Enhancement of TiO2 photocatalytic activity by structuring the photocatalyst film as photonic sponge

E. Carbonell; F. Ramiro-Manzano; I. Rodríguez; A. Corma; F. Meseguer; H. García

doi:10.1039/B801954A

Research · Publication & note

Enhancement of TiO2 photocatalytic activity by structuring the photocatalyst film as photonic sponge

Journal article

E. Carbonell, F. Ramiro-Manzano, I. Rodriguez, A. Corma, F. Meseguer and H. Garcia

Photochem. Photobiol. Sci. Volume 7 931-935 2008

Abstract

A TiO₂ film having "photonic sponge" architecture (ps-TiO₂) has been prepared using titania Degussa P25 nanoparticles that have been infiltrated into a template obtained through the arrangement of a mixture of different size latex spheres. The resulting photonic sponge exhibits 3.3 fold higher initial photocatalytic degradation rate for succinonitrile disappearance than analogous films of the same thickness made of unstructured P25 TiO₂ nanoparticles. When corrected for the three fold lower mass of the ps-TiO₂ films with respect to more dense P25, the enhancement of the photocatalytic activity by the photonic sponge morphology for the same mass of photocatalyst is about one order of magnitude. It was also observed that films of photonic sponge of 3 micrometres depth are more efficient than thicker films (5 and 7 micrometres).

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The published article is available from the Royal Society of Chemistry. RSC authors may reuse their own figures, diagrams, tables and images without a formal permission request when the material is fully acknowledged. The six figures and two schemes below were extracted as complete compositions from the local article PDF without enlargement.

Figures

Complete Scheme 1 illustrating the preparation process for titanium dioxide photonic sponges — **Scheme 1.** Process for the preparation of photonic sponges.

Complete Figure 1 cross-sectional SEM image of a titanium dioxide photonic sponge — **Figure 1.** SEM image of a cross section of a ps-TiO₂ sample showing its porous structure.

Complete two-panel Figure 2 with diffuse reflectance spectra of P25 and photonic sponge titanium dioxide films — **Figure 2.** Diffuse reflectance UV-Vis spectra of P25-3 and ps-TiO₂-3 samples.

Complete Figure 3 infrared spectrum of a photonic sponge film containing succinonitrile — **Figure 3.** IR spectrum of a ps-TiO₂-5 film containing succinonitrile.

Complete Figure 4 series of infrared spectra recorded during photocatalytic degradation — **Figure 4.** Series of IR spectra recorded at increasing irradiation times during succinonitrile photodegradation.

Complete Figure 5 time-photodegradation curves for photonic sponge films of different thicknesses — **Figure 5.** Time-photodegradation plots for ps-TiO₂ films as a function of film thickness.

Complete Figure 6 time-photodegradation curves for unstructured P25 films of different thicknesses — **Figure 6.** Time-photodegradation plots for P25 films as a function of film thickness.

Complete Scheme 2 comparing irradiation in thin and thick photocatalyst films — **Scheme 2.** Cross sections of thin and thick films illustrating inefficient irradiation when the film is too thick.

Research fields

Bottom-up

TiO₂ nanoparticles are assembled into a disordered hierarchical photonic sponge whose voids span the wavelength scale, while reference films use the same photocatalyst in a compact morphology. The comparison isolates structural light management from chemical composition.

Characterization

Cross-sectional SEM, optical absorbance and time-resolved infrared monitoring of succinonitrile degradation measure morphology, photon capture and catalytic outcome. Normalising for the lower TiO₂ mass shows that stronger activity follows enhanced absorption near the semiconductor edge.

RESEARCH TOPICS

HarvestingThe porous architecture improves how incident photons are used rather than changing the TiO₂ chemistry. For films of equal thickness, the photonic sponge accelerates succinonitrile degradation by a factor of 3.3; after correcting for its three-times lower TiO₂ mass, the intrinsic activity enhancement is about one order of magnitude. The 3 μm sponge performs better than the thicker 5 and 7 μm layers.

InterferenceThe mixture of latex-sphere sizes creates a disordered network of voids from roughly 200 to 2000 nm, broadening the range of optical paths instead of producing one narrow photonic stop band. Figure 2 shows the strongest absorbance gain near the TiO₂ absorption edge above 380 nm, where multiple scattering keeps weakly absorbed photons inside the catalyst for longer.

MaterialsDegussa P25 nanoparticles - approximately 80% anatase and 20% rutile - are packed around a sacrificial blend of latex spheres. Calcination removes the polymer and leaves a low-density, interconnected TiO₂ sponge. Because the comparison samples use the same P25 material and processing, the measured activity difference can be assigned to morphology and photon transport rather than to a different semiconductor composition.

ChemistryPhotocatalysis is followed through the disappearance of succinonitrile deposited on the films and irradiated with a medium-pressure mercury lamp above 300 nm. The porous sponge increases the reaction rate without metal or non-metal doping: the chemical gain comes from stronger absorption on the red side of the TiO₂ onset and from making more of the nanoparticle surface optically active.