Sol-gel Titania Films Research Articles

Correlation between the photocatalysed oxidation of methylene blue in solution and the reduction of resazurin in a photocatalyst activity indicator ink (Rz Paii )

Abstract The initial rate of the photocatalysed oxidation of methylene blue, MB, by dissolved oxygen in solution, ri(MB), is measured for a series of titania on glass samples exhibiting a wide range of activities. The samples used include two different types of commercial self-cleaning glass and a lab-made sol-gel titania film. The activities of these samples are also assessed using a resazurin-based photocatalyst activity indicator ink, i.e. Rz paii, for which the initial rates of the photocatalysed reduction of Rz were measured, ri(Rz). A plot of ri(MB) vs. ri(Rz) reveals a good straight line, thereby demonstrating a linear correlation (for TiO2 films on glass at least) between the slow (usually hours) photocatalysed oxidation of organic materials, such as MB, and the fast (typically min) photocatalysed irreversible reduction of a dye, like Rz, in a paii. The possible use of paii technology for assessing, in a simple, quick and inexpensive manner, photocatalytic films both in the laboratory and in situ is discussed briefly.

Insights into meso-structured photoanodes based on titanium oxide thin film with high dye adsorption ability

In this work, the absorption of N719 dye on titania films with significantly different morphologies has been investigated. By changing the synthetic approach, we have prepared sol–gel titania films with a flat surface (TiSG), mesoporous films with an ordered porous structure (TiMS) and a reference film from commercial titania P25 using the well-known doctor-blade technique (TiDB). The surface roughness and grain size distribution of titania films has been explored by atomic force microscopy. The effectiveness of the various type of titania films to absorb the N719 dye has been investigated by UV–vis spectroscopy, whereas the dye-titania interaction was examined by Raman spectroscopy. Further information on the structural and morphological properties of titania films were obtained by small angle X-ray diffraction and scanning electron microscopy. Both sol–gel and mesoporous titania films were deposited by spin-coating the solution of titania precursor, which produces films of about 300nm, much thinner than those afforded by the doctor-blade technique (∼6.8μm). When the thickness of TiMS film was increased up to 1μm, the absorbance of dye-sensitized titania became also higher, reaching levels similar to those of the reference N719-TiDB film in spite of its lower thickness. Our findings show that the morphological properties of titania substrates affect their ability to adsorb the N719 dye and, in particular, it was observed an enhanced dye adsorption onto meso-structured titania. N719 absorbance normalized to titania film thickness reveals that both the roughness and porosity of titania promote the dye adsorption. The much higher ability of meso-structured photoanodes to adsorb the dye can be attributed to the ordered and highly porous structure of TiMS. The photocurrent–voltage measurements of TiMS and TiDB dye sensitized films with similar thickness (∼500nm) have shown the superiority of N719-TiMS thus suggesting the key role of the mesoporous structure.

Photocatalytic degradation of Lissamine Green B dye by using nanostructured sol–gel TiO2 films

Nanostructured sol–gel TiO2 films were prepared on a glass substrate by means of the dip-coating technique with titanium tetraisopropoxide as a precursor. TiO2 sols were synthesized with and without the addition of polyethylene glycol (PEG) as a structure-directing agent. The synthesized sol–gel TiO2 were characterized by XRD, AFM, FTIR and Micro-Raman spectroscopy. The photocatalytic activity of the films was evaluated by the photocatalytic degradation of Lissamine Green B (LGB) dye (dissolved in water) as a model pollutant with the predominant irradiation wavelength of 365nm (UV-A). It was found that the addition of PEG to the initial sol affects the surface morphology and the photocatalytic properties of prepared sol–gel TiO2 films. AFM analysis confirmed the presence of nanostructured sol–gel titania films on the glass substrate. Roughness parameters (Ra, Rq, and Zmax) of the sol–gel TiO2 film with the addition of PEG are higher than the parameters of the sol–gel TiO2 film without the addition of PEG. The TiO2 film prepared with the addition of PEG has a higher surface density (a larger active surface area) and better photocatalytic activity in the degradation of the LGB dye solution than the TiO2 film prepared without the addition of PEG.

Influences of Acid on Molecular Forms of Fluorescein and Photoinduced Electron Transfer in Fluorescein‐Dispersing Sol–Gel Titania Films

Fluorescein-dispersing titania gel films were prepared by the acid-catalyzed sol-gel reaction using a titanium alkoxide solution containing fluorescein. The molecular forms of fluorescein in the films, depending on its acid-base equilibria, and the complex formation and photoinduced electron transfer process between the dye and titania surface were investigated by fluorescence and photoelectric measurements. The titanium species were coordinated to the carboxylate and phenolate-like groups of the fluorescein species. The quantum efficiencies of the fluorescence quenching and photoelectric conversion were higher upon excitation of the dianion species interacting with the titania, i.e. the dye-titania complex. This result indicated that the dianion form was the most favorable for formation of the dye-titania complex exhibiting the highest electron transfer efficiency. Using nitric acid as the catalyst, the titania surface bonded to the fluorescein instead of the adsorbed nitrate ion during the steam treatment. The dye-titania complex formation played an important role in the electron injection from the dye to the titania conduction band.

Photocatalytic organic synthesis in an NMR tube: C[sbnd]C coupling of phenoxyacetic acid and acrylamide

NMR was used to study the semiconductor photocatalytic (SPC) CC coupling of phenoxyacetic acid (PAA) with acrylamide (ACM) in an NMR tube photoreactor. Using an NMR tube with a sol–gel titania inner coating as a photoreactor, this reaction is relatively clean, forming only 1 product, 4-phenoxybutanamide (4-PB), in yields up to 78%. This SPC reaction is used to assess the activity of the sol–gel titania coating as a function of their annealing temperature, which alters the surface area and phase of the titania, and the general reusability of the TiO2 coated NMR tubes. The optimum temperature range for annealing the sol–gel titania films is between 450°C and 800°C, with the maximum yield and rate attained at 450°C. Despite a decrease in the initial rates of formation of 4-PB above an annealing temperature of 450°C, the final product yields remained similar, giving maximum yields within 60min of irradiation. The reusability study reveals that the activity of the sol–gel titania can quickly deteriorate with repeated use due to the adsorption of yellow/brown coloured, insoluble, most likely organic polymeric, material and its screening effect on the underlying photocatalyst. The titania can, however, be restored to its original activity by a simple heat treatment at 450°C for 30min.

Preparation and reactivity of sol-gel titania films on FeCrAl alloys and estimation of adhesion energy of alumina scales

The sol-gel TiO 2 coatings were applied in order to improve the rapid transformation of alumina scales formed on the FeCrAl alloys during oxidation at high temperatures. The titanium-containing sol-gel coatings were prepared from the precursor tetra-isopropyl-orthotitanate (TIPT). The preparation conditions such as solvent, concentration and pH were studied. The influence of pH and (solvent/precursor ratio) on gelification time was also studied. The adhesion energy was estimated using the tensile test in the SEM chamber for the samples oxidized in air at 850 and 950 °C for 72 h.

Sol-gel Titania-Coated Needles for Solid Phase Dynamic Extraction-GC/MS Analysis of Desomorphine and Desocodeine

Novel sol-gel titania film coated needles for solid-phase dynamic extraction (SPDE)-GC/MS analysis of desomorphine and desocodeine are described. The high thermal stability of titania film permits efficient extraction and analysis of poorly volatile opiate drugs. The influences of sol-gel reaction time, coating layer, extraction and desorption time and temperature on the SPDE needle performance were investigated. The deuterium labeled internal standard was introduced either during the extraction of analyte or directly injected to GC after the extraction process. The latter method was shown to be more sensitive for the analysis of water and urine samples containing opiate drugs. The proposed conditions provided a wide linear range (from 5-5000 ppb), and satisfactory linearity, with R(2) values from 0.9958 to 0.9999, and prominent sensitivity, LOQs (1.0-5.0 ng/g). The sol-gel titania film coated needle with SPDE-GC/MS will be a promising technique for desomorphine and desocodeine analysis in urine.

Structural Characterization and Corrosion Behavior of Stainless Steel Coated With Sol-Gel Titania

Sol-gel titania films were prepared from hydrolysis and condensation of titanium (IV) isopropoxide. Diethanolamine was used as chelant agent in titania synthesis. 316L stainless steel substrates were dip-coated at three different withdrawal speeds (6, 30, and 60 mm/min) and heated up to 400 °C. Thermogravimetry and differential thermal analyses of the titania gel solution evinced a continuous mass loss for temperatures up to 800 °C. The transition of anatase to the rutile phase begins at 610-650 °C, being the rutile transformation completed at 900 °C. The thicknesses of the films were determined as a function of the heat treatment and withdrawal speed. It was observed that their thicknesses varied from 130 to 770 nm. Scanning electron microscopy images of the composites revealed the glass-like microstructure of the films. The obtained sol-gel films were also characterized by energy dispersive spectroscopy. The chemical evolution of the films as a function of the heating temperature was evaluated by Fourier transform infrared spectroscopy (specular reflectance method). After performing the adhesion tests, the adherence of the titania films to the stainless steel substrate was excellent, rated 5B according to ASTM 3359. The hardness of the ceramic films obtained was measured by the Knoop microindentation hardness test with a 10 g load. We observed that the titania film became harder than the steel substrate when it was heated above 400 °C. The corrosion rates of the titania/steel composites, determined from potentiodynamic curves, were two orders of magnitude lower than that of the bare stainless steel. The presence of the sol-gel titania film contributed to the increase of the corrosion potential in ca. 650 mV and the passivation potential in ca. 720 mV.

Photocatalytic properties of different TiO 2 thin films of various porosity and titania loading

Thin sol–gel titania films with varying porosity but constant mass were coated on SiO 2/glass substrate. The mass was kept constant by changing the withdrawal rate of dipping from a titania sol. The porosity properties of the films were determined by the analysis of adsorption isotherms of Kr at ca 77 K. Photocatalytic performance of TiO 2 layers in the degradation of Acid orange 7 (AO7) and 4-chlorophenol (4CP) in aqueous solution depends on the mass of titania but not on the porosity/surface area of the films. On the contrary, in the degradation of thin layers of Resazurin (Rz) ink and methyl stearate (MS) deposited on the TiO 2 film surface, the activity of the films is clearly enhanced due to the porosity but does not depend on titania mass itself. Obtained data suggest that while in an aqueous solution the reaction rate depends primarily on the mass of titania and amount of absorbed light, in solid thin films test the porosity/surface area together with interfacial area of tested compound with titania layer is significant.

Infrared Spectroscopy of Titania Sol-Gel Coatings on 316L Stainless Steel

Sol-gel titania films were deposited on 316L stainless steel using titanium isopropoxide as a chemical precursor. Dip-coating was performed at withdrawal speeds of 6 mm/min, 30 mm/min, and 60 mm/min. Deposited gel films were heat treated in air at 80℃, 100℃, 300℃, and 400℃. The structural evolution of the coatings was evaluated by infrared reflection-absorption spectroscopy. The influence of the withdrawal speed and the heat treatment temperature on the structure of the films was studied by varying the reflectance incidence angle during the infrared experiments and by Glow Discharge Spectrometry. Free functional groups were detected. The results indicate the formation of bidendate bridging coordination of carboxylic acid to titanium. Titanium atoms can also be pentacoordinated according to the processing conditions of the films. We observed a tendency of increasing amounts of OH groups with decreasing reflectance incidence angle. The film hardness was measured via Knoop microindenation hardness test.

Sol–gel titania films on YHfAl foils and their influence on inhibition of transition aluminas during oxidation at high temperatures

In this experimental work, sol–gel technique was used as surface coating of YHfAl alloys to avoid the formation of transition aluminas (δ and θ). This technique has an advantage of low cost and rapidity. The preparation conditions of TiO2 sol–gel coatings were presented with a description of their evolution with temperature during the first stages of YHfAl oxidation. It is noted that the rutile is the only detected phase at 800°C. The sol–gel thin films were prepared in order to inhibit the formation of transition aluminas. An important gain mass was recorded at a temperature of 875°C and the first grains of aluminas were appeared after 0.5h of oxidation. A sol–gel TiO2 film of nearly 60nm thick has proven its efficiency and has allowed to reduce the presence of transition aluminas.

Excimer laser processing as a tool for photocatalytic design of sol–gel TiO 2 thin films

Nanostructured sol–gel TiO 2 thin films spin coated on silicate glass plates are subjected to excimer (KrF *) pulsed laser irradiation in order to tailor their structure and photocatalytic properties. The surface morphology of virgin and laser-processed films are followed applying electron optical imaging and atomic force microscopy. The evolution of the surface roughness and pore formation are shown to be accompanied by optical absorption edge shift to infrared wavelength range. Conventional X-ray diffraction analysis and high-resolution transmission electron imaging are applied in order to obtain information on the phase composition. Co-existence of amorphous and anatase TiO 2 phases in nonirradiated sol–gel films is found. It is established that after laser processing the most intense XRD anatase peak is shifted to lower 2 θ range. The analysis of high-resolution transmission electron images of film profiles evidences for the laser induced phase transitions. Formation of rutile and brookite TiO 2 accompanied by evolution of oxygen deficient Ti n O 2 n−1 phases are identified in the subsurface region. The contribution of laser processing for increasing the photocatalytic efficiency of laser-modified films toward the oxidation of methylene blue water solution is demonstrated. The results obtained reveal a novel-processing route for designing sol–gel titania films with improved photocatalytical activity.

Current and possible future methods of assessing the activities of photocatalyst films

The continuing interest in semiconductor photochemistry, SPC, and the emergence of commercial products that utilise films of photocatalyst materials, has created an urgent need to agree a set of methods for assessing photocatalytic activity and international committees are now meeting to address this issue. This article provides a brief overview of two of the most popular current methods employed by researchers for assessing SPC activity, and one which has been published just recently and might gain popularity in the future, given its ease of use. These tests are: the stearic acid (SA) test, the methylene blue (MB) test and the resazurin (Rz) ink test, respectively. The basic photochemical and chemical processes that underpin each of these tests are described, along with typical results for laboratory made sol–gel titania films and a commercial form of self-cleaning glass, Activ™. The pros and cons of their future use as possible standard assessment techniques are considered.

Simultaneous monitoring of the destruction of stearic acid and generation of carbon dioxide by self-cleaning semiconductor photocatalytic films

The destruction of stearic acid (SA), the SA test, is a popular approach used to evaluate the activities of photocatalytic films. The destruction of SA via semiconductor photocatalysis is monitored simultaneously, using FT-IR spectroscopy, via the disappearance of SA and the appearance of CO 2. Sol–gel and P25 films of titania are used as the semiconductor photocatalytic self-cleaning films. A conversion factor is used of 9.7 × 10 15 molecules of SA cm −2 ≡ 1 cm −1 integrated areas of the peaks in the FT-IR of SA over the range 2700–3000 cm −1, which is three times that reported previously by others. As the SA disappeared the concomitant amount of CO 2 generated was >90% that expected throughout the photomineralisation process for the sol–gel titania film. In contrast, the slightly more active, and scattering, P25 titania films generated CO 2 levels that dipped as low as 69% during the course of the photoreaction, but eventually recovered to ≅100% that expected based on the amount of SA present. The importance of these results with respect to SA test and the evaluation of new and existing self-cleaning films are discussed briefly.

Photocatalytic properties of sol–gel titania film under fluorescent-light irradiation improved by silver negative-ion implantation

We have investigated on improvement of photocatalytic properties under fluorescent light of anatase-type titanium oxide film by using silver negative-ion implantation. The anatase films were prepared in a sol–gel method on a glass plate with a commercial sol material by spin-coating and baking at 500°C for 30min. Typical thickness of films was about 500nm. Silver negative ions were implanted to these films at 30keV with a dose in 1–5×1016ions/cm2. For forming Ag nanoparticles and recovering ion-induced defects, the implanted samples were annealed at temperature in a range of 400–600°C. All the samples showed a strong optical absorption at photon energy of about 2.2eV after annealing at 300–500°C. This means the existence of Ag nanoparticles in the films. In reduction test of methylene blue solution under fluorescent light, all samples showed improved photocatalytic properties. The best efficiency was obtained for the Ag-implanted sol–gel film with 1×1016ions/cm2 after annealing at 400°C, and it was higher by a factor of 1.6 than that of single rutile crystalline plate. The increase in the efficiency is considered to be due to existence of Ag nanoparticles in the film.

Preparation and characterisation of novel thick sol-gel titania film photocatalysts.

The preparation and characterization of thick (9 microns), clear, mechanically robust and photocatalytically active films of nanocrystalline anatase titania are described. XRD and SEM analysis show the films comprise 13 nm particles of anatase TiO2. Thin (54 nm) films of the 'paste' TiO2, along with sol-gel titania films made by a more traditional route are also prepared and characterised. All titania films mediate the photocatalytic destruction of stearic acid with a quantum yield of 0.0016 +/- 0.0003, using either 365 nm (i.e. BLB) or 254 nm (germicidal) light. P25 TiO2 films also appear to mediate the same process with a similar formal quantum efficiency. Of all the films tested, the thick paste TiO2 films are the most ideally suited for use with near UV light, for reasons which are discussed. All the titania films tested exhibit photoinduced superhydrophilicity.

Characterization and Tribological Investigation of Sol–Gel Titania and Doped Titania Thin Films

Titania (TiO2) and doped TiO2 ceramic thin films were prepared on a glass substrate by a sol–gel and dip‐coating process from specially formulated sols, followed by annealing at 460°C. The morphologies of the original and worn surfaces of the films were analyzed with atomic force microscopy (AFM) and scanning electron microscopy. The chemical compositions of the obtained films were characterized by means of X‐ray photoelectron spectroscopy (XPS). The tribological properties of TiO2 and doped TiO2 thin films sliding against Si3N4 ball were evaluated on a one‐way reciprocating friction and wear tester. The AFM analysis shows that the morphologies of the resulting films are very different in nanoscale, which partly accounts for their tribological properties. XPS analysis reveals that the doped elements exist in different states, such as oxide and silicate, and diffusion took place between the film and the glass substrate. TiO2 films show an excellent ability to reduce friction and resist wear. A friction coefficient as low as 0.18 and a wear life of 2280 sliding passes at 3 N were recorded. Unfortunately, all the doped TiO2 films are inferior to the TiO2 films in friction reduction and wear resistance, primarily because of their differences in structures and chemical compositions caused by the doped elements. The wear of the glass is characteristic of brittle fracture and severe abrasion. The wear of the TiO2 thin film is characteristic of plastic deformation with slight abrasive and fatigue wear. The doped TiO2 thin films show lower plasticity than the TiO2 thin film, which leads to large cracks. The propagation of the cracks caused serious fracture and failure of the films.

Interrelationship between Densification, Crystallization, and Chemical Evolution in Sol‐Gel Titania Thin Films

The mechanisms contributing to densification during non‐isothermal heat treatments in sol‐gel titania films have been studied. At low temperatures, shrinkage is attributed to condensation reactions between hydroxyls located primarily within the oxide skeleton. At higher temperatures, densification is due to both continued condensation between surface hydroxyls and structural relaxation. Shrinkage stops when films attain a level of crystallinity corresponding to the percolation threshold. At conventional rates, densification is delayed by faster heating; however, at very fast heating rates provided by rapid thermal annealing (8000°C/min), condensation is highly arrested and densification is faster. Similarly, the onset of crystallization increases with conventional heating rates, but crosses over to a lower temperature with rapid thermal annealing.

Optical properties of CdS- and (Cd, Zn)S-doped sol-gel titania films

Microcrystalline CdS and (Cd, Zn)S were generated in sol-gel derived dip-coated amorphous TiO 2 films. The relevant properties, e.g. optical absorption, optical gap ( E opt ) and fluorescence were studied via variations of different preparative parameters, e.g. CdS: TiO 2 and CdS: ZnS stoichiometry, baking temperature and soaking time.

Raman Studies of Stress-Induced Phase Transformations in Titania Films

AbstractTime-resolved micro-Raman spectroscopy is used to follow the amorphous to crystalline phase transformation in sol-gel deposited titania films induced thermally or through the action of applied hydrostatic pressure in a diamond anvil cell. Time-dependent phonon intensities intrinsic to the growing phase are related to the volume fraction of crystallite present at any time. The sigmoidally generated curves can be modeled in terms of modified Avrami ingrowth kinetics in which diffusion of the amorphous phase to the nucleation center is restricted by the morphology of the evolving phase. Phonon frequency and linewidth measurements during the course of the transformation probe changes in film stress and particle size which are used to understand the mechanistics of the transformation. Raman measurements also are used to derive a phase stability diagram for titania films.