Existing Form of Zinc Oxide and Phase Transformation for Zinc Oxide Encapsulated in Mesoporous Silica

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1 Available online at SciVerse ScienceDirect J. Mater. Sci. Technol., 2013, 29(9), 841e845 Existing Form of Zinc Oxide and Phase Transformation for Zinc Oxide Encapsulated in Mesoporous Silica Qingshan Lu 1)*, Guohong Yun 1,2), Wenping Zhou 1), Jiangong Li 3) 1) Inner Mongolia Key Laboratory of Nanomagnetic and Functional Materials and College of Physical Science and Technology, Inner Mongolia University, Hohhot , China 2) College of Physics and Electronic Information, Inner Mongolia Normal University, Hohhot , China 3) Institute of Materials Science and Engineering, Lanzhou University, Lanzhou , China [Manuscript received February 22, 2012, in revised form October 18, 2012, Available online 29 June 2013] Nanocomposites of ZnO encapsulated in mesoporous silica were prepared by wetness impregnation and calcination. The samples were characterized by X-ray diffraction, transmission electron microscopy, nitrogen adsorptionedesorption isotherms, and X-ray photoelectron spectroscopy. The effects of ZnO content and thermal treatment on the existing form of ZnO as well as phase transformation were investigated. ZnO exists stably in the form of non-crystalline phase or cluster when crystallite size is small. With increasing ZnO content, as the size of ZnO reaches a critical size of crystalline phase, the non-crystalline ZnO or cluster transforms structurally to crystalline ZnO with low energy state. Besides, Zn 2 SiO 4 was obtained by solid-state reaction between ZnO and mesoporous silica. The mesoporous silica acts as not only a reactant but also a diffusion barrier which inhibits the phase transformation from b-zn 2 SiO 4 to a-zn 2 SiO 4. The formation temperature of Zn 2 SiO 4 is lower than that of conventional solid-state reaction because of the unique structure of mesoporous silica. KEY WORDS: Zinc oxide; Mesoporous silica; Solid-state reaction; Composites; Zinc silicate 1. Introduction Zinc oxide (ZnO), one of the most important semiconductors, has a wide bandgap of 3.37 ev and a high exciton binding energy of 60 mev at room temperature. During the last decades, ZnO has attracted great interests due to potential applications as ultraviolet lasers, solar cells, sensors, photocatalysts, piezotronics, etc [1e4]. ZnO nanostructures usually exhibit different properties from bulk counterparts [5]. Meanwhile, the size distribution and dispersion of ZnO are crucial for the performance [6]. All these have excited researches to develop new synthetic methods to prepare well-controlled ZnO nanomaterials. In recent years, a variety of chemical and physical growth techniques have been used to fabricate size and shape-controlled ZnO [2,7,8]. Among these techniques, the template-assisted synthesis involving the confined growth provides a simple, low-cost, and high yield synthetic route. Additionally, the template-assisted synthesis can * Corresponding author. Ph.D.; Tel.: þ ; address: luqs@imu.edu.cn (Q. Lu) /$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. not only limit the size of ZnO during the growth, but also avoid the formation of ZnO agglomerations. Mesoporous silica is an ideal hard template for the preparation of nanoparticles because of uniform pore size, ordered mesostructure, large specific surface areas, and high thermal and chemical stability [9]. Mesoporous silica could be regarded as a nanoreactor for constructing well dispersed nanomaterials with controlled size. The nanocomposites of ZnO encapsulated in mesoporous silica have been studied [10e14]. Up to date, three methods including wetness impregnation, surface modification, and two-solvent have been developed [15]. All these methods are based on the introduction of zinc precursor into silica pores, allowing the precursor to decompose by thermal treatment. Taking the photoluminescence properties of ZnO as an example, the results exhibit the most notable feature of the blue-shift due to the quantum size effect [12,13]. However, the studies show that ZnO encapsulated in mesoporous silica exists in different forms such as nanocrystalline, cluster or non-crystalline phase reported in literature [13,16]. Till now, the existing form of ZnO and the phase transformation for ZnO encapsulated in mesoporous silica toward ZnO content as well as thermal treatment have not been studied in detail. In this work, the nanocomposites of ZnO encapsulated in mesoporous silica (ZnO/MS) were prepared by wetness impregnation. The dependence of the existing form of ZnO and

2 842 Q. Lu et al.: J. Mater. Sci. Technol., 2013, 29(9), 841e845 the phase transformation on the ZnO content as well as thermal treatment was studied. 2. Experimental Parent mesoporous silica was synthesized according to the reported process [9]. The calcined mesoporous silica was rehydrated in water at 80 C for 1 h, and then heated in a vacuum at 150 C for 24 h to remove residual water. The ZnO/MS nanocomposites were prepared by conventional wetness impregnation. The procedure is as follows. An amount zinc nitrate hexahydrate was dissolved in water-ethanol solution with a water-to-ethanol volume ratio of 1. After the zinc nitrate was dissolved, the obtained mesoporous silica was added to the above zinc nitrate solution at once. The resulting solution was stirred at room temperature in a vacuum until all the solvents were evaporated in order to obtain dry powder. The dried powder was calcined at different temperatures for 4 h. The ZnO/MS nanocomposites with different ZnO contents are referred as x wt % ZnO/MS, where x represents the weight percentage of ZnO in the nanocomposites. X-ray diffraction (XRD) measurements were taken on a Rigaku D/Max-2400 X-ray diffractometer using CuKa radiation in qe2q scan mode. The measurement was performed at a scan rate of 5 /min with the step size of High-resolution transmission electron microscopy (HRTEM) observations were conducted on a JEOL JEM 2010 electron microscope operated at 200 kv. Nitrogen adsorptionedesorption isotherms were measured using a Micromeritics ASAP2010 surface area analyzer. BarretteEmmetteTeller (BET) method in the relative pressure p/p 0 range of 0.01e0.20 was used to calculate specific surface areas. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI-5702 spectrometer using an AlKa X-ray source. The energy scale was internally calibrated by referencing to the binding energy of the C 1s peak of a carbon contaminant at ev. 3. Results and Discussion Fig. 1 shows the XRD patterns of the ZnO/MS nanocomposites with different ZnO weight percentages calcined at 500 C for 4 h. Both of the 15 wt% and 30 wt% ZnO/MS nanocomposites exhibit the broad diffuse peaks attributed to the non-crystalline silica and ZnO. No diffraction peaks of the ZnO crystalline phase were detected, indicating that ZnO in the ZnO/ MS nanocomposites is non-crystalline or may exist as clusters with ultrafine particles [12]. This result is the same as other metal oxide clusters encapsulated in mesoporous silica [13,17]. As the weight percentage of ZnO increases to 40 wt%, the XRD pattern shows seven diffraction peaks overlapped on the broad diffuse peak of silica. The peaks can be attributed to the ZnO wurtzite [2], revealing the formation of crystalline ZnO in the 40 wt% ZnO/ MS nanocomposites calcined at 500 C. The intensity of ZnO diffraction peaks is weak due to poor crystallinity. The widths of these peaks are broad, indicating the small crystallite size of ZnO. This may be owing to the confined growth of ZnO inside the pores of mesoporous silica. As the weight percentage of ZnO reaches 60 wt%, the intense diffraction peaks confirm the formation of crystalline ZnO with high crystallinity. In addition, the diffraction peaks become sharp in comparison with that of the 40 wt% ZnO/MS nanocomposites, which can be ascribed to the increase of crystallite size. Based on the XRD results, the ZnO in ZnO/MS nanocomposites calcined at 500 C exists in the form of Fig. 1 X-ray diffraction patterns of the ZnO/MS nanocomposites with ZnO weight percentages of 15 wt% (a), 30 wt% (b), 40 wt% (c), and 60 wt% (d) calcined at 500 C for 2 h. non-crystalline phase or clusters at low weight percentage, and in the form of crystalline phase at high weight percentage. Veprek et al. [18] reported that nanocrystalline Si with the average crystallite size less than 3.5 nm can reduce its excess energy stored in the high density grain boundaries, if it transforms structurally from a nanocrystalline to a non-crystalline structure. That is to say, Si can exist stably in the form of non-crystalline phase when the crystallites are smaller than 3.5 nm and in the form of crystalline phase when the crystallites are larger than 3.5 nm. In fact, this type of structural transformation has also been observed for Pd [19]. In this study, the zinc nitrate solution was introduced into the pores of mesoporous silica due to capillary forces. During the decomposition of zinc nitrate at 500 C, the ZnO forms and disperses on the large surface of mesoporous silica. When the weight percentage of ZnO is below 30 wt%, the size of ZnO is ultrafine. In order to reduce the energy, the ZnO exists stably in the form of non-crystalline phase or clusters. With increasing the weight percentage of ZnO to 40 wt%, the XRD pattern shows the weak and broad diffraction peaks of ZnO, demonstrating the formation of crystalline ZnO. The high ZnO weight percentage results in the increase of the average crystallite size. When the size reaches a critical size of crystalline phase, the non-crystalline ZnO or cluster transforms to crystalline ZnO with a low energy state. With increasing the weight percentage of ZnO further to 60 wt%, the crystallite size of ZnO increases, and the crystallinity of ZnO is improved. These are supported by the intense and sharp diffraction peaks. Fig. 2(a) shows the TEM micrograph of 15 wt% ZnO/MS nanocomposites calcined at 500 C for 4 h. The 15 wt% ZnO/ MS nanocomposites have a hexagonal mesoporous structure the same as parent mesoporous silica [9]. The non-crystalline ZnO or clusters could not be observed by TEM. This may be due to the weak image contrast between the silica framework and ZnO cluster, as in the case of ZnO or other metal oxide clusters inside the micropores of zeolite [17]. The TEM micrograph of 40 wt% ZnO/MS nanocomposites calcined at 500 C for 4 h is shown in Fig. 2(b). The sample keeps hexagonal mesoporous structure, but the periodic structure is destroyed partly. This is resulted from the decomposition of zinc nitrate and the nucleation as well as growth of crystalline ZnO during calcination. According to the selected area electron diffraction pattern, the crystalline ZnO exists in the ZnO/MS nanocomposites, which is coincident with the XRD result. The ZnO agglomerations are also present in the nanocomposites. The size of ZnO agglomerations is larger than the pore size of mesoporous silica. This indicates that a part of

3 Q. Lu et al.: J. Mater. Sci. Technol., 2013, 29(9), 841e Fig. 2 Transmission electron microscopy micrographs of ZnO/MS nanocomposites calcined at 500 C for 4 h with ZnO weight percentages of 15 wt% (a), 40 wt% (b), and dark field micrograph of 40 wt% ZnO/MS nanocomposites (c) (the inset in (b) shows the corresponding SAED pattern). ZnO grows outside the pores of mesoporous silica to form agglomerations. The more zinc nitrate is added, the more ZnO would form and attach to the wall of pores. Parts of ZnO would block the pores when the weight percentage is above 40 wt%. This would lead to the formation of ZnO agglomerations outside the pores. Fig. 2(c) shows the dark-field TEM micrograph of the 40 wt% ZnO/MS nanocomposites. It is obvious that part of the images shows high bright contrast, indicating the presence of ZnO crystals in the ZnO/MS nanocomposites. To confirm the presence of ZnO clusters in the ZnO/MS nanocomposites with the ZnO weight percentage below 40 wt%, the XPS spectra of the 15 wt% ZnO/MS nanocomposites calcined at 500 C for 4 h were studied. As shown in Fig. 3(a), the peak at ev is ascribed to the core level of Zn 2p 3/2 for pure ZnO [4]. Fig. 3(b) shows the XPS spectra of O 1s at a binding energy of ev. The peak is asymmetric, and it can be fitted by two Gaussian peaks. The weak shoulder peak at about ev is related to the oxygen of ZnO [20]. The main peak at about ev can be assigned to the oxygen of the noncrystalline silica. It is well-known that the valence electron density of O in the SieOeSi bond is lower than that in the Zne OeZn bond, due to the higher electronegativity of Si (1.9) than that of Zn (1.65) [21]. The XPS results confirm that ZnO exists in the ZnO/MS nanocomposites calcined at 500 C, when the weight percentage of ZnO is below 40 wt%. In order to determine the effect of calcination temperature on the existing form of ZnO and the phase transformation, the 15 wt % ZnO/MS nanocomposites were calcined at various temperatures for 4 h. As shown in Fig. 4(a), the sample calcined at 500 C yields only a diffraction halo attributed to silica and ZnO. With increasing the calcination temperature to 700 C, there are no diffraction peaks of ZnO crystalline phase. The ZnO formed inside the mesoporous silica still exists stably in the form of noncrystalline state or cluster. The size of ZnO is too small to form crystallites. The thermal treatment does not induce the size of ZnO reaching the critical size of a crystalline phase. This may be attributed to the factor as follows. Mesoporous silica with large surface areas can disperse ultrafine ZnO clusters [12]. The lower weight percentage of ZnO is, the better dispersed ZnO clusters and the larger distance among ZnO clusters are. Herein mesoporous silica with large surface areas could be regarded as a diffusion barrier, which would increase the crystallinity activation energy. The ZnO clusters encapsulated in mesoporous silica exist stably at a high temperature of 700 C, which enlarges the scope of its applications. After calcination at 800 C, the XRD pattern shows that the b-phase of zinc silicate (b-zn 2 SiO 4 ) forms because the solid-state reaction between ZnO clusters and mesoporous silica takes place [22]. As well-known, b-zn 2 SiO 4 is a meta-stable phase. b-zn 2 SiO 4 is quite stable at room temperature and only at high temperature it transforms to a-phase of zinc silicate (a-zn 2 SiO 4 ). In our study, b-zn 2 SiO 4 does not transform to a-zn 2 SiO 4 at 800 C. For the 15 wt% ZnO/MS nanocomposites, the ZnO weight percentage is as low as 15 wt%, and the mesoporous silica is abundant in the solid-state reaction. The ZnO clusters react with silica, and the formed b-zn 2 SiO 4 disperses in the wall of mesoporous silica. The crystalline b-zn 2 SiO 4 is considered to be surrounded by non-crystalline silica. This may be attributed to the fact that non-crystalline silica can suppress the phase transformation from b-zn 2 SiO 4 to a-zn 2 SiO 4, which is similar to the reported result that the silica phase of mesoporous silica/titania nanocomposites could restrain the phase transformation from anatase to rutile TiO 2 and hinder the growth of rutile TiO [23] 2. Herein mesoporous silica acts as not only a reactant but also a diffusion barrier. Fig. 3 X-ray photoelectron spectroscopic results of the 15 wt% ZnO/MS nanocomposites calcined at 500 C for 4 h: (a) Zn 2p, (b) O 1s.

4 844 Q. Lu et al.: J. Mater. Sci. Technol., 2013, 29(9), 841e845 Fig. 4 X-ray diffraction patterns of the 15 wt% ZnO/MS nanocomposites calcined for 4 h at different temperatures (a), and low-angle X-ray diffraction pattern and nitrogen adsorptionedesorption isotherms (in the inset) of the 15 wt% ZnO/MS nanocomposites calcined at 800 C for 4 h (b). The low-angle XRD pattern of the 15 wt% ZnO/MS nanocomposites calcined at 800 C is shown in Fig. 4(b). There are three well-resolved diffraction peaks, similar to that of mesoporous silica [9,23]. The nitrogen adsorptionedesorption isotherms of the 15 wt% ZnO/MS nanocomposites calcined at 800 C is shown in the inset of Fig. 4(b). The sample shows the type IV isotherms with H1-type hysteresis loops, typical for mesoporous materials with two-dimensional hexagonal structures [9,11,13], which is coincident with the results obtained from the low-angle XRD analysis. Additionally, the BET surface area of the sample is 564 m 2 /g. The weight percentage of 15 wt% is relatively low, and the solid-state reaction between silica framework and ZnO does not destroy the mesoporous structure during thermal treatment. Crystalline b-zn 2 SiO 4 with mesoporous structure is firstly reported in our study, and it may have great potential applications in absorption, separation, catalysis, phosphors and so on. Fig. 5(a) exhibits the XRD patterns of the 60 wt% ZnO/MS nanocomposites calcined for 4 h at various temperatures. The diffraction peaks corresponding to ZnO can be observed [2,3]. This demonstrates that the crystalline ZnO are formed by the decomposition of zinc nitrate after 500 C. At 700 C, besides the diffraction peaks assigned to ZnO crystalline phase, some additional peaks are detected. The diffraction peaks are assigned to the a-zn 2 SiO [22,24] 4. This is ascribed to the fact that mesoporous silica reacts with crystalline ZnO. The a-zn 2 SiO 4 forms at 700 C while the ZnO weight percentage is as high as 60 wt%. The b-zn 2 SiO 4 was not detected by XRD, indicating that the meta-stable phase b-zn 2 SiO 4 has transformed to stable a- Zn 2 SiO 4 during thermal treatment [22]. After calcination at 800 C, the diffraction peaks of the crystalline ZnO disappear, the XRD pattern has only the diffraction peaks of a-zn 2 SiO 4. Recently, Cho and Chang reported that manganese doped zinc silicate was prepared with appropriate oxides by solid-state reaction at 1400 C [25]. Compared to the traditional solid-state reaction method, the a-zn 2 SiO 4 can be obtained at a low calcination temperature of 800 C in our study. The relatively low reaction temperature and the fast reaction kinetics may be attributed to two main factors. Firstly, mesoporous silica has high surface areas and hence a higher Gibbs free energy [26]. The higher Gibbs free energy can reduce the reaction activation energy. Secondly, the mesoporous silica with nanoscale pores can lead nitrate solution into the pores during mixing process. Zinc oxide disperses inside the pores in subsequent calcination. Due to the large surface areas of mesoporous silica [9], the reaction interfaces among the reactants are increased. The diffusion distance for the solid-state reaction is greatly reduced. Therefore, the reaction kinetics is enhanced and the reaction temperature is decreased. The low-angle XRD pattern of the 60 wt% ZnO/MS nanocomposites calcined at 800 C is shown in Fig. 5(b). No diffraction peaks are observed in the low-angle region, in comparison to diffraction peaks of mesoporous silica. The N 2 absorptionedesorption isotherms of the 60 wt% ZnO/MS Fig. 5 X-ray diffraction patterns of the 60 wt% ZnO/MS nanocomposites calcined for 4 h at different temperatures (a), and low-angle X-ray diffraction pattern and nitrogen adsorptionedesorption isotherms (in the inset) of the 60 wt% ZnO/MS nanocomposites calcined at 800 C for 4 h (b).

5 Q. Lu et al.: J. Mater. Sci. Technol., 2013, 29(9), 841e nanocomposites calcined at 800 C is displayed in the inset of Fig. 5(b). In contrast with mesoporous silica [9], the a-zn 2 SiO 4 sample shows straight lines; and the absorption and desorption isotherms overlap with each other. This implies that there is no capillary condensation step in the a-zn 2 SiO 4 sample [13]. The BET surface area of the sample is as low as 8 m 2 /g. The results indicate the disappearance of the mesoporous structure for the a-zn 2 SiO 4. During calcination, a-zn 2 SiO 4 particles grow along the silica framework in the consumption of the silica. As a result, after the mesoporous silica is mostly consumed, the mesoporous structure contracts and collapses; the dense crystalline a-zn 2 SiO 4 forms. 4. Conclusion The wetness impregnation was used to prepare the ZnO/MS nanocomposites. The weight percentage of ZnO is a key factor to control the existing form of ZnO and the phase transformation. The ZnO exists in the form of non-crystalline phase or clusters as the crystallite size is small. With increasing ZnO content, when the crystallite size reaches a critical size of the crystalline phase, the non-crystalline ZnO or cluster transforms structurally to crystalline ZnO in order to reduce the energy. By increasing the calcination temperature, the solid-state reaction between ZnO and mesoporous silica takes place, and the Zn 2 SiO 4 forms. The mesoporous silica acts as not only a reactant but also a diffusion barrier which can inhibit the phase transformation from b- Zn 2 SiO 4 to a-zn 2 SiO 4. The formation temperature of Zn 2 SiO 4 is much lower than that by means of the conventional solid-state reaction. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant No , the China Postdoctoral Science Foundation under Grant No. 2012M510789, the Natural Science Foundation of Inner Mongolia Autonomous Region under Grant No. 2011BS0804, and the Program of Higher-level Talents of Inner Mongolia University under Grant No. Z REFERENCES [1] J.W.P. Hsu, D.R. Tallant, R.L. Simpson, N.A. Missert, R.G. Copeland, Appl. Phys. Lett. 88 (2006) [2] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Angew. Chem. Int. Ed. 42 (2003) 3031e [3] R. Ranjusha, P. Lekha, K.R.V. Subramanian, V. Nair Shantikumar, A. Balakrishnan, J. Mater. Sci. Technol. 27 (2011) 961e966. [4] M.M. Rahman, M.K.R. Khan, M.R. Islam, M.A. Halim, M. Shahjahan, M.A. Hakim, D.K. Saha, J.U. Khan, J. Mater. Sci. Technol. 28 (2012) 329e335. [5] K.F. Lin, H.M. Cheng, H.C. Hsu, L.J. Lin, W.F. Hsieh, Chem. Phys. Lett. 409 (2005) 208e211. [6] M.L. Kahn, M.M. Monge, V. Collière, F. Senocq, A. Maisonnat, B. Chaudret, Adv. Funct. Mater. 15 (2005) 458e468. [7] Y.F. Gao, M. Nagai, Y. Masuda, F. Sato, K. Koumoto, J. Cryst. Growth 286 (2006) 445e450. [8] J. Gong, X. Zhang, Z. Pei, C. Sun, L. Wen, J. Mater. Sci. Technol. 27 (2011) 393e397. [9] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024e6036. [10] C. Cannas, M. Mainas, A. Musinu, G. Piccaluga, Compos. Sci. Technol. 63 (2003) 1187e1191. [11] W. Zeng, Z. Wang, X.F. Qian, J. Yin, Z.K. Zhu, Mater. Res. Bull. 41 (2006) 1155e1159. [12] H.G. Chen, J.L. Shi, H.R. Chen, J.N. Yan, Y.S. Li, Z.L. Hua, Y. Yang, D.S. Yan, Opt. Mater. 25 (2004) 79e84. [13] Q. Jiang, Z.Y. Wu, Y.M. Wang, Y. Cao, C. Zhou, J. Zhu, J. Mater. Chem. 16 (2006) 1536e1542. [14] C. Bouvy, E. Chelnokov, W. Marine, R. Sporken, B.L. Su, J. Non- Cryst. Solids 355 (2009) 1152e1156. [15] W. Yue, W. Zhou, Chem. Mater. 19 (2007) 2359e2363. [16] L.I. Burova, D.I. Petukhov, A.A. Eliseev, A.V. Lukashin, Y.D. Tretyakov, Superlattice Microstruct. 39 (2006) 257e266. [17] J. Chen, Z. Feng, P. Ying, C. Li, J. Phys. Chem. B 108 (2004) 12669e [18] S. Veprek, Z. Iqbal, F.A. Sarott, Philos. Mag. B 45 (1982) 137e145. [19] P. Keblinski, S.R. Phillpot, D. Wolf, H. Gleiter, Acta Mater. 45 (1997) 987e998. [20] J. Zheng, J. Song, Q. Jiang, J. Lian, J. Mater. Sci. Technol. 28 (2012) 103e108. [21] Z. Fu, B. Yang, L. Li, W. Dong, C. Jia, W. Wu, J. Phys. Condens. Matter 15 (2003) 2867e2873. [22] N. Taghavinia, G. Lerondel, H. Makino, A. Yamamoto, T. Yao, Y. Kawazoe1, T. Goto, Nanotechnology 12 (2001) 547e551. [23] W. Dong, Y. Sun, C.W. Lee, W. Hua, X. Lu, Y. Shi, S. Zhang, J. Chen, D. Zhao, J. Am. Chem. Soc. 129 (2007) 13894e [24] M. Takesue, A. Suino, K. Shimoyama, Y. Hakuta, H. Hayashi, R.L. Smith, J. Cryst. Growth 310 (2008) 4185e4189. [25] T.H. Cho, H.J. Chang, Ceram. Int. 29 (2003) 611e618. [26] P. Mohanty, S. Ram, Mater. Lett. 53 (2002) 287e295.