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Facile synthesize α-MoO3 nanobelts with high adsorption property
Materials Letters 157 (2015) 53–56
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesize α-MoO3 nanobelts with high adsorption property Ying Ma a,b, Yulong Jia a,b, Zhengbo Jiao a,b, Lina Wang a,b, Min Yang a, Yingpu Bi a,n, Yanxing Qi a,n a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 February 2015 Received in revised form 4 May 2015 Accepted 21 May 2015
A simple approach is explored to synthesize orthorhombic MoO3 (α-MoO3) nanobelts using sodium molybdate (Na2MoO4) and fluoboric acid (HBF4). In the reaction, HBF4 not only serves as a reagent to provide H þ , but also a surfactant to manipulate the morphology and crystalline phase of the as-prepared products. The results show that the as-prepared samples are MoO3 nanobelts with 300 nm in width, 50 nm in thickness and 2–5 μm in length. Moreover, the possible formation mechanism of α-MoO3 nanobelts with HBF4 is studied. Finally, devoted to the belt-like shape and high stability, the MoO3 nanobelts exhibit a good adsorption performance for methyl blue (MB). & 2015 Elsevier B.V. All rights reserved.
Keywords: α-MoO3 nanobelts h-MoO3 nanorods HBF4 Formation mechanism Adsorption
1. Introduction One-dimensional molybdenum oxides nanostructures have always drawn considerable attention for their superior physical and chemical properties. 1-D MoO3 nanostructures such as the nanowires [1], nanotubes [2], nanorods [3], and nanobelts [4] have been studied and applied extensively as chemical and biological sensors [5], catalysts [6], battery electrodes [7] and supercapacitors [4]. Significantly, many works have been conducted to fabricate MoO3 nanobelts ascribed to the layered strucutre and anisotropic growth. For example, Li et al. [8] have prepared nanobelts on a Si wafer by heating a Mo foil in air at 850 °C. Zhou and coworkers [9] have synthesized nanobelts by hydrothermal treatment of peroxomolybdic acid solution. Cao’s group [10] have obtained nanobelts through a hydrothermal method by adding inorganic salts. All above methods have obtained MoO3 nanobelts with superior characteristics, but the hard reaction conditions and expensive precursors restrict their wide application, and there remains much interest in the morphology-controllable synthesis of MoO3 nanobelts with a facile and economical method. Herein, a simple hydrothermal route is employed to prepare single-crystal α-MoO3 nanobelts. To the best of our knowledge, n
Corresponding authors. E-mail addresses: [email protected], [email protected] (Y. Qi).
http://dx.doi.org/10.1016/j.matlet.2015.05.095 0167-577X/& 2015 Elsevier B.V. All rights reserved.
there has been no report on the synthesis of well-crystallized nanobelts utilizing fluoboric acid. This is of particular interest because HBF4 not only acts as the H þ resource, but also as a surfactant to control the formation of the products. The probable mechanism of effect of HBF4 is proposed through the comparative tests. In addition, the surface adsorption property for degradation of Methylene bule (MB) on α-MoO3 nanobelts is investigated. As a result, it can nearly adsorb the 80% and 100% of MB in 5 min and 60 min respectively, suggesting an excellent adsorption property.
2. Experiment All the chemical reagents are analytical grade and they are used without further purification. In a typical synthesis, 2.5 g (0.01 mol) sodium molybdate is dissolved in 30 ml of distilled water under magnetic stirring. Then 12 ml HBF4 is gradually added into the solution. After stirring for another 10 min, the obtained solution is transferred and sealed in a Teflon-lined stainless autoclave with a capacity of 100 ml, and the autoclave is heated to 180 °C for 24 h. After the reaction, the autoclave is allowed to cool down to the room temperature naturally. Finally, the precipitate is washed several times with absolute ethanol and distilled water, respectively, and then dried at 60 °C for several hours and kept for further characterization. As a contrast experiment, 1.67 g H2MoO4 (0.01 mol) is dissolved in 30 ml of distilled water and then 12 ml
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Y. Ma et al. / Materials Letters 157 (2015) 53–56
HBF4 is gradually added into the solution or not. Finally, the obtained solution is kept at 180 °C for 24 h by a hydrothermal reaction.
3. Results and discussion The morphologies of the obtained samples are characterized by field-emission scanning electron microscope (FESEM) as shown in Fig. 1a. It is clearly can be seen that the samples consist entirely of nanobelts with length of 2–5 μm, width of 300 nm and the thickness of 50 nm (inset image). Furthermore, the X-ray diffraction (XRD) spectrum (Fig. 1b) demonstrates the crystallography of the as-prepared products. All of the reflection peaks can be readily assigned to the orthorhombic MoO3 (α-MoO3) (JCPDS 05-0508 with lattice constants constants a ¼3.966 Å, b ¼13.858 and c¼ 3.693). Moreover, the absent peaks of other phases indicate that the high purity and the stronger intensities of the (020), (040) and (060) diffraction peaks suggest the anisotropic growth of the obtained MoO3 nanobelts [4,9,11]. More specifically, the selected area electron diffraction (SAED) pattern (inset of Fig. 1c) recorded perpendicular to the growth axis of the single nanobelts can be ascribed to the [010] zone axis diffraction of α-MoO3, suggesting that the samples grow along the [001] direction. Furthermore, in Fig. 1d, two sets of parallel fringes with a spacing of 0.38 nm and 0.35 nm correspond to (100) and (001) planes, respectively. The fully scanned spectra (Fig. S1) shows that the Mo and O elements exist on the surface of the samples and no obvious impurity peaks can be detected. The binding energy at 233.7 eV and 236.9 eV are
ascribed to the Mo3d5/2 and Mo3d3/2 of Mo6 þ oxidation state, respectively. Moreover, the corresponding integral areas ratio of Mo3d5/2/Mo3d3/2 is about 3:2 and the energy gap between the two doublets is 3.2 eV which is in a good agreement with α-MoO3 on the the previous report [12]. In order to study the role of BF4 on controlling synthesis of the products further and eliminate the other influence factors, contrast experiments are conducted by hydrothermal treatment of H2MoO4 with HBF4 or not. As can be seen in Fig. 2, without HBF4, only h-MoO3 nanorods (JCPDS 02-0659) with length of several micrometers and diameter of about 300 nm can be obtained. While in the presence of HBF4, the α-MoO3 nanobelts with 2–3 μm long, 100–150 nm wide can be synthseized. Considering the above results, it is easy to suppose that the HBF4 make an important function in preparing the α-MoO3 nanobelts. Moreover, the possible formation mechanism of α-MoO3 nanobelts with HBF4 is proposed and the scheme illustration of the growth process is exhibited in the Scheme 1. Just as H2WO4 [13], the H2MoO4 consists of [MO6] octahedral, which share their four equatorial oxygen atoms. More specifically, the molybdenum atom resides at the center and six oxygen atoms are coordinated octahedrally, thus forming the basic building blocks of h-MoO3 [14]. Then the freshly formed octahedral crystal nuclei grows and assembles to form a large amount of h-MoO3. In the case of adding HBF4, BF4 can selectively adsorb onto the crystal plane parallel to the c-axis of the MoO3 crystal nucleus so as to influence anisotropic growth and aggregation, resulting in α-MoO3 nanobelts. More specifically, the BF4 has a distorted tetrahedron structure, in which four fluorine atoms occupy the apexes of a tetrahedron
Fig. 1. The FESEM image (a) (inset is the large image of somewhere from image (a)), XRD pattern (b), TEM image (c) (inset is the SAED image) and the HRTEM image (d) of the obtained α-MoO3 nanobelts.
Y. Ma et al. / Materials Letters 157 (2015) 53–56
55
Fig. 2. FESEM image (a) and XRD pattern (c) of the products by hydrothermal treatment of H2MoO4, FESEM image (b) and XRD pattern (d) of the products by hydrothermal treatment of H2MoO4 and HBF4.
Scheme 1. Schematic representation of the formation pathways of h-MoO3 and α-MoO3 under controlled conditions.
centering on each boron atom [15]. Then BF4 can be added to the hydrogen bonds system, which may induce the oriented arrangement of the BF4 on the (010) plane of H2MoO4. Thus, the growth of the (010) plane of H2MoO4 is inhibited by the adsorption of BF4 , and α-MoO3 nanobelts are produced. To evaluate the potential application in water treatment of the asprepared α-MoO3 nanobelts, the adsorption capacities for the organic pollutant-Methylene bule (MB) are investigated. Fig. 3A exhibits the UV–vis absorption spectroscopy recording the absorption behavior of
the solution after treatment by using characteristic light absorption of MB at 664 nm. It is vividly that the intensity of characteristic peak for MB remarkably decreases with treatment in 5 minutes and reaches to almost zero after 60 min treatment, suggesting the high adsorption property of the MoO3 nanobelts. Moreover, as shown in Fig. 3B, in the first 5 min, featuring a sharp variation in removal ratios, can be deemed as a quick adsorption stage. The subsequent second stage between 5 and 10 min displays a smaller removal ratio of 10%. While almost no obvious change can be seen after 30 min,
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Y. Ma et al. / Materials Letters 157 (2015) 53–56
Fig. 3. Absorption spectra (a) and adsorption rate (b) of an aqueous solution of MB (20 mg L 1, 30 ml) after treatment with α-MoO3 nanobelts (30 mg) at different time intervals.
and the removal ratio reaches to nearly 100% after 60 min indicating that the adsorption of MB completes. Considering the similarity with WO3, the high adsorption property of MoO3 may be largely attributed to the electrostatic attraction between the surface and the MB species [16]. However, the exact adsorption mechanism of MoO3 nanobelts can not be completely understood and a more detailed study is still underway.
4. Conclusions In summary, we report a facile one-pot hydrothermal route for synthesis of α-MoO3 nanobelts using HBF4 as the H þ resource and the surfactant controlling the morphology and crystalline phase of product. Through comparative experiments, the effect of BF4 is explored and it is found that BF4 can induce the formation of α-MoO3 nanobelts. Characterizations show that the prepared samples exhibit an applicable adsorptive capacity for methylene blue. Moreover, the exact adsorption mechanism of MoO3 nanobelts can be further developed.
Acknowledgments This work is supported by the Chinese Academy of Sciences and Technology Project (XBLZ-2011-013) and the Technologies R&D Program of Gansu Province (No. 1104FKCA156).
Appendix A. Suplementary Information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015.05.095.
References [1] R.Q. Song, A.W. Xu, B. Deng, Y.P. Fang, J. Phys. Chem. B 109 (2005) 22758–22766. [2] S. Hu, X. Wang, J. Am. Chem. Soc. 130 (2008) 8126–8127. [3] X.W. Lou, H.C. Zeng, Chem. Mater. 14 (2002) 4781–4789. [4] J. Li, X.H. Liu, Cryst.Eng.Comm. 16 (2014) 184–190. [5] J.N. Yao, K. Hashimoto, A. Fujishima, Nature 355 (1992) 624–626. [6] K.R. Reddy, K. Ramesh, K.K. Seela, V.V. Rao, K.V.R. Chary, Catal. Commun. 4 (2003) 112–117. [7] Z.Y. Wang, S.V. Madhavi, X.W. Lou, J. Phys. Chem. C 116 (2012) 12508–12513. [8] B.Y. Li, Y. Bando, D. Golberg, K. Kurashima, Appl. Phys. Lett. 81 (2002) 5048–5050. [9] L. Zhou, L.C. Yang, P. Yuan, J. Zou, J. Phys. Chem. C 114 (2012) 21868–21872. [10] T. Xia, Q. Li, X.D. Liu, J. Meng, J. Phys. Chem. B 110 (2006) 2006–2012. [11] Y. Chen, C. Lu, L. Xu, Y. Ma, W. Hou, J. Zhu, CrystEngComm 12 (2010) 3740–3747. [12] J. Zhou, S.Z. Deng, N.S. Xu, J. Chen, Appl. Phys. Lett. 83 (2003) 2653–2655. [13] J.M. Ma, J. Zhang, S.R. Wang, T.H. Wang, J. Phys. Chem. C 15 (2011) 18157–18163. [14] V.P. Kumar, X. Wang, Lee Pooi See, Cryst.Eng.Comm. 15 (2013) 7663–7669. [15] A. Chithambararaj, N.S. Sanjini, S. Velmathi, Phys. Chem. Chem. Phys. 15 (2013) 14761–14769. [16] J. Li, X.H. Liu, Q.F. Han, X.X. Yao, X. Wang, J. Mater. Chem. A 1 (2013) 1246–1253.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesize α-MoO3 nanobelts with high adsorption property Ying Ma a,b, Yulong Jia a,b, Zhengbo Jiao a,b, Lina Wang a,b, Min Yang a, Yingpu Bi a,n, Yanxing Qi a,n a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 February 2015 Received in revised form 4 May 2015 Accepted 21 May 2015
A simple approach is explored to synthesize orthorhombic MoO3 (α-MoO3) nanobelts using sodium molybdate (Na2MoO4) and fluoboric acid (HBF4). In the reaction, HBF4 not only serves as a reagent to provide H þ , but also a surfactant to manipulate the morphology and crystalline phase of the as-prepared products. The results show that the as-prepared samples are MoO3 nanobelts with 300 nm in width, 50 nm in thickness and 2–5 μm in length. Moreover, the possible formation mechanism of α-MoO3 nanobelts with HBF4 is studied. Finally, devoted to the belt-like shape and high stability, the MoO3 nanobelts exhibit a good adsorption performance for methyl blue (MB). & 2015 Elsevier B.V. All rights reserved.
Keywords: α-MoO3 nanobelts h-MoO3 nanorods HBF4 Formation mechanism Adsorption
1. Introduction One-dimensional molybdenum oxides nanostructures have always drawn considerable attention for their superior physical and chemical properties. 1-D MoO3 nanostructures such as the nanowires [1], nanotubes [2], nanorods [3], and nanobelts [4] have been studied and applied extensively as chemical and biological sensors [5], catalysts [6], battery electrodes [7] and supercapacitors [4]. Significantly, many works have been conducted to fabricate MoO3 nanobelts ascribed to the layered strucutre and anisotropic growth. For example, Li et al. [8] have prepared nanobelts on a Si wafer by heating a Mo foil in air at 850 °C. Zhou and coworkers [9] have synthesized nanobelts by hydrothermal treatment of peroxomolybdic acid solution. Cao’s group [10] have obtained nanobelts through a hydrothermal method by adding inorganic salts. All above methods have obtained MoO3 nanobelts with superior characteristics, but the hard reaction conditions and expensive precursors restrict their wide application, and there remains much interest in the morphology-controllable synthesis of MoO3 nanobelts with a facile and economical method. Herein, a simple hydrothermal route is employed to prepare single-crystal α-MoO3 nanobelts. To the best of our knowledge, n
Corresponding authors. E-mail addresses: [email protected], [email protected] (Y. Qi).
http://dx.doi.org/10.1016/j.matlet.2015.05.095 0167-577X/& 2015 Elsevier B.V. All rights reserved.
there has been no report on the synthesis of well-crystallized nanobelts utilizing fluoboric acid. This is of particular interest because HBF4 not only acts as the H þ resource, but also as a surfactant to control the formation of the products. The probable mechanism of effect of HBF4 is proposed through the comparative tests. In addition, the surface adsorption property for degradation of Methylene bule (MB) on α-MoO3 nanobelts is investigated. As a result, it can nearly adsorb the 80% and 100% of MB in 5 min and 60 min respectively, suggesting an excellent adsorption property.
2. Experiment All the chemical reagents are analytical grade and they are used without further purification. In a typical synthesis, 2.5 g (0.01 mol) sodium molybdate is dissolved in 30 ml of distilled water under magnetic stirring. Then 12 ml HBF4 is gradually added into the solution. After stirring for another 10 min, the obtained solution is transferred and sealed in a Teflon-lined stainless autoclave with a capacity of 100 ml, and the autoclave is heated to 180 °C for 24 h. After the reaction, the autoclave is allowed to cool down to the room temperature naturally. Finally, the precipitate is washed several times with absolute ethanol and distilled water, respectively, and then dried at 60 °C for several hours and kept for further characterization. As a contrast experiment, 1.67 g H2MoO4 (0.01 mol) is dissolved in 30 ml of distilled water and then 12 ml
54
Y. Ma et al. / Materials Letters 157 (2015) 53–56
HBF4 is gradually added into the solution or not. Finally, the obtained solution is kept at 180 °C for 24 h by a hydrothermal reaction.
3. Results and discussion The morphologies of the obtained samples are characterized by field-emission scanning electron microscope (FESEM) as shown in Fig. 1a. It is clearly can be seen that the samples consist entirely of nanobelts with length of 2–5 μm, width of 300 nm and the thickness of 50 nm (inset image). Furthermore, the X-ray diffraction (XRD) spectrum (Fig. 1b) demonstrates the crystallography of the as-prepared products. All of the reflection peaks can be readily assigned to the orthorhombic MoO3 (α-MoO3) (JCPDS 05-0508 with lattice constants constants a ¼3.966 Å, b ¼13.858 and c¼ 3.693). Moreover, the absent peaks of other phases indicate that the high purity and the stronger intensities of the (020), (040) and (060) diffraction peaks suggest the anisotropic growth of the obtained MoO3 nanobelts [4,9,11]. More specifically, the selected area electron diffraction (SAED) pattern (inset of Fig. 1c) recorded perpendicular to the growth axis of the single nanobelts can be ascribed to the [010] zone axis diffraction of α-MoO3, suggesting that the samples grow along the [001] direction. Furthermore, in Fig. 1d, two sets of parallel fringes with a spacing of 0.38 nm and 0.35 nm correspond to (100) and (001) planes, respectively. The fully scanned spectra (Fig. S1) shows that the Mo and O elements exist on the surface of the samples and no obvious impurity peaks can be detected. The binding energy at 233.7 eV and 236.9 eV are
ascribed to the Mo3d5/2 and Mo3d3/2 of Mo6 þ oxidation state, respectively. Moreover, the corresponding integral areas ratio of Mo3d5/2/Mo3d3/2 is about 3:2 and the energy gap between the two doublets is 3.2 eV which is in a good agreement with α-MoO3 on the the previous report [12]. In order to study the role of BF4 on controlling synthesis of the products further and eliminate the other influence factors, contrast experiments are conducted by hydrothermal treatment of H2MoO4 with HBF4 or not. As can be seen in Fig. 2, without HBF4, only h-MoO3 nanorods (JCPDS 02-0659) with length of several micrometers and diameter of about 300 nm can be obtained. While in the presence of HBF4, the α-MoO3 nanobelts with 2–3 μm long, 100–150 nm wide can be synthseized. Considering the above results, it is easy to suppose that the HBF4 make an important function in preparing the α-MoO3 nanobelts. Moreover, the possible formation mechanism of α-MoO3 nanobelts with HBF4 is proposed and the scheme illustration of the growth process is exhibited in the Scheme 1. Just as H2WO4 [13], the H2MoO4 consists of [MO6] octahedral, which share their four equatorial oxygen atoms. More specifically, the molybdenum atom resides at the center and six oxygen atoms are coordinated octahedrally, thus forming the basic building blocks of h-MoO3 [14]. Then the freshly formed octahedral crystal nuclei grows and assembles to form a large amount of h-MoO3. In the case of adding HBF4, BF4 can selectively adsorb onto the crystal plane parallel to the c-axis of the MoO3 crystal nucleus so as to influence anisotropic growth and aggregation, resulting in α-MoO3 nanobelts. More specifically, the BF4 has a distorted tetrahedron structure, in which four fluorine atoms occupy the apexes of a tetrahedron
Fig. 1. The FESEM image (a) (inset is the large image of somewhere from image (a)), XRD pattern (b), TEM image (c) (inset is the SAED image) and the HRTEM image (d) of the obtained α-MoO3 nanobelts.
Y. Ma et al. / Materials Letters 157 (2015) 53–56
55
Fig. 2. FESEM image (a) and XRD pattern (c) of the products by hydrothermal treatment of H2MoO4, FESEM image (b) and XRD pattern (d) of the products by hydrothermal treatment of H2MoO4 and HBF4.
Scheme 1. Schematic representation of the formation pathways of h-MoO3 and α-MoO3 under controlled conditions.
centering on each boron atom [15]. Then BF4 can be added to the hydrogen bonds system, which may induce the oriented arrangement of the BF4 on the (010) plane of H2MoO4. Thus, the growth of the (010) plane of H2MoO4 is inhibited by the adsorption of BF4 , and α-MoO3 nanobelts are produced. To evaluate the potential application in water treatment of the asprepared α-MoO3 nanobelts, the adsorption capacities for the organic pollutant-Methylene bule (MB) are investigated. Fig. 3A exhibits the UV–vis absorption spectroscopy recording the absorption behavior of
the solution after treatment by using characteristic light absorption of MB at 664 nm. It is vividly that the intensity of characteristic peak for MB remarkably decreases with treatment in 5 minutes and reaches to almost zero after 60 min treatment, suggesting the high adsorption property of the MoO3 nanobelts. Moreover, as shown in Fig. 3B, in the first 5 min, featuring a sharp variation in removal ratios, can be deemed as a quick adsorption stage. The subsequent second stage between 5 and 10 min displays a smaller removal ratio of 10%. While almost no obvious change can be seen after 30 min,
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Y. Ma et al. / Materials Letters 157 (2015) 53–56
Fig. 3. Absorption spectra (a) and adsorption rate (b) of an aqueous solution of MB (20 mg L 1, 30 ml) after treatment with α-MoO3 nanobelts (30 mg) at different time intervals.
and the removal ratio reaches to nearly 100% after 60 min indicating that the adsorption of MB completes. Considering the similarity with WO3, the high adsorption property of MoO3 may be largely attributed to the electrostatic attraction between the surface and the MB species [16]. However, the exact adsorption mechanism of MoO3 nanobelts can not be completely understood and a more detailed study is still underway.
4. Conclusions In summary, we report a facile one-pot hydrothermal route for synthesis of α-MoO3 nanobelts using HBF4 as the H þ resource and the surfactant controlling the morphology and crystalline phase of product. Through comparative experiments, the effect of BF4 is explored and it is found that BF4 can induce the formation of α-MoO3 nanobelts. Characterizations show that the prepared samples exhibit an applicable adsorptive capacity for methylene blue. Moreover, the exact adsorption mechanism of MoO3 nanobelts can be further developed.
Acknowledgments This work is supported by the Chinese Academy of Sciences and Technology Project (XBLZ-2011-013) and the Technologies R&D Program of Gansu Province (No. 1104FKCA156).
Appendix A. Suplementary Information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015.05.095.
References [1] R.Q. Song, A.W. Xu, B. Deng, Y.P. Fang, J. Phys. Chem. B 109 (2005) 22758–22766. [2] S. Hu, X. Wang, J. Am. Chem. Soc. 130 (2008) 8126–8127. [3] X.W. Lou, H.C. Zeng, Chem. Mater. 14 (2002) 4781–4789. [4] J. Li, X.H. Liu, Cryst.Eng.Comm. 16 (2014) 184–190. [5] J.N. Yao, K. Hashimoto, A. Fujishima, Nature 355 (1992) 624–626. [6] K.R. Reddy, K. Ramesh, K.K. Seela, V.V. Rao, K.V.R. Chary, Catal. Commun. 4 (2003) 112–117. [7] Z.Y. Wang, S.V. Madhavi, X.W. Lou, J. Phys. Chem. C 116 (2012) 12508–12513. [8] B.Y. Li, Y. Bando, D. Golberg, K. Kurashima, Appl. Phys. Lett. 81 (2002) 5048–5050. [9] L. Zhou, L.C. Yang, P. Yuan, J. Zou, J. Phys. Chem. C 114 (2012) 21868–21872. [10] T. Xia, Q. Li, X.D. Liu, J. Meng, J. Phys. Chem. B 110 (2006) 2006–2012. [11] Y. Chen, C. Lu, L. Xu, Y. Ma, W. Hou, J. Zhu, CrystEngComm 12 (2010) 3740–3747. [12] J. Zhou, S.Z. Deng, N.S. Xu, J. Chen, Appl. Phys. Lett. 83 (2003) 2653–2655. [13] J.M. Ma, J. Zhang, S.R. Wang, T.H. Wang, J. Phys. Chem. C 15 (2011) 18157–18163. [14] V.P. Kumar, X. Wang, Lee Pooi See, Cryst.Eng.Comm. 15 (2013) 7663–7669. [15] A. Chithambararaj, N.S. Sanjini, S. Velmathi, Phys. Chem. Chem. Phys. 15 (2013) 14761–14769. [16] J. Li, X.H. Liu, Q.F. Han, X.X. Yao, X. Wang, J. Mater. Chem. A 1 (2013) 1246–1253.