Effect of process parameters on laser damage threshold of TiO2 coatings

We investigate the laser damage behaviour of an electron-beam-deposited TiO2 monolayer at different process parameters. The optical properties, chemical composition, surface defects, absorption and laser-induced damage threshold (LIDT) of Elms are measured. It is found that TiO2 Elms with the minimum absorption and the highest LIDT can be fabricated using a TiO2 starting material after annealing. LIDT is mainly related to absorption and is influenced by the non-stoichiometric defects for TiO2 films. Surface defects show no evident effects on LIDT in this experiment.

EFFECT OF PH ON THE STABILITY OF TIO2 COATINGS ON GLASS PHOTOCATALYSIS REACTORS FOR WATER-PURIFICATION

TiO2 coated glass shows excellent stability in the range pH 2-9, however, there is rapid and complete stripping of the TiO2 coating between pH 11 and 12.

Suspension plasma sprayed coatings using dilute hydrothermally produced titania feedstocks for photocatalytic applications

Titanium dioxide coatings have potential applications including photocatalysts for solar assisted hydrogen production, solar water disinfection and self-cleaning windows. Herein, we report the use of suspension plasma spraying (SPS) for the deposition of conformal titanium dioxide coatings. The process utilises a nanoparticle slurry of TiO2 (ca. 6 and 12 nm respectively) in water, which is fed into a high temperature plasma jet (ca. 7000-20 000 K). This facilitated the deposition of adherent coatings of nanostructured titanium dioxide with predominantly anatase crystal structure. In this study, suspensions of nano-titanium dioxide, made via continuous hydrothermal flow synthesis (CHFS), were used directly as a feedstock for the SPS process. Coatings were produced by varying the feedstock crystallite size, spray distance and plasma conditions. The coatings produced exhibited ca. 90-100% anatase phase content with the remainder being rutile (demonstrated by XRD). Phase distribution was homogenous throughout the coatings as determined by micro-Raman spectroscopy. The coatings had a granular surface, with a high specific surface area and consisted of densely packed agglomerates interspersed with some melted material. All of the coatings were shown to be photoactive by means of a sacrificial hydrogen evolution test under UV radiation and compared favourably with reported values for CVD coatings and compressed discs of P25.

Effect of microstructure of TiO2 thin films on optical band gap energy

TiO2 coatings are prepared on fused silica with conventional electron beam evaporation deposition. After annealed at different temperatures for four hours, the spectra and XRD patterns of TiO2 thin film are obtained. XRD patterns reveal that only anatase phase can be observed in TiO2 coatings regardless of the different annealing temperatures, and with the increasing annealing temperature, the grain size gradually increases. The relationship between the energy gap and microstructure of anatase is determined and discussed. The quantum confinement effect is observed that with the increasing grain size of TiO2 thin film, the band gap energy shifts from 3.4 eV to 3.21 eV. Moreover, other possible influence of the TiO2 thin-film microstructure, such as surface roughness and thin film absorption, on band gap energy is also expected.

Investigation on microstructure and optical properties of titanium dioxide coatings annealed at various temperature

TiO2 coatings were prepared on fused silica with conventional electron beam evaporation deposition. After TiO2 thin films were annealed at different temperatures for 4 h, several properties were investigated by X-ray diffraction (XRD), spectrometer.. photoelectron spectroscopy (XPS) and AFM. It was found that with the annealing temperature increasing, the transmittance of TiO2 coatings decreased, and the cutoff wavelength shifted to long wavelength in near ultraviolet band. Especially, when coatings were annealed at high temperature, the optical loss is very serious, which can be attributed to the scattering and the absorption of TiO2 coatings. XRD patterns revealed that only anatase phase was observed in TiO2 coatings regardless of the different annealing temperatures. XPS results indicated that the fine chemical shift of TiO2 2p(1/2) should be attributed to existence of oxygen vacancies around Ti+4 ion. The investigation on surface morphology by AFM showed that the RMS of titania thin films gradually increases from less than 0.40 nm to 5.03 nm and it should be ascribed to the growth of titanium dioxide grain size with the increase of annealing temperature. (C) 2005 Elsevier B.V. All rights reserved.

Investigation of damage threshold to TiO2 coatings at different laser wavelength and pulse duration

Laser-induced damages to TiO2 single layers and TiO2/SiO2 high reflectors at laser wavelength of 1064 nm, 800 run, 532 urn, and pulse width of 12 ns, 220 ps, 50 fs, 8 ns are investigated. All films are prepared by electron beam evaporation. The relations among microstructure, chemical composition, optical properties and laser-induced damage threshold (LIDT), have been researched. The dependence of damage mechanism on laser wavelength and pulse width is discussed. It is found that from 1064 nm to 532 nm, LIDT is mainly absorption related, which is determined by film's extinction coefficient and stoichiometric defects. The rapid decrease of LIDT at 800 nm is due to the pulse width factor. TiO2 coatings are mainly thermally by damaged at long pulse (tau >= 220 ps). The damage shows ablation feature at 50 fs. (C) 2007 Elsevier B.V. All rights reserved.

Quantum yield measurements for the photocatalytic oxidation of Acid Orange 7 (AO7) and reduction of 2,6-dichloroindophenol (DCIP) on transparent TiO2 films of various thickness

This work comprises the photoactivity assessment of transparent sol–gel TiO2 coatings of various thickness using two test systems. The initial rates of both photocatalytic reactions, namely the oxidative bleaching of Acid Orange 7 (AO7) and the reductive bleaching of 2,6-dichlorindophenol (DCIP) increase linearly with increasing titania film thickness as well as with increasing absorbed light flux. The latter work revealed quantum yields (QY) of 0.19% and 92% for the AO7 and DCIP test system, respectively. The low QY for the AO7 oxidation is due to the combination of a slow irreversible reduction of oxygen and also for the oxidation of AO7, thus favouring the high efficiency for electron–hole recombination that is typical for aqueous organic pollutants. In contrast, the very high QY for the photocatalysed reduction of DCIP is due to the presence of a vast excess of glycerol which traps the photogenerated holes efficiently and so allow time for the slower reduction of dye to take place. Furthermore, the oxidation of glycerol results in the generation of highly reducing R-hydroxyalkyl radicals that are able to also reduce DCIP. As a consequence of this ‘current doubling’ effect, the observed QY (92%) is much higher than the apparent theoretical value of 50%.

Corrosion studies on the plasma-sprayed nanostructured titanium dioxide coating

Purpose: The purpose of this paper is to report the resistance of plasma-sprayed titanium dioxide (TiO2) nanostructured coatings in a corrosive environment.----- Design/methodology/approach: Weight loss studies are performed according to ASTM G31 specifications in 3.5?wt% NaCl. Electrochemical polarization resistance measurements are made according to ASTM G59-91 specifications. Corrosion resistance in a humid and corrosive environment is determined by exposing the samples in a salt spray chamber for 100?h. Microstructural studies are carried out using an atomic force microscope and scanning electron microscope.----- Findings: The nanostructured TiO2 coatings offer good resistance to corrosion, as shown by the results of immersion, electrochemical and salt spray studies. The corrosion resistance of the coating is dictated primarily by the geometry of splat lamellae, density of unmelted nanoparticles, magnitude of porosity and surface homogeneity.----- Practical implications: The TiO2 nanostructured coatings show promising potential for use as abrasion, wear-resistant and thermal barrier coatings for service in harsh environments.----- Originality/value: The paper relates the corrosion resistance of nanostructured TiO2 coatings to their structure and surface morphology.

Reactive plasmas as a versatile nanofabrication tool

The underlying physics of the application of low-temperature, low-pressure reactive plasmas in various nanoassembly processes is described. From the viewpoint of the "cause and effect" approach, this Colloquium focuses on the benefits and challenges of using plasma-based systems in nanofabrication of nanostructured silicon films, low-dimensional semiconducting quantum structures, ordered carbon nanotip arrays, highly crystalline TiO2 coatings, and nanostructured hydroxyapatite bioceramics. Other examples and future prospects of plasma-aided nanofabrication are also discussed. © 2005 The American Physical Society.

Titanium dioxide based nanomaterials for photocatalytic water treatment

Water treatment using photocatalysis has gained extensive attention in recent years. Photocatalysis is promising technology from green chemistry point of view. The most widely studied and used photocatalyst for decomposition of pollutants in water under ultraviolet irradiation is TiO2 because it is not toxic, relatively cheap and highly active in various reactions. Within this thesis unmodified and modified TiO2 materials (powders and thin films) were prepared. Physico-chemical properties of photocatalytic materials were characterized with UV-visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectrometry (XPS), inductively coupled plasma optical emission spectroscopy (ICP-OES), ellipsometry, time-of-flight secondary ion mass spectrometry (ToF-SIMS), Raman spectroscopy, goniometry, diffuse reflectance measurements, thermogravimetric analysis (TGA) and nitrogen adsorption/desorption. Photocatalytic activity of prepared samples in aqueous environment was tested using model compounds such as phenol, formic acid and metazachlor. Also purification of real pulp and paper wastewater effluent was studied. Concentration of chosen pollutants was measured with high pressure liquid chromatography (HPLC). Mineralization and oxidation of organic contaminants were monitored with total organic carbon (TOC) and chemical oxygen demand (COD) analysis. Titanium dioxide powders prepared via sol-gel method and doped with dysprosium and praseodymium were photocatalytically active for decomposition of metazachlor. The highest degradation rate of metazachlor was observed when Pr-TiO2 treated at 450ºC (8h) was used. The photocatalytic LED-based treatment of wastewater effluent from plywood mill using commercially available TiO2 was demonstrated to be promising post-treatment method (72% of COD and 60% of TOC was decreased after 60 min of irradiation). The TiO2 coatings prepared by atomic layer deposition technique on aluminium foam were photocatalytically active for degradation of formic and phenol, however suppression of activity was observed. Photocatalytic activity of TiO2/SiO2 films doped with gold bipyramid-like nanoparticles was about two times higher than reference, which was not the case when gold nanospheres were used.

Plasma-controlled nanocrystallinity and phase composition of TiO2 : a smart way to enhance biomimetic response

This contribution sheds light on the role of crystal size and phase composition in inducing biomimetic apatite growth on the surface of nanostructured titania films synthesized by reactive magnetron sputtering of Ti targets in Ar+O2 plasmas. Unlike most existing techniques, this method enables one to deposit highly crystalline titania films with a wide range of phase composition and nanocrystal size, without any substrate heating or postannealing. Moreover, by using this dry plasma-based method one can avoid surface hydroxylation at the deposition stage, almost inevitable in wet chemical processes. Results of this work show that high phase purity and optimum crystal size appear to be the essential requirement for efficient apatite formation on magnetron plasma-fabricated bioactive titania coatings. © 2006 Wiley Periodicals, Inc.

真空中有机污染物对TiO2/SiO2介质膜损伤阈值的影响

本文主要研究了真空中有机污染物对薄膜阈值的影响。采用电子枪蒸镀的方式制备了由TiO2/SiO2两种材料组合而成的1064nm高反膜，分别在大气和真空中对其损伤阈值进行了测量。发现在真空中污染物易黏附于光学元件表面，导致损伤；真空中的氧分子会加速损伤过程，根据破斑形貌的分析认为是由于在激光作用下氧分子与有机污染物发生反应造成的；真空中的有机污染物被去除后，真空和大气中的损伤阈值差别不大，破斑形貌相似。结果表明有机污染物是真空中损伤阈值降低的主要原因。

Comparison of TiO2 and ZrO2 films deposited by electron-beam evaporation and by sol-gel process

TiO2 and ZrO2 films are deposited by electron-beam (EB) evaporation and by sol-gel process. The film properties are characterized by visible and Fourier-transform infrared spectrometry, x-ray diffraction analysis, surface roughness measure, absorption and laser-induced damage threshold (LIDT) test. It is found that the sol-gel Elms have lower refractive index, packing density and roughness than EB deposited films due to their amorphous structure and high OH group concentration in the film. The high LIDT of sol-gel films is mainly due to their amorphous and porous structure, and low absorption. LIDT of EB deposited film is considerably affected by defects in the Elm, and LIDT of sol-gel deposited film is mainly effected by residual organic impurities and solvent trapped in the film.

Influence of annealing temperature on structure, optical loss and laser-induced damage threshold of TiO2 thin films

TiO2 thin films are prepared on fused silica with conventional electron beam evaporation deposition. After annealed at different temperatures for 4h, the spectra and XRD patterns of the TiO2 thin film are obtained. Weak absorption of coatings is measured by the surface thermal lensing technique, and laser-induced damage threshold (LIDT) is determined. It is found that with the increasing annealing temperature, the transmittance of TiO2 films decreases. Especially when coatings are annealed at high temperature over 1173K, the optical loss is very serious. Weak absorption detection indicates that the absorption of coatings decreases firstly and then increases, and the absorption and defects play major roles in the LIDT of TiO2 thin films.

Impact of organic contamination on laser-induced damage threshold of high reflectance coatings in vacuum

The influence of organic contamination in vacuum on the laser-induced damage threshold (LIDT) of coatings is studied. TiO2/SiO2 dielectric mirrors with high reflection at 1064 nm are deposited by the electron beam evaporation method. The LIDTs of mirrors are measured in vacuum and atmosphere, respectively. It is found that the contamination in vacuum is easily attracted to optical surfaces because of the low pressure and becomes the source of damage. LIDTs of mirrors have a little change in vacuum compared with in atmosphere when the organic contamination is wiped off. The results indicate that organic contamination is a significant reason to decrease the LIDT. N-2 molecules in vacuum can reduce the influence of the organic contaminations and prtectect high reflectance coatings. (C) 2008 Elsevier B.V. All rights reserved.