Visible light responsive TiO₂ photocatalysts for degradation of indoor acetaldehyde

Photocatalysis is a promising technique for developing sustainable and environmentally friendly materials to improve indoor air quality. Visible-light-responsive TiO₂ has been widely investigated but there are inconsistent results because photocatalytic properties depend strongly on synthetic methods. Herein, we synthesize TiO₂ doped with 10 different metal ions (M-TiO₂) by conducting a dialysis in a sol-gel method to obtain the best photocatalyst for the degradation of acetaldehyde under LED irradiation. Purification of a sol by dialysis enables us to discuss pure effects of dopants on the photocatalytic activity because impurities such as counter ions of metal salts are removed before sintering. Only Cr-, Pt-, V-, and Fe-TiO₂ show photocatalytic activity and the optimal doping amounts are 0.50-1.7, 0.10, 1.0, and 0.10 atom%, respectively. Such differences in the optimal amounts can be explained in terms of the dopant ions having different valence states, suggesting the formation of oxygen vacancies. The Cr-TiO₂ powder exhibits high activity even at the doping amount of 4.2 atom%. We also demonstrate that the Cr-TiO₂ film prepared on a glass substrate can be used to degrade acetaldehyde by changing the film thickness and the LED intensity depending on the degree of the indoor contamination.