Single-Color and Two-Color Femtosecond Pump–Probe Experiments on Graphitic Carbon Nitrides Revealing Their Charge Carrier Kinetics

Abstract

Graphitic carbon nitride related materials have been one type of the most promising photocatalysts. Comprehensive understanding of their genuine charge carrier kinetic aspects will strengthen their photocatalytic applications. Herein, we synthesized two samples with different termination/functionalization groups, CN and NCN, and performed both single-color and two-color femtosecond pump–probe experiments on them. We found that both the recovery kinetics obtained from single-color femtosecond pump–probe experiments (SCFPPE) and the decay kinetics obtained from two-color femtosecond pump–probe experiments (TCFPPE) for CN and NCN display stronger pump fluence dependences, indicating that their genuine charge carrier kinetics should be measured under very low pump fluence. The charge carrier kinetics obtained under higher pump fluence would introduce a dramatically short picosecond component due to carrier–carrier interactions. With the obtained pump fluence-dependent charge carrier kinetics, for the first time, we derived the absorption cross section of CN and NCN at pump wavelength. The derived absorption cross section of CN at ∼398 nm is ∼5.2 × 10–15 cm2, and the derived absorption cross section of NCN at ∼398 nm is ∼8.3 × 10–15 cm2. Under very low pump fluence, we also found that the recovery kinetics of CN and NCN obtained from SCFPPE is obviously faster than their decay kinetics obtained from TCFPPE, suggesting that an additional fast decay photophysical process of CN and NCN might not be detected in TCFPPE. On the base of those femtosecond studies, we proposed a model for CN and NCN to understand how the pump fluence affect their charge carrier kinetics. We also discussed how the introduced termination/functionalization group in CN and NCN affects their genuine charge carrier kinetics so as to modify their photocatalytic hydrogen production activities. FTIR, XPS, XRD, and SEM EDX of CN and NCN were also measured for characterizing their chemical compositions and structures.