12 research outputs found

    Effective Space Confinement by Inverse Miniemulsion for the Controlled Synthesis of Undoped and Eu3+^{3+} -Doped Calcium Molybdate Nanophosphors: A Systematic Comparison with Batch Synthesis

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    The possibility to precisely control reaction outcomes for pursuing materials with well-defined features is a main endeavor in the development of inorganic materials. Confining reactions within a confined space, such as nanoreactors, is an extremely promising methodology which allows to ensure control over the final properties of the material. An effective room temperature inverse miniemulsion approach for the controlled synthesis of undoped and Eu3+-doped calcium molybdate crystalline nanophosphors was developed. The advantages and the efficiency of confined space in terms of controlling nanoparticle features like size, shape, and functional properties are highlighted by systematically comparing miniemulsion products with calcium molybdate particles obtained without confinement from a typical batch synthesis. A relevant beneficial impact of space confinement by miniemulsion nanodroplets is observed on the control of size and shape of the final nanoparticles, resulting in 12 nm spherical nanoparticles with a narrow size distribution, as compared to the 58 nm irregularly shaped and aggregated particles from the batch approach (assessed by TEM analysis). Further considerable effects of the confined space for the miniemulsion samples are found on the doping effectiveness, leading to a more homogeneous distribution of the Eu3+^{3+} ions into the molybdate host matrix, without segregation (assessed by PXRD, XAS, and ICP-MS). These findings are finally related to the photoluminescence properties, which are evidenced to be closely dependent on the Eu3+^{3+} content for the miniemulsion samples, as an increase of the relative intensity of the direct f–f excitation and a shortening of the lifetime (from 0.901 ms for 1 at. % to 0.625 ms for 7 at. % samples) with increasing Eu3+^{3+} content are observed, whereas no relationship between these parameters and the Eu3+^{3+} content is evidenced for the batch samples. All these results are ascribed to the uniform and controlled crystallization occurring inside each miniemulsion nanodroplet, as opposed to the less controlled nucleation and growth for a classic nonconfined approach

    Work Function Tuning in Hydrothermally Synthesized Vanadium-Doped MoO3 and Co3O4 Mesostructures for Energy Conversion Devices

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    The wide interest in developing green energy technologies stimulates the scientific community to seek, for devices, new substitute material platforms with a low environmental impact, ease of production and processing and long-term stability. The synthesis of metal oxide (MO) semiconductors fulfils these requirements and efforts are addressed towards optimizing their functional properties through the improvement of charge mobility or energy level alignment. Two MOs have rising perspectives for application in light harvesting devices, mainly for the role of charge selective layers but also as light absorbers, namely MoO3 (an electron blocking layer) and Co3O4 (a small band gap semiconductor). The need to achieve better charge transport has prompted us to explore strategies for the doping of MoO3 and Co3O4 with vanadium (V) ions that, when combined with oxygen in V2O5, produce a high work function MO. We report on subcritical hydrothermal synthesis of V-doped mesostructures of MoO3 and of Co3O4, in which a tight control of the doping is exerted by tuning the relative amounts of reactants. We accomplished a full analytical characterization of these V-doped MOs that unambiguously demonstrates the incorporation of the vanadium ions in the host material, as well as the effects on the optical properties and work function. We foresee a promising future use of these materials as charge selective materials in energy devices based on multilayer structures

    Work Function Tuning in Hydrothermally Synthesized Vanadium-Doped MoO3 and Co3O4 Mesostructures for Energy Conversion Devices

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    The wide interest in developing green energy technologies stimulates the scientific community to seek, for devices, new substitute material platforms with a low environmental impact, ease of production and processing and long-term stability. The synthesis of metal oxide (MO) semiconductors fulfils these requirements and efforts are addressed towards optimizing their functional properties through the improvement of charge mobility or energy level alignment. Two MOs have rising perspectives for application in light harvesting devices, mainly for the role of charge selective layers but also as light absorbers, namely MoO3 (an electron blocking layer) and Co3O4 (a small band gap semiconductor). The need to achieve better charge transport has prompted us to explore strategies for the doping of MoO3 and Co3O4 with vanadium (V) ions that, when combined with oxygen in V2O5, produce a high work function MO. We report on subcritical hydrothermal synthesis of V-doped mesostructures of MoO3 and of Co3O4, in which a tight control of the doping is exerted by tuning the relative amounts of reactants. We accomplished a full analytical characterization of these V-doped MOs that unambiguously demonstrates the incorporation of the vanadium ions in the host material, as well as the effects on the optical properties and work function. We foresee a promising future use of these materials as charge selective materials in energy devices based on multilayer structures

    Space matters: crystallization of inorganic systems in confined spaces

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    Uno degli obiettivi più ambiziosi che possa porsi un chimico riguarda lo sviluppo di approcci sintetici che permettano un controllo fine sul meccanismo di una reazione, e quindi sui suoi esiti, e una fondamentale comprensione dei fenomeni ad essa correlati. Una strategia che è risultata promettente per perseguire questo obiettivo risulta essere il confinamento delle reazioni in uno spazio confinato, i.e. un volume delimitato di dimensione nanometrica con limitata accessibilità, utilizzato quindi come nanoreattore. In questo contesto, in questa Tesi di dottorato sono stati esplorati ambienti confinati di diversa dimensione come reattori per controllare da un punto di vista spaziale la sintesi di sistemi inorganici, con l’obiettivo di valutare gli effetti del confinamento spaziale sugli esiti delle sintesi. In particolare, sono stati sintetizzate diverse classi di materiali inorganici, i.e. ossidi metallici (MoO3 e CaMoO4 puro e drogato con ioni Eu3+) e nanoparticelle metalliche (Pd), in condizioni di confinamento crescente in i) un microreattore in flusso continuo, ii) nanogocce prodotte da miniemulsioni inverse (acqua in olio) e iii) nanopori di silice mesoporosa. Inoltre, gli effetti degli spazi confinati di diversa dimensione sono stati valutati per confronto degli esiti sintetici ottenuti in ambiente confinato con i risultati ottenuti effettuando le sintesi in un macroreattore (approccio di sintesi batch), con un approccio sperimentale sistematico ed esaustivo. I materiali ottenuti sono stati caratterizzati in dettaglio attraverso numerose tecniche di caratterizzazione, sia da un punto di vista composizionale, strutturale e dimensionale, che da quello funzionale, utilizzando sia tecniche di caratterizzazione ex situ di laboratorio che studi in situ più avanzati, effettuati in modo risolto nel tempo impiegando luce di sincrotrone.One of the most ambitious aims of a chemist is the development and comprehensive understanding of synthetic approaches to finely control reaction pathways and, ultimately, chemical reactions’ outcomes. A promising strategy that has been adopted to pursue this goal is the confinement of reactions within confined spaces, i.e. enclosed volumes in the nanometer scale range with limited accessibility, employed as nanoreactors. Within this framework, this Ph.D. Thesis investigated differently sized enclosed environments as reactors for the spatially controlled synthesis of inorganic systems, with the aim of evaluating the effects of space confinement on the syntheses outcomes. In particular, different classes of inorganic materials, encompassing metal oxides (MoO3 and undoped and Eu-doped CaMoO4) and metal nanoparticles (Pd), were synthesized in the increasingly constrained environment of i) a continuous-flow microreactor, ii) nanodroplets produced by inverse (water-in-oil) miniemulsions, and iii) nanopores of mesoporous silica materials. Moreover, the effects of the differently sized confined spaces were evaluated by comparing the constrained synthesis outcomes with those obtained in a macroreactor (batch approach). The systematic and comprehensive experimental approach was supported by a wide array of characterization techniques, from the compositional, structural, dimensional, and functional point of view, exploiting both ex situ laboratory techniques and more advanced in situ studies, performed at synchrotron facilities in a time-resolved fashion

    Comparing methods for comparing networks

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    With the impressive growth of available data and the flexibility of network modelling, the problem of devising effective quantitative methods for the comparison of networks arises. Plenty of such methods have been designed to accomplish this task: most of them deal with undirected and unweighted networks only, but a few are capable of handling directed and/or weighted networks too, thus properly exploiting richer information. In this work, we contribute to the effort of comparing the different methods for comparing networks and providing a guide for the selection of an appropriate one. First, we review and classify a collection of network comparison methods, highlighting the criteria they are based on and their advantages and drawbacks. The set includes methods requiring known node-correspondence, such as DeltaCon and Cut Distance, as well as methods not requiring a priori known node-correspondence, such as alignment-based, graphlet-based, and spectral methods, and the recently proposed Portrait Divergence and NetLSD. We test the above methods on synthetic networks and we assess their usability and the meaningfulness of the results they provide. Finally, we apply the methods to two real-world datasets, the European Air Transportation Network and the FAO Trade Network, in order to discuss the results that can be drawn from this type of analysis

    Pursuing unprecedented anisotropic morphologies of halide-free Pd nanoparticles by tuning their nucleation and growth

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    In this paper, a highly effective and scalable polyol-based modified procedure is reported, yielding shape-controlled Pd nanoparticles (NPs) formed via two distinct growth mechanisms as a function of apparent pH. Starting from tetraethylammonium tetrahydroxypalladate (TEA)(2)[Pd(OH)(4)], a halide-free precursor, the resulting shape of the NPs ranged from highly defective worm-like nanostructures to well-defined polyhedra (tetrahedra, octahedra and 5-fold twins) as shown by TEM, HRTEM, and STEM. The effect of the different synthesis parameters was thoroughly investigated, finding that apparent pH - modulated by adding diluted HNO3 - is the key parameter in determining the final size and shape of Pd NPs, whose evolution was followed during the reaction. A rational explanation of the observed shape modification as a function of apparent pH was proposed. The as-prepared Pd NPs, once dried, were analysed by means of XRD. DRIFT spectroscopy was used to show how CO binds on the Pd NPs after deposition on gamma-Al2O3 as catalytic support

    Exploring the Phase-Selective, Green, Hydrothermal Synthesis of Upconverting Doped Sodium Yttrium Fluoride: Effects of Temperature, Time, and Precursors

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    The aim of this work was i) to develop a hydrothermal, low-temperature synthesis protocol affording the upconverting hexagonal phase NaYF4 with suitable dopants while adhering to the "green chemistry" standards and ii) to explore the effect that different parameters have on the products. In optimizing the synthesis protocol, short reaction times and low temperatures (below 150 degrees C) were considered. Yb3+ and Er3+ ions were chosen as dopants for the NaYF4 material. Within the context of the second goal, parameters including nature of the precursors, treatment temperature, and treatment time were investigated to afford a pure hexagonal crystalline phase, both in the doped and undoped materials. To fully explore the synthesis results, the prepared materials were characterized from a structural (XRD), compositional (XPS, ICP-MS), and morphological (SEM) point of view. The upconverting properties of the compounds were confirmed by photoluminescence measurements

    Exploring the phase-selective, green hydrothermal synthesis of upconverting doped sodium yttrium fluoride: effects of temperature, time and precursors

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    The aim of this work was i) to develop a hydrothermal, low temperature synthesis protocol affording the upconverting hexagonal phase NaYF4 with suitable dopants while adhering to the \u201cgreen chemistry\u201d standards and ii) to explore the effect that different parameters have on the products. In optimizing the synthesis protocol, short reaction times and low temperatures (below 150 \ub0C) were considered. Yb3+ and Er3+ ions were chosen as dopants for the NaYF4 material. Within the context of the second goal, parameters including nature of the precursors, treatment temperature and treatment time were investigated in order to afford a pure hexagonal crystalline phase, both in the doped and undoped materials. To fully explore the synthesis results, the prepared materials were characterized from a structural (XRD), compositional (XPS, ICP\u2010MS) and morphological (SEM) point of view. The upconverting properties of the compounds were confirmed by photoluminescence measurements
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