Anodic TiO<Subscript>2</Subscript> nanotubes powder and its application in dye-sensitized solar cells

An increasing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions. TiO₂ semiconductor material is expected to play an important role in helping solve the energy crisis through effective utilization of solar energy based on photovoltaic devices. Dye-sensitized solar cells (DSSCs) are potentially lower cost alternative to inorganic silicon-based photovoltaic cells. In this study, we report on the fabrication of DSSCs from anodic TiO₂ nanotubes (NT) powder, produced by rapid breakdown potentiostatic anodization of Ti foil in 0.1 M HClO₄ electrolyte, as photoanode. TiO₂ NT powders with a typical NT outer diameter of approximately 40 nm, wall thickness of approximately 8–15 nm, and length of about 20–25 μm, have been synthesized. The counter electrode was made by electrodeposition of Pt from an aqueous solution of 5 mM H₂PtCl₆ onto fluorine-doped tin oxide (FTO) glass substrate. The above front-side illuminated DSSCs were compared with back-side illuminated DSSCs fabricated from anodic TiO₂ NTs that were grown on the top of Ti foil as photoanode. The highest cell efficiency was 3.54% under 100 mW/cm² light intensity (1 sun AM 1.5G light, Jsc = 14.3 mA/cm², Voc = 0.544 V, FF = 0.455). To the best of our knowledge, this is the first report on the fabrication of DSSC from anodic TiO₂ NTs powder. The TiO₂/FTO photoanodes were characterized by FE-SEM, XRD, and UV–Visible spectroscopy. The catalytic properties of Pt/FTO counter electrodes have been examined by cyclic voltammetry.     

Poly(vinyl chloride)-<Emphasis Type="Italic">graft</Emphasis>-poly(<Emphasis Type="Italic">N</Emphasis>-vinyl caprolactam) graft copolymer: synthesis and use as template for porous TiO<Subscript>2</Subscript> thin films in dye-sensitized solar cells

Poly(N-vinyl caprolactam) (PNVCL) side chains were grafted to a poly(vinyl chloride) (PVC) backbone via atom transfer radical polymerization. The synthesized PVC-g-PNVCL graft copolymer was templated for the preparation of porous TiO₂ thin films, which involved a sol–gel reaction and calcination process. The interaction of the carbonyl groups in the PVC-g-PNVCL with the titania was revealed by FT-IR spectroscopy. X-ray diffraction and transmission electron microscopy analysis showed the formation of porous TiO₂ thin films with the anatase phase. A series of porous TiO₂ thin films with different pore sizes and porosities was prepared by varying the solution compositions and were used as photoelectrodes in dye-sensitized solar cells (DSSC) with a polymer electrolyte. The DSSC performed best when using the TiO₂ film with higher porosity, lower interfacial resistance, and longer electron life time. The highest energy conversion efficiency, photovoltage (V _oc), photocurrent density (J _sc), and fill factor (FF) were 1.2%, 0.68 V, 3.2 mA/cm², and 0.57 at 100 mW/cm², respectively, for the quasi-solid state DSSC with a 730-nm-thick TiO₂ film.     

Effect of ethylene carbonate plasticizer and TiO<Subscript>2</Subscript> nanoparticles on 49% poly(methyl methacrylate) grafted natural rubber-based polymer electrolyte

The effect of plasticizer and TiO₂ nanoparticles on the conductivity, chemical interaction and surface morphology of polymer electrolyte of MG49–EC–LiClO₄–TiO₂ has been investigated. The electrolyte films were successfully prepared by solution casting technique. The ceramic filler, TiO₂, was synthesized in situ by sol-gel process and was added into the MG49–EC–LiClO₄ electrolyte system. Alternating current electrochemical impedance spectroscopy was employed to investigate the ionic conductivity of the electrolyte films at 25 °C, and the analysis showed that the addition of TiO₂ filler and ethylene carbonate (EC) plasticizer has increased the ionic conductivity of the electrolyte up to its optimum level. The highest conductivity of 1.1 × 10⁻³ Scm⁻¹ was obtained at 30 wt.% of EC. Fourier transform infrared spectroscopy measurement was employed to study the interactions between lithium ions and oxygen atoms that occurred at carbonyl (C=O) and ether (C-O-C) groups. The scanning electron microscopy micrograph shows that the electrolyte with 30 wt.% EC posses the smoothest surface for which the highest conductivity was obtained.     

Electrical properties,equivalent circuit,and dielectric relaxation studies on [(C<Subscript>3</Subscript>H<Subscript>7</Subscript>)<Subscript>4</Subscript>N]<Subscript>2</Subscript>Cd<Subscript>2</Subscript>Cl<Subscript>6</Subscript> polycrystalline

The Ac electrical conductivity and the dielectric relaxation properties of the [(C₃H₇)₄N]₂Cd₂Cl₆ polycrystalline sample have been investigated by means of impedance spectroscopy measurements over a wide range of frequencies and temperatures, 209 Hz–5 MHz and 361–418 K, respectively. The purpose is to make a difference between the electrical and dielectric properties of the polycrystalline sample and single crystal. Besides, a detailed analysis of the impedance spectrum suggests that the electrical properties of the material are strongly temperature-dependent. Plots of (Z" versus Z') are well fitted to an equivalent circuit model consisting of a series combination of grains and grains boundary elements. Moreover, the temperature dependence of the electrical conductivity in the different phases follows the Arrhenius law and the frequency dependence of σ (ω) follows the Jonscher’s universal dynamic law. Furthermore, the modulus plots can be characterized by full width at half height or in terms of a nonexperiential decay function φ(t) = exp(t/t)^β. Finally, the imaginary part of the permittivity constant is analyzed with the Cole–Cole formalism.     

Pseudo-capacitive properties of LiCoO<Subscript>2</Subscript>/AC electrochemical capacitor in various aqueous electrolytes

LiCoO₂ particles were synthesized by a sol-gel process. X-ray diffraction analysis reveals that the prepared sample is a single phase with layered structure. A hybrid electrochemical capacitor was fabricated with LiCoO₂ as a positive electrode and activated carbon (AC) as a negative electrode in various aqueous electrolytes. Pseudo-capacitive properties of the LiCoO₂/AC electrochemical capacitor were determined by cyclic voltammetry, charge–discharge test, and electrochemical impedance measurement. The charge storage mechanism of the LiCoO₂-positive electrode in aqueous electrolyte was discussed, too. The results showed that the potential range, scan rate, species of aqueous electrolyte, and current density had great effect on capacitive properties of the hybrid capacitor. In the potential range of 0–1.4 V, it delivered a discharge specific capacitance of 45.9 Fg^–1 (based on the active mass of the two electrodes) at a current density of 100 mAg^–1 in 1 molL^–1 Li₂SO₄ aqueous electrolyte. The specific capacitance remained 41.7 Fg^–1 after 600 cycles.     

Preparation of TiO<Subscript>2</Subscript>-coated LiCo<Subscript>1/3</Subscript>Ni<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> by electrostatic self-assembly method and its properties

A precursor of TiO₂–LiCo_1/3Ni_1/3Mn_1/3O₂ was prepared by electrostatic self-assembly method. The final product was obtained by heating the precursor at 400–450 °C for 4–6 h in air. X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical tests were used to examine the structural, morphology, elementary valence, and electrochemical characteristics. XRD indicated that the TiO₂-coated material can be indexed by α-NaFeO₂ layered structure, which belongs to hexagonal-type space group R3m. XPS results confirmed the existence of TiO₂ compound on the surface of the coated sample. The SEM image showed that the material had spherically porous morphology with the uniform size about 6 μm. The initial charge–discharge capacity of the TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ material was 168.8/160.0 mAh/g. After 60 cycles, the discharge capacity of the TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ sample was 147.0 mAh/g, and the coulombic efficiency was 94.0%. Compared with the uncoated sample, the electrochemical performance of TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ was improved.     

Hydrothermal growth of large-scale macroporous TiO<Subscript>2</Subscript> nanowires and its application in 3D dye-sensitized solar cells

Large-scale macroporous TiO₂ nanowires (MTN) were directly grown on spiral-shaped titanium wires as photoanodes of dye-sensitized solar cells (DSSCs) via a facile hydrothermal reaction without any seeds, templates, and TiO₂ powder. The MTN thin film was characterized by SEM, XRD and TEM. The studies revealed that the MTN thin film had better mechanical properties and provided an efficient pathway for the diffusion of liquid electrolyte. The efficiency of 0.86% for the 3D DSSC was obtained with a J _sc of 2.30 mA/cm², V _oc of 616 mV, and FF of 0.61. This MNT-based mini 3D DSSC is a promising photovoltaic device for applications in the fields of high-integrated micro-electronic equipment.     

Electrical characterization of the [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>][N(C<Subscript>2</Subscript>H<Subscript>5</Subscript>)<Subscript>4</Subscript>]ZnCl<Subscript>4</Subscript> compound

The [N(CH₃)₄][N(C₂H₅)₄]ZnCl₄ compound has been synthesized by a solution-based chemical method. The X-ray diffraction study at room temperature revealed an orthorhombic system with P2₁2₁2 space group. The complex impedance has been investigated in the temperature and frequency ranges 420–520 K and 200 Hz–5 MHz, respectively. The grain interior and grain boundary contribution to the electrical response in the material have been identified. Dielectric data were analyzed using the complex electrical modulus M ^* for the sample at various temperature. The modulus plots can be characterized by full width at half height or in terms of a non-exponential decay function ϕ(t) = exp[(−t/τ) ^β ]. The detailed conductivity study indicated that the electrical conduction in the material is a thermally activated process. The variation of the AC conductivity with frequency at different temperatures obeys the Almond and West universal law.     

Nanosized IrO<Subscript>2</Subscript> electrocatalysts for oxygen evolution reaction in an SPE electrolyzer

Nanosized IrO₂ electrocatalysts (d ~ 7–9 nm) with specific surface area up to 100 m² g⁻¹ were synthesized and characterized for the oxygen evolution reaction in a solid polymer electrolyte (SPE) electrolyzer. The catalysts were prepared by a colloidal method in aqueous solution and a subsequent thermal treatment. An iridium hydroxide hydrate precursor was obtained at ~100 °C, which was, successively, calcined at different temperatures from 200 to 500 °C. The physico-chemical characterization was carried out by X-ray diffraction (XRD), thermogravimetry–differential scanning calorimetry (TG–DSC) and transmission electron microscopy (TEM). IrO₂ catalysts were sprayed onto a Nafion 115 membrane up to a loading of 3 mg cm⁻². A Pt catalyst was used at the cathode compartment with a loading of 0.6 mg cm⁻². The electrochemical activity for water electrolysis of the membrane-electrode assemblies (MEAs) was investigated in a single cell SPE electrolyzer by steady-state polarization curves, impedance spectroscopy and chrono-amperometric measurements. A maximum current density of 1.3 A cm⁻² was obtained at 1.8 V and 80 °C for the IrO₂ catalyst calcined at 400 °C for 1 h. A stable performance was recorded in single cell for this anode catalyst at 80 °C. The suitable catalytic activity and stability of the most performing catalyst were interpreted in terms of proper combination between nanostructure and suitable morphology.     

Efficient visible light-induced photoelectrocatalytic degradation of rhodamine B by polyaniline-sensitized TiO<Subscript>2</Subscript> nanotube arrays

Self-organized anodic TiO₂ nanotube arrays were sensitized with polyaniline by a simple electrodeposite method. The morphological and structural properties studied by scanning electron microscopy and fourier transform infrared spectroscopy reveal the successful deposition of polyaniline on the nanotube arrays. The polyaniline-sensitized TiO₂ nanotube arrays exhibit a distinguishable red shift on the absorption spectrum. Electrochemical impedance investigation attested to a significant improvement of the interfacial electron-transfer kinetics for promoted electron–hole effective separation. The as-prepared samples showed a high efficiency for the photoelectrocatalytic degradation of rhodamine B under visible-light irradiation (λ > 400 nm). The enhanced photoelectrocatalytic activity could be attributed to the extended absorption in the visible-light region by the polyaniline and the effective separation of photogenerated carriers driven by the photoinduced potential difference generated at the polyaniline/TiO₂ nanotube arrays interface.     

Low-temperature synthesis of Cu-doped Li<Subscript>1.2</Subscript>V<Subscript>3</Subscript>O<Subscript>8</Subscript> as cathode for reversible lithium storage

Li_{1 .2}V₃O₈ and Cu-doped Li_1.2V₃O₈ were prepared at a temperature as low as 300 °C by a sol-gel method. The structure, morphology, and electrochemical performance of the as-prepared samples were characterized by means of X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscopy, and the galvanostatic discharge–charge techniques. It is found that the Cu-doped Li_1.2V₃O₈ sample exhibits less capacity loss during repeated cycling than the undoped one. The Cu-doped Li_1.2V₃O₈ sample demonstrates the first discharge capacity of 275.9 mAh/g in the range of 3.8–1.7 V at a current rate of 30 mA/g and remains at a stable discharge capacity of 264 mAh/g within 30 cycles. Furthermore, the possible role that copper plays in enhancing the cycleability of Li_1.2V₃O₈ has also been elucidated.     

Structural and electrical properties of pure and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> doped (PVA)<Subscript>0.7</Subscript>(NaI)<Subscript>0.3</Subscript> solid polymer electrolyte

Solid polymer electrolytes (SPE) based on poly-(vinyl alcohol) (PVA)_0.7 and sodium iodide (NaI)_0.3 complexed with sulfuric acid (SA) at different concentrations were prepared using solution casting technique. The structural properties of these electrolyte films were examined by X-ray diffraction (XRD) studies. The XRD data revealed that sulfuric acid disrupt the semi-crystalline nature of (PVA)_0.7(NaI)_0.3 and convert it into an amorphous phase. The proton conductivity and impedance of the electrolyte were studied with changing sulfuric acid concentration from 0 to 5.1 mol/liter (M). The highest conductivity of (PVA)_0.7(NaI)_0.3 matrix at room temperature was 10⁻⁵ S cm⁻¹ and this increased to 10⁻³ S cm⁻¹ with doping by 5.1 M sulfuric acid. The electrical conductivity (σ) and dielectric permittivity (ε′) of the solid polymer electrolyte in frequency range (500 Hz–1 MHz) and temperature range (300–400) K were carried out. The electrolyte with the highest electrical conductivity was used in the fabrication of a sodium battery with the configuration Na/SPE/MnO₂. The fabricated cells give open circuit voltage of 3.34 V and have an internal resistance of 4.5 kΩ.     

Luminescence of CaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>:Eu crystals

We have studied photoluminescence and thermoluminescence (PL and TL) in CaGa₂Se₄:Eu crystals in the temperature range 77–400 K. We have established that broadband photoluminescence with maximum at 571 nm is due to intracenter transitions 4f⁶ 5d–4f⁷ (⁸S_7/2) of the Eu²⁺ ions. From the temperature dependence of the intensity (log I–10³/T), we determined the activation energy (E _a = 0.04 eV) for thermal quenching of photoluminescence. From the thermoluminescence spectra, we determined the trap depths: 0.31, 0.44, 0.53, 0.59 eV. The lifetime of the excited state 4f6 5d of the Eu²⁺ ions in the CaGa₂Se₄ crystal found from the luminescence decay kinetics is 3.8 μsec. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 112–116, January–February, 2009.     

Electrical and electrochemical behaviour of several LiFe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript> solid solutions as cathode materials for lithium ion batteries

Several olivine phosphates were investigated in the last years as cathode materials for secondary lithium ion batteries. Among these compounds, LiFe _x Co_1 − x PO₄ solid solutions might be interesting candidates because they should combine the high potential value of Co³⁺/Co²⁺ (higher than 4.5 V vs Li⁺/Li) with the relatively high charge–discharge rate of LiFePO₄. Solid solutions were prepared by solid-state route and characterised by X-ray powder diffraction, cyclic voltammetry, impedance spectroscopy and the Hebb–Wagner method. The results show that also low amount of iron induces high electronic conductivity in the solid solutions.     

Biocorrosion studies of TiO<Subscript>2</Subscript> nanoparticle-coated Ti–6Al–4V implant in simulated biofluids

The corrosion behaviors of the TiO₂ nanoparticles coated bioimplant Ti–6Al–4V exposed to three different simulated biofluids (SBF), namely, (1) NaCl solution, (2) Hank’s solution, and (3) Cigada solution, were studied by using micro-Raman spectroscopy, electrochemical techniques, and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). The different electrochemical impedance spectroscopy models were applied to fit the data obtained from the implants before and after the coating of TiO₂ nanoparticles (50–100 nm). It was found that the TiO₂ nanoparticle coatings increased the thickness of the pre-existing oxide layer on the Ti–6Al–4V surface, serving to improve the bioimplant corrosion resistance.     

NMR study and electrical properties investigation of Zn<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

The α-Zn₂P₂O₇ compound was obtained by conventional solid-state reaction. The sample was characterized by X-ray powder diffraction, solid state ³¹P NMR MAS, and electrical impedance spectroscopy. The solid state ³¹P MAS NMR, performed at 121.49 MHz, shows three isotropic resonances at −21.1, −18.8, and −15.8 ppm, confirming the non-equivalency of the three PO₄ groups in the α-Zn₂P₂O₇ form. They are characterized by different chemical shift tensor parameters with the local geometrical features of the tetrahedra. Electrical impedance measurements of β-Zn₂P₂O₇, form stable for temperature greater than 403 K, were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The AC conductivity obeys the universal power law. The approximation type correlated barrier hopping model explains the universal behavior of the n exponent. The impedance plane plot shows semicircle arcs at different temperatures, and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance R _p and constant phase elements CPE. The simulated spectra show a good correlation with the experimental data.     

Synthesis and characterization of pristine Li<Subscript>2</Subscript>MnSiO<Subscript>4</Subscript> and Li<Subscript>2</Subscript>MnSiO<Subscript>4</Subscript>/C cathode materials for lithium ion batteries

The cathode materials, pristine Li₂MnSiO₄ and carbon-coated Li₂MnSiO₄ (Li₂MnSiO₄/C), were synthesized by the sol–gel method. Power X-ray diffraction and scanning electron microscopy analyses show that the presence of carbon during synthesis can weaken the formation of impurities in the final product and decrease the particle size of the final product. The effects of carbon coating on electrochemical characteristics were investigated by galvanostatic cycling test and electrochemical impedance spectroscopy. The galvanostatic cycling test results indicate that Li₂MnSiO₄/C cathode exhibits better electrochemical performance with an initial discharge capacity of 134.4 mAh g⁻¹ and a capacity retention of 63.9 mAh g⁻¹ after 20 cycles. Electrochemical impedance analyses confirm that carbon coating can increase electronic conductivity, which results in good electrochemical performance of Li₂MnSiO₄/C cathode. The two semicircles and the large arc obtained in this study can be attributed to the migration of lithium ions through the solid electrolyte interphase films, the electronic properties of the material, and the charge transfer step, respectively.     

Influence of carbon additive on the properties of spherical Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> and LiFePO<Subscript>4</Subscript> materials for lithium-ion batteries

Preparing spherical particles with carbon additive is considered as one effective way to improve both high rate performance and tap density of Li₄Ti₅O₁₂ and LiFePO₄ materials. Spherical Li₄Ti₅O₁₂/C and LiFePO₄/C composites are prepared by spray-drying–solid-state reaction method and controlled crystallization–carbothermal reduction method, respectively. The X-ray diffraction characterization, scanning electron microscope, Brunauer–Emmett–Teller, alternating current impedance analyzing, tap density testing, and electrochemical property measurements are investigated. After hybridizing carbon with a proper quantity, the crystal grain size of active materials is remarkably decreased and the electrochemical properties are obviously improved. The Li₄Ti₅O₁₂/C and LiFePO₄/C composites prepared in this work are spherical. The tap density and the specific surface area are as high as 1.71 g cm⁻³ and 8.26 m² g⁻¹ for spherical Li₄Ti₅O₁₂/C, which are 1.35 g cm⁻³ and 18.86 m² g⁻¹ for spherical LiFePO₄/C powders. Between 1.0 and 3.0 V versus Li, the reversible specific capacity of the Li₄Ti₅O₁₂/C is more than 150 mAh g⁻¹ at 1.0-C rate. Between 2.5 and 4.2 V versus Li, the reversible capacity of the LiFePO₄/C is close to 140 mAh g⁻¹ at 1.0-C rate.     

Preparation of MnO<Subscript>2</Subscript>/WMNT composite and MnO<Subscript>2</Subscript>/AB composite by redox deposition method and its comparative study as supercapacitive materials

The manganese oxide/multi-walled carbon nanotube (MnO₂/MWNT) composite and the manganese oxide/acetylene black (MnO₂/AB) composite were prepared by translating potassium permanganate into MnO₂ which formed the above composite with residual carbon material using the redox deposition method and carbon as a reducer. The products were characterized by X-ray diffraction, Fourier transform infrared, and scanning electron microscope. Electrochemical properties of both the MnO₂/MWNT and MnO₂/AB electrodes were studied by using cyclic voltammetry, electrochemical impedance measurement, and galvanostatic charge/discharge tests. The results show that the MnO₂/MWNT electrode has better electrochemical capacitance performance than the MnO₂/AB electrode. The charge–discharge test showed the specific capacitance of 182.3 F·g⁻¹ for the MnO₂/MWNT electrode, and the specific capacitance of 127.2 F·g⁻¹ for the MnO₂/AB electrode had obtained, within potential range of 0–1 V at a charge/discharge current density of 200 mA·g⁻¹ in 0.5 mol·L⁻¹ potassium sulfate electrolyte solution in the first cycle. The specific capacitance of both the MnO₂/MWNT and MnO₂/AB electrodes were 141.2 F·g⁻¹ and 78.5 F·g⁻¹ after 1,200 cycles, respectively. The MnO₂/MWNT electrode has better cycling performance. The effect of different morphologies was investigated for both MnO₂/MWNT and MnO₂/AB composites.     

Dielectric relaxation and ionic conductivity studies of Ag<Subscript>2</Subscript>ZnP<Subscript>2</Subscript>O<Subscript>7</Subscript>

The complex impedance of the Ag₂ZnP₂O₇ compound has been investigated in the temperature range 419–557 K and in the frequency range 200 Hz–5 MHz. The Z′ and Z′ versus frequency plots are well fitted to an equivalent circuit model. Dielectric data were analyzed using complex electrical modulus M* for the sample at various temperatures. The modulus plot can be characterized by full width at half-height or in terms of a non-exponential decay function f( \textt ) = exp( - \textt/t )^b \phi \left( {\text{t}} \right) = \exp {\left( { - {\text{t}}/\tau } \right)^\beta } . The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law: s( w) = s_\textdc + \textAwⁿ \sigma \left( \omega \right) = {\sigma_{\text{dc}}} + {\text{A}}{\omega^n} . The conductivity σ _dc follows the Arrhenius relation. The near value of activation energies obtained from the analysis of M″, conductivity data, and equivalent circuit confirms that the transport is through ion hopping mechanism dominated by the motion of the Ag⁺ ions in the structure of the investigated material.