Pseudocapacitive behaviour in sol-gel derived electrochromic titania nanostructures.

Nanostructured thin films are widely investigated for application in multifunctional devices thanks to their peculiar optoelectronic properties. In this work anatase TiO2 nanoparticles (average diameter 10 nm) synthesised by a green aqueous sol-gel route are exploited to fabricate optically active electrodes for pseudocapacitive-electrochromic devices. In our approach, highly transparent and homogeneous thin films having a good electronic coupling between nanoparticles are prepared. These electrodes present a spongy-like nanostructure in which the dimension of native nanoparticles is preserved, resulting in a huge surface area. Cyclic voltammetry studies reveal that there are significant contributions to the total stored charge from both intercalation capacitance and pseudocapacitance, with a remarkable 50% of the total charge deriving from this second effect. Fast and reversible colouration occurs, with an optical modulation of ∼60% in the range of 315-1660 nm, and a colouration efficiency of 25.1 cm2 C-1 at 550 nm. This combination of pseudocapacitance and electrochromism makes the sol-gel derived titania thin films promising candidates for multifunctional 'smart windows'.

Films of chitosan and natural modified hydroxyapatite as effective UV-protecting, biocompatible and antibacterial wound dressings.

Chitosan is a natural polysaccharide widely used in biomedicine, for instance for wound dressing. Hydroxyapatite is a very bioactive calcium phosphate which, if modified with an appropriate element (iron Fe), can also have UV-absorbing properties. In this work, we report the study of films of chitosan incorporated with iron-modified hydroxyapatite of natural origin (from cod fish bones); this combination led to an innovative chitosan-based material with excellent and advanced functional properties. The films showed very high UV absorption (Ultraviolet Protection Factor (UPF) value higher than 50). This is the first time that a chitosan-based material has shown such high UV protection properties. The films also showed to be non-cytotoxic, and possessed antimicrobial activity towards both Gram-positive and negative strains. Their mechanical properties, optimised with an experimental design approach, confirmed their potential use as multifunctional wound dressing, capable of reducing bacterial infections and, at the same time, protecting from UV light.

Exploring Tantalum as a Potential Dopant to Promote the Thermoelectric Performance of Zinc Oxide.

Zinc oxide (ZnO) has being recognised as a potentially interesting thermoelectric material, allowing flexible tuning of the electrical properties by donor doping. This work focuses on the assessment of tantalum doping effects on the relevant structural, microstructural, optical and thermoelectric properties of ZnO. Processing of the samples with a nominal composition Zn1-xTaxO by conventional solid-state route results in limited solubility of Ta in the wurtzite structure. Electronic doping is accompanied by the formation of other defects and dislocations as a compensation mechanism and simultaneous segregation of ZnTa2O6 at the grain boundaries. Highly defective structure and partial blocking of the grain boundaries suppress the electrical transport, while the evolution of Seebeck coefficient and band gap suggest that the charge carrier concentration continuously increases from x = 0 to 0.008. Thermal conductivity is almost not affected by the tantalum content. The highest ZT~0.07 at 1175 K observed for Zn0.998Ta0.002O is mainly provided by high Seebeck coefficient (-464 V/K) along with a moderate electrical conductivity of ~13 S/cm. The results suggest that tantalum may represent a suitable dopant for thermoelectric zinc oxide, but this requires the application of specific processing methods and compositional design to enhance the solubility of Ta in wurtzite lattice.

Hydrothermal Synthesis of Rare-Earth Modified Titania: Influence on Phase Composition, Optical Properties, and Photocatalytic Activity.

In order to expand the use of titania indoor as well as to increase its overall performance, narrowing the band gap is one of the possibilities to achieve this. Modifying with rare earths (REs) has been relatively unexplored, especially the modification of rutile with rare earth cations. The aim of this study was to find the influence of the modification of TiO2 with rare earths on its structural, optical, morphological, and photocatalytic properties. Titania was synthesized using TiOSO4 as the source of titanium via hydrothermal synthesis procedure at low temperature (200 °C) and modified with selected rare earth elements, namely, Ce, La, and Gd. Structural properties of samples were determined by X-ray powder diffraction (XRD), and the phase ratio was calculated using the Rietveld method. Optical properties were analyzed by ultraviolet and visible light (UV-Vis) spectroscopy. Field emission scanning electron microscope (FE-SEM) was used to determine the morphological properties of samples and to estimate the size of primary crystals. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical bonding properties of samples. Photocatalytic activity of the prepared photocatalysts as well as the titania available on the market (P25) was measured in three different setups, assessing volatile organic compound (VOC) degradation, NOx abatement, and water purification. It was found out that modification with rare earth elements slows down the transformation of anatase and brookite to rutile. Whereas the unmodified sample was composed of only rutile, La- and Gd-modified samples contained anatase and rutile, and Ce-modified samples consisted of anatase, brookite, and rutile. Modification with rare earth metals has turned out to be detrimental to photocatalytic activity. In all cases, pure TiO2 outperformed the modified samples. Cerium-modified TiO2 was the least active sample, despite having a light absorption tail up to 585 nm wavelength. La- and Gd-modified samples did not show a significant shift in light absorption when compared to the pure TiO2 sample. The reason for the lower activity of modified samples was attributed to a greater Ti3+/Ti4+ ratio and a large amount of hydroxyl oxygen found in pure TiO2. All the modified samples had a smaller Ti3+/Ti4+ ratio and less hydroxyl oxygen.

Solid-Gas Phase Photo-Catalytic Behaviour of Rutile and TiOn (1 < n < 2) Sub-Oxide Phases for Self-Cleaning Applications.

The solid-gas phase photo-catalytic activities of rutile TiO2 and TiOn (1 < n < 2) sub-oxide phases have been evaluated. Varying concentrations of Ti3+ defects were introduced into the rutile polymorph of titanium dioxide through carbo-thermal reduction at temperatures ranging from 350 °C to 1300 °C. The resulting sub-oxides formed were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, impedance spectroscopy and UV-visible diffuse reflectance spectroscopy. The presence of Ti3+ in rutile exposed to high reduction temperatures was confirmed by X-ray diffraction. In addition, a Ti3+-Ti4+ system was demonstrated to enhance the photo-catalytic properties of rutile for the degradation of the air pollutants NO2 and CO2 under UV irradiation of wavelengths (λ) 376⁻387 nm and 381⁻392 nm. The optimum reduction temperature for photo-catalytic activity was within the range 350⁻400 °C and attributed to improved charge-separation. The materials that were subject to carbo-thermal reduction at temperatures of 350 °C and 400 °C exhibited electrical conductivities over one hundred times higher compared to the non-reduced rutile. The results highlight that sub-oxide phases form an important alternative approach to doping with other elements to improve the photo-catalytic performance of TiO2. Such materials are important for applications such as self-cleaning where particles can be incorporated into surface coatings.

Smallest Bimetallic CoPt3 Superparamagnetic Nanoparticles.

We present for the first time a nonaqueous sol-gel route to produce ultrasmall (<2 nm) magnetic bimetallic CoPt3 nanoparticles (NPs). The one-pot procedure is carried out at low temperature (180 °C) using benzyl alcohol, acting as both reducing agent and solvent. The highly monodisperse CoPt3 NPs were investigated with innovative advanced X-ray methods (whole powder pattern modeling), HR-STEM, XPS, and SQUID magnetometry. XPS showed Co was mostly in metallic form, but with a very small amount of CoO on the NP surface. The spherical NPs had an ultrasmall diameter of 1.6 nm and could self-assemble in aligned linear chains, or nanobelts, of single NPs. They are superparamagnetic, with blocking temperature of ∼20 K and coercivity at 10 K of 27.9 kA m-1 (∼350 Oe). However, there is evidence of a second magnetic phase (probably CoO) in the ZFC magnetization curve, which enhances their magnetization values, without significantly affecting their superparamagnetism.

Alkali Niobate and Tantalate Perovskites as Alternative Photocatalysts.

Alkali tantalates and niobates are listed as important photocatalysts for the development of renewable energy technologies and environmental remediation. Herein, the photocatalytic degradation of methylene blue dye in aqueous solution by using highly crystalline particles with perovskite-like structures, LiTaO3 , LiNbO3 , NaTaO3 , NaNbO3 , KNbO3 , and KTaO3 , is investigated. It is demonstrated that ferroelectric KNbO3 is the most efficient photocatalyst of those tested because it combines an electronic band structure that can respond successfully to UVA light with a relatively high surface energy that enhances the catalytic properties. Additionally, the built-in electric field due to internal polarization of ferroelectric particles may contribute to the unique properties of this functional photocatalyst. This work provides an ideal platform for the rational design of more efficient ferroelectric-based photocatalytic devices.