Rapid Synthesis of Oxygen-Rich Covalent C2N (CNO) Nanosheets by

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Applications of Polymer, Composite, and Coating Materials

Rapid synthesis of oxygen-rich covalent C2N (CNO) nanosheets by sacrifice of HKUST-1: advanced metal-free nanofillers for polymers Yanbei Hou, Fukai Chu, Shicong Ma, Yuan Hu, Weizhao Hu, and Zhou Gui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11299 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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ACS Applied Materials & Interfaces

Rapid synthesis of oxygen-rich covalent C2N (CNO) nanosheets by sacrifice of HKUST-1: advanced metal-free nanofillers for polymers Yanbei Hou1, Fukai Chu1, Shicong Ma1, Yuan Hu1, Weizhao Hu1*, Zhou Gui1* State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China Abstract: Metal-organic framework (MOF, HKUST-1) derived covalent oxygen-rich C2N (CNO) network was innovatively synthesized by a rapid and green microwave irradiation method. This method can produce CNO multilayers efficiently, which paves a way of practical application for the nanosheets. Structural characterization and synthesis processes of CNO nanosheets were investigated to further understand the key role of HKUST-1. As-prepared CNO has layered feature, which theoretically favors to improve flame retardancy and mechanical performance of polymers. Desirable results were obtained as expected: the fire safety, anti-tensile, and impact resistance of polylactic acid (PLA) were prominently enhanced after adding CNO nanosheets, which can be attributed to the excellent dispersion and compatibility. PLA/CNO nanocomposite was self-distinguished at 2wt% content of CNO, while the tensile strength was increased more than 36% compared with pure PLA, as well as impact strength. This work broadens the application fields of CNO and endows it a possibility of actual application. Keywords: metal-organic framework; carbon nitrides; polylactic acid; fire safety; mechanical performance.

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1. Introduction Covalent networks of the carbon and nitrogen atoms, carbon nitrides (CxN, including C3N4, C3N, and C2N), have been in the limelight due to their colossal applications in the fields of energy1-2, catalysis3-5, and polymer enhancement6. Among the available preparation methods, the frontrunners include thermal annealing7-8, chemical vaporous deposition9, and supramolecular assembly10. These methods were novel and solved encountered problems at that time. However, congenital disadvantages limited their large-scale applications, including rigorous conditions, special equipment, even toxic precursors. To obtain attractive nanometer effects, some exfoliation and modification processes must be conducted in order to generate CxN nanostructure. Take C3N4 as an example, bulk C3N4 obtained by thermal-annealed method should be exfoliated by the similar processes of graphene and two-dimensional

transition-metal

dichalcogenides

(TMDs),

including

micromechanical11-12 and ion-intercalation13-14, which increase the complexity of preparation processes of C3N4 nanosheets. Thus, a lookout for a novel approach to obtain CxN nanostructure turns out to be a great challenge in the development of new application fields of CxN. C3N4 nanosheets have been introduced into polymers to obtain functional materials15-16, but few reports about other polymer/CxN nanocomposites were published. Compared with conductive graphene nanosheets, semi-conductive C3N4 can satisfy the requirement of mechanical and thermal properties of polymer composites, as well as maintain their electrical insulation property and high dielectric


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constant. Different from TMDs, metal-free C3N4 compound has little influence on the application fields of polymers and weaken the metal ion sensitivity to an extreme in certain conditions. Confidently, the mentioned advantages of C3N4 can be also reached by other CxN compounds. The two encountered problems that urgently need to be addressed for the application of CxN in polymeric composites are inferior compatibility and low preparation efficiency. Lacking in polar groups, CxN compounds theoretically separate with polymer matrixes. Some organic-modified C3N4 nanosheets showed better compatibility with polymers17. However, those modification processes generally wasted organic reagents, which increased costing of resource and time. Low preparation efficiency of CxN nanostructures limits their large-scale application in commercial fields. Therefore, preparing polymer-compatible CxN nano-materials by a low-cost and large-scale method is the critical link to practically apply them in polymer composites. Microwave irradiation has been employed to rapidly prepare inorganic and organic compounds18-19. Microwave causes internal heating, which significantly accelerates the kinetics of the organic/inorganic chemical reactions20-21. It is a time-saving and inexpensive method to synthesize compounds in large scale22. The utilization of microwave irradiation to prepare CxN probably resolve the predicaments mentioned above. It has reported that C3N4 was prepared in a layered host matrix (layered double hydroxides) by microwave treatment23. The successful preparation of C3N4 declared the enablement of synthesizing CxN using microwave irradiation. This method has not been employed to synthesize other CxN nanostructures.



Over the past decade, highly porous metal-organic frameworks (MOFs) have been widely used in the fields of gas storage/separation24-25, catalysis26, drug delivery27, and electrochemistry28. On account of their self-sacrificial nature, MOFs have been recently used to derive other functional materials, including porous carbon sources29 and metallic compounds30, thereby broadened research interests of these porous MOFs. Organic ligands in MOFs are generally coordinated with metal ions, which can be removed or immobilized by heating under different atmospheres, and thus generate different MOFs derivatives31. Extensive efforts have been endeavored to prepare MOFs derived materials, but few researches concentrated on the structure reengineering of MOFs, viz. replacing the metal ions with other chemicals. Crystalline MOFs not only provide active sites for catalytic applications, but also framework structures for chemical reactions32. Meanwhile, MOFs are able to accommodate guest species into their pore space, including metal complexes, organic compounds, as well as certain designed reactants33-34. These reactants attack immobilized ligands in MOFs probably generate new functional materials. In this work, HKUST-1 chose as precursor reacted with urea under microwave treatment to prepare CxN compounds. Urea attacked metal ions and reacted with organic ligands (1, 3, 5-benzenetricarboxylate, H3BTC), producing light yellow products within seconds scale. Various elemental analysis using different techniques confirmed the atomic ratio of C and N was 2:1, therefore chemical formula can be speculated as C2N. Meanwhile, abundant oxygen element was detected in the as-synthesized C2N nanosheets, which benefits to improve its compatibility with


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polymers. Structural characterizations of as-prepared oxygen-rich C2N (CNO) nanosheets were conducted and the results were obtained as expected. CNO nanosheets exhibited excellent dispersion state in polylactic acid (PLA) matrix, and prominently improved its mechanical properties, as well as flame retardancy. 2. Experimental All reagents are purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and analytical pure, used without further purification. Characterization techniques are listed in the supporting information file. 2.1 Preparation of CNO nanosheets CNO nanosheets were synthesized by a rapid microwave irradiation method. With assistance of ultrasonic equipment, 0.1 g HKUST-1 was mixed with 0.3 g urea in 5 mL distilled water. The mixture was placed in a domestic microwave oven with a rated output power of 700W. Continuous treatment for 6 min, the blue solution become light pink bulks. After washing with excess 0.5 M H2SO4, pink products became light yellow suspended solids, which gradually precipitated to bottom of beaker. Yellow products were collected and washed with distilled water for several times. 2.2 Preparation of PLA/CNO composites As synthesized CNO compounds were dispersed in 100 mL dichloromethane with ultrasonication treatment for 0.5 h, then corresponding weight of PLA added above solution to obtain PLA/CNO composites, in which the content of CNO were 0.5, 1, 2 wt%. After removing the solvent, obtained PLA/CNO composites were hot pressed



into specific shapes for later use. 3. Results and discussion From X-Ray Diffraction (XRD) curves (Figure 1a), characteristic diffraction peaks of HKUST-1 were clearly identified, indicating the successful synthesis of octahedral-structure of HKUST-135. The structure was identified by scanning electron microscope (SEM), shown in Figure S1a. After microwave treatment, the features of urea and HKUST-1 were disappeared, replaced by a 002 sharp diffraction peak located at 28.2o. The phenomenon is an indicative of that urea reacted with HKUST-1 and generated a new layered crystal with 0.32 nm interplanar distance (d-spacing), which is coincide with previous reports about CxN compounds1, 36. Figure 1b, c present SEM images of as-prepared products before and after acid washing. In both images, nanosheets could be easy to distinguish, confirmed layered structure of the crystal substance. Elemental composition and distribution were detected by SEM with mapping mode and portrayed in Figure 1d. Obviously, the compounds were composed by carbon, oxygen and nitrogen with the average weight ratio of C: N: O=54:29:17, the results are consistent with previous reports36. Transmission electron microscopy (TEM) image further confirmed the multilayer structure of the products (Figure 1e). At high resolution, the interplanar spacing was measured as 0.32 nm from clear lattice fringes and diffraction rings (Figure 1f), agreement with XRD result. The lateral height of the nanosheets was detected by atomic force microscope (AFM) and shown in Figure 1g. It is clear that the thickness of nanosheets was lower than 20 nm, confirmed the successful preparation of multilayer CNO nanosheets. The nitrogen


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adsorption–desorption isotherms of HKUST-1 and CNO nanosheets are portrayed in Figure 1h. HKUST-1 exhibited type 1 adsorption behaviors, indicating micro-porous structure existed in the MOF37. In contrast, CNO showed type 3 behavior, which is attributed to the weak adsorbent-adsorbent interaction38. Meanwhile, the type H3 hysteresis loop of CNO confirmed the appearance of narrow slit-shaped porous structure, which were constructed by the aggregation of CNO nanosheets39-40. X-ray photoelectron spectroscopy (XPS) measurement was employed to further probe the chemical composition of CNO (Figure 1i). No impurity elements were detected in all samples. The appearance of nitrogen at 399 eV confirmed the presence of sp2-hybridized nitrogen atoms in CNO nanosheets. Atomic percent of C 1s, N 1s, and O 1s were obtained as 47.6%, 27.6% and 24.1%, respectively. Carbon/nitrogen ratio is good agreement with EDX results, further confirmed the molecular formula. The relatively higher oxygen content might cause by the adsorption of oxygen/H2O on the surface of CNO. Cu 2p detected in HKUST-1 and CNO-BW (CNO before acid washing) showed different valence states (Figure 1j). Two characteristic peaks of divalent Cu2+ detected at 954.2 and 934.5 eV were corresponding to Cu 2p1/2 and Cu 2p3/2, respectively41. Moreover, the presence of satellite peaks located in the range of 930-965 eV was generally considered as an indication of the existence of Cu (+2) species42. Obviously, there were no obvious satellite peaks for Cu 2p in CNO-BW, indicating the absence of Cu2+. Characteristic peaks of Cu 2p1/2 and Cu 2p3/2 were left shifted to 952.7 and 932.8 eV respectively, indicating the presence of Cu (+1)43. Clearly, Cu ions were reduced to Cu2O and made CNO-BW in little pink. The high



resolution of N 1s and C 1s were displayed in Figure 1k and Figure 1l to further investigate the formation of CNO structure. N 1s spectrum was deconvoluted into two peaks, 399.6 eV for C=N and 400.5 eV for C-N44. Compared with HKUST-1, two extra carbon species, 288.7 eV for C=N and 286.3 eV for C-N, were probed in the C 1s of CNO, except 284.6 eV for C=C and 288.5 eV for C=O45-46. The disappearance of C-O carbon species (285.5 eV) in CNO nanosheets further confirmed the structural modification of organic ligands. All above results revealed that urea has reacted with HKUST-1 under microwave irradiation and produced CNO nanosheets and Cu2O. Furthermore, the coexistence of C=N, C=O, C-N groups in the CNO nanosheets provided basis for structural inference. Fourier Transform Infrared Spectroscopy (FTIR) spectrum showed more structural information of CNO (Figure S1b). Peaks located at 3000-3300, 1450-1600, and 1680 cm-1 could be ascribed to the vibration of –NH, -C=O, and -C-N, respectively47. The vibration of C=N band was detected at 1650 cm-1 (enlarged area) directly confirmed the reaction between urea and HKUST-148. Based on the above analysis, Figure 2a illustrates the synthesis process of CNO nanosheets. Gigantic HKUST-1 was etched by urea under microwave irradiation within minutes scale. After acid washing, CNO nanosheets were obtained. The process was time-saving and eco-friendly. Figure 2b schematically shows the structural modification of HKUST-1 and molecular structure the CNO nanosheets. Urea gradually broke the structure of MOF and composed new constructions with the organic ligands. Then condensation polymerization occurred between the new substances and generated CNO-BW. After removing the excess urea and by-products,


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little yellow CNO multilayers were obtained, which can be clearly identified from the digital image of CNO ethanol solution in Figure 2b. The possible chemical reactions between BTC and urea was supplied in Figure S2. To further understand the role of processing time and HKUST-1 in the preparation processes of CNO nanosheets, three sets of control samples were prepared: to investigate the influence of urea-to-HKUST-1 ratio on the products, different mass of urea were added to the reaction systems with the same microwave processing time; the second were samples synthesized by the microwave assistant reaction of urea-HKUST-1 with different processing time; the third were samples synthesized by urea+urea and urea+H3BTC with the same microwave processing time. As shown in the Figure 3, the dosages of urea obviously effected the morphologies and elemental composition of products. Without urea, pure HKUST-1 became dark blue after microwave processing, which was ascribed to the elimination of water or organic solvent in the pores49. From SEM image (middle), the characteristic octahedral structure of HKUST-1 was retained. Carbon, oxygen and copper were observed as expected in EDS curve (bottom). When 0.2 g urea was put into the reaction system, little blue products were obtained. After acid washing, little yellow products were precipitated. The products were agglomerated in SEM view with layered structures. But Cu element was detected in EDX curve, indicating the insufficient reaction of HKUST-1. The system could reach a ratio balance between urea and HKUST-1 when the amount of urea was 0.3 g. Yellow precipitates showed layered structure with a good dispersion state and designed elemental composition (EDX results). When the



dosage of urea increased to 0.4 g, the products became little yellow again. From SEM image, the layered products were stacked together by mushy substance, which could be generated by the reaction of excess urea. The abundant N element detected by EDX further confirmed the over-dose of urea. The amount of urea in reaction system made a big difference on the products. From Figure 4a, with time increasing, blue reaction system turned into pink solid. Meanwhile, the intensity of characteristic XRD peak of urea became weaker and weaker, even disappeared in CNO-BW, as shown in Figure 4b. Conversely, a new peak appeared at 28.2o was increasingly sharp. Figure 4c shows the SEM image of raw materials at 2 min. It is clear that the surfaces of HKUST-1 were covered by needle-like urea crystals. Continue processing, the spicules were disappeared and layered structure could be observed on the surfaces of HKUST-1 (Figure 4d). Under microwave irradiation, urea+urea and urea+H3BTC series can also occur chemical reactions and generate new crystals, confirmed by XRD pattern (Figure 4e). Obviously, no characteristic peaks of urea and H3BTC are detected in the products. Different from light yellow products of CNO, the products of urea+urea and urea+H3BTC were white and presented amorphous, fragmented morphologies (Figure 4f, g). Compared with preparation process of CNO nanosheets, the presence of HKUST-1 developed advantageous effects on the reaction between urea and organic ligands. Processing time also played a key role in the formation of CNO nanosheets. Based on above analysis data, the reaction mechanisms between BTC and urea can be speculated as follow: urea corroded the surface of HKUST-1 and permeated into its


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inner space. The reaction products composed by BTC and urea acted as precursors, which was further employed to construct layered CNO. The urea-to-HKUST-1 ratio, reactants and processing time significantly influence the structure of products. Thermostability of as-synthesized CNO nanosheets governed its practicability in polymers, because inferior thermal stability will increase the potential fire hazards of polymeric composites. As shown in Figure 5a, CNO exhibited the highest T-5% (temperature at 5 wt% weight loss) and char yield, implying the best thermostability of CNO. Meanwhile, the decomposition process of CNO was similar with pure PLA, shown in Figure 5b. It is desirable that the addition of CNO nanosheets has slight influence on the initial decomposition temperature of PLA/CNO composites, but obviously improves their char yields. More char yield is beneficial to decrease the value of total heat release (THR), for less gaseous products of the matrix are released and combusted. Figure 5c, d show the heat release behaviors of PLA and its composites. As expected, the introduction of CNO into PLA obviously reduced the values of peak heat release rate (PHRR) and THR of the composites. PHRR and THR are virtual factors to evaluate fire safety of materials50. The notably reduction of values of the both parameters directly confirmed the enhanced fire safety of PLA/CNO composites. Thermogravimetric analysis-infrared spectrometry (TG-IR) technique was applied to detect the pyrolysis products of CNO, PLA, and PLA/CNO-2.0, and thus further understand the effect of CNO on the enhancement of fire safety of PLA composites. Pyrolysis products, released from CNO, appeared earlier than that of PLA composites



in Gram-Schmidt curves, as shown in Figure S3a. The phenomenon can be attributed to the lower initial decomposition temperature of CNO. Moreover, the addition of CNO obviously reduced the total yield of pyrolysis products of PLA/CNO-2.0. Less organic products released from PLA was conducive to the suppression of fuel supply in combustion area, and thus inhibited combustion. FTIR spectra at maximum degradation rate were plotted in Figure 5e. There were two main peaks in CNO spectrum, located at 2380 and 3550 cm-1, which were corresponding to the vibrations of C=O (arise from CO2) and N-H bands (originate from NH3, corresponded broad peak at 750 cm-1 was ascribed to the out-of-plane bending vibration of N-H)51, respectively. It can be speculated that layered CNO was pyrolysed into char residue and non-flammable gases. These gases were ejected out of pyrolysis area of PLA matrix and diluted the concentration of oxygen in combustion region, resulting the suppression of flame propagation. Compare with pure PLA, the absorbance intensity of PLA/CNO-2.0 increased markedly, indicating the pyrolysis process of PLA matrix was suppressed by CNO nanosheets. Thermal conductivity (κ) of certain polymer is mainly determined by its structure. As shown in Figure S3b, the presence of CNO nanosheets markedly decreased the values of κ (more than 35% decrease at 2 wt% content of CNO), which can be attributed to the low κ of the multilayers. Compared with previous reports, as-prepared CNO in this work shows extremely low κ52-53. Different construction made a difference between these CxN compounds: oxygen-rich structure compromised the conjugated structure and thus decreased thermal conductivity of CNO 54. Homo-dispersed CNO nanosheets in PLA matrix performed


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good thermal insulation effect, thereby decrease the κ values of PLA/CNO composites. Figure S3c, d show the Raman spectra of char residues of PLA and PLA/CNO-2.0. Area ratio of D band (1399 cm-1) and G band (1600 cm-1) (ID/IG) was used to estimate defect density and graphitization degree of char residues by the Tuinstra-Koenig relation55. It has explained in our previous work that the value of ID/IG was in inversely proportion to the defect intensity and graphitization degree56. Due to the smaller ratio of ID/IG, the residues of PLA/CNO-2.0 performed higher degree of graphitization, which contributed to the well play of barrier effect of char layers. SEM images of char residues were observed and shown in Figure S3e, f. There were nearly no residues for pure PLA, but some white ashes, which were composed by individual particles in SEM view. In contrast, char residues of PLA/CNO-2.0 were continuous and unbroken. Scattered holes on the surface of char might be generated by the emission of these noncombustible gases, released by the degradation of CNO nanosheets. Therefore, the flame retardancy mechanisms can be concluded as follows. CNO nanosheets were pyrolysed ahead of PLA matrix and generated inflammable gases and char residues. These gases diluted the concentration of oxygen in combustion region, while char layers inhibited the transfer of pyrolysis products of PLA matrix. The two factors worked together to improve the fire safety of PLA/CNO composites. To confirm the speculated mechanisms, vertical flame tests were conducted and shown in Figure 5f. Pure PLA combusted stably and continuously until complete combustion. Inversely, PLA/CNO-2.0 was soon snuffed out with unstable combustion. Gases were ejected out and broke the original shape of flame. Both



samples were melting and dropping in combustion processes, but the ejected gases suppressed heat transfer and cut off dropping, thus promote self-extinguish of PLA matrix. 2D nano-materials have been proved to be attractive enhancers for polymers, which can improve the mechanical properties of polymeric materials for various engineering applications57. The dispersion state of fillers in matrix effects the physical performance of polymeric composites prominently. The fractured surface of PLA/CNO-2.0 was portrayed in Figure 6a. There were no obvious aggregation of CNO nanosheets can be observed on the section, indicating good dispersion of the fillers. Ultrathin section of the composite was obtained by TEM, from which the layered structure of CNO can be clearly identified. Excellent dispersion of CNO nanosheets can contribute superior performance to PLA/CNO composites. Meanwhile, folds and pull-out features could be observed on the rough surface of the composites, which are usually considered to be the characters of ductile fractures. Figure S4a shows the cross-section of pure PLA with characteristic brittle fracture morphologies. It is obvious that the addition of CNO nanosheets had great influence on the fracture pattern of PLA matrix. The viscoelastic response of polymers can be analyzed by dynamic mechanical analyzer (DMA) with the measured temperature range. Storage modulus is the contribution of elastic components of polymer composites and can be used to evaluate their stiffness58. It can be seen that CNO nanosheets had slight influence on the transition from glassy to rubbery state with temperature increasing (Figure 6b).


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However, the value of storage modulus at low temperature increased with increasing concentration of CNO nanosheets. The results might be attributed to the fact that oxygen-rich CNO nanosheets promoted the interfacial interactions between the matrix and fillers, thus reducing the mobility of polymer chains59. Two dimensional (2D) CNO also acted as interfacial bridge to transfer stress from molecular chains to nanosheets. Furthermore, the formation of hydrogen bonds is the accepted theory to explain the excellent mechanical properties of PLA incorporated with oxygen-rich CNO (like nanocellulose)60. The value of damping factor (tan δ) reflects the motion resistance of segments, as well as the energy absorption ability of polymers. As shown in Figure 6c, with the increasing concentration of CNO nanosheets in PLA, the value of tan δ peak and its corresponding glass-transition temperature (Tg) were progressively increased more than 20% and 6 oC respectively, reconfirmed the addition of CNO greatly inhibited the chain segment movement of PLA matrix. Well-dispersed CNO nanosheets intercalated into PLA molecular chains and acted as cross-linked points by forming strong intermolecular interactions, thus contributed to molecular restrictions at the interfaces. The width of tan δ peak for pure PLA became broader after incorporating with CNO nanosheets, which was ascribed to the increased volume of interface between matrix and fillers61. Therefore, the addition of oxygen-rich CNO decreased the mobility of polymer chains and thus improved the stiffness of PLA/CNO composites. It has been well reported that the crystal behaviors of polymers had close relationship with their mechanical performances62. Therefore, it is necessary to



investigate the influence of CNO nanosheets on the crystallization processes of PLA composites. Tg of the samples, obtained by differential scanning calorimetry (DSC) (Figure 6d), was gradually increased from 62.5 to 65.5 oC with the increasing content of CNO in PLA matrix, which was consistent with DMA results. It is clear that the introduction of CNO had little influence on melting temperature (Tm). In contrast, the crystallization temperature (Tc) increased from 98 oC (pure PLA) to 102.5 oC (PLA/CNO-0.5), which can be attributed to the heterogeneous nucleation induced by CNO nanosheets. The crystallinity (αc) of samples can be calculated by the follow equation = ∆ /∆ × 100% (1) where ∆ is the melting enthalpy of PLA matrix in the composites and ∆ is standard enthalpy of PLA (93.6 J/g). Interestingly, with the increasing concentration of CNO, the value of ∆ continuously decreased with the increasing content of the fillers, denoting the crystallinity was decreasing. The phenomenon could be ascribed to the restriction effect of CNO nanosheets on molecular motion of PLA matrix. Therefore, molecular chains were immobilized on the surfaces of CNO and difficult to form PLA crystals. Relative crystallinity (Xc) is defined as = ∆ /∆ where ∆ is melting heat of the sample at time t, while ∆ denotes the whole melting heat of the sample. From the Xc-t curves of PLA and PLA/CNO composites (Figure S4b), the half crystallization time (t50%) can be obtained, which is usually taken as a key parameter to estimate the overall crystallization kinetics. It is obvious that t50% exhibited similar tendency with above parameters, decreased with the increasing


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content of CNO nanosheets. These results confirmed that CNO showed a remarkable suspension effect on the crystallinity of PLA matrix, while promoted the overall crystallization rate of crystalline region in PLA matrix. The concrete impact of CNO nanosheets on the mechanical performance was investigated, including tensile (Figure 6e) and impact (Figure 6f) properties. It is unequivocal that the mechanical properties of PLA composites were improved by adding CNO nanosheets. More than 36% increase in tensile strength confirmed the reinforcement effect of CNO. The shape of strain-stress curves of the composites changed after combining CNO nanosheets with PLA matrix. A visible yield process appeared in the curve of PLA/CNO-2.0, which was different with the brittle fracture of pure PLA. Improved non-notch or notched impact strengths indicated superior shock resistance of PLA/CNO composites to that of pure PLA. The findings can be attributed to above analysis, oxygen-rich CNO nanosheets exhibited strong interfacial interactions with PLA chain segments, which restricted the molecular motion and thus improved the stiffness of PLA composites. Meanwhile, well-dispersed CNO nanosheets acted as cross-linked points, which promoted the interaction between molecular chains, thereby enhanced the toughness of PLA/CNO composites. To further understand the influence of CNO on the mechanical performance, the rheological behaviors of PLA and its composite were investigated by rotational rheometer and shown in Figure 7. It is clear that the addition of CNO nanosheets significantly improved the complex viscosity (η) of PLA/CNO composites. With the increasing content of CNO, the shear-thinning behaviors of the composites became



more obvious. Meanwhile, the values of storage modulus (Gʹ) and loss modulus (Gʺ) were also increased with the increasing content of CNO nanosheets. These phenomena indicated that oxygen-rich nanofillers had strong interaction with molecular chains of PLA, which increased the chain entanglement and immobilized the molecular. Weighted relaxation time (τ) can directly understand the relaxation behavior of composites, which can be calculated by follow equations63:

ʹ =

ʺ =

(2) (3)

As shown in Figure 4d, with the increasing content of CNO nanofillers in PLA composites, the peaks of τ shifted to high time domain. It can be speculated that CNO nanosheets acted as crosslinking sites in PLA matrix, resulting intermolecular cross-linking. Thus, it took longer to finish relaxation process for PLA/CNO composites. All results were well agreement the previous inference. 4. Conclusion Oxygen-rich C2N nanosheets were rapidly synthesized by a microwave irradiation method. This method can produce CNO in a large scale, which is time- and cost-saving. Compared with the preparation process of C3N4 nanosheets, one-step synthesized CNO multilayers had prominently advantages. The synthesis processes were investigated to reveal the key role of HKUST-1. The introduction of HKUST-1 promoted the reaction between BTC and urea, and thus produced CNO nanosheets. Oxygen-rich CNO was ingeniously introduced into PLA and showed excellent compatibility. The addition of CNO nanosheets obviously improved the fire safety of


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PLA composites. Meanwhile, the mechanical properties of the composites were also enhanced. This work firstly prepared MOF-derived covalent frameworks of carbon and nitrogen atoms in a green method, which paves a way of synthesizing carbon nitrides in a quick and effective way. Acknowledgements The authors would like to acknowledge the support from National Natural Science Foundation of China (51603200) and Fundamental Research Funds for the Central Universities (WK2320000036 and WK2320000037). Supporting information Characterization techniques; Figure S1 SEM images of HKUST-1 (a), FTIR spectrum of CNO multilayers (b); Figure S2 the possible chemical reactions between BTC and urea; Figure S3 Gram-Schmidt curves of CNO, PLA, and PLA/CNO-2.0 (a); thermal conductivity results of CNO, PLA and PLA/CNO composites (b); Raman spectra and SEM images of char residues of PLA (c, e) and PLA/CNO-2.0 (d, f); Figure S4 SEM image of fractured surface of PLA (a); Xc-t curves of PLA and PLA/CNO composites (b).



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Figure caption Figure 1 XRD curves of raw materials and CNO nanosheets (a); SEM images of CNO-BW (b), and CNO (c); element map of CNO nanosheets (d); TEM images of CNO multilayers (e) and its high-resolution images (f); AFM images of CNO nanosheets (g); nitrogen adsorption–desorption isotherms of HKUST-1 and CNO nanosheets (h); XPS spectra of HKUST-1, CNO-BW, and CNO (i); high resolution of Cu 2p (j), N 1s (k), and C 1s (l). Figure 2 Schematic illustrations of synthesis processes of CNO nanosheets (a) and structure transformation from HKUST-1 to CNO nanosheets (b) Figure 3 The digital images (top; solid, before washing; solution, after washing), SEM images (middle) and EDX curves (bottom) of the products generated by the chemical reactions between different amount of urea and HKUST-1 Figure 4 Digital images of reaction system of HKUST-1 and urea under different microwave processing time (a); XRD curves of the reaction systems at different time (b); SEM images of the reaction system at 2 minutes (c) and 4 minutes (d); XRD curves of different reaction systems (e); SEM images of the products of urea+urea (f) and urea+H3BTC (g) Figure 5 TG curves of the products of different reaction systems (a); TG curves of PLA and PLA/CNO composites under N2 atmosphere (b); HRR (c) and THR (d) release behaviors of PLA and its composites; FTIR spectra of CNO, PLA and PLA/CNO-2.0 at the maximum degradation rate (e); vertical flame tests (f) of PLA (top) and PLA/CNO-2.0 (bottom)


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Figure 6 SEM and TEM images of fractured surfaces of PLA/CNO-2.0 (a); storage modulus (b), tan δ (c) and DSC (d) curves of PLA and PLA/CNO composites; strain-stress curves (e) and results of impact results (f) of PLA and its composites Figure 7 Rheological behaviors of PLA and its composites: complex viscosity (a); storage modulus (b); loss modulus (c); weighted relaxation time (d).



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



Figure 7


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Abstract graphic


Rapid Synthesis of Oxygen-Rich Covalent C2N (CNO) Nanosheets by

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