Improving Photoelectrochemical Water Splitting Activity of TiO2

Article pubs.acs.org/JPCC

Improving Photoelectrochemical Water Splitting Activity of TiO2 Nanotube Arrays by Tuning Geometrical Parameters Suzhen Liang,† Jingfu He,† Zhihu Sun, Qinghua Liu,* Yong Jiang, Hao Cheng, Bo He, Zhi Xie, and Shiqiang Wei* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China ABSTRACT: Here we report a study to improve the solar water splitting activity of TiO2 photoanodes by tuning the porosity of the nanotube arrays. Through modifying the electrochemical anodization conditions, the average wall thickness and inner diameter of the synthesized TiO2 nanotube arrays were controlled in the range of 8−20 nm and 40−145 nm, respectively, corresponding to a variation of the porosity from 47.1% to 75.7%. The photoelectrochemical (PEC) measurements demonstrate that the sample with maximum porosity (75.7%) shows the peak ultraviolet conversion efficiency of 7.02%. Further analysis reveals that the photoconversion efficiency increases monotonously with porosity rather than with wall thickness and/or inner diameter. We suggest that the large porosity can ensure a much shorter hole diffusion path toward wall surface and accelerate ion migration in the tube to overcome the kinetic bottleneck, thus enhancing the PEC water splitting efficiency of the TiO2 nanotube arrays.

I. INTRODUCTION Photoelectrochemical (PEC) water splitting offers an elegant solution for collecting the energy of sunlight and storing it in the form of chemical bonds on a global scale.1−3 For this process of energy conversion, TiO2 has emerged as a strong candidate owing to its excellent chemical stability, low cost, nontoxicity and environment-friendly feature.4−7 It has been proven that nanostructured TiO2 electrodes have higher photochemical reactivity than the bulk form of TiO 2 materials,8−11 since bulk TiO2 suffers from a short diffusion length. In particular, TiO2 nanotube arrays film exhibits good oriented charge-transport property and overcomes the shortcoming of nanoparticulate TiO2 film that the structural disorder at the contact between two crystalline particles leads to an enlarged scattering of free electrons and reduces the electron mobility.4,12 Despite this advantage, practically reported photoconversion efficiencies of TiO2 nanotube arrays photoanodes are still substantially lower than the theoretical limit.13 It is recognized that the PEC water splitting property of TiO2 nanotube arrays are closely related to their geometric parameters.14−17 Over the past decades, various TiO2 nanotube arrays with nanotube length from 0.5 μm to 1000 μm,18,19 pore size from 12 nm to 350 nm,14,20 wall thicknesses from 5 nm to 40 nm,20,21 and tube-to-tube spacing from several tens of nanometers to nothing have been synthesized.22 Based on these geometric adjustments, the relationship between the performance and geometric dimensions of TiO2 nanotube arrays has been extensively investigated.23−27 For examples, Grimes and co-workers have reported that the photoconversion efficiency of TiO2 nanotube was increased from 2.4% to 3.8% with decreasing the pore size from 76 to 22 nm.28 Subsequently, the © 2012 American Chemical Society

same research group declared that the increase of wall thickness from 9 to 34 nm can enhance the photocurrent of TiO2 nanotube arrays, and the authors attributed the reason to the decreased surface recombination rates by the enhanced band bending of thicker walls.29 Paulose et al. have reported that the photoconversion efficiency increased with the length of TiO2 nanotube,30 but it was difficult to determine the role of pore size, because more than one geometrical feature was changing per given fabrication parameter. Recently, Zhu and co-workers have systematically investigated the PEC properties of TiO2 nanotubes influenced by pore size (115−145 nm) and wall thickness (14−22 nm).31 They suggested that a larger pore size and thinner wall were beneficial to the PEC activity.30 In spite of these great efforts, some of these results are controversial. A comprehensive picture connecting these parameters with geometry and PEC property is still lacking, which is however important to further optimize the PEC activity of TiO2 nanotube arrays films. In this work, we fabricated TiO2 nanotube arrays with various inner diameters and wall thicknesses by an electrochemical anodization method. Emphasis is given to the influences of these geometric parameters on the PEC water splitting activity of the TiO2 nanotube arrays photoanodes. By analyzing the structural, optical, and photochemical properties, we find that the photocurrent density and energy conversion efficiency are increased with the porosity of the nanotube arrays. These Received: January 17, 2012 Revised: February 23, 2012 Published: April 5, 2012 9049

dx.doi.org/10.1021/jp300552s | J. Phys. Chem. C 2012, 116, 9049−9053

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%), and water content (1.8−4.5 wt %), a series of TiO2 nanotube arrays films were fabricated with different inner diameters and wall thicknesses. A representative sample (denoted as S4 later), which was synthesized by the sonoelectrochemical anodization at a voltage of 60 V and electrolyte consisting of ammonium fluoride (NH4F, 0.5 wt %), deionized water (1.8 wt %), and ethylene glycol (97.7 wt %), was chosen to testify the morphology and structure of the TiO2 nanotube arrays. The FESEM images of this annealed sample and corresponding XRD pattern are presented in Figure 1. From

results are valuable for further optimization of the PEC properties of TiO2 nanotube arrays photoanodes.

II. EXPERIMENTAL SECTION Prior to anodization, the titanium foils (0.5 mm, 99.9% in purity) used in this study were immersed in diluted hydrochloric acid for 2 h and then degreased by sonicating in ethanol, acetone, and deionized (DI) water. After that, the foils were rinsed with DI water and then dried in nitrogen stream. Anodization was performed in a two-electrode configuration with titanium foil as the working electrode and platinum foil as the counter electrode under constant potential at room temperature (∼22 °C). The anodization electrolytes were prepared by mixing deionized water, ammonium fluoride (NH4F), and ethylene glycol. A direct current power supply was used as the voltage source to drive the anodization. After anodization, the TiO2 nanotube arrays samples were rinsed with deionized water and dried in nitrogen. Then all the TiO2 nanotube arrays samples were annealed in atmosphere at 450 °C for 2 h at a heating rate of 1 °C/min and processed for subsequent characterization. A field emission scanning electron microscope (FESEM; FEI, Sirion200) was used to analyze the tube morphology, including the length, inner diameter, and wall thickness. The crosssection and bottom images were taken from the cracked layers after mechanically bending the samples. Prior to imaging, the samples were sputtered with gold for 30 s. XRD patterns of the samples were recorded using a Rigaku TTR-III diffractometer, equipped with a graphite monochromator. Cu Kα radiation (λ = 1.540 56 Å) with a fixed power source (30 kV and 140 mA) was used. The samples were scanned at a rate of 8° (2θ)/min with the step size of 0.02° over a range of 20°−80°, which covered the main characteristic diffraction peaks of the titanate, anatase, and rutile. Ultraviolet−visible (UV−vis) absorption spectra of TiO2 nanotube array samples were measured using a UV−vis spectrophotometer (SolideSpec−3700 PC, Shimadzu). Fine BaSO4 powder was used as a standard for baseline, and all the spectra were recorded in the wavelength range from 300 to 600 nm. Current−voltage characteristics were measured using an CHI760D electrochemical platform in a standard threeelectrode optical cell with 1 M KOH as the electrolyte, a Pt sheet as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. Contacting to the photoanode was made by an iron clasp attached to the exposed titanium foil surface at the top of the anode, while the lower portion containing the sample was submerged in the electrolyte. The photocurrent density was measured by potentiodynamic scanning all photoanodes from −1.0 to 1.0 V vs Ag/ AgCl with a scanning rate of 50 mV s−1 under UV light illumination (280−400 nm, 100 mW/cm2). The illumination region of the working electrode was of the identical size of 1 × 1 cm2. Unless otherwise specified, all experiments were performed at room temperature in an air atmosphere.

Figure 1. (a−c) FESEM images of a nanotube arrays sample grown at 60 V in 0.5 wt % NH4F in ethylene glycol: (a) top surface, (b) crosssection view, and (c) view of bottom surface. (d) The corresponding XRD patterns of the nanotube arrays sample annealed at 450 °C for 2 h in air showing anatase phase.

Figure 1a−c, we can see that the TiO2 nanotube arrays exhibit hexagonal packing mode at the bottom and are still highly ordered and compacted with each other at the top. They are entirely smooth along the vertical direction, without ripples on the walls. As shown in Figure 1d, the XRD patterns for TiO2 nanotube arrays sample shows pure anatase phase, except some diffraction peaks of titanium which guarantees that the whole TiO2 oxide layer is detected. All nanotube samples present the similar morphology and structure, thus ensuring that the performance of these TiO2 nanotube arrays is mainly associated with their geometric parameters. Albu et al.32 have reported that annealing at 450 °C of TiO2 nanotubes with diameters smaller than 30 nm could result in a phase transition from amorphous to rutile while a transition from amorphous to anatase for larger diameters (more than 30 nm). Bauer et al.33 have reported that TiO2 nanotube layers annealed at 450 °C consisted of rutile in the thermal oxide, whereas the tube wall consisted fully of anatase. Since the diameters of our samples are all larger than 30 nm, the XRD results are consistent with the reported results.32 Shown in Figure 2b−h are the top-view FESEM images of the prepared TiO2 nanotube arrays with various planar structural parameters. It can be seen that the average inner diameters of these nanotubes are in the range from 40 to 145 nm, and the wall thicknesses are varied from 8 to 20 nm. For better comparison, in the following, we define the samples with the increasing inner diameter sizes of 40, 65, 85, 95, 105, 130, and 145 nm as S1, S2, S3, S4, S5, S6, and S7, respectively.

III. RESULTS AND DISCUSSION It is well-known that increasing the anodization voltage can increase the pore diameter, wall thickness, and the tube length, whereas the increase of fluoride or water content could lead to the decrease of wall thickness and tube length and the increase of pore diameter. Therefore, by adjusting the anodization voltage (30−60 V), ammonium fluoride content (0.2−0.5 wt 9050

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approximate equality of the tube lengths guarantees that the efficiency of the samples is only dependent on their planar construction. In order to investigate the influence of the geometric parameters on the optical absorption and PEC water splitting activity of the TiO2 nanotube arrays, UV−vis absorption spectra and photocurrent density of TiO2 nanotube arrays samples (S1−S7) were measured, with the results shown in Figure 3. From Figure 3a, it is clear that the band gap

Figure 2. (a) Schematic of calculated porosity for all samples (S1−S7). Top view FESEM images of the self-organized, highly ordered TiO2 nanotube arrays with various inner diameters: (b) 40 nm (S1); (c) 65 nm (S2); (d) 85 nm (S3); (e) 95 nm (S4); (f) 105 nm (S5); (g) 130 nm (S6); and (h) 145 nm (S7). (i) and (j) show cross-sectional FESEM images for S2 and S6, respectively.

Figure 3. (a) UV−vis absorption spectra of all nanotube arrays samples (S1−S7). (b) Photocurrent density (in 1 M KOH solution) vs measured potential vs Ag/AgCl for S1−S7, under a UV light illumination (280−400 nm, 100 mW/cm2). The dashed lines in (b) show the dark current densities for the samples. The inset in (b) shows the corresponding photoconversion efficiency η for the samples of S1−S7.

Correspondingly, the wall thicknesses for S1−S7 are 11, 11, 10, 8, 11, 16, and 20 nm in order. The detailed dimension information for the samples of S1−S7 is listed in Table 1. The tube lengths of these nanotubes are 10.7, 10.5, 10.6, 10.4, 10.3, 10.2, and 10.5 μm for S1−S7, respectively, all of which are close to 10 μm. Figures 2i and 2j and the inset of Figure 1b show three cross-sectional FESEM images for the representative samples of S2, S6, and S4, respectively. Therefore, the

absorption edges are all around 390 nm, corresponding to the anatase band gap energy of 3.2 eV. This result also confirms further the same morphology and structure for S1−S7. The slight absorption shoulder near the absorption edge of S4 is probably due to the superposition of interference effects as reported by Falaras et al.34 Since the light source we used for photocurrent measurement is the UV light illumination from 280 to 400 nm, the increase in absorption above 400 nm has little influence on the measured PEC property of our samples. It is observed from the Figure 3b that the photocurrent has a rapid rise at −0.9 V vs Ag/AgCl and achieves a current platform at about −0.6 V. Comparing the photocurrent density at −0.65 V (vs Ag/AgCl) of all samples, it is apparent that a volcano shape of photocurrent density distribution is exhibited from S1

Table 1. Parameters of Diameter, Wall Thickness, and Calculated Porosity for the Samples sample pore diameter (nm) wall thickness (nm) porosity (P, %)

S5

S6

S7

40

S1

65

S2

85

S3

95

S4

105

130

145

11

11

10

8

11

16

20

47.1

59.9

68.7

75.7

71.3

67.7

65.0 9051

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Recently, Kontos et al. have proposed the concept of “porosity”, which not only involves the inner diameter and wall thickness but also can give the quantitative morphological information on the nanotube arrays.34 The calculation formula of the porosity is P = 1 − 2πW(W + D)/[31/2(D + 2W)2], where W is the wall thickness and D the inner diameter. As listed in Table 1, the porosities for S1−S7 are calculated to be 47.1%, 59.9%, 68.7%, 75.7%, 71.3%, 67.7%, and 65.0%, respectively. The peak porosity is found for S4 (Figure 2a), which also shows the maximum photocurrent density and the maximum photoconversion efficiency. Therefore, to discover the relationship of the PEC property and the porosity, we depicted the relationship of the photoconversion efficiency and the porosity in Figure 4. It is readily seen that an interesting phenomenon is apparent: the photoconversion efficiency increases exponentially from 4.79% to 7.02% when the porosity increases from 47.1% to 75.7% (the porosity as the horizontal axis), and the maximum photoconversion efficiency of 7.02% corresponds to the maximum porosity (75.7%) in S4. Because the porosity is determined by the two one-dimensional geometric parameters of inner diameter and wall thickness, the PEC property of TiO2 nanotube arrays photoanode is actually determined by the two parameters’ coupling and competition. To understand the above phenomenon, the PEC water splitting processes of TiO2 nanotube arrays should be analyzed in depth. When TiO2 nanotube absorbs a photon, an electron− hole pair is excited. The photogenerated electrons migrate along TiO2 nanotubes to the counter electrode and reduce water to generate hydrogen. Meanwhile, the photogenerated holes transport to the surface of TiO2 nanotubes to generate oxygen and a cycle of water-splitting reaction is complete. Generally speaking, the photogenerated holes are expected to transport to the tube walls along the shortest path. If the tube wall thickness is quite less than the hole diffusion length (∼20 nm in TiO2),35 the holes losses will be marginal. This is the key reason for the good performance of TiO2 nanotubes compared with bulk TiO2. In practice, many researches on PEC reaction process show that the top of nanotubes is always the main reaction place.36−38 The reason is that the upward migration of holes along tube walls in electric field direction is easier than that of reactant/products (ion/molecular) in electrolyte. Therefore, in the deeper part of the tube, the accumulation of the products restrains the surface reaction of the hole, which compels the hole to the top part of the tube to react with the water. However, the long hole migration route leads to the photogenerated carriers recombination and restrain the PEC process. This explains the volcano shape of photocurrent density distribution for samples S1−S7. For S1−S4 which have similar and small tube wall thickness (less than half of the hole diffusion length), larger inner diameter is crucial to speed up ion migration in the tube. This effectively increases the available depth of nanotube for water splitting reaction and reduces the carriers recombination rate because of the vertical migration of photogenerated holes. On the contrary, for S4−S7, their inner diameters are large enough for reactant/products migration. As a result, the smallest wall thickness of S4 becomes a decisive factor. It is interesting that the average efficiency of S5−S7 is obviously higher than that of S1−S3, indicating that the inner diameter may play more significant role in affecting the PEC activities of TiO2 nanotubes. It is worth mentioning that the top of nanotubes is the main reaction places of water cleavage, which are generally well proportioned for the samples.

to S7, with S4 possessing the highest photocurrent of 7.14 mA/ cm2. It is worth noticing that all the photocurrents are in the region from 4.86 to 7.14 mA/cm2, indicating that all samples show similar properties and the change of structure parameter could be used to optimize the photoconversion property. The photoconversion efficiencies η were calculated to range from 4.79% to 4.02%,4 about half of the limit of the conversion efficiency of TiO2 and comparable with the state-of-the-art results in the literature. Thus, it is meaningful to discuss the principle of optimizing the PEC water splitting activity of TiO2 nanotubes in this efficiency region. To clarify the influence nature of geometric parameters on the PEC property, we drew in Figure 4 the dependence of

Figure 4. Influences of inner radius, wall thickness, and calculated porosity on the photoconversion efficiency for all samples (S1−S7).

photoconversion efficiency on the inner radius and wall thickness. We can see that there is no obvious monotonous variation tendency of inner radius with the increase of the photoconversion efficiency. Furthermore, the inner radius is not at an ultimate value but an intermediate one when the photoconversion efficiency of TiO2 nanotube arrays photoanode is optimal. As for the wall thickness, although it is minimum when the photoconversion efficiency of TiO2 nanotube arrays sample is maximum, it has no monotonous relationship with the photoconversion efficiency. It has been proved that the photogenerated carriers separation in the nanotube wall is mainly controlled by charge diffusion rather than the potential gradient over the space charge region, since the formation of a space charge layer is improbable because of the small wall thickness. The prominent PEC efficiency of S4 may be related with the smallest half-value of wall thickness as 4 nm which ensures an efficient charge carrier separation. However, the PEC property of TiO2 nanotube arrays photoanode is not directly linear or proportional with either of the two one-dimensional geometric parameters. In the related reports before, there are many controversial reports about the influence of these two parameters. For examples, Grimes and co-workers reported that the PEC property improved with the smaller inner diameter or thicker wall thickness,28,29 while Zhu and his co-workers reached exactly the opposite conclusion.31 The controversy may be resulted from the different processes of sample preparation in which one cannot control one geometric parameter by locking other parameters completely. 9052

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(8) Chen, Y. S.; Crittenden, J. C.; Hackney, S.; Sutter, L.; Hand, D. W. Environ. Sci. Technol. 2005, 39, 1201−1208. (9) Khan, M. A.; Jung, H. T.; Yang, O. B. J. Phys. Chem. B 2006, 110, 6626−6630. (10) Macak, J. M.; Zlamal, M.; Krysa, J.; Schmuki, P. Small 2007, 3, 300−304. (11) Barros, D. A. J. Appl. Phys. 2007, 101, 034309. (12) Yan, W. S.; Sun, Z. H.; Yao, T.; Pan, Z. Y.; Li, Z. R.; Liu, Q. H.; Wei, S. Q. J. Appl. Phys. 2009, 106, 123918. (13) Murphy, A. B.; Barnes, P. R. F.; Randeniya, L. K.; Plumb, I. C.; Grey, I. E.; Horne, M. D.; Glasscock, J. A. Int. J. Hydrogen Energy 2006, 31, 1999−2017. (14) Yoriya, S.; Grimes, C. A. Langmuir 2010, 26, 417−420. (15) Huang, L.; Peng, F.; Yu, H.; Wang, H. J.; Yang, J.; Li, Z. Mater. Res. Bull. 2010, 45, 200−204. (16) Liu, Y. B.; Li, J. H.; Zhou, B. X.; Bai, J.; Zheng, Q.; Zhang, J. L.; Cai, W. M. Environ. Chem. Lett. 2009, 7, 363−368. (17) Chen, J. F.; Lin, J.; Chen, X. F. J. Nanomater. 2010, 753253. (18) Gong, D.; Grimes, C. A.; Varghese, O. K.; Hu, W. C.; Singh, R. S.; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331−3334. (19) Paulose, M.; Prakasam, H. E.; Varghese, O. K.; Peng, L.; Popat, K. C.; Mor, G. K.; Desai, T. A.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 14992−14997. (20) Shankar, K.; Mor, G. K.; Fitzgerald, A.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 21−26. (21) Ruan, C. M.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. Chem. B 2005, 109, 15754−15759. (22) Yoriya, S.; Grimes, C. A. J. Mater. Chem. 2011, 21, 102−108. (23) Lynch, R. P.; Ghicov, A.; Schmuki, P. J. Electrochem. Soc. 2010, 157, G76−G84. (24) Cui, Q. A.; Feng, B.; Chen, W.; Wang, J. X.; Lu, X.; Weng, J. J. Inorg. Mater. 2010, 25, 916−920. (25) Kontos, A. G.; Kontos, A. I.; Tsoukleris, D. S.; Likodimos, V.; Kunze, J.; Schmuki, P.; Falaras, P. Nanotechnology 2009, 20, 045603. (26) Ghicov, A.; Albu, S. P.; Hahn, R.; Kim, D.; Stergiopoulos, T.; Kunze, J.; Schiller, C. A.; Falaras, P.; Schmuki, P. Chem.Asian J. 2009, 4, 520−525. (27) Beranek, R.; Macak, J. M.; Gartner, M.; Meyer, K.; Schmuki, P. Electrochim. Acta 2009, 54, 2640−2646. (28) Mor, G. K.; Shankar, K.; Varghese, O. K.; Grimes, C. A. J. Mater. Res. 2004, 19, 2989−2996. (29) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191−195. (30) Paulose, M.; Mor, G. K.; Varghese, O. K.; Shankar, K.; Grimes, C. A. J. Photochem. Photobiol., A 2006, 178, 8−15. (31) Zhu, W.; Liu, X.; Liu, H. Q.; Tong, D. L.; Yang, J. Y.; Peng, J. Y. Electrochim. Acta 2011, 56, 2618−2626. (32) Albu, S. P.; Tsuchiya, H.; Fujimoto, S.; Schmuki, P. Eur. J. Inorg. Chem. 2010, 4351−4356. (33) Bauer, S.; Pittrof, A.; Tsuchiya, H.; Schmuki, P. Electrochem. Commun. 2011, 13, 538−541. (34) Kontos, A. G.; Katsanaki, A.; Maggos, T.; Likodimos, V.; Ghico, A.; Kim, D.; Kunze, J.; Vasilakos, C.; Schmuki, P.; Falaras, P. Chem. Phys. Lett. 2010, 490, 58−62. (35) Allam, N. K.; Poncheri, A. J.; El-Sayed, M. A. ACS Nano 2011, 5, 5056−5066. (36) Zhu, W.; Liu, X.; Liu, H. Q.; Tong, D. L.; Yang, J. Y.; Peng, J. Y. J. Am. Chem. Soc. 2010, 132, 12619−12626. (37) Li, X. Y.; Hou, Y.; Zhao, Q. D.; Chen, G. H. Langmuir 2011, 27, 3113−3120. (38) Sun, W. T.; Yu, Y.; Pan, H. Y.; Gao, X. F.; Chen, Q.; Peng, L. M. J. Am. Chem. Soc. 2008, 130, 1124−1125.

Therefore, the influences of porosity on the PEC properties of either U- or V-shaped TiO2 nanotubes are almost identical. In short, as a two-dimensional geometrical parameter, the porosity can well express the competition relationship of inner diameter and wall thickness of the influence on the PEC property of TiO2 nanotube arrays photoanode.

IV. CONCLUSION We prepared highly ordered TiO2 nanotube arrays with tuned inner diameter (40−145 nm), wall thickness (8−20 nm), and fixed tube length (10 μm) by anodization of Ti foil in different ethylene glycol electrolyte. By utilization of FESEM and XRD, it is confirmed that all these samples are single anatase phase, highly ordered, and entirely smooth. The photoelectrochemical measurements show that all samples present photoconversion efficiencies around 7%, comparable with the state-of-the-art results. Further analysis reveals that the photoconversion efficiency has a monotonous variation with porosity rather than with wall thickness or inner diameter. We suggest that large porosity can benefit the photoconversion efficiency from two aspects. First, the large surface areas ensure a much shorter path toward wall surface than the hole diffusion length. Second, a larger inner space of nanotube is important to accelerate the ion migration in the tube and overcome the kinetic bottleneck. Since the porosity is well related to the PEC property of TiO2 nanotube arrays photoanode, it provides an alternative design principle toward high photoconversion efficiency of TiO2-based photoanodes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Q.H.L.), [email protected] (S.Q.W.). Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 11135008, 11105151, 10979044, 10905058, 11075164, ) and Knowledge Innovative Program of The Chinese Academy of Sciences (KJCX2-YW-N40), and the Fundamental Research Funds for the Central Universities (WK2310000014).

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REFERENCES

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