Photoinduced Crystallization and Activation of Amorphous
Titanium Dioxide
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Abstract
Titanium dioxide (TiO<sub>2</sub>) is one of the most common photosensitive
materials used in photocatalysis, solar cells, self-cleaning coatings,
and sunscreens. Although the crystalline TiO<sub>2</sub> phases such
as anatase and rutile are well-known to be photoactive, whether amorphous
TiO<sub>2</sub> is active in photocatalytic reactions is still controversial.
Here we show that amorphous TiO<sub>2</sub> prepared by the commonly
used sol–gel method of tetrabutyl titanate hydrolysis is active
in photocatalytic water reduction and methylene blue oxidation under
the irradiation of a xenon lamp. The amorphous TiO<sub>2</sub> gains
photoactivity after an induction period of approximately an hour,
suggesting that phase transition is involved. Using an extensive series
of microscopic and spectroscopic analyses, we further show that the
photoinduced crystallization by amorphous TiO<sub>2</sub> forms a
nanometer-thin layer of rutile nanocrystallites under the irradiation
in the middle ultraviolet range. The resulting core–shell nanoparticles
have a bandgap of 3.3 eV and are enriched with surface-active sites
including reduced titanium and oxygen vacancies. The revelation of
photoinduced crystallization raises the possibility of preparing photosensitive
TiO<sub>2</sub> using low-temperature radiation techniques that can
not only save energy but also incorporate heat-sensitive components
into manufacturing