Academic literature on the topic '2D carbides and nitrides (MXenes)'

A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.

The advance of research on 2D transition metal carbides, carbonitrides, and nitrides (collectively known as MXenes) has progressed rapidly since the introduction of Ti3C2 in 2011. Nowadays the number of MXene synthesized in lab has reached more than 20, while there are currently about 20 theoretically predicted structures. In this study, we calculate the electromagnetic interference shielding effectiveness of a series of MXene films in theory and find that the results are in good agreement with the measured data. From this, we can use this method to calculate electromagnetic properties of all kinds of 2D material films which are similar to Mxenes.

MXenes, 2D transition metal carbides/nitrides, with superior electrical, optical, and mechanical properties is a recent discovery and have already been deployed in a variety of fields such as batteries, composites, sensors and medical devices.

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (named as MXenes) have become of the fastest growing family of 2D materials in terms of compositions and their applications in different areas.

Two-dimensional (2D) materials such as graphene, graphene oxide (GO), metal carbides and nitrides (MXenes), transition metal dichalcogenides (TMDS), boron nitride (BN), and layered double hydroxide (LDH) metal–organic frameworks (MOFs) have been widely investigated as potential candidates in various separation applications because of their high mechanical strength, large surface area, ideal chemical and thermal stability, simplicity, ease of functionalization, environmental comparability, and good antibacterial performance. Recently, MXene as a new member of the 2D polymer family has attracted significant attention in water purification, desalination, gas separation, antibacterial, and antifouling applications. Herein, we review the most recent progress in the fabrication, preparation, and modification methods of MXene-based lamellar membranes with the emphasis on applications for water purification and desalination. Moreover, the antibacterial properties of MXene-based membranes show a significant potential for commercial use in water purification. Thus, this review provides a directional guide for future development in this emerging technology.

AbstractA growing family of two-dimensional (2D) transition metal carbides or nitrides, known as MXenes, have received increasing attention because of their unique properties, such as metallic conductivity and good hydrophilicity. The studies on MXenes have been widely pursued, given the composition diversity of the parent MAX phases. This review focuses on MXene films, an important form of MXene-based materials for practical applications. We summarized the synthesis methods of MXenes, focusing on emerging synthesis strategies and reaction mechanisms. The advanced assembly technologies of MXene films, including vacuum-assisted filtration, spin-coating methods, and several other approaches, were then highlighted. Finally, recent progress in the applications of MXene films in electrochemical energy storage, membrane separation, electromagnetic shielding fields, and burgeoning areas, as well as the correlation between compositions, architecture, and performance, was discussed.

In the past decade, a thriving family of 2D nanomaterials, transition-metal carbides/nitrides (MXenes), have garnered tremendous interest due to its intriguing physical/chemical properties, structural features, and versatile functionality. Integrating these 2D nanosheets into 3D monoliths offers an exciting and powerful platform for translating their fundamental advantages into practical applications. Introducing internal pores, such as isotropic pores and aligned channels, within the monoliths can not only address the restacking of MXenes, but also afford a series of novel and, in some cases, unique structural merits to advance the utility of the MXene-based materials. Here, a brief overview of the development of MXene-based porous monoliths, in terms of the types of microstructures, is provided, focusing on the pore design and how the porous microstructure affects the application performance.

L'azote joue un rôle indispensable pour toute vie sur terre et pour le développement des êtres humains. À l'heure actuelle, la seule technologie de synthèse de l'ammoniac à l'échelle industrielle est le procédé mis au point par Haber et Bosch au début du XXe siècle, qui utilise les phases gazeuses N2 et H2. Cependant, le procédé Haber-Bosch nécessite des conditions difficiles, des équipements complexes et une consommation d'énergie élevée, et fonctionne avec de faibles taux de conversion, ce qui est incompatible avec les exigences d’un développement durable. Par rapport à la méthode Haber-Bosch, l'électrocatalyse est l'une des voies prometteuses qui permet d'intégrer l'électricité produite à partir de technologies d'énergies renouvelables pour la production d'ammoniac à température ambiante et à pression ambiante. Un défi spécifique est lié au développement de nouveaux électrocatalyseurs/électrodes dans le but de parvenir à une production d'ammoniac à faible coût, à grande échelle et délocalisée. Compte tenu ces défis scientifiques , ce travail de doctorat se concentre sur trois aspects principaux de la réaction électrocatalytique de réduction de l'azote (NRR) : i) ingénierie et conception de l'électrocatalyseur, ii) conception de l'électrode et de la cellule du dispositif électrochimique et iii) amélioration et optimisation des conditions de réaction, afin d'améliorer les performances de la synthèse de l'ammoniac. La plupart des activités de recherche de ce travail de doctorat sur la synthèse et la caractérisation des matériaux électrocatalytiques et l'assemblage/le test des électrodes dans des dispositifs électrochimiques non conventionnels ont été menées au laboratoire CASPE de l'université de Messine. En outre, une période de 12 mois a été passée en cotutelle avec l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), où des voies de synthèse avancées ont été explorées pour la préparation d'électrocatalyseurs à base de composés organométalliques qui ont été utilisés comme électrodes plus actives dans la RRN. Cette thèse de doctorat est organisée en cinq grands chapitres. Le chapitre 1 se concentre sur les questions de fixation de l'azote et sur la description du processus industriel de Haber-Bosch, avec un aperçu des implications générales liées à ses besoins élevés en énergie. Le chapitre 2 fait référence aux matériaux électrocatalytiques développés pour la préparation des électrodes : 1) les matériaux hybrides organiques-inorganiques de type MOF, une classe de matériaux poreux très prometteurs pour leurs caractéristiques particulières de surface spécifique élevée et leurs propriétés ajustables ainsi que pour la possibilité de créer des sites catalytiques actifs spécifiques grâce aux groupes fonctionnels et aux centres d'ions métalliques ; 2) les MXènes, une classe de matériaux en carbure ou nitrure de métal à structure bidimensionnelle (2D), qui ont récemment suscité un grand intérêt pour un large éventail d'applications, notamment la catalyse et la fixation de N2, pour leurs propriétés uniques de conductivité métallique et de nature hydrophile des surfaces terminées par un hydroxyle ou un oxygène. Les chapitres 3 à 5 présentent et analysent les résultats expérimentaux. Le chapitre 3 concerne la préparation d'une série d'électrodes à base de Fe-MOF (Fe@Zn/SIM-1) et leur test dans la réaction NRR en utilisant un réacteur triphasé de pointe, fonctionnant en phase gazeuse. Dans le chapitre 4, une série de matériaux améliorés à base de Fe-MOF (incluant un dopage additionel par un métal alcalin du MOF UiO-66-(COOH)2), synthétisés par une technique de réaction d'échange de cations pour remplacer le proton de l'acide carboxylique par un cation de fer, sont présentés. Enfin, le chapitre 5 fait référence à l'exploration des matériaux avancés à base de MXène (Ti3C2 MXène) et à la tentative de synthèse d'une nanoarchitecture 3D à partir de catalyseurs à base de MXène en 2D
Nitrogen plays an indispensable role for all life on earth and for the development of human beings. Industrially, nitrogen gas is converted to ammonia (NH3) and nitrogen-rich fertilisers to supplement the amount of nitrogen fixed spontaneously by nature. At present, the only industrial-scale ammonia synthesis technology is the process developed by Haber and Bosch in the early 20th century using gas phase N2 and H2 as the feeding gases. However, the Haber-Bosch process requires harsh conditions, complex equipment and high energy consumption, and operates with low conversion rates, which are inconsistent with economic and social growing development requirements. Compared to the Haber-Bosch method, electrocatalysis is one of the promising routes that can integrate electricity produced from renewable energy technologies for the production of ammonia at room temperature and ambient pressure. A specific challenge is related to the development of novel electrocatalysts/electrodes with the aim to achieve a low-cost, large-scale and delocalized production of ammonia. In view of the above key scientific issues, this PhD work focuses on three main aspects of the electrocatalytic nitrogen reduction reaction (NRR): i) engineering and design of the electrocatalyst, ii) electrode and cell design of the electrochemical device and iii) improvement and optimization of the reaction conditions, to enhance the performances of ammonia synthesis. Most of the research activities of this PhD work about synthesis and characterization of the electrocatalytic materials and assembling/testing of the electrodes in unconventional electrochemical devices were carried out at the laboratory CASPE (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina. Moreover, during the three years, a period of 12 months was spent in cotutelle with the École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), where advanced synthesis routes were explored for the preparation of organometallic-based electrocatalysts to be used as more active electrodes in NRR. The PhD thesis is organized in five main chapters. Chapter 1 focuses on N2 fixation issues and on describing the industrial Haber-Bosch process, with an overview of the general implications related to its high energy requirements. Chapter 2, instead, refers to the electrocatalytic materials developed in this PhD work for the preparation of the electrodes: 1) the Metal-organic Frameworks (MOFs), a class of porous materials very promising for their peculiar characteristics of high surface area, tunable properties, organic functionality and porosity, as well as for the possibility of creating specific catalytic active sites thanks to both the functional groups and the metal ion centres; 2) the MXenes, a class of metal carbide or nitride materials with a two-dimensional (2D) structure, which have recently attracted a large interest for a broad range of applications, including catalysis and N2 fixation, for their unique properties of metallic conductivity and hydrophilic nature of the hydroxyl or oxygen terminated surfaces. In Chapters 3-5, the experimental results are presented and discussed. Chapter 3 concerns the preparation of a series of Fe-MOF-based (Fe@Zn/SIM-1) electrodes and their testing in NRR by using an advanced engineered three-phase reactor, working in gas-phase. In Chapter 4, a series of improved Fe-MOF-based materials (Fe-based and Fe-alkali metal-based MOF UiO-66-(COOH)2), synthesized by cation exchange reaction technique to replace the proton of carboxylic acid with an iron cation, are presented. Finally, Chapter 5 refers to the exploration of advanced MXene materials (Ti3C2 MXene) and to the attempt of synthesizing a 3D nanoarchitecture starting from 2D-dimensional MXene-based catalysts

Two dimensional (2D) materials have received growing interest because of their unique properties compared to their bulk counterparts. Graphene is the archetype 2D solid, but other materials beyond graphene, such as MoS2 and BN have become potential candidates for several applications. Recently, a new family of 2D materials of early transition metal carbides and carbonitrides (Ti2CTx, Ti3C2Tx, Ti3CNTx, Ta4C3Tx, and more), labelled MXenes, has been discovered, where T stands for the surface-terminating groups. Before the present work, MXenes had only been synthesized in the form of exfoliated and delaminated powders, which is not suitable for electronic applications. In this thesis, I demonstrate the synthesis of MXenes as epitaxial thin films, a more suitable form for electronic and photonic applications. Results show that 2D epitaxial Ti3C2Tx films - produced by HF and NH4HF2 etching of magnetron sputter-grown Ti3AlC2 - exhibit metallic conductive behaviour down to 100 K and are 90% transparent to light in the visible-infrared range. The results from this work may open the door for MXenes as potential candidates for transparent conductive electrodes as well as in electronic, photonic and sensing applications. MXenes have been shown to intercalate cations and molecules between their layers that in turn can alter the surface termination groups. There is therefore a need to study the surface chemistries of synthetized MXenes to be able to study the effect of intercalation as well as altering the surface termination groups on the electronic structure and chemical states of the elements present in MXene layers. X-ray Photoelectron Spectroscopy (XPS) in-depth characterization was used to investigate surface chemistries of Ti3C2Tx and Ti2CTx. This thesis includes the discussion of the effect of Ar+ sputtering and the number of layers on the surface chemistry of MXenes. This study serves as a baseline for chemical modification and tailoring of the surface chemistry groups to potential uses and applications. New MXene phases, Nb2CTx and V2CTx, are shown in this thesis to be produced from HF chemical etching of Nb2AlC and V2AlC powders. Characterization of the produced MXenes was carried out using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Transmission Electron Microscope (TEM) and XPS. Nb2CTx and V2CTx showed promising performance as electrodes for Li-ion batteries. In this thesis, electrochemical etching was used in an attempt to produce 2D metal carbides (MXene) from their ternary metal carbides, Ti3SiC2, Ti3AlC2 and Ti2AlC MAX phases. MAX phases in the form of highly dense bulk produced by Hot Isostatic Press. Several etching solutions were used such as HF, NaCl and HCl. Unlike the HF chemical etching of MAX phases, which results in MXenes, the electrochemical etching resulted in Carbide Derived Carbon (CDC). Here, I show the characterization of the produced CDC using several techniques such as XRD, TEM, Raman spectroscopy, and XPS. Electrochemical characterization was performed in the form of cyclic voltammetry, which sheds light on the etching mechanism.

The series name Linköping Studies in Science and Technology Licentiate Thesis in this publication is incorrect. Correct name is Linköping Studies in Science and Technology. Thesis.

Nanomaterials have been served as essential building blocks in the era of nanotechnology. Nanomaterials often exhibit different properties compared to their bulk phase, due to heavily enlarged portion of surface characteristics to the bulk. Beyond the simple size- effect, nanomaterials can be classified into 0D, 1D, and 2D materials depends on the number of restricted dimensionalities. They exhibit different unique properties and transport mechanism due to the quantum confinement effect. MXenes are one of the latest additions of 2D material family that can be obtained by selective chemical etching and exfoliation of layered ternary precursors (Mn+1AXn phases). Due to the unique etch process, surface functional groups (such as oxygen, hydroxyl, fluorine, etc) are formed at the surface of MXenes. This benefits MXenes for stable aqueous dispersions due to their hydrophilic surface. The coexistence of hydrophilicity and high electrical conductivity promised MXenes in superior performance in electrochemical energy storage and electromagnetic interference shielding applications. These characteristics are equally important for electronic applications. From the synthesis of MXene suspension to thin film deposition by spray-coating and photolithography patterning of MXene films are discussed for electronic device applications of MXenes. Vacuum-assisted filtration method was used for Mo-based MXene freestanding papers for investigation of thermoelectric energy harvesting performances. Both n-type ZnO and p-type SnO thin film transistors with MXene electrical contacts (gate, source, and drain electrodes) have been demonstrated by lift-off patterning method. Their complementary metal-oxide-semiconductor (CMOS) inverter exhibits a high gain value of 80 V/V at a supply voltage of 5 V. The lift-off patterning is simple but effective method for top-contact electrode patterning. However, it has a disadvantage of remaining sidewall-like MXene residue, resulting in leakage issues in the bottom-contact transistor structure. Hence, dry-etch patterning method is developed which allows direct patterning of MXene nanosheet thin films through conventional photolithography process. The conductive MXene electrode array was integrated into a quantum dot electric double layer transistors by all solution processes, which possess impressive performance including electron mobility of 3.3 cm2/V·s, current modulation of 104, threshold voltage as low as 0.36 V at low driving gate voltage range of only 1.25 V.

L'azoto gioca un ruolo indispensabile per la vita sulla terra e per lo sviluppo degli esseri umani. Industrialmente, è necessario convertire l'azoto gassoso in ammoniaca (NH3) per la produzione di fertilizzanti, in modo da integrare la quantità di azoto fissata spontaneamente in natura. Attualmente, l'unica tecnologia di sintesi dell'ammoniaca su scala industriale è il processo sviluppato da Haber e Bosch all'inizio del XX secolo che utilizza N2 e H2 come gas di alimentazione. Tuttavia, il processo Haber-Bosch richiede condizioni molto drastiche, apparecchiature complesse e porta ad un elevato consumo energetico, operando inoltre a bassi tassi di conversione che non sono coerenti con le esigenze sempre crescenti di sviluppo economico e sociale. In alternativa al metodo Haber-Bosch, l'elettrocatalisi rappresenta una delle vie più promettenti che possono integrare l'elettricità prodotta da tecnologie di energia rinnovabile con la produzione di ammoniaca a temperatura ambiente e a pressione atmosferica. Una sfida specifica è legata allo sviluppo di nuovi elettrocatalizzatori/elettrodi con l'obiettivo di ottenere una produzione di ammoniaca a basso costo, su larga scala e delocalizzata sul territorio. Alla luce delle suddette questioni scientifiche fondamentali, questo lavoro di dottorato si concentra su tre aspetti principali legati alla reazione elettrocatalitica di riduzione dell'azoto (NRR): i) l’ingegneria e la progettazione dell'elettrocatalizzatore, ii) la progettazione dell'elettrodo e del dispositivo elettrochimico e iii) il miglioramento e l’ottimizzazione delle condizioni di reazione, per migliorarne le prestazioni nella sintesi dell'ammoniaca. La maggior parte delle attività di ricerca di questo dottorato, dalla sintesi e caratterizzazione dei materiali elettrocatalitici all'assemblaggio/collaudo degli elettrodi in dispositivi elettrochimici non convenzionali, sono state svolte presso il laboratorio CASPE (Laboratorio di Catalisi per la Produzione e l'Energia Sostenibile) dell'Università di Messina. Durante i tre anni, un periodo di 12 mesi è stato inoltre trascorso in cotutela con l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), dove sono state studiate tecniche di sintesi avanzate per la preparazione di elettrocatalizzatori a base organometallica da utilizzare come elettrodi cataliticamente attivi nella NRR. La tesi di dottorato è organizzata in cinque capitoli principali. Il capitolo 1 si concentra sulle questioni di fissazione dell’azoto e sulla descrizione del processo industriale Haber-Bosch, con una panoramica sulle implicazioni generali relative al suo elevato fabbisogno energetico. Vengono poi presentati i metodi alternativi per la fissazione elettrochimica dell'azoto, con un'ampia descrizione dei vantaggi, legati alle condizioni più favorevoli (cioè temperatura ambiente e pressione atmosferica) e degli svantaggi, e discutendo gli elementi da sviluppare per una futura implementazione di questa tecnologia, includendo anche una descrizione del possibile meccanismo di reazione, ancora non del tutto chiaro in letteratura. Il capitolo 2, invece, si riferisce alla descrizione dei materiali elettrocatalitici sviluppati in questo lavoro di dottorato per la preparazione degli elettrodi: 1) i “Metal-Organic Frameworks” (MOF), una classe di materiali porosi molto promettenti per le loro caratteristiche peculiari di elevata superficie, proprietà adattabili, funzionalità organica e porosità, oltre che per la possibilità di creare specifici siti attivi catalitici grazie sia ai gruppi funzionali che ai centri ionici metallici; 2) i MXeni, una classe di materiali a base di carburi o nitruri metallici con struttura bidimensionale (2D), che hanno recentemente attirato un grande interesse per una vasta gamma di applicazioni, tra cui la catalisi e la fissazione di N2, per le loro proprietà uniche di conducibilità metallica e la natura idrofila delle superfici con terminali idrossilici o di ossigeno. Nei capitoli 3-5 vengono presentati e discussi i risultati sperimentali. Il capitolo 3 riguarda la preparazione di una serie di elettrodi di MOF a base di Fe (Fe@Zn/SIM-1) e il loro test nella NRR utilizzando un reattore trifasico avanzato, che lavora in fase gassosa. Questo nuovo dispositivo funziona a temperatura ambiente e a pressione atmosferica, e possiede il controelettrodo e l’elettrodo di riferimento immersi in una semicella anodica (dove avviene l'ossidazione di H2O a O2) contenente un elettrolita liquido (l'anolita), mentre la semicella catodica per la NRR opera in fase gassosa senza elettrolita liquido. Questo tipo di reattore elettrocatalitico è quindi molto diverso dai reattori elettrocatalitici convenzionali che operano in fase liquida, con il grande vantaggio di evitare problematiche legate alla bassa solubilità e al trasporto di N2 nell'elettrolita, e di permettere inoltre un più facile recupero dell'ammoniaca prodotta. I risultati ottenuti da questi test elettrocatalitici in fase gassosa sono stati molto utili per migliorare la progettazione degli elettrodi a base di MOF, evidenziando i limiti di questo tipo di materiali in termini di contenuto di N, stabilità e possibilità di preparare elettrocatalizzatori più avanzati mediante carbonizzazione. Un'ampia parte di questo capitolo è stata dedicata allo sviluppo di nuove strategie sperimentali per evitare i falsi positivi nella rilevazione dell'ammoniaca, che è uno degli argomenti più investigati negli ultimi due anni dai ricercatori che lavorano sulla NRR. Mentre in letteratura sono stati recentemente proposti protocolli molto accurati che utilizzano tecniche analitiche avanzate (basati sull’azoto marcato 15N), in questo lavoro viene invece suggerita una metodologia più semplice basata sull'analisi spettrofotometrica UV-visibile (accoppiata a test in bianco con gas inerti in luogo dell’azoto) che hanno permesso con successo di evitare contaminazioni da ammoniaca e identificare i falsi positivi, anche se tecniche analitiche più sofisticate sono sicuramente necessarie per confermare definitivamente la vera fonte di ammoniaca. Nel capitolo 4 viene presentata una serie di materiali MOF migliorati (MOF UiO-66-(COOH)2 a base di Fe o Fe e metalli alcalini), sintetizzati con la tecnica di reazione a scambio cationico per sostituire il protone dell'acido carbossilico con un catione di ferro. Rispetto ai materiali Fe@Zn/SIM-1, questa nuova classe di MOF è più stabile in acqua e non contiene atomi di azoto nella sua struttura. I risultati hanno dimostrato che il Fe@UiO-66-(COOH)2 ottenuto mediante l'80% di scambio cationico (con un contenuto effettivo di Fe di circa 8% in peso) è stato il miglior elettrocatalizzatore testato tra i vari materiali MOF a base di Fe sintetizzati. Le prestazioni nella NRR dipendono fortemente dal design della cella e dell'elettrodo. Più in dettaglio, è stato ottenuto un rendimento di ammoniaca di 1.19 μg•h-1•mgcat-2 con una configurazione di strati assemblati ed ordinati nel modo seguente: i) Nafion (la membrana), ii) MOF a base di Fe (l'elettrocatalizzatore), iii) il GDL (lo strato di diffusione gassosa a base di carbonio) e iv) un ulteriore strato di Fe-MOF. È stato anche esplorato l'effetto del voltaggio applicato, con un potenziale ottimale di -0.5 V vs RHE per massimizzare l'attività nella NRR e limitare la reazione collaterale di evoluzione dell'idrogeno. Inoltre, come attualmente utilizzato nei catalizzatori industriali per il processo Haber-Bosh, è stata studiata anche l'introduzione del potassio negli elettrocatalizzatori, al fine di facilitare il trasferimento di carica dagli ioni K- verso la superficie del catalizzatore a base di ferro, bilanciando il chemisorbimento dissociativo tra H2 e N2, e sopprimendo le reazioni collaterali, migliorandone così sia l'attività che la stabilità. I risultati ottenuti sono molto promettenti, anche se sono necessari ulteriori studi per migliorare le loro prestazioni nella NRR, per superare le limitazioni legate ai materiali MOF stessi, soprattutto a causa della loro bassa conducibilità e stabilità. Infine, il capitolo 5 si riferisce all'esplorazione di materiali avanzati, i MXeni (Ti3C2 MXeni), e al tentativo di sintetizzare una nanoarchitettura 3D partendo dalla loro forma bidimensionale. Per comprendere il ruolo della nanostruttura dei materiali MXeni nella NRR, “nanoribbons” (nano-nastri) di Ti3C2 sono stati trattati con KOH per ottenere una forma finale di strutture porose tridimensionali (3D). In particolare, l'obiettivo di questa parte di lavoro è stato quello di indagare come la conversione dei “nanoribbons” di Ti3C2 in strutture tridimensionali influenzi la reattività nella NRR condotta nel dispositivo elettrochimico in fase gassosa. È stata anche effettuata una caratterizzazione completa dei “nanoribbons” di MXeni (SEM, TEM, HRTEM, XRD, XPS e EDX). I risultati hanno mostrato che la nanostruttura tridimensionale porta ad un significativo miglioramento dell'attività di fissazione di N2 a causa della formazione di siti esposti di Ti-OH. È stata anche osservata una relazione lineare tra il tasso di formazione di ammoniaca e la quantità di ossigeno sulla superficie dei Ti3C2 MXeni.
Nitrogen plays an indispensable role for all life on earth and for the development of human beings. Industrially, nitrogen gas is converted to ammonia (NH3) and nitrogen-rich fertilisers to supplement the amount of nitrogen fixed spontaneously by nature. At present, the only industrial-scale ammonia synthesis technology is the process developed by Haber and Bosch in the early 20th century using gas phase N2 and H2 as the feeding gases. However, the Haber-Bosch process requires harsh conditions, complex equipment and high energy consumption, and operates with low conversion rates, which are inconsistent with economic and social growing development requirements. Compared to the Haber-Bosch method, electrocatalysis is one of the promising routes that can integrate electricity produced from renewable energy technologies for the production of ammonia at room temperature and ambient pressure. A specific challenge is related to the development of novel electrocatalysts/electrodes with the aim to achieve a low-cost, large-scale and delocalized production of ammonia. In view of the above key scientific issues, this PhD work focuses on three main aspects of the electrocatalytic nitrogen reduction reaction (NRR): i) engineering and design of the electrocatalyst, ii) electrode and cell design of the electrochemical device and iii) improvement and optimization of the reaction conditions, to enhance the performances of ammonia synthesis. Most of the research activities of this PhD work about synthesis and characterization of the electrocatalytic materials and assembling/testing of the electrodes in unconventional electrochemical devices were carried out at the laboratory CASPE (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina. Moreover, during the three years, a period of 12 months was spent in cotutelle with the École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), where advanced synthesis routes were explored for the preparation of organometallic-based electrocatalysts to be used as more active electrodes in NRR. The PhD thesis is organized in five main chapters. Chapter 1 focuses on N2 fixation issues and on describing the industrial Haber-Bosch process, with an overview of the general implications related to its high energy requirements. The alternative methods based on the electrochemical nitrogen fixation are then presented, with a wide description of pros and cons related to the milder conditions (i.e., room temperature and atmospheric pressure) and by discussing the elements to be developed for a future implementation of this technology, including a description of the possible reaction mechanism, which is still unclear in literature. Chapter 2, instead, refers to the electrocatalytic materials developed in this PhD work for the preparation of the electrodes: 1) the Metal-organic Frameworks (MOFs), a class of porous materials very promising for their peculiar characteristics of high surface area, tunable properties, organic functionality and porosity, as well as for the possibility of creating specific catalytic active sites thanks to both the functional groups and the metal ion centres; 2) the MXenes, a class of metal carbide or nitride materials with a two-dimensional (2D) structure, which have recently attracted a large interest for a broad range of applications, including catalysis and N2 fixation, for their unique properties of metallic conductivity and hydrophilic nature of the hydroxyl or oxygen terminated surfaces. In Chapters 3-5, the experimental results are presented and discussed. Chapter 3 concerns the preparation of a series of Fe-MOF-based (Fe@Zn/SIM-1) electrodes and their testing in NRR by using an advanced engineered three-phase reactor, working in gas-phase. This novel device operates at room temperature and atmospheric pressure, with counter and reference electrodes immersed into an anode half-cell (where the oxidation of H2O to O2 occurs) containing a liquid electrolyte (the anolyte), while the cathode half-cell for NRR operates in gas phase without a liquid electrolyte (electrolyte-less conditions). This type of electrocatalytic reactor is thus quite different from the conventional electrocatalytic reactors operating in liquid phase, with the main advantages of avoiding issues related to the low N2 solubility and transport in the electrolyte, and allowing an easier recovery of ammonia. The results obtained from these electrocatalytic tests in gas-phase were very useful to improve the design of the MOFs-based electrodes, evidencing the limits of these kinds of materials in terms of N content, stability and possibility to prepare more advanced electrocatalysts by carbonization. A wide part of this chapter was dedicated to the development of new experimental strategies for avoiding false positive in the detection of ammonia, which is one of the topics most studied from scientists working in NRR in the last two years. As accurate protocols were recently suggested in literature, also using advanced analytical techniques (i.e. using 15N labelled nitrogen), an easier methodology based on UV-visible spectrophotometric analysis (coupled with blank tests with inert gases) was suggested in this work to avoid ammonia contaminations and false positives, although more sophisticated analytical techniques may definitely confirm the real source of ammonia. In Chapter 4, a series of improved Fe-MOF-based materials (Fe-based and Fe-alkali metal-based MOF UiO-66-(COOH)2), synthesized by cation exchange reaction technique to replace the proton of carboxylic acid with an iron cation, are presented. With respect to Fe@Zn/SIM-1, this new class of MOFs are more stable in water and do not contain nitrogen atoms in their structure. Results evidenced that 80% cation exchange Fe@UiO-66-(COOH)2 (with an effective Fe content of around 8 wt.%) was the best electrocatalyst among the tested Fe-based MOF synthesized materials. The performances in NRR highly depended on cell and electrode design. More in detail, an ammonia yield of 1.19 μg•h-1•mgcat-2 was obtained with an assembling configuration of layers ordered as i) Nafion (the membrane), ii) Fe-based MOF (the electrocatalyst), iii) GDL (the carbon gas diffusion layer) and iv) a further layer of Fe-MOF. The effect of applied voltage was also explored, indicating an optimal voltage of -0.5 V vs. RHE to maximize activity in NRR and limiting the side hydrogen evolution reaction. Moreover, as currently used in the industrial catalysts for Haber-Bosh process, the introduction of potassium in the electrocatalysts was also investigated, in order to facilitate charge transfer from K- ions to the iron-based catalyst surface, balancing the dissociative chemisorption between H2 and N2, and suppressing side reactions, thus improving both activity and stability. These results were very promising, although a further experimentation is needed to improve their performances in NRR, to overcome limitations related to MOF materials themselves, majorly due to their low conductivity and stability. Finally, Chapter 5 refers to the exploration of advanced MXene materials (Ti3C2 MXene) and to the attempt of synthesizing a 3D nanoarchitecture starting from 2D-dimensional MXene-based catalysts. To understand the role of the nanostructure of MXene materials in NRR, Ti3C2 nanosheets were treated with KOH to obtain a final shape of three-dimensional (3D) porous frameworks nanoribbons. Specifically, the objective of this research was to investigate how the conversion of Ti3C2 nanosheets to 3D-like nanoribbons influence the NRR reactivity in the gas-phase electrochemical device. A full characterization of MXenes nanoribbons (SEM, TEM, HRTEM, XRD, XPS and EDX) was also presented. Results showed that the 3D-type nanostructure (nanoribbons) leads to a significant enhancement of the N2 fixation activity due to the formation of exposed Ti-OH sites. A linear relationship was observed between ammonia formation rate and amount of oxygen on the surface of Ti3C2 MXene.
L'azote joue un rôle indispensable pour toute vie sur terre et pour le développement des êtres humains. Industriellement, l'azote gazeux est converti en ammoniac (NH3) et en engrais riches en azote pour compléter la quantité d'azote fixée spontanément par la nature. À l'heure actuelle, la seule technologie de synthèse de l'ammoniac à l'échelle industrielle est le procédé mis au point par Haber et Bosch au début du XXe siècle, qui utilise les phases gazeuses N2 et H2. Cependant, le procédé Haber-Bosch nécessite des conditions difficiles, des équipements complexes et une consommation d'énergie élevée, et fonctionne avec de faibles taux de conversion, ce qui est incompatible avec les exigences d’un développement durable. Par rapport à la méthode Haber-Bosch, l'électrocatalyse est l'une des voies prometteuses qui permet d'intégrer l'électricité produite à partir de technologies d'énergies renouvelables pour la production d'ammoniac à température ambiante et à pression ambiante. Un défi spécifique est lié au développement de nouveaux électrocatalyseurs/électrodes dans le but de parvenir à une production d'ammoniac à faible coût, à grande échelle et délocalisée. Compte tenu ces défis scientifiques, ce travail de doctorat se concentre sur trois aspects principaux de la réaction électrocatalytique de réduction de l'azote (NRR) : i) ingénierie et conception de l'électrocatalyseur, ii) conception de l'électrode et de la cellule du dispositif électrochimique et iii) amélioration et optimisation des conditions de réaction, afin d'améliorer les performances de la synthèse de l'ammoniac. La plupart des activités de recherche de ce travail de doctorat sur la synthèse et la caractérisation des matériaux électrocatalytiques et l'assemblage/le test des électrodes dans des dispositifs électrochimiques non conventionnels ont été menées au laboratoire CASPE (Laboratory of Catalysis for Sustainable Production and Energy) de l'université de Messine. En outre, une période de 12 mois a été passée en cotutelle avec l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), où des voies de synthèse avancées ont été explorées pour la préparation d'électrocatalyseurs à base de composés organométalliques qui ont été utilisés comme électrodes plus actives dans la RRN. Cette thèse de doctorat est organisée en cinq grands chapitres. Le chapitre 1 se concentre sur les questions de fixation de l'azote et sur la description du processus industriel de Haber-Bosch, avec un aperçu des implications générales liées à ses besoins élevés en énergie. Les méthodes alternatives basées sur la fixation électrochimique de l'azote sont ensuite présentées, avec une large description des avantages et des inconvénients liés aux conditions plus douces (c'est-à-dire la température ambiante et la pression atmosphérique) et en discutant des éléments à développer pour une future mise en œuvre de cette technologie, y compris une description du mécanisme de réaction possible, encore débattu dans la littérature. Le chapitre 2 fait référence aux matériaux électrocatalytiques développés pour la préparation des électrodes : 1) les matériaux hybrides organiques-inorganiques de type MOF, une classe de matériaux poreux très prometteurs pour leurs caractéristiques particulières de surface spécifique élevée et leurs propriétés ajustables ainsi que pour la possibilité de créer des sites catalytiques actifs spécifiques grâce aux groupes fonctionnels et aux centres d'ions métalliques ; 2) les MXènes, une classe de matériaux en carbure ou nitrure de métal à structure bidimensionnelle (2D), qui ont récemment suscité un grand intérêt pour un large éventail d'applications, notamment la catalyse et la fixation de N2, pour leurs propriétés uniques de conductivité métallique et de nature hydrophile des surfaces terminées par un hydroxyle ou un oxygène. Les chapitres 3 à 5 présentent et analysent les résultats expérimentaux. Le chapitre 3 concerne la préparation d'une série d'électrodes à base de Fe-MOF (Fe@Zn/SIM-1) et leur test dans la réaction NRR en utilisant un réacteur triphasé de pointe, fonctionnant en phase gazeuse. Ce nouveau dispositif fonctionne à température ambiante et à la pression atmosphérique, avec des électrodes de comptage et de référence immergées dans une demi-cellule anodique (où se produit l'oxydation de H2O en O2) contenant un électrolyte liquide (l'anolyte), tandis que la demi-cellule cathodique pour le NRR fonctionne en phase gazeuse sans électrolyte liquide. Ce type de réacteur électrocatalytique est donc très différent des réacteurs électrocatalytiques classiques fonctionnant en phase liquide, avec les principaux avantages d'éviter les problèmes liés à la faible solubilité et au transport de N2 dans l'électrolyte, et de permettre une récupération plus facile de l'ammoniac. Les résultats obtenus lors de ces essais électrocatalytiques en phase gazeuse ont été très utiles pour améliorer la conception des électrodes à base de MOFs, mettant en évidence les limites de ce type de matériaux en termes de teneur en N, de stabilité et de possibilité de préparer des électrocatalyseurs plus avancés par carbonisation. Une grande partie du chapitre 3 a été consacrée au développement de nouvelles stratégies expérimentales pour éviter les faux positifs dans la détection de l'ammoniac, qui est l'un des sujets les plus étudiés par les scientifiques travaillant dans la NRR ces deux dernières années. Comme des protocoles précis ont été récemment suggérés dans la littérature, utilisant également des techniques analytiques avancées (c'est-à-dire utilisant de l'azote marqué à 15N), une méthodologie plus facile basée sur l'analyse spectrophotométrique UV-visible (couplée à des essais à blanc avec des gaz inertes) a été suggérée dans ce travail pour éviter les contaminations par l'ammoniac et les faux positifs, bien que des techniques analytiques plus sophistiquées puissent définitivement confirmer la source réelle d'ammoniac. Dans le chapitre 4, une série de matériaux améliorés à base de Fe-MOF (incluant un dopage additionel par un métal alcalin du MOF UiO-66-(COOH)2), synthétisés par une technique de réaction d'échange de cations pour remplacer le proton de l'acide carboxylique par un cation de fer, sont présentés. En ce qui concerne le Fe@Zn/SIM-1, cette nouvelle classe de MOF est plus stable dans l'eau et ne contient pas d'atomes d'azote dans sa structure. Les résultats ont montré que l'échange cationique à 80 % Fe@UiO-66-(COOH)2 (avec une teneur effective en Fe d'environ 8 % en poids) était le meilleur électrocatalyseur parmi les matériaux synthétisés de MOF à base de Fe testés. Les performances du NRR dépendaient fortement de la conception de la cellule et de l'électrode. Plus en détail, un rendement en ammoniac de 1.19 μg•h-1•mgcat-2 a été obtenu avec une configuration d'assemblage de couches ordonnées comme i) Nafion (la membrane), ii) MOF à base de Fe (l'électrocatalyseur), iii) GDL (la couche de diffusion de gaz carbonique) et iv) une autre couche de Fe-MOF. L'effet de la tension appliquée a également été exploré, indiquant une tension optimale de -0,5 V par rapport à la RHE pour maximiser l'activité dans le NRR et limiter la réaction latérale d'évolution de l'hydrogène. En outre, comme c'est le cas actuellement dans les catalyseurs industriels pour le procédé Haber-Bosh, l'introduction de potassium dans les électrocatalyseurs a également été étudiée, afin de faciliter le transfert de charge des ions K- à la surface du catalyseur à base de fer, en équilibrant la chimisorption dissociative entre H2 et N2, et en supprimant les réactions secondaires, ce qui améliore à la fois l'activité et la stabilité. Ces résultats étaient très prometteurs, bien qu'une nouvelle expérimentation soit nécessaire pour améliorer leurs performances dans les NRR, afin de surmonter les limitations liées aux matériaux MOF eux-mêmes, principalement en raison de leur faible conductivité et de leur stabilité. Enfin, le chapitre 5 fait référence à l'exploration des matériaux avancés à base de MXène (Ti3C2 MXène) et à la tentative de synthèse d'une nanoarchitecture 3D à partir de catalyseurs à base de MXène en 2D. Pour comprendre le rôle de la nanostructure des matériaux à base de MXène dans la NRR, des nanofeuilles de Ti3C2 ont été traitées au KOH pour obtenir une forme finale de nanorubans à armature poreuse tridimensionnelle (3D). Plus précisément, l'objectif de cette recherche était d'étudier comment la conversion des nanofeuilles de Ti3C2 en nanorubans tridimensionnels influençait la réactivité du NRR dans le dispositif électrochimique en phase gazeuse. Une caractérisation complète des nanorubans MXenes (SEM, TEM, HRTEM, XRD, XPS et EDX) a également été présentée. Les résultats ont montré que la nanostructure de type 3D (nanorubans) conduit à une amélioration significative de l'activité de fixation du N2 en raison de la formation de sites Ti-OH exposés. Une relation linéaire a été observée entre le taux de formation d'ammoniac et la quantité d'oxygène à la surface du Ti3C2 MXene.

Stretchable and wearable strain-sensing devices are appropriate for motion detection, biomedical monitoring, human-machine interaction. These pressure sensors are working based on numerous electrophysical phenomena's such as piezoelectric, capacitive and piezoresistive reactions towards mechanical stretching. Piezoresistive sensors are highly favored due to their features like high sensitivity, fast response, easy fabrication and low energy requirement. They are generally fabricated using a suitable polymeric matrix and electrically conductive fillers, such as graphite, graphene or carbon nanotubes. MXenes are a relatively new family of (2D) transition metal carbides, nitrides or carbonitrides, produced by the selective chemical etching of “A” from MAXphases, where M is a transition metal, A is a group IIIA or IVA element and X is C or N. These nanomaterials are first reported in 2011 by the Gogotsi and Barsoum groups. These materials have received tremendous attention from the scientific community due to their excellent physiochemical properties, electrical conductivity and hydrophilicity. Herein, we report the preparation, characterization and piezoresistive individualities of semiconductive, electrospun mats composed of copolyamide 6, 10 and Ti3C2Tx. We observed that the relative resistance of the sensor increased with an increase in the Ti3C2Tx content, and the materials with higher electrical conductivity showcased a significantly higher sensitivity to applied pressure until reaching the percolation limit (font size can be increased).

Abstract Two-dimensional (2D) materials have recently garnered significant interest due to their novel and emergent properties. A plethora of 2D materials have been discovered and intensively studied, such as graphene, hexagonal boron nitride, transitionmetal dichalcogenides (TMDCs), and other metallic compound MXenes (nitrides, phosphides, and hydroxides), as well as elemental 2D materials (borophene, germanene, phosphorene, silicene, etc.). Considering the widespread interest in conventional van der Waals 2D materials, two-dimensional metallic nanosheets (2DMNS), a recent addition to the 2D materials family, have exhibited diverse potential spanning optics, electronics, magnetics, catalysis, etc. However, the close-packed, non-layered structure and non-directional, isotropic bonding of metallic materials make it difficult to access metals in their 2D forms, unlike 2D van der Waals materials, which have intrinsically layered structure (strong in-plane bonding in addition to the weak interlayer interaction). Until now, conventional top-down and bottom-up synthesis schemes of these 2DMNS have encountered various limitations such as precursor availability, substrate incompatibility, difficulty of control over thickness and stoichiometry, limited thermal budget, etc. To overcome these manufacturing limitations of 2DMNS, here we report a facile, rapid, large-scale, and cost-effective fabrication technique of nanometer-scale copper (Cu) 2DMNS via iterative rolling, folding, and calendering (RFC) that is readily generalizable to other conventional elemental metallic materials. Overall, we successfully show a scalable fabrication technique of 2DMNS.