The development of efficient and stable photoelectrochemical (PEC) water splitting devices is crucial for sustainable energy production. Noble metal catalysts, particularly iridium (Ir), have shown promise for oxygen evolution reaction (OER), a key half-reaction in water splitting. However, the high cost and limited abundance of noble metals hinder their widespread application. Single-atom catalysts (SACs) have emerged as a potential solution, offering high catalytic activity and efficiency while minimizing material usage. SACs not only maintain the inherent catalytic efficiency of noble metals but also exhibit unique geometric and electronic structures due to strong metal-support interactions. The precise control over the coordination environment of single atoms is crucial for optimizing catalytic performance. Despite the advantages of SACs, their application in PEC water splitting remains limited due to challenges in creating stable anchor sites on photoelectrodes and analyzing the charge carrier kinetics at the single-atom level. Unlike electrochemical systems, PEC systems require efficient charge separation and transport to the electrolyte, making the evaluation of surface recombination crucial for performance optimization. This study aims to address these challenges by developing a novel PEC photoanode using atomically dispersed Ir SACs, investigating the charge carrier kinetics through IMPS and EIS, and validating the findings with DFT calculations.

Previous research has explored various strategies to enhance PEC water splitting efficiency, including the use of different semiconductor materials, surface modifications, and cocatalysts. Studies have demonstrated the effectiveness of metal-insulator-semiconductor (MIS) structures in improving photovoltage by reducing Fermi level pinning. The use of thin films of catalytic materials like NiO/Ni has shown promise for enhancing both catalytic activity and stability in alkaline environments. Recent work on SACs has highlighted their potential for various catalytic applications, demonstrating their superior activity compared to nanoparticles or bulk materials. However, the application of SACs in PEC water splitting has been limited, with few studies investigating the charge transfer kinetics at the single-atom level. This gap in understanding necessitates a comprehensive study that combines experimental characterization with theoretical modeling to elucidate the role of SACs in PEC water oxidation.

The researchers fabricated an Ir SAs/NiO/Ni/ZrO2/n-Si photoanode. A 2 nm thick Ni layer was deposited on n-type silicon, followed by a 1 nm ZrO2 layer using electron beam evaporation. Atomically dispersed Ir single atoms were then deposited onto the NiO/Ni surface using a single cycle of atomic layer deposition (ALD). The morphology, structure, and chemical state of the Ir catalysts were characterized using high-resolution transmission electron microscopy (HR-TEM), aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). The photoelectrochemical (PEC) performance of the fabricated photoanodes was evaluated using linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), and incident photon-to-current conversion efficiency (IPCE) measurements in 1 M NaOH electrolyte. Long-term stability was assessed by chronoamperometry. Density functional theory (DFT) calculations, including climbing image nudged elastic band (CI-NEB) calculations, were performed to elucidate the catalytic mechanism of Ir single atoms in OER. The DFT calculations used the Vienna Ab initio Simulation Package (VASP) with the projector augmented wave method and generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) for the exchange-correlation potentials. Hubbard-U correction was applied to account for on-site correlation effects.

HAADF-STEM images clearly showed the atomically dispersed Ir single atoms on the NiO surface. XPS and XAS analyses confirmed that the Ir atoms were primarily in the +3 to +4 oxidation states, indicating charge transfer between Ir and NiO. The Ir SAs/NiO/Ni/ZrO2/n-Si photoanode exhibited a record-high photocurrent density of 27.7 mA cm⁻² at 1.23 V vs. RHE, significantly outperforming photoanodes with Ir nanoclusters or films. The improved performance was attributed to the enhanced charge transfer efficiency and suppressed charge recombination, as evidenced by IMPS and EIS measurements. The Ir single atoms showed the lowest charge transfer resistance (Rct) and the highest charge transfer efficiency (ηtrans) compared to Ir nanoclusters and films. DFT calculations revealed that the isolated Ir atoms lower the thermodynamic energy barrier of the potential-determining step (PDS) in OER, specifically the conversion of O* to OOH*. The calculated overpotential for Ir SAs/NiO (100) was 0.621 V, lower than that of NiO (100) (1.09 V) and IrO2 (110) (0.633 V). Bader charge analysis indicated strong local polarization between Ir single atoms and OH intermediates, facilitating hole transport to the adsorbates. The photoanode exhibited remarkable stability, maintaining its performance for over 130 h, attributed to the robust NiO/Ni catalyst and the strong interaction between Ir single atoms and the support.

The results demonstrate that the synergistic combination of Ir single-atom catalysis, NiO/Ni cocatalyst, and ZrO2 insulating layer leads to significantly enhanced PEC performance. The atomic dispersion of Ir atoms maximizes the number of active sites while minimizing material usage. The efficient charge transfer and suppressed recombination are crucial for achieving high photocurrent density. The DFT calculations provide atomic-level insights into the catalytic mechanism, confirming the role of Ir single atoms in lowering the energy barrier for OER. The long-term stability highlights the potential of this approach for practical applications. The findings challenge the conventional approach of using high loadings of noble metal catalysts in PEC water splitting, suggesting a more sustainable and cost-effective alternative.

This study successfully demonstrates a high-performance photoanode for PEC water oxidation using atomically dispersed Ir single atoms as catalysts. The record-high photocurrent density, exceptional stability, and mechanistic understanding provided by DFT calculations establish this approach as a promising strategy for developing efficient and durable PEC devices. Future research could focus on exploring other SACs for PEC applications, investigating different support materials, and optimizing the photoanode architecture for further performance enhancement.

The study focused on a specific Ir precursor and ALD conditions, and the generalizability of the findings to other precursors or deposition methods requires further investigation. The DFT calculations were performed under specific conditions and might not fully capture the complex interactions present in the real system. The long-term stability test was conducted under specific conditions and further testing under various environmental conditions would be beneficial.