From ultrathin nanosheets, triangular plates to nanocrystals with exposed (102) facets, a morphology and phase transformation of sp² hybrid BN nanomaterials

Abstract

In this study, an “autoclave route” was applied for the synthesis of 2D sp² hybrid BN nanomaterials. By simply increasing the reaction temperature, a phase and morphology transformation occurs. The turbostratic BN nanosheets (BNNSs, 2–6 nm, 400–500 °C) transformed into the mixed phases (r-BN and h-BN) of triangular nanoplates (BNTPs) in the temperature range of 550–600 °C, and finally to h-BN nanocrystals (BNNCs, >730 °C). Interestingly, the intergrowths of r-BN and h-BN into the BNTP and BNNC plate was obtained in the range of 600–690 °C, which were determined via the selected area electron diffraction (SAED) pattern. Basically, the sizes (side length and thinness) of BNTPs and BNNCs can be tuned by adjusting the reaction temperature and time. The phase transformation temperature was much lower than that of previous reports. The in situ produced Fe nanoparticles, molten Na and autogenic pressure were considered to have positive effects on the phase transformation. Their band gap was estimated to be 5.6–5.8 eV according to the optical absorption spectrum, and Cathodoluminescence (CL) images show that as-synthesized BN nanomaterials had uniform optical properties in the ultraviolet region. Besides, the Ag–BN composite showed excellent antibacterial activities and therefore has potential applications in biomedical and related fields.

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1. Introduction

The successful isolation of graphene from graphite has inspired intense interest in a variety of two-dimensional (2D) nanostructures¹ due to their superior properties, including high specific surface area (SSA), large aspect ratio and sharp open edges.^2,3 Boron nitride (BN) is a structural analogue of carbon; it is an excellent complementary material to their carbon counterpart due to its high chemical stability, high thermal stability, anti-oxidation and biocompatible ability.⁴ It has attracted many researchers to engage in the preparation of 2D BN nanomaterials with a tailored morphology, which can meet varied needs, extending their applications and fine-tune properties. For example, the hydrogen uptake efficiency increases with the surface area of ultrathin BN nanosheets (BNNSs).⁵ BN nanoribbons were considered as ideal gas-sensing materials with a high number of unsaturated edge atoms.⁶

So far, a variety of methods, such as chemical vapour deposition (CVD),^7–9 chemical exfoliation,¹⁰ ball milling¹¹ and the “autoclave route”,^12b have been used to fabricate ultrathin BNNSs. However, the fabrication of uniform 2D BN structures, especially, triangular nanoplates (BNTPs) and hexagonal nanocrystals (BNNCs) was relatively limited. Though the CVD technique can fabricate BN triangular islands on various transition metals substrates (Ru,¹³ Co,¹⁴ Ni,¹⁵ Cu^7,16), the supported islands with limited yield have the difficulty of meeting the increasing sub-gram scale usages, such as catalyst support and polymer filler. Therefore, a larger scale synthetic route is required and aligned BNTPs,¹⁷ BNTPs and BNNCs^12,18 have been reported via the “autoclave route” or “surface segregation”. It is a pity that unsupported BNTPs and BNNCs with controlled size and high yields still remain a challenge.

It is expected that the physical chemical and electronic properties of the BN material are to be substantially affected by the stacking deviations.¹⁹ The two graphite-like BNs give a different stacking sequence of the basal planes (AA′AA′… for h-BN and ABCABC… for r-BN).²⁰ Though pure r-BNTPs²¹ and h-BNNCs^12a have been reported, it is strange that the dispersed metastable samples, especially the intergrowth of r-BN and h-BN BNTPs or BNNCs, have been paid little attention to. Besides, a phase evolution was absent¹⁸ or neglected¹² for the morphology evolution, such as BNNSs to BNTPs and BNTPs to BNNCs. To the best of our knowledge, little work has been focused on the phase and morphology transition between BNNSs, BNTPs and BNNCs. The lack of research hinders the further application of the BN nanocomposite, especially for Ag–BN composites, which have shown to be an excellent catalyst for CO oxidation,²² p-nitrophenol degradation²³ and SERS sensors.²⁴

Herein, an “autoclave route” was applied for the synthesis of sp² hybrid 2D BN nanomaterials. By simply increasing the reaction temperature, a phase and morphology transformation occurred. The ultrathin BNNSs, BNTPs and BNNCs were present in sequence. It was found that the onset temperature from dispersed r-BN to h-BN was no more than 730 °C. The BN nanomaterials with different sizes (side length and thinness) and SSA can be obtained by adjusting the reaction parameters. Those BN nanomaterials with varied morphologies and a different phase could be excellent supports or fillers for various needs. Our results show that the three types of Ag–BN composites have excellent antibacterial activities, therefore they have potential applications in biomedical and related fields.

2. Experimental

2.1. Materials

All the reagents used were purchased from the Sinopharm group Co. Ltd and used without purification. NaN₃ and FeCl₃ were of CP grade, all the other reagents used were of AR grade.

2.2. Sample preparation

In a typical procedure for BNTPs, 30 mmol NaBH₄, 6.2 mmol FeCl₃ and 40 mmol NaN₃ were mixed and placed in a 316L stainless steel autoclave with 20 mL capacity. The sealed autoclaves were heated in an electric stove from room temperature to 600 °C with an increasing rate of 10 °C min⁻¹ and then maintained at 600 °C for 12 h. After that, it was cooled to room temperature naturally. The as-obtained powders were treated with alcohol and distilled water in sequence, and then diluted in hydrochloric acid (2–3 M) overnight to remove the by-products. Then, the as-obtained products were filtered and dried at 80 °C. To investigate the morphology evolution as well as the formation mechanism, a series of experiments was carried out with a similar procedure. The reaction parameters, such as reactants and temperature, are summarized in Table 1.

Table 1 Synthesis parameters for BN with different morphologies

Sample no.	FeCl3 (mmol)	NaBH4 (mmol)	NaN3 (mmol)	Temperature (°C)	Reaction time (h)	Morphology	Average size (nm)		SSA (m^2 g^−1)
							Thinness	Side length
S400	6.2	30	40	400	12	BNNSs	1–6	—	233
S500	6.2	30	40	500	48	BNTPs	5	50	197
S600	6.2	30	40	600	12	BNTPs	10	150	59
S690	6.2	30	40	690	48	BNNCs	∼70	210	21
S730	6.2	30	40	730	12	BNNCs	∼80	250	13

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The 5 wt% Ag–BN composite was prepared by immersing 60 mg BN in a 0.25 mL 2% AgNO₃ solution (1_ethanol : 1_H2O, volume ratio) for several hours, which was then dried at 60 °C for 4 h and finally calcined at 500 °C for 3 h.

2.3. Sample characterization

X-ray powder diffraction (XRD) measurements were collected by a Bruker D8 advanced X-ray diffractometer. The morphology of the as obtained samples was investigated by a field emission scanning electron microscope (FE-SEM, FEI, Sirion200), transmission electron microscope (TEM, Hitachi, H-7650) and a High-resolution TEM (HRTEM, JEOL 2100). A CL spectrophotometer attached to the SU-70 FE-SEM was used. The Fourier transform infrared spectroscopy (FTIR) spectrum instrument was a Bruker VERTEX 70 with the resolution of 4 cm⁻¹. The optical absorption spectrum was recorded by a UV-vis-NIR spectrometer (Shimadzu, UV-3700). TG analysis was carried out using a TA SDT Q600 simultaneous thermogravimetric analyzer in ambient atmosphere. The specific surface area was estimated by the Brunauer–Emmet–Teller (BET) equation based on the nitrogen adsorption isotherm (77 K) using a NAVA 2000e surface area and pore size analyzer.

2.4. Antibacterial assay

The antibacterial activity of the three Ag–BN nanomaterial types was tested using Staphylococcus aureus (S. aureus). The bacteria were cultured under shaking conditions in a beef extract-peptone (BEP) medium at 37 °C for 18 h. Then the cell concentration was adjusted to 10⁵ CFU mL⁻¹. A 20 mg sample of each group was dispersed into 1 mL bacterial suspension in each well of a 24-well tissue culture plate and was shaken at 110 rpm at 37 °C for 12 h. Afterwards, 0.1 mL of the bacterial suspension was taken from each well of the plate and serial dilutions with PBS and spread plate counting method were performed. The blank control was carried out without Ag–BN additives under the same conditions. The antibacterial rate (R) was calculated using the following formula: R = (B − A)/B × 100%, where B is the average number of viable bacteria in the blank control and A indicates the average number of viable bacteria in suspensions with test samples.

3. Results and discussion

3.1. XRD analysis

Similar to that of sp² hybrid carbon materials, the B–N layers of BN materials are held together by weak van der Waals forces. There are two main modifications of graphite-like BN: hexagonal (h-BN) and rhombohedral (r-BN), which are different in the stacking sequence of identical layers. Especially when the degree of three-dimensional (3D) order was low, the B–N layers were roughly parallel, the (10) diffraction peak rather than (100) and (101) was present for the so-called “turbostratic structure” (t-BN),²⁵ and meanwhile the (102) and other peaks also disappeared.^26,27 Fig. 1 shows the XRD patterns of five samples prepared at different reaction temperatures. For the white powders obtained below 500 °C (S₄₀₀, S₅₀₀), we can attribute them as t-BN due to the presence of unsplit (10) peaks around 41°. Evidence for stacks of a few BN layers comes from the broad XRD out-of-plane graphitic reflections based on the Scherrer equation. The (10) diffraction of the BN nanomaterials would split into four peaks at elevated temperatures (i.e. S₆₀₀ and S₆₉₀), two of which can be attributed to the (101)_r and (012)_r reflections of r-BN (JCPDS card no. 45-1171, as indicated by the arrows in Fig. 1) and the other two peaks were attributed to the (100)_h and (101)_h reflections of h-BN (JCPDS card no. 34–0421). Therefore there was a transition from t-BN to the mixed phase of r-BN and h-BN with the temperature increase. From Fig. 1 we can see that the full width at half maximum (FWHM) of the (002) and the intensity of the (101)_r and (012)_r decreased with the increase of reaction temperatures, indicating that the grain size enlarged along the c-axis and a reduced content of the r-BN in the BN samples. Finally, when the reaction temperature was above 730 °C the r-BN could not be detected.

Fig. 1 Typical XRD patterns of the BN samples obtained at difference temperatures. t-BN (S₄₀₀ and S₅₀₀), mixed phases of r-BN and h-BN (S₆₀₀ and S₆₉₀) and pure h-BN (S₇₃₀). The standard JCPDS cards of h-BN (34-0421) and r-BN (45-1171) are shown at the bottom.

3.2. FTIR spectra

The FTIR spectra of the as-obtained samples are shown in Fig. 2. Two obvious absorption bands located around 1390 and 810 cm⁻¹ can be attributed to the in-plane B–N stretching vibrations and B–N–B out-of-plane bending vibrations, respectively. The peaks centred around 3469 cm⁻¹ can be ascribed to the stretching modes of the H–N–H or O–H groups.²⁸ The O–H group could be introduced during the treatment. According to Li et al.'s opinion,¹⁸ the nitrogen atoms of BN at the edge may bond with hydrogen atoms when the synthesis environment is rich in hydrogen. Since the N–H bond is weaker than the B–N bonds, the breaking of N–H bonds would happen at high temperatures. Obviously, the band got weaker as the temperature increased, indicating that the structural defects of BN have decreased.

Fig. 2 IR spectrum of BN samples obtained at different temperatures.

3.3. Morphology and structural analysis

Fig. 3 shows the typical TEM images of as-obtained S₅₀₀ and S₆₀₀ samples. The typical TEM image of S₅₀₀ is shown in Fig. 3a, BNTPs with side length around 50 nm can be obtained at 500 °C. By measuring over 100 plates, the average thinness of the plates was estimated to be 5 nm. Though the supported BNTPs or islands have been synthesized by the CVD method, little progress has been made for the larger scale synthesis of unsupported BNTPs with such a small size. When the reaction temperature was 550 °C, the small size BNTPs can also be obtained at 12 h. However, the BNTPs grow and their average side length was up to 150 nm at 600 °C, and their yield was up to 95% (see Fig. SI 1a–e† and Fig. 3b).

Fig. 3 TEM (a–c) and HRTEM (e) images of S₅₀₀ (a), S₆₀₀ (b–e), the SEAD pattern was recorded for the BNTPs in (c). (e) The d-spacings of 0.33 nm and 0.21 nm correspond to the (003)_r and (011)_r planes.

Fig. 3c is the TEM image of an upright BNTP whose thinness is around 10 nm. Its SEAD pattern (Fig. 3d) was a bit confused and can be indexed in the direction of either [1−10]_r or [1−10]_h. However, the HRTEM image (Fig. 3e) of an upright BNTP shows an ABCABC… stacking sequence. This result is in accordance with the XRD results that r-BN made up the majority. The appearance of weak streaks along the c*-axis in the FFT image (Fig. SI 1e†) and SEAD pattern (arrows in Fig. 3d) can be attributed to the stacking faults of {00l}.

Together with BNTPs, BNNCs can be found in S₆₉₀ (Fig. 4a). The average side length of BNNCs was 210 nm. The side view of the BNNCs gives an average thinness of ∼70 nm (Fig. 4b). The corresponding SEAD pattern (Fig. 4c) was in the [1−10] diffraction of h-BN and [100] diffraction of r-BN, which means BNNCs are comprised of intergrowth rhombohedral and hexagonal phases.^29,30 In other words, the forbidden 1/2{002}_h reflections (marked by an asterisk in Fig. 4c) are due to the stacking faults lying parallel to the (002)_h surface, which is similar to those of Au and Ag nanoplates with forbidden 1/3{422} diffraction spots.^31,32 The extra two spots marked by the arrows in Fig. 4c also indicate the presence of dislocation.

Fig. 4 SEM (a and d) and TEM (b) images of S₆₉₀ (a–c) and S₇₃₀ (d). The SEAD pattern (c) was collected in the framed area in (b). Cartoon scheme for BNNCs in the top (e1) and side (e2–4) views. The side view observed in the [110]_h (e3) and [110]_h (e4) direction, and the two planes at the tip could be (10 ± l) or (11 ± l), respectively.

Therefore, it is reasonable to consider that r-BN was dispersed within the hexagonal structure in both S₆₀₀ and S₆₉₀. Since inclined BNNCs were unavoidable in the TEM observation, the intersection angle of BNNCs tips give a range between 62° and 70°. Based on the h-BN phase, it is close to the intersection angle of (102)_h and (10−2)_h (66°), but smaller than that of (101)_h and (10−1)_h (72°) in [110]_h direction (Fig. 4e(3)). If it was observed in the [100]_h direction (Fig. 4e(4)), the intersection angle of (112)_h and (11−2)_h was only 51°. Therefore, the twelve side trapezoids can be indexed to {102}_h or {103}_r (Fig. 4b). The scheme for the BNNCs and the two projection directions is shown in Fig. 4e. It is reported that electrostatic forces dictate the optimal stacking sequence. Non-bulk stacking arrangements in 2D h-BN are strongly influenced by impurities and the sodium dopants can facilitate rotations of the layers in AA′ stacked bilayers.¹⁹ Though our stacking sequence seems different from the report, the presence of intergrowth should contribute to the sodium and other byproducts. On the other hand the relatively mild temperature made the intergrowth possible. Actually, the metastable samples will transform to h-BN because of its thermodynamic instability at high temperatures (>730 °C), and a high yield of h-BNNCs (90%) can then be observed (Fig. 4d).

3.4. Control experiment

To study the effect of the reaction parameters on the morphology, sets of experiments was carried out at different reaction times and temperatures, which are summarized in Fig. 5. From Fig. 5, the morphology profile can be divided into four zones: (1) unreacted zone, (2) BNNSs zone, (3) BNTPs zone and (4) BNNCs zone. As the reaction time was 12 h, the BNNSs can be produced at a relatively low temperature (400 °C, Fig. SI 1f and g†), the BNTPs was an intermediate morphology (500–650 °C), which is prone to turn into BNNCs at a high temperature (>690 °C). A similar trend can also be found when the reaction time changed. Taking the reactions at 600 °C as an example, BNNSs rather than BNTPs were obtained at 0.5 h (Fig. 6a) and BNTPs with the average side lengths of 60 nm were produced at 4 h (Fig. 6b). As the reaction time was more than 12 h, no morphology evolution was found, however, the average side lengths of the BNTPs increased to 250 nm at 48 h. Therefore, the sizes of the BN nanomaterials could be tuned in principle by adjusting the reaction parameters. The two types of conversion are indicated by dashed arrows in Fig. 5. It should be noted that the reaction near the boundary tends to generate the mixtures.

Fig. 5 The morphology profile of boron nitride nanomaterials obtained at different reaction times and temperatures. Area 1: the reaction couldn't be carried out. Area 2: ultrathin BNNSs. Area 3: sp² hybridized triangular BN. Area 4: BNNCs. The reaction near the boundary tends to generate the mixture of nearby BN nanomaterials. Notes: every point shows a reaction, the boundaries give a probable location.

Fig. 6 TEM and SEM images of BN samples prepared at 600 °C at different reaction times, 0.5 h (a), 4 h (b) and 48 h (c). The wrapped black particles in (a) are Fe nanoparticles, which is due to the inadequate HCl used in the treatment process.

Combined with previous XRD, SEM and TEM results, a morphology and phase evolution was present in this paper. By analyzing the experimental results, we found that BNNSs (t-BN) transformed into BNTPs (r-BN and h-BN), and then BNTPs can convert to BNNCs (h-BN). The basal stacking faults (Fig. 3d) and the special features of the SEAD patterns of the two phases (r-BN and h-BN) (Fig. 4c) point to the transformation occurring with the participation of the basal slip.²⁹ In previous reports, the phase transformation from t-BN to h-BN occurs at 1450 °C but completes at 1800 °C.³³ The onset temperature of the mixed-phase (r-BN + h-BN) samples and pyrolytic residues are as high as 1800 °C and 2500 °C, respectively.³⁴ However, in this study, the phase transformation occurred even below 730 °C.

3.5. Formation mechanisms of BNTPs and BNNCs

BNTPs and BNNCs are composed of B₃N₃ monolayers. Because of the strong in-plane bonding and the weak interplanar interactions combined by van der Waals forces, we took a monolayer as example. Though the reaction parameters were different, the chemical reaction can be described elsewhere.^12b Hydrogen and nitrogen were rich in our synthetic environment. The zigzag edges are terminated with an N atom and armchair edges with both N and B atoms.¹⁵ The BN monolayer would be terminated with the zigzag N edges (Fig. SI 2a†), superior to that of the armchair (Fig. SI 2b†). Owing to the small lattice mismatch between (100)_hBN and (111)_Fe, BN layers grow epitaxially onto the Fe particles formed in situ (Fig. SI2c†). Calculations indicate that triangular BN islands are more stable^21,35 than hexagonal ones. One N atom of the triangular islands can be saturated with one H atom at edge and two H atoms at the corner (Fig. SI 2a†). Two types of h-BN islands on the surface of the fcc or hcp domains of Fe (111) is shown in Fig. SI 2c.†¹³ As the reaction temperature was increased, BN nanomaterials grew larger. Then the exposed H saturated atoms at the edge decreased dramatically, which will then have minor effects on the morphology. BNNCs rather than BNTPs will be produced at a high temperature. In fact, few BNTPs with a side length over 800 nm could be observed in this study.

The transition metal (Fe, Ni) had a positive impact on the formation of BNTPs and BNNCs, which is accepted for the graphitization of the carbon analogue.³⁶ Without the aid of Fe species in our route, BNTPs and BNNCs could not be obtained. On the other hand, NaN₃ and NaBH₄ decomposed at a high temperature (350 °C for NaN₃ and 450 °C for NaBH₄) to generate the in situ autogenic pressure. Therefore, the phase transition below 730 °C was reasonable.

The fresh Na was in the liquid state when the temperature was above 98 °C. Therefore, an oriented aggregation mechanism in the solution systems was taken into account, in which the primary nanocrystals serve as “building blocks” for the growth of larger ones. The intermediate states of the triangular and hexagonal plates building from smaller blocks were captured under TEM observation, as shown in Fig. 7a and b, respectively. Because the bonding process reduces the overall energy, the process is energetically favoured. Since the defects were unavoidable during the attachment, unique, often symmetry-defying, twins, and/or intergrowths can possibly be formed according to Penn's opinion.³⁷ Fig. 7c shows a connection of two hexagonal plates. A connection of two broken BNNCs can occasionally be observed, which is shown in Fig. 7d, a twin boundary is indicated by arrows. It is believed that another traditional Ostwald ripening mechanism also acted to form the initial particles before attachment and smooth the particles during the course of attachment.³⁸

Fig. 7 TEM and SEM images of BNTPs (a) and BNNCs (b) intermediates, a connection of two hexagonal plates (c), a connection of two broken tetrakaidecahedron (d).

3.6. Thermal stability

The thermal stability of BN materials is important for various applications, such as, catalyst support³⁹ and recycled Surface-Enhanced Raman Scattering (SERS) support at elevated reaction temperatures.⁴⁰ In this study, the thermal stability of BN nanomaterials was investigated by TGA, and the typical curves were conducted at ambient atmosphere. From Fig. 8 we can see in all five curves that no significant weight gain appeared below 800 °C. As the temperature was further raised, the weight gain was observed due to the formation of B₂O₃. The TGA curves of those samples display different characteristics. Relatively minor changes can be found for S₆₉₀ and S₇₆₀ (<3%, at 1000 °C), showing a higher thermal stability at ambient atmosphere. Nevertheless, S₄₀₀ and S₅₀₀ show a fast weight gain above 800 °C (>23%, at 1000 °C), implying a fast oxidation.

Fig. 8 TGA curves of BN samples obtained at different temperatures (400–760 °C).

The different antioxidation ability of these BN nanomaterials seems to deal with varied crystallinity and specific surface areas (SSA). A large SSA can provide more reactive sites, favourable for the reaction between BN and O₂ in air. Therefore, compared with S₇₆₀ with small SSA (6 m² g⁻¹),^12a a fast weight gain can be observed for S₄₀₀ (233 m² g⁻¹) and S₅₀₀ (197 m² g⁻¹) with larger SSA.

3.7. Optical properties

The optical absorption properties reflect the electronic state of the materials and are widely used to determine the band gap of semiconductors. To investigate the optical properties of the as-obtained BN samples, they were suspended in ethanol followed by sonication. Then, thin films were deposited onto quartz substrates using an airbrush, as described elsewhere.⁴¹ The optical absorption spectrum of BN samples prepared at different temperatures is displayed in Fig. 9. The optical absorption spectrum of S₇₆₀ shows a single peak, whose position at 6.05 eV (205 nm) well correlates with the experimentally determined band-gap value of h-BNs. The band gap can be estimated to lie between 5.6 and 5.8 eV. Though the stacking sequences of S₆₀₀ and S₇₆₀ are different, the peak shift observed was insignificant. A shifting of the dominant absorption peak from 6.05 eV (205 nm) to 6.17 eV (201 nm) for S₄₀₀ and S₅₀₀ reveals how the crystal structure influences the electronic structure of BN species.

Fig. 9 Optical absorption spectrum of BN samples deposited onto quartz substrates.

Due to the different synthetic routes and defects type, CL emission properties varied in the previous reports. The structural changes will lead to changes in the electronic state, and the optical properties might be related to the microstructures.⁴² Compared with BNNCs,^12a a red shift can be found for BNTPs and the emission of ultrathin BNNSs is weaker and wider (Fig. SI 3†). Their corresponding SEM and CL images show uniform optical properties across the whole sample (Fig. SI 3b and c panel†). The different morphology and CL properties of the BN materials would give them potential applications in optoelectronic devices in the ultraviolet region.

3.8. Antibacterial activity

Fig. 10a shows a typical TEM image of 5 wt% Ag–BNTPs. We can see that the Ag nanoparticles (5–15 nm) were well dispersed on the surface of BNTPs. The TEM images of Ag–BNNSs and Ag–BNNCs can be found in Fig. SI 3a and b,† respectively. The XRD pattern can be indexed as Ag and BN (Fig. SI 3c†). The Ag–BN composites have been proved to be excellent SERS supports⁴⁰ and catalysts for both CO oxidation²² and 4-Np.²³ Though it has been demonstrated that BN was a biocompatible material, little attention has been paid to an antibacterial test. To evaluate the antibacterial activity of Ag–BN samples, the spread plate counting method was utilized. As shown in Fig. 10b, three kinds of Ag–BN samples show the antibacterial rate to be nearly 100%. A 5-log reduction (Ag–BNNSs), 4-log reduction (Ag–BNTPs) and 3-log reduction (Ag–BNNCs) in viable S. aureus bacteria were observed within 12 h of exposure using S₄₀₀, S₅₀₀ and S₆₉₀ as the support, respectively. Also, little difference can be found for S₆₉₀ and S₇₃₀ as the support. Those results indicate that all samples have excellent antibacterial activity. The supports can avoid the aggregation of active Ag nanoparticles. Because of their small size, the Ag nanoparticles have a high specific surface area in contact with the bacteria, therefore bactericidal contact may be the dominant role. However, bactericidal release may also contribute to the present results. No matter which bactericidal mechanism plays the major role, our results do suggest that the three kinds of BN materials can be considered as promising supports for biocides used in biomedical and related fields.

Fig. 10 TEM images of Ag–BNTPs (a) and viable cell numbers of S. aureus on the control group and different Ag–BN nanomaterials (b).

4. Conclusions

In this study, an “autoclave route” was applied for the synthesis of sp² hybrid 2D BN nanomaterials. By simply increasing the reaction temperature, a phase and morphology transformation occurred. It is surprising that the onset temperature was much lower than previously reported. t-BN ultrathin BNNSs (2–6 nm), mixed phase (r-BN and h-BN) BNTPs and pure h-BN BNNCs were obtained in sequence by increasing the temperature. Besides, an intergrowth of r-BN and h-BN for BNTPs and BNNCs was detected via SEAD pattern. It is thought that BN materials with different sizes, specific surface areas, morphology and high thermal stability would make them meet different needs. The band gap and CL images of the BN samples was also investigated, implying potential application as optoelectronics in the UV region. Ag decorated BN nanomaterials have excellent antibacterial activities so they can be used as supports in biomedical and related materials.

Acknowledgements

Financial support from the National Natural Science Foundation of China (no. 21201129) and the National Natural Science Foundation of Shanxi Province (no. 2013011012-3) are greatly appreciated. Thanks Dr Lulu Si for proofing the manuscript.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: SEM and TEM images of BNTPs and BNNSs, scheme for armchair, zigzag terminated triangular BN monolayers, CL properties and TEM images for Ag–BN composite. See DOI: 10.1039/c3ra47005a

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