Jing Linab,
Lulu Xua,
Yang Huang*a,
Jie Lia,
Weijia Wanga,
Congcong Fenga,
Zhenya Liua,
Xuewen Xua,
Jin Zoubc and
Chengchun Tang*a
aSchool of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China. E-mail: huangyang@hebut.edu.cn; tangcc@hebut.edu.cn
bMaterials Engineering, The University of Queensland, St Lucia, QLD 4072, Australia
cCentre for Microscopy and Microanalysis, The University of Queensland, St Lucia, QLD 4072, Australia
First published on 18th December 2015
One-dimensional (1D) boron nitride (BN) nanostructures with a high aspect ratio are of prime interest due to their importance in fundamental research and wide-range potential applications. Herein we developed a facile method for the first synthesis of ultrafine porous BN nanofibers in high purity and a high yield. The method included two-steps, freeze-drying of a hot melamine/boric acid solution and post pyrolysis of the as-obtained products. The extremely rapid cooling of the hot melamine/boric acid solution during the freeze-drying process resulted in the formation of ultrafine precursors, which was the key for the final synthesis of porous BN nanofibers with downsized diameters (20–60 nm) and high aspect ratios. The as-synthesized ultrafine BN nanofibers possessed a high specific surface area and a large pore volume, which could be tuned by the pyrolysis parameters. All of these characteristics make the porous BN nanofibers promising in the applications of water treatment, hydrogen storage, catalyst support, etc. Especially, ultrafast adsorption of methylene blue (MB) in water has been demonstrated using the present porous BN nanofibers as an adsorbent.
In recent years, BN in porous structures have been synthesized in many methods, i.e. replica fabrication using templates, elemental substitution, etc.13–17,19–23 However, the controlled synthesis of porous BN in 1D nanostructures has been seldom achieved. G. Lian et al. prepared BN ultrathin nanofiber network and hollow nanorods via a solvothermal process using NH4BF4 and NaN3 as source materials.18,26 The porous BN nanostructures presented promising adsorption and hydrogen uptake property. However, their harsh reaction conditions and using of toxic precursors will be a great concern when scale up. It is highly desired to develop a green and low-cost method to obtain 1D porous BN nanostructures with well-controlled morphology, high aspect ratios, high purity and large quantities.
In this paper, we report a green two-step method for the first synthesis of 1D porous BN nanostructures in a high yield. The product consists of novel ultrafine 1D nanofibers with high porosity, which is quite distinct from the well-known BN nanotubes. Readily available and inexpensive boric acid and melamine was used as the starting materials. A freeze-drying process has been designed to successfully downsize the melamine diborate (C3N6H6·2H3BO3, M·2B) precursor into ultrafine 1D nanostructures. After a high-temperature pyrolysis process, porous BN nanofibers with diameters of 20–60 nm can be obtained in a high yield and high purity. Especially, a rapid adsorption rate of the present nanofibers for methylene blue (MB) removal in water indicates a prospect for utilization of the present porous BN nanofibers for water treatment.
The final product obtained after a single experimental run is shown in Fig. 2a, indicating a large-scale production. Fig. 2b is a typical SEM image of the as-prepared product (pyrolysis treated at 1100 °C), displaying 1D nanofibers with high purity. The length of the nanofibers varies from several to tens of micrometers. The SEM images verify that the pyrolysis treated product remains similar morphology as the M·2B precursor, as shown in Fig. S1 (ESI†). A typical low-magnification TEM image (Fig. 2c) indicates that the nanofibers are very flexible and display high aspect ratios. Enlarged TEM image (Fig. 2d) shows nanofibers with diameters less than 100 nm usually crystallize as bundles. All of the nanofibers exhibit a high porosity structure, which is quite different from the normally reported BN nanotubes with smooth surfaces.
Detailed TEM analysis further reveals the porous nature of the nanofibers. Fig. 3a and b show a bright-field STEM image and the corresponding high-angular dark-field (HAADF) STEM image of the nanofibers, respectively. It is known that the contrast of HAADF image is strongly dependent on the average atomic number of atoms in the sample. In Fig. 3b, the nanofibers display as bright nanowires with randomly distributed dark points, which represent the micropores in the nanofibers. Fig. 3c and d depict representative TEM images of individual nanofibers, respectively. All of the nanofibers have randomly distributed pores and nanofibers with diameters of 20–60 nm have most frequently been observed. Fig. 3d shows a single nanofiber with downsized diameter of sub-10 nm. High resolution TEM image (Fig. 3e) clearly reveals that the nanofiber has disordered BN layers. The crystallized BN layers interlink and form many disordered cavities inside the nanofiber (marked by the arrows). The corresponding SAED pattern (shown inset of Fig. 3e) further indicates the polycrystalline characteristic of the BN nanofiber.
Fig. 4a–d show HRTEM images of BN nanofibers prepared at different pyrolysis temperatures (from 1000 to 1300 °C), respectively. The BN nanofibers obtained at 1000 °C contain both amorphous and crystalline phases (Fig. 4a). With an increase of the pyrolysis temperature, the crystalline phases gradually become dominant (Fig. 4b–d). In fact, a higher temperature is favourable for the growth of crystalline BN with ordered layers.21 This phenomenon can be confirmed by XRD investigation, as shown in Fig. 4e, in which gradually increased (0002) peaks are observed for the different products. It is noted that the (0002) peaks are located at 2θ = 24.2–25.4°, indicating an average lattice distance of 0.376–0.359 nm. These d0002 values are much larger than that in the BN nanotubes and bulk hexagonal-BN, revealing a turbostratic BN phase of the porous nanofibers. Besides, with increasing the pyrolysis temperature, a slight shift of the (0002) peaks toward to larger 2θ can be observed, reflecting the lattice distance decreases from 0.376 nm (nanofibers obtained at 1000 °C) to 0.359 nm (nanofibers obtained at 1300 °C). Fig. 4f presents the EELS profiles of as-synthesized BN nanofibers. All spectra show the intense B and N K-edges, respectively. The π* peaks (as marked) for B and N is a characteristic of the sp2 bonding configuration. Quantitative analysis confirms that the BN nanofibers mainly consist of B and N with a small amount of C and O. With increasing the pyrolysis temperature, the contents of C and O tend to decrease. The main composition of BN nanofibers prepared at 1300 °C is B and N with a molar ratio of ca. 1.0, and a very small content of C (∼2 at%) and O (<0.1 at%).
Then the specific surface areas and porosity of all pyrolysis treated products are investigated using N2 adsorption and desorption isotherms measured at 77 K, and the results are shown in Fig. 5a. As can be seen, a typical N2 adsorption/desorption isotherm of BN nanofibers synthesized at 1100 °C can be classified as type IV isotherm based on the IUPAC classification, and exhibits a H4 type hysteresis loop. In particular, the measured isotherm shows a rapid increase of N2 adsorptions at relative low pressure range, implying the presence of micropores in the nanofibers. Besides, the H4 hysteresis loop indicates that the BN nanofibers contain narrow slit-shaped mesopores. As the relative pressure increases to ∼1 (P/P0 ≈ 1), a great enhancement of N2 adsorptions can be observed, indicating the existence of macrospores in the product. We anticipate that the macrospores can be attributed to the accumulation of numerous nanofibers, which is in an excellent agreement with our SEM and low-magnification TEM observations. The isotherms for nanofibers obtained at 1000, 1200 and 1300 °C display similar results. The BET specific surface areas of all the samples were calculated using a multipoint BET method, and the results are summarized in Fig. 5b. Compared with BN nanofibers obtained at other temperatures, the 1100 °C-annealed sample displays a much higher specific surface area of 515 m2 g−1 and a high pore volume of 0.566 cm3 g−1. To understand this phenomenon, we calculated the pore size distribution (PSD) using a non-local density functional theory (NLDFT) method.27,28 As shown in Fig. 5c, the BN nanofibers have a broad PSD with the main characteristic pore size of ∼1.4 nm, ∼1.7 nm and ∼2.7 nm. In particular, the 1100 °C-annealed BN sample has the largest micropore volume (as shown in Fig. 5b). Combined with the HRTEM and XRD results, we can conclude that the decrease of specific area and pore volume for high-temperature-samples (above 1100 °C) can be attributed to the ordering or crystallization of layered BN materials at very high temperatures.
In order to further clarify the growth mechanism of the nanofibers, we synthesized porous BN without using the freeze-drying process for comparison. In detail, a naturally cooling process of hot H3BO3/C3N6H6 solution was utilized instead of the liquid nitrogen freeze-drying treatment. The final product consists of BN whiskers with diameters of 5–10 μm (Fig. S2, ESI†). This indicates that the freeze-drying process is the key factor for the ultrafine nanofiber formation. Based on the structural analysis and the synthesis approach, the growth mechanism of porous BN nanofibers can be clarified. Firstly, ultrafine M·2B nanofibers were synthesized by freeze-drying of hot aqueous solution of H3BO3 and C3N6H6 via the reaction: 2H3BO3 + C3N6H6 → C3N6H6·2H3BO3. The M·2B precursor was a hydrogen-bonded structure consisting of inter-linked planar triangular H3BO3 and C3N6H6 molecules.19 Here the M·2B precursors were made by cryodesiccation from the hot aqueous solutions of H3BO3 and C3N6H6, which was quite different from the natural cooling and precipitate drying process for porous BN whiskers.19 The liquid nitrogen freezing of hot H3BO3 and C3N6H6 solution led to a rapid precipitation of numerous M·2B nuclei. Then the M·2B nuclei continuously absorbed newly generated M·2B precipitates to form 1D M·2B nanofibers. The newly formed M·2B nanofibers were frozen in a very short period of time during the extremely rapid cooling process, rather than continuously grow up to micrometer sized whiskers (Fig. S2, ESI†). Then the M·2B nanofibers with hydrogen-bonded layered structure were changed into BN nanofibers by the high temperature pyrolysis treatment. It should be noted that, the specific surface area and pore volume of the samples depended on the pyrolysis temperature. There were two processes taking place during the annealing of precursors at around 1100 °C. One was the release of gaseous groups, such as H2O, NH3, N2 and CO from the M·2B precursors. Another was the crystallization process of the left BN. When the annealing temperature was lower or equal to 1100 °C, the former process was dominated and led to the increase of the number of porosities and specific surface area. When the temperature was above 1100 °C, the crystallization of BN began to speed up. The ordering of layered BN led to merging of the micropores and as a result, the total pore volumes shrunk and the specific surface areas decreased.
Due to their porous nature with high specific surface area, we anticipate that our porous BN nanofibers can be valuable adsorbent for wastewater treatment. Accordingly, we investigate the adsorption performance of our porous BN nanofibers for the removal of MB, one of the commonly used cationic dyes in the textile industry. After adding 50 mg of porous BN nanofibers (pyrolysis treated at 1100 °C) into 200 mL of MB solution with an initial concentration of 20 mg L−1, the UV-Vis spectra of the MB solution collected at different time intervals are shown in Fig. 6a. Fig. 6b shows the corresponding adsorption rate curve. As can be seen, ∼97.6% of MB was removed from the solution within 2 min of reaction, indicating a rapid adsorption capability of our BN nanofibers. Inset of Fig. 6b shows adsorption rate curves of BN nanofibers for the removal of MB with different concentrations. Although the removal of MB decreases with increasing in initial MB concentration, all of the adsorption equilibrium may be achieved in a very short time (almost within ∼2 min), indicating an ultrafast removal rate of the porous BN nanofibers. Fig. 6c shows the adsorption isotherm of MB on the porous BN nanofibers. The experimental data fit the Langmuir model29 well, with the correction coefficient of 0.989, corresponding to complete monolayer covering of the adsorbent (also see Fig. S3 and Table S1, ESI†). The maximum adsorption capacity of porous BN nanofibers for MB is 107.3 mg g−1. The as-prepared porous BN nanofibers display an ultrafast adsorption rate compared with many reported adsorbents, such as grapheme oxide nanosheets,30 MnO2 hollow nanostructures,31 NiO nanosheets,32 porous BN nanosheets,16 and BN hollow spheres.17 The fast adsorption rate can be attributed to the large external surface of the ultrafine 1D nanostructures, the high specific surface areas, and negatively charged BN surfaces.33 When porous BN crystallized in 1D nanostructure, a large amount of micropores tend to distribute on the external surface of the 1D nanofibers,34 resulting in a large number of adsorption sites for dye molecules. So the porous BN nanofibers show a faster diffusion rate compared with bulk or micrometer-sized porous materials. Besides, the strong electrostatic interaction between the cationic dye molecules to the external surfaces of negatively charged BN is also beneficial for their adsorption performances.18 Moreover, we believe that the unique 1D nanostructure with high aspect ratio is also a great advantage for the further assembly of the porous BN nanofibers into filtration membrane for their practical adsorbent applications. We have also studied the adsorption performances of porous BN nanofibers for another kind of dye, i.e. Rhodamine B (RhB). Similarly, an ultrafast adsorption rate was observed (Fig. S4, ESI†). Therefore, the present porous BN nanofibers can be considered as valuable adsorbent for highly efficient remove of hazardous dyes.
Footnote |
† Electronic supplementary information (ESI) available: SEM image of the M·2B precursor, SEM image of M·2B whiskers synthesized via a naturally cooling process, TEM images of porous BN whiskers, isotherm parameters for the adsorption of MB on porous BN nanofibers, and adsorption test of Rhodamine B using porous BN nanofibers. See DOI: 10.1039/c5ra23426c |
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