TiO₂ films obtained by sol-gel method and dip‑coating technique is a methodology flexible, economical and reproducible. It has been used in applications such as: photocatalysis, photoelectrochemical, hydrogen production, solar cells and many others. Particularly, for the photoelectrochemical water oxidation has received great attention, since this is the semi-reaction that occurs at the photoanode (TiO₂ film) during the production of hydrogen from water [1]. However, to ensure the good performance of these materials it is necessary to know their semiconductor properties. In this work, the visible light activity and photoelectrochemical properties of N,S-TiO2 films supported on metal meshes were evaluated for the water oxidation.

The N,S-TiO₂ films were prepared by sol–gel dip-coating on titanium grade 2 expanded mesh. The procedure was adapted from that described elsewhere [5]. Titanium (IV) butoxide was used as N,S-TiO₂ precursor and thiourea was used as nitrogen and sulfur dopant precursor. The volume ratio alkoxide : alcohol (solvent) was 1.0 : 4.6, the molar ratio alkoxide : acetylacetone (complexing agent) was 1.0 : 1.5 and the molar ratio alkoxide : water was 1.0:2.8 [2].

Morphology, crystalline phase and optical band gap energy were determined by FE-SEM with energy-dispersive X-ray spectroscopy (EDS), XRD, and diffuse reflectance UV-visible spectroscopy (DRS). Open circuit potential (OCP) measurements were conducted in the dark and under Vis illumination in order to evaluate the photocatalytic activity of the TiO2 film. The illumination was provided externally by a Xe lamp (Newport 66984-300XV-R1). The semiconducting properties of the films were obtained by open circuit potential (OCP), linear sweep voltammetry (LSV), Electrochemical Impedance Spectroscopy (EIS-OCP), Cyclic voltammetry (CV), Linear sweep voltamperometry (LSV-On/Off), Chronoamperometry (CA) with chopped light and the Mott-Schottky analysis. The (photo)electrochemical tests were carried out in a conventional three-electrode cell equipped with a borosilicate window to allow the UV light illumination of the entire portion (1 cm2) of theTiO2 film exposed to the electrolyte. An Ag/AgCl (3M KCl) electrode was employed as reference electrode and a graphite bar (99.999% Alfa Aesar) as counter electrode. The 0.1 M KClO4 aqueous electrolyte. Before each test, the electrolyte was bubbled with N2 gas during 20 min. The illumination was provided externally by a Xe lamp (Newport 66984-300XV-R1).

Changes in the OCP a semiconductor film of N,S TiO2 supported on a mesh of titanium under intermittent illumination are shown in Fig. 1.

In Fig. 1, it can be seen that when the light is on and the photoanode illuminate a displacement occurs in the potential to negative values, associated with the generation of charge carriers in the semiconductor in the presence of light (foto-huecos and photogenerated electrons) [3].

Under ilumination a potential variation towards more negative values is observed until a photostationary steady state is reached. When the illumination is interrupted, the OCP recovers its value in the dark.

The linear sweep voltammogram for N,S-TiO₂ in the dark and illumination is shown in Fig. 2

In Fig. 2 it can be see that under illumination an increase in the current occurs when the potential swep starts. It is due to the applied potential creates an electric field inside it that favors the separation of the photogenerated e ^- h ⁺ pairs

The chronoamperometry with chopped light for N,S TiO₂ films is shown in Fig. 3.

In Fig. 3, It can see that N-S TiO₂ films showed a constant photogenerated currents and also good stability as the illumination period was repeated. However, TiO₂ films the showed a decrease in photogenerated currents.

References

[1] P. Acevedo-Peña, I. González, J. Electrochem. Soc. 160 (2013) 452

[2] M.I. Jaramillo-Gutiérrez, E.P. Rivero, M.R. Cruz-Díaz, M.E. Niño-Gómez, J.A. Pedraza-Avella, Catal. Today. 266 (2016) 17.

[3] D. Ramirez-Ortega, A.M. Meléndez, P.Acevedo-Peña, I. González, R. Arroyo, Electrochim. Acta, 140 (2014) 541.