C₃N₅: A Low Bandgap Semiconductor Containing an Azo-Linked Carbon Nitride Framework for Photocatalytic, Photovoltaic and Adsorbent Applications

Abstract

Modification of carbon nitride based polymeric 2D materials for tailoring their optical, electronic and chemical properties for various applications has gained significant interest. The present report demonstrates the synthesis of a novel modified carbon nitride framework with a remarkable 3:5 C:N stoichiometry (C3N5) and an electronic bandgap of 1.76 eV, by thermal deammoniation of the melem hydrazine precursor. Characterization revealed that in the C3N5 polymer, two s-heptazine units are bridged together with azo linkage, which constitutes an entirely new and different bonding fashion from g-C3N4 where three heptazine units are linked together with tertiary nitrogen. Extended conjugation due to overlap of azo nitrogens and increased electron density on heptazine nucleus due to the aromatic π network of heptazine units lead to an upward shift of the valence band maximum resulting in bandgap reduction down to 1.76 eV. XRD, He-ion imaging, HR-TEM, EELS, PL, fluorescence lifetime imaging, Raman, FTIR, TGA, KPFM, XPS, NMR and EPR clearly show that the properties of C3N5 are distinct from pristine carbon nitride (g-C3N4). When used as an electron transport layer (ETL) in MAPbBr3 based halide perovskite solar cells, C3N5 outperformed g-C3N4, in particular generating an open circuit photovoltage as high as 1.3 V, while C3N5 blended with MA x FA1–x Pb­(I0.85Br0.15)3 perovskite active layer achieved a photoconversion efficiency (PCE) up to 16.7%. C3N5 was also shown to be an effective visible light sensitizer for TiO2 photoanodes in photoelectrochemical water splitting. Because of its electron-rich character, the C3N5 material displayed instantaneous adsorption of methylene blue from aqueous solution reaching complete equilibrium within 10 min, which is significantly faster than pristine g-C3N4 and other carbon based materials. C3N5 coupled with plasmonic silver nanocubes promotes plasmon-exciton coinduced surface catalytic reactions reaching completion at much low laser intensity (1.0 mW) than g-C3N4, which showed sluggish performance even at high laser power (10.0 mW). The relatively narrow bandgap and 2D structure of C3N5 make it an interesting air-stable and temperature-resistant semiconductor for optoelectronic applications while its electron-rich character and intrasheet cavity make it an attractive supramolecular adsorbent for environmental applications.

Direct Observation of Dynamic Bond Evolution in Single‐Atom Pt/C3N4 Catalysts

“…As compared with the relatively low SI‐DRIFTS spectrum of S‐Pt‐C 3 N 4 under dark conditions, the characteristic peaks were significantly enhanced under light irradiation (Figure C). More specifically, the peak at 821 cm −1 is attributed to the breathing vibration of heptazine rings, and the peak at 1006 cm −1 is indexed to the N−H bending vibration . The broad band at 1200–1700 cm −1 corresponds to the stretching vibration mode of CN heterocycles with peaks positioned at 1697, 1493, and 1341 cm −1 .…”

Thermodynamically Stable Mesoporous C3N7 and C3N6 with Ordered Structure and Their Excellent Performance for Oxygen Reduction Reaction

“…Because of its semiconducting property and diverse functionalities, the g‐C 3 N 4 attracts intense research interest for water splitting, redox catalysis, sensing and gas adsorption . Besides the g‐C 3 N 4 , nonstoichiometric and N‐rich carbon nitrides such as C 3 N 5 and C 3 N 6 have been explored by many researchers . Our group was successful in preparing mesoporous s‐heptazine‐based C 3 N 5 with terminal groups of triazole using a precursor of 3‐amino‐1,2,4‐triazole which showed excellent photocatalytic and sensing properties .…”

“…Our group was successful in preparing mesoporous s‐heptazine‐based C 3 N 5 with terminal groups of triazole using a precursor of 3‐amino‐1,2,4‐triazole which showed excellent photocatalytic and sensing properties . Very recently, Kumar et al used azide precursor of 2,5,8‐trihydrazino‐s‐heptazine to demonstrate azo‐linked but nonporous C 3 N 5 composed of s‐heptazine core structure . In comparison to g‐C 3 N 4 , the azo‐linked C 3 N 5 exhibits significantly modified physicochemical properties such as a narrow bandgap and electron‐rich basicity …”

“…Very recently, Kumar et al used azide precursor of 2,5,8‐trihydrazino‐s‐heptazine to demonstrate azo‐linked but nonporous C 3 N 5 composed of s‐heptazine core structure . In comparison to g‐C 3 N 4 , the azo‐linked C 3 N 5 exhibits significantly modified physicochemical properties such as a narrow bandgap and electron‐rich basicity …”

Exfoliation and Sensitization of 2D Carbon Nitride for Photoelectrochemical Biosensing under Red Light

“…To determine whether TsCuPc antennas were attached to the CNNS substrate by noncovalent interaction, FTIR and Raman spectroscopy were employed. The FTIR spectrum of CNNS‐TsCuPc (Figure a) showed strong peaks at 810 and 1200–1630 cm −1 , reminiscent of the typical tri‐ s ‐triazine ring sextant out‐of‐plane bending and stretching vibration modes (CN heterocycles) of bulk CN . Meanwhile, CNNS‐TsCuPc exhibited extra peaks in the range of 690–800 cm −1 , ascribed to the characteristic fingerprint region of TsCuPc .…”