Search

Sub-nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X-ray Absorption spectroscopy and Near Ambient Pressure X-ray Photoelectron spectroscopy results reveal that supported Cu5 clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion-corrected DFT and first principles thermochemistry revealing that most of the adsorbed O2 molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude “breathing” motions. A chemical phase diagram for Cu oxidation states of the Cu5-oxygen system is presented, clearly different from the already known bulk and nano-structured chemistry of Cu

Silver nanoclusters of five atoms (Ag5) display outstanding catalytic activities for the deactivation of radicals. Using 2,2-diphenyl-1‑picrylhydrazyl (DPPH) radical as a model system, we observed a fast radical reduction to DPPH anions using only [Ag5] 3 to 4 orders of magnitude less than [DPPH]. Moreover, nanoclusters remain stable at the end of the reaction, and can deactivate again DPPH radicals at the same rate, indicating that they act as anti-radical catalysts. The radical scavenger catalytic activity of Ag5 proceeds selectively through the oxidation of methanol (used to dissolve the radical) to formaldehyde, which is supported by DFT calculations. The obtained catalytic rate constants are almost 2 orders of magnitude higher than oxidases, and more than 4 orders of magnitude larger than graphene quantum dots. We also show that Ag5 not only catalyze the reduction of radicals but also their oxidation, promoting the inhibition of the autoxidation mechanisms of hydrocarbon polymers, which are very sensitive to the presence of radicals. For this purpose, thin films of two industrially relevant polymers (polyisoprene and acrylonitrile–butadienestyrene copolymer), were exposed to standard simulated photo-ageing conditions in the presence of Ag5. Using Attenuated Total Reflection-FTIR and DFT modeling we observed that, although Ag5 nanoclusters, with ≈ 15% surface coverage, do not totally inhibit the oxidation, they favour a decomposition that yields inactive products, in contrast with the more detrimental ketone formation pathway. These results not only open new possibilities for developing a post-process inhibition of polymer degradation, for which nowadays there are no efficient procedures, but also, they could be used as very efficient dual-redox catalytic radical scavengers for different industrial or biomedical purposes

Low atomicity clusters present properties dependent on the size, due to the quantum confinement, with well-defined electronic structures and high stability. Here it is shown that Ag5 clusters catalyze the complete oxidation of sulfur to S+6. Ag5 catalytic activity increases with different oxidant species in the order O2 ≪ H2O2 < OH•. Selective oxidation of thiols on the cysteine residues of glutathione and thioredoxin is the primary mechanism human cells have to maintain redox homeostasis. Contingent upon oxidant concentration, Ag5 catalyzes the irreversible oxidation of glutathione and thioredoxin, triggering apoptosis. Modification of the intracellular environment to a more oxidized state to mimic conditions within cancer cells through the expression of an activated oncogene (HRASG12V) or through ARID1A mutation, sensitizes cells to Ag5 mediated apoptosis. While cancers evolve to evade treatments designed to target pathways or genetic mutations that drive them, they cannot evade a treatment that takes advantage of aberrant redox homeostasis, which is essential for tumor progression and metastasis. Ag5 has antitumor activity in mice with orthotopic lung tumors reducing primary tumor size, and the burden of affected lymphatic nodes. The findings suggest the unique intracellular redox chemistry of Ag5 may lead to new redox-based approaches to cancer therapy

La presente tesis doctoral está dirigida hacia la obtención de clústeres de Ag a gran escala en ausencia de surfactantes o grupos protectores. Para ello, se estudiará la optimización de una síntesis fotoquímica que nos permita obtener estas especies subnanométricas en concentraciones elevadas mediante un método fácilmente escalable a nivel industrial, lo que supone sin duda un gran desafío. Además, se realizará una amplia investigación acerca de los métodos de caracterización y purificación de estos materiales cuyas propiedades, estrictamente dependientes de su tamaño, representan una gran novedad en diversas aplicaciones como catálisis, fotocatálisis y biomedicina, entre otras.

En el presente trabajo, se han sintetizado y caracterizado nanopartículas de SnO2 y TiO2 y materiales híbridos SnO2/TiO2 para utilizar en reacciones fotocatalíticas como sustratos para los clústeres cuánticos atómicos (AQCs). Los métodos de síntesis se han basado en aproximaciones bottom-up en fase líquida, como el método sol-gel o la precipitación química, los cuales son fáciles de escalar. Además, los materiales fueron caracterizados fisicoquímicamente por técnicas como difracción de rayos X en polvo (DRX), espectroscopías FTIR-ATR y Raman, análisis elemental (CHNS) y microscopía electrónica de transmisión (TEM) con el fin de determinar la estructura cristalina, morfología y composición química de estos materiales (es decir, cómo están los elementos distribuidos en los materiales híbridos y qué está adherido y protegiendo la superficie de las nanopartículas) así como predecir sus propiedades fotocatalíticas mediante espectroscopía de reflectancia difusa e isotermas de adsorción BET. Un segundo objetivo de este Trabajo fin de máster fue la utilización de estas nanopartículas semiconductoras como sustrato para la deposición de AQCs mediante impregnación seca o incipient wetness impregnation. Posteriormente, se estudió por espectroscopía UV-Vis cómo el proceso de deposición favorece las propiedades ópticas de los materiales, incrementando la absorción de luz visible mediante la aparición de midgaps, lo que hace a estos materiales muy interesantes para reacciones fotocatalíticas. Para ello, se llevaron a cabo estudios preliminares de actividad fotocatalítica con la degradación de un colorante en agua, a diferentes longitudes de onda y empleando un simulador solar., In this Master thesis, SnO2 and TiO2 nanoparticles and SnO2/TiO2 hybrid materials have been synthesized and characterized for use in photocatalytic reactions as substrates for atomic quantum clusters (AQCs). Synthesis methods were based on bottom-up liquid phase approach such as sol-gel method or chemical precipitation, which are easy to scale up. Furthermore, materials were characterized by physicochemical techniques such as powder X-ray diffraction (XRD), ATR-FTIR and Raman spectroscopies, elemental analysis (CHNS) and transmission electron microscopy (TEM) in order to determinate the crystalline structure, the morphology and the chemical composition of these nanomaterials (i.e., how the elements are distributed in the hybrid materials and what is attached and protecting the nanoparticles surface) as well as predict the photocatalytic properties using reflectance diffuse spectroscopy and BET sorption isotherms. A second objective of this Master thesis was the use of these semiconductor nanoparticles as substrate for the AQCs deposition using dry impregnation or incipient wetness impregnation. Then, it was studied by UV-Vis spectroscopy how deposition process improves the optical properties of the materials, increasing visible light absorption through the appearance of midgaps, which make these materials very interesting for photocatalytic reactions. For this, preliminary studies of photocatalytic activity were carried out to the dye degradation in water, at different wavelengths and using a solar simulator.

El objetivo a desarrollar en la presente tesis será la síntesis de clústeres de átomos metálicos monodispersos de Ag sin la presencia de ligandos enlazados fuertemente, así como el estudio de sus propiedades catalíticas y de técnicas de separación avanzadas. Debido a las potenciales aplicaciones de los clústeres en campos de gran importancia, como la catálisis, la fotocatálisis, las actividades biológicas, etc. estrictamente dependientes del tamaño y la concentración, se hace extremadamente necesario el desarrollo de técnicas de caracterización y purificación avanzadas. Dichas técnicas han de ser capaces de cuantificar y separar los clústeres atómicos entre si.

El objetivo a desarrollar en la presente tesis será la síntesis de clústeres de átomos metálicos monodispersos de Cu y Ag sin la presencia de ligandos enlazados fuertemente, así como el estudio de su purificación, análisis y sus propiedades catalíticas. Debido a las potenciales aplicaciones de los clústeres en campos de gran importancia, como la catálisis, la fotocatálisis, las actividades biológicas, etc. estrictamente dependientes del tamaño y la concentración, se hace extremadamente necesario el desarrollo de técnicas de caracterización y purificación avanzadas.

The aim to develop in this thesis is the synthesis of clusters of monodisperse metal atoms like Cu , Ag, Au and Pt without the presence of strong ligands, and the study of their biological properties , due to their potential applications in fields such as cancer and infectious diseases .

La presente tesis está enfocada a explorar el uso de clústeres cuánticos de 5 átomos como catalizadores en diferentes reacciones de gran interés social, ambiental e industrial, como son la búsqueda de combustibles renovables, eliminación de la contaminación, nuevos agentes anticancerígenos/antivirales, etc. Así, se trató de realizar una amplia exploración de propiedades catalíticas de estos clústeres, tanto en catálisis homogénea como heterogénea, en: procesos de eliminación de radicales libres, epoxidación de alquenos, eliminación de compuestos organosulfurados derivados del petróleo, oxidación de CO e hidrogenación de C02.

The enhancement of catalysts efficiency of bimetallic nanoparticles depends on the ability to exert control over surface composition. However, results relating surface composition and feeding solution of bimetallic nanoparticles synthesized in microemulsions are controversial and apparently contradictory. In order to comprehend how the resulting surface can be modified under different synthesis conditions and for different pairs of metals, a computer simulation study was carried out. The resulting surface compositions are explained based on the relative rates of deposition of the two metals, which depend on the particular metal pair, the concentration of reactants and the microemulsion composition. This study provides a satisfactory understanding of experimental results and allows us to identify the main factors affecting the nanoparticle’s surface composition. Consequently, concrete and practical guidelines can be established to facilitate the experimental synthesis of bimetallic nanoparticles with tailored surfaces

Emerging contaminants (ECs) represent a wide range of compounds, whose complete elimination from wastewaters by conventional methods is not always guaranteed, posing human and environmental risks. Advanced oxidation processes (AOPs), based on the generation of highly oxidizing species, lead to the degradation of these ECs. In this context, TiO2 and ZnO are the most widely used inorganic photocatalysts, mainly due to their low cost and wide availability. The addition of small amounts of nanoclusters may imply enhanced light absorption and an attenuation effect on the recombination rate of electron/hole pairs, resulting in improved photocatalytic activity. In this work, we propose the use of silver nanoclusters deposited on ZnO nanoparticles (ZnO–Ag), with a view to evaluating their catalytic activity under both ultraviolet A (UVA) and visible light, in order to reduce energetic requirements in prospective applications on a larger scale. The catalysts were produced and then characterized by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and inductively coupled plasma-optical emission spectrometry (ICP-OES). As proof of concept of the capacity of photocatalysts doped with nanoclusters, experiments were carried out to remove the azo dye Orange II (OII). The results demonstrated the high photocatalytic efficiency achieved thanks to the incorporation of nanoclusters, especially evident in the experiments performed under white light

Using a combination of first-principles modelling, X-ray absorption spectroscopy, and diffuse reflectance spectroscopy measurements, we explore the properties of Ag5-modified TiO2 surfaces. A general electron polarization phenomenon associated with surface polarons on TiO2 has been revealed theoretically and confirmed experimentally. First, the Ag5 cluster donates an electron to TiO2, leading to the formation of polaronic Ti3+ 3d1 states on the rutile TiO2(110) surface. The analysis of polarization effects in the nearby electronic structure accompanying the polaron formation is confirmed with X-ray absorption spectroscopy measurements at the Ti K-edge of TiO2 nanoparticles. Next, the UV-Vis optical absorption spectrum of the polaronic state is also computationally modelled and an enlargement of the polaron wavefunction is predicted. Moreover, we find an overall improvement of the UV-Vis optical response of the material through diffuse reflectance spectroscopy measurements. Finally, we predict that charge-transfer processes at the Ag5–TiO2 interface triggered by solar photons might allow for a photoinduced activation of CO2 by sunlight

The decoration of semiconductors with subnanometer-sized clusters of metal atoms can have a strong impact on the optical properties of the support. The changes induced differ greatly from effects known for their well-studied, metallic counterparts in the nanometer range. In this work, we study the deposition of Cu5 clusters on a TiO2 surface and investigate their influence on the photon-absorption properties of TiO2 nanoparticles via the computational modeling of a decorated rutile TiO2 (110) surface. Our findings are further supported by selected experiments using diffuse reflectance and X-ray absorption spectroscopy. The Cu5 cluster donates an electron to TiO2, leading to the formation of a small polaron Ti3+ 3d1 state and depopulation of Cu(3d) orbitals, successfully explaining the absorption spectroscopy measurements at the K-edge of copper. A monolayer of highly stable and well fixated Cu5 clusters is formed, which not only enhances the overall absorption, but also extends the absorption profile into the visible region of the solar spectrum via direct photo-induced electron transfer and formation of a charge-separated state

En el presente trabajo, se ha diseñado un nuevo método para la conversión de luz solar en energía eléctrica mediante el uso de un electrodo fotovoltaico basado en clústeres de Cu de baja atomicidad (AQCs). La idea es utilizar una monocapa de CuAQCs como semiconductores que se encuentren depositados sobre una superficie conductora de nanofibras de Ag como material conductor. En primer lugar, se funcionalizó una lámina de vidrio con 3-aminopropiltrietoxisilano y a continuación, las nanofibras de Ag se depositaron mediante spin coating. Por último, se depositaron encima los clústeres de Cu, sintetizados electroquímicamente en el laboratorio, utilizando la técnica de drop casting. La respuesta fotovoltaica se comprobó utilizando una celda electroquímica, customizada con una ventana de cuarzo, y que posee un hilo de Pt como electrodo de referencia y contraelectrodo. Se utilizó como fuente de luz un simulador solar. Se estudió la variación de la respuesta de la fotocorriente en función de diferentes electrolitos y hole scavengers. Mediante las medidas de voltaje de circuito abierto y corriente de corto-circuito se demostró la implicación de los CuAQCs en el incremento de la respuesta de la fotocorriente. Mediante el método propuesto, es posible cuantificar la cantidad de fotones absorbidos por los clústeres que son emitidos como electrones debido a la formación de pares electrónhueco. A pesar de que el método no está completamente optimizado, su desarrollo podría suponer una nueva estrategia para construir sistemas fotovoltaicos de tamaño atómico que sustituyan a los sistemas fotovoltaicos convencionales basados en óxidos metálicos., Here, we report a new method to convert solar light in electrical energy by designing a photovoltaic electrode based on Cu clusters of low atomicity (AQCs). The idea is to use a monolayer of CuAQCs as atomic level high efficient semiconductors deposited on Ag nanowires as conductive material. For this first a glass slide was functionalised with 3-aminopropyltriethylsilane and then Ag nanofibers were depossited by spin coating. Finally, the monolayer films of Cu clusters, which were synthesized electrochemically in the laboratory, were deposited using drop casting technique. The photovoltaic response was tested by using an homemade electrochemical cell (with a quartz window) and using Pt as counter and reference electrode. A solar simulator was used as light source. The measurements of photocurrent response were studied as a function of different electrolytes and hole scavengers. Both, open circuit potential and short-circuit amperograms were meassure and demonstrated that the CuAQCs increase the photocurrent response. By this method, we can quantify how many photons are absorbed by clusters and are emitted like electrons by the formation of electron-hole pairs. In conclusion, despite this method has not been completly optimizated, their development could suppose a new strategy to build atomic size photovoltaic devices and represent and alternative to the use of typical semiconductors (i.e.: cadmiun selenide, titanium dioxide) in the conversion of solar energy into electrical.

The factors that govern the reaction rate of Au/Pt bimetallic nanoparticles prepared in microemulsions by a one-pot method are examined in the light of a simulation model. Kinetic analysis proves that the intermicellar exchange has a strong effect on the reaction rates of the metal precursors. Relating to Au, reaction rate is controlled by the intermicellar exchange rate whenever concentration is high enough. With respect to Pt, the combination of a slower reduction rate and the confinement of the reactants inside micelles gives rise to an increase of local Pt salt concentration. Two main consequences must be emphasized: On one hand, Pt reduction may continue independently whether or not a new intermicellar exchange takes place. On the other hand, the accumulation of Pt reactants accelerates the reaction. As the reactant accumulation is larger when the exchange rate is faster, the resulting Pt rate increases. This results in a minor difference in the reduction rate of both metals. This difference is reflected in the metal distribution of the bimetallic nanoparticle, which shows a greater degree of mixture as the intermicellar exchange rate is faster.