This review summarizes recent advances in the design, synthesis, mechanistic understanding and multifunctional applications of g-C₃N₄based heterojunctions/heterostructures.

The photocatalytic performance of the star photocatalyst g-C₃N₄ was restricted by the low efficiency because of the fast charge recombination. The present work developed a facile in situ method to construct g-C₃N₄/g-C₃N₄ metal-free isotype heterojunction with molecular composite precursors with the aim to greatly promote the charge separation. Considering the fact that g-C₃N₄ samples prepared from urea and thiourea separately have different band structure, the molecular composite precursors of urea and thiourea were treated simultaneously under the same thermal conditions, in situ creating a novel layered g-C₃N₄/g-C₃N₄ metal-free heterojunction (g-g CN heterojunction). This synthesis method is facile, economic, and environmentally benign using easily available earth-abundant green precursors. The confirmation of isotype g-g CN heterojunction was based on XRD, HRTEM, valence band XPS, ns-level PL, photocurrent, and EIS measurement. Upon visible-light irradiation, the photogenerated electrons transfer from g-C₃N₄ (thiourea) to g-C₃N₄ (urea) driven by the conduction band offset of 0.10 eV, whereas the photogenerated holes transfer from g-C₃N₄ (urea) to g-C₃N₄ (thiourea) driven by the valence band offset of 0.40 eV. The potential difference between the two g-C₃N₄ components in the heterojunction is the main driving force for efficient charge separation and transfer. For the removal of NO in air, the g-g CN heterojunction exhibited significantly enhanced visible light photocatalytic activity over g-C₃N₄ alone and physical mixture of g-C₃N₄ samples. The enhanced photocatalytic performance of g-g CN isotype heterojunction can be directly ascribed to efficient charge separation and transfer across the heterojunction interface as well as prolonged lifetime of charge carriers. This work demonstrated that rational design and construction of isotype heterojunction could open up a new avenue for the development of new efficient visible-light photocatalysts.

In order to develop efficient visible light driven photocatalysts for environmental applications, novel polymeric g-C₃N₄ layered materials with high surface areas are synthesized efficiently from an oxygen-containing precursor by directly treating urea in air between 450 and 600 °C, without the assistance of a template for the first time. The as-prepared g-C₃N₄ materials with strong visible light absorption have a band gap around 2.7 eV. The crystallinity and specific surface areas of g-C₃N₄ increases simultaneously when the heating temperatures increases. The g-C₃N₄ materials are demonstrated to exhibit much higher visible light photocatalytic activity than that of C-doped TiO₂ and g-C₃N₄ prepared from dicyanamide for the degradation of aqueous RhB. The large surface areas, layered structure and band structure in all contributed to the efficient visible light photocatalytic activity. The efficient synthesis method for g-C₃N₄ combined with efficient photocatalytic activity is of significant interest for environmental pollutants degradation and solar energy conversion in large scale applications.

The recent progress, challenges and promising future on design, synthesis and fabrication of MnO ₂ for supercapacitors are reviewed and discussed.