TITANIUM DIOXIDE PHOTOACTIVITY: EFFECTS OF COMBINED STRUCTURAL, SURFACE AND MORPHOLOGICAL MODIFICATIONS

Titanium dioxide is the most widely used and studied photocatalyst for a variety of applications. However, its performance as a photocatalyst is limited by two main drawbacks: i) the fast recombination of photogenerated charge carriers and ii) a high band gap energy, requiring UV light irradiation to promote its electronic excitation. Several strategies have been investigated to overcome these limitations, such as surface modification with noble metal (NM) nanoparticles (NPs), Au, Pt, Pd, and doping with transition metals or p-block elements. The fine tuning of TiO2 crystal morphology represents another modification strategy that recently gained a lot of attention. By using suitable shape-controlling agents it is possible to modify the crystal habit of anatase, favoring the transition from its typical truncated bipyramidal structure, characterized by approximately 90% surface exposure of {101} facets, to the nanosheet-like morphology, mainly exposing the thermodynamically less stable (but potentially more reactive) {001} surfaces. At the same time, recent studies demonstrated that in-situ adsorption of fluoride ions has specific effects on the photocatalytic activity of TiO2, depending on the TiO2 crystal phase composition and on the mechanism of the photocatalytic reaction. A systematic evaluation of the combined effects produced on photoactivity by NM NPs deposition and in-situ TiO2 fluorination, as a function of the adopted deposition technique, the chosen NM and/or the semiconductor morphology, was still lacking. In the present PhD thesis, the photocatalytic properties of doped and/or morphology-controlled anatase TiO2 samples modified by surface deposition of Au NPs was firstly investigated in both photocatalytic oxidation and reduction reactions. In particular, the effects induced by Au NPs deposited on the TiO2 surface by different techniques were investigated, as well as those of the in-situ addition of fluoride ions, to highlight possible synergistic effects on TiO2 photoactivity induced by coupling surface fluorination, specific morphology, and deposited NM NPs. A systematic study was thus undertaken by studying at first the effects deriving from the use of two different Au NPs deposition methods, i.e., deposition-precipitation (DP) and photoreduction (P), on the photocatalytic activity of sol-gel prepared titanium dioxide samples bulk co-doped with different amounts of N and F. The photoactivity of the prepared composite Au/TiO2 materials was monitored in two different photocatalytic test reactions, i.e., formic acid oxidation and Cr(VI) reduction, each proceeding through completely different reaction paths. In particular, whereas in the case of formic acid oxidation no significant differences in the materials photoactivity was induced by the use of different Au NPs deposition techniques, regardless of the dopant content introduced in the TiO2 bulk structure, an outstanding photoactivity was instead attained in Cr(VI) to Cr(III) photoreduction upon coupling an optimal amount of bulk dopant in TiO2 with the photodeposition of NM NPS. The origin of such synergy between the use of a selective Au NPs deposition method (i.e., photodeposition) and the use of an optimal amount of dopant was then studied via Time-Resolved Photoluminescence Spectroscopy. Next, the effects induced upon using the above-mentioned Au NPs deposition techniques on the photocatalytic activity of differently shaped anatase powders, eventually also coupled with in-situ surface fluorination, were evaluated mainly in three different photocatalytic test reactions, i.e., formic acid (FA) oxidation, rhodamine B (RhB) bleaching and Cr(VI) to Cr(III) reduction. In particular, the deposition of Au NPs on TiO2 was beneficial in FA degradation only in the case of {101} facet-dominated materials, whereas coupling Au NPs photodeposition with a nanosheet-like morphology of TiO2 did not lead to better performing photocatalytic materials. Similar results were also obtained in the case of rhodamine B photobleaching. In particular, whereas in the case of metal-free titanium dioxide a positive synergy was observed upon coupling a nanosheet morphology (i.e., materials mainly exposing {001} facets) with in-situ surface fluorination, a positive effect on photoactivity induced by coupling the deposition of Au NPs and in-situ surface fluorination was obtained only when the materials were mainly dominated by the exposure of {101} facets (thus bearing a pseudo-spherical shape). Differently, the photoactivity of morphology controlled Au/TiO2 materials in Cr(VI) photoreduction was found to be only slightly affected by the deposition of NM NPs. Instead, the photoefficiency of anatase was largely improved upon shifting from a pseudo-spherical morphology towards nanosheet-shaped crystallites, producing materials which also significantly outperformed reference benchmark P25 TiO2. This is possibly due to an improved oxidative power of {001} facets which help to overcome the kinetically rate-determining anodic half-reaction in Cr(VI) reduction, i.e., water oxidation. Furthermore, an enhanced oxidative power of {001} surfaces in comparison to {101} facets was demonstrated by a higher production of HO• radicals in solution during the hydroxylation of terephthalic acid in the aqueous media. This effect was observed when the morphology of TiO2 was shifted from a pseudo-spherical shape to a nanosheet-like structure, providing further confirmation of the mentioned findings. Lastly, the performance of the materials was assessed in hydrogen production via methanol photo steam reforming. Our results revealed significant enhancements in the photoactivity of the powders upon deposition of NM NPs and, notably, the utilization of pseudo-spherically shaped TiO2 materials. It is possible that the increased exposure of {101} facets, which act as reduction sites, facilitates a greater availability of reduction centres, thereby promoting H2 evolution. The obtained photoactivity results in the investigated test reactions allowed us to hypothesize a reaction mechanism based on charge migration in differently shaped Au/TiO2 composite materials able to rationalize the effects on photoactivity induced upon coupling Au deposition with differently shaped TiO2 materials, also under in-situ fluorinated conditions.