Heberton Wender

Self-Organized TiO2 Nanotube Arrays: Synthesis by Anodization in an Ionic Liquid and Assessment of Photocatalytic Properties

Self-organized TiO(2) nanotube (NT) arrays were produced by anodization in ethylene glycol (EG) electrolytes containing 1-n-butyl-3-methyl-imidazolium tetrafluoroborate (BMI.BF(4)) ionic liquid and water. The morphology of the as-formed NTs was considerably affected by changing the anodization time, voltage, and water and ionic liquid electrolyte concentrations. In general, a nanoporous layer was formed on the top surface of the TiO(2) NTs, except for anodization at 100 V with 1 vol % of BMI.BF(4), where the NTs mouth was revealed. The length and bottom diameter of the NTs as well as the pore diameter of the top layer showed a linear relationship with increased anodization voltage. These TiO(2) NTs were tested as photocatalysts for methyl orange photodegradation and hydrogen evolution from water/methanol solutions by UV light irradiation. The results show that the TiO(2) NTs obtained by anodization in EG/H(2)O/BMI.BF(4) electrolytes are active and efficient for both applications.

Photocatalytic hydrogen production of Co(OH)2 nanoparticle-coated α-Fe2O3 nanorings

The production of hydrogen from water using only a catalyst and solar energy is one of the most challenging and promising outlets for the generation of clean and renewable energy. Semiconductor photocatalysts for solar hydrogen production by water photolysis must employ stable, non-toxic, abundant and inexpensive visible-light absorbers capable of harvesting light photons with adequate potential to reduce water. Here, we show that α-Fe₂O₃ can meet these requirements by means of using hydrothermally prepared nanorings. These iron oxide nanoring photocatalysts proved capable of producing hydrogen efficiently without application of an external bias. In addition, Co(OH)₂ nanoparticles were shown to be efficient co-catalysts on the nanoring surface by improving the efficiency of hydrogen generation. Both nanoparticle-coated and uncoated nanorings displayed superior photocatalytic activity for hydrogen evolution when compared with TiO₂ nanoparticles, showing themselves to be promising materials for water-splitting using only solar light.

Easy Access to Metallic Copper Nanoparticles with High Activity and Stability for CO Oxidation.

Copper catalysts are very promising, affordable alternatives for noble metals in CO oxidation; however, the nature of the active species remains unclear and differs throughout previous reports. Here, we report the preparation of 8 nm copper nanoparticles (Cu NPs), with high metallic content, directly deposited onto the surface of silica nanopowders by magnetron sputtering deposition. The as-prepared Cu/SiO2 contains 85% Cu0 and 15% Cu2+ and was enriched in the Cu0 phase by H2 soft pretreatment (96% Cu0 and 4% Cu2+) or further oxidized after treatment with O2 (33% Cu0 and 67% Cu2+). These catalysts were studied in the catalytic oxidation of CO under dry and humid conditions. Higher activity was observed for the sample previously reduced with H2, suggesting that the presence of Cu-metal species enhances CO oxidation performance. Inversely, a poorer performance was observed for the sample previously oxidized with O2. The presence of water vapor caused only a small increase in the temperature require for the reaction to reach 100% conversion. Under dry conditions, the Cu NP catalyst was able to maintain full conversion for up to 45 h at 350 °C, but it deactivated with time on stream in the presence of water vapor.

On the crystallization of Ta2O5 nanotubes: structural and local atomic properties investigated by EXAFS and XRD

Metal oxide nanotubes (NTs) semiconductors prepared by anodization are promising materials due to their expected unique optical and electric properties. However, most of the work reported to date did not find photoelectrochemical devices with higher efficiency than those assembled with nanoparticles. Moreover, this behavior is due to the difficulty of having non-defective crystalline structures and the disruption of the tubular shape during thermal treatment while trying to reduce oxygen vacancies. This work describes in detail the local atomic configuration, surface area and morphology properties of Ta2O5 NTs prepared by anodization as a function of the temperature and the crystallization time by using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The crystallization process of adhered and freestanding Ta2O5 NTs is discussed. Adhered NTs crystallized at 550 °C due to the oxidation of the Ta metal during annealing in the air atmosphere and not the NT array itself. Freestanding Ta2O5 NTs crystallized after annealing at 800 °C. Rietveld refinements were performed to investigate the effects of the temperature and the annealing time on the grain size and microstrain and obtain information about Ta–O interatomic distances. The local structure of amorphous and crystalline Ta2O5 NTs was investigated with EXAFS. Low coordination numbers were found in the as-anodized samples as well as the samples annealed for 30 min at 800 °C. The coordination number increased when annealing was performed above 800 °C or when the annealing time was longer than 30 min. Moreover, the decrease of defects was followed by an increase in the crystal size and collapse of the tubular shape due to the increase in internal stress generated by the increase in the crystallinity of the tubes and the orthorhombic Ta2O5 crystal size.

Third-order nonlinear optical response of colloidal gold nanoparticles prepared by sputtering deposition

The nonlinear optical responses of gold nanoparticles dispersed in castor oil produced by sputtering deposition were investigated, using the thermally managed Z-scan technique. Particles with spherical shape and 2.6 nm of average diameter were obtained and characterized by transmission electron microscopy and small angle X-ray scattering. This colloid was highly stable, without the presence of chemical impurities, neither stabilizers. It was observed that this system presents a large refractive third-order nonlinear response and a negligible nonlinear absorption. Moreover, the evaluation of the all-optical switching figures of merit demonstrated that the colloidal nanoparticles prepared by sputtering deposition have a good potential for the development of ultrafast photonic devices.

Photocatalytic Water Splitting by Suspended Semiconductor Particles

Starting and emphasizing the importance for future demand on Earth energy feed, this chapter discusses the state-of-the-art of photocatalytic water splitting (PWS) using suspended particles for hydrogen evolution. Herein, the thermodynamics requirements for photocatalyst semiconductors for water splitting for efficient hydrogen and oxygen production, to date overview on important photocatalysts and related fine-tuning processes adopted to meet the desired properties for PWS with improved photocatalytic activities, a profound discussion and review about tandem photocatalyst systems are covered. A descriptive discussion on efficiency determination including solar-to-hydrogen efficiency and apparent quantum efficiency to compare the photocatalytic performance of the photocatalysts is presented.