Fabrication of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> double layer thin films with self-cleaning and photocatalytic properties

Transparent antireflective SiO₂/TiO₂ double layer thin films were prepared using a sol–gel method and deposited on glass substrate by spin coating technique. Thin films were characterized using XRD, FE-SEM, AFM, UV–Vis spectroscopy and water contact angle measurements. XRD analysis reveals that the existence of pure anatase phase TiO₂ crystallites in the thin films. FE-SEM analysis confirms the homogeneous dispersion of TiO₂ on SiO₂ layer. Water contact angle on the thin films was measured by a contact angle analyzer under UV light irradiation. The photocatalytic performance of the TiO₂ and SiO₂/TiO₂ thin films was studied by the degradation of methylene blue under UV irradiation. The effect of an intermediate SiO₂ layer on the photocatalytic performance of TiO₂ thin films was examined. SiO₂/TiO₂ double layer thin films showed enhanced photocatalytic activity towards methylene blue dye.     

Visible light photocatalytic degradation of paraoxon and parathion pesticides on carbon-doped TiO<Subscript>2</Subscript> nanorod thin films

In the present work the nanostructured carbon-doped TiO₂ thin films with nanorod morphology were deposited on glass substrate by a combination of ultrasonic and chemical vapor deposition methods, and for the first time were applied for the photocatalytic degradation of paraoxon and parathion organophosphorus pesticides under visible light irradiation. X-ray Diffraction, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, and scanning electron microscopy techniques were used for characterization of the prepared thin films. Obtained results show that presence of carbon element and also special nanorod morphology of the thin films remarkably improve the optical properties of TiO₂ in visible light region and results in the good visible light photocatalytic activity of the thin films for degradation of the pesticides. The photonic efficiencies of the prepared thin films were also examined based on the international ISO-10678:2010 standard protocol for photocatalytic degradation of methylene blue under UV light irradiation. The results show a maximum photonic efficiency of 0.0312% for the carbon-doped TiO₂ thin film with 570 nm thickness, which compared to a reference standard TiO₂ films indicates a 30% improvement in photonic efficiency.     

Preparation and characterization of Co-doped TiO2 materials for solar light induced current and photocatalytic applications

Different amounts of Co-doped TiO₂ powders and thin films were prepared by following a conventional co-precipitation and sol–gel dip coating technique, respectively. The synthesized powders and thin films were subjected to thermal treatments from 400 to 800 °C and were thoroughly investigated by means of X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive analysis with X-rays, FT-infrared, FT-Raman, diffuse reflectance spectroscopy, ultraviolet–visible spectroscopy, BET surface area, zeta potential, flat band potential measurements, band-gap energy, etc. The photocatalytic ability of the powders was evaluated by methylene blue (MB) degradation studies. The thin films were characterized by photocurrent and ultraviolet–visible (UV–Vis) spectroscopy techniques. The characterization results suggest that the Co-doped TiO₂ powders synthesized in this study consist mainly anatase phase, and possess reasonably high specific surface area, low band gap energy and flat band potentials amenable to water oxidation in photoelectrochemical (PEC) cells. The photocatalytic degradation of MB over Co-doped TiO₂ powders followed the Langmuir–Hinshelwood first order reaction rate relationship. The 0.1 wt.% Co-doped TiO₂ composition provided the higher photocurrent, n-type semi-conducting behavior and higher photocatalytic activity among various Co-doped TiO₂ compositions and pure TiO₂ investigated.     

Surface properties of doped and undoped TiO2 thin films deposited by magnetron sputtering

In this work, transparent titanium dioxide (TiO₂) thin films were deposited onto microscope glass slides by means of the d.c. reactive magnetron sputtering method. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible spectroscopy (UV) and contact angle analysis using the Owens-Wendt method for the surface energy calculation. The photocatalytic activity of the films was tested by measuring the photodegradation of Rhodamine-B (RhB) dye under radiation of UV light. Iron-doped TiO₂ films were also prepared in order to study the Fe-doping effect on TiO₂ photocatalytic activity. The influences of different iron concentrations on the contact angle of the series of Fe-doped TiO₂ thin films, were investigated. The influences of total sputtering pressures on TiO₂ photocatalytic activity were also investigated. It was observed that the photocatalytic activity of the TiO₂ thin films was slightly improved by increasing the total sputtering pressure. Moreover, it was also observed that in general, iron-doping was detrimental for photocatalytic activity, nevertheless the films with low iron concentrations showed better photocatalytic activity than those with high iron concentrations. It was found that iron-doping has changed the wettability appetency of TiO₂ coated surfaces.     

Preparation of anatase TiO2 thin films by vacuum arc plasma evaporation

Anatase titanium dioxide (TiO₂) thin films with high photocatalytic activity have been prepared with deposition rates as high as 16 nm/min by a newly developed vacuum arc plasma evaporation (VAPE) method using sintered TiO₂ pellets as the source material. Highly transparent TiO₂ thin films prepared at substrate temperatures from room temperature to 400 °C exhibited photocatalytic activity, regardless whether oxygen (O₂) gas was introduced during the VAPE deposition. The highest photocatalytic activity and photo-induced hydrophilicity were obtained in anatase TiO₂ thin films prepared at 300 °C, which correlated to the best crystallinity of the films, as evidenced from X-ray diffraction. In addition, a transparent and conductive anatase TiO₂ thin film with a resistivity of 2.6 × 10^− 1 Ω cm was prepared at a substrate temperature of 400 °C without the introduction of O₂ gas.     

An efficient visible light photocatalyst prepared from TiO<Subscript>2</Subscript> and polyvinyl chloride

An efficient visible light photocatalyst has been prepared from TiO₂ nanoparticles and a partly conjugated polymer derived from polyvinyl chloride (PVC). It was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (UV–Vis DRS), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The visible light photocatalytic activity of the as-prepared photocatalyst was evaluated by the photocatalytic degradation of Rhodamine B (RhB) under visible light irradiation. The XPS, FT-IR, and Raman spectra show that the partly conjugated polymer derived from PVC exists on the surface of the TiO₂ nanoparticles. The UV–Vis DRS, XRD, and TEM results reveal that the modification of the partly conjugated polymer can obviously improve the absorbance of the TiO₂ nanoparticles in the range of visible light and hardly affect their size and crystallinity. The visible light photocatalytic activity of the as-prepared TiO₂ nanocomposites is higher than that of commercial TiO₂ (Degussa P25) and comparable with those of visible light photocatalysts reported in the literature. Their visible light photocatalytic stability is also good. The reasons for their excellent visible light photocatalytic activity and the major factors affecting their photocatalytic activity are discussed.     

Photocatalytic activity of polypyrrole/TiO<Subscript>2</Subscript> nanocomposites under visible and UV light

A series of polypyrrole (PPy)/titanium dioxide (TiO₂) nanocomposites were prepared in different polymerization conditions by ‘in situ’ chemical oxidative polymerization. The nanocomposites were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy spectra (XPS), and UV–Vis diffuse reflectance spectra. The photocatalytic degradation of methyl orange (MO) was chosen as a model reaction to evaluate the photocatalytic activities of TiO₂/PPy catalysts. The results show that a strong interaction exists at the interface between TiO₂ and PPy, the deposition of PPy on TiO₂ nanoparticles can alleviate their agglomeration, PPy/TiO₂ nanocomposites show stronger absorbance than neat TiO₂ under the whole range of visible light. The obtained PPy/TiO₂ nanocomposites exhibit significantly higher photocatalytic activity than the neat TiO₂ on the degradation of MO aqueous solution under visible and UV light illumination. The reasons for improving the photocatalytic activity were also discussed.     

Low-temperature processing of thin films based on rutile TiO2 nanoparticles for UV photocatalysis and bacteria inactivation

Using a low-temperature, simple, and economic processing technique, TiO₂ nanoparticles (rutile phase) are immobilized in an inorganic matrix and then deposited on glass for bacteria inactivation in water. Using this low thermal budget method (maximum processing temperature of 220 °C), thin films of immobilized TiO₂ nanoparticles are obtained so that practical water decontamination after UV radiation is possible by avoiding the additional step of catalyst separation from treated water. In order to validate the photocatalytic activities of these TiO₂ nanoparticles (prepared as thin films), they were tested for bacteria inactivation in water under UV–A radiation (λ > 365 nm), while extensive characterizations by dynamic light scattering, X-ray diffraction, ultra violet–visible absorption spectroscopy, fourier-transform infra red spectroscopy, and profilometry were also carried out. Despite previous reports on the low or lack of photocatalytic activity of rutile-phase TiO₂, inactivation of Escherichia coli in water was observed when thin films of this material were used when compared with the application of UV radiation alone. Physical characterization of the films suggests that size and concentration-related effects may allow the existence of photocatalytic activity for rutile-TiO₂ as long as they are exposed under UV–A radiation, whereas no effect on bacteria inactivation was observed for thin films in the absence of TiO₂ or radiation. In brief, a low thermal budget process applied to thin films based on TiO₂ nanoparticles has shown to be useful for bacteria inactivation, while possible application of these films on widely available substrates like polyethylene terephthalate materials is expected.     

Fabrication and characterization of nano TiO2 thin films at low temperature

Anatase TiO₂ thin films were successfully prepared on glass slide substrates via a sol–gel method from refluxed sol (RS) containing anatase TiO₂ crystals at low temperature of 100 °C. The influences of various refluxing time on crystallinity, morphology and size of the RS sol and dried TiO₂ films particles were discussed. These samples were characterized by infrared absorption spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission-scanning electron microscopy (FE-SEM) and UV–vis absorption spectroscopy (UV–vis). The photocatalytic activities of the TiO₂ thin films were assessed by the degradation of methyl orange in aqueous solution. The results indicated that titania films thus obtained were transparent and their maximal light transmittance exceeded 80% under visible light region. The TiO₂ thin films prepared from RS-6 sol showed the highest photocatalytic activity, when the calcination temperature is higher than 300 °C. The degradation of methyl orange of RS-6 thin films reached 99% after irradiated for 120 min, the results suggested that the TiO₂ thin films prepared from RS sol exhibited high photoactivities.     

Effects of samarium dopant on photocatalytic activity of TiO<Subscript>2</Subscript> nanocrystallite for methylene blue degradation

Sm³⁺-doped TiO₂ nanocrystalline was synthesized by a sol–gel auto-combustion method and characterized by X-ray diffraction, Brunauer-Emmett-Teller method (BET), UV–vis diffuse reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The photocatalytic activity of Sm³⁺–TiO₂ catalyst was evaluated by measuring degradation rates of methylene blue (MB) under either UV or visible light. The results showed that doping with the samarium ions significantly enhanced the photocatalytic activity for MB degradation under UV or visible light irradiation. This was ascribed to the fact that a small amount of samarium dopant simultaneously increased MB adsorption capacity and separation efficiency of electron-hole pairs. The results of DRS showed that Sm³⁺-doped TiO₂ had significant absorption between 400 nm and 500 nm, which increased with the increase of samarium ion content. The adsorption experimental demonstrated that Sm³⁺–TiO₂ had a higher MB adsorption capacity than undoped TiO₂ and adsorption capacity of MB increased with the increase of samarium ion content. It is found that the stronger the PL intensity, the higher the photocatalytic activity. This could be explained by the points that PL spectra mainly resulted from surface oxygen vacancies and defects during the process of PL, while surface oxygen vacancies and defects could be favorable in capturing the photoinduced electrons during the process of photocatalytic reactions, so that the recombination of photoinduced electrons and holes could be effectively inhibited.     

Ag surfactant effects of TiO2 films prepared by sputter deposition

The surfactant effect of Ag on the thin film structure of TiO₂ by radio frequency magnetron sputtering has been investigated. Comparisons between the atomic force microscopy images revealed that the surface roughness of TiO₂ film mediated by Ag was smaller than that of the TiO₂ film without Ag. The surface segregation effect of Ag was confirmed using X-ray photoelectron spectroscopy. The results of X-ray diffraction revealed that the initial deposition of a 0.4 nm thick Ag surfactant layer onto a Fe buffer layer prior to the deposition of the TiO₂ film reduced the rutile (110) growth and enhanced the anatase (100) growth. It was concluded that Ag was an effective surfactant for changing the thin film structure of TiO₂ on the Fe buffer layer. The photocatalytic effect of the fabricated TiO₂ film was also investigated using the remote oxidation process. TiO₂ films with the Ag surfactant exhibited higher photocatalytic activity than conventionally deposited TiO₂ films.     

Ag surfactant effects of TiO2 films prepared by sputter deposition

The surfactant effect of Ag on the thin film structure of TiO₂ by radio frequency magnetron sputtering has been investigated. Comparisons between the atomic force microscopy images revealed that the surface roughness of TiO₂ film mediated by Ag was smaller than that of the TiO₂ film without Ag. The surface segregation effect of Ag was confirmed using X-ray photoelectron spectroscopy. The results of X-ray diffraction revealed that the initial deposition of a 0.4 nm thick Ag surfactant layer onto a Fe buffer layer prior to the deposition of the TiO₂ film reduced the rutile (110) growth and enhanced the anatase (100) growth. It was concluded that Ag was an effective surfactant for changing the thin film structure of TiO₂ on the Fe buffer layer. The photocatalytic effect of the fabricated TiO₂ film was also investigated using the remote oxidation process. TiO₂ films with the Ag surfactant exhibited higher photocatalytic activity than conventionally deposited TiO₂ films.     

Mussel‐Inspired Immobilization of Catalysts for Microchemical Applications

Immobilization of photocatalytic TiO₂ nanoparticles inside PDMS‐based microreactors was successfully attempted by sequential mussel‐inspired surface engineering of microchannels by using CA‐PVP and commercially available TiO₂ nanoparticles, respectively. TiO₂‐immobilized microreactors accomplished continuous photocatalytic degradation of MB for up to 30 days without releasing TiO₂ nanoparticles from the surface of the microchannels, confirming the robustness of TiO₂‐immobilization in photocatalysis media. Regeneration of microreactors with diminished photocatalytic activities was also possible with simple strong acid treatment, enabling efficient cleaning of used microreactors.     

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.     

Nitrogen-substituted TiO<Subscript>2</Subscript>: investigation on the photocatalytic activity in the visible light range

Nitrogen-doped titanium oxides are attractive materials for the degradation of organic pollutants in water due to their photocatalytic activity in the visible light range. The evolution of the photocatalytic properties was studied on a number of TiO _x N _y powder samples where x varied from 2 to 0 (TiO₂ to TiN) through increasing the nitrogen content (y = 0–1). X-ray diffraction and Raman spectroscopy showed that an anatase type TiO _x N _y was obtained at low nitrogen contents (<2 wt%). With increasing nitrogen content a structural transition from anatase to cubic TiN was observed. Electron spin resonance measurements of the TiO _x N _y samples confirmed the presence of unpaired electrons and defects for the TiO _x N _y materials with low nitrogen content (<2 wt%). The photo-induced activity of the TiO _x N _y materials was evaluated under VIS illumination of solutions containing methylene blue as an organic probe. The TiO _x N _y samples exhibited an improved photocatalytic activity under visible light illumination compared to TiO₂ at nitrogen levels lower than 2 wt%. A photocatalytic activity could not be detected at nitrogen levels higher than 10 wt% and after conversion of the TiO _x N _y into the cubic phase. Optimum photocatalytic activity in the visible range can be achieved at nitrogen levels lower than 2 wt% for TiO _x N _y materials.     

Preparation of nitrogen co-doped SiO2/TiO2 thin films on ceramic with enhanced photocatalytic activity under visible-light irradiation

In this study, we have successfully deposited N-doped SiO₂/TiO₂ thin films on ceramic tile substrates by sol–gel method for auto cleaning purpose. After dip coating and annealing process the film was transparent, smooth and had a strong adhesion on the ceramic tile surface. The synthesised catalysts were then characterised by using several analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscope (AFM) and UV-vis absorption spectroscopy (UV-vis). The analytical results revealed that the optical response of the synthesised N-doped SiO₂/TiO₂ thin films was shifted from the ultraviolet to the visible light region. The nitrogen substituted some of the lattice oxygen atoms. The surface area of co-doped catalyst increased, and its photocatalytic efficiency was enhanced. The photocatalytic tests indicated that nitrogen co-doped SiO₂/TiO₂ thin films demonstrated higher than of the SiO₂/TiO₂ activity in decolouring of methylene blue under visible light. The enhanced photocatalytic activity was attributed to an increasing of the surface area and a forming of more hydroxyl groups in the doped catalyst.     

Photocatalytic activities of Ion doped TiO2 thin films when prepared on different substrates

Pure and ion doped TiO₂ thin films were prepared by sol-gel dip coating process on metallic and non-metallic substrates. Test metal ion concentration ranged from 0.000002 to 0.4 at.%. The resulting films were annealed in air and characterized by optical spectroscopy and X-ray diffraction. The photodegradation of methyl orange under UV irradiation by pristine and ion-doped TiO₂ films was quantified in a photocatalytic reactor developed in this study. In general, both doped and undoped TiO₂ crystals appeared in anatase phase and the photocatalytic activities of the TiO₂ thin films varied with substrates, calcination temperature, doping ions and their concentrations. The best calcination temperature for different substrates ranged from 450 to 580 °C. Films prepared on the metallic substrates resulted in higher photocatalytic activities, while ion doping lowered their efficiencies. On the contrary, for non-metallic substrates except ceramic the photocatalytic efficiencies of undoped films were much lower (< 30%), while ion doping was shown to increase the photocatalytic efficiencies remarkably in some cases, e.g., Cr³⁺ with the tile substrate. Overall, ion doping affected the photocatalytic efficiency of TiO₂ films, and an optimal doping concentration of between 0.0002 and 0.002 at.%, close to an estimate by the Debye length equation, resulted in the highest efficiency for most substrates.     

One-pot synthesis and photocatalytic activity of Fe-doped TiO<Subscript>2</Subscript> films with anatase–rutile nanojunction prepared by plasma electrolytic oxidation

A novel one-pot low temperature preparation of Fe-doped anatase–rutile TiO₂ (Fe-TiO₂) films is demonstrated using plasma electrolytic oxidation (PEO). Pale yellow TiO₂ films are obtained by PEO treatment of Ti metals in the electrolyte dispersing TiO₂ and Fe₂O₃ particles. The oxidized layer on Ti metal have a sponge-like structure with thickness and pore size of 10 and 0.1–1 μm, respectively. Investigation by X-ray diffraction, X-ray photoelectron spectroscopy, and UV–Vis absorption spectroscopy all indicate that dissolved Fe³⁺ ions in the strong acidic electrolyte are doped into the TiO₂ structure during PEO. The photocatalytic activity of Fe-TiO₂ samples was investigated by studying the photocatalytic decomposition of acetaldehyde. Fe-TiO₂ samples doped with optimum Fe content show visible light photocatalytic activity and further increased photocatalytic activity under UV illumination compared with that of pure TiO₂ films.     

Enhanced photocatalytic activity of silver nanoparticles modified TiO2 thin films prepared by RF magnetron sputtering

Ag-TiO₂ nanostructured thin films with silver volume fraction of 0–40% were prepared by RF magnetron sputtering. The microstructure, surface topography, and optical properties of the films were characterized by X-ray diffractometer, transmission electron microscope, and ultraviolet–visible spectrophotometer. Photocatalytic activity of the films was evaluated by light-induced degradation of methyl orange (C₁₄H₁₄N₃NaO₃S) solution using a high pressure mercury lamp as lamp-house. The relation of photocatalytic activity and silver content was studied in detail. It is found that silver content influences microstructure of TiO₂ thin films, and silver in the films is metallic Ag (Ag⁰). Photocatalytic activity of the films increases with increasing silver content up to 5 vol.% Ag and then decreases to values significantly still bigger than that of pure TiO₂ thin films. Silver nanoparticles significantly enhance the photocatalytic activity of TiO₂ films. The better separation between electrons and holes on silver modified TiO₂ thin films surface allowed more efficiency for the oxidation and reduction reactions. The enhanced photocatalytic activity was mainly attributed to the decrease of energy gap of the films and the increase of oxygen anion radicals O₂⁻ and reactive center of surface Ti³⁺ on silver modified TiO₂ thin films surface.     

Synthesis of TiO2/Pt/TiO2 multilayer films via radio frequency magnetron sputtering and their enhanced photocatalytic activity

TiO₂/Pt/TiO₂ (TPT) multilayered films with different thicknesses of Pt layers from about 0.75 to 12 nm were prepared by radio frequency magnetron sputtering method. The as-prepared films were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, UV-visible diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The photocatalytic activity of the samples was evaluated by the photocatalytic decolorization of methyl blue aqueous solution under ultraviolet light irradiation. The photocatalytic activities of all TPT multilayer films were higher than that of the pure TiO₂ films. When Pt thickness was increased to 3 nm, the measured photocatalytic activity of the TPT film was highest, and exceeded that of the pure TiO₂ films by a factor of more than three times. Such enhancement was ascribed to the presence of Pt layer, which inhibits the recombination of the photogenerated charge and carriers, as well as modifies the crystallinity of the TiO₂ top layer.