Transient Absorption Spectroscopy Reveals Performance-Limiting Factors in a Narrow-Bandgap Oxysulfide La₅(Ti_0.99Mg_0.01)₂CuS₅O_6.99 Photocatalyst for H₂ Generation

Mg-doped La₅(Ti_0.99Mg_0.01)₂CuS₅O_6.99 (Mg-LTC) is an oxysulfide-based H₂ evolution photocatalyst with a bandgap of ≈1.84 eV. In addition, Mg-LTC has also been successfully integrated as a photocathode in a Z-scheme-based overall water-splitting process. Despite 15 years of intensive research, LTC-based photocatalysts are yet to demonstrate high photoconversion efficiency. To this end, transient absorption spectroscopy was employed to unveil the key loss processes reducing the efficiency of H₂ evolution in Mg-LTC. Charge carrier dynamics from sub-picosecond to microsecond was probed over a broad spectral region (visible to IR) for both the Mg-LTC powder and Mg-LTC/Au photocathode prepared by the particle-transfer method with Au back contact. A faster decay of hole dynamics (700 nm probe) for the Mg-LTC/Au photocathode than Mg-LTC powder indicated hole transfer from Mg-LTC to Au with a time constant of 0.5 ps and 12.5% efficiency. At least 50% of the electrons probed at 3435 nm decayed by trapping to defects in tens of picosecond. As a result, the key electron-transfer process from Mg-LTC to the Pt cocatalyst, which determines the H₂ evolution efficiency, occurred from the microsecond long-lived electrons with a time constant of 0.26 μs and 30.5% efficiency. The results provide insight into the perspective of material-design and photocathode fabrication procedure to further advance the H₂-generation efficiency in Mg-LTC.