Tune the Fluorescence and Electrochemiluminescence of Graphitic Carbon Nitride Nanosheets by Controlling the Defect States

Abstract

The effects of defect states on the fluorescence (FL) and electrochemiluminescence (ECL) properties of graphite phase carbon nitride (g-CN) are systematically investigated for the first time. The g-CN nanosheets (CNNSs) obtained at different condensation temperatures are used as the study models. It can be found that all the CNNSs have two kinds of defect states, one is originated from the edge of CNNSs (labeled as CN-defect) and the other is attributed to the partially carbonization regions (labeled as C-defect). Both two kinds of defect states substantially affect the luminescent properties of CNNSs. Both the FL and ECL signals of CNNSs contain a band gap emission and two defect emissions. For the FL of CNNSs, decreasing the density of defect states can increase efficiently the FL quantum yield, while increasing the density of defect states can make the FL spectra red shift. For the ECL of CNNSs, increasing the density of CN-defect states and decreasing the density of C-defect states are greatly important to improve the ECL activity. This work provides a deep insight into the FL and ECL mechanisms of g-CN, and is of significance in tuning the FL and ECL properties of g-CN. Also, it will greatly promote the applications of CNNSs based on the FL and ECL properties.

Activity Enhancement of Defective Carbon Nitride for Photocatalytic Ammonia Generation by Modification with Pyrite

“…C3N4 as a polymeric material with a structure in between that of molecules and solid crystals, generally exhibits a prominent blue fluorescence, that has been shown to be significantly influenced by the introduction of defects. 30,93 Upon irradiation at wavelengths below the band gap energy, a broad fluorescence signal between 410 and 640 nm is observed for both VN-C3N4 and the composites, with a maximum emission at 470 nm and a slightly less intense emission at 445 nm, as shown in Figure 9 and Figure S38. The samples could be excited over a wide energy range from 250 to 450 nm (Figure S37) for both emission signals, signifying that radiative recombination proceeds from the same energy levels.…”

“…An additional tail towards higher wavelengths includes contributions from defect emissions, such as transitions from lone pair states of nitrogen atoms in the s-triazine unit. 94,30 The significantly increased inner band gap emission in relation to defect emission in the composites might indicate the inhibition of radiative recombination at defect sites, likely due to interaction of VN-C3N4 with FeS2 at the defect sites. Charge transfer might assist in exciton separation and retard recombination.…”

“…Mostly, emission peaks for C3N4 are decomposed into three signals. 30,74 We additionally used four Gauss functions to account for deviations at higher wavelengths that are apparent in the fit with three species (Figure S40). The determined emission signals were located at ~437 nm, 465 nm and 505 nm, or at 438 nm, 464 nm, 488 nm and 528 nm, respectively.…”

“…They likely correspond to inner band gap emission, emission from N-(C)3 sites and possibly either transition from electron lone pairs in NH-bridging nitrogen/ N-C bridging nitrogen, or from graphitized areas to the valence band. 94,30,93 The additional defect state emission at 488 nm could perhaps be related to either lone pairs in imine nitrogen in the vicinity of cyano groups/ nitrogen vacancies, or in the cyano groups themselves.…”

“…22 Another important type of vacancies in C3N4 for NRR are carbon vacancies, graphitized areas and possibly hydroxyl groups. 30,21 Furthermore, although thermal polymerization is an easy and versatile method, residual intermediates were observed the resulting C3N4, causing a high degrees of disorder. 31 Apart from defect engineering, the formation of heterojunctions, preferably of type II, can lead to an improvement of the photocatalytic activity with C3N4.…”