Adsorption of H2 molecules on B/N-doped defected graphene sheets—a DFT study

Abstract

Adsorption of hydrogen molecules on the surface of two-dimensional sheets is extremely challenging due to the non-polar nature of the molecules. Though the adsorption occurs on the surface of the two-dimensional sheets, the adsorption energy tends to be weak. There are a few factors such as delocalized electrons, adsorption sites, the curvature of adsorbent and adsorption orientation which play a vital role in the adsorption of hydrogen molecule. Thus, the primary objective of the present work is focusing on the adsorption factors and giving a range of ideas to increase the adsorption of hydrogen molecules on the surface of two-dimensional defected graphene sheets. In order to explore in a wide range, four different graphene defects such as 5-8-5, 55-77, 555-777 and 5555-6-7777 sheets are taken and each sheet is doped with B and N atoms. Interestingly, the obtained results show that hydrogen orientation and adsorption sites are playing a vital role in hydrogen storage. Addition of N atom in the sheets increases the delocalized electrons and the addition of B atoms increase the localized electrons in the system. The N-doped sheets are behaving as a donor (n-type semiconductor) and B-doped sheets are behaving as acceptors (p-type semi-conductors). The bandgap of H-terminated bare 5-8-5, 55-77, 555-777 and 5555-6-7777 sheets are in the order of 5-8-5 and 55-77 > 5555-6-7777 > 555-777. The hydrogen adsorption values on the specific sheet with the perpendicular orientation of hydrogen molecule provide maximum adsorption energy value of − 80 meV. Similarly, the specific sheet with a parallel orientation of hydrogen provides the minimal adsorption energy value of − 9 meV. The overall B/N-doped 5-8-5, 55-77, 555-777 and 5555-6-7777 ranges from − 9 to − 80 meV which is lesser than the required adsorption energy value of 100 meV. Though the results are slightly low, the exploration provides clear insights on the Hydrogen adsorption on the surface of graphene defects and a possible way to enlarge the adsorption energy. The obtained results further show that the fusion points of pentagonal and hexagonal rings possess more delocalized electrons which are favourable for hydrogen adsorption.