Facile one-step and high-yield synthesis of few-layered and hierarchically porous boron nitride nanosheets

Abstract

Few-layered boron nitride nanosheets (BNNSs) have attracted increasing research interest in the past few years due to their unique material properties. However, the lack of a reliable scale-up production method is an inhibiting issue for their practical applications. In this work, we report a facile one-step and high-yield method for the synthesis of few-layered and hierarchically porous BNNSs through simultaneous etching and in situ nitridation of calcium hexaboride (CaB₆) by ammonium chloride under moderate conditions. The output of the few-layered BNNSs is as high as 1.4 g with respect to 1.06 g of starting CaB₆ crystals. Transmission electron microscopy and atomic force microscopy characterizations confirm the successful synthesis of few-layered BNNSs, most of which are layered with a thickness less than 3 nm (layer number < 10). The as-prepared BNNSs exhibit a high specific surface area (492–795 m² g⁻¹) and a high pore volume (0.34–0.50 cm³ g⁻¹). In addition, the as-resulted BNNSs exhibit high and tuneable H₂ uptakes from 1.48 to 2.18 wt% at 77 K and at a relatively low pressure of 1.0 MPa, thus guiding the further search of materials for H₂ storage. Our results suggest that the simultaneous etching and in situ nitridation of metallic borides is a facile and effective method for reliable production of few-layered BNNSs with hierarchical porosity for potential applications such as gas storage and functional composites.

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1. Introduction

Motivated by the discovery and extensive research of graphene, two-dimensional (2D) nanomaterials with a thickness below 20 nm have become a hot topic of current research.^1–6 Among these 2D nanomaterials, hexagonal boron nitride nanosheets (BNNSs) have attracted ever-increasing research interest in the past few years, owing to their specific properties as well as promising applications.^7–11 As a structural analogue of graphene, BNNSs were first synthesized in 2004.¹² Since then, it has become one of the most intriguing 2D inorganic nanomaterials.^13–17 BNNSs possess superb chemical and thermal stability, excellent mechanical properties, and high thermal conductivity. Such unique features make BNNSs promising in a variety of applications such as nanodevices, functional composites, hydrogen accumulators, water purification, and electrically insulating substrates perfectly matching the graphene lattices, and so on.^18–25

Compared with the extensive research on graphene, however, the properties and applications of few-layered BNNSs have remained largely unexplored. This may be partially due to the fact that there is a lake of well-defined synthetic method for BNNSs. The synthesis and characterization of 2D nanosheets in general and few-layered BNNSs in particular, are rather new topics in materials science and the related reports are still very limited.²⁶ The pioneering procedure to obtain atomic sheets of hexagonal BN (hBN) was the micromechanical cleavage techniques.^12,27 The mechanically peeled 2D nanosheets often have fewer defects than those produced by chemical methods. Wet chemical methods including exfoliation and reaction methods have been developed to produce few-layered BNNSs. It has been demonstrated that, however, for effective exfoliation of the layered hBN and stabilization of BNNSs, expensive and toxic organic solvents are frequently involved.^28–30 Lin et al. reported that water is effective to exfoliate the layered hBN to nanosheets with the assistance of bath sonication without the use of surfactants or organic functionalization,³¹ however, the low yield and efficiency limited its further application. Alternatively, wet chemical reactions, e.g. “chemical blowing” of thin-walled bubbles, usually require critical conditions such as high temperature (above 900 °C) and/or expensive boron precursors.^32,33 To date, a few so-called bulk quantity methods, including substrate growth by chemical vapour deposition (CVD),^19,34–40 high-energy electron irradiation,^41,42 and ball milling,^43–45 have been developed for the production of BNNSs. However, the large-scale application of BNNSs still suffers from the high cost, low yield (usually in milligram scale), and complicated fabrication procedures. A reliable, simple, green, and low-cost synthesis method is still a great challenge and of significance for further studies and applications.

In this work, we demonstrate a facile one-step method for the synthesis of few-layered BNNSs with hierarchical porosity by using a simultaneous etching and in situ nitridation method using metallic borides (e.g., CaB₆) and ammonium chloride (NH₄Cl) as reactants under moderate conditions. The as-prepared few-layered BNNSs exhibit a high specific surface area and hierarchical porosity nature, and possess enhanced hydrogen (H₂) uptake capacity. These results suggest that the simultaneous etching and in situ nitridation of metallic borides is an efficient approach for the reliable synthesis of few-layered BNNSs with a potential for fine control over number of layers, shape, and structural quality.

2. Experimental

2.1 Synthesis of few-layered and hierarchically porous BNNSs

The reactions were carried out in a sealed stainless steel autoclave of 50 mL in capacity with a quartz liner. Analytical pure grade ammonium chloride (NH₄Cl) and commercial CaB₆ powders were used without further purification. First, 0.01 mol of CaB₆ and 0.12 mol of NH₄Cl were mixed by grinding and loaded into the autoclave in argon atmosphere. The autoclave was sealed and then heated at 600 °C for 1–48 h in a furnace. After that, the autoclave was cooled to room temperature naturally and then opened. The contents were collected, and washed with distilled water for several times. The final products were obtained by drying in vacuum at 80 °C for 6 h. Finally, ca. 2.8 g of white powders were obtained, indicating the high yield of as high as 1.4 g of BNNSs with respect to 1.06 g of the starting CaB₆ raw material.

2.2 Structure and composition characterization

The morphology and structure characterizations were carried out by using scanning electron microscope (SEM, JSM-6330F), transmission electron microscope (TEM, Tecnai-10) and high-resolution TEM (HRTEM, JEM-3000F) operated at 200 kV, and atomic force microscopy (AFM, SPM Dimension 3100). Chemical compositions were determined by energy-dispersive X-ray spectroscopy (EDS, Oxford INCA), electron energy loss spectroscopy (EELS, Gatan 766 2D-DigiPEELS), and X-ray photoelectron spectroscopy (XPS, ESCALAB 250). X-ray diffraction (XRD) data were collected on a MSAL-XD2 X-ray diffractometer (Cu Kα). Raman spectra of the samples were acquired by a Renishaw inVia microspectrometer using a 514.5 nm excitation laser. Fourier transform infrared (FTIR) spectra were recorded with a Nicolet Model Fourier transform infrared spectrophotometer using a KBr wafer.

2.3 Specific surface area, porosity, and H₂ uptake capacity measurements

The nitrogen adsorption–desorption isotherms were recorded at 77 K on a Micromeritics ASAP 2020 analyzer. Specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) equation using the relative pressure range from 0.02 to 0.12. The pore size distribution, combining respective pore volume, and average pore size were calculated based on the nonlocal density functional theory (NLDFT). The H₂ uptake isotherms were taken on a Sievert-type apparatus using H₂ (99.9999%) at 77 K and at a relatively low pressure of 0–1.0 MPa. Before the measurements, all samples were outgassed at 300 °C for 24 h.

3. Results and discussion

3.1 Morphology, structure, and chemical composition of the as-prepared BNNSs

Our synthesis strategy is based on simultaneous etching and in situ nitridation of metal borides under moderate conditions. We first employed commercial CaB₆ crystals as an example owing to their high content of boron element (ca. 62 wt%). The XRD pattern of the employed commercial CaB₆ crystals clearly shows a cubic phase (Fig. S1, see ESI†). It is well known that cubic CaB₆ has a layered structure, in which boron atoms arrange in a three-dimensionally extended layered framework of B₆ octahedral cages, and the Ca atoms occupy the voids between the cages. Each Ca atom has 24 coordinating with boron atoms (Fig. S1 inset, ESI†). It has been reported that at elevated temperatures, oxidation and cracks occurred on the surface of CaB₆ crystals, and the outer oxide layers of CaO and B₂O₃ were peeled off when the reaction preceded inwards.⁴⁶ Therefore, it is expected that the etching and in situ nitridation of the crystals can lead to the formation of layered BN nanostructures as long as certain effective and powerful etching and nitridation agents are available. In the present work, we employed inexpensive NH₄Cl as the etching and nitridation precursor because it can be decomposed easily to generate fresh hydrogen chloride (HCl) and ammonia (NH₃), which act as etching and nitridation agents, respectively.

The morphology and structure of the as-prepared few-layered BNNSs were investigated by SEM, TEM, and AFM. Representative images of the nanosheets shown in Fig. 1 demonstrate a rosette structure, a typical BN morphology derived from CaB₆ precursor at 600 °C for 24 h (denoted as BNNSs-600-24). The low-magnification SEM image shows that the as-prepared sample contains a large number of aligned petal-like nanosheets (Fig. 1a). High-magnification SEM image shown in Fig. 1b further verifies that the petal width is typically in the range of several micrometers. According to the TEM images (Fig. 1c) the nanosheets are vertically aligned, in agreement with the SEM observations. The selected area electron diffraction (SAED) reveals the hBN hexagonal structure of the nanosheets with a layered structure (inset in Fig. 1c). The TEM image of the dispersed nanosheets also indicates the few layered characteristic (Fig. 1d). Fig. 1e shows a high-resolution TEM (HRTEM) image of the as-prepared BNNSs. The edge region of the nanosheets exhibits 3–6 parallel fringes corresponding to 3–6 stacked BN layers (Fig. 1e, indicated by black arrows). It is noteworthy that the average spacing between adjacent (002) fringes is 0.35 nm, larger than the (002) interplanar distance in bulk hBN.⁹ Accordingly, the AFM image and height profile shows the thickness of one nanosheet is ca. 0.6 nm (Fig. 1f). It should be mentioned that some defects can also be observed at the edge and within the nanosheets of the resultant BNNSs, as shown in Fig. 1e and f.

Fig. 1 Morphology and nanostructure of few-layered BNNSs obtained at 600 °C for 24 h. (a) Low-magnification SEM image of the few-layered BNNSs. Scale bar, 2 μm. (b) High-magnification SEM image revealing the rosette structure. Scale bar, 200 nm. (c) TEM image of vertically aligned BNNSs; corresponding SAED pattern indicating a layered BN structure (inset). Scale bar, 200 nm. (d) TEM image of dispersed BNNSs. Scale bar, 100 nm. (e) High-resolution TEM image of the edge folding of a nanosheet with 3–6 BN layer domains highlighted by black arrows. Scale bar, 2 nm. (f) AFM image of a nanosheet and the inserted height profile showing typical size and thickness of a single nanosheet.

Fig. 2a shows the representative XRD patterns of the as-obtained BNNSs and commercial bulk hBN for comparison. All diffraction peaks of bulk hBN and BNNSs coincide well with the standard data (JCPDS card no. 89-1068). The main peak at around 26° corresponds to the characteristic (002) reflection and another reflection in the range of 40–50° covering the unresolved reflections of (100) and (101). The most characteristic change in XRD pattern of BNNSs compared to the bulk hBN is the broadened and less intensive (002) and (100) peaks. The broadening and weakening of the diffraction peaks indicate that the as-prepared BNNSs crystallized in the hexagonal structure with a low crystallinity and exhibited a decrease in structure ordering.⁴⁷ The lowered crystallinity and structure ordering is probably because of the low reaction temperature of 600 °C, leading to a structure similar to randomly stacked hBN layers along the c-axis.⁴⁸ It is noted that the diffraction peak (002) of BNNSs shifted down to 25.21° compared to that of bulk hBN (25.85°), corresponding to an interlayer distance d₀₀₂ change from 0.353 nm to 0.344 nm. To obtain further information on the microstructure of the hBN powders, the sample was examined by FTIR spectroscopy. Fig. 2b shows the FTIR spectra of the as-prepared BNNSs and the bulk hBN. In FTIR, BNNSs show peaks at ca. 1390 and 790 cm⁻¹, corresponding to the B–N stretching and bending modes, respectively. These two peaks are considered to be the fingerprints of sp² bonds within hBN, which is important evidence of the formation of hBN.^30,49

Fig. 2 Phase structure and chemical composition of few-layered BNNSs. (a) XRD patterns of the as-obtained BNNSs and commercial bulk hBN. (b) FT-IR spectrum of the as-prepared BNNS sample compared to that of the commercial bulk hBN. (c) XPS spectra (Gaussian fitting shown in red) of B and N1s core levels. (d) Raman spectra showing the E_2g peak of as-obtained BNNS samples obtained at 600 °C for different reaction time in comparison to that of the commercial hBN crystals.

The chemical composition of BNNSs was investigated by EDS and EELS spectroscopy. The accumulated EDS spectrum is displayed in Fig. S2a,† indicating that the BNNS sample is mainly composed of B and N elements. A quantitative analysis based on the EDS results gave an atomic ratio of B and N of about 0.98. An EELS spectrum of BNNSs-600-24 sample reveals distinct B and N core loss K-edges at 189 and 400 eV, respectively (Fig. S2b, ESI†). The π* and σ* bonds in the fine structure at each core-edge are characteristics of sp²-hybridized states in the layered hexagonal structure.^18,33 The chemical states of B and N were further studied by XPS spectroscopy, as shown in Fig. 2c. The B1s spectrum has a dominant peak (∼189.9 eV) attributable to B–N bonds, which are also verified by the N1s core level (∼397.5 eV). The XPS survey spectrum yields a formula B_1.0N_0.98O_0.01 (Fig. S3, ESI†). The very low oxygen content shows that the BNNSs are insensitive to oxidation upon exposure to air.^50–52 These results indicate the present simultaneous etching and in situ nitridation method can produce BNNSs with a high purity.

Raman spectroscopy was further employed to explore the phase purity of the as-prepared BNNSs. As shown in Fig. 2d, a characteristic peak centred at ca. 1375 cm⁻¹ for BNNSs-600-24 sample can be observed, ascribed to the high-frequency E_2g vibrational mode of hBN.⁵³ Commonly, the Raman shift and FWHM (full width at half maximum) of the E_2g vibration were used to evaluate the crystalline structure and ordering of layered BN structures. It is well-known that high-quality hBN single crystals display an intrinsic E_2g vibrational peak at 1367 cm⁻¹ with the FWHM of 9.1 cm⁻¹.⁵⁴ It has been reported that Raman shift and FWHM are sensitive to the number of layers in hBN.^9,54 With a decrease in this number, to a monolayer, the E_2g mode shifts to a higher frequency (∼1369 cm⁻¹) with a slight increase of FWHM to 10–12 cm⁻¹.⁵⁵ In the present work, the E_2g peaks at 1375, 1373, and 1372 cm⁻¹ of the as-prepared BN samples obtained at 600 °C for 24, 12, and 6 h, as shown in Fig. 2b; and these have FWHM values of ∼15, 13, and 13 cm⁻¹, respectively. An upshift of E_2g vibrational mode to the higher frequency compared to the bulk hBN (∼1369 cm⁻¹) indicates a weaker interaction between interlayers in BNNSs, whereas the obvious broadening of the E_2g peak reflects clear size shrinking of the layered BN phase.⁵⁵ These results indicate that the number of BN layers decreases with an increase of reaction time (from 6 to 24 h), offering a control over the number of BN layers. Compared with the results from CVD and chemical exfoliation methods where most cases yield a high level of impurities, the present simultaneous etching and in situ nitridation technique could produce BNNSs with a high purity.

3.2 Porosity structures of the as-prepared few-layered BNNSs

The BET specific surface areas (SSAs) and porosity structures of the resultant few-layered BNNSs were determined by nitrogen (N₂) adsorption–desorption. As shown in Fig. 3a, the isotherms and hysteresis loops of all BNNS samples obtained for different reaction time belong to the type-I isotherm and H₄ loop, indicating the microporosity and mesopores that are associated with capillary condensation.⁵⁶ The calculated BET SSAs were 795, 561, and 491 m² g⁻¹ for the sample of BNNSs-600-24, BNNSs-600-12, and BNNSs-600-6, respectively. Obviously, the as-resulted BNNSs in the present work exhibit much a higher surface area than that of bulk hBN (about 25 m² g⁻¹). The surface area increases with an increasing reaction time from 6 to 24 h. We consider this to reflect the decrease in the number of layers, consistent with the Raman results shown in Fig. 2d. It should be noted that the BET SSA decreased to 745 m² g⁻¹ when the reaction time was further prolonged to 48 h, which is possibly due to the collapse of the porosity and/or the restack of the BN nanosheets under too long a reaction time. Previously, porous BN with a high SSA has been prepared by template methods.^57–63 For example, mesoporous hBN with a SSA ranging from 140 to 565 m² g⁻¹ were prepared from the template of silica,^58–60 activated carbons,^61,62 and mesoporous carbon (CMK-3).⁶³ Compared to the reported templating methods of porous BN, no template and harmful reagents for the removal of template were involved, and no expensive raw materials were employed in the present work. Therefore, the present work may provide a facile, efficient, cost effective, and green method for the synthesis of few-layered BNNSs with a high SSA and hierarchical porosity.

Fig. 3 (a) N₂ adsorption–desorption isotherms and (b) pore size distribution profiles of few-layered BNNSs obtained at 600 °C for different reaction time (BNNSs-600-6, BNNSs-600-12, BNNSs-600-24, and BNNSs-600-48).

The pore size distribution profiles clearly reveal the as-prepared BNNSs exhibit hierarchical porosity characteristic that consist of micropores with a pore size less than 2 nm, mesopores with a pore size of 2–50 nm, and macropores with a size larger than 50 nm (Fig. 3b). The detailed textural characteristics of the samples are shown in Table S1 (ESI†). All the samples present a hierarchical porosity with a high microporosity (0.15–0.27 cm³ g⁻¹) as well as meso- and macroporosity (0.16–0.29 cm³ g⁻¹), and with an average micropore diameter of about 1.2 nm. The total pore and micropore volumes increase with an increase of the reaction time, and BNNSs-600-24 exhibits the largest total pore volume (0.50 cm³ g⁻¹) and micropore volume (0.27 cm³ g⁻¹). Such a high SSA and microporosity of these BNNSs may be resulted from the removal of adducts and the dislocation structures.

3.3 Formation mechanism of few-layered and hierarchically porous BNNSs

To investigate the growth mechanism of the few-layered BNNSs, a series of time-dependent experiments were conducted. Fig. 4 shows the evolution of the few-layered BNNSs with growth time from 1 to 12 h. In order to obtain high-quality few-layered BNNSs, all these experiments were performed at temperature of 600 °C. As shown in Fig. 4a, the employed CaB₆ crystals have a multilayered morphology with smooth surfaces and particle size of several hundred of micrometers. SEM image of the sample reacted for 1 h shows that the surface of CaB₆ crystals become rough (Fig. 4b), indicating that the surfaces of the layered CaB₆ particles were etched. In a longer reaction time, the layered structures of CaB₆ particles can be retained well (Fig. 4c), but a large number of boron nanosheets and more rough surfaces can be observed from the surface of CaB₆ layered particles (inset in Fig. 4c). With increasing reaction time, the etching of CaB₆ particles, and the size and density of BNNSs increase dramatically.

Fig. 4 SEM top-view images of BNNSs grown for different durations: (a) 0 h, CaB₆ crystalline materials. Scale bar, 10 μm. (b) 1 h. Scale bar, 10 μm. (c) 3 h. Scale bar, 1 μm. Inset in (c) shows the growth of BNNSs begins from the surface of CaB₆ crystals. Inset scale bar, 500 nm. (d) 3 h. Scale bar, 1 μm. (e) 6 h. Scale bar, 1 μm. (f) 12 h. Scale bar, 2 μm.

As the growth begins, the petals of BNNSs are very small platelets and loosely distributed on the surfaces of the CaB₆ particles, and then these petals grow larger and become in contact with each other, generating increasing pressure in between adjacent circular petals which inevitably curve and crack the circular platelets as time passes (Fig. 4d). The etching of the layered CaB₆ particles is undergoing from outer surface to inner, and the growth of BNNSs is roughly perpendicular to the CaB₆ layer (Fig. 4e). After growing for 12 h, the BNNS clusters with certain interspaces were obtained, and the layered CaB₆ particles were destroyed completely (Fig. 4f). After 24 h, a flourishing array of vertically aligned BN nanosheets was formed (Fig. 1a and b). By varying the reaction time, both the BNNSs' features and the amount of open space in the array between BNNS clusters can be tuned.

Based on the above experimental results, a simultaneous etching and in situ nitridation process was proposed to demonstrate the possible formation mechanism of the few-layered and hierarchically porous BNNSs, as described in Scheme 1. The crystal structure of CaB₆ is a cubic lattice with calcium at the cell center and compact, regular octahedrons of boron atoms linked at the vertices by B–B bonds to give a three dimensional boron network, in which each Ca atom has 24 nearest-neighbor boron atoms.⁶⁴ The Ca atoms are arranged in simple cubic packing so that there is a 3D central void among the eight Ca atoms situated at the vertices of a cube.⁶⁵ Scheme 1a shows the layered 3D structure of the primitive cubic phase CaB₆. With an increase of temperature, NH₄Cl is first decomposed into HCl and NH₃ (eqn (1)):

NH₄Cl → NH₃ + HCl (1)

Scheme 1 Schematic illustration of the formation of few-layered and hierarchically porous BNNSs.

Exposing in the mixed gases of NH₃ and HCl, the inter-octahedral boron bonds that link up the compact boron octahedral were first attacked by the HCl molecules, forming CaCl₂ and [B₆H₂] clusters (eqn (2)):

CaB₆ + 2HCl → CaCl₂ + [B₆H₂] (2)

After this interfacial etching process, the reaction proceeds until all CaB₆ in inner particle are consumed. It was reported that the boron clusters had two-dimensional geometry,^66,67 therefore it is reasonable that BNNSs can be produced by in situ nitridation of the [B₆H₂] clusters. As shown in Scheme 1b, the [B₆H₂] clusters in (001)-oriented direction will arrange in a layered structure. The in situ reaction between the formed [B₆H₂] clusters and NH₃ results in the formation of BN (eqn (3)):

[B₆H₂] + 6NH₃ → 6BN + 10H₂ (3)

It should be noted that the etching of Ca atoms by HCl is concurrent with the in situ nitridation of B₆ clusters by NH₃. It is known that the resulted CaCl₂ can react with NH₃, and a series of adducts such as CaCl₂·NH₃, CaCl₂·2NH₃, CaCl₂·4NH₃, and CaCl₂·8NH₃, will be formed.⁶⁸ With the continuous etching and in situ nitridation, parallel layers of BNNSs will be formed upon in situ nitridation of [B₆H₂] clusters. As shown in Scheme 1c, the inner-layers are composed of hBN thin sheets, and the thin sheets have a lamellar structure of honeycomb-like hBN molecules stacking on the (002) planes. During this process, the resultant adducts of CaCl₂ and NH₃ serve as layer separators to insert into the parallel BN layers, and prevent extensive stack of BN layers to bulk BN, resulting in few-layered BNNSs. In addition, the resulted Ca adducts will also occupy the space in the BN layer as well as the interlayers between the nanosheets, and will result in the formation of hierarchical pores after removal of these adducts, which is similar to the previous report.⁶⁹ Experimentally, providing excessive NH₄Cl would favour the sufficient etch and nitridation of CaB₆ crystals and effectively inhibit the stack of BN layers to produce few-layered BNNSs.

To verify the above proposed simultaneous etching and in situ nitridation mechanism for few-layered and hierarchically porous BNNSs, other metal borides (e.g., LaB₆) were employed as boron precursor. Fig. 5 shows the morphology and structure of BNNSs obtained from the simultaneous etching and in situ nitridation of LaB₆ at 600 °C for different reaction time. As shown in the inset in Fig. 5a, LaB₆ also presents a layered structure, which is similar to that of CaB₆. With an increase of reaction time, BNNSs grow along to the vertical direction of LaB₆ surface (Fig. 5a–d). Prolonging the time to 24 h, the etching and in situ nitriding reaction was almost completed, and few-layered BNNSs were obtained, similar to the growth of BNNSs obtained from CaB₆. Vertically aligned BNNSs can be observed clearly form the TEM image in Fig. 5e. The HRTEM image shown in Fig. 5f reveals that the edge region of the nanosheets with 6–9 parallel fringes corresponding to 6–9 stacked BN layers, which is larger than that of the BNNSs obtained from CaB₆. The possible reason is that the formation of the adducts of the generated LaCl₃ and NH₃ from LaB₆ is not as easy as that from CaB₆, and cannot be as effective as to inhibit the stack of BN layers, and thus thicker BNNSs with 6–9 layers were produced. These results indicate that the simultaneous etching and in situ nitridation route presented here is a simple and effective method for the synthesis of few-layered BNNSs.

Fig. 5 (a–d) SEM images of the few-layered BNNSs obtained from LaB₆ and NH₄Cl at 600 °C for different reaction time. (a) 3 h. Scale bar, 300 nm. The inset shows the layered structure of a LaB₆ crystal. Scale bar, 25 μm. (b) 6 h. Scale bar, 1 μm. (c) 12 h. Scale bar, 200 nm. (d) 24 h. Scale bar, 1 μm. TEM (e) and HRTEM (f) of the BNNSs obtained from LaB₆ and NH₄Cl at 600 °C for 24 h. Scale bars in (e) and (f) are 200 and 5 nm, respectively.

3.4 H₂ uptake capacity of the resultant few-layered BNNSs

Owing to the structural characteristics, such as high SSA and microporosity, the as-prepared few-layered and hierarchically porous BNNSs are promising for H₂ storage. The H₂ uptake capacity of the few-layered BNNSs was measured by the volumetric method at 77 K and relatively low pressure of 1.0 MPa. Fig. 6 shows the H₂ uptake isotherms of the as-prepared BNNS samples. With the increase of reaction time, the BNNSs present an improved H₂ uptake capacity (1.48–2.18 wt%), which obviously follows their textural properties, such as SSA, micropore volume, and the number of BN layer. BNNSs-600-24 exhibits the highest H₂ uptake capacity of 2.18 wt%. It is noteworthy that, as for the BNNSs-600-48, the slight decrease of H₂ uptake was observed compared to that of BNNSs-600-24, possible owing to the decrease of microporosity. The H₂ uptake capacity of the as-prepared BNNSs is higher than the reported graphene analogues of BN³² and that of bulk hBN (only 0.30 wt% at 77 K and 1.0 MPa, Fig. 4). It should be mentioned that the H₂ uptake is slightly lower than that of previously reported BN porous belts (2.3 wt% at 77 K and at pressure of 1 MPa) that exhibit a much higher SSA (1488 m² g⁻¹) than the as-resulted BNNSs.⁵⁶ In consideration of the H₂ uptake capacity per SSA, the as-prepared BNNSs manifest higher H₂ uptake efficiency than BN porous belts. This improvement may be attributed to the novel nanostructures including vertically aligned BN nanosheets with a thickness of less than 3 nm, high SSA, and hierarchical porosity. Although the mechanism of hydrogen storage for BNNSs has not been completely understood, the enhanced H₂ uptake capacity presented herein indicates that the resultant few-layered and hierarchically porous BNNSs have potential in the field of gas storage.

Fig. 6 H₂ uptake isotherms at 77 K and 1 MPa of the resultant few-layered BNNSs and commercial bulk hBN.

4. Conclusions

In conclusion, few-layered and hierarchically porous BNNSs were successfully prepared by a simultaneous etching and in situ nitridation method using metallic borides and NH₄Cl as reactants under moderate conditions. The TEM and AFM characterizations confirmed the successful fabrication of few-layered BNNSs, most of which are of a thickness of less than 3 nm. As an example, we used CaB₆ as boron source to fabricate few-layered BNNSs, and the output of BNNSs is as high as 1.4 g with respect to 1.06 g of CaB₆. The surface area of the BN layers increases with a decrease in layer thickness as expected, and the sample with the lowest layer thickness exhibiting a surface area of 795 m² g⁻¹ and a high H₂ uptake capacity of 2.18 wt% at temperature of 77 K and under pressure of 1.0 MPa. The present work provides a facile, high-yield, and green pathway for the reliable production of few-layered and hierarchically porous BNNSs, which is expected to promote widespread application of 2D BN nanomaterials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (U1501242, 21571066, 21401057, and 21371061), the Key Program of Science Technology Innovation Foundation of Universities (cxzd1113), the Science and Technology Plan Projects of Guangdong Province (2012B010200030), and the Natural Science Foundation of Guangdong Province (S2013030012842). M. Z. also appreciates the financial support from the China Scholarship Council.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07455c

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