Desenvolupament de models per nanopartícules de TiO2 i ZnO en fotocatàlisis

Abstract

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.

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 photocatalyticactivity.