The reduction of oxygen on titanium oxide electrodes

Enhanced Electrocatalytic Oxygen Reduction Reaction of TiO2 Nanotubes by Combining Surface Oxygen Vacancy Engineering and Zr Doping

This work examines the cooperative effect between Zr doping and oxygen vacancy engineering in anodized TiO2 nanotubes (TNTs) for enhanced oxygen reduction reactions (ORRs). Zr dopant and annealing conditions significantly affected the electrocatalytic characteristics of grown TNTs. Zr doping results in Zr4+ substituted for Ti4+ species, which indirectly creates oxygen vacancy donors that enhance charge transfer kinetics and reduce carrier recombination in TNT bulk. Moreover, oxygen vacancies promote the creation of unsaturated Ti3+(Zr3+) sites at the surface, which also boosts the ORR interfacial process. Annealing at reductive atmospheres (e.g., H2, vacuum) resulted in a larger increase in oxygen vacancies, which greatly enhanced the ORR activity. In comparison to bare TNTs, Zr doping and vacuum treatment (Zr:TNT-Vac) significantly improved the conductivity and activity of ORRs in alkaline media. The finding also provides selective hydrogen peroxide production by the electrochemical reduction of oxygen.

Adapting Aluminum-Doped Zinc Oxide for Electrically Conductive Membranes Fabricated by Atomic Layer Deposition

The use of electrically conductive membranes has recently drawn great interest in water treatment as an approach to reduce biofouling. Most conductive membranes are made by binding nanoparticles (carbon nanotubes or graphene) to a polymeric membrane using additional polymers, but this method risks leaching these nanomaterials into the environment. A new approach was developed here based on producing an electrically conductive layer of aluminum-doped zinc oxide (AZO) by atomic layer deposition. The aqueous instability of AZO, which is a critical challenge for water applications, was solved by capping the AZO layer with an ultrathin (∼11 nm) TiO₂ layer (AZO/TiO₂). The combined film exhibited prolonged stability in water and had a low sheet resistance of 67 Ω/sq with a 120 nm-thick coating, while the noncapped AZO coating quickly deteriorated as shown by a large increase in membrane resistance. The AZO/TiO₂ membranes had enhanced resistance to biofouling, with a 72% reduction in bacterial counts in the absence of an applied current due to its higher hydrophilicity than the bare polymeric membrane, and it achieved an additional 50% reduction in bacterial colonization with an applied voltage. The use of TiO₂-capped AZO layers provides a new approach for producing conductive membranes using abundant materials, and it avoids the risk of releasing nanoparticles into the environment.

Scavenging activity of Magnéli phases as a function of Ti4+/Ti3+ ratios

TiO₂ is able to scavenge reactive oxygen and nitrogen species (ROS and RNS) in the absence of light. The scavenging mechanism has been related to the chemistry of defects (oxygen vacancy reduced oxidation states of Ti) but it is still unknown. This study describes the ROS scavenging activity of different titanium oxide phases and relates their scavenging activities with the Ti⁴⁺/Ti³⁺ molar ratio as well as the band gap value. The Ti₅O₉ phase, with a mixture of both oxidation states, presented a substantially higher percentage of 2,2-diphenyl-1-picrylhydracyl radicals (DPPH˙) eliminated per m² of specific surface area in comparison to phases with predominant oxidation states Ti⁴⁺ or Ti³⁺ such as TiO₂ and Ti₂O₃, respectively. The obtained results indicate that the DPPH˙ scavenging mechanism corresponds to a catalytic process on the Ti₅O₉ surface which is facilitated by the presence of charges that can easily move through the material. The mobility of charges and electrons in the semiconductor surface, related to the presence of oxidation states Ti⁴⁺ and Ti³⁺ and a small band gap, could create an attractive surface for radical species such as DPPH˙. This puts forward Ti₅O₉ as a promising candidate coating for implantable biomedical devices, as an electrode, since it can cushion inflammatory processes which could lead to device encapsulation and, consequently, failure.

Noncrystalline Titanium Oxide Catalysts for Electrochemical Oxygen Reduction Reactions

Titanium oxides crystals are widely used in a variety of fields, but little has been reported on the functionalities of noncrystalline intermediates formed in their structural transformation. We measured the oxygen reduction reaction activity of titanium oxide nanoparticles heat-treated for a different time and found that the activity abruptly increased at a certain time of the treatment. We analyzed their structures by using X-ray pair distribution functions with the help of high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy and ascertained that the abrupt increase in the activity corresponded to a structural transformation from a reduced lepidocrocite-type layered titanate to a disordered structure consisting of domains of brookite-like TiO₆ octahedral linkages. The further treatment transformed these brookite-like domains into another phase having more edge-sharing sites like the TiO-type cubic structure. This finding would position noncrystalline, disordered structure as a possible origin of the catalytic activity, though nanocrystalline rutile particles might be also considered as the origin.

Electrochemical Behavior of TiO(x)C(y) as Catalyst Support for Direct Ethanol Fuel Cells at Intermediate Temperature: From Planar Systems to Powders

To achieve complete oxidation of ethanol (EOR) to CO2, higher operating temperatures (often called intermediate-T, 150-200 °C) and appropriate catalysts are required. We examine here titanium oxycarbide (hereafter TiOxCy) as a possible alternative to standard carbon-based supports to enhance the stability of the catalyst/support assembly at intermediate-T. To test this material as electrocatalyst support, a systematic study of its behavior under electrochemical conditions was carried out. To have a clear description of the chemical changes of TiOxCy induced by electrochemical polarization of the material, a special setup that allows the combination of X-ray photoelectron spectroscopy and electrochemical measurements was used. Subsequently, an electrochemical study was carried out on TiOxCy powders, both at room temperature and at 150 °C. The present study has revealed that TiOxCy is a sufficiently conductive material whose surface is passivated by a TiO2 film under working conditions, which prevents the full oxidation of the TiOxCy and can thus be considered a stable electrode material for EOR working conditions. This result has also been confirmed through density functional theory (DFT) calculations on a simplified model system. Furthermore, it has been experimentally observed that ethanol molecules adsorb on the TiOxCy surface, inhibiting its oxidation. This result has been confirmed by using in situ Fourier transform infrared spectroscopy (FTIRS). The adsorption of ethanol is expected to favor the EOR in the presence of suitable catalyst nanoparticles supported on TiOxCy.

A silver nanoparticle loaded TiO2 nanoporous layer for visible light induced antimicrobial applications

A nanoporous TiO2 layer was formed on commercially pure titanium by a simple anodization method in aqueous hydrofluoric acid (HF) medium. Silver nanoparticles (AgNP) were loaded into the nanoporous TiO2 layer by UV light irradiation. The morphology, chemical composition and photocatalytic activity of the modified titanium surfaces were characterized by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and UV-vis absorption spectroscopy techniques. The redox behavior of the AgNP loaded TiO2 layer was analyzed by cyclic voltammetry (CV) studies. The impedance behavior of the nanoporous TiO2 layer with and without AgNP was investigated by electrochemical impedance spectroscopy (EIS). The antibacterial effect of the AgNP loaded TiO2 layer was evaluated using Pseudomonas sp. and Bacillus sp. cultures. The efficacy of this modified layer to act as an antibacterial agent to minimize biofouling of titanium is demonstrated in this investigation.

Oxygen reduction catalyzed by Au-TiO2 nanocomposites in alkaline media

Au-TiO2 nanocomposites were prepared by chemical deposition of gold nanoparticles onto TiO2 nanocolloids that were synthesized by a hydrothermal method. Transmission electron microscopic measurements showed that the TiO2 colloids exhibited an average diameter of about 5 nm and clearly defined lattice fringes that were consistent with those of anatase TiO2 and formed rather large agglomerates that spanned a few hundred nanometers in length. Additionally, gold nanoparticles were found to be embedded within the TiO2 matrices, and the size increased with increasing gold loading but all ranged from 10 to 50 nm in diameter. Consistent results were obtained in X-ray diffraction measurements. Electrochemical studies demonstrated that the resulting Au-TiO2 nanocomposites exhibited apparent electrocatalytic activity in oxygen reduction that was markedly improved as compared to that of TiO2 particles alone, as reflected in the onset potential, number of electron transfers involved, and kinetic current density. Among the series, the best catalyst for oxygen reduction was identified with the Au/Ti atomic ratio of 5.2%. The enhanced oxygen reduction kinetics was ascribed to the dissociation of water and formation of surface-adsorbed hydroxyl moieties that was facilitated by the loading of gold nanoparticles onto the TiO2 colloids.

The cathodic behavior of titanium: Serum effect

The cathodic behavior of titanium was investigated in inorganic buffer solutions and in fetal bovine serum through potential sweep techniques. Under cathodic polarization, the oxygen reduction and the hydrogen evolution reactions were observed. It was found that the activity of the electrode toward the electron transfer increased with decreasing the electrode surface charge. The polarization curves recorded in serum displayed the same features as those observed in inorganic buffered solutions. However, organic molecules were likely adsorbed onto the titanium surface and the adsorption reactions were potential dependant. Additionally, it was noticed that the buffer capacity of serum could be overwhelmed by the progressive interface alkalinization during cathodic polarization. Furthermore, serum affected the corrosion current of the implant materials and the effect was pH-dependent.