Visible light photocatalytic activity of macro-mesoporous TiO₂-CeO₂ inverse opals

Macro-mesoporous TiO₂ inverse opal materials were synthesized and they were tested as photocatalysts under visible light irradiation. The influence of cerium oxide addition towards the Rhodamine B (RhB) photodegradation activity was evaluated. Structural, textural, spectral and surface properties of the TiO₂-CeO₂ inverse opal nanocomposites were studied by XRD, XPS, SEM, TEM, N₂ adsorption-desorption, Diffuse Reflectance UV–vis and Photoluminescence spectroscopies. Compared to commercial TiO₂ anatase, the macro-mesoporous TiO₂ inverse opal exhibited six times higher kinetic rate constant in the RhB degradation under visible light irradiation. The good photocatalytic activity was related to the peculiar structure of this material, providing higher active surface area and enhancement of the mass transfer phenomena due to a very significant porosity. A positive effect of ceria addition was observed in terms of increased photocatalytic activity (73% of RhB degradation after 120 min of irradiation) when the amount of the CeO₂ was low (up to 3% wt). Moreover the TiO₂-3%CeO₂ exhibited the lowest photoluminescence band intensity among all samples, indicating that the best efficiency in the charge carriers separation occurs in this catalyst. The presence of Ce³⁺ species was favoured for small amounts of cerium oxide, resulting in a positive effect on the photoactivity. The mutual interaction between Ti and Ce metal cations promotes an easier charge transfer on the surface, accelerating in particular the Ce⁴⁺/Ce³⁺ redox process that is beneficial for the oxidation reactions. On the contrary the use of high amounts of cerium oxide (>5% wt) led to a progressive agglomeration of CeO₂, thus increasing the crystal size of TiO₂-CeO₂ particles (from 27 to 33 nm) and favouring the coverage of TiO₂ active sites. This work reports the preparation and some photoactivity tests of some visible light responsive nanomaterials for an efficient solar energy utilization.