9 “Decreasing the time response of calibration-free pH sensors based on tungsten bronze nanocrystals” Regina Cisternas, María Luisa Valenzuela, Luis Ballesteros, Heike Kahlert, Fritz Scholz, Journal of Electroanalytical Chemistry 81, (2017) 315-318
Abstract: A considerable improvement of time constants of tungsten bronze pH sensors was achieved by decreasing the crystal size of the tungsten bronze previously reported by the authors. Experiments have been performed on the basis of a previously developed calibration-free pH tungsten bronze electrode. The nano-sizing of the tungsten bronze was realized by addition of sodium chloride to the Na2WO4/WO3 melt in which tungsten wires were oxidized. With increasing NaCl concentration, the crystals size decreased
8 “Electroanalytical analysis of glassy carbon electrode modified with COOH- and NO2- functionalized polyphosphazenes” María Luisa Valenzuela, Regina Cisternas, Paola Jara- Ulloa, Lissette Rodriguez, Journal of the Chilean Chemical Society 62 (2), (2017) 3515-3518
Abstract: Glassy carbon electrode modified with COOH− and NO2−functionalized polyspyrophosphazenes were used to study the redox process of a electroactive probe (ferrocene) in non-aqueous electrolyte. The electrochemical behavior of polyphosphazenes substituted with COOH and NO2 groups has been studied using cyclic voltammetry. Their blocking properties depend on the substituent group and on the substitution/nitration degree. The oxoreduction of the couple Fc/Fc+ occurs with rapid redox kinetic at the modified glassy carbon electrode with high nitration degree NO2−functionalized polyspyrophosphazene. The polymers were immobilized on the glassy carbon surface at different concentrations to study surface charge density effect. In this report were studied the reproducibility, repeatability and stability of the modified electrodes.
7 “The electrode responses of a tungsten bronze electrode differ in potentiometry and voltammetry and give access to the individual contributions of electron and proton transfer” Regina Cisternas, Heike Kahlert, Harm Wull, Fritz Scholz, Electrochemistry Communications 56, (2015) 34-37
Abstract: A tungsten wire covered with Na0.75WO3 acts in potentiometry as a reversible pH electrode having a pH dependent open-circuit potential Eocp with nernstian slope. The mid-peak potential Emp of cyclic voltammograms also depends on pH. At low pH (e.g., pH 2) and slow scan rates (e.g., 2 mV s-1) the voltammetric response is almost completely reversible. At higher pH and faster scan rates, the voltammetric systems exhibit features of increasing irreversibility. Under the conditions of reversibility, the Eocp and Emp differ significantly. Eocp is determined by the proton transfer at the electrode surface; whereas Emp is determined by the electron transfer equilibrium tungsten(VI)/tungsten(V) and the proton transfer at the electrode surface. The difference between Eocp and Emp provides the individual thermodynamic contributions of electron and proton transfer to the overall pH dependent redox electrode. This is the first time that both contributions can be separated for an insertion electrochemical system (thin surface layer). It is also shown for the first time that the mechanism of an ion-sensitive electrode can differ in potentiometry and voltammetry.
6 “Direct contact tungsten bronze electrode for calibration free potentiometric pH measurements” Regina Cisternas, Heike Kahlert, Harm Wull, Fritz Scholz, Electrochemistry Communications 60 (2015) 17-20
Abstract: Tungsten wires were oxidized in a Na2WO4/WO3 melt, resulting in pure well-ordered crystal layers of Na0.75WO3 with cubic perovskite structure. These electrodes exhibited an excellent linear behavior of the open-circuit potential Eocp depending on the pH of the electrolyte solution with an almost nernstian slope in the pH range 1 to 10 at 25 °C. Because of the direct contact between tungsten and the bronze of well-defined stoichiometry, the electrode potentials are highly reproducible allowing calibration-free pH measurements. The electrodes showed fast response times, and long life times.
5 “Electrochemical synthesis and photoelectrochemical characterization of copper hexacyanoferrate (II) deposited on n-Si(100)” Regina Cisternas, Ricardo Córdova, Rodrigo Henríquez, Ricardo Schrebler, Eduardo Muñoz, Journal of Electroanalytical Chemistry 741 (2015) 120- 126
Abstract: In this study, we examined the synthesis and characterization of copper hexacyanoferrate (II) on an n-Si(1 0 0) substrate. The copper hexacyanoferrate (II) was formed in the following stages: (i) deposition of metallic copper on an n-Si(1 0 0) electrode using an electroless process and (ii) electrooxidation of the copper film in a solution containing Fe(CN)6 4- ions. In the first stage, a study using a potential step method was performed, and the corresponding nucleation and growth mechanisms were determined. A morphologic analysis of the deposits obtained at different potentials was performed using atomic force microscopy. The results were consistent with a 3D progressive nucleation with diffusion-controlled growth. Electrochemical characterization under illumination demonstrated a charge transfer process due to the photogenerated holes in the valence band of silicon. Afterwards, the diffusion coefficient of the potassium ions and rate constants of the electrochemical processes were determined. Finally, this research is designed to increase the understanding of this system for its possible use in an electrochemical storage device that can be loaded in situ via the photovoltaic action of the doped silicon.
4 “Electrochemical synthesis and nucleation and growth mechanism of Prussian blue films on p-Si(100) electrodes” Eduardo Muñoz, Ricardo Córdova, Rodrigo Henríquez, Ricardo Schrebler, Regina Cisternas, Ricardo Marotti, Journal of Solid State Electrochemistry 16, 1 (2012) 93-100
Abstract: In this study, we examined the synthesis of Prussian blue onto p-Si(100). The Prussian blue formation was carried out by means of the deposition of a Fe film and then its dissolution in presence of potassium hexacyanoferrate(II). In the first stage, a study by cyclic voltammetry was carried out, and then, using the potential step method, the corresponding nucleation and growth mechanism were determined. Likewise, a morphologic analysis of the deposits obtained at different potential values by means of atomic force microscopy was carried out. The results are consistent with a 3D progressive nucleation with diffusion-controlled growth. Finally, this research is oriented to construct electrochemical storage devices which can be in situ loaded by the photovoltaic action of the semiconductor base material doped silicon.
3 “Photoelectrochemical and optical characterization of Prussian blue films on p-Si(100) electrodes” Eduardo Muñoz, Rodrigo Henríquez, Ricardo Córdova, Ricardo Schrebler, Regina Cisternas, Luis Ballesteros, Ricardo Marotti, Enrique Dalchiele, Journal of Solid State Electrochemistry 16, 1 (2012) 165-171
Abstract: In this study, we examined the characterization of Prussian blue deposited onto p-Si(100). A cyclic voltammetry analysis was carried out under illumination showing quasi-reversibility responses of high and low-spin iron centres in the deposit. Optical measurements were done, where XRD analysis allowed to determine crystallinity while EDS analysis indicated that there is influence in the number of cycles on the film composition. Reflectance measurements confirm the coloration observed in the films. However a Kubelka–Munk analysis demonstrates the presence of blue greenish coloration which is an indication of a mix between Prussian blue and Berlin green films. Finally, this research is oriented to construct electrochemical storage devices which can be in situ loaded by the photovoltaic action of the semiconductor base material-doped silicon.
2 “Irreversible electrostatic deposition of Prussian blue from colloidal solutions” Regina Cisternas, Eduardo Muñoz, Rodrigo Henríquez, Ricardo Córdova, Heike Kahlert, Ulrich Hasse, Fritz Scholz, Journal of Solid State Electrochemistry 15, 11-12 (2011) 2461-2468
Abstract: Prussian blue (PB) can be deposited from colloidal solutions (5.4 × 10−3 molPB L-1, 0.01 mol L-1KNO3) on glassy carbon, either by potential cycling or potentiostatically, provided that the deposition potential is more positive than −0.2 V vs. Hg/Hg2Cl2. Depending on the deposition potential, the PB particles form either a single layer of Everitt’s salt, of PB, or multilayers of Berlin green. Also depending on the electrode potential, the deposition was accompanied by currents which were either only of capacitive nature, or represent the sum of capacitive and faradaic currents. The currents were always limited by the diffusion of the colloidal particles to the electrode surface, i.e., they obeyed the Cottrell equation. The PB layers were characterized by in situ atomic force microscopy.
1 “Characterization of natural tetrahedrite/aqueous solutions system by electrochemical techniques” Paula Grez, Ricardo Schrebler, Rodrigo Henríquez, Regina Cisternas, Marcos Orellana, Ricardo Córdova, Journal of Electroanalytical Chemistry 628, 1-2 (2009) 134-143
Abstract: The electrochemical behavior of the natural tetrahedrite in quiescent solutions covering a wide pH range was investigated using cyclic voltammetry (CV), open circuit potential (OCP), stationary polarization (SP) and electrochemical impedance spectroscopy (EIS) techniques. During the electrooxidation, the following processes might occur: (i) a redox internal process through which Cu(I) ions, present in the mineral tetrahedrite structure, are oxidized to Cu(II) ions, the possible mechanism for this process would be a hopping electron diffusion, (ii) the formation of a surface copper-depleted polysulfide film together with a release of Cu(I) from the mineral structure, and (iii) the Cu(I) ions freed from the tetrahedrite structure, according to pH solution and potential applied to the interface electrode/solution, can evolve to cupric ion or oxygen-containing Cu(II) species (oxo–hydroxo compounds). When the formation of oxo–hydroxo compounds takes place, the surface film becomes more complex but the electrode surface does not become fully passive possibly due to an irregular arrangement of its constituent species. During the electroreduction of tetrahedrite, the release of sulfur occurs and the consequent formation of Cu2S and antimony sulfurs with different stoichiometry would take place.