**4.2 Thermal exfoliation**

For the fundamental studies, mechanical exfoliation can produce small samples of a single h-BN sheet. However, the thermal exfoliation route is more convenient if some large production is needed, such as micro- or nanofillers in polymer composites. Therefore, Cui et al. attempted a large-scale thermal exfoliation of h-BN using the easy and scalable thermal oxidation approach [59]. They observed that heating h-BN in the presence of air adds oxygen to the lattice. After heating, the material was stirred in deionized water for several minutes resulting in a thick mixture that exfoliates to form hydroxylated boron nitride without the need for sonication (or mild sonication required to increase the yield). Furthermore, Yu et al. prepared the hydroxylated boron nitride by heating h-BN powder to 1000°C inside the tubular furnace. This

process yields a similar material as Cui et al.; the prepared material was collected, mixed with binders, and stirred to obtain flakes of h-BN [60].

## **4.3 Chemical vapor deposition**

Chemical vapor deposition (CVD) is widely used to grow various materials. **Figure 6** shows the schematic diagram of the basic CVD growth technique. CVD is the industrialized large-area growth process, which uses liquid precursors and process gases to grow the material on the desired surface at elevated temperatures. In earlier studies, researchers used diborane and ammonia as precursors for the deposition of h-BN nanosheets on various metal substrates [61]. CVD growth of BN nanosheets is the primary approach to achieving large-area growth. The large-area growth involves suppressing the nucleation sites and enhancing the 2D growth mode. The nucleation could further be suppressed using atomically flat surfaces and optimized CVD parameters [32, 62].

Additionally, using a metal substrate can enhance the surface migration, further enhancing the domains' size by suppressing nucleation and growth rate due to solid gas reactions involving the metal surface chemistry [63]. The type of precursors could be a separate boron or nitrogen source, e.g. for boron source BF3 and NH3, BCl3 and NH3, and B6H6 and NH3). Otherwise, it could also be a single precursor like ammonium borane and borazine [64–66]. **Table 1** lists some conventional and advanced precursors in the trend to grow BN through the CVD technique. **Figure 6** illustrates the basic schematic of the horizontal CVD apparatus [76]. The apparatus consists of a horizontal quartz tube with three heater zones that provide even temperature gradient control throughout the reacting zone (inside the tube). From one side, gaseous precursors are introduced, which take part in the growth zone and precipitate (or epitaxially deposited) onto the surface of the substrate. The epitaxial growth mechanism is governed by the boundary layer adsorption phenomenon. For growth,


#### **Table 1.**

*List of conventional precursors used in CVD method for BN nanosheets/nanostructure growth.*

**Figure 6.** *Schematic diagram of basic CVD growth apparatus.*

the substrate is placed over an inclined susceptor generally made up of graphite, and the inclined angle may vary from 7° to 15°. This inclination provides uniform gas flow over the susceptor and suppresses the parasitic gas-phase reactions [76, 77].
