Photocatalytic Nb2O5-doped TiO2 nanoparticles for glazed ceramic tiles

TiO₂ nanoparticles are currently used as coating for self-cleaning building products. In order to achieve high self-cleaning efficiency for outdoor applications, it is important that titania is present as anatase phase. Moreover, it is desirable that the particle sizes are in nano-range, so that a large enough surface area is available for enhanced catalytic performance. In this work, TiO₂ nanoparticles doped with 0–5 mol% Nb₂O₅ were synthesized by co-precipitation. Nb₂O₅ postponed the anatase to rutile transformation of TiO₂ by about 200 °C, such that after calcination at 700 °C, no rutile was detected for 5 mol% Nb₂O₅-doped TiO₂, while undoped TiO₂ presented 90 wt% of the rutile phase. A systematic decreasing on crystallite size and increasing on specific surface area of TiO₂ were observed with higher concentration of Nb₂O₅ dopant. Photocatalytic activity of anatase polymorph was measured by the decomposition rate of methylene blue under ultraviolet and daylight illumination and compared to commercial standard catalyst (P25). The results showed enhanced catalysis under UV and visible light for Nb₂O₅-doped TiO₂ as compared to pure TiO₂. In addition, 5 mol% Nb₂O₅-doped TiO₂ presented higher photocatalytic activity than P25 under visible light. The enhanced performance was attributed to surface chemistry change associated with a slight shift in the band gap.     

Ferromagnetism in Rutile Structure Cr Doped VO<Subscript>2</Subscript> Thin Films Prepared by Reactive-Bias Target Ion Beam Deposition

First generation spintronics has entered the mainstream of information technology through its utilization of the magnetic tunnel junction in applicable devices such as read head sensors for hard disk drives and magnetic random access memory. The future of spintronic devices requires next generation spintronic materials (Wolf et al. in IBM J. Res. Dev. 9:101, [2006]). Here we report on the structural, transport, and magnetic characteristics of V_1−x Cr _x O₂ (0.1≤x≤0.2) thin films deposited on (001) Al₂O₃ substrates. We show that the metal-insulator transition of VO₂ is suppressed and the rutile structure is stable down to 100 K. The films are remarkably smooth having a root-mean squared surface roughness of 0.3 nm. Films are conductive at room temperature and appear to follow a variable-range-hopping conduction mechanism below that. Ferromagnetism is observed at room temperature and is dependent on Cr concentration. The combination of these characteristics makes V_1−x Cr _x O₂ a viable candidate material for next generation spintronic multilayer devices.     

Non-Stoichiometric Oxide Surfaces and Ultra-thin Films: Characterisation of TiO2

Reducible transition metal oxides are key components in many catalytic, sensing and device systems however relatively little quantitative information exists on the nature of non-stoichiometric phases. In this study, we show how substrates can be prepared in a controlled manner with designed levels of non-stoichiometry. These in turn will facilitate quantitative studies of the interaction of non-stoichiometric oxide materials with adsorbates, reactants and supported metallic particles. We grow stoichiometric ultra-thin films of TiO₂ in the rutile phase and (110) orientation on W(100) which represent our baseline material. Films are then doped with submonolayer excess Ti and annealed to create films with quantified levels of non-stoichiometry. The study relies heavily on previous scanning tunnelling microscopy work, which shows the dominant nature of Ti interstitial defects in the surface chemistry of TiO₂, combined with detailed and quantitative X-ray photoelectron spectroscopy measurements.     

New fine structures resolved at the ELNES Ti-L2,3 edge spectra of anatase and rutile: Comparison between experiment and calculation

Anatase and rutile Ti-L_2,3 edge spectra were measured in electron energy loss spectroscopy (EELS) using a transmission electron microscope (TEM) coupled to a CEOS Cs-probe corrector, an omega-type monochromator and an in-column omega-type energy filter fully corrected for 2nd order aberrations. Thanks to the high energy resolution, high electron probe current and high stability achieved under this instrumental configuration, new fine structures, never reported before, were resolved at the L₃ band of both rutile and anatase. The data suggest that new peaks also exist in the L₂ e_g band. The experimental spectra are compared with multichannel multiple scattering (MMS) calculations. Good agreement is found for number, energy position and intensity of the newly resolved spectral features. Up to now, the L₃ e_g band splitting could not be well described by theory not even through the crystal field multiplet approach. We show that the L₃ e_g band splitting is due to long range band structure effects, contrary to the usual interpretations in terms of local ligand field or near-neighbour hybridization effects.     

Classical molecular dynamics simulations of gold clusters deposited on rutile TiO2(1 1 0) surface

Classical molecular dynamic simulations of Au clusters supported on the non-defective rutile TiO₂(1 1 0) surface are reported. The oxide surface is represented by a slab obtained by imposing periodic boundary conditions to a 12 × 12 × 1 supercell. The dynamics of the system is accounted for thorough classical pair potentials describing both the metal–metal and the metal–surface interactions, determined from periodic density functional theory model calculations. Deposited particles show a well defined structure and can be described as hexagonal truncated pyramids mainly exhibiting (1 1 1) facets in agreement with scanning tunnel microscopy experiments conducted under atomic resolution.     

Morphological and crystallite size impact on electrochemical performance of electrospun rutile and rutile/multiwall carbon nanotube nanofibers for lithium ion batteries

A sol–gel based route was used to produce TiO₂ based nanocomposites. Sols were electrospun into continuous nanofibers and calcined to get rutile phase. Fibers with diameter around 100 nm and crystallites size between 10 and 50 nm were obtained. The morphological impact and crystallites size dependence of the electrochemical performance for as-synthesized materials are reported. Enhancements using inert calcination atmosphere and incorporation of multi-wall carbon nanotubes (MWCNT) into the system are also presented.     

Photocatalytic oxidation of methyl orange by natural V-bearing rutile under visible light

Natural rutile, which contains substituting metal ions as V⁵⁺ and Fe³⁺, was found to exhibit good photoactivity in visible light. The result showed that 60.59% of methyl orange (MO) was photodegraded after 1 h visible light irradiation. Compared with P25 TiO₂, the efficiency and mechanism of MO degradation by natural rutile was discussed. The visible light response was due to the substituting ions altering the band structure, and the effective photoactivity was related to the presence of H₂O₂ and surface hydroxyl groups. Moreover, the substituting metal ions in their high oxidation states could capture photogenerated electrons to suppress electron–hole combination.     

Rutile TiO2 nanorod arrays directly grown on Ti foil substrates towards lithium-ion micro-batteries

Nanosized rutile TiO₂ is one of the most promising candidates for anode material in lithium-ion micro-batteries owing to their smaller dimension in ab-plane resulting in an enhanced performance for area capacity. However, few reports have yet emerged up to date of rutile TiO₂ nanorod arrays growing along c-axis for Li-ion battery electrode application. In this study, single-crystalline rutile TiO₂ nanorod arrays growing directly on Ti foil substrates have been fabricated using a template-free method. These nanorods can significantly improve the electrochemical performance of rutile TiO₂ in Li-ion batteries. The capacity increase is about 10 times in comparison with rutile TiO₂ compact layer.     

Microscale meshes of Ti3O5 nano- and microfibers prepared via annealing of C-doped TiO2 thin films

A new technique to produce microscale Ti₃O₅ nano- and microfiber meshes is proposed. When a 3 wt% carbon-doped TiO₂ film on Si(1 0 0) was annealed at 1000 °C in wet nitrogen (0.8%H₂O), the amorphous TiO₂ phase gave rise to crystalline phases of λ-Ti₃O₅ (75%) and rutile + trace of TiO_2−xC_x (25%). From Raman and FTIR Spectroscopy results, it was concluded that rutile is formed at the inner layer located at the interface between the mesh and the Si that was located away from the surface such that the meshes of nano- and microfibers are predominantly composed of Ti₃O₅ grown from the reaction of rutile with Si to form Ti₃O₅ and SiO₂. On the other hand, it was noteworthy that the microscale mesh of nano- and microfibers showed increased photoluminescence compared with amorphous TiO₂. The PL spectrum which had a broad band in the visible spectrum, fitted as three broad Gaussian distributions centered at 571.6 nm (∼2.2 eV), 623.0 nm (∼2.0 eV) and 661.9 nm (∼1.9 eV).     

Transparent conducting oxide films of heavily Nb-doped titania by reactive co-sputtering

Niobium-doped titania (TNO) films of various Nb content were deposited on glass and silicon substrates by reactive co-sputtering of Ti and Nb metal targets. Nb content in the TNO films was varied from 0 to ∼13 at.% (atomic percent), corresponding to Ti_1−xNb_xO₂ with x = 0-0.52, by modulating the Nb target power from 0 to 150 W (Watts). The influence of ion bombardment on the TNO films was investigated by applying an RF substrate bias from 0 to 25 W. The as-deposited TNO films were all amorphous and insulating, but after annealing at 600 °C for 1 h in hydrogen, they became crystalline and conductive. The annealed films crystallized into either pure anatase or mixed anatase and rutile structures. The as-deposited and the annealed films were transparent, with an average transmittance above 70%. Anatase TNO film (Ti_1−0.39Nb_0.39O₂) with Nb 9.7 at.% exhibited a dramatically reduced resistivity of 9.2 × 10⁻⁴ Ω cm, a carrier density of 6.6 × 10²¹ cm⁻³ and a carrier mobility around 1.0 cm² V⁻¹ s⁻¹. In contrast, the mixed-phase Ti_1−0.39Nb_0.39O₂ showed a higher resistivity of 1.2 × 10⁻¹ Ω cm. This work demonstrates that the anatase phase, oxygen vacancies, and Nb dopants are all important factors in achieving high conductivities in TNO films.