Photocatalytic reduction of Cr(VI) on WO3 doped long TiO2 nanotube arrays in the presence of citric acid

Photoreduction of Cr(VI) was successfully achieved on WO3 doped TiO2 nanotube (NT) arrays in the presence of citric acid. WO3 doped TiO2 NT arrays were fabricated by annealing anodic titania NTs that were preloaded with peroxotungstic acid. Driven by electrostatic force, the negatively charged peroxotungstic acid sol uniformly clung to the positively charged titania NT arrays, causing a solid solution of WO3 and TiO2 after annealing. The citric acid plays an important role in the photoreduction of Cr(VI): (i) it works as a sacrificial electron donor to deplete the photogenerated holes from the excited TiO2; and (ii) it takes part in the redox with Cr(VI) under UV illumination because of its electron rich property. The photochemical-reduction rate of Cr(VI) in citric acid under simulated solar light was 6.9 μg/L min cm2. Subtracting the part mentioned above, the reduction rate of Cr(VI) due to photogenerated electrons from excited photocatalysts increased from 8.3 μg/L min cm2 on TiO2 NTs to 31.6 μg/L min cm2 on WO3/TiO2 NTs containing 1 at% of W. The net Cr(VI) reduction rate on WO3/TiO2 NT arrays is 3.76 times that on the unmodified TiO2 NTs. Since both WO3 and TiO2 can be excited by UV light, the enhanced photoactivity of WO3/TiO2 is attributed to the increased probability of charge–carrier separation and the extended photo-response spectrum in visible light region due to the doping of WO3 (band gap: 2.6 eV).