Hydrothermal-synthesized NiO nanowall array for lithium ion batteries

Journal of Alloys and Compounds 556 (2013) 56–61

Contents lists available at SciVerse ScienceDirect

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

Hydrothermal-synthesized NiO nanowall array for lithium ion batteries Xiaoyan Yan a,⇑, Xili Tong b,⇑, Jian Wang a, Changwei Gong a, Mingang Zhang a, Liping Liang a a b

Institute of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, PR China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS, Taiyuan 030001, PR China

a r t i c l e

i n f o

Article history: Received 12 October 2012 Received in revised form 20 December 2012 Accepted 21 December 2012 Available online 29 December 2012 Keywords: Nickel oxides Porous films Energy storage Li ion battery Array

a b s t r a c t We report a self-supported NiO nanowall array prepared by a facile hydrothermal synthesis method. The microstructure and morphology of the sample are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The hydrothermalsynthesized NiO nanowalls with thicknesses of 20 nm arrange vertically to the substrate forming a net-like nanowall array structure. As anode material for lithium ion batteries, the NiO nanowall array exhibits better electrochemical performances with higher coulombic efficiency and better cycling performance as compared to the dense NiO film. The NiO nanowall array shows an initial coulombic efficiency of 76%, as well as good cycling stability with a capacity of 567 mAh g1 at 0.3 A g1 after 50 cycles, higher than those of the dense polycrystalline NiO film (361 mAh g1). The superior electrochemical performance is mainly due to the unique nanowall array structure with shorter diffusion length for mass and charge transport. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction NiO is an important material with many applications such as electrochromics [1], supercapacitors [2], and lithium–ion batteries [3–5]. Inspired by the pioneering work of Tarascon et al. [6], great efforts are devoted to nanoscale transition metal oxides for lithium–ion batteries [7–10]. Among these transition metal materials, NiO is considered to be promising anode due to its high capacity and excellent reversibility. Despite its high capacity (theoretic capacity of 718 mAh g1), the practical use of NiO for lithium–ion batteries is still hindered by large initial irreversible loss and poor capacity retention resulting from the large specific volume change causing pulverization and deterioration of active materials over extended cycling [11,12]. To overcome these problems, an effective approach is to create NiO with nanoporous structures to buffer the volume change upon cycles. Prompted by these interests, many NiO nanostructures, including nanotube [13], nanowall [14,15], nanosheet [16], nanobowel [17], nanocone[18], and mesoporous structure [19] have been synthesized by various routes and applied for lithium–ion batteries. These nanoporous structures have been demonstrated to exhibit superior performance for lithium–ion batteries due to their nanoscale porous architecture in which the Li ion diffusion path could be shortened and the inner stress caused by Li insertion/desertion could be lowered. Recently, fabrication of vertically aligned transition metal oxide arrays directly grown on conductive substrate represents the feasi⇑ Corresponding authors. Tel.: +86 351 4605282. E-mail address: [email protected] (X. Yan). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.124

bility of improving the electrode kinetics. In particular, self-supported nanowall structures have been considered as one of the most promising structures due to their higher surface-to-volume ratio than other one-dimensional nanostructures such as nanowires and more difficult for aggregation in comparison with nanoparticles. To date, several methods such as plasma assisted oxidation method [9], anodic electrodeposition [20], chemical bath deposition method [21], and thermal oxidation method [22,23], have been developed to prepare NiO nanowall arrays. Previously, Varghese et al. [14] reported vertically aligned NiO nanowalls on nickel foils by using a plasma assisted oxidation method and its superior electrochemical performance with a capacity of 638 mA h g1 after cycling for 85 cycles. Wu and Lin [20] reported anodic electrodeposited NiO nanowalls with ultrahigh capacity of 1250 mA h g1 and large initial irreversible loss of 56%. Tu’s group has pioneered the NiO nanowall arrays and their composites prepared by a chemical bath deposition and their application for lithium–ion batteries [4,23]. In the present work, we report a NiO nanowall array grown on nickel foam prepared via a facile hydrothermal method and its electrochemical performance toward lithium is analyzed. The as-prepared NiO nanowall array exhibits superior performance with high specific capacity and quite good cycle life due to the unique two-dimensional porous structure. 2. Experimental All solvents and chemicals were of reagent quality and used without further purification. The nickel nitrate, ammonium fluoride and urea were obtained from Shanghai Chemical Reagent Co. All aqueous solutions were freshly prepared with high purity water (18 MX cm resistance). In a typical synthesis of NiO nanowall

X. Yan et al. / Journal of Alloys and Compounds 556 (2013) 56–61

♦

Ni

♦

(220)

♦

Intensity / a.u.

array on nickel foam, 1 mmol Ni(NO3)2, 2 mmol of NH4F, and 5 mmol of CO(NH2)2 were dissolved in 50 mL of water under stirring, respectively. Then, the homogeneous solution was transferred into a Teflon-lined stainless steel autoclave. Then a piece of clean nickel foam with a size of 3  7 cm2 was immersed into the reaction solution. The top side was protected from solution contamination by uniformly coating with a polytetrafluorethylene tape. Hereafter, the autoclave was sealed and maintained at 110 °C for 5 h, and allowed to cool down to room temperature spontaneously. After the reaction, the substrate was taken out, completely washed with deionized water, and dried in the air. Finally, the substrate was annealed at 350 °C in argon for 2 h. The involved chemical reactions were illustrated as follows [24].

NiO

(200)

(111)

NiO nanowall array Dense NiO film

Ni

♦

Ni

2þ

Ni

20

30

40

50

60

70

57

80

2θ / degree Fig. 1. XRD patterns of NiO nanowall array and dense NiO film grown on nickel foam.

þ xF ! ½NiFx ðx2Þ

ð1Þ

H2 NCoNH2 þ H2 O ! 2NH3 þ CO2

ð2Þ

þ CO2 þ H2 O ! CO2 3 þ 2H

ð3Þ

NH3  H2 O ! NHþ4 þ OH

ð4Þ

2½NiFx 

ðx2Þ

  þ 2CO2 3 þ 2OH þ nH2 O ! Ni2 ðOHÞ2 CO3  nH2 O þ 2xF

ð5Þ

The NiO forms after the annealing process as below.

Fig. 2. (a and b) SEM images of NiO nanowall array (side view of the array in inset); (c) TEM image of individual nanowall; (d) Selected area electronic diffraction (SAED) pattern of the NiO nanowall; (e) EDX spectrum of the NiO nanowalls.

58

Ni2 ðOHÞ2 CO3  nH2 O ! 2NiO þ ð1 þ nÞH2 O þ CO2

X. Yan et al. / Journal of Alloys and Compounds 556 (2013) 56–61

ð6Þ

The NH4F acts as the chelating agent to control the growth speed and density of the NiO nanowall arrays. This reaction mechanism is different from previous work of Chen et al. [25]. For the sake of comparison, a dense NiO film was prepared by an electrodeposition method carried out at anodic constant current of 2.5 mA cm2 for 400 s in electrolyte of 1 M Ni(NO3)2 and 0.1 M NaNO3. The sample was annealed at 400 °C in argon for 2 h. The thickness of both samples was approximately 2 lm. The morphology and microstructure of the sample were characterized by a field emission scanning electron microscopy (FESEM, Hitachi S-4700), transmission electron microscopy (TEM, JEM 200 CX 200 kV), and X-ray diffraction (XRD, Philips PC– APD with Cu Ka radiation). The load weight for the NiO nanowall array and dense NiO film was approximately 1.8 mg cm2 and 2.5 mg cm2, respectively. Test cells were assembled in a glove box filled with argon using the NiO grown on nickel foam as the working electrode, Li foil as the counter electrode, and polypropylene film as the separator. The electrolyte was a mixed solution containing ethylene carbonate and diethyl carbonate, in which dissolved 1 mol L1 of LiPF6. The galvanostatic charge/discharge tests were conducted on LAND battery program-control test system from 0.02 to 3.0 V (versus Li/Li+) at room temperature (25 ± 1 °C). Cyclic voltammetry (CV) tests were carried out using the CHI660C electrochemical workshop at a scanning rate of 0.1 mV s1. The electrochemical impedance spectroscopy (EIS) measurements were made with a superimposed 5 mV sinusoidal voltage in the frequency range of 100 kHz–0.01 Hz.

3. Results and discussion Fig. 1 shows the XRD patterns of both NiO samples prepared by hydrothermal synthesis and electrodeposition methods, respectively. Obviously, except for three representative peaks from nickel foam substrate, the other diffraction peaks at 37.3°, 43.3° and 62.9° correspond to (1 1 1), (2 0 0) and (2 2 0) crystal planes of cubic NiO phase (JCPDS 47-1049), respectively, indicating that crystalline NiO films have formed after heat treatment. Fig. 2 shows the SEM and TEM images of the hydrothermal-synthesized NiO film. It is observed that the entire surface of nickel foam is uniformly covered by free-standing two-dimensional (2D) nanowall array (Fig. 2a). The as-prepared NiO nanowall array consists of intercon-

nected NiO nanowalls with thicknesses of 20 nm (Fig. 2b). According to the cross-sectional SEM image (inset in Fig. 2a), the NiO nanowalls arrange vertically to the substrate resulting in the formation of extended porous net-like structure. Further insight into the microstructure of the NiO nanowall is performed by TEM (Fig. 2c). The individual NiO nanoflake presents a flat and smooth texture. Selected area electronic diffraction (SAED) pattern of the NiO nanowall in Fig. 2d shows that the NiO nanowall is polycrystalline in nature. The composition of the NiO nanowall array is confirmed by the EDX result (Fig. 2e), which indicates that the obtained NiO is stoichiometric. On the contrary, the electrodeposited NiO film shows a dense structure (Fig. 3a). The surface of the electrodeposited NiO film consists of numerous nanoparticles with sizes of 20–30 nm (Fig. 3b). The TEM image confirms its relatively coarse structure (Fig. 3c). Besides, the SAED pattern indicates that the dense NiO is also polycrystalline (Fig. 3d). As anode for lithium ion batteries, the electrochemical properties of the NiO nanowall array are evaluated by cyclic voltammetry (CV) and galvanostatic discharge/charge cycling. The reaction mechanism between NiO and Li has been well demonstrated in previous reports [6,26]. It involves the formation and decomposition of Li2O accompanying the reduction and oxidation of metal Ni. The electrochemical reaction can be described as follows [6]: þ

NiO þ 2Li þ 2e $ Li2 O þ Ni

ð7Þ

Fig. 4 shows the CV curves of both NiO films at a scanning rate of 0.1 mV s1. Both NiO films exhibit similar CV behavior with two reduction peaks in the cathodic process and two oxidation peaks in the anodic process, respectively. For the NiO nanowall array, the reduction peaks at 0.77 and 1.41 V correspond to the reduction of NiO to metallic Ni and the formation of a partially reversible solid electrolyte interphase (SEI) layer whose composition includes Li2CO3, ethyleneoxide-based oligomers, LiF, and lithium alkyl car-

Fig. 3. (a and b) SEM images of dense NiO film (side view in inset); (c) TEM image of the dense NiO film; (d) Selected area electronic diffraction (SAED) pattern of the dense NiO film.

59

X. Yan et al. / Journal of Alloys and Compounds 556 (2013) 56–61

-2

3.0

1.55 V 2.30 V

Potential (vs. Li/Li ) / V

0.5

2.5

+

1.73 V

0.0

2.45 V

1.26 V 0.70 V

-0.5

1.41 V

-1.0 0.5

1.0

NiO nanowall array Dense NiO film

1.5 1.0 0.5 0.0

0.77 V

0.0

2.0

1.5

2.0

2.5

3.0

0

200

400

+

(b)

1000

1000 Dense NiO film NiO nanowall arrray

Specific capacity / mAhg

800

-1

0.5 A g

-1

1Ag

600

-1

1.5 A g

-1

2A g

400

200

0

0

5

10

15

20

25

30

35

Cycle number

(c)

1000

Discharge Capacity / mAh g-1

bonate (ROCO2Li) [27–33]. The oxidation peak around 1.55 V corresponds to the partial decomposition of the SEI, and the main oxidation peak located at 2.3 V corresponds to the decomposition of Li2O and nickel leading to the reformation of NiO [14,29–32]. The CV behaviors are consistent with those in the literature [14,29–33]. For the dense polycrystalline NiO film, two lower reduction peak potentials (0.7 V and 1.26 V) and higher oxidation peak potentials (1.73 and 2.45 V) are observed in the CV curve. Obviously, the NiO nanowall array shortens the peak potential separation between the oxidation peak and the reduction peak. The open porous structure facilitates electrolyte penetration to every part of the film and shortens diffusion paths for both electrons and lithium ion within the oxides. This morphology is beneficial to reduce polarization with lower oxidation peak potentials and higher reduction peak potentials. Additionally, the NiO nanowall array exhibits higher peak currents than those of dense polycrystalline NiO film. Take the above results together, it is concluded that the NiO nanowall array has better reaction reversibility and enhanced electrochemical activity than the dense NiO film. The discharge/charge curves of both NiO films at 0.5 A g1 are shown in Fig. 5a. Two discharge voltage plateaus and a sloping potential range are noticed during the first discharge. The long discharge plateau at 0.65 V corresponds to the reduction of NiO to metallic nickel nanodomains embedded in the Li2O matrix, which is consistent with the CV result. The first discharge capacity of the NiO nanowall array is 934 mAh g1, higher than the dense NiO film (820 mAh g1). The first discharge values are higher than the theoretical value (718 mAh g1), which is due to the formation of solid electrolyte interphase (SEI) in the first discharge. This phenomenon happens in all transition metal oxides, including NiO, FeO, CoO, Co3O4 and CuO [26–33]. The first charge process of the NiO nanowall array exhibits a capacity of 710 mAh g1, less than that of the first discharge. The irreversible capacity loss is attributed to the incomplete decomposition of both of the SEI and Li2O. The corresponding initial coulombic efficiency is 76%. In contrast, the dense NiO film exhibits much lower first charge capacity and initial coulombic efficiency (550 mAh g1 and 67%). It is noticed that the NiO nanowall array exhibits lower charge plateau and higher discharge plateau, indicating its lower polarization than the dense NiO film, consistent with the CV result. The values obtained from the NiO nanowall array are comparable with the hierarchically ordered porous NiO film, NiO nanowall electrode obtained by Varghese et al. [14], Co-doped NiO thin film electrode [32], but superior to NiO/PEDOT film [23] and other NiO powder materials [34,35,13,36]. These capacities are also a little higher than those of Chen et al. [25].

800

Specific capacity / mA h g

-1

Fig. 4. Cyclic voltammograms of NiO nanowall array and dense NiO film at a scanning rate of 0.1 mV s1 at the second cycle.

600

-1

Potential (vs. Li/Li ) / V

100

800

NiO nanowall array Dense NiO film

80

600

60

400

40

200

20

0

0

10

20

30

40

50

Efficienty / %

Current density / mA cm

(a)

Dense NiO film NiO nanowall array

1.0

0

Cycle Number Fig. 5. (a) First discharge/charge curves of NiO nanowall array and dense NiO film; (b) Rate capability and (c) Cycling performances of NiO nanowall array and dense NiO film at 0.3 A g1.

The rate capabilities of both NiO films are presented in Fig. 5b. The NiO nanowall array shows much better rate capability than the dense NiO film. The NiO nanowall array delivers 602 mAh g1 at 0.5 A g1, 549 mAh g1 at 1 A g1, 494 mAh g1 at 1.5 A g1, and 435 mAh g1 at 2 A g1, higher than the those obtained from the dense NiO film (453 mAh g1 at 0.5 A g1, 362 mAh g1 at 1 A g1, 311 mAh g1 at 1.5 A g1, and 262 mAh g1 at 2 A g1), demonstrating the capability for a high cycling rate. Furthermore, the NiO nanowall array exhibits quite good cycling stability with a capacity of 567 mAh g1 at 0.3 A g1 after 50 cycles, maintaining

60

X. Yan et al. / Journal of Alloys and Compounds 556 (2013) 56–61

300

NiO nanowall array Dense NiO film

250

-Zim / Ω

200 150 100 50 0 0

50

100

150

200

250

300

Zre/ Ω Fig. 6. Nyquist plots of both NiO film electrodes at 100% depth of discharge.

semicircle in high-frequency region and an inclined line in low frequency region. The semicircle represents the charge-transfer impedance on electrode/electrolyte interface, and the inclined line in low frequency region is due to the ion diffusion process within electrodes. The NiO nanowall array exhibits smaller semicircle and higher line slope than the dense NiO film, implying that the NiO nanowall array has lower charge-transfer impedance and ion diffusion impedance, leading to smaller polarization and lower internal resistance. This EIS result is in accordance with the enhanced rate capability. The enhanced electrochemical performance of the NiO nanowall array comes from the unique porous nanowall array structure. Each nanoflake directly grown on nickel foam has its own contact with the substrate and ensures every nanoflake participating in the electrochemical reaction. Besides, the nanowall array configuration provides fast ion/electron transfer, sufficient contact between active materials and electrolyte, and enhanced flexibility, resulting in reduced internal resistance and improved high-power performance [37]. Moreover, the NiO nanowall array shows better morphological stability than the dense NiO film. The NiO nanowall array basically keeps the porous structure integrity after 10 cycles (Fig. 7a), but the dense NiO film loses its integrity and leaves many cracks in the film with aggregated particles (Fig. 7b), which cause activity deterioration on electrode materials. It is indicated that the porous NiO nanowall array is beneficial to relax the volume expansion and alleviates the structure damage during the cycling process. This feature is particularly helpful for high rate applications, resulting in better cycling performance.

4. Conclusions Self-supported NiO nanowall array grown on nickel foam has been prepared by a facile hydrothermal synthesis method and tested as the anode for lithium–ion batteries. The as-prepared NiO nanowall array has shown weaker polarization, higher capacity, better cycling stability and enhanced high rate capability compared to the dense polycrystalline NiO film. The improved electrochemical performances are attributed to the porous nanowall array architecture, which provides high electrode–electrolyte contact area and fast electron/lithium ion diffusion path and alleviates structure damage during the cycling process. References

Fig. 7. SEM images of the fully discharged NiO nanowall array (a) and dense NiO film (b) at 10th cycle.

79% of the theoretical value, higher than the dense NiO film (361 mAh g1 at 0.3 A g1 with 51%) (Fig. 5c). The charge–discharge efficiency of the porous NiO film (nearly 99%) is higher than the dense counterpart (96.5%). To check the reason for the enhanced electrochemical performance, electrochemical impedance spectroscopy measurements (EIS) were carried out after the cycling (Fig. 6). Nyquist plots of both NiO films are composed of a

[1] X.H. Xia, J.P. Tu, J. Zhang, X.L. Wang, W.K. Zhang, H. Huang, Sol. Energ. Mater. Sol. Cells 92 (2008) 628. [2] X.H. Xia, J.P. Tu, X.L. Wang, C.D. Gu, X.B. Zhao, J. Mater. Chem. 21 (2011) 671. [3] J. Zhong, X.L. Wang, X.H. Xia, C.D. Gu, J.Y. Xiang, J. Zhang, J.P. Tu, J. Alloys Compd. 509 (2011) 3889. [4] X.H. Huang, Y.F. Yuan, Z. Wang, S.Y. Zhang, F. Zhou, J. Alloys Compd. 509 (2011) 3425. [5] Y.Z. Wu, R. Balakrishna, M.V. Reddy, A.S. Nair, B.V.R. Chowdari, S. Ramakrishna, J. Alloys Compd. 517 (2012) 69. [6] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496. [7] N. Du, H. Zhang, X. Fan, J.X. Yu, D.R. Yang, J. Alloys Compd. 526 (2012) 53. [8] B. Wang, J.L. Cheng, Y.P. Wu, J. Alloys Compd. 527 (2012) 132. [9] Q.Q. Xiong, Y. Lu, X.L. Wang, C.D. Gu, Y.Q. Qiao, J.P. Tu, J. Alloys Compd. 536 (2012) 219. [10] Y. Qi, N. Du, H. Zhang, J.Z. Wang, Y. Yang, D.R. Yang, J. Alloys Compd. 521 (2012) 83. [11] X.H. Xia, J.P. Tu, J. Zhang, X.L. Wang, W.K. Zhang, H. Huang, Electrochim. Acta 53 (2008) 5721. [12] X.H. Huang, J.P. Tu, X.H. Xia, X.L. Wang, J.Y. Xiang, Electrochem. Commun. 10 (2008) 1288. [13] S.A. Needham, G.X. Wang, H.K. Liu, J. Power Sources 159 (2006) 254. [14] B. Varghese, M.V. Reddy, Z. Yanwu, C.S. Lit, T.C. Hoong, G.V.S. Rao, B.V.R. Chowdari, A.T.S. Wee, C.T. Lim, C.H. Sow, Chem. Mater. 20 (2008) 3360. [15] Y.J. Mai, X.H. Xia, R. Chen, C.D. Gu, X.L. Wang, J.P. Tu, Electrochim. Acta 67 (2012) 73. [16] Y. Wang, Y.Q. Zou, Nanoscale 3 (2011) 2615.

X. Yan et al. / Journal of Alloys and Compounds 556 (2013) 56–61 [17] Y.F. Yuan, X.H. Xia, J.B. Wu, J.L. Yang, Y.B. Chen, S.Y. Guo, Electrochem. Commun. 12 (2010) 890. [18] X. Wang, Z. Yang, X. Sun, X. Li, D. Wang, P. Wang, D. He, J. Mater. Chem. 21 (2011) 9988. [19] G.X. Wang, H.L. Liu, H.J. Liu, S.Z. Qiao, H.J. Ahn, J. Mater. Chem. 21 (2011) 3046. [20] M.S. Wu, Y.P. Lin, Electrochim. Acta 56 (2011) 2068. [21] A.C. Gandhi, C.Y. Huang, C.C. Yang, T.S. Chan, C.L. Cheng, Y.R. Ma, S.Y. Wu, Nanoscale Res. Lett. 6 (2011) 485. [22] K.L. Zhang, C. Rossi, P. Alphonse, C. Tenailleau, Nanotechnology 19 (2008) 155605. [23] X.H. Huang, J.P. Tu, X.H. Xia, X.L. Wang, J.Y. Xiang, L. Zhang, J. Power Sources 195 (2010) 1207. [24] X.H. Xia, J.P. Tu, Y.Q. Zhang, X.L. Wang, C.D. Gu, X.B. Zhao, H.J. Fan, ACS Nano 6 (2012) 5531. [25] Z. Chen, A. Xiao, Y. Chen, C. Zuo, S. Zhou, L. Li, Mater. Res. Bull. 47 (2012) 1987. [26] J.-M. Tarascon, M. Armand, Nature 414 (2001) 359. [27] S. Grugeon, S. Laruelle, R. Herrera-Urbina, L. Dupont, P. Poizot, J.M. Tarascon, J. Electrochem. Soc. 148 (2001) A285.

61

[28] G. Gachot, S. Grugeon, M. Armand, S. Pilard, P. Guenot, J.-M. Tarascon, S. Laruelle, J. Power Sources 178 (2008) 409. [29] S. Grugeon, S. Laruelle, R. Herrera-Urbina, L. Dupont, P. Poizot, J.-M. Trarscon, J. Electrochem. Soc. 148 (2001) A285. [30] A. Debart, L. Dupont, P. Poizot, J.-B. Leriche, J.-M. Tarascon, J. Electrochem. Soc. 148 (2001) A1266. [31] P. Poizot, S. Laruelle, S. Grugeon, J.-M. Tarascon, J. Electrochem. Soc. 149 (2002) A1212. [32] Y.J. Mai, J.P. Tu, X.H. Xia, C.D. Gu, X.L. Wang, J. Power Sources 196 (2011) 6388. [33] X.H. Huang, J.P. Tu, Z.Y. Zeng, J.Y. Xiang, X.B. Zhao, J. Electrochem. Soc. 155 (2008) A438. [34] X.H. Huang, J.P. Tu, C.Q. Zhang, X.T. Chen, Y.F. Yuan, H.M. Wu, Electrochim. Acta 52 (2007) 4177. [35] M.M. Rahman, S.L. Chou, C. Zhong, J.Z. Wang, D. Wexler, H.K. Liu, Solid State Ionics 180 (2010) 1646. [36] H.W. Lu, D. Li, K. Sun, Y.S. Li, Z.W. Fu, Solid State Sci. 11 (2009) 982. [37] X.H. Xia, J.P. Tu, X.L. Wang, C.D. Gu, X.B. Zhao, Chem. Commun. 47 (2011) 5786.