All-inorganic Pb-free bismuth (Bi) halogen perovskite quantum dots (PQDs) with distinct structural and photoelectric properties provide plenty of room for selective photoreduction of CO 2 . However, the efficient conversion of CO 2 -to-CO with high selectivity on Bi-based PQDs driven by solar light remains unachieved, and the precise reaction path/ mechanism promoted by the surface halogen-associated active sites is still poorly understood. Herein, we screen a series of nontoxic and stable Cs 3 Bi 2 X 9 (X = Cl, Br, I) PQDs for selective photocatalytic reduction of CO 2 -to-CO at the gas−solid interface. Among all the reported pure-phase PQDs, the assynthesized Cs 3 Bi 2 Br 9 PQDs exhibited the highest CO 2 -to-CO conversion efficiency generating 134.76 μmol g −1 of CO yield with 98.7% selectivity under AM 1.5G simulated solar illumination. The surface halogen-associated active sites and reaction intermediates were dynamically monitored and precisely unraveled based on in situ DRIFTS investigation. In combination with the DFT calculation, it was revealed that the surface Br sites allow for optimizing the coordination modes of surfacebound intermediate species and reducing the reaction energy of the rate-limiting step of COOH − intermediate formation from • CO 2 − . This work presents a mechanistic insight into the halogen-involved catalytic reaction mechanism in solar fuel production.

Lead (Pb) halide perovskite quantum dots (PQDs) are promising candidates for the photochemical reduction of CO 2 . However, the intrinsically weak adsorption and activation toward inert CO 2 molecules have seriously hindered their practical application. This study reports alternative Cs 2 CuBr 4 PQDs for gas−solid phase photocatalytic CO 2 reduction under simulated solar irradiation. Cs 2 CuBr 4 PQDs exhibited CO 2 photoreduction performance with CH 4 and CO yields of 74.81 and 148.98 μmol g −1 , respectively. In situ diffuse reflectance infrared Fourier transform spectra and density functional theory calculations cooperatively revealed the synergistic strengthening of microelectronic polarization in Cs 2 CuBr 4 PQDs induced by surface-frustrated Lewis pair-like properties and intrinsic Cu d-band properties facilitated robust CO 2 adsorption and activation. This study demonstrated the potential of Cs 2 CuBr 4 PQDs as a platform for highly efficient CO 2 photoreduction and provided a distinct concept for CO 2 adsorption and activation based on the catalytic mechanism of Cu-based PQDs. KEYWORDS: photocatalytic CO 2 reduction, Cs 2 CuBr 4 , perovskite quantum dots, CO 2 adsorption and activation, catalytic mechanism

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In this work, hydrothermally prepared p−n heterojunction BiOBr/SnO 2 photocatalysts were applied to eliminate NO in visible light. The as-synthesized BiOBr/SnO 2 photocatalysts exhibit superior photocatalytic activity and stability through the establishment of a p−n heterojunction, resulting in a significant improvement in charge separation and transfer properties. The morphological structure and optical property of the BiOBr/ SnO 2 heterojunction were also investigated comprehensively. Extended light absorption into the visible range was achieved by SnO 2 coating on the surface of the BiOBr microsphere through the constructed heterojunction between BiOBr and SnO 2 , thus achieving efficient NO removal. Moreover, the transfer channels and directions of charge at the BiOBr/SnO 2 interface were determined by a combination of theoretical calculations and experimental studies. Within this p− n heterojunction, the charge in SnO 2 migrates into BiOBr through the preformed electron transfer channels, thus generating an internal electric field (IEF) between SnO 2 and BiOBr. Under the influence of IEF, the photogenerated electrons of BiOBr migrate from the conduction band (CB) to the CB of SnO 2 , thus promoting the separation of electrons (e − )−holes (h + ) pairs. The intermediates and final products were monitored by the in situ DRIFTS technology in the process of removal of NO in visible light; hence, the oxidation pathways of NO were reasonably proposed. Meanwhile, the construction of the heterojunction not only achieves more efficient NO photocatalytic oxidation but also inhibits the production of more toxic NO 2 . This work provides mechanistic insights into the interfacial charge transfer for heterojunction photocatalysts and reaction mechanism for efficient air purification.

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