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Chemical synthesis and functional properties of monodispersed lanthanum phosphate nanorods
Materials Letters 112 (2013) 16–19
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Chemical synthesis and functional properties of monodispersed lanthanum phosphate nanorods R. Annie Sujatha a, M. Navaneethan b, J. Archana b,n, P. Stalin Joseph c, S. Ponnusamy a, C. Muthamizhchelvan a, Y. Hayakawa b a
Department of Physics and Nanotechnology, SRM University, Kattankulathur, Chennai 603203, Tamil Nadu, India Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan c Department of Physics, Kaveri College of Engineering and technology, Tiruchirapalli, Tamil Nadu, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2013 Accepted 25 August 2013 Available online 31 August 2013
Monodispersed LaPO4 nanorods were synthesized by wet chemical method without the aid of organic additives. The effect of concentration of the precursors on the formation of nanorods was investigated. XRD studies revealed the formation of monoclinic phased LaPO4 nanorods. The effect of concentration of the precursor on the electronic state of LaPO4 nanorods was studied by XPS analysis. TEM analysis confirmed the monodispersed LaPO4 nanorods with an average diameter of 10 nm and length of 150 nm. HRTEM analysis revealed the higher crystallinity of the synthesized LaPO4 nanorods. The obtained results demonstrated that the monodispersed nanorods with dominant luminescence properties were obtained for the precursor ratio of 1:2 (lanthanum nitrate:phosphoric acid). & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanorods Lanthanum phosphate Morphology Optical properties
1. Introduction In recent years, rare earth doped nanostructures have been greatly focused owing to their high luminescent nature. Among the various rare earth elements, lanthanide receives a special attention due to its unique electron configuration (4f electrons) and other optical and magnetic properties [1,2]. Lanthanide nanocrystals have various applications such as solid-state lasers, luminescent markers, biolabels and displays [3–5]. On the other hand, there were number of host materials available; in which phosphate provides some advantages of having high quality fluorescent lighting due to its quantum fluorescent lighting and it retains the stability at higher temperature [6,7]. Therefore, lanthanum phosphate (LaPO4) has been highly investigated to fabricate the luminescent devices. However, the size confinement and dimensionality are considered to be an important key factor which governs the properties of the nanomaterials owing to their unique optical, thermal, electronic and chemical properties. The one-dimensional (1D) structures [8–12] like nanowires, nanorods, nanotubes, nanobelts were expected to be more efficient comparing to zero-dimensional (0D) structures. Various methods have been adopted for the synthesis of LaPO4 nanostructures such as hydrothermal, wet chemical method, etc. [13–16]. Kai et al. prepared the LaPO4 nanorods by water and ethylene glycol mixed solvothermal route. They observed that the increase in the ratio of ethylene glycol
n
Corresponding author. Tel./fax: þ 81 53 478 1338. E-mail address: [email protected] (J. Archana).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.104
favors the formation of nanorods [17]. Shankar et al. synthesized the LaPO4 nanorods by sol–gel method and the aligned nanorods with size of 25–100 nm and average length of about 50 nm were obtained [18]. Ekthammathat group synthesized the nanorods in de-ionized water by microwave irradiation. LaPO4 nanorods were obtained with the average length of 600–1000 nm and 20–40 nm in diameter [19]. Xiajuan et al. prepared LaPO4 nanorods with the dimensions of 8 nm in diameter and 80 nm in length by oil bath method at 100 1C [20]. Among these methods, wet chemical method is considered to be simple and inexpensive route. However, the excess amount of organic additives during the chemical synthesis of the inorganic material results the quenching of luminescence and increase of defect level emission [21]. Since, there are no reports on the organic additives free wet chemical synthesis of well-defined monodispersed LaPO4 nanorods. It is very important to optimize the growth parameters to obtain the monodispersed LaPO4 nanorods. In this paper we report the synthesis of LaPO4 nanorods by wet chemical method. The effect of precursor concentration on the formation of nanorods and functional properties were investigated. The possible growth mechanism of the LaPO4 nanorods is discussed. 2. Experimental All the chemicals were purchased from Aldrich Company and used without further purification. In a typical procedure 0.1 mol of lanthanum nitrate (LaNO3)3 and 0.1 mol of phosphoric acid
R.A. Sujatha et al. / Materials Letters 112 (2013) 16–19
17
(H3PO4) were added to the 50 ml of de-ionized water. The mixture of the solution was allowed to stir for 5 h. Then the resultant solution was annealed at 200 1C for 3 h. Finally, the samples were dried at 90 1C for 12 h and collected; this sample was labeled as A. The similar procedure was followed for sample B with the variation of LaNO3 from 0.1 to 0.2 mol, whereas the H3PO4 was fixed at 0.1 mol. For sample C the LaNO3 concentration was fixed at 0.1 mol and the concentration of H3PO4 was varied from 0.1 to 0.2 mol. XRD pattern was recorded using a Rigaku (Japan) X-ray diffractometer (RINT-2200, Cu Kα radiation) at 0.02 deg/s as the step interval. UV–visible analysis was performed by a Shimadzu (Japan) 3100 PC spectrophotometer using ethanol as a dispersing medium. The analysis of X-ray photoelectron spectroscopy (XPS) was performed via a Shimadzu ESCA 3100. The transmission electron microscope (TEM) images were recorded by a JEOL JEM 2100F microscope at an accelerating voltage of 200 kV. Photoluminescence spectra were recorded using a spectrometer (Hamamatsu Photonics) with 325-nm He–Cd laser excitation.
3. Results and discussion Fig. 1 shows the XRD pattern of the samples. All the diffraction peaks of LaPO4 can be indexed as the monoclinic phase of LaPO4 with the lattice constants of a ¼6.84, b¼ 7.07, c¼ 6.54 Å and β¼10.851. It has good agreement with the JCPDS card no. 35-0731. No other peak related to the secondary phase of impurity was detected. Fig. 2 shows the XPS spectra of monoclinic LaPO4 1D nanorods. In Fig. 2(a) two strong peaks locating at 835.5 eV and 855.5 eV can be assigned to the binding energy of (La 3d5/2). All three samples exhibited similar peak position for the La 3d5/2. The peaks observed at 534 eV (Fig. 2(b)) and 285.5 eV (Fig. 2(c)) correspond to the binding energies of (O1s) and (P2s) core level respectively. There was no shift in the peak position of O1s and P2s states of sample A and B. Whereas, sample C showed slight shifts in the O1s and P2s state. This can be attributed to the presence of more amounts of O and P from the PO4 group. Fig. 3(a), (c), and (e) represents the TEM images of LaPO4 in the ratio of (1:1), (2:1) and (1:2) respectively. When La(NO3)3:H3PO4 was in equal ratio, LaPO4 indicated rod-like morphology with the average length of 100–150 nm with the diameter of 10–20 nm. Fig. 3(a) shows that the nanorods are not well defined and not uniform. It indicates the initial stage for the formation of nanorods
Fig. 2. XPS analysis of (a) La state, (b) O state and (c) P state of samples A, B and C.
Fig. 1. XRD patterns of samples A, B and C.
by the primary crystals attaching with its neighboring crystal by oriented attachment mechanism. When the ratio of La(NO3)3: H3PO4 was 2:1. There was a monodispersed formation of LaPO4 nanorods with the average length of 100–150 nm and the diameter of 15–20 nm as shown in Fig. 3(c). Moreover, when the ratio of La(NO3)3:H3PO4 is 1:2. The rod shape got ruined and somewhat agglomerated irregular structure was observed. The magnified TEM images were shown in Fig. 3(b), (d), and (f) for sample A, B and C, respectively. HRTEM images were shown as inset.
18
R.A. Sujatha et al. / Materials Letters 112 (2013) 16–19
Fig. 3. (a) and (b) TEM images of sample A (inset: HRTEM), (c) and (d) TEM images of sample B (inset: HRTEM) and (e) and (f) TEM images of sample C (inset: HRTEM).
These images revealed that all the samples had good crystalline nature. From the above experimental fact, it is clear that the 2:1 ratio of La(NO3)3:H3PO4 favors the monodispersed formation of nanorods. In the present work, the concentration ratio of La(NO3)3: H3PO4 plays a critical role in the formation of nanorods. In general, the lanthanum has a significant affinity towards the oxygen atom. La3 þ ions readily form a co-ordinate bond with the PO4 ions to form LaPO4 nuclei. The concentration of
La(NO3)3:H3PO4 is (1:1), La3 þ establishes the co-ordination and initiates the LaPO4 nuclei formation. When the concentration ratio of La(NO3)3:H3PO4 is (2:1) excess of La3 þ interacts with the PO4 ions and it extends to form a boundary over the nuclei to avoid the sterical hindrance, thus it favors the growth of nanorod without any agglomeration. When the concentration ratio of La(NO3)3: H3PO4 is (1:2) the excess of PO4 ions do not have the ability to form the co-ordination bond due to the lack of La3 þ , thus it retards
R.A. Sujatha et al. / Materials Letters 112 (2013) 16–19
19
than other two samples. It can be attributed to the presence of fewer defects in sample B. Whereas sample A and sample C have more defects due to the lack and excess of La and PO4 ions. Sample B exhibited the strong emission under UV light illumination compared to other two samples. Moreover, the obtained results were well agreed with TEM analysis. The well-defined and monodispersed nature of sample B exhibited the higher intensity in the photoluminescence.
4. Conclusions Monodispersed LaPO4 nanorods have been successfully synthesized by optimized precursor concentrations without the aid of organic additives. 1:2 M concentration ratio of La(NO3)3:H3PO4 yielded the well-defined monodispersed nanorods. XRD studies reveal the formation of monoclinic phased LaPO4 nanorods. TEM analysis confirms the monodispersed LaPO4 nanorods with an average diameter of 10 nm and length of 150 nm. Photoluminescence studies revealed the higher intensity for the ratio of 1:2.
References
Fig. 4. (a) UV visible absorption and (b) photoluminescence spectra of samples A, B and C.
the formation of nanorods by disturbing the LaPO4 nuclei. The chemical equation for the formation of LaPO4 nanorods was expressed as La(NO3)3 þH3PO4-LaPO4 þ3HNO3
(1)
UV visible absorption spectra of the samples were shown in Fig. 4(a). All the three samples exhibited the absorption in the region of 300–450 nm. The absorption onsets of the samples were 300 nm for sample A, 298 nm for sample B and 302 nm for sample C. The photoluminescence spectra of the samples were shown in Fig. 4(b) and inset shows the samples under UV light illumination. All the three samples showed two distinct peaks with the peak center of 400 nm and 516 nm. Sample B shows the higher intensity
[1] Lehmann O, Meyssamy H, Kömpe K, Schnablegger H, Haase M. Journal of Physical Chemistry B 2003;107:7449. [2] Huignard A, Buissette V, Laurent G, Gacoin T, Boilot JP. Chemistry of Materials 2002;14:2264. [3] Heer S, Lehmann O, Haase M, Gudel HU. Angewandte Chemie International Edition 2003;42:3179. [4] Sivakumar S, Diamente PR, Van Veggel FCJM. Chemistry – A European Journal 2006;12:5878. [5] Stouwdam JW, Van Veggel FCJM. Nano Letters 2002;2:733. [6] Lucas S, Champion D, Bernache A, Leroy G. Journal of Solid State Chemistry 2004;177:1312. [7] Rao RP, Devine DJ. Journal of Luminescence 2000;87:1260. [8] Gao R, Qian D, Li W. Transactions of Nonferrous Metals Society of China 2010;20:432. [9] LiXin Y, Hongwei S, Shaozhe L, Zhongxin L, Linmei Y, Tie W, et al. Materials Research Bulletin 2004;39:2083. [10] Mary JF, Wehzhong W, Peter KD, Ellen RF. Journal of Physical Chemistry C 2008;112:1901. [11] Xiaojuan W, Mingyuan G. Journal of Materials Chemistry 2006;16:1360. [12] Lin X, Hongwei S, Biao D, Yu W, Xue B, Guolei W, et al. Journal of Physical Chemistry C 2009;113:9609. [13] Brown SS, Im H, Rondinone AJ, Dai S. Journal of Colloid and Interface Science 2005;292:127. [14] Xu L, Wei H, Dong B, Wang Y, Bal X, Wang GL, et al. Journal of Physical Chemistry C 2009;113:9609. [15] Zhu HL, Zhu E, Yang H, Wang L, Jin DL, Yao KH. Journal of the American Ceramic Society 2008;91:1682. [16] Wu XY, You HP, Cui HT, Zeng XQ, Hong GY, Kim CH, et al. Materials Research Bulletin 2002;37:1531. [17] Kai M, Yonghong N, Yanwei X, Xiang M, Jianming H. Journal of Colloid and Interface Science 2011;356:490. [18] Sankar S, Warrier KG. Journal of Sol–Gel Science and Technology 2011;58:195. [19] Ekthammathat N, Thongtem T, Phuruangrat A, Thontem S. Journal of Experimental Nanoscience 2012;7:616. [20] Wang X, Gao M. Journal of Materials Chemistry 2006;16:1360. [21] Navaneethan M, Archana J, Arivanandhan M, Hayakawa Y. Chemical Engineering Journal 2012;231:70.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Chemical synthesis and functional properties of monodispersed lanthanum phosphate nanorods R. Annie Sujatha a, M. Navaneethan b, J. Archana b,n, P. Stalin Joseph c, S. Ponnusamy a, C. Muthamizhchelvan a, Y. Hayakawa b a
Department of Physics and Nanotechnology, SRM University, Kattankulathur, Chennai 603203, Tamil Nadu, India Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan c Department of Physics, Kaveri College of Engineering and technology, Tiruchirapalli, Tamil Nadu, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2013 Accepted 25 August 2013 Available online 31 August 2013
Monodispersed LaPO4 nanorods were synthesized by wet chemical method without the aid of organic additives. The effect of concentration of the precursors on the formation of nanorods was investigated. XRD studies revealed the formation of monoclinic phased LaPO4 nanorods. The effect of concentration of the precursor on the electronic state of LaPO4 nanorods was studied by XPS analysis. TEM analysis confirmed the monodispersed LaPO4 nanorods with an average diameter of 10 nm and length of 150 nm. HRTEM analysis revealed the higher crystallinity of the synthesized LaPO4 nanorods. The obtained results demonstrated that the monodispersed nanorods with dominant luminescence properties were obtained for the precursor ratio of 1:2 (lanthanum nitrate:phosphoric acid). & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanorods Lanthanum phosphate Morphology Optical properties
1. Introduction In recent years, rare earth doped nanostructures have been greatly focused owing to their high luminescent nature. Among the various rare earth elements, lanthanide receives a special attention due to its unique electron configuration (4f electrons) and other optical and magnetic properties [1,2]. Lanthanide nanocrystals have various applications such as solid-state lasers, luminescent markers, biolabels and displays [3–5]. On the other hand, there were number of host materials available; in which phosphate provides some advantages of having high quality fluorescent lighting due to its quantum fluorescent lighting and it retains the stability at higher temperature [6,7]. Therefore, lanthanum phosphate (LaPO4) has been highly investigated to fabricate the luminescent devices. However, the size confinement and dimensionality are considered to be an important key factor which governs the properties of the nanomaterials owing to their unique optical, thermal, electronic and chemical properties. The one-dimensional (1D) structures [8–12] like nanowires, nanorods, nanotubes, nanobelts were expected to be more efficient comparing to zero-dimensional (0D) structures. Various methods have been adopted for the synthesis of LaPO4 nanostructures such as hydrothermal, wet chemical method, etc. [13–16]. Kai et al. prepared the LaPO4 nanorods by water and ethylene glycol mixed solvothermal route. They observed that the increase in the ratio of ethylene glycol
n
Corresponding author. Tel./fax: þ 81 53 478 1338. E-mail address: [email protected] (J. Archana).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.104
favors the formation of nanorods [17]. Shankar et al. synthesized the LaPO4 nanorods by sol–gel method and the aligned nanorods with size of 25–100 nm and average length of about 50 nm were obtained [18]. Ekthammathat group synthesized the nanorods in de-ionized water by microwave irradiation. LaPO4 nanorods were obtained with the average length of 600–1000 nm and 20–40 nm in diameter [19]. Xiajuan et al. prepared LaPO4 nanorods with the dimensions of 8 nm in diameter and 80 nm in length by oil bath method at 100 1C [20]. Among these methods, wet chemical method is considered to be simple and inexpensive route. However, the excess amount of organic additives during the chemical synthesis of the inorganic material results the quenching of luminescence and increase of defect level emission [21]. Since, there are no reports on the organic additives free wet chemical synthesis of well-defined monodispersed LaPO4 nanorods. It is very important to optimize the growth parameters to obtain the monodispersed LaPO4 nanorods. In this paper we report the synthesis of LaPO4 nanorods by wet chemical method. The effect of precursor concentration on the formation of nanorods and functional properties were investigated. The possible growth mechanism of the LaPO4 nanorods is discussed. 2. Experimental All the chemicals were purchased from Aldrich Company and used without further purification. In a typical procedure 0.1 mol of lanthanum nitrate (LaNO3)3 and 0.1 mol of phosphoric acid
R.A. Sujatha et al. / Materials Letters 112 (2013) 16–19
17
(H3PO4) were added to the 50 ml of de-ionized water. The mixture of the solution was allowed to stir for 5 h. Then the resultant solution was annealed at 200 1C for 3 h. Finally, the samples were dried at 90 1C for 12 h and collected; this sample was labeled as A. The similar procedure was followed for sample B with the variation of LaNO3 from 0.1 to 0.2 mol, whereas the H3PO4 was fixed at 0.1 mol. For sample C the LaNO3 concentration was fixed at 0.1 mol and the concentration of H3PO4 was varied from 0.1 to 0.2 mol. XRD pattern was recorded using a Rigaku (Japan) X-ray diffractometer (RINT-2200, Cu Kα radiation) at 0.02 deg/s as the step interval. UV–visible analysis was performed by a Shimadzu (Japan) 3100 PC spectrophotometer using ethanol as a dispersing medium. The analysis of X-ray photoelectron spectroscopy (XPS) was performed via a Shimadzu ESCA 3100. The transmission electron microscope (TEM) images were recorded by a JEOL JEM 2100F microscope at an accelerating voltage of 200 kV. Photoluminescence spectra were recorded using a spectrometer (Hamamatsu Photonics) with 325-nm He–Cd laser excitation.
3. Results and discussion Fig. 1 shows the XRD pattern of the samples. All the diffraction peaks of LaPO4 can be indexed as the monoclinic phase of LaPO4 with the lattice constants of a ¼6.84, b¼ 7.07, c¼ 6.54 Å and β¼10.851. It has good agreement with the JCPDS card no. 35-0731. No other peak related to the secondary phase of impurity was detected. Fig. 2 shows the XPS spectra of monoclinic LaPO4 1D nanorods. In Fig. 2(a) two strong peaks locating at 835.5 eV and 855.5 eV can be assigned to the binding energy of (La 3d5/2). All three samples exhibited similar peak position for the La 3d5/2. The peaks observed at 534 eV (Fig. 2(b)) and 285.5 eV (Fig. 2(c)) correspond to the binding energies of (O1s) and (P2s) core level respectively. There was no shift in the peak position of O1s and P2s states of sample A and B. Whereas, sample C showed slight shifts in the O1s and P2s state. This can be attributed to the presence of more amounts of O and P from the PO4 group. Fig. 3(a), (c), and (e) represents the TEM images of LaPO4 in the ratio of (1:1), (2:1) and (1:2) respectively. When La(NO3)3:H3PO4 was in equal ratio, LaPO4 indicated rod-like morphology with the average length of 100–150 nm with the diameter of 10–20 nm. Fig. 3(a) shows that the nanorods are not well defined and not uniform. It indicates the initial stage for the formation of nanorods
Fig. 2. XPS analysis of (a) La state, (b) O state and (c) P state of samples A, B and C.
Fig. 1. XRD patterns of samples A, B and C.
by the primary crystals attaching with its neighboring crystal by oriented attachment mechanism. When the ratio of La(NO3)3: H3PO4 was 2:1. There was a monodispersed formation of LaPO4 nanorods with the average length of 100–150 nm and the diameter of 15–20 nm as shown in Fig. 3(c). Moreover, when the ratio of La(NO3)3:H3PO4 is 1:2. The rod shape got ruined and somewhat agglomerated irregular structure was observed. The magnified TEM images were shown in Fig. 3(b), (d), and (f) for sample A, B and C, respectively. HRTEM images were shown as inset.
18
R.A. Sujatha et al. / Materials Letters 112 (2013) 16–19
Fig. 3. (a) and (b) TEM images of sample A (inset: HRTEM), (c) and (d) TEM images of sample B (inset: HRTEM) and (e) and (f) TEM images of sample C (inset: HRTEM).
These images revealed that all the samples had good crystalline nature. From the above experimental fact, it is clear that the 2:1 ratio of La(NO3)3:H3PO4 favors the monodispersed formation of nanorods. In the present work, the concentration ratio of La(NO3)3: H3PO4 plays a critical role in the formation of nanorods. In general, the lanthanum has a significant affinity towards the oxygen atom. La3 þ ions readily form a co-ordinate bond with the PO4 ions to form LaPO4 nuclei. The concentration of
La(NO3)3:H3PO4 is (1:1), La3 þ establishes the co-ordination and initiates the LaPO4 nuclei formation. When the concentration ratio of La(NO3)3:H3PO4 is (2:1) excess of La3 þ interacts with the PO4 ions and it extends to form a boundary over the nuclei to avoid the sterical hindrance, thus it favors the growth of nanorod without any agglomeration. When the concentration ratio of La(NO3)3: H3PO4 is (1:2) the excess of PO4 ions do not have the ability to form the co-ordination bond due to the lack of La3 þ , thus it retards
R.A. Sujatha et al. / Materials Letters 112 (2013) 16–19
19
than other two samples. It can be attributed to the presence of fewer defects in sample B. Whereas sample A and sample C have more defects due to the lack and excess of La and PO4 ions. Sample B exhibited the strong emission under UV light illumination compared to other two samples. Moreover, the obtained results were well agreed with TEM analysis. The well-defined and monodispersed nature of sample B exhibited the higher intensity in the photoluminescence.
4. Conclusions Monodispersed LaPO4 nanorods have been successfully synthesized by optimized precursor concentrations without the aid of organic additives. 1:2 M concentration ratio of La(NO3)3:H3PO4 yielded the well-defined monodispersed nanorods. XRD studies reveal the formation of monoclinic phased LaPO4 nanorods. TEM analysis confirms the monodispersed LaPO4 nanorods with an average diameter of 10 nm and length of 150 nm. Photoluminescence studies revealed the higher intensity for the ratio of 1:2.
References
Fig. 4. (a) UV visible absorption and (b) photoluminescence spectra of samples A, B and C.
the formation of nanorods by disturbing the LaPO4 nuclei. The chemical equation for the formation of LaPO4 nanorods was expressed as La(NO3)3 þH3PO4-LaPO4 þ3HNO3
(1)
UV visible absorption spectra of the samples were shown in Fig. 4(a). All the three samples exhibited the absorption in the region of 300–450 nm. The absorption onsets of the samples were 300 nm for sample A, 298 nm for sample B and 302 nm for sample C. The photoluminescence spectra of the samples were shown in Fig. 4(b) and inset shows the samples under UV light illumination. All the three samples showed two distinct peaks with the peak center of 400 nm and 516 nm. Sample B shows the higher intensity
[1] Lehmann O, Meyssamy H, Kömpe K, Schnablegger H, Haase M. Journal of Physical Chemistry B 2003;107:7449. [2] Huignard A, Buissette V, Laurent G, Gacoin T, Boilot JP. Chemistry of Materials 2002;14:2264. [3] Heer S, Lehmann O, Haase M, Gudel HU. Angewandte Chemie International Edition 2003;42:3179. [4] Sivakumar S, Diamente PR, Van Veggel FCJM. Chemistry – A European Journal 2006;12:5878. [5] Stouwdam JW, Van Veggel FCJM. Nano Letters 2002;2:733. [6] Lucas S, Champion D, Bernache A, Leroy G. Journal of Solid State Chemistry 2004;177:1312. [7] Rao RP, Devine DJ. Journal of Luminescence 2000;87:1260. [8] Gao R, Qian D, Li W. Transactions of Nonferrous Metals Society of China 2010;20:432. [9] LiXin Y, Hongwei S, Shaozhe L, Zhongxin L, Linmei Y, Tie W, et al. Materials Research Bulletin 2004;39:2083. [10] Mary JF, Wehzhong W, Peter KD, Ellen RF. Journal of Physical Chemistry C 2008;112:1901. [11] Xiaojuan W, Mingyuan G. Journal of Materials Chemistry 2006;16:1360. [12] Lin X, Hongwei S, Biao D, Yu W, Xue B, Guolei W, et al. Journal of Physical Chemistry C 2009;113:9609. [13] Brown SS, Im H, Rondinone AJ, Dai S. Journal of Colloid and Interface Science 2005;292:127. [14] Xu L, Wei H, Dong B, Wang Y, Bal X, Wang GL, et al. Journal of Physical Chemistry C 2009;113:9609. [15] Zhu HL, Zhu E, Yang H, Wang L, Jin DL, Yao KH. Journal of the American Ceramic Society 2008;91:1682. [16] Wu XY, You HP, Cui HT, Zeng XQ, Hong GY, Kim CH, et al. Materials Research Bulletin 2002;37:1531. [17] Kai M, Yonghong N, Yanwei X, Xiang M, Jianming H. Journal of Colloid and Interface Science 2011;356:490. [18] Sankar S, Warrier KG. Journal of Sol–Gel Science and Technology 2011;58:195. [19] Ekthammathat N, Thongtem T, Phuruangrat A, Thontem S. Journal of Experimental Nanoscience 2012;7:616. [20] Wang X, Gao M. Journal of Materials Chemistry 2006;16:1360. [21] Navaneethan M, Archana J, Arivanandhan M, Hayakawa Y. Chemical Engineering Journal 2012;231:70.