Hybrid heterostructures and materials based on transition metal dichalcogenides

La irrupción de los nanomateriales en la vida moderna es un claro indicador del avance científico. El diseño de materiales a la carta permite establecer un control preciso sobre las propiedades finales de estos. El objetivo principal que ha motivado la presente Tesis es la combinación y funcionalización de láminas de MoS2 ultrafinas químicamente exfoliadas (ce-MoS2) con diferentes sistemas de base molecular. La Tesis se divide en los siguientes Capítulos: Capítulo I: En primer lugar, se introducen brevemente los nanomateriales. En segundo lugar, se describen los materiales bidimensionales (2D). A continuación, se detallan algunos aspectos generales de los dicalcogenuros de metales de transición (TMDCs). Luego, la atención se dirige al MoS2, el TMDC más estudiado. En este bloque, se ofrece información detallada sobre los politipos más conocidos, los principales métodos de exfoliación y la preparación de materiales de tipo composite (composite) basados en láminas de MoS2. Finalmente, se profundiza en la funcionalización del MoS2. Capítulo II: Se centra en la preparación de un composite a partir de la combinación de láminas de ce-MoS2 con precursores de azul de Prusia (PB). La estrategia sintética se basa en el doble rol del ce-MoS2 (como agente reductor y plataforma 2D de nucleación). Se demuestra que las láminas de MoS2 se oxidan completamente como consecuencia de su funcionalización con el PB. El composite resultante exhibe un excelente rendimiento como material catódico en baterías de iones sodio y potasio (SIBs and KIBs, respectivamente). Finalmente, se explica cómo reducir el contenido de óxido en el composite final. Capítulo III: Se basa en la preparación de un nuevo composite a partir de la combinación de ce MoS2 y cubo molecular diamagnético de Fe y Co. Como consecuencia de la transferencia electrónica del ce-MoS2 al cubo diamagnético de partida, el sistema diamagnético se transforma en un compuesto paramagnético. Capítulo IV: Tiene por objetivo la funcionalización covalente de ce-MoS2 con un sistema fotocrómico (derivado de ditienileteno) tanto en su forma abierta como en la cerrada. Los resultados obtenidos mediante diferentes técnicas de caracterización confirman la formación de nuevos enlaces covalentes (C-S) entre los dos componentes. Además, parece que la fotoluminiscencia (PL) inherente al sistema fotocrómico decae después de la funcionalización, indicando una posible transferencia de carga entre el material 2D y el derivado de ditienileteno. Capítulo V: Trata sobre la formación de heterostructuras de WS2/MoS2. El método que se propone consta de 2 partes: i) funcionalización de ce-MoS2 con un clúster de tungsteno a través de interacciones electrostáticas, ii) tratamiento térmico del composite resultante a fin de inducir la formación de nanodominos de WS2 sobre las láminas de MoS2. Esta estrategia sintética combina las ventajas de un método en disolución (sencillez, escalabilidad y bajo coste) con la formación de interfases de alta calidad. Además, como consecuencia del fuerte acoplamiento entre ambos dicalcogenuros, se observa una atenuación de la PL correspondiente a la heterostructura. Capítulo VI: Recoge las conclusiones principales de la Tesis.The outbreak of nanomaterials is a clear sensor of the scientific advance. The design of nano-sized materials at will permits to tailor the fascinating novel properties resulting from a drastic reduction in dimensionality. In this vein, we have done our particular contribution to the field of the so-called 2D materials. The main goal that has motivated the development of this Thesis is the functionalization and combination of ultrathin MoS2 flakes obtained by chemical exfoliation (abbreviated as ce-MoS2 flakes) with different molecular-based systems. This Thesis is divided into the following blocks: Chapter I: It begins with a brief introduction to nanomaterials. After that, two-dimensional (2D) materials are succinctly described. Subsequently, the family of layered transition metal dichalcogenides (TMDCs) is presented. Next, the focus turns to MoS2, the most studied layered TMDC. In this latter section, we can find detailed information about the most common polytypes, dry and wet exfoliation methods, and composites based on MoS2 layers. Finally, the molecular functionalization of MoS2 is described. Chapter II: It is focused on the preparation of a new MoS2-based composite by combining ce MoS2 flakes and Prussian blue (PB) precursors. The synthetic strategy here described takes profit from negative-charged MoS2 flakes as both reducing agent and 2D platform for the PB nucleation. We show that MoS2 layers are fully oxidized after functionalization. Interestingly, the resultant composite exhibits an excellent performance as cathode material for sodium- and potassium-ion batteries (SIBs and KIBs, respectively). Finally, we give some insights about how to reduce the oxidation extent into our final composite. Chapter III: It aims to the preparation of a novel composite by combining ce-MoS2 flakes and a preformed diamagnetic FeCo molecular cube. We provide evidence to support the idea that the diamagnetic system is transformed into a paramagnetic compound, as a result of the electron transfer from negative-charged MoS2 flakes to the starting FeCo cube. Chapter IV: It talks about the covalent functionalization of ce-MoS2 flakes via the well-known diazonium strategy, starting from a photochromic dithienylethene derivative (in its open- and closed structure forms) bearing two reactive sites. The results obtained from several characterization techniques confirm the formation of new C–S bonds between both interacting components. Interestingly, it appears that the inherent photoluminescence (PL) of the organic molecule is quenched after functionalization, indicating a possible charge transfer between the 2D material and the attached dithienylethene derivative. Chapter V: It tackles the formation of WS2/MoS2 heterostructures through the thermal treatment of MoS2 layers electrostatically functionalized with W3S4-core clusters. This chemical strategy combines the advantages of a solution approach (simplicity, scalability, and low cost) with the formation of good quality interfaces (usually, reached by more complex physical methods). The PL of both counterparts is significantly quenched, which would confirm an efficient interlayer coupling between them. Chapter VI: It summarizes the main conclusions of this Thesis

DoE-Assisted Development of a 2H-MoS2 -Catalyzed Approach for the Production of Indole Derivatives

2H-MoS2 is an appealing semiconductor because of its Earth-abundant nature, cheapness, and low toxicity. This material has shown promising catalytic activity for various energy-related processes, but its use in catalysis for C-C bond forming reactions towards useful organic compounds is still largely unexplored. The lack of examples in organic synthesis is mainly due to the intrinsic difficulties of using bulk 2H-MoS2 (e. g., low surface area), which implies the reliance on high catalytic loadings for obtaining acceptable yields. This makes the optimization process more expensive and tedious. Here, we report the development of a 2H-MoS2 -mediated synthesis of valuable bis(indolyl)methane derivatives, using indoles and benzaldehydes as starting materials. Exploiting the Design of Experiments (DoE) method, we identified the critical parameters affecting the catalytic performance of commercial 2H-MoS2 powder and optimized the reaction conditions. Lastly, we demonstrated that the catalytic system has versatility and good tolerance towards functional group variations of the reagents

Functionalisation of MoS2 2D layers with diarylethene molecules

Functionalisation of two dimensional (2D) materials with stimuli-responsive molecules has been scarcely investigated. Here, MoS2 layers obtained by chemical exfoliation are covalently and non-covalently functionalised using two photoswitchable diarylethene derivatives under their open- and closed-ring isomers. The choice of these light-responsive molecules is based on their excellent thermal irreversibility and fatigue resistance. The characterisation of the resultant molecular/2D heterostructures proves the successful anchoring of the molecules by both approaches as well as the influence that the driving interaction has in the photoswitching behaviour of the diarylethene isomers after their deposition on the 2D layer

Fast Polymeric Functionalization Approach for the Covalent Coating of MoS2 Layers

We present the covalent coating of chemically exfoliated molybdenum disulfide (MoS2) based on the polymerization of functional acryl molecules. The method relies on the efficient diazonium anchoring reaction to provoke the in situ radical polymerization and covalent adhesion of functional coatings. In particular, we successfully implement hydrophobicity on the exfoliated MoS2 in a direct, fast, and quantitative synthetic approach. The covalent functionalization is proved by multiple techniques including X-ray photoelectron spectroscopy and TGA-MS. This approach represents a simple and general protocol to reach dense and homogeneous functional coatings on 2D materials

Molecular stabilization of chemically exfoliated bare MnPS3 layers

Transition metal chalcogenophosphates of general formula MPX3 have attracted recent interest in the field of 2D materials due to the possibility of tuning their properties upon reaching the 2D limit. Several works address this challenge by dry mechanical exfoliation. However, only a few of them use a scalable approach. In this work, we apply a general chemical protocol to exfoliate MnPS3. The method employs in the first step chemical intercalation and liquid phase exfoliation and in the second step the addition of molecules used as capping agents on the inorganic layers. Therefore, molecules of different nature prompt the quality of the exfoliated material and its stabilization in an aqueous solution, opening the possibility of using these functionalized layers in several fields. Here we illustrate this possibility in electrochemistry. Thus, we show that when polyethylenimine is used as the capping agent, it is possible to reach a compromise between the stability of high quality MnPS3 flakes in aqueous suspension and their optimum performance as an electrocatalytic system for HER activity

Spin-crossover nanoparticles anchored on MoS2 layers for heterostructures with tunable strain driven by thermal or light-induced spin switching

In the last few years, the effect of strain on the optical and electronic properties of MoS2 layers has been deeply studied. Complex devices have been designed where strain is externally applied on the 2D material. However, so far, the preparation of a reversible self-strainable system based on MoS2 layers has remained elusive. In this work, spin-crossover nanoparticles are covalently grafted onto functionalized layers of semiconducting MoS2 to form a hybrid heterostructure. We use the ability of spin-crossover molecules to switch between two spin states upon the application of external stimuli to generate strain over the MoS2 layer. This spin crossover is accompanied by a volume change and induces strain and a substantial and reversible change of the electrical and optical properties of the heterostructure. This strategy opens the way towards a next generation of hybrid multifunctional materials and devices of direct application in highly topical fields like electronics, spintronics or molecular sensing

WS2/MoS2 Heterostructures through Thermal Treatment of MoS2 Layers Electrostatically Functionalized with W3S4 Molecular Clusters

The preparation of 2D stacked layers that combine flakes of different nature, gives rise to countless number of heterostructures where new band alignments, defined at the interfaces, control the electronic properties of the system. Among the large family of 2D/2D heterostructures, the one formed by the combination of the most common semiconducting transition metal dichalcogenides WS2/MoS2, has awaken great interest due to its photovoltaic and photoelectrochemical properties. Solution as well as dry physical methods have been developed to optimize the synthesis of these heterostructures. Here a suspension of negatively charged MoS2 flakes is mixed with a methanolic solution of a cationic W3S4-core cluster, giving rise to a homogeneous distribution of the clusters over the layers. In a second step, a calcination under N2 of this molecular/2D heterostructure leads to the formation of clean WS2/MoS2 heterostructures where the photoluminescence of both counterparts is quenched, proving an efficient interlayer coupling. Thus, this chemical method combines the advantages of a solution approach (simple, scalable and low-cost) with the good quality interfaces reached by using more complicated traditional physical methods

Fast Polymeric Functionalization Approach for the Covalent Coating of MoS2 Layers

We present the covalent coating of chemically exfoliated molybdenum disulfide (MoS2) based on the polymerization of functional acryl molecules. The method relies on the efficient diazonium anchoring reaction to provoke the in situ radical polymerization and covalent adhesion of functional coatings. In particular, we successfully implement hydrophobicity on the exfoliated MoS2 in a direct, fast, and quantitative synthetic approach. This approach represents a simple and general protocol to reach dense and homogeneous functional coatings on 2D materials

Molecular stabilization of chemically exfoliated bare MnPS3 layers

Transition metal chalcogenophosphates of general formula MPX3 have attracted recent interest in the field of 2D materials due to the possibility of tuning their properties when reaching the 2D limit. Several works address this challenge by dry mechanical exfoliation. However, only a few of them use a scalable approach. In this work, we apply a general chemical protocol to exfoliate MnPS3. The method uses in a first step chemical intercalation and liquid phase exfoliation, followed in a second step by the addition of molecules used as capping agents on the inorganic layers. Therefore, molecules of different nature prompts the quality of the exfoliated material and its stabilization in aqueous solution, opening the possibility of using these functionalized layers in several fields. Here we illustrate this possibility in electrochemistry. Thus, we show that when polyethylenimine is used as capping agent, it is possible to reach a compromise between the stability of high quality MnPS3 flakes in aqueous suspension and their optimum performance as an electrocatalytic system for HER activity

Hybrid Heterostructures of a Spin Crossover Coordination Polymer on MoS2: Elucidating the Role of the 2D Substrate

Controlling the deposition of spin-crossover (SCO) materials constitutes a crucial step for the integration of these bistable molecular systems in electronic devices. Moreover, the influence of functional surfaces, such as 2D materials, can be determinant on the properties of the deposited SCO film. In this work, ultrathin films of the SCO Hofmann-type coordination polymer [Fe(py)2{Pt(CN)4}] (py = pyridine) onto monolayers of 1T and 2H MoS2 polytypes are grown. The resulting hybrid heterostructures are characterized by GIXRD, XAS, XPS, and EXAFS to get information on the structure and the specific interactions generated at the interface, as well as on the spin transition. The use of a layer-by-layer results in SCO/2D heterostructures, with crystalline and well-oriented [Fe(py)2{Pt(CN)4}]. Unlike with conventional Au or SiO2 substrates, no intermediate self-assembled monolayer is required, thanks to the surface S atoms. Furthermore, it is observed that the higher presence of Fe3+ in the 2H heterostructures hinders an effective spin transition for [Fe(py)2{Pt(CN)4}] films thinner than 8 nm. Remarkably, when using 1T MoS2, this transition is preserved in films as thin as 4 nm, due to the reducing character of this metallic substrate. These results highlight the active role that 2D materials play as substrates in hybrid molecular/2D heterostructures