Growth and characterization of porous sp2-BN films with hollow spheres under hydrogen etching effect via borazane thermal CVD

Porous sp2-boron nitride (sp2-BN) films were directly grown on c-plane Al2O3 substrates by low pressure thermal chemical vapor deposition (CVD) using borazane (NH3-BH3) as a single precursor and without any catalyst. It was found that the BN films grown under 20 mbar are porous and consist of nano-scaled hollow BN spheres of various sizes, with outer radius (ro) ranging from 7.20 nm to 13.32 nm. The B-N bonding was confirmed to be sp2-bonded by the Raman scattering whereas, the crystal structure was determined by TEM and XRD to be turbostratic BN (t-BN) having a lattice constant 'c' larger than the hexagonal BN (h-BN) bulk crystal. Homogenous nucleation played a vital role in the porous sp2-BN formation that is contrary to normal CVD growth under such low pressure i.e. 2 mbar for denser BN films. Additionally, because of the H2 chemical etching effect, the H2/N2 ratio shows a strong influence on the film growth. All of the as-grown films have shown good ultraviolet (UV) absorption edge near 210 nm, hence providing a feasible route towards the synthesis of porous sp2-BN with higher surface-volume ratio for high performance photo sensors and smart energy storage devices.