Haroldo G. Oliveira

Reducing Staphylococcus aureus growth on Ti alloy nanostructured surfaces through the addition of Sn

β-type Ti alloys containing Nb are exciting materials for numerous orthopedic and dental applications due to their exceptional mechanical properties. To improve their cytocompatibility properties (such as increasing bone growth and decreasing infection), the surfaces of such materials can be optimized by adding elements and/or nanotexturing through anodization. Because of the increasing prevalence of orthopedic implant infections, the objective of this in vitro study was to add Sn and create unique nanoscale surface features on β-type Ti alloys. Nanotubes and nanofeatures on Ti-35Nb and Ti-35Nb-4Sn alloys were created by anodization in a HF-based electrolyte and then heat treated in a furnace to promote amorphous structures and phases such as anatase, a mixture of anatase-rutile, and rutile. Samples were characterized by SEM, which indicated different morphologies dependent on the oxide content and method of modification. XPS experiments identified the oxide content which resulted in a phase transformation in the oxide layer formed onto Ti-35Nb and Ti-35Nb-4Sn alloys. Most importantly, regardless of the resulting nanostructures (nanotubes or nanofeatures) and crystalline phase, this study showed for the first time that adding Sn to β-type Ti alloys strongly decreased the adhesion of Staphylococcus aureus (S. aureus; a bacteria which commonly infects orthopedic implants leading to their failure). Thus, this study demonstrated that β-type Ti alloys with Nb and Sn have great promise to improve numerous orthopedic applications where infection may be a concern.

Characterization of Fe-TiO2 films synthesized by sol-gel method for application in energy conversion devices

Fe-TiO2 particles were synthesized by sol-gel process from hydrolysis of titanium tetra-isopropoxide with nitric acid and ferric nitrate aqueous solutions (relative Fe:Ti molar ratio ranging from 1 to 6 at %) followed by hydrothermal treatment. Thin films were deposited onto conducting glass electrodes from a suspension with polyethylene glycol and heating at 450 °C for 30 min, which resulted in 1.5 μm thick transparent porous films. Crystalline samples, 93 % anatase and 7 % brookite, were obtained. Increasing the iron amount, the crystallite size estimated from XRD patterns ranged from 18 to 11 nm and the color varied from slightly yellow to brown. The optical properties have also changed; the absorption edge shifted towards longer wavelengths, with band gap energy decreasing from 3.0 to 2.7 eV. The films exhibited photocatalytic activity for phenol degradation that indicates promising applications in solar energy conversion.

Photoelectrochemical and photocatalytic properties of nanocrystalline TiO2 electrodes

The electrochemical and photocatalytic properties of a TiO2 film deposited on transparent electrodes were investigated. Its electrochemical behavior was typical of an n-type semiconductor electrode. Its photocatalytic activity, investigated for phenol degradation on an optical bench (area of 1 cm2, 5 mL of solution), revealed small currents (3 μA) and poor total organic carbon (TOC) removal (5 %) when the electrode was biased at + 1.1 V in the dark for 3 h. Under polychromatic irradiation, the electrode presented 25 μA of current and 12 % of phenol degradation. A better performance was achieved for photoelectrocatalytic configuration, when the electrode was irradiated and biased with + 0.6 V. Experiments done under irradiation with a metallic vapor lamp using 9 cm2 electrodes and 10 mL of solution revealed that heterogeneous photocatalysis configuration (HPC) resulted in 50 % of TOC removal, while 85 % was achieved by the electro-assisted process (EHPC). Both the configurations exhibited pseudo-first order kinetics for phenol degradation, but the rate constant was two times that of EHPC. The application of a potential bias to the TiO2 porous electrode must enhance the photogenerated electron/hole separation, which minimize the charge recombination and increases its photocatalytic activity towards organic pollutant degradation.

Electrochemical and photocatalytic properties of TiO2/WO3 photoelectrodes

Porous films of TiO2 and TiO2/WO3 were deposited onto transparent electrodes from aqueous suspensions with polyethylene glycol, TiO2 particles and different amounts of tungistic acid. After annealing, crystalline samples were obtained. The band gap energy, approximately 3.1 eV for TiO2, decreased from 2.9 to 2.7 eV for varying W/Ti molar ratios from 3 to 12 %. The electrochemical properties were investigated in Na2SO4 aqueous solution; for the TiO2 electrode, the open circuit potential changed from 0.18 V in the dark to -0.25 V under irradiation from a solar simulator. For hybrid TiO2/WO3 electrodes, the VOC values were almost independent of the WO3 content and corresponded to 0.3 V in the dark and -0.1 V under irradiation; however, photocurrent and interfacial capacitance increased with a higher WO3 concentration. The electrodes were then used as photocatalysts for 17-α-etinylestradiol removal from water, and the mixed TiO2/WO3 exhibited better performance for photocatalytic oxidation of estradiol than TiO2. Adding WO3 enhances the visible light harvesting and minimizes the charge recombination resulting in higher efficiency for solar energy conversion.

TiO2 and TiO2/WO3 porous film electrodes for application in solar energy conversion

TiO2 and TiO2/WO3 porous films were deposited onto transparent conducting glass electrodes, resulting in uniform films consisted of agglomerated particles with average diameters ranging from 50 to 200 nm; Ti, O and W atoms were homogeneously distributed at the surface of hybrid film. Comparable electrochemical properties were observed in the dark, with small capacitive currents and similar potentials for O2 and H2 evolution reactions in aqueous solution. Under polychromatic irradiation, the hybrid film electrode, molar ratio WO3/TiO2 = 12 %, reveled higher photocurrent and photocatalytic activity for oxidation of phenol and 17-α-ethinylestradiol. The visible light harvesting ability of hybrid film, with band gap energy estimated as 2.3 eV, and the relative position of conduction and valence band edges that inhibits charge recombination, should improve its photocatalytic activity for organic pollutant removal.