  Titanium dioxide has been extensively studied in recent decades for its
important photocatalytic application in environmental purification. The search
for a method to narrow the optical band-gap of TiO2 plays a key role for
enhancing its photocatalytic application. The optical band gap of epitaxial
rutile and anatase TiO2 thin films deposited by helicon magnetron sputtering on
sapphire and on SrTiO3 substrates was correlated to the lattice constants
estimated from HRTEM images and SAED. The optical band-gap of 3.03 eV for
bulk-rutile increased for the thin films to 3.37 on sapphire. The band gap of
3.20 eV for bulk-anatase increases to 3.51 on SrTiO3. In order to interpret the
optical band gap expansion for both phases, ab-initio calculations were
performed using the Vienna ab-initio software. The calculations for rutile as
well anatase show an almost linear increase of the band gap width with
decreasing volume or increasing lattice constant a. The calculated band gap
fits well with the experimental values. The conclusion from these calculations
is, in order to achieve a smaller band-gap for both, rutile or anatase, the
lattice constants c has to be compressed, and a has to be expanded.
