Wetting angles and photocatalytic activities of illuminated TiO2 thin films

TiO₂ thin films have been prepared by physical vapour deposition (PVD) and plasma enhanced chemical vapour deposition (PECVD) to study the UV-induced photo-activity of this material. Wetting angle variations and photo-catalytic activity for the degradation of dyes upon UV illumination have been compared for thin films with different crystalline structure (amorphous, rutile and anatase), microstructure (columnar, compact, etc.) and porosities as estimated from the values of their refraction indices and their direct assessment with a quartz crystal monitor. The surface of the thin films became superhydrophilic upon UV light irradiation and then it recovered its original state by keeping the samples in the dark. Wetting angle decays follow very similar kinetics for amorphous and crystalline films, independently of their actual porosities. By contrast the photo-catalytic activity was very dependent on the crystalline structure of the films (anatase > rutile > amorphous) and on their porosities. The different behaviour depicted by the films with regard to these two properties suggests that they respond to different though related mechanisms and that they cannot be considered as equivalent when trying to prove the photo-activity of TiO₂.     

Physical and electrochemical characterization of nanostructured composites formed by TiO2 templates and PEDOT-PPS films

We report the physical and electrochemical characterization of nanostructured composites formed by TiO₂ templates and PEDOT-PPS films. TiO₂ templates were prepared by sol-gel techniques using TiCl₄ and TiF₄ as the titania precursors, and also by adding a soluble polymer at the initial stages of the TiCl₄ sol-gel synthesis to induce porous oxide networks. The effect of different precursors and annealing environments in the crystallite and particle size of the templates was followed by XRD and SEM studies, and correlated to the electrochemical properties of TiO₂ templates dip-coated in a conducting polymer solution (PEDOT-PSS 1.3%). We found that the microstructure had a strong influence in the adsorption of the conductive polymer, which shows superior electrochemical activity in matrices with abundant surface hydroxylation, broad particle size distribution and residual carbon. Cyclic voltammetry and electrochemical impedance data revealed the differences between the several TiO₂/PEDOT-PSS systems, with the best performance observed in systems based on TiO₂ templates obtained from the TiCl₄ sol-gel route and containing residual mesoporous carbon.     

Inkjet printing of photocatalytically active TiO2 thin films from water based precursor solutions

In this work, aqueous chemical solution deposition route suited for inkjet printing is used for the synthesis of photocatalytically active TiO₂ coatings. Environmentally friendly precursor solutions with electromagnetic ink-jet printing, allows cheap and simple processing of TiO₂ films on glass. The hydrolysis reaction of water sensitive titanium alkoxide (Ti-alkoxide) precursor is controlled by adding complexing agents as citric acid and triethanolamine prior to water addition, and aqueous stable solutions are achieved. The pH of the solutions is brought to neutral to guarantee flexible processing, avoid damage to substrates and equipment. Solution parameters are adapted to obtain optimal gelation conditions and good jettability. The influence of processing parameters on the phase formation and surface morphology is studied by thermogravimetric analysis and differential thermal analysis (TGA/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The photocatalytic activity of the films is evaluated by the degradation of methyl orange.     

Morphology control of cathodically deposited TiO2 films

This work demonstrates that the microstructure of TiO₂ film can be designed and controlled by adjusting the temperature and cycle number of cathodic deposition in a solution containing TiCl₃ and NaNO₃. The redox interactions between TiCl₃ and NO₃⁻ are investigated by in situ ultraviolet-visible (UV-vis) absorption spectroscopy. Linear sweep voltammetry (LSV) is employed to study the NO₃⁻ reduction and to clarify the deposition behavior of TiO₂ in the designed plating solution. The decrease in TiO₂ deposition rate with the TiO₂ thickness may be due to the poor electron conductivity of TiO₂ depressing the generation rate of OH⁻ from the NO₃⁻ reduction. The morphology and size of TiO₂ aggregates are strongly influenced by varying the deposition temperature from 5 to 50 °C and a maximal rate of TiO₂ deposition is obtained at 25-35 °C. TiO₂ deposited at 25 °C is the roughest with a roughness factor (Ra) of ca. 67 nm. This study provides a useful method to control the morphology and deposition rate of TiO₂ film for practical photoelectrochemical applications.     

Dynamic behaviour of viologen-activated nanostructured TiO2: correlation between kinetics of charging and coloration

The dynamic response of viologen-activated nanostructured titanium dioxide has been studied by means of electrical and electro-optical measurements. We show that the state of charge of the semiconductor network is the key factor mediating between the electrode potential and colouration of viologen. Theoretically, we relate the electrode potential to the statistics of occupancy of both TiO₂ nanoparticles and oxidized viologen molecules attached to the surface, on the assumption of quasi-equilibrium of Fermi levels in these contacting phases. Experimentally, we determine the statistical function from steady-state measurements (electrochemical impedance spectroscopy) of the capacitance of the semiconductor film. From this understanding we explain the main features that correlate the simultaneous voltammetry and transmittance responses. Finally, the redox process of viologen is resolved separately from the TiO₂ response by means of transmittance data.     

The synthesis and morphology characteristic study of BAO-ODPA polyimide/TiO2 nano hybrid films

Hybrid nanocomposite films of titanium dioxide (TiO₂) in polyimide (PI) from 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (BAO) and 4,4′-oxydiphthalic anhydride (ODPA) have been successfully fabricated by an in situ sol-gel process. These nanocomposite films exhibit fair good optical transparency up to 40 wt% of TiO₂ content. X-ray diffraction spectroscopy shows three sharp peaks in pure BAO-ODPA PI. It results from the intermolecular regularity. However, the intermolecular regularity in the hybrid film is disrupted by the introduction of TiO₂ nanoparticles with no sharp peak in XRD spectra. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results confirm the formation of TiO₂ particles in PI matrix. The surface Ti content is much lower than the theoretical bulk content in all hybrid films. The ratio of the former to the latter increases with the TiO₂ content and levels off at TiO₂ wt%≥20. Transmission electron microscope (TEM) images show that the TiO₂ phase is well dispersed in the polymer matrix. The size of the TiO₂ phase increases from 10 to 40 nm when the TiO₂ content is 5-30 wt%, respectively.     

Synthesis and characterization of mesoporous TiO2 nanostructured films prepared by a modified sol-gel method for application in dye solar cells

Anatase TiO₂ colloidal dispersions were obtained by hydrothermal synthesis at 200 °C from titanium isopropoxide gels modified with acetic acid in the presence of a non-ionic surfactant. Absolute ethanol, anhydrous terpineol and ethyl cellulose were added to this anatase dispersion resulting in a 23 wt% TiO₂ paste. Mesoporous films for application as working electrodes in dye-sensitized solar cells were prepared by the screen-printing method, yielding reproducible films with thicknesses about 10 μm and desired porosity levels in a single printing operation. An average energy conversion efficiency of 5.2%, and a fill factor of 0.66 were achieved with anatase particle sizes ranging between 15 and 20 nm. The reproducibility of the results was confirmed by electrochemical impedance spectroscopy analysis.     

TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis

TiO₂, TiO₂/Ag and TiO₂/Au photocatalysts exhibiting a hollow spherical morphology were prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors in one-step. Effects of precursor concentration and spray pyrolysis temperature were investigated. By subsequent heat treatment, photocatalysts with phase compositions from 10 to 100% rutile and crystallite sizes from 12 to 120 nm were obtained. A correlation between precursor concentration and size of the hollow spherical agglomerates obtained during spray pyrolysis was established. The anatase to rutile transformation was enhanced with metal incorporations and increased precursor concentration. The photocatalytic activity was evaluated by oxidation of methylene blue under UV-irradiation. As-prepared TiO₂ particles with large amounts of amorphous phase and organic residuals showed similar photocatalytic activity as the commercial Degussa P25. The metal incorporated samples showed comparable photocatalytic activity to the pure TiO₂ photocatalysts.     

Preparation of UO2/TiO2 composite ceramic fuel by sol gel-press forming method

This paper studies the preparation of UO₂/TiO₂ composite ceramic fuel through sol gel-press forming. The research shows that at 1200–1300 °C, UO₂/TiO₂ composite ceramic grains are small, uniform and compact. Among them, UO₂–0.5 wt%TiO₂ has the highest density and good mechanical strength at 1250 °C.     

Effect of different precursor sols on the properties of CeO2-TiO2 films for electrochromic window applications

The properties of spin coated CeO₂-TiO₂ films derived from three different sols containing equimolar quantities of cerium and titanium, fired at 500 °C have been investigated. The films have been deposited using cerium chloride and two different alkoxides and the influence of acetic acid added as a catalyst and modifier of microstructure of the coatings has also been studied. Optical, structural, thermal, and electrochemical properties have been studied and compared. Although gelation time of sols is dependent on the precursor material, enhanced transparency is exhibited by the films prepared with all the sols that have reached the state of gelation. The crystallization behavior and the porosity of the films are highly influenced by the precursor material. Acetic acid derived films with the highest porosity exhibit the highest diffusion coefficient for Li ions. Amorphicity prevailing in films derived from Ti propoxide based precursor sol as against the nanocrystalline films derived from the other sols endow higher ion insertion capacity to former films and highest coloration efficiency is attained when these films are incorporated into an electrochromic device. The highest reversibility for the charging and discharging processes and excellent electrochemical properties observed for the film derived from titanium propoxide prove its practical utility in electrochemical applications. Besides, the highest optical modulation for the electrochromic device comprising WO₃ (electrochromic electrode) and titanium propoxide derived counter electrode is a manifestation of the suitability of the latter electrode in electrochromic window applications.     

Phase content controlled TiO2 nanoparticles using the MicroJetReactor technology

In this paper, a method for the continuous preparation of nanoscaled titania with controlled phase content is presented. The method bases on the MicroJetReactor technology. The synthesis process was carried out by using the hydrolysis of titanium tetraethylate (TET). Synthesis with flow rates to 14 ml/min are implemented, and temperatures are varied between 20 and 210 °C. Particle size distribution measurements by dynamic light scattering (DLS) show monomodal particle size distributions from 1 to 10 nm, stable for more than 24 h. There is no correlation between hydrolysis temperature and the particle size distributions.XRD (X-ray diffractometry) investigations showed, that crystal structures of anatase, brookite, rutile and an amorphous content can be detected in all samples. Quantitative analysis using the Rietveld refinement shows a significant effect of the synthesis temperature on the phase content. The relative phase content of anatase can be raised from 40 wt% up to 75 wt%, accompanied by a loss of all other phases.     

Operando FTIR study of the photocatalytic oxidation of acetone in air over TiO2–ZrO2 thin films

Operando FTIR spectroscopy has been used to study the photocatalytic oxidation of acetone vapors over semiconductors films containing TiO₂ and ZrO₂. Preparation of these coatings was carried out by dipping a silicon wafer in stable sols containing particles of TiO₂, Ti_1−xZr_xO₂, or a mixture of ZrO₂ and TiO₂. These differences in chemical composition and phase homogeneity were selected in order to determine their effect on the photocatalytic performance. A transmission cell specifically designed for in situ studies of photocatalytic coatings was utilized for the FTIR experiments under reaction conditions. In contrast with investigations with powdered photocatalysts, the use of thin films guarantees that the whole semiconductor is irradiated, and for that reason purely photochemical reactions are monitored. Acetone adsorption takes place molecularly and is higher on the Ti_1−xZr_xO₂ coating. This fact is very likely related to the higher specific surface of the samples containing Zr. However, the maximum photocatalytic rate for acetone degradation corresponds to the films composed by a binary mixture of TiO₂ and ZrO₂. On the other hand, remarkable differences on the type and concentration of intermediates appearing as a result of the photocatalytic oxidation of acetone are found for the coatings studied. A simple kinetic model was applied to analyze the evolution of both gas phase and surface species. The parameters obtained indicate that each specific surface process is affected in a different way by the variation in the composition of the photoactive films.     

Photocatalytic properties of TiO2 sol-gel modified nanocomposite films

TiO₂ nanocomposite films with different concentrations of TiO₂ MT-150A nanoparticles were immobilized on glass substrates using a dip coating process. The crystalline structure and surface chemical state of nanocomposite film properties were examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The specific surface area and morphology of TiO₂ MT-150A nanoparticles were evaluated by the BET method and Field Emission Scanning Electron Microscopy (FE-SEM). The photocatalytic activities of films were evaluated by the methyl orange decoloring rate. XPS measurements showed that the oxygen amount (%) was related to the film composition. The composite film with 10 g/L MT-150A loading yielded the highest amount of surface oxygen (26.82%) and TiO₂ rutile showed the lowest amount of surface oxygen (13.67%) in the form of surface hydroxyl groups. The remaining oxygen was identified as lattice oxygen. In addition, the nanocomposite film with 10 g/L MT-150A loading yielded the highest photocatalytic activity.     

Breakage of TiO2 agglomerates in electrostatically stabilized aqueous dispersions

Nanoparticles of metal oxides have applications as additives in thin nanocomposite films. For optical applications that include transparent films and coatings, nanoparticles should be uniformly dispersed in the polymer film. Most commercially available nanoparticles are large agglomerates about 1 μm in maximum dimensions composed of primary particles with sizes ranging from 5 to 50 nm. The large agglomerates scatter light and are not directly suitable for optical systems. Ultrasonication of liquid suspensions was used to prepare stable dispersions from commercial titania nanopowders. The mean diameter of sonicated titania nanopowders was correlated inversely to the specific energy. After a rapid initial size reduction, continued ultrasonication lead to insignificant reduction and even reagglomeration of the particles. Both erosion and fracture mechanisms were observed. None of the commercial nanopowders were successfully broken to their primary particle sizes. Reagglomeration of the dispersion could be prevented by electrostatic stabilization with nitric acid or ammonium hydroxide when its zeta potential value was less than − 30 mV or greater than + 30 mV.     

Dispersion of TiO2 nanopowders to obtain homogeneous nanostructured granules by spray-drying

This work deals with the dispersion and stabilisation of nanosized TiO₂ particles in an aqueous medium as a first step in the preparation of spray-dried nanostructured powders.A colloidal route leading to the production of titania nanostructured feedstocks to obtain nanostructured powders was developed. The process was based on the production of homogeneous and concentrated TiO₂ nanosuspensions dispersed in deionised water with a suitable control of pH and the use of an appropriate anionic dispersant. Concentrated suspensions could be obtained by mixing with an ultrasounds probe at different times depending on the dispersing conditions.Homogeneous suspensions prepared were then reconstituted by spray drying into free-flowing powders with an adequate granule size distribution for diverse purposes, such as atmospheric plasma spraying coatings.     

Synthesis, characterization and photocatalytic properties of sol-gel TiO2 films

The application of heterogeneous photocatalysis is described as an advanced oxidation process (AOP) for the degradation of the diazo reactive dye using immobilized TiO₂ as a photocatalyst. Starting TiO₂ solutions were prepared with and without the addition of polyethylene glycol (PEG) and TiO₂ films were directly deposited on a borosilicate glass substrate using the sol-gel dip-coating method. The surface morphology and the nanoscale roughness of TiO₂ films were studied by means of atomic force microscopy (AFM). Structural properties of TiO₂ were identified by X-ray diffraction (XRD). The decomposition behaviour of organic compounds from the gels was investigated using thermal gravimetry (TG) and differential scanning calorimetry (DSC). Photocatalytic activities of TiO₂ films in the process of degradation of the commercial diazo textile dye Congo red (CR), used as a model pollutant, were monitored by means of UV/vis spectrophotometry. The kinetics of the degradation of the CR dye was described with the Langmuir-Hinshelwood (L-H) kinetic model.The addition of PEG to the TiO₂ solution resulted in the changes in the film surface morphology, and affected the ratio of anatase-rutile crystal phases and the photocatalytic activity of TiO₂. The TiO₂ film prepared with PEG is characterized by higher roughness parameters (R_a, R_max, R_q, R_z and Z_max), a lower amount of the rutile phase of TiO₂, a higher amount of the anatase phase of TiO₂ and a better photocatalytic activity compared to the TiO₂ film without the addition of PEG.     

Dye-sensitized solar cells based on double-layered TiO2 composite films and enhanced photovoltaic performance

Dye-sensitized solar cells (DSSCs) are fabricated based on double-layered composite films of TiO₂ nanoparticles and hollow spheres. The photoelectric conversion performances of DSSCs based on nanoparticles/nanoparticles (PP), hollow spheres/hollow spheres (HH), hollow spheres/nanoparticles (HP), and nanoparticles/hollow spheres (PH) double-layered films are investigated, and their photo-electric conversion efficiencies are 4.33, 4.72, 4.93 and 5.28%, respectively. The enhanced performance of TiO₂ nanoparticles/hollow spheres double-layered composite film solar cells can be attributed to the combined effect of following factors. The light scattering of overlayer hollow spheres enhances harvesting light of the DSSCs and the underlayer TiO₂ nanoparticle layer ensures good electronic contact between film electrode and the F-doped tin oxide (FTO) glass substrate. Furthermore, the high surface areas and pore volume of TiO₂ hollow spheres are respectively beneficial to adsorption of dye molecules and transfer of electrolyte solution.     

Influence of TiO2 and CeO2 on the hydrogenation activity of bulk Ni2P

TiO₂- and CeO₂-promoted bulk Ni₂P catalysts were prepared by impregnation and in-situ H₂ temperature-programmed reduction method. The prepared catalysts were characterized by XRD and XPS. The hydrogenation activities of the catalysts were studied using 1.5 wt.% 1-heptene in toluene and 1.0 wt.% phenylacetylene in ethanol as the model feeds. The results indicate that bulk Ni₂P possesses low hydrogenation activity but is tunable by simply controlling the content of the additives (TiO₂ or CeO₂), suggesting that TiO₂ and CeO₂ are effective promoters to enhance the hydrogenation activity of Ni₂P.