Artificial photosynthesis, specifically solar-driven CO₂ reduction to renewable fuels like CO, is a crucial strategy for addressing climate change and energy security. Developing highly active and selective photocatalysts is a major challenge. Atomically dispersed catalysts, such as single-atom catalysts (SACs), offer maximum atom utilization and excellent photoelectric performance, but their limited light absorption and generation of byproducts hinder their widespread application. SACs also suffer from poorly defined active sites, inadequate HER suppression, and weak substrate interactions. The CO₂RR involves three key processes: photoabsorption, charge separation, and CO₂ reduction. Ideal photocatalysts need sufficient optically active centers for charge generation and efficient pathways to transfer these charges to selective catalytic centers. Dual-atom catalysts (DACs), with bimetallic centers, offer significant advantages over SACs due to their combined atom-specific characteristics and optimized charge carrier transfer, leading to enhanced charge generation and catalytic activity while maintaining high atom utilization and stability. Suitable scaffolds for integrating DACs include porous materials like MOFs and COFs, known for their well-defined porosity, high surface area, and predetermined structures. These properties, coupled with the confining influence of the pore structure, provide a favorable environment for metal atom anchoring. However, the role of the substrate in the CO₂RR process requires further investigation, particularly regarding the interaction between catalyst and substrate which influences carrier separation and transfer, thus impacting catalytic performance. This study focuses on developing a photocatalyst incorporating atomically dispersed La-Ni sites within a COF-5 colloid using an electrostatically driven self-assembly approach assisted by phenanthroline ligands. The La-Ni structure is designed to enable efficient charge generation and transfer between optically active (La) and catalytically active (Ni) centers.

The literature extensively explores various strategies for photocatalytic CO₂ reduction. Studies highlight the potential of single-atom catalysts (SACs) for selective CO₂ reduction, but their limitations in light absorption and byproduct formation have spurred research into alternative approaches. Dual-atom catalysts (DACs), offering synergistic effects from bimetallic sites, have emerged as promising candidates. Several studies demonstrate enhanced activity and selectivity in CO₂ reduction using DACs supported on various substrates, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). The use of COFs as supports is particularly attractive due to their well-defined porosity, high surface area, and tunable properties, which allow for precise control over the active sites. However, research on COF-based DACs for CO₂ reduction is still in its early stages, and a thorough understanding of the substrate's role and the charge transfer mechanisms within these systems is needed.

This study employed an electrostatically driven self-assembly method to integrate atomically dispersed La-Ni sites (LaNi-Phen, where Phen = phenanthroline) into a conjugated boronate-ester-linked COF-5 colloid. The synthesis involved a multi-step process: initially, COF-5 colloid was synthesized using a modified reported procedure involving HHTP and PBBA precursors in a solvent mixture. Subsequently, La and Ni ions were incorporated via adsorption and chelation with LaCl₃·6H₂O, NiCl₂·6H₂O, and 1,10-phenanthroline. The order of addition (Ni then Phen then La) was crucial for preventing Ni aggregation. The resulting LaNi-Phen/COF-5 was characterized extensively using various techniques. Powder X-ray diffraction (PXRD) confirmed the COF-5 structure's integrity. High-resolution transmission electron microscopy (HRTEM) and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) visualized the atomic dispersion of La and Ni, revealing La-Ni distances of ~2.8 Å, indicative of La-Ni double-atomic sites. X-ray photoelectron spectroscopy (XPS) determined the valence states of Ni (+2) and La (+3). X-ray absorption fine structure (XAFS) analysis confirmed the coordination environment of La and Ni, showing dominant Ni-N and La-N peaks. UV-vis diffuse reflectance spectroscopy (DRS) and ultraviolet photoelectron spectroscopy (UPS) determined the optical and electronic properties of the material. The photocatalytic activity was evaluated by measuring CO and H₂ evolution during CO₂ reduction under simulated solar irradiation, using BIH as a sacrificial electron donor and H₂O as a proton source. Control experiments under different conditions (N₂ atmosphere, dark, absence of photocatalyst, COF-5, LaNi-Phen, or BIH) were performed to confirm the CO₂RR process's authenticity. Isotope-labeled (¹³CO₂) experiments were done to confirm the origin of CO. Cycling tests evaluated the catalyst's durability. Steady-state and time-resolved photoluminescence (PL) spectroscopy and electrochemical impedance spectroscopy (EIS) and photocurrent-time (I-t) measurements investigated the photogenerated charge carrier dynamics. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) identified reaction intermediates, and density functional theory (DFT) calculations investigated the reaction mechanism.

The LaNi-Phen/COF-5 photocatalyst exhibited significantly enhanced CO₂ reduction activity compared to the pristine COF-5 colloid, La-Phen/COF-5, Ni-Phen/COF-5, and a physical mixture of La-Phen/COF-5 and Ni-Phen/COF-5. The optimized LaNi-Phen/COF-5 achieved a CO evolution rate of 605.8 µmol·g⁻¹h⁻¹, representing a 15.2-fold improvement over COF-5 and high CO selectivity (98.2%). Characterizations confirmed the atomic dispersion of La and Ni within the COF-5 structure, forming La-Ni double-atomic sites with a distance of ~2.8 Å. In situ DRIFTS experiments showed the formation of key intermediates like *COOH and CO*, while DFT calculations suggested a low energy barrier for COOH formation (0.65 eV) and CO desorption (0.72 eV), explaining the high CO selectivity. Photoelectrochemical measurements revealed efficient charge separation and transfer in LaNi-Phen/COF-5, with La acting as the optically active center and Ni as the catalytic center. Time-resolved PL showed that the carrier lifetime decreased from 4.75 ns (pure COF-5) to 0.53 ns (LaNi-Phen/COF-5) indicating less charge recombination. The EIS analysis indicated that the electron transfer resistance was reduced significantly in LaNi-Phen/COF-5 compared to COF-5, and La-Phen/COF-5 or Ni-Phen/COF-5.

The significantly enhanced photocatalytic CO₂ reduction performance of LaNi-Phen/COF-5 stems from the synergistic combination of the optically active La sites and the catalytically active Ni sites within the highly porous COF-5 framework. The observed directional charge transfer between the La and Ni atoms, facilitated by the COF structure, lowers the energy barriers for key reaction steps, leading to faster CO₂ conversion and improved CO selectivity. The superior performance compared to single-metal-site catalysts and the physical mixture demonstrates the importance of the bimetallic structure in promoting both light absorption and catalytic activity. The COF-5 colloid plays a critical role in providing a stable support and facilitating efficient charge transport. This study provides valuable insights into the design and synthesis of efficient photocatalysts for CO₂ reduction, highlighting the potential of COF-supported DACs for addressing global energy and environmental challenges.

This work successfully demonstrated a facile and scalable strategy for fabricating a high-performance photocatalyst for CO₂ reduction using electrostatically driven self-assembly to integrate La-Ni dual-atom active centers into a COF-5 scaffold. The La-Ni structure effectively combined light absorption and catalytic activity, leading to a remarkable 15.2-fold increase in CO₂ reduction rate and high CO selectivity. This approach opens new avenues for designing and synthesizing efficient and stable photocatalysts for CO₂ conversion to valuable fuels. Future research could focus on exploring other bimetallic combinations and COF structures to further optimize the catalyst's performance and explore the potential of this approach for other catalytic applications.

While the LaNi-Phen/COF-5 photocatalyst showed excellent performance, further optimization of the synthesis parameters may be needed to achieve even higher activity and selectivity. The long-term stability of the catalyst under continuous operation needs more extensive investigation. The study focused on CO₂ reduction to CO; exploring other products, such as methanol, would enhance the system's versatility. The use of BIH as a sacrificial electron donor limits the overall efficiency; future research could investigate more sustainable electron donors.