Probing the role of surface energetics of electrons and their accumulation in photoreduction processes on TiO₂

Composite CdS/TiO2 Powders for the Selective Reduction of 4-Nitrobenzaldehyde by Visible Light: Relation between Preparation, Morphology and Photocatalytic Activity

A series of composite CdS/TiO2 powders was obtained by nucleation of TiO2 on CdS nanoseeds. This combination presents the appropriate band edge position for photocatalytic redox reactions: visible light irradiation of CdS allows the injection of electrons into dark TiO2, increasing the lifetimes of separated charges. The electrons have been used for the quantitative photoreduction of 4-nitrobenzaldehyde to 4-aminobenzaldehyde, whose formation was pointed out by 1H NMR and ESI-MS positive ion mode. Concomitant sacrificial oxidation of 2-propanol, which was also the proton source, occurred. The use of characterization techniques (XRD, N2 adsorption-desorption) evidenced the principal factors driving the photocatalytic reaction: the nanometric size of anatase crystalline domains, the presence of dispersed CdS to form an extended active junction CdS/anatase, and the presence of mesopores as nanoreactors. The result is an efficient photocatalytic system that uses visible light. In addition, the presence of TiO2 in combination with CdS improves the stability of the photoactive material, enabling its recyclability.

Turning Waste into Useful Products by Photocatalysis with Nanocrystalline TiO2 Thin Films: Reductive Cleavage of Azo Bond in the Presence of Aqueous Formate

UV-photoexcitation of TiO₂ in contact with aqueous solutions of azo dyes does not imply only its photocatalytic degradation, but the reaction fate of the dye depends on the experimental conditions. In fact, we demonstrate that the presence of sodium formate is the switch from a degradative pathway of the dye to its transformation into useful products. Laser flash photolysis experiments show that charge separation is extremely long lived in nanostructured TiO₂ thin films, making them suitable to drive both oxidation and reduction reactions. ESR spin trapping and photoluminescence experiments demonstrate that formate anions are very efficient in intercepting holes, thereby inhibiting OH radicals formation. Under these conditions, electrons promoted in the conduction band of TiO₂ and protons deriving from the oxidation of formate on photogenerated holes lead to the reductive cleavage of N=N bonds with formation and accumulation of reduced intermediates. Negative ion ESI–MS findings provide clear support to point out this new mechanism. This study provides a facile solution for realizing together wastewater purification and photocatalytic conversion of a waste (discharged dye) into useful products (such as sulfanilic acid used again for synthesis of new azo dyes). Moreover, the use of TiO₂ deposited on an FTO (Fluorine Tin Oxide) glass circumvents all the difficulties related to the use of slurries. The obtained photocatalyst is easy to handle and to recover and shows an excellent stability allowing complete recyclability.

Modification of Charge Trapping at Particle/Particle Interfaces by Electrochemical Hydrogen Doping of Nanocrystalline TiO2

Particle/particle interfaces play a crucial role in the functionality and performance of nanocrystalline materials such as mesoporous metal oxide electrodes. Defects at these interfaces are known to impede charge separation via slow-down of transport and increase of charge recombination, but can be passivated via electrochemical doping (i.e., incorporation of electron/proton pairs), leading to transient but large enhancement of photoelectrode performance. Although this process is technologically very relevant, it is still poorly understood. Here we report on the electrochemical characterization and the theoretical modeling of electron traps in nanocrystalline rutile TiO₂ films. Significant changes in the electrochemical response of porous films consisting of a random network of TiO₂ particles are observed upon the electrochemical accumulation of electron/proton pairs. The reversible shift of a capacitive peak in the voltammetric profile of the electrode is assigned to an energetic modification of trap states at particle/particle interfaces. This hypothesis is supported by first-principles theoretical calculations on a TiO₂ grain boundary, providing a simple model for particle/particle interfaces. In particular, it is shown how protons readily segregate to the grain boundary (being up to 0.6 eV more stable than in the TiO₂ bulk), modifying its structure and electron-trapping properties. The presence of hydrogen at the grain boundary increases the average depth of traps while at the same time reducing their number compared to the undoped situation. This provides an explanation for the transient enhancement of the photoelectrocatalytic activity toward methanol photooxidation which is observed following electrochemical hydrogen doping of rutile TiO₂ nanoparticle electrodes.

EPR spin trapping evidence of radical intermediates in the photo-reduction of bicarbonate/CO2 in TiO2 aqueous suspensions

Storage of CO₂ as carbonates is a strategy for its reductive activation. Carbonates are reduced at pH 5.5–7.5 with photoexcited TiO₂. EPR is convenient for detecting reaction intermediates.

Radical intermediates in photoinduced reactions on TiO2 (an EPR spin trapping study)

The radical intermediates formed upon UVA irradiation of titanium dioxide suspensions in aqueous and non-aqueous environments were investigated applying the EPR spin trapping technique. The results showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties (e.g., polarity, solubility of molecular oxygen, rate constant for the reaction of hydroxyl radicals with the solvent). The formation of hydroxyl radicals, evidenced as the corresponding spin-adducts, dominated in the irradiated TiO₂ aqueous suspensions. The addition of ¹⁷O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the ¹⁷O nucleus. The photoexcitation of TiO₂ in non-aqueous solvents (dimethylsulfoxide, acetonitrile, methanol and ethanol) in the presence of 5,5-dimethyl-1-pyrroline N-oxide spin trap displayed a stabilization of the superoxide radical anions generated via electron transfer reaction to molecular oxygen, and various oxygen- and carbon-centered radicals from the solvents were generated. The character and origin of the carbon-centered spin-adducts was confirmed using nitroso spin trapping agents.