We report here a series of nontoxic and stable bismuth-based perovskite nanocrystals (PeNCs) with applications for photocatalytic reduction of carbon dioxide to methane and carbon monoxide. Three bismuth-based PeNCs of general chemical formulas A 3 Bi 2 I 9 , in which cation A + = Rb + or Cs + or CH 3 NH 3 + (MA + ), were synthesized with a novel ultrasonication top-down method. PeNC of Cs 3 Bi 2 I 9 had the best photocatalytic activity for the reduction of CO 2 at the gas−solid interface with formation yields 14.9 μmol g −1 of methane and 77.6 μmol g −1 of CO, representing a much more effective catalyst than TiO 2 (P25) under the same experimental conditions. The products of the photocatalytic reactions were analyzed using a gas chromatograph coupled with a mass spectrometer. According to electron paramagnetic resonance and diffuse-reflectance infrared spectra, we propose a reaction mechanism for photoreduction of CO 2 via Bi-based PeNC photocatalysts to form CO, CH 4 , and other possible side products.

Despite severe charge recombination occurring within the bulk lattice, sol–gel-derived amorphous TiO2 particles with abundant OH groups (81.6 mg/g) and high surface area (274 m2/g) were for the first time demonstrated to exhibit respectively 8–14 and 9 times higher photocatalytic activity for CO2 reduction than their thermally derived anatase crystals and the commercial P25 powder. Moreover, the high density of the OH groups (12.45/nm2) enabled the amorphous oxide to exhibit higher specific surface reactivity than the crystals. The OH groups not only converted CO2 molecules into bonded bicarbonate/carbonate species to improve CO2 chemisorption but also trapped holes to form Ti–O–O–Ti species when the OH density was higher than a threshold value of 8.74/nm2, which synergistically promoted interfacial charge transfer. Bidentate carbonate and ·CO2 – were two active species that were able to underwent CO3 2–→ Ti–OOCH2 → Ti–O–CH3 → CH4 and ·CO2 – → CO2 2– → Ti–COOH → CO sequences on the hydroxylated surface to produce CH4 and CO products, respectively. High coverage of the chemisorbed carbonate species selected for CO2 reduction rather than H2 evolution to proceed. Moreover, it led with CH4 as the major product. Oxygen vacancies were the major active sites on the anatase crystals. Their influences on the surface transformations were also characterized to comprehensively understand the surface-controlled activity and selectivity.

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Femtosecond transient absorption spectral (TAS) investigations were performed to understand the carrier relaxation mechanism for perovskite nanocrystals Cs1–x FA x PbBr3 (CF, x = 0.45) and CsPbBr3 (CS), which served as efficient photocatalysts for splitting of CO2 into CO and O2 in the absence of water. Upon light irradiation for 12 h, formation of deep trap states was found for both CS and CF samples with spectral characteristics of the TAS photobleach (PB) band showing a long spectral tail extending to the long wavelength region. The charge recombination rates at the shallow surface states, bulk states, and deep-trapped surface state were found to be significantly retarded for the CF sample than for the CS sample, in agreement with the photocatalytic performances for CO product yields of the CF catalyst being greater by a factor of 3 compared to those of the CS catalyst.