Green synthesis of rectangular hollow tubular carbon nitride via in-situ self-assembly strategy to enhance the degradation of tetracycline hydrochloride under visible light irradiation.

It has been an urgent requirement for materials with remarkable performance in the photocatalytic degradation of organic contaminants by photocatalytic technology. Limited surface area and speedy recombination rate of photogenerated charge carriers seriously restrain the application of g-C3N4. Morphology control is a powerful approach to enhance the photocatalytic efficiency of g-C3N4. Herein, we reported a method to attain graphitic carbon nitride with rectangular hollow tubular morphology and asperous surface (TUM-CN-2) which is prepared from urea-melamine hydrothermal products and trithiocyanuric acid by self-assembling without using organic solvents or template agents. The specific surface area, photocatalytic activity, and photo-generated carriers migration and separation rate of the obtained photocatalyst TUM-CN-2 are vastly improved. Contrasted with pure g-C3N4, the degradation rate of tetracycline hydrochloride (TCH) and Rhodamine B (RhB) was enhanced about 3.04 and 13.96 times in visible light irradiation, respectively. Moreover, the interference parameters, active free radicals, potential degradation mechanism, and degradation paths of TCH were researched systematically. This work provides a green way to acquire the modified g-C3N4 with splendid catalytic activity through the self-assembly method.

Multi-Color Fluorescent Carbon Dots: Graphitized sp2 Conjugated Domains and Surface State Energy Level Co-Modulate Band Gap Rather Than Size Effects.

Four types of carbon dots (CDs) with various color (blue, green, yellow, and red) emissions have been synthesized under solvent-free conditions from citric acid and different nitrogen sources (DMF, urea, ethanamide, and formamide). By detailed characterization and comparison, it is confirmed that the graphitized sp2 conjugated domain and surface functional groups such as C-O and C=N play synergetic roles in adjusting the fluorescence properties. Notably, the size effect is not the dominant mechanism to achieve multi-color fluorescence emissions in this work. The structural configuration of the carbon dots further influences the energy band structure, as demonstrated in simplified energy level diagrams. An absorption peak at approximately 560 nm appears in the visible light region for red-emitting CDs, assigned to an n→π* transition of the aromatic structure, thus introducing a new surface state energy level, resulting in a reduction in the energy of electron transition and the expansion into the visible region of the UV/Vis spectrum. Taking advantage of the diverse absorption and emission properties, different CDs/TiO2 binary composites are obtained for photocatalytic degradation of organic dyes, and it is found that the absorption range in terms of visible light and the band gap of the carbon dots make a difference to the photocatalytic performance of the composites.