A general method for type I and type II g-C3N4/g-C3N4 metal-free isotype heterostructures with enhanced visible light photocatalysis

Abstract

In order to address the fast charge recombination of pristine g-C₃N₄, easily available composite precursors such as dicyandiamide (melamine) and urea were used and thermally treated in situ creating type I and type II g-C₃N₄/g-C₃N₄ metal-free isotype heterostructures. The construction of these heterostructures was based on different band-alignment patterns (staggered and straddled band alignments). The confirmation of isotype g-C₃N₄/g-C₃N₄ heterostructures was based on X-ray diffraction, photoluminescence, transmission electron microscopy, and valence band X-ray photoelectron spectroscopy. For g-C₃N₄/g-C₃N₄ heterostructures from dicyandiamide and urea (type II, staggered band alignments) under visible light, the photogenerated electrons in the conduction band of CN-D (g-C₃N₄ from dicyandiamide) could transfer to the conduction band of CN-U (g-C₃N₄ from urea) driven by an offset of 0.04 eV, whereas the photogenerated holes could transfer from CN-U to CN-D driven by a valence band offset of 0.36 eV, thus photogenerated electrons and holes could be separated effectively. For g-C₃N₄/g-C₃N₄ heterostructures from melamine and urea (type I, straddled band alignments), the photo-induced electrons could transfer from CN-U to CN-M (g-C₃N₄ from melamine) driven by a conduction band offset of 0.17 eV, while photogenerated holes could not be transported from one side to another side, also promoting the separation of photo-induced electrons and holes. The intrinsic drawback of fast charge recombination of pristine g-C₃N₄ was overcome by formation of type I and type II g-C₃N₄/g-C₃N₄ heterostructures. For the removal of ppb-level NO in air, the type I and type II g-C₃N₄ based heterostructures demonstrated highly enhanced photocatalytic activity and stability in comparison with g-C₃N₄ alone, which could be directly ascribed to the promoted charge separation. The rational design and construction of type I and type II isotype heterojunctions was general and powerful for the development of efficient visible-light photocatalysts with potential large scale environmental and energetic applications. The present work could also enrich new types of visible-light heterostructured photocatalysts, which may find wide application in other areas.