sp2/sp3 bonding ratio dependence of the band-gap in graphene oxide Ebrahim Akbari, Iman Akbari and Mohammad Reza Ebrahimi Eur. Phys. J. B, 92 4 (2019) 71 Published online: 03 April 2019 DOI: 10.1140/epjb/e2019-90675-y