Oxygen Vacancies in Oxide Nanoclusters: When Silica Is More Reducible Than Titania

Oxygen vacancies are related to specific optical, conductivity and magnetic properties in macroscopic SiO2 and TiO2 compounds. As such, the ease with which oxygen vacancies form often determines the application potential of these materials in many technological fields. However, little is known about the role of oxygen vacancies in nanosized materials. In this work we compute the energies to create oxygen vacancies in highly stable nanoclusters of (TiO2)N, (SiO2)N, and mixed (TixSi1−xO2)N for sizes between N = 2 and N = 24 units. Contrary to the results for bulk and surfaces, we predict that removing an oxygen atom from global minima silica clusters is energetically more favorable than from the respective titania species. This unexpected chemical behavior is clearly linked to the inherent presence of terminal unsaturated oxygens at these nanoscale systems. In order to fully characterize our findings, we provide an extensive set of descriptors (oxygen vacancy formation energy, electron localization, density of states, relaxation energy, and geometry) that can be used to compare our results with those for other compositions and sizes. Our results will help in the search of novel nanomaterials for technological and scientific applications such as heterogeneous catalysis, electronics, and cluster chemistry

Global optimisation of hydroxylated silica clusters: a cascade Monte Carlo Basin Hopping approach

We report on a global optimisation study of hydroxylated silica nanoclusters (SiO2)/w(H2O)(N) with sizes M = 6, 8, 10 12, and for each size with a variable number of dissociatively chemisorbed water molecules (N = 1, 2, 3...). Due to the high structural complexity of these systems and the associated ruggedness of the underlying potential energy landscape, we employ a 'cascade' global optimisation approach. Specifically, we use Monte Carlo Basin Hopping (MCBH) where for each step we employ two energy minimisations with: (i) a lightly parameterised but computationally efficient interatomic potential (IP) which does not distinguish between H-bonded conformational isomers, and then (ii) a more sophisticated IP which accounts for polarisation and H-bonding. Final energies from the MCBH search are then refined with optimisations using density functional theory. The reliability of our approach is first established via comparison with previously reported results for the (SiO2)(8).(H2O)(N) case, and then applied to the M = 6, 10 and 12 systems. For all systems studied our results follow the trend in hydroxylation energy versus N, whereby the energy gain with hydroxylation is found to level off at a point where the average tetrahedral distortion of the SiO4 centres is minimised. This optimal hydroxylation point is further found to follow an inverse power law with increasing cluster size (M) with an exponent close to -2/3, further confirming work in previous studies for other cluster sizes. (C) 2016 Elsevier B.V. All rights reserved

Predicting size-dependent emergence of crystallinity in nanomaterials: titania nanoclusters versus nanocrystals

Bottom-up and top-down derived nanoparticle structures refined by accurate ab initio calculations are used to investigate the size dependent emergence of crystallinity in titania from the monomer upwards. Global optimisation and data mining are used to provide a series of ( TiO2) N global minima candidates in the range N = 1-38, where our approach provides many new low energy structures for N > 10. A range of nanocrystal cuts from the anatase crystal structure are also considered up to a size of over 250 atoms. All nanocrystals considered are predicted to be metastable with respect to non-crystalline nanoclusters, which has implications with respect to the limitations of the cluster approach to modelling large titania nanosystems. Extrapolating both data sets using a generalised expansion of a top-down derived energy expression for nanoparticles, we obtain an estimate of the non-crystalline to crystalline crossover size for titania. Our results compare well with the available experimental results and imply that anatase-like crystallinity emerges in titania nanoparticles of approximately 2-3 nm diameter

Modelling Nano-Oxide Materials with Technological and Environmental Relevance: Silica, Titania and Titanosilicates

[eng] Properties of nanomaterials are known to be size dependent and generally are very different from those of the corresponding bulk. Such behaviour, which is strongly system and structure dependent, allows one to tune material’s properties by varying their dimensions. This tunability opens up many possibilities in nanotechnology for manufacturing materials with tailored properties for specific applications. Thus, understanding size-dependent properties of nanoparticles and mechanisms taking place at the nanoscale is fundamental for the improvement of existing materials and for the designing of more efficient and optimized ones. However, the synthesis of nanomaterials and their experimental characterization is difficult, especially for very small sizes. Here, theoretical modelling plays a fundamental role in the characterization of small nanoparticles for both helping experimental interpretation and predicting novel and potentially synthesizable materials with new properties. In this thesis we focus on modelling of titania, silica and titanosilicate based materials because of their technological and environmental importance as they are employed in heterogeneous (photo-)catalysis, electronics and gas sensing to cite a few. For such systems, we firstly performed global optimization studies in gas-phase and water containing environments in order to identify the structures of nanoparticles. Secondly, we studied structural, energetic and electronic size-dependent properties of such nanoparticles as well as their reducibility, extrapolating up to the bulk macroscopic level in some cases. For such characterization we used accurate quantum mechanical methods based on Density Functional Theory (DFT). Our results point to a series of important predictions, such as for instance: (i) the crystallinity of titania nanoparticles, which is the key property for the photoactivity of such material, is predicted to emerge when nanoparticles become larger than 2.0-2.5 nm; (ii) the mixing of titania and silica to form titanosilicates, which are an important class of materials used in industry as catalyst, is found be thermodynamically favorable at the nanoscale, contrary to the bulk; (iii) the hydration of silica and titania nanoclusters, which plays an important role in the aggregation and nucleation process during the synthesis of larger nanoparticles, is controlled by environmental factors such as temperature and water vapor pressure as predicted from calculated phase diagrams; iv) the oxygen vacancy formation energy, which is an indicator of the system reducibility, is found to be less energetically costly in small nanosilica clusters rather than in nanotitania which is the opposite of what happens at the corresponding bulk level. We hope to inspire experimental studies to address the synthesis of novel titanosilicates materials with potentially enhanced properties by using as building blocks the nanoparticles predicted here.[spa] Es bien sabido que las propiedades de los nanomateriales dependen del tamaño y son muy diferentes de las del correspondiente cristal, o bulk. Tal comportamiento, que depende fuertemente del sistema y de la estructura, permite ajustar las propiedades del material variando sus dimensiones. Esta flexibilidad abre muchas posibilidades en nanotecnología para la fabricación de materiales con propiedades adaptadas para aplicaciones específicas. Por lo tanto, comprender las propiedades dependientes del tamaño de las nanopartículas y los mecanismos que tienen lugar a escala nanométrica es fundamental para la mejora de los materiales existentes, diseñándolos más eficientes y optimizados. Sin embargo, la síntesis de nanomateriales y su caracterización experimental es difícil, especialmente para tamaños muy pequeños. Aquí, la modelización teórica juega un papel fundamental en la caracterización de pequeñas nanopartículas, tanto para ayudar a la interpretación experimental como para predecir materiales novedosos y potencialmente sintetizables con nuevas propiedades. En esta tesis nos enfocamos en el modelado de materiales basados en titania, sílice y titanosilicates debido a su importancia tecnológica y ambiental, ya que se emplean en (foto-)catálisis heterogénea, electrónica y detección de gases, por citar algunos. Para tales sistemas, primero realizamos estudios de optimización global de agregados en fase de gas y en presencia de agua para identificar las estructuras de las nanopartículas. En segundo lugar, estudiamos las propiedades estructurales, energéticas y electrónicas dependientes del tamaño de tales nanopartículas, así como su reducibilidad, extrapolando hasta el nivel macroscópico en algunos casos. Para tal caracterización utilizamos métodos mecano-cuánticos precisos basados en la Teoría del Funcional de la Densidad (DFT). Nuestros resultados apuntan a una serie de predicciones importantes, como por ejemplo: (i) se predice que la cristalinidad de las nanopartículas de titania, que es la propiedad clave para la fotoactividad de dicho material, emerge cuando las nanopartículas llegan a tamaños de 2.0-2.5 nm; (ii) la mezcla de titania y sílice para formar titanosilicatos, que son una clase importante de materiales utilizados en la industria como catalizadores, se considera que es termodinámicamente favorable a escala nanométrica, contrariamente al material cristalino; (iii) la hidratación de nanoagregados de sílice y titania, que juega un papel importante en el proceso de agregación y nucleación durante la síntesis de las nanopartículas más grandes, está controlada por factores ambientales como la temperatura y presión de vapor de agua según los diagramas de fase calculados; iv) la formación de vacantes de oxígeno, que es un indicador de la reducibilidad del sistema, resulta ser energéticamente menos costosa en pequeños agregados de nanosílice que de nanotitania, que es lo contrario de lo que ocurre al nivel macroscópico. Con nuestro trabajo, esperamos inspirar estudios experimentales para abordar la síntesis de nuevos materiales de titanosilicatos con propiedades potencialmente mejoradas mediante el uso de nanopartículas predichas aquí como bloques de construcción

Modélisation de nanomatériaux à base d’oxyde avec intérêt technologique et environnemental : oxydes de titane, silice et titanosilicates

In this thesis we focus on modelling of titania, silica and titanosilicate based nano materials because of their technological importance as they are employed in heterogeneous (photo-)catalysis, in electronics gas-sensing etc. to cite a few. For such systems, we firstly performed global optimization studies in gas-phase and water containing environments in order to identify the structures of nanoparticles. Secondly, we studied structural, energetic and electronic size-dependent properties of such nanoparticles as well as their reducibility, extrapolating up to the bulk macroscopic level in some cases. For such characterization we use accurate quantum mechanical methods based on the Density Functional Theory (DFT). Our results point to a series of important predictions such us: i) the crystallinity of titania nanoparticles, which is the key property for the photoactivity, is predicted to emerge when nanoparticles become larger than 2.0-2.5 nm; ii) the mixing of titania and silica to form titanosilicates is found be thermodynamically favorable at the nanoscale, contrary to the bulk; iii) the hydration of silica and titania nanoclusters, which plays an important role in the aggregation and nucleation process during the synthesis of larger nanoparticles, is controlled by environmental factors such as temperature and water vapor pressure as predicted from calculated phase diagrams.Dans cette thèse, nous nous concentrons sur la modélisation des nano matériaux à base de titane, de silice et de titanosilicate en raison de leur importance technologique. Pour de tels systèmes, d'abord, nous avons identifiez les structures plus stables au moyen des méthodes d'optimisation global. Dans un deuxième temps, nous avons étudié les propriétés structurelles, énergétiques et électroniques de ces nanoparticules en fonction de leur taille, en extrapolant parfois jusqu'à l'échelle macroscopique. Pour une telle caractérisation, nous avons utilisé des méthodes quantiques basées sur la Théorie Fonctionnelle de la Densité. Des résultats obtenus, nous pouvons prédire que: (i) la cristallinité des nanoparticules d'oxyde de titane, qui est la propriété clé pour son activité photocatalytique, émergerait lorsque les nanoparticules atteignent une taille supérieure à 2,0-2,5 nm; (ii) le mélange d'oxyde de titane et de silice pour former des titanosilicates, se révèle thermodynamiquement favorable à l'échelle nanométrique, contrairement à l'échelle macroscopique; (iii) l'hydratation des nanoparticules de silice et de titane, qui joue un rôle important dans le processus d'agrégation et de nucléation pendant la synthèse de nanoparticules plus grandes, est contrôlée par les facteurs environnementaux tels que la température et la pression de vapeur d'eau

Stability of mixed-oxide titanosilicates: dependency on size and composition from nanocluster to bulk

International audienceNanostructured titanosilicate materials based upon interfacing nano-TiO 2 with nano-SiO 2 have drawn much attention due to their huge potential for applications in a diverse range of important fields including gas sensing, (photo)catalysis, solar cells, photonics/optical components, tailored multi-(bio)functional supports and self-cleaning coatings. In each case it is the specific mixed combination of the two SiO 2 and TiO 2 nanophases that determines the unique properties of the final nanomaterial. In the bulk, stoichiometric mixing of TiO 2 with SiO 2 is limited by formation of segregated TiO 2 nanoparticles or metastable glassy phases and more controlled disperse crystalline mixings only occur at small fractions of TiO 2 (< 15 wt%). In order to more fully understand the stability nano-SiO 2 and nano-TiO 2 combinations with respect to composition and size, we employ accurate all-electron density functional calculations to evaluate the mixing energy in (Ti x Si 1-x O 2) n nanoclusters with a range of sizes (n = 2-24) having different titania molar fractions (x = 0-1). We derive all nanoclusters from a dedicated global optimisation procedure to help ensure that they are the most energetically stable structures for their size and composition. We also consider a selection of representative intimately mixed crystalline solid phase (Ti x Si 1-x O 2) bulk systems for comparison. In agreement with experiment, we find that intimate mixing of SiO 2 and TiO 2 in bulk crystalline phases is energetically unfavourable. Conversely, we find that SiO 2-TiO 2 mixing is energetically favoured in small (Ti x Si 1-x O 2) n nanoclusters. Following the evolution of mixing energy with nanocluster size and composition we find that mixing is most favoured in nanoclusters with a diameter of 1 nm with a TiO 2 molar fraction of 0.3-0.4. Thereafter, mixed nanoclusters with increasing size have progressively less negative mixing energies up to diameters of approximately 1.5 nm. We propose some chemical-structural principles to help rationale this energetically favourable nanoscale mixing. As a guide fo

Global optimisation of hydroxylated silica clusters: a cascade Monte Carlo Basin Hopping approach

We report on a global optimisation study of hydroxylated silica nanoclusters (SiO2)/w(H2O)(N) with sizes M = 6, 8, 10 12, and for each size with a variable number of dissociatively chemisorbed water molecules (N = 1, 2, 3...). Due to the high structural complexity of these systems and the associated ruggedness of the underlying potential energy landscape, we employ a 'cascade' global optimisation approach. Specifically, we use Monte Carlo Basin Hopping (MCBH) where for each step we employ two energy minimisations with: (i) a lightly parameterised but computationally efficient interatomic potential (IP) which does not distinguish between H-bonded conformational isomers, and then (ii) a more sophisticated IP which accounts for polarisation and H-bonding. Final energies from the MCBH search are then refined with optimisations using density functional theory. The reliability of our approach is first established via comparison with previously reported results for the (SiO2)(8).(H2O)(N) case, and then applied to the M = 6, 10 and 12 systems. For all systems studied our results follow the trend in hydroxylation energy versus N, whereby the energy gain with hydroxylation is found to level off at a point where the average tetrahedral distortion of the SiO4 centres is minimised. This optimal hydroxylation point is further found to follow an inverse power law with increasing cluster size (M) with an exponent close to -2/3, further confirming work in previous studies for other cluster sizes. (C) 2016 Elsevier B.V. All rights reserved

Predicting size-dependent emergence of crystallinity in nanomaterials: titania nanoclusters versus nanocrystals

International audienceBottom-up and top-down derived nanoparticle structures refined by accurate ab initio calculations are used to investigate the size dependent emergence of crystallinity in titania from the monomer upwards. Global optimisation and data mining are used to provide a series of (TiO 2) N global minima candidates in the range N = 1–38, where our approach provides many new low energy structures for N > 10. A range of nanocrystal cuts from the anatase crystal structure are also considered up to a size of over 250 atoms. All nanocrystals considered are predicted to be metastable with respect to non-crystalline nanoclusters, which has implications with respect to the limitations of the cluster approach to modelling large titania nanosystems. Extrapolating both data sets using a generalised expansion of a top-down derived energy expression for nanoparticles, we obtain an estimate of the non-crystalline to crystalline crossover size for titania. Our results compare well with the available experimental results and imply that anatase-like crys-tallinity emerges in titania nanoparticles of approximately 2–3 nm diameter

Predicting size-dependent emergence of crystallinity in nanomaterials: titania nanoclusters versus nanocrystals

Bottom-up and top-down derived nanoparticle structures refined by accurate ab initio calculations are used to investigate the size dependent emergence of crystallinity in titania from the monomer upwards. Global optimisation and data mining are used to provide a series of ( TiO2) N global minima candidates in the range N = 1-38, where our approach provides many new low energy structures for N > 10. A range of nanocrystal cuts from the anatase crystal structure are also considered up to a size of over 250 atoms. All nanocrystals considered are predicted to be metastable with respect to non-crystalline nanoclusters, which has implications with respect to the limitations of the cluster approach to modelling large titania nanosystems. Extrapolating both data sets using a generalised expansion of a top-down derived energy expression for nanoparticles, we obtain an estimate of the non-crystalline to crystalline crossover size for titania. Our results compare well with the available experimental results and imply that anatase-like crystallinity emerges in titania nanoparticles of approximately 2-3 nm diameter