Enhanced photocatalytic activity of TiO2 nano-structured thin film with a silver hierarchical configuration

TiO₂ sol-gels with various Ag/TiO₂ molar ratios from 0 to 0.9% were used to fabricate silver-modified nano-structured TiO₂ thin films using a layer-by-layer dip-coating (LLDC) technique. This technique allows obtaining TiO₂ nano-structured thin films with a silver hierarchical configuration. The coating of pure TiO₂ sol-gel and Ag-modified sol-gel was marked as T and A, respectively. According to the coating order and the nature of the TiO₂ sol-gel, four types of the TiO₂ thin films were constructed, and marked as AT (bottom layer was Ag modified, surface layer was pure TiO₂), TA (bottom layer was pure TiO₂, surface layer was Ag modified), TT (pure TiO₂ thin film) and AA (TiO₂ thin film was uniformly Ag modified). These thin films were characterized by means of linear sweep voltammetry (LSV), X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy and transient photocurrent (I_ph). LSV confirmed the existence of Ag⁰ state in the TiO₂ thin film. SEM and XRD experiments indicated that the sizes of the TiO₂ nanoparticles of the resulting films were in the order of TT > AT > TA > AA, suggesting the gradient Ag distribution in the films. The SEM and XRD results also confirmed that Ag had an inhibition effect on the size growth of anatase nanoparticles. Photocatalytic activities of the resulting thin films were also evaluated in the photocatalytic degradation process of methyl orange. The preliminary results demonstrated the sequence of the photocatalytic activity of the resulting films was AT > TA > AA > TT. This suggested that the silver hierarchical configuration can be used to improve the photocatalytic activity of TiO₂ thin film.     

Transparent nano composite PVA–TiO2 and PMMA–TiO2 thin films: Optical and dielectric properties

Transparent nano composite PVA–TiO₂ and PMMA–TiO₂ thin films were prepared by an easy and cost effective dip coating method. Al/PVA–TiO₂/Al and Al/PMMA–TiO₂/Al sandwich structures were prepared to study the dielectric behavior. The presence of metal–oxide (Ti–O) bond in the prepared films was confirmed by Fourier transform infrared spectroscopy. X-ray diffraction pattern indicated that the prepared films were predominantly amorphous in nature. Scanning electron micrographs showed cluster of TiO₂ nanoparticles distributed over the film surface and also there were no cracks and pin holes on the surface. The transmittance of the films was above 80% in the visible region and the optical band gap was estimated to be about 3.77 eV and 3.78 eV respectively for PVA–TiO₂ and PMMA–TiO₂ films by using Tauc's plot. The determined refractive index (n) values were between 1.6 and 2.3. High value of dielectric constant (?′ = 24.6 and ?′ = 26.8) was obtained for the prepared composite films. The conduction in the composite films was found to be due to electrons. The observed amorphous structure, good optical properties and dielectric behavior of the prepared nano composite thin films indicated that these films could be used in opto-electronic devices and in thin film transistors.     

Nanostructure of photocatalytic TiO2 films sputtered at temperatures below 200 °C

The article reports on correlations between the process parameters of reactive pulsed dc magnetron sputtering, physical properties and the photocatalytic activity (PCA) of TiO₂ films sputtered at substrate surface temperature T_surf ≤ 180 °C. Films were deposited using a dual magnetron system equipped with Ti (Ø50 mm) targets in Ar + O₂ atmosphere in oxide mode of sputtering. The TiO₂ films with highly photoactive anatase phase were prepared without a post-deposition thermal annealing. The decomposition rate of the acid orange 7 (AO7) solution during the photoactivation of the TiO₂ film with UV light was used for characterization of the film PCA. It was found that (i) the partial pressure of oxygen pO₂ and the total sputtering gas pressure p_T are the key deposition parameters influencing the TiO₂ film phase composition that directly affects its PCA, (ii) the structure of sputtered TiO₂ films varies along the growth direction from the film/substrate interface to the film surface, (iii) ∼500 nm thick anatase TiO₂ films with high PCA were prepared and (iv) the structure of sputtered TiO₂ films is not affected by the substrate surface temperature T_surf when T_surf < 180 °C. The interruption of the sputtering process and deposition in long (tens of minutes) pulses alternating with cooling pauses has no effect on the structure and the PCA of TiO₂ films and results in a decrease of maximum value of T_surf necessary for the creation of nanocrystalline nc-TiO₂ film. It was demonstrated that crystalline TiO₂ films with high PCA can be sputtered at T_surf ≤ 130 °C. Based on obtained results a phase zone model of TiO₂ films was developed.     

TiO2 activation using acid-treated vermiculite as a support: Characteristics and photoreactivity

Vermiculite was treated by sulfuric or nitric acid aqueous solutions with different concentration. These modified materials as the promising supports, were used to immobilize TiO₂. TiO₂ was prepared by the precursor, which was obtained by substituting partly isopropyl alcohol with Cl⁻ in titanium chloride {[Ti(IV)(OR)_nCl_m] (n = 2-3, m = 4 − n)}. The TiO₂/vermiculite composites were characterized by X-ray diffraction, scanning electron microscopy, and the nitrogen absorption. Their photocatalytic activity was evaluated by removal of methylene blue (MB). The pure anatase type crystalline phase was well deposited on the supports. The concentrations of acid for treatment had a significant influence on pore sizes and surface area of vermiculite. The treatment process changed microstructure of vermiculite, modified its characteristics, and farther improved the catalytic activity and absorption capacity of TiO₂/vermiculite composites. The treatment effect of nitric acid was superior to that of sulfuric acid.     

Effect of surface ethoxy groups on photoactivity of TiO2 nanocrystals

TiO₂ nanocrystals modified by ethoxy groups were prepared by a facile nonhydrolytic solvothermal method and characterized by XRD, TEM, TG-DTA and XPS, which showed an enhanced visible-light photocatalytic activity on the degradation of Rhodamine B compared with TiO₂ modified by benzyloxy groups and the “naked” TiO₂. The adsorption and degradation pathway of Rhodamine B on TiO₂ modified by ethoxy groups were also investigated. The zeta-potential (ζ) results showed that the TiO₂ modified by ethoxy groups had high negative surface charge, which incited the positive -N(Et)₂ group of RhB absorbing on the TiO₂ surface and preferably led the N-dealkylation pathway under visible light irradiation.     

A new method to characterizing surface roughness of TiO2 thin films

Titanium dioxide (TiO₂) thin films have been widely coated in the self-cleaning glass for facade application. The benefit of these glasses is its ability to actively decompose organic compounds with the help of ultraviolet light. Understanding the surface roughness of TiO₂ thin films is important before manufacturing of self-cleaning glasses using TiO₂ thin films because surface roughness of TiO₂ thin films has highly significant influence on the photocatalytic performance. Traditional approach for measuring surface roughness of TiO₂ thin films is atomic force microscopy. The disadvantage of this approach include long lead-time and slow measurement speed. To solve this problem, an optical inspection system for rapidly measuring the surface roughness of TiO₂ thin films is developed in this study. It is found that the incident angle of 60° is a good candidate for measuring surface roughness of TiO₂ thin films and y=90.391x+0.5123 is a trend equation for predicting the surface roughness of TiO₂ thin films. Roughness average (Ra) of TiO₂ thin films (y) can be directly determined from the peak power density (x) using the optical inspection system developed. The results were verified by white-light interferometer. The measurement error rate of the optical inspection system developed can be controlled by about 8.8%. The saving in inspection time of the surface roughness of TiO₂ thin films is up to 83%.     

Influence of the OH groups on the photocatalytic activity and photoinduced hydrophilicity of microwave assisted sol-gel TiO2 film

In the present work the influence of the OH groups on the photocatalytic activity and the photoinduced hydrophilicity of microwave assisted sol-gel TiO₂ films was investigated. The prepared TiO₂ films were characterized using XRD and AFM. Furthermore, the surface of the TiO₂ films was examined by help of XPS in order to determine the amount of OH groups before and after UV irradiation at different humidities. The activity of the TiO₂ films was determined using stearic acid as a model compound and the photoinduced superhydrophilicity was investigated through contact angle measurements.The results of this investigation showed that the microwave assisted sol-gel technique produces highly homogeneous and efficient TiO₂ films without the need for heat treatment for crystallization. Based on the conducted experiments it is suggested that the amount of OH groups on the TiO₂ surface highly influence the photocatalytic activity and the photoinduced superhydrophilicity and that the two mechanisms may be closely related. It is suggested that the superhydrophilicity is obtained through a combination of photocatalytic degradation of organic contaminants and surface structural changes in form of an increased amount of OH-groups.     

TiO2 thick films supported on reticulated macroporous Al2O3 foams and their photoactivity in phenol mineralization

TiO₂ thick films deposited on macroporous reticulated Al₂O₃ foams with pore size of 10 ppi and 15 ppi were prepared using dip coating from slurries of Aeroxide^® P25 nanopowder and precipitated titania. All prepared films have sufficiently good adhesion to the surface of the substrate also in case of strongly cracked films. No measurable release of deposited TiO₂ after repeated photocatalytic cycles was observed. The photocatalytic activity was characterized as the rate of mineralization of aqueous phenol solution under irradiation of UVA light by TOC technique. The best activity was obtained with Aeroxide^® P25 coated Al₂O₃ foam with the pore size of 10 ppi, annealed at 600 °C. The optimal annealing temperature for preparation of films from precipitated titania could be determined at 700 °C. Films prepared by sol-gel deposition technique were considerably thinner compared to coatings made of suspensions and their photocatalytic activity was significantly smaller.     

Tunable photocatalytic selectivity of fluoropolymer PVDF modified TiO2

Fluoropolymer poly-vinylidene-fluoride modified TiO₂ (PVDF/TiO₂) were prepared via a simple chemisorption approach and characterized by thermo gravimetric analyse, transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy and photoluminescence spectra. The modified mechanism and the photocatalytic selectivity of the PVDF/TiO₂ were studied. The existence of Ti-F coordination bond on the interface between TiO₂ and PVDF was confirmed. For the PVDF modification, the photocatalytic degradation (PCD) of cationic dye was greatly enhanced, and the PCD of anionic dye was obviously inhibited. PVDF/TiO₂ shows high photocatalytic selectivity than that of TiO₂ by degrading mixed solution of cationic dyes MB and anionic dyes MO. The selectivity can be tuned by changing the PVDF modification amount.     

XPS characterization of sensitized n-TiO2 thin films for dye-sensitized solar cell applications

TiO₂ thin films, employed in dye-sensitized solar cells, were prepared by the sol-gel method or directly by Degussa P25 oxide and their surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The effect of adsorption of the cis-[Ru(dcbH₂)₂(NCS)₂] dye, N3, on the surface of films was investigated. From XPS spectra taken before and after argon-ion sputtering procedure, the surface composition of inner and outer layers of sensitized films was obtained and a preferential etching of Ru peak in relation to the Ti and N ones was identified. The photoelectrochemical parameters were also evaluated and rationalized in terms of the morphological characteristics of the films.     

In situ Radical Copolymerization in Presence of Surface‐Modified TiO2 Nanoparticles: Influence of a Double Modification on Properties of Unsaturated Polyester (UP) Nanocomposites

In the preparation of nanocomposites, there is competition between the dispersion of nanoparticles and the formation of agglomerates. In this study, radical copolymerization of ethyl acrylate and methyl methacrylate initiated by 2,2‐azobis (isobutyro) nitrile (AIBN) was performed, in the presence of titanium oxide (TiO₂) nanoparticles modified in a new approach; a good dispersion of the nanoparticles in the unsaturated polyester (UP) matrix was obtained. The TiO₂ nanoparticles were exposed to 3‐(methacryloxy) propyl trimethoxy silane as the coupling agent. The presence of coupling agent‐grafted TiO₂ nanoparticles in the copolymerization process resulted in the formation of a polymeric layer on the surface of the TiO₂ nanoparticles (doubly modified‐TiO₂). The grafting of coupling agent molecules and consequently copolymer macromolecular chains onto the surface of TiO₂ nanoparticles was investigated using Fourier transform infrared (FTIR) analysis. It found that the formation of an acrylate layer on the surface of nanoparticles was successful. Then, unsaturated polyester (UP)/TiO₂ nanocomposites were prepared. The morphology was studied using transmission electron microscopy (TEM). Mechanical properties and ultraviolet visible (UV/VIS) spectroscopy of various samples, including the doubly modified‐TiO₂ nanoparticles, with different nanoparticle inclusions and the unmodified‐TiO₂ nanoparticles, were also investigated. The results showed the doubly modified‐TiO₂ nanoparticles, compared to those of unmodified‐TiO₂, had better nanoparticle dispersion causing improvement in the mechanical properties and UV shielding.     

Optical properties of ITO/TiO2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO₂ substrates either at room temperatures (RT) or heated to 200-400 °C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 °C enhances the transmission of TiO₂ films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO₂ with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated. n ≈ 2.03 and 2.04, respectively for ITO films and TiO₂ film deposited at 400 °C in the visible region. Post-annealing of the TiO₂ films for 3 h at 300 and 500 °C was performed to enhance n. The refractive index of the TiO₂ films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 °C, respectively. The TiO₂ films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO₂ films and when annealed at 400 °C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO₂ films deposited at 200 °C. The shift decreases by half when the TiO₂ film was deposited at 400 °C. Post-annealing was also performed on double layer ITO/TiO₂.     

Super-hydrophilic properties of TiO2-DLC nanocomposite films fabricated by the simple electrochemical process

Anatase TiO₂ nanoparticles incorporated DLC films were successfully deposited on single crystalline silicon substrates by the electrolysis of TiO₂-methanol solution under ambient atmospheric pressure and low temperature. Anatase TiO₂ nanoparticles were embedded into amorphous carbon matrix, forming the typical nanocrystalline/amorphous nanocomposite films, confirmed by transmission electron microscopy (TEM). TiO₂ incorporation effectively increased the sp³-hybridized carbon concentration in the composite film, and further regulated the microstructure and surface morphology. Furthermore, the static contact testing completely displayed, TiO₂ incorporation got the composite films super-hydrophilic, which fundamentally improved the wetting ability of DLC film.     

Deposition of Au/TiO2 film by pulsed laser

Au nanoparticles, which were photoreduced by a Nd:YAG laser in HAuCl₄ solution containing TiO₂ colloid and accompanied by the TiO₂ particles, were deposited on the substrate surface. The film consisting of Au/TiO₂ particles was characterized by the absorption spectra, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The adhesion between the film and substrate was evaluated by using adhesive tape test. It was found that the presence of TiO₂ dramatically enhanced the adhesion strength between the film and the substrate, as well as the deposition rate of film. The mechanism for the deposition of Au/TiO₂ film was also discussed.     

Reactive sputtering TiO2 films for surface coating of poly(dimethylsiloxane)

Titanium dioxide (TiO₂) films were prepared on poly(dimethylsiloxane) (PDMS) substrate by direct current (DC) reactive sputtering to change surface physical properties of PDMS. The effects of the changes were investigated by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) analysis, water contact angle measurements and protein adsorption tests. Improved wettability and reduced adsorption properties were observed on PDMS surface coated TiO₂ films.     

Preparation, structures and photoluminescent enhancement of CdWO4-TiO2 composite nanofilms

For the first time, Cadmium tungstate (CdWO₄)-TiO₂ composite nanofilms on a glass substrate were prepared by means of the dip-coating technique, in which collodion was used as a dispersant and film-forming agent. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo gravimetric and thermal analyses (TG/DTA), FTIR and photoluminescence (PL) methods, respectively. SEM and XRD characterization of these films indicated that CdWO₄ particles crystallized in a monoclinic wolframite-type structure whereas TiO₂ particles were Anatase phase; and both of them were well distributed in the nanofilms. FTIR spectra proved the presence of CdWO₄ on the nanofilms. Photoluminescent results showed that the emitting peak of CdWO₄ films blue shifted slightly relative to that of CdWO₄ crystal. Moreover, the PL intensity of CdWO₄-TiO₂ composite nanofilm was much higher than that of CdWO₄ nanofilm. We ascribed that the introduction of TiO₂ should be responsible for the PL enhancement.     

Surface modification of PMMA/O-MMT composite microfibers by TiO2 coating

In the present work, poly(methyl methacrylate) (PMMA)/organically modified montmorillonite (O-MMT) composite microfibers were firstly prepared by emulsion polymerization combined with electrospinning, and then coated by nanosize titanium dioxide (TiO₂) using RF magnetron sputter technique. The modified surfaces of PMMA/O-MMT composite microfibers were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), UV-vis spectroscopy and drop shape analyzer. Finally, the photocatalytic properties of TiO₂ coated PMMA/O-MMT composite microfiber membranes were evaluated by degradation of methylene blue(MB) under UV illumination. The experimental results revealed that anatase-TiO₂ and rutile-TiO₂ nanoparticles were well spread and physically deposited on the surface of PMMA/O-MMT microfibers, and the wettability of the PMMA/O-MMT composite microfibers was improved after surface modification by sputter coating. Furthermore, the PMMA/O-MMT microfibers membrane coated with TiO₂ performed well in photocatalytic degradation of MB.     

Structural properties of single and multilayer ITO and TiO2 films deposited by reactive pulsed laser ablation deposition technique

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser. The films were deposited on SiO₂ substrates heated at 200 and 400 °C. ITO and TiO₂ films with uniform thicknesses of about 400 and 800 nm, respectively, over large areas were prepared. X-ray diffraction (XRD) analysis revealed that the ITO films are formed of highly orientated nanocrystals with an average particle size of 10-15 nm. Atomic force microscopy (AFM) observations indicate rough ITO films surfaces with average roughness of 26-30 nm. Pores were also observed. TiO₂ films deposited on the prepared ITO films result less crystalline. Annealing at 300 and 500 °C for three consecutive hours promoted formation of TiO₂ anatase phase, with crystal size of ∼6-7 nm. From the scanning transmission electron microscope (STEM) images, it can be seen that the TiO₂ films deposited onto the prepared ITO films present a relatively high pore sizes with an average pore diameter of ∼40 nm and excellent uniformity. In addition, STEM cross-sectional analysis of our films showed a columnar structure but no evidence of voids in the structure. Therefore, films exhibited large surface area, well suited for dye-sensitized solar cells (DSSC) applications.     

Photoelectrocatalytic degradation of organic contaminants at Bi2O3/TiO2 nanotube array electrode

In the current work, TiO₂ nanotube array was prepared via electrochemical anode method. Then the Bi₂O₃ nanoparticles were deposited onto the TiO₂ nanotube array via dip-coating method from an amorphous complex precursor. The crystal structures were characterized via X-ray diffraction analysis. Their surface textures were observed via electron-scanning microscope. The prepared composite array electrode exhibited high photoelectrocatalytic activities towards degrading organic contaminants under visible light irradiation. High photoelectrocatalytic activities were also exhibited under UV light irradiation. The catalytic mechanism was discussed based on the analysis of electrochemical and degradation kinetics results. It is suggested a P (Bi₂O₃)-N (TiO₂) junction was formed to increase the catalytic activates. The stability of the electrode materials was confirmed finally.     

TiO2 nanoparticle thin film deposition by matrix assisted pulsed laser evaporation for sensing applications

The MAPLE technique has been used for the deposition of nanostructured titania (TiO₂) nanoparticles thin films to be used for gas sensors applications. An aqueous solution of TiO₂ nanoparticles, synthesised by a novel chemical route, was frozen at liquid nitrogen temperature and irradiated with a pulsed ArF excimer laser in a vacuum chamber. A uniform distribution of TiO₂ nanoparticles with an average size of about 10 nm was deposited on Si and interdigitated Al₂O₃ substrates as demonstrated by high resolution scanning electron microscopy-field emission gun inspection (SEM-FEG). Energy dispersive X-ray (EDX) analysis revealed the presence of only the titanium and oxygen signals and FTIR (Fourier transform infra-red) revealed the TiO₂ characteristic composition and bond. A comparison with a spin coated thin film obtained from the same solution of TiO₂ nanoparticles is reported. The sensing properties of the films deposited on interdigitated substrates were investigated, too.