Tunable Type I and II heterojunction of CoO_x nanoparticles confined in g-C₃N₄ nanotubes for photocatalytic hydrogen production

This work reports tunable heterojunction architectures of cobalt oxides (CoO_x) nanoparticles confined on well-arrayed graphitic carbon nitride nanotubes (C₃N₄ NTs) by using a facile one-pot method but under different annealing atmospheres. A Type II heterojunction of cobalt monoxide nanoparticles (CoO NPs)/C₃N₄ NTs was obtained after annealing under vacuum, and fine CoO NPs less than 8 nm in size were homogeneously anchored on the surface of C₃N₄ NTs. A Type I heterojunction of tricobalt tetraoxide (Co₃O₄)/C₃N₄ NTs were formed under air condition, and Co₃O₄ NPs in the size range of 20 to 80 nm were aggregated on the surface. The photocatalytic activities of these two heterojunctions were evaluated with hydrogen production from water splitting. The strategically developed CoO/C₃N₄ NTs with a 7 wt. % CoO shows the highest H₂ yield under visible light irradiation and the best stability among the photocatalysts studied in this work. Comprehensive characterization results reveal that the superior performance of CoO/C₃N₄ NTs may be attributed to the uniformly distributed smaller nanoparticles on the well-arrayed nanotubes, the longer lifetime of excited electrons, the faster charge transfer and the stronger electronic interaction between the heterojunctions. Our Kelvin probe force microscopy results firmly verify that the CoO/C₃N₄ NT and Co₃O₄/C₃N₄ NT nanocomposites form a Type II and Type I heterojunction, respectively, and charge transfer pathways and reaction mechanisms are therefore established.