Enhanced detoxification of Cr6+ by Shewanella oneidensis via adsorption on spherical and flower-like manganese ferrite nanostructures

Maximizing the safe removal of hexavalent chromium (Cr6+) from waste streams is an increasing demand due to the environmental, economic and health benefits. The integrated adsorption and bio-reduction method can be applied for the elimination of the highly toxic Cr6+ and its detoxification. This work describes a synthetic method for achieving the best chemical composition of spherical and flower-like manganese ferrite (MnxFe3-xO4) nanostructures (NS) for Cr6+ adsorption. We selected NS with the highest adsorption performance to study its efficiency in the extracellular reduction of Cr6+ into a trivalent state (Cr3+) by Shewanella oneidensis (S. oneidensis) MR-1. MnxFe3-xO4 NS were prepared by a polyol solvothermal synthesis process. They were characterised by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectrometry (XPS), dynamic light scattering (DLS) and Fourier transform-infrared (FTIR) spectroscopy. The elemental composition of MnxFe3-xO4 was evaluated by inductively coupled plasma atomic emission spectroscopy. Our results reveal that the oxidation state of the manganese precursor significantly affects the Cr6+ adsorption efficiency of MnxFe3-xO4 NS. The best adsorption capacity for Cr6+ is 16.8 ± 1.6 mg Cr6+/g by the spherical Mn0.22+Fe2.83+O4 nanoparticles at pH 7, which is 1.4 times higher than that of Mn0.8Fe2.2O4 nanoflowers. This was attributed to the relative excess of divalent manganese in Mn0.22+Fe2.83+O4 based on our XPS analysis. The lethal concentration of Cr6+ for S. oneidensis MR-1 was 60 mg L-1 (determined by flow cytometry). The addition of Mn0.22+Fe2.83+O4 nanoparticles to S. oneidensis MR-1 enhanced the bio-reduction of Cr6+ 2.66 times compared to the presence of the bacteria alone. This work provides a cost-effective method for the removal of Cr6+ with a minimum amount of sludge production

Photo-engineered optoelectronic properties of indium tin oxide via reactive laser annealing

Abstract: Transparent conductive oxides are appealing materials for optoelectronic and plasmonic applications as, amongst other advantages, their properties can be modulated by engineering their defects. Optimisation of this adjustment is, however, a complex design problem. This work examined the modification of the carrier transport properties of sputtered tin-doped indium oxide (ITO) via laser annealing in reactive environments. We relate the optical modifications to the structural, compositional, and electronic properties to elucidate the precise mechanisms behind the reactive laser annealing (ReLA) process. For sufficiently high laser fluence, we reveal an ambient-dependent and purely compositional modulation of the carrier concentration of ITO thin films. Hereby, we demonstrate that ReLA utilises the precise energy delivery of photonic processing to enhance the carrier mobility and finely tune the carrier concentration without significantly affecting the crystal structure. Exploitation of this phenomena may enable one to selectively engineer the optoelectronic properties of ITO, promising an alternative to the exploration of new materials for optoelectronic and photonic applications

Growth, structural and magnetic characterization of advanced, nanometric dimensioned heterostructures applied in magnon - spintronics

Αn introduction of the magnetic properties of mater is presented In the theoretical part of the thesis, while special emphasis is attributed to ferromagnetism, as Fe was the main material used experimentally. The principles of the spintronic technologies lie in the electron interactions, that determine the material magnetization state. The combination of the standard electronics with spin transfer effects, is achieved through magnetization dynamics and the theory of the spin wave propagation (spin current). The spin current production requires the excitation of the ordered magnetic moments, via e.g. ferromagnetic resonance (FMR). In order to utilize the spin waves (magnons) generated in the volume of a magnetic material, interfaces for the spin injection into a non-magnetic (NM) spin detector are required. The spin transmittance through the interfaces is described by the spin pumping (SP) mechanism. The lack of polarity in NM materials, results in the inverse spin-Hall effect (ISHE), which converts spin currents into a detectable charge current on the NM film. The recent advancements in the field of spintronics, studying thin films of magnetic heterostructures, revealed the strong influence of the structural properties and interfacial morphology of interfaces in spin dynamics. Conforming to the international scientific trends, Fe/Pt was the main system studied in the specific research. Molecular beam epitaxy (MBE) and electron beam evaporation (e-beam), were applied for the thin film deposition on MgO substrates. Contemporary characterization methods were combined for the study of the structural properties, such as X – ray diffraction (XRD), X – ray reflectance (XRR), X – ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM/HRTEM). Static and dynamic magnetic properties were examined via mangeto-optic Kerr effect (MOKE), SQUID magnetometry, electrical ISHE measurements, SP and FMR experiments.During the experimental process, an investigation of the most proper NM material was initially performed. At first, the MgO/Fe heteroepitaxy on the substrates was examined and used as reference. The results from the Fe/Au, Fe/Pd and Fe/Pt study were then compared. The stratification type proved to be crucial for the structural properties of the systems. Local diffusion between Fe and Au atoms, which caused a significant, unexpected increase of the magnetic harness, while the insufficient protection from Fe oxidation of the Pd cup layer, were undesired effects, that appointed the Fe/Pt systems as a safer option. Further studies on the system, disclosed a sufficiently small value for the spin diffusion length (λSD ≃ 1.1 nm). Fe/Pt heterostructures were investigated about their NM thickness effects on their properties, while Fe received the stable thickness of 12 nm. The Pt thickness of 6 nm was defined as crucial for the magnetic properties studied by MOKE measurements, as below that limit, a notable increase of the coercive field was recorded, followed by an induced uniaxial magnetic anisotropy on the four-fold cubic anisotropy. The structural characterization and HRTEM analysis, attributed the previous results to a local symmetry breaking on the heterostructures, performing increased interfacial roughness, which enhanced magnetic proximity effects. The experiments were supported by molecular dynamic simulations with Monte Carlo, revealing the presence of Pt psedomorphic epitaxial growth on its first monolayers. In the next experimental step, the effect of growth temperature during the thin film deposition was studied. Both structural and magnetic properties revealed the 300οC as the most propriate temperature for the Fe/Pt epitaxy. The combined structural characterization, in contrast with molecular dynamics simulations, derived the strain model of the bilayers. The Fe layer was presented relaxed from internal elastic strain with the increase of the growth temperature. Under the same conditions, Pt formed grain boundaries and stacking faults. The epitaxial model provided the opportunity for interface modification, where the magnetic “impurities” of the Fe3O4 nanoparticles enhanced the coercivity of the systems intensively, potentiating the four-foul cubic anisotropy of Fe. FMR measurements and SP experiments provided a large range of the resonant peaks’ width and a widening on the magnetic field ΔΗ for the detectable VDC voltage. Τhe introduction of an ultra-thin buffer layer between the metallic layers was the second effort of an interfacial modification. The main target was the study of the spin injection via SP effect through the atomic layers of the MgO. The samples that were examined received a stable Pt thickness of 6 nm and Fe at 12 nm, while the investigation was performed as a function of the MgO interlayer’s thicknesses (0.5 – 2 nm). It was proven that spin currents can be injected to Pt, up to 2 nm of the dielectric thickness and can produce significant voltages in the Pt layer. The electrical detection of spin-pumping, furthermore, reveals the critical role of rectification and shunting effects on the generated voltages. As a barrier material, the selection of MgO intended not only to investigate the role of tunneling properties of the conduction electrons at the Fe/MgO/Pt interfaces but also to probe the flow of spin current through the MgO/Fe interface of the Fe/Pt bilayers.By the completion of the dissertation the main goal of the project was accomplished, which was the determination of the spin dynamics dependence on the structural characteristics that the spintronic bilayers exhibit. The goal was succeeded by the structural and elemental analysis methods, in combination with techniques determining their magnetic properties.Στο εισαγωγικό μέρος της διατριβής, παρουσιάζεται μια αναφορά στις μαγνητικές ιδιότητες της ύλης, ενώ δίδεται ιδιαίτερη έμφαση στο σιδηρομαγνητισμό, καθώς το βασικό υλικό που χρησιμοποιήθηκε πειραματικά, ήταν ο Fe. Η αρχή λειτουργίας της σπιντρονικής εντοπίζεται στις ηλεκτρονιακές αλληλεπιδράσεις, που καθορίζουν τη μαγνήτιση των υλικών. Η σύνδεση της βασικής ηλεκτρονικής με τη σπιντρονική επιτυγχάνεται μέσω της δυναμικής του μαγνητισμού και τη διάδοση (ρεύμα) των κυμάτων spin. Για την παραγωγή ρευμάτων spin απαιτείται η διέγερση των διατεταγμένων μαγνητικών ροπών, που είναι δυνατό να επιτευχθεί μέσω του σιδηρομαγνητικού συντονισμού (FMR). Με σκοπό να αξιοποιηθούν τα κύματα spin (μαγνόνια) που δημιουργούνται στα μαγνητικά υλικά, απαιτείται η επαφή ενός μη-μαγνητικού (ΝΜ) ανιχνευτή ρευμάτων, όπου αντλούνται οι ροπές δια του φαινομένου spin-pumping (SP). Η φύση των NM υλικών έχει ως συνέπεια την εμφάνιση του αντιστρόφου φαινόμενο spin-Hall (ISHE), δημιουργώντας ένα ανιχνεύσιμο ρεύμα φόρτου στην επιφάνεια τους. Οι τελευταίες εξελίξεις στον τομέα της σπιντρονικής, με τη μελέτη λεπτών υμενίων μαγνητικών ετεροδομών, ανέδειξαν μια άμεση εξάρτηση των δυναμικών φαινομένων spin με τις δομικές ιδιότητες και τη μορφολογία των διεπιφανειών. Ακολουθώντας τις ενδείξεις της διεθνούς έρευνας, αναπτύχθηκε και μελετήθηκε κυρίως το σύστημα λεπτών υμενίων Fe/Pt. Για την ανάπτυξη των υμενίων χρησιμοποιήθηκε η επιταξία μοριακής δέσμης (ΜΒΕ) και εξάχνωσης με δέσμη ηλεκτρονίων (e-beam), σε υποστρώματα MgO. Για το δομικό χαρακτηρισμό των ετεροδομών, συνδυάστηκαν σύγχρονες μέθοδοι, όπως η περίθλαση και ανακλαστικότητα ακτίνων–Χ (XRD και XRR), η φασματοσκοπία φωτοηλεκτρονίων ακτίνων–Χ (XPS), και η ηλεκτρονική μικροσκοπία διέλευσης (ΤΕΜ/HRTEM). Ο στατικός και δυναμικός μαγνητικός χαρακτηρισμός ολοκληρώθηκε με μαγνητομετρία μαγνητο-οπτικού φαινομένου Kerr (MOKE) και στατικού πεδίου (SQUID), μετρήσεις ρεύματος ISHE, πειράματα SP και FMR.Κατά την πειραματική εργασία, πραγματοποιήθηκε αρχικά, μια μελέτη διερεύνησης του καταλληλότερου NM υλικού σε επαφή με τον Fe. Εξετάστηκε η ετεροεπιταξία του Fe σε υποστρώματα MgO, που λήφθηκε ως αναφορά, ενώ συγκρίθηκαν τα αποτελέσματα χαρακτηρισμού με επικάλυψη την Pt, τον Au και το Pd. Η διαστρωμάτωση αποδείχθηκε καίριας σημασίας για τα δομικά χαρακτηριστικά, που είχαν επίδραση στις μαγνητικές ιδιότητες των υμενίων. Ανεπιθύμητα αποτελέσματα, όπως η τοπική αλληλοδιάχυση μεταξύ Αu και Fe, αύξησαν έντονα τη μαγνητική σκληρότητα των συστημάτων, ενώ η αδυναμία του Pd να προστατέψει το μαγνητικό υλικό από οξείδωση, ανέδειξαν ως πιο ασφαλή επιλογή το σύστημα Fe/Pt. Η περαιτέρω μελέτη, κατέληξε για το σύστημα σε μια ικανοποιητικά μικρή τιμή του μήκους διάχυσης spin (λSD ≃ 1,1 nm).Για τα υμένια Fe/Pt, μελετήθηκε η επίδραση του πάχους του NM υμενίου στις ιδιότητες των συστημάτων, με σταθερό πάχος Fe 12 nm. Βρέθηκε ότι το πάχος των 6 nm της Pt, αποτελεί ένα όριο, κάτω του οποίου καταγράφηκε αξιοσημείωτη αύξηση του συνεκτικού πεδίου στα πειράματα μαγνητομετρίας MOKE, με ταυτόχρονη εισαγωγή μιας επιπρόσθετης μονοαξονικής ανισοτροπίας επί της κυβικής ανισοτροπίας της δομής του Fe. Ο δομικός χαρακτηρισμός και η ανάλυση HRΤΕΜ, απέδωσε τα φαινόμενα στη τοπική διακοπή της συμμετρίας των ετεροδομών, με αύξηση της διεπιφανειακής τραχύτητας, που ενίσχυσε τα φαινόμενα μαγνητικής γειτνίασης. Τα αποτελέσματα επιβεβαιώθηκαν με την υποστήριξη προσομοιώσεων μοριακής δυναμικής, που αποκάλυψαν την παρουσία ψευδομορφικής ανάπτυξης της Pt για τα πρώτα ατομικά επίπεδά της. Στη συνέχεια, μελετήθηκε η επίδραση θερμοκρασίας κατά την ανάπτυξη των υμενίων Fe/Pt. Ο δομικός και μαγνητικός χαρακτηρισμός ανέδειξε τη θερμοκρασία ανάπτυξης των 300οC ως ιδανική για την ομαλή επιταξία. Ο συνολικός πειραματικός χαρακτηρισμός σε συνδυασμό με τα αποτελέσματα που λήφθηκαν από τις προσομοιώσεις μοριακής δυναμικής εξήγαγαν το μοντέλο εφηρέμησης των εσωτερικών παραμορφώσεων των υμενίων, με το στρώμα του Fe να εκτονώνεται με την αύξηση της θερμοκρασίας ανάπτυξης και του στρώματος της Pt με τον σχηματισμό ορίων κρυσταλλιτών και σφαλμάτων επιστοίβασης. Η εξαγωγή του επιταξιακού μοντέλου παρείχε την ευκαιρία για ελεγχόμενη παραμετροποίηση των διεπιφανειών, όπου οι μαγνητικές «ατέλειες» νανοσωματιδίων Fe3O4 ενίσχυσαν το συνεκτικό πεδίο των συστημάτων, αυξάνοντας ταυτοχρόνως την κυβική ανισοτροπία του Fe. Oι μετρήσεις FMR και τα περάματα SP, προσέδωσαν ένα μεγάλο εύρος στις καμπύλες συντονισμού, παρέχοντας επιπροσθέτως και μεγάλο εύρος μαγνητικού πεδίου για την παραγή τάσης φόρτου στις επιφάνειες. H εισαγωγή ενός υπέρλεπτου στρώματος MgO στις μεταλλικές διεπιφάνειες, αποτέλεσε μια επιπρόσθετη προσπάθεια παραμετροποίησης των διεπιφανειών, με σκοπό τη μελέτη της άντλησης των ιδιοστροφορμών δια μέσου του διηλεκτρικού φράγματος. Τα δείγματα που εξετάστηκαν είχαν σταθερό πάχος Pt 6 nm και Fe 12 nm, ενώ η μελέτη πραγματοποιήθηκε για τα διαφορετικά πάχη (0,5 – 2 nm) ενδιάμεσου MgO. Αποδείχθηκε ότι τα ρεύματα spin μπορούν να διαδοθούν εντός του στρώματος της Pt για πάχη έως 2 nm MgΟ. Η ηλεκτρική ανίχνευση του SP επιπλέον, αναδεικνύει την υψηλή σημασία των φαινομένων επανόρθωσης και παρέκκλισης του ρεύματος στις επαγόμενες τάσεις. Η επιλογή του MgO, ως ενδιάμεσο στρώμα παρείχε επιπλέον πληροφορίες για την επίδραση της παρουσίας του ως υπόστρωμα στα συστήματα Fe/Pt. Με την ολοκλήρωση της διατριβής εκπληρώθηκε ο βασικός στόχος της εργασίας, που ήταν ο προσδιορισμός της εξάρτησης της δυναμικής των spin από τα δομικά χαρακτηριστικά των σπιντρονικών διατάξεων˙ επιτεύχθηκε δε, με το συνδυασμό των μεθόδων δομικού και στοιχειακού χαρακτηρισμού με τις τεχνικές προσδιορισμού των μαγνητικών ιδιοτήτων

Oxygen Evolution Reaction at IrO2/Ir(Ni) Film Electrodes Prepared by Galvanic Replacement and Anodization: Effect of Precursor Ni Film Thickness

IrO2/Ir(Ni) film electrodes of variable Ni content have been prepared via a galvanic replacement method, whereby surface layers of pre-deposited Ni are replaced by Ir, followed by electrochemical anodization. Electrodeposition of Ni on a glassy carbon electrode support has been carried out at constant potential and the charge of electrodeposited Ni controlled so as to investigate the effect of precursor Ni layer thickness on the electrocatalytic activity of the corresponding IrO2/Ir(Ni)/GC electrodes for the oxygen evolution reaction (OER). After their preparation, these electrodes were characterized by microscopic (SEM) and spectroscopic (EDS, XPS) techniques, revealing the formation of Ir deposits on the Ni support and a thin IrO2 layer on their surfaces. To determine the electroactive surface area of the IrO2 coatings, cyclic voltammograms were recorded in the potential range between hydrogen and oxygen evolution and the charge under the anodic part of the curves, corresponding to Ir surface oxide formation, served as an indicator of the quantity of active IrO2 in the film. The electrocatalytic activity of the coatings for OER was investigated by current–potential curves under steady state conditions, revealing that the catalysts prepared from thinner Ni films exhibited enhanced electrocatalytic performance

Incorporating Graphene Nanoplatelets and Carbon Nanotubes in Biobased Poly(ethylene 2,5-furandicarboxylate): Fillers’ Effect on the Matrix’s Structure and Lifetime

Poly(ethylene 2,5-furandicarboxylate) (PEF) nanocomposites reinforced with Graphene nanoplatelets (GNPs) and Carbon nanotubes (CNTs) were in situ synthesized in this work. PEF is a biobased polyester with physical properties and is the sustainable counterpart of Polyethylene Terephthalate (PET). Its low crystallizability affects the processing of the material, limiting its use to packaging, films, and textile applications. The crystallization promotion and the reinforcement of PEF can lead to broadening its potential applications. Therefore, PEF nanocomposites reinforced with various loadings of GNPs, CNTs, and hybrids containing both fillers were prepared, and the effect of each filler on their structural characteristics was investigated by X-ray Diffraction (XRD), Fourier transform infrared spectroscopy—attenuated total reflectance (FTIR–ATR), and X-Ray Photoelectron Spectroscopy (XPS). The morphology and structural properties of a hybrid PEF nanocomposite were evaluated by Transmission Electron Microscopy (TEM). The thermo-oxidative degradation, as well as lifetime predictions of PEF nanocomposites, in an ambient atmosphere, were studied using Thermogravimetric Analysis (TGA). Results showed that the fillers’ incorporation in the PEF matrix induced changes in the lamellar thickness and increased crystallinity up to 27%. TEM analysis indicated the formation of large CNTs aggregates in the case of the hybrid PEF nanocomposite as a result of the ultrasonication process. Finally, the presence of CNTs caused the retardation of PEF’s carbonization process. This led to a slightly longer lifetime under isothermal conditions at higher temperatures, while at ambient temperature the PEF nanocomposites’ lifetime is shorter, compared to neat PEF

Thiol-Functionalization Carbonaceous Adsorbents for the Removal of Methyl-Mercury from Water in the ppb Levels

Mercury is a highly toxic pollutant of major public health concern, and human exposure is mainly related to the aqueous phase, where its dominant form is methyl-mercury (MeHg). In the current work, two carbon-based adsorbents, i.e., a commercial activated carbon and a sunflower seeds’ biochar, were modified by the introduction of thiol-active groups onto their surfaces for the MeHg removal from natural-like water in ppb concentration levels. The examined thiol-functionalization was a two-step process, since the raw materials were initially treated with nitric acid (6 N), which is a reagent that favors the formation of surface carboxyl groups, and subsequently by the thiol surface bonding groups through an esterification reaction in methanol matrix. The adsorbents’ capacity was evaluated toward the Hgtotal legislative regulation limit (1 μg/L) in drinking water (denoted as Q1). The respective isothermal adsorption results revealed an increased affinity between MeHg and thiol-functionalized materials, where the commercial carbon showed slightly higher capacity (0.116 μg Hg/mg) compared with the biochar (0.108 μg Hg/mg). This variation can be attributed to the respective higher surface area, resulting, also, to higher thiol groups loading. Regarding the proposed mechanism, it was proved that the S-Hg bond was formed, based on the characterization of the best performed saturated adsorbent

Tuning the Fe(II)/hydroxide Ratio during Synthesis of Magnetite Nanoparticles to Maximize Cr(VI) Uptake Capacity

The impact of hydroxyl excess as defined by the Fe(II)/hydroxide ratio during the synthesis of Fe3O4 nanoparticles by oxidative precipitation of FeSO4 was examined as a critical parameter determining the potential for Cr(VI) uptake from polluted water. Various samples were prepared by varying the OH− excess in the range of −0.10 up to +0.03 M and characterized according to their composition, morphology, and surface configuration. Their efficiency for Cr(VI) removal was evaluated by batch adsorption tests, carried out under similar conditions with drinking water purification in the concentration range below 10 mg/L. Results indicate that near the zero-excess point for hydroxyl balance, the uptake capacity for residual Cr(VI) concentration equal to 25 μg/L remains at very low levels (2+/Fe3+ = 0.42). In addition, utilizing negative excess values below −0.05 M triggers a similar efficiency rise, although the morphology of the obtained aggregates is rather different. Such finding is attributed to a possible exchange mechanism between adsorbed sulfates and chromate anions that assist approach of Cr(VI) to the material’s surface. Overall, proper tuning of hydroxyl excess offers multiple options for the implementation of monodisperse magnetically responsive nanoparticles or larger aggregates with optimized purification efficiency in water technology

An Optimized Cr(VI)-Removal System Using Sn-based Reducing Adsorbents

Despite significant risks to human health due to elevated Cr(VI) concentrations in drinking water, a selective adsorbent capable of purifying water before consumption is still not commercially available. This work introduces an integrated household water filtration setup, for point-of-use applications, loaded with a tin-based Cr(VI)-oriented adsorbent that was tested under various contact times, pH values and Cr(VI) concentrations. The adsorbent comprises a chloride-substituted stannous oxy-hydroxide with a structure resembling that of the mineral abhurite. It demonstrated high reducing capacity that triggered the formation of insoluble Cr(III) hydroxides and the complete removal of Cr(VI) in considerably high volumes of polluted water. Test operation of the filtration system verified its ability to produce Cr(VI)-free water in compliance with the impending drinking water regulation, even for extreme initial concentrations (1000 μg/L). Apart from its high efficiency, the potential of the studied material is enhanced by its minimal-cost synthesis method carried out in a continuous-flow reactor by tin chloride precipitation under acidic conditions

Tuning the Fe(II)/hydroxide Ratio during Synthesis of Magnetite Nanoparticles to Maximize Cr(VI) Uptake Capacity

The impact of hydroxyl excess as defined by the Fe(II)/hydroxide ratio during the synthesis of Fe3O4 nanoparticles by oxidative precipitation of FeSO4 was examined as a critical parameter determining the potential for Cr(VI) uptake from polluted water. Various samples were prepared by varying the OH− excess in the range of −0.10 up to +0.03 M and characterized according to their composition, morphology, and surface configuration. Their efficiency for Cr(VI) removal was evaluated by batch adsorption tests, carried out under similar conditions with drinking water purification in the concentration range below 10 mg/L. Results indicate that near the zero-excess point for hydroxyl balance, the uptake capacity for residual Cr(VI) concentration equal to 25 μg/L remains at very low levels (<0.5 mg/g). However, a small increase above +0.02 M features synthesized nanoparticles with an uptake capacity of 2.5 mg/g owed to the decrease in particles size (28 nm) and enhancement of the reducing potential (Fe2+/Fe3+ = 0.42). In addition, utilizing negative excess values below −0.05 M triggers a similar efficiency rise, although the morphology of the obtained aggregates is rather different. Such finding is attributed to a possible exchange mechanism between adsorbed sulfates and chromate anions that assist approach of Cr(VI) to the material’s surface. Overall, proper tuning of hydroxyl excess offers multiple options for the implementation of monodisperse magnetically responsive nanoparticles or larger aggregates with optimized purification efficiency in water technology