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.     

Nanoporous TiO2 thin film based conductometric H2 sensor

Nanoporous titanium dioxide (TiO₂) based conductometric sensors have been fabricated and their sensitivity to hydrogen (H₂) gas has been investigated. A filtered cathodic vacuum arc (FCVA) system was used to deposit ultra-smooth Ti thin films on a transducer having patterned inter-digital gold electrodes (IDTs). Nanoporous TiO₂ films were obtained by anodization of the titanium (Ti) thin films using a neutral 0.5% (wt) NH₄F in ethylene glycol solution at 5 V for 1 h. After anodization, the films were annealed at 600 °C for 8 h to convert the remaining Ti into TiO₂. The scanning electron microscopy (SEM) images revealed that the average diameters of the nanopores are in the range of 20 to 25 nm. The sensor was exposed to different concentrations of H₂ in synthetic air at operating temperatures between 100 °C and 300 °C. The sensor responded with a highest sensitivity of 1.24 to 1% of H₂ gas at 225 °C.     

Photoactivity of anatase–rutile TiO2 nanotubes formed by anodization method

Titania (TiO₂) nanotubes were prepared by anodizing titanium (Ti) foils in an electrochemical bath consisting of 1 M glycerol with 0.5 wt.% NH₄F.The pH of the bath was kept constant at 6 and the anodization voltage was varied from 5 V, 20 V to 30 V. It is found that the morphology of the anodized titanium is a function of anodization voltage with pits-like oxide formed for the sample made at 5 V and samples made at 20 V and 30 V consisted of well-aligned nanotubes growing perpendicularly on the titanium foil. However, the nanotubes formed on the samples made at 30 V were not uniform in terms of the nanotubes' diameter and wall thickness. Regardless of the anodization voltage, as anodised samples were amorphous. The crystal structure evolution was studied as a function of annealing temperatures and was characterised by X-ray diffraction and Raman spectroscopy analyses. Crystallization of the nanotubes to anatase phase occurred at 400 °C while rutile formation occurred at 700 °C. Disintegration of the nanotube arrays was observed at 600 °C and the structure completely vanished at 700 °C. TiO₂ nanotube annealed at 400 °C and containing 100% anatase revealed the highest photocatalytic activity for the degradation of methyl orange. Consequently, these results indicate that diameter, wall thickness, crystal structure and degree of crystallinity of the TiO₂ nanotube arrays are the important factors influencing the efficiency of the photocatalytic activity.     

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.     

Untersuchungen an Ta2N-oxid-schichten

Reactively sputtered Ta₂N films have been oxidized anodically and basic data were measured. The density of the Ta₂O₅N turned out to be 6.1 g/cm³. A Ta₂N thickness anodization voltage ratio of 0.43 nm/V was determined which corresponds to an oxide build-up of 1.40 nm/V. From the measured anodization rate, i.e. the rise in voltage per unit of time at constant current, of 0.54 V/s (j = 1 mA/cm²) and stoichiometric considerations it is concluded, that the nitrogen is not bonded chemically in Ta₂N and Ta₂O₅N.     

Solar cells with Sb2S3 absorber films

CdS/Sb₂S₃/PbS structures were prepared by sequential chemical deposition of CdS, Sb₂S₃ and PbS thin films on TEC-8 (Pilkington) transparent electrically conductive SnO₂ (TCO) coatings. CdS thin films (100 nm) were deposited with hexagonal structure from Cd-citrate bath and of cubic structure from Cd-ammine/triethanolamine bath. Sb₂S₃ thin films were deposited at 40 °C from a solution mixture of potassium antimony tartrate, triethanolamine, ammonia and thioacetamide(TA) or at 1 to 10 °C from a mixture of antimony trichloride and thiosulfate (TS). These films were made photoconductive by heating at temperatures 250 to 300 °C. When heated in the presence of a chemically deposited Se thin film of 300 nm, a solid solution Sb₂S_1.8Se_1.2 resulted. PbS thin films of 100-200 nm thickness were deposited on the TCO/CdS/Sb₂S₃ or TCO/CdS/Sb₂S_1.8Se_1.2 structure. Graphite paint was applied on the PbS film prior to applying a silver epoxy paint. The cell structures were of area 0.4 cm². The best results reported here is for a cell: TCO/CdS(hex-100 nm)/Sb₂S₃(TS-100 nm)/PbS(200 nm) with open circuit voltage (V_oc) 640 mV, short circuit current density 3.73 mA/cm², fill factor 0.29, and conversion efficiency 0.7% under 1000 Wm^− 2 sunlight. Four series-connected cells of area 1 cm² each gave V_oc of 2 V and short circuit current of 1.15 mA.     

Fabrication of TiO2 nanotube arrays by electrochemical anodization in an NH4F/H3PO4 electrolyte

Highly ordered TiO₂ nanotube arrays were fabricated by electrochemical anodization of titanium in an NH₄F/H₃PO₄ electrolyte. A TiO₂ crystal phase was identified by X-ray diffraction, and the morphology, length and pore diameter of the TiO₂ nanotube arrays were determined by field-emission scanning electron microscopy (FE-SEM). The anodization parameters including the rate of magnetic stirring, F⁻ concentration, calcination temperature, anodization voltage and anodization time were investigated in detail. The results show that the as-prepared TiO₂ nanotube arrays possessed good uniformity, a well-aligned morphology with a length of 750 nm and an average pore diameter of 62 nm at a 150 rpm rate of magnetic stirring for 120 min at 20 V in an electrolyte mixture of 0.2 M H₃PO₄ and 0.3 M NH₄F with a 500 °C calcination to obtain 100% anatase phase. The adsorption of N-719 dye at different tube lengths was determined by UV-vis analysis and found to increase with increasing tube length. We also discuss the formation mechanism of the TiO₂ nanotube arrays. The findings indicate that the formation of the TiO₂ nanotube arrays proceeds by the combined action of the electrochemical etching and chemical dissolution.     

Synthesis and characterization of P-doped TiO2 nanotubes

Titanium dioxide (TiO₂) doped with phosphorus (P) was synthesized by anodization of Ti in the mixed acid electrolyte of H₃PO₄ and HF and characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis spectrum. The morphology greatly depends on the applied voltage. The as-formed nanotubes under the optimized condition, at 20 V for 2 h, are highly ordered with ~ 200 nm in length and the average tube diameter is about 100 nm. By annealing the initial samples at different temperatures, the importance of the crystalline nature is confirmed. Significantly, the peak positions of anatase in XRD patterns shifts to lower diffraction angles with an increase in the amount of H₃PO₄ ion. A remarkable red shift of the absorption edge has been observed for the sample formed in the electrolyte of HF and H₃PO₄, which is related to the introduction of P⁵⁺ into TiO₂ crystallization and the possible impurity energy level formed in the TiO₂ band gap. The presence of P 2p state in XPS spectrum can further confirm the P⁵⁺ which can replace a part of Ti⁴⁺ has been introduced into TiO₂ crystallization. The present convenient synthesis technique can be considered to the composition of other doped oxide materials.     

Fabrication of 10 nm diameter TiO2 nanotube arrays by titanium anodization

We report the fabrication of TiO₂ nanotube thin films using anodization method with HCl electrolyte and copper cathode. The process represents an alternative electrochemical approach using a non-noble metal cathode along with a safer electrolyte. In addition to the choice of electrolyte, the electrolyte concentration, anodization voltage, and anodization time all affect nanotube morphology. TiO₂ nanotubes with diameters as small as 10 nm were achieved.     

Voltage tunable Ba0.6Sr0.4TiO3 thin films and coplanar phase shifters

Not only the ferroelectric properties of Ba_0.6Sr_0.4TiO₃ thin films but also the design, fabrication and microwave properties of coplanar waveguide (CPW) phase shifter were investigated. When the applied voltage changes from zero to 20 V range at a frequency of 1 kHz, the dielectric constant, tunability, remanent polarization (2P_r) and coercive electric field (Ec) of the BST films are 1266, 29.5%, 2.29 μC/cm² and 22.27 kV/cm, respectively. The structure of CPW phase shifter was designed by ANSOFT High Frequency Structure Simulator (HFSS) and Agilent Advanced Design System (ADS). The designed phase shifter consists of 56 same sections. It is observed from the photograph of the fabricated device that the lines of electrodes are regular and the widths of bottom electrodes overlapped by top electrodes are about 5 μm. It was found that the phase shift gradually increases as the voltage increases from 0 V to 20 V, which may be due to the tunability of BST films varies with external dc field. About 168° phase shift was achieved at 28 GHz with a bias voltage of 20 V.     

Microstructure, optical and optoelectrical properties of mesoporous nc-TiO2 films by hydrolysis-limited sol-gel process with different inhibitors

Mesoporous nanocrystalline titania (nc-TiO₂) films were synthesized by sol-gel route using poly (alkylene oxide) block copolymer as template and tetrabutyl orthotitanate as titanium source. Influence of acetylacetone (AcAc), glacial acetic acid, HCl and AcAc-HCl as inhibitors on hydrolysis/condensation reaction was investigated. The mesopore structure, crystalline phase and optical characteristics were analyzed by N₂ adsorption, X-ray diffraction, transmission electron microscopy and UV-VIS-NIR spectrum. Photoaction measurements were performed on dye-sensitized solar cells (DSSCs) using the mesoporous nc-TiO₂ as photoanodes. The results show that the formed films are anatase phase and mesoporous structure. Different restraining mechanisms of these inhibitors to hydrolysis/condensation process lead to different mesoporous structure whose specific surface area (S_BET) varies from 34 to 176 m²/g, and crystal size is from 5.6 to 38.2 nm. The mesoporous photoanode with high S_BET and large crystalline grains may be responsible for increasing conversion efficiency of the DSSCs.     

Titanium oxide films produced on commercially pure titanium by anodic oxidation with different voltages

Titanium oxide films produced on commercially pure Ti by anodic oxidation with different voltages were analyzed. Anodic oxidation was carried out at room temperature using 1.4 M H₃PO₄ electrolyte and a platinum counter-electrode, in potentiostatic mode under the following conditions: 50 V, 100 V, 150 V, 200 V and 250 V. It was observed that porous titanium layers were formed at all voltage values but morphological differences were observed. Initially, the film was thin but with increasing voltage it broke down locally and porous regions became evident due to the dielectric breakdown. The porosity and the pore size increased with the increasing voltage. The surface morphology in samples formed with 200 V had substantially different porous structures than those formed with other voltage values. The anodic film surface displayed pores and craters formed on the relatively flat ground oxide surface. AFM images showed that higher voltages produced thicker titanium oxide films.     

Titanium-oxide interface structures formed by degassing and anodization processes

High-resolution electron microscopy was used to investigate two types of titanium-oxide interface structures. The first type was generated by thermal oxidation during the degassing process, which is one step in the process of porcelain-fused-to-metal systems. The thermal oxidation was performed for 1 min at a temperature of 1073 K in a porcelain furnace under a reduced pressure at 27 hPa. Columnar and granular rutile oxide formed on the titanium, and the surface oxide layer was almost 1 μ m thick. On an atomic scale, the crystal size of the rutile adjacent to the interface decreased about 10 nm. In addition, a very thin transitional layer 2–3 nm thick formed at the titanium-oxide interface. The crystal structure of the thin layer seemed to be the T{i}O phase with a NaCl-type structure. The interface between the hcp titanium and T{i}O phases was coherent through the close-packed planes ((0 0 0 2)_hcp and (1 1 1)_TiO). Partial coherency was observed at the interface between the T{i}O and T{i}O₂ phases. The second type of titanium-oxide interface was generated by anodization on a screw-type titanium implant. The morphology of the surface suggested that the titanium implant had been treated by spark anodization. The surface oxide, which was estimated to be about 10 μ m thick, was a mixture of the anatase-type T{i}O₂ phase and the amorphous phase. The crystal size of the anatase varied from less than 10 nm to more than 100 nm. A phosphate anion was concentrated in the amorphous phase. Between the surface oxide and the titanium base, macroscopic defects like cracks and voids were observed. Microscopic observation could not confirm the formation of a clear interface and lattice coherency between the titanium and oxide as a result of the degassing process.     

Microstructure control of (Pb,Sr)TiO3 films on Pt/Ti/SiO2/Si substrates by a TiO2 buffer layer

A thin TiO₂ buffer layer was used to control the microstructure and electrical properties of the polycrystalline (Pb,Sr)TiO₃ (PST) films produced by a Sol-Gel method on Pt(111)/Ti/SiO₂/Si(100) substrates. The PST films included (Pb_0.6Sr_0.4)TiO₃ (PST40) and (Pb_0.4Sr_0.6)TiO₃ (PST60). It was found that a crystallized TiO₂ buffer layer with a thickness of nearly 5 nm was critical for improving the crystallinity and surface morphology of both the thinner (about 40 nm) and thicker (about 330 nm) PST films, which exhibited a (l00) preferred orientation and much smoother surface comparing with those without the buffer layer. The electrical properties of the PST films having TiO₂ buffer layer were also improved. For 330-nm-thick PST40 films, the dielectric constant and its tunability by dc voltage were increased from 482 and 26.8% at 10 kHz to 590 and 51.2%, while the loss and leakage current density were reduced from 0.04 and 4.26 × 10⁻⁴ A/cm² at 100 kV/cm to 0.034 and 7.63 × 10⁻⁶ A/cm², respectively. Similar results were also found in the PST60 films.     

Study on TiO2 thin films grown by advanced pulsed laser deposition on ITO

TiO₂ films were grown by an advanced pulsed laser deposition method (PLD) on ITO substrates to be used as functional electrodes in the manufacturing of solar cells. A pure titanium target (99.99%) was irradiated by a Nd:YAG laser (355 and 532 nm, 5 ns, 35 mJ, 3 J/cm²) in an oxygen atmosphere at different pressures (20-160 mTorr) and at room temperature. After deposition, the films were subjected to an annealing process at 350 °C. The film structure, surface morphology, thickness, roughness, and optical transmission were investigated. Regardless of the wavelength used, the films deposited at room temperature presented only Ti₂O and TiO peaks. After thermal treatment, the TiO₂ films became strongly crystalline, with a tetragonal structure and in the anatase phase; the threshold temperature value was 250 °C. The deposition rate was in the range of 0.035-0.250 nm/pulse, and the roughness was 135-305 nm. Optical transmission of the films in the visible range was between 40% and 60%.     

Structural and mechanical properties of titanium and titanium diboride monolayers and Ti/TiB2 multilayers

Nano-multilayers represent a new class of engineering materials that are made up of alternating nanometer scale layers of two different components. In the present work a titanium (Ti) monolayer was combined with titanium diboride (TiB₂) to form a Ti/TiB₂ nano-multilayer. Designed experimental parameters enabled an evaluation of the effects of direct current bias voltage (U_b) and bilayer thickness (Λ) during multilayer deposition on the mechanical properties of reactively sputtered Ti/TiB₂ multilayer films. Their nanostructures and mechanical properties were characterized and analyzed using X-ray photoelectron spectroscopy (XPS), low-angle and high-angle X-ray diffraction (XRD), plan-view and cross-sectional high-resolution transmission electron microscopy (HRTEM), and microindentation measurements. Under the optimal bias voltage of U_b = − 60 V, it was found that Λ (varied from 1.1 to 9.8 nm) was the most important factor which dominated the nanostructure and hardness. The hardness values obtained varied from 12 GPa for Ti and 15 GPa for TiB₂ monolayers, up to 33 GPa for the hardest Ti/TiB₂ multilayer at Λ = 1.9 nm. The observed hardness enhancement correlated to the layer thickness, followed a relation similar to the Hall-Petch strengthening dependence, with a generalized power of ∼ 0.6. In addition, the structural barriers between two materials (hcp Ti/amorphous TiB₂) and stress relaxation at interfaces within multilayer films resulted in a reduction of crack propagation and high-hardness.     

Dye-sensitized solar cells using TiO<Subscript>2</Subscript> nanoparticles transformed from nanotube arrays

Dye-sensitized solar cells (DSSCs) were fabricated using TiO₂ mesoporous layers obtained by very simple method—transformation of TiO₂ nanotube (NT) films grown by electrochemical oxidation to nanoparticle (NP) films. This transformation is based on thermal annealing of TiO₂ NT arrays formed by anodization of titanium foil in fluorine ambient. Performance of DSSCs fabricated with different size NPs was studied in the range from 35 to 350 nm. Highest nominal efficiency (9.05%) was achieved for DSSC with NP size 65 nm while the lowest nominal efficiency (1.48%) was observed for DSSCs with NP size 350 nm. The dependence of the solar cell parameters with NP size is discussed.     

Sputter deposition and surface treatment of TiO2 films for dye-sensitized solar cells using reactive RF plasma

Sputter deposition followed by surface treatment was studied using reactive RF plasma as a method for preparing titanium oxide (TiO₂) films on indium tin oxide (ITO) coated glass substrate for dye-sensitized solar cells (DSCs). Anatase structure TiO₂ films deposited by reactive RF magnetron sputtering under the conditions of Ar/O₂(5%) mixtures, RF power of 600 W and substrate temperature of 400 °C were surface-treated by inductive coupled plasma (ICP) with Ar/O₂ mixtures at substrate temperature of 400 °C, and thus the films were applied to the DSCs. The TiO₂ films made on these experimental bases exhibited the BET specific surface area of 95 m²/g, the pore volume of 0.3 cm²/g and the TEM particle size of ∼ 25 nm. The DSCs made of this TiO₂ material exhibited an energy conversion efficiency of about 2.25% at 100 mW/cm² light intensity.     

Binary semiconductor In2Te3 for the application of phase-change memory device

Nonvolatile phase-change memory devices with 500 nm contact hole based on In₂Te₃ were successfully fabricated by using focused ion beam, pulsed laser deposition, and dc magnetic sputtering techniques. In₂Te₃ films were characterized by using differential thermal analysis, X-ray diffraction, and UV–vis diffuse absorption spectroscopy, respectively. The devices can be switched between high and low resistance states repeatedly with the programmed voltage pulses. The reset operation (crystalline to amorphous) was done by the voltage pulse with a magnitude of 3.5 V and a duration of 30 ns, and the set operation (amorphous to crystalline) was done by the voltage pulse with a magnitude of 1.4 V and a duration of 100 ns. A dynamic resistance switching ratio (OFF/ON ratio) of 3.2 × 10³ has been obtained.     

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.