Improving Formation Conditions and Properties of hBN Nanosheets Through BaF2-assisted Polymer Derived Ceramics (PDCs) Technique

Abstract

Hexagonal boron nitrite (hBN) is an attractive material for many applications such as in electronics as a complement to graphene, in anti-oxidation coatings, light emitters, etc. However, the synthesis of high-quality hBN at cost-effective conditions is still a great challenge. Thus, this work reports on the synthesis of large-area and crystalline hBN nanosheets via the modified polymer derived ceramics (PDCs) process. The addition of both the BaF2 and Li3N, as melting-point reduction and crystallization agents, respectively, led to the production of hBN powders with excellent physicochemical properties at relatively low temperatures and atmospheric pressure conditions. For instance, XRD, Raman, and XPS data revealed improved crystallinity and quality at a decreased formation temperature of 1200 °C upon the addition of 5 wt% of BaF2. Moreover, morphological determination illustrated the formation of multi-layered nanocrystalline and well-defined shaped hBN powders with crystal sizes of 2.74–8.41 ± 0.71 µm in diameter. Despite the compromised thermal stability, as shown by the ease of oxidation at high temperatures, this work paves way for the production of large-scale and high-quality hBN crystals at a relatively low temperature and atmospheric pressure conditions.

Chemical Sensing Properties of BaF2-Modified hBN Flakes towards Detection of Volatile Organic Compounds

“…The hexagonal boron nitride (hBN) nanosheets were synthesised based on the reported procedure, whereby borazine was used a source for boron and nitrogen atoms, lithium nitride (Li 3 N, 99.4%, Alfa Aesar) [26,27], and barium fluoride (BaF 2 , 99%, Alfa Aesar) [28]. Typically, pure borazine monomer was polymerized at 55 • C to generate a colourless liquid polyborazylene (PBN) [28,29]. Then, 5 wt.% Li 3 N and 0-10 wt.% BaF 2 [30] were added to PBN, and the suspension was homogenised for 10 min.…”

“…The morphology of the device active materials was investigated using transmission electron microscopy (TEM), which showed that the hBN samples were mainly overlapping, well-defined, and plate-like nanostructures of 0.89 ± 0.01, 2.9 ± 0.7, 3.3 ± 0.3, and 3.2 ± 0.7 µm in dimension for pristine hBN nanoflakes as well as the modified hBN samples after modification with 2.5, 5, and 10 wt.% of BaF 2 , respectively (Figure 1). The improvement in hBN nanosheets size upon addition of BaF 2 to the pre-ceramization mixture can be attributed to the faster melting of Li 3 N. This was facilitated by BaF 2 [28], consequently leading to improved crystallisation of hBN from PBN. As such, the effect of the addition of BaF 2 to crystallinity and textual properties of the hBN nanosheets was studied using Raman spectroscopy and BET analysis.…”

“…However, the results still indicate that the samples had sufficient adsorption sites for the chemical vapours, thus exhibiting potential for application in the chemical sensor technology. The crystalline structural properties of the samples by Raman spectroscopy revealed formation of hBN nanostructures as evidenced by the presence of the first-order active Raman vibrating mode of hBN (E 2g ) centred at −1365.4 ± 1.6 cm −1 (Figure 2a) [28,36]. Additionally, correlation of the finite-size effects within hBN with the inherent broadening of the Raman vibrational modes (Figure 2a, inset) showed the dependence of crystallinity of the hBN nanosheets to the addition of BaF 2 .…”

“…Based on the multi-point Brunauer-Emmet-Teller (BET) method shown in Figure 2b, the samples exhibited a type II isotherm-an indication of the formation of macroporous or non-porous materials. As a result, the specific surface area was then determined to be 8.7 m 2 /g for the pristine hBN sample, whilst modification with 2.5, 5, and 10 wt % of BaF 2 led to surface areas of 3.5, 3.6, and 2.9 m 2 /g, respectively [28]. The decreasing surface area was expectedly due to the formation of larger crystalline planer flakes, thus limiting the amount of adsorption sites for N 2 and resulting in adsorption and desorption of the monolayer of N 2 predominantly on the material's external rough surface on the basal planes.…”

“…Figure 2. (a) Raman plots and (b) adsorption isotherms of pristine and 2.5, 5, and 10 wt% BaF2-modified hBN samples.Inset: bandwidth of the Raman spectra as a function of BaF2 wt%[28].…”