Solar spectrum photocatalytic conversion of CO₂ to CH₄ utilizing TiO₂ nanotube arrays embedded with graphene quantum dots

TiO₂ nanotube arrays (TNT) offer an exciting prospect as a photocatalytic material architecture due to the combined properties of high surface area, 1-D vectorial charge transfer, and reduced photogenerated charge recombination. However the TiO₂ band gap (≈3.2eV) limits light absorption to the UV region, which comprises but a small fraction of the solar spectrum energy. Graphene is known to effectively absorb visible light, and due to its high conductivity promote efficient charge transfer. Herein, we present a novel photocatalyst composed of TNTs sensitized with electrodeposited graphene quantum dots (GQDs). GQDs electrodeposition-duration is varied to optimize photocatalytic performance of the resulting nanostructured graphene-TNT (G-TNT) films. Under solar spectrum illumination we find optimal G-TNT samples promote a CO₂ to CH₄ photocatalytic conversion rate of 1.98ppmcm^-2 h^-1, with carbon origin confirmed by ¹³CO₂ isotopic test.