*2.1. Borazine-Derived BN Nanoparticles*

Borazine (BZ) had been originally discovered by Alfred Stock in 1926 [17]. It displays a chemical formula H3B3N3H3. It is a preformed B-N-like ring structure and has the correct B-to-N ratio. Furthermore, it is economically competitive and attractive from a technical point of view, based on its reaction starting from cheap compounds, such as (NH4)2SO4 and (NaBH4), reacting in tetraglyme at low temperature (120–140 °C) [18]. Borazine offers the advantage of being liquid with an adequate vapor pressure to be applied in gas phase pyrolysis processes to prepare nanostructured BN. As an illustration, we have demonstrated the interest of BZ to produce BN nanoparticles by spray-pyrolysis [19–22]. In our process, BZ is nebulized into an aerosol, and the stream consisting of tiny BZ droplets suspended in the carrier gas is transported by the carrier gas to be passed through the preheated tubular furnace at 1400 °C under nitrogen. In the hot-zone, the conversion of the nebulized precursor occurs through molecular condensation and ring-opening mechanisms involving the evolution of dihydrogen and producing vapors of BN ring-based species. The latter, reacting to form the consolidated boron nitride network, are swept by the nitrogen carrier-gas flow and, then, condensed into a white product getting collected into the cooling traps near the outlet of the furnace. The as-obtained product is stored inside an argon-filled glove-box. The scanning electron microscopy (SEM) images in Figure 1a show that the sample consists of particles with a relatively homogeneous size. This indicates that the most important operating factors, including the properties of the starting precursor, the pyrolysis temperature, the nitrogen flow rate, the residence time and heating rate of the droplet particles, are controlled during processing.

**Figure 1.** SEM (**a**), TEM (**b**) and HRTEM (**c**) images of samples obtained by spray-pyrolysis of borazine (BZ).

The low-magnification transmission electron microscope (TEM) bright field image of the sample (Figure 1b) show elementary blocks that are composed of slightly agglomerated nanoparticles (NPs). The particle size ranges from 55 to 120 nm. The high resolution TEM (HRTEM) image (Figure 1c) of the particle core demonstrates that the specimen consists of very fine BN crystallites in which *sp*<sup>2</sup> layers are significantly buckled in a disordered stacking sequence, exhibiting a size corresponding to less than six atomic basal planes, whereas their length does not exceed 50 Å. This points to the fact that BN is poorly crystallized similarly to a turbostratic structure. The TEM data are reinforced by the X-ray diffraction (XRD) experiments (Figure 2). The corresponding XRD patterns show very broad peaks at the *h*-BN (002), (100)/(101)/(004) and (110) positions. In particular, the (002) peak slightly shifts to lower angles in such samples, and the (100), (101) and (004) peaks merge into a single broad peak. Finally, the samples displayed a chemical formula of B1.0N0.9. Their specific surface area is 34.6 m2 g<sup>í</sup><sup>1</sup> , and the helium density is 1.95 g cm<sup>í</sup><sup>3</sup> , as measured by Brunauer-Emmett-Teller (BET) and helium pycnometry, respectively.

B1.0N0.9-NPs are stable in air below 850 °C in which only surface oxidation proceeds [21]. Here, we report the evolution of the nanostructural organization of B1.0N0.9-NPs in the temperature range of 1450–2000 °C under nitrogen. The XRD patterns in Figure 2 range from 10° to 90° for heat-treated B1.0N0.9-NPs.

**Figure 2.** XRD patterns of borazine-derived B1.0N0.9-NPs and annealed at a temperature ranging from 1450 to 2000 °C.

The XRD patterns of samples heat-treated in the temperature range of 1450–1600 °C display features similar to the ones recorded for B1.0N0.9-NPs, indicating a turbostratic structure. For the sample annealed at 1700 °C, the (002) peak at 25.30° is sharpened, suggesting that the crystallite size became larger in the *c*-axis direction, although the shoulder-shaped broad feature remained on the low-angle side of the peak. This is also shown for the sharper (100)/(101)/(004) peak, which tends to be separated into the (004) peak and the (100)/(101) peak. The increase of the heat-treatment temperature to 1800 °C and 2000 °C resulted in an increased resolution of the XRD patterns. We can clearly distinguish the (002), (100)/(101), (004) and (110) peak positions. According to the sharpening of the (002) and (100)/(101) peaks, we suggest that the crystallite size continuously increased in the *c*- and *a*-axes directions from 1400 °C to 2000 °C. However, no clear peaks corresponding to the (102) and (112) planes were observed. These findings tend to demonstrate that B1.0N0.9-NPs annealed at 2000 °C exhibit a turbostratic structure. The variation of the average crystallite size in the *c*-axis from the (002) peak ( Lc) and the interlayer d002 spacing of the samples during heat-treatment is shown in Figure 3. The dimension d002 is calculated from Bragg's law using the diffraction angle of the (002) peak. Lc represents the average number of stacked layers in the crystallites. The average stack height Lc is calculated from the Scherrer relation ( Lc = 0.9Ȝ/(B2 í B'<sup>2</sup> ) 1/2cosșwhere Ȝ is the CuKĮ<sup>1</sup> wavelength (Ȝ = nm), ș the Bragg angle of the (002) diffraction peak, B the full width at half maximum intensity (FWHM) of the peak and B' the instrumental contribution).

In the range of 1450 °C ( Lc = 1.10 nm; d002 = 0.367 nm)–1600 °C (Lc = 1.42 nm; d002 = 0.363 nm), there is no major modification in both the apparent average grain size ( Lc(002)) and the value of the interlayer d-spacing d002. Values are close to those calculated for as-prepared B1.0N0.9-NPs ( Lc = 1.06 nm; d002 = 0.376 nm). This indicates a relatively high amount of disorder in the structure of the corresponding samples. At 1700 °C, the apparent average grain size increases slightly ( Lc = 2.23 nm). Although the crystallization state in NPs heat-treated at 1700 °C is slightly improved, the BN phase remains poorly ordered as confirmed by the value of d002 (d002 = 0.351 nm), higher than

that in a *h*-BN crystal (0.3327 nm). At 1800 °C, Lc increases to 4.63 nm and the interlayer d002 spacing is found to be 0.345 nm, which are values characteristic of a turbostratic phase. The minor changes in the XRD patterns of samples heat-treated at 2000 °C is reflected in the values of Lc (4.65 nm) and d002 (0.346 nm). In addition to XRD studies, we investigated TEM (Figure 4) and HRTEM (Figure 5) experiments to follow the evolution of the nanostructural organization in the temperature range of 1450–2000 °C.

**Figure 3.** Evolution of Lc(002) and d002 *vs.* annealing temperature.

**Figure 4.** TEM images of the samples annealed at (**a**) 1450 °C; (**b**) 1600 °C; (**c**) 1700 °C; (**d**) 1800 °C and (**e**) 2000 °C.

**Figure 5.** HRTEM images of the samples annealed at (**a**) 1450 °C; (**b**) 1600 °C; (**c**) 1700 °C; (**d**) 1800 °C; (**e**) 2000 °C; (**f**) evidence of a core-shell structure generated at 2000 °C.

The annealed samples form elementary blocks composed of nanosized particles that are round in shape and slightly agglomerated. Both the average size of annealed particles and the agglomeration seem to increase with the temperature of the annealing, which is in good agreement. This is clear for the samples annealed at 1800 °C (Figure 4d) and 2000 °C (Figure 4e), respectively. We therefore extended our analysis, by performing high resolution TEM (HRTEM), in order to refine/emphasize the structural information.

Figure 5 reports HRTEM images of the same samples.

Clear differences appear between the samples annealed in the range of 1450–2000 °C. After heat-treatment to 1450 °C (Figure 5a), the sample displays a turbostratic BN structure with more distinct (002) layers in comparison to the nanostructure observed in pristine B1.0N0.9-NPs (Figure 1c). In the sample annealed at 1600 °C (Figure 5b) and 1700 °C (Figure 5c), we can also observe the formation of nanodomains made of BN layers surrounding voids. The HRTEM image reveals the formation of concentric shelled nanodomains. The lattices of these BN nanostructures have a local interlayer spacing of 3.51 Å. Annealing at a temperature of 1800 °C (Figure 5d) and 2000 °C (Figure 5e,f) leads to hollow-cored BN-NPs that are strongly facetted, forming polygonal particles with an interlayer spacing of 3.34 Å. We investigated the potential of samples heat-treated at 2000 °C to confine H2 storage materials.
