Similarly to alizarin molecules, 3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonate (alizarin red, AR), chelates TiO 2 nanoparticles through the catechol moiety, and shifts the absorption threshold of the semiconductor to the visible region. The photoinduced reactivity of the coupled system AR@TiO 2 was investigated through quantum yields determinations in nonscattering sols of TiO 2 modified nanoparticles. In contrast with the behavior observed in TiO 2 microparticulated systems, the chemisorbed ligand has a high stability under aerated visible light irradiation. The quantum yield for alizarin red oxidation Φ -AR ) 4 × 10 -4 correlates with the negligible efficiency for oxygen reduction in the constrained environment of the smaller particles. Conversely, reduction of Cr(VI) to Cr(V) in the coupled AR@TiO 2 system, confirmed by electron paramagnetic resonance spectroscopy, utilizes a high fraction of the photogenerated electrons and induces the degradation of the complex. Quantum efficiencies for chromium(VI) disappearance, Φ -Cr(VI) , approaches 37% at [Cr(VI)] 0 ) 200 µM. The interactions between Cr(VI)/AR and Cr(VI)/TiO 2 are analyzed in detail. Spectroscopic evidence is presented for the first time that Cr(VI) forms a charge-transfer complex with TiO 2 nanoparticles that could be excited by visible light (λ e 440 nm). The environmental implications of the above findings are briefly discussed.

Electron accumulation in TiO 2 ethanolic sols prepared by the HCl hydrolysis of a titanium alkoxide has been scrutinized by UV−vis and EPR spectroscopy. Unexpectedly, Ti(III) centers, g = 1.9551, formed af ter controlled monochromatic irradiation of the sols could be detected at room temperature by EPR spectroscopy. The yield of the paramagnetic signal and the number of accumulated reducing species, detected in dark titration experiments, increase as the water to titanium molar ratio, h, used in the synthesis diminishes. A 3.8% Ti(III) production efficiency was estimated for h = 6.5. Bidentate ethoxide coordination to the titanium dioxide surface and the replacement of surface hydroxylic groups by chloride ions is directly inferred by FTIR and EPR spectroscopies. Both findings are proposed to account for the room temperature detection of the Ti(III) species, and the higher electron storage capacity of the colloids prepared with lower h values.

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Reduction of alizarin molecules coupled to TiO(2) nanoparticles (A@TiO(2)) occurs on UV irradiation in the presence of a sacrificial electron donor. Evidence is presented that reduction is mediated by conduction-band electrons and yields a 1,2,9,10-tetrahydroxyanthracene species which remains coupled to the TiO(2) nanoparticles. The spectrum of the reduced complex displays two overlapping broad bands centred at 480 and 650 nm which can harvest visible photons besides 900 nm, in agreement with theoretical predictions by TDDFT. The potential relevance of the dual-redox behaviour of strongly TiO(2) coupled anthraquinone dyes in the field of photocatalysis and in connection with their utilization in the development of dye-sensitized TiO(2) solar cells is briefly discussed.

Hydroxoaluminiumtricarboxymonoamide phthalocyanine (AlTCPc) adsorbed at different loadings on TiO(2) Degussa P-25 was tested for Cr(vi) photocatalytic reduction under visible irradiation in the presence of 4-chlorophenol (4-CP) as sacrificial donor. A rapid reaction takes place in spite of the presumable aggregation of the dye on the TiO(2) surface. The removal of Cr(vi) is fairly negligible under visible-light irradiation, either without photocatalyst or in the presence of bare TiO(2). The fast capture of conduction band electrons by Cr(vi), which forms a surface complex with TiO(2), inhibits the formation of reactive oxygen species in the reductive pathway. This fact and the easier oxidation of 4-CP as compared to AlTCPc hinder the photobleaching of the dye and make feasible Cr(vi) reduction under visible irradiation. The consumption of Cr(vi) follows a pseudo-first order kinetics; the decay constant depends, in the studied range, on the photocatalyst mass, but it is barely affected by dye loading. The presence of 4-CP is essential, but its concentration has no effect on the Cr(vi) decay rate. Oxidation products of 4-CP, such as hydroquinone, catechol or benzoquinone, are not observed. Direct evidence of the one-electron reduction of Cr(vi) to Cr(v) was obtained by EPR spectroscopy using citric acid as Cr(v) trapping agent. In this case, disappearance of Cr(v) also follows a first order decay, but conduction band electrons do not seem to be involved. The fact that oxidation products of 4-CP are not observed is consistent with the fast dark removal of reaction intermediates by Cr(v), proved by EPR.

Photocurrent voltage curves obtained under visible light excitation of alizarin molecules chemisorbed to nanoporous TiO 2 films show both anodic and cathodic currents. The potential at which the sign reversal occurs depends on the electrolyte pH, the presence of acceptors, and the dye coverage, but as a general rule, it occurs at potentials ca. 600-700 mV more positive than the flat band potential. Negative photocurrents are accounted by efficient electron discharge to the electrolyte mediated by the ligand. Cathodic photocurrents are only observed at pH values higher than ca. 4.0 and go through a maximum at intermediate alizarin loadings. This phenomenon is ascribed to the progressive reparation of surface states by alizarin which hampers carrier transport through the TiO 2 matrix and decreases electron discharge to the electrolyte solution.

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Phone: þ54 223 481 6600, Fax: þ54 223 481 0046 A recent accomplishment in the preparation of a highly efficient thin film solar cell is reported. This superstrate cell is composed of FTO/TiO 2 /In 2 S 3 /Cu 2 ZnSnS 4 /graphite. A maximum conversion efficiency of 3.5% has been achieved for the first time using this configuration and materials. The device includes low cost methods and nontoxic components. Details of the experimental procedures are provided and the device characterization data are presented and analyzed. Left: Current-voltage response of the best cell FTO/TiO 2 / In 2 S 3 /Cu 2 ZnSnS 4 /graphite in the dark (dotted line) and under simulated solar irradiation (solid line). Right: Cross-sectional view of the cell. Inset: Profilometric scan of the device.Rapid Research Letter 1700144 (3 of 5) M. Berruet et al.: Highly-efficient superstrate Cu 2 ZnSnS 4 solar cell ß