Single-atom catalysts (SACs), characterized by 100% metal dispersity, maximize atom efficiency, leading to cost-effective catalysts. Unlike conventional metal catalysts, SACs exhibit unique active sites and catalytic pathways, demonstrating superior activity and selectivity in various reactions, including oxygen reduction, CO oxidation, hydrogenation reactions, and others. Methods for achieving atomic dispersion of metals on solid supports have been developed, including utilizing vacancy defects, fabricating metal-organic frameworks (MOFs), employing spatial confinement in zeolites, and enhancing metal-support interactions. However, these methods often involve complex synthetic steps, sensitive conditions, and may not completely prevent metal aggregation, resulting in low reproducibility and low metal loading. The development of a practical and direct approach for constructing SACs with high metal loading is crucial. Strong coordination bonds between metal species and high-electronegativity heteroatoms (N, O, S) are vital in preventing agglomeration and achieving atomic-level dispersion. These bonds, however, alter the electronic structure of metal atoms (d-band center) through ligand effects, impacting catalytic activity. For reduction reactions, electron-rich centers are beneficial. The challenge lies in creating support materials with well-defined structures to stabilize catalytic metal atoms without relying on strong heteroatom coordination. This paper presents a room-temperature photochemical strategy using hydrogen to produce stable, high-loading SACs (Cu, Co) on 2D black phosphorus (BP) without strong heteroatom coordination.

The literature extensively documents the synthesis and application of single-atom catalysts (SACs) across diverse catalytic reactions. Studies have highlighted the importance of maximizing atom utilization efficiency for cost-effectiveness and the role of strong metal-support interactions in preventing aggregation. Various techniques, such as defect engineering, MOF synthesis, and zeolite confinement, have been explored to achieve atomic dispersion. However, these methods often face challenges related to complexity, sensitivity to conditions, and the risk of metal aggregation, frequently resulting in low metal loading. The literature also emphasizes the impact of electronic structure modification by ligand effects on catalytic activity, particularly the importance of electron-rich centers for reduction reactions. Existing strategies for SAC synthesis, while successful in some cases, often struggle to achieve high metal loading while maintaining stability. This work addresses these limitations by employing a novel room-temperature photochemical approach.

The synthesis of Cu/BP and Co/BP catalysts involved liquid exfoliation of bulk BP in N-methyl-2-pyrrolidone (NMP) under an Ar atmosphere. The exfoliated BP layers, with an average thickness of ~2.4 nm (four layers), were confirmed by TEM, Raman spectroscopy, and AFM. Cu(Ac)2·H2O and Co(Ac)2·4H2O were introduced into the BP-NMP dispersion, followed by photoinduced reduction assisted by hydrogen. Visible light irradiation (300 W Xe arc lamp, λ > 400 nm) for 3 hours generated hydrogen radicals (H·) on the BP surface, crucial for the formation of high-loading neighboring Cu single atoms (n-Cu/BP). The catalysts were characterized using various techniques: * **XRD:** To confirm the high dispersion of Cu and Co atoms. * **STEM and HAADF-STEM:** For direct observation of isolated atoms and neighboring atom groups (n-M/BP). * **EDS:** To confirm the even dispersion of metal atoms. * **ICP-OES:** To determine the metal loading (11.3 wt% for Cu, 5.2 wt% for Co). * **XAFS:** For detailed analysis of atomic structure and coordination state, including XANES and FT-EXAFS. * **UV-Vis DRS:** To investigate the light-harvesting capability of BP and Cu/BP. * **UPS:** To determine the valence band values (EVB). * **ESR:** To detect the presence of hydrogen radicals (H·) using DMPO as a radical trapping agent. * **XPS:** To analyze the valence states of M/BP. Electrocatalytic HER activity was assessed using a three-electrode system in 1 M KOH solution. LSV scans, Tafel plots, TOF calculations, mass activity measurements, and EIS analysis were performed to evaluate the catalytic performance. Long-term stability was tested using chronoamperometry and CV cycles. Density Functional Theory (DFT) calculations were used to understand the electronic structure, adsorption energies, charge transfer, free energies of hydrogen adsorption (ΔGH*), and water dissociation pathways.

The photochemical strategy yielded high-loading n-Cu/BP (11.3 wt%) and n-Co/BP (5.2 wt%), significantly exceeding most reported SACs. Characterizations revealed the formation of moderately coordinated Cu-P3 and Co-P3 structures. The single-atom Cu sites exhibited electron-rich features, with ΔGH* close to zero, favorable for catalysis. Neighboring Cu atoms synergistically enhanced water dissociation activity, surpassing both isolated Cu SACs and Cu nanoclusters. n-Cu/BP demonstrated exceptional HER performance: a low overpotential of 41 mV at 10 mA cm−2, a Tafel slope of 53.4 mV dec−1, and a high TOF of 0.53 H2 s−1 at 150 mV overpotential. Even when Co was introduced, n-Cu/BP showed superior TOF compared to n-Co/BP and bi-atomic CuCo/BP. Electrochemical impedance spectroscopy (EIS) revealed lower charge transfer resistance for n-Cu/BP than for CuCo/BP and n-Co/BP. The catalyst demonstrated excellent stability over 22 hours of chronoamperometric testing and retained 90% of its initial activity after 2500 CV cycles. DFT calculations confirmed the electron-rich nature of Cu sites, the optimal ΔGH* for Cu/BP, and the enhanced water dissociation activity due to the synergistic effect of neighboring Cu atoms.

The findings demonstrate a successful strategy for synthesizing high-loading, stable SACs through a room-temperature photochemical method. The use of hydrogen radicals plays a crucial role in achieving high metal loading. The superior HER activity of n-Cu/BP compared to isolated Cu atoms and other metal combinations highlights the importance of neighboring Cu atoms' synergistic effects. The nearly zero Gibbs free energy of hydrogen adsorption and low activation energy for water dissociation contribute to the exceptional performance. These results offer a new direction in SAC design, emphasizing the importance of moderate coordination and neighboring atom effects for enhanced reaction kinetics. The success of this approach extends beyond Cu, suggesting its applicability to other metal SACs, thus providing a path for designing highly active and stable catalysts for various applications.

This research introduces a novel room-temperature photochemical strategy for synthesizing high-loading, stable single-atom catalysts (SACs). This method utilizes the synergistic effect of neighboring Cu atoms on a black phosphorus support, achieving an exceptional hydrogen evolution reaction (HER) performance. The findings highlight the importance of controlled coordination and neighboring atom interactions in optimizing SAC activity and stability, presenting a promising avenue for the future design of advanced catalytic materials. Future research could explore the application of this method to other catalytic reactions and investigate the effects of different support materials and metal combinations.

While this study demonstrates the high activity and stability of the n-Cu/BP catalyst, further investigations are needed to assess its long-term durability under more demanding conditions. The study focuses on alkaline HER; exploring its performance in acidic environments would broaden its applicability. The synthesis procedure could be optimized to further enhance the control over the density and arrangement of neighboring single atoms, leading to potential improvements in catalytic performance. Moreover, scaling up the synthesis process for industrial applications requires further study.