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

Graphitic carbon nitride related materials have beenone type ofthe most promising photocatalysts. Comprehensive understanding oftheir genuine charge carrier kinetic aspects will strengthen theirphotocatalytic applications. Herein, we synthesized two samples withdifferent termination/functionalization groups, CN and NCN, and performedboth single-color and two-color femtosecond pump-probe experimentson them. We found that both the recovery kinetics obtained from single-colorfemtosecond pump-probe experiments (SCFPPE) and the decay kineticsobtained 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 measuredunder very low pump fluence. The charge carrier kinetics obtainedunder higher pump fluence would introduce a dramatically short picosecondcomponent due to carrier-carrier interactions. With the obtainedpump 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 similar to 398 nm is similar to 5.2x 10(-15) cm(2), and the derived absorptioncross section of NCN at similar to 398 nm is similar to 8.3 x 10(-15) cm(2). Under very low pump fluence, wealso found that the recovery kinetics of CN and NCN obtained fromSCFPPE is obviously faster than their decay kinetics obtained fromTCFPPE, suggesting that an additional fast decay photophysical processof CN and NCN might not be detected in TCFPPE. On the base of thosefemtosecond studies, we proposed a model for CN and NCN to understandhow the pump fluence affect their charge carrier kinetics. We alsodiscussed how the introduced termination/functionalization group inCN and NCN affects their genuine charge carrier kinetics so as tomodify their photocatalytic hydrogen production activities. FTIR,XPS, XRD, and SEM EDX of CN and NCN were also measured for characterizingtheir chemical compositions and structures.

Automated summary

Graphitic carbon nitride related materials are promising photocatalysts. Understanding their genuine charge carrier kinetics is important for their photocatalytic applications. Two samples with different termination/functionalization groups were synthesized and studied using single-color and two-color femtosecond pump-probe experiments. The results showed that the charge carrier kinetics of CN and NCN should be measured under very low pump fluence, and higher pump fluence would introduce a short picosecond component due to carrier-carrier interactions. The derived absorption cross section of CN and NCN at pump wavelength were also determined. Overall, this study provides insights into the charge carrier kinetics of CN and NCN and their photocatalytic activities.