Facile preparation of MXene/BP nanocomposite films with high thermal conductivity and excellent flame retardancy

Abstract

Black phoshrene (BP) nanosheets can compensate each other between the MXene layers, thus preventing the agglomeration of MXene, and forming a more compact microstructure. This study successfully synthesizes f-MXenes non-covalently modified by the strong electrolyte PHMB. A series of MXene/BP nanocomposite films are prepared through the strong electrostatic interaction between f-MXenes and BP nanosheets and verified by FTIR, XRD, and XPS. Cross-sectional SEM imaging further reveals the formation of densely ordered aligned microstructures. Owing to the compensation effect of BP nanosheets between f-MXene layers, more efficient thermal conductivity pathways are constructed. At a low amount (5 wt%) of BP, the in-plane and through-plane thermal conductivity of the 5-M/BP film increased to 12.71 and 0.37 W m⁻¹ K⁻¹, respectively. TGA and MCC results demonstrate that the flexible 5-M/BP film exhibited an ultra-low total release rate (5.8 kJ g⁻¹) and excellent thermal stability (66.74 wt% residual carbon). Considering their improved thermal conductive performance, the 5-M/BP nanocomposite film can be the most promising candidate for thermal management applications.

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New concepts

In this manuscript, many continuous thermal conductivity paths are constructed in a microstructure through strong electrostatic interactions, resulting in nanocomposites with high thermal conductivity. It has been verified that small amounts of BP nanosheets can compensate for each other between the f-MXene layers, which prevents MXene agglomeration and forms a more compact microstructure. This dense and orderly arrangement of microstructures can minimize phonon scattering, significantly improving thermal conductivity and flame retardant ability. MXene thermally conductive nanocomposites have been studied for some time. As is well known, nano-2D materials (including MXenes, BP, and h-BN nanosheets) cannot be molded alone, except for graphene. Therefore, polymers added to thermal conductive nanocomposites are indispensable. This has limitations in improving the thermal conductivity of nanocomposites. This study highlights that positively charged f-MXenes and negatively charged BP can form nanocomposite films with good mechanical properties by adding only a small amount of a crosslinker. Consequently, it demonstrates the significance of rationally designing microscopic nanostructures to enhance the versatility of nanocomposites. Moreover, it has been confirmed that nanofilms can be obtained by combining two-dimensional nanomaterials through electrostatic interactions. This breakthrough discovery opens up more possibilities for the practical application of nanocomposites.

1. Introduction

Nowadays, the new generation of microelectronic devices is developing rapidly.^1,2 As electronic components are used more frequently, heat dissipation problems are becoming more serious. Using high thermal conductivity fillers is the most straightforward and effective strategy to overcome thermal management problems. The phonon transport of ferroelectric materials, ferromagnetic materials, nanomaterials, nanostructures, polymers, and phase change materials should be discussed in depth in the application of thermal management materials.³ Polymers are regarded as ideal materials for microelectronic packaging due to their electrical insulation, corrosion resistance, ease of processing, and good mechanical strength.⁴ However, they have low thermal conductivity. As the dissipation frequency and power density increase dramatically, the excess heat generated by electronic devices cannot be eliminated in time, which directly affects their normal operation. Thermal management issues face serious challenges.⁵

Recently, many researchers have tried to use thermally conductive fillers in the field of photothermal conversion. Photothermal conversion is an advanced, environmentally friendly, and sustainable technology that is widely used in the field of solar-driven evaporation. However, the technology still presents significant challenges due to the low solar energy storage capacity and the low efficiency of heat conversion. Graphene has an atomic layer structure and ultra-high thermal conductivity among most nano-two-dimensional materials. Liu et al.⁶ discussed the application of graphene-based materials in thermally conductive materials by combining theoretical calculations and molecular dynamics simulations and the hydrodynamics from ballistic and diffusive regimes. Chan et al.⁷ created a mushroom-like graphene heterogeneous architecture, and a hydrophilic gradient hydrogel evaporator (GHE) was prepared using PVA, which achieved an ultra-high evaporation rate (3.6 kg m⁻² h⁻¹) under sunlight, greatly expanding the water production capacity. Inspired by the tree transport system, Zhao et al.⁸ constructed a structurally graded aerogel to achieve a two-way transport mechanism for water and salt, which could quickly absorb water and desalinate. The aerogel was prepared by blending a GO-CNT dispersion with PVA by the freeze-drying method. The structure of the aerogel maximizes solar energy absorption and reduces heat loss. An impressive evaporation rate of 1.94 kg m⁻² h⁻¹ was achieved in a 20 wt% NaCl solution for 8 h with no salt accumulation. Dong et al.⁹ fabricated an insulation-radiation-evaporative cooler with anisotropic synergistic execution for the coordinated transfer of heat and water energy. The cooler consisted of a PVA cross-linked network on the inside and a vertically arranged hydrophobic aerogel of h-BN on the outside, achieving a cooling power of 311 W m⁻² at a temperature ∼8.2 °C below ambient temperature.

Recently, 2D transition metal carbon/carbonitrides and nitrides (MXenes) have been regarded as some of the best candidates to enhance the thermal conductivity of nanocomposites owing to their layered structure and inherently high thermal conductivity.^10,11 Compared to the thermally conductive fillers mentioned above, MXene edges have a large number of functional groups and excellent hydrophilicity. Unfortunately, the strong van der Waals interaction–caused self-stacking of MXene nanosheets is inevitable.¹² This defect mainly leads to a serious reduction in phonon transmission efficiency and an unsatisfactory thermal conductivity utilization of MXene.¹³ Many researchers have developed several strategies to prevent MXene self-stacking to improve the thermal conductivity of nanocomposites. Jin et al.¹⁴ prepared PVA/MXenes by the multilayer casting method to obtain PVA/MXene films with an increased thermal conductivity of 4.57 W m⁻¹ K⁻¹ and a good droplet resistance property. Zhan et al.¹⁵ prepared a series of cellulose-based films with a filler (SiO₂@MXene) via the in situ method. The nanocomposite film, owing to the construction of a cross-linked skeleton, achieved high thermal conductivity and excellent flame retardancy. Wu et al.¹⁶ provided a strategy to utilize the strong intermolecular hydrogen bonds between MXene and aramid nanofibers, realizing the fabrication of large-scale, flexible, and thermally conductive nanocomposite films. However, the results so far are still unsatisfactory. It is still challenging to further improve the thermal conductivity of MXene nanocomposites owing to the high interfacial thermal resistance between the nanosheets.

As a new member of the two-dimensional (2D) material family, black phosphorene (BP) nanosheets have attracted much attention owing to their unique bandgap chemical structure and superior charge mobility.^17–20 It has been reported that the thermal conductivity of BP in the monolayer along the zigzag crystallization direction and through-plane direction is about 101 and 5.5 W m⁻¹ K⁻¹, respectively.^21–23 At present, BP has been widely applied in many fields such as optoelectronics, thermoelectric devices, and sensors.^24–27 However, there are few studies on thermal management materials. To a certain extent, all nanosheet materials have some limitations, for example, the above-mentioned MXenes easily self-agglomerate. According to previous reports, the exfoliation BP has the advantages of better size and environmental safety.^28,29 We intend to combine the respective features of MXene and BP nanosheets for hybridization to prepare nanocomposite fillers with better thermal conductivity for thermal management devices.

However, before integrating thermally conductive nanocomposites into thermal management materials, extensive research is required to ensure their thermal conductive performance and other functional properties.^30,31 Based on the above background, this research focuses on synthesizing a hybrid nanocomposite of MXene with BP nanosheets for thermal management materials. The MXene is non-covalently modified by the strong electrolyte polyhexamethylene biguanide (PHMB), and the functionalized MXene (f-MXene) is electropositive. A small amount of BP nanosheets serves as a barrier to prevent the MXene nanosheet self-agglomeration. Furthermore, more high-efficiency thermal conductive pathways are constructed by the electrostatic interaction between f-MXene and BP. Various characterization studies are applied to analyze the effects of the microstructure of the MXene/BP nanocomposites on thermal conductivity and thermal behavior. In addition, the flame retardant and mechanical properties of nanocomposite materials are also studied in detail. The findings might verify the intercalation compensation of BP with MXene nanosheets to improve the thermal conductivity.

2. Experimental section

2.1 Materials

The Ti₃AlC₂ bulk with an average particle size of 400 mesh was obtained from Shandong Xiyan New Materials Tech. Co., Ltd, China. Lithium fluoride (LiF) (AR, 99%), isopropanol (IPA, AR, >99%), and glutaraldehyde (AR, 50% in H₂O) were provided by Aladdin Reagent, China. Hydrochloric acid (HCl, 36%) was bought from Sinopharm Chemical Reagent Company, China. Polyhexamethylene biguanide (PHMB, 99%) was purchased from Macklin Reagent, China. The black phosphorene (BP) bulk (99.9%) was provided by Black Phosphorene Technology Service Co., Ltd, China. Deionized (DI) water was used for purification in this study. All chemicals were used without further purification.

2.2 Preparation of functionalized Ti₃C₂T_x (f-MXene) nanosheets

Firstly, the delaminated MXene nanosheet was prepared by a previously published method.³² LiF (1.6 g) was added to HCl (20 mL) and stirred for 5 min to obtain a homogeneous solution. Then, the Ti₃AlC₂ bulk (1 g) was slowly added to the premixed solution in 10 min, and the reaction was continuously stirred for 24 h at a temperature of 42 °C. Purified MXene nanosheets were obtained by repeated washing and centrifugation in DI water until the pH of the supernatant was ∼6. The collected sediment was re-dispersed in DI water by sonication and centrifuged for 30 min to obtain the MXene aqueous solution of 5 mg ml⁻¹. Then, 20 ml of a PHMB solution (1 mg ml⁻¹) was added to 20 ml of an MXene aqueous solution, and the mixture was magnetically stirred at room temperature for 6 h to ensure PHMB contacted the MXene nanosheets. The mixture was named f-MXene.

2.3 Preparation of black phosphorene (BP) nanosheets

BP nanosheets were prepared by a top-down liquid-phase exfoliation method.¹⁷ First, 0.5 g of the BP bulk was ground to the smallest particle size powder. Then, the BP powder was dispersed in 500 ml of an IPA solution and sonicated by an ultrasonicator in an ice-water bath for at least 9 h. The residual BP bulk and IPA were removed utilizing vacuum-assisted filtration. The collected sediment was left overnight in a vacuum oven at 40 °C to obtain dried BP nanosheets. During synthesis, the duration of sonication will directly affect the size of the BP nanosheets.

2.4 Preparation of MXene/BP nanocomposite films

Different mass fractions of BP to MXene (MXene/BP) were prepared and denoted as 1 wt% MXene/BP (1-M/BP), 3 wt% MXene/BP (3-M/BP), and 5 wt% MXene/BP (5-M/BP). In brief, different weight percentages of BP were added to 20 ml of the f-MXene dispersion, which were 1, 3, and 5 wt%. In the meantime, 3 ml of a glutaraldehyde solution was slowly added to the MXene/BP mixture and magnetically stirred at room temperature for 3 h. Then, the mixture was made homogeneous by 30-min ultrasonic treatment. The MXene/BP nanocomposite films were obtained by the vacuum-assisted filtration (VAF) method. As a reference, we also prepared the f-MXene film in the same way. Details of the preparation of nanocomposite films are provided in Table S1.

2.5 Characterization

The microscopic morphologies of MXene and BP nanosheets were observed by atomic force microscopy (AFM, Dimension Edge). The morphological features of the as-prepared MXene/BP nanocomposite films were characterized by scanning electron microscopy (SEM, Zeiss Sigma 330) and high-resolution transmission electron microscopy (HR-TEM, G2F20 S-TWTN-TEM, an accelerating voltage of 400 kV). The morphological analysis and corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mappings of the samples were collected by SEM at an acceleration voltage of 10 kV. The chemical structures of the samples were recorded and analyzed by FTIR spectra using a Bruker TENSOR27 Fourier transform infrared spectrometer (Thermo Fisher). X-ray diffraction (XRD) spectroscopy was performed by using a Cu target (λ = 1.5406 Å) at a scan rate of 5° min⁻¹ from 5 to 90° (Bruker 8, Germany). X-ray photoelectron spectroscopy (XPS) was characterized by Thermo SCIENTIFIC K-Alpha from Thermo Fisher.

The thermal conductivity (λ, W m⁻¹ K⁻¹) was obtained by multiplying the thermal diffusivity (α, mm² s⁻¹), heat capacity (C_p, J g⁻¹ K⁻¹), and density (ρ, g cm⁻³). eqn (1) is as follows:

λ = α × C_p × ρ (1)

The in-plane and through-plane thermal diffusivities (α) were measured by the laser flash method at room temperature (Netzsch, LFA467). The density (ρ) was measured by the Archimedes method (Sartorious, Quintix224-1SKR). The specific heat (C_p) was measured by differential scanning calorimetry (METTLER DSC1). The surface temperature of the nanocomposite films was recorded by a thermal imaging camera (FLIRONE PRO), which was attached to the LED chip by thermal grease under the action of direct current and 12 V voltage. Thermal conductivity enhancement (TCE) was also explored to evaluate the in-plane thermal conductive properties of the MXene/BP nanocomposite films. Eqn (2), for calculating the thermal conductivity enhancement efficiency (TCE) of nanocomposites, is as follows:

where λ_com is the thermal conductivity of the composite, and λ_m is the thermal conductivity of the matrix material. w_com is the mass fraction of the filler in the MXene/BP nanocomposite film.

The thermal stability of f-MXene and nanocomposite films was investigated by thermal gravimetric analysis (TGA, TG 209F3 Netzsch, a heating rate of 10 min⁻¹ from 30 to 800 °C, nitrogen (N₂) atmosphere). The fire resistance test was performed by placing the sample on a lighter, and the video was recorded simultaneously by a smartphone. The combustion behaviors of the samples were further analyzed by a microcombustion calorimeter (MCC-2, GOVMARK) at a heating rate of 1 °C s⁻¹ from 80 to 750 °C under a N₂ (80%) and O₂ (20%) atmosphere. The tensile test of the samples was carried out on a universal material testing machine (CMT4503, MTS) at a tensile speed of 1 mm min⁻¹. Each sample was cut into a strip of 6 mm × 30 mm and tested five times.

3. Results and discussion

3.1 Morphology and structure of MXene and BP nanosheets

In this study, MXene and BP nanosheets were prepared by well-reported methods, respectively.^17,32 MXene nanosheets with a typical structure were obtained by a traditional etching and ultrasound-assisted delamination approach, as shown in Fig. 1(a). The AFM image of the prepared MXene shows that the length is approximately 1 μm (Fig. 1(b)). The surface SEM and AFM images of the few-layer BP nanosheets prepared by liquid-phase exfoliation are shown in Fig. 1(c) and (d). BP nanosheets have a thickness of about 7.2 nm, about 2–3 layers, and an average size of 80–100 nm. Morphological analysis shows that the MXene and BP nanosheets with few layers have been successfully prepared.

Fig. 1 (a), (b) SEM and AFM images of MXene nanosheets. (c), (d) SEM and AFM images of BP nanosheets.

XRD and Raman spectroscopies were further used to characterize the chemical composition and structure of MXene and BP nanosheets. Fig. 2(a) provides the XRD spectra for analyzing changes in the crystallographic structure of MAX phases and MXene nanosheets. Compared with MXenes, after the selective etching treatment, the diffraction peaks of MAX characteristic peaks (0 0 4), (1 0 1), (0 0 8) and (1 0 4) disappear at 19.1°, 34.1°, 38.9° and 41.7°, respectively, indicating that all Al elements in the MAX phase have been eliminated.³³ Besides, the interplanar distance of MXene is 0.24 nm, in good agreement with the (0 2 0) crystal plane of MXene.³⁴ In Fig. 2(b), the diffraction peaks observed at 17.0°, 26.6°, 34.3°, 35.1°, 43.4°, and 52.5° correspond to the (0 2 0), (0 2 1), (0 4 0), (1 1 1), (1 3 1), and (1 1 2) crystal planes of BP nanosheets, respectively.^18,22,23 The Raman spectra of MXene, BP, and BP/M are exhibited in Fig. 2(c). The three characteristic peaks observed at 358.4, 431.9, and 459.2 cm⁻¹ are identified as A_g¹, B_2g, and A_g² Raman modes on the surface of BP, respectively. The appearance of A_g¹ (154.4 cm⁻¹), E_g (380.8 cm⁻¹), and E_g (621.0 cm⁻¹) modes is assigned to the characteristic peaks of MXene.²⁴ The MXene and MXene functionalized by PHMB were analyzed by zeta potential analysis, as shown in Fig. 2(d). The zeta potentials of MXene before and after modification are −23.3 and 50.1 mV, respectively. MXene has a stable negative ζ potential owing to a large number of hydrophilic hydroxyl groups (–OH) on the surface.^35,36 PHMB ionizes in aqueous solutions and is strongly electropositive.^37–39 The non-covalent bonding of MXene by a large number of positively charged groups of PHMB makes f-MXene electropositive. The zeta potential of BP is −6.19 mV because −OH is adsorbed on the surface of BP, making it negatively charged, during the fabrication process of exfoliated BP in the IPA solvent.²¹ The results of the zeta potential analysis confirm the successful preparation of f-MXene. During electrostatic self-assembly, BP and f-MXene are bonded together by electrostatic adsorption.

Fig. 2 (a) XRD spectra of MAX and MXene. (b) XRD spectra of MXenes, BP and 5-M/BP. (c) Raman spectra of MXenes, BP and 5-M/BP. (d) Zeta potentials of PHMB, MXenes, f-MXene, BP and the 5-M/BP nanocomposite.

3.2 Morphological and structural characterizations of MXene/BP nanocomposite films

The nanocomposite films of BP nanosheets with different contents were prepared by the electrostatic interaction between BP nanosheets and functionalized MXene (f-MXene). The digital image of all samples is shown in Fig. 3(a). To visualize the morphology and microstructure of the MXene/BP nanocomposite film, SEM and HR-TEM characterizations were systematically carried out. As shown in Fig. 3(b), a small amount of BP nanosheets is distributed in the surface-sectional SEM image of the 5-M/BP nanocomposite film, and the related elemental distribution is shown in Fig. 3(c). Additionally, the HR-TEM image and corresponding selected area electron diffraction (SAED) pattern of the 5-M/BP exhibit both crystal and amorphous characteristics, as indicated in Fig. 3(d) and (e). It can be seen that the BP nanosheets are dispersed on the surface of the MXene nanosheets with an average size of 80–100 nm. In Fig. 3(f), the lattice fringe distance of 0.26 nm with the (0 4 0) crystal plane of BP confirms the existence of the BP material.^18,19 Therefore, these BP nanosheets facilitate cross-intercalation assembly with MXene nanosheets, interacting to form thermal conduct pathways.

Fig. 3 (a) The digital image of all the samples. (b) The surface-sectional SEM image of 5-M/BP. (c) The distribution of phosphorus (P), titanium (Ti), carbon (C) and nitrogen (N) elements in the 5-M/BP EDS mapping image. (d) HR-TEM image of 5-M/BP. (e) The magnified view of the BP nanosheet in the 5-M/BP HR-TEM image. (f) The SAED pattern of the BP.

The XRD pattern of the 5-M/BP is shown in Fig. 2(b), and it is observed that all the characteristic peaks of MXene are also observed in the 5-M/BP nanocomposite, which indicates that the hybridization of BP has little impact on the crystal structure of MXene. The new peaks observed in 5-M/BP at 17.0 and 34.3° are in accordance with the (0 2 0) and (0 4 0) planes of BP, respectively. This phenomenon is consistent with the HR-TEM image of the 5-M/BP nanocomposites (Fig. 3(f)). In the Raman spectrum of the 5-M/BP nanocomposite material, two prominent peaks at E_g (421.3 cm⁻¹) and E_g (622.5 cm⁻¹) redshift and become broader as a result of the successful hybridization of BP nanosheets.^40,41 Therefore, the aforementioned results confirm that BP and MXene can form a nanocomposite by electrostatic self-assembly without damage to their crystal structures. Additionally, the zeta potential of the 5-M/BP nanocomposite is 84.1 mV (Fig. 2(d)).

To further elucidate the interaction between f-MXene and BP nanosheets, XPS was also used to study the chemical bonding of MXene/BP nanocomposites. For comparison, the full XPS survey of raw MXene identifies the presence of only C, Ti, and O elements, while the spectra of the 5-M/BP nanocomposite film indicate the successful introduction of P and N elements (Fig. 4(a)).^37,38 Thereinto, the characteristic peak of N 1s is due to the amine group in PHMG.³⁹ The major feature peaks centered at 531.8, 530.6, and 529.3 eV are observed in the high-resolution O 1s XPS spectrum of MXene (Fig. 4(c)), corresponding to C–Ti–(OH)_x, C–Ti–O_x, and Ti–O, respectively.¹⁵ Because of the electrostatic interaction between f-MXene and BP nanosheets, a new characteristic peak appears in the O 1s spectrum of the 5-M/BP nanocomposite film, which is assigned to Ti–O–P at 531.2 eV.^42,43 Meanwhile, after the hybridization of BP nanosheets, the characteristic peaks of C–Ti–O_x and Ti–O are shifted to lower binding energies, and the intensities are greatly enhanced.⁴⁴ These phenomena verify the formation of strong interactions between f-MXene and BP nanosheets in the nanocomposite film.

Fig. 4 (a) XPS wide-scan spectra of MXenes and 5-M/BP. (b) Three-dimensional XPS wide-scan spectra of MXenes, 5-M/BP and burned 5-M/BP. (c) O 1s spectra of MXenes and 5-M/BP. (d) Ti 2p spectra of MXenes and 5-M/BP.

To investigate the effect of the internal structure of the MXene/BP nanocomposite film on thermal conductivity, the SEM cross-sectional morphology of the nanocomposite film was observed. In the f-MXene film, a layer-by-layer structure consisting of MXene nanosheets can be seen, while a few void defects are observed (Fig. 5(a)). Notably, the existence of agglomeration and voids caused by the stacked nanosheets prevents the channels of heat transfer, which increases interfacial thermal resistance and reduces thermal diffusion efficiency. In the nanocomposite film, the BP nanosheet achieves the in-plane orientation through electrostatic interactions with f-MXene. As shown in SEM images (Fig. 5(b)–(d)), the BP nanosheets greatly increase the contact area between the nanosheets, building more efficient thermal conduction paths. In this work, the prepared BP nanosheets are 80–100 nm, which are appropriately filled with the MXene intermediate layer. In particular, after the addition of the 5 wt% filler, the images show BP nanosheets and f-MXene nanosheets tightly stacked with each other to construct densely aligned microstructures (Fig. 5(d)). A continuous and ordered pathway of “f-MXene-BP-f-MXene” is formed. This is beneficial to the transmission of phonons and further improves the thermal conductivity of the nanocomposite films. Compared with 5-M/BP films, the lower thermal conductivity of the f-MXene film is due to the presence of some voids in its microstructure, resulting in the lack of heat transfer channels and aggravating the scattering of phonon propagation.⁴⁵ These results demonstrate that the BP nanosheets not only fill the voids in the microstructure but also effectively realize the connection of f-MXene nanosheets and enhance the long-range highly ordered orientation. Thus, BP nanosheets exhibit great superiority as a thermally conductive filler. Furthermore, the distribution of the P element in the EDS image (Fig. 5(b′)–(d′) and (b′′)–(d′′)) reveals that BP nanosheets are uniformly dispersed in f-MXene, and no agglomerates are found. These findings demonstrate the great advantage of BP in making up for MXene's structural shortcomings.

Fig. 5 (a), (b), (c) and (d) The cross-sectional SEM images of f-MXenes, 1-M/BP, 3-M/BP and 5-M/BP, respectively. (a′), (b′), (c′) and (d′) The EDS mapping images of f-MXenes, 1-M/BP, 3-M/BP and 5-M/BP, respectively. (a′′), (b′′), (c′′) and (d′′) The distributions of the P element in the EDS mappings of f-MXenes, 1-M/BP, 3-M/BP and 5-M/BP, respectively.

3.3 Thermal conductivity and application for thermal management facility of MXene/BP nanocomposite films

To assess the effect of BP nanosheets on the thermal conductivity of MXene, the in-plane and through-plane thermal conductivities of different nanocomposite films were measured using the laser flash method. Fig. 6(a) exhibits the in-plane and through-plane thermal diffusivity of f-MXene and MXene/BP nanocomposite films. The thermal diffusivity of the f-MXene film is 5.45 mm² s⁻¹. It is visible that as the filler amount increases from 1 wt% to 5 wt%, the thermal diffusivity of the MXene/BP nanocomposite film increases dramatically from 6.01 to 8.16 mm² s⁻¹, which is due to the fact that BP increases the establishment of an effective thermal conductive pathway. However, as the filler content further increases to 5 wt%, the MXene/BP nanocomposite film exhibits a slight increase in thermal diffusion. The in-plane and through-plane thermal conductivities are shown in Fig. 6(b). The in-plane thermal conductivities of 1-M/BP, 3-M/BP, and 5-M/BP are 8.49, 12.47, and 12.71 W m⁻¹ K⁻¹, respectively. As a control, the f-MXene film possesses a poor in-plane thermal conductivity of 8.76 W m⁻¹ K⁻¹. Interestingly, at a 1 wt% loading, the in-plane thermal conductivity is unexpectedly lower than that of the f-MXene film. The small amount (1 wt%) of introduced BP nanosheets is too small to establish an efficient thermal conductivity path between the layers of the f-MXene nanosheet. In addition, a small number of BP nanosheets are disordered and dispersed between the f-MXene nanosheet layers, further increasing the presence of 1-M/BP nanocomposite film voids. This is consistent with the 1-M/BP cross-sectional SEM image, as shown in Fig. 5(b) and (b′). With the gradual increase in BP introduction, more thermal conductivity pathways are formed between the f-MXene layers of BP nanosheets, and the voids are reduced. Obviously, the thermal conductivity of 3-M/BP and 5-M/BP is improved. With the introduction of 5 wt% BP nanosheets, the in-plane thermal conductivity value is further improved to 12.71 W m⁻¹ K⁻¹, which is 1.6 times higher than that of the f-MXene film. As shown in Fig. 6(c), the largest TCE of the 5-M/BP nanocomposite film is 901%. This is consistent with the results of the fracture morphology of the 5-M/BP film, which reveals that some BP nanosheets can fill the voids of f-MXene.

Fig. 6 (a) In-plane and through-plane thermal conductivities of f-MXenes, 1-M/BP, 3-M/BP and 5-M/BP. (b) In-plane and through-plane thermal diffusion coefficients of f-MXenes, 1-M/BP, 3-M/BP and 5-M/BP. (c) Temperature–time (T–t) curves of f-MXenes and 5-M/BP. (d) The infrared thermal images of the f-MXenes and 5-M/BP nanocomposite films.

Apart from the in-plane thermal conductivity, the through-plane thermal conductivities of the MXene/BP nanocomposite films were also investigated. There is no significant increase in the through-plane conductivity, but the through-plane thermal conductivity has a similar growth trend as the in-plane thermal conductivity. The through-plane thermal conductivity of the f-MXene film is 0.25 W m⁻¹ K⁻¹, while that of the 5-M/BP film is 0.37 W m⁻¹ K⁻¹. The primary reason is that the thermal conductive path constructed between the BP nanosheet and f-MXene in the through-plane direction is limited, and the heat transfer efficiency is inadequate. Details of the in-plane and through-plane thermal conductivities of nanocomposite films are provided in Tables S2 and S3.

Herein, we compared the in-plane and through-plane thermal conductivity of MXene/BP films with previously reported thermal conductivity values for MXene nanocomposites, as shown in Table 1. These results indicate that the 5-M/BP composite films have good thermal conductivity properties. Furthermore, we applied f-MXene and 5-M/BP nanocomposite films on LED chips to more intuitively observe their changes during the heating and cooling processes. The images from the infrared camera record the temperature change in the center of the LED chips, as shown in Fig. 6(d). It also displays the obtained temperature vs time curves (Fig. 6(c)). After heating for 30 seconds, the 5-M/BP nanocomposite film reaches a higher temperature (70 °C) compared to the f-MXene film. Then, after 50 seconds of cooling, the 5-M/BP nanocomposite film reaches a lower temperature than earlier, directly indicating that the heat transfer efficiency of the 5-M/BP nanocomposite film is higher than that of f-MXene. This is attributed to the 5-M/BP film having a high thermal conductivity in the in-plane direction, where heat can be quickly transferred out. Thus, the 5-M/BP film is beneficial for reducing interfacial thermal resistance and improving thermal conductivity during practical applications (Fig. 7).

Table 1 Comparison of the thermal conductivity of previous MXene-based nanocomposites

Matrix	Fillers	Filler	Testing	Thermal conductivity/W m^−1 K^−1		Year^ref.
		Loading	Method	In-plane	Through-plane
Epoxy	MXene	30%	LFA	3.14	0.294	2020^46
Epoxy	Ti3C2Tx@PS	10%	LFA	0.32	—	2023^47
PVA	BNNS/Ti3C2Tx	30%	LFA	8.54	1.74	2023^48
Polyimide	BNNS/Ti3C2Tx	6%	LFA	4.73	0.86	2024^49
Bacterial cellulose	Ti3C2Tx/LM	55%	LFA	10.44	0.67	2024^50
MXene	BP	5%	LFA	12.71	0.37	This work

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Fig. 7 Schematic of the MXene/BP nanocomposite film thermal conductivity mechanism.

3.4 Flame retardant property of MXene/BP nanocomposite films

TGA was used to analyze the thermal stability of f-MXene and 5-M/BP nanocomposites, and the results are shown in Fig. 8(a). Combined with the TGA curve and DTG curve, it can be found that the main thermal degradation process of the 5-M/BP film occurs in the range from 234 °C to 540 °C due to the good thermal stability of MXene and BP nanosheets. For f-MXene nanocomposite films, firstly, due to the presence of stored water and surface groups of MXene nanosheets, a mass loss of about 4.0 wt% occurs below 163 °C, and the possible chemical reactions of MXene at this temperature are shown in reaction (1):

Ti₃C₂Ti → Ti₃C₂ + Ti_x (1)

Fig. 8 (a) TGA and DTG curves of f-MXenes and 5-M/BP. (b) HRR curves of f-MXenes and 5-M/BP. (c) The surface-sectional SEM image of burned 5-M/BP. (d) The distribution of P, Ti, C, and N in the burned 5-M/BP EDS mapping image.

Between 273 °C and 388 °C, a mass loss of about 7.70 wt% occurs. In this temperature range, reaction (1) and the partial oxidation of Ti₃C₂ to TiO₂ occur simultaneously, and reaction (2) is as follows.⁵¹ Among them, these two reactions are shown to compete with each other in this range:

Ti₃C₂ + 3O₂ → 3TiO₂ + 2C (2)

In addition, the thermal degradation of PHMB and glutaraldehyde also occurs in this temperature range. The third stage of thermal degradation occurs between 388 °C and 540 °C, and all Ti₃C₂ is oxidized to TiO₂.⁵² Due to the release of gases, the f-MXene carbon residue is 66.74 wt%. The carbon residue in 5-M/BP is slightly higher than that in f-MXene, where the carbon residue is 69.99 wt%. BP nanosheets can hinder the diffusion of combustible gases on the surface of 5-M/BP nanocomposite films, further improving the thermal stability of 5-M/BP. The above results confirm that the synergistic effect of MXene and BP nanosheets significantly improves the thermal stability of the 5-M/BP nanocomposite film.

Microcalorimetry (MCC) is one of the effective methods to characterize the combustion capacity of materials, and the combustion properties of f-MXene and 5-M/BP were studied (Fig. 8(b)). Detailed data of MCC, including heat release capacity (HRC), total heat release capacity (THR), and peak heat release rate (PHRR), are summarized in Table 2. The heat release rate (HRR) of f-MXene at 436 °C is 82.7 W g⁻¹, which is due to the degradation of the carbon skeleton of MXene at high temperatures.^53,54 Compared with f-MXene, the PHRR of 5-M/BP decreases slightly, and the HRR decreases to 76.5 W g⁻¹ at 434 °C, which is 6.2 W g⁻¹ lower than that of f-MXene. The results further clarify that the introduction of BP nanosheets can effectively inhibit the heat release of 5-M/BP composites during combustion. The results of the MCC are consistent with the data from the TGA. In addition, the SEM images and EDS mapping results (Fig. 8(c) and (d)) of the 5-M/BP film after combustion tests demonstrate that the synergistic effect of MXene and BP nanosheets contributes to flame retardancy. The XPS spectrum was further used to study the flame retardancy properties. The XPS spectra of the 5-M/BP and burned 5-M/BP nanocomposite films are shown in Fig. 9(c). The feature peaks centered at 532.8, 531.4, 530.0 and 529.3 eV are observed in the high-resolution O 1s XPS spectrum of burned 5-M/BP (Fig. 9(a)), corresponding to C–Ti–(OH)_x, C–Ti–O_x, Ti–O–P and Ti–O, respectively. Due to the combustion experiment, the BP nanosheets with better flame retardant properties are wrapped on the surface of f-MXene nanosheets. The P 2p characteristic peak strength of burned 5-M/BP is significantly enhanced, which is mainly attributed to the BP nanosheets with thermal stability (Fig. 9(b)). The above-mentioned data illustrate that the MXene/BP nanocomposite not only effectively improves the thermal conductivity, but also endows the material with excellent flame retardant properties.

Table 2 Microscale combustion calorimeter data of f-MXenes and 5-MXene/BP films

Sample	HRC/J g^−1 K^−1	THR/kJ g^−1	PHRR/W g^−1	Temperature/°C
f-MXene	78	6.2	82.7	436
5-MXene/BP	63	5.8	76.5	434

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Fig. 9 (a) O 1s spectra of 5-M/BP and burned 5-M/BP. (b) P 2p spectra of 5-M/BP and burned 5-M/BP. (c) XPS wide-scan spectra of 5-M/BP and burned 5-M/BP.

3.5 Mechanical properties of MXene/BP nanocomposite films

Favorable mechanical properties are necessary for the practical application of thermal management materials in flexible electronics. Most 2D materials exhibit significant brittleness, such as MXene with a tensile strength of about 15 MPa. In addition, pure BP nanosheets cannot be made into films individually. In this work, glutaraldehyde acts as a cross-linking agent to connect f-MXene and BP nanosheets, making the internal structure more compactly stacked. Significantly, the stability of the compact nanostructure is provided by the formation of amide bonds between the amine groups of PHMB and the aldehyde groups of glutaraldehyde.

Herein, the tensile properties of MXene/BP nanocomposite films were investigated in detail. Fig. 10(a) presents the stress–strain curves of the f-MXene and MXene/BP nanocomposite films, and the corresponding maximum tensile strength is summarized in Fig. 10(b). As exhibited in Fig. 10(a), f-MXene shows a tensile strength of 31.2 MPa. With the addition of BP nanosheets, the mechanical properties of the MXene/BP nanocomposite films decrease slightly. The 5-BP/M film has the lowest tensile strength of 20.7 MPa. Although the tensile strength of 5-M/BP is a bit lower than that of the f-MXene film, the MXene/BP nanocomposite film still maintains acceptable tensile strength for high thermal conductivity thermal management materials. As shown in Fig. 10(c), the 5-M/BP nanocomposite film can be folded, showing good flexibility. Notably, the strain of the f-MXene film is only 1.3%, while the strain rate increases to 1.6% and 1.9% for 1-M/BP and 3-M/BP, respectively. This is attributed to BP and MXene building a long-range ordered structure with significantly enhanced strain.^40,41 Meanwhile, the BP nanosheets on both sides of the f-MXene act as sliding layers, enhancing the tensile strain of the nanocomposite film. Such flexibility and excellent mechanical properties of the 5-M/BP nanocomposite film are suited for application in next-generation electronic products.

Fig. 10 (a) Strain-stress curves and (b) maximum tensile strength of f-MXenes and MXene/BP nanocomposite films. (c) The digital image of the 5-M/BP nanocomposite film.

4. Conclusions

In this study, MXene is functionalized by electropositive PHMB to obtain the f-MXene. Through the strong electrostatic interaction between f-MXene and BP and vacuum-assisted self-assembly, the MXene/BP films were prepared. The in-plane and through-plane thermal conductivities of the 5-M/BP reached 12.71 and 0.37 W m⁻¹ K⁻¹, respectively, thanks to the formation of ordered thermal conductivity pathways by f-MXene and BP. At the same time, a small amount of BP nanosheets made up for the voids and stacking defects in the f-MXene structure, which increased the heat transfer efficiency. The 5-M/BP also exhibited excellent flame retardant properties and good mechanical properties, with an ultra-low THR of 5.8 kJ g⁻¹. This superior combination of high thermal conductivity, flame retardancy, and flexibility in the MXene/BP nanocomposite will benefit the next generation of microelectronic devices.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and SI.

Supplementary information available: Formulation of n-MXene/BP nanocomposite film. Detail information about the in-plane thermal conductivity of the f-MXene film and MXene/BP nanocomposite films. Detail information about the through-plane thermal conductivity of the f-MXene film and MXene/BP nanocomposite films. TEM image of MXene nanosheets. See DOI: https://doi.org/10.1039/d5nh00174a

Acknowledgements

This work was supported by the Shaoguan Science and Technology planning project, China (210906134536798).

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