Modified Electrodes for Energy and Sensing Applications

This thesis describes the research focused on the study of different electrode supports modified with layered double hydroxides (LDHs) on Co or Ni as M(II) and Al or Fe as M(III) or conducting polymers for energy applications. The LDHs were characterized by electrochemical techniques, FE-SEM, XRD, XPS and XAS. Glassy carbon and Pt electrodes modified with electrosynthesized LDHs were employed in order to investigate their performances as oxygen evolution reaction catalysts and as pseudocapacitor materials. Moreover, the electrochemical synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) on indium tin oxide (ITO) was carried out in order to exploit an alternative route to fabricate bulk heterojunction solar cells with similar performances but less expensive than those obtained by casting. The photoactive layer was composed by [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) as electron acceptor, while as donor polymer it was employed either the commonly employed rr-poly(3-hexylthiophene) or a polythiophene copolymer, functionalized with a porphyrin derivative in order to improve the absorption in the UV/Vis region. In the second part of the thesis, the LDHs modified electrodes were employed for sensing, taking into account the electrocatalytic oxidation of sugars. Ni/Al or Ni/Fe LDHs were studied with the aim to investigate again the effect of Fe on the electrocatalysis. LDHs prepared both by chemical and electrochemical syntheses were employed with the aim of studying the effect of the order degree on the LDHs performance since this parameter is crucial to improve the “sensing” properties. Furthermore, a sensor for the amperometric detection of sugars in flow systems, based on Co/Al LDH electrosynthesized on Pt electrodes, was developed. A mixture of sugars was submitted to high performance anion chromatography with amperometric detection, using the modified electrode as the working electrode. Moreover, to assess the applicability of the device glucose, fructose, and sucrose content in real samples were successfully determined

Platinum-decorated graphene: Experimental insight into growth mechanisms and hydrogen adsorption properties

Pt-functionalized graphene shows promise for near-ambient hydrogen storage due to graphene's potential as a hydrogen host and platinum's role as a catalyst for the hydrogen evolution reaction and spillover effect. This study explores Pt cluster formation on epitaxial graphene and its suitability for hydrogen storage. Scanning Tunneling Microscopy reveals two growth pathways. Initially, up to similar to 1 monolayer of Pt coverage, Pt tends to randomly disperse and cover the graphene surface, whereas the cluster height remains unchanged. Beyond a coverage of 3 monolayer, the nucleation of new layers on existing clusters becomes predominant, and the clusters mainly grow in height. Thermal Desorption Spectroscopy on hydrogenated Pt-decorated graphene reveals the presence of multiple hydrogen adsorption mechanisms. Two Gaussian peaks, which we attribute to hydrogen physisorbed (peak at 155 degrees C) and chemisorbed (peak at 430 degrees C) on the surface of Pt clusters are superimoposed on a linearly increasing background assigned to hydrogen bonded in the bulk of the Pt clusters. These measurements demonstrate the ability of Pt-functionalized graphene to store molecular hydrogen at temperatures that are high enough for stable hydrogen binding at room temperature

Platinum-Decorated Graphene: Experimental Insight into Growth Mechanisms and Hydrogen Adsorption Properties

Pt-functionalized graphene shows promise for near-ambient hydrogen storage due to graphene’s potential as a hydrogen host and platinum’s role as a catalyst for the hydrogen evolution reaction and spillover effect. This study explores Pt cluster formation on epitaxial graphene and its suitability for hydrogen storage. Scanning Tunneling Microscopy reveals two growth pathways. Initially, up to 1 ML of Pt coverage, Pt tends to randomly disperse and cover the graphene surface, whereas the cluster height remains unchanged. Beyond a coverage of 3 ML, the nucleation of new layers on existing clusters becomes predominant, the clusters mainly grow in height. Thermal Desorption Spectroscopy on hydrogenated Pt-decorated graphene reveals the presence of multiple hydrogen adsorption mechanisms. Two Gaussian peaks which we attribute to hydrogen physisorbed (peak at 155°C) and chemisorbed (peak at 430°C) on the surface of Pt clusters, superimoposed on a linearly increasing background assigned to hydrogen bonded in the bulk of the Pt clusters. These measurements demonstrate the ability of Pt-functionalized graphene to store molecular hydrogen at temperatures that are high enough for stable hydrogen binding at room temperature

Three-dimensional graphene on a nano-porous 4H-SiC backbone: a novel material for food sensing applications

Sensors which are sensitive to volatile organic compounds and thus able to monitor the conservation state of food, are precious because they work non-destructively and allow to avoid direct contact with the food, ensuring hygienic conditions. In particular, the monitoring of rancidity would solve a widespread issue in food storage. The sensor discussed here is produced utilizing a novel three-dimensional arrangement of graphene, which is grown on a crystalline silicon carbide (SiC) wafer previously porousified by chemical etching. This approach allows a very high surface-to.volume ratio. Furthermore, the structure of the sensor surface features a large amount of edges, dangling bounds, and active sites, which make the sensor, on a chemically robust skeleton, chemically active, particularly to hydrogenated molecules. The interaction of the sensor with such compounds is read out by measuring the sensor resistance in a four wire configuration. The sensor performance has been assessed on three hazelnut samples: sound hazelnuts, spoiled hazelnuts, and stink bug hazelnuts. A resistance variation of about DeltaR = 0.13 (0.02) Ohm between sound and damaged hazelnuts has been detected. Our measurements confirm the ability of the sensor to discriminate between sound and damaged hazelnuts. The sensor signal is stable for days, providing the possibility to use this sensor for the monitoring of the storage state of fats and foods in general.Comment: 11 pages, 6 figures 1 tabl

Metal Nanoparticle-Functionalized Three-Dimensional Graphene: a versatile platform towards sensors and energy-related applications

We demonstrate the first successful functionalization of epitaxial three-dimensional graphene with metal nanoparticles. The functionalization is obtained by immersing the 3D graphene in a nanoparticle colloidal solution. This method is versatile and here is demonstrated for gold and palladium, but can be extended to other types and shapes of nanoparticles. We have measured the nanoparticle density on the top-surface and in the porous layer volume by Scanning Electron Microscopy and Scanning Transmission Electron Microscopy. Samples exhibit a high coverage of nanoparticles with minimal clustering. High quality graphene has been demonstrated to promote the functionalization leading to higher nanoparticle density, both on the surface and in the pores. X-ray Photoelectron Spectroscopy allowed to verify the absence of contamination after the functionalization process. Moreover, it confirmed the thermal stability of the Au- and Pd-functionalized three-dimensional graphene up to 530{\deg}C. Our approach opens up new avenues for utilizing three-dimensional graphene as a versatile platform for catalytic applications, sensors, and energy storage and conversion

Pseudocapacitors based on layered double hydroxides electrodeposited on Pt electrodes

Electrochemical capacitors also known as supercapacitors can be divided into two categories, namely, electric double layer capacitors (EDLCs), founded on non-Faradic charge storage process, and pseudocapacitors, which use metal oxides/hydroxides as the main electrodes since their capacitance arises from redox processes occurring at or near the solid electrode surface. Layered double hydroxides (LDHs) especially those containing transition metals are considered as ideal pseudocapacitive materials due to their peculiar properties such as efficient anion exchange capacity and high redox activity[1,2]. LDHs have the general formula [M(II)1 xM(III)x(OH)2]x+[Xqx/q−· nH2O] where M(II) and M(III) are bivalent and trivalent metal cations and X is the charge-balancing interlayer anion. Electrosynthesis is an efficient method to prepare LDH thin films suitable for sensing applications and in the last few years our group has optimised the one-step electrodeposition, mainly on Pt electrodes, of LDHs based on redox active metals as Ni or Co and Al [3]. The applications of LDH modified electrodes requires the formation of well adherent thin films and this result can be achieved if Pt surface is electrochemically pre-treated in 0.1 M H2SO4 [4]. In this work four LDHs containing Co and Ni, as bivalent and Fe and Al as trivalent cations have been synthesized on Pt by electrochemical reduction, at –0.90 V vs SCE for 30 s, of the proper electrolytic solution [5]. All the LDHs have been characterized in basic solution (0.1 and 1 M NaOH) to investigate if they behave as pseudocapacitive materials by using cyclic voltammetry and galvanostatic charge/discharge curves. The calculation of capacitance per gram of material is very important when evaluating materials for this application, so the mass deposited during the synthesis was determined using the electrochemical quartz crystal microbalance. As an example in Fig. 1 a and b the CV and the galvanostatic charge/discharge curves recorded for the LDH containing Al and Co, are shown. All the LDHs displayed good performances both in terms of specific capacitance and life cycles, as estimated by galvanostatic charge/discharge curves. As conductive support also glassy carbon was investigated in order to fabricate cheaper devices

Deposizione elettrochimica di film polimerici foto-attivi per la realizzazione di celle solari

Plastic solar cells bear the potential for large-scale power generation based on flexible, lightweight, inexpensive materials. Since the discovery of the photo-induced electron transfer from a conjugated polymer (electron-donor) to fullerene or its derivatives molecules (electron-acceptors), followed by the introduction of the bulk heterojunction concept which means donors and acceptors blended together to realize the fotoactive layer, materials and deposition techniques have been extensively studied. In this work, electrochemical-deposition methods of polymeric conductive films were studied in order to realize bulk heterojunction solar cells. Indium Tin Oxide (ITO) glass electrodes modified with a thin layer of poly(3,4-ethylenedioxythiophene) (PEDOT) were electrochemically prepared under potentiodynamic and potentiostatic conditions; then those techniques were applied for the electrochemical co-deposition of donor and acceptor on modified ITO electrode to produce the active layer (blend). For the deposition of the electron-donor polymer the electropolymerization of many functionalized thiophene monomers was investigated while, as regards acceptors, fullerene was used first, then the study was focused on its derivative PCBM ([6,6]-phenyl-C61-butyric acid methyl ester). The polymeric films obtained (PEDOT and blend) were electrochemically and spectrophotometrically characterized and the film thicknesses were evaluated by atomic force microscopy (AFM). Finally, to check the performances and the efficiency of the realized solar cells, tests were carried out under standard conditions. Nowadays bulk heterojunction solar cells are still poorly efficient to be competitively commercialized. A challenge will be to find new materials and better deposition techniques in order to obtain better performances. The research has led to several breakthroughs in efficiency, with a power conversion efficiency approaching 5 %. The efficiency of the solar cells produced in this work is even lower (lower than 1 %). Despite all, solar cells of this type are interesting and may represent a cheaper and easier alternative to traditional silicon-based solar panels

Bringing Again Noble Metal Nanoparticles to the Forefront of Cancer Therapy

Nanomaterials have attracted increasing interest for their potentiality to revolutionize the diagnosis and treatment of many diseases, especially neoplasms. Interestingly, there is a huge imbalance between the number of proposed nanoplatforms and the few ones approved for clinical applications. This disequilibrium affects in particular noble metal nanoparticles (NPs), that present no-approved platform and very few candidates in clinical trials because of the issue of persistence. In this perspective, we discuss if nanomedicine is generally keeping its promises with a focus on the approach that could fill the gap between NPs and oncology in the next future: the ultrasmall-in-nano

GLASSY CARBON MODIFIED ELECTRODES WITH LAYERED DOUBLE HYDROXIDES FOR THE OXYGEN EVOLUTION REACTION

Glassy carbon electrodes (GCE) were modified with a thin film of Ni/Al and Ni/Fe Layered Double Hydroxides (LDHs). The LDHs were chemically synthesized, by the double-microemulsion method according to literature data1. That modified electrodes were found to be active towards the oxygen evolution reaction (OER). The OER activity of LDHs modified electrodes was investigated in 0,1 M NaOH, at a slow scan rate of 5 mV/s, between 0 V and the OER onset potential (Ag/AgCl/3 M was used as reference electrode). During the measurements the working electrode was rotating at 1600 rpm to remove the generated oxygen bubbles from it surface. The electrochemical characterization of the Ni/Al and Ni/Fe-LDHs evidenced that the OER onset potential of those modified electrodes occurred respectively at +0.55 V and +0.53 V, showing a considerably reduced potential, compared to the values recorded in the same conditions with the bare GCE or GCE modified with a film of not redox active Mg/Al and Mg/Fe-LDHs (onset of OER current at ~1.20 V). A similar behaviour to the chemically synthesized active catalysts, was observed for the ones electrochemically synthesized from a solution of Ni(NO3)2 and Fe(NO3)3 or Al(NO3)3 (molar ratio 3:1), by applying an anodic potential of -0,9 V for 90 seconds. Furthermore the active LDHs exhibited a good stability in alkaline solution, since the chronopotentiometry curves, recorded applying a current density of 2,5 mA/cm2 for five minutes, showed that the catalysts had a nearly constant operating potential

Reduced Graphene Oxide and Carbon Nanotubes for the development of polyphenols amperometric biosensors

Biosensors for sensitive, rapid and precise determination of phenolic compounds are attracting growing interest in environmental control and protection as well as in food industry. Laccase (Lac) and tyrosinase (Tyr) are multicopper oxidase enzymes that catalyze the oxidation of phenol derivatives to the relevant quinones with the concomitant reduction of oxygen directly to water without the formation of reactive oxygen intermediates. Here we present the development of amperometric biosensors based on Lac or Tyr physically adsorbed on glassy carbon electrodes modified with carbon-based nanomaterials. Carbon-based nanomaterials are often employed in electrochemistry for their beneficial properties and in the last few years they have been frequently used in the development of biosensors to enhance the electron transfer between the electrode and the enzyme. Graphene represents an excellent material for sensing applications due to properties like fast heterogeneous electron transfer, large surface area, high mechanical strength, ease of functionalization, high conductivity and good biocompatibility. The electron transfer rates on graphene sheets obtained by electrochemical reduction of graphene oxide (GO) are similar to those observed for carbon nanotubes and higher than those of glassy carbon electrodes since the reduction of oxygen functional groups at edge and basal planes produces defect sites. The electrochemical reduction of GO is a versatile method to obtain a graphene layer (rGO) on the electrode surface which still displays some controllable oxygen-containing functionalities, usable for covalent or physical immobilization of enzymes. The enzyme immobilization represents a rather critical issue because the methods used for this procedure significantly influence the biosensor properties, operability and long-term stability. Among the methods available in the case of tyrosinase and laccase, the most commonly employed is the physical entrapment, obtained by a cross-linking with bovine serum albumin (BSA) and glutaraldehyde (GA)