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Colloidal sol-gel is a common method used for the preparation of stable and homogeneous nanoparticulate sols. In this work, TiO2 and Er3+ doped-TiO2 sols (1, 2 and 3 mole % Er3+) have been prepared. The peptization time, viscosity and isoelectric point increase at higher Er3+ concentration indicating that erbium (III) delays the peptization process, increases the viscosity and adsorbs onto the surface of the TiO2 nanoparticles. When the sols of TiO2 are doped with Er3+ lower values of the band gap are obtained. After drying, in all cases, anatase was obtained as major phase although traces of brookite are also detected. The microstructure was observed by TEM and shows direct evidence that the xerogels are polycrystalline.
1 D TiO2 nanomaterials (nanotubes, nanowires) were synthesized through hydrothermal treatment of TiO2 powder (P25) in concentrated alkaline solutions (NaOH for nanotubes, KOH for nanowires) followed by calcination at varying temperatures between 400°C and 700°C. Samples were characterized by HRTEM, XRD, Raman spectroscopy, and N2 adsorption-desorption isotherms. High surface area nanotubular TiO2 materials can maintain their 1D morphology up to a temperature of calcination of 400°C while changing their phase from hydrogenotitanate to anatase. The use of KOH leads to a retarded formation of anatase. Photocatalytic results showed that TiO2 anatase nanotubes calcined at 400°C can degrade formic acid with a rate constant four times higher than for P25. A direct correlation between surface area and photocatalytic activity explains the much higher activity of TiO2 anatase nanotubes. On the opposite, for the degradation of phenol, P25 remains more active. In the disinfection of water, contrary to P25, the high surface area of TiO2 nanotubes allows the simultaneous degradation of formic acid and the inactivation of pathogen fungus showing the interest of such materials for the treatment of wastewater.
Mismatched or shadowed individual cells in a module can operate in the Reverse Bias (RB) regime. Subjecting a dye solar cell (DSC) to an accelerated RB stress by forcing a constant current equal to 2-fold its ISC, produced significant alterations on the current-voltage (I-V) characteristics in RB with time and a severe loss of cell efficiency in 32h. We investigated and identified a key mechanism for RB charge transfer and degradation in DSCs. I-V characteristics in RB were found to be sensitive to the type of dye utilized and to TiCl4 substrate treatment.
Highly mesoporous TiO2 nanoparticles (NPs) were synthesized by an aero-sol-gel process in this approach. By varying the mass fraction of inorganic templates, the formation of mesoporous TiO2 NPs with optimized surface area and pore volume distributions was examined. Then, the photovoltaic properties of the resulting mesoporous TiO2 NPs were systematically investigated by applying them into the photoanode of dye-sensitized solar cells (DSSCs). The mesoporous TiO2 NP-based DSSCs fabricated in this study showed an improved short circuit current density and power conversion efficiency compared with solid TiO2 NP-based DSSCs due to the increase of the amount of inorganic dye (N719) adsorption in the mesoporous TiO2 NPs. These mesoporous TiO2 NPs fabricated have a strong potential as an effective dye supporting and electron transfer medium to improve the photovoltaic performance of DSSCs.
Compressibility of anatase nano particles of TiO2 changes from the bulk counterpart. This has been associated with amorphization and compaction. The behavior of such systems under extreme conditions is examined using a shell partial distribution function and some comparison made with rutile and baddeylite polytypes based nano structures. Particle energies of rutile and baddeylite nano particles appear to be rather size independent as compared to the anatase polytypes. The latter is associated with large relaxations and re-bonding in the relatively soft anatase phase of nano TiO2.
UV exposure of dye-sensitized solar cells (DSCs) results in a loss of triiodide from the electrolyte and this is as a result of direct band gap excitation of the TiO2 semiconductor. The UV stability of a typical electrolyte composition is explored further and the results show that the electrolyte is very stable to UV irradiation in the absence of TiO2 but experiences rapid triiodide loss in its presence. Furthermore, the effect of a periodic triiodide regeneration technique, applied to UV exposed DSCs, is investigated and whilst this treatment does not appear to be able to permanently reverse triiodide loss in UV exposed cells, devices that are periodically regenerated, maintain higher average photocurrents over the UV exposure period.
In this paper, a simple process to fabricate free of disorder nanostructures, large area, flat and mechanically robust free-standing TiO2 nanotube (TNT) membranes was developed. Self-organized TNTs with ultrahigh aspect ratio (∼2000) were fabricated via anodization of Ti foil in fluorine containing ethylene glycol. Then by controlling the evaporation rate of rinsing solvent on the as-anodized TNT films in atmosphere, large area TNT membranes were self-detached uniformly from the metallic Ti substrate during the drying process. These free-standing membranes may exhibit many potential applications for optoelectronic devices.
A novel polymer dispersant, poly(oxyethylene)-segment imide (POEM) in the structure was incorporated in the nanocrystalline TiO2 film as the electrode. The uses of the dispersants could disperse TiO2 by decreasing the van der waals force among the nanoparticles, observed by TEM. The resultant TiO2/POEM film as the photoanode rendered the dye-sensitized solar cell (DSSC) with enhanced performance. By comparing to the traditional photoanode composing of polyethylene glycol (PEG) dispersed TiO2, the POEM dispersed TiO2 provided large surface area and high roughness in the dye adsorbed film. Furthermore, the fabricated TiO2/POEM photoanode has a better light-scattering property which contributes to the improvement for the short-circuit current density (Jsc) and the power-conversion efficiency (_) of the DSSC to be 19.1 mA cm-2 and 8.7%, respectively. The performance is superior to 13.2 mA cm-2 and 7.34% for a DSSC with the photoanode containing TiO2/PEG.
Nanostructured fiber-mats have large surface area, high reactivity, low weight and low agglomeration tendency. These are advantages if compared with nanoparticles for photocatalytic application. Fiber-mats can be used not only as a photocatalytic material on their own, but also incorporated in different surfaces or fabrics and as well as a filtration membrane. In this work, high temperature stable anatase titanium dioxide fiber-mats doped with silica (0.5 to 30 %) or doped with tin (0.5 to 15 %) were produced by electrospinning technology. The precursors used were titanium propoxide (TiP), tetrapropoxysilane (TPS) and tin 2-ethylhexanoate. They were hydrolyzed in acetic acid and mixed with an alcoholic solution of 10 wt% polyvinylpirrolidone. The effect of heat treatment on the microstructure characteristics and the photocatalytic activity of the fiber-mats in comparison with a commercial TiO2 powder (Evonik P-25) were studied. After the electrospinning process, a thin, porous fiber-mat was obtained. This material was dried in air at room temperature for 24h. These fibers were then heat treated from 500 to 800°C for 3 hours at a heating rate of 1.4°C/min. The fiber-mats were then characterized using N2 adsorption (BET method) for surface area measurements, X-ray diffraction for phase determination, SEM and TEM analyses for morphological characterization. The photocatalytic activity was studied using as model system the degradation of methyl orange in water (20ppm) under UV-A light. As-obtained fibers are amorphous but become crystalline after heat treatment. As the heat treatment temperature increases the surface area decreases significantly. Quite the opposite happens with the rutile to anatase ratio and the anatase and rutile crystallite sizes, which increase with higher heat treatment temperatures. The photoactivity increases with the increment in heat treatment temperature until 650°C, when the fibers start to become denser and the surface area drops due to sintering. Fibers produced at higher temperatures and with lower amounts of Si and Sn are predominantly anatase and are generally more photoactive under UV-A radiation.
Visible (λ > 420 nm) light-driven photooxidation of water at TiO2-polyheptazine (TiO2-PH) hybrid photoanodes loaded with two different metal oxide co-catalysts was investigated in a twoelectrode setup. As compared to TiO2-PH photoanodes loaded with colloidal IrO2, photoelectrodes modified with photodeposited CoOx oxygen-evolving co-catalyst (Co-Pi) showed both higher photocurrents and more efficient oxygen evolution. The minimum external electric bias needed to observe complete photooxidation of water to dioxygen at TiO2-PH photoanodes modified with Co-Pi was estimated to be ca. 0.6 V at pH 7.
The photoelectric properties of oxygen-deficient titanium dioxide (TiO2) nanotube arrays are investigated in this study. The TiO2 nanotube arrays are prepared by anodization, followed by annealing at 450 to 750 °C for 3 h in air to form different crystalline phase mixtures. When the annealing temperature is increased, several phenomena are observed: (1) the ratio of anatase to rutile decreases, (2) the anatase nanotubes are shortened and (3) the thickness of the dense rutile film layer underneath the anatase nanotubes increases. The efficiency of visible light absorption of the nanotube arrays is enhanced with increasing annealing temperature. This is believed to be caused by the ionic defects, especially the oxygen vacancies, generated during the annealing procedure, enabling the absorption of low-energy radiations. The X-ray photoelectron spectroscopy (XPS) depth profile analysis provides the supporting evidence on the chemical nonstoichiometry (i.e., oxygen-deficiency) of the TiO2 nanotube arrays annealed at high temperature. With increasing annealing temperature, a decrease and an increase in the photocurrent density of the nanotube arrays under UV and visible light (wavelength > 500 nm) irradiations, respectively, are detected. The decrease of the photocurrent density under UV irradiation is caused by the reduction in the specific surface area (i.e., anatase nanotubes transform into rutile film with vigorous annealing). In contrast, the increase of the photocurrent density under visible light irradiation is contributed to the oxygen vacancies in the nanostructure, providing extra electron energy levels (locating below the conduction band of TiO2) within the band structure.