7 resultados para Nanostructured systems

em AMS Tesi di Dottorato - Alm@DL - Università di Bologna


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The object of the present study is the process of gas transport in nano-sized materials, i.e. systems having structural elements of the order of nanometers. The aim of this work is to advance the understanding of the gas transport mechanism in such materials, for which traditional models are not often suitable, by providing a correct interpretation of the relationship between diffusive phenomena and structural features. This result would allow the development new materials with permeation properties tailored on the specific application, especially in packaging systems. The methods used to achieve this goal were a detailed experimental characterization and different simulation methods. The experimental campaign regarded the determination of oxygen permeability and diffusivity in different sets of organic-inorganic hybrid coatings prepared via sol-gel technique. The polymeric samples coated with these hybrid layers experienced a remarkable enhancement of the barrier properties, which was explained by the strong interconnection at the nano-scale between the organic moiety and silica domains. An analogous characterization was performed on microfibrillated cellulose films, which presented remarkable barrier effect toward oxygen when it is dry, while in the presence of water the performance significantly drops. The very low value of water diffusivity at low activities is also an interesting characteristic which deals with its structural properties. Two different approaches of simulation were then considered: the diffusion of oxygen through polymer-layered silicates was modeled on a continuum scale with a CFD software, while the properties of n-alkanthiolate self assembled monolayers on gold were analyzed from a molecular point of view by means of a molecular dynamics algorithm. Modeling transport properties in layered nanocomposites, resulting from the ordered dispersion of impermeable flakes in a 2-D matrix, allowed the calculation of the enhancement of barrier effect in relation with platelets structural parameters leading to derive a new expression. On this basis, randomly distributed systems were simulated and the results were analyzed to evaluate the different contributions to the overall effect. The study of more realistic three-dimensional geometries revealed a prefect correspondence with the 2-D approximation. A completely different approach was applied to simulate the effect of temperature on the oxygen transport through self assembled monolayers; the structural information obtained from equilibrium MD simulations showed that raising the temperature, makes the monolayer less ordered and consequently less crystalline. This disorder produces a decrease in the barrier free energy and it lowers the overall resistance to oxygen diffusion, making the monolayer more permeable to small molecules.

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Oxygen Reduction Reaction (ORR) requires a platinum-based catalyst to reduce the activation barrier. One of the most promising materials as alternative catalysts are carbon-based, graphene and carbon nanotubes (CNT) derivatives. ORR on a carbon-based substrate involves the less efficient two electrons process and the optimal four electrons process. New synthetic strategies to produce tunable graphene-based materials utilizing graphene oxide (GO) as a base inspired the first part of this work. Hydrogen Evolution Reaction (HER) is a slow process requiring also platinum or palladium as catalyst. In the second part of this work, we develop and use a technique for Ni nanoparticles electrodeposition using NiCl2 as precursor in the presence of ascorbate ligands. Electrodeposition of nano-nickel onto flat glassy carbon (GC) and onto nitrogen-doped reduced graphene oxide (rGO-N) substrates are studied. State of the art catalysts for CO2RR requires rare metals rhenium or rhodium. In recent years significant research has been done on non-noble metals and molecular systems to use as electro and photo-catalysts (artificial photosynthesis). As Cu-Zn alloys show good CO2RR performance, here we applied the same nanoparticle electrosynthesis technique using as precursors CuCl2 and Cl2Zn and observed successful formation of the nanoparticles and a notable activity in presence of CO2. Using rhenium complexes as catalysts is another popular approach and di-nuclear complexes have a positive cooperative effect. More recently a growing family of pre-catalysts based on the earth-abundant metal manganese, has emerged as a promising, cheaper alternative. Here we study the cooperative effects of di-nuclear manganese complexes derivatives when used as homogeneous electrocatalysts, as well as a rhenium functionalized polymer used as heterogeneous electrocatalyst.

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Polycyclic aromatic hydrocarbons (PAHs) are a large class of π-conjugated organic molecules with fused aromatic rings, which can be considered as fragments of 2D-graphene and have been extensively studied for their unique optical and electronic properties. The aim of this study is to understand the complex electrochemical behaviour of planar, curved, and heteroatom doped polycyclic aromatic molecules, particularly focusing on the oxidative coupling of their radical cations and the electrochemically induced cyclodehydrogenation reactions. In the first part of this thesis, the class of PAHs and aromatic nanostructures are introduced, and the reactivity of electrogenerated species is discussed, focusing on the electrochemical approach for the synthesis of extended π-conjugated structures. Subsequently, the electrochemical properties and reactivity of electrogenerated radical ions of planar and curved polyaromatics are correlated to their structures. In the third chapter, electrochemical cyclodehydrogenation of hexaphenylbenzene is used to prepare self-assembled hexabenzocoronene, directly deposited on an interdigitated electrode, which was characterised as organic electrochemical transistor. In the fourth chapter, the electrochemical behaviour of a family of azapyrene derivatives has been carefully investigated together with the electrogenerated chemiluminescence (ECL), both by ion-annihilation and co-reactant methods. Two structural azapyrene isomers with different nitrogen positions are thoroughly discussed in terms of redox and ECL properties. Interestingly, the ECL of only one of them showed a double emission with excimer formation. A detailed mechanism is discussed for the ECL by co-reactant benzoyl peroxide, to rationalise the different ECL behaviours of the two isomers on the basis of their topologically modulated electronic properties. In conclusion, the different electrochemical behaviours of PAHs were shown, focussing on the chemical reactivity of the electrogenerated species and taking advantage of it for important processes spanning from unconventional synthesis methods for carbon nanostructures to the exploitation of self-assembled nanostructured systems in organic electronics, to novel organic emitters in ECL.

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Nanomaterials are nowadays widely recognised as advantageous sensing tools due to their unique properties. Some natural nanomaterials, such as DNA or hyaluronic acid analysed in this PhD thesis, have an intrinsic biocompatibility that overcomes a series of issues in the field of sensing in biological environments. Therefore, the main aim of this project was to derivatize HA chains with luminescent dyes - both organic and metal complexes - in order to obtain natural polymer-based optical sensors. A derivatization of HA with these moieties was obtained and a photophysical characterization was provided. To prove their sensing ability towards nanomaterials, the interaction with. PluS Nanoparticles, featuring an outer PEG shell, was tested. It was mostly demonstrated that the main features of the luminophores used were present in the HA nanogels as well. For example, HA@Dansyl was proven to be a luminescent probe able to sense different environment polarities. Furthermore, in HA@PA the amount of excimers/monomers emission was found to be relatable to the degree of entanglement of HA chains, that changes upon interactions with nanoparticles. Moreover, two ruthenium bipyridyl derivatives were linked to HA and it was found out that HA interacts with long DNA sequences. Also, the presence of BPA, a small molecule of environmental concern, was detected using (i) an already studied hyaluronic acid derivative with rhodamine (HA@RB) , (ii) a dizinc ruthenium complex coordinating BPA to the metal centres, and (iii) a new probe constituted by PluSNPs@DEAC and HA@RB. Despite all the systems were found to be able to detect BPA, the latter probe presented advantages in terms of sensitivity. Furthermore, the chapter 2 of this thesis is focused on the detection of a NF-κB protein in PC3 cancer cells. via confocal microscopy by following a FRET signal variation on a triplex-hairpin derivatized with a FRET couple of dyes.

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This Ph.D. thesis concerns the synthesis of nanostructured Cu-containing materials to be used as electrode modifiers for the CO2 electroreduction in aqueous phase and the evaluation of their catalytic performances. Inspired by the fascinating concept of the artificial photosynthesis-oriented systems, several catalytic layers were electrochemically loaded on carbonaceous gas diffusion membranes, i.e., 3D structures that allow the design of eco-friendly materials for applications in green carbon recycling processes. In particular, early studies on Cu(I-II)-Cu(0) nanostructured materials were carried out to produce films on 4 cm2 sized supports by means of a fast and low-cost electrochemical procedure. Besides, through a screening of potentials, it was possible to find out a selective value for the CH3COOH production at -0.4 V vs RHE with a maximum productivity (1h reaction), ensured by the presence of the Cu+/Cu0 active redox couple (0.31 mmol gcat-1 h-1). On the basis of these results, further optimisations of the electrocatalyst chemical composition were carried out with the aim of (i) facilitating the interaction with CO2, (ii) increasing the dispersion of the catalytic active phase, and (iii) enhancing the CH3COOH productivity. To this aim, novel electrocatalysts based on layered double hydroxides (LDHs) were optimised, having as a final goal the formation of a new Cu2O-Cu0 based electrocatalyst derived from electrochemically achieved CuMgAl LDHs, subjected to calcination and reduction processes. The as-obtained electrocatalysts were tested for the selective production of CH3COOH and unprecedented results were obtained with the pristine CuMgAl LDH (2.0 mmol gcat-1 h-1). Additional characterisations of such an electrocatalyst have highlighted the possibility to achieve a ternary LDH in intimate contact with Cu2O-Cu0 species starting from the electrochemical deposition. The presence of these species, along with an alkaline environment on the electrode surface, were essential to preserve the selectivity towards the desired product, as confirmed by further operando studies.

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This thesis aims to investigate the fundamental processes governing the performance of different types of photoelectrodes used in photoelectrochemical (PEC) applications, such as unbiased water splitting for hydrogen production. Unraveling the transport and recombination phenomena in nanostructured and surface-modified heterojunctions at a semiconductor/electrolyte interface is not trivial. To approach this task, the work presented here first focus on a hydrogen-terminated p-silicon photocathode in acetonitrile, considered as a standard reference for PEC studies. Steady-state and time-resolved excitation at long wavelength provided clear evidence of the formation of an inversion layer and revealed that the most optimal photovoltage and the longest electron-hole pair lifetime occurs when the reduction potential for the species in solution lies within the unfilled conduction band states. Understanding more complex systems is not as straight-forward and a complete characterization that combine time- and frequency-resolved techniques is needed. Intensity modulated photocurrent spectroscopy and transient absorption spectroscopy are used here on WO3/BiVO4 heterojunctions. By selectively probing the two layers of the heterojunction, the occurrence of interfacial recombination was identified. Then, the addition of Co-Fe based overlayers resulted in passivation of surface states and charge storage at the overlayer active sites, providing higher charge separation efficiency and suppression of recombination in time scales that go from picoseconds to seconds. Finally, the charge carrier kinetics of several different Cu(In,Ga)Se2 (CIGS)-based architectures used for water reduction was investigated. The efficiency of a CIGS photocathode is severely limited by charge transfer at the electrode/electrolyte interface compared to the same absorber layer used as a photovoltaic cell. A NiMo binary alloy deposited on the photocathode surface showed a remarkable enhancement in the transfer rate of electrons in solution. An external CIGS photovoltaic module assisting a NiMo dark cathode displayed optimal absorption and charge separation properties and a highly performing interface with the solution.