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Tailoring Active Sites via Synergy between Graphitic and Pyridinic N for Enhanced Catalytic Efficiency of a Carbocatalyst Jieyuan Li,† Shi Yin,† Fan Dong,§ Wanglai Cen,*,‡ and Yinghao Chu*,† †

College of Architecture and Environment and ‡Institute of New Energy and Low Carbon Technology, Sichuan University, Chengdu Shi, Sichuan Sheng 610000, P. R. China § Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, P. R. China S Supporting Information *

ABSTRACT: Because of the limited characterization methods of the structures and morphology of N-doped carbocatalysts that are available at the atomic level, the detailed promotion mechanism of the catalytic efficiency is unspecific and the particular active sites introduced by the N atoms require further evaluation. Herein, this challenging issue is tackled by extensive theoretical simulation. It is first proposed that the active sites, wherein O2 molecules become adsorbed and activated, be tailored by synergistic graphitic and pyridinic N atoms (GrN and PyN, respectively), which remarkably accelerate the generation of highly chemically reactive O-containing species. The boosted catalytic efficiency is essentially contributed by the electron donor and acceptor of the two active sites, which are induced by PyN and GrN, respectively. These active sites steer the electron transfer between O2 molecules, and the reaction centers in a one-way transmission manner along the PyN → O1 → O2 → C → GrN path. This work provides a feasible protocol for the modification of generally practical carbocatalysts and sheds new light on the understanding of the catalysis mechanism. KEYWORDS: N doping, carbocatalyst, DFT, O2 activation, active site, reaction rate, NEB, AIMD

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areas.16,17 These novel properties enable NCs to become desirable metal-free catalysts in the extensive applications of catalysis (electrocatalysis), energy generation, and contaminant removal. By introducing N atoms, the spin density of the adjacent C atoms can be tailored to preferably create increased defects and active sites.18 Moreover, the electronegativity difference between N and C atoms is responsible for the extension of the conjugated π bonds of the sp2-hybridized carbon planes, which guides electrons to deviate from the planes for intense charge transfer with the reactant. To better understand the promotion mechanism of catalytic efficiency induced by N doping, it is imperative to identify the nature of the N species that overwhelmingly leads to the enhanced activity. In the recent decade, two types of N atoms have been at the center of controversy regarding the origin of active sites, that is, pyridinic N (PyN, N bonded to two C atoms) and graphitic N (GrN, N bonded to three C atoms). Previous researchers have agreed that the introduction of N atoms dominantly contributes to the formation of surfaceactivated O-containing species, which is the essential intermediate in a general catalysis process.5,19−21 However,

INTRODUCTION Carbon materials have long been utilized as excellent catalysts in the broad range of heterogeneous oxidation, O2 reduction, and hydrogenation reactions.1,2 Despite that, carbocatalysts still suffer from the limitation of the nanoarchitecture not being facilely designed for targeted catalytic use. Especially, the surface active sites, which provide reaction centers for O2 adsorption and activation, require further tailoring for specific needs.3 Foreign-element incorporation is considered to be one of the most effective approaches for accelerating the catalytic efficiency of carbocatalysts. The improved activity is widely thought to originate from the charge redistribution and altered active sites of the carbon planes.4−6 Carbocatalysts have been generally used in heterogeneous reactions as supports for stabilizing and coordinating with the deposited metal atoms.7,8 However, confronted with the issues of leaching and sintering of metal-supported carbocatalysts, numerous researchers have devoted time to the investigation of metal-free alternatives, whose prominent advantages and potential applications have been widely proposed.9−11 The development of nitrogen-doped carbocatalysts (NCs) has recently become a key task in the catalysis and materials science fields, due to their excellent stability,12,13 ease of fabrication,14,15 and state-of-the-art performance in various © 2017 American Chemical Society

Received: March 22, 2017 Accepted: May 23, 2017 Published: May 23, 2017 19861

DOI: 10.1021/acsami.7b04026 ACS Appl. Mater. Interfaces 2017, 9, 19861−19869

Research Article

ACS Applied Materials & Interfaces

Figure 1. Optimized local structures (a−d) and time evolution in 10 ps for Gr0Py3 (e) and Gr1Py3 (f) at 800 and 1000 K, respectively, obtained using the ab initio molecular dynamics (AIMD) method. Blue and gray spheres depict N and C atoms. Ed stands for the doping energy of N atoms and the negative sign denotes heat release. All of the energies and lengths are given in eV and Å.

of the essential intermediate critically contributes toward the fruitful efficiency enhancement of the generally practical carbocatalysts. Calculation Details. A 3√3 × 6 supercell of GP that includes 72 C atoms was used as a model catalyst, with lattice parameters relaxed to 12.78 × 14.76 Å2. A vacuum region of 15 Å was applied to minimize the interaction between different layers.31 The PyN atoms were designed to be saturated at the boundary of a single C-atom vacancy (VG). GrN- and PyNdoped VGs are denoted as GrxPyy (Figure 1), where x and y stand for the number of N atoms, respectively. On the basis of our test calculations (Figure S1), GrN and PyN tend to substitute C atoms in a hexagon at para sites. Pyrrolic N, which is much more rare and requires an ultrahigh binding energy to be generated compared to that for the generation of PyN and GrN,2,32 is not considered in this work. All of the spin-polarized density functional theory-D2 (DFTD2) calculations were performed with the “Vienna ab initio simulation package” (VASP 5.3 code),33,34 by applying a generalized gradient approximation with the Perdew−Burke− Ernzerhof (PBE) exchange and correlation function.35 A planewave basis set with a cutoff energy of 400 eV within the framework of the projector-augmented wave method was employed.36−38 The Gaussian smearing width was set to 0.2 eV. The van der Waals correction was described by the Grimme method,3939 with default parameters. The Brillouin zone was sampled with a 3 × 3 × 1 k-points mesh. All atoms, except those at the boundary, were allowed to relax and converged to 0.01 eV Å−1 for all systems. The calculated bond lengths are

the key roles of PyN and GrN are still under debate. On the one hand, some works have proposed the idea that PyN is devoted to the formation of active sites via generation of PyN− O/PyN−OH species22−24 or enhancement of the Lewis basicity of adjacent C atoms to accommodate the absorbed O2 molecules.16 On the other hand, it is also revealed that catalysis takes place at GrN sites.25−28 The researchers have asserted that the adsorption and activation of O2 molecules over the GrN sites produces highly chemically reactive N−O2 and C−O2 intermediates.29,30 Despite the advances, conclusions on active sites with specific N species may not be readily drawn due to the limitations of the characterization methods, especially the structures and morphologies at the atomic level. Furthermore, the potential synergy between PyN and GrN is vital but regrettably neglected, which urgently requires comprehensive investigation for the effective design of NCs. By conducting extensive theoretical calculations, we first conclusively reveal that the facilitated catalytic efficiency of carbocatalysts is through the contribution of the synergistic GrN and PyN. This work sheds new light on understanding the catalytic mechanism and provides a feasible protocol for modification of the abundant carbocatalysts. As an unprecedented result, the active sites are tailored by these two types of N atoms, which boosts the generation of highly efficient surface O-containing groups. Under mild conditions (∼150 °C), O2 molecules become facilely activated and convert to •O2−, which subsequently dissociates into epoxy and PyN−O groups. Mediated by synergistic GrN and PyN, the mass production 19862


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Figure 2. PDOS of PyN (a) and C adjacent to GrN (b) in Gr0Py3 and Gr1Py3, respectively. The charge difference density and Bader effective charge for PyN in Gr0Py3 (c) and Gr1Py3 (d) and C adjacent to GrN in Gr0Py3 (e) and Gr1Py3 (f). The Fermi level is set to 0 eV. Charge accumulation is in blue and depletion, in yellow. The isosurfaces are all set to 0.005 eV Å−3, and the color coding of atoms is the same as that in Figure 1.

consistent with the published values.40,41 The climbing image nudged elastic band (NEB) method was utilized to locate the minimum energy pathways (MEPs) from an initial state (IS) to its final state, and the transition state (TS) was localized using the climbing image method and verified with a single imaginary frequency.42,43 The kinetic properties were calculated via the harmonic transition state theory.44,45 AIMD simulations were carried out to verify the thermostability of the doped configurations using the Born−Oppenheimer approximation.46,47 The doping energy (Ed) is defined as Ed = Edoped − (Eundoped − EC × n + E N × n)

In addition, the solvent effect of water is not considered in the calculations, although it can result in more convincing conclusions. However, according to several advanced works,16,48,49 it is still believable that the results and perspectives here can be extended to the applications of carbocatalysts in the aqueous phase.

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Materials Design and Thermostability. Three PyN atoms are designed to saturately substitute relative C atoms at the boundary of a single-atom vacancy (labeled Gr0Py3; Figure 1a), which impedes the influence of the unsaturated C atoms.50 Then, GrN atoms are added at the para sites of PyN in an atom by atom manner (Gr1Py3, Gr2Py3, and Gr3Py3; Figure 1b−d, respectively). After the optimization, all four structures are rather stable, confirmed by the apparent heat release under the standard DFT condition (1 atm, 0 K). Notably, the doping energy (Ed) for N atoms decreases in the order Gr0Py3 (−4.46 eV) > Gr1Py3 (−4.23 eV) > Gr2Py3 (−3.49 eV) > Gr3Py3 (−2.59 eV). This result implies that the doping procedure of PyN is thermodynamically favorable, whereas the incorporation

(1)

where EC and EN are the total energies of individual C and N atoms, which can be calculated from the pristine GP and isolated N2 molecule, respectively. n Refers to the number of doped N atoms. The adsorption energy (ΔEads) is defined as ΔEads = Etot − (Emol + Eslab)

RESULTS AND DISCUSSION

(2)

where Etot, Emol, and Eslab are the total energies of the adsorption complex, isolated molecule, and GP slab, respectively. 19863


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Figure 3. Reaction pathways (a) and relative reaction rates (b) for the OARs. The electronic localization function (ELF) and charge difference for O2 molecules in the IS and TS of the OAR in Gr0Py3 (c) and Gr1Py3 (d). Charge accumulation is in blue and depletion, in yellow. All isosurfaces are set to 0.002 eV Å−3. Red, blue, and gray spheres depict O, N, and C atoms, respectively.

adsorption and activation.50 However, the key role of GrN and the potential synergistic effect between these two types of N atoms require further investigation, especially on the essential contribution to the generation of surface O-containing species. Hence, the projected density of states (PDOS) for the PyN atom in Gr0Py3 and Gr1Py3 are first calculated (Figure 2a). It is observed that the introduction of GrN barely influences the electrons in the outer orbit (2p) of PyN, due to the overlap of the two main peaks (α and β) in the valence band (VB) of Gr0Py3 and Gr1Py3. Besides, the x + y plane and z direction decomposed PDOS of PyN (the x + y plane and z direction donate the GP planes and the direction vertical to the GP planes, respectively), maintain the overlap, and further illustrate that the conjugated π bonds are not further extended by the incorporation of GrN. Therefore, this specific active site is predominantly determined solely by PyN.

of GrN requires heat adsorption, which matches well with the experimental results.48,49,51 However, the thermostability should be further verified under the general fabrication temperatures of NC (ca. 800− 1000 K9,16,52). Thus, the time evolution in 10 ps is calculated over the course of the AIMD method to identify the thermostability of these structures in the specific temperature window. As can been seen in Figure 1e, Gr0Py3 is stable at temperatures from 800 to 1000 K because the equilibrium distances between PyN/C and the C atoms [dPyN−C(equ), dC−C(equ)] show no significant variation from their respective values at 0 K. In addition, Gr1Py3 retains similar patterns (Figure 1f). Hence, little surface reorganization is induced during the heating process, indicating that GrN- and PyNcodoped GP are successfully designed. Generation and Roles of the Active Sites. Our previous work demonstrated that PyN is the prior active site for O2 19864


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the energy barriers are distinctly decreased from 0.68 eV to approximately 0.50 eV and the heat release is increased from −0.64 eV to approximately −0.96 eV, induced by GrN [refer Figure S3 in the Supporting Information for the detailed MEP in relative OARs]. Moreover, taking the temperature of 420 K as an example, the respective reaction rates increase in the order Gr0Py3 (2.65 mol s−1 m−2) < Gr1Py3 (2.99 mol s−1 m−2) < Gr2Py3 (4.89 mol s−1 m−2) < Gr3Py3 (7.12 mol s−1 m−2). Concretely, a honeycomb containing both the active sites plays the role of a reaction center for OAR. The reaction rate is determined both by the activities and numbers of the reaction centers. It can be concluded that the efficiency of OAR is significantly accelerated via GrN and PyN codoping. Detailed analysis of the electronic structures is performed to further explain the enhanced efficiency (Figure 3c−d). At their relative IS’s, it is observed that the O−O bond length is increased from 1.28 Å (Gr0Py3) to 1.30 Å (Gr1Py3) and the covalent interaction in the O2 molecule is weakened, as depicted by their respective ELFs (refer Figure S3 for the full range and line profile of the respective ELFs). In addition, O2 over Gr1Py3 receives more electrons (−0.26e) than that over Gr0Py3 (−0.19e). Hence, more intense preactivation of O2 over Gr1Py3 is verified compared to that over Gr0Py3 and pristine GP (Table 1), which is intrinsically correlated with the synergy

As mentioned above, the C atom adjacent to GrN may act as another surface active site. Also, it is found from the band structures and total density of states (TDOS) that the major peak (β) at the valence band (VB) edge (−2 to 0 eV) of Gr1Py3 upshifts compared to that of Gr0Py3 (Figure S2). This should be attributed to the charge redistribution in the GrN−C area. Hence, the respective PDOS of this C atom in Gr0Py3 and Gr1Py3 are subsequently calculated (Figure 2b) to determine the role of the C atom. Interestingly, simultaneous upshifts of the major peaks at the valence bond (α and β) are identified after GrN doping. The increased peak height is essentially contributed by more electrons hopping from a lower energy level to a higher one, which approaches the VB edge.53,54 Thus, the electron mobility of the C atom in Gr1Py3 is obviously accelerated compared to that in Gr0Py3. This result matches the electron paramagnetic resonance patterns in some experimental work55 and further identifies that the incorporation of GrN leads the electrons of the adjacent C atom to a shift to a higher energy level, approaching the VB maximum (VBM) to become readily heat/electro-excited. Moreover, this shift in the two main peaks is decomposed into the x + y plane and z direction. It is observed that the β peak is upshifted in Gr1Py3 compared to that in Gr0Py3. However, this shift in the x + y plane may not contribute to the efficiency enhancement, subject to the electron constraint in the conjugated π system. Notably, the β peak is also upshifted along the z direction, which affirms that the delocalized π bonds of the C atom have been extended to deviate from the GP planes via GrN doping. Thus, evidence is provided that the C atom adjacent to GrN is indeed an active site and that there exists potential electron delivery between the C atom and the isolated O2 molecules. Most importantly, the charge density difference and Bader effective charge56 are combined to determine the pivotal roles of the two active sites (Figure 2c−f). Deriving from the charge distribution ranges and the carried charge of PyN atoms in Gr0Py3 (Figure 2c) and Gr1Py3 (Figure 2d), it is again demonstrated that the charge of PyN is not quantitatively changed by the introduction of GrN. Therefore, the PyN atom undoubtedly manifests the strongest Lewis basicity. The extra unpaired electrons in PyN could be preferentially polarized with the approach of the O2 molecules.57 It could be assumed that the PyN atom is the electron donor. Notably, surfacecharge redistribution around GrN is observed in Gr1Py3 (Figure 2f) compared to that in Gr0Py3 (Figure 2e). Considering the electronegativity difference between C and N atoms, the electrons of the adjacent C atom are overwhelmingly guided to GrN, which costs the C atom 0.43e of electrons to manifest the strongest Lewis acidity. Thus, the electron acceptor is conceivably formed at the C site. Herein, we first propose the perspective that the adsorption and activation of O2 molecules on the NC surface could be ascribed to the electron donor−acceptor mechanism. Specifically, upon injection of O2, PyN preferably donates its electrons to O2 molecule for its adsorption at one end. During the activation process, the C atom adjacent to GrN receives electrons from the O2 to accommodate it at the other end, which holds a Lewis equilibrium of acid and base at the local active center. After that, the O2 molecule continuously obtains electrons from the GP planes to convert to •O2− and then dissociates into PyN−O and epoxy species. To verify the function of the donor−acceptor mechanism, a series of calculations on the O2 activation reaction (OAR) are carried out via the NEB method (Figure 3a). It is suggested that

Table 1. Summary of the Calculated Results for O2 Adsorption on Different Surfaces configuration

dO−Oa (Å)

dO−Cb (Å)

dO−Nb (Å)

ΔEads (eV)

Δqc (e)

ELFd

Gr0Py3 Gr1Py3 Gr2Py3 Gr3Py3

1.28 1.30 1.29 1.29

2.00 1.98 1.96 1.99

1.98 2.01 2.07 2.01

−0.33 −0.41 −0.41 −0.40

−0.19 −0.26 −0.26 −0.26

0.45 0.43 0.42 0.43

a

Bond length of the O2 molecule. bDistance between O and the closest C/N atom on the surface. cBader effective charge for the O2 molecule. dIntensity of the covalent interaction between the two O atoms, which increases from 0.00 to 1.00.

between GrN and PyN. Because of the boosted electron transfer during the preactivation of O2 over Gr1Py3, the O2 molecule requires less energy to climb over the barrier to dissociate on Gr1Py3. However, the O−O bond requires to be completely broken to reach the TS over Gr0Py3, which triggers a higher energy barrier (0.68 eV) than that of Gr1Py3 (0.5 eV). This discrepancy is primarily ascribed to the particular electron delivery channel in the steered direction of PyN → O1 → O2 → C → GrN in Gr1Py3, which leads to more electron transfer between O2 and the Gr1Py3 planes in a one-way transmission manner, exceeding that in Gr0Py3 and pristine GP. A local Lewis equilibrium of acid and base is generated at the reaction center of Gr1Py3, which confirms our conjecture. Subsequently, as the covalent interaction in O2 becomes weaker, the two O atoms separately receive electrons from the GP planes to convert to •O2− and then dissociate into PyN−O and epoxy species. It is illustrated that the enhanced efficiency of OAR on carbocatalysts is attributed to the contribution of the electron donor−acceptor mechanism, which is due to GrN and PyN functioning synergetically. This result provides a unified mechanism for O2 activation on NC in general catalytic applications. Oxidation Reactions via O-Containing Species. Because of the clarified oxidation mechanism of SO2 and the 19865


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Figure 4. Reaction pathways and relative reaction rates for the first SOR (SOR1; a, b) and second SOR (SOR2; c, d).

broad applications of carbocatalysts in desulfuration,32 the SO2 oxidation reaction (SOR) is considered to be the model reaction to evaluate the catalytic activity of the aforementioned O-containing species. Both the dissociated O atoms need to be consumed for the recovery of catalysis in a complete catalytic loop. SO2 oxidation via the PyN−O species (SOR1) is carried out first (Figure 4a; refer to Figures S5 and S6 in the Supporting Information for detailed MEP in relative SORs). It is observed that SOR1 on the four complexes manifests little difference, as expected, due to the almost same energy barriers (about 0.50 eV) and heat release (about −1.00 eV). It is again affirmed that the introduction of GrN barely influences PyN, which still acts as the primary active site. Also, there is no obvious increase of the reaction rates from Gr0Py3 to Gr1Py3 (Figure 4b). SO2 oxidation by epoxy groups (SOR2) is subsequently performed (Figure 4c). Interestingly, the energy barriers of Gr1−3Py3 show a sharp decrease from 0.47 eV (Gr0Py3) to ca. 0.10 eV, which suggests that GrN not only effectively facilitates the formation of epoxy groups but also shows a remarkably enhanced catalytic activity for the oxidation reaction. Moreover, the reaction rate of SOR2 is dramatically increased by more

than 40 times via GrN induction. It can be concluded that the reaction rates of the catalytic oxidation reaction are apparently increased in the order OAR < SOR1 < SOR2, which suggests that the adsorption and activation of O2 molecules is the ratedetermining step of the catalysis reactions. The effect of GrN on the O-containing groups could be well addressed by employing PDOS and the corresponding partial decomposed charge density (PDCD; Figure 5). For PyN−O in Gr1Py3, an inconspicuous upshift and downshift are demonstrated in the α and β peak range, respectively, compared to those in Gr0Py3. Similar patterns are also acquired in the zdirection-decomposed PDOS, which implies that the introduction of GrN causes little difference to the electronic structures of the PyN−O groups in Gr0Py3 and Gr1Py3. Moreover, the PDCD in the β-peak range of the corresponding PyN−O groups (Figure 5c,d) confirms the results of PDOS because the carried charge and charge orientation are not obviously altered. On the contrary, the activity of epoxy is evidently boosted via GrN doping. It can be observed in Figure 5b that the α and β peaks of epoxy in Gr1Py3 are closer to the VBM, especially in the z direction, which illustrates that the epoxy in Gr1Py3 is more likely to be polarized by SO2 molecules compared to that 19866


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Figure 5. PDOS of O atoms in PyN−O (a) and epoxy groups (b) of Gr0Py3 and Gr1Py3. PDCD of O atoms in PyN−O and epoxy groups of Gr0Py3 (c) and Gr1Py3 (d) in the β diffraction region. The Fermi level is set to 0 eV. The isosurfaces are all set to 0.001 eV Å−3, and the color coding of atoms is the same as that in Figure 3.

the targeted design of abundant carbon-based materials for extensive application of heterogeneous catalytic oxidation, O2 reduction, and hydrogenation reactions.

in Gr0Py3. In addition, the PDCD results confirm that the activity of epoxy in Gr1Py3 is enhanced, because of more carried charge in the β peak range than that of Gr0Py3. It could be concluded that the activity of the PyN−O and epoxy groups is conclusively determined by the PyN and GrN atoms, correspondingly.

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ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04026. Initial calculation model, doping site tests of GrN, band structures and TDOS, ELF for O2 molecules in the IS and TS of the OAR, detailed MEP of O2 activation, the first and second SORs (PDF)

CONCLUSIONS Using GrN and PyN synergetic doping, we developed a novel modification strategy to accelerate the catalytic efficiency of carbocatalysts and opened the door to a more detailed understanding of the promotion mechanism. It is first proposed that the enhanced catalytic efficiency of NCs is crucially contributed by the synergy between GrN and PyN atoms. Specifically, PyN donates its electrons from the outer orbit to the absorbed O2 molecule at one end; then, the C atom adjacent to GrN accepts electrons from the absorbed O2 at the other end, generating an electron-delivery channel in the steered direction of PyN → O1 → O2 → C → GrN. This electron donor−acceptor mechanism essentially accelerates the formation of highly chemically reactive O-containing species, thus significantly promoting the catalytic efficiency of carbocatalysts. This work could provide new possibilities for

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.C.). *E-mail: [email protected] (Y.C.). ORCID

Jieyuan Li: 0000-0003-3666-9796 Wanglai Cen: 0000-0002-2854-964X Notes

The authors declare no competing financial interest. 19867


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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51508356), the Science and Technology Support Program of Sichuan Province (2014GZ0213, 2016GZ0045), and the Science and Technology Support Program of Chengdu (2015-HM01-00255-SF). The authors also acknowledge the National Supercomputer Center in Shanghai of China and the Institute of New Energy and Low Carbon Technology of Sichuan University for computational support.

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