The following sections report the most recent progress in
defect chemistry of TiO2, including the contribution made by
the authors in derivation of defect disorder diagrams and the
determination of defect-related properties of TiO2. It will be
shown that the photocatalytic properties of nonstoichiometric
compounds, such as TiO2, are dependent on defect disorder in
much greater extent than the commonly studied properties, such
as crystal structure and surface area. The topical sections will
be concluded with the remarks on the impact of specific
properties on photocatalysis.