Tailoring of crystalline structure of carbon nitride for superior photocatalytic hydrogen evolution

Template-free preparation of carbon nitride hollow spheres with adjustable sizes for photocatalytic hydrogen generation

Carbon nitride hollow spheres (CNHS) with adjustable sizes were successfully fabricated via a template-free supramolecular pre-assembly strategy, in which melamine-cyanuric acid (MCA) hollow spheres were constructed through hydrogen bonds. A feasible formation mechanism was proposed, which coupled an inside-out Ostwald ripening with the supramolecular pre-assembly process. Interestingly, the sizes of MCA could be manipulated by changing the pre-assembly temperature. Consequently, the sizes of CNHS were adjustable. The optimal CNHS exhibited excellent photocatalytic hydrogen evolution rate (98.6 μmol/h) in the visible-light region, which was approximately 11 times higher than that of bulk carbon nitride calcined by melamine. The significantly improved performance was due to the contributions including: the unique architectures with remarkable light absorption ability, high electrical conductivity, relatively narrowed band gap, fast charge separation. This work provides a facile template-free supramolecular pre-assembly strategy to fabricate carbon nitride hollow spheres with adjustable sizes for the first time.

The tremendous boost for photocatalytic properties of g-C3N4: regulation from polymerization kinetics to crystal structure engineering

Graphite carbon nitride (g-C3N4) has become research hotspot owing to its special electronic structure and excellent chemical stability. Although g-C3N4 has made great progress in the field of photocatalysis, its...

Structure modulation of g-C3N4 in TiO2{001}/g-C3N4 hetero-structures for boosting photocatalytic hydrogen evolution

Structure design of photocatalysts is highly desirable for taking full advantage of their abilities for H2 evolution. Herein, the highly-efficient TiO2{001}/g-C3N4 (TCN) heterostructures have been fabricated successfully via an in situ ethanol-thermal method. And the structure of g-C3N4 in the TCN heterostructures could be exfoliated from bulk g-C3N4 to nanosheets, nanocrystals and quantum dots with the increase of the synthetic temperature. Through detailed characterization, the structural evolution of g-C3N4 could be attributed to the enhanced temperature of the ethanol-thermal treatment with the shear effects of HF acid. As expected, the optimal TCN-2 heterostructure shows excellent photocatalytic H2 evolution efficiency (1.78 mmol h-1 g-1) under visible-light irradiation. Except for the formed built-in electric field, the significantly enhanced photocatalytic activity of TCN-2 could be ascribed to the enhanced crystallinity of TiO2{001} nanosheets and the formed g-C3N4 nanocrystals with large surface area, which could extend the visible light absorption, and expedite the transfer of photo-generated charge carriers further. Our work could provide guidance on designing TCN heterostructures with the desired structure for highly-efficient photocatalytic water splitting.

Ultra-small molybdenum sulfide nanodot-coupled graphitic carbon nitride nanosheets: Trifunctional ammonium tetrathiomolybdate-assisted synthesis and high photocatalytic hydrogen evolution

The preparation of nanoscale molybdenum sulfide (MoS₂)-modified graphitic carbon nitride (g-C₃N₄) nanosheets usually contains complex and multiple-step operations, including the separate synthesis of nanoscale MoS₂ and g-C₃N₄ nanosheet, and their subsequent composite process. To effectively overcome the above drawbacks, herein, a facile one-step trifunctional ammonium tetrathiomolybdate ((NH₄)₂MoS₄)-assisted approach has been designed to produce ultra-small MoS_x nanodot-coupled g-C₃N₄ nanosheet photocatalyst, including the first addition of ammonium chloride (NH₄Cl) and (NH₄)₂MoS₄ into melamine precursors and their following one-step calcination. During high-temperature calcination, except for the promoting generation of the g-C₃N₄ nanosheets by produced ammonia (NH₃) and hydrogen sulfide (H₂S) gases, the above (NH₄)₂MoS₄ decomposition not only can efficiently clip the s-heptazine framework to produce more terminal amino groups and cyano groups, but also can produce ultra-small MoS_x nanodots on the resultant g-C₃N₄ nanosheet surface, resulting in the final production of ultra-small MoS_x nanodot-coupled g-C₃N₄ nanosheets. The resultant MoS_x nanodot-coupled g-C₃N₄ nanosheets evidently exhibit increased photocatalytic hydrogen (H₂)-generation rate, about 8-fold increase to the traditional MoS₂-modified g-C₃N₄ photocatalyst. The increased H₂-generation rate can be mainly attributed to the synergism of MoS_x nanodots and cyano group on the g-C₃N₄ nanosheet surface. The current facile technology could open the sights for the preparation of other high-efficiency photocatalysts.