Maria Vittoria Dozzi

Effects of gold nanoparticles deposition on the photocatalytic activity of titanium dioxide under visible light

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.

A Luminescent Poly(amidoamine)-Iridium Complex as a New Singlet-Oxygen Sensitizer for Photodynamic Therapy

A polymer complex (1P) was synthesized by binding bis(cyclometalated) Ir(ppy)2(+) fragments (ppy = 2-phenylpyridyl) to phenanthroline (phen) pendants of a poly(amidoamine) copolymer (PhenISA, in which the phen pendants involved ∼6% of the repeating units). The corresponding molecular complex [Ir(ppy)2(bap)](+) (1M, bap = 4-(butyl-4-amino)-1,10-phenanthroline) was also prepared for comparison. In water solution 1P gives nanoaggregates with a hydrodynamic diameter of 30 nm in which the lipophilic metal centers are presumed to be segregated within polymer tasks to reduce their interaction with water. Such confinement, combined with the dilution of triplet emitters along the polymer chains, led to 1P having a photoluminescence quantum yield greater than that of 1M (0.061 vs 0.034, respectively, in an aerated water solution) with a longer lifetime of the (3)MLCT excited states and a blue-shifted emission (595 nm vs 604 nm, respectively). NMR data supported segregation of the metal centers. Photoreaction of O2 with 1,5-dihydroxynaphthalene showed that 1P is able to sensitize (1)O2 generation but with half the quantum yield of 1M. Cellular uptake experiments showed that both 1M and 1P are efficient cell staining agents endowed with two-photon excitation (TPE) imaging capability. TPE microscopy at 840 nm indicated that both complexes penetrate the cellular membrane of HeLa cells, localizing in the perinuclear region. Cellular photodynamic therapy tests showed that both 1M and 1P are able to induce cell apoptosis upon exposure to Xe lamp irradiation. The fraction of apoptotic cells for 1M was higher than that for 1P (74 and 38%, respectively) 6 h after being irradiated for 5 min, but cells incubated with 1P showed much lower levels of necrosis as well as lower toxicity in the absence of irradiation. More generally, the results indicate that cell damage induced by 1M was avoided by binding the iridium sensitizers to the poly(amidoamine).

Absorption and action spectra analysis of ammonium fluoride-doped titania photocatalysts

The photocatalytic behaviour of a series of ammonium fluoride (NH(4)F)-doped titania (TiO(2)) photocatalysts was investigated in the decomposition of acetic acid in aqueous suspensions and in the gas phase mineralization of acetaldehyde. Very similar photocatalytic activity trends, usually increasing with increasing the calcination temperature for a given nominal dopant amount, were obtained for the two test reactions. Moderately doped TiO(2) calcined at 700 °C, consisting of pure anatase, was the best performing photocatalyst in both reactions. The photocatalytic oxidation of acetic acid was investigated systematically as a function of irradiation wavelength, by collecting so-called action spectra. By comparing the shapes of the action spectra with those of the absorption spectra of the investigated photocatalysts a model is proposed, based on spectral features deconvolution, which allows a clear distinction between inactive light absorption and effective photoactivity in acetic acid decomposition.

Influence of TiO2 electronic structure and strong metal–support interaction on plasmonic Au photocatalytic oxidations

Aiming at understanding how plasmonic reactions depend on important parameters such as metal loading and strong metal–support interaction (SMSI), we report the plasmonic photodegradation of formic acid (FA) under green LED irradiation employing three TiO2 supports (stoichiometric TiO2, N-doped TiO2, black TiO2) modified with Au nanoparticles (NPs) 3–6 nm in size. The rate of FA photooxidation follows different trends depending on Au loading for stoichiometric and doped Au/TiO2 materials. In the first case, the only contribution of hot electron transfer produces a volcano-shaped curve of photoreaction rates with increasing Au loading. When TiO2 contains intra-bandgap states the photoactivity increases linearly with the amount of Au NPs due to the concomitant enhancement produced by hot electron transfer and plasmon resonant energy transfer (PRET). The role of PRET is supported by finite element method simulations, which show that the increase in both Au NP inter-distance and SMSI enhances the probability of charge carrier generation at the Au/TiO2 interface.

Unraveling the Multiple Effects Originating the Increased Oxidative Photoactivity of {001}-Facet Enriched Anatase TiO2

Crystal shape control on a series of anatase photocatalysts was achieved by varying the amount of HF employed as a capping agent in their hydrothermal synthesis. A systematic comparison between their physicochemical properties, determined by several complementary surface and bulk techniques before and after thermal treatment at 500 °C, allowed one to discern the influence of the relative amount of exposed {001} crystal facets among a series of effects simultaneously affecting their oxidative photocatalytic activity. The results of both formic acid and terephthalic acid photo-oxidation test reactions point to the primary role played by calcination in making {001} facets effectively photoactive. Annealing not only removes most of the residual fluorine capping agent from the photocatalyst surface, thus favoring substrate adsorption, but also produces morphological modifications to a crystal packing that makes accessible a larger portion of surface {001} facets due to the unpiling of platelike crystals. The photocatalyst bearing the highest amount of exposed {001} facets (60%) shows the highest photoactivity in both the direct and the (•)OH-radical-mediated photocatalytic test reaction.

Effects of Surface Modification on the Photocatalytic Activity of TiO2

Abstract The photocatalytic activity of titanium dioxide modified by surface fluorination (F-TiO2) and by differently deposited gold nanoparticles (Au/TiO2) was tested in the degradation of organic substrates, i.e. mainly formic acid and the azo dye Acid Red 1 (AR1), in comparison to that of unmodified titania. The effect of different noble metals deposition on TiO2 was also investigated in the photocatalytic production of hydrogen from water solutions. Surface fluorination increases the rate of photocatalytic oxidation proceeding through a hydroxyl radical-mediated path. These radicals accumulated in higher amount on F-TiO2, because of the shielding effect of adsorbed fluoride. By contrast, the degradation of substrates implying direct interaction with the photogenerated holes occurred at a lower rate on F-TiO2, because of their hampered absorption on the fluorinated surface. 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 surface properties of TiO2. The presence of gold on TiO2 facilitates both the electron transfer to O2 and the mineralization of formic acid, proceeding mainly through direct interaction with photoproduced valence band holes. For substrates undergoing mainly oxidation through a hydroxyl radical mediated mechanism, the photogenerated holes may partly oxidize gold nanoparticles, which consequently act as recombination centers of photoproduced charge carriers. Photocatalytic hydrogen production tests, either from water or from methanol reforming, performed in a special set up, closed recirculation laboratory scale photoreactor employing titanium dioxide and noble-metals-modified titanium dioxide samples prepared by different techniques, confirmed the main role of noble metals nanoparticles on TiO2 in facilitating electron transfer reactions also under anaerobic conditions. Higher hydrogen production rates were attained with the photocatalysts in contact with the vapor phase, i.e. in the absence of the liquid phase mass transfer limitations of aqueous suspensions. The activity of metal nanoparticles-containing photocatalysts prepared by one step flame spray pyrolysis was always higher than that of photocatalysts prepared by deposition on pre-formed gold nanoparticles on TiO2 , most probably because of the better noble metal dispersion on the photocatalyst surface.

SPIO@SiO2–Re@PEG nanoparticles as magneto-optical dual probes and sensitizers for photodynamic therapy

A dual magneto-optical nanoprobe, endowed with properties useful for photodynamic therapy, has been prepared. It is constituted by a superparamagnetic iron oxide (SPIO) core (diameter size distribution centered at ca. 10 nm), obtained by thermal decomposition of iron oleate, coated by a compact silica shell, grown in a reverse microemulsion environment. Luminescent [Re(phen)(CO)3(py)]CF3SO3 complexes were covalently anchored to the silica shell, by functionalizing the pyridine ligand with a triethoxysilane moiety. Finally, the surface of the nanoparticles (NPs) was coated with a layer of polyethylene glycol (PEG), functionalized with triethoxysilane, to improve stability and stealthiness in physiological conditions. Transmission electron microscopy of these SPIO@SiO2–Re@PEG nanocomposites showed a single population, with size distribution centered at ca. 40 nm. NPs showed nuclear relaxivity values that guarantee an appreciable contrast in magnetic resonance imaging (r2 > 30 s−1 mM−1 at frequencies higher than 5 MHz). The presence of the Re complexes imparted photoluminescence to the NPs, with blue shifted emission and higher photoluminescence quantum yields with respect to the free [Re(phen)(CO)3(py-upts)]+ complex (λem 553 vs. 580 nm, Φ 0.060 vs. 0.038, in aerated water solution). The complexes embedded into the NPs maintained a satisfactory efficiency toward 1O2 generation (quantum yields 0.21 vs. 0.26 for the free complex, as assessed using 1,5-dihydroxynaphthalene as indirect marker of the 1O2 presence). Preliminary cell penetration tests were performed on human lung adenocarcinoma A549 cells. Two photon excitation confocal microscopy showed that the NPs were easily internalized and accumulated in the perinuclear region of the cells already after 4 h of incubation. Increased cytotoxicity upon irradiation with respect to the dark was also observed, showing the potential of the nanocomposite for photodynamic therapy applications.

A Ni-2,2′-bisdipyrrinato complex as a potential sensitizer: synthesis and photoelectrochemical characterization

A novel Ni-based 2,2′-bisdipyrrinato complex bearing peripheral benzoic acid groups (Ni-bisdpmCOOH) has been synthesized and characterized. Electrochemical investigations revealed a high energy LUMO level that, combined with its absorption properties extending up to the NIR region, is compatible with its use as a sensitizer in thermodynamically uphill photocatalytic reactions, such as proton and carbon dioxide photoreduction. A sensitized photo-anode was prepared by adsorbing Ni-bisdpmCOOH onto a TiO2 film, which showed an incident photon to current efficiency, measured at 0.62 V vs. RHE, extending up to the visible light region. This demonstrates that electronically excited Ni-bisdpmCOOH is effectively able to inject electrons into the conduction band of TiO2, thus opening the way for effective TiO2 sensitization or MOF implementation of the molecule itself for energetic applications.

Morphological Characterization of Shape-Controlled TiO2 Anatase through XRPD Analysis

Abstract Preferential growth of anatase crystallites along different directions is known to deeply affect their photocatalytic properties, especially with respect to the exposure of the reactive {001} facets. Its extent can be easily quantified through simple geometric calculations, on the basis of crystal sizes extracted for specific directions by means of X-Ray Diffraction data analysis. Nevertheless, the actual results depend on the method employed for such a quantification. Here we report on a comparative morphological investigation, performed by employing the Scherrer equation and the line profile from Rietveld refinements, on shape-controlled anatase photocatalysts produced by employing HF as capping agent. Compared to the Rietveld-based method, the use of the Scherrer equation produces a systematic underestimation of crystallite dimensions, especially concerning the [100] direction, which in turn causes the percentage of exposed {001} crystal facets to be underestimated. Neglecting instrumental-related effects may further reduce the estimate.

Photoinduced Charge-Transfer Dynamics in WO3/BiVO4 Photoanodes Probed through Midinfrared Transient Absorption Spectroscopy

The dynamics of photopromoted electrons in BiVO4, WO3, and WO3/BiVO4 heterojunction electrodes has been directly probed by transient absorption (TA) midinfrared (mid-IR) spectroscopy in the picosecond to microsecond time range. By comparison of 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 BiVO4 and WO3 have been directly tracked for the first time. These results support the charge carrier interactions which were previously hypothesized by probing the BiVO4 hole dynamics through TA spectroscopy in the visible range. Nanosecond mid-IR TA experiments confirmed that charge carrier separation occurs in WO3/BiVO4 electrodes under visible-light excitation, persisting up to the microsecond time scale.