Synthesis and upconversion luminescence properties of CaF2:Yb ,Er ...

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nanoparticles obtained from SBA-15 template. Zhiguo Xiaa) and ... CaF2:Yb3ю,Er3ю upconversion (UC) luminescence nanoparticles have been synthesized.
Synthesis and upconversion luminescence properties of CaF2:Yb3+,Er3+ nanoparticles obtained from SBA-15 template Zhiguo Xiaa) and Peng Du School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, People’s Republic of China (Received 7 March 2010; accepted 14 April 2010)

CaF2:Yb3þ,Er3þ upconversion (UC) luminescence nanoparticles have been synthesized using mesoporous silica (SBA-15) as a hard template. The samples were characterized by x-ray diffraction, Fourier transform infrared spectra, field-emission scanning electron microscopy, transmission electron microscopy, and UC emission spectra, respectively. Highly crystalline cubic phase CaF2:Yb3þ,Er3þ nanoparticles are uniformly distributed with an average diameter of about 40–50 nm, and the formation process is also demonstrated. The UC fluorescence has been realized in the as-prepared CaF2:Yb3þ,Er3þ nanoparticles on 980-nm excitation. The UC emission transitions for 4F9/2–4I15/2 (red), 2 H11/2–4I15/2 (green), 4S3/2–4I15/2 (green), and 2H9/2–4I15/2 (violet) in the Yb3þ/Er3þ codoped CaF2 nanoparticles depending on pumping power and temperature have been discussed. The UC mechanism, especially the origin on the temperature-dependent UC emission intensities ratio between 2H11/2 and 4S3/2 levels, have been proposed. I. INTRODUCTION

It is well known that metal fluorides are efficient host lattices for luminescent centers because of their low phonon energy, wide optical transparency, and favorable chemical and mechanical proprieties.1,2 In particular, the CaF2 crystal with an optically isotropic fluorite structure, is suitable as a phosphor host; therefore, lanthanide (III)doped CaF2 compounds have been extensively studied for their prospective application potential in high density optical media for scintillators and fluorescent labeling materials.3–5 Many reports on lanthanide (III)-doped CaF2 bulk and powder materials became a host issue from the consideration of the synthesis method, crystal structure, and luminescent behavior.6–8 The reported phosphors in CaF2 host include the downconversion type CaF2 materials doped with Eu3þ, Eu2þ, Ce3þ, Tb3þ, Pr3þ, Dy3þ, and the upconversion (UC) type CaF2 materials doped with Yb3þ, Er3þ, Tm3þ, respectively.3–10 It is found that CaF2 nanoparticles attracted great attention particularly for the UC process, and many new methods have been explored to obtain CaF2 nanoparticles with UC emission properties. Cao et al.11 reported the synthesis of UC white light emission Tm3þ/Er3þ/Yb3þ tri-doped CaF2 nanoparticles using a simple hydrothermal method. Wang et al.12 successfully prepared sub-10 nm monodispersed CaF2:Yb3þ/Er3þ nanocrystals according to a so-called liquid/solid/solution (LSS) strategy. Bensalah et al.13 reported the synthesis a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0255 J. Mater. Res., Vol. 25, No. 10, Oct 2010

of undoped and Yb3þ/Er3þ ions codoped CaF2 nanoparticles with an average size of about 20 nm by the Igepal/cyclohexane/water reverse micelles method. Recently, Zhang et al.14 reported the synthesis of ordered mesostructured LaF3 nanoarrays by a nanocasting process using La(CF3COO)3 as a precursor and mesoporous silica (SBA-15) as a hard template, which has merits of one-step and easy operation. On the basis of this report, we consider that SBA-15 can be used as a hard template for confining the growth of nanoparticles by a wet impregnation method. Accordingly, in this work, CaF2:Yb3þ/Er3þ UC luminescence nanoparticles have been synthesized using SBA-15 as a hard template, and their phase formation, structure, and luminescence behavior are described in detail. II. EXPERIMENTAL

All the reagents including Ca(NO3)2 analytical reagent (A.R.), NH4BF4 (A.R.), Yb2O3 (99.99%), Er2O3 (99.99%), ethanol (A.R.), and dilute HNO3 (A.R.) were received without further purification. High-quality SBA-15 was obtained from Zhao’s group, which was prepared according to their previously reported method.15 For a typical synthesis of 0.002 mol Ca0.78F2:0.2Yb3þ,0.02Er3þ nanoparticles, the received SBA-15 was first dried in a vacuum oven at 80  C for 12 h. The 0.6 g dried sample was then dispersed in a mixed solution containing 3 mL distilled water and 15 mL ethanol, hereafter denoted as solution A. In addition, 0.0765 g Yb2O3 and 0.0077 g Er2O3 were dissolved in dilute HNO3 under vigorous stirring, resulting in the formation of a weak pink color solution. After evaporation of excessive HNO3, solution B © 2010 Materials Research Society

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was obtained by adding 0.2560 g Ca(NO3)2 and 7 mL distilled water. Solution C was prepared by dissolving 0.4204 g NH4BF4 in 7 mL distilled water, and stirring homogenously. Solution B was added dropwise to solution A with vigorous magnetic stirring for 2 h. Solution C was then added dropwise to the mixture of solution A and solution B with vigorous magnetic stirring for another 2 h. The formed suspension was then transferred to a Teflonlined (DuPont, Wilmington, DE) autoclave of 40 mL capacity, sealed, and heated at 160  C for 8 h. It was then cooled to room temperature naturally. The white precipitate was separated centrifugation and washed with distilled water and ethanol several times. The final products hereafter denoted as SBA-15/CaF2:Yb3þ,Er3þ composites were obtained after drying at 80  C for 12 h. Finally, silica (SBA-15) was dissolved by a 10 wt% HF solution and washed with sufficient distilled water to obtain the template-free CaF2:Yb3þ,Er3þ nanoparticles. As a reference, pure CaF2:Yb3þ,Er3þ nanoparticles were also prepared following the previous procedure without addition of SBA-15 template. The phase structure of the as-synthesized UC materials were determined by a Shimadzu XRD-6000 x-ray powder diffraction spectroscopy (Kyoto, Japan) operating at Cu Ka radiation, 40 kV, 30 mA, and a scan speed of 2.0 (2y)/min. Fourier transform infrared (FTIR) spectroscopy data were collected on a FTIT-AVATAR370 spectrophotometer (Madison, WI) over the range of wave number 4000400 cm1, and the standard KBr pellet technique was used. A field-emission scanning electron microscope (FE-SEM, JEOL, JSM-6330F, Tokyo, Japan) was used to analyze the size and shape of the selected samples. Transmission electron microscope (TEM) images were observed using a JEOL-2010 electron microscope with an acceleration voltage of 120 kV. The UC luminescence spectra were recorded on a Hitachi F-4500 spectrophotometer (Norwalk, CT) equipped with an external power-controllable 980-nm semiconductor laser (Beijing Viasho Technology Company, China) as the excitation source, which is connected with an optic fiber accessory. The temperature dependence UC spectra were measured by the same equipment previously mentioned, which was then combined with a self-made heating attachment and a computer-controlled electric furnace. III. RESULTS AND DISCUSSION

Figures 1(a) and 1(b) are XRD patterns of the samples SBA-15/CaF2:Yb3þ,Er3þ composites and CaF2:Yb3þ, Er3þ nanoparticles, respectively. All characteristic diffraction peaks at 2y ¼ 28.1 (111), 32.5 (200), 46.7 (220), 55.4 (311), and 58.5 (222), are presented, which are in good agreement with the standard values for the bulk cubic CaF2 (JCPDS 87-0791). In particular, except for the characteristic diffraction peaks of cubic CaF2, an obvi2036

FIG. 1. XRD patterns for (a) SBA-15/CaF2:Yb3þ,Er3þ composites and (b) CaF2:Yb3þ,Er3þ nanoparticles, and the JCPDS card 87-0971 for CaF2 is also given.

ous broad band centered at 2y ¼ 22.00 is observed for the sample of SBA-15/CaF2:Yb3þ,Er3þ composites, which is the characteristic peak for amorphous SiO2 (JCPDS 29-0085). This indicates that the CaF2:Yb3þ,Er3þ nanoparticles have crystallized on the surface of mesoporous silica (SBA-15). Furthermore, the lattice parameters of CaF2:Yb3þ,Er3þ were calculated by the UnitCell program in the cubic system based on the given XRD data in Fig. 1(b),16 and it has a cubic phase and lattice constants ˚ , u (cell volume) ¼ 165.1613  of aC ¼ 5.4866  0.0032 A 3 ˚ 0.2897 A . In general, the nanocrystallite size can be estimated from the Scherrer formula17: Dhkl ¼ Kl=ðb cos yÞ

;

ð1Þ

where l is the x-ray wavelength (0.15418 nm), b is the full width at half-maximum, y is the diffraction angle, K is a constant (0.89), and Dhkl is the size along the (hkl) direction. Here, we use diffraction data at 28.1 , 46.7 , and 55.4 to calculate the nanocrystallite size, which gives an estimated average size of 37 nm. Figures 2(a) and 2(b) give the FTIR spectra of asprepared SBA-15/CaF2:Yb3þ,Er3þ composites and CaF2: Yb3þ,Er3þ nanoparticles, respectively. As shown in Figs. 2(a) and 2(b), for the as-prepared SBA-15/CaF2: Yb3þ,Er3þ composites, the bands due to OH and H2O stretching are observed at 3437 and 1621 cm1, respectively.18 Furthermore, the characteristic absorption peaks of Si–O–Si bond at 1115 cm1 and the bending vibration of Si–O bond at 471 cm1 for amorphous SiO2 [Fig. 2(a)] have been observed clearly, which demonstrate the main phase of SBA-15 in SBA-15/CaF2:Yb3þ,Er3þ composites. As a comparison, no obvious absorption peaks from Sicontaining groups can be detected, and it indicates the main phase of CaF2:Yb3þ,Er3þ nanoparticles.

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Figure 3 displays the FE-SEM images of as-prepared SBA-15/CaF2:Yb3þ,Er3þ composites [Figs. 3(a) and 3(b)] and CaF2:Yb3þ,Er3þ nanoparticles [Figs. 3(c) and 3(d)] with different scale. It can be seen from Fig. 3(a) that some cubic CaF2:Yb3þ,Er3þ grains are irregularly distributed on the surface and wall edge of bend-like SBA-15 grains. Furthermore, Fig. 3(b) gives the FE-SEM image of SBA-15/CaF2:Yb3þ,Er3þ composites in 100-nm scale, which can be used to easily observe the existed position and morphology of CaF2:Yb3þ,Er3þ nanoparticles and SBA-15 template. It is found that most CaF2:Yb3þ,Er3þ

FIG. 2. FTIR spectra of as-prepared (a) SBA-15/CaF2:Yb3þ,Er3þ composites and (b) CaF2:Yb3þ,Er3þ nanoparticles.

nanoparticles grew along the nanochannels, not the inner pores of the SBA-15 template, suggesting that the confined growth of CaF2:Yb3þ,Er3þ nanoparticles takes place outside the wall edge of bend-like SBA-15 grains, not the channels of the SBA-15 template. We proposed that it is because of the larger grain size of CaF2:Yb3þ,Er3þ nanoparticles obtained by the chemical reaction of Ca(NO3)2 and NH4BF4 in the present hydrothermal conditions, which cannot enter inside SBA-15 nanochannels. In aqueous solution, NH4BF4 was hydrolyzed to produce BO33, HF, and F anions, and then F anions react with Ca2þ to form CaF2.19 Because the pore size of SBA-15 is so small (tens of nanometers), it is hard for as-prepared CaF2:Yb3þ,Er3þ nanoparticles to permeate into the pores. Furthermore, it is proved that the surface energy or surface functional groups (not the nanochannels) of SBA-15 controls the morphology of the as-prepared CaF2:Yb3þ,Er3þ nanoparticles. As given in Figs. 3(c) and 3(d), the asprepared CaF2:Yb3þ,Er3þ nanoparticles show a highly crystalline cubic phase, which are uniformly distributed with an average diameter of about 40–50 nm. The uniform size testifies the good template effect for the primary host of mesoporous silica SBA-15, which is presumably helpful to the nucleation and growth of the CaF2 nanoparticles in the controlled cubic structure and morphology. Figure 4 shows the TEM images of as-prepared SBA15/CaF2:Yb3þ,Er3þ composites viewed from [001] and [110] direction and CaF2:Yb3þ,Er3þ nanoparticles, respectively. As shown in Fig. 4(a), SBA-15/CaF2:Yb3þ, Er3þ composites clearly show a regular mesostructure in large domains of the [001] planes, and the inset gives a

FIG. 3. FE-SEM images of as-prepared (a, b) SBA-15/ CaF2:Yb3þ,Er3þ composites and (c, d) CaF2:Yb3þ,Er3þ nanoparticles with different scale. J. Mater. Res., Vol. 25, No. 10, Oct 2010

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continuous framework with ordered pipelike mesopores of the [110] direction.15,20 However, we can only find the SBA-15 grains and the above mesostructure in this scale for SBA-15/CaF2:Yb3þ,Er3þ composites. It is also found that the size of the nanochannels of SBA-15 is about 10 nm, so we cannot fill the larger CaF2:Yb3þ,Er3þ nanoparticles into the smaller nanochannels in the present experimental conditions. Figure 4(b) gives the TEM

FIG. 4. TEM images of as-prepared (a) SBA-15/CaF2:Yb3þ,Er3þ composites viewed from [001] and [110] direction and (b) CaF2: Yb3þ,Er3þ nanoparticles; the inset in (b) shows the single cubic phase CaF2:Yb3þ,Er3þ nanocrystal. 2038

image of CaF2:Yb3þ,Er3þ nanoparticles after the removal of the SBA-15 template, and the inset shows the single cubic phase CaF2:Yb3þ,Er3þ nanocrystal. It is found that the samples are of cubic shape with an average pore size of about 40–50 nm. Furthermore, to prove the template effect of SBA-15, the XRD pattern, FE-SEM images, and TEM image of CaF2:Yb3þ,Er3þ nanoparticles prepared by the hydrothermal method without addition of SBA-15 template are shown in Figs. A1–A3 in the Appendix. Although pure phase structure CaF2 can be obtained without SBA-15 template, the as-prepared CaF2:Yb3þ,Er3þ nanoparticles show some irregular shape not in cubic structure, and many grains are in conglobation. Figure 5 shows the UC emission spectra of asprepared SBA-15/CaF2:Yb3þ,Er3þ composites and CaF2: Yb3þ,Er3þ nanoparticles on an excitation wavelength of 980 nm, both of which have similar spectra profile corresponding to the same wavelength positions except for different emission intensities. The UC intensity of CaF2:Yb3þ,Er3þ nanoparticles is more than two times larger than that of SBA-15/CaF2:Yb3þ,Er3þ composites because of the existence of the main SBA-15 host. The weak violet emission centered at 410 nm, also enlarged in the inset, is attributed to the 2H9/2–4I15/2 transition of Er3þ ions. The obvious emission peaks observed at 524 and 540 nm are assigned to the (2H11/2, 4S3/2)–4I15/2 transitions of the Er3þ ions, while the observed red emission peak centered at 653 nm is attributed to the Er3þ4F9/2–4I15/2 transition.21 Furthermore, Fig. 6 gives the pumping power dependent upconversion spectra of CaF2:Yb3þ,Er3þ nanoparticles, suggesting that UC emission intensities increased gradually with increasing pumping power. To investigate the fundamental UC mechanism of the sample, the inset presents the pumping

FIG. 5. Upconversion spectra of as-prepared (a) SBA-15/CaF2:Yb3þ, Er3þ composites and (b) CaF2:Yb3þ,Er3þ nanoparticles. Inset: enlarged 2 H9/2–4I15/2 transition near 410 nm.

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FIG. 7. Schematic energy levels of Yb3þ and Er3þ ions in the as-prepared CaF2:Yb3þ,Er3þ nanoparticles obtained from SBA-15 template. FIG. 6. Power dependent upconversion spectra of CaF2:Yb3þ,Er3þ nanoparticles, and the inset shows ln–ln plots of 2H9/2–4I15/2 (violet), 2 H11/2(4S3/2)–4I15/2 (green), and 4F9/2–4I15/2 (red) transition emissions of CaF2:Yb3þ,Er3þ sample versus pump power.

power dependence of the 2H9/2–4I15/2 (violet), 2H11/2 (4S3/2)–4I15/2 (green), and 4F9/2–4I15/2 (red) transition emissions for CaF2:Yb3þ,Er3þ sample. It is well known that the emission intensity (If) will be proportional to some power (n) of the infrared excitation power (P)22: If / Pn

;

ð2Þ

where n is the number of photons required to populate the emitting state. As given in the inset, n ¼ 2.46, 1.74, and 1.84 for the 2H9/2–4I15/2, 2H11/2(4S3/2)–4I15/2, and 4 F9/2–4I15/2 emissions, respectively. This means that the population of the states 2H9/2, 2H11/2/4S3/2, and 4F9/2 came from three-, two-, and two-photon UC processes, respectively. The energy-level diagrams of Er3þ and Yb3þ ions in the as-prepared CaF2:Yb3þ,Er3þ nanoparticles obtained from the SBA-15 template are presented in Fig. 7. In the complex Yb3þ/Er3þ codoped CaF2 system, laser excitation of Yb3þ ions is only considered, and Yb3þ ions can efficiently sensitize Er3þ ions. The violet UC emission comes from the Yb3þ/ Er3þ pairs via a three-photon process. First, two energy transfers from Yb3þ ions can promote the Er3þ ion to 4 F7/2 state, and then part of the excited Er3þ ions at 4F7/2 state relax to 4S3/2 level, and another energy transfer from Yb3þ can pump the Er3þ ion from the 4S3/2 to 2 G7/2 level.23 Violet UC emission can be observed from the 2H9/2 level to the ground 4I15/2 level as a three-photon process after the nonradiative relaxation process from 2 G7/2 to 2H9/2 state. Since smaller electrons can reach 4 S3/2 level, and even smaller electrons can be pumped to the 2G7/2 level, we can observe weak violet emission. The green band from Yb3þ/Er3þ pairs is a two-photon process, which involves the two energy transfers from Yb3þ ions pumping Er3þ ion to the 4F7/2 state, as given

FIG. 8. Temperature dependent upconversion spectra of CaF2:Yb3þ, Er3þ nanoparticles, and the inset shows the relative intensities of 2 H11/2–4I15/2 and 4S3/2–4I15/2 transitions as a function of temperature.

in Fig. 7. Because the excited electrons of Er3þ ions at F7/2 state can relax to 2H11/2 and 4S3/2 levels, respectively, the green emission peak at 524 and 540 nm can be observed. Additionally, the electrons of Er3þ in the 4 F7/2 state can also decay nonradiatively to a lower energy state 4F9/2, and then the red UC emission is produced through the transition from 4F9/2 !4I15/2. To further investigate the possible application at high temperature, as well as the detailed UC mechanism, Fig. 8 gives the temperature dependent UC spectra of CaF2:Yb3þ,Er3þ nanoparticles. It is found that the overall UC emission intensities decreased sharply with increasing operating temperature. In particular, as given in the inset, it shows the corresponding intensities of 2H11/2– 4 I15/2 and 4S3/2–4I15/2 transitions as a function of temperature. It is clear that the intensity of 4S3/2–4I15/2 transition decreased faster than that of 2H11/2–4I15/2 transition. The UC emission intensity ratio between 4S3/2–4I15/2 and 4

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2

H11/2–4I15/2 transitions changes from about four times to one time. This phenomenon can be explained by the relative nonradiative relaxation possibility from 4F7/2 state to 4 S3/2 and 2H11/2 level. As is well known, increasing nonradiative relaxation acts as one of the major factors responsible for the decrease of the UC emission with increasing temperature.24 Considering that the energy difference between 4F7/2 and 4S3/2 level is smaller than that between 4F7/2 and 2H11/2 level, it is hard to promote more electrons to 4S3/2 state and form the 4S3/2–4I15/2 transition with increasing temperature. Therefore, the UC emission intensity of 4S3/2–4I15/2 decreases sharply, while that of 2 H11/2–4I15/2 keeps nearly invariable. The observation also proved the UC mechanism of Yb3þ/Er3þ pairs given in Fig. 7. IV. CONCLUSIONS

In conclusion, CaF2:Yb3þ,Er3þ UC luminescence nanoparticles have been synthesized using mesoporous silica (SBA-15) as a hard template. Pure cubic phase CaF2 can be obtained in the as-prepared SBA-15/CaF2:Yb3þ,Er3þ composites. Highly crystalline cubic phase CaF2:Yb3þ, Er3þ nanoparticles are uniformly distributed with an average diameter of about 40–50 nm after the removal of the SBA-15 template. The UC fluorescence has been realized in the as-prepared CaF2:Yb3þ,Er3þ nanoparticles on 980-nm excitation. The UC emission transitions for 2H9/2–4I15/2 (violet), 2H11/2(4S3/2)–4I15/2 (green), and 4 F9/2–4I15/2 (red) in the CaF2:Yb3þ,Er3þ nanoparticles came from three-, two-, and two-photon UC processes, respectively. The UC mechanisms, especially the origin on the temperature dependent UC emission intensities ratio between 2H11/2 and 4S3/2 levels have been proposed. ACKNOWLEDGMENTS

This work was supported by the Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20090022120002), the Fundamental Research Funds for the Central Universities (2010ZY35), and the College Student Research Innovation Program of China University of Geosciences, Beijing. We would also like to thank Prof. Libing Liao for his suggestions and for his financial support. REFERENCES 1. F. Wang and X.G. Liu: Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals. Chem. Soc. Rev. 38, 976 (2009). 2. J.H. Zeng, Z.H. Li, J. Su, L.Y. Wang, and Y.D. Li: Synthesis of complex rare earth fluoride nanocrystal phosphors. Nanotechnology 17, 3549 (2006). 3. Z.W. Quan, D.M. Yang, P.P. Yang, X.M. Zhang, H.Z. Lian, X.M. Liu, and J. Lin: Uniform colloidal alkaline earth metal fluoride nanocrystals: Nonhydrolytic synthesis and luminescence properties. Inorg. Chem. 47, 9509 (2008).

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APPENDIX

CaF2:Yb3þ,Er3þ nanoparticles can also be prepared by the same hydrothermal method without the SBA-15 template. As given in Fig. A1, pure phase structure CaF2 can be obtained from the observed XRD pattern. All characteristic diffraction peaks for the bulk cubic CaF2 (JCPDS 87-0791) have been clearly demonstrated. Figures A2(a) and A2(b) give the FE-SEM images of as-prepared CaF2: Yb3þ,Er3þ nanoparticles without SBA-15 under different scales, 1 mm and 200 nm, respectively. Compared to CaF2:Yb3þ,Er3þ nanoparticles prepared by the SBA-15 template method [Figs. 3(c) and 3(d)], the as-prepared CaF2:Yb3þ,Er3þ nanoparticles without SBA-15 template show some irregular shape, and many grains are in conglobation, which also testifies the effect of SBA-15 template. In addition, Fig. A3 gives the TEM images of as-prepared CaF2:Yb3þ,Er3þ nanoparticles without SBA-15. It is found that there is no obvious cubic structure CaF2:Yb3þ,Er3þ nanoparticles formed in the direct hydrothermal method. The previous results indicate that SBA-15 template play an important role in the preparation of uniform, cubic structure CaF2:Yb3þ,Er3þ nanoparticles; therefore, CaF2:Yb3þ,Er3þ nanoparticles prepared by SBA-15 template act as the key content in our present study.

FIG. A1. XRD pattern of as-prepared CaF2:Yb3þ,Er3þ nanoparticles without SBA-15-assisted synthesis.

FIG. A2. FE-SEM images of as-prepared CaF2:Yb3þ,Er3þ nanoparticles without SBA-15-assisted synthesis under different scale (a) 1 mm and (b) 200 nm.

FIG. A3. TEM images of as-prepared CaF2:Yb3þ,Er3þ nanoparticles without SBA-15-assisted synthesis.

J. Mater. Res., Vol. 25, No. 10, Oct 2010

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