Chemical vapor deposition (CVD) technology is of great importance to fabricate hexagonal boron nitride (h-BN) micro-/nano-materials.^[1–5] The traditional plasma-enhanced and thermal CVD technologies are still widely applied to h-BN growth,^[6,7] and the most commonly used precursors include multi-source precursors with ammonium (NH₃), such as NH₃–boron halide (BX₃, X = F, Cl, or I), NH₃–boron (B_xH_y), NH₃–organoborane (R_zB_xH_y, R = C, F, Cl, or I), NH₃–organoborate [B(OR)₃, R = C_nH_{2n + 1}], etc;^[2,8–10] and single-source ones, i.e., some organic or inorganic molecules which contain boron and nitrogen components, for instance, borazine (B₃N₃H₆), chloroborazine (B₃N₃H_{6 − x}Cl_x), ammonia borane (BNH₆), and so on.^[5,11–13] Generally speaking, almost all of the multi-source CVD routes possess considerable complexity, and the final h-BN properties depend strongly on the vapor phase composition, the controlled target temperature, and the selective substrate in the reaction system.^[5,9–13] In contrast, the single-source precursors can not only make the synthetic route much easier, but also improve the h-BN product.^[5,11–13] Actually, both the frequently used single- and multi-source precursor materials are disadvantageous to the controllable CVD routes and the final h-BN micro-/nano-materials because of their deucedly-instable, highly-flammable/explosive, extremely-poisonous, and carbon-contaminated properties.^[5,11–13] Furthermore, the above-mentioned single-source materials and their synthetics have been quite costly to date. Therefore, a stable, hazard-free, and carbon-free single-source precursor is highly desirable for the h-BN CVD system, and designing its feasible CVD route is also of great significance for the BN growth technology.

As a commonly-used chemical, ammonium tetrafluoroborate (NH₄BF₄), which is a molecule containing boron and nitrogen, is highly-stable in the ambient atmosphere and very cheap on the market. As reported by Yang et al., a reaction was carried out in an autoclave in the temperature range of 400–600 °C via NH₄BF₄ pyrolysis in metallic Na flux,^[14] suggesting that NH₄BF₄ would be able to be used as a single-source precursor in the h-BN CVD system due to its nature of high-temperature sublimation. However, an explosion occurred in the autoclave, easily at such a high temperature in experiments, and it could not be controlled precisely either. MgCl₂-assisted NH₄BF₄ CVD was subsequently carried out to successfully obtain a large-scale h-BN phase,^[15] indicating the feasibility of using NH₄BF₄ in the single-source CVD reaction. As elaborated in that investigation, the key to h-BN formation in an NH₄BF₄ single-source CVD system is to stabilize the by-product HF generated from the NH₄BF₄ decomposition. Due to the chemical-cycled equilibrium, cycle reaction of NH₄BF₄ occurs in the high-temperature phase (NH₄BF₄ → HF + NH₃BF₃; 4NH₃BF₃ → BN + 3NH₄BF₄; 4HF + BN → NH₄BF₄), revealing that h-BN formation only happens by effectively depressing the HF content in the reaction zone. The HF can be efficiently limited by forming a stable MgF₂ phase (via a reaction of HF with MgCl₂ vapor), resulting in the chemical equilibrium towards h-BN direction and forming h-BN in large-scale. Unfortunately, other chemicals that could play a similar role as MgCl₂ have not yet been found or reported for the NH₄BF₄ CVD system.

In this study, mesoporous SiO₂ (SBA-15) is utilized to effectively stabilize and transfer the HF in the single-source NH₄BF₄ CVD reaction system. As a result, a hollow-capsular h-BN structure is successfully obtained. The formation mechanism is proposed for the first time.

After the NH₄BF₄ CVD process (at 1050 °C), the as-collected residues exhibit the collapsed surfaces that differed from the smooth ones of raw SBA-15, as shown in Figs. 2(a) and 2(b). The thickness of the obtained hollow-capsular h-BN is about 1 μm, as shown in Fig. 2(b). Actually, the coarse and collapsed surface is composed of numerous disordered ultra-thin sheets (Fig. 2(c)). The clearly visible edges of the super-thin nanosheets are displayed in twists and turns (Fig. 2(d)).

Fig. 2.

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	Fig. 2. SEM images of (a) SBA-15 and (b)–(d) the samples obtained after NH4BF4 CVD reaction.

Further TEM measurements indicate that the washed samples completely convert to hollow capsular microstructures from the pristine solid micro-capsules (unwashed) (Figs. 3(a)–3(c)). The detailed HRTEM images reveal the wall of the hollow structure made up of several layers that are ∼ 0.34 nm separated from each other (Fig. 3(d) and its inset), and collapsed-morphology nanosheets possessing less than 10 layers (Fig. 3(e), the inset shows a 6-layered nanosheet, single-layered nanosheets can be found frequently). The corresponding EELS analysis demonstrates a nearly 1/1 molar ratio of B/N; no other element was detected (Fig. 3(f)). The XRD pattern can be indexed into the h-BN phase with (002) and (100) planes located at 2θ = 26.1° and 42.4°, respectively. The distance between the (100) and (002) (Fig. 3(g)) planes is calculated to be ∼ 0.34 nm by 2d sin θ = nλ. Therefore, the as-indexed 0.34 nm distance (inset of Fig. 3(d)) can be assuredly speculated as being the interplanar spacing of the h-BN (002) plane family.^[17–22]

Fig. 3.

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	Fig. 3. Characterization of the samples obtained at 1050 °C: (a) TEM image of the sample without washing by NaOH solution, (b) and (c) TEM images of the sample after washing; (d) and (e) HRTEM images of the sample in (c); (f) EELS spectrum of the washed sample; and (g) XRD pattern of the washed sample.

In order to obtain the optimum temperature, we did a lot of work. From the XRD patterns of different-temperature samples, we know that the optimum temperature should be fixed at 1050 °C in our experiments, in light of a completely-washed-removal SiO₂ treatment for the relatively pure h-BN (Fig. 4). The XRD patterns illustrate the crystal variations of promoter SBA-15 in the NH₄BF₄ CVD reaction at different temperatures, and reveal the crystal phase of the hollow capsule structure, as shown in Fig. 4. It is clear that two diffraction peaks centered at 25.88° and 42.13° become more and more obvious with increasing temperature. The washed samples have the two peaks at ∼ 26.05° and 42.22°, as shown in Fig. 4(b), which are indexed into h-BN lattice planes (002) and (100), respectively (Fig. 4(a)). At 1100 °C, the sharp diffraction peaks that are attributed to SiO₂ crystallization from its amorphous state are observed due to the elevated temperature near the SiO₂ melting point. After an identical washing process by using a concentrated NaOH solution, h-BN crystal phase is determined, especially for the samples fabricated at 1050 °C (without impurity peaks found) (Fig. 4(b)), revealing the obtained samples with a hollow capsule structure corresponding to a pure h-BN phase.

Fig. 4.

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	Fig. 4. XRD patterns of the samples: (a) SBA-15 treated with the gaseous ammonium fluoroborate at different temperatures; (b) the corresponding samples with identical washing process by concentrated NaOH solution.

In Fig. 5, we can see that when the set temperature is between 800 °C and 850 °C, the ultrathin h-BN deposits on mesoporous SiO₂ surfaces to form thin nanolayers with smooth surfaces, especially the washed samples have smooth surfaces. Due to HF corroding the as-formed wall layers in the relative high temperature range of 900–950 °C, a handful of nanosheets start to grow along the layered-wall breakages. When the temperature is in the range of 1000–1050 °C, the decomposed HF erodes the formed h-BN surface layers seriously, resulting in more collapsed breakages on the surface. The h-BN growth occurs along the fracture edges by a CVD process of NH₃ · BF₃ [(4NH₃ · BF₃(v) + 3SiO₂(s) → BN(s) + 3SiF₄(v) + 6H₂O(v)], leading to a disordered surface with a dense few-layered h-BN structure. When the temperature is higher than 1100 °C, the amorphous SiO₂ is crystallized rapidly before the deposition starts, resulting in numerous broken surfaces.

Fig. 5.

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Fig. 5. SEM and TEM images of the samples obtained at different temperatures: (a), (d), and (g) SEM, temperature in the ranges of 800–850 °C, 900–950 °C, and 1000–1050 °C, the washed samples; (b) and (c) TEM, temperature in the range of 800–850 °C, the unwashed and washed samples; (e) and (f) TEM, temperature in the range of 900–950 °C, the washed samples; (h) TEM, temperature in the range of 1000–1050 °C, the washed samples; (i) SEM, the unwashed samples obtained at 1100 °C.

Figure 6(a) shows the XRD patterns of raw SBA-15, unwashed, and washed samples. The swell-like single peak (centered at ∼ 22.09°) in the SBA-15 pattern reveals its amorphous property. The washed sample is indexed into h-BN pure phase and the indexed (002) peak is located at ∼ 2θ = 26.10°. The unwashed one has mixed phases composed of amorphous SiO₂ (SBA-15) and h-BN (see inset in Fig. 6(a), abbreviated as BN/SiO₂). The FTIR further confirms that the hollow capsules with collapsed surfaces are pure h-BN, as shown in Fig. 6(b). Excluding the two typical h-BN vibration bands (centered at 1387 cm^{− 1} and 808 cm^{− 1}),^[17,23] many additional bands (∼ 3415 cm^{− 1}, 2970 cm^{− 1}, 2887 cm^{− 1}, 1087 cm^{− 1}, 1051 cm^{− 1}, and 465 cm^{− 1}, in the unwashed sample) are attributed to the related SBA-15 (amorphous SiO₂) vibration bands (–OH, −CH₂, Si–O–Si, and Si–O–C, corresponding to the C, B, and A frame areas in Fig. 6(b)).^[24] These results correspond to the mixed phases (BN/SiO₂) found in the unwashed sample, and the washed one (only two vibration bands located at 1387 cm^{− 1} and 808 cm^{− 1}) again demonstrates the highly-pure h-BN property. The BET nitrogen adsorption–desorption iso-therm analyses of the samples (obtained at 1050 °C) and raw SBA-15 are also comparatively shown in Fig. 6(c). According to the International Union of Pure and Applied Chemistry (IUPAC) classifications, the isotherm of SBA-15 is attributed to type H1 hysteresis loop, which is associated with porous properties corresponding to well-defined cylindrical-like pore channels. Contrastively, isotherms of the washed and unwashed samples are attributed to the H3 type, indicating a non-rigid aggregation of plate-like particles that can give rise to numerous slit-shaped pores in their aggregation matrix. The BET specific surface area of the hollow capsule is 151 m²/g, which is far below that of SBA-15 (832 m²/g), but slightly above that of the unwashed one (134 m²/g). Thus, the specific surface areas are sharply decreased due to the raw ordered pore channels being seriously destroyed in SBA-15 itself.

Fig. 6.

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	Fig. 6. (a) XRD patterns, (b) FTIR, and (c) nitrogen adsorption-desorption isothermals of the SBA-15, washed, and unwashed samples obtained at 1050 °C. The inset in panel (a) shows the bimodal fitting.

The formation mechanism is proposed and demonstrated in Fig. 7. For the NH₄BF₄-cycle reaction, NH₄BF₄ sublimation and decomposition occur simultaneously at high temperature

Fig. 7.

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	Fig. 7. Schematic diagram of the formation mechanism.

The formed NH₃·BF₃ vapor not only has a high thermal stability, but also works well as a CVD precursor for h-BN formation^[15]

However, the decomposed HF by-product can strongly corrode the pre-formed BN, especially at a relatively high temperature (the as-formed BN can be completely consumed by HF), again generating NH₄BF₄

Therefore, the chemical vapor balance by the cycle reactions (Eqs. (1)–(3)) leads to a final conclusion that NH₄BF₄ cannot be used as a single-source precursor for the h-BN CVD route. Actually, the formed HF in the reaction balance strongly restricts the BN formation in this CVD reaction. Here, the mesoporous SiO₂ (SBA-15) can effectively stabilize and transfer the HF due to the following related chemical reaction:

In order to verify the SiF₄ generation in the SBA-15-assisted NH₄BF₄ CVD reaction system, we added a water-cooling-trap system (with a plastic bottle) in the exhaust gas before the NaOH-solution-cooling-trap system. When the reaction system cooled to room temperature naturally, the water solution of the plastic bottle was dried to obtain a small quantity of white powder. We tested the EDS of this powder, and found that the Si element emerged (Fig. 8). This result can indirectly verify that the chemical reaction 4HF(v) + SiO₂(s) → SiF₄(v) + 2H₂O(v) occurs in the SBA-15-assisted NH₄BF₄ CVD reaction system.

Fig. 8.

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	Fig. 8. EDS of the sample obtained from the water cooling system.

The as-introduced SBA-15 can disequilibrate the NH₄BF₄-cycle reactions by effectively stabilizing and transferring the formed HF in the cycle process. It is predicted that when the temperature is fixed at 1050 °C, the decomposed HF seriously erodes the formed h-BN surface layers, resulting in more collapsed breakages on the surface while h-BN growth occurs along the fracture edges by a CVD process of NH₃·BF₃ [proposed as (4NH₃· BF₃(v) + 3SiO₂(s) → BN(s) + 3SiF₄(v) + 6H₂O(v)], leading to a disordered surface with a dense few-layered h-BN structure. Actually, SBA-15 has acted as an in-situ template for the h-BN deposition. Although SiF₄ could react with H₂O to generate SiO₂ again [SiF₄(v) + 2H₂O(v) → 4HF(v) + SiO₂(s)], the constant Ar flow could effectively remove SiF₄ and H₂O from the depositing reaction region, which does not influence the h-BN formation on the SBA-15 surfaces. Here, the F⁻ can be transferred by forming an intermediate SiF₄ in the CVD system, benefiting for the h-BN formation.

We successfully synthesized a hollow h-BN capsule structure by using the novel SBA-15-assisted NH₄BF₄ CVD system. The hollow capsule shells were composed of relatively well-aligned h-BN few-layers, and the collapsed surfaces were made up of disordered ultrafine h-BN nanosheets (one-layered and few-layered). The effective F⁻ stabilization and transference of the HF are vital for h-BN deposition and growth in the NH₄BF₄ CVD reaction.