Studies on Ultrafast Photocarrier Recombination Mechanisms of the Rh-Doped BaTiO3 Photocatalyst

Rh-doped BaTiO₃ (BTO:Rh) is an emerging photocatalyst for solar hydrogen production by means of water splitting. Nanosecond time-resolved studies on the photocarrier relaxation dynamics have implied that the defects introduced by Rh doping will decrease the carrier lifetime, thus hindering the improvement of water-splitting efficiency with BTO:Rh. Given that these previous studies are measured with nanosecond or millisecond time intervals, while the photon-induced charge separations in fact occur within femtosecond time scales, one crucial question yet to be answered is as follows: what are the initial carrier relaxation mechanisms spanning from femtoseconds to picoseconds? Here, we employ the femtosecond ultrafast time-resolved optical pump–probe technique to investigate the relaxation dynamics of photocarriers generated in BTO:Rh specimens and compare them with undoped pure BTOs. Our results confirm that Rh defects indeed accelerate the trap-assisted recombination process and further reveal that the second-order recombination mechanism is also enhanced due to the doping of Rh. Therefore, there are two major mechanisms responsible for the lifetime of photocarriers in BTO:Rh. These findings may throw light on material engineering toward an enhanced water-splitting efficiency with BTO:Rh by extending its carrier lifetime.