Femtosecond relaxation dynamics of two-dimensional BiOI nanoplatelets as efficient photocatalysts

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.