Transient Absorption Spectroscopy Reveals Performance-Limiting Factors in a Narrow-Bandgap Oxysulfide La5(Ti0.99Mg0.01)2CuS5O6.99 Photocatalyst for H2 Generation

Abstract

Mg-doped La5(Ti0.99Mg0.01)2CuS5O6.99 (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.