Gas-Solid Phase Reaction-Driven CO₂ Reduction : Exploring the Photocatalytic Potential of Silver-Bismuth-Iodide Rudorffite

In recent decades, researchers have been striving to tackle climate change originated from global warming and searching a solution for CO₂ reduction. Photocatalytic CO₂ reduction is a promising solution to balance reduction greenhouse gas and creating energy sources. However, the main challenges of photocatalysts for CO₂ reduction are their wide band gaps and short lifetime of photo-induced electron-hole pairs. That not only limits their visible-light absorption but also inhibits the transportation of electron-hole pairs for catalytic reactions. Silver bismuth iodide rudorffite materials (SBI) having a relatively lower band gap, < 1.8 eV, and high absorption coefficient are selected for photocatalytic CO₂ reduction. Herein, we synthesize various SBI photocatalysts including AgBi₂I₇, AgBiI₄, Ag₂BiI₅, and Ag₃BiI₆ through a gas-solid phase strategy. Owing to the different crystal structures, SBI photocatalysts perform photocatalytic activity differently. To realize the correlation between crystal structure of SBI and photocatalytic activity, X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and photo-assisted Kelvin Probe Force Microscopy (Photo-assisted KPFM) are used to frame the mechanism of their photocatalytic CO₂ reduction. Through gas-solid phase reaction, the crystal structures of SBI productions can be easily manipulated by controlling the stoichiometry of their reactants, silver iodide and bismuth iodide. For Ag-rich compositions, Ag/Bi ratio ≥1, the SBI photocatalyst tends to crystalize in a hexagonal structure. The partial Ag ions are delocalized in the lattice and occupy the vacant tetrahedral sites between AgI₆ and BiI₆ octahedrons. That results in the formation of AgI secondary phase. On the other hand, Bi-rich composition, Ag/Bi ratio <1, prefers to form a cubic structure. Bi ions tend to partially substitute the central site cation, Ag, of AgI₆ octahedron. To insight the speculation above, XPS and XAS were applied to realize the electronic structure, valence state changes, and atomic environment surrounding Ag atoms in SBI materials. In terms of the Ag-rich components, they all exhibit a high density of uncoordinated silver, strong Ag-I bonding, and short bond length between Ag and I. The photocatalytic CO₂ reduction rate shows a significant relative to increasing the ratio of silver in SBI photocatalysts, Ag₃BiI₆ > Ag₂BiI₅ > AgBiI₄ > AgBi₂I₇. The same tendency of photocatalytic activity and electron configuration of SBI materials points out that the metallic Ag in SBI photocatalysts is the active sites for CO₂ photoreduction. Among the SBI materials, Ag₃BiI₆ shows the highest photocatalytic activity, achieving average CO and CH₄ production rates of 0.23 and 0.10 µmol●g^-1h^-1. Furthermore, the results of Photo-assisted KPFM reveals the photo-induced potential change, contact potential difference (CPD), of SBI materials under different LED light sources including UV at 365 nm, blue at 470 nm, green at 530 nm, and red at 656 nm. The highest CPD of Ag₃BiI₆ validates that the photocatalytic CO₂ reduction rate is proportional to the evolution of CPD. The high CPD can be ascribed to the effective electron-hole pair separation that are originated from active Ag sites. The effective separation rate of electron-hole pairs promotes the photocatalytic CO₂ reduction activity. In summary, our study sheds the light in the curial role of SBI crystal structure in photocatalytic CO₂ reduction. By validating the active sites within SBI and unraveling its photocatalytic mechanism, we offer a promising solution to address the challenges of environmental pollution and the increasing demand for sustainable energy. The insights gained from our research pave the way for further advancements in the field of photocatalysis and contribute to the development of efficient and environmentally friendly CO₂ reduction technologies.