Direct experimental evidence of the higher concentration of hydroxyl radicals generated on fluorinated titanium dioxide (F–TiO₂) under irradiation was obtained by spin-trapping EPR measurements. The faster photoinduced bleaching of the azo dye Acid Red 1 (AR1) observed in the presence of F–TiO₂ was explained by the high affinity of the azo double bond towards ˙OH radicals. Moreover, the pronounced decrease of the AR1 bleaching rate by addition of 2-propanol, as hydroxyl radicals scavenger, on F–TiO₂ and not on naked TiO₂ demonstrated that on fluorinated titania AR1 is mainly degraded via ˙OH radical attack.

The effects of gold nanoparticles deposited on titanium dioxide on the photocatalytic oxidative degradation of two organic substrates, i.e.formic acid and the azo dye Acid Red 1, and on the parallel O₂reduction yielding hydrogen peroxide have been investigated under visible light irradiation. The method employed to reduce Au(III) to metallic gold in the preparation of Au/TiO₂ photocatalysts was found to affect their photoactivity, also by modifying the properties of TiO₂. The presence of gold on TiO₂ facilitates both the electron transfer to O₂ and the mineralization of formic acid, which mainly proceeds through direct interaction with photoproduced valence band holes. The so-formed highly reductant CO₂˙⁻ intermediate species may contribute in maintaining gold in metallic form. The controversial results obtained in the photocatalytic degradation of the dye were rationalised by taking into account that with this substrate, which mainly undergoes oxidation through a hydroxyl radical mediated mechanism, the photogenerated holes may partly oxidise gold nanoparticles, which consequently act as recombination centres of photoproduced charge carriers.

Due to its ∼2.4 eV band gap, BiVO₄ is a very promising photoanode material for harvesting the blue portion of the solar light for photoelectrochemical (PEC) water splitting applications. In WO₃/BiVO₄ heterojunction films, the electrons photoexcited in BiVO₄ are injected into WO₃, overcoming the lower charge carriers’ diffusion properties limiting the PEC performance of BiVO₄ photoanodes. Here, we investigate by ultrafast transient absorption spectroscopy the charge carrier interactions occurring at the interface between the two oxides in heterojunction systems to directly unveil their wavelength dependence. Under selective BiVO₄ excitation, a favorable electron transfer from photoexcited BiVO₄ to WO₃ occurs immediately after excitation and leads to an increase of the trapped holes’ lifetime in BiVO₄. However, a recombination channel opens when both oxides are simultaneously excited, evidenced by a shorter lifetime of trapped holes in BiVO₄. PEC measurements reveal the implication of these wavelength-dependent ultrafast interactions on the performances of the WO₃/BiVO₄ heterojunction.

Photocatalysis and sonolysis exhibit synergistic effects in the degradation of organic molecules in aqueous suspension, when low ultrasound frequency (i.e. 20 kHz) is employed. The degradation of the azo dye Acid Orange 8 in aqueous suspensions was systematically evaluated under sonolysis, photocatalysis and sonophotocatalysis as a function of dye concentration, amount and type of photocatalyst (TiO₂ or ZnO) and catalase addition, in terms of pseudo-first-order degradation rate constants. The evolution of hydrogen peroxide was also monitored. Synergistic effects of the combination of ultrasound and photocatalysis should mainly involve the aqueous phase and be due to an increased concentration of reactive radicals, consequent to the action of ultrasound on the peroxide species produced by both photocatalysis and sonolysis.

Semiconductors crystal facet engineering has become an important strategy for properly tuning and optimizing both the physicochemical properties and the reactivity of photocatalysts. In this review, a concise survey of recent results obtained in the field of specific surface-oriented anatase TiO₂ crystals preparation is presented. The attention is mainly focused on the fluorine-mediated hydrothermal and/or solvothermal processes employed for the synthesis and the assembly of anatase micro/nanostructures with dominant {001} facets. Their peculiar photocatalytic properties and potential applications are also presented, with a particular focus on photocatalysis-based environmental clean up and solar energy conversion applications. Finally, the most promising results obtained in the engineering of TiO₂ anatase crystal facets obtained by employing alternative, possibly more environmentally friendly methods are critically compared.