Visible-light-absorbing Evonik P-25 nanoparticles modified with tungstophosphoric acid and their photocatalytic activity on different wavelengths

• Tungstophosphoric acid onto P-25 nanoparticles led to visible light absorption.
• Keggin anion-TiO2 surface complex should be related with visible light absorption.
• Sample P25-TPA-500 exhibited the highest photocatalytic activity.
• Upon UV light P25-TPA-500 showed evidence about a classic photocatalytic mechanism.
• Upon visible light photocatalytic activity could follow a photosensitized mechanism

For the first time, Evonik P-25 nanoparticles were impregnated with tungstophosphoric acid (TPA) (30% w/w) and annealed at two different temperatures, 200 °C (P25-TPA-200) and 500 °C (P25-TPA-500) for 1 h. Samples were characterized by different analytical techniques. Their photocatalytic activity was evaluated in the degradation of malachite green (MG) solutions at pH 5.0 upon three different irradiations: λ1 > 320 nm, λ2 > 450 nm and λ3 > 590 nm. Only sample P25-TPA-500 showed visible-light absorption. This absorption could be related to the formation of a surface complex TPA Keggin anion-TiO2. P25-TPA-500 and P25-TPA-200 samples showed an important dark adsorption of MG dye (∼30%); however, sample P25-TPA-500 exhibited the highest photocatalytic activity either under irradiation λ1 or λ2. The presence of tert-butyl alcohol (TBA), an OH scavenger affected the photocatalytic reaction upon irradiation λ1, but just slightly upon irradiation λ2. Finally, P25-TPA-200 exhibited a higher leaching of TPA than P25-TPA-500 samples.

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