Ultrathin two-dimensional (2D) nanomaterials (UTNs), with thicknesses down to a few nanometers, offer significant advantages for energy catalysis, environmental remediation, and optoelectronics. Their large surface area, well-defined interfaces, quantum-confined electrons, and tunable band structures make them promising photocatalysts for CO₂ reduction reaction (CO₂RR). Metal oxyhalide UTNs are particularly interesting for CO₂RR because of the coexistence of covalent metal-oxygen and ionic metal-halide bonding within their 2D layers. This heterogeneity creates an internal electric field that aids charge separation. Defect engineering, such as removing halide or oxygen atoms, further enhances this effect. Bismuth oxyhalides are especially promising due to their anisotropic electronic structure stemming from the arrangement of [Bi_mO_n](3m-2n)+ layers and halogen layers. However, bimetallic oxyhalide UTNs with well-defined elemental ratios and dual-metallic active centers are rarely reported. This study focuses on developing a novel top-down desalination strategy to unlock these bimetallic active sites from a complex parental structure and assess its impact on CO₂RR efficiency.

Extensive research has explored the synthesis and applications of ultrathin 2D materials, particularly in photocatalysis. Monometallic oxyhalide UTNs, especially bismuth-based ones, have shown considerable promise due to their unique electronic structure and internal electric field. Various synthetic approaches, including chemical vapor deposition, wet-chemical exfoliation, and acid-assisted etching, have been employed to create high-quality monometallic oxyhalide UTNs. However, the synthesis of bimetallic oxyhalide UTNs with precisely controlled elemental ratios and well-defined dual-metallic active centers has been less explored. This gap highlights the need for novel synthetic strategies that can unlock the potential of these materials for advanced photocatalysis.

This study introduces a top-down desalination strategy to synthesize ultrathin bimetallic oxyhalide layered material Pb_0.6Bi_1.4O₂Cl_1.4 (PBOC) from its parental structure Pb_0.6Bi_1.4Cs_0.6O₂Cl₂ (PBCOC). The strategy leverages the significantly lower hydration energy of the ionic bonding between alkali metal cations (Cs+) and halide anions (Cl-) compared to the metal-oxygen and metal-halide bonds within the PBCOC structure. Ultrasonication in deionized water preferentially dissolves the Cs-Cl layer, delaminating the PBOC layers and forming ultrathin sheets. The resulting material was characterized using various techniques, including powder X-ray diffraction (PXRD), atomic force microscopy (AFM), Brunauer-Emmett-Teller (BET) surface area analysis, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS) mapping, and X-ray photoelectron spectroscopy (XPS). Inductively coupled plasma-mass spectrometry (ICP-MS) and anion chromatography (AC) were used to track the dissolution of CsCl. The crystal structure was further analyzed using STEM and Le Bail refinement against PXRD. Density functional theory (DFT) calculations were employed to investigate the dissolution energy of Cs-Cl in PBOC. The electronic structure and photoelectric properties of PBOC were investigated using UV-vis diffuse reflectance spectroscopy, Mott-Schottky measurements, electrochemical impedance spectroscopy (EIS), transient photocurrent density measurements, and time-resolved fluorescence spectroscopy. Photocatalytic CO2RR performance was evaluated under full-spectrum and visible light irradiation using various CO2 concentrations, including atmospheric air. Isotopic tracing experiments using 13C and 18O were conducted to confirm the reaction mechanisms. DFT calculations were performed to elucidate the CO2RR reaction dynamics on the PBOC interface, including the electronic structure of the outer layer and the reaction pathways for CO, CH3OH, and CH4 formation.

The top-down desalination strategy successfully produced ultrathin PBOC sheets with an average thickness of 3.2 nm. The BET surface area of PBOC (2.561 m²/g) was 4.3 times larger than that of PBCOC (0.5963 m²/g). STEM-EDS mapping confirmed the removal of Cs and a reduction in Cl concentration. XPS analysis showed that the valence states of Bi, Pb, O, and Cl remained unchanged after desalination. ICP-MS and AC analysis showed nearly complete removal of Cs and 30% of Cl. The Le Bail refinement confirmed a tetragonal unit cell for PBOC with lattice parameters consistent with STEM observations. DFT calculations showed that the desalination process is thermodynamically feasible due to the large energy required to separate CsCl. PBOC exhibited a narrower bandgap (2.82 eV) than PBCOC (3.14 eV), improved charge transfer efficiency as evidenced by EIS and PL, and faster carrier dynamics as shown by transient photocurrent and time-resolved fluorescence measurements. In photocatalytic CO2RR experiments, PBOC showed significantly enhanced activity compared to PBCOC and BiOCl, producing significantly higher amounts of CO, CH3OH, and CH4. Isotopic tracing confirmed that the carbon and oxygen in the products originated from CO2 and H2O. PBOC maintained consistent activity over four cycles (16 h), suggesting good photostability. DFT calculations revealed that the co-occupied Pb in PBOC contributes to bandgap reduction, enhanced polarization, and improved charge separation. The co-occupied Pb and Bi atoms show strong electrostatic attraction towards CO2, facilitating CO2 activation and subsequent reduction reactions. DFT analysis of CO2RR pathways suggested that CO2 is activated to CO2* on the PBOC interface, then transformed into *COO* and *CO*. The reduction of CO to *CHO*, then CH3OH or CH4, is energetically favorable.

The findings demonstrate the effectiveness of the top-down desalination strategy in creating a high-performance photocatalyst for CO₂ reduction. The enhanced activity of PBOC compared to PBCOC and other state-of-the-art photocatalysts is attributed to the synergistic effects of increased surface area, unlocked bimetallic active sites, improved light absorption, and efficient charge separation and transfer. The DFT calculations provide a deeper understanding of the reaction mechanism, highlighting the roles of Pb and Bi in CO₂ activation and reduction. The results showcase the potential of this desalination strategy for designing other high-performance 2D photocatalysts for various energy and environmental applications. The superior performance of PBOC opens new avenues for designing and synthesizing efficient and stable photocatalysts for atmospheric CO₂ reduction.

This study successfully demonstrated a novel top-down desalination approach for synthesizing a high-performance ultrathin bimetallic oxyhalide photocatalyst (PBOC) for CO₂ reduction. PBOC exhibited significantly enhanced activity compared to its parental material and other benchmark photocatalysts. The strategy leverages the difference in hydration energies of ionic and covalent bonds to selectively remove an interlayer, thus unlocking bimetallic active sites. This work highlights the potential of this method for designing other advanced 2D materials for various applications and contributes significantly to the development of efficient and sustainable CO₂ reduction technologies. Future research could explore other bimetallic oxyhalides and investigate the potential for further enhancing catalytic activity through controlled defect engineering or heterostructure formation.

While the study demonstrates the high efficiency of PBOC for CO₂ reduction, further investigations are needed to assess the long-term stability and scalability of the desalination process. The DFT calculations provide insights into the reaction mechanism; however, experimental validation of specific reaction intermediates and pathways would strengthen the conclusions. The study primarily focused on CO₂ reduction; future work could explore the potential applications of PBOC in other photocatalytic reactions.