Reactive Molecular Dynamics Study of Effects of Small-Molecule

Oct 26, 2018 - Reactive molecular dynamics was used to investigate the atomic-level mechanism of formic-acid-accelerated deterioration of meta-aramid ...
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Reactive Molecular Dynamics Study of Effects of Small-Molecule Organic Acids on PMIA Thermal Decomposition Fei Yin, Chao Tang,* Yujing Tang, Yingang Gui, and Zhongyong Zhao

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College of Engineering and Technology, Southwest University, Chongqing 400715, China ABSTRACT: Reactive molecular dynamics was used to investigate the atomic-level mechanism of formic acidaccelerated deterioration of meta-aramid (PMIA) fibers. The simulation results showed that formic acid promoted PMIA decomposition. The activation energy of a composite system (PF) consisting of formic acid and PMIA was 106.94 kJ/mol at 2000−3000 K, which is 11.95% lower than that of pure PMIA. The main small-molecule products of the PF system were H/C/O-containing molecules (H2O, CO, and CO2), hydrocarbon molecules (e.g., CH4, •C2H, C2H4, and C3H4), N-containing molecules (N2, NH3, and HCN), H2, and various free radicals. Formic acid can promote the production of small molecules such as CO, CO2, and H2O. The N−H bonds, C−N bonds and the amide CO double bond of PMIA were vulnerable to CO, H ions, and free radicals produced by formic acid decomposition, and this decreased the PMIA stability. Temperature is an important factor in the thermal decomposition of PMIA and can accelerate reactions in the PF system. The initial reaction rate of PMIA at 3000 K was 8.1 times that at 2000 K, and the intermediate reaction rate was 6.2 times that at 2200 K; temperature also affects the types of pyrolysis products, for example, hydrocarbons are high-temperature products.

1. INTRODUCTION In the long-term operation of a transformer, insulating paper gradually ages because of a combination of external and internal causes, resulting in degradation of its electrical and mechanical performances. The aging of transformer insulating paper is an irreversible process, and the insulating paper performance directly affects the operation of the transformer.1 Oil−paper insulating materials produce a large amount of organic acids during aging. These are mainly small-molecule organic acids, and it is easy for these to be absorbed by the insulating paper, resulting in further aging of the paper. The amount of acids produced increases continuously with increasing operating time of the transformer, and acid and water have a synergistic effect on the aging of insulating paper. This leads to a rapid decrease in the polymerization degree of the insulating material and shortens the safe use time of the transformer.2−5 The aging mechanism of oil−paper insulating materials has been widely studied experimentally.6,7 The aging behaviors of transformer oil−paper insulating materials modified with various amines have been studied.1 The effects of small-molecule organic acids on the aging of oil−paper insulating materials have also been studied.3,8 The results show that when the formic acid content reached 0.1 × 10−3 mg· KOH/g, it had a catalytic effect on the aging of oil−paper insulating materials. Wang et al.9 studied the decomposition mechanism of formic acid on cellulose insulating paper. The study showed that the unsaturation degree of the produced small-molecule hydrocarbons increased with increasing simulation temperature, and formic acid contributed to the © XXXX American Chemical Society

formation of H2. It is clear that the effects of acids on transformer insulating materials cannot be ignored, and the mechanisms of the effects of acids on the deterioration of transformer oil−paper insulating materials need to be studied at the microscopic level. The atomic-level reactive molecular dynamics method can be used to calculate various properties of materials precisely and to simulate the breakage and formation of chemical bonds. The main reactive force fields currently used include optimized potentials for liquid simulations, 10 empirical valence bonds,11,12 RWFF,13 and ReaxFF.14 Among these, ReaxFF is a typical reactive force field. Unlike quantum chemical methods, the ReaxFF method does not need a preset reaction path. It can simulate larger-scale chemical reactions, and the computational cost is lower.15 The ReaxFF force field has been successfully applied to studies of energetic materials,16−18 combustion,19,20 new batteries,21,22 nanomaterials,23,24 high polymers15,25,26 and other areas since it was developed in 2001. For example, ReaxFF has been widely used to study the detonation and chemical reactions of energetic materials in different environments.17,27,28 The initial pyrolysis of Illinois No. 6 coal at high temperatures was studied by reactive molecular dynamics.29,30 The effects of supercritical water on coal pyrolysis and hydrogen generation were investigated by using a combination Received: September 25, 2018 Revised: October 26, 2018 Published: October 26, 2018 A

DOI: 10.1021/acs.jpcb.8b09343 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B of ReaxFF and density functional theory (DFT) methods.31 Ding et al.32 used the ReaxFF to study the mechanism of nheptane pyrolysis, systematically analyzed the intermediate reactions involved in n-heptane pyrolysis at the atomic level, and calculated and analyzed the dynamic behaviors of the main intermediate products/generated products. Zhang et al.33 used the ReaxFF to study the mechanisms of the thermal decompositions of various types of epoxy resins and the formation law of small-molecule gases at 1300 K. Compared with traditional classical force fields, a reactive force field has the advantages of fitting the breakage and formation of chemical bonds. On the basis of this, the stress−strain behavior of para-aramid (PPTA) fiber crystals under reactive force fields has been studied.34,35 The simulation results show that the C− N bond is the weakest chemical bond in the main chain of PPTA. Chenoweth et al.36 simulated and studied the thermal decomposition mechanisms of pure polyethylene and polyethylene containing various additives at 2500 K. It can be seen that the molecular dynamics method based on the ReaxFF has been widely used to study the microscopic mechanisms of chemical reactions. Meta-aramid (PMIA) is a typical aramid fiber. It has excellent properties such as good mechanical properties and heat resistance, and its combustion is difficult. It is therefore widely used in transformer insulating materials.37 Current research on PMIA is mainly focused on its modification and physical properties,38,39 and studies of the effects of acid on the mechanism of microdeterioration during thermal aging have rarely been reported. In this study, the ReaxFF was used to study the mechanism of thermal aging of PMIA in a formic acid environment at the atomic level, the thermal aging products of compound models were calculated and analyzed, and possible reaction paths for formic acid acceleration of the decomposition of PMIA were identified.

Table 1. Comparison of the Equilibrium Structures of Prominent Fragments Observed during the Molecular Dynamics Simulationa

a RM and θM are the bond length (Å) and angle (degree) using ReaxFF, respectively. RQ and θQ represent the bond length (Å) and angle (degree) using DFT, respectively. bThe bond length and bond angle of the molecule are the results of the ref 40.

2. METHODS 2.1. Quantum Mechanical Calculations and Molecular Dynamics Simulation. To evaluate the accuracy of ReaxFF force field, the simulation error of selected force field is calculated by using quantum mechanics (QM). QM calculations are performed using the B3LYP40 DFT algorithm, and all calculations are made by Materials Studio software. On the one hand, equilibrium structures [bond length (R), bond angle (θ)] of potentially important substances (small molecules and free radicals) generated by the thermal decomposition of the composite system were analyzed and are showed in Table 1, and also were compared with that showed in ref 40. On the other hand, the change of potential energy in the chemical reaction of ReaxFF force field was tested and compared with the calculation results of the QM method, which were presented in Figures 1 and 2. From Table 1, it can be seen that the calculated results adopted by DFT are basically consistent with the simulation results of ReaxFF force field and accord with the conclusions showed in ref 40. For example, the bond length errors of the C−H bond of free-radical •CH, •CH2, and •CH3 are 0.015, 0.032, and 0.029, respectively. The bond angle errors of •CH2 and •CH3 are 6.743 and 0.756, respectively. By analyzing the change of molecular potential energy with the breaking of bonds in Figures 1 and 2, it shows that the system potential energy can be fitted correctly by using ReaxFF force field, which proved the accuracy of simulation results of reactive molecular dynamics.

Figure 1. Comparison of potential energy curves for formic acid using DFT and ReaxFF.

Figure 2. Comparison of potential energy curves for PMIA using DFT and ReaxFF.

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DOI: 10.1021/acs.jpcb.8b09343 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Figure 3. Configuration of PF model (a) before and (b) after optimization.

2.2. Model Construction. Transformer oil−paper insulating materials produce a large amount of organic acids during service, among which formic acid has the greatest impact on the degradation of insulating paper. Small-molecule organic acids such as formic acid and acetic acid have similar properties.8 In this study, the effects of formic acid, which is a typical small-molecule organic acid, on the thermal decomposition of PMIA were studied. First, the PMIA monomer structure was constructed with Materials Studio software, and then, a compound model (PF) with 30 PMIA and 10 formic acid molecules was constructed with the Amorphous Cell module. The initial density of the model was set at 1.0 g/cm3, the total number of atoms was 950, and the unit cell size was 23.4 × 23.4 × 23.4 Å3. The initial model was constructed and then the energy of the model was minimized in 5000 steps. Relaxation for 10 ps was then performed at 100 K with the NPT ensemble (with a certain particle number N, pressure P, and temperature T), to obtain the final density of the PF equilibrium model, namely, 1.27 g/ cm3. Figure 3 shows the system configuration before and after relaxation optimization. The reactive molecular dynamics calculation of the equilibrium PF model at a target temperature of 2000−3000 K and an interval of 200 K was performed with the NVT ensemble (with a certain particle number N, volume V, and temperature T). The simulation step was set at 0.25 fs. Periodic boundary conditions were used for all calculations, the force field was ReaxFF, and the Verlet velocity method was used for integration.

Figure 4. Changes in number of PMIA molecules with time at various temperatures.

where N0 is the initial number of PMIA molecules, t is the initial PMIA decomposition time, and Nt is the number of PMIA at time t. The fitting results for the initial reaction rate k at various temperatures are shown in Table 2. Table 2. First-Order Reaction Rate Parameters T (K) 103 1/T k (ps−1) ln k

2200 0.45 0.111 25.44

2400 0.42 0.162 25.81

2600 0.38 0.297 26.42

2800 0.36 0.431 26.79

3000 0.33 0.588 27.10

The data in Table 2 show that increasing the temperature accelerated the initial reaction. For example, the PMIA reaction rate at 3000 K was 6.2 times that at 2000 K. The activation energy and pre-exponential factor for the reaction can be obtained by fitting the k values at various temperatures to the Arrhenius equation

3. RESULTS AND DISCUSSION 3.1. Pyrolysis Kinetics Analysis. Figure 4 shows that the number of PMIA molecules at different temperatures varied with time. From Figure 4, it can be concluded that the time required for thorough thermal decomposition of PMIA decreased continuously with the increasing temperature. The total time required for thermal decomposition of PMIA at 2000, 2200, 2400, 2600, 2800, and 3000 K were 66.2, 48.5, 18.5, 17.5, 8.0, and 7.5 ps, respectively. The effects of formic acid on the thermal decomposition of PMIA at different temperatures were explained by fitting the first-order decay expression to the molecular dynamic model;41,42 the initial reaction rates (k) of PMIA at various temperatures were obtained. ln N0 − ln Nt = kt

2000 0.50 0.073 25.01

Ea (2) RT where A is the pre-exponential factor, Ea is the activation energy, T is the temperature, and R is the standard molar gas constant. The reaction rate and chemical kinetic parameters for the initial thermal decomposition of the PMIA molecule in a formic acid environment can be obtained from eqs 1 and 2. The results are shown in Figure 5. The intercept and slope of the fitting line in the Arrhenius curve were 31.34 and −12 862.48, respectively, and the preexponential factor and activation energy of the PF composite ln k = ln A −

(1) C

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where U∞ is the asymptotic value of the potential energy, ΔU is the heat released by the reaction, and tmax is the time needed for the potential energy to reach the maximum value. In Figure 6, there is no downward trend in the potential energy change at 2000 K. This is because at this temperature the main chemical reaction was the initial decomposition reaction; there was little intermediate decomposition reaction, and the PF compound model did not show an exothermic state. In this study, only potential energy change curves in the range 2200−3000 K were fitted. Fitting showed that the intermediate reaction rate increased with increasing temperature. For example, the PMIA reaction rate at 3000 K was 6.2 times that at 2200 K and 1.6 times that at 2400 K, indicating that increasing the temperature increased the reaction rate in the intermediate stage. Kinetic analysis of PF pyrolysis shows that formic acid can promote the thermal decomposition of PMIA molecules. In the presence of formic acid, the activation energy for PMIA decomposition is 11.95% lower than that for pure PMIA. An increase in temperature not only accelerates the initial stage of the thermal decomposition of PMIA but also accelerates the intermediate reaction stage. For example, the initial reaction rate for PMIA at 3000 K is 8.1 times that at 2000 K, and the intermediate reaction rate is 6.2 times that at 2200 K, indicating that temperature is an important factor in the thermal decomposition of the compound system. 3.2. Analysis of Main Thermal Decomposition Products. The mechanism of the intermediate decomposition stage was studied by investigating the changes in the intermediates produced with time at various temperatures. The results are shown in Figure 7. The main intermediates produced at different temperatures were C6H6N, C6H7N, C6H7N2, C6H8N2, and C8H5O2. This indicates that initially the C−N amide bonds of PMIA were broken. Figure 7 shows that the number of intermediate products formed increased with increasing temperature in the initial stage of the reaction. After reaching a maximum value, the intermediate products further reacted to form other substances. The time needed for intermediate product formation to reach the middle value and the time for complete conversion of the intermediate products decreased continuously with increasing temperature. Figure 7 also shows that the formation of the intermediate product C6H8N2 started later than the formation of C6H7N2. In the initial stage, the amount of C6H8N2 increased with decreasing amount of C6H7N2, which indicates that there was a correlation between them. Monitoring the trajectories of C6H7N2 and C6H8N2 fragments showed that the C−N bond of PMIA was broken to form C6H7N2. The N in the imino group on C6H7N2 has a lone pair of electrons; therefore, it can easily capture an H atom in the system to form C6H8N2. Figure 8 shows the changes with time in the number of fragments produced by the decomposition of PMIA molecules in the PF compound model at various temperatures. The Figure 8 shows that the overall trend in the number of fragments produced increased with increasing temperature, and the faster the system produced new substances, the more the types of new substances were produced. This indicates that temperature is an important factor in the thermal decomposition reaction. Monitoring the trend in the product quantity changes with time showed that fragment production could be divided into three stages, in terms of numbers produced: an initial stage, intermediate stage, and a final stage. In the initial

Figure 5. Arrhenius equation fitting curve for compound model PMIA.

system were 4.08 × 1013 s−1 and 106.94 kJ/mol, respectively. This activation energy is 11.95% lower than that for pure PMIA under the same reaction conditions.41 This implies that formic acid promotes the thermal decomposition of PMIA. The potential energy changes of the compound model at different temperatures were calculated; the results are shown in Figure 6. The figure shows that the trend in the potential

Figure 6. Trends in potential energy at various temperatures.

energy change at different temperatures can be divided into three stages. First, it increased rapidly, and then decreased gradually after reaching a maximum value, and finally reached a constant value, that is, the chemical reaction reached dynamic equilibrium. The increase in potential energy in the initial stage of the chemical reaction indicates an endothermic reaction. When the maximum value was reached, the total potential energy decreased. This is the intermediate stage of the chemical reaction. It is an exothermic reaction because of chemical bond formation, which releases a large amount of energy. It is also accompanied by a number of small molecules during this process. Figure 6 also shows that the potential energy increased with increasing reaction temperature, that is, an endothermic process, and the time to reach dynamic equilibrium of the chemical reaction was shorter. The reaction rate (k1) in the intermediate reaction stage was determined from eq 3.16,43 The results are shown in Table 3. U (t ) = U∞ + ΔU × exp( −k1 × (t − tmax ))

(3)

Table 3. Intermediate Reaction Rate Parameters T (K) tmax (ps) k1 (ps−1)

2000

2200 52.0 0.005

2400 22.2 0.019

2600 16.3 0.023

2800 12.7 0.027

3000 8.7 0.031 D

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Figure 7. Changes in number of intermediates in PF system: (a) 2000; (b) 2200; (c) 2400; (d) 2600; (e) 2800; and (f) 3000 K.

The main small-molecule products formed in the PF compound system at various temperatures were calculated and the statistical results are shown in Figure 9. The figure shows that H2, H/C/O-containing molecules (H2O, CO, CO2), hydrocarbon molecules (e.g., CH4, •C2H, C2H4, and C3H4), N-containing molecules (N2, NH3, and HCN), and free radicals were the main fragments produced in the thermal decomposition of the PF system. As the temperature increased, the time needed for the emergence of various products decreased, and the products were more complex. For example, after reaction for 100 ps at temperatures lower than 2400 K, hydrocarbons were not detected, but a large number of hydrocarbon fragments such as CH4, •C2H, C2H2, and C3H4 appeared at 3000 K. Hydrocarbons were produced during thermal decomposition at high temperatures; a similar result in the ref 44 has been reported for the decomposition of PMIA. The complexity of the produced hydrocarbons increased with increasing temperature. Figure 9 shows that small-molecule hydrocarbon formation preceded formation of macromolecular hydrocarbons. For example, only CH4 and C2H2 appeared at 2600 K, C3H4 appeared at 2800 K, and C4H2 appeared at 3000 K. In terms of product quantity, the yields of low-molecularweight hydrocarbons were higher than those of highmolecular-weight hydrocarbons (CH4 > C2H2 > C3H4 > C4H2). The amount of water molecules produced by thermal decomposition of the PF system, that is, in the presence of formic acid, was significantly higher than in the case of pure PMIA, that is, without acid.41 Research has shown that moisture affects the mechanical and electrical performances of oil−paper insulating materials and can also accelerate their aging.45,46 The results of this study show that the amount of H2O molecules produced increased continuously with increasing temperature. Studies of phenolic resins and cellulose showed that the number of water molecules produced increased rapidly with increasing temperature,45,47 which is consistent with the results of this study. In practical

Figure 8. Changes in decomposition products of PF system with time at various temperatures.

stage, the number of fragments increased rapidly. The main activity was breakage of the amide C−N bond in PMIA. In the intermediate stage, the number of fragments fluctuated after an initial rapid increase. This is because of the presence of a large number of benzene rings after the breakage of the C−N bonds of PMIA. It is not easy to break the bonds in the stable benzene ring structure; therefore, the main activities in this stage were transformation and recombination of substances. In the final stage, thermal decomposition continued, benzene rings were opened, and some fragments underwent secondary or multiple decompositions to produce stable products and small-molecule products. In this stage, the amount of products again increased. Figure 8 shows that the higher the temperature is, the more obvious the trend is. At low temperature (2000 K), the amount of thermal decomposition products increased rapidly to a maximum value (about 58 molecules), and the number of molecules basically reached a dynamic equilibrium state. At higher temperatures (≥2600 K), the changes in the product quantity clearly reflected the above three stages. E

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Figure 9. Statistics for small molecules in PF model at various temperatures: (a) 2000; (b) 2200; (c) 2400; (d) 2600; (e) 2800; and (f) 3000 K.

applications of PMIA, the effect of moisture on physical and chemical properties of PMIA should be paid great attention. 3.3. Reaction Mechanism and Product Formation Path for Formic Acid. Tracing the atomic trajectory of the system enables the product formation path to be identified. This is helpful in clarifying the chemical bond breakage and formation processes, which cannot be observed experimentally. The presence of formic acid affects the thermal decomposition of PMIA; therefore, its decomposition path in the presence of formic acid is not entirely the same as that for pure PMIA. Figure 10 shows the thermal decomposition of formic acid at various temperatures. The figure shows that with increasing

free radicals; and (3) the H on the carbon atoms or the H atom of the hydroxyl group of formic acid shifted. Taking the reaction at 3000 K as an example, the initial reaction mechanism for formic acid and PMIA (Figure 11, path 1) involves formation of NH3, CO, and other molecules by the reaction of formic acid molecules with the amino group at the end of PMIA. In Figure 11, path 1 shows that the amino group at one end of PMIA breaks away from the main chain after heating for 1.75 ps and binds to a formic acid molecule at 3.75 ps to form the intermediate HCOOH−NH2. After a short period of inactivity, the H on formic acid is captured by the imino group to form NH3 and a •COOH free radical at 3.90 ps, and this free radical decomposes to CO and an •OH free radical. In Figure 11, path 2 shows the reaction of formic acid with PMIA to produce H2O and other molecules. At 3.5 ps, the C−O bond is broken because heating of formic acid molecules produces •OH and CHO• free radicals. The •OH free radical then approaches the amide bond H atom. At 4.75 ps, the OH free radical begins to attack the H atom because • OH free radicals are strongly oxidizing. After a short period of inactivity, H2O molecules break off from the main chain before 5.1 ps. In Figure 11, path 3 shows reduction of the formic acid carbonyl to form a hydroxyl and acceleration of the thermal decomposition of PMIA. At 2.5 ps, the H on the formic acid hydroxyl breaks away and reduces the amide CO to a hydroxyl. Next, at 2.75 ps, the hydroxyl captures the H on the adjacent N−H and intramolecular dehydration occurs. At 3.25 ps, the C−N bond of PMIA breaks and reaction with the HCOO• free radical forms CO2, aniline (C6H7N), and 3cyanobenzaldehyde (C8H5NO). The paths for generation of H2O molecules can be summarized as follows. The main initial paths are as follows: (1) heating of formic acid breaks the C−O bond and hydroxyl groups are formed, which capture adjacent H atoms or combine with free H in the system to form H2O molecules and (2) capture by PMIA carbonyls of free H atoms or intramolecular hydrogen to form hydroxyl groups, which

Figure 10. Changes in number of formic acid molecules with time at various temperatures.

temperature, the decomposition rate of formic acid increased continuously. At 2000 K, there were two formic acid molecules left after 100 ps; at 3000 K, all the formic acid molecules decomposed completely in 33.1 ps. Tracing the trajectories of formic acid molecules in the PF system showed that the main initial bond breakage positions in formic acid were as follows. (1) Heating of formic acid caused the H of the hydroxyl group to break away to form free radicals, that is, •COOH and •H; (2) formic acid hydroxyls broke away to form •OH and CHO• F

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between formic acid and PMIA shows that the main positions for attack of PMIA by formic acid decomposition fragments are N−H bonds, amide C−N bonds, and CO bonds.

4. CONCLUSIONS In this study, the thermal aging mechanism of PMIA in a formic acid environment was simulated by the reactive molecular dynamics method. It is important to understand the deterioration of PMIA materials caused by small-molecule organic acids. The pyrolysis kinetics of a PF compound system was fitted and calculated. The results show that formic acid can promote the thermal decomposition of PMIA. The activation energy of the PF system PMIA at 2000−3000 K was 106.94 kJ/mol, which is 11.95% lower than that of pure PMIA. Formic acid easily decomposes and produces large amounts of CO, H ions, and free radicals. These species are highly active and accelerate the thermal decomposition of PMIA. Tracing the path of the reaction of formic acid with PMIA showed that the main positions at which the formic acid decomposition fragments attacked PMIA were the N−H bond, amide C−N bond, and CO bond. The final products of thermal decomposition of the PF system were calculated and analyzed. The results show that the main small-molecule products were H/C/O-containing molecules (H2O, CO, and CO2), H2, hydrocarbon molecules (e.g., CH4, •C2H, C2H4, and C3H4), N-containing molecules (N2, NH3, and HCN), and various free radicals. Formic acid can promote the formation of small molecules, that is, H2O, CO, and CO2. In practical applications, these molecules can be identified by various techniques and can be used for diagnosing thermal faults in environments with small-molecule organic acids. Temperature is an important factor in the thermal decomposition of PMIA. The initial reaction rate and the intermediate reaction rate increase with the increasing temperature. For example, the initial reaction rate for PMIA at 3000 K is 8.1 times that at 2000 K, and the intermediate reaction rate is 6.2 times that at 2200 K. The higher the temperature, the more severe the reaction of the compound model is and the more complex the types of fragments produced by the thermal decomposition of PMIA is. Hydrocarbons are produced at high temperatures.

Figure 11. Main paths in initial stage of chemical reaction of formic acid and PMIA (a) reaction path 1; (b) reaction path 2; and (c) reaction path 3.

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then break away from the main chain to form OH free radicals, which then capture amino H atoms or free H atoms to form H2O. The main source of NH3 molecules is neighboring H atom capture by PMIA amino groups that break away from the main chain or combining of •NH with H in the system to form • NH2, followed by capture of neighboring H atoms to form NH3 molecules. A large amount of CO is generated under the action of formic acid. Formic acid dehydrogenation generates CO, and the decomposes of C8H5O2 which come from the breakage of PMIA C−N bonds also generates a large amount of CO. Because CO has strong reducibility, it reacts easily with PMIA. In summary, formic acid produces a large number of H ions, free radicals (formic acid group, •OH, and aldehyde), CO, and other fragments during thermal degradation. Because of the high activities of these fragments, they easily react with PMIA, and this decreases the stability of PMIA and promotes its thermal decomposition. Tracing the paths of the reaction

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-023-68251265. ORCID

Fei Yin: 0000-0002-2207-0582 Yingang Gui: 0000-0003-1424-7082 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the National Key R&D Program of China (grant nos. 2017YFB0902700, 2017YBF0902702) for the financial support. G

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DOI: 10.1021/acs.jpcb.8b09343 J. Phys. Chem. B XXXX, XXX, XXX−XXX