devices (SAW), gas sensor devices, laser and optoelectronic devices [2–4,7]. Zinc oxide nanoparticles can be prepared by different methods. From these methods, the sol–gel method is more popular because of its cheapness, reliability, repeatability, and simplicity. In addition, the nanoparticles which are produced by this route, show good optical properties. But, this can be achieved only by good control of the size and morphology of the particles. That is, we have to carefully control the sol–gel process and the growth of nuclei must be prevented. One of the ways that this can be done is by providing good absorption of the surfactant onto the surface of the particles. For this purpose, in the preparation of ZnO nanoparticles, surfactants such as MEA (Monoethanolamine) and DEA (Diethanolamine) are employed [5–7]. There are three procedures which have been frequently chosen for the ⁎ Corresponding author. E-mail address: [email protected] (M.S. Ghamsari). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.089
preparation of zinc oxide nanoparticles. These procedures are categorized as {ZnAc (zinc acetate dihydrate), 2-Methoxyethanol, MEA}, {ZnAc, 2-Propanol, DEA} and {ZnAc, Ethylene Glycol, glycerol, 1-Propanol,}. In this study and for the first time, we have used TEA (triethanolamine) as a surfactant. There is no other research result on the preparation of ZnO nanoparticle in the presence of TEA. The best ratio of each component was selected based on ZnO that gave better optical properties. According to FTIR analyses, consequent reactions from dissolving of the ZnAc to formation of ZnO were suggested. Excitation of ZnO nanoparticles was studied based on their photoluminescence spectra and their morphologies were presented by TEM analysis. 2. Experimental In the preparation of ZnO sols, the following procedure was carried out. First, the appropriate ratio of triethanolamine was added to ethanol. After complete mixing by a magnetic stirrer, zinc acetate dihydrate (ZnAc) was added as precursor. The amounts of ZnAc in sols were varied to 0.25, 0.5 and 0.75 M in order to assess the best ratio, and also the molar ratio of TEA to ZnAc was adjusted to 3:5, 6:5 and 9:5 with the same aim. The
3266
M. Vafaee, M.S. Ghamsari / Materials Letters 61 (2007) 3265–3268
Fig. 1. Fresh sols absorption spectra. The molar ratios of TEA:ZnAc are for (-·-·-): 3:5, (·····): 6:5 and (full line): 9:5.
solutions were stirred in about 30 min by heating to 50–60 °C. Each sol was studied considering their stability by aging them for one to two months. Absorption spectroscopy and Fouriertransform infrared (FTIR) spectroscopy were performed using Hitachi U-3410 and Bruker (Vector 22) instruments respectively on sols. The ability of emission excitation of ZnO nanoparticles was investigated using Cary Eclipse photoluminescence analyzer. Transmission electron microscopy was carried out using Philips (3M200) instrument. 3. Result and discussion This study was performed with the hope that ZnO nanoparticles with good optical properties can be obtained. This can be achieved by preparing ZnO nanoparticles which will have good characteristic absorption and sharp luminescence spectrum. Three different ratios of both ZnAc and TEA were chosen to determine the best sol considering their optical properties. First, for ZnAc of 0.5 M three different amounts of TEA were added. After completion, the sol's absorption was measured at various wavelengths. The absorption spectra are shown in Fig. 1. The sol whose TEA:ZnAc molar ratio was 3:5 shows a characteristic absorption spectrum around 275 nm. But the others have no peak and their absorption spectra arise drastically. This peak corresponds to electron–hole conjunctions [8]. According to quantum calculations by Brus [9] we can estimate the particle radius based on
Fig. 2. FTIR spectra obtained from fresh sols. The amounts of ZnAc are indicated above each spectrum.
Fig. 3. The effect of adding each component on chemical vibration bonds (black colour: EtOH–TEA–ZnAc; blue colour: EtOH–TEA; green colour: EtOH). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
their absorption onset. The particle size for all sols is around 2 to 3 nm. The two other sols did not show this particular phenomenon which is due to the semiconductors special ability. It is possible that these sols contain particles with a low percentage of ZnO and surely a huge amount of organic complexes. To sum up, the ratio of TEA to ZnAc must be 3:5 according to absorption spectra. In order to demonstrate the influence of ZnAc concentration on the optical properties of sols three other sols have been prepared which contained various molarities of ZnAc (0.25, 0.5 and 0.75). But the molar ratio of TEA was fixed to 3:5 similar to the first experiment. To evaluate the bonds energy differences between sols with different amounts of ZnAc and absolutely TEA, FTIR analyses were carried out on fresh sols. The resultant spectra are shown in Fig. 2. TEA to ZnAc molar ratio was fixed to 3:5. The broad peak in the range of 3000 to 3500 cm− 1 is attributed to water of hydration present in the solution. The peaks which are located at 2885 and 2937 cm− 1 are due to symmetric and asymmetric C–H bonds respectively. C–H bonds are present in monoacetate groups as intermediate products [5]. 1565 and 1380 cm− 1 peaks are attributed to asymmetric and symmetric C_O bonds vibrations respectively, and these bonds modified form vibrate at 1330 and 1260 cm− 1. C–N bonds vibrate at 1010, 1060 and 1140 cm− 1 [5,10–13]. O–H vibrations become sharper and all the other vibrations are moderated due to the increase in ZnAc molar ratio. According to these curves, it seems that the nature of the bonds in sols changes when the molar ratios increase. In Fig. 3, the effect of adding each component is represented in order through FTIR analyses. There is no important vibration in ethanol FTIR spectrum except in the range of C–H bonds, but when TEA is being added, three O–H bonds of TEA can be recognized with regard to vibrations in the range of 3000 to 3500 cm− 1. When TEA is added, it will bring three numbers of O–H into the solution, so vibrations at this state will be boosted. C–H
Fig. 4. Photoluminescence spectrum of ZnO nanoparticles in 0.75 M ZnAc sol.
M. Vafaee, M.S. Ghamsari / Materials Letters 61 (2007) 3265–3268
Fig. 5. Transmission electron microscope micrograph of ZnO nanoparticles.
and C_O bonds are also formed. It is clear from the curves that ethanol and triethanolamine do not react. By ZnAc addition C–N and C_O bonds shifted to higher wave numbers in comparison with FTIR spectrum related to ZnAc. It is proposed that in the process of ZnO synthesis of ZnAc and a surfactant, an intermediate product called zinc monoacetate is obtained. The following reactions which represent the process of ZnO formation from sol's component are suggested considering the FTIR spectra. CH3 –COðH2 OÞ–O–Zn–O–COðH2 OÞ–CH3 →½CH3 –CO–O–Znþ þ ½O–CO–CH3 − þ 2Hþ þ 2ðOHÞ− ð1Þ In the above reaction, zinc acetate dihydrate was changed to zinc monoacetate. But in the following reaction, a new complex is formed. Complexes such as this one will connect together chemically and so the polycondensation process will occur. ½CH3 –CO–O–Znþ þ NðCH2 –CH2 –OHÞ3 →½CH3 –CO–O–ZnNðCH2 –CH2 –OHÞ2 þ ½CH2 –CH2 –OH− ð2Þ C–N bond vibrations in Fig. 4 are shifted to the left side of the diagram, and this means that movements of C–N bonds became easier; one of the ethanol chains which is connected to nitrogen, therefore, must leave its place for the monoacetate group. The sol which has the highest amount of ZnAc (0.75 M) was subjected to analysis by photoluminescence spectroscopy (Fig. 4). The emission light wavelength was adapted to 258 nm in regard to the intensity of the absorption of the peaks. The green emission peak which appeared at 520 nm is a sharp peak. In other studies, this emission always appears as a broad peak [13–15]. However in TEA, ZnAc and ethanol system, it appeared in a sharp form peak. But the UV emission which always appears as a sharp peak, in our study is seen as a broad peak at 360 nm and also has a shoulder at 330 nm [13–15]. Although the wavelengths at which these two peaks occurred are the same as those of other studies, the shapes of the peaks are completely different. This enables us to use these nanoparticles in applications where a monochromatic emission is needed. The UV emissions are mostly assigned to radiative recombination of electrons and holes; in addition, UV emissions always show the quantum confinement size effect. With the increase of particles size, energy of the band gap decreases and also absorption onset shifts to longer wavelengths. The
3267
green emissions, on the other hand, are assigned to transition of photogenerated electrons from the conduction band edge to a trap level near the conduction band [16]. These peaks do not obey the quantum confinement size effect and their intensity and shape are dominated by surface structure of particles [17]. In order to elucidate the as-prepared ZnO nanoparticles morphologies, transmission electron microscopy (TEM) was performed on the sol with 0.75 M ZnAc content. For the purpose of having a better chance to obtain the appropriate number of particles, the amount of ZnO nanoparticles related to solvent in the sol was increased by evaporating some amount of the solvent in air. As it can be seen in Fig. 5, there is no aggregation between nanoparticles, but the dense structure due to evaporation process was formed. The particles tend to have a minimum value of energy according to their spherical shape. The average particle size in this image was estimated to be 4 nm by applying the cross line method. After measuring the radius of the circles and comparing it to JCPDS data [18], the crystal phase of these particles was recognized. Nanoparticles in such sols were crystallized in wurtzite ZnO.
4. Conclusion According to absorption spectra, the best ratio for sols with good optical properties was chosen. Possible reactions which may occur during ZnO nanoparticles synthesis were proposed based on FTIR spectra. Different shapes of UV and green peaks in the photoluminescence spectra of these particles suggest the possible use of these particles in monochromatic excitation applications. The morphology of the particles and their crystal structure were evaluated via TEM micrographs and diffraction pattern respectively. References [1] Y.S. Kim, W.P. Tai, S.J. Shu, Effect of preheating temperature on structural and optical properties of ZnO thin films by sol–gel process, Thin Solid Films 491 (2005) 153–160. [2] M. Fonrodona, J. Escarre, F. Villar, D. Soler, J.M. Asensi, J. Bertomeu, J. Andreu, PEN as substrate for new solar cell technologies, Sol. Energy Mater. Sol. Cells 89 (2005) 37–47. [3] E. Fortunato, P. Barquinha, A. Pimentel, A. Goncalves, A. Marques, L. Pereira, R. Martins, Recent advances in ZnO transparent thin film transistors, Thin Solid Films 487 (2005) 205–211. [4] R.P. Wanga, H. Muto, X. Gang, P. Jin, M. Tazawa, Ultraviolet lasing with low excitation intensity in deep-level emission free ZnO films, J. Cryst. Growth 282 (2005) 359–364. [5] R.C. Perez, O.J. Sandoval, S.J. Sandoval, J.M. Marın, A.M. Galvan, G.T. Delgado, Influence of annealing temperature on the formation and characteristics of sol–gel prepared ZnO films, J. Vac. Sci. Technol., A, Vac. Surf. Films 17 (1999) 1811–1816. [6] H. Li, J. Wang, H. Liu, H. Zhang, X. Li, Zinc oxide films prepared by sol– gel method, J. Cryst. Growth 275 (2005) 943–946. [7] X.L. Cheng, H. Zhao, L.H. Huo, S. Gao, J.G. Zhao, ZnO nanoparticulate thin film: preparation, characterization and gas-sensing property, Sens. Actuators, B, Chem. 102 (2004) 248–252. [8] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, The luminescence of nanocrystalline ZnO particles: the mechanism of the ultraviolet and visible emission, J. Lumin. 87–89 (2000) 454–456. [9] L. Brus, Electronic wave functions in semiconductor clusters: experiment and theory, J. Phys. Chem. 90 (1986) 2555–2560. [10] M. Wang, J. Wang, W. Chen, Y. Cui, L. Wang, Effect of preheating and annealing temperatures on quality characteristics of ZnO thin film prepared by sol–gel method, Mater. Chem. Phys. xxx (2005) xxx.
3268
M. Vafaee, M.S. Ghamsari / Materials Letters 61 (2007) 3265–3268
[11] S. Bandyopadhyay, G.K. Paul, R. Roy, S.K. Sen, S. Sen, Study of structural and electrical properties of grain-boundary modified ZnO films prepared by sol–gel technique, Mater. Chem. Phys. 74 (2002) 83–91. [12] Zh. Liu, Zh. Jin, W. Li, J. Qui, Preparation of ZnO porous thin films by sol–gel method using PEG template, Mater. Lett. 59 (2005) 3620–3625. [13] D.W. Bahnemann, C. Kormann, M.R. Hoffmann, Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study, J. Phys. Chem. 91 (1987) 3789–3798. [14] L. Spanhel, M.A. Anderson, Semiconductor clusters in the sol–gel process: quantized aggregation, gelation, and crystal growth in concentrated ZnO colloids, J. Am. Chem. Soc. 113 (1991) 2826–2833.
[15] M. Haase, H. Weller, A. Henglein, Photochemistry and radiation chemistry of colloidal semiconductors. 23. Electron storage on ZnO particles and size quantization, J. Phys. Chem. 92 (1988) 482–487. [16] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, Identification of the transition responsible for the visible emission in ZnO using quantum size effects, J. Lumin. 90 (2000) 123–128. [17] S. Sakohara, L.D. Tickanen, M.A. Anderson, Luminescence properties of thin zinc oxide membranes prepared by the sol–gel technique: change in visible luminescence during firing, J. Phys. Chem. 96 (1992) 11086–11091. [18] JCPDS cards, no. 36-1451.
Preparation and characterization of ZnO nanoparticles by a novel sol–gel route M. Vafaee a , M. Sasani Ghamsari b,⁎ a
Department of Material Science and Engineering, Sharif University of Technology, 11365-9898, Tehran, Iran b Solid State Laser Division, Laser Research Center, 11365-8486, Tehran, Iran Received 27 September 2006; accepted 9 November 2006 Available online 5 December 2006
Abstract In this study, ZnO nanoparticles with 3 to 4 nm size and spherical shape have been prepared. For the first time, TEA (triethanolamine) as a surfactant has been used for the preparation of ZnO nanoparticles. The best concentration of each component was adjusted through comparison between different sol absorption spectra. The best sol regarding its optical property was subjected to analysis by photoluminescence spectroscopy. TEM micrograph and electron diffraction pattern of these particles were obtained to represent the morphology and crystalline phase of the particles, respectively. Experimental results have shown that the prepared zinc oxide nanoparticles by this method have higher photoluminescence spectra as compared to other methods. © 2006 Elsevier B.V. All rights reserved. Keywords: ZnO nanoparticles; TEA; Surfactant; Photoluminescence spectra
1. Introduction Zinc oxide is a well-known semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy of 60 meV at room temperature [1]. It is used in many applications such as surface acoustic wave devices (SAW), gas sensor devices, laser and optoelectronic devices [2–4,7]. Zinc oxide nanoparticles can be prepared by different methods. From these methods, the sol–gel method is more popular because of its cheapness, reliability, repeatability, and simplicity. In addition, the nanoparticles which are produced by this route, show good optical properties. But, this can be achieved only by good control of the size and morphology of the particles. That is, we have to carefully control the sol–gel process and the growth of nuclei must be prevented. One of the ways that this can be done is by providing good absorption of the surfactant onto the surface of the particles. For this purpose, in the preparation of ZnO nanoparticles, surfactants such as MEA (Monoethanolamine) and DEA (Diethanolamine) are employed [5–7]. There are three procedures which have been frequently chosen for the ⁎ Corresponding author. E-mail address: [email protected] (M.S. Ghamsari). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.089
preparation of zinc oxide nanoparticles. These procedures are categorized as {ZnAc (zinc acetate dihydrate), 2-Methoxyethanol, MEA}, {ZnAc, 2-Propanol, DEA} and {ZnAc, Ethylene Glycol, glycerol, 1-Propanol,}. In this study and for the first time, we have used TEA (triethanolamine) as a surfactant. There is no other research result on the preparation of ZnO nanoparticle in the presence of TEA. The best ratio of each component was selected based on ZnO that gave better optical properties. According to FTIR analyses, consequent reactions from dissolving of the ZnAc to formation of ZnO were suggested. Excitation of ZnO nanoparticles was studied based on their photoluminescence spectra and their morphologies were presented by TEM analysis. 2. Experimental In the preparation of ZnO sols, the following procedure was carried out. First, the appropriate ratio of triethanolamine was added to ethanol. After complete mixing by a magnetic stirrer, zinc acetate dihydrate (ZnAc) was added as precursor. The amounts of ZnAc in sols were varied to 0.25, 0.5 and 0.75 M in order to assess the best ratio, and also the molar ratio of TEA to ZnAc was adjusted to 3:5, 6:5 and 9:5 with the same aim. The
3266
M. Vafaee, M.S. Ghamsari / Materials Letters 61 (2007) 3265–3268
Fig. 1. Fresh sols absorption spectra. The molar ratios of TEA:ZnAc are for (-·-·-): 3:5, (·····): 6:5 and (full line): 9:5.
solutions were stirred in about 30 min by heating to 50–60 °C. Each sol was studied considering their stability by aging them for one to two months. Absorption spectroscopy and Fouriertransform infrared (FTIR) spectroscopy were performed using Hitachi U-3410 and Bruker (Vector 22) instruments respectively on sols. The ability of emission excitation of ZnO nanoparticles was investigated using Cary Eclipse photoluminescence analyzer. Transmission electron microscopy was carried out using Philips (3M200) instrument. 3. Result and discussion This study was performed with the hope that ZnO nanoparticles with good optical properties can be obtained. This can be achieved by preparing ZnO nanoparticles which will have good characteristic absorption and sharp luminescence spectrum. Three different ratios of both ZnAc and TEA were chosen to determine the best sol considering their optical properties. First, for ZnAc of 0.5 M three different amounts of TEA were added. After completion, the sol's absorption was measured at various wavelengths. The absorption spectra are shown in Fig. 1. The sol whose TEA:ZnAc molar ratio was 3:5 shows a characteristic absorption spectrum around 275 nm. But the others have no peak and their absorption spectra arise drastically. This peak corresponds to electron–hole conjunctions [8]. According to quantum calculations by Brus [9] we can estimate the particle radius based on
Fig. 2. FTIR spectra obtained from fresh sols. The amounts of ZnAc are indicated above each spectrum.
Fig. 3. The effect of adding each component on chemical vibration bonds (black colour: EtOH–TEA–ZnAc; blue colour: EtOH–TEA; green colour: EtOH). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
their absorption onset. The particle size for all sols is around 2 to 3 nm. The two other sols did not show this particular phenomenon which is due to the semiconductors special ability. It is possible that these sols contain particles with a low percentage of ZnO and surely a huge amount of organic complexes. To sum up, the ratio of TEA to ZnAc must be 3:5 according to absorption spectra. In order to demonstrate the influence of ZnAc concentration on the optical properties of sols three other sols have been prepared which contained various molarities of ZnAc (0.25, 0.5 and 0.75). But the molar ratio of TEA was fixed to 3:5 similar to the first experiment. To evaluate the bonds energy differences between sols with different amounts of ZnAc and absolutely TEA, FTIR analyses were carried out on fresh sols. The resultant spectra are shown in Fig. 2. TEA to ZnAc molar ratio was fixed to 3:5. The broad peak in the range of 3000 to 3500 cm− 1 is attributed to water of hydration present in the solution. The peaks which are located at 2885 and 2937 cm− 1 are due to symmetric and asymmetric C–H bonds respectively. C–H bonds are present in monoacetate groups as intermediate products [5]. 1565 and 1380 cm− 1 peaks are attributed to asymmetric and symmetric C_O bonds vibrations respectively, and these bonds modified form vibrate at 1330 and 1260 cm− 1. C–N bonds vibrate at 1010, 1060 and 1140 cm− 1 [5,10–13]. O–H vibrations become sharper and all the other vibrations are moderated due to the increase in ZnAc molar ratio. According to these curves, it seems that the nature of the bonds in sols changes when the molar ratios increase. In Fig. 3, the effect of adding each component is represented in order through FTIR analyses. There is no important vibration in ethanol FTIR spectrum except in the range of C–H bonds, but when TEA is being added, three O–H bonds of TEA can be recognized with regard to vibrations in the range of 3000 to 3500 cm− 1. When TEA is added, it will bring three numbers of O–H into the solution, so vibrations at this state will be boosted. C–H
Fig. 4. Photoluminescence spectrum of ZnO nanoparticles in 0.75 M ZnAc sol.
M. Vafaee, M.S. Ghamsari / Materials Letters 61 (2007) 3265–3268
Fig. 5. Transmission electron microscope micrograph of ZnO nanoparticles.
and C_O bonds are also formed. It is clear from the curves that ethanol and triethanolamine do not react. By ZnAc addition C–N and C_O bonds shifted to higher wave numbers in comparison with FTIR spectrum related to ZnAc. It is proposed that in the process of ZnO synthesis of ZnAc and a surfactant, an intermediate product called zinc monoacetate is obtained. The following reactions which represent the process of ZnO formation from sol's component are suggested considering the FTIR spectra. CH3 –COðH2 OÞ–O–Zn–O–COðH2 OÞ–CH3 →½CH3 –CO–O–Znþ þ ½O–CO–CH3 − þ 2Hþ þ 2ðOHÞ− ð1Þ In the above reaction, zinc acetate dihydrate was changed to zinc monoacetate. But in the following reaction, a new complex is formed. Complexes such as this one will connect together chemically and so the polycondensation process will occur. ½CH3 –CO–O–Znþ þ NðCH2 –CH2 –OHÞ3 →½CH3 –CO–O–ZnNðCH2 –CH2 –OHÞ2 þ ½CH2 –CH2 –OH− ð2Þ C–N bond vibrations in Fig. 4 are shifted to the left side of the diagram, and this means that movements of C–N bonds became easier; one of the ethanol chains which is connected to nitrogen, therefore, must leave its place for the monoacetate group. The sol which has the highest amount of ZnAc (0.75 M) was subjected to analysis by photoluminescence spectroscopy (Fig. 4). The emission light wavelength was adapted to 258 nm in regard to the intensity of the absorption of the peaks. The green emission peak which appeared at 520 nm is a sharp peak. In other studies, this emission always appears as a broad peak [13–15]. However in TEA, ZnAc and ethanol system, it appeared in a sharp form peak. But the UV emission which always appears as a sharp peak, in our study is seen as a broad peak at 360 nm and also has a shoulder at 330 nm [13–15]. Although the wavelengths at which these two peaks occurred are the same as those of other studies, the shapes of the peaks are completely different. This enables us to use these nanoparticles in applications where a monochromatic emission is needed. The UV emissions are mostly assigned to radiative recombination of electrons and holes; in addition, UV emissions always show the quantum confinement size effect. With the increase of particles size, energy of the band gap decreases and also absorption onset shifts to longer wavelengths. The
3267
green emissions, on the other hand, are assigned to transition of photogenerated electrons from the conduction band edge to a trap level near the conduction band [16]. These peaks do not obey the quantum confinement size effect and their intensity and shape are dominated by surface structure of particles [17]. In order to elucidate the as-prepared ZnO nanoparticles morphologies, transmission electron microscopy (TEM) was performed on the sol with 0.75 M ZnAc content. For the purpose of having a better chance to obtain the appropriate number of particles, the amount of ZnO nanoparticles related to solvent in the sol was increased by evaporating some amount of the solvent in air. As it can be seen in Fig. 5, there is no aggregation between nanoparticles, but the dense structure due to evaporation process was formed. The particles tend to have a minimum value of energy according to their spherical shape. The average particle size in this image was estimated to be 4 nm by applying the cross line method. After measuring the radius of the circles and comparing it to JCPDS data [18], the crystal phase of these particles was recognized. Nanoparticles in such sols were crystallized in wurtzite ZnO.
4. Conclusion According to absorption spectra, the best ratio for sols with good optical properties was chosen. Possible reactions which may occur during ZnO nanoparticles synthesis were proposed based on FTIR spectra. Different shapes of UV and green peaks in the photoluminescence spectra of these particles suggest the possible use of these particles in monochromatic excitation applications. The morphology of the particles and their crystal structure were evaluated via TEM micrographs and diffraction pattern respectively. References [1] Y.S. Kim, W.P. Tai, S.J. Shu, Effect of preheating temperature on structural and optical properties of ZnO thin films by sol–gel process, Thin Solid Films 491 (2005) 153–160. [2] M. Fonrodona, J. Escarre, F. Villar, D. Soler, J.M. Asensi, J. Bertomeu, J. Andreu, PEN as substrate for new solar cell technologies, Sol. Energy Mater. Sol. Cells 89 (2005) 37–47. [3] E. Fortunato, P. Barquinha, A. Pimentel, A. Goncalves, A. Marques, L. Pereira, R. Martins, Recent advances in ZnO transparent thin film transistors, Thin Solid Films 487 (2005) 205–211. [4] R.P. Wanga, H. Muto, X. Gang, P. Jin, M. Tazawa, Ultraviolet lasing with low excitation intensity in deep-level emission free ZnO films, J. Cryst. Growth 282 (2005) 359–364. [5] R.C. Perez, O.J. Sandoval, S.J. Sandoval, J.M. Marın, A.M. Galvan, G.T. Delgado, Influence of annealing temperature on the formation and characteristics of sol–gel prepared ZnO films, J. Vac. Sci. Technol., A, Vac. Surf. Films 17 (1999) 1811–1816. [6] H. Li, J. Wang, H. Liu, H. Zhang, X. Li, Zinc oxide films prepared by sol– gel method, J. Cryst. Growth 275 (2005) 943–946. [7] X.L. Cheng, H. Zhao, L.H. Huo, S. Gao, J.G. Zhao, ZnO nanoparticulate thin film: preparation, characterization and gas-sensing property, Sens. Actuators, B, Chem. 102 (2004) 248–252. [8] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, The luminescence of nanocrystalline ZnO particles: the mechanism of the ultraviolet and visible emission, J. Lumin. 87–89 (2000) 454–456. [9] L. Brus, Electronic wave functions in semiconductor clusters: experiment and theory, J. Phys. Chem. 90 (1986) 2555–2560. [10] M. Wang, J. Wang, W. Chen, Y. Cui, L. Wang, Effect of preheating and annealing temperatures on quality characteristics of ZnO thin film prepared by sol–gel method, Mater. Chem. Phys. xxx (2005) xxx.
3268
M. Vafaee, M.S. Ghamsari / Materials Letters 61 (2007) 3265–3268
[11] S. Bandyopadhyay, G.K. Paul, R. Roy, S.K. Sen, S. Sen, Study of structural and electrical properties of grain-boundary modified ZnO films prepared by sol–gel technique, Mater. Chem. Phys. 74 (2002) 83–91. [12] Zh. Liu, Zh. Jin, W. Li, J. Qui, Preparation of ZnO porous thin films by sol–gel method using PEG template, Mater. Lett. 59 (2005) 3620–3625. [13] D.W. Bahnemann, C. Kormann, M.R. Hoffmann, Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study, J. Phys. Chem. 91 (1987) 3789–3798. [14] L. Spanhel, M.A. Anderson, Semiconductor clusters in the sol–gel process: quantized aggregation, gelation, and crystal growth in concentrated ZnO colloids, J. Am. Chem. Soc. 113 (1991) 2826–2833.
[15] M. Haase, H. Weller, A. Henglein, Photochemistry and radiation chemistry of colloidal semiconductors. 23. Electron storage on ZnO particles and size quantization, J. Phys. Chem. 92 (1988) 482–487. [16] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, Identification of the transition responsible for the visible emission in ZnO using quantum size effects, J. Lumin. 90 (2000) 123–128. [17] S. Sakohara, L.D. Tickanen, M.A. Anderson, Luminescence properties of thin zinc oxide membranes prepared by the sol–gel technique: change in visible luminescence during firing, J. Phys. Chem. 96 (1992) 11086–11091. [18] JCPDS cards, no. 36-1451.