For Mn3O4 membranes to be useful as a FNG, they must be able to produce charge separation without screening from free electrons. Mn3O4 nanoparticles have an experimental band gap in the range of ~3.2 to 3.8 eV, which varies with size. Figures and show the electronic density of states (DOS) plotted relative to the Fermi energy for monolayer and bulk geometries of Mn3O4, respectively. They show similar band gap of 0.9 eV for the monolayer and 1 eV for the bulk. While the predicted band gaps are expected to be underestimated due to the use of semi-local generalized-gradient approximation exchange-correlation functional, our calculations indicate minimal changes in the band gap upon exfoliation. This is confirmed by experiments wherein bulk Mn3O4 and the corresponding exfoliated membranes show a band gap of 3.8 and 4.0 eV, respectively (Figure ).