Impact of Silicon Resistivity on the Performance of Silicon Photoanode

Mar 31, 2017 - In this work, deposition thickness set points of the NiFe alloy is 2 nm using electron ... with Si resistivity of 0.5 Ω·cm–1 exhibi...
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Impact of Silicon Resistivity on the Performance of Silicon Photoanode for Efficient Water Oxidation Reaction Qian Cai, Wenting Hong, Chuanyong Jian, Jing Li, and Wei Liu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on March 31, 2017

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Impact of Silicon Resistivity on the Performance of Silicon Photoanode for Efficient Water Oxidation Reaction Qian Cai, † Wenting Hong, † Chuanyong Jian, † Jing Li, † Wei Liu*†‡ †

Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China ‡ Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China ABSTRACT: Exploitation of efficient water oxidation catalysts with cost-effective and high activity is a prerequisite to enable water splitting as an alternative pathway for a renewable energy source. Silicon has the potential for high-efficiency oxygen evolution reaction (OER). However, an important factor which is the doping concentration of the silicon influence on the OER performance has not been studied. Our results show that the performance of silicon photoanode is significantly influenced by its resistivity, which is directly relation to the doping concentration. Combining with ultra-thin NiFe alloy nanoflakes (2 nm thick) deposited by electron beam evaporation method, NixFe(1-x)/TiO2/n-Si photoanode for water oxidation has been demonstrated. Our results show that the prepared Ni80Fe20/TiO2/n-Si photoanode with Si resistivity of 0.5 Ω·cm-1 exhibits high catalytic performance with low onset potential of 1.06 V versus RHE (η0 = -0.17 V), which are comparable to the state-of-the-art photoanodes using Ir or Ru as a catalyst. In addition, the photocurrent density at the reversible potential for water oxidation (1.23 V versus RHE) is around 21.5 mA·cm2 , which is higher than most of the metal/TiO2/n-Si photoanodes. Therefore, our Ni80Fe20/TiO2/n-Si (0.5) photoanode shows the great potential to replace precious metal for high efficient large-scale water splitting.

KEYWORDS: Oxygen evolution reaction (OER), photoanode, photoelectrochemical, resistivity, nickel iron alloy

Photoelectrochemical splitting of water is a practical and environmentally friendly strategy for efficient renewable energy production. However, oxygen evolution reaction (OER), as a critical half reaction, is still regarded as a bottleneck in improving the water splitting efficiency.1-3 OER process is kinetically sluggish with large overpotential due to the multistep four-electron oxidation (4OH- → 2H2O + O2 + 4e-, in base solution) process.4 Therefore, highly active metal catalysts and semiconductor anode are employed to overcome this kinetic limitation by providing a strong driving force, and thus achieving high efficient water oxidation. Silicon (Si) is a promising semiconductor that can be used to efficiently absorb light because its narrow bandgap (1.12 eV) which matches the solar spectrum. It is worth to note that Si is the second most abundant element in the earth and has emerged as one of the most attractive photoanode semiconductors.5,6 However, Si is extremely prone to photocorrosion under strongly alkaline conditions. Si is mainly used in the photoanode with a metalinsulator-semiconductor (MIS) structure, in which metal catalyst and a thin insulator (few nanometers thick) layer are deposited on top of Si to prevent the photocorrosion, thereby enhancing OER performance and stability.7,8 The Ir/TiO2/n-Si metal-insulator-semiconductor (MIS) photoanode was firstly reported by Chen et al.,9 in which, a large photovoltage of ∼550 mV and the small onset overpotential of -0.21 V demonstrate its high OER activity. It has been confirmed that silicon passivation methods can significantly influence the OER performance of silicon-based photoanodes.1,5 On the other hand, using a Si P-N diode can also improve the OER performance.

Scheunemann used Ir as a catalyst and achieved a maximum photovoltage of 630 mV on a p+n-Si diode based TiO2/SiO2/p+n-Si photoanode.10 In particular, we noticed that a series of Si wafer with variation of resistivity (doping concentration) was selected as substrate, including 0.3-0.5 Ω·cm-1,11 2.06-2.18 Ω·cm-1,5 3.0-10 Ω·cm-1,12 0.1-0.2 Ω·cm-1.10 However, the doping concentration of the Si influence on the OER performance has not been systematicly studied. It is well known that doping concentration of Si can affect the electronhole recombination and charge transport, which may eventually determine the OER performance. Hence, the study of the doping concentration of Si on OER performance is critical for high-performance water splitting catalyst design. Ir and Ru are known as the best OER catalysts.13-15 However, the low abundance and high cost of Ir and Ru severely restrict their applications in large scale. Consequently, enormous efforts have been dedicated to exploring the nonprecious metal alternatives with low cost and high catalytic capability.16,17 The first-row transition metal (e.g., Fe, Co, Ni) and their hydroxides/oxides have attracted considerable attention for OER.18-20 Among them, NiFe alloy is one of the most promising candidates with earth-abundant, cost-effective, long-term stability and remarkable OER activitiy.21-23 In this work, using ultra-thin NiFe nanoflakes as catalyst, the effect of the doping concentration of Si substrate on the OER performance was systematically investigated. The doping concentration of silicon can be reflected on the resistivity. A low resistivity indicates high doping concentration. Herein,

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the resistivity of 0.01 Ω·cm-1, 0.5 Ω·cm-1 and 3.5 Ω·cm-1 were selected as Si substrate. We denote the various resistivity of Si substrate as Si (0.01), Si (0.5) and Si (3.5), respectively. A high-performance Ni80Fe20/TiO2/n-Si photoanode is fabricated using the conventional complementary metal oxide semiconductor (CMOS) fabrication technology which can significantly reduce the manufacturing cost of the Si photoanode.

Figure 1. (a) Schematic of Ni80Fe20/TiO2/n-Si photoanode. (b) Energy band diagram of Ni80Fe20/TiO2/n-Si photoanode under illumination in 1 M KOH electrolyte (pH=14). (c) Atomic Force Microscopy (AFM) image of NiFe catalysts on TiO2 surface. (d) The corresponding height profiles of NiFe catalysts. The numbers from 1 to 3 correspond to the red dash line in (c).

The structure and energy schematic diagram of Ni80Fe20/TiO2/n-Si photoanode for water oxidation are shown in Figure 1a, 1b. A thin TiO2 layer (2 nm) deposited on the surface of Si wafer by atomic layer deposition (ALD) serves as the electron filter, which is thin enough for holes generated from Si substrate tunneling through TiO2 to the surface of NiFe catalyst. The fabrication method of NiFe/TiO2/n-Si photoanode is described in Supplementary information S1. In this work, deposition thickness set points of the NiFe alloy is 2 nm using electron beam evaporation. However, the deposited 2 nm NiFe cannot form a continuous film observed from AFM image in Figure 1c. NiFe flakes have an ultrathin thickness around 2.1-2.8 nm (Figure 1d). Ultra-thin NiFe nanoflakes will effectively be optically “transparent” and not significantly affect the light absorption of the Si substrate (Supplementary information S2 and Figure S1). In addition, this atomic thick NiFe alloy will substantially reduce the charge transfer distance and eventually enhance the OER performance.21 Furthermore, this ultra-thin NiFe catalyst deposition method is quite simple compared with the wet chemistry method and can significantly reduce photoanode fabrication cost.

Figure 2. Electrochemical characterizations of Ni80Fe20/TiO2/n-Si photoanodes with different Si resistivity. (a) LSV curves. (b) Potential required for 10 mA·cm-2 for the Si substrates with different resistivity based on Ni80Fe20 and Ni60Fe40 catalysts. (c) Transient photocurrents recorded at constant bias of 1.4 V versus RHE under chopped simulated irradiation (AM 1.5G) (green: 0.01 Ω·cm-1; red: 0.5 Ω·cm-1; blue: 3.5 Ω·cm-1). (d)Tafel plots. (e) The electrochemical active surface area (ECSA). (f) Incident-photonconversion efficiency (IPCE).

The OER performance of Ni80Fe20/TiO2/n-Si photoanode is characterized by electrochemical workstation and the detailed operation method is described in Supplementary information S3. The linear sweep voltammetry (LSV) curves of asprepared Ni80Fe20/TiO2/n-Si photoanodes with different Si resistivity are conducted in 1 M KOH electrolytes under the simulated AM 1.5G (100 mW·cm-2) illumination. As shown in the insert image of Figure 2a, the similar oxidation peak for all the photoanodes before OER potential can be assigned to the quasi-reversible redox couple (Ni2+/Ni3+) occurring on the surface of catalysts. Ni3+or NiOOH is believed to be the active site for the OER.24,25 It is apparent that Ni80Fe20/TiO2/n-Si (0.5) photoanode with Si resistivity of 0.5 Ω·cm-1 exhibits the highest OER activity. Ni80Fe20/TiO2/n-Si (0.5) photoanode has the lowest onset potential at ~ 1.06 V versus RHE (overpotential, η0 = -0.17 V) (Figure 2a, red line), which is about 70 mV and 290 mV lower than Ni80Fe20/TiO2/n-Si (3.5) and Ni80Fe20/TiO2/n-Si (0.01) photoanodes, respectively. In addition, Ni80Fe20/TiO2/n-Si (0.5) photoanode requires a potential of merely 1.16 V versus RHE (η= -0.07 V) to achieve a current density of 10 mA·cm-2. To further confirm this phenomenon, another ratio of Ni60Fe40 catalysts was deposited onto three types of Si substrates for comparison. As presented in Figure 2b, the photoanode with the Si resistivity of 0.5 Ω·cm-1 displays the lowest potential to reach the current density of 10 mA·cm2 for both the Ni80Fe20 and Ni60Fe40 catalysts. Furthermore, the high catalytic activity is also reflected by the photocurrent density at 1.23 V versus

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RHE (reversible potential for water oxidation). The photocurrent at 1.23 V versus RHE for Ni80Fe20/TiO2/n-Si (0.5) photoanode is up to 21.5 mA·cm-2 indicating its high OER activity, while this value is higher than most of the metal/TiO2/n-Si photoanodes as summarized in supplementary information S4 and Table S1. The photocurrent of various photoanodes are measured under the same light intensity (100 mW·cm-2). To explicitly compare the photocurrent of Ni80Fe20/TiO2/n-Si photoanodes, the transient photocurrents (Figure 2c) were recorded at a constant bias of 1.4 V versus RHE under chopped simulated irradiation (AM 1.5G). It is obviously observed that the Ni80Fe20/TiO2/n-Si (0.5) photoanode has the highest photocurrent density (39 mA·cm-2), which is approximately 1.5 times and 28.8 times greater than Ni80Fe20/TiO2/n-Si (3.5) photoanode (26.3 mA·cm-2) and Ni80Fe20/TiO2/n-Si (0.01) photoanode (1.57 mA·cm-2), respectively. Besides, the enlarged portions of transient photocurrents are shown in Supplementary information S3.1 and Figure S2. Photocurrent spikes are typically observed when the light is switched both on and off for Ni80Fe20/TiO2/n-Si (0.01) photoanode. The sharp current spike is attributed to the accumulated photogenerated holes at the semiconductor-liquid junction due to the slow oxygen evolution reaction kinetics or high sensitivity to the surface states.26 For Ni80Fe20/TiO2/n-Si (0.5) photoanode, the negligible spike current and the quick reach steady state imply the effective charge separation and the reduced charge recombination. The excellent catalytic activity of Ni80Fe20/TiO2/n-Si (0.5) is also revealed by Tafel slope per the Tafel equation η = b × log (j/j0), where η is the overpotential, b is the Tafel slope, j is the current density, and j0 is the exchange current density. As shown in Figure 2d, the Ni80Fe20/TiO2/n-Si (0.5) photoanode exhibits the lowest Tafel slope of 64.8 mV·dec−1, while the Tafel slopes of Ni80Fe20/TiO2/n-Si (0.01) and Ni80Fe20/TiO2/nSi (3.5) are 74.8 and 77.8 mV·dec−1, respectively. Consequently, a lower Tafel slope implies a faster OER kinetics and will lead to a significantly boosted current density at a higher potential.27 The electrochemical active surface area (ECSA) is another key factor for catalyst in OER process. It is well known that a large ESCA is responsible for the enhanced catalytic activity.28 The ECSA can be estimated from the cyclic voltammetry (CV) curves in 1 M KOH solution in the dark (supplementary information S3.2, Figure S3). The linear slope of capacitive current versus scan rate (Figure 2e), equivalent to the twice of the electrochemical double-layer capacitance (Cdl), is used to represent the ECSA. The results show that NiFe/TiO2/n-Si photoanodes with different Si resistivity have the similar electrocatalyst surface area. Therefore, one can conclude that the change of electrochemical activity is solely from the effect of Si dopant concentration. IPCE as a function of wavelength for the photoanodes at 1.3 V versus RHE is shown in Figure 2f. The IPCE was calculated by equation (1): IPCE %=

1240×Jph Pmono × λ

×100% (1)

where Jph is the photocurrent density (mA·cm-2), Pmono is the intensity of the incident monochromatic light (mW·cm-2), and λ is the wavelength of the monochromatic light. It is observed that the Ni80Fe20/TiO2/n-Si (0.5) photoanode has the highest IPCE over the entire visible light wavelength range, which is in good agreement with the band gap of Si (1.12 eV). However, the heavily doped Si (0.01 Ω·cm-1) presents a

reduced efficiency for photons absorption, which is usually attributed to increased surface recombination possibility. It was reported that photoresponse is effectively affected by the surface recombination from surface textures and Auger recombination from dopants. By reducing doping level to suppress Auger recombination process, the IPCE of Si photoanode can be dramatically improved.29 The photovoltage was defined as the difference between the overpotential to drive 1 mA·cm-2 through the illuminated n-Si photoanode and the dark p+Si anode. The Ni80Fe20/TiO2/n-Si (0.5) anodes has a high photovoltage around 545 mV as shown in Figure S5. Additionally, the stability of OER catalyst is a critical issue due to the highly corrosive and oxidative reaction conditions. The Ni80Fe20/TiO2/n-Si (0.5) photoanode retain the most of its OER activity after 20 hours continuous operation at a constant potential of 1.16 V vs RHE in 1 M KOH as shown in Support information S6 and Figure S6. The effect of Si substrate resistivity on the OER performance of photoanodes can be attributed to the variation of the doping concentration (ND) in the Si substrate. The doping concentration in a semiconductor can be approximately expressed by equation (2): ρ=

1 ND eu

2

Where ρ, e, and u are the resistivity, electronic charge, and carrier mobility, respectively.30 The calculated values of ND are listed in Table 1. It is apparently shows that the ND of Si is inversely proportion to resistivity.

Figure 3. The effect of the doping concentration on the band bending and photogenerated carrier behavior of Si. (a) Large Si resistivity (low doping concentration): depletion layer length (D) > photon penetration (Dp). (b) Medium Si resistivity (medium doping concentration): depletion layer length (D) = photon penetration (Dp). (c) Small Si resistivity (heavily doping concentration): depletion layer length (D) < photon penetration (DP).

The band bending in silicon is determined by the doping concentration. The Si with low concentration doping (low ND, 3.5 Ω·cm-1) will result in a weak band bending as shown in Figure 3a. Therefore, the excited electron/hole pair in the photon penetration (Dp) region is hard to be separated and mostly recombine in the bulk or the surface region due to the relative flat energy band, eventually, leading to a poor OER activity. When the doping concentration increase (medium ND, 0.5 Ω·cm-1), the degree of band bending will increase (Figure 3b). The depletion layer depth (D) of Si with various doping concentration is obtained by equation (3) and summarized in Table 1. 

 / 





  

 

ln 

 

/ 3

Where ε0, ε is the dielectric constant of vacuum and silicon, respectively; K0 is Boltzmann constant (1.38×10-23 J/K);

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the value of T is 273K; ni is the intrinsic carrier concentration of Si. The general propagation of illuminated light is determined by the Beer–Lambert law:31     ! "#$ 4 Where I0 is the incident light intensity and I is the axial dependence of the intensity on the absorption depth x into the material; α is the absorption coefficient which is strongly dependent on wavelength (λ). In general, x can be defined as the photon penetration depth (Dp), while x is equal to 1/α.32 The α of single-crystal silicon in the visible light region is measured by Wang.33 The α of Si is decreased from 6310 cm -1 to 1940 cm-1 with the light wavelengths ranging from 500 nm to 700 nm. Hence, the calculated Dp value of Si is in the range of 0.16 µm to 0.51 µm in visible light region which is close to the depletion layer depth of silicon with a resistivity of 0.5 Ω·cm-1. It is known that the photogenerated electron-hole pair can be separated with maximum efficiency leading to the lowest recombination rate and reaching the highest OER efficiency when the depletion layer depth (D) close to the photon penetration depth (Dp). Hence, silicon with a resistivity of 0.5 Ω·cm-1 has high OER efficiency. As the Si became heavily doped (high ND, 0.01 Ω·cm-1), the depletion layer depth become less than the photon penetration depth, and only the photogenerated electron-hole pair in the depletion region can be separated efficiently (Figure 3c). Consequently, a further increase of dopant concentration will decrease the OER efficiency. In addition, the dopant can also act as a recombination site, which may also reduce the photocatalytic efficiency.34,35 Hence, it is crucial to select a substrate with proper resistivity for a high-performance OER photoanode design and fabrication. Table 1. The parameters of different Si substrates by theoretical calculation Resistivity (Ω·cm-1)

Doping concentration(ND) (cm-3)

Depletion depth (D) (µm)

0.01

4.31 × 1017

0.051

0.5

8.62 × 1015

0.313

3.5

15

0.769

1.23 × 10

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NiFe catalysts, which is consistent with the previous reports. 36-38

It is known that the rapid charge transfer is very critical for efficient OER catalysts. To explore the charge transport and recombination properties of the photoanodes, electrochemical impedance spectroscopy (EIS) measurements are conducted with the frequency ranging from 0.1 Hz to 10000 Hz. The equivalent circuit (inset, Figure 4d) can be modeled as a parallel combination of charge transfer resistance (Rct) and a double electric layer capacitance (CPE1). Rct can be described by semicircle in the Nyquist plots, while the Rct value is inversely proportional to the conductivity of photoanode. The EIS of the different photoanodes shown in Figure 4d indicates that Ni80Fe20/TiO2/n-Si photoanode has the smallest Rct, leading to the efficient separation of photogenerated electron-hole pairs, and hence the fast-interfacial charge transfer, which is consistent with the best OER catalytic performance of Ni80Fe20/TiO2/n-Si photoanode. The electrochemical performance of catalysts with different Ni/Fe ratios indicates that an optimal Fe content plays a major role in influencing the OER performance of NixFe(1-x)/TiO2/n-Si photoanodes, which is consistent with previous reports.24,38,39 Previous studies reveal that the charge exchange process occurred between Fe and Ni atoms increases the number of activated Ni centers, thereby significantly enhancing the catalytic activity of Ni.40,41

layer

Based on the above discussion, the effect of the silicon doping concentration on the OER performance was revealed. It provides the useful guidance for the high-performance Si OER anode design. On the other hand, the catalyst is another important factor for OER performance. The optimal ratio of NixFe(1-x) catalyst was also studied to achieve highperformance OER photoanode. The LSV curves of asprepared NixFe(1-x)/TiO2/n-Si (0.5) photoanodes with Fe weight percentages ranging from 0 to 100 wt. % are presented in Figure 4a. Pure Fe catalyst shows the worst performance with the positive onset potential and weak photocurrent density. It is apparent that Ni80Fe20/TiO2/n-Si photoanode exhibits the highest OER activity. From Figure 4b and 4c, the Ni80Fe20/TiO2/nSi photoanode exhibits the lowest Tafel slope around 64.8 mV·dec−1 and the most negative overpotential. All these results demonstrate that the Ni80Fe20/TiO2/n-Si photoanode has the highest OER activity compare with another component of

Figure 4. Electrochemical characterizations of NixFe(1-x)/TiO2/nSi photoanodes with different Fe weight percentage. (a) LSV curves. (b) The corresponding Tafel slop. (c) the plots of the overpotential required for 10 mA·cm-2 and the Tafel slop. (d) Electrochemical impedance spectroscopy (EIS) spectra, the inset shows the equivalent circuit model.

The surface composition of the Ni80Fe20/TiO2/n-Si photoanode before and after OER test was investigated by X-ray photoelectron spectroscopy (XPS). Ni, Fe, C and O signals are detected from the survey spectra as shown in Supplementary information S7 and Figure S7. To further collect the information of the surface of Ni80Fe20/TiO2/n-Si photoanode, the high-resolution Ni 2p, Fe 2p and O 1s core-level spectra are fitted to investigate the electronic states. As shown in highresolution Ni 2p spectra (Figure 5a), it is obviously that the peak of Ni (∼852.8 eV) vanished after OER testing, confirming that a complete Ni oxidation occurred during the electrochemistry process. The fitted peak at 854.3 eV presences only observed on pristine photoanode is attributed to the nickel oxides (NiOx). The peak near 855.8 eV shows negligible

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change after OER testing, while this peak exhibited coexisting valence states of Ni2+ (2p3/2, ~856.1 eV) and Ni3+ (2p3/2, ~855.6 eV), which can be assigned to the NiOOH phase. These spectral differences (Figure 5a, upper and lower curves) indicate a transformation from NiOx to NiOOH after the OER. The change in the binding environment of Ni can also be reflected by the O1s XPS spectra (Supplementary information S7, Figure S8).42,43

Ni80Fe20/TiO2/n-Si (0.5) photoanode only requires a potential of 1.06 V versus RHE (η0 = -0.17 V) to initiate the reaction and is capable of reaching the current density of 10 mA·cm-2 at the potential of 1.16 V RHE (η0 = -0.07 V). The photocurrent density of Ni80Fe20/TiO2/n-Si (0.5) photoanode is up to 21.5 mA·cm-2 at 1.23 V versus RHE, which is higher than most of the other metal/TiO2/n-Si photoanodes. Regarding high costeffective, long-term stability and unusual OER activity, our Ni80Fe20/TiO2/n-Si photoanode has shown the great potential to replace the noble-metal-based (IrO2, RuO2) catalysts towards the efficient renewable energy production. In addition, this work also provides the useful guidence for the design of high performance OER photoanode.

ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Detailed descriptions of the experimental and characterization methods or additional data, such as UV-Vis spectra, IT curve, XPS spectra are presented in Supporting Information.

AUTHOR INFORMATION Corresponding Author * [email protected] Figure 5. High-resolution XPS spectra of the Ni80Fe20/TiO2/n-Si photoanode before and after OER tests. (a) High-resolution Ni 2p spectra. (b) XPS depth profiles spectra of Ni 2p after OER test. (c) High-resolution Fe 2p spectra. (d) XPS depth profiles spectra of Fe 2p after OER test.

Using ion milling method, the depth profile of highresolution Ni spectra (Figure 5b) reveals that Ni on the surface is oxidation state (Ni3+) because of the anodization of Ni in 1 M KOH solution. As the depth increased, the appearance of metallic Ni species is due to the in situ reducing oxides by ion milling. The high-resolution Fe 2p XPS spectra of Ni80Fe20/TiO2/n-Si photoanode before and after OER testing are shown in Figure 5c. It is observed that the spectra have negligible changes after OER testing, which is consistent with the absence of the Fe2+/Fe3+ redox peak in LSV curves (Figure 2a). The peaks of Fe 2p can be fitted into several peaks, which are attributed to Fe-O and Fe-OH, respectively. The observation of Fe 2p3/2 and 2p1/2 at the binding energy of 712.8 eV and 725.9 eV implies that Fe is mostly in Fe3+ oxidation state, which is consistent with the observed XPS spectra of Fe2O3 and FeOOH.44 Similar changes are observed in the depth profile of Fe 2p (Figure 5d), the metallic Fe is only found under the surface of 1 nm, while the active oxidation states dominant on the surface. It has been shown in Figure 3a that Fe2O3 and FeOOH have minimal OER activity. Hence, the XPS spectra study of the NiFe catalyst further indicates that Ni3+ on the surface of the alloy is the main active center for water splitting. In summary, the effect of the resistivity of Si substrate on the performace of OER is systematically studied. The proper dopant concentration can minimize the recombination of the photogenerated electrons and holes as well as improve the OER activity. The electrochemistry characterizations indicate the prepared Ni80Fe20/TiO2/n-Si photoanode with Si resistivity of 0.5 Ω·cm-1 shows the highest OER performance. The

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences and National Natural Science Foundation of China (No. 61674152).

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