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Larissa Kunz1 Matteo Cargnello1 Arun Majumdar1

1, Stanford University, Stanford, California, United States

Nanostructured photocatalysts have the potential of combining large potential-driven chemical driving forces with the near-volumetric system scaling achieved by surface reactions on dispersed, nanoscale particles. With increased biodiesel production over the past 20 years having saturated the glycerol market and thereby devaluing biodiesel, glycerol photoreforming presents a useful model photocatalytic reaction. Titania remains the dominant photocatalyst today but, despite nanostructuring and phase engineering efforts, continues to suffer from low photocatalytic efficiencies. Previous work has shown brookite-phase TiO2 nanorods to have improved photoreforming efficiency in comparison to other TiO2 phases and morphologies. In this contribution, we present the synthesis of composites of these TiO2 nanorods with graphitic carbon nitride (g-C3N4), a 2D semiconducting polymer, to improve upon this relatively high performance by introducing a heterojunction with favorable band positioning (based on bulk material properties) to suppress electron-hole recombination. These composites, in combination with platinum nanoparticles, demonstrate much larger steady-state hydrogen production rates from glycerol photoreforming than do either of the individual components alone or a physical mixture of the two components, highlighting the synergism between the two phases that is related to band positioning and intimate contact between the two building blocks. Controlled synthesis of the composites, including the ability to tune TiO2 nanorod length and exfoliate and etch sheets of g-C3N4, enables a more controlled study of electron excitation and charge transfer in these materials to then understand what limits their photocatalytic performance. By first studying the electronic structure and optical properties of the TiO2 nanorods and the g-C3N4 individually, a more detailed mechanistic picture can be put together of process energetics and relevant energy barriers. This mechanistic understanding will be useful in designing higher efficiency photocatalytic systems moving forwards.

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