  This topical review focuses on the recently active debate on the band
alignment between two polymorphs of TiO$_2$, rutile and anatase. A summary is
given to the popular methods for measurement and calculation of band alignment
between materials. We point out, through examination of recently experimental
and theoretical reports, that the outstanding discrepancy in the band alignment
between two TiO$_2$ phases is attributed to factors that influence band
alignment rather than needs a definite answer of which band alignment is right.
According to an important factor, the presence of an interface, a new
classification of band alignment is proposed as the coupled and intrinsic band
alignments. This classification indeed reveals that the rutile/anatase
interface can qualitatively change the type of their band alignment. However,
further systematic information of the interface and other factors that
influence band alignment will be needed to understand changes in energy bands
of materials better. The results obtained from discussion of the band alignment
between rutile and anatase may also work for the band alignment between other
semiconductors.
