Fabrication of Tubular g‐C3N4 with N‐Defects and Extended π‐Conjugated System for Promoted Photocatalytic Hydrogen Production

Abstract Semiconductor photocatalytic hydrogen evolution from water splitting using solar energy is one of the most potential strategies for settling the energy crisis, but a bottleneck is occurred in developing a highly efficient photocatalyst. Herein, we successfully fabricated a tubular g‐C 3 N 4 with nitrogen defects (including both nitrogen vacancies and cyano groups) and extended π‐conjugated system, wherein the nitrogen defects and extended π‐conjugated system were conveniently introduced during thermal polymerization of the pyridinium alkaline ionic liquid‐modified hydrogen‐bonding melamine‐cyanuric acid supramolecular (PHMCS) pre‐aggregates towards carbon nitride with hexagonal cylinder morphology formed by the pyridinium alkaline ionic liquid‐assisted hydrothermal process of low‐cost melamine, in which the added trace pyridinium alkaline ionic liquid serves as an all‐rounder with three roles including the promoting agent towards the transformation of melamine to cyanuric acid for constructing PHMCS, nitrogen defects making agent, and the electronic‐tuning agent regarding of the as‐formed carbon nitride. The tubular structure enhances the light scattering and improves the accessibility of active sites. Meanwhile, the high charge transfer efficiency, broad optical absorption region, and suitable energy band positions are related to both the introduction of nitrogen defects by alkali of ionic liquid and the extended π‐electron delocalization in the conjunction carbon nitride networks. As a consequence, the resulted carbon nitride using optimized amount of pyridinium alkaline ionic liquid (AIL‐2.0‐CN) photocatalyst exhibited a high hydrogen evolution rate of 1284 μmol h −1 g −1 (λ>420 nm) with an apparent quantum efficiency of 4.2 % at 420 nm, which is 13.6 times higher than that of bulk g‐C 3 N 4 (B−CN), and even 3.5 folds much higher than the one (SHPY−CN) prepared by the similar process to that for AIL‐2.0‐CN except for the replacement of pyridinium alkaline ionic liquid by a mixture containing a corresponding amount of pyridine and sodium hydroxide, suggesting the promoting effect resulting from the unique characteristic of pyridinium alkaline ionic liquid. Based on the unique physicochemical properties and the molecules‐ adjustability, the developed alkaline ionic liquid assisted strategy can open an avenue for designing and preparing excellent g‐C 3 N 4 based photocatalysts with tunable geometric and electronic structures for solar hydrogen production.