Titanium Suboxide-Based Composite Electrocatalysts: Physico-Chemical and Semiconductor Properties

Abstract

Titanium dioxide is one of the main products of chemical industry. Due to its optical properties, it is most widely used in the paint and varnish industry and the production of pigments. Its sensory, adsorption, optical, electrical, and catalytic properties are widely recognized as the objects of close attention of researchers [1].Due to its high chemical inertness, lack of toxicity and low cost, titanium dioxide is increasingly used as a photocatalyst, while it has a number of significant disadvantages: low quantum efficiency of the process due to weak separation of the electron-hole pair, limited absorption spectrum in the ultraviolet region, which makes it impossible to use the energy of sunlight [2,3]. Scientists in all leading countries of the world are engaged in solving these problems.It is known that nanosized TiO2 particles (<50 nm) have the highest photocatalytic activity; therefore, the preparation of TiO2 nanoparticles is one of the ways to reduce the degree of charge recombination and increase the active surface area of the oxide [4].We propose to use a combined electrochemical-pyrolytic method of nanotube synthesis. This method will allow one to create a porous developed surface of the matrix for electrodeposition of catalytic layers of platinum and palladium; and their subsequent heat treatment at different partial pressures of oxygen will allow one to design composites with different composition. The high number of cationic vacancies in the matrix and the deficiency of oxygen ions will significantly increase the mobility of platinum and palladium atoms during heat treatment, and the resulting composite will have practically metal conductivity, high catalytic activity, selectivity and extended service life.Naked Ti/TiO2 contain a significant amount of X-ray amorphous compounds on the surface, which are most likely hydrated titanium oxides. The main crystalline phase is titanium dioxide in the allotropic anatase form. Metallic titanium is present on the surface in trace amounts. Thermal treatment of this material at a temperature of 500 ºC for 3 hours in an air atmosphere leads to an increase in the proportion of the crystalline phase. The content of metallic titanium increases significantly, reaching about a third. A partial electrochemical reduction of nanotubes allows one to obtain more electrically conductive titanium suboxides. After cathodic reduction of nanotubes for one hour, a galvanic coating with metallic platinum is uniformly deposited on the surface of the material. Thermal treated Ti/TiO2 nanotubes is an n-type semiconductor with a flat-band potential equal to –0.589 V and a carrier concentration of 6×1020 cm-3. Such a high concentration of carriers is obviously due to the small thickness of the oxide film and its nonstoichiometry, as a result of which the surface is not very depleted in electrons, since titanium metal acts as their donor.An original technique was developed for the deposition of platinized Ti/TiO2 nanotubes, including the stage of thermal treatment of the coating in an air atmosphere. It has been shown that the deposition of platinum on the previously reduced surface of nanotubes allows one to obtain composite coatings with a higher electrical conductivity, and the heat treatment of such a coating is characterized by the content of a larger fraction of TiO2, increased adhesion to the current collector, and an increase in the crystallinity of the coating. At the same time, the internal stresses of the coating are reduced by several times. References V.R.A. Ferreira, P.R.M. Santos, C.I.Q. Silva, M.A. Azenha. Latest developments on TiO2-based photocatalysis: a special focus on selectivity and hollownes for enhanced photonic efficiency, Appl. Catal., A, 623, 118243 (2021).S. Palmas, L. Mais, M. Mascia, A. Vacca. Trend in using TiO2nanotubes as photoelectrodes in PEC processes for wastewater treatment, Curr. Opin. Electrochem., 28, 100699 (2021).E. Brillas. A critical review on ibuprofen removal from synthetic waters, natural waters, and real wastewaters by advanced oxidation processes, Chemosphere, 286, 131849 (2022).M. Bellardita, A. Di Paola, L. Palmisano, F. Parrini, G. Buscarino, R. Amadelli, Preparation and photoactivity of samarium loaded anatase, brookite and rutile catalysts, Appl. Catal., B, 104, 291-299 (2011).