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Improved Photoelectrocatalytic Performance for Water Oxidation by Earthabundant Cobalt Molecular Porphyrin Complex-Integrated BiVO4 Photoanode Bin Liu, Jian Li, Hao-Lin Wu, Wen-Qiang Liu, Xin Jiang, Zhi-Jun Li, Bin Chen, Chen-Ho Tung, and Li-Zhu Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04510 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 4, 2016

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ACS Applied Materials & Interfaces

Improved Photoelectrocatalytic Performance for Water Oxidation by Earth-abundant Cobalt Molecular Porphyrin Complex-Integrated BiVO4 Photoanode Bin Liu,† Jian Li,† Hao-Lin Wu, Wen-Qiang Liu, Xin Jiang, Zhi-Jun Li, Bin Chen, Chen-Ho Tung, Li-Zhu Wu* Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ABSTRACT: An earth-abundant, low-cost cobalt porphyrin complex (CoTCPP) is designed as a molecular catalyst to work on three-dimensional BiVO4 film electrode for water oxidation for the first time. Under illumination of 100 mW cm-2 Xe lamp, the CoTCPP-functionalized BiVO4 photoanode exhibits a 2-fold enhancement in photocurrent density at 1.23 V vs RHE, and nearly 450 mV cathodic shift at 0.5 mA/cm2 photocurrent density relative to bare BiVO4 in 0.1 M Na2SO4 (pH = 6.8). Simultaneously, stoichiometric oxygen and hydrogen are generated with a faradic efficiency of 80% over 4 h. The activity and stability of the BiVO4 photoanode is dramatically increased by molecular CoTCPP, giving rise to higher performance than previously reported noble metal ruthenium complex modified-BiVO4 photoanode. By using hydrogen peroxide as the hole scavenger, we demonstrate that molecular CoTCPP catalyst greatly suppresses the hole-electron recombination on the surface of BiVO4 semiconductor, which offers a promising route toward high efficiency, low-cost, practical solar fuel generation device.

KEYWORDS: molecular porphyrin complex, BiVO4, photoanode, charge separation efficiency, water oxidation kinetic

bands of semiconductor, the molecular-semiconductor INTRODUCTION interface comprises unique electronic structure,31-32 which Photoelectrochemical (PEC) water splitting into could further overcome the kinetic barrier for OER at molecular oxygen and hydrogen using semiconductor semiconductor surface without losing light absorption materials offers an attractive route to achieve the efficiency. However, it was not until recently that a PEC conversion and storage of solar energy.1-4 In such process, photoanode combining molecular ruthenium catalyst the oxygen evolution reaction (OER) occurring on the with BiVO4 was reported.33 photoanode is a multistep and four-electron process, thus In that work, the ruthenium-based molecular catalyst requiring high overpotential.5-7 Developing photoanodes [(cy)Ru(L2bpy)OH2]+ (L = 4,4-dicarboxylic acid) with advantages of high efficiency, low cost and superb functionalized BiVO4 photoanode showed an enhanced stability is crucial for the successful construction of photocurrent of approximate 1.5 mA cm-2 at 1.23 V vs RHE high-performance, commercially viable PEC devices. and about 200 mV cathodic shift of onset potential BiVO4 is considered as one of the most promising relative to bare BiVO4 photoanode in 0.1 M phosphate photoanode materials,8-14 due to its proper band gap of 2.4 buffer (pH 7.1) under simulated AM 1.5 solar illumination. eV, excellent light absorption depth up to 500 nm and low However, no oxygen evolution was presented, which was cost of synthesis. However, the OER activity and stability unfavorable to evaluate the real oxygen evolution ability. achieved by BiVO4 photoanode to date have been far In addition, the scarcity and relative high price of below what are expected due to suffering from high ruthenium make such electrode adverse to large-scale electron-hole recombination and poor water oxidation application. kinetics. Surface modification with metal ions,15-16 With this recognition, we wish to report a BiVO4 doping,17-18 construction of heterojunctions19-20 and loading photoanode employing earth-abundant cobalt molecular 21-28 cocatalysts have been employed to alleviate these porphyrin (CoTCPP, the structure is shown in Figure S1) restrictions. Among them, coupling with OER catalyst is as the OER catalyst to improve the charge separation and regarded as an effective approach. Compared to the most water oxidation kinetic. The reason for choosing CoTCPP frequently used inorganic heterogeneous OER complex is due to the following advantages: i) CoTCPP is catalysts,21-28 the recent studies have shown higher water composed of non-precious, earth-abundant elements; ii) oxidation rates by using surface-functionalized molecular porphyrin ligand can be modified by carboxyl or 29-30 electrochemical systems. Owing to distinct molecular phosphate groups easily, vital to the fabrication of orbitals of the catalyst that can overlap the electronic ACS Paragon Plus Environment


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assembled electrode;34 iii) a few examples using cobalt porphyrin complexes as electrocatalysts have shown excellent performance for electrocatalytic OER in neutral water.35-36 It is expected that molecular CoTCPP complex can be introduced into BiVO4 photoanode to serve as an inexpensive photoelectrochemical OER catalyst, and to improve the performance of BiVO4 photoanode. Moreover, Al2O3 thin layer was introduced to bind with more catalyst molecules. In contrast to traditional loading methods of inorganic catalysts, this molecular CoTCPP modified BiVO4 photoanode is simply made by solution processing assembly, ease of complicated construction. Under illumination of 100 mW cm-2 of Xe lamp, the assembled molecule–semiconductor hybrid photoanode exhibits a 2-fold enhancement in photocurrent density at 1.23 V vs RHE, nearly 450 mV cathodic shift at 0.5 mA/cm2 photocurrent density and enables a 0.45% half-cell photoconversion efficiency relative to bare BiVO4 in 0.1 M Na2SO4 (pH = 6.8). Simultaneously, stoichiometric oxygen and hydrogen are generated with a faradic efficiency of 80% over 4 h. Both OER activity and stability are higher than that of previously reported noble metal ruthenium molecular complex modified BiVO4 photoanode.33 These results demonstrate that low-cost molecular catalyst-semiconductor hybrid photoanodes could have the potential in constructing high-efficiency and practical PEC device.

EXPERIMENTAL SECTION Chemicals and Reagents. All chemicals were analytical grade and used as received without further purification. The organic reactions were performed in air and the prepared chemicals are dried in air and stored in vacuum. Characterization. The UV-Vis diffuse reflectance spectra (UV-Vis DRS) were performed by Cary 5000 UV-visible-NIR spectrophotometer employing a lab-sphere diffuse reflectance accessory in the range of 200-2000 nm. The morphologies and energy dispersive X-ray (EDX) mapping of the electrodes were recorded by a HITACHI S-4800 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (accelerating voltage of 15 kV). Transmission electron microscope (TEM) was acquired on JEM 2100F operated at 200 kV. Element content was determined by an inductively coupled plasma mass spectroscopy (ICP, Varian710-ES). Raman spectra were collected by raman spectrometer (Horiba HR800 Raman system) with laser excitation wavelength of 532 nm. The X-ray photoelectron spectroscopy (XPS) was obtained on an ESCALAB 250 spectrophommeter with Al-Kα radiation. Photoelectrochemical Test. Photoelectrochemical measurements were performed in a three-electrode with the working electrode of the sample film electrode, counter electrode of platinum disk and a Ag/AgCl electrode (3 M KCl) as the reference (ERHE = EAg/AgCl + 0.194 + 0.059 × pH). The electrolyte is 0.1 M Na2SO4 degassed for 30 minutes by flushing high purity

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argon prior to and during each measurement. For photocurrent measurement, the light source was a 300 W Xe-lamp and the light intensity at the surface of the electrode is 100 mW cm-2. A Zennium electrochemical workstation (Germany, Zahner Company) was used to record the J-V and Electrochemical Impedance Spectroscopy (EIS) curves. The produced hydrogen and oxygen were measured using a gas chromatography with CH4 as internal standard for quantitative analysis. Fabrication of molecular catalyst–semiconductor hybrid photoanode. BiVO4 electrode was prepared via electrodeposition.24 Firstly, Bi(NO3)3∙5H2O (0.04 M), KI (0.4 M) were dissolved in 5o mL distilled water forming a clear solution and pH was adjusted to 1.7 by adding HNO3. Then 20 mL of absolute ethanol containing 0.23 M p-benzoquinone was added to the above mixed solution, followed by vigorously stirring for 20 minutes. A typical three-electrode cell using clean fluorine doped tin oxide (FTO) as working electrode was used for electrodeposition. The deposition was performed potentiostatically at -0.1 V vs. Ag/AgCl at room temperature (RT) for 10 minutes, resulting in the film of BiOI on the FTO. After being taken out and washed with distilled water, the dried BiOI film was covered through dipping 200 µL of solution containing 0.2 M vanadylacetylacetonate (VO(acac)2) dissolved in DMSO, which was then heated in a muffle furnace at 450 o C for 1 h in air to convert BiOI to BiVO4. After being cooled down to RT naturally, the electrode was soaked in 1.0 M NaOH solution for 30 minutes to remove excess V2O5 present in the BiVO4 film. The produced pure BiVO4 film electrode was rinsed with distilled water and dried at RT. This BiVO4 film electrode is in monoclinic phase (Figure S2, JCPDS#: 14-0688) with good crystallinity. The as-prepared BiVO4 electrode was immersed into the solution containing 0.3 mL aluminum tri-sec-butoxide and 48.5 mL isopropanol, which was then heated at 60 oC for 0.5 h. After that, the electrode was taken out and heated in an oven at 200 oC for 1 h in air, obtaining Al2O3 thin layer coated BiVO4 electrode. CoTCPP complex was synthesized according to the previous literature.37 Briefly, CoCl2∙6H2O (0.24 M) and TCPP (0.04 M) were dissolved in 25 mL DMSO, followed by refluxing for 24 h. After the solution was cooled to RT, HCl (1 M, ca. 75 mL) was added to the solution which precipitated the crude product. The crude product was collected by filtration, washed with distilled water, and dried in vacuum. Then this was redissolved in NaOH (0.1 M, ca. 20 mL) followed by addition of HCl (1 M, ca. 40 mL) to cause deposition of the product as a purple solid. The assembled BiVO4/Al2O3/CoTCPP electrode was finally obtained by soaking BiVO4/Al2O3 electrode into 0.5 mM CoTCPP in acetonitrile at RT for 10 hours in the dark, followed by rinsing with excess acetonitrile and drying with N2.

RESULTS AND DISCUSSION

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Figure 1. SEM (a), (b) and TEM (c) images of CoTCPP assembled BiVO4 electrode with Al2O3 layer; (d) the elemental mapping of bismuth, vanadium, aluminium and cobalt by EDX spectroscopy. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) were employed to characterize the morphology of the as-prepared electrodes. As shown in Figure S3, BiVO4 showed an appearance of highly interconnected and aligned nanosheets, composed of plenty of nanoparticles around 80 nm, vertically grew on the surface of conductive substrate with uniform morphology and dense loading, forming a three-dimensional (3D) network with a highly open and interstitial structure. The obvious difference in appearance after the coating of Al2O3 (Figure S4) was not observed by SEM due to its ultrathin feature, but could be detected by TEM images (Figure S5). Bare BiVO4 had smooth surface, while many dense nanoparticles with good dispersion were deposited on the surface of BiVO4 after Al2O3 modification, forming a thin layer with the thickness of about 3.5 nm. When molecular CoTCPP was further assembled, the electrode was still very well retained (Figure 1(a-c)). Elemental mapping (Figure 1d) from energy dispersive X-ray (EDX) spectroscopy suggested that Co was uniformly dispersed on the surface of BiVO4, indicating the successful assembly of CoTCPP on the surface of BiVO4. This result was further confirmed by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), where the signal of CoTCPP (Figure S6) and Co element (Figure S7) were observed, respectively. Additionally, CoTCPP had almost no effect on the characteristic absorption of the original electrode (Figure S8), which is one of the most advantages of using molecular catalyst. This transparency is in stark contrast to other inorganic catalysts on BiVO4 electrodes, which significantly block or reflect the relevant photons in the visible light range.

Figure 2. (a) Photocurrent density vs applied potential curves and (b) Half cell photoconversion efficiency for BiVO4/Al2O3/CoTCPP, BiVO4/CoTCPP, BiVO4/Al2O3 and BiVO4 electrodes under Xe lamp illumination. Confident that BiVO4 electrode was modified with CoTCPP, the photoelectrochemical performance of the assembled photoanode was directly compared to the same films prior to modification in a water splitting setup, with the sample photoelectrode as working electrode, Ag/AgCl (3 M KCl) as reference electrode, platinum disk as counter electrode in 0.1 M Na2SO4 (pH = 6.8). The water oxidation activity was measured by linear sweep voltammetry (LSV). As shown in Figure 2a, all of the cases with negligible current in the dark (Figure S9) displayed obvious photocurrents under the irradiation of 100 mW cm-2 Xe lamp. In addition, the coating of Al2O3 on BiVO4 produced a slightly higher photocurrent relative to the bare BiVO4 due to passivation effect.38 Through further tethering molecular CoTCPP catalyst on BiVO4 electrode with Al2O3 layer, the photocurrent was found to be obviously increased at all potentials, achieving a photocurrent of 2.1 mA cm-2 at 1.23 V vs RHE, nearly two times higher than that of bare BiVO4, and a 450 mV cathodic potential shift at 0.5 mA/cm2 photocurrent density was observed, demonstrating that molecular CoTCPP could be used as an efficient OER catalyst, which was further confirmed by the FTO/Al2O3/CoTCPP electrode in the dark (Figure S10), where an obvious catalytic current was produced. It is worthwhile to mention that both the increase in photocurrent and the cathodic potential shift brought about by the modification of CoTCPP are much higher than that of previously reported noble metal ruthenium molecular catalyst modified BiVO4 photoanode.33 The half-cell photoconversion efficiency calculated from the LSV results in Figure 2b reached 0.45%, which belonged to average level among the reported single-photon photoanodes.


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Figure 3. (a) Hole injection yield for BiVO4, BiVO4/Al2O3 and BiVO4/Al2O3/CoTCPP electrodes; (b) Relative charge separation yield between BiVO4 and BiVO4/Al2O3/CoTCPP electrodes; (c) Schematic diagrams illustrating the reaction at the surface of BiVO4; (d) The reaction at the surface of BiVO4/Al2O3/CoTCPP electrode (abs: photon absorption and charge carrier generation; br: bulk recombination; sr: surface recombination; inj: hole injection into electrode/electrolyte interface).

Meanwhile, it should be noted that Al2O3 layer was vital to the adsorption of CoTCPP molecules. According to the results of inductively coupled plasma-atomic emission spectrometry (ICP-AES), the amount of Co bound to the electrode with Al2O3 layer was about 3.8 nmol cm-2, yet only 1.2 nmol cm-2 for that without Al2O3 layer, suggesting that the surface of Al2O3 may be helpful for binding with the carboxylate group of CoTCPP molecules.39-40 This was accordant with the very small increase in photocurrent and photoconversion efficiency produced on the modified electrode without Al2O3 middle layer. In addition, we carried out control experiments: If a BiVO4/Al2O3 electrode was soaked in solution absent of molecular CoTCPP with only TCPP ligand or with only Co ion present, there was no increase in photocurrent density at all (Figure S11). Therefore, anchoring molecular CoTCPP is necessary to improve photocurrent density on the electrode. The remarkable reduction in onset potential and increase in photocurrent were indicative of a fast water oxidation reaction in the presence of CoTCPP. The improved photoelectrochemical performance resulted from the efficient charge separation and reduced hole-electron recombination. The recombination may occur in the bulk or the surface of BiVO4 semiconductor. To better assess the effectiveness of CoTCPP on the surface recombination, we introduced an easily oxidized hole scavenger to eliminate the injection barrier for holes. Hydrogen peroxide (H2O2) is such a hole scavenger owing to its faster oxidation kinetics and lower reduction θ potential (E = 0.68 V vs NHE for O2/H2O2) compared to 1.23 V vs NHE.41-42 The photoelectrochemical H2O2 oxidation was performed in 0.1 M Na2SO4 mixed with 1

mM H2O2. Figure S12 plots showed the photocurrent produced by BiVO4 and BiVO4/Al2O3/CoTCPP electrodes in the presence or absence of H2O2. As for BiVO4 electrode, we observed obvious increase of photocurrent and reduction of onset potential in the presence of H2O2 indicating that H2O2 eliminated the surface recombination and promoted the injection of holes to the electrolyte. Unlike the BiVO4 electrode, the increase of photocurrent produced on BiVO4/Al2O3/CoTCPP electrode due to H2O2 addition was not as much as that of BiVO4 and exhibited nearly the same onset potentials with and without H2O2. Thus, it implies that CoTCPP has promoted the hole transfer of BiVO4 to electrode/electrolyte interface in the low applied potential in the absence of H2O2. In addition, the photocurrent generated by BiVO4/Al2O3/CoTCPP photoanode without H2O2 was close to that of BiVO4 electrode with almost 100% charge injection yield in the presence of H2O2. That is to say, CoTCPP improves the rate of water oxidation reactions over the entire potential range due to efficient injection of holes to the electrode/electrolyte interface. For more quantitative discussion, the holes injection yield for water oxidation reaction could be extracted from Figure S12 by the equation (1):42 Pcharge injection = Jphotocurrent(H2O)/Jphotocurrent(H2O2)

(1)

where Pcharge injection represents the injection yield of photogenerated holes to electrolyte for water oxidation reaction; Jphotocurrent(H2O) and Jphotocurrent(H2O2) represent the photocurrent from water oxidation and H2O2 oxidation, respectively. More details of the equation is in the ESI. As could be seen from Figure 3a, the hole injection yield of


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BiVO4 and BiVO4/Al2O3 electrodes kept below 50%, suggesting that half of the holes were consumed by the surface recombination rather than dedicating to the water oxidation reaction. On the contrary, the holes injection of BiVO4/Al2O3/CoTCPP photoanode, higher than that of BiVO4/Al2O3 at all bias potential, started from 40% at 0.6 V, to 70% at 1.0 V and even to nearly 90% at 1.23 V (all of the potentials are relative to RHE), suggesting that the surface recombination was greatly restrained by the CoTCPP. The process can be illustrated in Figure 3c-3d. Upon illumination, BiVO4 is excited to generate holes and electrons, some of which suffer from recombination before migrating to the semiconductor/electrolyte interface (normally called bulk recombination), while part of which are able to arrive at interface. The holes reaching the interface, on one hand, can be injected into electrolyte to participate in the OER, but also suffered from surface recombination due to its sluggish water oxidation kinetics. Molecular CoTCPP can accelerate holes transfer generated from BiVO4 to the electrolyte electrode/electrolyte interface to participate in water oxidation reaction, which is further confirmed by the fitting results of EIS (Figure S13). CoTCPP modified electrode exhibited much smaller Rct (7.081 KΩ) than the pristine electrode (31.260 KΩ), indicating the more favorable environment for hole transfer to electrolyte. We also obtained the relative charge separation yield between BiVO4 and BiVO4/Al2O3/CoTCPP electrodes by dividing JH2O2 of BiVO4 by JH2O2 of BiVO4/Al2O3/CoTCPP, shown in Figure 3b. It showed about 90%, even at the low applied potential region and nearly reached 100% at higher potential, indicating that bulk recombination occurred to almost the same extent in both photoanodes, and CoTCPP mainly decreased the surface hole-electron recombination on BiVO4 layer.

Figure 4. (a) Oxygen and hydrogen evolution curves for BiVO4/Al2O3/CoTCPP electrode; (b) for BiVO4 electrode in 0.1 M Na2SO4 at 1.4 V vs RHE.

It is important to exactly evaluate the influence of molecular CoTCPP on the true oxygen evolution, in addition to the behavior of photocurrent. To this end, we detected oxygen using the gas chromatography. As shown in Figure 4a, under continuous irradiation at 1.4 V vs RHE, the oxygen and hydrogen evolved on the BiVO4/Al2O3/CoTCPP photoanode and its counter electrode in the expected stoichiometric ratio with generation rates of 8.3 μmol h−1 for oxygen and 16.8 μmol h−1 for hydrogen within a 4-hour period of testing time, which corresponded to a turnover number (TON) of 4370 for oxygen based on molecular CoTCPP and all of these data were considerably higher than that of bare BiVO4 (Figure 4b). Moreover, the faradaic efficiency of BiVO4/Al2O3/CoTCPP was calculated to be about 80%, higher than that of bare BiVO4 (≈ 60%) and the relevant results were summarized in Table S1.

Figure 5. Current density-time curves obtained at 1.4 V vs RHE for BiVO4, BiVO4/Al2O3 and BiVO4/Al2O3/CoTCPP electrodes. The PEC stability of the photoelectrode is another very important issue for the PEC device. Thus we investigated the stability of the photoanodes against anodic photocorrosion. To our great delight, molecular CoTCPP catalyst could remarkably improve the stability of the photoanode by kinetically directing a greater percentage of the photogenerated holes to oxidize water rather than BiVO4 itself which can cause photocorrosion. Just as shown in Figure 5, the pure BiVO4 electrode at an applied potential of 1.4 V vs RHE exhibited a rapid decrease in photocurrent, (over 50% within 2 hours) with the running of operation, while the decline (less than 5%), was strongly inhibited in the presence of CoTCPP for BiVO4/Al2O3/CoTCPP electrode. Additionally, SEM images (Figure S14) of the electrodes after 2 h irradiation showed that the structure of regular nanosheets collapsed in the BiVO4 film without CoTCPP due to the photocorrosion, while the morphology of the film with CoTCPP displayed no appreciable change, indicating the avoidance of photocorrosion. It has been proposed that the surface atomic ratio of V/Bi decreased significantly during irradiation, giving rise to a bismush rich surface and a considerable drop in photocurrent.15,28 The XPS analysis of BiVO4 and BiVO4/Al2O3/CoTCPP electrodes before illumination showed that the surface atomic ratio of V/Bi was about 0.38. After 2 h PEC test, the V/Bi ratio decreased to 0.16 for BiVO4 and 0.28 for BiVO4/Al2O3/CoTCPP. All of the above results clearly


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demonstrated that CoTCPP facilitated the extraction of accumulated holes from BiVO4 to oxidize water efficiently, thus stabilizing BiVO4 against corrosion. Recently, the studies carried out by Groves35 and Du36 revealed that cobalt porphyrin complex kept unchanged during electrocatalytic OER, neither becoming other kinds of cobalt complex nor being converted to heterogeneous cobalt oxide materials. Sakai et al37 found cobalt porphyrin decomposed into several less catalytically active (or inactive) species within 15 min, but the cobalt ion remained ligated, instead of generating CoOX nanoparticles in photocatalytic OER. Unlike the loss of catalytic activity due to the decomposition of cobalt porphyrin into other less catalytically active cobalt complex, negligible change in photocurrent for our CoTCPP assembled BiVO4 photoanode was observed, suggesting that the transformation of CoTCPP to other less catalytically species was avoided in 2 h PEC measurement，which was further confirmed by the XPS spectra of Co after photoelectrolysis (Figure S15). Moreover, TEM and HRTEM images (Figure S16) showed no obvious evidence of new nanoparticles grown on the BiVO4 surface or change in the morphology of BiVO4 after photoelectrolysis, indicating that the cobalt porphyrin complex did not decompose to form heterogeneous cobalt oxide materials under illumination. Based on the previous reports and the present results in our work, we speculate that most molecular CoTCPP catalyst may be unchanged, and more direct and strong evidences is still under research in our lab.

Conclusion In conclusion, we have for the first time successfully combined an earth-abundant, low-cost molecular catalyst CoTCPP with BiVO4 electrode to construct a photoanode system for OER in neutral water. The introduction of molecular CoTCPP dramatically increases the rate and stability of PEC water oxidation. Mechanistic study reveals that such enhancement of performance is derived from almost complete elimination of surface electron hole recombination. These results provide a promising route to achieve water oxidation efficiently by anchoring molecular catalysts to solar-responsive semiconductors. ASSOCIATED CONTENT Supporting Information. Experimental procedures, methods, and product characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Author contributions †These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT We are grateful for financial support from the Ministry of Science and Technology of China (2014CB239402, 2013CB834505 and 2013CB834804), the National Science Foundation of China (91427303, 21390404 and 51373193), the Key Research Programme of the Chinese Academy of Sciences (XDB17030201) and the Chinese Academy of Sciences.

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