Accessibility of grafted functional groups limits reactivity of covalent graphene derivatives

Graphene derivatives are an emerging and important class of promising materials because they can bear a wide variety of functional groups, rendering them suitable for a plethora of applications, ranging from energy storage to sensorics. Further functionalisation of these materials requires a thorough understanding of their reactivity at the molecular level because the organic functional groups are close to an effectively infinite surface, which may affect their reactivity. Nitrile groups grafted on a graphene can be easily hydrolysed to carboxyl groups, but they are resistant to reduction by LiAlH4. Here, we combine theoretical and experimental methods to explain the resistance of CN groups grafted on the graphene surface in terms of the limited accessibility of these groups for the reduction agent. We highlight that such mechanistic aspects, i.e., steric hindrance of the reaction centres and surface-solvent interactions, play a crucial role in the reactivity of 2D materials.Web of Science598art. no. 15379

Carbon nanostructures derived through hypergolic reaction of conductive polymers with fuming nitric acid at ambient conditions

Hypergolic systems rely on organic fuel and a powerful oxidizer that spontaneously ignites upon contact without any external ignition source. Although their main utilization pertains to rocket fuels and propellants, it is only recently that hypergolics has been established from our group as a new general method for the synthesis of different morphologies of carbon nanostructures depending on the hypergolic pair (organic fuel-oxidizer). In search of new pairs, the hypergolic mixture described here contains polyaniline as the organic source of carbon and fuming nitric acid as strong oxidizer. Specifically, the two reagents react rapidly and spontaneously upon contact at ambient conditions to afford carbon nanosheets. Further liquid-phase exfoliation of the nanosheets in dimethylformamide results in dispersed single layers exhibiting strong Tyndall effect. The method can be extended to other conductive polymers, such as polythiophene and polypyrrole, leading to the formation of different type carbon nanostructures (e.g., photolumincent carbon dots). Apart from being a new synthesis pathway towards carbon nanomaterials and a new type of reaction for conductive polymers, the present hypergolic pairs also provide a novel set of rocket bipropellants based on conductive polymers.Web of Science266art. no. 159

Microwave synthesis, characterization and perspectives of wood pencil-derived carbon

More than 14 billion pencils are manufactured and used globally every year. On average, a pencil is discarded after 60% of its original length has been depleted. In the present work we propose a simple and affordable way of converting this non-neglectable amount of waste into added value carbon product. In particular, we demonstrate the microwave synthesis of carbon from the wood pencil with and without chemical activation. This could be a process stage before the final recycling of the expensive graphite core. In the latter case, irradiation of the wood pencil in a domestic microwave oven heats up the pencil's graphite core, thus inducing carbonization of its wood casing. The carbonized product consists of amorphous carbon nanosheets having relatively low surface area. However, if the wood pencil is soaked in 50% KOH aqueous solution prior to microwave irradiation, a significantly higher surface area of carbon is obtained, consisting of irregular-shaped porous particles. Consequently, the obtained carbon can easily decolorize a methylene blue aqueous solution, can be used to make pocket warmers or gunpowder, and lastly, serves as an excellent adsorbent towards Cr(VI) removal from water, showing a maximum adsorption capacity of 70-75 mg/g within 24 h at 23 degrees C, pH = 3.Web of Science121art. no. 41

Study of materials with photocatalytic properties and energy applications

In this thesis, the photo(electro)catalytic decomposition of organic and inorganic substances, which are either pollutants or byproducts of biomass was studied, with simultaneous production of hydrogen and electricity. The production of electricity and hydrogen, by the photodegradation of organic wastes in a photoelectrochemical cell is an attractive field of study with double environmental benefit, which is the waste degradation while simultaneously producing renewable energy.In particular, the photo(electro)catalytic degradation of some organic and inorganic substances has been studied. These substances are divided in two main categories: substances that are recognized pollutants of the aquatic environment and substances that are byproducts of biomass, which when accumulated could potentially cause environmental problems. Τhe following types of compounds were tested: nitrogen compounds such as urea, which is found in large amounts in human urine and its accumulation could cause environmental problems, drugs that are detected in wastewater and their presence in large quantities causes pollution, organic dyes which are widely used in industry and a part of them escapes in the environment causing a significant environmental problem, alcohols such as ethanol, which is a biomass product and is a model of sacrificial compounds, and finally saccharides which are found in large quantities in food industry wastes. Degradation of those substances was obtained in photoelectrochemical cells of various configurations in order to improve the photocatalytic activity. Thus, in the experiments of this thesis, the benefits of photoelectrocatalytic against the photocatalytic degradation were studied and a combination electrocatalytic degradation followed by photocatalysis to decompose azo-dyes was proposed. Moreover, the development of a novel electrode for hydrogen production which carries both the photocatalyst (TiO2) and the electrocatalyst (platinum nanoparticles dispersed on a conductive carbon paste) was prepared, highlighting simplicity of the device and some attractive results in the production of hydrogen. Also, an alternative TiO2 photoanode was synthesized by flame spray pyrolysis, a technology that produces titania nanoparticles with high specific surface, which increases active interface yielding higher photocurrent values. Finally, an alternative photocatalyst, hematite, was used to produce hydrogen by the degradation of sugars both in its original form and after doping with metallic impurities.Στην παρούσα διδακτορική διατριβή μελετήθηκε η φωτο(ηλεκτρο)καταλυτική διάσπαση οργανικών και ανόργανων ουσιών, που είτε συνιστούν ρύπους είτε είναι προϊόντα της βιομάζας, με ταυτόχρονη παραγωγή υδρογόνου και ηλεκτρικής ενέργειας. Η παραγωγή ηλεκτρικής ενέργειας και υδρογόνου, με φωτοαποικοδόμηση οργανικών αποβλήτων σε ένα φωτοηλεκτροχημικό κελί αποτελεί μία ελκυστική διεργασία με διπλό περιβαλλοντικό όφελος δηλαδή, την αποικοδόμηση αποβλήτων με ταυτόχρονη παραγωγή χρήσιμων μορφών ενέργειας.Πιο συγκεκριμένα, μελετήθηκε η φωτο(ηλεκτρο)καταλυτική διάσπαση μιας σειράς οργανικών και ανόργανων ουσιών που χρησιμοποιήθηκαν ως θυσιαζόμενες ενώσεις. Οι ουσίες αυτές διακρίνονται σε δύο κύριες κατηγορίες: ουσίες που αποτελούν αναγνωρισμένους ρύπους του υδάτινου περιβάλλοντος και ουσίες που είναι παραπροϊόντα της βιομάζας που όταν συσσωρεύονται μπορούν, εν δυνάμει, να προκαλέσουν περιβαλλοντικό πρόβλημα. Οι κατηγορίες ενώσεων που μελετήθηκαν ήταν οι εξής: αζωτούχες ενώσεις όπως η ουρία που βρίσκεται σε μεγάλες ποσότητες στα ούρα και η συσσώρευση της αποτελεί περιβαλλοντικό πρόβλημα, φαρμακευτικές ουσίες οι οποίες ανιχνεύονται σε υδάτινα λύματα και η παρουσία τους σε αυτά σε μεγάλες ποσότητες προκαλεί ρύπανση, χρωστικές ενώσεις οι οποίες χρησιμοποιούνται ευρέως στη βιομηχανία και ένα μέρος τους διαφεύγει στο περιβάλλον προκαλώντας σημαντικό περιβαλλοντικό πρόβλημα, αλκοόλες όπως ή αιθανόλη, η οποία αποτελεί προϊόν βιομάζας και είναι πρότυπη θυσιαζόμενη ένωση, και τέλος σάκχαρα τα οποία βρίσκονται σε μεγάλες ποσότητες σε λύματα της βιομηχανίας τροφίμων. Η διάσπαση των ουσιών αυτών επετεύχθη σε φωτοηλεκτροχημικά κελιά διαφόρων διαμορφώσεων με σκοπό τη βελτίωση της φωτοκαταλυτικής δράσης. Έτσι, στα πειράματα της παρούσας διατριβής μελετήθηκαν τα οφέλη της φωτοηλεκτροκαταλυτικής έναντι της φωτοκαταλυτικής αποικοδόμησης και προτάθηκε ένας συνδυασμός ηλεκτροκαταλυτικής διάσπασης ακολουθούμενη από φωτοκατάλυση για τη διάσπαση αζω-χρωμάτων. Επίσης, αναπτύχθηκε ένα καινοτόμο ηλεκτρόδιο για την παραγωγή υδρογόνου που φέρει τόσο τον φωτοκαταλύτη (TiO2) όσο και τον ηλεκτροκαταλύτη (νανοσωματίδια λευκόχρυσου διεσπαρμένα σε αγώγιμη ανθρακική πάστα) προφέροντας εκτός από την απλότητα της διάταξης και κάποια ελκυστικά αποτελέσματα όσον αφορά την παραγωγή υδρογόνου. Επιπλέον, μελετήθηκαν καινοτόμες φωτοάνοδοι TiO2 που συντέθηκαν με τη μέθοδο πυρόλυσης ψεκασμού φλόγας, μια τεχνολογία που παράγει νανοσωματίδια τιτάνιας με μεγάλη ειδική επιφάνεια, πράγμα που επιτρέπει στην τιτάνια να προσροφήσει μεγαλύτερες ποσότητες κβαντικών τελειών κατά την φωτοευαισθητοποίηση αποδίδοντας υψηλότερες τιμές φωτορεύματος και τελικής απόδοσης του κελιού. Τέλος, ένας εναλλακτικός φωτοκαταλύτης, ο αιματίτης, χρησιμοποιήθηκε για την παραγωγή υδρογόνου από τη διάσπαση σακχαρούχων ενώσεων τόσο στην αρχική του μορφή όσο και μετά από ενίσχυση με μεταλλικές προσμίξεις

Photocurrent increase by metal modification of Fe2O3 photoanodes and its effect on photoelectrocatalytic hydrogen production by degradation of organic substances

The present work reports the investigation of photocurrent increase by metal modification of Fe2O3 photoanodes and its effect on photoelectrocatalytic hydrogen production using aqueous solutions containing various organic compounds. Fe2O3 photoanodes were prepared by the electrodeposition method. The efficiency of various metal modifiers of the hematite structure (Ti, Ni, Sn, Co and Cu) has been tested by monitoring the photoelectrochemical behavior of the ensuing photoanodes. Hydrogen production was monitored in an H-shaped reactor using pure and metal-modified hematite films deposited on FTO electrodes as photocatalyst while a combination of commercial carbon paste with dispersed Pt nanoparticles was used as electrocatalyst. In all cases, hydrogen production was obtained by application of a small external electric bias (in the range 0.5- 0.7 V vs Ag/AgCl electrode). Highest photocurrent production has been achieved with a Ti-modified Fe2O3 photoanode in the presence of glucose as sacrificial agent

Carbon nanotube based metal-organic framework hybrids from fundamentals toward applications

Metal-organic frameworks (MOFs) materials constructed by the coordination chemistry of metal ions and organic ligands are important members of the crystalline materials family. Owing to their exceptional properties, for example, high porosity, tunable pore size, and large surface area, MOFs have been applied in several fields such as gas or liquid adsorbents, sensors, batteries, and supercapacitors. However, poor conductivity and low stability hamper their potential applications in several attractive fields such as energy and gas storage. The integration of MOFs with carbon nanotubes (CNTs), a well-established carbon allotrope that exhibits high conductivity and stability, has been proposed as an efficient strategy to overcome such limitations. By combining the advantages of MOFs and CNTs, a wide variety of composites can be prepared with properties superior to their parent materials. This review provides a comprehensive summary of the preparation of CNT@MOF composites and focuses on their recent applications in several important fields, such as water purification, gas storage and separation, sensing, electrocatalysis, and energy storage (supercapacitors and batteries). Future challenges and prospects for CNT@MOF composites are also discussed.Web of Science184art. no. 210462

Rapid Carbon Formation from Spontaneous Reaction of Ferrocene and Liquid Bromine at Ambient Conditions

Herein, we present an interesting route to carbon derived from ferrocene without pyrolysis. Specifically, the direct contact of the metallocene with liquid bromine at ambient conditions released rapidly and spontaneously carbon soot, the latter containing dense spheres, nanosheets, and hollow spheres. The derived carbon carried surface C-Br bonds that permitted postfunctionalization of the solid through nucleophilic substitution. For instance, treatment with diglycolamine led to covalent attachment of the amine onto the carbon surface, thus conferring aqueous dispersability to t he solid. The dispersed solid exhibited visible photoluminescence under UV irradiation as a result of surface passivation by the amine. Hence, the present method not only allowed a rapid and spontaneous carbon formation at ambient conditions, but also surface engineering of the particles to impart new properties (e.g., photoluminescence)

Covalently interlinked graphene sheets with sulfur-chains enable superior lithium-sulfur battery cathodes at full-mass level

Sulfur represents a low-cost, sustainable, and high theoretical capacity cathode material for lithium-sulfur batteries, which can meet the growing demand in portable power sources, such as in electric vehicles and mobile information technologies. However, the shuttling effect of the formed lithium polysulfides, as well as their low conductivity, compromise the electrochemical performance of lithium-sulfur cells. To tackle this challenge, a so far unexplored cathode, composed of sulfur covalently bonded directly on graphene is developed. This is achieved by leveraging the nucleophilicity of polysulfide chains, which react readily with the electrophilic centers in fluorographene, as experimental and theoretical data unveil. The reaction leads to the formation of carbon-sulfur covalent bonds and a particularly high sulfur content of 80 mass%. Owing to these features, the developed cathode exhibits excellent performance with only 5 mass% of conductive carbon additive, delivering very high full-cathode-mass capacities and rate capability, combined with superior cycling stability. In combination with a fluorinated ether as electrolyte additive, the capacity persists at approximate to 700 mAh g(-1) after 100 cycles at 0.1 C, and at approximate to 644 mAh g(-1) after 250 cycles at 0.2 C, keeping approximate to 470 mAh g(-1) even after 500 cycles.Web of Scienceart. no. 210132

The use of activated bio-carbon derived from “Posidonia oceanica” sea-waste for Lithium-Sulfur batteries development

Lithium-Sulfur Batteries is promising energy storage systems due to their superior capacity and energy density. A promising solution for drawbacks such as low sulfur utilization and cycling stability is the use of porous carbon as sulfur carrier. On the other hand, cyclic economy and green ideas is of great importance nowadays. Carbon-sulfur cathodes from waste valorization, abundant, and low-cost precursors is an attractive approach. Herein, an activated carbon (AC-Poc) derived from "Posidonia oceanica" sea-waste, was studied as a matrix for the development of a novel carbon-sulfur composite cathode (AC-Poc/S) for the first time. AC-Poc can be used as an effective sulfur host, due to its high specific BET surface area (1264 m(2).g(-1)) hierarchical porous structure, and total pore volume 0.81 cm(3).g(-1). AC-Poc/S reveals an outstanding initial capacity of 1539 mAh.g(-1) as cathode material, combined with high reversible capacity at 0.2 C. Furthermore, the discharge capacity of 390 mAh/g at 2 C reveals good rate capability, even at increased C-rate. AC-Poc/S composite exhibits excellent sulfur utilization (92 %) alongside with sufficiently well electrochemical performance. These results combined with the easy synthesis method of the activated carbon from an abundant and low-cost precursor make AC-Poc/S a very promising material for LSBs applications.Web of Science53art. no. 10274

Synthesis of new photosensitive H2BBQ2+[ZnCl4]2−/[(ZnCl)2(μ-BBH)] complexes, through selective oxidation of H2O to H2O2

A new two-electron photosensitizer, H2BBQ2+[ZnCl4]2−/[(ZnCl)2(μ-BBH)] (BBQ stands for 2,5-bis[bis(pyridin-2-ylmethyl)amino]-1,4-quinone and BBH stands for 2,5-bis[bis(pyridin-2-ylmethyl)amino]-1,4-hydroquinone), has been synthesized and the oxidation state of the ligand was determined by X-ray crystallography and NMR spectroscopy. Under light illumination the H2BBQ2+[ZnCl4]2− + ZnCl2 is reduced quantitatively to [(ZnCl)2(μ-BBH)] (pH ∼ 5) oxidizing H2O to H2O2 as is evident by trap experiments. Electrochemistry gave a reversible two-electron ligand-centered redox wave for [(ZnCl)2(μ-BBH)]. UV-Vis, luminescence and EPR spectroscopies reveal the slow formation of a stable quinone diradical, intermediate of the reaction. DFT calculations are in agreement with the proposed mechanism. Based on this property an aqueous {[(ZnCl)2(μ-BBH)]||H2O2} solar rechargeable galvanic cell has been constructed