Strategies for the Fabrication of 2D Carbon Nitride Nanosheets

Layered graphitic carbon nitride (g-C3N4) is a typical polymeric semiconductor with an sp(2) pi-conjugated system having great potential in energy conversion, environmental purification, materials science, etc., owing to its unique physicochemical and electrical properties. However, bulk g-C3N4 obtained by calcination suffers from a low specific surface area, rapid charge carrier recombination, and poor dispersion in aqueous solutions, which limit its practical applications. Controlling the size of g-C3N4 (e.g., preparing g-C3N4 nanosheets) can effectively solve the above problems. Compared with the bulk material, g-C3N4 nanosheets have a larger specific surface area, richer active sites, and a larger band gap due to the quantum confinement effect. As g-C3N4 has a layered structure with strong in-plane C-N covalent bonds and weak van der Waals forces between the layers, g-C3N4 nanosheets can be prepared by exfoliating bulk g-C3N4. Alternatively, g-C3N4 nanosheets can otherwise be obtained through the anisotropic assembly of organic precursors. Nevertheless, some of these methods have various limitations, such as high energy consumption, are time consuming, and have low yield. Accordingly, developing green and cost-effective exfoliation and preparation strategies for g-C3N4 nanosheets is necessary. Herein, the research progress of the exfoliation and preparation strategies (including the thermal oxidation etching process, the ultrasound-assisted route, the chemical exfoliation, the mechanical method, and the template method) for two-dimensional C3N4 nanosheets are introduced. Their features are systematically analyzed and the perspectives and challenges in the preparation of g-C3N4 nanosheets are discussed. This study emphasizes the following: (1) The preparation method of g-C3N4 nanosheets should be properly selected according to the practical application needs. Additionally, various strategies (such as chemical method and ultrasonic method) can be combined to exfoliate nanosheets from bulk g-C3N4; (2) More reasonable nano- or even subnanostructured g-C3N4 nanosheets should be continuously explored; (3) Novel modification strategies, such as defective engineering, heterojunction construction, and surface functional group regulation, should be introduced to improve the reactivity and selectivity of the g-C3N4 nanosheets; (4) The application of in situ characterization techniques (such as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), electron spin resonance (ESR) spectroscopy, and Raman spectroscopy) should also be strengthened to monitor the detailed catalytic process and investigate the g-C3N4 nanosheet structure-efficiency relationship. (5) To gain a deeper understanding of the relationship between the macroscopic properties and the microscopic structure, the combination of theoretical calculations and experimental results should be strengthened, which will be beneficial for exploiting high-quality g-C3N4 nanosheets.