Controlling the secondary pollutant on B-doped g-C3N4 during photocatalytic NO removal: a combined DRIFTS and DFT investigation

Jiarui Liab, Maoxi Ranab, Peng Chenab, Wen Cuibc, Jieyuan Libd, Yanjuan Sun*ab, Guangming Jiangab, Ying Zhoubc and Fan Dong*ab
aEngineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education, Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, China. E-mail: dfctbu@126.com
bResearch Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 611731, China
cThe Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China
dCollege of Architecture and Environment, Sichuan University, Chengdu, Sichuan 610065, China

Received 26th May 2019 , Accepted 17th July 2019

First published on 17th July 2019


The accumulation of a toxic intermediate (NO2) during photocatalytic NO oxidation would lead to secondary pollution and largely limit the application of photocatalytic technology in air purification. Accordingly, the problems can be well tackled by employing a novel co-pyrolysis method to construct B-doped g-C3N4 (CNB), which could efficiently enhance the charge separation and dramatically inhibit the generation of the secondary pollutant (NO2) on CNB in contrast to the pristine CN. The effects of B-doping on the local electronic structure, reactant activation, and optical properties of CN were revealed. O2 activation was promoted on CNB owing to the localized electrons. With the combination of in situ DRIFTS verification and DFT simulation, it was revealed that the activation barrier of NO oxidation on CNB was significantly reduced, which facilitated the conversion of NO into the final product (NO3). Subsequently, the mechanism for the inhibition of toxic NO2 was unraveled and the conversion pathways of photocatalytic NO oxidation on B-doped g-C3N4 have been proposed. The present work demonstrated a new strategy to optimize the photocatalytic performance for safer and efficient air purification.


1. Introduction

Atmospheric pollution (NOx, VOCs, and PM2.5) is a major challenge faced by developing countries.1,2 Among the various pollutants, the nitrogen oxide (NO) emitted from the combustion of fossil fuels acts as the key factor to form smog and haze.3–5 Thus, it is essential to develop a feasible technology to purify NO in an efficient and safe way. Photocatalysis technology has emerged as a new method utilizing solar energy to remove NO in air.6–10 It is environmentally friendly and efficient for the removal of NO at low concentrations in comparison with selective catalytic reduction and biological methods. For the photocatalytic purification of NO, the toxic intermediate NO2 is progressively generated during the photocatalytic NO oxidation process in some cases, which could lead to secondary pollution and limit the photocatalytic performance.11

Metal-free graphitic carbon nitride (labelled CN) is an attractive photocatalyst because of its appropriate band gap (2.7 eV), adjustable electronic structure, high chemical stability and facile synthesis.12–15 Over the past few years, researchers have witnessed a booming trend in the exploration of CN-based photocatalysts for environmental remediation16–18 and solar energy conversion.19–23

However, the pristine CN exhibits a relatively high charge carrier recombination rate and insufficient oxidation capacity that could result in limited photocatalytic activity and generation of secondary pollutants during photocatalytic purification. As one of the most significant strategies to boost the photocatalytic activity of CN, nonmetal elemental doping on CN is deemed as an effective method that could narrow the band gap and further enhance its photocatalytic performance.24–27 Although doping elements have been applied in the modification of CN,28–31 the mechanisms of photocatalytic NO oxidation pathways and inhibition of the NO2 intermediate have not been revealed, which need a systematic and urgent investigation.

Herein, we design and construct B element-doped CN (CNB) as a model photocatalyst that not only obviously boosts the activity performance but also efficiently decreases the accumulation percentages of toxic intermediate species (NO2) from 79.6% to 39.3% in photocatalytic NO oxidation. Besides, the conduction band and valence band edges of CNB are upward shifted relative to those of the pristine CN, resulting in photoinduced electrons with stronger reduction ability and facilitating the activation of O2 molecules to generate more reactive oxygen species. Most importantly, combining the methods of in situ DRIFTS and DFT simulations, detailed mechanistic insights into the inhibition of the toxic intermediate (NO2) and the involved conversion pathways of photocatalytic NO oxidation on B-doped g-C3N4 have been proposed. The activation barrier for the NO oxidation process on CNB is reduced which would facilitate the reactant activation and further transform NO into the final product (NO3) with NO2 being suppressed. The present work demonstrates a new strategy to optimize the photocatalytic performance for safe and efficient air cleaning.

2. Experimental

2.1 Catalyst preparation

All chemicals used in this study were of analytical grade and used without further treatment. B-doped g-C3N4 was synthesized by co-pyrolysis of urea and H3BO3. 10.0 g urea and a known amount of H3BO3 (0.08, 0.10, and 0.11 mmol) were added into an alumina crucible (50 mL) with 25 mL distilled water, respectively. The obtained solution was dried in an oven at 60 °C to re-crystallize the solid precursors. Then, the precursors were calcined at 550 °C for 2 h at a heating rate of 15 °C min−1 in air. After cooling down to room temperature, the samples of the pristine g-C3N4 and B-doped g-C3N4 were labeled CN, CNB-0.08, CNB-0.10 and CNB-0.11, respectively.

2.2 Characterization

The as-obtained catalysts were characterized by XRD, XPS, SEM, TEM, UV-vis DRS, PL and EPR measurements. Detailed information on the characterization methods are given in the ESI.

2.3 Visible light photocatalytic NO removal

The photocatalytic activity was investigated by determining the removal ratio of NO at ppb levels (500 ppb) in a continuous-flow reactor (rectangular reactor, 30 × 15 × 10 cm). The concentration of NO was continuously detected using a NOx analyzer (Thermo Environmental Instruments Inc., Model 42c-TL), which can monitor the concentrations of NO, NO2, and NOx (NOx represents NO + NO2). Detailed descriptions of the NO removal experiments are available in the ESI.

2.4 In situ DRIFTS study on the photocatalytic NO oxidation process

The in situ DRIFTS measurements (Scheme S1) were conducted using a Tensor II FT-IR spectrometer (Bruker) equipped with an in situ diffuse reflectance cell (Harrick). The experimental conditions for DRIFTS measurements are provided in the ESI.

2.5 Density functional theory calculations

Density functional theory (DFT) calculations were carried out using the Vienna ab initio simulation package (VASP. 5.4),32,33 applying a generalized gradient correlation functional.34 A plane-wave basis set with a cut-off energy of 450 eV within the framework of the projector-augmented wave method was employed.35 The Gaussian smearing width was set to be 0.2 eV. The Brillouin zone was sampled with a 3 × 3 × 1 Monkhorst–Pack grid. All atoms were converged to 0.01 eV Å−1. Hybrid functionals based on the Heyd–Scuseria–Ernzerhof (HSE06) method36,37 were applied to estimate the exact band structures. The adsorption energy (Eads) is defined as
Eads = Etot − (ECN + Emol)
where Etot, ECN and Emol are the total energy of the adsorption complex, the pure CN and the isolated molecules, respectively.

3. Results and discussion

3.1 Catalyst microstructure and chemical composition

The XRD patterns of the CN and CNB catalysts (Fig. 1a) exhibit two characteristic diffraction peaks at 13.1° and 27.2°, respectively, which are indexed to the typical crystal planes {100} and {002} of the graphite structure.38 For the CNB samples, the intensity of the {100} peaks at 13.1° is decreased obviously after the introduction of boron atoms caused by the lower crystallinity.30 Simultaneously, the {002} peaks of CNB broaden gradually as the B doping content is increased, indicating that the planar layered structure is expanded to some extent after B doping.
image file: c9cy01030k-f1.tif
Fig. 1 Crystal structures of the photocatalyst. XRD patterns of the four as-prepared samples (a); XPS spectra of B1s of CNB-0.10, and the inset is the optimized local structure of B-doped CN (b). Gray, brown and green spheres represent N, C and B atoms, respectively; N 1s (c); C 1s (d).

Subsequently, in order to reveal the doping site of boron atoms and verify their chemical state in the CNB, XPS combined with DFT calculation was carried out. In Fig. 1b, the peak at 191.4 eV can be attributed to the B-doping in the layered structure,31 and the binding energy in this peak is between that of h-BN (190.1 eV) and Kawaguchi's BCN(H) (192.1 eV), where boron is replaced by a C atom and surrounded with three N atoms.31,39,40 In order to proclaim the most possible doping site of the B element in CN, we employed DFT calculation to optimize four possible crystal structures. On the basis of the simulated results, the final doping site of the B element in the heptazine heterocyclic ring is confirmed (insert in Fig. 1b) because of its lowest doping energy (E0 = −0.47281 × 103 eV) compared to that of other sites (Fig. S1). As shown in Fig. 1c, four binding energies can be fitted at 398.0, 399.0, 400.1 and 404.8 eV, indicating the presence of sp2-bonded nitrogen in the N-containing aromatic rings (C–N[double bond, length as m-dash]C),12 the N–B bond, and the tertiary nitrogen groups N–(C)3 and N-oxide.41 In the C 1s spectra (Fig. 1d), the C–N–C bond can be observed at 286.2 eV. In addition, the peaks at 284.6 and 288.2 eV can be ascribed to the sp2 C–C bonds and N–C[double bond, length as m-dash]N bonds, respectively.19

In Fig. 2a–c, the microstructures of CN and CNB were revealed by SEM and TEM. The SEM image of CN (Fig. 2a) shows the typical layered structure with thick nanosheets. Interestingly, curved surfaces with nanoholes can be observed in CNB-0.10 in Fig. 2b, illustrating that B-doping on CN could promote the formation of this unique nanotube structure owing to the borate melting, dehydration and evaporation processes at 550 °C.42 Because of the nanosheets curling up and the formation of many more well-ordered channels in CNB-0.10, the transport of reactant and product species will be further facilitated more efficiently. Subsequently, the stacked and curved hollow nanotubes were further confirmed by the TEM results (Fig. 2c) for CNB-0.10. The N2 adsorption–desorption isotherms of the samples exhibit type IV and H3 hysteresis loops, respectively (Fig, S2), indicating the existence of mesopores formed by the porous and curved nanosheets.43 The pore size distribution curves of the as-prepared photocatalysts display broad intervals from 2 to 100 nm (Fig. S3). In addition, due to its unique morphological structure (curved surface with nanoholes) (Fig. 2b), the CNB-0.10 sample has higher BET surface areas (SBET) and pore volumes than the other samples (CNB-0.08 and CNB-0.11) (Table 1), which will further facilitate the transport of reactant and product species more efficiently, thus improving the photocatalytic NO oxidation performance of CNB-0.10 to some extent, in comparison with that of CNB-0.08 and CNB-0.11. Furthermore, as shown in Fig. 2d–g, EDX elemental mapping of CNB-0.10 demonstrates that the C, N, O and B elements are uniformly dispersed on the catalyst.


image file: c9cy01030k-f2.tif
Fig. 2 Morphologies and structures of the as-prepared samples. SEM images of CN (a) and CNB-0.10 (b); TEM image of CNB-0.10 (c); FESEM-EDX elemental mapping of CNB-0.10 (d–g).
Table 1 The BET surface areas (SBET), pore volumes and NO removal ratios of the samples
Sample SBET, m2 g−1 Pore volume (cm3 g−1) η (%)
CN 46.12 0.16 30.00
CNB-0.08 53.89 0.25 34.70
CNB-0.10 61.86 0.29 41.40
CNB-0.11 47.07 0.21 29.10


3.2 Optical properties and charge separation

The light absorption properties of the as-prepared photocatalysts were analyzed by UV-vis DRS. In Fig. 3a, CNB-0.10 shows the most noticeable red shift effect on its absorption edge among all the samples, which could extend the photo-response range in the visible light region. The results suggest that the appropriate B-doping amount could improve the light-harvesting capacity of CN, and benefit the generation of abundant photo-excited carriers. Meanwhile, the band gaps of CN, CNB-0.08, CNB-0.10 and CNB-0.11 estimated from the intercept of the tangents to the plots vs. photon energy (Fig. 3a insert) are calculated to be 2.71, 2.66, 2.50 and 2.62 eV, respectively. The B-doped CN displays a narrowed band gap. Subsequently, the calculated absorption spectra are presented in Fig. 3b, which are also consistent with those from UV-vis DRS.
image file: c9cy01030k-f3.tif
Fig. 3 The UV-vis spectra (a); the calculated absorption spectra (b); PL spectra of the four as-prepared samples (c); room temperature solid state EPR spectra of CN and CNB-0.10 (d); gray, brown and green spheres represent N, C and B atoms. The carried electrons on CN (e) and CNB (f).

The charge separation properties were investigated by a combined experimental and theoretical method. In Fig. 3c, the peak intensity of CNB-0.10 is much lower than those of CNB-0.08 and CNB-0.11, which implies that the CNB-0.10 charge recombination has been largely inhibited. Fig. 3d shows the EPR signals at a g value of 1.90 for all the samples due to the unpaired electrons on the π-conjugated CN aromatic rings.6 In the dark and under visible light irradiation, the EPR signal intensity of CNB-0.10 is stronger than that of CN significantly, which implies that the B-doping in the graphitic π-conjugated structure could accelerate the electron mobility and promote the charge transfer effectively. Moreover, the local electron distributions of the CN and CNB photocatalysts are calculated by the Bader methods.44 The atoms near the B atom in CNB (Fig. 3f) carry more electrons than those in CN (Fig. 3e), verifying that the introduction of boron atoms enables electron localization. The localized electrons could have increased probability of participating in the activation of reactants and intermediates.

3.3 Photocatalytic NO removal and reactive species determination

The photocatalytic performance of the CN and CNB samples has been evaluated for NO removal under visible light irradiation (λ > 420 nm). In Fig. 4a, the maximum NO removal ratios reached are in the order CNB-0.10 (41.4%) > CNB-0.08 (34.7%) > CN (30.0%) > CNB-0.11 (29.1%), respectively, which exhibits that boron doping would evidently promote the oxidation and conversion of NO. Meanwhile, the secondary pollutant (NO2) production rates of the samples are also recorded in Fig. 4a (insert). Among all the catalysts, CNB-0.10 displays the most efficient capacity for suppressing the production of NO2, which avoids the covering of the active sites on the surface of CNB-0.10 by increased NO2 pollutants. Thus, the above results confirm that B-doping not only could effectively improve the transformation of NO but also highly control the generation of the secondary pollutant (NO2).
image file: c9cy01030k-f4.tif
Fig. 4 Photocatalytic activities for NO removal and the inset is the NO2 production rate of the as-prepared samples (a); DMPO ESR spectra in the dark and under visible-light (λ ≥ 420 nm) for 15 min in methanol dispersion for ˙O2 (b) and for ˙OH (f), respectively; calculated differences in charge density distributions for O2 activation on CN (c) and CNB (d). Charge accumulation is in blue and depletion is in yellow with the isosurfaces set to 0.005 eV Å−3; density of states (DOS), the Fermi level is set to 0 eV (e).

The superoxide radicals (˙O2) and hydroxyl radicals (˙OH) are the key factors for the NO oxidation process, which can be proven by the DMPO spin-trapping experiments (ESR), subsequently. As shown in Fig. 4b, the stronger ˙O2 signals of CNB-0.10 are more notable than those of CN during the visible light irradiation, implying that O2 molecules on the surface of CNB-0.10 could be activated to generate the ˙O2 species more efficiently. In order to intuitively demonstrate this phenomenon, the production of ˙O2 radicals is simulated with the DFT method by calculating O2 adsorption on the catalyst surface. Compared to the CN sample (Fig. 4c), the O–O bond length has been enlarged from 1.24 (CN) to 1.25 Å (CNB), and the adsorption energy is increased from −0.34 (CN) to −0.42 eV (CNB) (Fig. 4d), respectively. This fact certifies that the O2 could gain abundant localized electrons and further facilitate the production of ˙O2 radicals on the CNB surface.

Simultaneously, the density of states (DOS) has been calculated by the DFT method to explore the band energy (Fig. 4e). After B-doping in CN, the CNB band gap (1.15 eV) is narrowed relative to that of CN (1.30 eV), which shows that the light-harvesting capacity of the catalyst has been improved. This calculation result is consistent with the observation in Fig. 3a. Otherwise, from Fig. 4e, the valence band and conduction band edges of CNB are shifted up obviously, resulting in a valence band hole with stronger oxidation ability to generate ˙OH radicals on CNB. Enhanced signals of ˙OH are detected in CNB-0.10 (Fig. 4f).

Otherwise, by using the valence band XPS technique, in Fig. S4, the EVB values of the as-prepared samples are further determined in the order CNB-0.10 (2.02 eV) > CNB-0.08 (2.00 eV) > CNB-0.11 (1.99 eV) > CN (1.52 eV). Notably, for the CNB samples, CNB-0.11 exhibits the most positive valence band edge, indicating that the oxidation capacity of CNB-0.10 is the strongest among them. Because of the significant oxidation potential of CNB-0.10, in contrast to those of CNB-0.08 and CNB-0.11, NO could be oxidized more easily and the reaction intermediates could transform into the final product (NO3) rather than the secondary pollutant (NO2) on CNB-0.10. Otherwise, it should be noted that the ˙OH species can be directly generated on CNB-0.10 because the potential energy of the valence band holes (2.02 eV) is higher than the potential energy of OH/˙OH (1.99 eV) (Fig. S5). Thus, CNB-0.10 more efficiently donates reactive oxygen species (˙O2 and ˙OH radicals) than CN, implying that the appropriate B-doping amount could promote the charge separation and tune the band structure simultaneously, and further facilitate the photocatalytic NO oxidation process.

3.4 Photocatalysis mechanism and inhibition of toxic intermediates

In order to probe the conversion process and reaction mechanism of photocatalytic NO oxidation, in situ DRIFTS is utilized to dynamically monitor the adsorbed reaction intermediates and final products in a time sequence. In Fig. 5a, the baseline is recorded before NO is injected into the reaction chamber. Subsequently, the NO absorption bands appear once the NO molecules come into contact with the CN surface under dark conditions. The peaks of NO (1095 and 2001 cm−1)45 are observed owing to the physical adsorption. Simultaneously, the peaks of NO2 (2084 cm−1), N2O (2223 cm−1) and N2O4 (917 cm−1) can be obviously detected, which can be ascribed to the chemical adsorption of NO on CN.45,46 In addition, the gradual accumulation of chelated nitrites (857 and 1158 cm−1),47,48 monodentate nitrates (1024 cm−1)49 and bidentate nitrates (1218 cm−1)48 has been identified respectively. The formation of these species is associated with the active two-coordinated N atoms on CN,50 which could provide extra lone-pair electrons to the active O2 for the generation of ˙O2-.51 On the surface of CNB-0.10 (Fig. 5b), similar adsorption species are also observed, namely NO (1891 cm−1), NO2 (2089 cm−1), N2O (2223 cm−1), chelated nitrites (857, 1170, 1102, and 1192 cm−1), NO2 (1324 and 1340 cm−1) and bidentate nitrates (981 cm−1), respectively.6,47,48,52–55
image file: c9cy01030k-f5.tif
Fig. 5 In situ DRIFTS spectra of the adsorption process of NO + O2 on CN (a) and CNB-0.10 (b); visible light irradiated reaction processes on CN (c) and CNB-0.10 (d).

The photocatalytic NO oxidation process on CN has been measured progressively under visible light irradiation conditions (Fig. 5c) after achieving the adsorption equilibrium. Intermediate bands appear at 2001, 918, 2086 and 2223 cm−1, which can be assigned to NO, N2O4, NO2, and N2O.45,46 Meanwhile, the peaks of the chelated nitrites (860, 1101, and 1163 cm−1),6,47,48 NO2 (1340 cm−1),53 monodentate nitrates (1027 cm−1)49 and bidentate nitrates (1220 cm−1)55 are also observed on CN, illustrating that NO could be oxidized to the nitrate species.

In the case of CNB-0.10, similar IR bands for NO oxidation are correspondingly noted for CNB-0.10 (Fig. 5d). The absorption peaks of the reaction species such as NO (1881 cm−1),45 N2O (2223 cm−1),46 NO2 (2096 cm−1),45,46 NO2 (1340 cm−1),53 chelated nitrites (860 and 1101 cm−1),47,48 and bidentate nitrates (928 and 978 cm−1)54,55 are clearly observed. In contrast to CN, the band of N2O4 is not recorded for CNB-0.10. Meanwhile, the normalized absorbance of NO2 reveals the differences of the reaction processes on CN and CNB. In contrast to CN (NO2 accumulation ratio: 79.6%), the quantity of generated NO2 on CNB-0.10 (NO2 accumulation ratio: 39.3%) is decreased significantly (Fig. 6a). Simultaneously, a higher quantity of the final species (NO3) is produced on CNB-0.10 (NO3 accumulation ratio: 85.2%) than that on CN (NO3 accumulation ratio: 41.7%), as shown in Fig. 6b. These results suggest that NO2 tends to oxidize into the final product (NO3) more efficiently on CNB-0.10, demonstrating that the unique electronic structure of B-doped CN could inhibit the toxic intermediates and improve the oxidation performance of NO, simultaneously.


image file: c9cy01030k-f6.tif
Fig. 6 The changes in normalized absorbance of the NO2 species (a) and the NO3 species (b) on CN and CNB-0.10 during the visible light irradiated reaction process, respectively.

Furthermore, in order to reveal the essential reasons for the suppressed toxic intermediate production, DFT calculations have been performed to analyze the adsorption energies (Fig. 7). In the case of CN (Fig. 7a), the activation energy of the NO to NO2 pathway exhibits an ascending and endothermic tendency due to the high activation barrier, which means that the pathway should be triggered by higher energy. Differently, for the same pathway of NO to NO2 on CNB (Fig. 7b), a descending and exothermic tendency is observed, which implies that the activation barrier disappears and thus the oxidation process can advance smoothly in comparison with pure CN. After that, the adsorption energies of NO2 to NO3 on CNB (−0.43–−1.39 eV) are more negative than those on CN (−0.42–−1.17 eV) relatively, which could promote the transformation of toxic NO2 adsorbed on the CNB surface into the final product (NO3) by the active radicals. Simultaneously, all the bond lengths of the adsorbed products on CNB are longer than those on CN, representing that the ability of the adsorbed species on CNB is also stronger than on CN. All the absorption and reaction IR bands of the photocatalysts are listed in Tables S1 and S2.


image file: c9cy01030k-f7.tif
Fig. 7 DFT calculated adsorption energies and bond lengths of the major intermediate adsorption species on CN (a) and CNB-0.10 (b), the lengths are given in Å.

By combining the in situ DRIFTS spectra and DFT simulation results, the photocatalytic reaction pathways on B-doped g-C3N4 have been proposed. Under visible light irradiation, ˙O2 and ˙OH can be generated on the conduction band (eqn (1)) and valence band (eqn (2)), respectively. NO was oxidized to NO2 by O2 (eqn (3)). Then, NO2 receives the localized electrons originating from the B-doping sites and transforms into more stable bridging nitrites (eqn (4)). After that, h+ and ˙O2 could oxidize the bridging nitrites to nitrates (eqn (5)). On the other hand, the bridging nitrites could also be subsequently oxidized by ˙OH to the nitrate species (eqn (6)).

 
e + O2 → ˙O2 (1)
 
h+ + OH → ˙OH (2)
 
NO + O2 → NO2 (3)
 
NO2 + e → NO2 (4)
 
2NO2 + ˙O2 + h+ → 2NO3 (5)
 
NO2 + 2˙OH → NO3 + H2O (6)

The mechanisms of enhanced photocatalysis efficiency and suppression of toxic intermediate (NO2) production during photocatalytic NO oxidation on B-doped g-C3N4 were illustrated in Fig. 8.


image file: c9cy01030k-f8.tif
Fig. 8 Schematic diagram of the mechanism of enhanced photocatalysis efficiency and suppression of toxic intermediate production.

4. Conclusion

In order to boost the performance and control the secondary pollutant (NO2) during photocatalytic NO oxidation, we construct B-doped g-C3N4 as a model photocatalyst by a facile method. B-doping could simultaneously enhance the photocatalytic NO removal ratio and suppress the production of toxic intermediates. As revealed by DFT simulation, reactant activation is promoted and the reaction activation barrier on CNB is significantly reduced during the NO oxidation process, which would facilitate the transformation of NO into the final product (NO3−) with NO2 being suppressed. By combining the methods of in situ DRIFTS and ESR, the detailed mechanisms for the inhibition of toxic intermediates and the conversion pathway of photocatalytic NO oxidation on CNB have been proposed. The present work elucidates a new strategy to control the generation of toxic intermediates and boost the photocatalysis performance for safe air purification.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21822601, 21501016 and 21777011), the National R&D Program of China (2016YFC02047), the Innovative Research Team of Chongqing (CXTDG201602014), the Natural Science Foundation of Chongqing (cstc2017jcyjBX0052), and the Plan for “National Youth Talents” of the Organization Department of the Central Committee. The authors also acknowledge AM-HPC in Suzhou, China for computational support.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cy01030k

This journal is © The Royal Society of Chemistry 2019