Metal-free hybrid heterostructures based on double-walled carbon nanotubes and graphitic carbon nitride for efficient hydrogen evolution in photocatalytic water-splitting reaction

Solar hydrogen production from water has emerged as a viable method for sustainable energy production. The search for efficient and sustainable photocatalysts has led to considerable interest in graphitic carbon nitride (g-C 3 N 4 ) due to its suitable band gap ( Eg ) in the visible region and electronic properties. Certain limitations of the high recombination rate of g-C 3 N 4 paved the way for the development of nano-hybrid structures with carbon nanotubes (CNT) to increase the separation efficiency of the photogenerated electrons and holes. In this study, we used a facile synthesis process for g-C 3 N 4 :CNT hybrids. We varied both the ratio between 2 usual precursors (melamine and urea) for the synthesis of g-C 3 N 4 and the ratio of CNT, while comparing the influence of the surface chemistry of the nanotubes (as-synthesised or oxidised), with the assumption that oxidation would improve the dispersion of carbon nanotubes and thus increase the interactions between the two nanomaterials, ultimately improving the performance of the system. The samples were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller analysis (BET), elemental analysis (EA), X-ray photoelectron spectroscopy (XPS) and UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). Finally, samples were tested as photocatalysts for hydrogen evolution under simulated solar illumination. A photocatalytic activity of ca. 260 μmol/g in ca. 3 h was obtained for the most active catalyst. This photoactivity was attributed to the enhanced charge transfer effect at the interface between g-C 3 N 4 and CNT, which effectively reduced the recombination rate of photogenerated electrons and holes.