Cesium Salts as Mild Chemical Scissors To Trim Carbon Nitride forPhotocatalytic H2 EvolutionWenming Xu,† Xianghui An,† Qinggang Zhang,† Zhen Li,† Qinhua Zhang,† Zheng Yao,† Xiaokai Wang,†

Sha Wang,‡ Jingtang Zheng,† Jing Zhang,§ Wenting Wu,*,† and Mingbo Wu*,†

†State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (East China),No.66 Changjiang West Road, Qingdao 266580, P.R. China‡Jiangsu Provincial Key Laboratory of Pulp and Paper Science and Technology, Nanjing Forestry University, No.159 Longpan Road,Nanjing 210037, China§State Key Laboratory of Safety and Control for Chemicals, SINOPEC Research Institute of Safety Engineering, No.339 SonglingRoad, Qingdao 266071, China

*S Supporting Information

ABSTRACT: Carbon nitride (CN) is considered to be one of the most promising materials for solar photocatalytic hydrogenevolution. However, its low crystallinity degree, to some extent, limited its photocatalytic activity. Unlike previous reports, wedeveloped a feasible method to improve the crystallinity of carbon nitride using unmelted CsCl as mild chemical scissors to trimpristine carbon nitride instead of its precursor or the intermediates with incomplete structure. As a result, the regularpoly(heptazine imides) structures with higher π conjugation were exposed to the surface, wherein the bonded Cs+ ions on thesurface of carbon nitride changed the charge distribution. The high regular structure can wipe out the defect sites, reducingrecombined sites of electron and hole, and poly(heptazine imides) structure can greatly reserve the visible light absorptionability from pristine carbon nitride. Benefiting from these excellent features, the resultant product exhibits excellentphotocatalytic hydrogen evolution activity, which is about 23 times higher than that of bulk carbon nitride.

KEYWORDS: Carbon nitride, Unmelted salt, Crystallinity, Electron−hole separation, Photocatalytic hydrogen evolution

■ INTRODUCTION

A large number of scientists concentrate on the use of solarenergy,1,2 especially on solar-light-driven water splitting toproduce hydrogen gas using photocatalysts, which is one of thenoble strategies to solve global energy and environmentissues.3−5 Carbon nitride, as a representative polymer, isconsidered to be one of the most promising materials forphotocatalytic hydrogen evolution because of its suitable bandgap, visible absorption capacity, abundant and cheapprecursors, and so on.6−10 The photocatalysis always happenedat the surface of the photocatalyst. However, it is difficult toform a regular structure on the surface of carbon nitride,mainly because of the defects, fold structure, incompletepolymerization, and so forth.11,12 These particularly limit the

efficiency of electron−hole pairs trapping, migration, andtransfer for carbon nitride in photocatalysis.13−16

Generally, pristine carbon nitride has a common structure,like heptazine and/or triazine. Using a chemical scissor, to trimand expose the regular structure of carbon nitride to thesurface, can provide a new vision for fabricating carbon nitridefor photocatalysis. Previously, eutectic salt mixtures, like LiCland KCl,17,18 LiBr and KBr,19 LiCl, NaCl, and KCl,20 andMCl/SnCl2 (M = Na, K, Cs),21 reacting with precursors orcondensation intermediates to improve the crystallinity ofcarbon nitride have been explored. However, most of them

Received: March 27, 2019Revised: May 22, 2019Published: June 14, 2019

Research Article

pubs.acs.org/journal/ascecgCite This: ACS Sustainable Chem. Eng. 2019, 7, 12351−12357

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bring about poly(triazine imide) structures instead of poly-(heptazine imide) structures; the lower π conjugation ofpoly(triazine imides) may reduce the visible light absorptionability in photoctalysis.22−26 It indicates that, to some extent,the molten salt can prevent the polymerization process, anddestroy the carbon nitride structure, especially LiCl.27 Fromanother perspective, the salt can be a good candidate as achemical scissor under relative mild reaction conditions.In this work, we would like to challenge the common

method from the precursors or intermediates to obtain theregular structure of carbon nitride. Cesium ions are less activethan lithium ions, and CsCl cannot break the carbon nitridestructure such as poly(heptazine imides). We demonstrate thatusing a single salt with common carbon nitride product canimprove its crystallinity, even though the subsequentmodifying temperature is below the melting point of salt(645 °C). Carbon nitride and cesium chloride are first groundand then heated in a tubular furnace for 2 h at 550 °C under anitrogen atmosphere. Finally, the samples were achieved bycentrifugation with water and drying overnight (Figure 1).With clipping of cesium chloride, carbon nitride was trimmedwith crystallinity structure for the catalysis system, and itssurface area was improved remarkably. These excellentproperties enable the resultant product to exhibit effectiveelectron−hole pairs trapping, migration, transfer properties,and subsequent excellent photocatalytic hydrogen evolution

activity, which is about 23 times higher than that of bulkcarbon nitride.

■ EXPERIMENTAL SECTIONChemicals. Dicyandiamide (99%) and CsCl (99%) were

purchased from Aladdin Industrial Corporation and used withoutfurther purification.

Preparation of Photocatalysts. Bulk g-C3N4 was prepared byheating 10 g of dicyandiamide up to 550 °C with a ramp rate of 2.3°C/min and kept for 4 h in a tube furnace covered by aluminum foilunder a N2 atmosphere. The resultant product was denoted as CN.After that, 500 mg of CN and 1000 mg of CsCl were mixed and fullyground. Then the mixture was put into a tube furnace and heated to550 °C for 2 h with a ramp rate of 5 °C/min in a nitrogenatmosphere. After cooling to room temperature, the product wasfurther ground and ultrasonically dispersed in deionized water.Finally, it was washed with deionized water by centrifugation severaltimes and dried at 60 °C overnight. This sample was denoted as Cs-CN-2h.

For comparison, bulk g-C3N4 was ground fully and heated furtherto 550 °C for 2 h under the same conditions, the product of whichwas referred to as CN-2h. Dicyandiamide (1000 mg) was ground with1000 mg of CsCl and then heated to 550 °C for 4 h with a ramp rateof 2.3 °C/min. After cooling to room temperature, the product wasfurther ground and ultrasonically dispersed in deionized water.Finally, it was washed with deionized water by centrifugation severaltimes and dried at 60 °C overnight.The sample was denoted as D-Cs-CN.

Figure 1. Schematic diagram for the preparation process of modified carbon nitride using CsCl as a chemical scissor and its application inphotocatalytic H2 evolution.

Figure 2. (a) SEM image of Cs-CN-2h. (b) TEM mapping images of Cs-CN-2h. (c) High-resolution TEM image shows the interlayer latticeimages of Cs-CN-2h. (d) In-plane lattice and the enlarged scale image with the superimposed structural model (inset) from image (e). (e) High-resolution TEM images of Cs-CN-2h. (f) In-plane mesocrystalline lattice and the enlarged scale image with the superimposed structural model(inset) from image (e).

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Image and Spectroscopic Characterization. The samples werecharacterized by X-ray diffraction (XRD) (X’Pert PRO MPD,Holland), scanning electron microscopy (SEM) (Hitachi SU8010,Japan), Fourier transform infrared spectrometry (FT-IR) (ThermoNicolet NEXUS670, USA), and X-ray photoelectron spectroscopy(XPS) using a Kratos AXIS Ultra spectrometer equipped with aprereduction chamber, Elementar Vario EL III instrument (Ele-mentar, Germany). The UV−vis diffused reflectance spectra wereobtained from the dry-pressed disk samples using a Scan UV−visspectrophotometer (UV−vis DRS UV-2700, Shimadzu, Japan)equipped with an integrating sphere assembly, using BaSO4 as areflectance sample. Brunauer−Emmett−Teller (BET) surface areaswere obtained on a nitrogen adsorption apparatus (MicromeriticsASAP 2020M) with all samples degassed at 423 K for 12 h prior tomeasurements. Time-resolved fluorescence decay spectra wereobtained with an Edinburgh FLS980 spectrophotometer with theexcitation wavelength at 375 nm and the emission wavelength at 450nm. Photoluminescence spectra (PL) were measured on afluorospectrophotometer (F97pro, Lengguang Tech, China).Photocatalytic Hydrogen Activity Test. Photocatalytic hydro-

gen evolution was performed as follows. Four milligrams of thecatalyst powder was dispersed in 4 mL of aqueous solution containing10 vol % triethanolamine (TEOA) scavengers flushed with Ar gas. Pt(2 wt %) was loaded on the surface of the photocatalyst as a cocatalystusing an in situ photodeposition method with K2PtCl6. The solutionwas then irradiated with a 300 W xenon lamp equipped with a 420 nmcutoff filter at room temperature. The concentration of hydrogen gasin a headspace was quantified by a Shimadzu GC-2014 gaschromatograph (Ar carrier, a capillary column with molecular sieves5A) equipped with a thermal conductivity detector.

■ RESULTS AND DISCUSSION

The morphology was first studied by SEM and TEM. Thepristine carbon nitride is bulk material but nonuniform. Withthe help of CsCl, it shows that the pristine carbon nitride wastrimmed into small particles (Cs-CN-2h) with the averagerange of 50−300 nm in diameter (Figure 2a and Figure S1 inthe Supporting Information). The surface area of Cs-CN-2h(40.82 m2 g−1) is 4 times higher than that of CN (10.10 m2

g−1) (Table S1). In addition, TEM mapping images (Figure2b) show that the elements (e.g., Cs, C, and N) were

uniformly distributed. All these indicate that CsCl can gentlyand uniformly control the morphology of carbon nitride. Fromthe high-resolution TEM images of Cs-CN-2h (Figures 2c−f),there are obvious lattice structures on both the interlayer andin-plane . In Figure 2c, the lattice spacing is 0.32 nm, whichcorresponds to the interlayer structure.28 Figure 2d shows thatit has a clear hexagonal lattice structure, and its lattice spacingis 1.05 nm, which is likely from the in-plane periodicity.Actually, the preparation of carbon nitride is hardly obtainedwith completely perfect structure; to some extent, it may resultin the formation of mesocrystalline structure (Figure 2f).Therefore, similar lattice structure with average lattice spacing(≈1.05 nm) can be seen, suggesting that it is also from the in-plane periodicity. These results are consistent with XRD tests(vide infra). These improvements on the lattice structure mayefficiently enhance the charge transfer and reduce detrimentalrecombination defect sites.28−30

The chemical unit structure of this photocatalyst wascarefully studied by XRD, solid-state 13C CP-MAS NMR,FT-IR, XPS, and elemental analysis. The XRD of CN-2hexhibits a similar pattern to that of CN, indicating that onlypostannealing has little influence on improving the crystalphase structure and crystallinity (Figure 3a). But there is anoticeable decrease in the peak intensities of Cs-CN-2h afterdealing with CsCl, which may be due to the scattering effect bythe position of cesium ions lacking a strict regularity in thelattice.30 The main pattern of Cs-CN-2h resembles that of apoly(heptazine imide) phase.31 The solid-state 13C CP-MASNMR were used to carefully analyze the structure of Cs-CN-2has shown in Figure 3b. Two obvious resonance peaks werefound at 165.2 and 159.5 ppm for Cs-CN-2h. The former peakis due to the C(e) atoms (N2−CN or terminal CN2(NHx))from heptazine or triazine. Compared to that of CN (156.8ppm), the latter peak (159.5 ppm) has a little shift, which maybe ascribed to the presence of Cs ion changing the electrondensity.32 For the latter peak, it can be attributed to the C(i)atoms (C−N3) from heptazine.30 Therefore, Cs-CN-2h maybe the mixture of heptazine/triazine or only heptazine in the

Figure 3. (a) XRD patterns of the samples. (b) Solid-state 13C CP-MAS NMR of the samples. Inset: the structure of triazine (left) and heptazine(right). (c) FT-IR spectra of the samples. The high-resolution XPS of C 1s (d) and N 1s (e) of the samples. (f) Raman spectra (325 nm excitation)of CN and Cs-CN-2h.

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Cs-CN-2h framework. In FT-IR spectra, the peak at 670 cm−1

is the typical peak for poly(triazine imides). But it is worthnoting that there is no obvious peak for Cs-CN-2h, indicatingthat Cs-CN-2h mainly contains heptazine unit structures(Figure 3c).30

For the photocatalysis, it always happened at the surface ofcatalysis materials, so the XPS spectra can be used to analyzethe surface structure in the resultant products (Figure 3d,e andFigure S2). Signals for Cs-CN-2h and D-Cs-CN correspondingto the elements C, N, Cs, and O were observed in the widespectrum survey. The C 1s spectra of CN in Figure 3d can beresolved into two peaks; however, the C 1s spectra of Cs-CN-2h and D-Cs-CN can be resolved into three peaks centered at284.6, 286.4, and 288.3 eV. The peaks at 284.6 and 288.3 eVcorrespond to sp2 C−C bonding in the standard referencecarbon and CN3 carbon atoms in the heterocycle ring,respectively. And the additional peak at 286.4 eV arises fromsurface C−OH, wherein the oxygen in the resultant productscomes from the tiny amounts (3 ppm V) of O2 in the N2 flowand/or from the water adsorbed on the surface of the rawmaterial. The N 1s signal of Cs-CN-2h and D-Cs-CN (Figure3e) consists of five peaks at 397.6, 398.7, 400.5, 401.3, and404.0 eV. The first signal at 397.6 eV is due to thedeprotonated nitrogen atoms N⊖,30 which more easily absorbCs+ ions and trim the carbon nitride. The last four peaks aresimilar to those of CN, and can correspond to the sp2-hybridized nitrogen (C−NC) and the tertiary nitrogen N−(C)3 groups involved in the heptazine rings, the aminofunctions (C−N−H), and the charging effects, respectively.33

The area and scale of the peaks in N 1s spectrum survey areshown in Table S2 and Table 1, which could further illustratethe heptazine structure of then samples. N1, N2, and N3represent the peaks of C−NC, N−C3, and C−N−H,respectively. The ratio of N1 to N2 is partly indicative of thepolymerization degree of the samples. The ratio of ideal C3N4is 3.00, and the smaller ratio means the higher degree ofpolymerization. The smallest ratio, 5.72 for Cs-CN-2h,indicates the highest polymerization degree. The percentageof N3 indicates the amount of terminal amino on the surface ofthe samples. The percentage of N3 for CN is the smallest(4.30%) among the prepared materials, probably because someof the small molecules that are not polymerized are encased inthe polymerized carbon nitride. The percentage for Cs-CN-2his 5.98%, which is smaller than that for D-Cs-CN (8.27%). Itindicates Cs-CN-2h has a higher degree of polymerization andless terminal amino groups. The ratio of N2 to N3, to someextent, could explain the structure of the samples. The higherthe ratio, the larger the polymerized carbon nitride. The ratio

for Cs-CN-2h is 2.16, which is larger than that for D-Cs-CN(1.35). That is to say, the size of polymerization for Cs-CN-2his larger than that of D-CN-2h, which is consistent with theTEM images. The ratio for D-Cs-CN is very near 1.00, whichis the value for the melon polymer, suggesting that thestructure of D-Cs-CN is on the verge of being a melonpolymer. The molar ratio of C/N can be used to determine thestructural perfection of carbon nitride. The C/N molar ratio ofCs-CN-2h modified by CsCl is about 0.705 (Table S3), whichis close to the expected poly(heptazine imide) (0.71), and isbetween the melon polymer (0.68) and the ideal C3N4(0.75).30 The value for D-Cs-CN (0.700) is also in agreementwith the above analysis that D-Cs-CN is on the verge of beinga melon polymer. This phenomenon may be because CsCltrimmed the carbon nitride into small fragments.CsCl plays an important role in this modification of carbon

nitride. Interestingly, few signals corresponding to Cl weredetected (Figure S2a), even in the higher-resolution Cl 2pspectra of XPS (Figure S2b). On the basis of XPS results(Table S4), the surface Cl content of resultant productsexhibits 0.27 at. % for Cs-CN-2h and 0.25 at. % for D-Cs-CN,which can be ignored. Figure S3 is the XRD pattern of thebyproduct of Cs-CN-2h deposited on the inside surface of thequartz tube, indicating that NH4Cl was present in thebyproduct. These results can further explain the decrease ofN and Cl elements formed NH4Cl and then got away with N2,just as found in our previous study.32 Figure S2c shows Cs 3dspectrum with the Cs+ signals contribution. The surface Cscontents are 4.80 at. % for Cs-CN-2h and 4.11 at. % for D-Cs-CN, respectively (Table S4). The zeta potential of pristine CNis −25.2 mV (Figure S4), while the zeta potential of Cs-CN-2his −35.8 mV, indicating that Cs-CN-2h is more likely tocoordinate with positively charged ions. Besides, the FT-IR(Figure 3c) absorption bands at 996 and 1156 cm−1 for Cs-CN-2h are due to the symmetric and asymmetric vibrations ofNC2 bonds and of metal−NC2 groups, respectively.30 Asshown in Raman spectra (Figure 3f), there is further chargetransfer between carbon nitride and Cs atoms. Three obviouspeaks were observed at 707, 764, and 978 cm−1 in Figure 3f.The peak at 978 cm−1 is due to the breathing modes ofheptazine units.18,34 The other two peaks at 707 and 764 cm−1

are a doubly degenerate mode associated with in-planebending vibrations of the C−NC linked heptazine link-ages.18,34 The densities of the peak decrease after CsClmodification, and the peak of Cs-CN-2h shifts from 707 to 726cm−1. All these indicate that charge transfer is occurringbetween carbon nitride and Cs atoms, and Cs ion with positivecharge coordinated with the negatively charged nitrogen in the

Table 1. Peak Analysis from High-Resolution XPS N 1sa

aN1, N2, and N3 come from Table S2; ∞ means infinite.

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heptazine unit in the form of metal−NC2 group, as depicted inFigure S5.On the basis of the structure modification, the optical

properties of the resultant products have been improved forphotocatalysis. As shown in the diffuse reflectance spectra(DRS, Figure S6), it is obvious that the absorption edge of Cs-CN-2h shows a red shift in comparison to the other samples,mainly because of higher π conjugation and charge transferbetween Cs+ and heptazine. The color of Cs-CN-2h is yellowgreen (Figure S6b). The band gaps were further determinedfrom the transformed Kubelka−Munk function (Figure S7).Combined with the valence band (VB) calculated fromultraviolet photoelectron spectrometry (UPS) (Figure S8),the VB, CB, and band gaps are summarized in Figure S9. Theband gaps narrowed from 2.76 eV (CN) to 2.67 eV (Cs-CN-2h). The suitable CB (−1.04 eV) to standard potential of H+/H2 of Cs-CN-2h portend the higher activity of photocatalytichydrogen evolution when compared to other resultantproducts. These are beneficial for enhancing visible absorptionability and band structures of photocatalysts.The photoluminescence (PL) spectra and time-resolved

florescence decay spectra could reveal the efficiency ofelectron−hole pairs trapping, migration, and transfer, whichare the key parameters in determining the photocatalyticperformance.35−39 As shown in Figure 4a inset, Cs-CN-2h has

very weak PL intensity compared to the others. This PLquenching could be due to the mesocrystalline structure andlower surface defect density (surface defects acting asrecombination centers for the separated electron−hole).15These results were further confirmed by time-resolvedflorescence decay spectra. As shown in Figure 4a, the florescentlifetime of Cs-CN-2h (4.7 ns) shows quick decay kinetics ascompared to that of the CN (9.00 ns) counterpart. Thelifetime implies that the relaxation of a small fraction of Cs-CN-2h excitons occurs via nonradiative paths, presumably by

charge transfer of electrons and holes with high mobility tonew localized states.40 These results indicate that Cs-CN-2hgreatly represses the electron−hole pair recombination, whichis advantageous to photocatalytic hydrogen evolution.Photocurrent response and electrochemical impedance

spectroscopy (EIS) are common methods to reflect thetransfers of photoexcited charges.41−43 The photocurrentsignal (Figure 4b) produced in Cs-CN-2h was large ascompared to that in other samples, distinctly proving theefficient separation of photogenerated charge carriers and areformative radiative charge mobility in Cs-CN-2h.44 Thephenomenon was further corroborated by EIS in Figure 4c. Cs-CN-2h with a decreased semicircular arc radius suggests asmaller impedance.45

It is reasonable to expect that Cs-CN-2h is a promisingphotocatalyst because of its larger surface area and efficientcharge separation. To this end, the photocatalytic propertieswere evaluated by photocatalytic hydrogen evolution (PHE),which was measured in water using visible light irradiation (λ >420 nm), triethanolamine (10 v%) as sacrificial agent, and Pt(2 wt %) as cocatalyst. Figure 4d shows the PHE activities ofthe resultant products in the first 4 h. The cycle stability test ofCs-CN-2h was also tested and the results are shown in FigureS10. The resultant products modified with CsCl havesignificantly increased H2 production, especially Cs-CN-2h.Cs-CN-2h exhibits about 23 times higher hydrogen evolution(6.17 μmol) than that of the bulk CN (0.27 μmol). Thesignificantly enhanced hydrogen production for Cs-CN-2h isprimarily attributed to the significant improvement in themesocrystalline and relatively perfect heptazine structure,which can efficiently enhance the charge transfer and reducedetrimental recombination defect sites. In addition, the largersurface area and Cs+ dopant are also favorable conditions.Moreover, when the PHE test was carried out with CN byadding CsCl (5 wt % Cs of CN, just like the Cs content of Cs-CN-2h) to the reaction solution, no obvious difference wasobserved (Figure S11), which indicates that Cs+ ion in thesolution does not influence the activity of PHE. However, thestructure of the Cs+ ion on the surface may change the surfaceproperties to speed up the transfer of the photogeneratedcarriers. Figure S12 shows the H2 production of Cs-CN-2h inthe first 4 h under dark conditions, indicating the catalyst is aphotocatalyst. Moreover, the H2 evolution rate of Cs-CN-2hwas strongly dependent on the wavelength of the incident light(Figure S13).

■ CONCLUSION

In summary, we have reported a feasible method to preparepoly(heptazine imides) materials possessing high crystallinitystructure using unmelted CsCl and stable carbon nitride.Unlike previous report, the heptazine structure mainly reservedand processed well visible light absorption ability. Bymodification of cesium chloride, the resultant product hasbonded Cs+, and the crystallinity and surface area have alsobeen improved remarkably. The Cs+ ions on the surface ofcarbon nitride changed the charge distribution, and the highcrystallinity wiped out the defect sites, which may be therecombined site for electron and hole. Because of highercrystallinity, a larger surface area, and fewer defect sites, theresultant product shows effective electron−hole pairs trapping,migration, and transfer properties. Benefiting from theseexcellent features, the resultant product exhibits excellent

Figure 4. (a) Time-resolved photoluminescence spectra and photo-luminescence spectra (inset) under 350 nm excitation. (b) Transientphotocurrent response under λ > 420 nm light irradiation of samples.(c) Electrical impedance spectra (EIS) Nyquist plots of samples. (d)Product H2 for samples in first 4 h using 4 mg samples, visible lightirradiation (λ > 420 nm), triethanolamine (10 vol %) as sacrificialagent, and Pt (2 wt %) as cocatalyst.

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photocatalytic hydrogen evolution activity, which is about 23times higher than that of bulk carbon nitride.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acssusche-meng.9b01717.

SEM and TEM images, BET surface areas, XPS,elements contents, the XRD of byproduct, zeta potential,diffuse reflectance spectra, electronic band structure ofsamples, cycle stability test of Cs-CN-2h, and discussionof some experimental data (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: wuwt@upc.edu.cn (W.W.).*E-mail:wumb@upc.edu.cn (M.W.).

ORCIDWenting Wu: 0000-0002-8380-7904Mingbo Wu: 0000-0003-0048-778XNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by NSFC (51672309,21503279, and 51372277) and the Fundamental ResearchFunds for Central Universities (18CX07009A). We alsoacknowledge the State Key Laboratory of Molecular Engineer-ing of Polymers (Fudan University, K2109-29), the YoungTaishan Scholars Program of Shandong Province(tsqn20182027), Initiative Funds of Scientific Research forMetasequoia Talent (163105049), and Technological LeadingScholar of 10000 Talent Project (W03020508).

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