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Defects and Interfaces on PtPb Nanoplates Boost Fuel Cell Electrocatalysis Yingjun Sun, Yanxia Liang, Mingchuan Luo, Fan Lv, Yingnan Qin, Lei Wang, Chuan Xu, Engang Fu,* and Shaojun Guo* accelerating the oxygen reduction reaction (ORR) at cathode of fuel cells and oxidation reaction of small molecules like hydrogen, methanol, and ethanol at anode.[9–12] Therefore, tremendous research interests have been devoted to tuning/optimizing Ptbased catalysts for achieving the extra-high activity and enhanced durability for fuel cell reactions. Previous studies reveal that the catalytic behaviors of metallic nanocrystals are heavily influenced by their composition, shape, and structure.[13–16] This has stimulated the researchers to explore new procedures for making different types of Pt-based nanocrystals with the optimized parameters for enhancing catalytic activity through adding control agents during the growth, including wires,[6] cages,[17] plates,[18] cubes,[19] spheres,[20] octahedral,[21] and icosahedra.[22] However, these well-known controls still show the limited ability to maximize the Pt utilization efficiency for achieving more efficient fuel cell catalysis. Actually, the crystal defect occupies a decisive position in the research field of material science, and has been widely studied in bulk materials and carbon materials for many years.[23–25] In brief, the defects include point defects (e.g., interstitials, vacancies), line defects (e.g., dislocations), and plane defects

Nanostructured Pt is the most efficient single-metal catalyst for fuel cell technology. Great efforts have been devoted to optimizing the Pt-based alloy nanocrystals with desired structure, composition, and shape for boosting the electrocatalytic activity. However, these well-known controls still show the limited ability in maximizing the Pt utilization efficiency for achieving more efficient fuel cell catalysis. Herein, a new strategy for maximizing the fuel cell catalysis by controlling/tuning the defects and interfaces of PtPb nanoplates using ion irradiation technique is reported. The defects and interfaces on PtPb nanoplates, controlled by the fluence of incident C+ ions, make them exhibit the volcano-like electrocatalytic activity for methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), and oxygen reduction reaction (ORR) as a function of ion irradiation fluence. The optimized PtPb nanoplates with the mixed structure of dislocations, subgrain boundaries, and small amorphous domains are the most active for MOR, EOR, and ORR. They can also maintain high catalytic stability in acid solution. This work highlights the impact and significance of inducing/controlling the defects and interfaces on Pt-based nanocrystals toward maximizing the catalytic performance by advanced ion irradiation strategy. The growing amount of exhaust emission produced by the vehicles is a major source of air pollution.[1–3] Fuel cells are considered as one of the cleanest and efficient devices to solve this environmental problem.[4–8] Platinum (Pt) as an omnipotent metal catalyst exhibits the topping electrocatalytic properties in Y. Sun, Dr. M. Luo, F. Lv, Y. Qin, Prof. S. Guo Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, China E-mail: [email protected] Y. Sun, Y. Qin, Prof. L. Wang College of Chemistry and Molecular Engineering Qingdao University of Science and Technology Qingdao 266042, China Y. Liang, C. Xu, Prof. E. Fu State Key Laboratory of Nuclear Physics and Technology School of Physics Peking University Beijing 100871, China E-mail: [email protected]

Prof. S. Guo BIC-ESAT College of Engineering Peking University Beijing 100871, China Prof. S. Guo Department of Energy and Resources Engineering College of Engineering Peking University Beijing 100871, China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201702259.

DOI: 10.1002/smll.201702259

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(e.g., grain boundaries, twin boundaries, sub-boundaries, and phase boundaries). Both experimental and theoretical results reveal that the change of the crystal structure enabled by the defects can undoubtedly break the electron–hole symmetry, probably leading to a different catalytic property compared to the original one.[26–28] However, there are three limitations in systematically studying the role of crystal detects in Pt-based nanocrystals toward the catalytic activity tuning: (1) the used Ptbased catalysts are generally at nanoscale, which are difficult to induce the controllable defects via traditional chemistry strategies; (2) the common method for producing defects is usually not efficient on Pt-based catalysis, perhaps because Pt cannot be alloyed with most of heteroatoms; (3) it is not clear whether the induced defects and the defect-related interfaces are the main factors in catalytic activity enhancement or not. Herein, we report an effective strategy for tuning/optimizing the defects and interfaces of intermetallic PtPb nanoplates by ion irradiation to achieve more efficient fuel cell catalysis. The radiation, an advanced technique, is extensively used in the field of aerial materials, semiconductors, modified foods, and medical treatment, which can cause the microstructure and properties change via transferring high energies to the target compounds by the process of ionization, electronic excitation, and collision cascade.[29–32] It can control the energy and variety of incident ions as well as the location of beam spot to tune the site, type, and quantity of defects on target materials. The induced defects and interfaces on PtPb nanoplates can be controlled at atom level through adjusting the fluence of incident C+ ion. To the best of our knowledge, this is the first attempt to induce/ control the defects and the defects-related interfaces on Pt-based nanocrystals by ion irradiation, and also the first attempt to apply ion irradiation approach to electrocatalytic fields. Compared to the original PtPb nanoplates, the structure due to ion irradiation changes from single crystal to multicrystal with the defects like the dislocations, subgrain boundaries, amorphization, and also multiple defects-related interfaces. The defects and interfacesengineered PtPb nanoplates show the volcano-like electrocatalytic activity for methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), and oxygen reduction reaction (ORR) as a function of ion irradiation fluence. The optimized PtPb nanoplates exhibit 1.87 and 1.91 times higher specific and mass activities for MOR, 1.82 and 1.85 times higher specific and mass activities for EOR, and 2.97 and 3.00 times higher specific and mass activities for ORR than original PtPb nanoplates. They are also very stable for MOR, EOR, and ORR after severe durability measurements in acid solution. The present work highlights the necessity and importance of ion irradiation technique in inducing/tuning the defects and interfaces of PtPb nanoplates for achieving more efficient energy catalysis, and may shed light on designing different defects and interfaces of multimetallic nanocrystals in developing more efficient industrial catalysts. The ion irradiation strategy was used to control/tune the defects and interfaces of PtPb nanoplates by controlling the radiation conditions (see the Experimental Section, Supporting information). Figure 1a shows the schematic diagram of accelerator. In current study, the carbon ions (C+) are ionized through ion source by sputtering, then accelerated after high-voltage electric field in accelerating cavity, and ultimately pass through the target materials. The final PtPb nanoplates

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in the sample chamber subjected to 3 MeV C+ ion irradiation were modified through partial atomic rearrangement due to the interactions between energetic ions and atoms. The incident ions (3 MeV C+) pass entirely through the sample with almost no residual concentration (Figures S1 and S2, Supporting Information), which could make the influence of residual carbons on catalysis be ignored.[33] Figure 1b shows the typical transmission electron microscopy (TEM) image of pristine Pt64Pb36 nanoplates. They have a hexagonal nanoplate structure, with monodisperse edge length of about 16 nm and average thickness of 5 nm (as measured from the vertical nanoplates on the grid). The high-resolution TEM (HRTEM) was used to investigate the crystalline structure of PtPb nanoplates before and after the ion irradiation. Before the ion irradiation, the edge has a different structure from the interior (Figure 1c). Figure 1c1,2 is the fast Fourier transform (FFT) of partial area in a nanoplate, respectively, and Figure 1c3 is the FFT of the whole nanoplate. The Pt and PtPb phases can be identified from their diffraction patterns, indicating a core/shell nanostructure with intermetallic PtPb core and Pt shell. The FFT of the whole nanoplate suggests its perfect singe-crystalline structure, and the relationship of crystal orientation is Pt[110]//PtPb[001]. When the PtPb nanoplates were irradiated by 3 MeV C+ ions with a total fluence of 2 × 1016 ions cm−2 (denoted as PtPb/C irra-2), a different structure from original nanoplates was obtained, as shown in Figure 1d. The FFT indicates that the grains with different crystalline orientations are formed in a nanoplate. The enlarged image (Figure 1e) of red square in Figure 1d reveals that high-density dislocations (red mark) are formed in the nanoplate, along with the subgrain boundaries (blue line). Some disordered areas (yellow ellipse) are produced at the interface of crystals with different orientations. Meanwhile, the nanoplate edge exhibits the partial amorphous features (orange area) interfacing with dislocations and boundaries (Figure 1f,g). The enlarged image (Figure 1g) of red square in Figure 1f shows that subgrain boundaries (blue line) are formed between subgrains with different crystalline orientations (white line), and the interface (orange line) between crystalline structure and amorphous structure is observed. More interestingly, some edges of hexagonal nanoplate were concaved inordinately and the angles of the hexagonal nanoplate were obtuse compared with pristine nanoplate. The volume of interfacial area in the nanoplate is largely increased by the dislocations, subgrain boundaries, and the interface between crystalline structure and amorphous structure due to ion irradiation. The composition of Pt to Pb in PtPb nanoplates after the irradiation is determined to be 66/34 by TEM energy-dispersive X-ray spectroscopy (TEM-EDS) (Figure S3, Supporting Information), similar to that before the irradiation (64/36). This means there are almost no changes in the PtPb composition before and after ion irradiation. The alloyed structure was further confirmed by the scanning transmission electron microscopy (STEM) elemental mapping analysis, where only Pt and Pb are observed through the analyzed area (Figure S4, Supporting Information). We further investigate how the damage level of nanoplate controlled by the different incident ion fluences affect the transition of crystal structure. Figure 2a,b shows the HRTEM images of PtPb nanoplates irradiated by 3 MeV C+ ion at fluence of (1 × 1016 ions cm−2, denoted as PtPb/C irra-1). The FFT

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Figure 1.  a) The schematic diagram of ion accelerator. b) TEM image of PtPb nanoplates. c) HRTEM of PtPb/C nanoplate and d–g) PtPb/C nanoplate irradiated with the ion fluence of 2 × 1016 ions cm−2 (PtPb/C irra-2). The insets in (c) are FFT patterns of the area of (c1), (c2) and whole nanoplate of (c3). The inset in (d) is FFT pattern of whole nanoplate. The inset in (f) is FFT pattern of orange areas. The red “T” represents dislocation, blue line represents subgrain boundary, white line represents crystal orientation, areas surrounded by yellow curve represent disordered regions, and orange area represents amorphous domain.

of the image (inset in Figure 2a) reveals that the crystal structure is polycrystalline, and the enlarged image of red square in Figure 2a reveals that some defects such as dislocations and subgrain boundaries without amorphous phase are observed in the nanoplate (Figure 2b). The edges of the nanoplate irradiated at fluence of 1 × 1016 ions cm−2 are not obviously deformed. While the fluence of C+ ion was increased to 3 × 1016 ions cm−2 (denoted as PtPb/C irra-3), the PtPb nanoplate emerges larger amorphous regions both in edge and in center (Figure 2c,d), and part of edges are obviously deformed after ion irradiation. It should be noted after irradiation, the alloy phase of PtPb could be retained, proved by TEM image in Figure S5 (Supporting Information), which shows the typical (110) plane of PtPb phase. The X-ray diffraction patterns (XRD) of original PtPb nanoplates and irradiated nanoplates further prove that there was no phase transformation after irradiation (Figure S6, Supporting Information). These results confirm that accurately controlling the damage level through fluence of incident ions can significantly tune the defects of PtPb nanoplates and thus control the volume of interface in the nanoplates.

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We first used the electro-oxidation of MOR and EOR as the model reactions to show how the defects and related interfaces can tune the electrocatalytic activity of PtPb nanoplates. The cyclic voltammograms (CVs) of Pt/C, PtPb/C, and ion-irradiated PtPb/C were carried out in a 0.1 m HClO4 solution with or without 0.1 m CH3OH. The electrochemical active surface areas (ECSAs) were estimated to be 81.3 m2 g−1 for PtPb/C, 78.3 m2 g−1 for PtPb/C irra-1, 82.8 m2 g−1 for PtPb/C irra-2, 86.7 m2 g−1 for PtPb/C irra-3, and 77.6 m2 g−1 for Pt/C, respectively (Figure S7a, Supporting Information), indicating the ion irradiation can basically not impact the surface area of nanoplates. But, the ion-irradiated PtPb/C catalysts show much higher specific activity than the one without ion irradiation (Figure 3a). The peak current density increases in the following sequence: Pt/C < PtPb/C < PtPb/C irra-1 < PtPb/C irra-3 < PtPb/C irra-2. The PtPb/C irra-2 shows the highest ECSAs-normalized peak current density of 6.22 mA cm−2, which is 1.87 and 25.9 times higher than those of PtPb/C and commercial Pt/C (Figure 3b). And the PtPb/C irra-2 also displays the highest mass activity,

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performance for EOR with high mass activity of 4.37 A mg−1, 1.85 and 19.0 times higher than those of PtPb/C (2.36 A mg−1) and Pt/C (0.23 A mg−1) catalysts, respectively. Meanwhile, the PtPb/C irra-2 shows a high specific activity of 5.28 mA cm−2, 1.82 and 17.6 times higher than those of the PtPb/C (2.90 mA cm−2) and Pt/C (0.30 mA cm−2) catalysts. These results reveal the defects have little effect in reducing the overpotential of EOR, but are important for in enhancing the current density of EOR.[34,35] The ionirradiated PtPb/C catalysts were also stable toward EOR, proved by the chronoamperometric experiments (Figure S11b, Supporting Information) and long-term CV sweeps (Figures S11c and S12, Supporting Information). Similarly, the PtPb/C irra-2 still exhibits high durability with much higher activity retention of 64.9% than that of commercial Pt/C (5.5%) (Table S1, Supporting Information). The defects and related interfaces-engineered Pt64Pb36/C can also be applied to enhance ORR. Figure 4a shows the ORR polarization curves of the different catalysts in O2-saturated 0.1 m HClO4 solution at a rotation rate of 1600 rpm. The specific Figure 2.  a,b) HRTEM images of PtPb/C with the ion fluence of 1 × 1016 ions cm−2 (PtPb/C and mass activities of PtPb/C irra-2 are 16 −2 irra-1) and c,d) PtPb/C with the ion fluence of 3 × 10 ions cm (PtPb/C irra-3). The inset in 1.01 mA cm−2 and 0.84 A mg−1 at 0.95 V versus (a) is FFT pattern of whole nanoplate. RHE, 2.97 and 3.00 times higher than those of PtPb/C, and 21.5 and 23.3 times higher than those of the commercial Pt/C, respectively (Figure 4b). The electron transfer number on PtPb/C ≈25.8 times higher than that of commercial Pt/C (Figure S7b, irra-2 is determined to be ≈4 using rotating ring disk electrode Supporting Information). (Figure S13, Supporting Information). The electrochemical The chronoamperometric experiments were used to study durability of the PtPb/C irra-2 was evaluated at the potential the stability of the ion-irradiated PtPb nanoplates in 0.1 m between 0.6 and 1.1 V versus RHE in 0.1 m HClO4 solution. HClO4 + 0.1 m CH3OH solution at 0.65 V versus reversible hydrogen electrode (RHE) (Figure 3c). After 4500 s, the ionFigure 4c shows that there is no change in ORR polarization irradiated PtPb/C still shows much higher currents than those curves after 50 000 long-term cycles, and only 10.7 and 8.9% of original PtPb/C and Pt/C catalysts. The catalysts were also loss in mass and specific activities (Figure 4d). Similarly, the tested by using CV for 2500 cycles at the scan rate of 50 mV s−1 other ion-irradiated PtPb/C (PtPb/C irra-1, PtPb/C irra-3) also exhibits much higher ORR stability compared to the commerin 0.1 m HClO4 + 0.1 m CH3OH solution (Figure 3d and cial Pt/C (Figure S14 and Table S2, Supporting Information). Figure S8, Supporting Information). The results reveal that the We propose and prove that the induced defects and interPtPb/C irra-2 exhibits the current retention of 52.4%, higher faces on Pt-based catalysis by C+ ion irradiation are an efficient than those of original PtPb/C (40.0%) and commercial Pt/C (18.2%) (Table S1, Supporting Information). The TEM images strategy for boosting electrocatalysis using MOR, EOR, and (Figures S9 and S10, Supporting Information) show that there ORR as the model reactions. The catalytic activity behavior is is no change of the nanoplate structure in the ion-irradiated deeply influenced by the damage level through ion fluence. As PtPb/C and PtPb/C, while obvious expansion and aggregation the energetic incident ions pass through the sample in the prooccurs at the Pt/C catalyst. All the data confirm the high duracess of ion irradiation, ions make violent collision with lattice bility of the ion-irradiated PtPb nanoplates. atoms, displacing them from lattice sites. And the displaced To prove our conjecture, we also investigated the electroatoms can in turn displace others, thus creating a cascade of catalytic performance of the ion-irradiated PtPb/C toward atomic collision around the ion track. Therefore, the structure EOR. As shown in Figure 3e and Figure S11a (Supporting of the sample undergoes change.[36] The microstructure evoInformation), the EOR oxidation peak potentials of PtPb/C, lution process of original nanoplate can be divided into three PtPb/C irra-1, PtPb/C irra-2, and PtPb/C irra-3 are 0.987, stages with the increase of the ion fluence (Figure 5a). In the 0.992, 1.011, and 1.015 V, respectively. Figure 3e,f reveals first stage, the microstructure was transformed from single that the PtPb/C irra-2 also exhibits the best electrocatalytic crystalline to polycrystalline due to the collision of incident ion.

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Figure 3. a) Specific-normalized CVs for MOR. b) Histogram of mass and specific activities for MOR. c) Chronoamperometric (CA) tests and d) CV durability of different catalysts in 0.1 m HClO4 solution containing 0.1 m methanol. The CAs were recorded at the constant potential of 0.65 V versus RHE. e) Specific-normalized CVs for EOR; f) Histogram of mass and specific activities for EOR.

The point defects created by collision could locally destabilize the crystalline structure, producing many dislocations. The subgrain boundaries were formed with the dislocations aggregation and rearrangement.[37] As a result, single-crystalline nanoplates were transformed to the polycrystalline nanoplates with dislocations and subgrain boundaries. In the second stage, partial amorphous regions and crystal-amorphous interfaces were first produced. The formation of the amorphous regions could be due to the constant expansion of disorder areas at the subgrain boundaries, as well as the defect density increased to an extent with the increase of ion fluence.[38] When the structure was transformed from intermetallic form to amorphization, the electron modulation would be different from the ordered one. What’s more, the edges were rougher compared with original nanoplates, causing more active sites. In the final stage, the volume fraction of amorphous region further expands with the increase of ion fluence. The above results demonstrate that with increasing the fluence, the number of dislocations increases,

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followed by producing the subgrain boundaries and amorphous phase with multiple interfaces among them. This means that the fractions of dislocations, subgrain boundaries, and phase interface could be controlled by the ion fluence. The corresponding specific and mass activities of MOR, EOR, and ORR at three typical stages are shown in Figure 5b,c, and Figure S15 and Tables S3–S5 (Supporting Information). Clearly, that the catalytic activity shows the volcano-like trends for MOR, EOR, and ORR with the partial amorphous phase in PtPb nanoplates being more favorable for catalysis. Remarkably, the ion irradiation was indeed able to increase the volume of interfacial area by controlling/tuning the defects and related interfaces, which largely enhance the catalysis in the PtPb nanoplates. The defect and interface engineering on nanoplates could undoubtedly create active sites for catalyst reactions by breaking the electron–hole symmetry, which could be related to the abrupt change in the thermodynamic and kinetic condition of the reaction.[25] For the PtPb/C

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Figure 4.  a) ORR polarization curves and b) specific activities and mass activities of different catalysts at 0.95 V versus RHE. The ORR polarization curves were recorded at room temperature in an O2-saturated 0.1 m HClO4 solution. c) ORR polarization curves of the PtPb /C irra-2 catalyst before and after 50 K cycles between 0.6 and 1.1 V versus RHE. d) The changes on mass and specific activities of the PtPb/C irra-2 before and after 50 K cycles.

irra-2 catalyst, it contains all kinds of defects and interfaces (dislocations, subgrain boundaries, phase boundaries) produced during ion irradiation process. The dislocations and subgrain boundaries, also known as topological defects, can alter bond lengths, long-term deformations, and rehybridize the electron orbitals due to more electron dumplings produced by the defective structure, leading to a considerable modification of electron trajectories, and thereby a higher catalytic activity.[25,39,40] Compared to crystallized counterpart, the metal atoms in amorphous regions can further mediate the electronic structure or tune the atomic arrangement due to more lattice defects produced.[41–43] Furthermore, the migration and formation energies of the atoms at interfaces are more favorable for enhancing the ORR catalytic activity than those of perfect nanocrystals.[44,45] The highest catalytic activity in PtPb/C irra-2 is probably caused by the optimized mixture of dislocations, subgrain boundaries, and amorphous region, which enables the maximum volume of interfacial area, and therefore maximizes the catalytic activity. The PtPb/C irra-3 with serious amorphization displays a bit lower activity but still much higher than pristine PtPb/C, perhaps because the increase of amorphization proportion leads to the decrease of volume of interfacial area, breaking the optimized state maintained by the mixed amorphous and crystalline phases. In addition, the irradiated PtPb nanoplates show a little bit higher ORR, MOR, and EOR stability than the PtPb nanoplates without the irradiation, much better ORR, MOR, and EOR stability than commercial C/Pt, indicating the large

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lateral size of PtPb nanoplates is the key in boosting the durability herein.[18,46] To summarize, we report the first example on tuning/optimizing the defects and defects-related interfaces on PtPb nanoplates by ion irradiation to achieve more efficient catalysis for MOR, EOR, and ORR. The defects induced by C+ ion irradiation include dislocations, subgrain boundaries, and interface of crystal phase and amorphous phase. We found that by controlling the ion fluence, the PtPb nanoplates could be well transformed from single crystal to polycrystal with varying degrees of dislocations, subgrain boundaries, and partial amorphization. Such interesting strategy of controlling/tuning the defects and interfaces on the PtPb nanoplates enables them expose more active sites for boosting the MOR, EOR, and ORR catalysis. The defects and the related interfaces engineered PtPb nanoplates show the volcano-like electrocatalytic activities for MOR, EOR, and ORR with the one with the proper amorphous phase being more favorable for catalysis due to more defects and interfaces. The optimized PtPb nanoplates can largely maximize the Pt catalytic efficiency and utilization. Their mass and specific activities are ≈1.91 and 1.87 times higher for MOR, 1.85 and 1.82 times higher for EOR, and 3.00 and 2.97 times higher for ORR than those of pristine PtPb nanoplates. The current work presents an advanced defect-engineering approach to maximize the catalytic activity of Pt-based catalysis by ion irradiation, and may provide a new guidance for the rational design of highly efficient defect catalysts for future practical fuel-cells applications.

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Figure 5.  a) The simulation models of PtPb nanoplates structure evolution by controlling the C+ ion fluences. The red line represents interface, red “T” represents dislocation, and yellow line represents crystal orientation. The line charts of specific (b) and mass activities (c) for MOR, EOR, and ORR of PtPb nanoplates, irradiated PtPb/C and commercial Pt/C catalysts under the different ion irradiations.

Supporting Information

Conflict of Interest

Supporting Information is available from the Wiley Online Library or from the author.

The authors declare no conflict of interest.

Keywords

Acknowledgements

defects, fuel cells, interfaces, irradiation, PtPb nanoplates

Yingjun Sun and Yanxia Liang contributed equally. This work was financially supported by the National Natural Science Foundation of China (Nos. 51671003, 11375018, and 11528508), the National Key Research and Development Program of China (No. 2016YEB0100201), and the start-up funding from Peking University and National Young Thousand Talented Program. The authors appreciate the National Magnetic Confinement Fusion Energy Research Project with Award No. 2015GB121004 from Ministry of Science and Technology of China, the Ion Beam Materials Lab at Peking University, and the Instrumental Analysis Fund of Peking University.

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Received: July 3, 2017 Revised: September 9, 2017 Published online:

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