Properties of composite films of titania nanofibers and Safranin O dye

Titania (TiO₂) nanofibers and composite thin films of titania nanofibers and Safranin O dye (SAF) were studied. TiO₂ nanofibers were prepared by electrospinning technique from titanium tetra-isopropoxide precursor solution in ethanol. Surface topology of the nanofibers was observed using scanning electron microscopy (SEM), their crystal structure was studied by X-ray diffraction (XRD) and the chemical composition by X-ray photoelectron spectroscopy (XPS). Properties of the TiO₂ nanofibers were studied in dependence on the values of relative air humidity in the range from 15% to 55%. It was necessary to maintain the relative humidity lower than 30% during electrospinning in order to obtain high quality nanofiber films. The average minimum diameter of the as-prepared TiO₂ nanofibers was found to be around 100 nm. Nanofiber diameter diminishes to about 50 nm after annealing at 420 °C for 1 h. The as-prepared titania nanofiber films were completely amorphous while anatase crystal phase was detected in the films after annealing. In order to prepare the composite films, solution of SAF dye with poly(vinylpyrrolidone) in ethanol/water was dropped off on the prepared titania nanofibers surface. Opto-electrical properties of SAF dye and the resulting nanocomposite films were studied by UV–Vis spectroscopy and current–voltage characteristics. Safranin O is characterized by two strong absorption peaks; one at 274 nm and a wide band with splitting between 420 nm and 600 nm. The optical energy band gap of titania nanofibers was estimated from the UV–Vis measurements to be 3.4 eV. The charge transport in the composite films is influenced by the space charge limited currents due to the very high resistance of the materials.     

Effects of anchoring groups in multi-anchoring organic dyes with thiophene bridge for dye-sensitized solar cells

Organic photo-sensitizers were designed and synthesized based on a phenothiazine framework containing single- and double-electron acceptors that were bridged with thiophene for the dye sensitized solar cells (DSSCs). The optimized geometries were determined with density functional theory (DFT) calculations to estimate the photovoltaic properties of the dyes in the design stage. The organic dye with the double electron acceptors exhibited a better light absorption at long wavelength and an effective electron extraction pathway from the electron donor to the TiO₂ surface, leading to an improved short-circuit current (11.6 mA cm^?2), compared with that of the dye with the single electron acceptor (10.2 mA cm^?2) and the conventional N3 Ru-dye (10.4 mA cm^?2). Contrarily, the open-circuit voltage of the organic dye with the double electron acceptors decreased because the additional protonated carboxylic groups caused a positive shift in the Fermi level of TiO₂.     

Microstructural studies and electrical properties of Mg-doped SrTiO3 thin films

The structure–property relations of Mg-doped SrTiO₃ (ST) sol–gel thin films deposited on Pt/TiO₂/SiO₂/Si substrates have been investigated in order to determine the effect that Mg dopants have on the dielectric properties of SrTiO₃. It has been predicted that Mg-doped SrTiO₃ should exhibit a dielectric anomaly similar to that observed recently in Bi doped SrTiO₃ but, to date, no polar state has been reported. It has been suggested that this may relate to the low solubility of Mg on the A-site in bulk ceramics (<0.05 at.%). However, for Sr_1−xMg_xTiO₃ (SMT) (x ⩽ 0.30) films annealed at 750 °C, all Mg ions were accommodated in the perovskite lattice and for SMT films annealed at 900 °C, the solubility limit of Mg was x = 0.10, above which a Mg-rich ilmenite second phase was observed. Irrespective of the higher solid solubility limit of Mg in the ST lattice for sol–gel ST films compared to equivalent ceramics, no ferroelectric or relaxor phase transition was observed, refuting previous predictions for this dopant.     

Elaboration and characterization of nanocrystalline TiO2 thin films prepared by sol–gel dip-coating

Nanocrystalline TiO₂ thin films were deposited on a ITO coated glass substrate by sol–gel dip coating technique, the layers undergo a heat treatment at temperatures varying from 300 to 450 °C. The structural, morphological and optical characterizations of the as deposited and annealed films were carried out using X-ray diffraction (XRD), Raman spectroscopy, Atomic Force Microscopy (AFM), visible, (Fourier-Transform) infrared and ultraviolet spectroscopy, Fluorescence and spectroscopic ellipsometry. The results indicate that an anatase phase structure TiO₂ thin film with nanocrystallite size of about 15 nm can be obtained at the heat treatment temperature of 350 °C or above, that is to say, at the heat treatment temperature below 300 °C, the thin films grow in amorphous phase; while the heat treatment temperature is increased up to 400 °C or above, the thin film develops a crystalline phase corresponding to the titanium oxide anatase phase. We have accurately determined the layer thickness, refractive index and extinction coefficient of the TiO₂ thin films by the ellipsometric analysis. The optical gap decreases from 3.9 to 3.5 eV when the annealing temperature increases. Photocatalytic activity of the TiO₂ films was studied by monitoring the degradation of aqueous methylene blue under UV light irradiation and was observed that films annealed above 350 °C had good photocatalytic activity which is explained as due to the structural and morphological properties of the films.     

Dye sensitization of nanocrystalline TiO2 by perylene derivatives

New sensitizing dye–semiconductor system comprised of 3,4,9,10-perylene tetracarboxylic acid (PTCA) or perylene 3,4-dicarboxylic-9,10-(n-butylamine) carboximide (PNDCA) on titanium dioxide (TiO₂) had been prepared and characterized. Good energy conversion efficiency was obtained. The film of PTCA on TiO₂ yielded an incident photo-to-current efficiency (IPCE) of approximately 40% in the wavelength region of 440 to 530 nm, while that for PNDCA was approximately 14% at the wavelength region from 460 to 510 nm. Further studies concentrated on the doping of this system with bromine (Br₂). The results indicated that the sensitizing wavelength region was red shifted, which was advantageous for solar light harvesting. Furthermore, the doped films gave lower quantum conversion efficiency than the non-doped films.     

Preparation and characterization of Ca0.18Na0.32Bi0.50TiO3 ferroelectric thin films by metalorganic solution deposition

Ca_0.18Na_0.32Bi_0.50TiO₃ (CNBT) ferroelectric thin films were prepared by metalorganic solution deposition on silicon substrate and annealed at different temperatures. The morphology and structure of the films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The crystal structure of Ca-doped Na_0.50Bi_0.50TiO₃ films shows no obvious lattice distortion compared with that of un-doped one. The optimal heat treatment process for CNBT films were determined to be high-temperature drying at 400 °C for no less than 15 min followed by annealing at 600 °C for 5 min, which leads to the formation of compact films with uniform grains of 30–50 nm. Ferroelectric property measurement shows that the remanent polarization of CNBT films is 18 times higher than that of un-doped Na_0.50Bi_0.50TiO₃ (NBT) thin films.     

Different effects of alkyl sulfate and alkylbenzene sulfonate surfactants on the synthesis and properties of CuPc/TiO2 composite films by the liquid-phase deposition (LPD) method

The insoluble copper phthalocyanine (CuPc)/TiO₂ composite thin film has been successfully prepared in an aqueous solution by liquid-phase deposition (LPD) method, applying two surfactants as the solubilizing agents. The two surfactants are sodium dodecyl benzylsulfonate (SDBS) and sodium lauryl sulfate (SDS), selected as the representative of their groups. The deposited films were characterized by UV–vis, SEM, FT-IR, XRD and ICP–AES. Comparing the two composite thin films, it revealed a series of interesting facts. The films showed excellent adherence to the substrate with particle diameter ranging from 20–50 nm in, and 90 and 280 nm in thickness. It is identified that CuPc is coexisting of dimers and monomers form, mainly in dimeric form for both the composite thin films by UV–vis spectroscopy, illustrating that the CuPc is not further aggregated by LPD process. The dependences of deposited amount of Ti and Cu with the reaction time suggested the growing of the films can be easily controlled, the SDBS seems to be hindrance to the growing of the films. After heat treatment at 250 °C for 2 h, the SDS–CuPc/TiO₂ thin film becomes crystallized, but no obvious XRD peak is observed in the SDBS–CuPc/TiO₂ composite thin film.     

Porphyrin dye-sensitization of polythiophene in a conjugated polymer/TiO2 p–n hetero-junction solar cell

In the blended solid of poly(3-hexylthiophene-2,5-diyl) (P3HT) and porphyrin (TPP)/TiO₂ p–n hetero-junction solar cells, a photo-induced charge transfer between P3HT and TPP accelerated the charge separation in the depletion layer formed at the P3HT + TPP/TiO₂ interface, enhancing the photovoltaic properties. For the blended cell containing zinc porphyrin as TPP, the energy conversion yield of 0.26% was obtained under the illumination of solar simulated light AM1.5–100 mW/cm².     

Micro arc oxidation of S-containing TiO2 films by sulfur bearing electrolytes

Porous S-containing titania (TiO₂) films were fabricated by micro arc oxidation (MAO) in sulfur bearing electrolytes. Two kinds of electrolytes, Na₂SO₃ solution and Na₂S₂O₃ solution, were used in the experiment. The influence of electrolyte and applied voltage on the structure and surface morphology of prepared TiO₂ films has been evaluated. The critical voltage for film generation is 150 V in Na₂SO₃ solution and 100 V in Na₂S₂O₃ solution. Under applied voltage of 250 V, the thickness of the TiO₂ film is 15 μm for Na₂SO₃ electrolyte and 25 μm for Na₂S₂O₃ electrolyte, respectively. The films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) later. Two main phases, anatase and rutile, are contained in the porous film. Detailed reaction mechanism of MAO process was discussed later, and it is concluded that TiO₂ films are more easily prepared in Na₂S₂O₃ solution due to sulfur deposition.     

Semiconductor TiO2–Ga2O3 thin film gas sensors derived from particulate sol–gel route

Nanostructured and mesoporous TiO₂–Ga₂O₃ thin films with various Ti:Ga atomic ratios were prepared by a new straightforward particulate sol–gel route. Titanium isopropoxide and gallium (III) nitrate hydrate were used as precursors, and hydroxypropyl cellulose (HPC) was used as a polymeric fugitive agent (PFA) in order to increase the specific surface area (SSA). XRD and TEM analysis of the powders revealed that the Ga₂O₃ formed from the nitrate precursor retarded anatase-to-rutile transformation, crystallization and crystal growth. The average crystallite size of pure TiO₂ powder annealed at 600–1000 °C were in the range 4–10 nm; the values that could be decreased to 2–6 nm for TiO₂–Ga₂O₃ powders. Furthermore, one of the highest SSA was obtained by introducing Ga₂O₃ into TiO₂, being 305 m² g⁻¹ for TG11 (Ti:Ga = 50:50 atomic ratio) binary oxide annealed at 600 °C. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO and NO₂ gases at low operating temperature of 200 °C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption. TG11 sensor showed the highest response towards all CO and NO₂ concentrations operated at 200 °C. The response magnitude of 13.7 and 4.3 with response times of 30 s and 108 s were achieved for TG11 sensor towards 400 ppm CO and 10 ppm NO₂, respectively. Furthermore, calibration curves revealed that TiO₂–Ga₂O₃ sensors follow the power law (S = A[gas]^B) (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration) for the two types of gases, and they have excellent capability for the detection of low gas concentrations (25 ppm CO and 0.5 ppm NO₂). The maximum response of TiO₂–Ga₂O₃ sensors towards CO and NO₂ was measured at 450 and 400 °C, respectively. The sensor response decreased with increasing film annealing temperature owing to sintering of the particles. The response magnitude and response time of the sensors obtained in this work is superior to TiO₂-based sensors reported in previous studies.     

Novel 1,3,4-oxadiazole derivatives as efficient sensitizers for dye-sensitized solar cells: A combined experimental and computational study

We have synthesized and characterized donor–π–spacer–acceptor type molecules in which 1,3,4-oxadiazoles are π-spacers, triphenylamines are the donors and cyanoacetic acid are the acceptors for use as sensitizers in dye-sensitized solar cells (DSSCs). Detailed absorption, emission, electrochemical, photoelectrochemical and computational studies have been carried out on five novel derivatives. The dyes have an absorption range of 377–388 nm, and an emission in the range of 494–540 nm. There is a large charge transfer from the donor side to the acceptor side on excitation. The propeller shape of the triphenylamine and the bulky substituents on it help in reducing the dye-aggregation on TiO₂ surface. The dyes exhibited good overall conversion efficiency (2.79–3.21%). Plane wave calculations indicate that the dye has a reasonably strong binding to the TiO₂ surface and the generated DOS picture shows an overlap of the molecular orbitals of the dye and the TiO₂ bands. We conclude that the dyes have a promising role as sensitizers in DSSC.     

Titanium-supported Ce-, Zr-containing oxide coatings modified by platinum or nickel and copper oxides and their catalytic activity in CO oxidation

A combination of plasma electrolytic oxidation (PEO) and impregnation techniques followed by annealing in air has been used to obtain composites Pt/nZrO₂ + pTiO₂/Ti, Pt/nZrO₂ + pTiO₂ + zCeO_x/Ti, NiO + CuO/nZrO₂ + pTiO₂/Ti, NiO + CuO/nZrO₂ + pTiO₂ + zCeO_x/Ti with different zirconium and titanium contents and ZrO₂/TiO₂ phase ratio. The composites have been investigated by means of XRD, XPS and SEM/XSA methods. According to the XPS data, the platinum content on the coating surface is ~ 0.4 at.%, whereas the XSA measurements have shown that the nickel and copper contents in coatings attain 16 and 8 at.%, respectively, depending on the initial oxide coatings composition. Nickel and copper oxides form either extended islets or solid layers (“crusts”) on the coating surface. Both the composites promoted with platinum and those with the “crust” built from nickel and copper oxides are active in CO oxidation at the temperatures above 200 °C and 300 °C, respectively.     

Stability and dewetting kinetics of thin gold films on Ti,TiOx and ZnO adhesion layers

We present an in situ high-temperature confocal laser microscopy study on the thermal stability of 40 nm thick gold thin films grown on 40 nm Ti, TiO_x and ZnO adhesion layers on (0 0 1) Si. In situ observation of the dewetting process was performed over a wide range of set temperatures (400–800 °C) and ramp rates (10–50 °C min^?1) for each gold/adhesion layer combination. We found that significant dewetting and subsequent formation of gold islands occurs only at and above 700 °C for all adhesion layers. The dewetting is driven to equilibrium for gold/ZnO compared to gold/Ti and gold/TiO_x as confirmed by ex situ X-ray diffraction and scanning electron microscopy characterization. Quantification of the in situ data through stretched exponential kinetic models reveals an underlying apparent activation energy of the dewetting process. This energy barrier for dewetting is higher for gold/Ti and gold/TiO_x compared to gold/ZnO, thus confirming the ex situ observations. We rationalize that these apparent activation energies correspond to the underlying thermal stability of each gold/adhesion layer system.     

Thin-film solid-state reactions of solid BaCO3 and BaO vapor with (1 0 0) rutile substrates

Thin-film solid-state reactions in the BaCO₃–TiO₂ and BaO–TiO₂ systems have been experimentally studied in air and in vacuum, respectively, on (1 0 0) TiO₂ substrates. The solid–solid reactions in vacuum and in air showed that different processes occur during BaTiO₃ formation: an intermediate Ba₂TiO₄ compound is always observed in vacuum but not in air. The crystallographic orientations between the reaction products and the TiO₂ substrate were investigated by X-ray diffractometry and transmission electron microscopy. One orientation relationship has been found for BaTiO₃ after a solid–solid reaction, whereas different orientation relationships for BaTiO₃ have been identified after a vapor–solid reaction. At high temperatures, Ti-rich polytitanates were grown in both types of solid-state reactions but with quite different textures. The crystallographic relationships and their origins are analyzed.     

Surface modification of NiTi/PZT heterostructure thin films using various protective layers for potential MEMS applications

The present study explored the in-situ deposition of hard and adherent nanocrystalline protective coatings on NiTi/PZT/TiO_x thin film heterostructure prepared by dc/rf magnetron sputtering. Protective layers (AlN, CrN and TiCrN) of approximate thickness (~ 200 nm) were used to improve the surface, mechanical and corrosion properties of NiTi/PZT/TiO_x heterostructure without sacrificing the shape memory effect and ferroelectricity of the NiTi and PZT layers, respectively. The influence of the protective layer on structural, electrical and mechanical properties of NiTi/PZT/TiO_x heterostructure was systematically investigated and the results were compared. Nanoindentation studies were performed at room temperature to determine the hardness and reduced modulus. The surface modified NiTi/PZT/TiO_x heterostructures were found to exhibit high hardness, high elastic modulus and thereby better wear resistance as compared to pure NiTi/PZT/TiO_x films. From the results of potentiodynamic polarization test conducted in 1 M NaCl solution, the CrTiN coated NiTi/PZT/TiO_x heterostructure showed the best corrosion resistance with the lowest corrosion current density (1.52 × 10^? 8 A cm^? 2) and the highest protective efficiency (96.8%). The results presented here prove the potential of a surface modified NiTi/PZT/TiO_x heterostructure to be used in various microelectromechanical (MEMS) applications.     

Nanoscale surficial films and a surface transition in V2O5–TiO2-based ternary oxide systems

Nanoscale, vanadia-based, quasi-liquid films of self-selecting (equilibrium) thickness were observed on TiO₂ surfaces in six ternary oxide systems (Ti–V–X–O; X = P, Na, K, Nb, Mo or W). It is demonstrated that the film appearance and thickness could be tailored via co-doping or changing the equilibration temperature. Furthermore, the observed discontinuous changes in film thickness, hysteresis and bimodal thickness distributions indicate a first-order monolayer-to-multilayer adsorption transition, which is interpreted as a coupled prewetting and premelting transition. The film thickness and stability are measured as functions of equilibration temperature, anneal time, thermal treatment history, co-doping, overall composition and TiO₂ phase and orientation. The characterization of more than 850 independent films represents the most systematic measurement of similar interfacial films to date, providing insights into the formation mechanisms and the thermodynamic stability of equilibrium-thickness surficial films and analogous intergranular films.     

Electrical properties of TiO2-filled polyimide nanocomposite films prepared via an in situ polymerization process

To fully understand the reliable electrical properties of the nanocomposite films, their physical and chemical behaviors as well as dielectric properties were analyzed. Polyimide/TiO₂ (PI/TiO₂) nanocomposite films were prepared using in situ dispersion polymerization process. Influences of frequency, temperature and the nano-TiO₂ particles loading concentration on dielectric permittivities of the PI/TiO₂ nanocomposite films were studied. The dielectric relaxation behavior of the PI/TiO₂ nanocomposite films has been investigated with dielectric relaxation spectrum. It was found that interfacial polarization, namely Maxwell–Wagner–Sillars, existed in the PI/TiO₂ nanocomposite films with different loading concentration of the nano-TiO₂ particles. Dielectric permittivities of the PI/TiO₂ nanocomposite films fluctuation with temperature (?50–150 °C) were attributed to the mobility and thermal expansion of the polymer matrix, for which the Cole–Davidson model, or modified Cole–Cole model, was brought to account.     

Self-assembled polyaniline nanotubes and nanoribbons/titanium dioxide nanocomposites

Self-assembled polyaniline (PANI) nanotubes, accompanied with nanoribbons, were synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in an aqueous medium, in the presence of colloidal titanium dioxide (TiO₂) nanoparticles of 4.5 nm size, without added acid. The morphology, structure, and physicochemical properties of the PANI/TiO₂ nanocomposites, prepared at various initial aniline/TiO₂ mole ratios, were studied by scanning (SEM) and transmission (TEM) electron microscopies, FTIR, Raman and inductively coupled plasma optical emission (ICP-OES) spectroscopies, elemental analysis, X-ray powder diffraction (XRPD), conductivity measurements, and thermogravimetric analysis (TGA). The electrical conductivity of PANI/TiO₂ nanocomposites increases in the range 3.8 × 10^?4 to 1.1 × 10^?3 S cm^?1 by increasing aniline/TiO₂ mole ratio from 1 to 10. The morphology of PANI/TiO₂ nanocomposites significantly depends on the initial aniline/TiO₂ mole ratio. In the morphology of the nanocomposite synthesized using aniline/TiO₂ mole ratio 10, nanotubes accompanied with nanosheets prevail. The nanocomposite synthesized at aniline/TiO₂ mole ratio 5 consists of the network of nanotubes (an outer diameter 30–40 nm, an inner diameter 4–7 nm) and nanorods (diameter 50–90 nm), accompanied with nanoribbons (a thickness, width, and length in the range of 50–70 nm, 160–350 nm, and ～1–3 μm, respectively). The PANI/TiO₂ nanocomposite synthesized at aniline/TiO₂ mole ratio 2 contains polyhedral submicrometre particles accompanied with nanotubes, while the nanocomposite prepared at aniline/TiO₂ mole ratio 1 consists of agglomerated nanofibers, submicrometre and nanoparticles. The presence of emeraldine salt form of PANI, linear and branched PANI chains, and phenazine units in PANI/TiO₂ nanocomposites was proved by FTIR and Raman spectroscopies. The improved thermal stability of PANI matrix in all PANI/TiO₂ nanocomposites was observed.     

Enhanced ferroelectric properties of Pb(Zr0.52Ti0.48)O3 films on Pt/TiO2/SiO2/Si(001) using ZnO buffer layer

(0 0 1)-textured Pb(Zr_0.52Ti_0.48)O₃ films have been grown on Pt/TiO₂/SiO₂/Si(0 0 1) substrates by pulsed laser deposition using Al-doped ZnO as a buffer layer and bottom electrode. The as-fabricated ferroelectric capacitors exhibit large remanent polarization and excellent fatigue and retention properties.     

Depletion- and ambipolar-mode field-effect transistors based on the organic heterojunction composed of pentacene and hexadecafluorophtholocyaninatocopper

We investigated heterojunction organic field-effect transistors (OFETs) using pentacene and hexadecafluorophtholocyaninatocopper (F₁₆CuPc) as double active layers. Two operation modes including depletion- and ambipolar-type were observed depending on the deposition order of two organic films. Depletion-mode was firstly observed from that devices with pentacene as the bottom layer and F₁₆CuPc as the top layer, which was attributed to dipole effects originated from the pentacene/F₁₆CuPc interface. Then improved device performances were obtained with mobility from 0.87 to 1.06 cm²/V s, and threshold voltage shifted from ?20 to +25 V as compared with conventional pentacene-based devices. Furthermore, the heterojunction OFETs exhibited typical ambipolar transport when alternating the deposition order of two films, which exhibited ambipolar mobilities with 0.06 cm²/V s for electron and 0.0025 cm²/V s for hole, respectively. All results implied the utilization of heterojunction can effectively improve the device performances of OFET, and operation mode strongly on the deposition order of two films.