The photophysical properties of carbon nitride (C 3 N 4 ) are very basic and important in the understanding of its photocatalytic activity. Herein, we measured the UV−vis diffuse reflectance (UVDR) and fluorescence spectra of C 3 N 4 prepared at different temperatures and studied their fluorescence decay kinetics under different wavelengths and different fluences of light excitation. We found, first, that the fluorescence lifetime of C 3 N 4 under visible (465 nm) light excitation is shorter than that under UV (395 nm) light excitation; second, that the fluorescence lifetime of C 3 N 4 under 465 nm light excitation decreases as increasing its processing temperature; and third, that the fluorescence lifetime of C 3 N 4 under 395 nm light excitation decreases with the increase of the light excitation fluence. These findings revealed that the two distinct absorption bands in the UVDR spectrum of C 3 N 4 arise from two different transitions of C 3 N 4 and that the origin of the fluorescence emission of C 3 N 4 arises from its singlet exciton. Besides, the photocatalytic H 2 evolutions of C 3 N 4 synthesized at different temperatures under visible light irradiation were also measured and discussed to correlate with the obtained photophysical properties of C 3 N 4 .
Carbon nitrides (CN) have been widely used in photocatalytic applications. However, the charge carrier kinetics of CN after light excitation remains unclear. Herein, we prepared a stable and transparent CN colloid in an aqueous tetraethylammonium hydroxide solution and investigated its carrier kinetics using both femtosecond transient absorption and picosecond time-resolved fluorescence spectroscopy. We found that a new and positive absorption band appears in the femtosecond transient absorption spectrum of the CN colloid, which could be attributed to the absorption of the photogenerated electron/hole pairs (or the electronic excited state) of the CN colloid after light excitation. Moreover, we found that the charge carrier kinetics obtained from the femtosecond transient absorption measurements is dramatically different from that obtained from the picosecond time-resolved fluorescence measurements, indicating that the photophysical process of the CN colloid after light excitation is complicated. With the results obtained from both the femtosecond transient absorption and picosecond time-resolved fluorescence measurements, we proposed a schematic to understand the photophysics and charge carrier kinetics of the CN colloid. We believe that the current study is also significant for researchers to understand the photophysics and charge carrier kinetics of bulk CN.
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