Three-dimensionally interconnected porous boron nitride foam derived from polymeric foams

Abstract

In this work, for the first time, we report the successful synthesis of three-dimensionally interconnected porous boron nitride foams (BNFs) with a high degree of crystallinity using porous sacrificial polymeric hard templates. Ammonia borane/CTAB solution were infiltrated inside highly porous preforms of poly(styrene-co-divinylbenzene) (PS), poly(acrylonitrile-co-divinylbenzene) (PAN) and poly(ethylhexyl acrylate-co-divinylbenzene) (PEHA) made using a high internal phase emulsion process. These were later subjected to pyrolysis under an ammonia atmosphere at the rather low temperature of 1150 °C for 90 minutes. The obtained products were characterized using X-ray diffraction, Fourier transformed infrared spectroscopy, N₂ sorption analysis, scanning electron microscopy, scanning transmission electron microscopy and high resolution transmission electron microscopy (HR-TEM). The synthesized BNFs closely replicated and retained the open-cellular interconnected microstructure of the polymeric templates. The HR-TEM results revealed the formation of highly crystalline BN stack layers in small domains. The prepared BNF using the PS template showed superhydrophobic behavior which was typical for all of the prepared samples, with a water contact angle of ∼144° and a high adsorption capacity of 1800% for used engine oil.

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1. Introduction

Three dimensional (3D) interconnected porous frameworks such as aerogels, foams, and sponges are interesting final shape structures which exhibit continuously interconnected macroporous structures, large surface area, ultra-light bulk densities and excellent transport kinetics. These structures can be used for various applications such as chemical and mechanical sensing, electromagnetic interference shielding, energy devices and oil adsorption and separation.^1–6 To obtain such frameworks, 3D organic/inorganic porous foams have been usually utilized as hard templates.

Metallic foams such as nickel and copper have been used as templates for the preparation of 3D interconnected porous graphene, MnO₂, MnO₂/graphene and carbon nanotube/graphene composite foams.^7–11 However, as obtained frameworks are usually prepared with CVD or particle assembly on the metallic foams, the final structures are usually composed of macropores rather than mesopores or micropores in the framework.^8,11 Further, the process of metallic template removal is another shortcoming of such processes.

Polymeric foams (PFs) coated with inorganic compounds lend themselves as convenient templates for appropriate replication of their highly interconnected porous structures into the rigid inorganic bodies after being subjected to suitable pyrolysis conditions. Particle-stabilized high internal phase emulsions systems (HIPEs) have already been utilized for the preparation of 3D interconnected porous PFs. HIPEs are commonly defined as very concentrated emulsions, in which the volume fraction of the internal phase is more than 74% of the emulsion volume.¹² The prepared porous polymers with this route are known as polyHIPEs.¹² In this approach, oil phase of the emulsion gradually polymerizes into a bulk porous polymer, while the aqueous phase, initially serving as the liquid droplet template gradually evaporates, leading to the interconnected macropores framework. This procedure was also employed for the synthesis of ZnO, TiO₂, graphene oxide/polymer composite and silica foamy structures.^13–16 3D interconnected porous PFs have been also utilized for the synthesis of porous carbon foams (CFs) after applying appropriate carbonization process, directly.^17,18

Interesting properties of boron nitride (BN) such as low density, oxidation resistance, high thermal conductivity, low dielectric constant and loss and wide band gap have attracted attentions of scientists to develop its applications for high technology devices.^19–22 Various BN morphologies like nanotubes,²³ nanosheets,²⁴ nanoparticles,²⁵ microbelts,²⁶ BN aerogels^27,28 and meso/microporous powders²⁹ have been prepared with different preparation methods. In the case of the latter, fabrication of highly interconnected macro, meso and microporous structures lead to enhanced/fast diffusion and transport behavior of the adsorbed species. In this respect, 3D foamy structures by containing the above mentioned pore structures satisfy those conditions. However, despite many research works focusing on the synthesis of BN powders by various preparation routes, there has been less attention to the synthesis of BNF of highly interconnected pores. For this purpose, previous works have mainly employed hard template approach.

M. Rousseas et al. to obtain a high crystalline BN aerogel via carbothermal reduction of boron oxide in the presence of graphene aerogel at high temperatures (1600–1800 °C).³⁰ This strategy has also been already employed for the synthesis of porous BN nanourchins, nanocages and mesoporous BN powders using porous carbon powders.^31–33 Compared to CVD methods with hard templates such as carbon and nickel foam to make a BNF, carbothermal reduction at high temperature does not need extra washing or treatment for the removal of hard template.³⁰ However, due to reaction between template and boron oxide at high temperatures, the resulting BN product form carbothermal reduction did not retain the original morphology of the carbon templates very well.

Well-organized 3D interconnected BNF was prepared through chemical vapor deposition of ammonia borane (AB) on the surface of hard template (nickel foam) at 1000 °C.³⁴ Further, CFs derived from silica foams have also been employed to obtain BNFs. In this approach, polyborazylene had been infiltrated inside of CFs and followed by pyrolysis of the infiltrated CFs under nitrogen atmosphere which led to the formation of BN–C composite. Then, carbon was eliminated under ammonia atmosphere to form BNF.³⁵ However, despite the successful synthesis of open-cellular interconnected BNF structure based on such approach, the removal of template, the use of toxic/volatile precursors and various processing stages reflect themselves as the major drawbacks for such preparation route.³⁵

The use of sacrificial PFs as the hard template along with an appropriate BN precursor offer a novel solution to overcome such problems for the preparation of the foamy BN for different applications. In this communication, for the first time, we have utilized a new class of removable polyHIPEs templates: poly(styrene-co-divinylbenzene) (PS), poly(acrylonitrile-co-divinylbenzene) (PAN) and poly(ethylhexyl acrylate-co-divinylbenzene) (PEHA) for the synthesis of well-organized 3D interconnected porous BNFs at rather low temperature of 1150 °C.

2. Experimental

2.1. Materials

All chemical reagents were purchased from Merck Co. (Darmstadt, Germany), unless otherwise stated. Styrene (St), acrylonitrile (AN), divinylbenzene (DVB) and 2-ethylhexylacrylate (EHA) were distilled under vacuum to remove the inhibitor and stored at 5 °C before use. Sorbitan monooleate (Span 80), 1,2-dichlorobenzene, potassium persulfate (K₂S₂O₈), benzoyl peroxide (BPO) and calcium chloride dihydrate (CaCl₂·2H₂O) were used without any purification. Polyglycerol polyricinoleate (PGPR, Palsgaard, Denmark) was kindly donated by Pishgaman Pakhsh Sedigh Co. Cetyltrimethylammonium bromide (CTAB), ethanol and ammonia borane complex (97%) were purchased from Sigma-Aldrich and used without any further purification.

2.2. Synthesis of poly(HIPE) foams

The aqueous phase of HIPEs containing CaCl₂·2H₂O as electrolyte and potassium persulfate as initiator were added into the organic continuous phase. The organic phase consisted of monomers, crosslinker, oil-soluble surfactant, initiator (if required) and a porogenic solvent. Table 1 displays the recipes for the preparation of HIPEs using various monomers. The prepared concentrated emulsion was transferred into the mold and polymerized at 60 °C in a circulating oven for 24 h. The polymerized emulsion was then dried at 70 °C for 24 h. The surfactant and unreacted initiator were respectively extracted by methanol and water for 24 h in a Soxhlet apparatus.³⁶

Table 1 Composition of high internal phase emulsions (HIPEs)

Materials	Weight (g)
	Polymeric foam
	PS	PAN	PEHA
Water	90	85	90
CaCl2·2H2O	0.99	0.94	0.99
Potassium persulfate	0.30	0.425	0.425
Styrene	4.83	—	—
Acrylonitrile	—	4.86	—
2-Ethylhexylacrylate	—	—	6.88
Divinylbenzene	1.21	3.64	1.82
1,2-Dichlorobenzene	4.37	6.55	—
Benzoyl peroxide	—	0.425	—
Span 80	2	—	2
PGPR	—	1.5	—

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2.3. Synthesis of BNFs

Typically, 0.2 g of CTAB was dissolved in 4.5 g of ethanol at room temperature. After complete dissolving of CTAB, certain amount of AB was added to the prepared solution followed by stirring at room temperature. The resulting homogeneous solution of CTAB/AB was coated onto polyHIPE foams (PS, PAN and PEHA). After evaporation of the solvent, the products were aged at 75 °C for 16 h in a vacuum oven. The as-prepared composite was then pyrolyzed at 1150 °C for 90 min with heating rate of 1.5 °C min⁻¹ under ammonia atmosphere to remove of carbonaceous products and BN formation in one step. The details of the prepared samples are shown in Table 2.

Table 2 Experimental conditions for preparation of BN foams

Sample	CTAB	AB	Tpolymerization (°C)	PolyHIPE	tpolymerization (h)	Theat treatment (°C)
BN.PS	0.2	0.1	75	PS	16	1150
BN.PS.A	—	0.1	150	PS	2	1150
BN.PEHA	0.2	0.1	75	PEHA	16	1150
BN.PAN	0.2	0.1	75	PAN	16	1150

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3. Characterization

X-ray diffraction (XRD) patterns were recorded using an X'Pert Pro MPD diffractometer (Philips, Cu Kα, 1.54 Å). The infrared spectra of powders were studied by using a Fourier transformed infrared (FT-IR) spectra (PerkinElmer) apparatus. All spectra in the range of 500–4000 cm⁻¹ with a 2 cm⁻¹ spectral resolution were obtained from compressed KBr pellets in which the powder samples were dispersed. Nitrogen physisorption was measured using a Belsorb system at −196 °C. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area (S_BET) using the adsorption branch. Scanning electron microscope (SEM) micrographs were obtained by using a TESCAN VEGA//XMU microscope. The microstructural analyses were carried out using a high resolution transmission electron microscope (HRTEM, JEOL, JEM-2100) operated at 200 kV. The mean size of the cells and windows were calculated approximately from the SEM micrographs of 50 cells or windows using the following equations:³⁷where D̄_n and D̄_w are the number and weight-average cell sizes, respectively and N_i is the number of cells or windows with a diameter of D_i. The polydispersity index (PDI) which is a criterion of the cell or window size distribution within the polymeric foam was also determined for the studied samples.

Contact angles were measured by a contact-angle meter (OCA 15 plus, Dataphysics Co., Ltd) using a 4 μL droplet of water at room temperature. Average values of contact angle were obtained from three measurements per sponge.

In a typical sorption test, a BNF was placed in contact with an organic liquid until the foam was completely filled with the organic liquid, and then was taken out for weight measurement. The saturated foams were weighed quickly to prevent evaporation of the adsorbed organic liquid.

4. Results and discussion

4.1. PolyHIPE template foams

Fig. 1 shows the SEM micrographs of the polyHIPE foams used as polymeric templates. PS and PAN foams were prepared with 1,2-dichlorobenzene as the porogenic solvent. While the continuous organic phase containing monomer, crosslinker and porogenic solvent and oil-soluble surfactant is added to the aqueous phase comprised of water, initiator and electrolyte, the water droplets are gradually entrapped within a thin layer of the organic phase. The use of an optimal amount of oil-soluble surfactant guaranteed the stability of the resulting HIPEs. The electrolyte (CaCl₂) used to adjust the ionic strength of the aqueous phase prevented the water droplets from coalescence through Ostwald ripening. As can be seen in Fig. 1a–d the phase separation induced by the porogenic solvent occurred within the continuous organic phase during the polymerization process resulted in PS and PAN polyHIPEs with rough surfaces, respectively.³⁶ In contrast, the PEHA foam prepared in the absence of 1,2-dichlorobenzene showed a smooth cell wall with no the observable porosity on the wall surface (Fig. 1e and f).

Fig. 1 SEM micrographs of (a and b) PS, (c and d) PEHA and (e and f) PAN foams.

4.2. BNF formation

The prepared foams after pyrolysis were white. The Fig. 2a shows the XRD pattern of the BN.PS derived from AB/CATB/PS hybrid after pyrolysis at 1150 °C under ammonia atmosphere. The pattern shows the existence of two broad peaks at 2θ around 24–26° and 41–43°. These peaks are similar to (0002) and (101̄0) reflections of h-BN structure, respectively.³⁸ This reflects the good crystalline nature of the as-synthesized BN foam. However, rather low intensity and sharpness of (101̄0) peak suggests the lack of long-range order in c direction.²⁸ Thus, the crystalline structure could be assigned to the turbostratic BN (t-BN) phase.²⁸

Fig. 2 (a) XRD and (b) FTIR spectra of the BNF prepared from PS.

The FT-IR spectrum of the BN.PS is displayed in Fig. 2b. The strong peak appeared at 1385 cm⁻¹ can be attributed to the stretching vibration of the B–N bonds, while that peak appeared at 810 cm⁻¹ can be assigned to the out-of-plane bending vibrations of B–N–B bonds.³⁹ The weak bands around 3433 cm⁻¹ can be also assigned to ν(N–H) or ν(BOH) vibrations.⁴⁰ The lack of peaks related to B–C or C–N bonds in FTIR spectrum (C–N: 1254 cm⁻¹, C=N: 1632 cm⁻¹, and C≡N: 2162 cm⁻¹, B–C: 1100, 1200 cm⁻¹) demonstrate the elimination of carbonaceous template materials.^41,42 CHN analysis revealed low carbon content (2 wt%) in the prepared BNF (BN.PS).

4.3. BN foam morphology

The SEM micrographs of the BN.PS foam prepared from the PS polyHIPE support foam are shown in Fig. 3. As shown in the micrographs (Fig. 3a–c), the as-made BNF replicated and retained the open-cellular interconnected polyHIPE foam. Fig. 3c display the rough surface of BNF resulted from PS template surface.

Fig. 3 (a–c) SEM micrographs, (d–f) TEM and HRTEM images, (g) STEM micrograph, (h) B map and (i) N map of the BNF.PS.

TEM and STEM micrographs (Fig. 3d, e and g) reveal the porous structure of BN.PS product. In harmony with SEM images, TEM images show the presence of many pores with different sizes distributed on the surface of BNF. The high porosity of the structures is indeed obvious. In addition to large pores, mesopores and micropores can be seen in TEM images. Micropores are clearly visible on the walls around mesopores in Fig. 3e. HR-TEM characterization of BNF (Fig. 3f) indicates that BN layers are parallel in small domains of 5–7 nm. Distorted layers and discontinuities can be seen in HR-TEM image. The interlayer spacing between adjacent (002) fringes is in the range of 0.34–0.36 nm and changes from one domain to another. However, these observations are larger than the (0002) interplanar distance in bulk h-BN (0.33 nm).⁴³ The ordering type reported for highly porous BN structures is t-BN as an intermediate phase between classical h-BN and amorphous (a-BN) phases.²⁸ The different d-spacing in different points could be related to misorientation of BN stack layers. In fact, lacking long-range order in c-axis is the character of t-BN phase so that some layers are rotated relative to other ones.²⁸

The crystalline nature of the product can be attributed to poly(aminoborane) and borazine formation. D. P. Kim et al. had reported that poly(aminoborane) is graphitizable precursor in formation of BN.⁴⁴ The formation of poly(aminoborane) from AB decomposition was observed when AB was aged at 70–80 °C.⁴⁵ Therefore, poly(aminoborane) formation in the pores of template resulted from pretreatment at 75 °C before pyrolysis could be possibly lead to high degree of crystallinity in the obtained products. Furthermore, the use of AB as a BN precursor in CVD procedures led to high crystalline BN structures. G. Kim et al. obtained h-BN with high degree of crystallinity through low pressure chemical vapor deposition of AB on a platinum foil at 1100 °C.⁴⁶ Further, high crystalline BN layers were grown on graphene layers by borazine-derived from AB.⁴⁷ The formation of highly crystalline structures obtained by CVD methods presumably are due to similar conformation of polymerized borazine and BN.⁴⁸ For the prepared BNFs in this work, the initial precursor sol can penetrate in the macropores and mesopores of the template. As a result of increasing the temperature in pyrolysis program, the borazine could also be formed and trapped in the highly porous template.⁴⁹ The trapped borazine in the highly porous structure of polyHIPE may increase the crystallinity of the product. Fig. 3 h and i revealed the elemental mapping of boron and nitrogen. These images approved homogenous distribution of boron and nitrogen in the sample as well.

Since rapid heating of AB is accompanied with foaming and swelling,⁴⁵ the sample of PS.BN.A was prepared through fast heating of AB infiltrated in PS without CTAB. For this purpose, the PS foam infiltrated with AB heated to 150 °C very fast under nitrogen atmosphere to make a continuous layer of AB decomposition products on the PS walls through AB foaming. BN.PS.A sample did not retain the microstructure of initial PS foam very well compared to BN.PS after pyrolysis at ammonia atmosphere (1150 °C) as shown in Fig. 4a. The prepared BN.PS.A sample also revealed higher shrinkage and it was more fragile to handle compared to PS.BN sample. Mokoya et al. also used AB as a BN precursor and mesoporous silica as a hard template to prepare mesoporous BN.⁵⁰ Porous structure of silica was not replicated by BN even under so long heat treatment under ammonia atmosphere. This could be due to direct pyrolysis of AB without pretreatment.^45,51,52

Fig. 4 SEM micrographs of (a) BN.PS.A, (b) BN.PEHA and (c) BN.PAN.

After successful synthesis of BNF with PS as the porous polymeric template, the PEHA and PAN polyHIPE foams were also utilized to prepare BNFs. The white BNFs were obtained by PEHA and PAN polyHIPE foams. SEM and TEM images in Fig. 4b and S1a and b† demonstrated that the BN.PAN prepared using PAN polyHIPE successfully replicated the microstructure of the polyHIPE foam. Despite dimensional stability of the bulk BN.PEHA, the resulting BN foam did not retain the original open-cellular microstructure of the PEHA foam very well (Fig. 4c and S1c†). The mean size of the cells and windows of the templates and the obtained BNFs are summarized in Table 3. Following pyrolysis, the mean cell and window diameter of the PS foam decreased from 82.17 to 67.35 μm and from 20.62 to 17.04 μm, respectively. The mean cell and window diameter of the BN.PAN foam decreased 4.3% and 15%, respectively. In addition, the size distribution of the cells and windows in both BN foams is almost identical to those of the polyHIPE foams. The smaller pore size of the BN foams can be attributed to shrinkage taken place during pyrolysis at high temperature.

Table 3 Characteristics of polyHIPEs and BN foams

Foam	Cells			Windows
	D̄n (μm)	D̄w (μm)	PDI	d̄n (μm)	d̄w (μm)	PDI
PS	82.17	97.78	1.19	20.62	24.25	1.17
PEHA	9.10	9.74	1.07	3.16	3.90	1.23
PAN	13.47	15.21	1.13	2.42	2.89	1.19
BF.PS	67.35	80.17	1.20	17.04	20.19	1.18
BF.PEHA	Not determinable
BF.PAN	12.89	13.73	1.06	2.08	3.36	1.61

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4.4. Superhydrophobicity and oil absorption

The BN.PS foam showed a low affinity to water. To investigate the wetting behavior of the BN.PS foam, static water CA measurements were performed at room temperature. As seen in Fig. 5a, the water droplets retain a nearly spherical shape on the surface of BN.PS with an approximate contact angle of 144° conforming the superhydrophobic behavior of BNF surfaces. For further investigation of the non-wetting properties, the water droplet was approached and withdrawn by a metallic needle from the surface of BN.PS. As it is clearly seen in Fig. 5b, no visible trace of water was left on the BN.PS surface indicating a very weaker adhesion force between the BN.PS and the water droplet as compared to that between the needle and the droplet.

Fig. 5 (a) Superhydrophobic behavior of the BN.PS. The inset show contact angle of BNF, (b) withdrawal of water droplet from the BN.PS surface, (c) and (d) engine oil adsorption by BN.PS.

According to literature, BN films displays the hydrophilic behavior due to ionic nature and interaction of the B–N bond with polar water molecules.²⁸ However, superhydrophobic behavior has been observed owing to surface chemistry and surface roughness in BN nanostructures. Boinovich et al. found an increase in the water contact angle on BN film with adsorption of organic pollutant from air.⁵³ Recently, Zettl research group verified this phenomenon for the appearance of superhydrophobicity in BN aerogel.²⁸ Furthermore, surface roughness with a certain size leads to air entrapment between the droplet and solid surface. Such an air pocket can lead to superhydrophobicity.⁵⁴ A similar behavior could also be expected for the superhydrophobicity in BNF prepared in this study as well.

When the superhydrophobic BN.PS was immersed in water, it could easily float on the surface of water. By application/removal of an external force, the foam refloats on the surface of water. This implies that the superhydrophobic BN.PS foam can be used to remove organic pollutants or solvents on water surface or for separation of oil/water mixtures. Further, it was reported that porous BN nanosheets could adsorb organic solvents and oils from the water surface.⁵⁵ To examine the adsorption capacity of BN.PS foam prepared in this work, the used engine oil, cyclohexane, ethanol, gasoline, ethylene glycol, glycerin, toluene, n-propanol and dichlorobenzene were utilized. The BNF quickly adsorbed used engine oil from the surface of water (Fig. 5c and d and ESI,† Movie). Once the BN.PS foam was dropped on the oil–water surface, it immediately adsorbed the oil engine and became dark brown. Based on various experiments run for the prepared BNF samples, it was confirmed that samples can approximately absorb up to 19 times of their own weight by used engine oil. As can be seen, the BN.PS still float on the surface of water. Although the adsorption capacity was lower than that of BN nanosheets,⁵⁵ the BN.PS does not suffer from the main drawback of nanosheets, i.e. powders skimming from the water surface.

The prepared BN.PS showed reasonable adsorption capacities for other oils and organic solvents. As shown in Fig. 6, used engine oil, cyclohexane, ethanol, gasoline, ethylene glycol, glycerin, toluene, n-propanol and dichlorobenzene 600 to 1900 wt% (relative to the BNF dry weight). The use of BNF seems to act as a container to pull organic compounds in its highly porous structure with high surface area (180 m² g⁻¹, S2†) compared to conventional BN powders. The high adsorption capacity of BNF can be due to capillarity effects and filling of the interconnected pores in the highly porous BNF. This structure with abundant pores in different sizes not only leads to a high surface roughness, but also provides enough sites to adsorb large amounts of oil.

Fig. 6 Adsorption capacity of the BN.PS foam.

5. Conclusion

New polymeric templates, poly(styrene-co-divinylbenzene) and poly(acrylonitrile-co-divinylbenzene) polyHIPE foams were successfully used for the synthesis BN foams. TEM and SEM observation studies demonstrated the 3D interconnected porous structure of the obtained BNFs products from polyHIPEs templates. The prepared BNF showed high superhydrophobicity (water contact angle: 144°) and oil uptake. The obtained BNF showed reasonable adsorption capacities for used engine oil, cyclohexane, ethanol, gasoline, ethylene glycol, glycerin, toluene, n-propanol and dichlorobenzene from 600 to 1900 wt% relative to the BNF dry weight.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07751j

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