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

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 xFA1- xPb(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.