Próspero Acevedo-Peña

Effect of water and fluoride content on morphology and barrier layer properties of TiO2 nanotubes grown in ethylene glycol-based electrolytes

This paper studies the effect of H2O and NH4F content on morphology and barrier layer properties of TiO2 nanotubes grown by potentiostatic anodization in ethylene glycol-based electrolytes. The increase in these two variables leads to an increase in the chemical attack of the formed oxide. However, each of these variables plays a different role in the formation of TiO2 nanotubes. On the one hand, a higher percentage of H2O in the electrolyte leads to a transition from a nanoporous to a nanotubular structure, as well as to a greater diameter of the tubes and a decrease in their length and barrier layer thickness. In contrast, a higher NH4F concentration decreases nanotube diameter and increases their length modifying barrier layer properties due to insertion of F− ions into the lattice. This diminishes the barrier layer resistance, but increases both the adsorption and the diffusion coefficient of F− ions. The different roles of H2O and NH4F in film formation are also associated with the presence of sub-oxides detected by XPS.

Electrochemical Characterization of TiO2 Films Formed by Cathodic EPD in Aqueous Media

The present work studies the influence of applied electric field on the cathodic electrophoretic formation of TiO 2 films in an aqueous medium. The growth of the film was followed by measuring current density, deposited mass, and thickness under imposed electric field (ϕ). In scanning electronic microscopy images of the TiO 2 films formed under different electric fields, three regions can be seen: (i) formation of nuclei that are converted into agglomerates, (ii) uniform and one-sided growth, and (iii) growth by the formation of agglomerates on the TiO 2 surface. The characterization of semiconductor properties showed that increased ϕ for film growth leads to an increase in the density of donors and to a displacement of the flatband potential toward less negative values. The photoelectrochemical characterization of the oxide films revealed that the photoelectrochemical performance of the TiO 2 films decreases as a function of the electric field used to carry out the electrophoretic deposition (EPD) process. The electrochemical behavior observed for these films seems closely related to the content of Ti 3+ doping sites electrogenerated in situ during the EPD, which act as electron traps negatively affecting the electron transport toward the indium tin oxide collector, also making their photoelectrochemical activity inefficient.

Formation of TiO2 photoanodes by simultaneous electrophoretic deposition of anatase and rutile particles for photoassisted electrolytic copper ions removal

The influence of Anatasa/Rutile ratio on TiO2 films, grown by electrophoretic deposition was studied in the photoassisted electrolytic copper ions removal from cyanide solutions. The proper dispersant dosage allowing the simultaneous electrophoretic deposition of Anatase and Rutile was chosen based on electrokinetic measurements; evidenced by the XRD spectra of the formed films. The evaluation of films photoassisted electrolytic copper ion removal showeds that it is possible to enhance the activity of Anatase films by adding some Rutile exploiting the synergetic interaction between these two materials, achieve by its proper deposition.

Photo-assisted electrochemical copper removal from cyanide solutions using porous TiO2 thin film photo-anodes

TiO2 porous films were prepared on ITO coated glass slides by the sol-gel dip-coating method assisted with Polyethylene glycol (PEG). The films were used as photo-anodes in the photo-assisted electrolytic removal of cuprous ions in cyanide media. These were characterized by SEM, UV-Visible spectroscopy and XRD. The PEG modified films were free of cracks and developed a porous structure after heat treatment at 500 oC, due to the thermal decomposition of the structure associated PEG. It was demonstrated that the photo-assisted electrochemical reduction of copper is promoted by the use of modified TiO2 films as photo-anodes, thanks to the greater surface area given by the PEG decomposition. However, the film thickness was found to be a critical factor in the process, to such an extent that films composed of 5 layers were completely inefficient, meaning that despite the open porosity, multilayered films acted as a barrier within the photo-electrolytic process.

TiO2 photoanodes prepared by cathodic electrophoretic deposition in 2-propanol: effect of the electric field and deposition time

This paper evaluates the influence of electric field and deposition time applied on cathodic electrophoretic formation of TiO2 films in organic medium (2-propanol). The film morphology was tracked by measuring the deposited mass and film thickness. The variation in film porosity was correlated with the apparition of surface states distribution in the cyclic voltammetric characterization in the dark, due to grain boundaries defects generated in the contact of the TiO2 particles. The open-circuit voltage decay curves showed that there is no formation of deep energy states inside the band gap of the TiO2. The photopotential of the films increased until a critical thickness but the photocurrents showed to be dependent on operational variables, due to the fact that anodic polarization in thin films increases the electric field generated by the illumination at the ITO/TiO2 interface, favoring the transport of the photogenerated electrons to the rear contact.

Improving photocatalytic reduction of 4-nitrophenol over ZrO2–TiO2 by synergistic interaction between methanol and sulfite ions

The effect of two sacrificial agents (methanol and sodium sulfite) on the photocatalytic reduction of 4-nitrophenol employing a ZrO2–TiO2 photocatalyst is reported. The experimental results showed a decrease in the generation of OH˙ radicals and diminution in the charge transfer resistance that favor the electron transfer toward 4-nitrophenolate ions. Based on the results, it is possible to establish that methanol acts as a hole scavenger while sulfite ions act as radical scavengers, improving the reduction from 4-nitrophenol to 4-aminophenol. The synergistic effect has been corroborated by photoluminescence and photoelectrochemical measurements.

Generación de estados superficiales durante la formación electroforética catódica de películas de TiO2 sobre ito

In the present work TiO2 films were formed over Indium Tin Oxide (ITO) employing cathodic electrophoretic deposition (Cathodic-EPD) and Dr. Blade Technique. The films were characterized by electrochemical techniques in order to compare their electronic properties; as well as, their photoelectrochemical behavior. The electrochemical performance showed by the films, allowed to relate the modification occurring during the Cathodic-EPD, with the partial reduction of TiO2 nanoparticles, generating Ti3+ defects. These trapping states are modifying the electronic properties of the film, and diminishing the transport of the photoelectrogenerated electrons toward ITO.