<p>Metal halide perovskites with high absorption
coefficient, direct generation of free charge carriers, excellent ambipolar
charge carrier transport properties, point-defect tolerance, compositional
versatility and solution processability are potentially transforming the
photovoltaics and optoelectronics industries. However their limited ambient
stability, particularly those of iodide perovskites, obscures their use as
photocatalysts especially in aqueous medium. In an unprecedented approach we
have exploited the photo-absorption property of the less toxic lead-free Cs<sub>3</sub>Bi<sub>2</sub>X<sub>9
</sub>(X = Br, I) nanocrystals (NCs) to catalyse the degradation of water
pollutant organic dye, methylene blue (MB) in presence of visible light at room
temperature. After providing a proof-of-concept with bromide perovskites in
isopropanol, the perovskites are employed as photocatalysts in water medium by
designing perovskite/Ag<sub>2</sub>S and perovskite/TiO<sub>2 </sub>composite
systems, with Type I (or quasi Type II) and Type II alignments, respectively. Ag<sub>2</sub>S
and TiO<sub>2</sub> coatings decelerate penetration of water into the perovskite
layer while facilitating charge carrier extraction. With a minimal NC loading,
Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub>/Ag<sub>2</sub>S degrades ~90% MB
within an hour. Our approach has the potential to unravel the photocatalytic
properties of metal halide perovskites for a wide spectrum of real-life
applications. </p>
Breakdown of the Static Picture of Defect Energetics in Halide Perovskites: The Case of the Br Vacancy in CsPbBr3