Construction of bimetallic ZIFs derived Co-Ni LDHs on the surfaces of

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Applications of Polymer, Composite, and Coating Materials

Construction of bimetallic ZIFs derived Co-Ni LDHs on the surfaces of GO or CNTs with a recyclable method: towards reduce toxicity of gaseous thermal decomposition products of unsaturated polyester resin Yanbei Hou, Shuilai Qiu, Yuan Hu, Chanchal Kumar Kundu, Zhou Gui, and Weizhao Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04340 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Applied Materials & Interfaces

Construction of bimetallic ZIFs derived Co-Ni LDHs on the surfaces of GO or CNTs with a recyclable method: towards reduce toxicity of gaseous thermal decomposition products of unsaturated polyester resin Yanbei Hou1, Shuilai Qiu1, Yuan Hu1, Chanchal Kumar Kundu1, Zhou Gui1∗, Weizhao Hu1∗ 1

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China

Abstract: This work proposed an idea of recycling in preparing Co-Ni layered double hydroxides (LDHs) derived flame retardants. A novel and feasible method was developed to synthesize CO-Ni LDHs decorated graphene oxide (GO) and carbon nanotubes (CNTs), by sacrificing bimetal zeolitic imidazolate frameworks (ZIFs). Organic ligands departed from ZIFs were recyclable and can be reused to synthesis ZIFs. ZIFs, as transitional objects, in-situ synthesized on the surfaces of GO or CNTs directly suppressed the re-stacking of the carbides and facilitated the preparation of GO@LDHs and CNTs@LDHs. As prepared hybrids catalytically reduced toxic CO yield during thermal decomposition of unsaturated polyester resin (UPR). What’s more, the release behaviors of aromatic compounds were also suppressed during the pyrolysis process of UPR composites. The addition of GO@LDHs and CNTs@LDHs obviously inhibited the heat release and smoke emission behaviors of UPR matrix during combustion. Mechanical properties of UPR matrix also improved by inclusion of the carbides derivatives. This work paved a feasible method to prepare ∗

Corresponding author. Tel/Fax: +86 551 63601669 (Z. Gui), +86 551 63602353 (W.Z. Hu). E-mail address: [email protected](Z. Gui), [email protected] (W.Z. Hu).

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well-dispersed carbides@Co-Ni LDHs nanocomposites with a more environmentally friendly method. Keywords: Co-Ni layered double hydroxides; metal-organic frameworks; graphene and carbon nanotubes; unsaturated polyester resin; gaseous toxicity. 1. Introduction Since 1995, metal-organic frameworks (MOFs) have been developed more than 20000 different structures1. The preparation of these MOFs with nano-/micro-porous structures has been a subject of great interest because of their potential applications in the fields of gas storage2, catalysis3, drug delivery4, and sensor5. Except for direct application, MOFs can be used as precursors or sacrificial templates for other functional materials, due to their well-defined pore structures, large surface area and adjustable physical chemistry properties6. Zeolitic imidazolate framework (ZIFs) are a subclass of MOFs, which are formed by transition metal ions with imidazolate ligands7. Recently, ZIFs derived materials also have been intensively studied to explore specific applications. For example, as a novel anode material, ZIF-67 derived CoSe/C composite exhibited high capacity and superior rate capability8. ZIF-8 has been exploited as template to prepare porous carbon materials, which can be used as metal-free catalysts9. A hollow Co-based bimetallic sulfide polyhedral with superior hydrogen evolution reaction activity and stability was synthesized form bimetal ZIFs (Co, Zn, Ni, or Cu), which was highly active, universal, and inexpensive10. The desirable performance of the derivatives can be easily achieved by adjusting the design scheme correspondingly. The application fields of MOFs-derived materials


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have a promising prospect, which should not be confined to limited area. In our previous reports, different MOFs have been used as flame retardants to improve the fire resistance of polymers11-12. However, considering the flame retardancy efficiency and mechanical properties, it still needs to be improved for MOFs flame-retarded polymers. There were nearly no reports about the application of MOFs-derived materials in this fields. The ignored point may be a key to solve the predicament MOFs encountered. Graphene oxide (GO) and carbon nanotubes (CNTs), as

morphologically

distinct

two-dimensional and

one

dimensional

carbon

nanomaterials, have been widely studied in the last decade due to their brilliant application prospects13-15, including electrochemical energy storage16, gas storage17, and used as supports for metal nanoparticles18. Besides, both carbides and their derivatives have been intensively investigated as flame retardants, including metal oxide decorated carbides19, nitrogen-/phosphorus-doped carbides20, and organic modified carbides21. Raw carbides have an intendancy to form frameworks and then promote the char-forming process of polymers during combustion22, and thus the flame retardancy of polymeric materials will be enhanced. Untreated carbides have a restacked or reagglomeration tendency in the polymeric matrix. Therefore, appropriate treatment is necessary. It has reported that under appropriate processing conditions, MOFs can be regularly arrayed on the surfaces of GO sheets or CNTs23-24, such as Ni-based MOF/GO25, MOF-5/GO26, UIO-66/GO27, ZIF-8/CNTs28, etc. MOFs isolated sheets or tubes and thus prevented the agglomeration of the carbides. A better dispersion state of these compounds is beneficial to the improvement of combination



properties for polymers. Layered double hydroxides (LDHs) are another environmentally friendly halogen-free flame retardants additives with layered structure, which have been extensively used in polymers29. Cobaltic and nickelous compounds based LDHs have been intensively studied for their excellent catalytic effect on toxicity reduction and smoke suppression during combustion of polymer composites. However, nearly no reports about Co-Ni LDHs as flame retardants have been published. Considering synergism effect between Ni ions and Co ions in catalytic oxidation field30-31, the influence of Co-Ni LDHs on toxicity reduction and smoke suppression should be studied. Co-Ni LDHs have been reported and prepared in a strictly inert environment and higher temperature32-33, which was complicated. Therefore, a feasible method of preparing Co-Ni LDHs is a critical step to broaden its applications as flame retardants. Meanwhile, considering low flame retardancy efficiency of LDHs, some modifications should be conducted. Recycling of raw materials can reduce cost of production and provide more environmentally friendly method to synthesize function composites. In this work, ZIFs were used as sacrifice to synthesize Co-Ni LDHs on the surfaces of GO (GO@LDHs) and CNTs (CNTs@LDHs). During the preparation process, organic ligands (2-methylimidazole) departed from ZIFs were recyclable and can be reused to prepare ZIFs. With using ZIFs as transitional objects, Co-Ni LDHs can be prepared at ambient temperature and without heat treatment. Abundant structure characterization techniques were applied to generate the successful preparation of GO@LDHs and


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CNTs@LDHs. Both samples were used to investigate their influence on the toxic products production, smoke emission and heat release behaviors of unsaturated polyester resin (UPR) matrix during combustion. The possible mechanisms of toxic reduction and flame retardancy were proposed based on the results obtained from the analysis of gas and condensed phases. 2. Experimental To ensure the reliability of the experiment, Co-Ni LDHs were prepared in a traditional method, and detailed synthesis process can be seen in supporting information. All chemical reagents were purchased and analytical degree. 2.1 Construction of ZIFs on the surfaces of CNTs or GO GO used in this work was prepared with a modified Hummer’s method34. Briefly, GO or CNTs (0.1 g) and polymethylpyrrolidone (PVP, 0.3 g) were dispersed in 50 mL methanol with ultrasound for 30 min. Then 0.595 g Zn(NO3)2·6H2O and 1.164 g Co(NO3)2·6H2O were added into above solution and continued to ultrasonic for another 30 min. 2-methylimidazole was dispersed in 20 mL methanol and poured into the system with vigorously stirring for 10 min. After keeping still for 8 h, the product was washed by distilled water for 3 times and dried in an oven at 80 oC overnight. 2.2 Preparation of Co-Ni LDHs on the surfaces of CNTs or GO 0.3 g prepared CNTS@ZIFs or GO@ZIFs was dispersed 100 mL ethanol. Then 0.6 g Ni(NO3)2·6H2O was added into the solution with the assistant of ultrasonic treatment for 3 h (the power of the ultrasonic machine is 400W). The atropurpureus solution generally became light-green. The product was collected by centrifugation



with rotate speed of 3000 r/min and washed by ethanol for several times. Then the product was dried in a vacuum desiccator at room temperature. The liquid supernatants were also collected for later use. 2.3 Collection and recycling of 2-methylimidazole Excess NaHCO3 was dissolved in few water and added into the collected supernatant with stirring for 30 min. White sediments were removed by suction filtration and the light-green solution became transparency. The solution was put into refrigerator and kept -15 oC overnight to recrystallize and further remove NaHCO3. Ethanol was separated from 2-methylimidazole by rotary evaporators. To confirm the repeatability of collected 2-methylimidazole, quantitative white product was added into 50 mL methanol contained Zn(NO3)2·6H2O and Co(NO3)2·6H2O. After magnetic stirring for 10 min, the purple product was collected, washed several times, and dried in an 80 oC oven overnight. 2.4 Preparation of UPR/GO@LDHs and UPR/CNTs@LDHs UPR composites were prepared by a facile solution method. Briefly, the preparation of UPR composites with 2 wt% GO, CNTs, Co-Ni LDHs, GO@LDHs, and CNTs@LDHs were performed as follow: to obtain homogeneous dispersed additives in matrix, a given mass of filler was dispersed in corresponding mass of UPR with sonication for 0.5 h (below 35 oC). Recrystallized benzoyl peroxide (BPO) was poured into the system and stirred several minutes. The viscous UPR pre-polymers cured at 70 oC for 2 h and post cured at 120 oC for another 2 h, and then the UPR composites were obtained.


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2.5 Characterization X-ray diffraction (XRD) patterns, conducted by a Japan Rigaku Dmax X-ray diffractometer, were used to study the crystal structure of the carbides and their derivatives.

The

instrument

is

equipped

with

graphite

monochromatized

high-intensity Cu-Kα radiation (λ = 1.54178 Å). Morphologies of the derivatives and char residues obtained from UPR composites were studied by a PHILIPS XL30E scanning electron microscope (SEM) and transmission electron microscopy (TEM) (JEM-2100F, Japan Electron Optics Laboratory Co., Ltd.), which was also employed to observe the fractured surfaces of UPR and its composites. X-ray photoelectron spectroscopy (XPS) was used to characterize the elemental composition of the derivatives, recorded using a Kratos Axis Ultra DLD spectrometer. Nitrogen adsorption–desorption isotherms at -196 °C were recorded on a Micromeritics ASAP2010C nitrogen adsorption instrument. The degassing process were conducted at 100 oC. Thermogravimetric analysis (TGA) was performed by a Q5000 thermoanalyzer instrument (TA Instruments Inc., USA) under air/N2 flow of 25 mL·min-1 and heated from room temperature to 800 oC at a linear heating rate of 20

o

C·min-1.

Thermogravimetric analysis-infrared spectrometry (TG-IR) was conducted using a TGA that was linked to a Nicolet 6700 FTIR spectrophotometer. The work atmosphere is nitrogen atmosphere, flow rate of 50 ml·min-1. Flammability of the samples was characterized using a cone calorimeter (Fire



Testing Technology, UK) according to ISO 5660. Specimens (100×100×3 mm3) were irradiated at a heat flux of 35 kW·m-2, corresponding to a mild fire scenario. Smoke measurement under static conditions has been carried out according to ISO 5659 using a smoke chamber (Fire Testing Technology Ltd., UK). 75×75×3 mm3 samples were each exposed to a radiant heat source of 35 kW/m2 in a closed chamber. Py-GC/MS analysis was carried out by a system combined with a commercial pyroprobe pyrolyzer (CDS5250, CDS, USA) and a GC/MS equipment (Trace DQS Ⅱ, Thermo Scientific, USA). The pyrolysis temperature is 800 oC, being hold for 10 s. All samples were repeated at least three times to guarantee the date reproducibility. Pyrolysis products were identified by software and previous literatures. Laser Raman spectroscopy (LRS) with a SPEX-1403 laser Raman spectrometer (USA) was performed at room temperature to investigate graphitization degree of char residues of UPR composites. Tensile testing was measured by an electronic universal testing instrument (WD-20D, Changchun Intelligent Instrument Co. Ltd., China) at a tensile speed of 5 mm•min-1. 3. Results and discussion The preparation procedure of CNTs@LDHs and GO@LDHs are depicted in Scheme 1a. The possible synthesis mechanism is illustrated in Scheme 1b. ZIF-67 crystals directly deposited on the GO sheet via the direct growth route usually show a large crystal size, for the fast growth kinetics35. As a result, ZIF-67 crystals usually inhomogeneous deposited on the surfaces of GO, which is adverse to the retainment


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of 3D GO/ZIF construction. The addition of zinc ions can reduce the size of ZIF-67 and thus forming a bimetallic ZIFs (ZIF-8/ZIF-67). Several studies have recently proven that bimetallic ZIFs with different ratios of Zn/Co exhibit higher catalytic performance than sole metal contained MOFs36-37. Moreover, the non-covalent adsorption of zinc ions on the surfaces GO or CNTs is feasible for anchoring 2-methylimidazole in the growth-initiating MOFs38. The well arrayed ZIFs on the surfaces of carbides paved a feasible way to prepare Co-Ni LDHs decorated GO or CNTs. 3.1 Characterization of GO@LDHs and CNTs@LDHs Fig. 1 shows the XRD patterns of GO@LDHs (Fig. 1a) and CNTs@LDHs (Fig. 1b). All characteristic peaks of ZIFs were well agreement with previous reports, indicating the successful synthesis of ZIF-8/ZIF-6739-40. It is clear that both materials revealed the typical diffraction peaks of ZIFs and no characteristic peaks of GO and CNTs were detected in the XRD curves, which can be attributed to the loss of long-range order in GO sheets for the growth of ZIFs on GO surfaces41 and excellent dispersion of CNTs42, that is, carbon backbone was closely embedded in the ZIFs43. After ultrasonic processing, the characteristic peaks of LDHs were evidently appeared in both compounds, indicative of the crystal feature of Co-Ni LDHs which was in line with the previous report44. The additional peak in CNTs@LDHs located at 26.2o was well in agreement with the main peak of CNTs, indicating the presence of CNTs in the composites. The disappeared peaks of GO was possibly ascribed to its excellent dispersed state. Additionally, compared with neat Co-Ni LDHs, the location of (003)



peak shifted from 11.5o to 9.6o for GO@LDHs and 9.5o for CNTs@LDHs. The phenomenon was an indication of that the interlayer distances of LDH sheets became larger after combined with the carbides45, implying thinner Co-Ni LDHs sheets were obtained with less agglomeration. Morphologies of raw Co-Ni LDHs, GO and CNTs were observed by TEM and SEM, shown in Fig. S1: Co-Ni LDH sheets were stacking together (Fig. S1a); intertwined CNTs knitted together to form a huge net (Fig. S1b); GO sheets with large area were bespreaded on copper grid (Fig. S1c). These images confirmed the inferior dispersion state of raw Co-Ni LDHs and carbides. In the enlarged area of Fig. S1a, Co-Ni LDH nanosheets can be identified, which shows similar features with previous report46. The layered structure of GO was retained after assembling with ZIFs (Fig. 2a). ZIFs particles uniformly grew on both side of GO sheets, endowed the GO/ZIFs a sandwich-like structure. It is clearly that ZIFs were arranged in the cocked edge of GO (image inert in Fig. 2a). The phenomenon is even obvious in the TEM image (Fig.2b). From the enlarged image, a gradient structure of GO/ZIFs can be found, further indicating a sandwich structure of GO/ZIFs. The EDX results of GO/ZIFs (Fig.2c) revealed that all elements were existed as designed, further confirmed the successful synthesis of ZIFs.

After ultrasonic treatment at higher power, ZIFs

particles were replaced by closely arrayed nanosheets on the surfaces of GO (Fig.2d), which are corresponding to Co-Ni LDHs, confirmed by XRD pattern. Curved sheets became more distinct in the enlarged area. The formation mechanism of LDHs can be speculated as follows. ZIFs are etched by protons generated from the hydrolysis of


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Ni2+ and release Co2+ ions, which can be partially oxidized into Co3+ by O2 and NO3ions in the solution47. The consumption of protons will generate amount of hydroxyl ions. Therefore, Co-Ni LDHs will be formed when metal ions precipitate with hydroxyl ions. Meanwhile, ZIFs becomes vulnerable and continuously release Co2+ ions, which eventually leads to formation LHDs on the surfaces of carbides44. The morphologies of GO sheets were difficult to identify. It can be speculated that GO was well dispersed and thus no characteristic peak of GO appeared in the XRD curve of GO@LDHs. Different from agglomerated raw Co-Ni LDHs, these nanosheets were grown on the surfaces of GO in a better dispersed state. Porous structure of GO@LDHs is evident in TEM images (Fig. 2e), indicating that as-synthesized GO@LDHs had a large porosity. The voids were generated from the decomposition of ZIFs, similar with previous report48. Mapping images were portrayed in Fig. 2f and Fig. S2 to detect the elemental composition of GO@LDHs. It is desirable that all elements identified in GO@LDHs were well in line with expected. Microstructures of CNTs/ZIFs and CNTs@LDHs were also characterized by SEM and TEM. ZIFs particles grew orderly in line with the direction of CNTs, like pearl necklace (Fig. 3a). In the magnified image, CNTs can be observed discontinuously, proving the existing of CNTs. ZIFs were overlapped on the surfaces of CNTs and thus covered CNTs tightly. At higher voltage, CNTs became obviously in TEM image (Fig. 3b). Long carbon tubes passed through ZIFs successively, further confirmed the supporter property of CNTs. From Fig.3c, CNTs/ZIFs showed similar EDX results with GO/ZIFs, and revealed that all elements were existed as designed, further



confirmed the successful synthesis of ZIFs. The morphologies of CNTs@LDHs were similar with GO@LDHs: Co-Ni LDHs were closely arrayed on the surfaces of CNTs, shown in Fig. 3d. Porous structure also can be observed in the TEM image of CNTs@LDHs (Fig. 3e), which was composed by the stacking of LDHs. Compared with raw Co-Ni LDHs, a better dispersed state was obtained after oriented grown on the surfaces of CNTs. Fig. 3f and Fig. S2 exhibit the SEM images with mapping mode. The detected elements were the same as GO@LDHs, indicating the elemental composition of CNTs@LDHs obtained as designed. To further investigate the elementary composition of GO@LDHs and CNTs@LDHs, XPS technique was applied. Both samples had similar element composition. XPS spectra showed that the sharp N 1s peaks at 399 eV were disappeared while O 1s peaks became visible (Fig. 4). The phenomenon indicated the decomposition of ZIFs and the formation of Co-Ni LDHs, for N element mainly arose from the organic ligands in ZIFs and O element originated from LDHs. All elements were well in line with above results obtained by SEM with mapping mode. Ni 2p, as a new element signal, appeared in XPS spectra of both carbides supported LDHs, confirmed Ni element also participated in the construction of LDHs. Compared with Co 2P in GO/ZIFs and CNTs/ZIFs, the peak of Co element exhibited quiet different in GO@LDHs and CNTs@LDHs (Fig. 4c, d). The change of valence state can be identified from high-resolution XPS spectra of elements. The Co 2p core lines of GO@ZIFs and CNTs@ZIFs are very similar to that of Co(OH)2, which are splited into Co 2p3/2 (781.8 eV) and Co 2p1/2 (798.0 eV) parts accompanied by satellite lines


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at 787.5 and 803.0 eV respectively49. The spin-orbit slitting value is calculated as 16.2 eV. After the ultrasonic processing, the spin-orbit splitting values are reduced to 15.3 and 15.4 eV for GO@LDHs and CNTs@LDHs respectively. Meanwhile, the intensity of the Co2p3/2 satellite line is noticeably lowered, indicating the coexistence of Co2+ and Co3+ in GO@LDHs and CNTs@LDHs50-51. It is evident that no obvious different can be observed in the high-resolution XPS spectra of Zn 2p (Fig. S3a, b) and Ni 2p (Fig. S3c). The binding energy values of Zn 2p3/2 and Zn 2p1/2 are observed at 1021.7 and 1044.8 eV, respectively, confirming the zinc ions maintain the divalent state during the reaction52. Meanwhile, the Ni 2p peaks of both two samples appeared at the same location (Ni 2p3/2: 855.7 eV, Ni 2p3/2 satellite: 861.8 eV, Ni 2p1/2: 873.3 eV, Ni 2p1/2 satellite: 879.5 eV), which is well agreement with previous report53. The elemental content of GO@LDHs and CNTs@LDHs detected by XPS were also provided in Table S1. It is evident that C and O accounted for a large proportion of the derivatives and the content of N was negligible. GO@LDHs and CNTs@LDHs were investigated by Brunauer-Emmett-Teller test to reveal the properties of porous structure. The isotherms of both samples could be classified as type Ⅳ (IUPAC classification) and showed high adsorption at high relative pressures (Fig. 5a, b). The shape of hysteresis loops was identified as type H3, which was ascribed to the narrow slit-shaped pores formed by the aggregation of Co-Ni LDH sheets11, 54. This feature implied the micro- and mesoporous existed in both samples. The pore radium distribution curves plotted in Fig. 5 confirmed the presence of macropores and mesoporous in GO@LDHs and CNTs@LDHs. It is



evident that both curves were composed by a sharp peak at 3-5 nm and a broad peak at about 10 nm. Surface area and pore volume are another important features of porous materials. GO@LDHs showed a larger surface area (229.4 m2/g) than that of CNTs@LDHs (138.2 m2/g). Ascribed to the stratified nature, GO can carry more LDHs nanosheets. Meanwhile, the pore volume of GO@LDHs was also larger than that of CNTs@LDHs, corresponding to 0.749 and 0.404 cc/g respectively. It has reported that porous structure was conducive to the absorption of gaseous products during the pyrolysis of polymers and thus delayed the transportation of gases from inner space to combustion area55. Larger surface area provides more place for metallic compounds to locate and then perform catalytic oxidation reaction. Therefore, porous structure of LDHs modified carbides is beneficial to the application as flame retardants. 3.2 Characterization of ZIFs prepared by recycled organic ligands It is interesting that N element was disappeared at EDX and XPS spectra, which is denoting that organic ligands (2-methylimidazole) were removed during the ultrasonic treatment. The collected white products from the waste solution were used to prepare ZIFs to test the recoverability of 2-methylimidazole. Fig. S4 shows the XRD curve and SEM images of as synthesized ZIFs. It is obvious that characteristic XRD peaks of ZIFs (Fig. S4a) were well in agreement with previous report. Digital images of collected supernatant also provided in Fig. S4a. It is clear that little green solution became transparency after treating with HCO3-, in which the concentration of metal ions were extremely low, 0.053, 0.118, and 0.174 ug/mL for bivalent Co, Ni, and Zn


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respectively. Cubic ZIFs randomly scattered in the view of SEM images (Fig. S4b). The morphology of ZIFs in the enlarged SEM image exhibited typical shape of ZIF-8/ZIF-67 composite, further confirmed the successful synthesis of ZIFs and recoverability of 2-methylimidazole. 3.3 Characterization of UPR composites The morphologies of section surfaces can reflect the fracture patterns of polymers and extrapolate the influence of fillers on the mechanical properties of composites. It also can be used to evaluate the surface interaction between matrix and fillers. Fig. 6 and Fig. S5 show the fractured surfaces of UPR and its composites with different magnifications. Neat UPR exhibited an extremely smooth section surfaces (Fig. 6a, d), implying a characteristic brittle fracture. With the addition of the fillers, fractured surfaces became relatively rough but still had features of nonplastic fracture. It is clear that the addition of the fillers had little influence on the break forms of UPR composites. Meanwhile, the agglomeration of GO, CNTs and Co-Ni LDHs can be clearly observed in Fig. S5 under low magnification, which could function as stress concentration points and thus adverse to the mechanical properties of polymers. The magnified images visibly showed the interfaces between matrix and fillers. It is evident that distinct interfaces can be detected between UPR matrix and agglomerated GO, CNTs and Co-Ni LDHs. There were no agglomerative GO@LDHs and CNTs@LDHs can be seen under low magnification, implying better dispersion state. Besides, the features of GO@LDHs and CNTs@LDHs were hard to identify at high magnification. To further evaluate the dispersed state of the derivatives, ultrathin



sections of UPR/GO@LDHs (Fig. 7a) and UPR/CNTs@LDHs (Fig. 7b) were observed by TEM. It is clear that GO@LDHs was well dispersed in UPR matrix. Layered structure of GO@LDHs can be observed under larger magnification. The same phenomenon was also observed in UPR/CNTs@LDHs, well dispersed CNTs were surrounded by layered structure. In contrast with individual components in UPR matrix, the combination of Co-Ni LDHs and the carbides directly suppressed their self-agglomeration. The essential reason for this results can be ascribed to the preparation process of GO@LDHs and CNTs@LDHs. For the growth of ZIFs departed each nanosheet or nanotube, and thus the LDHs decorated carbides with good dispersion state can be obtained. Meanwhile, oriented grown Co-Ni LDHs had less trend to stacking together. Therefore, GO@LDHs and CNTs@LDHs showed superior dispersed state than their original materials. A better dispersion state of fillers generally provides composites better performance. 3.4 Thermostability of fillers and UPR composites Fig. 8a shows a visible weight loss of both samples occurred below 250 oC except CNTs, which can be attributed to the loss of oxygen-containing groups from GO and external surface water or gallery water from Co-Ni LDHs. The further weight loss (above 250 oC) for GO@LDHs and CNTs@LDHs can be associated with the dehydroxylation of the LDH layers, resulting in complete destruction of LDHs. Plentiful oxygen-containing groups existed in the surface of GO sheets contributed to lots of active sites for Co-Ni LDHs anchoring, but also leaded to inferior thermostability for GO@LDHs. The decomposition products under air and N2 were


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analyzed by XRD, as shown in Fig. S6. Characteristic peaks of metal oxides including zinc oxide (ZnO), nickel oxide (NiO), and cobalt oxide (Co3O4), can be identified in the XRD curves of LDHs decorated carbides (air condition, Fig. S6a). While the decomposed products of GO@LDHs and CNTs@LDHs were mainly zinc oxide (ZnO), nickel oxide (NiO), and cobalt oxides (CoO, Co3O4) under N2 conditions (Fig. S6b). Obviously, valence state of Co changed under different atmosphere, which was beneficial to the catalytic oxidation of CO and other toxic products generated during combustion of polymers. TG curves of UPR and its composites obtained under N2 condition were portrayed in Fig. 8b. All samples exhibited distinct one-step decomposition processes. It is evident that adding the fillers into UPR matrix was adverse to the T-5% (temperature at 5wt% weight loss) of composites, which can be ascribed to the lower T-5% of the fillers. Compared with neat UPR, all samples exhibited higher yield of char residues at 800 oC. GO@LDHs and CNTs@LDHs obviously promoted higher char yields than that of individual materials. The char yields increased from about 3.5 wt% for neat UPR to around 11.3 wt% for UPR/GO@LDHs and 9.8 wt% for UPR/CNTs@LDHs. TG curves of UPR composites obtained under air atmosphere were portrayed in Fig. S7. It is obvious that increased char yields were achieved after combing LDHs decorated carbides with UPR matrix, similar with the results collected under N2 condition. All above results confirmed that the addition of all fillers could improve the char-forming process of UPR composites. Combination of Co-Ni LDHs and the carbides made the effect more obvious. Generally, more yield of char residues is



denoting less heat release, and thus better flame retardancy. Because char residue plays a critical role in suppressing transform of heat and fuel in condense phase. 3.5 Heat release and smoke emission behaviors of UPR and its composites When it comes to flame retardancy of polymeric materials, heat release behaviors and smoke emission behaviors should not be ignored. Cone calorimeter is a standard and efficient technique to evaluate the flame retardancy of polymers. Fig. 9 gives the heat release and smoke emission curves of UPR and its composites. Peak heat release rate (PHRR) is an important factor to estimate the fire safety of materials. It is obvious that the addition of all fillers reduced the value of PHRR compared with neat UPR (Fig. 9a). The decline of PHRR values for UPR/GO@LDHs and UPR/CNTs@LDHs are more prominent. There were more than 35.5% and 30.5% decrease in PHRR value for UPR/GO@LDHs and UPR/CNTs@LDHs respectively, compared with neat UPR, indicating Co-Ni LDHs decorated carbides could provide superior fire safety for UPR composites than their individual components, which can be ascribed to their superior dispersed state. Total heat release (THR) is also a vital data related to condensed phase, used to estimate the flame retardancy of materials. The decrease in value of THR is evident for UPR composites, implying the addition of all the fillers can enhance the flame retardancy of UPR. GO@LDHs and CNTs@LDHs took obviously advantages in reducing THR values than original carbides and Co-Ni LDHs, denoting that the combination of LDHs and carbides further improved flame retardancy of UPR composites. In contrast to neat UPR, more than 27.4% and 24.6% of THR value decreased for UPR/GO@LDHs and


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UPR/CNTs@LDHs respectively. These phenomena can be caused by the different dispersed state of the fillers. Barrier effect of layered materials during combustion of polymers cannot well achieved for agglomerative GO and Co-Ni LDHs, but well obtained for GO@LDHs. Co-Ni LDHs, grew on the surfaces of the carbides, also had a superior dispersion state and shown an acceleration effect on improving flame retardancy of UPR. It has demonstrated that linear CNTs tended to establish network in the condensed phase, thus enhanced char-forming process during combustion and flame retardancy of polymers22. Well dispersed CNTs@LDHs further promoted the char formation process. Smoke release during the combustion of polymers is even the most important factor, which directly puts people to death by suffocation and/or inhalation of the toxic gases56. Smoke emission behaviors shown in Fig. 9c, d directly confirmed that after combining with Co-Ni LDHs, carbides’ derivatives had significant influence on smoke suppression. Peaks of smoke production rate (PSPR) for UPR composites were weakened obviously after integrating the carbides derivatives with UPR matrix. What to be noted, PSPR values of UPR were also decreased after only combing matrix with the Co-Ni LDHs and carbides. However, the decrease degree was incomparable with that of GO@LDHs and CNTs@LDHs. Correspondingly, the yields of total smoke production (TSP) for UPR/GO@LDHs and UPR/CNTs@LDHs were also suppressed. Compared with neat UPR, more than 27.7% and 22.7% decreases in the value of TSP were obtained after cooperation with GO@LDHs and CNTs@LDHs respectively. While UPR/Co-Ni LDHs exhibited passable performance in smoke suppression.



Cobaltic and nickelous compounds had significant effect on smoke or toxic gases suppression55, 57. It can be speculated that the carbides functioned in the condensed phase to improve char formation of UPR matrix, while Co-Ni LDHs decomposed cobaltic and nickel compounds could catalytically promote char-forming processes58. Char layers blocked the release of decomposition products and thus suppressed the smoke emission rate of UPR composites. Weakened smoke release behaviors are beneficial to people's evacuation and firemen's rescue. Therefore, the addition of Co-Ni LDHs decorated carbides can improve the fire safety of UPR. CO/CO2 release behaviors were also investigated and shown in Fig. 10. It is delighted to find that the inclusion of GO@LDHs and CNTs@LDHs functioned superior CO suppression effect than that of Co-Ni LDHs, GO and CNTs, implying Co-Ni LDHs improved the performance of raw carbides (Fig. 10a). In contrast with neat UPR, more than 46.1% and 33.9% decreases in total CO yield were obtained for UPR/GO@LDHs and UPR/CNTs@LDHs, respectively. Toxic CO forcibly occupies hemoglobin and causes people death in a fire. The decrease of CO reduced the toxicity of gaseous decomposition products, probably caused by catalytic oxidation of cobalt and nickel ions proved before59-60. Theoretically, CO is absorbed on the surfaces of metallic oxides (cobaltous oxide and nickel oxide), which lower the oxidizing potential and promote the reaction between CO and active oxygen, forming CO2 in the gaseous phase. Due to coefficient effect between cobaltous oxide and nickel oxide, the suppression effect of Co-Ni LDHs on CO release was even more significant. The inferior performance of Co-Ni LDHs can be attributed to its


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agglomeration in UPR matrix, which suppressed well play of catalytic effect. Correspondingly, CO2 yield was slightly increased after adding raw carbides and their derivatives, especially for GO@LDHs and CNTs@LDHs (Fig. 10c, d). 3.6 Analysis of gaseous pyrolysis products To investigate the change of pyrolysis products after adding GO@LDHs and CNTs@LDHs, FTIR spectra of decomposed products for UPR and its composites versus temperature are plotted in Fig. 11. The absorbance intensity of pyrolysis products for neat UPR was obviously higher than that of UPR/GO@LDHs and UPR/CNTs@LDHs, including carbonyl compounds, hydrocarbons, CO and aromatic compounds. It can be concluded that the addition of the Co-Ni LDHs decorated carbides suppressed pyrolysis process of UPR matrix, and thus released less organic compounds. Those organic compounds not only supported combustion but also aggregated to form smoke particles. Thus the decrease in release amount of organic compounds can promote the flame retardancy and fire safety of UPR composites. Considering TG results, reduced organic compounds probably be catalyzed into char residues and thus improved char yield at high temperature. What to be noted, peaks intensities of CO and aromatic compounds were obviously decreased with the addition of GO@LDHs and CNTs@LDHs. As mentioned above, CO is a toxic gas in a fire and the reduction of CO is beneficial to the fire safety of UPR composites. Generally, partial aromatic compounds, especially benzene, are potentially carcinogenic. The decreased aromatic compounds production obviously reduced the possibility of secondary damage for humans.



More particular knowledge of the pyrolysis products was studied by Py-GC-MC to further understand decomposition process of UPR composites. The decreases of relative abundance for the composites are distinct in contrast with neat UPR shown in Fig. 12, implying that the addition of Ni-Co LDHs decorated carbides promoted the generation of condensed phase and thus reduced the gaseous pyrolysis products. The detailed data of analysis results for primary products were listed in Table S2-4. It is evident that the main products were consisted of carbonyl and aromatic compounds, and the difference between them was unconspicuous, but high molecular weight products appeared with the addition of GO@LDHs and CNTs@LDHs. Meanwhile, the obvious decrease in relative abundance can directly confirm the positive effect of GO@LDHs and CNTs@LDHs on flame retarding UPR composites. It has confirmed that benzene compounds are dangerous substance, which are high toxicity and induce canner and abnormality61. Peaks appeared at 3.65, 6.14 and 10.17 min are corresponding to benzene, methylbenzene and styrene, respectively. It is inspiring that the addition of GO@LDHs and CNTs@LDHs intensively decreased the relative abundance of these aromatic compounds, indicating the reduction of toxicity of pyrolysis products. In addition, peak intensities of phthalic acid decreased visibly after cooperating with carbides derived Co-Ni LDHs, implying the production of these flammable compounds possibly transferred into other macromolecular compounds or char residues, for all peaks intensities were decreased. As mentioned above, the addition of GO@LDHs and CNTs@LDHs can suppresses the pyrolysis processes of UPR matrix, and thus decreases the yield of aromatic compounds and


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other organic compounds. Therefore, the toxicity of these gaseous pyrolysis products was decreased, and the flame retardancy of UPR composites was enhanced. 3.7 Analysis of char residues Condensed phase is another significant perspective to understand flame retardant mechanism. Residues of neat UPR presented a broken morphology and a fluffy surface (Fig. 13). In contrast, the surfaces of UPR/GO, UPR/CNTs and UPR/Co-Ni LDHs were integrated but decorated with some holes (Fig. S8). The surfaces of char layers for UPR/GO@LDHs and UPR/CNTs@LDHs exhibited smoother surfaces, which appeared more intact and less fly ash. Intact char layers can well-play the effect of barrier, which is beneficial to suppress the transform of organic decomposition products and heat between flame area and pyrolysis region. Raman spectra of D and G regions of UPR samples are shown in the bottom of Fig. 13 and Fig. S8. The Tuinstra-Koenig relation was employed to estimate the defect density of char residues62 LD= [2.4×10-10nm-3] • λ4 • (ID/IG)-1. (Ⅳ) From the equation (Ⅳ) we can see, the distance between point-like defects (LD) is in inversely proportion to the intensity ration of D and G peaks (ID/IG) under the same Raman excitation wavelength (λ). As the values of ID/IG followed the sequence of UPR>

UPR/Co-Ni

LDHs >UPR/GO>UPR/CNTs>UPR/GO@LDHs>UPR/CNTs@LDHs, the values of LD lined in reverse order. Defects damage the degree of graphitization and further effect the heat resistance of char residues. In summary, the addition of carbides not



only promoted the char-forming process, but improved the graphitization degree of char resides. This phenomenon was more distinct for GO@LDHs and CNTs@LDHs, that is, more residues were obtained with less defects/higher graphitization degree, which probably ascribed to the catalytic effect of metallic compounds and promotion of carbides in the char residues. On the basis of the above analysis, Scheme 2 shows the possible mechanism of flame retardancy and toxicity reduction. Decomposed products of GO@LDHs and CNTs@LDHs existed in char layer not only promoted char forming with higher graphitization degree, but catalyzed organic compounds into char residues, and thus suppressed the smoke emission. Derivatives of carbides in the pyrolysis region also performed double effect, including barrier effect of layered components and catalytic effect of metallic components. Compact char layers and layered fillers worked together to inhibit the transform of heat and pyrolysis products. Less heat received in pyrolysis area is adverse to the decomposition of matrix, while less fuel arrived in combustion region is beneficial to flame inhibition. The release of toxic gases generated during pyrolysis process of UPR matrix, including CO and aromatic compounds, also catalytically suppressed by Co-Ni LDHs. In summary, the addition of GO@LDHs and CNTs@LDHs not merely enhanced flame retardancy of UPR composites, but also recued gaseous toxicity. 3.8 Mechanical performance The influence of different fillers on the mechanical properties of UPR composites were investigated, including tensile and impact experiments. Stress-strain curves of


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neat UPR and its composites are shown in Fig. 14a, corresponding data of tensile strength and tensile modulus are plotted in the figure too. Interestingly, the tensile properties of UPR/GO, UPR/CNTs and UPR/Co-Ni LDHs were weakened by the fillers, compared with neat UPR. This finding probably results from the agglomeration of the fillers (confirmed by SEM), which brought stress-focusing effect to the composites, and thus diminished the energy dissipation capability of fillers. In contrast, after incorporating with GO@LDHs and CNTs@LDHs, the tensile properties of the UPR matrix were obviously improved. For UPR/GO@LDHs, maximum increase of 31% and 50% in tensile strength and tensile modulus, respectively. Similar results also obtained from UPR/CNTs@LDHs. Those improvements in tensile properties indicated efficient load transfer between LDHs decorated carbides and UPR matrix, which can be attributed to the superior dispersion state of GO@LDHs and CNTs@LDHs. As mentioned above, the synthesis process endowed GO@LDHs and CNTs@LDHs this character, for the growth of ZIFs on surfaces directly weakened the stacking of GO or CNTs, and then the agglomeration of the fillers. The data of impact strength exhibited similar tendency with tensile strength (Fig. 14b), indicating dispersion state also made a difference in the impact properties. Conclusion In this work, GO@LDHs and CNTs@LDHs were synthesized by recycling 2-methylimidazole. Compared with traditional method, the pre-growth of ZIFs on the surfaces not only facilitated the preparation of Co-Ni LDHs on the surfaces of



carbides in open environment, but directly suppressed the agglomeration of GO or CNTs. The whole preparation processes were feasible and time-saving. Different from the agglomeration of Co-Ni LDHs, GO or CNTs in UPR matrix, GO@LDHs and CNTs@LDHs exhibited superior dispersion state in the matrix. Barrier effect of GO or Co-Ni LDHs and network-forming effect of CNTs can be efficiently performed during the combustion of UPR composites, which promoted the char forming process of matrix, and thus enhanced the flame retardancy of UPR. Meanwhile, as transition metal elements (Co, Ni) existed in LDHs decorated carbides, the fillers also exhibited better smoke suppression and toxicity reduction performance in contrast with raw GO or CNTs. Superior dispersion also had positive effect on mechanical performance of UPR/GO@LDHs and UPR/CNTs@LDHs. This work provided another feasible and green method to prepare Co-Ni LDHs derived composites, which will be beneficial to the preparation of functional polymeric composites. Acknowledgment The work was financially supported by National Key Research and Development Program of China (2016YFB0302104), National Natural Science Foundation of China (51603200) and Fundamental Research Funds for the Central Universities (WK2320000037). Supporting Information. Experimental: Preparation of Co-Ni LDHs; Fig. S1 SEM image of Co-Ni LDHs (a), CNTs (b) and TEM images of GO (c); Fig. S2 Distribution diagram of each element detected from GO@LDHs and CNTs@LDHs; Fig. S3 High-resolution XPS spectra of


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Zn 2p in GO derivatives (a) and CNTs derivatives (b), and high-resolution XPS spectra of Ni 2p in both samples; Fig. S4 XRD curve (a) and SEM images (b) of prepared ZIFs; Fig. S5 Fractured surfaces of UPR/GO (a, d), UPR/CNTs (d, e) and UPR/Co-Ni LDHs (c, f); Fig. S6 XRD patterns of decomposition compounds of GO and CNTs derivatives obtain under air (a) and N2 (b) atmospheres; Fig. S7 TG curves of UPR composites under air atmosphere; Fig. S8 SEM images (top/low magnification and middle/high magnification) and Raman spectra (bottom) of UPR composites. Table 1 Element composition and content of GO@LDHs and CNTs@LDHs; Table 2 Analysis results of UPR by Py-GC-MS; Table 3 Analysis results of UPR/GO@LDHs by Py-GC-MS; Table 4 Analysis results of UPR/CNTs@LDHs by Py-GC-MS.



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Figure caption Scheme 1 Schematic illustration of the synthetic procedure for GO@LDHs and CNTs@LDHs, and possible growth mechanism (b). Fig. 1 XRD curves of GO (a) and CNTs (b) derivatives Fig. 2 SEM and TEM images of GO/ZIFs (a, b) and GO@LDHs (d, e); EDX curves of GO/ZIFs (c); SEM image of GO@LDHs with mapping mode (f) Fig. 3 SEM and TEM images of CNTs/ZIFs (a, b) and CNTs@LDHs (d, e); EDX curves of CNTs/ZIFs (c); SEM image of CNTs@LDHs with mapping mode (f) Fig. 4 XPS spectra of GO (a) and CNTs (b) derivatives; high-resolution XPS spectra of CO 2p in all samples (c, d) Fig. 5 N2 adsorption-desorption isotherms of GO@LDHs (a) and CNTs@LDHs (b) Fig. 6 Fractured surfaces of UPR (a, d), UPR/GO@LDHs (d, e) and UPR/CNTs@LDHs (c, f) Fig. 7 TEM images of ultrathin sections of UPR/GO@LDHs (a) and UPR/CNTs@LDHs (b) Fig. 8 TG curves of the fillers (a) and UPR composites (b) under N2 conditions Fig. 9 Heat release behaviors (a, b) and Smoke emission behaviors (c, d) of UPR and its composites Fig. 10 CO (a, b) and CO2 (c, d) release behaviors of UPR and its composites Fig. 11 Comparison of the absorbance of pyrolysis products of UPR, UPR/GO@LDHs, and UPR/CNTs@LDHs Fig.

12

Py-GC-MS

chromatograms

of

UPR,


UPR/GO@LDHs

and

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UPR/CNTs@LDHs at 700 oC Fig. 13 SEM images (top/low magnification and middle/high magnification) and Raman spectra (bottom) of UPR and its composites Scheme

2.

Schematic

illustration

of

the

flaming

UPR/GO@LDHs,

UPR/CNTs@LDHs Fig. 14 Stress-strain curves (a) and impact strength data (b) of UPR composites


and


Scheme 1


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Fig. 1



Fig. 2


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Fig. 3



Fig. 4


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Fig. 5



Fig. 6


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Fig. 7



Fig. 8


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Fig. 9



Fig. 10


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Fig. 11



Fig. 12


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Fig. 13



Scheme 2


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Fig. 14



Abstract Graphic


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Construction of bimetallic ZIFs derived Co-Ni LDHs on the surfaces of

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