Photo-induced properties of anodic oxide films on Ti6Al4V

Photocatalytic activity and wettability of the anodic oxide layer on Ti6Al4V prepared by anodization in a sulfuric acid electrolyte are explored. The oxide is composed mainly of TiO₂ with V₂O₅, VO₂ and Al₂O₃. The crystal structure of the TiO₂ varies from anatase to rutile with the sulfuric acid concentration in the electrolyte. Anatase exhibits better photocatalytic activity compared with rutile, which is different from those on Ti and Ti-Nb-Sn alloy. Contact angles of the oxides decrease with ultraviolet light illumination, and hydrophilicity is observed in the rutile oxide. Both photocatalytic activity and hydrophilicity are inferior to the corresponding Ti and Ti-Nb-Sn alloy, which is explained by the presence of Al₂O₃ in the anodic oxide.     

Photocatalytic activities of TiO2 thin films prepared on Galvanized Iron substrate by plasma-enhanced atomic layer deposition

Titanium dioxide (TiO₂) thin films were prepared on Galvanized Iron (GI) substrate by plasma-enhanced atomic layer deposition (PE-ALD) using tetrakis-dimethylamido titanium and O₂ plasma to investigate the photocatalytic activities. The PE-ALD TiO₂ thin films exhibited relatively high growth rate and the crystal structures of TiO₂ thin films depended on the growth temperatures. TiO₂ thin films deposited at 200 °C have amorphous phase, whereas those with anatase phase and bandgap energy about 3.2 eV were deposited at growth temperature of 250 °C and 300 °C. From contact angles measurement of water droplet, TiO₂ thin films with anatase phase and Activ™ glass exhibited superhydrophilic surfaces after UV light exposure. And from photo-induced degradation test of organic solution, anatase TiO₂ thin films and Activ™ glass decomposed organic solution under UV illumination. The anatase TiO₂ thin film on GI substrate showed higher photocatalytic efficiency than Activ™ glass after 5 h UV light exposure. Thus, we suggest that the anatase phase in TiO₂ thin film contributes to both superhydrophilicity and photocatalytic decomposition of 4-chlorophenol solution and anatase TiO₂ thin films are suitable for self-cleaning applications.     

Hydrophilicity of anatase TiO2/Cr-doped TiO2 thin films with different band gaps

Based on the concept that the electron-hole separation effect caused by a different band-gap structure would improve its hydrophilicity, anatase-TiO₂/Cr-doped TiO₂ thin films were synthesized by DC magnetron sputtering. The optical band gaps of TiO₂ thin films decreased from 3.23 to 2.95 eV with increasing Cr-doping content. Multilayer TiO₂ thin films with different band gaps exhibited a superhydrophilicity under UV illumination. In particular, in anatase TiO₂ (3.23 eV)/4.8% Cr-doped TiO₂ (2.95 eV), the hydrophilicity, which indicated a contact angle of less than 20°, lasted for 48 h in the dark after UV illumination was discontinued. This outstanding result has rarely been reported for TiO₂ thin films, which confirmed that the prominent superhydrophilicity of anatase TiO₂/Cr-doped TiO₂/glass could be attributed to the retardation of electron-hole recombination caused by the band-gap difference.     

Superhydrophilicity of TiO2 thin films using TiCl4 as a precursor

TiO₂ thin films on soda lime glass were prepared by the sol-gel method and spin coating process using TiCl₄ as a precursor. The AFM images indicate that the surface morphology of the films is granular with 72 nm particle size. The roughness and thickness of the films are about 3 nm and 140 nm, respectively. The XRD spectrum shows polycrystalline anatase phase without any considerable impurity phase. The UV-vis spectroscopy of the films show 80-90% transmission in the visible region. The absorption edge is at 370 nm, which corresponds to 3.3 eV energy band gap. The films have a high superhydrophilicity character after being exposed to UV illumination for about 10 min. The surfaces, which were synthesized by this method, can retain their superhydrophilicity property for at least 24 h. Our results are consistent with the idea that UV-induced wetting of TiO₂ surface is caused by the removal of hydrophobic layers of hydrocarbons by TiO₂-mediated photooxidation, which leads to the attractive interaction of water with clean TiO₂ surface. TiO₂ thin films on Si(1 1 1), Si(1 0 0), and quartz substrates need less time than glass and polycrystalline Si substrates to be converted to superhydrophilic surface.     

Photocatalytic activity of pulsed laser deposited TiO2 thin films in N2, O2 and CH4

We report on pulsed laser deposition of TiO₂ films on glass substrates in oxygen, methane, nitrogen and mixture of oxygen and nitrogen atmosphere. The nitrogen incorporation into TiO₂ lattice was successfully achieved, as demonstrated by optical absorption and XPS measurements. The absorption edge of the N-doped TiO₂ films was red-shifted up to ∼ 480 nm from 360 nm in case of undoped ones.The photocatalytic activity of TiO₂ films was investigated during toxic Cr(VI) ions photoreduction to Cr(III) state in aqueous media under irradiation with visible and UV light. Under visible light irradiation, TiO₂ films deposited in nitrogen atmosphere showed the highest photocatalytic activity, whereas by UV light exposure the best results were obtained for the TiO₂ structures deposited in pure methane and oxygen atmosphere.     

Determination of photo-catalytic activity of un-doped and Mn-doped TiO2 anatase powders on acetaldehyde under UV and visible light

Titanium dioxide (TiO₂) photocatalytic powder materials doped with various levels of manganese (Mn) were synthesized to be used as additives to wall painting in combating indoor and outdoor air pollution. The heterogeneous photocatalytic degradation of gaseous acetaldehyde (CH₃CHO) on Mn-TiO₂ surfaces under ultraviolet and visible (UV/Vis) irradiation was investigated, by employing the Photochemical Static Reactor coupled with Fourier-Transformed Infrared spectroscopy (PSR/FTIR) technique. Experiments were performed by exposing acetaldehyde (~ 400 Pa) and synthetic air mixtures (~ 1.01 × 10⁵ Pa total pressure) on un-doped TiO₂ and doped with various levels of Mn (0.1-33% mole percentage) under UV and visible irradiation at room temperature. Photoactivation was initiated using either UV or visible light sources with known emission spectra. Initially, the photo-activity of CH₃CHO under the above light sources, and the physical adsorption of CH₃CHO on Mn-TiO₂ samples in the absence of light were determined prior to the photocatalytic experiments. The photocatalytic loss of CH₃CHO on un-doped TiO₂ and Mn-TiO₂ samples in the absence and presence of UV or visible irradiation was measured over a long time period (≈ 60 min), to evaluate their relative photocatalytic activity. The gaseous photocatalytic end products were also determined using absorption FTIR spectroscopy. Carbon dioxide (CO₂) was identified as the main photocatalysis product. It was found that 0.1% Mn-TiO₂ samples resulted in the highest photocatalytic loss of CH₃CHO under visible irradiation. This efficiency was drastically diminished at higher levels of Mn doping (1-33%). The CO₂ yields were the highest for 0.1% Mn-TiO₂ samples under UV irradiation, in agreement with the observed highest CH₃CHO decomposition rates. It was demonstrated that low-level (0.1%) doping of TiO₂ with Mn results in a significant increase of their photocatalytic activity in the visible range, compared to un-doped TiO₂. This elevated activity is lost at high doping levels (1-33%). Finally, the photocatalytic degradation mechanism of CH₃CHO on 0.1% Mn-TiO₂ surfaces under visible irradiation leading to low CO₂ yields is different than that under UV irradiation resulting to high CO₂ yields.     

Photocatalytic degradation of 2,4-dichlorophenol wastewater using porphyrin/TiO2 complexes activated by visible light

The use of TiO₂ as photocatalyst to degrade the organic compounds is an effective method of oxidation process and has been widely studied in environmental engineering. However, TiO₂ absorbed the UV light which is only small part of sunlight reaching earth surface to activate photocatalytic procedure effectively is a major disadvantage. Therefore, studies on the development of new TiO₂ wherein its photocatalytic activity can be activated by visible light which is the major part of sunlight will be valuable for field application. In this study, we evaluate the photocatalytic degrading efficiency of porphyrins/TiO₂ complexes on the organic pollutants under irradiation with visible light (λ = 419 nm). The results showed that the photodecomposition efficiency of 2,4-dichlorophenol (2,4-DCP) wastewater by using porphyrin/TiO₂ irradiated under visible light for 4 h was up to 42-81% at pH 10. These evidences reveal that the system of porphyrin/TiO₂ complexes has also significantly efficiency of photocatalytic degradation for some hazardous or recalcitrant pollutants under visible light irradiation.     

Photocatalysis and characterization of the gel-derived TiO2 and P-TiO2 transparent thin films

The gel-derived TiO₂ and P-TiO₂ transparent films coated on fused-SiO₂ substrates were prepared using a spin-coating technique. Effects of phosphorus dopants and calcination temperature on crystal structure, crystallite size, microstructure, light transmittance and photocatalytic activity of the films were investigated. By introducing P atoms to Ti-O framework, the growth of anatase crystallites was hindered and the crystal structure of anatase-TiO₂ could withstand temperature up to 900 °C. The photocatalytic activities of the prepared films were characterized using the characteristic time constant (τ) for the photocatalytic reaction. The titania film with a smaller τ value possesses a higher photocatalytic ability. After exposing to 365-nm UV light for 12 h, the P-TiO₂ films calcined between 600 °C and 900 °C can photocatalytically decomposed ≥ 84 mol% of the methylene blue in water with corresponding τ ≤ 7.1 h, which were better than the pure TiO₂ films prepared at the same calcination temperature.     

Nanoscale characterization of anodic oxide films on Ti-6Al-4V alloy

The paper analyses, at nanoscale levels, the chemical composition and mechanical properties of the anodic oxide films formed on Ti-6Al-4V alloy by galvanostatic polarization at maximum final voltages of 12-100 V. For the investigations Auger Electron Spectroscopy, Photoelectron Spectroscopy and nanoindentation measurements have been used. The results have shown that anodizing the Ti-6Al-4V alloy produces an oxide film whose thickness depends on the final voltage. The chemical composition is not significantly dependent on the thickness, the film consists of TiO₂ and Al₂O₃. However, the best insulating properties of the films, determined from the growth parameter nm/V, are achieved with a final voltage between 30 and 65 V. Nanohardness and Young's modulus measurements have shown that the anodic films formed by different voltages exhibit similar mechanical properties which is consistent with the results of the surface analysis.     

Photocatalytic performance of iron (III) and niobium (V)-codoped TiO2 nanopowders synthesized by a radio frequency thermal plasma process

Iron (III) and niobium (V)-codoped TiO₂ nanopowders have been synthesized by Ar/O₂ RF thermal plasma. Phase composition, morphology, and photocatalytic performance of the plasma-generated powders have been investigated by the combined means of XRD, FE-SEM/TEM, and UV-vis absorption spectroscopy. Rutile formation in the plasma-produced phase composition of anatase and rutile was promoted by Fe³⁺ addition but was inhibited by Nb⁵⁺ doping. The resultant powders consisted of a majority of fine crystallites (several nanometers) and a small portion of coarse particles (~ 100 nm). In comparison with TiO₂ singly doped with 0.1 at.% of Fe³⁺, photocatalytic reactivity of codoped TiO₂ was improved at 2.0 at.% of Nb⁵⁺ but was depressed at 6.0 at.% under the UV irradiation, indicating that UV-induced photocatalytic capability was dominated by Nb⁵⁺ doping concentration. In contrast to the case of 1.0 at.% of Fe³⁺ single addition, the codoped sample obtained the decreased photocatalytic performance with increasing Nb⁵⁺ content under the visible light irradiation, due to the low visible light absorption resulting from a broadened band gap.     

Photocatalytic properties of Cr-doped TiO2 films prepared by oxygen cluster ion beam assisted deposition

We prepared Cr-doped titanium dioxide (TiO₂) films by oxygen (O₂) cluster ion beam assisted deposition method, and investigated photocatalytic properties of the films as well as crystallographic property, optical property and surface morphology. The films prepared at a substrate temperature below 200 °C were found to be amorphous from the X-ray diffraction measurement. For the substrate temperatures such as 300 °C and 400 °C, the films exhibited rutile and/or anatase structures. The film surface measured by the atomic force microscope (AFM) was smooth at an atomic level. Furthermore, the optical band gap decreased with increase of Cr-composition, and it was approximately 3.3 eV for the non-doped films, 3.2 eV for the 1% Cr-doped films and 3.1 eV for the 10% Cr-doped films, respectively. With regard to the photocatalytic properties of the Cr-doped TiO₂ films, we measured the change of contact angle as well as the photocatalytic degradation of methylene blue by the UV light irradiation. Compared with the non-doped films, the 1% Cr-doped films prepared at a substrate temperature of 400 °C showed high degradation efficiency. In addition, the contact angle of the 1% Cr-doped films with an initial value of 60° decreased to 10° by the UV light irradiation for 20 min, and the films exhibited the predominant properties of photocatalytic hydrophilicity even for the UV light irradiation with longer wavelengths.     

Glycine assisted synthesis of flower-like TiO2 hierarchical spheres and its application in photocatalysis

Flower-like anatase TiO₂ hierarchical spheres assembled by nanosheets were synthesized by glycine assistant via a simple hydrothermal approach and after-annealing process. These flower-like spheres are about 2 μm in diameter with sheet thickness about 20 nm. Results showed reaction time, temperature, solution pH and glycine dosage all played an important role in control of shape and size of the as-synthesized TiO₂ nanocrystals. The photocatalytic activity of this nano-TiO₂ was evaluated by the photocatalytic oxidation decomposition of methyl orange under sunlight illumination in the presence of hydrogen peroxide (H₂O₂). The photocatalytic activity of the obtained TiO₂ was higher than that of commercial TiO₂.     

Chemical vapor deposition and characterization of nitrogen doped TiO2 thin films on glass substrates

Photocatalytically active, N-doped TiO₂ thin films were prepared by low pressure metalorganic chemical vapor deposition (MOCVD) using titanium tetra-iso-propoxide (TTIP) as a precursor and NH₃ as a reactive doping gas. We present the influence of the growth parameters (temperature, reactive gas phase composition) on the microstructural and physico-chemical characteristics of the films, as deduced from X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and ultra-violet and visible (UV/Vis) spectroscopy analysis. The N-doping level was controlled by the partial pressure ratio R = [NH₃]/[TTIP] at the entrance of the reactor and by the substrate temperature. For R = 2200, the N-doped TiO₂ layers are transparent and exhibit significant visible light photocatalytic activity (PA) in a narrow growth temperature range (375-400 °C). The optimum N-doping level is approximately 0.8 at.%. However, the PA activity of these N-doped films, under UV light radiation, is lower than that of undoped TiO₂ films of comparable thickness.     

Photocatalytic properties of Au/TiO2 thin films prepared by RF magnetron co-sputtering

Au/TiO₂ thin films with various Au doping contents were deposited on quartz substrates by radio frequency (RF) magnetron co-sputtering. The as-deposited Au/TiO₂ films were characterized by energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), XRD, and UV-vis techniques. Au doping and UV treatment enhanced the photocatalytic efficiency of TiO₂ thin films. The optimal RF power of the Au target and UV treatment time were 5 W and 1 h, respectively. The enhanced photoactivity of Au(5 W)/TiO₂ thin films with UV treatment is found to result from the increased hydroxyl concentration.     

Rapid erasing of wettability patterns based on TiO2-PDMS composite films

TiO₂-polydimethylsiloxane (TiO₂-PDMS) composite films are prepared using the sol–gel method from a Ti(OBu)₄–benzoylacetone solution containing PDMS. The prepared films are cured by irradiation with ultraviolet (UV) light. Structural changes in the films after UV irradiation are confirmed by UV–vis absorption experiments, which show that an absorption band characteristic of the benzoylacetonate chelate rings disappears. This finding is ascribed to structural changes associated with the dissociation of the chelate rings. The IR spectra of the thin films exhibit a broad absorption band after UV irradiation, indicating that a Ti–O–Ti network forms in the thin film. Contact angles are measured for the TiO₂-PDMS thin films, showing wettability conversion from hydrophobic to superhydrophilic states by irradiation with oxygen plasma for 1 s. This phenomenon is explained by XPS experiments which reveal that the number of carbon atoms decreases, whereas the number of oxygen atoms increases on the surface of the TiO₂-PDMS composite films. Finally, hydrophobic–superhydrophilic patterns are fabricated based on a patterned TiO₂-PDMS composite film. The film displays a rapid change to superhydrophilicity over the whole film surface upon plasma irradiation for 1 s, which means that the wettability patterns are rapidly erasable.     

Hydrothermal synthesis and characterization of TiO2 nanorod arrays on glass substrates

Large-scale, well-aligned single crystalline TiO₂ nanorod arrays were prepared on the pre-treated glass substrate by a hydrothermal approach. The as-prepared TiO₂ nanorod arrays were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. X-ray diffraction results show that the main phase of TiO₂ is rutile. Scanning electron microscopy and transmission electron microscopy results demonstrate that the large-scale TiO₂ nanorod arrays grown on the pre-treated glass substrate are well-aligned single crystal and grow along [0 0 1] direction. The average diameter and length of the nanorods are approximately 21 and 400 nm, respectively. The photocatalytic activity of TiO₂ nanorod arrays was investigated by measuring the photodegradation rate of methyl blue aqueous solution under UV irradiation (254 nm). And the results indicate that TiO₂ nanorod arrays exhibit relatively higher photocatalytic activity.     

Mn-doped TiO2 nanopowders with remarkable visible light photocatalytic activity

TiO₂ nanocrystalline powders with various Mn-doping levels were synthesized by the sol-gel process using tetrabutyl titanate and manganese nitrate as precursors. The crystal structure, morphology, doping concentration, optical absorption property, and elemental state of the obtained samples were analyzed. TEM results showed that the synthesized TiO₂ powders were anatase nanoparticles about 7 nm in size. EDX and XPS analyses proved the incorporation of Mn ions into the TiO₂ lattice. A remarkable red shift of the absorption edge was achievable by increased Mn content, leading to gigantically narrowed energy gap to permit absorption well into the infrared spectral region. The dramatic optical absorbance of the doped TiO₂ nanopowders in the visible spectral region led to strong photocatalytic activity under visible light illumination, which was observed by measuring the degradation of methylene blue. In contrast, little degradation was observed for the pure TiO₂ powder. The optimum Mn/Ti ratio was observed to be 0.2 at.% for photocatalytic applications.     

Nitrogen and europium doped TiO2 anodized films with applications in photocatalysis

Micro-arc oxidation method is a useful process for mesoporous titanium dioxide films. In order to improve the photocatalytic activity of the TiO₂ film, N-Eu co-doped titania catalyst was synthesized by micro-arc oxidation in the H₂SO₄/Eu(NO₃)₃ mixture solution.The specific surface area and the roughness of the anodic titania film fabricated in the H₂SO₄/Eu(NO₃)₃ electrolyte, were increased compared to that of the anodic TiO₂ film prepared in H₂SO₄ solution. The absorbance response of N-Eu titania film shows a higher adsorption onset toward visible light region, and the incorporated N and Eu ions during anodization as a dopant in the anodic TiO₂ film significantly enhanced the photocatalytic activity for dye degradation. After dye decomposition test for 3 h, dye removal rates for the anodic TiO₂ film were 60.7% and 90.1% for the N-Eu doped titania film. The improvement of the photocatalytic activity was ascribed to the synergistic effects of the surface enlargement and the new electronic state of the TiO₂ band gap by N and Eu co-doping.     

Study of the deposition parameters and Fe-dopant effect in the photocatalytic activity of TiO2 films prepared by dc reactive magnetron sputtering

The reactive magnetron sputtering method was used to prepare pure and Fe-doped titanium dioxide thin films. The films were deposited onto microscope glass slides and polycarbonate plates at different total pressure and Fe-doping concentrations. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-visible spectroscopy (UV). For glass substrates a polycrystalline TiO₂ structure was verified with X-ray diffraction, which showed typical characteristic anatase reflections. An iron phase appeared in the highly Fe-doped samples. The absorption edges of the Fe-doped TiO₂ films shifted to visible region with increasing concentration of iron. For the polycarbonate substrate an amorphous TiO₂ structure was revealed for all deposition conditions. The effects of different Fe-doping and total pressure levels on the photocatalytic activity were obtained by the degradation rates of Rhodamine-B (RoB) dye under UV light irradiation. For the deposition conditions considered in this study the highest photodegradation rates were achieved for films deposited on the polymer substrates. Of these overall highest rates was achieved for deposition at 0.4 Pa and without doping. However, for both substrates, films prepared at the particular total pressure of 0.5 Pa and a low iron concentration showed better photocatalytic activity than the pure TiO₂ films prepared under the same deposition parameters. On the contrary, the photocatalytic degradation rates of RoB on the highly Fe-doped TiO₂ films decreased strongly.     

A photoelectrochemical study of CdS modified TiO2 nanotube arrays as photoanodes for cathodic protection of stainless steel

An electrodeposited CdS nanoparticles-modified highly-ordered TiO₂ nanotube arrays (CdS-TNs) photoelectrode and its performance of photocathodic protection are reported. The self-organized TiO₂ nanotube arrays are fabricated by electrochemical anodization in an organic-inorganic mixed electrolyte and sensitized with CdS nanoparticles by electrodeposition via a single-step direct current. The morphology, crystalline phase, and composition of the CdS-TNs films were characterized systematically by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and ultraviolet-visible (UV-Vis) spectroscopy, respectively. The photoelectrochemical performances of the CdS-TNs film under illumination and dark conditions in 0.5 M NaCl solution were evaluated through the electrochemical measurements. It is indicated that the TNs incorporated by CdS effectively harvest solar light in the UV as well as the visible light (up to 480 nm) region. It is supposed that the high photoelectro-response activity of the CdS-TNs is attributed to the increased efficiency of charge separation and transport of electrons. The electrode potentials of 304 stainless steel coupled with the CdS-TNs is found to be negatively shifted for about 246 mV and 215 mV under UV and white light irradiation, respectively, which can be remained for 24 h even in darkness. It is implied that the CdS-TNs are able to effectively function a photogenerated cathodic protection for metals both under the UV and visible light illumination.