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Aiming at discerning the role of fluorine from that of nitrogen as a dopant in N,F-codoped TiO 2 , a series of HF-doped TiO 2 photocatalysts were investigated in the decomposition of formic and acetic acid in aqueous suspensions, also as a function of the irradiation wavelength (action spectra analysis), in comparison with recent results obtained with an analogous series of NH 4 F-doped TiO 2 photocatalysts. Visible light absorption around 420 nm, which was found to be inactive in acetic acid decomposition, is definitely shown to be associated with nitrogen doping, whereas the enhanced absorption at ca. 365 nm, increasing with increasing calcination temperature, can be unambiguously attributed to structural modifications induced by fluorine doping. Action spectra analysis confirms that this absorption is active in acetic acid decomposition, in both HF-and NH 4 F-doped TiO 2 photocatalysts. From time-resolved photoluminescence (PL) spectroscopy analysis, a clear correlation is outlined between the photoactivity of the materials and the long-lasting component of the PL signal, which increases with the calcination temperature and is related to the formation of surface defects. Thus, fluorine doping, followed by calcination at high temperature, increases the amount of surface traps originating the long-lasting PL signal, which are beneficial in photoactivity by ensuring long-living photoproduced charge couples.

The effect of noble metal (Pt and Au) nanoparticle photodeposition on a series of NH4F-doped TiO2 photocatalysts calcined at 700 °C was investigated both in a thermodynamically down-hill reaction, i.e. the degradation of formic acid in aqueous suspension, and in an up-hill reaction, i.e. hydrogen production from methanol-water vapour mixtures. All photocatalysts were characterized by BET, XRD, UV-vis absorption and HRTEM analysis. Intriguing synergistic effects of simultaneous bulk and surface TiO2 modification were evidenced in both photocatalytic reactions, which can be interpreted in relation to the structural features of the materials. On one hand NH4F doping guarantees that the most active TiO2 anatase phase is stabilised up to high calcination temperature, ensuring high crystallinity and good photoinduced charge carriers production, on the other hand noble metal nanoparticles contribute in increasing the separation of photoproduced charge carriers, resulting in enhanced photocatalytic performances of the surface- and bulk-modified photocatalyst systems.

Due to its ∼2.4 eV band gap, BiVO 4 is a very promising photoanode material for harvesting the blue portion of the solar light for photoelectrochemical (PEC) water splitting applications. In WO 3 /BiVO 4 heterojunction films, the electrons photoexcited in BiVO 4 are injected into WO 3 , overcoming the lower charge carriers' diffusion properties limiting the PEC performance of BiVO 4 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 BiVO 4 excitation, a favorable electron transfer from photoexcited BiVO 4 to WO 3 occurs immediately after excitation and leads to an increase of the trapped holes' lifetime in BiVO 4 . However, a recombination channel opens when both oxides are simultaneously excited, evidenced by a shorter lifetime of trapped holes in BiVO 4 . PEC measurements reveal the implication of these wavelengthdependent ultrafast interactions on the performances of the WO 3 /BiVO 4 heterojunction.

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 O(2) reduction 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/TiO(2) photocatalysts was found to affect their photoactivity, also by modifying the properties of TiO(2). The presence of gold on TiO(2) facilitates both the electron transfer to O(2) and the mineralization of formic acid, which mainly proceeds through direct interaction with photoproduced valence band holes. The so-formed highly reductant CO(2)*(-) 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.

Two series of doped titanium dioxide samples (S-TiO(2) and F-TiO(2)) were prepared by the sol-gel method in the presence of different amounts of dopant source (thiourea and NH(4)F, respectively), followed by calcination at 500, 600 or 700 °C, and characterised by BET, UV-vis absorption, XPS, HRTEM, XRD and EPR analyses. Reference undoped materials were prepared by the same synthetic procedure. Their photocatalytic activity under visible light was investigated employing the photocatalytic degradation of formic acid in aqueous suspension as test reaction. S-doped TiO(2) showed a photocatalytic activity quite similar to that of undoped materials. In this regard, the insertion of S, characterised by a relatively large ionic radius, into the TiO(2) crystalline structure appears rather difficult, as confirmed by XPS analysis. On the contrary, moderate F doping was beneficial in increasing the rate of formic acid photocatalytic degradation, especially for photocatalysts calcined at high temperature, consisting of highly crystalline pure anatase, in which the rate of detrimental charge carrier recombination was reduced. For both series of doped materials, high doping levels appear to limit the semiconductor photoactivity, probably due to the formation of a progressively increasing number of charge recombination centres. The EPR characterisation of the investigated doped TiO(2) samples evidenced the presence of nitrogen containing species (nitric oxide radical encapsulated in micro-void, with no photoactivity, and N(b)˙ species, active in visible light sensitisation) and of titanium reduced centres Ti(3+), due to charge imbalance consequent to dopant introduction in the TiO(2) lattice either in anionic (F(-)) or in cationic form (S(6+)).

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The dynamics of photopromoted electrons in BiVO 4 , WO 3 and WO 3 /BiVO 4 heterojunction electrodes has been directly probed by transient absorption (TA) mid-infrared (mid-IR) spectroscopy in the picosecond to microsecond time range. By comparing the dynamics recorded with the two individual oxides at 2050 cm-1 with that of the heterojunction system after excitation at different wavelengths, electron transfer processes between selectively excited BiVO 4 and WO 3 have been directly tracked for the first time. These results support the charge carrier interactions which were previously hypothesized by probing the BiVO 4 hole dynamics through TA spectroscopy in the visible range. Nanosecond mid-IR TA experiments confirmed that charge carrier separation occurs in WO 3 /BiVO 4 electrodes under visible light excitation, persisting up to the microsecond timescale.