A 3D Nanostructure Based on Transition-Metal Phosphide Decorated

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A 3D nanostructure based on transition metal phosphide decorated heteroatom-doped mesoporous nanospheres interconnected with graphene: Synthesis and applications Shuilai Qiu, Weiyi Xing, Xiaowei Mu, Xiaming Feng, Chao Ma, Richard K.K. Yuen, and Yuan Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11101 • Publication Date (Web): 09 Nov 2016 Downloaded from http://pubs.acs.org on November 11, 2016

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ACS Applied Materials & Interfaces

A 3D nanostructure based on transition metal phosphide decorated heteroatom-doped mesoporous nanospheres interconnected with graphene: Synthesis and applications

Shuilai Qiu ab, Weiyi Xing *a, Xiaowei Mu a, Xiaming Feng ab, Chao Ma c, Richard K. K. Yuen bd and Yuan Hu *ab

a

State Key Laboratory of Fire Science, University of Science and Technology of

China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China b

USTC-CityU Joint Advanced Research Centre, Suzhou Key Laboratory of Urban

Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, P. R. China c

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and

Engineering, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China d

Department of Architecture and Civil Engineering, City University of Hong Kong,

Tat Chee Avenue, Kowloon, Hong Kong

Corresponding Authors *Weiyi Xing. Fax/Tel: +86-551-63602353. E-mail: [email protected]. *Yuan Hu. Fax/Tel: +86-551-63601664. E-mail: [email protected].

ACS Paragon Plus Environment


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Abstract: A novel three-dimensional nanostructure based on cobalt phosphide nanoparticles (Co2P NPs) and heteroatom-doped mesoporous carbon spheres interconnected with graphene (3D PZM@Co2P@RGO) was facilely synthesized for the first time, and used for enhancing the flame retardancy and toxicity suppression of epoxy resins (EP) via synergistic effect. Herein, the cross-linked polyphosphazene hollow spheres (PZM) were used as templates for the fabrication of 3D architecture. The 3D architecture based on Co2P decorated heteroatom-doped carbon sphere and reduced graphene oxide was prepared via a carbonization procedure followed by hydrothermal self-assembly strategy. The as-prepared material exhibits excellent catalytic activity with regards to combustion process. Notably, inclusion of incorporating PZM@Co2P@RGO resulted in dramatically reduction on fire hazards of EP, such as 47.9% maximum decrease in peak heat release rate and 29.2% maximum decrease in total heat release, lower toxic CO yield and formation of high graphitized protective char layer. In addition, the mechanism for flame retardancy and toxicity suppression was proposed. It is reasonable to know that the improved flame retardant performance for EP nanocomposites is attributed to tripartite cooperative effect from respective components (Co2P NPs and RGO) plus the heteroatom-doped carbon spheres.

Keywords: cobalt phosphide; heteroatom-doped mesoporous spheres; graphene; nanocomposites; flame retardancy


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1. Introduction Since the successful exfoliation of single graphene layers a dozen years ago, carbon

nanomaterials

have

attracted

the

growing

interest

of

researchers

from various fields.1,2 Novel carbon nanomaterials with geometrical nanoarchitecture and specific surface chemistry may contribute to the rapid development of energy materials.3 Especially, chemical functionalization such as surface heteroatoms doping can endow these nanomaterials with modifiable surface chemistry, good conductivity and structure stability.4,5 Heteroatom (P, N, O, B, S or their combination)-doped carbon nanomaterials have been reported as efficient catalysts in the oxygen reduction reaction.6,7 Besides, these materials that enable high efficiency of catalytic activity and surface utilization require specific surface structural features such as geometrical nanoarchitecture and mesopore structures.8,9 In previous cases, however, carbon nanomaterials involve specific nanoarchitecture were prepared with the assistance of additional templates or activating agents, which are complicated to achieve.10,11 Polyphosphazenes are a family of versatile hybrid organic–inorganic materials with the phosphazene unit (–P=N–) in the backbone, the polyphosphazenes include a large amount of heteroatom elements such as P, N and O, which can provide heteroatom sources in the carbonation process to obtain desired products. 12-14 Based on this, the present work utilized cross-linked polyphosphazene nanospheres to prepare heteroatom-doped carbon materials by a simple technique. Transition metal phosphides (TMPs), which are intrinsically metallic materials, have been used as efficient electrocatalysts with increasing research interest worldwide.15 The typical TMPs have been reported, such as CoP,16 Co2P,17 Ni2P,18 MoP19 and FeP,20 these materials exhibit potential application as cocatalysts for photocatalytic hydrogen evolution reaction.21 The TMPs have been constructed into different nanostructures with controlled morphologies such as nanoparticles, nanotubes, nanowires, nanorods and hollow spheres.22-24 Such nanostructures, possessing better accommodation and higher interfacial, are expected to achieve a stronger catalytic ability. Among these catalysts, cobalt phosphide is the most



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researched TMP with notable catalytic efficiency, due to their metallic character, specific surface and good thermal stability.25,26 With the development of nanotechnology, many strategies have been adopted for the synthesis of cobalt phosphides, such as combination of the elements, reactions with phosphine, disproportionation of phosphides, hydrogen reduction of phosphates, and so on.27-29 However, these synthetic methods to cobalt phosphides used toxic phosphides as raw materials. Furthermore the nanocrystals are difficult to grow under such conditions. Adjusting carbon species structures and active phase of TMP catalysts can significantly improve their catalytic efficiency.30,31 For example, it has been reported that the catalytic activity of the Co2P might be correlated with the charged natures of Co and P.32 Pan et al.33 prepared a series of TMPs based electrocatalysts, including Co2P and CoP. The results showed that nanostructured CoP exhibited higher catalytic activity than that of Co2P. On the other hand, the hybridization of carbon nanomaterials and TMPs may improve the catalytic efficiency owing to the large surface area and high electrical conductivity of the carbon nanomaterials. For example, Hou et al.34 reported that the ultrafine CoP nanoparticles (CoP NPs) exhibited enhanced catalytic activity by introduction of N-doped multiwalled carbon nanotubes. Graphene, a two-dimensional (2D) carbon nanomaterial with extraordinary properties, including large specific surface area, excellent thermal conductivity and superior mechanical flexibility, has wide application prospects in the energy storage, optics, electrosorption and composite materials, etc.35-37 It is well known that the graphene or surface functionalized graphene plays crucial roles on the integrated performance

enhancement

of

polymeric

nanocomposites.

For

example,

functionalization of graphene with grafted hexachlorocyclotriphosphazene leads to the strong interfacial interaction and good dispersion between graphene and epoxy matrix, and also improves the electrical conductivity and flame retardancy of epoxy nanocomposites.38 Based on aforementioned analysis, if a composite nanostructure connecting heteroatom carbon material with graphene was built, which may not only endow efficient catalytic activity and enhancement of integrated properties, but also provide novel peculiarities such as 3D hybrid nanostructures, hierarchical porous


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structures and flexible mechanical scaffolds. Herein, we propose a reasonable design and preparation of a novel 3D nanostructure with interlinked connections of heteroatom-doped mesoporous carbon nanospheres with graphene sheets. First, specifically, the polyphosphazenes coated on silica spheres (PZM@SiO2) prepared through a condensation polymerization reaction and SiO2 cores were etched by hydrofluoric acid, these hollow spheres designed as the models for the fabrication of 3D nanostructures. Then, the heteroatom-doped mesoporous nanospheres decorated with Co2P NPs (PZM@Co2P) were fabricated via carbonization of the composite of highly cross-linked poly-(cyclotriphospazene-co-4,4′-diaminodiphenyl ether) hollow spheres (PZM) and the cobalt precursor. Heteroatoms (P, N, O) deriving from the PZM were maintained in the hybrid materials and offered flame retardant elements for the combustion process. Subsequently, the hydrothermal procedure was performed to connect the PZM@Co2P and graphene oxide (GO) homogeneously due to the electrostatic interactions. The resulting composites (3D PZM@Co2P@RGO) were realized (Scheme 1). The composites combined all advantage of the Co2P NPs, mesoporous nanospheres and RGO. The good distribution of spheres wrapped by the RGO nanosheets owing to the electrostatic self-assembly reduced the aggregation of graphene. PZM@Co2P@RGO composites then were introduced into the epoxy resin (EP) for investigation of their flame retardancy and toxicity suppression performances during combustion. It is expected that the combination of Co2P NPs, mesoporous carbon spheres and RGO sheets could effectively enhance flame retardant and toxicity elimination properties during the EP combustion.

2. Experimental section 2.1 Materials Hexachlorocyclotriphosphazene (HCCP), 4,4′-diaminodiphenyl ether (ODA) and 4,4′-diaminodiphenylmethane (DDM) were purchased from Aldrich Chemical Co. Ltd. (U.S.). Anhydrous ethanol, ammonium hydroxide (NH3·H2O), ascorbic acid, tetraethyl orthosilicate (TEOS), triethylamine (TEA), acetonitrile, acetone, cobaltous acetate, graphite powder, sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl),



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potassium permanganate (KMnO4) and hydrogen peroxide (H2O2, 30% aq.) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). EP (DGEBA, E-44) was supplied by Hefei Jiangfeng Chemical Industry Co. Ltd. (China). Acetonitrile and TEA were dried over 4 ˚ A molecular sieves before use. 2.2 Synthesis of PZM hollow spheres as templates SiO2 nanospheres as templates were prepared according to a previous method with slight modification.39 SiO2 nanospheres were dispersed in 100 mL acetonitrile (0.2 mg/mL) with ultrasonic-assistance. Subsequently, ODA (80 mg) and TEA (6 mL) were added to the mixture. Then the solution of HCCP (40 mg) in 5 mL of acetonitrile was added dropwise into the system within 30 min. The suspension was maintained at 40 oC with ultrasonic-assistance (53 kHz) for an additional 6 h. Finally, the resulting SiO2@PZM were collected by centrifugation at 8500 rpm for 7 min, and washed with distilled water and ethanol (20 mL × 3), respectively, then dried under vacuum at 60 °C, followed by etching of HF solution to obtain the PZM hollow spheres. 2.3 Synthesis of PZM hollow spheres loaded with Co precursors In a typical synthesis procedure of PZM-Co, 0.2 g PZM was dispersed in 20 ml anhydrous ethanol for 30 min. Subsequently, cobaltous acetate aqueous solution (0.2 ml, 0.6 M) was injected into the PZM/ethanol suspension. The suspension was stirred under ultrasonication for an additional 2 h, 0.3 ml of NH3·H2O (20% solution) was injected to the mixture, and the system temperature was controlled and maintained for 1 h. Ultimately, PZM-Co was collected by centrifugation at 8500 rpm for 3 min, washed with distilled water and freeze-dried. 2.4 Synthesis of Co2P NPs modified heteroatom-doped carbon nanospheres The Co2P NPs modified heteroatom-doped carbon nanospheres was prepared by direct carbonization of PZM-Co in a tube type furnace under a pure nitrogen condition. The synthesis process as follow, 1.0 g of PZM-Co in a quartz boat was inserted in the tube furnace. Afterward, the system temperature was rose to 500 °C under dynamic pure nitrogen atmosphere with a heating rate of 10 °C min-1, and kept at that temperature for 2 h. In the second stage, the system temperature was ramped to 800 °C under nitrogen with a rate of 10 °C min-1, followed by an isothermal process


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for 1 h. After the tube furnace was cooled to room temperature, the powders were collected and washed. The resulting samples are denoted as PZM@Co2P. 2.5 Preparation of 3D PZM@Co2P@RGO composites GO was prepared from graphite powder adopting a modified Hummers' method.40 Firstly, 2.0 g PZM@Co2P were dispersed in 80 mL water under ultrasonication for 30 min. Afterward, GO aqueous solution (30 mL, 20 mg mL-1) was added dropwise and dispersed for 2 hour. Simultaneously, 1.0 g of ascorbic acid dissolved in water (30 mL) was added, finally the suspension solution was maintained at 180 °C for 24 h in a sealed steel autoclave to generate PZM@Co2P@RGO gel. The obtained gel was collected by centrifugation and dried under vacuum at 65 °C for 24 h. The 3D PZM@Co2P@RGO composites were obtained. 2.6 Preparation of EP/PZM@Co2P@RGO nanocomposite A typical preparation procedure of EP composite containing 2 wt% PZM@Co2P@RGO is illustrated below: A quantity of 0.9 g of PZM@Co2P@RGO (2.0 wt%) was dispersed in 30 mL of acetone solution with ultrasonic-assistance for 30 min. Subsequently, 36.2 g of EP was poured into the above mixture with mechanical stirring for 1 h. The solvent was removed in a vacuum oven at 70 oC for 6 h. Then, 7.9 g of DDM was melt and mixed with the above mixture by a vigorous stirring for 1 min. Finally, the sample named as EP/ PZM@[email protected] was cured at 100 and 150 °C for 2 h, respectively. After the completion of curing process, the sample was allowed to cool to room temperature. For the preparation of pure EP, EP/PZM2.0 (2.0 wt%), EP/[email protected] (2.0 wt%) and EP/PZM@[email protected] (3.0 wt%) composites, an similar prepared process was utilized except the type of the nanoadditives. 2.7 Characterization Fourier transform infrared (FTIR) spectra were conducted on a Nicolet 6700 spectrometer (Nicolet Instrument Co., USA). X-ray diffraction (XRD) were performed on an X-ray diffractometer (Rigaku Co., Japan), using Cu Kα radiation (λ= 0.15418 nm). X-Ray photoelectron spectroscopy (XPS) spectra were obtained from a VG ESCALAB MK-II electron spectrometer (V.G. Scientific Ltd., UK). Transmission



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electron microscopy (TEM) was carried out using a JEM-2100F transmission electron microscopy (Japan Electron Optics Laboratory Co., Ltd., Japan). Nitrogen adsorption-desorption isotherms were measured with a Micromeritics Coulter (USA) instrument. Thermogravimetric analysis (TGA) was performed on a Q5000 thermo-analyzer instrument (TA Instruments Inc., USA), at a linear heating rate of 20 o

C min-1 from 20 to 800 oC. The fire performance of EP and its nanocomposites were

conducted on a cone calorimeter based on ASTM E1354/ISO 5660. Every specimen was exposed horizontally under a heat flux of 35 kW/m2. Microstructures of the residual char and fracture surface were investigated on a PHILIPS XL30E scanning electron microscope (SEM). The structure and components of the residual char of EP composites were evaluated by a LABRAM-HR laser confocal microRaman spectrometer (Jobin Yvon Co., Ltd., France) with an argon laser of 514.5 nm. Thermogravimetric analysis/infrared spectrometry (TG-IR) was carried out on a TGA Q5000 thermogravimetric analyzer, which used a stainless steel transfer pipe to combined with a Nicolet 6700 FT-IR spectrophotometer. The real time Fourier transform infrared spectra (RTFTIR) were conducted using a Nicolet MAGNA-IR 750 spectrophotometer.

Scheme 1. Synthetic route of 3D PZM@Co2P@RGO.


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3. Results and discussion 3.1 Characterization of 3D PZM@Co2P@RGO composites The morphologies of PZM nanospheres, PZM@Co2P and PZM@Co2P@RGO composites were studied by TEM and SEM (Figure 1). The SiO2 cores in SiO2@PZM core-shell structures were etched by HF solution to form hollow PZM nanospheres. Figure 1a shows that the TEM image of the PZM nanoshells has a high electron beam transmitted area, revealing the formation of a hollow nanostructure. The PZM nanoshells are fairly uniform in shape and size with diameters of about 500 nm. After heat treatment of PZM-Co precursors at 800 oC under nitrogen atmosphere, part of surface

polyphosphazene

phase

was

carbonized

into

amorphous

carbon.

Simultaneously, Co2P NPs were formed on the surface of mesoporous PZM nanoshells (Figure 1b and c). In addition, the Co2P components supported on the hollow polyphosphazene nanospheres shows well dispersion (Figure 1d). It is correspondingly due to the coordination behavior between transition metal compounds and polyphosphazene components. Figure 1e and f shows the nanostructure of the 3D PZM@Co2P@RGO composites, in most cases the reduced GO nanosheets are wrapped around the surface of the PZM@Co2P composites, due to the presence of ultrathin and flexible graphene, and the rough PZM spheres play a separation role as nano-spacer of graphene sheets. As shown in SEM image, Figure 1g presents the typical structure of the 3D PZM@Co2P@RGO composites, in which the PZM@Co2P was derived after heat treatment and a thin graphene network formed by hydrothermal reduction. In Figure 1h, the high-resolution TEM (HRTEM) image adopted from single nanoparticle indicates good crystallinity of the Co2P NPs, with a lattice parameter of 0.205 nm, corresponding to the (211) diffraction plane of Co2P.41 The results are consistent with the relevant XRD patterns (Figure 2b). The presence of Co2P NPs and polyphosphazene shells has been confirmed by using EDX, shown in Figure 1i.



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Figure 1. TEM images of the (a) PZM and (b, c, d) PZM@Co2P and (e, f) PZM@Co2P@RGO; SEM image of the (g) PZM@Co2P@RGO; (h) HRTEM image of Co2P and (i) the EDX result of PZM@Co2P@RGO.

FTIR analysis provides crucial structural information of PZM, PZM@Co2P and PZM@Co2P@RGO, as presented in Figure 2a. From the IR spectrum of PZM nanoshells, it can be observed that two distinct peaks at 1500 and 1606 cm−1 are attributed to the absorption of C=C groups in the phenol of ODA units. Besides, The characteristic peaks at 848 and 1203 cm-1 are assigned to the stretching vibration of P–N and P=N groups, respectively. The sharp peak at 1103 cm-1 is attributed to the stretching vibration of the ether bond, and the absorption peak at 3370 cm-1 corresponds to the amino groups in ODA units, are observed in the spectra of all samples. The existence of Co2P NPs can be determined by EDX characterization in the TEM test. The XRD method further confirmed the above results. Figure 2b exhibits the comparable XRD patterns of PZM, PZM@Co2P and PZM@Co2P@RGO. For


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PZM, a broad peak at 2θ value of around 20.0° is assigned to the reflection peak of amorphous polyphosphazene phase. In the XRD pattern of PZM@Co2P and PZM@Co2P@RGO, the characteristic peaks at 2θ = 40.3, 43.3° can be ascribed to the (121) and (211) reflection planes of Co2P, revealing the formation of Co2P NPs on the surface of PZM with hexagonal phase.42 In addition, the diffraction peak at 2θ = 25.3° can be ascribed to the (002) reflection planes of reduced GO, indicating the graphitic structure of the RGO.43

Figure 2. (a) FT-IR spectra; (b) wide-angle XRD patterns; (c) TGA curves; (d) XPS survey spectra of the PZM, PZM@Co2P and PZM@Co2P@RGO.

The thermal properties of PZM, PZM@Co2P and PZM@Co2P@RGO were investigated by TGA in air. The temperatures at which 5% mass loss occurs are defined as the initial degradation temperature (T−5%). As can be seen in Figure 2c, PZM exhibits a one-step weight loss under air. After carbonization and decorating with Co2P, the T−5% of PZM@Co2P is over 526 °C, with a char yield of 54 wt% at 800 °C, indicating that the PZM@Co2P composites possess outstanding thermal stability. For PZM@Co2P@RGO, the T−5% is less than 320 °C, with a char yield of 16



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wt% at 800 °C, due to the presence of unstable RGO in air atmosphere. These results indicate that Co2P exhibits superior thermal stability at high temperature. XPS offers diversified information about surface composition and chemical state of PZM, PZM@Co2P and PZM@Co2P@RGO composites. The XPS survey spectra (Figure 2d) show that the surface of all samples are composed of C, O, P and N elements, the PZM@Co2P and PZM@Co2P@RGO samples exhibit Co elements. Figure 3 presents the high-resolution XPS spectra of Co2P in the P 2p and Co 2p regions, the peaks at 130.0 and 782.3 eV correspond to the P and reduced Co species of Co2P NPs, which are in accordance with binding energies in prior work for P (129.4-129.8 eV) and Co (778.2 eV) in Co2P.44 As can be observed from Figure 3a, the Co 2p XPS spectrum exhibits two typical peaks and the peaks located at 782.3 and 798.0 eV assigned to Co 2p3/2 and Co 2p1/2, respectively. In Figure 3b, the characteristic peak at 130.0 eV can be ascribed to P species of the Co2P NPs, revealing the emergence of trace Co2P. Besides, the other peaks at 133.1 and 135.2 eV are ascribed to the P–C and P–O of the carbon nanomaterials, respectively, suggesting that the Co2P phase was formed in the PZM@Co2P composites. PZM@Co2P@RGO shows similar spectra to PZM@Co2P in the Co 2p (Figure 3c) and P 2p (Figure 3d) regions. Nitrogen sorption isotherm of the PZM@Co2P@RGO composites belonged clearly to type IV curve, and the hysteresis loops implying characteristics of mesoporous structure (Figure 3e).45 The pore volume and surface area values are calculated to be as high as about 0.24 cm3 g−1 and 59.7 m2 g−1. The detailed pore size distribution

calculated

on

the

basis

of the

BJH

equation

exhibits

the

PZM@Co2P@RGO has mesopores with an average pore size of around 1.7 nm (Figure 3f).


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Figure 3. High-resolution XPS spectra of: PZM@Co2P in the (a) Co 2p and (b) P 2p regions; PZM@Co2P@RGO in the (c) Co 2p and (d) P 2p regions; (e) Nitrogen adsorption–desorption isotherms and the (f) pore size distribution curve of the PZM@Co2P@RGO.

3.2 Thermal stability and fractured surface characteristic of EP nanocomposites. For exploring the effect of PZM@Co2P@RGO on the thermal properties of EP, thermal stability of EP nanocomposites was characterized with TGA under nitrogen, the corresponding TGA and DTG curves displayed in Figure 4. The detailed data from TGA curves are shown in Table S1. All of the EP nanocomposites present a one-stage main degradation process (Figure 4a), exhibit the similar decomposition behaviors to bare EP. As can be observed from Figure 4c, with the incorporation of PZM@Co2P@RGO,

the

initial

decomposition

temperature

(T−5%)

of

EP/PZM@Co2P@RGO sample decreased slightly. It is attributed to the earlier



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thermal decomposition of EP triggered by the catalytic action of Co2P NPs and high heat conductivity of graphene. However, the char residues of EP/PZM@Co2P@RGO sample at 800 °C obviously increased compared to that of pure EP (Figure 4c). Furthermore, from the DTG profiles (Figure 4b and d), the maximum mass loss rates of EP/PZM, EP/PZM@Co2P and EP/PZM@Co2P@RGO composites are lower than that of pure EP, revealing the enhanced thermal resistance of the EP nanocomposites.

Figure 4. (a) TGA curves; (b) DTG curves; (c) the TGA curves at 250-400 °C and 500-800 °C regions; (d) the DGA curves at 325-395 °C.

As is well known, the interfacial compatibility between nanoadditives and polymer matrix acts a significant role in improving the integrated performance of composites.

The

freeze-fractured

surface

microstructures

for

EP and

its

nanocomposites were evaluated by SEM. For neat EP, the fractured surface is quite smooth with no feature in Figure 5a. After introducing the PZM@Co2P and PZM@Co2P@RGO, it can be observed from Figure 5b and c that distinct difference between the surfaces of neat EP, EP/[email protected] and EP/PZM@[email protected] nanocomposites, all of them exhibit the rough and large-crinkled morphologies. It reveals that incorporating PZM@Co2P and PZM@Co2P@RGO have a great impact on the fractured surface characteristic of the EP nanocomposites. As stated above, the


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active amino groups on the surface of PZM spheres, greatly decreasing the interfacial tension between PZM@Co2P@RGO and EP matrix, which is the primary cause for strong interfacial adhesion, resulting in the enhancement for properties of EP nanocomposites.

Figure 5. SEM images of the fractured sections of (a) neat EP, (b) EP/[email protected] and (c) EP/PZM@[email protected] nanocomposites under different magnifications.

3.3 Fire properties of EP nanocomposites. Cone calorimeter is a commonly used tool for evaluating combustion behaviors of polymer materials under real-world fire condition. Heat release rate (HRR) and Total heat release (THR) vs. time curves of EP composites are displayed in Figure 6a and b, several crucial parameters, such as the time to peak heat release rate (TPHRR), PHRR, THR, the CO production (COP), the CO2 production (CO2P) and maximum average heat rate emission (MAHRE) values obtained from cone calorimeter are listed in Table S2. Compared to pure EP, the incorporation of 3 wt% PZM@Co2P@RGO brings about a 47.9% maximum decrease in PHRR, a 29.2% maximum decrease in THR, indicating the high flame retarding efficiency of the nanofiller. In particular, apparent decreases in the PHRR of 24.9% and THR of 14.5% were achieved with the introduction of 2 wt% PZM@Co2P into EP. Addition of 2 wt%



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PZM@Co2P@RGO into EP further significantly decreases the PHRR and THR values. As a result, the PHRR and THR of EP/PZM@[email protected] are decreased by 37.8% and 16.6% than those of EP, respectively, revealing the higher flame retardant performance among the same loading samples. Carbon monoxide (CO) is the nonnegligible toxic gas produced from polymers including EP during combustion. Therefore, CO production is also an important factor on assessing the fire safety of polymer materials. CO production (COP) and CO2 production (CO2P) curves of EP and its nanocomposites are shown in Figure 6c and d. As can be seen from Figure 6c that the peak COP of EP is decreased from 0.025 g/s to 0.020 g/s after loading of 2 wt% PZM; however, it brings a maximum reduction of 0.008 g/s only by addition of 3 wt% PZM@Co2P@RGO. PZM working on flame inhibition results in the decrease of CO, in addition, it can be assigned to the physical barrier effect of the graphene nanosheets and the protective char layer originated from synergistic charring effect of the Co2P. Figure 6d indicates that the CO2P values of EP nanocomposites exhibit a similar trend to that of the COP. it can be observed that the CO2P values for all samples are reduced, EP/PZM@[email protected] exhibits a lowest CO2P value among them. The MAHRE values

of

the

EP

nanocomposites

are

decreased

upon

increasing

the

PZM@Co2P@RGO loading. These improvements were attributed to the incomplete combustion of EP and the protective layer of denser char residue.


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Figure 6. (a) HRR and (b) THR vs. time curves; (c) COP and CO2P (d) vs. time curves of EP and its nanocomposites obtained from cone calorimeter.

To further investigate the thermal pyrolysis behavior of EP nanocomposites, the toxic gases released from EP, EP/[email protected] and EP/PZM@[email protected] decomposition were detected by using a TG-FTIR technique. As shown in Figure 7a and c, 3D TG-FTIR spectra are obtained from thermal decomposition process of EP nanocomposites. Several toxic pyrolysis products are remarkably identified by typical FTIR signals. The characteristic peaks of the gaseous decomposition products appear in the regions of 3500–4000 cm-1, 2750–3200 cm-1, 2200–2400 cm-1, 1750–1900 cm-1, 1250–1600 cm-1 and 600–1000 cm-1, there is an obvious reduction in peak value as compared to the 3D images for neat EP and EP/PZM@[email protected]. FTIR spectra of the pyrolysis products for EP and EP/PZM@[email protected] at different temperatures are shown in Figure 7b and d. It can be observed that the FTIR spectra of EP/PZM@[email protected] are similar to pure EP. Several characteristic peaks are ascribed to the main pyrolysis products of EP, such as 1510 cm-1 (aromatic compounds), 1740 cm-1 (carbonyl compounds), 2190 cm-1 (CO), 2360 cm-1 (CO2), 2930 cm-1 (hydrocarbons) and 3650 cm-1 (absorbed water).46 Moreover, the



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EP/PZM@[email protected] composites release the pyrolysis products relative earlier than that of pure EP, revealing that introduction of PZM@Co2P@RGO catalyzes the thermal decomposition process of EP.

Figure 7. Three-dimensional (3D) TG-FTIR spectra of gasified pyrolysis products for (a) EP and (c) EP/PZM@[email protected]; FTIR spectra of the pyrolysis products for (b) EP and (d) EP/PZM@[email protected] at different temperatures.

For the purpose of comparing the change of evolved gaseous products, the absorbance intensities of the representative pyrolysis products for EP composites are shown in Figure 8. Total pyrolysis volatiles of EP/PZM@[email protected] are lower than that of pure EP, EP/[email protected] samples, indicating the lower smoke toxicity. With the incorporation of 2.0 wt% PZM@Co2P@RGO, the maximum absorbance intensity of pyrolysis products are shifted to lower values, including hydrocarbons, carbonyl compounds, aromatic compounds, CO2 and CO, comparing with those of pure EP, EP/[email protected] samples. The decrease in flammable volatiles further contributes to the reduction in heat release and suppression of smoke. Meanwhile, the reduction in CO leads to the reduction in smoke toxicity, which is beneficial for the enhancement of fire safety. The decreased intensity of pyrolysis products is attributed


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to more compact and cohesive char layer as reinforced barrier, retards the escape of pyrolysis products.

Figure 8. Absorbance of pyrolysis products for EP and its nanocomposites vs. time: (a) total pyrolysis products; (b) CO; (c) CO2; (d) hydrocarbons; (e) carbonyl and (f) aromatic compounds.

3.4 Condensed phase flame retardation analysis The real time FTIR (RTFTIR) is performed to further evaluate the thermal degradation process of pure EP and EP/PZM@[email protected] composite. As shown in Figure 9a, it can be observed that the peaks at 3409, 2961, 2870, 1609, 1508, 1459, 1363, 1240, 1180, 1040 and 829 cm-1 are the characteristic absorption peaks of pure EP.47 The absorption peak at 1363 cm-1, corresponds to the C(CH3)2 group. With the temperature increasing from initial temperature to 350 °C, the intensity of the peak gradually decreases and disappears completely over 380 °C, which can be assigned to the release of methyl groups in EP. The peak at 3409 cm-1 nearly disappears at 250 °C, revealing the absorbed water of EP released. When the temperature increases to 380 °C, it can be observed that the characteristic peaks at 2961, 2870, 1609, 1508, 1240, 1180 and 1040 cm-1 disappear, revealing that the main degradation of EP occurs in this stage, in accordance with the TGA results. By contrast, the RTFTIR spectrum of EP/PZM@[email protected] is shown in Figure 9b. The characteristic peaks of EP/PZM@[email protected] are same as pure EP. It can be observed that the intensity of the peaks at 2961, 2870 cm-1, correspond to CH3 stretching vibration, and the peak



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at 1363 cm-1, attributed to CH3 deformation vibration, respectively, gradually decrease from room temperature to 350 °C and disappear nearly over 380 °C. Furthermore, the absorption peak at 904 cm-1, which is ascribed to P–N bond decreases rapidly over 350 °C and then disappears nearly over 380 °C, revealing that P–N bonds in the PZM are stable under heating. The absorption peak at 1295 cm-1 attributed to the C-O-C groups, decreases rapidly over 350 °C and then disappears entirely over 380 °C. It is worth attention that the characteristic peaks at 1609, 1508, 829 cm-1 still exist at high temperature range (over 430 °C), indicating the formation of aromatic structure. The presence of PZM@Co2P@RGO in EP matrix enhances the thermal stability of EP nanocomposites, which is in accordance with TGA results.

Figure 9. Real time FTIR spectra of (a) EP and (b) EP/PZM@[email protected] at different pyrolysis temperatures.

To understand the condensed phase flame-retardant mechanism, the char residues of EP and its nanocomposites from cone calorimeter test were investigated. Figure 10 presents macro-morphologies of the residue chars of selected samples by using digital camera. During the combustion, neat EP was completely burned and a part of char residue was left in Figure 10a. After incorporating 2 wt% PZM@Co2P, the amount of char residue of EP/[email protected] nanocomposites is remarkably increased (Figure 10b). In the case of EP/PZM@[email protected] (Figure 10c), far more than 20 wt% char residue are formed the sample could swell into a foam-like structure with a continuous and compact surface when exposed to fire, corresponding to the TGA results. As well known, only the char layer with higher graphitization degree can


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effectively protect the inner materials exposed to fire. Figure 10 shows the SEM images of microstructures of external residual char for EP, EP/[email protected] and EP/PZM@[email protected] nanocomposites. For neat EP, part of fragile and flaky char layer can be observed in Figure 10d. As for the EP/[email protected] sample, exhibits a higher dense whole char left (Figure 10e). Moreover, it is obviously observed that a continuous and compact char surface is generated after the EP/PZM@[email protected] combustion. Thus, the char surface with a more compact and cohesive layer is conductive to inhibiting the heat and mass transfer between condensed and vapor-phase, thereby enhancing the flame retardancy. Raman spectroscopy was utilized to evaluate the specific components and structure of the char residues. The spectrum for EP (Figure 10g) depicts two bands at 1365 and 1596 cm-1, which are defined as D and G peak, respectively. The area ratio of D to G band (ID/IG) is adopted to evaluate the graphitization degree of the char residue. Relative lower ID/IG value indicates higher graphitization degree.48 The EP/[email protected] (Figure 10h) and EP/PZM@[email protected] (Figure 10i) samples exhibit the similar spectra to pure sample. The value of ID/IG for neat EP is 2.96, whereas the EP/[email protected] and EP/PZM@[email protected] samples exhibit lower value (2.76 and 2.58), respectively, revealing the higher graphitization degree and thermal stability. It can be assigned to the synergistic catalyzing carbonization from respective components (Co2P NPs and PZM) plus the RGO in thermal decomposition process of EP.



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Figure 10. Digital photos of the char residues of (a) EP, (b) EP/[email protected] and (c) EP/PZM@[email protected]; SEM images of the char residues from (d) EP, (e) EP/[email protected] and (f) EP/PZM@[email protected]; Raman spectra of the char residues of (g) EP, (h) EP/[email protected] and (i) EP/PZM@[email protected].

Figure 11 presents FTIR spectra (a) and XRD patterns (b) of the residual char for EP and its nanocomposites after cone tests. As shown in FTIR spectra, the peaks at 1585 and 734 cm-1 is assigned to multi-aromatic structure. EP nanocomposites show the similar char structure as pure EP. For the char residues of EP/PZM2.0, EP/[email protected] and EP/PZM@[email protected], a weak absorption peak at 877 cm−1 appeared, correspond to P–N characteristic absorption of polyphosphazenes, implying the formation of crosslinked phosphorus oxynitride.49 The XRD pattern of neat EP shows a broad diffraction peak at about 23°, corresponds to (002) diffraction of graphite. The XRD patterns for all of EP nanocomposites are similar to that of neat EP, revealing the formation of graphitized carbon. For EP/[email protected] and EP/PZM@[email protected] samples, the diffraction peaks at around 2θ = 25° can be ascribed to the (002) reflection planes of reduced GO, In addition, the characteristic peak at 2θ = 43.3° can be ascribed to (211) reflection planes of Co2P is clearly


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observed, indicating the remaining Co2P phase in the residual char of EP nanocomposites, in accordance with the mechanism proposed in scheme 2. XPS analysis was utilized to provide more detail information about elemental composition of the char residues. The XPS survey spectrum of PZM@Co2P@RGO (Figure 11c) shows that the surface of the sample are found to be composed of C, O, P, N and Co elements (Table S3), the relative content of element C in the char of EP/PZM@[email protected] is higher than that of neat EP, owing to the thermal stability of RGO in high temperature and catalytic charring effect of Co2P NPs. Figure 11d shows the high-resolution XPS spectra of EP/PZM@[email protected] in the Co 2p regions, The Co 2p BE of 797.1 eV is negatively shifted from that of Co metal (798.6 eV) in Figure 3c, revealing the transfer of electron density between P and Co element. Therefore, Co2P plays an important role in redox reaction during combustion of EP nanocomposites. Hence, the Co2P, stable RGO and crosslinked phosphorus oxynitride on the surface of char layer for EP/PZM@[email protected] present high thermal stability and act as a barrier to inhibit mass and heat transfer between condensed and gas phase.



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Figure 11. (a) FTIR spectra; (b) XRD patterns; (c) XPS survey spectra and (d) high-resolution spectra of Co 2p regions of the char residues for EP and its nanocomposites after cone calorimeter tests.

3.5 Flame retardant and toxicity suppression mechanism Based on the analysis aforementioned, the possible mechanism for the flame retardancy and reduced toxicity of PZM@Co2P@RGO in EP matrix is proposed in scheme 2. On one hand, during the earlier stage of combustion, thermal stable RGO nanosheets act as a mass barrier to retard the permeation of flammable gaseous products. Meanwhile, the metal nanoparticles have been reported to be effective additives to reduce toxic products during combustion.50 Co2P may play a role in conversion of CO to CO2 through a redox cycle, and catalytic charring effect of Co2P NPs leads to the formation of P-rich carbonaceous char residue. Then the formed char particles combine with the high aspect ratio graphene sheets together to generate a compact char shield on the surface of inner materials. On the other hand, for PZM, on the basis of the formation of carbonized aromatic networks and crosslinked phosphorus oxynitride during combustion into account, we propose an intumescent flame-retardant mechanism for the polyphosphazenes polymers, due to the PZM@Co2P@RGO could swell into a foam-like structure with a continuous and compact surface when exposed to fire, as shown in Figure 10c. Meanwhile, phosphorus elements in P-N structure have acted as a role in the condensed phase to promote char formation. Overall, it is reasonable to believe that the tripartite cooperative mechanism of the PZM@Co2P@RGO is the main contribution of the enhanced flame retardant property and toxicity elimination for EP nanocomposites.


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Scheme 2. Schematic illustration of mechanism for flame retardancy and toxicity elimination of PZM@Co2P@RGO in flaming EP composites.

4. Conclusions In conclusion, Co2P NPs decorated heteroatom-doped mesoporous spheres were fabricated via the carbonization of cobalt metal precursors and highly cross-linked polyphosphazene hollow spheres. Throughout the procedure, polyphosphazene served as heteroatom precursors and provided phosphorus elements to form transition metal phosphides. Follow by interconnecting with graphene during hydrothermal procedure to generate 3D nanostructure denoted as PZM@Co2P@RGO. This well-designed composite material exhibited multi-functional effect on enhancing integrated performance of EP nanocomposites. PZM@Co2P@RGO was dispersed in EP and formed strong interfacial interaction with matrix. Compared to neat EP, incorporation of PZM@Co2P@RGO into EP matrix decreased the maximum mass loss rate and enhanced the char yield at 800 oC, revealing the enhanced thermal stability. Furthermore, the presence of PZM@Co2P@RGO significantly decreased the PHRR and THR values by 47.9% and 29.2%, respectively; the yield of toxic CO and other volatile products from the EP decomposition apparently reduced, implying a reduced toxicity. The distinct improvement in the fire hazards was primarily attributed to the tripartite cooperative mechanism (catalytic charring performance of heteroatomdoped carbon spheres and catalytic activity of Co2P/C system, physical barrier effect



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of graphene) of the PZM@Co2P@RGO. Our work provides a novel strategy for the design of 3D architectures for achieving their potential applications in polymer composites.

Supporting Information. TGA data for EP and its nanocomposites in nitrogen, cone calorimeter data of EP and its nanocomposites, and XPS data of the residual char of pure EP and EP/PZM@[email protected] nanocomposite.

Acknowledgements The work was financially supported by the National Natural Science Foundation of China (No. 21374111 and No. 51675502), the Anhui Provincial Natural Science Foundation (1608085QE99), and the grants from the Research Grant Council of the Hong Kong Special Administrative Region, China (GRF Project number CityU 11301015 and Theme-based Research Scheme Project Number T32-101/15-R, respectively).


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A 3D Nanostructure Based on Transition-Metal Phosphide Decorated

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