TiO2-based nanopowders and thin films for photocatalytical applications

TiO2-Based Nanomaterials for Gas Sensing-Influence of Anatase and Rutile Contributions

The paper deals with application of three nanomaterial systems: undoped TiO₂, chromium-doped TiO₂:Cr and TiO₂-SnO₂ synthesized by flame spray synthesis (FSS) technique for hydrogen sensing. The emphasis is put on the role of anatase and rutile polymorphic forms of TiO₂ in enhancing sensitivity towards reducing gases. Anatase-to-rutile transformation is achieved by annealing of undoped TiO₂ in air at 700 °C, specific Cr doping and modification with SnO₂. Undoped TiO₂ and TiO₂-SnO₂ exhibit n-type behaviour and while TiO₂: 5 at.% Cr is a p-type semiconductor. X-ray diffraction (XRD) has been applied to determine anatase-to-rutile weight ratio as well as anatase and rutile crystal size. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been used to characterize the structure and morphological parameters. Optical reflectometry enabled to find and compare the band gaps E _g of anatase and rutile predominated compositions. Electrical properties, i.e. the electrical conductivity and values of constant phase element (CPE), have been established on the basis of impedance spectroscopy. Dynamic responses of the electrical resistance as a function of hydrogen concentration revealed that predominance of rutile in anatase/rutile mixture is beneficial for gas sensing. Partial transformation to rutile in all three material systems under study resulted in an increased sensitivity towards hydrogen. It is proposed that this effect can be explained in a similar way as in photocatalysis, i.e. by specific band alignment and electron transfer from rutile to anatase to facilitate oxygen preadsorption on the surface of anatase grains.

Nanocrystalline TiO2/SnO2 heterostructures for gas sensing

The aim of this research is to study the role of nanocrystalline TiO₂/SnO₂ n-n heterojunctions for hydrogen sensing. Nanopowders of pure SnO₂, 90 mol % SnO₂/10 mol % TiO₂, 10 mol % SnO₂/90 mol % TiO₂ and pure TiO₂ have been obtained using flame spray synthesis (FSS). The samples have been characterized by BET, XRD, SEM, HR-TEM, Mössbauer effect and impedance spectroscopy. Gas-sensing experiments have been performed for H₂ concentrations of 1-3000 ppm at 200-400 °C. The nanomaterials are well-crystallized, anatase TiO₂, rutile TiO₂ and cassiterite SnO₂ polymorphic forms are present depending on the chemical composition of the powders. The crystallite sizes from XRD peak analysis are within the range of 3-27 nm. Tin exhibits only the oxidation state 4+. The H₂ detection threshold for the studied TiO₂/SnO₂ heterostructures is lower than 1 ppm especially in the case of SnO₂-rich samples. The recovery time of SnO₂-based heterostructures, despite their large responses over the whole measuring range, is much longer than that of TiO₂-rich samples at higher H₂ flows. TiO₂/SnO₂ heterostructures can be intentionally modified for the improved H₂ detection within both the small (1-50 ppm) and the large (50-3000 ppm) concentration range. The temperature T_max at which the semiconducting behavior begins to prevail upon water desorption/oxygen adsorption depends on the TiO₂/SnO₂ composition. The electrical resistance of sensing materials exhibits a power-law dependence on the H₂ partial pressure. This allows us to draw a conclusion about the first step in the gas sensing mechanism related to the adsorption of oxygen ions at the surface of nanomaterials.

Effect of Nb doping on structural, optical and photocatalytic properties of flame-made TiO2 nanopowder

TiO(2):Nb nanopowders within a dopant concentration in the range of 0.1-15 at.% were prepared by one-step flame spray synthesis. Effect of niobium doping on structural, optical and photocatalytic properties of titanium dioxide nanopowders was studied. Morphology and structure were investigated by means of Brunauer-Emmett-Teller isotherm, X-ray diffraction and transmission electron microscopy. Diffuse reflectance and the resulting band gap energy were determined by diffuse reflectance spectroscopy. Photocatalytic activity of the investigated nanopowders was revised for the photodecomposition of methylene blue (MB), methyl orange (MO) and 4-chlorophenol under UVA and VIS light irradiation. Commercial TiO(2)-P25 nanopowder was used as a reference. The specific surface area of the powders was ranging from 42.9 m(2)/g for TiO(2):0.1 at.% Nb to 90.0 m(2)/g for TiO(2):15 at.% Nb. TiO(2):Nb particles were nanosized, spherically shaped and polycrystalline. Anatase was the predominant phase in all samples. The anatase-related transition was at 3.31 eV and rutile-related one at 3.14 eV. TiO(2):Nb nanopowders exhibited additional absorption in the visible range. In comparison to TiO(2)-P25, improved photocatalytic activity of TiO(2):Nb was observed for the degradation of MB and MO under both UVA and VIS irradiation, where low doping level (Nb < 1 at.%) was the most effective. Niobium doping affected structural, optical and photocatalytic properties of TiO(2). Low dopant level enhanced photocatalytic performance under UVA and VIS irradiation. Therefore, TiO(2):Nb (Nb < 1 at.%) can be proposed as an efficient selective solar light photocatalyst.