The structural and optical properties of pristine and photoactivated surfaces of facet [001] of 2D BiOI nanoplatelet powders synthesized with an antisolvent method were studied using X-ray diffraction (XRD), a scanning electron microscope, X-ray photoelectron spectra (XPS), visible-near infrared (vis-NIR) absorption, and transient absorption spectroscopic techniques. The synthesized BiOI nanoplatelets possessed a tetragonal structure with length 200-400 nm and thickness less than 50 nm. XRD analysis showed that the photoactivation did not affect the crystal structure. In contrast, the XPS analysis showed vacancies associated with elements Bi, I, and O. The band gaps estimated from the diffuse reflectance spectra of pristine and photoactivated samples are 1.83 and 1.80 eV, respectively. Femtosecond transient-absorption spectra measured in the vis-NIR (2.25-0.9 eV) region showed photobleach bands associated with the band edge, shallow trap, and deep trap states. A global fit of the transient spectral profiles showed that all bands decay synchronously for both pristine and photoactivated samples, but the photoactivated samples showed a greater magnitude of carrier-carrier annihilation following photoexcitation due to photodoping caused by defects. A carrier-relaxation model involving thermal equilibrium between band-edge, shallow, and deep trap states is proposed to explain the synchronous decay of all bands.

We designed an S-heterojunction system with a perovskite nanocrystal, Cs 1−x FA x PbBr 3 (CF), coupled with a bismuth oxyiodide (BiOI) nanosheet to form a perovskite heterojunction (PHJ) photocatalyst. On the basis of femtosecond transient absorption measurements, the pristine CF sample has two charge recombination periods, 100 and 900 ps, corresponding to surface and bulk trap-state relaxations, respectively. When CF was in contact with BiOI to form an S-heterojunction, rapid interfacial charge recombination occurred to show two decay components with time coefficients 1 and 35 ps, responsible for the electron−hole recombination in the surface and bulk states, respectively. We observed a new photoinduced absorption band on the blue side of the photobleach band of PHJ that gives relaxation more rapid than that of pristine CF, presumably due to doping of bismuth cations creating defect states to enhance the charge recombination that leads to photocatalytic performance for the PHJ catalyst poorer than for the pristine CF sample.