Fabrication of TiO2 nanotubes by anodization of Ti thin films for VOC sensing

Ti thin films were anodized in aqueous HF (0.5 wt.%) and in polar organic (0.5 wt.% NH₄F + ethylene glycol) electrolytes to form TiO₂ nanotube arrays. Ti thin films were deposited on microscope glass substrates and then anodized. Anodization was performed at potentials ranging from 5 V to 20 V for the aqueous HF and from 20 V to 60 V for the polar organic electrolytes over the temperatures range from 0 to 20 °C. The TiO₂ nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). It has been observed that anodization of the deposited Ti thin films with aqueous HF solution at 0 °C resulted in nanotube-type structures with diameters in the range of 30-80 nm for an applied voltage of 10 V. In addition, the nanotube-type structure is observed for polar organic electrolyte at room temperature at the anodization voltage higher than 40 V. The volatile organic compound (VOC) sensing properties of TiO₂ nanotubes fabricated using different electrolytes were investigated at 200 °C. The maximum sensor response is obtained for carbon tetrachloride. The sensor response is dependent on porosity of TiO₂. The highest sensor response is observed for TiO₂ nanotubes which are synthesized using aqueous HF electrolyte and have very high porosity.     

Influence of effective surface area on gas sensing properties of WO3 sputtered thin films

WO₃ thin films having different effective surface areas were deposited under various discharge gas pressures at room temperature by using reactive magnetron sputtering. The microstructure of WO₃ thin films was investigated by X-ray diffraction, scanning electron microscopy, and by the measurement of physical adsorption isotherms. The effective surface area and pore volume of WO₃ thin films increase with increasing discharge gas pressure from 0.4 to 12 Pa. Gas sensors based on WO₃ thin films show reversible response to NO₂ gas and H₂ gas at an operating temperature of 50-300 °C. The peak sensitivity is found at 200 °C for NO₂ gas and the peak sensitivity appears at 300 °C for H₂ gas. For both kinds of detected gases, the sensor sensitivity increases linearly with an increase of effective surface area of WO₃ thin films. The results demonstrate the importance of achieving high effective surface area on improving the gas sensing performance.     

TiO2-SnO2 system for gas sensing—Photodegradation of organic contaminants

Catalytic photodegradation of organic contaminants by means of UV light has been demonstrated for gas sensors based on composites of TiO₂-SnO₂. Thin film resistive-type gas sensors of TiO₂-SnO₂ have been deposited at 350 °C by RF sputtering from a Ti-SnO₂ target with varying surface ratio of SnO₂/Ti. Photodegradation experiments of bromothymol blue by UV light have been performed by means of the optical spectrophotometry over the wavelength range extending from 300 nm to 600 nm. The influence of the UV illumination on the sensor response to 100-20,000 ppm of H₂ has been investigated in situ on motor oil contaminated sensors. It has been found that sequential exposures to UV light lead to a partial recovery of the sensor signal to hydrogen.     

Field emission from the structure of well-aligned TiO2/Ti nanotube arrays

Well-aligned TiO₂/Ti nanotube arrays were synthesized by anodic oxidation of titanium foil in 0.5 wt.% HF in various anodization voltages. The images of filed emission scanning electron microscopy indicate that the nanotubes structure parameters, such as diameter, wall thickness and density, can be controlled by adjusting the anodization voltage. The peaks at 25.3° and 48.0° of X-ray diffraction pattern illuminate that the TiO₂ nanotube arrays annealed at 500 °C are mainly in anatase phase. The filed emission (FE) properties of the samples were investigated. A turn-on electric field 7.8 V/µm, a field enhancement factors approximately 870 and a highest FE current density 3.4 mA/cm² were obtained. The emission current (2.3 mA/cm² at 18.8 V/µm) was quite stable within 480 min. The results show that the FE properties of TiO₂/Ti have much relation to the structure parameters.     

Nanoporous titanium oxide synthesized from anodized Filtered Cathodic Vacuum Arc Ti thin films

This paper describes the formation of self-organized nanopores in thin films of titanium prepared using a Filtered Cathodic Vacuum Arc (FCVA) deposition system. The post-deposition anodization was performed using 0.5% (wt) NH₄F in ethylene glycol and an aqueous based solution containing 0.5% (wt) NH₄F and 1 M (NH₄)₂SO₄ electrolytes. Homogenously distributed nanopores with dimensions in the range of 10 to 20 nm were obtained. Nanoporous TiO₂ thin films were obtained after annealing the anodized samples at 600 °C for 4 h. Scanning electron microscopy (SEM) and Raman spectroscopy were used to characterize these nanoporous films. Raman measurements revealed that the rutile TiO₂ polymorph dominates these structures along with imperfect titanium oxidation resulting in the formation of defect structures, particularly when aqueous electrolyte was used for the anodization.     

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.     

Influence of metal plasma ion implantation on photo-sensitivity of anatase TiO2 thin films

Nano-scale TiO₂ thin films were synthesized by using sol-gel and spin-coating techniques on glass substrates for photo-catalytic applications. The Ti(IV) butoxide-based TiO₂ thin films were optimized for transforming into the high-purity crystalline anatase phase when calcined at 500 °C. To further enhance the photo-catalysis sensitivity of TiO₂ thin films for use in visible light environments, a metal plasma ion implantation process was implemented to modify the band gap electron configuration of Ti. Various transition metal atoms such as Ni, Cu, V, and Fe were ionized and accelerated at 20 keV to impinge on the surface of TiO₂ substrates at a dosage of 5 × 10¹⁵ ions/cm². ESCA analysis confirmed the binding energy shift of Ti by 0.8-1.2 eV, which accounted for the increased effective positive charge of Ti, resulting in more effective electron trapping capability and, thus, the electron-hole pair separation. In addition, the absorption spectroscopy demonstrated that optical absorption in the visible light regime occurred in specimens implanted with transition metal ions, likely due to the formation of extra impurity energy levels within the original TiO₂ band gap energy structure. Among all tested implant materials, the band gap energy of TiO₂ was effectively reduced by Cu and Fe ion implantation by 0.9-1.0 eV, which was sufficient enough to excite valence electrons over the band gap in visible light environments. The feasibility of the metal-doped TiO₂ thin films for effective applications under visible light irradiation was further confirmed by using super-hydrophilicity contact-angle measurement.     

Phase transformation in semi-transparent TiO2 films irradiated with CO2 laser

Anatase (TiO₂) thin films were obtained by immersion of glass plates into a titanium sol-gel precursor followed by calcination at 450 °C for 3 h. The Raman results for the CO₂ laser irradiated TiO₂ films show that laser radiation is able to promote favorable changes of anatase phase in anatase/rutile mixtures. Nevertheless, the transformation process level depends on laser characteristics and scan speed of the radiation treatment.     

Oxygen plasma induced hydrophilicity of TiO2 thin films

Anatase nano-TiO₂ thin films were fabricated by reactive magnetron sputtering metal Ti target followed by thermal annealing in air at 450 °C for 2 hrs. The crystalline structure of the sample films were characterized by X-ray diffraction (XRD) and the hydrophilicity was characterized with the diameters of 1 μl water drop. The films were irradiated by oxygen plasmas and the effects of the radio frequency (rf) power, the gas pressure and the irradiation time of the oxygen plasmas on the hydrophilicity of the TiO₂ thin films were investigated. Hydrophilicity can be induced by oxygen plasmas and further more the hydrophilicity shows high stability whenever under the natural light or in dark.     

Nanocrystalline chemically modified CdIn2O4 thick films for H2S gas sensor

CdIn₂O₄ sensor with high sensitivity and excellent selectivity for H₂S gas was synthesized by using sol-gel technique. Structural, electrical and gas sensing properties of doped and undoped CdIn₂O₄ thick films were studied. XRD revealed the single-phase polycrystalline nature of the synthesized CdIn₂O₄ nanomaterials. Since the resistance change of a sensing material is the measure of its response, selectivity and sensitivity was found to be enhanced by doping different concentrations of cobalt in CdIn₂O₄ thick films. The sensor exhibits high response and selectivity toward H₂S for 10 wt.% Co doped CdIn₂O₄ thick films. The current-voltage characteristics of 10 wt.% Co doped CdIn₂O₄ calcined at 650 °C shows one order increase in current with change in the bias voltage at an operating temperature of 200 °C for 1000 ppm H₂S gas.     

The gas sensitivity of nano TiO2-SnO2 film doped with Cd(II) ion

The nanocomposite oxide (0.2TiO₂-0.8SnO₂) doped with Cd²⁺ powder have been prepared and characterized by XRD and their gas-sensing sensitivity were characterized using gas sensing measurement. Experimental results show that, bicomponent nano anatase TiO₂ and rutile SnO₂ particulate thick film doped with Cd²⁺ behaves with good sensitivity to formaldehyde gas of 200 ppm in the air, and the optimum sensing temperature was reduced from 360 °C to 320 °C compared with the undoped Cd²⁺ thick film. The gas sensing thick films doped with Cd²⁺ also show good selectivity to formaldehyde among benzene, toluene, xylene and ammonia as disturbed gas and could be effectively used as an indoor formaldehyde sensor.     

Synthesis and characterization of CdO-doped nanocrystalline ZnO:TiO2-based H2S gas sensor

This paper reports the preparation and gas-sensing characteristic of ZnO:TiO₂-based hydrogen sulfide (H₂S) gas sensor with different mol% of CdO by polymerized complex method. The structural and gas-sensing properties of ZnO:TiO₂ materials have been characterized using X-ray diffraction and gas-sensing measurement. The electrical resistance response of the sensor based on the materials was investigated at different operating temperatures and different gas concentrations. The sensor with 10 mol% CdO-doped ZnO:TiO₂ shows excellent electrical resistance response toward H₂S gas. The cross sensitivity was also checked for reducing gases like CH₄, CO and H₂ gas. The selectivity and sensitivity of ZnO:TiO₂-based H₂S gas sensor were improved by the addition of 10 mol% of CdO at an operating temperature of 250 °C.     

Fast response detection of H2S by CuO-doped SnO2 films prepared by electrodeposition and oxidization at low temperature

Fast response detection of H₂S by CuO-doped SnO₂ films prepared was prepared by a simple two-step process: electrodeposition from aqueous solutions of SnCl₂ and CuCl₂, and oxidization at 600 °C. The phase constitution and morphology of the CuO-doped SnO₂ films were characterized by X-ray diffraction and scanning electron microscopy. In all cases, a polycrystalline porous film of SnO₂ was the product, with the CuO deposited on the individual SnO₂ particles. Two types of CuO-doped SnO₂ films with different microstructures were obtained via control of oxidation time: nanosized CuO dotted island doped SnO₂ and ultra-uniform, porous, and thin CuO film coated SnO₂. The sensor response of the CuO doped SnO₂ films to H₂S gas at 50–300 ppm was investigated within the temperature range of 25–125 °C. Both of the CuO-doped SnO₂ films show fast response and recovery properties. The response time of the ultra-uniform, porous, and thin CuO coated SnO₂ to H₂S gas at 50 ppm was 34 s at 100 °C, and its corresponding recovery time was about 1/3 of the response time.     

Photocatalytic properties of porous TiO2/Ag thin films

In this study, nanocrystalline TiO₂/Ag composite thin films were prepared by a sol-gel spin-coating technique. By introducing polystyrene (PS) spheres into the precursor solution, porous TiO₂/Ag thin films were prepared after calcination at a temperature of 500 °C for 4 h. Three different sizes (50, 200, and 400 nm) of PS spheres were used to prepare porous TiO₂ films. The as-prepared TiO₂ and TiO₂/Ag thin films were characterized by X-ray diffractometry (XRD) and by scanning electron microscopy to reveal structural and morphological differences. In addition, the photocatalytic properties of these films were investigated by degrading methylene blue under UV irradiation.When PS spheres of different sizes were introduced after calcination, the as-prepared TiO₂ films exhibited different porous structures. XRD results showed that all TiO₂/Ag films exhibited a major anatase phase. The photodegradation of porous TiO₂ thin films prepared with 200 nm PS spheres and doped with 1 mol% Ag exhibited the best photocatalytic efficiency where ∼ 100% methylene blue was decomposed within 8 h under UV exposure.     

Synthesis and characterization of beta barium borate thin films obtained from the BaO-B2O3-TiO2 ternary system

This work reports on the synthesis and the structural and optical characterization of beta barium borate (β-BBO) thin films containing 4, 8 and 16 mol% of titanium oxide (TiO₂) deposited on fused silica and silicon (0 0 1) substrates using the polymeric precursor method. The thin films were characterized by X-ray diffraction, Raman spectroscopy, atomic force microscopy and scanning electron microscopy techniques. The optical transmission spectra of the thin films were measured over a wavelength range of 800-200 nm. A decrease was observed in the band gap energy as the TiO₂ content was raised to 16 mol%. Only the β-BBO phase with a preferential orientation in the (0 0 l) direction was obtained in the sample containing 4 mol% of TiO₂ and crystallized at 650 °C for 2 h.     

Preparation of anatase TiO2 thin films for dye-sensitized solar cells by DC reactive magnetron sputtering technique

Anatase titanium dioxide (TiO₂) thin films are prepared by DC reactive magnetron sputtering using Ti target as the source material. In this work argon and oxygen are used as sputtering and reactive gas respectively. DC power is used at 100 W per 1 h. The distance between the target and substrate is fixed at 4 cm. The glass substrate temperature value varies from room temperature to 400 °C. The crystalline structure of the films is determined by X-ray diffraction analysis. All the films deposited at temperatures lower than 300 °C were amorphous, whereas films obtained at higher temperature grew in crystalline anatase phase. Phase transition from amorphous to anatase is observed at 400 °C annealing temperature. Transmittances of the TiO₂ thin films were measured using UV-visible NIR spectrophotometer. The direct and indirect optical band gap for room temperature and substrate temperature at 400 °C is found to be 3.50, 3.41 eV and 3.50, 3.54 eV respectively. The transmittance of TiO₂ thin films is noted higher than 75%. A comparison among all the films obtained at room temperature showed a transmittance value higher for films obtained at substrate temperature of 400 °C. The morphology of the films and the identification of the surface chemical stoichiometry of the deposited film at 400 °C were studied respectively, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The surface roughness and the grain size are measured using AFM.     

Mesoporous TiO2 thin films embedded with Au nanoparticles for the enhancement of the photocatalytic properties

Mesoporous TiO₂ thin films were prepared by hydrothermal-oxidation of titanium metal thin films, which were obtained by DC magnetron sputtering technique. Gold nanoparticles, which were prepared by reduction of HAuCl₄, were embedded into the holes of the mesoporous TiO₂ films by capillary method followed by annealing in air up to 400 °C. The size of pore of TiO₂ films is about 100 nm and that of Au nanoparticles is about 10 nm in average. The morphology of the films was analyzed by field emission scanning electron microscope (FE-SEM) and scanning probe microscopes (SPMs). Subsequently, the photocatalytic performances of the obtained nanosystems in the decomposition of methylene blue solution are discussed. The obtained results show that the dispersion of Au nanoparticles on the mesoporous TiO2 matrix will help enhancing the photocatalytic activity with respect to pure TiO₂ under visible light irradiation.     

Structural, thermal and optical characterization of TiO2:ZrO2 thin films prepared by sol-gel method

We have studied the structural and optical properties of thin films of TiO₂, doped with 5% ZrO₂ and deposited on glass substrate (by the sol-gel method). The dip-coated thin films have been examined at different annealing temperatures (350 to 450 °C) and for various layer thicknesses (63-286 nm). Refractive index and porosity were calculated from the measured transmittance spectrum. The values of the index of refraction are in the range of 1.62-2.29 and the porosity is in the range of 0.21-0.70. The coefficient of transmission varies from 50 to 90%. In the case of the powder of TiO₂, doped with 5% ZrO₂, and aged for 3 months in ambient temperature, we have noticed the formation of the anatase phase (tetragonal structure with 14.8 nm grains). However, the undoped TiO₂ exhibits an amorphous phase. After heat treatments of thin films, titanium oxide starts to crystallize at the annealing temperature 350 °C. The obtained structures are anatase and brookite. The calculated grain size, depending on the annealing temperature and the layer thickness, is in the range (8.58-20.56 nm).     

Influence of annealing treatments for ZnO-doped Zr0.8Sn0.2TiO4 thin films by RF magnetron sputtering

Zirconium tin titanium oxide doped 1 wt.% ZnO thin films on n-type Si substrate were deposited by rf magnetron sputtering at a fixed rf power of 300 W, a substrate temperature of 450 °C, a deposition pressure of 5 mTorr and an Ar/O₂ ratio of 100/0 with various annealing temperatures and annealing times. Electrical properties and microstructures of 1 wt.% ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films prepared by rf magnetron sputtering on n-type Si(100) substrates at different annealing temperatures (500 °C-700 °C) and annealing times (2 h-6 h) have been investigated. The structural and morphological characteristics analyzed by X-ray diffraction (XRD) and atomic force microscope (AFM) were sensitive to the treatment conditions such as annealing temperature and annealing time. At an annealing temperature of 600 °C and an annealing time of 6 h, the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films possess a dielectric constant of 46 (at f = 10 MHz), a dissipation factor of 0.059 (at f = 10 MHz), and a low leakage current density of 3.8 × 10^− 9 A/cm² at an electrical field of 1 kV/cm.     

Natural and persistent superhydrophilicity of SiO2/TiO2 and TiO2/SiO2 bi-layer films

Sol-gel SiO₂/TiO₂ and TiO₂/SiO₂ bi-layer films have been deposited from a polymeric SiO₂ solution and either a polymeric TiO₂ mother solution (MS) or a derived TiO₂ crystalline suspension (CS). The chemical and structural properties of MS and CS bi-layer films heat-treated at 500 °C have been investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscospy. Water contact angle measurements show that MS SiO₂/TiO₂ and CS TiO₂/SiO₂ bi-layer films exhibit a natural superhydrophilicity, but cannot maintain a zero contact angle for a long time over film aging. In contrast, CS SiO₂/TiO₂ bi-layer films exhibit a natural, persistent, and regenerable superhydrophilicity without the need of UV light. Superhydrophilic properties of bi-layer films are discussed with respect to the nature of the TiO₂ single-layer component and arrangement of the bi-layer structure, i.e. TiO₂ underlayer or overlayer.