Biosilica from sea water diatoms algae—electrochemical impedance spectroscopy study

Here, we report on an electrochemical impedance study of silica of organic origin as an active electrode material. The electrode material obtained from carbonized marine biomass containing nanoporous diatoms has been characterised by means of XRD, IR, SEM and EIS. Different kinds of crystallographic phases of silica as a result of thermal treatment have been found. The electrode is electrochemically stable during subsequent cyclic voltammetry measurements taken in the potential range from 0.005 up to 3.0 V vs. Li/Li⁺. The material has been found to exhibit high charge capacitance of 521 mAh g⁻¹ being cycled at a rate C/20 with capacity retention of about 97%. Electrochemical impedance spectroscopy performed at an equilibrated potential E = 0.1 V in the temperature range 288–294 K discloses low charge transfer resistivity and low diffusional impedance.

     

Reactions of solute species at an electrode modified with titanocene functionalized polypyrrole film: ferrocene and titanocene dichloride

We have studied for the first time the ability of a conducting polymer film, p(Tc3Py), representing a polypyrrole matrix with covalently attached titanocene dichloride (TcCl₂) centers, to serve as an intermediator for the electron charge transport between the electrode and the reaction sites of solute reactants. The standard potential of the first of these electroactive species, ferrocene (Fc), is in the range where the polymer matrix is in its slightly oxidized state so that solute Fc species give a reversible response at the surface of this modified electrode. Another solute reactant, TcCl₂, was studied in solutions in which it demonstrates a (quasi)reversible behavior at bare electrode surfaces, THF+TBAPF₆ and AN+TEACl. The standard redox potential of this species belongs to the range of the electroactivity of immobilized TcCl₂ centers (where the matrix is in its non-conducting state) so that the electron charge has to be transported via stepwise redox reactions between neighboring centers inside the film. The combination, solute reactant+film, results in a greater CV current compared to the response of the film in background solution or of the solute species at the bare electrode surface. This current for THF solution even exceeds the sum of separate currents for the film and the reactant. This finding is attributed to a catalytic effect of solute species as redox intermediators for the transformation of immobilized electroactive centers leading to a greater degree of the film reduction. The presence of solute TcCl₂ species results in a much greater stability of immobilized centers (compared to the corresponding reactant-free solution), both in the course of CV with the passage of the range of their response and in experiments with the film holding at the potential within this range. This holding leads to an almost constant current related to the reaction of solute species at the film/solution interface. Our estimate shows that immobilized centers undergo above 10,000 reversible transformations (without an observed tendency to the degradation) to ensure the passage of this current. The conclusion has been drawn that immobilized TcCl₂ centers are able to serve as sufficiently stable redox intermediators for the electron charge transport across the film, a prerequisite for the catalytic applications of such films.Abbreviations AN acetonitrile - THF tetrahydrofuran - Cp cyclopentadienyl, C₅H₅ - Cp cyclopentadienyl radical, C₅H₄ - Fc ferrocene, Cp₂Fe - TcCl₂ titanocene=bis(cyclopentadienyl)titanium dichloride, Cp₂TiCl₂ or its radical CpCpTiCl₂ - PPy polypyrrole - Tc3Py titanocene-propyl-pyrrole, Cl₂TiCpCp(CH₂)₃NC₄H₄ - p(Tc3Py) polymer obtained from Tc3Py - TBAPF₆ tetrabuthylammonium hexafluorophosphate - TEACl tetraethylammonium chlorideDedicated to Zbigniew Galus on the occasion of his 70th birthday.     

Does the low optical band gap of yellow Bi<Subscript>3</Subscript>YO<Subscript>6</Subscript> guarantee the photocatalytical activity under visible light illumination?

Bi₃YO₆, which is known as an ionic conductor, was tested here as an electrode and photoanode in contact with aqueous electrolytes. Bi₃YO₆ was deposited onto the Pt substrate and the such prepared electrode was polarized in various aqueous electrolytes. The optical energy band gap of the material equal to 1.89 eV was determined using the Kubelka-Munk function resulting from the UV-Vis spectrum (allowed indirect transition) and also was calculated using the semi-empirical PM7 method (3.38 eV of HOMO-LUMO energy gap). Despite the yellow color of Bi₃YO₆, the tested material exhibits photoelectroactivity only in the UV range of electromagnetic radiation. The anodic photocurrent characteristic for n-type metal oxide semiconductors was recorded. The electrode exhibits diffusion-controlled cathodic activity while polarized in chloride-free aqueous electrolytes.     

Optimization of boron-doping process of titania nanotubes via electrochemical method toward enhanced photoactivity

In this work, we were focused on the development of the electrochemical approach resulting in a stable boron doping of titania nanotubes. The doping procedure concerns anodic polarization of as-anodized titania in a H₃BO₃ solution acting as n boron precursor. The series of attempts were taken in order to elaborate the most beneficial doping conditions. The parameters of electrochemical doping allowing to obtain boron-doped titania characterized by the highest photoconversion efficiency are as follows: reaction voltage 1.8 V, process duration 0.5 h, and the concentration of boric acid 0.5 M. Spectroscopy techniques such as UV-vis, X-ray diffraction, photoluminescence emission, and X-ray photoelectron spectroscopy were used to characterize the absorbance capability and the crystalline phase, to confirm the presence of boron atoms and to study the nature of chemical compounds, respectively. The well-ordered structure of titania and resistance of its morphology toward electrochemical treatment in H₃BO₃ were confirmed by scanning electron microscopy images. However, cyclic voltammetry and electrochemical impedance spectroscopy studies showed the significant difference in conductivity and capacitance between doped and pristine titania. Moreover, the photocurrent densities of the B-doped sample were about seven times higher in comparison with those generated by the pure titania nanotube electrode.

     

Spectroelectrochemical characteristics of poly(3,4-ethylenedioxythiophene)/iron hexacyanoferrate film-modified electrodes

Spectroelectrochemical measurements of poly(3,4-ethylenedioxythiophene) (pEDOT) modified by iron hexacyanoferrate (Fehcf) network, chloride (Cl), polystyrenesulfonate (PSS), and hexacyanoferrate (FeCN) ions were shown. Depending on the electrode potential, three main maxima absorbance were recorded. The first related to, the π–π* transition in undoped state of pEDOT, the second and the third are ascribed to transitions between the valence band and the polaron and bipolaron bands, respectively. There is also identified spectrophotometric response from the ligand–metal charge transfer of hexacyanoferrates from pEDOT modified with Fehcf and FeCN. The energy band gap (E _g) was evaluated from the spectroelectrochemical curves of the undoped pEDOT films. The pEDOT/Fehcf material exhibits the band gap of 1.40 eV which is the lowest among measured E _g values equal 1.55, 1.53, and 1.58 eV for pEDOT/FeCN, pEDOT/Cl, and pEDOT/PSS, respectively. Thus, synergetic effect of polymer and Prussian blue is proved as a significant decrease of the E _g value.     

Optimization of electrochemical doping approach resulting in highly photoactive iodine-doped titania nanotubes

The paper focuses on the optimization procedure concerning the synthesis method resulting in highly ordered titania nanotubes doped with iodine atoms. The doping process was based on the electrochemical treatment of a titania nanotube layer immersed in a potassium iodide (KI) solution acting as an iodine precursor. A number of endeavors were undertaken in order to optimize the doping conditions. Electrolyte concentration, reaction voltage, and time/duration were the main factors that influenced the iodine (I)-doping effect on the photoactivity. The parameters of electrochemical doping that result in a material characterized by the highest photocurrent density are as follows: reaction voltage of 1.5 V, duration of 15 min, and 0.1 M KI. Different spectroscopic techniques, i.e., UV–Vis spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the absorbance capability and the crystalline phase, to confirm the presence of iodine atoms and to study the nature of chemical compounds. The morphology inspection performed by means of scanning electron microscopy shows that the doping process does not affect the ordered tubular architecture. The photocurrent densities of the I-doped sample were six times higher in comparison to those generated by the pure titania nanotube electrode. Moreover, doped samples act as a much better catalyst in the photodegradation process of methylene blue and formation of hydroxyl radicals (•OH) than undoped samples.