In situ growth of vanadia-titania nano/micro-porous layers with enhanced photocatalytic performance by micro-arc oxidation

Micro-arc oxidation process was used to synthesize V₂O₅-TiO₂ porous layers for the first time. Surface morphology and topography of the layers were investigated by scanning electron microscope (SEM) and atomic force microscope (AFM). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were also employed to evaluate phase structure and chemical composition of the layers. It was found that the V₂O₅-TiO₂ layers consisted of anatase, rutile, and vanadium pentoxide phases fraction of which varied with the applied voltage and the electrolyte concentration. It was also revealed that pore size and surface roughness increased with the applied voltage and the electrolyte concentration. Optical properties of the layers were studied by a UV-vis spectrophotometer, and the band gap energies of the MAO-grown pure TiO₂ and V₂O₅-TiO₂ layers were respectively calculated as 3.21 and 2.56 eV. Furthermore, the composite layers exhibited a significantly enhanced photo-activity when compared to pure TiO₂ layers. The photocatalytic reaction rate constants of degradation of methylene blue on the surface of the V₂O₅-TiO₂ layers under ultraviolet and visible irradiations were measured as 0.0228 and 0.0117 min⁻¹, respectively. As a consequence, micro-arc oxidation was deduced to be an appropriate and efficient method for synthesis of V₂O₅-TiO₂ porous layers.     

Self-organized porous titanium oxide prepared in Na2SO4/NaF electrolytes

The anodic formation of nanoporous TiO₂ on titanium was investigated in Na₂SO₄ electrolytes containing low concentrations of NaF (0.1-1 wt.%). It was found that under optimized electrolyte conditions and extended polarization, a self-organized nanostructure consisting of porous TiO₂ is obtained. The porous structure is arranged in sections of arrays with single pore diameters of typically 100 nm and an average spacing of 150 nm. The pores are open at the top and covered by oxide at the bottom. Compared with earlier work, we show that using a neutral NaF electrolyte significantly thicker porous layers can be obtained than in acidic solutions.     

Electrical properties of TiO2/SEO nanocomposites: From macro to nano

Tunneling atomic force microscopy (TUNA) was successfully used to confirm that electro-devices based on TiO₂-SEO nanocomposites can find possible application in solar power conversion field. Investigated electro-devices show different current flow depending on the layer combination. The highest capacity was shown by the electro-devices with a PEDOT-PSS layer on the top, being the average current values ∼200 pA at 10 V applied voltage. The conductivity value measured by Keithley indicated that the system ITO/1:PEDOT-PSS/1:TiO₂-SEO electro-device shows the highest conductivity level, 0.16 S/cm. Thus, these systems have high potential to find application as solar cell devices.     

Chlorobenzene total oxidation over palladium supported on ZrO2, TiO2 nanostructured supports

Two series of supported Pd catalysts were synthesized on new mesoporous–macroporous supports (ZrO₂, TiO₂) labelled M (Zr and Ti). The deposition of palladium was carried out by wet impregnation on the calcined TiO₂ and ZrO₂ supports at 400 °C (Pd/Zr₄, Pd/Ti₄) and 600 °C (Pd/Zr₆, Pd/Ti₆) and followed by a calcination at 400 °C for 4 h. The pre-reduced Pd/MX catalysts were investigated for the chlorobenzene total oxidation and their catalytic properties where compared to those of a reference catalyst Pd/Ti-Ref (TiO₂ from Huntsman Tioxide recalcined at 500 °C) and of a palladium supported on the fresh mesoporous–macroporous TiO₂ (Pd/Ti). Based on the activity determined by T₅₀, the Pd/Ti and Pd/Ti₄ catalysts have been found to be more active than the reference one. Moreover activity decreased owing to the sequence: Pd/TiX  Pd/ZrX and in each series when the temperature of calcination of the support was raised. The overall results clearly showed that the activity was dependant on the nature of the support. The better activity of Pd/TiX compared to Pd/ZrX was likely due to a better reducibility of the TiO₂ support (Ti⁴⁺ into Ti³⁺) leading to an enhancement of the oxygen mobility. Production of polychlorinated benzenes PhCl_x (x = 2–6) and of Cl₂ was also observed. Nevertheless at 500 °C the selectivity in HCl was higher than 90% for the best catalysts.     

Improving the stability and bioactivity of micro-arc oxidized calcium phosphate/titania porous coatings by high energy shot peening pretreatment

Micro-arc oxidation (MAO) is one of the conventional methods for surface modification of titanium (Ti)-based dental materials. However, in the process of MAO, the surface porous titania (TiO₂) coating is prone to micro-crack, which affects the stability of coating and the long-term service life of the implant. In this work, high-energy shot peening (HESP) pretreatment would be used to improve the stability and bioactivity of the MAO coatings and increase the effective doping of calcium and phosphorus (Ca & P) elements in surfaces. We verified that the MAO specimens pretreated by HESP (S-MAO) had larger pore size (~ 4.0 μm), coating thickness (~ 8.5 μm) and critical load (~ 11.1 N) than the samples without pretreatment (MAO, ~ 3.1 μm, 6.2 μm & 6.2 N); and the content of Ca and P in S-MAO group (Ca: ~ 6.3 wt%; P: ~ 9.4 wt%) was higher than that of MAO group (Ca: ~ 5.8 wt%; P: ~ 6.7 wt%. Meanwhile, it was also proved that compared with Ti and MAO groups, MC3T3-E1 cells on S-MAO substrates had better spreading, viability, ALP activity, and osteogenic gene expression. All the above results indicate that the HESP pretreatment has excellent potential to improve the coating stability and bioactivity of MAO-treated substrates.     

Modified titanium electrodes: Application to Ti/TiO2/PbO2 dimensionally stable anodes

Titanium is a valve metal able to withstand corrosion, due to the presence of a passivating layer of titanium oxide on its surface. But, due to that more or less insulating layer, titanium cannot be used directly as an anodic material. However, modification of the surface of a Ti/TiO₂ substrate may lead to the formation of new structures: Ti/TiO₂/M or Ti/TiO₂/OX, in which M is a metal such as platinum and OX a conducting oxide exhibiting electrocatalytic properties. These structures have interesting electrochemical properties and may be used as efficient electrode materials.In this paper, after a review of the electrochemical behaviour of these structures, we give new results concerning the selective electrodeposition of lead dioxide on Ti/TiO₂ substrates and we propose an interpretation of the results taking into account the dielectric properties of the underlying TiO₂. It is shown that there is a dramatic decrease of the resistance of the electrode when a PbO₂ layer is electrodeposited onto a Ti/TiO₂ structure. That effect allows the preparation of electrodes (low-cost DSAs) that may be used as anodes in spite of the presence of the underlying TiO₂ layer, that layer being useful to avoid corrosion of the titanium substrate. At last, the effect of stabilization of the underlying TiO₂ layer is discussed.     

TiO2 optical coating layers for self-cleaning applications

TiO₂ films deposited by various coating techniques were investigated for self-cleaning applications. The optical coating layers of TiO₂ films prepared from a sol–gel precursor were deposited on glass substrates using spin coating, dip coating and screen printing techniques. Effects of film deposition techniques on crystal structure, microstructure, thickness, photocatalytic activity, hydrophilicity and optical properties of the films were investigated using XRD, AFM, SEM, surface profilometer, UV–vis spectrophotometer and contact angle measurement. Dip coating the TiO₂ optical film two and three times resulted in superhydrophilic surfaces. Increasing number of dipping times was found to increase the photocatalytic activity.     

Development and study of anodic Ti/TiO2 electrodes and their potential use as impedimetric immunosensors

Titanium dioxide films were anodically formed at various potentials up to 65 V in 1 M H₂SO₄. Oxide films were characterized by performing various techniques, including electrochemical impedance spectroscopy, scanning electron microscopy, Raman spectroscopy, ellipsometry and diffuse reflectance FT-IR spectroscopy. Low voltage anodization (up to 10 V) results to amorphous TiO₂, whereas at higher applied potentials (up to 65 V), anatase is the predominant form. Anatase films were further hydroxylated with an acidic agent and the effect of this treatment on the overall impedance of the electrodes was studied with impedance spectroscopy. The potential use of anodic (anatase) Ti/TiO₂ electrodes in the development of impedimetric immunosensors is also demonstrated by monitoring the immunoreaction of avidin/anti-avidin with different instrumental approaches based on a FRA analyzer, an LCR-meter and a home-built charge integrator (Multipulser).     

Photo-induced electrochemical functionality of the TiO2 nanoscale films

Titanium(IV) oxide semiconducting thin films were deposited in the environment of chemically active plasma by means of the PVD hollow cathode plasma jet sputtering, PVD magnetron sputtering and the PECVD technique (surfatron produced plasma discharge as a jet type). In parallel the sol-gel process carried out in the reverse micelle environment was utilized as a purely chemical procedure. Prepared films were described by a series of characterization methods involving atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and ultraviolet-visible spectroscopy (UV-vis). All layers were tested then for their photo-induced electrochemical properties based on evaluation of polarization curves and corresponding photocurrents. These experiments were carried out in the opto-electrical apparatus supplemented with a system of highly precious spectral filters. The differences among photocurrent values were discussed.     

Characterization and property of bifunctional Zn-incorporated TiO2 micro-arc oxidation coatings: The influence of different Zn sources

In the present work, Zn-incorporated TiO₂ coatings are prepared through a one-step micro-arc oxidation (MAO) method on a grade 4 pure titanium with the addition of either Na₂Zn-EDTA solution or ZnO nanoparticles (NPs) as Zn sources. The microstructural features of both Zn-incorporated TiO₂ coatings were systematically examined. It is revealed that different Zn sources result in significant difference of phase component, chemical state, composition and morphology between the resultant Zn-incorporated MAO coatings. Zn species could be present as ZnO and Zn(OH)₂ in the coating when Na₂Zn-EDTA was used as Zn source whereas the presence of ZnO nano-clusters is obvious on the coating surface with ZnO NPs as Zn source. The addition of ZnO NPs during the MAO process also leads to a lower Zn content of the resultant coating, which is more defective with increased thickness in comparison to that of Na₂Zn-EDTA. Further, antibacterial property and osteogenic activity of both Zn-incorporated coatings were examined. Both Zn-incorporated coatings exhibit favourable bacterial inhibition ability and bone formability, suggesting the successful synthesis of bifunctional coatings through the facile one-step micro-arc oxidation method.     

Photocatalytic activity of Cu2O/TiO2, Bi2O3/TiO2 and ZnMn2O4/TiO2 heterojunctions

Cu₂O/TiO₂, Bi₂O₃/TiO₂ and ZnMn₂O₄/TiO₂ heterojunctions were studied for potential applications in water decontamination technology and their capacity to induce an oxidation process under VIS light. UV–vis spectroscopy analysis showed that the junctions-based Cu₂O, Bi₂O₃ and ZnMn₂O₄ are able to absorb a large part of visible light (respectively, up to 650, 460 and 1000 nm). This fact was confirmed in the case of Cu₂O/TiO₂ and Bi₂O₃/TiO₂ by photocatalytic experiments performed under visible light. A part of the charge recombination that can take place when both semiconductors are excited was observed when a photocatalytic experiment was performed under UV–vis illumination. Orange II, 4-hydroxybenzoic and benzamide were used as pollutants in the experiment. Photoactivity of the junctions was found to be strongly dependent on the substrate. The different phenomena that were observed in each case are discussed.     

Charge transfer properties and photoelectrocatalytic activity of TiO2/MWCNT hybrid

The vertically aligned multiwalled carbon nanotube (MWCNT) arrays on tantalum foils were successfully coated with TiO₂ nanoparticles by a hydrothermal process. The prepared TiO₂/MWCNT hybrid was characterized by scanning electron microscopy and transmission electron microscopy. The charge transfer properties and photocatalytic degradation of rhodamine B with and without bias potential under UV irradiation were investigated. The MWCNTs promoted the separation of photoinduced carriers in the TiO₂, thus enhanced photocatalytic activity. Applying bias potential on the photoanode further enhanced its catalytic activity. The efficient charge transportation and high photoelectrocatalytic activity towards degradation of rhodamine B made this hybrid material promising for photocatalyst and for the development of photoelectrical devices.     

Characteristics of in-situ synthesized Hydroxyapatite on TiO2 ceramic via plasma electrolytic oxidation

The biocompatibility properties of Ti scaffolds can be improved significantly by hydroxyapatite (HA) composite coating. We successfully coated the surface of the Ti substrates by in-situ formation of HA nanocrystals on TiO₂ sublayer under calcium acetate and trisodium phosphate electrolytes through the plasma electrolytic oxidation (PEO) process. The effects of the process parameters and passivation on the characteristics of the coated substrates were studied using X–ray diffraction, Fourier-transform infrared spectroscopy, Field emission scanning electron microscopy, and Energy–dispersive X–ray spectroscopy. The systematically controlled experimental studies indicated that using the higher calcium/phosphorous ratio in the electrolyte enhances the micro arcs power and consequently, thickens the synthesized HA layer. The HA nanocrystals were tailored on the walls and edges of the discharge channels due to the locally concentrated heating zones, which consequently resulted in noticeable amounts of Ca and P dopants in porous TiO₂.     

Effect of gold nanoparticles on the performances of TiO2 dye-sensitised solar cell

TiO₂ blended with Au nanoparticles (NP) was hydrothermally synthesised under controlled conditions and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM). The dye-sensitised solar cells (DSSC) were characterized using the UV–vis spectrometry and current-voltage (I-V) testing. The performance of TiO₂-Au DSSC is significantly higher than TiO₂ DSSC. The addition of Au NPs improves the light harvesting efficiency of the DSSC, thus improving the short circuit current density, J_sc, and the conversion efficiency of the DSSC. The excitation of localised surface plasmon resonance of the Au nanoparticles utilised the strong local field enhancement around the Au NPs to increase the absorption in the surrounding TiO₂ layer. It was demonstrated that the power conversion efficiency of the DSSC was improved by 28% with the addition of Au NPs.     

Study on the photocatalytic activities of TiO2 films prepared by reactive RF sputtering

Titanium oxide (TiO₂) films were deposited on non-alkali glass by reactive radio frequency (RF) magnetron sputtering using a Ti metal target in this study. The deposition parameters employed to realize the photocatalytic activities of TiO₂ films include RF power, deposition time, argon–oxygen ratio (O₂/(Ar + O₂)) and substrate temperature. The orthogonal array and analysis of variance (ANOVA) were adopted to determine the effect of the deposition variables on characteristic properties and the optimal conditions. The results indicated that a higher photocatalytic activity of TiO₂ films could be achieved under RF power of 150 W, deposition time of 3 h, argon–oxygen ratio of 40% and substrate temperature of 80 °C. RF power and argon–oxygen ratio had a higher effect on the methylene blue (MB) absorbance. The validation experiments show an improved photocatalytic activities of 5% when the Taguchi method is used.     

Preparation and characterization of Cu2O/TiO2 nano–nano heterostructure photocatalysts

Cu₂O/TiO₂ nano–nano heterostructures with different concentrations of Cu₂O were prepared by an alcohol-aqueous based chemical precipitation method, and were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and UV–vis diffuse reflection absorption spectra. The photocatalytic efficiency of the Cu₂O/TiO₂ heterostructures was evaluated by degradation of Acid Orange II in water under UV–vis light and visible light irradiation. The results show that the heterostructures have dramatically improved photocatalytic activity comparing with pure TiO₂ (P25). The prepared Cu₂O/TiO₂ heterostructures with the Cu₂O concentrations of 30% and 70% have the best photocatalytic efficiencies, which are 6 times and 27 times higher than that of pure TiO₂ (P25) under UV–vis light and visible light irradiation, respectively.     

Photoelectrochemical properties of thermally oxidized TiO2

The photoelectrochemical properties of polycrystalline TiO₂ prepared at high temperature and doped polycrystalline TiO₂ with noble metals have been investigated. The polycrystalline TiO₂ prepared at high temperature give a cathodic photocurrent as well as a visible light response. These phenomena can be explained by a model based on the d-band formed by the interstitial Ti ion in TiO₂ lattice. The doped polycrystalline TiO₂ with noble metals (Rh, Ru, Pt, Au) prepared at low temperature also show a cathodic photocurrent and a visible light response. These are based on the impurity band formed by the doping metals. It is judged that the impurity band is near the π* conduction band for the doped TiO₂ with Rh, Ru and Pt, but is near the π valence band for the Au doped TiO₂ in energy position. It is found that there exist overlap potentials of the anodic and cathodic photocurrents at the doped TiO₂ with noble metals. This will provide evidence on the mechanism of the enhancement of the photocatalysis on TiO₂ owing to the doping of noble metals.     

TiO2 films prepared by micro-plasma oxidation method for dye-sensitized solar cell

Nanocrystalline TiO₂ films are widely investigated as the electrodes of dye-sensitized solar cell(s) with different preparation methods. In this paper, thin titanium dioxide films have been prepared on titanium plates by the micro-plasma oxidation method in the sulfuric acid solution. The thin TiO₂ films were sensitized with a cis-RuL₂(SCN)₂·2H₂O (L = cis-2,2′-bipyridine-4,4′-dicarboxylic acid) ruthenium complex and implemented into a dye-sensitized solar cell configuration. The influence of reaction current density (10, 15, 20, 25 and 30 A dm⁻²) on the structural and the surface morphology of the films was investigated by X-ray diffraction, scanning electron microscopy, atom force microscopy and X-ray photoelectricity spectroscopy. Impedance analysis for dye-sensitized solar cells was carried out by electrochemical impedance spectroscopy. The results show that the rise of current density leads to the increase in the amount of rutile and the thickness of the TiO₂ film, which makes the TiO₂ films have different photovoltages and photocurrents. The relatively higher photoelectricity properties were obtained in the TiO₂ films prepared at a current density of 20 A dm⁻². The open-circuit voltage and the short-circuit current are 605 mV and 165 μA cm⁻², respectively.     

Photocatalytic reduction of carbon dioxide into gaseous hydrocarbon using TiO2 pellets

It has been shown that CO₂ could be transformed into hydrocarbons when it is in contact with water vapour and catalysts under UV irradiation. This paper presents an experimental set-up to study the process employing a new approach of heterogeneous photocatalysis using pellet form of catalyst instead of immobilized catalysts on solid substrates. In the experiment, CO₂ mixed with water vapour in saturation state was discharged into a quartz reactor containing porous TiO₂ pellets and illuminated by various UV lamps of different wavelengths for 48 h continuously. The gaseous products extracted were identified using gas chromatography. The results confirmed that CO₂ could be reformed in the presence of water vapour and TiO₂ pellets into CH₄ under continuous UV irradiation at room conditions. It showed that when UVC (253.7 nm) light was used, total yield of methane was approximately 200 ppm which was a fairly good reduction yield as compared to those obtained from the processes using immobilized catalysts through thin-film technique and anchoring method. CO and H₂ were also detected. Switching from UVC to UVA (365 nm) resulted in significant decrease in the product yields. The pellet form of catalyst has been found to be attractive for use in further research on photocatalytic reduction of CO₂.     

Characterization of poly(vinylidenefluoride-co-hexafluoropropylene)-based polymer electrolyte filled with TiO2 nanoparticles

Nanoscale TiO₂ particle filled poly(vinylidenefluoride-co-hexafluoropropylene) film is characterized by investigating some properties such as surface morphology, thermal and crystalline properties, swelling behavior after absorbing electrolyte solution, chemical and electrochemical stabilities, ionic conductivity, and compatibility with lithium electrode. Decent self-supporting polymer electrolyte film can be obtained at the range of <50 wt% TiO₂. Different optimal TiO₂ contents showing maximum liquid uptake may exist by adopting other electrolyte solution. Room temperature ionic conductivity of the polymer electrolyte placed surely on the region of >10⁻³ S/cm, and thus the film is very applicable to rechargeable lithium batteries. An emphasis is also be paid on that much lower interfacial resistance between the polymer electrolyte and lithium metal electrode can be obtained by the solid-solvent role of nanoscale TiO₂ filler.