The effect of silver concentration and calcination temperature on structural and optical properties of ZnO:Ag nanoparticles

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1 Modern Physics Letters B Vol. 29, No. 1 (2015) (11 pages) c World Scientific Publishing Company DOI: /S The effect of silver concentration and calcination temperature on structural and optical properties of ZnO:Ag nanoparticles M. Shayani Rad and A. Kompany Department of Physics, Ferdowsi University of Mashhad, Mashhad, Iran kompany@um.ac.ir A. Khorsand Zak and M. E. Abrishami Esfarayen University of Technology, Esfarayen, North Khorasan, Iran Received 20 July 2014 Accepted 14 November 2014 Published 16 January 2015 Pure and silver added zinc oxide nanoparticles (ZnO-NPs and ZnO:Ag-NPs) were synthesized through a modified sol gel method. The prepared samples were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. In the XRD patterns, silver diffracted peaks were also observed for the samples synthesized at different calcination temperatures of 500 C, 700 C, 900 C except 1100 C, in addition to ZnO. TEM images indicated that the average size of ZnO:Ag-NPs increases with the amount of Ag concentration. The PL spectra of the samples revealed that the increase of Ag concentration results in the increase of the visible emission intensity, whereas by increasing the calcination temperature the intensity of visible emission of the samples decreases. Keywords: Zinc oxide; ZnO:Ag; nanoparticles; calcination temperature; structural properties; optical properties. 1. Introduction Zinc oxide (ZnO) is a susceptible material that carries several unique properties, including semiconducting and piezoelectric behaviors. 1 Nowadays, it is used widely in variety of applications such as fabricating sensors and actuators, transparent conductive coating, electrodes for dye-sensitized solar cells, 2,3 gas sensors, 4 and as electro and photoluminescent materials. 5 Moreover, ZnO has been employed extensively as a cheap and nontoxic material for medical applications. 6 In the nanosized structure range, ZnO has proved to own remarkable physical and chemical properties, different from its bulk. 7 Several methods have been used to prepare ZnO Corresponding author

2 M. S. Rad et al. nanostructures and nanoparticles such as: sonochemical, 8 oxidation process, 9 sol gel, polymerization, 13 precipitation, 14,15 solvothermal and hydrothermal, 16 CVD, 17 laser ablation 18 and gel combustion. 19 Among these methods, sol gel is rather a simple approach that has been employed extensively to synthesize pure zinc oxide nanoparticles (ZnO-NPs). One of the most important issues regarding this method is to achieve nanoparticles with a narrow size distribution. Using a suitable stabilizer and polymerization agent is one of the tracks to control the size and morphology of the final products. Polyethylene glycol (PEG) 20 and cetyltrimethyl ammonium bromide (CTAB) 21 are two common polymers that have been used for this purpose. Doping ZnO, in both nano and ceramic forms, with some selective elements has been carried out as an effective attempt to enhance and control its physical properties. For instance, to control the optical properties Au 2+,Ce 3+,Eu 3+, In 3+,Li 1+,Na 1+,Ag + and Mg 2+ have been used as dopants and Mn 2+,Cr 2+, Co 2+,Ni 2+,Fe 3+,Cu 2+,V 5+ to enhance the magnetic properties of ZnO In this work, a modified sol gel method was performed to synthesize pure ZnO-NPs and silver-added zinc oxide nanoparticles (ZnO:Ag-NPs) at different calcination temperatures and silver concentrations, using gelatin as a polymerization and stabilizer agent. The effects of silver concentration and calcination temperature on size, morphology and crystallinity as well as the optical properties of the synthesized samples were investigated. 2. Materials and Method For the synthesis 5 g of the final product (ZnO and ZnO:Ag powders), a solution of gelatin (type B from bovine skin, Sigma Aldrich) was first prepared by dissolving 10 g gelatin in 150 ml deionized water at 60 C. Meanwhile, appropriate amounts of zinc nitrate (Zn(NO 3 ) 2 6H 2 O, Merck 99%) and silver nitrate (AgNO 3, Merck 99%) were dissolved in a minimum volume of deionized water at room temperature. The amounts of zinc and silver nitrates were chosen according to Zn 1 x Ag x O(x =0, 0.01, 0.03, 0.05) formula. After that, the two prepared solutions were mixed and stirred for 8 hours at 80 C in an oil bath to achieve the brown resins. Due to zinc cation mobility limitation, the growth of the particles terminates in the presence of gelatin. 27 Moreover, gelatin acts as a reduction agent giving an electron to Ag + which reduces it to Ag atom, so that the color of the gel becomes darker, as shown in Fig. 1. Zn 2+ and Ag + gain O 2 and e, respectively, from gelatin giving ZnO and Ag atoms, according to the following reactions, Eq. (1): Zn 2+ +O 2 Ag + +e gelatin gelatin Calcination ZnO+CO 2 +NO C Ag. There are negative sides in the gelatin chains which attract the Zn 2+ and Ag + just needs one electron to be reduced. Due to the reduction ability of gelatin, the Ag + obtains the needed electron and changes to metallic Ag atoms. During the calcination process, the Ag atoms are stable against the temperature, while Zn (1)

3 The effect of silver concentration and calcination temperature Fig. 1. The prepared gels for the synthesis of (a) ZnO-NPs and (b) ZnO:Ag-NPs. atoms get oxidized to ZnO-NPs. The pure ZnO-NPs and ZnO:Ag-NPs were finally produced by calcinating the resins at different temperatures of 500 C, 700 C, 900 C and 1100 C. 3. Results and Discussion 3.1. X-ray diffraction analysis For the pure gel, the lowest needed calcination temperature was found to be 500 C. The X-ray diffraction (XRD) pattern of the prepared powder at this temperature confirmed the formation of ZnO wurtzite structure. The prepared pure and Agadded gels were then calcined at 500 C, 700 C, 900 C and 1100 C temperatures. The XRD patterns of the gels calcined at 500 C are presented in Fig. 2. All the Fig. 2. (Color online) The XRD patterns of ZnO-NPs and ZnO:Ag-NPs calcined at 500 C

4 M. S. Rad et al. Fig. 3. (Color online) The XRD patterns of ZnO-NPs and ZnO:Ag-NPs calcined at 1100 C. peaks indexed ZnO in the XRD patterns corresponding to the JCPDS In addition to ZnO, some other diffraction peaks are also observed, which are attributed to the cubic structure of Ag atoms (JCPDS ). Therefore, it seems that the Ag atoms have kept their metallic phase and did not diffuse into ZnO lattice at this temperature. The intensity of the diffraction peaks of the metallic Ag increases as the result of increasing Ag concentration. Hence, it can be concluded that the ZnO and Ag structures have grown independently. In order to help the Ag atoms to diffuse in the ZnO matrix, the calcination temperature was increased to 700 C, 900 C and 1100 C. Figure 3 illustrates the XRD patterns of the pure and Ag-added gels calcined at 1100 C, in which no silver peaks can be observed for 1% and 3% Ag concentrations. However, Ag peaks can still be detected in the XRD pattern of ZnO:5%Ag-NPs, at about 2θ =38. The XRD patterns of ZnO:1%Ag-NPs calcined at 500 C, 700 C, 900 Cand 1100 C are shown in Fig. 4. As it can be observed, the diffracted peaks related to Ag are present in the samples calcined at 500 C, 700 C and 900 C, but disappears at 1100 C calcination temperature. This reveals that Ag atoms may have diffused in ZnO matrix. Energy dispersive X-ray analysis of ZnO:1%Ag-NPs calcined at 1100 Cconfirmed the presence of Ag atoms in ZnO lattice which is in agreement with the XRD results, Fig. 5. Figure 6 shows the XRD patterns of (a) ZnO-NPs calcined at different temperatures of 500 C, 700 C, 900 C and 1100 C, (b) pure ZnO-NPs and ZnO:Ag-NPs

5 The effect of silver concentration and calcination temperature Fig. 4. (Color online) The XRD patterns of ZnO:1%Ag-NPs calcined at 500 C, 700 C, 900 C and 1100 C. Fig. 5. (Color online) The EDX spectrum of ZnO:1%Ag-NPs calcined at 1100 C. with different Ag concentrations of 1%, 3% and 5%, calcined at 500 C and (c) pure ZnO-NPs and ZnO:Ag-NPs with different Ag concentrations of 1%, 3% and 5%, calcined at 1100 C, in the range of 2θ = The value of the FWHM, shown in Fig. 6(a), was used to estimate the crystallite size of the prepared samples. For ZnO samples, increasing the calcination temperature reduces this value. However, this reduction is negligible for the Ag-added samples calcined at 500 C with different Ag concentrations, Fig. 6(b). This figure indicates that ZnO and Ag crystals have grown separately, as

6 M. S. Rad et al. Fig. 6. (Color online) The XRD patterns of (a) pure ZnO-NPs calcined at 500 C, 700 C, 900 C and 1100 C, (b) pure ZnO-NPs and ZnO:Ag-NPs calcined at 500 C and (c) pure ZnO-NPs and ZnO:Ag-NPs calcined at 1100 C, in the range of 2θ = mentioned earlier. However, as it is observed in Fig. 6(c), when the calcination temperature is raised to 1100 C, the majority of Ag atoms diffuse into the ZnO matrix. Therefore, the increase of Ag concentration affects the FWHM of the presented peaks. These observations prove that Ag and ZnO crystallites grow independently at low calcination temperatures

7 The effect of silver concentration and calcination temperature Table 1. The crystallite size (in nanometer) of ZnO and ZnO:Ag-NPs calcined at 500 C, 700 C, 900 Cand 1100 C. Calcination temperature Sample 500 C 700 C 900 C 1100 C ZnO ZnO:1%Ag ZnO:3%Ag ZnO:5%Ag The (101) peak was selected to calculate the crystallite size of the samples, using Scherrer s formula, Eq. (2): D = Kλ β cos θ, (2) where K is the shape factor equal to 0.98 and β is the FWHM of the peaks which were used in our calculation. The results reveal that the crystallite size of the ZnO- NPs has increased slightly by adding silver to the system, as presented in Table 1. However, this is more significant at higher calcination temperatures Morphology study The transmission electron microscopy (TEM) image of pure ZnO-NPs calcined at 500 C and its corresponding size distribution diagram are shown in Fig. 7, which illustrates the rather nanospherical shape of the particles with the average size of 30 nm. In order to see the effect of silver concentration on the particle size of the synthesized nanopowders calcined at 700 C, the TEM images are given in Fig. 8. It is found out that the average particle size of the Ag-added samples are 63, 78 Fig. 7. (Color online) The TEM image and the corresponding size distribution diagram of ZnO- NPs calcined at 500 C

8 M. S. Rad et al. Fig. 8. (Color online) The TEM images and the corresponding size distribution diagrams of the ZnO:Ag-NPs calcined at 700 C. and 88 nm for 1%, 3% and 5% Ag concentration, respectively. It has been shown that some metals such as Au and Pt play as catalyst for growing ZnO crystals. 18 So, by adding Ag atoms into ZnO matrix, one expects to attain bigger ZnO-NPs, whichisingoodagreementwithourresults Photoluminescence spectroscopy Figure 9 shows the room temperature photoluminescence (PL) spectra of pure and Ag-added ZnO-NPs synthesized at 500 C. The dominant peaks of all samples are mostly in the visible region, due to the presence of oxygen vacancies and defects. 33 The enlarged image of PL, in the range of nm wavelengths, shows that there are no UV emission peaks for the pure ZnO, while observed for the Ag-added samples. It is seen that the intensity of these peaks have increased by increasing the Ag concentration. This confirms that Ag and ZnO structures have been formed separately, in agreement with the XRD results. Therefore, both Ag and ZnO emission peaks exist in the PL spectra. From the literature, it has been reported that there are two distinct emission peaks for Ag which are located in the UV and visible regions. 34 So, in UV region similar to visible, intensity of the peaks have increased by the increase of Ag concentration. The PL spectra of 1% ZnO:Ag-NPs calcined at different temperatures are given in Fig. 10. It is observed that the intensity of the visible emission of the PL spectrum has decreased with the increase of the calcination temperature. In the UV region,

9 The effect of silver concentration and calcination temperature Fig. 9. (Color online) The room temperature PL spectra of ZnO-NPs and ZnO:Ag-NPs calcined at 500 C. Fig. 10. (Color online) The room temperature PL spectra of ZnO:1%Ag-NPs calcined at 500 C, 700 C, 900 C and 1100 C. similar intensity decrease attributed to Ag crystals occurs and interestingly reaches zero for 1% Ag-added ZnO-NPs calcined at 1100 C. Now, the question is what happened to Ag atoms? To find the answer, EDX was carried out for Ag-added sample calcined at 1100 C. The result showed that Ag atoms are present in the ZnO matrix (Fig. 5). Furthermore, as it was mentioned earlier, no other peaks related to Ag lattice was detected in the XRD pattern of this sample. Therefore, from XRD, PL and EDX observations it can be concluded that the Ag atoms have been doped into the ZnO lattice structure at 1100 C which results in the formation of Zn (1 x) Ag x O

10 M. S. Rad et al. The emission enhancement in ZnO/Ag 2 O can be explained through the interaction between the Ag 2 O-NPs and the surface defects of the matrix, which changes the defect structures, results in the passivation of deep defect activities. The reduction of surface defects increases the number of energetic electrons in the conduction band, which recombine with the holes in the valence band leading to the enhancement of the near band-edge emission Conclusion ZnO and ZnO:Ag nanoparticles were synthesized by a modified sol gel method and the effects of silver concentration and calcination temperature on the structural and optical properties of the prepared samples were investigated. It was found that the lowest needed calcination temperature for the formation of ZnO wurtzite structure is 500 C. In order to investigate the effect of calcination temperature on the structural and optical properties of the prepared samples, the pure and Ag-added gels were calcined at higher temperatures of 700 C, 900 C and 1100 C. The XRD results revealed that ZnO and Ag nanocrystallites grow independently at calcination temperatures lower than 1100 C. Silver diffraction peaks were not detected in the XRD patterns of ZnO:1%Ag calcined at 1100 C. Moreover, the result obtained from EDX confirmed the existence of Ag atoms in the prepared sample at this temperature, indicating that ZnO lattice has been doped by Ag atoms. TEM images showed that the average particle size of pure ZnO-NPs calcined at 500 C is 30 nm. For 1%, 3% and 5% silver concentrations the average particle size of the ZnO:Ag particles calcined at 700 C were found to be 63, 78 and 88 nm, respectively. The room temperature PL spectra revealed that the emission peaks are mostly located in the visible region. However, for the pure ZnO and ZnO:1%Ag there is no peak in UV region. Therefore, it can be concluded that the Ag atoms were doped into ZnO lattice calcined at 1100 C. Acknowledgments This work was supported by Ferdowsi University of Mashhad (Vice President for Research and Technology), under code number 1/ References 1. C. Karunakaran, V. Rajeswari and P. Gomathisankar, Solid State Sci. 13 (2011) M. Pudukudy and Z. Yaakob, Solid State Sci. 30 (2014) C. K. N. Peh, L. Ke and G. W. Ho, Mater. Lett. 64 (2010) Y. Zong, Y. Cao, D. Jia, S. Bao and Y. Lu, Mater. Lett. 64 (2010) Y. Chen, D. M. Bagnall, H.-J. Koh, K.-T. Park, K. Hiraga, Z. Zhu and T. Yao, J. Appl. Phys. 84 (1998) A. K. Zak, R. Razali, W. H. A. Majid and M. Darroudi, Int. J. Nanomed. 6 (2011)

11 The effect of silver concentration and calcination temperature 7. M. Law, J. Goldberger and P. Yang, Annu. Rev. Mater. Res. 34 (2004) C. Deng, H. Hu, G. Shao and C. Han, Mater. Lett. 64 (2010) Z. H. Wang, D. Y. Geng, Z. Han and Z. D. Zhang, Mater. Lett. 63 (2009) A. K. Zak, W. H. Abd. Majid, M. R. Mahmoudian, M. Darroudi and R. Yousefi, Adv. Powder Technol. 24 (2013) F. Bigdeli and A. Morsali, Mater. Lett. 64 (2010) J. García-Serrano, E. Gómez-Hernández, M. Ocampo-Fernández and U. Pal, Curr. Appl. Phys. 9 (2009) P. Jajarmi, Mater. Lett. 63 (2009) C.-L. Kuo, C.-L. Wang, H.-H. Ko, W.-S. Hwang, K.-M. Chang, W.-L. Li, H.-H. Huang, Y.-H. Chang and M.-C. Wang, Ceram. Int. 36 (2010) R. Song, Y. Liu and L. He, Solid State Sci. 10 (2008) R. Razali, A. K. Zak, W. H. A. Majid and M. Darroudi, Ceram. Int. 37 (2011) R. Yousefi, M. R. Muhamad and A. K. Zak, Curr. Appl. Phys. 11 (2011) R.Zamiri,A.Zakaria,H.A.Ahangar,M.Darroudi,A.K.ZakandG.P.C.Drummen, J. Alloys Compd. 516 (2012) A. K. Zak, M. E. Abrishami, W. H. A. Majid, R. Yousefi and S. M. Hosseini, Ceram. Int. 37 (2011) B. Cheng and E. T. Samulski, Chem. Commun. (2004) H.-J. Zhai, W.-H. Wu, F. Lu, H.-S. Wang and C. Wang, Mater. Chem. Phys. 112 (2008) A. George, S. K. Sharma, S. Chawla, M. M. Malik and M. S. Qureshi, J. Alloys Compd. 509 (2011) N. Hongsith, C. Viriyaworasakul, P. Mangkorntong, N. Mangkorntong and S. Choopun, Ceram. Int. 34 (2008) O. D. Jayakumar, I. K. Gopalakrishnan, C. Sudakar, R. M. Kadam and S. K. Kulshreshtha, J. Alloys Compd. 438 (2007) A. K. Zak, W. H. A. Majid, M. E. Abrishami, R. Yousefi and R. Parvizi, Solid State Sci. 14 (2012) A.K.Zak,R.Yousefi,W.H.A.MajidandM.R.Muhamad,Ceram. Int. 38 (2012) Y.-B. Li, Y. Li, M.-Y. Zhu, T. Yang, J. Huang, H.-M. Jin and Y.-M. Hu, Solid State Commun. 150 (2010) J. M. Wesselinowa and A. T. Apostolov, J. Appl. Phys. 107 (2010) R. Yousefi, A. K. Zak and F. Jamali-Sheini, Ceram. Int. 39 (2013) Y. Yu, D. Chen, P. Huang, H. Lin and Y. Wang, Ceram. Int. 36 (2010) A. Escobedo-Morales and U. Pal, Curr. Appl. Phys. 11 (2011) R. S. Zeferino, M. B. Flores and U. Pal, J. Appl. Phys. 109 (2011) R. Yousefi, F. Jamali-Sheini, A. K. Zak and M. R. Mahmoudian, Ceram. Int. 38 (2012) S. L. Smitha, K. M. Nissamudeen, D. Philip and K. G. Gopchandran, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 71 (2008) Ma. De Lourdes Ruiz Peralta, U. Pal and R. S. Zeferino, ACS Appl. Mater. Interf. 4(9) (2012)