Faculty of Engineering and Natural Sciences
Titania nanostructures from first-principles calculations for photocatalytic and photovoltaic applications
Bilkent University, Department of Physics, Bilkent
We have systematically investigated structural, electronic and magnetic properties of various TiO nanostructures such as small (TiO2)n (n=1-10) clusters, very thin TiOx (x=1,2) nanowires as well as bulk-like (110) rutile nanowires and anatase surfaces by using the first--principles plane wave pseudopotential calculations based on density functional theory. A large number of different possible structures have been searched via total energy calculations in order to find the ground state structures of these nanostructures.
In general, ground state structures of TiO2 nanoclusters have at least one dangling or pendant O atom. Only lowest lying structure of n=10 cluster does not have any pendant O atom. In ground state structures, Ti atoms are at least 4--fold coordinated for n > 4. Clusters prefer to form three dimensional and compact structures. Formation energy increases with the size of cluster. All clusters have singlet ground state. Interaction of ground state structure of each (TiO2)n clusters with H2O and various transition metal (TM) atoms like V, Co and Pt also have been investigated. All these elements interact with clusters forming strong chemisorption bonds, and permanent magnetic moment is induced upon the adsorption of Co or V atoms.
Similarly, the stability of TiOx nanowires enhances with its increasing radius, thus reaching sufficient coordination number of Ti and O atoms. Three dimensional structures are more energetic than planar ones for both of the stoichiometries. All stoichiometric TiO2 nanowires studied exhibit semiconducting behavior and have nonmagnetic ground state. In TiO nanowires, both metallic and semiconductor nanowires are resulted. In this case, in addition to paramagnetic TiO nanowires, there are also ferromagnetic ones. In addition, there is a correlation between binding energy (Eb) and energy band gap (Eg) of TiO2 nanowires. In general, Eb increases with increasing Eg. We have also studied the structural and electronic properties of bulk-like rutile (110) nanowires. There is a crossover in terms of energetics, and bulk-like nanowires are more stable than the thin nanowires for larger radius wires after a critical diameter. These (110) rutile nanowires are all semiconductors.
Last, we systematically investigated the interaction of perylenediimide (PDI)-based dye compounds (BrPDI, BrGly, and BrAsp) with both unreconstructed (
UR) and reconstructed (RC) anatase TiO2 (001) surfaces. All dye molecules form strong chemical bonds with surface in the most favorable adsorption structures. In
UR--BrGly, RC--BrGly and RC--BrAsp cases, we have observed that HOMO and LUMO levels of molecules appear within band gap and conduction band region, respectively. Moreover, we have obtained a gap narrowing upon adsorption of BrPDI on the RC surface. Because of the reduction in effective band gap of surface--dye system and possibly achieving the visible light activity, these results are valuable for photovoltaic and photocatalytic applications.
May 21, 2009, 10:40, FENS L055