CVD Grown Sub 10 nm Size g-C₃N₄ Particle-Decorated TiO₂ Nanotube Array Composites for Enhanced Photocatalytic H₂ Production

TiO₂ nanotube arrays (NTA) have attracted much attention among photocatalysts because of their large specific surface area and easy surface transfer of excited electrons, and in recent years, attempts have been made to further improve their properties by forming Z-schemes when they are composited with other photocatalysts. However, as the spacing within and between nanotubes is only a few nanometers, the formation of heterojunctions is extremely difficult when TiO₂-NTA is composited with other photocatalytic materials with larger grain sizes. Creating nanoparticle photocatalysts with dimensions smaller than those of the nanotube system is thus required to effectively form heterojunctions. We have constructed an original vacuum chemical vapor deposition (CVD) system with fine temperature control, an attribute that we believe is necessary for the preparation of small nanoparticles. Using this system, it is possible to greatly reduce the polymerization rate of melamine, the precursor of the carbon nitride (g-C₃N₄) photocatalyst, which offers the benefits of increased reduction power and a metal-free composition. As a result, g-C₃N₄ small nanoparticles with particle sizes of about 10 nm were successfully prepared, and heterojunctions could be formed even inside TiO₂-NTA. The fabricated TiO₂-NTA/g-C₃N₄ composite structure exhibited significantly improved redox power and photocatalytic hydrogen production compared to TiO₂-NTA and g-C₃N₄ alone. In addition, while the hydrogen production rates for TiO₂-NTA and g-C₃N₄ were almost constant, TiO₂-NTA/g-C₃N₄ showed a rapid increase in the hydrogen production rate after a certain period of light irradiation, which was presumably caused by oxygen desorption from g-C₃N₄. The results of this study provide a method for supporting small nanoparticle materials on nanotube substrates and their importance in improving photocatalytic properties, and will also make a significant contribution not only to the field of photocatalysis but also to other fields requiring small nanoparticle materials.