Annelise Kopp Alves

Electrospun TiO2 Fiber Composite Photoelectrodes for Water Splitting

This work has focused on the development of electrospun TiO2 fiber composite photoelectrodes for hydrogen production by water splitting. For comparison, similar photoelectrodes were also developed using commercial TiO2 (Aeroxide P25) nanoparticles (NPs). Dispersions of either fibers or P25 NPs were used to make homogenous TiO2 films on fluorine-doped SnO2 (FTO) glass substrates by a doctor blade (DB) technique. Scanning electron microscopy (SEM) analysis revealed a much lower packing density of the DB fibers, with respect to DB-P25 TiO2 NPs; this was also directly reflected by the higher photocurrent measured for the NPs when irradiating the photoelectrodes at a light intensity of 1.5AM (1 sun, 1000 W/m(2)). For a better comparison of fibers vs. NPs, composite photoelectrodes by dip-coating (onto FTO) TiO2 sol-gel (SG) matrixes containing an equal amount (5 or 20 wt %) of either fibers or P25 NPs were also investigated. It emerged that the photoactivity of the fibers was significantly higher. For composites containing 5 wt % TiO2 fibers, a photocurrent of 0.5 mA/cm(2) (at 0.23 V vs Ag/AgCl) was measured, whereas 5 wt % P25 NPs only provided 0.2 mA/cm(2). When increasing to 20 wt % fibers or NPs, the photocurrent decreased, because of the formation of microcracks in the photoelectrodes, because of the shrinkage of the sol-gel. The high photoactivity of the fiber-based electrodes could be confirmed by incident photon to current efficiency (IPCE) measurements. Remarkably, the IPCE of composites containing 5 wt % fibers was between 35% and 40% in the region of 380-320 nm, and when accounting for transmission/reflection losses, the absorbed photon to current efficiency (APCE) was consistently over 60% between 380 nm and 320 nm. The superior photoactivity is attributed to the enhanced electron transport in the electrospun fibers, with respect to P25 NPs. According to this study, it is clear that the electronic connectivity ensured by the sol-gel also contributes positively to the enhanced photocurrent.

Solution-processable exfoliation and suspension of atomically thin WSe2.

HYPOTHESIS A scalable method for the production of low-dimensional semiconductor materials, such as tungsten diselenide (WSe2), is crucial for applications in flexible and transparent optoelectronic devices. Liquid exfoliation technique can be employed to fabricate a colloidal suspension of WSe2 nanosheets as a low-temperature and solution-processable route. EXPERIMENTS The production of atomically thin WSe2 by liquid exfoliation is investigated, focusing the maximization of the concentration in the resulting suspension by selecting the most suitable solvent for this process. Three different strategies are compared: exfoliation in pure solvents, mixed solvent and aqueous solutions with surfactant. FINDINGS N-methylpyrrolidone, previously reported as suitable solvent for suspension of other layered materials, results in the highest concentration among the pure solvents tested in this work; however a simple mixture of 30% of propan-2-ol in water surpasses its performance offering a low cost alternative. An optimal surface tension of 28 mN m(-1) is proposed and the influence of the molecular size of the solvent is also considered. The use of anionic surfactant in water makes feasible the exfoliation of WSe2 in aqueous solutions. The analysis of different exfoliation approaches under same processing conditions permits adequate comparison between different solvent systems and reveal optimal parameters for high yield liquid exfoliation of WSe2.

Synthesis of CeO2 and Y2O3-Doped CeO2 Composite Fibers by Electrospinning

Electrospinning was employed to produce homogeneous inorganic-organic composite fibers from alcohol solutions containing polyvinyl butyral (PVB) and precursor of yttrium and cerium ions. Upon heat treatment, ceria and yttria-doped ceria fibers were obtained. The fibers retained the original morphology observed in the as-spun composition. X-ray diffraction was used to identify the crystalline phases of the final products. Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential thermal analysis (DTA), and BET analysis were employed to study the ceramic-phase formation and the morphological evolution of the fibers. Thus, uniform ceria and yttria-doped ceria fibers several micrometers long of high-phase purity were produced. The CeO2 and the CeO2 with Y2O3 fibers presented average diameters that ranged from 0.8 to 1.7 µm, and the distribution of specific surface ranged from 24 to 127 m2/g after heat treatment at 1000°C.

Application of titania fibers obtained by electrospinning in photocatalytic degradation of methyl orange

Composite fibers of polyvinylpirrolidone and titanium propoxide were prepared using the electrospinning technique. Titanium oxide fibers were obtained on heat treatment of this composite. These fibers were characterized by means of specific surface area, thermal analysis, X-ray diffraction and electronic microscopy processes. Evaluation of their photocatalytic activity was carried out in comparison with the nanometric powder TiO2 P25 from Degussa as reference. The photo-oxidative decomposition of methyl orange was followed by ultraviolet-visible spectroscopy to determine the acid- and basic pH values. The heat treatment of these fibers at higher temperatures led to a decrease in the anatase phase responsible for their photoactivity. The heat treatment, the phases of the material, the pH of the medium, and the surface area affected the physical-chemical and photocatalytic properties of the titania fibers.

Designing of TiO2/MWCNT Nanocomposites for Photocatalytic Degradation of Organic Dye

The nanocomposites were obtained from commercial multi-walled carbon nanotubes (MWCNTs) and with titanium tetra propoxide as TiO2 precursor, and heat treated at 400°C and 500°C to form crystalline phases of TiO2. The nanocomposites were investigated for their photocatalytic activity, employing them as catalysts in the degradation of organic methyl orange dye in an aqueous solution under UV radiation. The results were associated with the characteristics of the nanocomposite structure, using techniques such as x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Optical characterization was obtained by photoluminescence spectroscopy and diffuse reflectance spectroscopy. The highest photocatalytic efficiency was observed in the TiO2–MWCNT nanocomposites obtained in an acid medium and heat treated at 500°C. These results could be associated with the lower transition energy and level of defects in the TiO2 of these nanocomposites when compared to the other samples.

Structural and photocatalytic characterization of BaFe2O4 obtained at low temperatures

Barium monoferrite BaFe2O4 classified as permanent magnet stands out among other ceramic magnets due to its high chemical stability, corrosion resistance and low production cost. In addition, experiments conducted on photocatalytic degradation of methyl orange and UV transmittance by spectrophotometry have shown that this material has photocatalytic properties. The spinel ferrite is of importance in many technological areas such as computing, communications and security. Several techniques for synthesis have been studied to optimize the properties of this material. The synthesis of BaFe2O4 by conventional processes often occurs at temperatures above 1000 oC. In this work, we obtained the phase BaFe2O4 at low temperatures (600 oC) from the combustion reaction using nitrates and maleic anhydride as metal complexing agent. Techniques of X-ray diffraction, specific surface area, thermogravimetry analysis and photocatalytic analysis were employed to characterize the products obtained.

Evaluation of a methodology of biodiesel purification: study of the contaminant removal capacity

ABSTRACT We studied biodiesel purification based on the adsorption on rice husk ash (RHA) for the removal of biodiesel production contaminants; we examined biodiesel produced using soybean oil and alkaline transesterification with methanol and KOH. We carried out a detailed analytical study by adding known amounts of contaminants to a standard sample of biodiesel and purifying it afterward. The results were compared with acid washing and adsorption on Magnesol® (commercial magnesium silicate). RHA was equal or superior to Magnesol® and acid washing for removing biodiesel production contaminants.

CNTs/TiO2 Composites

Titanium dioxide (TiO2) is a semiconductor material that is widely used in many different areas, such as gas sensors, air purification, catalysis, solar to electric energy conversion, photoelectrochemical systems and photocatalyst for degrading a wide range of organic pollutants because of its nontoxicity, photochemical stability, and low cost. There are reports that show that the heterojunction of TiO2 and carbon nanotubes (CNTs) improves the efficiency of the photocatalytic activity, mainly because the recombination of the photogenerated electron–hole pairs becomes more difficult in the presence of nanotubes. Multi-wall carbon nanotubes/TiO2 (MWCNT/TiO2) composite materials have been attracting attention in relation to their use in the treatment of contaminated water and air by heterogeneous photocatalysis, hydrogen evolution, CO2 photo-reduction, and dye sensitized solar cells. Nevertheless, functionalization routes to aggregate these materials and characterization methods need to be studied; since they have direct influence on properties and potential applications.

High-Energy Milling

Nanoparticles can be produced by mechanical attrition. Such nanoparticles are formed in a mill in which energy is used to transform course-grained materials into nanostructured powders. The particles themselves can have a size distribution of less than 100 nm, called a nanoparticle, or, as is common in materials milled using mechanical attrition, the product is highly crystalline, and the crystallite size after milling is between 1 and 10 nm in diameter, called nanocrystalline materials [1].

Nanomaterials and Catalysis

Catalysts are typical nanomaterials, perhaps the first nanomaterials in wide applications. Catalysis is a nanoscale phenomenon that has been the subject of research and development for many years, but only recently has it become a nanoscale science of materials and chemistry involving more investigations on the molecular level. Nanomaterial-based catalysts are usually heterogeneous catalysts. The extremely small size of the particles maximizes surface area exposed to the reactant, allowing more reactions to occur. However, thermal stability of these nanomaterials is limited by their critical sizes; the smaller the crystallite size, the lower the thermal stability. In this chapter the characterization of metal oxides such as CeO2, TiO2, and ZnO and some of their applications as catalysts for methane combustion and photocatalysis is described. The effects of mixed oxides, and mixed phases were investigated.