Influence of precursors on the morphology and performance of TiO2 photoanodes

The preparation of TiO₂ photoanodes for dye-sensitized electrochemical devices was investigated. TiO₂ precursors with different rates of hydrolysis were used to modify the microstructures of the resulting TiO₂ photoanodes. The photoelectrochemical quantum yield for electrodes prepared from citrate remained low. Acetate precursors gave rise to electrodes with high fill factors and high open circuit voltages. Titanium isopropoxide hydrolysed very rapidly, leaving a powderous, poorly compacted deposit. Photoanodes from TiCl₄ precursors exhibit high surface roughness, which is necessary for a high degree of dye adsorption. With high hydrolysis rates, isopropoxide precursors produced powdery, poorly coherent films. A composite electrode with an acetate-based substrate and TiCl₄-based surface layers appears to be a suitable compromise.     

Characterization of the aqueous peroxomethod for the synthesis of transparent TiO2 thin films

This article summarizes our progress made on an aqueous chemical solution deposition method used for the deposition of photocatalytically active TiO₂ thin films. Starting from Titanium(IV)butoxide we achieved a stable titanium precursor solution containing titanium-peroxo compounds by reaction between Titanium(IV)butoxide and hydrogen peroxide. We were able to deposit anatase TiO₂ films with good optical transparency and abrasion resistance. Dip-coating was used to deposit thin films on glass substrates from the solution. The occurring reaction mechanism was examined via thermal analysis, mass spectrometry and Raman spectroscopy. Decomposition of organic polluents was confirmed by the breakdown of ethanol. The obtained results show promising possibilities of this low-carbon containing synthesis method towards transparent, photocatalytic coatings. Presence of carbon was minimized by avoiding organic complexing agents. These materials are of great importance in the synthesis of self-cleaning materials.     

Laser-induced vapour-phase synthesis of titanium dioxide

Small-diameter titanium dioxide powders were synthesized from vapour-phase reactants that were heated with 10.591m infrared radiation from a CO₂ laser. Two reactants, titanium isopropoxide and titanium butoxide, were evaluated. Anatase powder was generated from titanium isopropoxide in both static gas and flowing gas configurations. The addition of oxygen to the titanium isopropoxide gas stream reduced the percentage of volatile components in the anatase powder. Detailed characterizations of the product powders are presented.     

Improvement of the Zirconia shell in nanostructured titania core–shell working electrodes for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were constructed using nanostructured core–shell working electrodes produced by a sol–gel method. The precursor chemistry was studied by comparing zirconium butoxide and zirconium isopropoxide. Additionally, the concentrations in polar solvents were altered and used on two different titania nanostructures. The electrodes were characterized by photoelectrochemical measurements (PECM). The best efficiencies were recorded for the working electrodes modified with 0.05 M zirconium butoxide in n-butanol at 2.29% from 1.36% (uncoated TiO₂) for P25 films and 2.06% from 1.38% (uncoated TiO₂) for anatase films under a light intensity of 0.1 Sun.     

The influence of surfactants on the roughness of titania sol–gel films

Substrate dipping in a composite sol–gel solution was used to prepare both smooth and rough thin films of titanium dioxide (TiO₂) on commercial fiberglass. The deposition of a composite film was done in a beaker using a solution of titanium (IV) isopropoxide as the sol–gel precursor and cetyltrimethyl ammonium bromide as the surfactant. In order to establish a correlation between experimental conditions and the titanium oxide produced, as well as the film quality, the calcined samples were characterized using Raman spectroscopy, UV–vis spectrophotometry, scanning electron microscopy and atomic force microscopy. One of the most important results is that a 61-nm TiO₂ film was obtained with a short immersion of fiberglass into the sol–gel without surfactant. In other cases, the deposited film consisted of a titanium precursor gel encapsulating micelles of surfactant. The gel films were converted to only the anatase phase by calcining them at 500 °C. The resulting films were crystalline and exhibited a uniform surface topography. In the present paper, it was found that the TiO₂ films prepared from the sol–gel with a surfactant showed a granular microstructure, and are composed of irregular particles between 1.5 and 3 μm. Smooth TiO₂ films could have useful optical and corrosion-protective properties and, on other hand, roughness on the TiO₂ films can enhance the inherent photocatalytic activity.     

Structural and mechanical properties of titanium oxide thin films for biomedical application

Titanium oxide thin films were deposited by radiofrequency reactive sputtering in Ar-O₂ atmosphere on silicon (100) wafers and titanium alloy plates (Ti-6Al-4V). Thin films structural characterization was carried out by grazing incidence X-ray diffraction, atomic force microscopy, scanning and transmission electron microscopies. Chemical composition was checked by X-ray wavelength dispersive spectroscopy. Mechanical assessment was achieved by nano-indentation and nano-scratch measurements. The films deposited on silicon substrates are over-stoechiometric in oxygen, with an oxygen to titanium ratio of about 2.2. The growth of anatase and rutile phases was promoted by ranging the total and oxygen partial pressures between 0.17-1.47 Pa and 35-85%. The growth rate of films, determined by grazing incidence X-ray reflectivity, was ranging from 35 to 55 nm/h. The rutile single-phased films possess a hardness of about 2.5 times higher and a lower friction coefficient than the anatase films. The films which contain anatase possess a high surface root-mean-square roughness and a reduced elastic modulus of around 120 GPa close to reduced elastic moduli of hydroxyapatite bioceramic and titanium alloy. So the anatase film could be the best candidate as a titanium oxide intermediate layer between hydroxyapatite and titanium alloy in the field of biomedical implants.     

TiO2 anatase thin films deposited by spray pyrolysis of an aerosol of titanium diisopropoxide

Titanium dioxide thin films were deposited on crystalline silicon (100) and fused quartz substrates by spray pyrolysis (SP) of an aerosol, generated ultrasonically, of titanium diisopropoxide. The evolution of the crystallization, studied by X-ray diffraction (XRD), atomic force (AFM) and scanning electron microscopy (SEM), reflection and transmission spectroscopies, shows that the deposition process is nearly close to the classical chemical vapor deposition (CVD) technique, producing films with smooth surface and good crystalline properties. At deposition temperatures below 400 °C, the films grow in amorphous phase with a flat surface (roughness∼0.5 nm); while for equal or higher values to this temperature, the films develop a crystalline phase corresponding to the TiO₂ anatase phase and the surface roughness is increased. After annealing at 750 °C, the samples deposited on Si show a transition to the rutile phase oriented in (111) direction, while for those films deposited on fused quartz no phase transition is observed.     

AFM characterisation of sol‐gel TiO2 films deposited on stainless steel

One‐layer, two‐layer and three‐layer titania films on AISI 304 stainless steel were deposited by sol‐gel process and dip‐coating method. Two sols were prepared by using titanium isopropoxide as a precursor, propanol as a solvent, nitric acid as a catalyst and acetylacetone for peptization. Both of the prepared sols contained the same amount of mentioned components, the only difference was in the addition of polyethylene glycol (PEG) as templating reagent to one of the sols. After calcination at 550°C, deposited films on the stainless steel substrate were characterized by atomic force microscope (AFM). Influence of layer number as well as addition of polyethylene glycol on morphology of titania films was analysed and discussed. Generally, by increasing the number of layers and by addition of polyethylene glycol, roughness parameters increase by changing surface topography. The surface topography analysis is very important when choosing the adequate industrial application of prepared layers.     

Atomic force microscopy study of TiO2 sol–gel films thermally treated under NH3 atmosphere

Multilayered TiO₂ films were obtained by sol–gel and dipping deposition on quartz substrate followed by thermal treatment under NH₃ atmosphere. In an attempt to understand the close relationship between microstructural characteristics and the synthesis parameters, a systematic research of the structure and the morphology of NH₃ modified TiO₂ sol–gel films by XRD and Atomic Force Microscopy is reported. The surface morphology has been evaluated in terms of grains size, fractal dimension and surface roughness. For each surface, it was found a self-similar behavior (with mean fractal dimension in the range of 2.67–3.00) related to an optimum morphology favorable to maintain a nano-size distribution of the grains. The root mean square (RMS) roughness of the samples was found to be in the range of 0.72–6.02 nm.     

Influence of nanostructure on titanium corrosion resistance in fluoridated medium

ABSTRACT

Nanocrystalline titanium, mainly owing to its high corrosion resistance, mechanical strength to density ratio and biocompatibility, has a great application potential in dental implantology. However, fluoridated agents commonly used for oral hygiene could have a destructive influence on the titanium protective passive films and lead to the formation of local corrosion damages. In this work, the effect of nanostructuring on titanium corrosion resistance in the concentration of F^? which is typical for toothpastes, was evaluated by different electrochemical and surface characterisation techniques. It was found that nanostructure influences beneficially on titanium corrosion resistance in fluoride solution. Furthermore, the lower increase in nanocrystalline titanium surface roughness in corrosion solutions indicates better stability of passive film formed on its surface.

This paper is part of a thematic issue on Titanium.     

Structural characterization and photocatalytic activity of ultrathin TiO2 films fabricated by Langmuir-Blodgett technique with octadecylamine

TiO₂ thin films with nanometer-scale thicknesses were prepared by the hydrolysis of titanium potassium oxalate using octadecylamine (ODA) Langmuir-Blodgett (LB) films as templates. After optimizing conditions in immersion process, the amount of TiO₂ generated in the ODA LB film was found to be precisely controlled by the number of deposited ODA layers. Morphological measurements showed that uniform TiO₂ film with surface roughness of less than 1.3 nm could be prepared from the monolayer LB films through subsequent heat-treatment process, while generation of cracks became less noticeable on the 5-layer film after heat-treatment at lower holding temperature with slow heating rate. In addition, photocatalytic activities of the TiO₂ films were examined from the decomposition of cadmium stearate (CdSt) LB films and stearic acid (SA) cast films for different time intervals of irradiation with UV light. Atomic force microscopy measurements showed that an almost flat surface of the CdSt LB film changes to a moth-eaten appearance as a result of decomposition under UV light irradiation. Furthermore, the post-heat-treatment at higher temperatures resulted in decreased photocatalytic activity of the TiO₂ film for the decomposition of SA cast film.     

Characterization of surface oxide films on titanium and bioactivity

Biological properties of titanium implant depend on its surface oxide film. In the present study, the surface oxide films on titanium were characterized and the relationship between the characterization and bioactivity of titanium was studied. The surface oxide films on titanium were obtained by heat-treatment in different oxidation atmospheres, such as air, oxygen and water vapor. The bioactivity of heat-treated titanium plates was investigated by immersion test in a supersaturated calcium phosphate solution. The surface roughness, energy morphology, chemical composition and crystal structure were used to characterize the titanium surfaces. The characterization was performed using profilometer, scanning electronic microscopy, ssesile drop method, X-ray photoelectron spectroscopy, common Bragg X-ray diffraction and sample tilting X-ray diffraction. Percentage of surface hydroxyl groups was determined by X-ray photoelectron spectroscopy analysis for titanium plates and density of surface hydroxyl groups was measured by chemical method for titanium powders. The results indicated that heat-treatment uniformly roughened the titanium surface and increased surface energy. After heat-treatment the surface titanium oxide was predominantly rutile TiO₂, and crystal planes in the rutile films preferentially orientated in (1 1 0) plane with the highest density of titanium ions. Heat-treatment increased the amount of surface hydroxyl groups on titanium. The different oxidation atmospheres resulted in different percentages of oxygen species in TiO₂, in physisorbed water and acidic hydroxyl groups, and in basic hydroxyl groups on the titanium surfaces. The immersion test in the supersaturated calcium phosphate solution showed that apatite spontaneously formed on to the rutile films. This revealed that rutile could be bioactivated. The analyses for the apatite coatings confirmed that the surface characterization of titanium has strong effect on bioactivity of titanium. The bioactivity of the rutile films on titanium was related not only to their surface basic hydroxyl groups, but also to acidic hydroxyl groups, and surface energy. Heat-treatment endowed titanium with bioactivity by increasing the amount of surface hydroxyl groups on titanium and its surface energy.     

Broad band optical characterization of sol-gel TiO2 thin film microstructure evolution with temperature

Titanium dioxide (TiO₂) thin films have been produced by spin coating a titanium isopropoxide sol on silicon wafer substrates. The structural evolution of the thin films in terms of decomposition, crystallization and densification has been monitored as a function of annealing temperature from 100 to 700 °C using optical characterization and other techniques. The effect of annealing temperature on the refractive index and extinction coefficient of these TiO₂ thin films was studied in the range of 0.62 to 4.96 eV photon energy (250-2000 nm wavelength) using spectroscopic ellipsometry. Thermal gravimetric analysis and atomic force microscopy support the ellipsometry data and provide information about structural transformations in the titania thin films with respect to different annealing temperatures. These data help construct a coherent picture of the decomposition of the sol-gel precursors and the creation of dense layers of TiO₂. It was observed that the refractive index increased from 2.02 to 2.45 at 2.48 eV (500 nm) in sol-gel spin coated titania films for annealing temperatures from 100 °C to 700 °C.     

Preparation of nanoporous TiO2 film with large surface area using aqueous sol with trehalose

The precursor solutions containing TiO₂ sol for nanoporous films were prepared by hydrolyzing titanium(IV) isopropoxide and adding trehalose dihydrate. The porous and thick TiO₂ film was prepared by dip-coating technique on glass substrate and heating at 500 °C. The maximum thickness of the film prepared by one-run dip-coating was ca. 740 nm. The film was composed of nanosized particles (10–20 nm) and pores (7 nm). The specific surface area and porosity of the film were 163 m²/g and 65%, respectively.     

Dip-coating of TiO2 films using a sol derived from Ti(O-i-Pr)4-diethanolamine-H2O-i-PrOH system

Diethanolamine (DEA) can suppress the precipitation of oxides from the alcoholic titanium isopropoxide solution in its hydrolysis so that much water can be added to the Ti(O-i-Pr)₄-diethanolamine-i-propanol the solution to give a clear solution. Addition of excess water converted the solution to the gel. The conditions for the formation of the clear solution and gel are examined. Uniform transparent TiO₂ films can be prepared by dip-coating with the clear solution. The limit of the thickness of uniform films was 200 to 240 nm. Films thicker than 1 m can be prepared by repeating the coating cycle. These films were well densified. Diethanolamine has some positive effect on the densification of the TiO₂ crystals.     

Poly(methyl methacrylate)–titania hybrid materials by sol–gel processing

Sol–gel derived poly(methyl methacrylate)–titania hybrid materials were synthesized by using acrylic acid or allyl acetylacetone (3-allyl-2,4-pentanedione) as coupling agent. Titanium butoxide modified with acrylic acid (or titanium isopropoxide modified with allyl acetylacetone) was hydrolysed to produce a titania network, and then poly(methyl methacrylate) (PMMA) chains, formed in situ through a radical polymerization, were chemically bonded to the forming titania network to synthesize a hybrid material. Transparent hybrid materials with different contents of titania were achieved. With increase of the titania content, the colours of the products changed from yellow to dark red. The synthesis process was investigated step by step by using Fourier transform–infrared spectroscopy, and the experimental results demonstrated that acrylate or acetylacetonato groups bound to titanium remain in the final hybrid materials. The thermal stability of the hybrid materials was considerably improved relative to pure PMMA. Field emission scanning electron microscopy analyses showed the hybrid materials are porous and pore diameters vary from 10–100 nm. The hybrid materials using allyl acetylacetone as coupling agent exhibited a thermochromic effect that neither pure PMMA nor titania exhibit. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electronic characterization and photocatalytic properties of TiO<Subscript>2</Subscript>/CdO electrospun nanofibers

Titanium dioxide-cadmium oxide (TiO₂/CdO) nanofibers were prepared by the electrospinning technique followed by a single-step calcination from a solution of titanium isopropoxide and cadmium acetate dihydrate. Scanning electron microscopy, transmission electron microscopy, and the Brunauer–Emmett–Teller technique were employed to characterize the as-spun nanofibers as well as the calcined product. The specific surface area of the calcined product was calculated to be 65.3067 m² g⁻¹. X-ray powder diffractometry analysis was conducted on the samples to study their chemical composition as well as their crystallographic structure. The results obtained indicated that the prepared nanostructure product can eliminate all of the methyl orange dye within about 75 min, whereas the pristine titanium dioxide nanofibers could not eliminate more than 50% even after 180 min.     

Photocatalytic activity of titania fibers obtained by electrospinning

Composite fibers of polyvinyl pyrrolidone and titanium isopropoxide were prepared in this study using the electrospinning technique. Titanium oxide fibers were obtained after subsequent heat treatment at high temperatures. The photocatalytic activity of these fibers is reported herein. The fibers were characterized using the BET model, thermogravimetry, X-ray diffraction, and scanning electron microscopy. The photocatalytic activity of the titania fibers was investigated using ultraviolet-visible absorbance by following the photooxidative decomposition of methylene blue in comparison to the reference TiO₂ powder Degussa P25. The temperature of heat treatment, the crystalline structure, and the surface area affected the physical and chemical properties of the as-synthesized titania fibers. Increasing the temperature of heat treatment resulted in a decrease in both the fraction of anatase phase and the surface area, thereby leading to reduced photocatalytic activity.     

Ti-O/Ti-N复合薄膜的离子束合成及人工心脏瓣膜材料表面改性研究

采用离子束增强沉积（ＩＢＥＤ）等方法在热解碳、钛及钴合金等人工心脏瓣膜材料表面制备Ｔｉ－Ｏ、Ｔｉ－Ｎ及其复合薄膜。对薄膜的成分、结构进行了研究，测定了材料的电阻率，对薄膜材料的血液相容性和力学性能进行了系统的研究。结果表明：合成薄膜具有优于热解碳的血液相容性和力学性能，提出了材料的血液相容性机理模型。     

利用前后处理技术改进钛／硅基板上的类金刚石场发射特性

采用微波等离子体化学气相沉积系统存钛/硅基板上沉积类金刚石薄膜,并利用拉曼光谱仪、扫瞄式电子显微镜及原子力显微镜研究了氢等离子体前处理及快速退火后处理对类金刚石薄膜场发射特性之影响.在沉积类金刚石薄膜之前,钛/硅基板使用了两种前处理技术:第一种为研磨金刚石粉末,第二种为研磨金刚石粉末后外加氢等离子体刻蚀处理.成长类金刚石薄膜后进行快速退火处理.发现不论是氢等离子体前处理还是快速退火后处理皆能改善场发射特性,其中经退火后处理的场发射特性比氢等离子体前处理的场发射特性改善更明显.其因之一在于快速退火后处理可在类金刚石薄膜表而形成sp2丛聚,提供了很多的场发射子,也同时增加了表面粗糙度;另一个原因可能是在快速退火后处理期间会使类金刚石薄膜进一步石墨化,因而提供了许多电子在通过类金刚石薄膜时的传输路径.研究结果表明:利用适当的前后处理技术可改进类金刚石薄膜的场发射特性,进而做为冷阴极材料之应用.