Synthesis of Layered Carbonitrides from Biotic Molecules for Photoredoxtransformations Can Yang,Bowang,Linzhu Zhang,Ling Yin, and Xinchen Wang*

Angewandte Communications Chemie

International Edition:DOI:10.1002/anie.201702213 Photocatalysis German Edition:DOI:10.1002/ange.201702213 Synthesis of Layered Carbonitrides from Biotic Molecules for PhotoredoxTransformations Can Yang,BoWang,Linzhu Zhang,Ling Yin, and Xinchen Wang*

Abstract: The construction of layered covalent carbon nitride pounds such as cyanamide,dicyanamide,triazine and hepta- polymers based on tri-s-triazine units has been achieved by zine derivatives.Further modifications on the chemical and using nucleobases (adenine,guanine,cytosine,thymine and physical properties by doping and extending its electron uracil) and urea to establish atwo-dimensional semiconduct- delocalization were achieved by using aromatic precursors ing structure that allows band-gap engineering applications. containing heteroatoms,including B, S, Nand P. Forexample, This biomolecule-derived binary carbon nitride polymer efficient g-CN photocatalysts have been prepared by the enables the generation of energized charge carrier with light- polymerization of dicyandiamide with 2-aminobenzonitrile or irradiation to induce photoredoxreactions for stable hydrogen 2,4-diaminoquinazoline,[12] and the condensation of urea with production and heterogeneous organosynthesis of C O, C C, barbituric acid, 2-aminothiophene-3-carbonitrile or diamino- À À C Nand N Nbonds,which may enrich discussion on maleonitrile.[14] Considering the diverse array of organic À À chemical reactions in prebiotic conditions by taking account precursors with different chemical compositions and elec- of the photoredox function of conjugated carbonitride semi- tronic structures,itpractically allows the one-pot design of conductors that have long been considered to be stable HCN- polymer photocatalysts on amolecular level in arational derived organic macromolecules in space. manner,but also enables band-gap engineering,donor/ acceptor design, topological fabrication and modifications of Conjugated polymers both in their neutral and charged their physical properties such as p/n characteristics.[24] states have gained great attention due to their promising In contract to ordinary organic precursors,here we move applications in electronic devices,such as light-emitting one step forward by applying biotic compounds to construct diodes,lasers,photovoltaic cells,field-effect transistors,and g-CN semiconductors using nucleobases and urea, to answer biology.[1–6] More recently,the applications have experienced the question if g-CN can be produced from lifesmolecules an extension to photoredox catalysis by acting as flexible light (Scheme 1).[25] All these compounds are existent in biotic transducers to enable the construction of soft photosynthetic machinery/device by using easy solution and thermal pro- cesses.[7–10] Current interest in the emerging field of organic photocatalysis has focused on the molecular design and modification of conjugated semiconductors,such as graphitic carbon nitrides (g-CN) and covalent organic frameworks with spatially extended p-bonding systems.[11–13] Being astable photocatalyst with atwo-dimensional conjugated structure and reduced exciton-binding energy,[14] g-CN has shown great [15] [16] promise for water splitting, CO2 reduction, and organosynthesis,[17,18] which has triggered much research devoted to organic photocatalysis for energy and environmental applications. Many synthetic strategies,including molten-salt growth[19–21] templating[22] and (solvo)thermal condensation[23] syntheses,have been applied to obtain g-CN nanostructures Scheme 1. The chemical production of polymeric carbon nitride semiconductors from nucleobases and urea. with high performance.Inparticular, thermal condensation is afacile and widely adopted approach to drive the construction and connection of triazine and heptazine tectons to poly- conditions as lifesbuilding blocks from simple precursors in conjugated systems,mostly using nitrogen-containing com- the primordial soup.[26, 27] We are therefore interested and encouraged to enrich the studies on prebiotic (photo)chemistry,where HCN polymeric clusters (including triazine-based [*] C. Yang, B. Wang, L. Zhang, L. Yin, Prof. X. Wang super carbonitride tectons) have been considered as the most State Key Laboratory of PhotocatalysisonEnergy and Environment, readily formed and most stable organic macromolecules in College of Chemistry,Fuzhou University [28] Fuzhou 350002 (China) space. Theutilization of these biological precursors for the E-mail:[email protected] chemical synthesis of g-CN-based semiconductors by ther- Homepage: http://wanglab.fzu.edu.cn mally induced condensation is representative of the extreme Supportinginformation for this article can be found under https:// terrestrial and thalassic thermal/hydrothermal conditions of doi.org/10.1002/anie.201702213. primordial Earth;for example the temperatures in hydro-

Angew.Chem. Int.Ed. 2017, 56,6627 –6631 2017 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 6627 Angewandte Communications Chemie

thermal vents close to the volcanic edifices have been found to urea-derived g-CN under visible light irradiation. More- to range from about 608Cto5008C. Thepotential existence of over, the photocatalyst synthesized from urea with cytosine photoactive g-CN semiconductors in primordial environ- (CNC) exhibits the best activity for producing hydrogen

ments to induce photoredox transformations of water, CO2 among all these samples.The superior photocatalytic perfor- and organics would enrich the discussion on the chemical mance of CNC is probably due to the better interaction and evolution of life.Furthermore,the study is also of relevance structural matching between cytosine and urea. Since one for the artificial photosynthesis field. cytosine molecule could break into two molecules with the Nucleobases [adenine (A), guanine (G), cytosine (C), similar structure to urea under thermal treatment, the thymine (T) and uracil (U)] are readily available and stable chemical structure of cytosine enables closer combination precursors.Surprisingly,according to our knowledge,there is with urea and better conjugation into the covalent carbon no report on the synthesis of carbon nitride from biomass- nitride frameworks.Onthe contrary,the polymerization of derived building blocks.Here,wepropose afacile synthesis of urea with other nucleobases with more functional groups is carbon nitride based materials by thermal condensation of less straightforward due to steric hindrance.Therefore, 1) urea with 2) A, G, C, Tand U, respectively.Weaim to cytosine,which could polymerize with the urea best, was demonstrate that the incorporation of such building blocks selected as the monomer to synthesize modified carbon

can lead to the development of carbon nitride for H2 nitride polymers with high efficiencyfor photocatalytic production, and potentially to induce photoredox transfor- reactions. mations under prebiotic conditions. Aseries of photocatalysts were prepared from mixtures of In atypical synthesis,acertain amount of nucleobase (0, 5, urea and different amounts of cytosine.These materials are

15, 30, 50, 80 mg) and urea (10 g) were dissolved in water with denoted as CNCy,where y stands for the amount of cytosine stirring, followed by heating at 808Ctoevaporate water. (y mg). As shown in Figure 1b and Figure S3 the XRD

Afterwards,the obtained mixture was calcined at 400–5508C pattern and FT-IR spectrum of CNCy,respectively,are very in air.The obtained samples are denoted as CNX, where X similar to those of urea-derived g-CN,which demonstrates represents the nucleobase.The reference sample from urea is that the graphitic carbon nitride based structure is maintained denoted as g-CN. with increasing amount of cytosine.The surface morphology

Thechemical structure and composition of CNX samples and texture of the CNC30 sample (after Pt deposition) were were characterized by X-ray diffraction (XRD) (Figure S2a in investigated by scanning electron microscopy (SEM) and the Supporting Information) and Fourier transform infrared transmission electron microscopy (TEM) (Figure 1a). In the spectroscopy (FT-IR) (Figures S2b and S3). TheXRD TEM image smooth, flat layers can be seen, and the Pt patterns of the samples in Figure S2a show astrong peak at particles were distributed uniformly on the surface after 27.48 related to the (002) interlayer reflection of alayered reaction (Figure 1a). There was no obvious difference crystal, plus aweak reflection at 138 due to the in-plane repeating unit of heptazine.The FT-IR spectrum in Figure S2b features distinct 1 peaks from 1200 to 1600 cmÀ corresponding to the stretch modes of aromatic CN heterocycles,while the breathing mode of the triazine 1 units corresponds to 810 cmÀ .The broad peaks between 3500 and 1 3100 cmÀ originate from N H À stretches on the surface of the carbon nitride due to the surface defective sites as aresult of incom- plete condensation. These results clearly confirmed that all of the CNX materials are featuring similar crystal and chemical structure of graphitic carbon nitride. Then, we investigated the photocatalytic activities of CNX samples in water splitting and revealed the effect of different nucleobase monomers on the photocatalytic performances.Asshown in Fig- ure S4, the hydrogen evolution

rates (HER) of CNX materials Figure 1. a) TEM of Pt@CNC30 after reaction. Inset:HR-TEM of Pt nanoparticles. b) XRD patterns of 13 increase by 6–8 times as compared different CNC samples. c) XPS analysis of CNC30 sample. d) Solid-state CNMR spectrum of CNC30.

6628 www.angewandte.org 2017 Wiley-VCH VerlagGmbH &Co. KGaA,Weinheim Angew.Chem.Int. Ed. 2017, 56,6627 –6631 Angewandte Communications Chemie between the morphologies of the sample before and after reaction. During the process of photo-reduction, the Pt nanoparticles were dis- persed on the surface of g-C3N4,and the size distribution of Pt NPs ranged from 3to5nm (Figure 1a, inset), the lattice distance of Pt NPs is 2.28 . Thesolid-state 13CNMR spectra of CNC30 (Figure 1d)shows two peaks,the first peak at 164.3 ppm is ascribed to the C(e) atoms [CN2-

(NHx)],whereas the second one at 155.6 ppm is attributed to the C(i) atoms of melem (CN3). These sig- nals confirm the existence of poly(- tri-s-triazine) structure in CNC30.In the XPS survey spectrum (Fig- ure 1c and S5b–d), there are three elements (C,Nand O), similar to those of g-CN.The O1s peak present in CNC30 is due to the surface- absorbed H2OorCO2,ascross- checked by the FT-IR analysis Figure 2. a) UV/Vis DRS spectra. Inset:photograph of the samples. b) Photoluminescence spectra (Figure 1). By increasing the reso- under 400 nm excitation. c) EPR spectra of CNC30 in the dark (dashed) or with visible light (solid), g- lution of XPS analysis,weobserved CN as areference. d) EIS Nyquist plots. Inset:photocurrent of g-CN and CNC30,on/off photocurrent two main carbon species in the C1s response at 0.2 Vbias potential vs. Ag/AgCl in 0.2m Na SO solution. À 2 4 spectrum (Figure S5). One carbon species with abinding energy (BE) of 287.9 eV is identified as sp2-bound carbon (C C=C), the conjugated system (see Figure 2c dashed dot line), thus À other with BE of 284.6 eV was due to carbon impurities.The resulting in an enhanced EPR signal in dark condition. When N1s XPS spectrum can be deconvoluted into four peaks at the samples were irradiated with visible light, the signal could 398.4, 399.6, 400.8, and 404.2 eV.The strongest N1s peak at be enhanced further (Figure 2c,solid lines).

398.4 eV was assigned to sp2-bound NinN-containing The(photo)electrochemical properties of the CNC30 aromatic rings (C N=C), whereas the weak peak at sample was examined by electrochemical impedance spec- À 399.6 eV is attributed to the tertiary nitrogen N-(C)3 groups. troscopy (EIS) and photocurrent test. Amarked decrease of

Thepeak at 400.7 eV indicated the presence of amino groups Nyquist plots diameter for CNC30 is observed in Figure 2d, (C N H) and the peak at 404.2 eV was attributed to charging which demonstrated that the electronic resistance of CNC is À À 30 effects or positive charge localization in heterocycles. smaller compared to pristine g-CN.Atthe same time,the

Figure 2a displays the UV/Vis diffuse reflectance spectra photocurrent of CNC30 increased by afactor of four compared (DRS) of the sample.The optical absorption is red-shifted to g-CN (Figure 2d inset). This photocurrent enhancement from 460 nm for g-CN to 470 nm for CNC30 and finally to illustrates that the mobility of the photo-excited charge

480 nm for CNC80,corresponding to the change of sample carriers is promoted. color from pale yellow to deep yellow and to orange.These Thesamples were evaluated in aphotocatalytic hydrogen changes are attributed to the combination of cytosine evolution assay by loading 3wt.%Ptasco-catalyst and using monomer in the CN that effectively broadens the conjugated triethanolamine (TEOA) as ahole scavenger. Figure 3a p-electron system and thus narrows the semiconductor band shows that all CNC samples exhibit better catalytic activity gap,which is also reflected by PL analysis (Figure 2b) in hydrogen evolution than the pure g-CN.Notably,the showing gradually red-shift of the emission peaks with CNC30 sample shows the highest activity for hydrogen increasing amount of cytosine added. Theeffect of cytosine production. With increasing amount (from 5mgto30mg) addition on the band structure (valence band and conduction of cytosine in the CNC samples,HER becomes gradually band level) is shown in Figure S6. faster. Theactivity decreased when further increasing the 1 With gradual integration of cytosine into the CN network, amount of cytosine to 80 mg (60 mmolhÀ ), but is still higher 1 electron paramagnetic resonance (EPR) intensity increases than with pure g-CN (37 mmolhÀ ). With 30 mg cytosin an 1 progressively,due to awell-evolved electronic structure with optimum HER (282 mmolhÀ )isachieved, which is nearly 8 extended delocalization of the p-conjugated system (Fig- times faster than with pure g-CN. ure S7). As expected, the generation of photochemical radical Thephotocatalytic performance of CNC30 is well consis- pairs can be efficiently promoted by this extended p- tent with its optical absorption (Figure 3b), and the amount of

Angew.Chem. Int.Ed. 2017, 56,6627 –6631 2017 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim www.angewandte.org 6629 Angewandte Communications Chemie

(32.3 mmol) and H2 (8.02 mmol), with aselectivity for CO

production of 80.1%(entry 1inTable S1). CNC30 gives the highest yield and selectivity for CO production, and there is no detectable production of CO in the absence of either

CNC30 or light (entries 2and 3inTable S1). Theevolution of

CO was not observed when CO2 was replaced with Ar gas (entry 6inTable S1), which together with the isotopic experi-

ments (Figure S14) confirmed that the source of CO is CO2 and not organic sources present in the system.

Besides H2Oand CO2,the process of photochemical evolution involves aseries of organic reactions.Wetherefore checked the potential capability of the synthetic g-CN in photocatalyzing the redox transformation of organic molecules,including amines,alcohols and the coupling of C C, C À À O, C Nand N Nbonds (Table 1). As shown in entry 1, À À

Table 1: Different organic reactions driven by the CNC30 photocatalyst. Entry Reaction Conv.[%] Sel. [%]

1[a] 79 98

2[b] 19.8 96.5

3[c] 56 94 4[d] 99.3 98

5[e] 61.4 90 Figure 3. Photocatalytic hydrogen evolution activity of the samples. a) The effect of cytosine amount on HER. b) Wavelengthdependence Reaction conditions: [a] Substrate (1 mmol), catalyst (50 mg), CH3CN (10 mL) as solvent, 808C, O2 (1 MPa). [b] Substrate (1 mmol), catalyst of the HER with CNC30 loaded with 3wt.%Pt. Inset:time-dependence (50 mg), CH3CN (4 mL) as solvent, 1608C, O2 (1 MPa), 24 h. [c] Sub- of the HER with CNC30 at different irradiationwavelengths. strate (0.1 mmol), catalyst (5 mg), trifluorotoluene (1.5 mL) as solvent,

608C, O2 (1 MPa), 3hunder visible light irradiation.[d] Iodobenzene (0.15 mmol) and benzeneboronic acid (0.6 mmol), catalyst (10 mg with

H2 increased linearly with time under different wavelengths 3wt.%Pd), 2.5 mL of water and 2.5 mL of ethanol,K2CO3 (1 mmol), (Figure 3b,inset), suggesting that the main driving force of 308C, 2hunder visible light irradiation.[e] 30 Vol.%substrate aqueous the photocatalytic reaction is the harvested visible photons. solution, catalyst (100 mg), 1wt.%Pt, 24 hunder UV/vis light illumination. All products were detected by GC-MS. Thestability of CNC30 was examined by operating the experiments under the same reaction conditions for several runs (Figure S8). Except for the first run, the reaction was amines can be photooxidized into imines by g-CN under

treated without light for one hour to ensure there was no H2 oxygen atmosphere,which are regarded as important electro- gas in the reaction system. Aslight deactivation was noticed philic intermediates in organic synthesis.The conversion is in the first four runs.When an appropriate amount of TEOA 79%and the selectivity 98%. Activation of sp3 C Hbonds À was added to the reaction solution, the activity of H2 can also be achieved, for instance,inentry 2, the CH2 group evolution improved in the fifth run. This indicates that the was oxygenated to C=Oselectively.The C Obond in benzyl À decrease in activity after the first run is mainly due to the alcohol could be oxidized to C=O(entry 3). In addition, as decreased concentration of TEOA. It is noted that no obvious shown in entries 4and 5, the synthesized g-CN is also an structural changes were observed (Figure S9–S10). active photocatalyst for the coupling of aromatic halides and Next, the apparent quantum yield (AQY) of the best the cross-linking of ketone and alcohol. Of particular note, sample was examined by studying catalytic kinetics using the reactions in entries 4and 5are both anaerobic reactions different amount of photocalalysts.Figure S11 shows that under light-irradiation, suggesting alink to the evolution of

AQYinitially increases with increasing amounts of CNC30, algae for photosynthesis.All reactions mentioned above then reaches aplateau at amaximum value of 7.1%before proceed with excellent selectivity and yield, which proves that decreasing slightly upon further increasing the amount of heterogeneous photocatalysis offers apromising route to catalyst. We therefore applied 75–100 mg sample to check the realize green organic syntheses under solar irradiation in intrinsic AQYwhere the reaction is limited by charge ambient conditions. separation at the interface to rule out mass diffusion effects. In conclusion, we have demonstrated abiotic precursor

We further studied the photocatalytic activity of CNC30 in approach to manipulate the texture,surface and optical

CO2 reduction (Figure S12–S13), using Co(bpy)3Cl2 as redox properties of g-CNX polymers synthesized from urea and mediator and TEOAaselectron donor under visible light (l > nucleobases.Byoptimizing the synthetic recipe,hydrogen

420 nm). CNC30 photocatalyzed the generation of CO evolution reaction under visible light (l > 420 nm) reached

6630 www.angewandte.org 2017 Wiley-VCH VerlagGmbH &Co. KGaA,Weinheim Angew.Chem.Int. Ed. 2017, 56,6627 –6631 Angewandte Communications Chemie

1 282 mmolhÀ with an apparent quantum yield of 7.1%, which [8] X. Liu, Y. Xu, D. Jiang, J. Am. Chem. Soc. 2012, 134,8738 –8741. is 8-fold higher than with g-CN produced from urea alone. [9] X. Wu,H.Li, B. Xu, H. Tong,L.Wang, Polym. Chem. 2014, 5, This polymeric catalysts are promising for water splitting, by 4521 –4525. [10] Y. Xu, L. Chen, Z. Guo,A.Nagai, D. Jiang, J. Am. Chem. Soc. coupling with other semiconductors and deposition of an 2011, 133,17622 –17625. appropriate co-catalyst. Thediversity of biomass molecules [11] R. S. Sprick, J.-X. Jiang,B.Bonillo,S.Ren, T. Ratvijitvech, P. and the flexibility of bottom-up synthesis will enable the Guiglion, M. A. Zwijnenburg, D. J. Adams,A.I.Cooper, J. Am. rational development of efficient polymeric light-harvesting Chem. Soc. 2015, 137,3265 –3270. transducers,which could potentially have emerged in nature [12] J. S. Zhang,G.G.Zhang,X.F.Chen, S. Lin, L. Mohlmann, G. by thermal assembly of lifesbuilding blocks from simple Dolega, G. Lipner,M.Antonietti, S. Blechert, X. C. Wang, compounds.This work may enrich the discussion on chemical Angew.Chem.Int. Ed. 2012, 51,3183 –3187; Angew.Chem. 2012, 124,3237 –3241. reactions under prebiotic conditions by considering arole of [13] Z. Z. Lin, X. C. Wang, Angew.Chem. Int. Ed. 2013, 52,1735 – naturally occurring carbonitride semiconductors.This study 1738; Angew.Chem. 2013, 125,1779 –1782. could also extend g-CN photocatalysis to redox transforma- [14] M. W. Zhang,X.C.Wang, Energy Environ. Sci. 2014, 7,1902 – tions of organic molecules including amines,alcohols and the 1906. coupling of C C, C O, C Nand N Nbonds. [15] X. C. Wang,K.Maeda, A. Thomas,K.Takanabe,G.Xin, J. M. À À À À Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8,76–80. [16] J. L. Lin, Z. M. Pan, X. C. Wang, ACSSustainable Chem. Eng. 2014, 2,353 –358. Acknowledgements [17] F. Z. Su, S. C. Mathew,G.Lipner,X.Z.Fu, M. Antonietti, S. Blechert, X. C. Wang, J. Am. Chem. Soc. 2010, 132,16299 – This work was financially supported by the National Basic 16301. Research Program of China (2013CB632405), the National [18] X.-H. Li, M. Baar, S. Blechert, M. Antonietti, Sci. Rep. 2013, 3, Natural Science Foundation of China (21425309 and 1743. 21761132002) and the 111 Project. [19] M. J. Bojdys,J.O.Müller,M.Antonietti, A. Thomas, Chem. Eur. J. 2008, 14,8177 –8182. [20] K. Schwinghammer,M.B.Mesch, V. Duppel, C. Ziegler,J.R. Senker,B.V.Lotsch, J. Am. Chem. Soc. 2014, 136,1730 –1733. Conflict of interest [21] G. Algara-Siller,N.Severin, S. Y. Chong,T.Bjçrkman, R. G. Palgrave, A. Laybourn,M.Antonietti,Y.Z.Khimyak, A. V. Theauthors declare no conflict of interest. Krasheninnikov,J.P.Rabe, Angew.Chem. Int. Ed. 2014, 53, 7450 –7455; Angew.Chem. 2014, 126,7580 –7585. Keywords: carbon nitride semiconductors ·chemical evolution · [22] Y. Zheng,L.H.Lin, B. Wang,X.C.Wang, Angew.Chem. Int. Ed. 2015, 54,12868 –12884; Angew.Chem. 2015, 127,13060 – nucleobases ·photocatalysis ·urea 13077. [23] Y. J. Cui, Z. X. Ding,X.Z.Fu, X. C. Wang, Angew.Chem. Int. Howtocite: Angew.Chem. Int. Ed. 2017, 56,6627–6631 Ed. 2012, 51,11814 –11818; Angew.Chem. 2012, 124,11984 – Angew.Chem. 2017, 129,6727–6731 11988. [24] G. Liu, P. Niu, L. Yin, H.-M. Cheng, J. Am. Chem. Soc. 2012, 134, [1] A. G. Slater,A.I.Cooper, Science 2015, 348,aaa8075. 9070 –9073. [2] Y. Xu, S. Jin, H. Xu, A. Nagai, D. Jiang, Chem. Soc.Rev. 2013, 42, [25] J. Oró, Biochem. Biophys.Res.Commun. 1960, 2,407 –412. 8012 –8031. [26] P. J. Bracher, Nat. Chem. 2015, 7,273 –274. [3] Y. Wang,X.C.Wang,M.Antonietti, Angew.Chem. Int. Ed. [27] M. W. Powner,B.Gerland, J. D. Sutherland, Nature 2009, 459, 2012, 51,68–89; Angew.Chem. 2012, 124,70–92. 239 –242. [4] P. Bujak, I. Kulszewicz-Bajer, M. Zagorska, V. Maurel, I. [28] C. N. Matthews, Planet. Space Sci. 1995, 43,1365 –1370. Wielgus,A.Pron, Chem. Soc.Rev. 2013, 42,8895 –8999. [5] C. Duan, K. Zhang,C.Zhong,F.Huang,Y.Cao, Chem. Soc.Rev. 2013, 42,9071 –9104. [6] G. G. Zhang,Z.A.Lan, X. C. Wang, Angew.Chem. Int. Ed. 2016, 55,15712 –15727; Angew.Chem. 2016, 128,15940 –15956. Manuscript received:March 1, 2017 [7] J.-S.Wu, S.-W.Cheng,Y.-J.Cheng,C.-S.Hsu, Chem. Soc.Rev. Revised manuscript received: March 27, 2017 2015, 44,1113 –1154. Version of record online: May 4, 2017

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