Synthesis, characterisation and modelling of two-dimensional hexagonal boron nitride nanosheets for gas sensing

Abstract

The gas sensing performance of two-dimensional (2D) hexagonal boron nitride nanosheets (h-BNNSs) has being studied by means of computational and experimental methods. The structural, stability and vacancies properties of both defect free and defected 2D h-BNNSs were studied using the classical molecular dynamics (MD) approach. The calculations were performed in the NVT Evans and NPT hoover ensembles using the Tersoff potentials with the Verlet leapfrog algorithm to obtain reliable structural properties and energies for defect free, boron (B) and nitrogen (N) vacancies. B and N defect energies were calculated relative to the bulk defect free total energies, and the results suggest that N vacancy is the most stable vacancy as compared to the B vacancy. The radial distribution functions and structure factors were used to predict the most probable structural form. Mean square displacements suggests the mobility of B and N atoms in the system is increasing with an increase in the surface area of the nanosheets. Results obtained are compared with the bulk defect free h-BNNSs. Experimentally, 2D h-BNNSs were synthesised using the wet chemical reaction method through chemical vapour deposition (CVD) catalyst free approach. The X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy (RM), UV-visible Spectroscopy (UV-VIS), dynamic light scattering (DLS), Energy Dispersion Spectroscopy (EDS) and Brunauer-Emmett Teller (BET) were adopted to attain the structural properties of the nanosheets. Each spectroscopic technique affirmed unique features about the surface morphology of h BNNSs. The crystallinity of the nanosheets with the stacking of the B and N vii honeycomb lattice was validated by the XRD, while the TEM disclosed the specimen orientations and chemical compositions of phases with the number of layers of a planar honeycomb BN sheet, the EDS express the atoms present in the samples and BET validated the surface area of the materials. The FTIR, RM, DLS and the UV-vis expressed the formation of the in-plane, out-of-plane h-BN vibrations and, the nature of the surface with the thickness, particles stability together with the optical properties of the nanosheets. From TEM, FTIR, RS and BET the material fabricated at 800°C showed different morphologies, large number of disordering together with high surface area, which enhances the sensing properties of the nanosheets. However, with an increase in temperature the sensitivity of the nanosheets was found to decrease. Additionally, the UV-vis results, confirmed a lower energy band gap of 4.79, 4.55 and 4.70 eV for materials fabricated at 800, 900 and 1000 °C, that improved the semiconducting properties of the materials, which in return enhanced the sensing properties of the nanosheets. The gas sensing properties of the 2D h BNNSs were also investigated on hydrogen sulphide (H2S) and carbon monoxide (CO). The fabricated sensor based on 800 – 900 °C h-BNNSs showed good sensitivity towards ppm of H2S at 250 °C. The excellent gas sensing properties could be attributed to high surface area, small crystallite size, defect/disordering of h BNNSs. Overall, the h-BNNSs were found to be more sensitive to H2S over CO.