Attaching titania clusters of various size to reduced graphene oxide and its impact on the conceivable photocatalytic behavior of the junctions—a DFT/D + U and TD DFTB modeling
Witold Piskorz1, Filip Zasada1, Gabriela Wójtowicz1, Antoni Morawski2, Wojciech Macyk1 and Zbigniew Sojka1
1 Faculty of Chemistry, Jagiellonian University, ul. Gronostajowa 2, 30-387 Kraków, Poland
2 Faculty of Chemical Technology and Engineering, Institute of Chemical and Environment Engineering, West Pomeranian University of Technology, ul. Pułaskiego 10, 70-322 Szczecin, Poland
E-mail: firstname.lastname@example.org and email@example.com
Received 17 April 2019, revised 7 June 2019. Accepted for publication 21 June 2019. Published 15 July 2019.
DFT/D + U and density functional based tight binding (DFTB) molecular modeling was used to investigate the role of the structural, electronic and optical properties of reduced graphene oxide surface (r-GO), hybridized with hydrated TiO2 moieties of various size, ranging from small molecular Ti2O4 clusters into extended Ti43O86 rutile type nanocrystals of ~5 nm diameter. The calculated adhesion energies, varying from −5.048 eV (r-GO|Ti2O4), −12.159 eV (r-GO|Ti5O10), −18.499 eV (r-GO|Ti15O30) to −42.484 eV (r-GO|Ti43O86), indicate high stability of these composites. It was shown that electronic interactions at the r-GO|(1 1 0)TiO2 interface give rise to net charge flow from the r-GO substrate towards the TiO2 moieties, analyzed in terms of the partial charge density 3D plots and an interfacial dipole moment formation. The DOS structure of the composites was calculated by means of the time dependent DFTB approach, and the position and composition of the VB and CB edges, along with the presence of weak mid-gap 2p C states originating from the intact graphene-like patches in the r-GO substrate were discussed in detail in the context of conceivable photocatalytic activity of the composites. The constructed band alignment diagram implies formation of the staggered type II scheme, with the electric field offset that is sensitive to the titania cluster size. In the case of the nano-reticular TiO2, where only a fraction of the Ti atoms is engaged in the Ti–O–C linkers formation, recombination of the photogenerated charges is inhibited owing to favorable spatial separation effect. For small molecular TiO2 clusters with all Ti cations anchored to the r-GO layer fast cross-relaxation quenches the beneficial interfacial charge separation effect, since the strong hybridization of the oxygen and carbon states provides a convenient pathway for the efficient electronic coupling between the CB edge states of r-GO and the VB edge states of the TiO2 moieties. A phenomenological model of the molecular r-GO|Ti2O4 and the reticular r-GO|Ti43O86composites was constructed in account for different photocatalytic behavior of both junctions