Mechanochemical Synthesis of Nitrogen-Deficient Mesopore-Rich Polymeric Carbon Nitride with Highly Enhanced Photocatalytic Performance

Polymeric carbon nitride is a non-toxic, low-cost, and chemically stable light-responsive photocatalyst that has attracted significant research interest stemming from its successful applications in carbon dioxide reduction, water splitting, organic synthesis, and contaminant degradation. Carbon nitride can be classified into crystalline carbon nitride and melon-type carbon nitride. However, the melon-type carbon nitride has a low specific surface area resulting in low surface reactive sites, which restrict photo-induced charge transfer across its layered structure. These two features are to blame for carbon nitride’s poor photocatalytic performance.

The creation of carbon and nitrogen defects to improve photo absorption and charge separation has been fronted as one of the methods suitable for improving the photoactivity of carbon nitride. As a result, the introduction of defects and mesopores in carbon nitride has been extensively studied in recent years as a modification strategy that’s high on relevant effective means.

Simultaneous introduction of nitrogen and/or carbon defects and microscopic morphologies in carbon nitride can enhance its photocatalytic performance, but effective methods are rarely reported. Moreover, virtually all reported strategies for defect creation and morphological control in carbon nitride have been based on single chemical treatment. Physical methods are hardly involved, though they should from cost-effectiveness and environmental protection points of view.

Mechanochemical or mechanical ball-milling is an important method used to synthesize nanomaterials, but it has hardly been used to prepare defect-rich carbon nitride. Researchers Guiming Ba (graduate student), Tingting Huo (graduate student), Dr. Quanhua Deng, Dr. Haiping Li, and led by Professor Wanguo Hou from Shandong University, China, successfully used a mechanochemical method to prepare mesopore-rich carbon nitride with nitrogen defects by mechanical ball-milling the intermediate with subsequent calcination. The prepared mesopore-rich carbon nitride with nitrogen vacancies exhibited improved photocatalytic activity compared with the bulk carbon nitride. The original research article is published in the journal Sustainable Chemistry & Engineering.

In their study carbon nitride was synthesized by direct pyrolysis of melamine. To prepare mesopore-rich defective carbon nitride, intermediates were prepared by calcining melamine. The resultant precursors were further processed by pyrolysis of melamine to obtain mesopore-rich carbon nitride with nitrogen defects. The photocatalysts were then subjected to photocatalytic hydrogen evolution tests.

The simultaneous formation of nitrogen defects and special morphology in carbon nitride resulted from structural distortion due to ball milling and breakage of intermolecular hydrogen bonds of the intermediate. Defective carbon nitride was obtained by ball-milling melamine and other intermediates like melam. The obtained mesopore-rich carbon nitride contained ultrathin nanosheets and exhibited increased specific surface area, improved charge separation, and increased visible light absorption compared with the bulk carbon nitride. Improvements in visible light absorption and charge separation resulted from the nitrogen vacancy-introduced defect level in the bandgap, reduced nanosheets thickness that favored charge separation, and increased specific surface area.

Defective carbon nitride exhibited an approximately 6.3-fold higher photocatalytic hydrogen evolution rate than the bulk carbon nitride under visible light. The mesopore-rich carbon nitride’s apparent quantum yields could reach up to 5.4 and 2.9% at 400 and 420nm wavelengths, respectively. The synthesized mesopore-rich carbon nitride with nitrogen defects also exhibited significantly high chemical stability during the photocatalytic test. Defective carbon nitride could also be mechanochemically synthesized using cyanamide and dicyandiamide as feedstocks and still exhibit high hydrogen evolution rates compared to bulk carbon nitride.

This study by Guiming Ba and colleagues provides a cost-effective yet environmentally-friendly industrializable method for preparing defective carbon nitride with high photoactivity. These findings pave the way for the mechanochemical synthesis of mesopore-rich nitrogen-deficient carbon nitride catalysts.