Protoporphyrin IX sensitized titanium oxide gel electrode

Titanium oxide (Ti(O)) xerogel films functionalized by protoporphyrin IX (PPIX) and ferrocene carboxylic acid (FCA) were deposited on indium tin oxide (ITO) electrodes following a sol–gel synthesis. PPIX and FCA were first complexed to titanium oxide precursors, which were then subjected to hydrolysis to obtain a homogenous Ti(O) polymeric network gel doped with PPIX and FCA. The Ti(O) film cast on the ITO electrode has been characterized by UV–Vis absorption, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and cyclic voltammetry. Illumination of the PPIX doped Ti(O) films on the ITO electrode immersed in aqueous electrolytes onsets photoinduced electron transfer reactions, and a cathodic photocurrent was observed in most cases. This photocurrent response was investigated in detail using a kinetic model. Preliminary investigations of oxygen reduction, lithium and proton insertion into the Ti(O) film have also been carried out.

Oxygen Plasma/Bismuth Modified Inkjet Printed Graphene Electrode for the Sensitive Simultaneous Detection of Lead and Cadmium

Hubert Girault, Léopoldine Guenang Sonfack, Prachi Gupta, Milica Jovic, Andreas Stephan Lesch

In this work, a simple procedure for the preparation of an inkjet printed disposable graphene electrode is reported. Commercial graphene ink was printed on a kapton substrate and the resulting electrode was 30 min treated by oxygen plasma, then modified by a bismuth salt. The as prepared electrode was characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), laser scanning microscopy (LSM) and scanning electron microscopy (SEM) coupled to energy-dispersive X-ray spectroscopy (EDX). The sensing properties of the characterized electrodes were then investigated using cyclic voltammetry and Electrochemical Impedance Spectroscopy (EIS). Afterwards, these electrodes were exploited in a comparative way for the electroanalysis of Cadmium(II) and Lead(II) ions. An increase in the electrode sensitivity due to its modification and to the presence of bismuth was observed. Some preliminary experiments based on stripping square wave voltammetry highlighted the interest of using the proposed disposable inkjet printed electrodes for the electrochemical detection of heavy metals in tap water.

2020

Selective light induced chemical vapour deposition of titanium dioxide thin films

Estelle Wagner

Light Induced Chemical Vapour Deposition (LICVD) of titanium dioxide thin films is studied in this work. It is shown that this technique enables to deposit locally and selectively a chosen crystalline phase with a precise controlled thickness at low substrate temperature, allowing even the use of polymer substrates. A home made LICVD reactor was set up, consisting of a main chamber in which the substrate was placed on a temperature controlled plate and could be irradiated perpendicularly through a window by a 250 ns long pulse XeCl excimer laser (308 nm) with a controlled fluence (energy per surface area per pulse). A liquid precursor (titanium tetra-isopropoxide) was brought by an oxygen carrier gas flow in the chamber and a nitrogen flow was added to prevent deposition on the window. The total pressure in the chamber was maintained at 10 mbar. Projection of a mask image on the substrate was realized. Titanium dioxide films could be deposited locally by this technique with a thickness ranging from few nanometers to several micrometers. Deposits were realized at substrate temperatures as low as room temperature and with fluences between 1 and 400 mJ/cm2. The deposited films were characterized by profilometry, Atomic Force Microscopy, optical microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, UV-vis spectroscopy and ellipsometry. A detailed and systematic study of the influence of some selected process parameters (substrate temperature and irradiation dose principally) on the deposition rate and material properties was realized on glass and silicon substrates. An empirical equation of the growth rate as a function of the different experimental parameters was derived and was tested successfully to obtain a precise control of the deposited thickness (a precision better than 10 nm was demonstrated for films below 300 nm), with growth rates up to 300 nm/min. From this equation, a deposition mechanism involving both a photolytic reaction and a thermal contribution of the substrate temperature was proposed. The deposited material was shown to consist of TiO2 with a possible very low additional carbon contamination. Depending on the deposition conditions, amorphous, anatase or rutile material was deposited. One parameter explaining the variation of the crystalline state of the material was the variation of the laser induced temperature rise which was simulated theoretically in parallel. Surface temperature rises in the order of several hundreds of degrees were calculated during the laser pulse, varying with the substrate, the fluence and the film thickness. The optical properties of the deposited material vary in a correlated way with the crystalline state of the material, and high indexes of refraction (up to 2.6 at 633 nm) were measured. Selective deposition in the irradiated area was studied. Provided substrate temperature was kept below 150°C, no thermal deposition was observed, and deposits were strictly located in the irradiated area. The edge resolution (around 10 micrometers) was demonstrated to be due to the optical aberrations of the mask projection set up. No effect of substrate temperature and irradiation conditions on the resolution could be evidenced, but resolution decreased with increasing deposited thickness due to optical effects. Thanks to the low substrate temperature required for the deposition process, deposition on thermo- fragile substrates (such as polymers, in particular PMMA) was demonstrated. However, in the case of polymer substrates, the fluence had to be kept below 15 mJ/cm2 not to damage or ablate the polymers, resulting in very low growth rates (in the order of 10-3 nm/pulse). In these conditions, only amorphous films with about 10% of carbon contamination could be obtained. Additionally, films were shown to crack above a certain deposited thickness, which is attributed to thermal effects. As a matter of fact, calculations showed a high temperature rise up to 70°C of the polymer/oxide interface. Deposition on polymers coated with transparent conductive oxide was also demonstrated.

EPFL2003

Electrochemical promotion of Pt catalysts for gas phase reactions

Arnaud Jaccoud

The subject of this work is the study of the electrochemical promotion of catalysis (EPOC), also called non-Faradaic electrochemical modification of catalytic activity (NEMCA). In this phenomenon, application of small currents or potentials on catalysts in contact with solid electrolytes leads to very pronounced strongly non-Faradaic and reversible changes in catalytic activity and selectivity. This work concerns the system composed of platinum film electrodes, deposited on the YSZ electrolyte (ZrO2 doped with 8% Y2O3), in contact with an oxygen containing atmosphere (the O2(g),Pt/YSZ system). The aim of this work was to focus on two points in the phenomenon of EPOC: 1) The nature of the species present on the surface of the catalyst and their impact on the catalytic activity. 2) The phenomenon of permanent promotion of catalysis (P-EPOC), for which the catalytic rate enhancement is not totally reversible, leading to a long lasting activated state. The Pt electrodes were prepared via three different methods (sputtering, thermal decomposition and screen-printing) and, as expected, the resulting microstructure, observed by scanning electron microscopy (SEM), was found to depend greatly on the preparation technique. The electrochemical response of each electrode was characterized by the technique of cyclic voltammetry. Due to the occurrence of different site-related electrochemical reactions, the voltammogram shape was highly influenced by the microstructure of the Pt/YSZ contact. Based on the preliminary studies, the cermet electrode was chosen for additional electrochemical experiments, using chronoamperometry, chronopotentiometry, cyclic voltammetry, steady state polarization and impedance spectroscopy. The results of all these techniques suggest that under anodic polarization, two parallel reactions take place. The first reaction is the oxygen evolution, giving rise to a steady state current under a constant applied potential. The other reaction is the oxygen storage (oxygen injection) in the form of Pt-O species. This second process gives rise to a time dependant current under a constant applied potential, and is therefore responsible for the pseudocapacitive behavior of the electrode. Moreover, linear sweep voltammetry measurements indicated that, by application of an anodic potential, at least three types of Pt-O species were stored, following distinct kinetics. Based on the amount of stored Pt-O and on the corresponding storage kinetics, they were attributed to three different locations on the electrode: 1) at the Pt/YSZ interface, 2) diffusing from the tpb toward the Pt/gas interface, 3) diffusing from the Pt/YSZ interface toward the bulk of the platinum electrode. It has been shown that the Pt film exhibited various activity states toward the catalytic oxidation of ethylene, depending on the oxygen species present at its surface. As evidenced by XPS measurements and in agreement with the thermodynamic data for the given system, the inactive (deactivated) state has been attributed to the formation of surface platinum oxide at T > 550°C. Decreasing the reaction temperature to 525°C caused the slow spontaneous reactivation of the catalyst and several days were necessary to reach the active state, where most of the catalyst was present in its metallic state. The catalyst activity recovery could be accelerated by the application of anodic potential pulses. In agreement with the mechanism of EPOC, this was explained by the oxide destabilizing effect of the anodically produced Oδ- species. Anodic polarization also caused reversible electrochemical promotion of catalysis at unusual high temperature (600°C). In addition, very intriguing catalytic relaxation transients were observed after prolonged anodic polarization times. Indeed, it was found that enhanced catalytic activity could be maintained for up to 10 hours after current interruption. Combination of these latter results, the related literature, the state of the art model of EPOC, and the results obtained by electrochemical techniques led to the proposition of an original model based on the processes of Pt-O storage/release at various locations of the O2(g),Pt/YSZ system. According to this model, anodic polarization produces Oδ- back-spillover species, promoting the catalytic activity, in agreement with the mechanism of EPOC. In parallel, hidden oxygen species are stored at the Pt/YSZ interface and diffuse toward the Pt phase. When the polarization is switched off, these hidden oxygen species diffuse to the gas-exposed surface and cause non-Faradaic promotion, as back-spillover Oδ- do. The large amount of stored charge and its slow diffusion controlled emergence causes the rate enhancement to last for hours.