Template-free synthesis of LaPO4:Eu3+ hollow spheres with enhanced luminescent properties

Template-free synthesis of LaPO4:Eu3+ hollow spheres with enhanced luminescent properties

Journal of Alloys and Compounds 532 (2012) 72–77 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage...

1MB Sizes 0 Downloads 3 Views

Journal of Alloys and Compounds 532 (2012) 72–77

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Template-free synthesis of LaPO4 :Eu3+ hollow spheres with enhanced luminescent properties Xia Li ∗ , Huifang Bi College of Materials Science and Technology, Qingdao University of Science & Technology, Qingdao 266042, PR China

a r t i c l e

i n f o

Article history: Received 26 December 2011 Received in revised form 23 March 2012 Accepted 26 March 2012 Available online 11 April 2012 Keywords: Phosphors Chemical synthesis Luminescence

a b s t r a c t Uniform LaPO4 :Eu micrometer-sized hollow spheres were synthesized through a simple solution-based hydrothermal method followed by a subsequent calcination process without using any surfactant, catalyst, or template. The phase composition and the microstructure of as-prepared products were characterized by field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), Fourier transform infrared spectrum (FT-IR) and photoluminescence spectroscope (PL). FE-SEM characterization showed that mono-disperse micrometer-sized hollow spheres with an average diameter of 3 ␮m were obtained. The formation mechanism for LaPO4 :Eu micrometer-sized hollow sphere was proposed based on the reaction results. Studies on photoluminescence indicated that the micrometer-sized hollow spheres had a stronger orange–red emission corresponding to 5 D0 → 7 F1 transition than that of the corresponding nanorods. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Over the past decades, inorganic nanostructures with welldefined shapes and sizes have attracted growing attention because of their unique size- and shape- dependent properties as well as their widespread potential applications in miniaturized photonic, electronic, and magnetic sensor devices [1–5]. Among these inorganic nanostructured materials, lanthanide phosphate (LnPO4 ) nanostructures are particularly important because of their potential application in phosphor powder, advanced flat panel displays, biological and chemical labels, and so on [6–11]. Different kinds of morphologies or structures of lanthanide phosphates, such as zero-dimensional nanoparticles [6], onedimensional [7,8], self-assembly micro-architectures [9], core/shell [10] and polygonal structures [11] have been obtained by a variety of methods in the past few years. Remarkably, hollow nano/microspheres currently represent one of the fastest growing areas compared with other structural and geometric features. The fabrication of hollow microspheres or nano-spheres with varying sizes has attracted fascinated interest, due to their distinct low effective densities, high specific surface areas, and potential scale-dependent applications in photonic devices, drug delivery, lightweight fillers, active materials encapsulation and robust catalysts/carriers. The hollow spheres of rare earth doped phosphors

∗ Corresponding author. E-mail address: [email protected] (X. Li). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.03.109

can reduce the use of expensive rare earth materials. Additionally, due to the low density of hollow spherical materials, when coating screens in display applications, the phosphors can dispersed well, enhance the uniformity, and give high packing densities of the coating [12,13]. Recently, many efforts have been made in the development of different methods for the design and preparation of hollow nano- or microspheres. One is to use hard templates [14,15], such as polymer latex particles, silica spheres, and metal nanoparticles. The other relies on soft templates [16,17], such as emulsion droplets, micelles, and gas bubble. However, these approaches are usually multistep processes, involving at least shell formation and core removal steps. Compared with the significant progress in the preparation of hollow nano- or microspheres of other systems, such as SiO2 [18,19], TiO2 [20,21], Y2 O3 (Gd2 O3 ) [22], etc., the synthesis of well-defined rare earth phosphate hollow spheres has been rarely studied. To our best knowledge, there are few reports on the synthesis of LaPO4 :Eu microsized spheres as well as corresponding optical properties [23,24]. Therefore, it is desirable to explore feasible, easily controllable, and highly repeatable methods for the synthesis of spherical hollow structures of rare earth phosphates with promising novel properties is highly demanded. In this paper, we report a simple synthesis of nearly mono-dispersed LaPO4 hollow microspheres through one-pot hydrothermal synthesis without applying any templates. The morphology, formation mechanism, and luminescent property of doped samples have been systemically studied and compared.

X. Li, H. Bi / Journal of Alloys and Compounds 532 (2012) 72–77


2. Experimental 2.1. Materials The initial chemicals used were polyphosphoric acid (H6 P4 O13 , A.R., Sinopharm Chemical Reagent Co. Ltd), La(NO3 )3 ·6H2 O (Aldrich, in purity 99.99%), Eu2 O3 (99.99%, Shanghai Yuelong Non-Ferrous Metals Limited, China), NaH2 PO4 and HNO3 (A.R., Beijing Fine Chemical Company, China). All chemicals were used as received. Eu(NO3 )3 aqueous solution was prepared by dissolving Eu2 O3 in dilute HNO3 (25%) solution under heating(40–50 ◦ C) with agitation.

2.2. Synthesis procedures For a typical microsized hollow spheres synthesis, 1.9 mmol of La(NO3 )3 ·6H2 O and 0.1 mmol Eu(NO3 )3 aqueous solutions were mixed together with 35 ml deionized water. Then a 5 ml aqueous solution of H6 P4 O13 (P2 O5 ≥ 80.0%) was added into the above mixture under vigorous stirring at 50 ◦ C. After vigorous stirring for 1 h, a white colloidal suspension was obtained and then was transferred to a 50 ml Teflon-lined autoclave. It was heated to 120 ◦ C for 12 h, and was then cooled to room temperature naturally. The resulting product was collected using a centrifuge, washed several times with distilled water, and dried at 60 ◦ C for 12 h. The corresponding product was labeled as MS1. The above product was retrieved through a heat treatment at 700 ◦ C in air for 2 h, and then the final product was labeled as MS2. For comparison, the nanorods were obtained by the same procedure [25]. To prepare the precursor solution, 1.9 mmol La(NO3 )3 ·6H2 O and 0.1 mmol Eu(NO3 )3 were dissolved in distilled water, then 5 ml NaH2 PO4 aqueous solution (1.00 M) was added dropwise under vigorous stirring. The pH value of the resultant solution was adjusted to 1.0 with dilute HNO3 (1.0 M). The resulting precursor colloidal was poured into a 50 ml Teflon-lined autoclave, which was filled with distilled water up to 80% of the total volume, sealed, and heated to the set temperature. The system was then cooled to room temperature. The resulting product was collected using a centrifuge, washed several times with distilled water, and dried at 60 ◦ C for 12 h. The corresponding products, i.e., precursor by hydrothermal synthesis and post heat treatment sample were labeled as NR1 and NR2, respectively.

2.3. Measurements and characterization The as-prepared samples were characterized by powder X-ray diffraction (XRD) performed on a Rigaku/Max-3A X-ray diffractometer, operating at 40 kV/50 mA ˚ Infrared specusing graphite monochromatized Cu K␣ radiation ( = 1.5406 A). troscopy spectra were measured with PerkingElmer 580B Fourier transform infrared spectrophotometer using the KBr pellet technique. The morphology and the structure of the samples were inspected using a field emission scanning electron microscope (FE-SEM) JSM-6700F equipped with energydispersive X-ray (EDX). Low-resolution transmission electron microscope (TEM) images were collected on JEM 2000EX with an accelerating voltage of 200 kV. Highresolution transmission electron microscope (HRTEM) images and fast FFT patterns were recorded on the JEM-2100 transmission electron microscope, operating at 200 kV. Photo-luminescent (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer, equipped with a 150 W xenon lamp as the excitation source, at room temperature. Cathodoluminescent measurements were carried out in an ultrahigh vacuum chamber, where the samples were excited by an electron beam at a voltage range of 1–5 kV with different filament currents, and the spectra were recorded on an F-4500 spectrophotometer.

3. Results and discussion Fig. 1 shows the XRD patterns of as-prepared samples. All the samples are well crystallized. According to the XRD results, the reaction at 120 ◦ C and pH 1–2 led to the formation of a pure hexagonal phase [10] [space group: P31 21 (152)] of LaPO4 ·0.5H2 O with lattice constants a = 0.710 nm and c = 0.649 nm (JCPDS no. 46-1439), either for MS1 or NR1. Consequently, these precursors were subjected to calcinations at the relatively moderate temperature of 700 ◦ C. Fig. 1c and d shows the XRD patterns of the final calcined samples. All reflection peaks can be easily indexed to the monoclinic LaPO4 (space group: P21 /n (14)), with cell parameters a = 0.683 nm, b = 0.707 nm, and c = 0.645 nm (JCPDS12-0283), which is in good agreement with that of the LaPO4 bulk crystal. No crystalline impurity, apart from the lanthanum phosphates, can be determined from the XRD analysis. As the reaction temperature increased, the significant improvement of the crystallinity was evident from the higher diffraction peak intensity and the smaller full width at half maximum (FWHM).

Fig. 1. XRD patterns of the obtained samples.

The morphology and the structure of the obtained MS1 product were investigated by FE-SEM. Fig. 2a gives the general SEM image of MS1, which shows that as-synthesized products are composed of nearly mono-dispersed spherical particles whose average size is about 2–3 ␮m. Some broken spheres observed from the SEM image confirmed the existence of the shell structure. A detailed structure of a few single particles with a broken shell is shown in Fig. 2b, which indicates the thickness of the shell is about 300 nm. A typical bright field TEM image and a selected area electron diffraction (SAED) pattern for the LaPO4 :Eu hollow spheres are shown in Fig. 2c, which consistent with the value shown in the SEM images. The SAED pattern image contains partial ring and dot patterns, indicating that the LaPO4 :Eu is of polycrystalline nature. This point can be confirmed by the XRD patterns further. Fig. 2d shows the obtained LaPO4 :Eu products after being annealed at 700 ◦ C. The post heat treated LaPO4 samples inherited their parent’s morphology, but they are slightly shrunken compared with the corresponding precursors, wherein the density of the former is higher than of the latter [10]. Fig. 2e and f shows a typical SEM image of NR1 an NR2, which displays the mormology of nanorods with uniform size, with diameters of ca.50–100 nm and lengths ranging from 1 to 3 ␮m. The growth mechanism was given in our previous report [25]. The hexagonal LaPO4 precursors were converted to monoclinic ones during the subsequent calcination process, as H2 O was completely eliminated. Nevertheless, the conversion did not lead to a change in the morphology. Such a transformation was common for lanthanum compounds and other decomposition [10,26]. To further investigate the formation mechanism of LaPO4 hollow spheres, time- dependent experiments were performed to gain insight into the evolution process. The morphology of the intermediate products collected at different synthesis time are shown in Fig. 3. At the early reaction stage (1 h), the primary nano-sized crystallites tend to aggregate together to form large solid spheres as presented in Fig. 3a. At this stage, the exterior of the spheres are packed much looser than the interior, indicating the intrinsic density variation inside these starting solid aggregates. When the reaction duration was prolonged to 2 h, solid evacuation started at a particular region underneath the immediate surface layer, which divided most of the solid spheres into discrete regions, resulting in the formation of semi-hollow homogenous core-shell structures, as shown in Fig. 3b and c. At 4 h, core and shell structures became separated (Fig. 3c), although the inter-space between the shell and the core was still small. At 12 h, complete hollow spheres were obtained (i.e., core was totally gone), as shown in Fig. 3d.


X. Li, H. Bi / Journal of Alloys and Compounds 532 (2012) 72–77

Fig. 2. FE-SEM and TEM images of the obtained samples. (a) Low-magnification SEM images (b) high-magnification SEM images (c) TEM image of MS1 (d) SEM and MS2 (e) SEM images of NR1 and (f) SEM images of NR2.

The homogeneous and well-crystallized microspheres are probably formed through a mechanism similar to those proposed for the hydrothermal preparation of SnO2 , anatase TiO2 , CdSe, and Y2 O3 hollow nanospheres [26–28], in which Ostwald ripening controlled the growth of spherical particles and the formation of hollow interiors. We also found that H6 P4 O13 played an important role in the micro-spheres formation in this process. In a controlled experiment, we used NaH2 PO4 instead of H6 P4 O13 as the PO4 3− source, but only LaPO4 nanorods were obtained. The reactivity and the association structure of NaH2 PO4 and H6 P4 O13 in solution were quite different. Adding Eu(NO3 )3 aqueous solution to the H6 P4 O13 solution, stirring for 2 h at 50 ◦ C lead to a transparent resulting solution, whereas adding Eu(NO3 )3 aqueous solution to the NaH2 PO4 solution led to an opalescent colloidal dispersion. This suggesting that the reactivity of H6 P4 O13 is much lower than that of NaH2 PO4 because of the slow generation of PO4 3− via the hydrolysis of H6 P4 O13 . Therefore, we think that H6 P4 O13 plays dual

roles. On one hand, it was a reactant to provide PO4 3− ; on the other hand it was a protective agent to control the growth rate of the precursor nano-particles in different directions [29]. We therefore hypothesize that La3+ ions should firstly coordinate with the P O group of H6 P4 O13 to form complexes in an aqueous solution, then a large amount of LaPO4 precursor nanocrystallites nucleate and quickly congregate to form larger solid spherical particles to decrease the surface energy. As a result of the presence of the association structure of H6 P4 O13 in the aqueous phase, the PO4 3− concentration of inside and outside the association structure results in smaller nanoparticles inside and in larger ones outside. Owing to the higher surface energy, smaller particle prefers to dissolve during Ostwald ripening. Mass transport thus gives rise to a core-shell structure, and then the diameter of the core gradually decreases with reaction time increasing. In the end, the core disappears and the hollow spheres form. As a result, the formation of the resultant hollow microspheres originates from the mass transfer from small

X. Li, H. Bi / Journal of Alloys and Compounds 532 (2012) 72–77


Fig. 3. SEM images of intermediate product synthesized with different reaction times: (a) 1 h, (b) 2 h, (c) 4 h and (d) 12 h.

crystallites inside the spheres to their outside because of Ostwald ripening, which does not require any additional reagent or template. On the basis of the above results and analysis, the detailed formation mechanism of hollow microspheres is depicted in Scheme 1. The FTIR spectra of all samples were studied in the range of 4000–500 cm−1 . As shown in Fig. 4, the broad band in the region of 3300–3600 cm−1 is caused by the O H (not hydrogen bond) stretching vibrations, with corresponding bending vibrations located at 1630 cm−1 . By comparing the strengths of the OH− bond in different samples, we can conclude that MS1 and MS2 adsorbed much water compared with NR1 and NR2. Hydroxyl content is

a reason for the decrease in emission intensity. The prominent peaks around 1060 cm−1 are from the phosphate P O stretching, which are typical features of the lanthanum phosphate in the monoclinic phase. This can be confirmed by XRD of MS2 and NR2. In the monoclinic form, the tetrahedral phosphate groups are distorted in the 9-fold coordination of La atoms [30,31]. The sharp band at approximately 620 cm−1 is attributed to the symmetrical stretching vibration (4) of the P O groups. The bands at about 540 cm−1 and 570 cm−1 are attributed to the bending vibration of the O P O groups. We can conclude that no vibration of P2 O7 groups is observed and that the product we synthesized is pure normal salts of phosphate.

Scheme 1. Schematic illustration of the formation process of LaPO4 hollow microspheres.


X. Li, H. Bi / Journal of Alloys and Compounds 532 (2012) 72–77 5 D –7 F (J = 0–2) can be easily seen, whereas the peaks of 5 D –7 F J 0 0 3 and 5 D0 –7 F4 are too weak to be detected. The magnetic-dipole transition 5 D0 –7 F1 (591 nm) is the strongest group compared with

Fig. 4. FTIR spectra of samples (a) MS1, (b) NR1, (c) MS2, and (d) NR2.

Room-temperature excitation spectra, at 611 nm emission for different samples are presented in Fig. 5 (left section). In the emission spectra, the broad band extending from 220 to 280 nm is associated with the charge transfer (CT) between 2p electrons of O2− and 4f electrons of Eu3+ . The CT bands in MS1 and MS2 have no shift compared with the bulk powders, whereas the CT bands for the nanorods (NR1 and NR2) red shifted. The intensity of the CT band in LaPO4 :Eu3+ nanorods also decreased by UV light irradiation just like the Y2 O3 :Eu3+ NPs, which was attributed to a local structure change surrounding Eu3+ ions at/near the surface [32]. Fig. 5 (right section) shows the excitation spectrum of Eu3+ doped LaPO4 with different morphologies at the excitation wavelength ex = 260 nm. The red 5 D0 –7 FJ (J = 0–4) transition is observed, as marked in the figure. For all samples, the peaks of

the others, and is characterized by an orange–red emission. The intensity of transitions between different J-number levels depends on the symmetry of the local environment of the europium ions and can be described in terms of the Judd–Ofelt theory [33]. The spectroscopic properties of the Eu3+ ion correspond to transitions between f-electron levels of the europium ions. The position and the intensity of these transitions are determined by the symmetry and the strength of the crystal field at the europium site, so Eu3+ ion is good for the chemical environment of the lanthanide ion. As it is well known, 5 D0 –7 F1 (591 nm) and 5 D0 –7 F2 (612 nm) emission are typical magnetic and electronic dipole–dipole transition, respectively, depending strongly on the local symmetry of the Eu3+ ion. It can be seen that electronic-dipole transition (5 D0 –7 F2 ) is not as intense as the magnetic-dipole transition (5 D0 –7 F1 ). Above luminescent properties of Eu3+ in the LaPO4 microspheres and nanorods are basically in agreement with those of nanowires and bulk materials reported previously, indicating that Eu3+ ions have been successfully doped into the host samples and the energy is effectively transferred from Eu3+ to La3+ [8]. Although the transition energies are the same for all samples, the intensity of their luminescence spectra show big differences. Compared with hexagonal LaPO4 (MS1 and NR1), the intensity of the peaks assigned to intra-4f transition increased greatly in monoclinic LaPO4 (MS2 and NR2). The emission spectra revealed the phase structure has a big effect on the luminescent intensity. The crystal structure of the sample obtained at a lower temperature was somewhat disordered, and quenching centers (such as OH− ions and adsorptive water on the surface as revealed in the FTIR) led to weakened emissions. High-temperature heat treatment greatly reduced these quenching centers, thereby improved the emissions. It worth noting that micro-sized hollow LaPO4 :Eu3+ spheres (i.e., MS2) exhibited stronger emission than nanorods (NR2) although they have the same phase structure. Such difference could be attributed to the following factors or even their

Fig. 5. Excitation and emission spectra of different samples. (a) NR1, (b) MS1, (c) NR2, and (d) MS2. (For interpretation of the references to color in the text for Fig. 5, the reader is referred to the web version of the article.)

X. Li, H. Bi / Journal of Alloys and Compounds 532 (2012) 72–77

combination. First, the nanorods prepared were highly agglomerated, as shown in the FE-SEM results. Second, and probably more importantly, MS2 possessed perfect morphology, i.e., monodispersed micro-spheres, which have high packing density and low scattering of light. 4. Conclusions In summary, microsized spheres of phosphors were synthesized using a one step hydrothermal method without templates and surfactants. On the basis of the hollowing evolution of LaPO4 spheres observed by SEM, the possible formation mechanism was proposed. The microsized hollow LaPO4 :Eu3+ spheres showed a strong orange–red emission corresponding to the 5 D0 –7 F1 transition of Eu3+ ions, which may find potential applications such as fields in phosphor powders, advanced flat panel displays, field emission display devices, and biological labeling. Acknowledgement This research was supported by the National Natural Science Foundation of China (Grant nos. 51072086, 50990303, 50872070). References [1] D.J. Milliron, S.M. Hughes, Y. Cui, L. Manna, J. Li, L.W. Wang, A.P. Alivisatos, Nature 430 (2004) 190. [2] C.C. Lin, R.S. Liu, Journal of Physical Chemistry Letters 2 (2011) 1268. [3] S.T. Mukherjeea, V. Sudarsana, P.U. Sastryb, A.K. Patrab, A.K. Tyagia, Journal of Alloys and Compounds 519 (2012) 9. [4] S.R. Gowda, A.L.M. Reddy, X.B. Zhan, P.M. Ajayan, Nano Letters 11 (2011) 3329. [5] D.Y. Kim, K.R. Choudhury, J.W. Lee, D.W. Song, G.o. Sarasqueta, F. So, Nano Letters 11 (2011) 2109. [6] X.W. Zhang, M.F. Zhang, Y.C. Zhu, P.F. Wang, F. Xue, J. Gu, H.Y. Bi, Materials Research Bulletin 45 (2010) 1324. [7] Z. Huo, C. Chen, D. Chu, H. Li, Y. Li, Chemistry A European Journal 13 (2007) 7708.


[8] Y.P. Fang, A.W. Xu, R.Q. Song, H.X. Zhang, You L.P. Yu, H.Q. Liu, Journal of the American Chemical Society 125 (2003) 16025. [9] M. Yang, H.P. You, Y.H. Song, Y. Huang, K. Liu, Y. Zheng, L. Zhang, H.J. Zhang, Journal of Physical Chemistry C 113 (2010) 20173. [10] Y.P. Fang, A.W. Xu, W.F. Dong, Small 1 (2005) 967. [11] W.W.S. Yu, E. Chang, J.C. Falkner, J. Zhang, A.M. Al-Somali, C.M. Sayes, J. Johns, R. Drezek, V.L. Colvin, Journal of the American Chemical Society 129 (2007) 2871. [12] J.H. Yang, X. Li, J.H. Lang, L.L. Yang, M. Gao, X.Y. Liu, M.B. Wei, Y. Liu, R. Wang, Journal of Alloys and Compounds 509 (2011) 10025. [13] G. Jia, M. Yang, Y.H. Song, H.P. You, H.J. Zhang, Crystal Growth and Design 9 (2009) 301. [14] G. Jia, C.M. Zhang, L.Y. Wang, S.W. Ding, H.P. You, Journal of Alloys and Compounds 509 (2011) 6418. [15] L.S Zhu, X.M. Liu, X.D. Liu, Q. Li, J.Y. Li, S.Y. Zhang, J. Meng, X.Q. Cao, Nanotechnology 17 (2006) 4217. [16] M. Todea, R.V.F. Turcu, B. Frentiu, M. Tamasan, H. Mocuta, O. Ponta, S. Simon, Journal of Molecular Structure 1000 (2011) 62. [17] X.J. Wang, F.Q. Wan, J. Liu, Y.J. Gao, K. Jiang, Journal of Alloys and Compounds 474 (2009) 233. [18] M. Chen, L. Wu, S. Zhou, B. You, Advanced Materials 18 (2006) 801. [19] P.M. Arnel, C. Weidenthaler, F. Schuth, Chemistry of Materials 18 (2006) 2733. [20] J.W. Hou, X.C. Yang, X.Y. Lv, M. Huang, Q.Y. Wang, J. Wang, Journal of Alloys and Compounds 511 (2012) 202. [21] J.H. Pan, Z.Y. Cai, Y. Yu, X.S. Zhao, Chemical Communications 47 (2011) 6942. [22] W.H. Di, X.G. Ren, L.H. Zhang, C.X. Liu, S.Z. Lu, Crystal Engineering Communications 13 (2011) 4831. [23] Z. Zhao, X. Zhang, J. Zuo, Z. Ding, Journal of Nanoscience and Nanotechnology 10 (2010) 7791. [24] M.S. Guan, F. Tao, J. Sun, Z. Xu, Langmuir 24 (2008) 8280. [25] X. Li, J. Ma, Journal of Luminescence 131 (2011) 1355. [26] J. Huo, L. Wang, E. Irran, H.J. Yu, J.M. Gao, D.S. Fan, B. Li, J.J. Wang, W.B. Ding, A.M. Amin, C. Li, L. Ma, Angewandte Chemie International Edition 122 (2010) 9423. [27] B. Liu, H.C. Zeng, Small 1 (2005) 566. [28] X.W. Lou, Y. Wang, C.L. Yuan, J.Y. Lee, L.A. Archer, Advanced Materials 18 (2006) 2325. [29] J. Yang, C.X. Li, Z.W. Quan, C.M. Zhang, P.P. Yang, X.Y. Li, C.C. Yu, J. Lin, Journal of Physical Chemistry C 112 (2008) 12777. [30] P.S. Ghosh, J. Oliva, E.D.I. Rose, K.K. Haldar, D. Solos, A. Patra, Journal of Physical Chemistry C 112 (2008) 9650. [31] L. Xu, H.W. Song, B. Dong, Y. Wang, G.L. Wang, X. Baiand, Journal of Physical Chemistry C 113 (2009) 9609. [32] H. Song, B. Chen, H. Peng, J. Zhang, Applied Physics Letters 81 (2002) 1776. [33] B.R. Judd, Physical Review 127 (1962) 750.