Υβριδικά υλικά βασισμένα σε γραφένιο και χαλκογενίδια μετάλλων για ενεργειακές εφαρμογές

Η παρούσα διδακτορική διατριβή πραγματεύεται την παρασκευή υβριδικών υλικών τα οποία έχουν ως βάση χαλκογενίδια μετάλλων ή/και γραφένιο και παρουσιάζουν φωτοφυσικές και ηλεκτροκαταλυτικές ιδιότητες με στόχο την ανάπτυξη της λειτουργικότητάς τους σε εφαρμογές μετατροπής ενέργειας. Το πρώτο κομμάτι της Διδακτορικής Διατριβής περιλαμβάνει υβριδικά υλικά τα οποία βασίστηκαν σε διχαλκογενίδια μετάλλων μετάπτωσης τα οποία παρασκευάστηκαν με τεχνικές από «κάτω προς τα πάνω». Στο πλαίσιο αυτό, πραγματοποιήθηκε η ταυτόχρονη παρασκευή και τροποποίηση του MoS2 με πολυμεθακρυλικό οξύ με ακτινοβόληση με μικροκυμάματα. Επιπλέον, αναπτύχθηκαν συστήματα δότη-δέκτη ηλεκτρονίων MoS2 με κβαντικές τελείες άνθρακα τα οποία εξετάστηκαν ως ηλεκτροκαταλύτες για την παραγωγή υδρογόνου. Επιπρόσθετα, αναπτύχθηκαν και μελετήθηκαν νανοδομές MoS2 σε σκληρή και αναλισκόμενη βάση GO και CaCO3 αντίστοιχα, ενώ εξετάστηκε η δραστικότητά τους για την ηλεκτροκαταλυτική παραγωγή υδρογόνου. Ακόμη, το GO ντοπαρίστηκε με άτομα θείου προκειμένου να χρησιμοποιηθεί ως υπόστρωμα για την in situ ανάπτυξη νανοδομών MoS2 και WS2 ώστε να χρησιμοποιηθούν ως ηλεκτροκαταλύτες για την παραγωγή υδρογόνου. Στο δεύτερο μέρος της Διδακτορικής Διατριβής αναλύονται υβριδικά υλικά τα οποία έχουν προκύψει χρησιμοποιώντας διχαλκογενίδια μετάλλων μετάπτωσης τα οποία παρασκευάστηκαν με τεχνικές από «πάνω προς τα κάτω». Σε αυτό το πλαίσιο, πραγματοποιήθηκε η αποφλοίωση MoS2 από το υλικό βάσης και η περαιτέρω τροποποίησή του με λιποϊκό οξύ για την in situ ανάπτυξη και ακινητοποίηση νανοσωματιδίων θειούχου καδμίου για την κατασκευή φωτοηλεκτροχημικών κελιών. Τέλος, παρασκευάστηκε υλικό τύπου πυρήνα/κελύφους αποτελούμενο από νανοσωματίδια πολυ(3-εξυλοθειοφαινίου και κβαντικές τελείες τελλουριούχου καδμίου δότη δέκτη ηλεκτρονίων για την κατασκευή φωτοηλεκτροχημικών κελιών.This current thesis deals with the preparation and characterization of hybrid materials based on graphene and/or metal chalcogenides that possess interesting photophysical and electrochemical properties aiming to energy conversion schemes. The first part of the thesis involves the preparation and study of hybrid materials that are based on transition metal chalcogenides prepared by employing bottom-up approaches. In this frame, MoS2 nanostructures were prepared and functionalized at the same time with poly(methacrylic acid) by using microwave irradiation. Moreover, donor-acceptor hybrids were prepared between MoS2 and carbon nanodots and were studied as electrocatalysts for hydrogen evolution reaction. Furthermore, MoS2 nanostructures were developed and studied by employing either a sacrificial soft template (CaCO3) or a non-sacrificial hard template (GO) while their electrocatalytic performance towards hydrogen evolution was tested. Moreover, GO was doped with sulfur and was used as template for the in situ growth of MoS2 and WS2 nanostructures that were used as catalysts for electrocatalytic hydrogen production. At the second part of this thesis we developed hybrid materials based on transition metal chalcogenides that were prepared by employing top-down approaches. In this frame, MoS2 nanosheets were prepared through exfoliation of the bulk and they were further functionalized with lipoic acid for the in situ growth and stabilization of cadmium sulfide nanoparticles for the preparation of photoelectrochemical cells. Finally, a donor acceptor core/shell material was prepared consisting of poly(3-hexylthiophene) nanoparticles and semiconducting quantum dots for the preparation of photoelectrochemical cells

Semicrystalline Polymer Micro/Nanostructures Formed by Droplet Evaporation of Aqueous Poly(ethylene oxide) Solutions: Effect of Solution Concentration

[Image: see text] Deposits formed after evaporation of sessile droplets, containing aqueous solutions of poly(ethylene oxide), on hydrophilic glass substrates were studied experimentally and mathematically as a function of the initial solution concentration. The macrostructure and micro/nanostructures of deposits were studied using stereo microscopy and atomic force microscopy. A model, based on thin-film lubrication theory, was developed to evaluate the deposit macrostructure by estimating the droplet final height. Moreover, the model was extended to evaluate the micro/nanostructure of deposits by estimating the rate of supersaturation development in connection with the driving force of crystallization. Previous studies had only described the macrostructure of poly(ethylene oxide) deposits formed after droplet evaporation, whereas the focus of our study was the deposit micro/nanostructures. Our atomic force microscopy study showed that regions close to the deposit periphery were composed of predominantly semicrystalline micro/nanostructures in the form of out-of-plane lamellae, which require a high driving force of crystallization. However, deposit central areas presented semicrystalline micro/nanostructures in the form of in-plane terraces and spirals, which require a lower driving force of crystallization. Increasing the initial concentration of solutions led to an increase in the lengths and thicknesses of the out-of-plane lamellae at the deposits’ periphery and enhanced the tendency to form spirals in the central areas. Our numerical study suggested that the rate of supersaturation development and thus the driving force of crystallization increased from the center toward the periphery of droplets, and the supersaturation rate was lower for solutions with higher initial concentrations at each radius. Therefore, periphery areas of droplets with lower initial concentrations were suitable for the formation of micro/nanostructures which require higher driving forces, whereas central areas of droplets with higher initial concentration were desirable for the formation of micro/nanostructures which require lower driving forces. These numerical results were in good qualitative agreement with the experimental findings

Phase change and complex phenomena in drops and bubbles of pure and binary fluids

Evaporation, wetting and multiphase flows of drops and bubbles are everyday life phenomena with potential impact in many industrial, biological, medical or engineering applications. The understanding and controlling of the physical and chemical mechanisms governing these phenomena have become of paramount importance. This thesis encompasses three topics: evaporation of sessile droplets of polymer solutions, the role of thermocapillarity on self-rewetting fluid dynamics and migration of bubbles in liquid flows. Firstly, the evaporative behaviour of sessile droplets of aqueous polymer solutions and the effect of different molecular weights on the drying process has been studied. Drop shape analysis allowed monitoring the evolution of all stages during drying and indicating the transitions between stages. The mechanisms taking place during the crucial stages of pinning and depinning were illustrated, revealing the effects of adhesion and contact line friction forces on the final morphology of the dried polymeric deposits. Additionally, the effect of varying substrates from hydrophilic to hydrophobic was examined demonstrating the importance of interfacial interaction phenomena. The initial spreading dynamics of binary alcohol mixtures (and pure liquids) deposited on different substrates in partially wetting situations, under non-isothermal conditions was systematically investigated. Moreover, the temporal and spatial thermal dynamics within pure droplets and alcohol mixtures using IR thermography revealed the existence of characteristic thermal patterns due to thermal and/or solutal instabilities. The contribution of the Marangoni effect as an important heat transport mechanism within the evaporating droplets was investigated. The motion of buoyancy-driven bubbles in a vertical microchannel and the significant role of thermocapillarity was reported in this series of experiments. The behaviour of the bubbles in self-rewetting fluid flows departed considerably from that of pure liquids flows. Furthermore, heat transfer coefficient calculations in the single and two phase flows demonstrated that the presence of Marangoni (surface tension) stresses resulted in the enhancement of the heat transfer distribution in the self-rewetting fluid flows compared with the pure ones

Hybrid materials based on graphene and metal chalcogenides for energy applications

This current thesis deals with the preparation and characterization of hybrid materials based on graphene and/or metal chalcogenides that possess interesting photophysical and electrochemical properties aiming to energy conversion schemes.The first part of the thesis involves the preparation and study of hybrid materials that are based on transition metal chalcogenides prepared by employing bottom-up approaches. In this frame, MoS2 nanostructures were prepared and functionalized at the same time with poly(methacrylic acid) by using microwave irradiation. Moreover, donor-acceptor hybrids were prepared between MoS2 and carbon nanodots and were studied as electrocatalysts for hydrogen evolution reaction. Furthermore, MoS2 nanostructures were developed and studied by employing either a sacrificial soft template (CaCO3) or a non-sacrificial hard template (graphene oxide-GO) while their electrocatalytic performance towards hydrogen evolution was tested. Moreover, GO was doped with sulfur and was used as template for the in situ growth of MoS2 and WS2 nanostructures that were used as catalysts for electrocatalytic hydrogen production.At the second part of this thesis we developed hybrid materials based on transition metal chalcogenides that were prepared by employing top-down approaches. In this frame, MoS2 nanosheets were prepared through exfoliation of the bulk and they were further functionalized with lipoic acid for the in situ growth and stabilization of cadmium sulfide nanoparticles for the preparation of photoelectrochemical cells. Finally, a donor acceptor core/shell material was prepared consisting of poly(3-hexylthiophene) nanoparticles and semiconducting quantum dots for the preparation of photoelectrochemical cells.SUBJECT AREA: Hybrid materialsKEYWORDS: graphene, transition metal dichalcogenides, hybrids, electron transferΗ παρούσα διδακτορική διατριβή πραγματεύεται την παρασκευή υβριδικών υλικών τα οποία έχουν ως βάση χαλκογενίδια μετάλλων ή/και γραφένιο και παρουσιάζουν φωτοφυσικές και ηλεκτροκαταλυτικές ιδιότητες, με στόχο την ανάπτυξη της λειτουργικότητάς τους σε εφαρμογές μετατροπής ενέργειας. Το πρώτο μέρος της Διδακτορικής Διατριβής περιλαμβάνει υβριδικά υλικά τα οποία βασίστηκαν σε διχαλκογενίδια μετάλλων μετάπτωσης τα οποία παρασκευάστηκαν με τεχνικές από «κάτω προς τα πάνω». Στο πλαίσιο αυτό, πραγματοποιήθηκε η ταυτόχρονη παρασκευή και τροποποίηση του MoS2 με πολυμεθακρυλικό οξύ με ακτινοβόληση με μικροκύματα. Επιπλέον, αναπτύχθηκαν συστήματα δότη-δέκτη ηλεκτρονίων MoS2 με κβαντικές τελείες άνθρακα τα οποία εξετάστηκαν ως ηλεκτροκαταλύτες για την παραγωγή υδρογόνου. Επιπρόσθετα, αναπτύχθηκαν και μελετήθηκαν νανοδομές MoS2 σε σκληρή και αναλισκόμενη βάση οξειδίου του γραφενίου (GO) και CaCO3, αντίστοιχα, ενώ εξετάστηκε η δραστικότητά τους για την ηλεκτροκαταλυτική παραγωγή υδρογόνου. Ακόμη, το GO ντοπαρίστηκε με άτομα θείου προκειμένου να χρησιμοποιηθεί ως υπόστρωμα για την in situ ανάπτυξη νανοδομών MoS2 και WS2 ώστε να χρησιμοποιηθούν ως ηλεκτροκαταλύτες για την παραγωγή υδρογόνου.Στο δεύτερο μέρος της Διδακτορικής Διατριβής αναλύονται υβριδικά υλικά τα οποία έχουν προκύψει χρησιμοποιώντας διχαλκογενίδια μετάλλων μετάπτωσης τα οποία παρασκευάστηκαν με τεχνικές από «πάνω προς τα κάτω». Σε αυτό το πλαίσιο, πραγματοποιήθηκε η αποφλοίωση MoS2 από το υλικό βάσης και η περαιτέρω τροποποίησή του με λιποϊκό οξύ για την in situ ανάπτυξη και ακινητοποίηση νανοσωματιδίων θειούχου καδμίου για την κατασκευή φωτοηλεκτροχημικών κελιών. Τέλος, παρασκευάστηκε υλικό τύπου πυρήνα/κελύφους αποτελούμενο από νανοσωματίδια πολυ(3-εξυλοθειοφαινίου) και κβαντικές τελείες τελλουριούχου καδμίου δότη δέκτη ηλεκτρονίων για την κατασκευή φωτοηλεκτροχημικών κελιών.ΘΕΜΑΤΙΚΗ ΠΕΡΙΟΧΗ: Υβριδικά υλικά ΛΕΞΕΙΣ ΚΛΕΙΔΙΑ: γραφένιο, διχαλκογενίδια μετάλλων μετάπτωσης, υβριδικά υλικά, μεταφορά ηλεκτρονίω

Carbon Nanohorn-Based Electrocatalysts for Energy Conversion

In the context of even more growing energy demands, the investigation of alternative environmentally friendly solutions, like fuel cells, is essential. Given their outstanding properties, carbon nanohorns (CNHs) have come forth as promising electrocatalysts within the nanocarbon family. Carbon nanohorns are conical nanostructures made of sp2 carbon sheets that form aggregated superstructures during their synthesis. They require no metal catalyst during their preparation and they are inexpensively produced in industrial quantities, affording a favorable candidate for electrocatalytic reactions. The aim of this article is to provide a comprehensive overview regarding CNHs in the field of electrocatalysis and especially, in oxygen reduction, methanol oxidation, and hydrogen evolution, as well as oxygen evolution from water splitting, underlining the progress made so far, and pointing out the areas where significant improvement can be achieved

Controlled chemical functionalization toward 3D-2D carbon nanohorn-MoS2 heterostructures with enhanced electrocatalytic activity for protons reduction

The realization of novel heterostructures arising from the combination of nanomaterials is an effective way to modify their physicochemical and electrocatalytic properties, giving them enhanced characteristics stemming from their individual constituents. Interfacing carbon nanohorns (CNHs) possessing high porosity, large specific surface area, and good electrical conductivity, with MoS2 owning multiple electrocatalytic active sites but lacking significant conductivity, robust interactions, and effective structure, can be a strategy to boost the electrocatalytic reduction of protons to molecular hydrogen. Herein, in a stepwise approach, complementary functional groups are covalently introduced at the conical tips and sidewalls of CNHs, along with the basal plane of MoS2, en route the construction of 3D-2D CNH-MoS2 heterostructures. The increased MoS2 loading onto CNHs, improving and facilitating charge delocalization and transfer in neighboring CNHs, along with the plethora of active sites, results in excellent electrocatalytic activity for protons reduction, same as that of commercial Pt/C. Minute overpotential is registered, low Tafel slope and small charge-transfer resistance for electrocatalyzing the evolution of hydrogen from the newly prepared heterostructure of 0.029 V, 71 mV dec−1, and 34.5 Ω, respectively. Furthermore, the stability of the 3D-2D CNH-MoS2 heterostructure is validated after performing 10 000 ongoing electrocatalytic cycles.This research was co-financed by Greece and the European Union (European Social Fund) through the Operational Programme “Human Resources Development, Education and Lifelong Learning” in the context of the project “Reinforcement of Postdoctoral Researchers—2nd Cycle” (MIS 5033021), implemented by the State Scholarships Foundation (IKY). R.A. gratefully acknowledges the support from the Spanish MICINN (PID2019-104739GB-100/AEI/10.13039/501100011033), Government of Aragon (project DGA E13-20R (FEDER, EU)) and from the European Union H2020 programs “ESTEEM3” (Grant number 823717) and Graphene Flagship (881603). Partial financial support from the project “National Infrastructure in Nanotechnology, Advanced Materials and Micro-/Nanoelectronics” (MIS 5002772), which was implemented under the Action “Reinforcement of the Research and Innovation Infrastructures”, funded by the Operational Program “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014–2020), Ministry of Development and Investments, and co-financed by Greece and the European Union (European Regional Development Fund) was also acknowledged. TEM and XPS measurements were performed at the Laboratorio de Microscopias Avanzadas (LMA) – Universidad de Zaragoza (Spain).Peer reviewe

Molybdenum diselenide and tungsten diselenide interfacing cobalt-porphyrin for electrocatalytic hydrogen evolution in alkaline and acidic media

Easy and effective modification approaches for transition metal dichalcogenides are highly desired in order to make them active toward electrocatalysis. In this manner, we report functionalized molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) via metal-ligand coordination with pyridine rings for the subsequent covalent grafting of a cobalt-porphyrin. The new hybrid materials were tested towards an electrocatalytic hydrogen evolution reaction in both acidic and alkaline media and showed enhanced activity compared to intact MoSe2 and WSe2. Hybrids exhibited lower overpotential, easier reaction kinetics, higher conductivity, and excellent stability after 10,000 ongoing cycles in acidic and alkaline electrolytes compared to MoSe2 and WSe2. Markedly, MoSe2-based hybrid material showed the best performance and marked a significantly low onset potential of −0.17 V vs RHE for acidic hydrogen evolution reaction. All in all, the ease and fast modification route provides a versatile functionalization procedure, extendable to other transition metal dichalcogenides, and can open new pathways for the realization of functional nanomaterials suitable in electrocatalysis

Molybdenum diselenide-manganese porphyrin bifunctional electrocatalyst for the hydrogen evolution reaction and selective hydrogen peroxide production

Electrochemical reactions for hydrogen and hydrogen peroxide production are essential for energy conversion to diminish the energy crisis but still lack efficient electrocatalysts. Development of non-noble metal bifunctional electrocatalysts for the hydrogen evolution reaction and 2e– oxygen reduction reaction to ease reaction kinetics is a challenging task. Integration of single components by employing easy strategies provides a key step toward the realization of highly active electrocatalysts. In this vein, MoSe2 owns catalytic active sites and a high specific surface area but suffers from insufficient conductivity and high catalytic performance that noble metals provide. Herein, MoSe2 was used as a platform for the incorporation of manganese-metalated porphyrin (MnP). The developed hybrid, namely, MoSe2–MnP, obtained by the initial metal–ligand coordination and the subsequent grafting with MnP was fully characterized and electrochemically assessed. The bifunctional electrocatalyst lowered the overpotential toward hydrogen evolution, improved the reaction kinetics and charge transfer processes, and was extremely stable after 10,000 ongoing cycles. Simultaneously, rotating ring disk electrode analysis showed that oxygen reduction proceeds through the 2e– pathway for the selective production of hydrogen peroxide with a high yield of 97%. The new facile modification route can be applied in diverse transition metal dichalcogenides and will help the development of new advanced functional materials.R.A. acknowledges support from Spanish MICINN (PID2019-104739GB-100/AEI/10.13039/501100011033), the Government of Aragon (projects DGA E13-20R) and from EU H2020 “ESTEEM3” (grant number 823717) and Graphene Flagship (881603). The TEM studies were performed in the Laboratorio de Microscopias Avanzadas (LMA), Universidad de Zaragoza (Spain). R.A. acknowledges support from Spanish MICINN (PID2019-104739GB-100/AEI/10.13039/501100011033), the Government of Aragon (projects DGA E13-20R) and from EU H2020 “ESTEEM3” (grant number 823717) and Graphene Flagship (881603).Peer reviewe

Molybdenum Diselenide and Tungsten Diselenide Interfacing Cobalt-Porphyrin for Electrocatalytic Hydrogen Evolution in Alkaline and Acidic Media

Easy and effective modification approaches for transition metal dichalcogenides are highly desired in order to make them active toward electrocatalysis. In this manner, we report functionalized molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) via metal-ligand coordination with pyridine rings for the subsequent covalent grafting of a cobalt-porphyrin. The new hybrid materials were tested towards an electrocatalytic hydrogen evolution reaction in both acidic and alkaline media and showed enhanced activity compared to intact MoSe2 and WSe2. Hybrids exhibited lower overpotential, easier reaction kinetics, higher conductivity, and excellent stability after 10,000 ongoing cycles in acidic and alkaline electrolytes compared to MoSe2 and WSe2. Markedly, MoSe2-based hybrid material showed the best performance and marked a significantly low onset potential of −0.17 V vs RHE for acidic hydrogen evolution reaction. All in all, the ease and fast modification route provides a versatile functionalization procedure, extendable to other transition metal dichalcogenides, and can open new pathways for the realization of functional nanomaterials suitable in electrocatalysis

Sulfur-doped graphene/transition metal dichalcogenide heterostructured hybrids with electrocatalytic activity toward the hydrogen evolution reaction

A facile route for the preparation of molybdenum disulfide (MoS2) and tungsten disulfide (WS2), uniformly deposited onto sulfur-doped graphene (SG), is reported. The realization of the SG/MoS2 and SG/WS2 heterostructured hybrids was accomplished by employing microwave irradiation for the thermal decomposition of ammonium tetrathiomolybdate and tetrathiotungstate, respectively, in the presence of SG. Two different weight ratios between SG and the inorganic species were used, namely 3:1 and 1:1, yielding SG/MoS2 (3:1), SG/MoS2 (1:1), SG/WS2 (3:1) and SG/WS2 (1:1). SG and all newly developed hybrid materials were characterized by ATR-IR and Raman spectroscopy, TGA, HR-TEM and EELS. The electrocatalytic activity of the SG/MoS2 and SG/WS2 heterostructured hybrids was examined against the hydrogen evolution reaction (HER) and it was found that the presence of SG not only significantly improved the catalytic activity of MoS2 and WS2 but also made it comparable to that of commercial Pt/C. Specifically, hybrids containing higher amounts of SG, namely SG/MoS2 (3:1) and SG/WS2 (3:1), exhibited extremely low onset overpotentials of 26 and 140 mV vs. RHE, respectively. The latter results highlighted the beneficial role of SG as a substrate for immobilizing MoS2 and WS2 and stressed its significance for achieving optimum electrocatalytic performance toward the HER. Finally, examination of the Tafel slopes as extracted from the electrocatalytic polarization curves, manifested the adsorption of hydrogen as the rate-limiting step for SG/MoS2 (3:1), while for SG/WS2 (3:1) the electrochemical desorption of adsorbed hydrogen atoms to generate hydrogen was revealed to be the rate-limiting step