Photoelectrochemical properties of sensitized thin films

In recent years, the fabrication of novel ultrathin film systems featuring multi-functionalities has been extensively studied due to their potential applications in photonics, sensors and solar cells. In a general sense, the primary goal of the present thesis was to develop dye-sensitized ultrathin film-modified electrodes based on the layer-by-layer methodology for future light energy conversion applications. In this work, we show that stable and uniform ultrathin multilayer films were successfully assembled by the electrostatic adsorption of alternating monolayer of poly-L-lysine (pLys) and poly-L-glutamic acid onto modified gold electrodes. The multilayer films become electroactive when ferri/ferrocyanide ions are loaded into the film and behave as photoactive when sensitized by Cadmium selenide (CdSe) quantum dots. Photocurrent responses originating from the CdSe sensitized ultrathin multilayer film are observed and were dependent on the applied potential, the thickness of the film and the presence of quenchers in the organic phase. From the obtained photocurrent and the kinetic analysis, two different regimes were observed for the potential distribution in the film: (i) the strong potential dependence regime observed in case of thin films where the system reacts as a modified electrode and (ii) the weak potential dependence regime observed in case of thick films where the potential drop occurs mainly in the first 5 layers and the system behave as supported Two Immiscible Electrolyte Solutions (ITIES). Thereafter, three-dimensional assembly of nanoparticles and pLys was found to be an efficient way to increase the photocurrent efficiency of the system. Large photocathodic and photoanodic currents were recorded in the case of CdSe and CdSe/CdS sensitized films, respectively. The kinetic analysis of photoinduced processes and coupled reactions revealed that the multilayer films behave as one electronic unit, i.e. homogeneous nanoporous semiconducting photoelectrodes. When sensitizing the multilayer film by porphyrin molecules instead of CdSe quantum dots, mainly cathodic photocurrent is observed and is dependent on the applied potential and the thickness of the film. Depending its thickness, the films vary from that of a modified electrode to that of a supported liquid film. Under illumination, the maximum photocurrent responses were reached only after five bilayers and the photocurrent efficiency was weaker than in case of CdSe nanoparticles. Furthermore, another way to increase the photocurrent efficiency is by increasing the specific surface area of the substrate. Reticulated Vitreous Carbon (RVC) foam with properties of high porosity was used as the substrate and the photocurrent efficiency is increased by an order of magnitude in comparison with a planar electrode. The photostability of CdSe nanoparticles is studied in different solvent and under different environment. In presence of light and oxygen, the photo-oxidation of CdSe particles occurs. When the particles are synthesized in water and present lattice defects, a luminescence photo-enhancement is observed due to the photo-oxidation. On the other hand, the photo-oxidation of organic soluble CdSe particles with high quantum yield and with no structure defects is translated by a photobleaching. Consequently, in both cases, the size of particles is decreased and a large absorption and emission blue shift is observed. The size decreases and the blue-shift can be precisely controlled by controlling the irradiation time by a pulsed nano-laser. The presence of ZnS shell around the particles can decrease the photo-oxidation processes. Moreover, adding a sacrificial holes acceptor (n-butylamine) is shown to be an efficient way to prevent the photo-oxidation of the particles even in presence of oxygen. The photophysical properties of cadmium telluride (CdTe) and gold particles are studied at liquid|liquid interfaces. First, time-resolved fluorescence of CdTe nanoparticles adsorbed at the liquid/liquid interface were probed by Time Correlated Single Photon Counting (TCSPC), which was compared with CdTe nanoparticles originally synthesized in aqueous solution and those transferred from the aqueous to organic solvents by using tetraoctylammonium bromide (TOAB) as a phase transfer reagent. Finally, the formation of metal-like liquid films using gold nanoparticles at liquid|liquid interfaces was successfully obtained. The optical properties of the meltal-like liquid film are studied by Surface Plasmon Resonance (SPR) technique at liquid|liquid interface. General experiments and simulations were carried out in order to determine the best parameters to optimize the SPR technique at liquid|liquid interface.

Assembly and reactivity of nanoparticles at liquid / liquid interfaces

Bin Su

The interface between two immiscible electrolyte solutions (ITIES), or liquid|liquid interface, provides a defect-free junction for fundamental studies of adsorption phenomena, heterogeneous charge transfer reactions, and phase-formation processes. Recent research interests include the organization and reactivity of metallic and semiconducting nanoparticles (NPs) at ITIES. In a general sense, the present thesis is devoted to addressing this issue, including the voltage-induced assembly of charged NPs, the photoreactivity of semiconducting NPs and the redox properties of alkanethiolate monolayer-protected gold nanoclusters. The adsorption of ionic species at the liquid|liquid interface was first modeled theoretically. The aim of the simulations was to envisage the effect of the adsorption of charged NPs on the interfacial charge distribution across the liquid|liquid interface. For simplicity, in the calculations the charged NPs were considered as large ions, which were further taken as point charges. Various adsorption isotherms were considered, including potential independent, Langmuir and Frumkin models. The simulation indicates that the ionic adsorption has a strong influence on the potential distribution across the interface. Under certain conditions, it results in a non-monotonous potential profile with a trap at the interface. The voltage induced assembly of mercaptosuccinic acid (MSA) stabilized gold and cadmium selenide (CdSe) NPs were investigated at the polarizable water|1,2-dichloroethane (DCE) interface. The studies reveal that the surface concentration of NPs at the liquid|liquid boundary is reversibly controlled by the applied bias potential. There was no evidence of irreversible aggregation or deposition of the particles at the interface. Furthermore, the adsorbed CdSe NPs at the water|DCE interface demonstrated the photoreactivity characteristic of a self-assembled ultrathin p-type semiconductor photoelectrode. The surface structure also has significant effect on the optical properties of CdSe NPs. The redox properties of hexanethiolate monolayer-protected gold nanoclusters (MPCs) were studied in detail at electrochemical interfaces. First, the absolute standard redox potential of MPCs in solutions was theoretically derived from electrostatic considerations. The significant effect of the solvent polarity was verified experimentally by studies in various organic solvents. Second, the redox properties of self-assembled MPCs on a gold electrode were studied in organic electrolytes and room temperature ionic liquids (RTILS). The effect of the electrostatic interaction between MPCs and electrode on the redox properties of MPCs was theoretically considered in terms of the method of images in classical electrostatics and justified experimentally. In RTILS, the rectification of the successive oxidation of self-assembled MPCs by the anionic component of RTILS was observed. Third, MPCs were further used as redox quenchers at the polarizable water|DCE interface. Photocurrent responses originating from the heterogeneous quenching of photoexcited porphyrins by MPCs dissolved in the DCE phase were observed. As MPCs can function as both electron acceptors and electron donors, the photocurrent results from the superposition of two simultaneous processes, which correspond to the oxidation and reduction of MPCs. The magnitude of the net photocurrent is essentially determined by the balance of the kinetics of these two processes, which can be controlled by tuning the Galvani potential difference between the two phases. We show that, within the available potential window, the apparent electron transfer rate constants follow the classical Butler-Volmer dependence on the applied potential difference.

EPFL2006

Préparation et caractérisation de nanoparticules bimétalliques (Pt-Au) sur un substrat en diamant dopé au bore

Bahaa El Roustom

Using petrol as a main source of energy, during the 20th century, has caused considerable amounts of damage among which are atmospheric pollution and global warming. At the same time, many geologists and economists expect that the discovery rate of new petrol sources will not follow the market's demand causing a serious shortage of petrol in the near future. These alarming perspectives have motivated scientists around the world to develop new clean and renewable energy sources. It is in that objective that electrochemists intensified their research in order to develop the fuel cell technology. This latter consists in directly converting chemical energy into electricity. The main advantage of this technology over traditional energy production is that the fuel cell energy efficiency is Carnot cycle independent. The theoretical efficiency of such an energy source is nearly 90%. Direct Alcohol Fuel Cell (DAFC), fed by ethanol on one hand and by air (oxygen) in the other hand, is a very interesting option for such kind of energy production. Ethanol is a renewable energy source of vegetal origin whose CO2 production is consumed by plants during the cycle. Even though Pt has long been known to be a perfect catalyst for DAFC, it has some principal limitations, the most important ones being poisoning by intermediate products (OH on the cathodic side and CO on the anodic side) and a relatively high cost compared to other materials. These problems have pushed the experts in the field to focalize their research on the development of more suitable and better performing materials. Gold nanoparticles have shown a surprising catalytic activity, very different from the bulk, making them a preferential candidate to replace Pt. A system based on using two or more metals as catalysts (example Au-Pt) dispersed on an appropriate substrate seems to be also an interesting candidate to enhance the cell's efficiency. The substrate should be chemically inert, non-oxydable and should not present any interference with the electrochemical and electrocatalytic behaviour of the nanoparticles. In the first part, the employed substrate, which is boron doped diamond (BDD) is characterized. The substrate's main characteristics are its chemical inertness, its weak residual current and its resistance to corrosion. These characteristics contribute to making the substrate an ideal support for nanoparticles. In this work, the substrate is a diamond film, but for fuel cell applications diamond should be used as powder or dispersed particles. In chapters III and IV, we studied the effect of boron incorporation on the morphology and the behaviour of the diamond film. For this purpose, samples with different ratios of boron/carbon (B/C) in the vapour phase during preparation were characterized (B/C = 300, 1000, 2500 and 6000 ppm). Results showed that the grain size decreases with increasing boron content in the film. This is accompanied by a transition towards a quasi-metallic character. It was also found that graphitic impurities in the diamond film increase when the B/C ratio increases. We also established some graphitic impurities increase in the diamond film when the ratio of B/C increases. The identity of these impurities was established in chapter III as being surface redox couples (semi-quinone/semi-hydroquinone). Using cyclic voltametry we were able to observe a total elimination (B/C=300 ppm) or partial elimination (B/C=6000 ppm) of these surface redox couples. We also found in chapter IV that these surface redox couples are responsible of the electrochemical activity of the BDD electrode. BDD electrodes with different doping level B/C=300, 1000 and 6000 ppm) in the vapour phase are modified with IrO2 nanoparticles using the method of thermal decomposition. Our results showed that, at low potentials, the behaviour of the composite electrode IrO2/BDD with respect to the reaction of oxygen evolution is independent of the doping level. On the other hand, at high potentials, the weakly doped electrode (B/C=300 ppm) presents a supplementary overpotential due to the interface BDD-IrO2. An approach based on the double junction is developed in this work describing two distinct behaviours depending on the boron concentration in the diamond film. A new method for Au nanoparticles deposition on highly doped BDD electrodes (B/C= 2500 ppm) is presented in chapter VI. This method involves two steps: the first step consists on sputtering the equivalent of few nanolayers of Au on the diamond surface; the second step is a heat treatment in air at high temperatures. Several parameters that could influence the dispersion, the stability as well as the diameter of the Au nanoparticles are studied. In chapter VII, the behaviour of the composite electrode Au/BDD is studied in a solution containing redox couples in acid media. This electrode was simulated as an arrangement of active Au microelectrodes dispersed on a less active BDD substrate. The overall behaviour of the composite electrode (Au/BDD) has been related to the electrochemical rate constants on each material. In chapter VIII, another new synthesis method for a bimetallic system formed by Au and Pt nanoparticles dispersed on a BDD substrate is developed. This method consists in electrodepositing a small amount of Pt on a composite electrode Au/BDD. Our results showed that Pt was preferentially deposited on Au covering its whole surface. In fact, Au nanoparticles serve as germ of nucleation for Pt. In the following step, the bimetallic PtbAu/BDD is heated to 400°C in air to enhance the interaction between Au and Pt. The experiments showed that Au reappeared on the surface. The electrochemical behaviour of the bi-metallic electrode is characterized by a negative shift in the peak potential of reduction of Pt oxides. This is not the case of heterogeneous alloys obtained by the fusion of two metals at high temperatures. The composite electrode Au/BDD as well as the bi-metallic electrode Pt-Au/BDD are used for electrocatalytic applications. The studied reaction in chapter IX is the oxygen reduction reaction (ORR) in an acid medium. We found that the number of exchanged electrons increases with increasing the amount of Pt at the surface. Oxygen is reduced into H2O2 on the Au/BDD electrode and into H2O on the Pt-Au/BDD electrode. In chapter X we studied the reaction of oxidation of oxalic acid. The behaviour of both Au and Pt electrodes seemed very similar for this reaction. The bi-metallic electrode containing both Pt and Au on its surface has shown an intermediate behaviour between the behaviour of the two metals.

EPFL2006

Novel catalytic applications of carbon nanofibers on sintered metal fibers filters as structured supports

Marina Ruta

Supported metal catalysts are important from both an industrial and a scientific point of view. They are used, amongst others, in large-scale processes such as catalytic reforming, hydrotreating, polymerization reactions and hydrogenations. Often, these catalysts consist of nanosized metal particles deposited on a suitable support, which acts as an anchor for the active phase and, in several cases, contributes to improve the overall catalyst performances. The growth of carbon nanofibers on sintered metal fibers filters (CNF/SMF) by hydrocarbons catalytic decomposition results in a structured composite material presenting all the suitable characteristics for its application as catalyst support. The main objective of this thesis was to develop novel catalytic systems based on CNF/SMF as catalytic support for continuous gas-phase hydrogenations. At first, the synthesis of CNF/SMF was optimized to tailor the properties of the material for further applications as catalytic support. The use of ethylene as carbon precursor and high synthesis temperature (973K) provided high yields of well-ordered CNF with increased specific surface area (SSA), up to 516 m2/g. On the other hand, the synthesis of CNF by ethane decomposition, which is less reactive than ethylene, resulted in a support with higher permeability, minimizing the pressure drop and being more suitable in a continuous-flow reactor. After the synthesis, the CNF surface was activated by treatment with O3 or H2O2. A novel technique, which consist of XPS analysis after step-wise TPD in UHV conditions, was developed and applied for the surface characterization. It allowed to assess the nature of the functional groups created on the CNF surface. The O3-activation was found the most effective for increasing the acidity of the CNF/SMF surface, yielding a relatively high amount of carboxylic functional groups. These groups are important because involved in the chemical anchoring and stabilization of the active metal deposited on the support. Industrially, many hydrogenations are performed over supported metal catalyst. The majority of these reactions are known to be structure sensitive reactions, so the control of the metal particle size is crucial in order to study the catalyst activity and selectivity. We prepared monodisperse-sized Pd nanoparticles (8, 11 and 13 nm) via the reverse microemulsion method and deposited on CNF/SMF and activated-CNF/SMF for a two-fold study: the effect of the nanoparticles size and the effect of the support nature on the selective hydrogenation of acetylene. The antipathetic size dependence of the TOF disappeared at particle size bigger than 11 nm. The initial selectivity to ethylene (∼60%) was found size-independent. The structure-sensitivity relations have been discussed in terms of "geometric" and "electronic" nature of the size-effect and rationalized assuming a Pd-Cx phase formation which is known to be size-dependent. CNF/SMF supports with increased acidity diminished the formation of coke and changed the by-products distribution, diminishing the catalyst deactivation. Subsequently, CNF/SMF was applied to develop a new catalytic system: the Structured Supported Ionic Liquid Phase (SSILP) catalyst, embedding an active Rh complex. The selective gas-phase hydrogenation of 1,3-cyclohexadiene to cyclohexene was used as model reaction. The SSILP based on CNF/SMF supports showed a high turnover frequency, up to 250 h-1 and selectivity > 96%. Multiple advantages of supporting a homogeneous active phase, dissolved in IL, on CNF/SMF have been demonstrated: the thin layer of IL was homogeneously distributed over the mesoporous support, so mass transfer limitations could be avoided, furthermore the chemical inertness of the CNF prevented any interaction between the active phase dissolved in IL ensuring an efficient use of the Rh complex. The SSILP concept can be applied not only for the "heterogenization" of a transition metal catalyst, but also to obtain metal nanoparticles with monodispersed size via the reduction of a metal precursor dissolved in IL. In this case, IL provides a suitable environment for the nucleation and growth of the nanoparticles. By applying the SSILP concept to the selective hydrogenation of acetylene, the influence of IL on the stability of the nanoparticles was investigated. Moreover, the different solubilities of reactants and products in the IL phase allowed to improve the selectivity toward ethylene. Monodispersed Pd nanoparticles of 5 and 10 nm in SILP on CNF/SMF showed excellent long-term stability. The lower solubility of ethylene than acetylene in the IL had the beneficial effect of allowing high selectivity to ethylene up to 85%, due to the inhibition of its consecutive hydrogenation to ethane. In conclusion, a novel catalytic system for continuous-flow, gas-phase hydrogenations was conceived, taking advantage of the suitable characteristics of CNF/SMF supports. The SSILP concept, involving both homogeneous and heterogeneous active phases has been demonstrated, indicating a great potential and flexibility for continuous-flow industrial applications.

EPFL2008

Novel catalytic applications of carbon nanofibers on sintered metal fibers filters as structured supports

Marina Ruta

EPFL2008

Préparation et caractérisation de nanoparticules bimétalliques (Pt-Au) sur un substrat en diamant dopé au bore

Bahaa El Roustom

EPFL2006

Assembly and reactivity of nanoparticles at liquid / liquid interfaces

Bin Su

EPFL2006