| 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 MAₓFA₁₋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. |
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