  Photocatalytic materials are pivotal for the implementation of disruptive
clean energy applications such as conversion of H$_{2}$O and CO$_{2}$ into
fuels and chemicals driven by solar energy. However, efficient and
cost-effective materials able to catalyze the chemical reactions of interest
when exposed to visible light are scarce due to the stringent electronic
conditions that they must satisfy. Chemical and nanostructuring approaches are
capable of improving the catalytic performance of known photoactive compounds
however the complexity of the synthesized nanomaterials and sophistication of
the employed methods make systematic design of photocatalysts difficult. Here,
we show by means of first-principles simulation methods that application of
biaxial stress, $\eta$, on semiconductor oxide thin films can modify their
optoelectronic and catalytic properties in a significant and predictable
manner. In particular, we show that upon moderate tensile strains CeO$_{2}$ and
TiO$_{2}$ thin films become suitable materials for photocatalytic conversion of
H$_{2}$O into H$_{2}$ and CO$_{2}$ into CH$_{4}$ under sunlight. The band gap
shifts induced by $\eta$ are reproduced qualitatively by a simple analytical
model that depends only on structural and dielectric susceptibility changes.
Thus, epitaxial strain represents a promising route for methodical screening
and rational design of photocatalytic materials.
