Desenvolupament de models per nanopartícules de TiO2 i ZnO en fotocatàlisis

[cat] En aquesta tesi s’han realitzat càlculs DFT per tal d’estudiar els sistemes TiO2 i ZnO i poder analitzar-ne les característiques de l’estructura electrònica, geometria i estabilitat energètica principalment. Primerament s’ha estudiat el funcionament de diferents metodologies computacionals per veure quina d’elles es la optima per realitzar aquests estudis. Aquesta metodologia ha estat provada i posteriorment utilitzada per els càlculs realitzats sobre el TiO2 i ZnO. Motivats per l’interès en l’estructura electrònica del òxid de titani amb intenció de poder augmentar-ne la seva activitat fotocatalítica. Hem realitzat diferents estudis sobre l’efecte de les vacants d’oxigen i el dòping amb fluor en el band gap del material. Observant com en tots dos casos apareixen estats electrònics nous al band gap del material. També s’ha estudiat el perfil energètic de la difusió de fluor en el sinus del material i l’efecte de l’adsorció de fluor en l’estabilitat de les superfícies de TiO2. Observant com la difusió es á energèticament favorable en certes direccions i que l’adsorció de fluor, apart de generar nous estats electrònics, també canvia l’estabilitat de les diferents superfícies. Fent que les nanopartícules presentin formes diferents i una millor activitat fotocatalítica. Hem extret similars conclusions en el cas de l’adsorció d’ àcid trifluoroacetic, ja que també afavoreix l’increment de la superfície mes reactiva per el TiO2 en la fase anatasa. L’estabilitat relativa i l’evolució de les propietats electròniques de les nanopartícules de TiO2 també ha estat font d’estudi. Observant com per mides inferiors a 125 unitats de TiO2 les nanopartícules no cristal·lines resulten mes estables. Però pera mides superiors, les nanopartícules cristal·lines son mes estables i les seves propietats electròniques convergeixen cap a les del sòlid. Analitzant els nostres resultats hem pogut predir que nanopartícules amb mides compreses entre 18 i 23 nm ja haurien de mostrar propietats i electròniques molt similars a les del sòlid bulk. De forma similar també s’ha estudiat el perfil d’estabilitats relatives de nanopartícules d’òxid de zinc, permetent-nos crear així una imatge del perfil d’estabilitat que presenten cinc famílies de nanopartícules amb diferent forma derivades de les fases mes estables per aquest material. L’estudi s’ha realitzat per un rang de nanopartícules que va des dels pocs àtoms fins a nanocristalls amb mes de 1000, Reportant-ne així les variacions d’estabilitat en front a la mida de partícula. Aquests estudis presentats en aquesta tesi poden ser útils de cara a millorar el coneixement d’aquests materials tan prometedors i poder trobar diferents estratègies per tal de millorar-ne la seva activitat fotocatalítica.[eng] In this thesis DFT based methods have been used in order to study TiO2 and ZnO systems, by analysing their electronic structure, geometrical features and energetic stability. A systematic study of the performance of the different computational methodologies has been carried on in order to find a suitable methodology able to describe the electronic features of these semiconductors. This is a key point because the electronic structure is directly related with its photocatalytic activity. One of its more interesting properties from an energetic and technologic point of view. Once the more suitable methodology to describe TiO2 electronic features was found, we tested it with a set of metal oxides, including ZnO. Obtaining also good results. Interested in the electronic structure of TiO2, and with the aim to improve the description of its photocatalytic activity. We performed several studies about the effect of oxygen vacancies,and fluor doping,on the band gap of the different TiO2 bulk polymorphs. Observing in both cases the new electronics states that lay in the band gap. For the oxygen vacancies the new states where found to be close to the conduction band,meanwhile for the fluordoping they were found to be near the valence band. This last case is very interesting because it can increase the photocatalytical activity of this material by shifting to the visible the type of wavelength absorbed by TiO2. Continuing with the study of the interaction between fluor and titanium oxide, we investigated the diffusion paths of fluor trough the TiO2 lattice and how the adsorption of fluor and trifluoroacetic acid adsorption can affect the electronic structure of the and specially the stability of the different surfaces. Finding how the fluor adsorption have similar effects to doping generating electronic states in the band gap while at same time change the order of stability between the (101) and (001) surfaces. Becoming the second one, which seem to be more active, more stable and subsequently more exposed. The same effect on the surface stability was found for the fluoroacetic acid adsorption. We also studied the stability and electronic structure of TiO2 nanostructures sampling a range that goes from a few atoms to more than 1000. Analysing the data obtained we found that the electronic properties depend on the shape significantly for the smaller nanoclusters but not for the larger nanoparticles. Where the size has a stronger effect on the electronic structure. It was also observed the non-crystalline nanoparticles to be more stable than the crystalline ones up to a size approximately 125 TiO2 units. This is an important point to predict the different properties expected for particles of a certain size. Also from the results obtained for the larger crystalline nanoparticles we observed how the electronic properties evolve the bulk ones as the size increases. From this data we could extrapolate that probably nanoparticles with sizes between 18 and 23 nm could present bulk-like electronic structure and subsequently photoactivity. A study in collaboration with experimental co-workers was done in order to explain theoretically the different activity presented by ZnO nanoparticles of different shape. These nanoparticles exposed different proportions of polar and non-polar surfaces. After analysing electronic structure and energetic stability of the different surfaces we found that in this case the higher activity was not closely related with the different electronic features. In the case of these large ZnO nanoparticles the activity was more related about the presence of a larger amount of polar surface exposed. This surface seem to stabilize the holes generated in the process of light adsorption. One last study is included in this thesis. The study about the relative stability of different ZnO nanostructures and its evolution with size. Five families of nanostructures where studied. In a range that goes from few ZnO units to more than 1000. The type of nanostructures studied are. Nano-cages, Multi-layered nano-cages, Sodalite bulk cuts, BCT bulk cuts and Wurtzite bulk cuts. Finding the nano-cages and multi-layered nano- cages especially stable for smaller sizes. When the diameter of the nanoparticles reach the region around 2.6 nm both types of nanocages, sodalite bulk cuts and BCT bulk cuts present very similar stability creating a transition zone. As the size of the particles increases,the BCT bulk cuts become the most stable nanoparticles up to sizes about 4.7 nm where the Wurtzite nanoparticle become the most stable. All these studies presented in this thesis are useful to increase the knowledge about these very promising materials and allow to develop different strategies to improve their photocatalytic activity

Approaching the quantitative description of enantioselective adsorption by the density functional theory means

The applications of enantiopure organic compounds range from medicine to green agrochemistry. Their racemic or enantioselective synthesis permits their acquisition beyond the extraction from life forms. These procedures need chiral resolution steps to achieve the required degrees of enantiomeric purity, though. Many research endeavours are addressed at finding chiral materials able to separate the enantiomers by their selective adsorption upon. Transition metal chiral surfaces have been found to reach enantiomeric excess degrees of purity outperforming surfaces of naturally existing chiral materials. Future research can be driven by high-throughput computational screening, given the employed methodology is able to discern the subtle enantiomeric differences of free energies of adsorption. The capabilities of density functional theory methods are here evaluated on the textbook case of D/L-aspartic acid adsorption on chiral Cu(3,1,17)R&S metal surfaces. Results show that dispersive forces are a prerequisite to properly describe the enantioselective adsorption, whereas the inclusion of fundamental vibrational energy and adsorbate vibrational free energies are key ingredients to approach a quantitative description. Simulated X-ray photoemission and infrared spectra indicate that the adsorption conformations can be qualitatively recognized

Performance of a modified hybrid functional in the simultaneous description of stoichiometric and reduced TiO2 polymorphs

Conventional density functionals with either the local density approximation (LDA) or the generalized gradient approximation (GGA) form of the exchange-correlation potential fail to describe the electronic structure of a large number of metal oxides. Both the LDA and the GGA grossly underestimate the band gaps of these materials which severely affect the description of oxygen vacancy point defect states in reduced samples. To find a pragmatic approach to simultaneously and accurately describe the atomic and electronic structures of the most common TiO2 polymorphs, we explore the effect of the percentage of exact, non-local, Fock exchange on the electronic structure of stoichiometric rutile and anatase. From these results, a modified hybrid functional is proposed to properly describe the atomic structures, formation enthalpies and electronic structures of rutile and anatase and, at the same time, the results of reduced samples are also in good agreement with the available experimental results. The present approach can be safely used to accurately describe numerous TiO2 based materials containing defects or realistic nanoparticles for which the required large unit cells or system sizes hinder the use of GW related techniques

Size dependent structural and polymorphic transitions in ZnO: from nanocluster to bulk

We report on an extensive survey of (ZnO)(N) nanostructures ranging from bottom-up generated nanoclusters to top-down nanoparticles cuts from bulk polymorphs. The obtained results enable us to follow the energetic preferences of structure and polymorphism in (ZnO)(N) systems with N varying between 10-1026. This size range encompasses small nanoclusters with 10s of atoms and nanoparticles with 100s of atoms, which we also compare with appropriate bulk limits. In all cases the nanostructures and bulk systems are optimized using accurate all-electron, relativistic density functional theory based calculations with numeric atom centered orbital basis sets. Specifically, sets of five families of (ZnO)(N) species are considered: single-layered and multi-layered nanocages, and bulk cut nanoparticles from the sodalite (SOD), body centered tetragonal (BCT), and wurtzite (WZ) ZnO polymorphs. Using suitable fits to interpolate and extrapolate these data allows us to assess the size-dependent energetic stabilities of each family. With increasing size our results indicate a progressive change in energetic stability from single-layered to multi-layered cage-like nanoclusters. For nanoparticles of around 2.6 nm diameter we identify a transitional region where multi-layered cages, SOD, and BCT nanostructures are very similar in energetic stability. This transition size also marks the size regime at which bottom-up nanoclusters give way to top-down bulk-cut nanoparticles. Eventually, a final crossover is found where the most stable WZ-ZnO polymorph begins to energetically dominate at N similar to 2200. This size corresponds to an approximate nanoparticle diameter of 4.7 nm, in line with experiments reporting the observation of wurtzite crystallinity in isolated ligand-free ZnO nanoparticles of 4-5 nm size or larger

Morphology effects in photoactive ZnO nanostructures: photooxidative activity of polar surfaces

A series of ZnO nanostructures with variable morphology were prepared by a microemulsion method and their structural, morphological, and electronic properties were investigated by a combined experimental and theoretical approach using microscopy (high resolution transmission electron microscopy) and spectroscopic (X-ray diffraction, Raman, and UV-visible) tools, together with density functional theory calculations. The present experimental and computational study provides a detailed insight into the relationship between surface-related physicochemical properties and the photochemical response of ZnO nanostructures. Specifically, the present results provide evidence that the light-triggered photochemical activity of ZnO nanostructures is related to the predominance of highly-active (polar) surfaces, in particular, the amount of Zn-terminated (0001) surfaces, rather than band gap sizes, carrier mobilities, and other variables usually mentioned in the literature. The computational results highlight the oxidative capability of polar surfaces, independently of the degree of hydration

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

When anatase nanoparticles become bulk-like: properties of realistic TiO2 nanoparticles in the 1-6 nm size range from all electron relativistic density functional theory based calculations

All electron relativistic density functional theory (DFT) based calculations using numerical atom-centered orbitals have been carried out to explore the relative stability, atomic, and electronic structure of a series of stoichiometric TiO2 anatase nanoparticles explicitly containing up to 1365 atoms as a function of size and morphology. The nanoparticles under scrutiny exhibit octahedral or truncated octahedral structures and span the 1-6 nm diameter size range. Initial structures were obtained using the Wulff construction, thus exhibiting the most stable (101) and (001) anatase surfaces. Final structures were obtained from geometry optimization with full relaxation of all structural parameters using both generalized gradient approximation (GGA) and hybrid density functionals. Results show that, for nanoparticles of a similar size, octahedral and truncated octahedral morphologies have comparable energetic stabilities. The electronic structure properties exhibit a clear trend converging:to the bulk values as the size of the nanoparticles increases but with a marked influence of the density functional employed. Our results suggest that electronic structure properties, and hence reactivity, for the largest anatase nanoparticles considered in this study will be similar to those exhibited by even larger mesoscale particles or by bulk systems. Finally, we present compelling evidence that anatase nanoparticles become effectively bulklike when reaching a size of similar to 20 nm diameter

Systematic study of the effect of HSE functional internal parameters on the electronic structure and band gap of a representative set of metal oxides

The effect of the amount of Hartree-Fock mixing parameter (α) and of the screening parameter (w) defining the range separated HSE type hybrid functional is systematically studied for a series of seven metal oxides: TiO2, ZrO2, CuO2, ZnO, MgO, SnO2, and SrTiO3. First, reliable band gap values were determined by comparing the optimal α reproducing the experiment with the inverse of the experimental dielectric constant. Then, the effect of the w in the HSE functional on the calculated band gap was explored in detail. Results evidence the existence of a virtually infinite number of combinations of the two parameters which are able to reproduce the experimental band gap, without a unique pair able to describe the full studied set of materials. Nevertheless, the results point out the possibility of describing the electronic structure of these materials through a functional including a screened HF exchange and an appropriate correlation contribution

Properties of single oxygen vacancies on a realistic (TiO2)(84) nanoparticle: a challenge for density functionals

Based on all electron relativistic density functional theory calculations, the properties of single oxygen vacancies in TiO2 nanoparticles (NPs) have been obtained using a suitable representative model consisting of an octahedral (TiO2)(84) nano particle of similar to 3 nm size terminated with (101) facets. This nanoparticle can be safely considered at the onset of the so-called scalable regime where properties scale linearly with size toward bulklike limit, and hence results can be more directly compared to experiment. A set of reduced Ti84O167 nanoparticles are selected to investigate the geometric, energetic, and electronic properties by using PBE semilocal functional with three different amounts of Fock exchange: 0% (PBE), 12.5% (PBEx), and 25% (PBEO). In particular, using the PBEx hybrid functional, previously validated for bulk anatase and rutile, it is predicted that the highly (three)-coordinated oxygen atom, located in the subsurface, and the least coordinated one at top sites are energetically the most suitable candidate for generating the oxygen vacancy. The subsurface case is in line with conclusions from experiments carried out on (101) single crystal anatase surfaces. The electronic structure of the reduced particles suggests that these would have better photocatalytic activity than their stoichiometric counterparts. Nevertheless, several properties of reduced TiO2 NPs are strongly affected by the choice of the exchange-correlation functional, implying that, in absence of validation by comparison to experiment, predictions must be taken with caution

Performance of a modified hybrid functional in the simultaneous description of stoichiometric and reduced TiO2 polymorphs

Conventional density functionals with either the local density approximation (LDA) or the generalized gradient approximation (GGA) form of the exchange-correlation potential fail to describe the electronic structure of a large number of metal oxides. Both the LDA and the GGA grossly underestimate the band gaps of these materials which severely affect the description of oxygen vacancy point defect states in reduced samples. To find a pragmatic approach to simultaneously and accurately describe the atomic and electronic structures of the most common TiO2 polymorphs, we explore the effect of the percentage of exact, non-local, Fock exchange on the electronic structure of stoichiometric rutile and anatase. From these results, a modified hybrid functional is proposed to properly describe the atomic structures, formation enthalpies and electronic structures of rutile and anatase and, at the same time, the results of reduced samples are also in good agreement with the available experimental results. The present approach can be safely used to accurately describe numerous TiO2 based materials containing defects or realistic nanoparticles for which the required large unit cells or system sizes hinder the use of GW related techniques