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

Abstract

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.

Interface-Enhanced Charge Recombination in the Heterojunction between Perovskite Nanocrystals and BiOI Nanosheets Serves as an S-Scheme Photocatalyst for CO2 Reduction

“…The pristine BiOI sample shows two lines at 159.8 and 165.1 eV associated with Bi 3+ 4f 7/2 and 4f 5/2 , respectively. 21,28 Similarly, the pristine CF sample displays two lines at 137.8 and 142.7 eV associated with Pb 2+ 4f 7/2 and 4f 5/2 , respectively. 14,19 For the PHJ sample, the XPS signals of Bi 4f shifted to smaller binding energies, whereas those of Pb 4f shifted to greater binding energies, which indicates that electron transfer occurs from CF PeNC to BiOI nanosheets upon formation of the heterojunction through the difference in their work functions.…”

“…The TAS signatures of BiOI nanosheets synthesized from Rb PeNC precursors resemble those of earlier reports. 21,37 The TAS of a PHJ sample show both depletion features of a pristine sample with enhanced transient signals, indicating that both species are excited by the femtosecond pulse with an interface-enhanced spectral feature.…”

“…The rapid and slow recoveries of the PB bands are assigned to shallow and deep trap states, respectively, because the TAS signals were broadly extended to the near-IR region. 21 The PB band recovery of a pristine CF sample at 515 nm shows time coefficients 100 and 900 ps for charge recombination, whereas the PB band recovery of the pristine BiOI sample at 650 nm shows much smaller charge recombination time coefficients, 7 and 70 ps (Figure 4a), because the layered BiOI might involve more surface defects than the CF sample. When CF and BiOI are coupled in the heterojunction sample, the transient of the PB bands of PHJ monitored at 530 nm (to compare with pristine CF at 515 nm) shows significant quenching, with two decay coefficients, 1 and 35 ps (Figure 3a).…”

“…Cs 1−x FA x PbBr 3 perovskite nanocrystals were synthesized by the method of hot addition; 22,23 ultrathin BiOI nanosheets were synthesized from rubidium bismuth iodide 24 (Rb 3 Bi 2 I 9 ) PeNC on washing rubidium (Rb) and oxidizing BiI 6 octahedra of Rb PeNC with isopropyl alcohol (IPA) and water. 21 Stirring CF and BiOI powders together for 48 h in toluene produced the CF:BiOI heterojunction samples. Because of the low surface energy of PeNC and strong interaction of the Bi cation with the Br anion of the perovskite, a heterojunction between CF and BiOI was created by simple stirring.…”

“…Because several examples have been provided for S-scheme photocatalysts to work properly for CO 2 reduction, ,,, it is timely to understand the ultrarapid kinetics of interfacial charge recombination in relation to their photocatalytic performances in a well-defined S-scheme heterojunction system. For this purpose, we designed an S-heterojunction system with a co-cationic cesium formamidinium lead halide perovskite (Cs 1– x FA x PbBr 3 , abbreviated CF with x ∼ 0.45) PeNC, a direct-band-gap semiconductor, coupled with ultrathin bismuth oxyiodide (BiOI) nanosheets, an indirect-band-gap semiconductor . The formation of the S-type heterojunction is confirmed by X-ray photoelectron spectra (XPS).…”