ABSTRACT
Graphene-based sensors exhibit high sensitivity, fast response, and good selectivity towards toxic gases but have low mechanical stability. The combination of graphene and two-dimensional hexagonal boron nitride (h-BN) is expected to increase the mechanical stability and enhance the adsorption performance of these gas sensors. Using first-principles calculations, we demonstrate that two-dimensional graphene/h-BN double layers can be used as good substrates for gas sensors with a small lattice mismatch of only 1.78%. Moreover, the presence of a h-BN layer widens the band gap by about 38 meV and considerably increases the work function, thus positively affecting the gas adsorption performance. Although these graphene/h-BN heterostructures do not change the physical adsorption mechanism of these sensors concerning the graphene-based materials, these bilayers significantly enhance the sensitivity of these sensors for detecting CO2, CO, NO, and NO2 toxic gases. Particularly, compared to the pristine graphene-based materials, the gas adsorption energies of graphene/h-BN increased by up to 13.78% for the adsorption of NO, and the shortest distances between the graphene/h-BN substrates and adsorbed gas molecules decreased. We also show that the graphene/h-BN heterostructure is more selective towards NOx gases while more inert towards COx gases, based on the different amounts of charge transferred from the substrate to the adsorbed gas molecules. Using the non-equilibrium Green functions in the context of density functional theory, we quantitatively associated these charge transfers with the reduction of the current passing through these scattering regions. These results demonstrate that graphene/h-BN heterostructures can be exploited as highly sensitive and selective room-temperature gas sensors for detecting toxic gases.
ABSTRACT
The adsorption mechanism of individual volatile organic compounds (VOCs) on the surface of graphene is investigated using nonempirical van der Waals (vdW) density functional theory. The VOCs chosen as adsorbates are ethanol, benzene, and toluene, which are found in the exhaled breath of lung cancer patients. The most energetically favorable configurations of the adsorbed systems, adsorption energy profiles, charge transfer, and work function are calculated. The fundamental insight into the interactions between the considered VOC molecules and graphene through molecular doping, i.e., charge transfer, is estimated. It is found that the adsorption energy is highly sensitive to the vdW functionals. Adsorption energies calculated by revPBE-vdW are in good agreement with the available experimental data, and the revPBE-vdW functional can cover well the physical phenomena behind the adsorption of these VOCs on graphene. Bader charge analysis shows that 0.064, 0.042, and 0.061e of charge were transferred from the graphene surface to ethanol, benzene, and toluene, respectively. All of the considered VOCs act as electron acceptors from graphene. By analyzing the electronic structure of the adsorption systems, we found that the energy level of the highest occupied molecular orbitals of these considered VOCs is shifted backward toward the Fermi level. The interaction of the VOCs with the π and π* states of the C atoms in graphene breaks the symmetry of graphene, leading to the opening of a band gap at the Fermi level. The adsorption of these considered VOCs onto the pristine graphene produces a band gap of 5-12 meV.
