  TiO2 is a wide band-gap semiconductor that has been intensively investigated
for photocatalysis and water-spiting. However, weak light absorption in the
visible region of the spectrum poses stringent limitation to its practical
application. Doping of TiO2 with N or transition-metal impurities has been
explored to shift the onset of optical absorption to the visible region, yet
with limited success. Based on hybrid density functional calculations, we
propose adding Bi to TiO2, in the form of dilute Ti_(1-x)Bi_(x)O2 alloys, to
efficiently shift the optical absorption to the visible region. Compared to N,
Bi introduces an intermediate valence band that is significantly higher in the
band gap, and leaves the conduction band almost unchanged, leading to a
remarkable redshift in the absorption coefficient to cover almost all the
visible-light spectrum. Comparing formation enthalpies, our results show that
adding Bi costs significantly less energy than N in oxidizing conditions, and
that Ti_(1-x)Bi_(x)O2 might make a much more efficient photocatalyst than
TiO_(2-y)N_(y) for water splitting.
