Synthesis and dielectric properties of polyaniline/titanium dioxide nanocomposites

Two series of polyaniline–TiO₂ nanocomposite materials were prepared in base form by in situ polymerization of aniline with inorganic fillers using TiO₂ nanoparticles (P25) and TiO₂ colloids (Hombikat), respectively. The effect of particle sizes and contents of TiO₂ materials on their dielectric properties was evaluated. The as-synthesized polyaniline–TiO₂ nanocomposite materials were characterized by transmission electron microscopy (TEM), Fourier transform infrared (FTIR), thermal analysis (DTA/TGA), and X-ray diffraction (XRD). Dielectric properties of polyaniline–TiO₂ nanocomposites in the form of films were measured at 1 KHz–1 MHz and a temperature range of 35–150 °C. Higher dielectric constants and dielectric losses of polyaniline–TiO₂ nanocomposites than those of neat PANI were found. PANI–TiO₂ nanocomposites derived from P25 exhibited higher dielectric constants and losses than those from Hombikat TiO₂ colloids. Electrical conductivity measurements indicate that the conductivity of nanocomposites is increased with TiO₂ content. The dielectric properties and conductivities are considered to be enhanced due to the addition of TiO₂, which might induce the formation of a more efficient network for charge transport in the base polyaniline matrix.     

Stress enhanced TiO2 nanowire growth on Ti–6Al–4V particles by thermal oxidation

Titanium dioxide (TiO₂) nanowires were grown on Ti - 6wt% Al - 4wt% V (Ti64) particles by thermal oxidation. To investigate the effect of stress on nanowire growth, the particles were milled in a planetary ball mill prior to the thermal oxidation. Thermal oxidation of the Ti64 particles was carried out in a horizontal tube furnace in a controlled oxygen atmosphere in the temperature range of 700–900 °C. The oxygen concentration was varied from 20 ppm to 80 ppm in Ar atmosphere. Nanostructures were characterized by high resolution field emission scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. TiO₂ nanowires grew on the surface of Ti64 particles and exhibited a square/rectangular cross sectional shape with thicknesses of 20–40 nm and lengths of 2–3 μm. Residual stress was found to play a significant role in nanowire growth. This was confirmed by growing TiO₂ nanowires on Ti64 alloy sheet with an induced stress gradient along its length. An improvement in nanowire coverage was observed in regions of high residual stress. A stress-induced growth mechanism is suggested to explain the confinement of nanowire growth to one dimension during thermal oxidation.     

Enhanced photocatalytic activity of Ag doped TiO2 nanoparticles synthesized by a microwave assisted method

Pure anatase TiO₂ photocatalyst with different Ag contents was prepared via a controlled and energy efficient microwave assisted method. The prepared material was further characterized by several analytical techniques like X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), surface area measurement (BET), Fourier transform-infrared spectroscopy (FT-IR), diffused reflectance spectroscopy (DRS), and thermogravimetric–differential thermal analysis (TGA–DTA). A 10 nm average crystallite size with nano-crystals of pseudo-cube like morphology was obtained for optimal (0.25 mol%) Ag doped TiO₂. The present research work is mainly focused on the enhancement of degradation efficiency of methyl orange (MO) by doping of Ag in TiO₂ matrix using UV light (365 nm). A 99.5% photodegradation efficiency of methyl orange was achieved by utilizing 0.25 mol% Ag doped TiO₂ (1 g/dm³) at pH=3 within 70 min. Recyclability of photocatalyst was also studied, with the material being found to be stable up to five runs.     

An approach to laminated flexible Dye sensitized solar cells

We have built TiO₂ Dye sensitized solar cells (DSSCs) that combined flexible TiO₂ photoanodes coated on ITO/PET substrates with a gel electrolyte based on PVDF-HFP-SiO₂ films. Titanium isopropoxide (TiP₄) was used as additive to TiO₂ nanoparticles for increasing power conversion efficiency in Dye sensitized solar cell electrodes prepared at low-temperature (130 °C). An efficiency η_AM1.5G = 3.55% on ITO/PET substrates is obtained at 48 mW/cm² illumination with a standard liquid electrolyte based on methoxypropionitrile. Among several solvents forming gels with PVDF-HFP-SiO₂, N-methyl (pyrrolidone) (NMP) was found to enable the most stable devices. A power conversion efficiency η_AM1.5G = 2% was obtained under 10 mW/cm² with flexible TiO₂-ITO-PET photoanodes and the PVDF-HFP-SiO₂ + NMP gel electrolyte.     

Fabrication of morphology controllable rutile TiO2 nanowire arrays by solvothermal route for dye-sensitized solar cells

Highly ordered, vertically oriented TiO₂ nanowire arrays (TNAs) are synthesized directly on transparent conducting substrate by solvothermal procedure without any template. The X-ray diffraction (XRD) pattern shows that TiO₂ array is in rutile phase growing along the (0 0 2) direction. The field-emission scanning electron microscopy (FE-SEM) images of the samples indicate that the TiO₂ array surface morphology and orientation are highly dependent on the synthesis conditions. In a typical condition of solvothermal at 180 °C for 2 h, the TNAs are composed of nanowires 10 ± 2 nm in width, and several nanowires bunch together to form a larger secondary structure of 60 ± 10 nm wide. Dye-sensitized solar cell (DSSC) assembled with the TNAs grown on the FTO glass as photoanode under illumination of simulated AM 1.5G solar light (100 mW cm⁻²) achieves an overall photoelectric conversion efficiency of 1.64%.     

EIS analysis on low temperature fabrication of TiO2 porous films for dye-sensitized solar cells

A low temperature (<150 °C) fabrication method for preparation of TiO₂ porous films with high efficiency in dye-sensitized solar cells (DSSCs) has been developed. The Ti(IV) tetraisopropoxide (TTIP) was added to the paste of TiO₂ nanoparticles to interconnect the TiO₂ particles. The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) and electron lifetime in the TiO₂ film (τ_e) under different molar ratios of TTIP/TiO₂ and also at various TiO₂ thicknesses. It was found that the R_ct2 decreased as the molar ratio increased from 0.02 to 0.08, however, it increased at a molar ratio of 0.2 due to the reduction in surface area for dye adsorption. In addition, the characteristic frequency peak shifted to lower frequency at a molar ratio of 0.08, indicating the longer electron lifetime. As for the thickness effect, TiO₂ film with a thickness around 17 μm achieved the best cell efficiency. EIS study also confirmed that, under illumination, the smallest R_ct2 was associated with a TiO₂ thickness of 17 μm, with the R_ct2 increased as the thickness of TiO₂ film increased. In the Bode plots, the characteristic frequency peaks shifted to higher frequency when the thickness of TiO₂ increased from 17.2 to 48.2 μm, indicating the electron recombination increases as the thickness of the TiO₂ electrode increases.Finally, to make better use of longer wavelength light, 30 wt% of larger TiO₂ particle (300 nm) was mixed with P25 TiO₂ as light scattering particles. It effectively increased the short-circuit current density and cell conversion efficiency from 7.44 to 8.80 mA cm⁻² and 3.75 to 4.20%, respectively.     

Aerosol assisted chemical vapour deposited (AACVD) of TiO2 thin film as compact layer for dye-sensitised solar cell

Compact TiO₂ has been introduced onto the surface of an indium tin oxide glass slide (ITO), using an aerosol-assisted chemical vapour deposition method. This serves as a blocking layer for a dye-sensitised solar cell (DSSC). The thickness of the compact TiO₂ could be controlled by deposition time. X-ray diffraction and Raman spectroscopy analyses reveal that the compact TiO₂ is made up of mixed anatase and rutile phases. The field emission scanning electron microscopy image displays a pyramidal morphology of the compact TiO₂. A layer of P25 paste was then smeared onto the compact TiO₂-modified ITO, using the doctor's blade method. A post-treatment procedure was applied to remove the contaminants from the prepared hybrid film, by immersing in a hydrochloric acid solution. The photoelectrochemical measurements and J–V characterisation of the hybrid film show an approximately fourfold increase in photocurrent density generation (114.22 µA/cm²), and approximately 25% enhancement of DSSC conversion efficiency (4.63%), compared to the acid-treated P25 paste alone (3.68%).     

Preparation and electrochemical characterization of TiO2 nanowires as an electrode material for lithium-ion batteries

Anatase TiO₂ nanowires containing minor TiO₂(B) phase were prepared by a hydrothermal chemical reaction followed by the post-heat treatment at 400 °C. The phase structure and morphology were analyzed by X-ray diffraction, Raman scattering, transmission electron microscope, and field-emission scanning electron microscopy. The electrochemical properties were investigated by employing constant current discharge-charge test, cyclic voltammetry, and electrochemical impedance techniques. These nanowires exhibited high rate capacity of 280 mAh g⁻¹ even after 40 cycles, and the coulombic efficiency was approximately 98%, indicating excellent cycling stability and reversibility. The electrochemical impedance spectra showed a stable kinetic process of the electrode reaction. These results indicated that the TiO₂ nanowires have promising application for high energy density lithium-ion batteries.     

Visible light photocatalytic degradation of thiophene using Ag–TiO2/multi-walled carbon nanotubes nanocomposite

Ag–TiO₂ nanocatalyst, supported on multi-walled carbon nanotubes, was synthesized successfully via a modified sol–gel method, and the prepared photocatalyst was used to remediate aqueous thiophene environmentally by photocatalytic oxidation under visible light. The prepared Ag–TiO₂/multi-walled carbon nanotubes nanocomposite photocatalyst was characterized through X-ray diffraction, Brunauer–Emmett–Teller (BET), transmission electron microscopy, and UV–vis spectra (UV–vis). The results showed that both Ag and TiO₂ nanoparticles were well-dispersed over the MWCNTs and formed a uniform nanocomposite. Ag doping can eliminate the recombination of electron–hole pairs in the catalyst, and the presence of MWCNTs in the TiO₂ composite can change surface properties to achieve sensitivity to visible light. The optimum mass ratio of MWCNT:TiO₂:Ag was 0.02:1.0:0.05, which resulted in the photocatalyst's experimental performance in oxidizing about 100% of the thiophene in a 600 mg/L solution within 30 min and with 1.4 g L⁻¹ amount of catalyst used.     

Synthesis and characterization of bionanocomposites of poly(lactic acid) and TiO2 nanowires by in situ polymerization

Bionanocomposites from biopolymers and inorganic nanoparticles are of great interest for packaging materials due to their enhanced physical, thermal, mechanical, and processing characteristics. In this study, poly(lactic acid) (PLA) nanocomposites with covalent bonding between TiO₂ nanowire surface and PLA chains were synthesized through in situ melt polycondensation. Molecular weight, structure, morphology, and thermal properties were characterized. Fourier transform infrared spectroscopy confirmed that PLA chains were covalently grafted onto TiO₂ nanowire surface. Transmission electron microscopy images also revealed clearly a third phase presence on the nanowires after the grafting process. Those grafted PLA chains exhibited significantly increased glass transition temperature and thermal stability, compared with pure PLA. The weight-average molecular weight of PLA/2% TiO₂ nanowire bulk nanocomposites increased by 66% compared with that of pure PLA. The electron microscopy results showed that strong interfacial interaction and homogeneous distribution were achieved between inorganic nanowires and organic PLA matrix in the bulk nanocomposites. The PLA matrix in bulk nanocomposites exhibited elevated glass transition temperature and decreased crystallization ability as the TiO₂ nanowire concentrations were increased from 0 to 2%.     

The preparation and characterization of Au@TiO2 nanoparticles and their catalytic activity for CO oxidation

Au@TiO₂ core/shell nanoparticles were synthesized by a simple and efficient one-step method using tetrabutyl titanate as TiO₂ precursor. The samples were characterized by TEM, XRD, UV–vis and XPS. The experiments demonstrated that the average particles size of Au was 10–15 nm, and the thickness of TiO₂ shell was 1–3 nm. TiO₂ shell induced a red-shift of the absorption peak of Au. This material exhibited catalytic activity for CO oxidation. This study offered an approach for CO oxidation by using Au@TiO₂ model catalysts.     

Gas sensing properties of multiple networked GaN/WO3 core-shell nanowire sensors

GaN nanowires and GaN-core/WO₃-shell nanowires were synthesized by the thermal evaporation of GaN powders followed by the sputter-deposition of WO₃ and their gas sensing properties were examined. The multiple networked pristine GaN nanowire sensors showed responses of approximately 125%, 140%, 146%, 159%, and 183% to 1, 2, 3, 4, and 5 ppm NO₂ gases, respectively. These responses are comparable to those obtained previously using metal oxide semiconductor one-dimensional nanostructure sensors. The responses of the nanowires to 1, 2, 3, 4, and 5 ppm NO₂ gases were improved 1.3, 1.4, 1.6, 1.7 and 1.8 fold, respectively, further through the encapsulation of GaN nanowires with a WO₃ thin film. The improvement in the response of GaN nanowires to NO₂ gas by encapsulation is attributed to the modulation of electron transport at GaN–WO₃ heterojunction. The electron transport in the core-shell nanowires is modulated by the heterojunction with an adjustable energy barrier height, resulting in an enhanced sensing property of the core-shell nanostructures.     

Preparation of high solids content nano-titania suspensions to obtain spray-dried nanostructured powders for atmospheric plasma spraying

Nanosized TiO₂ powder with an average primary size of ∼20 nm and surface area of ∼50 m²/g (Aeroxide^® P25, Degussa-Evonik, Germany) was used as starting material. A colloidal titania suspension from the same supplier was also used (W740X). The dispersing conditions were studied as a function of pH, dispersant content, and solids loading. Well-dispersed TiO₂ nanosuspensions with solids contents up to 30 vol.% (62 wt%) were obtained by dispersing the powder with 4 wt% PAA. Suspensions with solids contents as high as 35 vol.% were prepared by adding the TiO₂ nanoparticles to the TiO₂ colloidal suspension under optimised dispersing conditions.TiO₂ powder reconstitution was performed by spray drying both types of nanosuspensions to obtain free-flowing micrometre-sized nanostructured granules. The spray-dried nanostructured TiO₂ granules were deposited on austenitic stainless steel coupons using atmospheric plasma spraying. Coating microstructure and phase composition were characterised using scanning electron microscopy and X-ray diffraction techniques.     

Electrophoretic deposition of nanocrystalline TiO2 films on Ti substrates for use in flexible dye-sensitized solar cells

Nanocrystalline TiO₂ films were prepared on flexible Ti-metal sheets by electrophoretic deposition followed by chemical treatment with tetra-n-butyl titanate (TBT) and sintering at 450 °C. X-ray diffraction (XRD) analysis indicates that TBT treatment led to the formation of additional anatase TiO₂, which plays an important role in improving the interconnection between TiO₂ particles, as well as the adherence of the film to the substrate, and in modifying the surface properties of the nanocrystalline particles. The effect of TBT treatment on the electron transport in the nanocrystalline films was studied by intensity-modulated photocurrent spectroscopy (IMPS). An increase in the conversion efficiency was obtained for the dye-sensitized solar cells with TBT-treated nanocrystalline TiO₂ films. The cell performance was further optimized by designing nanocrystalline TiO₂ films with a double-layer structure composed of a light-scattering layer and a transparent layer. The light-scattering effect of the double-layer nanocrystalline films was evaluated by diffuse reflectance spectra. Employing the double-layer nanocrystalline films as the photoelectrodes resulted in a significant improvement in the incident photo-to-current conversion efficiency of the corresponding cells due to enhanced solar absorption by light scattering. A high conversion efficiency of 6.33% was measured under illumination with 100 mW cm⁻² (AM 1.5) simulated sunlight.     

Visible photocatalytic activity and photoelectrochemical behavior of TiO2 nanoparticles modified with metal porphyrins containing hydroxyl group

TiO₂ nanoparticles modified with 5-(p-hydroxylphenyl)-10,15,20-triphenylporphyrin (HTPP), 5-(p-hydroxylphenyl)-10,15,20-triphenylporphyrin zinc (ZnHTPP) and trans-dichloro-5-(p-hydroxylphenyl)-10,15,20-triphenylporphyrin tin (SnHTPP) were prepared in order to improve the visible photocatalytic activity of TiO₂ nanoparticles. The photocatalytic activity of the modified TiO₂ nanoparticles was investigated by carrying out the photodegradation of methyl orange in aqueous solution under visible light irradiation. The TiO₂ nanoparticles modified with SnHTPP show the highest visible photocatalytic activity with a degradation ratio of 86% of methyl orange after 180 min irradiation among three catalysts. This result indicates that the central metal ions in porphyrins can significantly influence the sensitization efficiency of porphyrins. In addition, the photoelectrochemical behavior of the modified TiO₂ nanoparticles was examined and related to their photocatalytic activity. Finally, the photocatalytic mechanism was discussed preliminarily.     

Hierarchical structured TiO2 nano-tubes for formaldehyde sensing

Hierarchical structured TiO₂ nano-tubes were prepared following a two-step method: the highly ordered uniform TiO₂ nanotube arrays were first grown by the conventional electrochemical anodization of the Ti metal sheet followed by mechanical milling of the as-fabricated TiO₂ nanotube arrays. The obtained nanotubes with a length around 400 nm and opening diameter ∼100 nm were formed mixed with the spherical TiO₂ single crystals with a diameter around 10 nm indicating hierarchical nanostructure. The as-synthesized TiO₂ hierarchical nanotubes based resistive-type chemical sensor exhibits good sensitivity to formaldehyde at room temperatures with or without UV-irradiation. The response of the sensor increased almost linearly as a function of the concentration of formaldehyde from 10–50 ppm under UV irradiation. The response of the sensor to different relative humidity and other possible interferents such as ammonia, methanol and alcohol was investigated. The larger response of the sensor to formaldehyde relative to these interferents is suggested to be due to the deeper diffusion of formaldehyde into the TiO₂ nanotubes.     

Anatase TiO2 spheres with high surface area and mesoporous structure via a hydrothermal process for dye-sensitized solar cells

Spherical pure anatase TiO₂ spheres with a mesoporous structure and high surface area of up to 116.5 m² g⁻¹ were prepared by a simple urea-assisted hydrothermal process and investigated as dye-sensitized solar-cell electrodes. Although the particle diameters of the prepared TiO₂ spheres ranged from 0.4 to 1.3 μm, due to the high specific surface area, mesoporous TiO₂ sphere electrode was obtained with enhanced light harvesting and a larger amount of dye loading. An overall light conversion efficiency of 7.54% under illumination of simulated AM 1.5G solar light (100 mW cm⁻²) was achieved using the mesoporous TiO₂ spheres electrode, which was significantly higher than a commercial Degussa P25 TiO₂ nanocrystals electrode (5.69%).     

Onset coarsening-coalescence kinetics of ZrO2 and less refractory TiO2 nanoparticles: Effect of homologous temperature and phase transformation

Specific surface area change of ZrO₂ (predominant tetragonal - (t) symmetry, 30-50 nm) and less refractory TiO₂ anatase nanoparticles (20-50 nm) upon isothermal firing at 700-1000 °C in air was determined by N₂ adsorption-desorption hysteresis isotherm. The nanoparticles underwent onset coarsening-coalescence within minutes without appreciable phase transformation for TiO₂, but with extensive transformation into monoclinic (m-) symmetry for ZrO₂. The apparent activation energy of such a process being not much higher for ZrO₂ (77 ± 23 kJ/mol) than TiO₂ (56 ± 3 kJ/mol) nanoparticles can be attributed to transformation plasticity. The minimum temperature for coarsening/coalescence of the present ZrO₂ and TiO₂ nanoparticles was estimated as 710 and 641 °C, respectively.     

A kinetic study on the electrophoretic deposition of hydroxyapatite–titania nanocomposite based on a statistical approach

In the present study, the electrophoretic deposition (EPD) process of hydroxyapatite–titania nanocomposite was kinetically described by the use of response surface methodology (RSM). The electrostatic interaction between particles in ethanol based suspensions was determined by Zeta potential and particle size analyses. After successful electrophoretic deposition from hydroxyapatite–titania suspensions with 0, 10 and 20 wt% of titania nanoparticles, it was shown that Baldisserri model can well reproduce the experimental data among the other semi-empirical kinetic equations. The as-deposited hydroxyapatite–titania nanocomposites were characterized employing SEM, AFM, XRD, and FT-IR analyses. Then, the effects of deposition voltage, deposition time and wt% TiO₂ on the kinetic of EPD at two time intervals (10–60 s and 60–300 s) were identified and quantified via RSM based on a central composite design (CCD). According to the results obtained from the statistical analysis, it was found that the deposition rate decreases by an increase in wt% TiO₂ and time. Also, a transition in deposition mechanism from linear to parabolic mode was observed and two second order polynomial equations were fitted to the response (deposit weight) at each time intervals.     

Synthesis,microstructure and optical characterization of zirconium oxide nanostructures

Zirconium oxide (ZrO₂) nanostructures were synthesized by hydrothermal route. Surface morphology analysis depicts the formation of the nanobars and hexagonal-shaped nanodiscs at different synthesis conditions. The structural analysis confirms that the as-synthesized ZrO₂ product is of pure monoclinic phase (m-ZrO₂) with crystallite size of about 25 nm. The product consists of monodispersed nanoparticles of uniform composition, high purity, and crystallinity. The Raman spectra are quantitatively analyzed and the observed peaks are attributed to various vibration modes of the m-ZrO₂. The UV–vis absorption spectrum showed a strong absorption peak at about 292 nm and the estimated optical band gap was around 3.57 eV. Photoluminescence (PL) spectrum of ZrO₂ nanostructure showed a strong and broad emission peak at around 410 nm at room temperature, which can be attributed to the ionized oxygen vacancy in the material.