Carbon nanotubes as an auxiliary catalyst in heterojunction photocatalysis for solar hydrogen

Abstract

This study demonstrates that multi-walled carbon nanotubes (CNTs) effectively catalyzes photocatalytic hydrogen production in heterojunction suspensions under solar visible light (AM 1.5G; λ>420nm). Due to the high catalytic activity of CNTs, use of Pt can be significantly reduced. For this, quaternary composites (CdS/TiO2/Pt/CNTs) are prepared by the creation of CdS on platinized TiO2 (TiO2/Pt) subsequently to which chemically oxidized CNTs are loaded. A binary (CdS/TiO2) and two ternaries (CdS/TiO2/Pt and CdS/TiO2/CNTs) are also prepared for comparison. A TEM analysis for the quaternary sample shows that TiO2 is a central component that holds Pt nanoparticles, CNTs, and CdS clusters, while the last is spatially away from the catalysts. Photoluminescence (PL) emission bands of the binary excited at 325nm and 410nm are reduced by loading either Pt or CNTs, and further by co-loading of both catalysts. This suggests that the recombination of photogenerated charges under UV or visible light is inhibited due to cascaded charge transfer between TiO2 and CdS, which is further decreased by Pt and/or CNTs. Photolysis confirms that either Pt or CNTs catalyzes effectively photocatalytic H2 production in aqueous CdS/TiO2 suspensions with sulfide/sulfite electron donor under visible light. Such activity is significantly enhanced by over 50% by co-loading of Pt and CNTs. It is found that the Pt amount can be reduced to approximately five- or one-tenth by additional loading of CNTs under an optimal condition. The maximized performance of the quaternary is also found in the significantly enhanced photocurrent generation compared to the two ternaries. The detailed mechanism and implications are discussed.