Quantum chemical characterization of oxide nanoparticles and interactions on their surfaces

Abstract: Popular Abstract in English Nanotechnology is a quickly developing field, which applies objects of extremely small sizes to novel technological applications. These objects, so-called nanomaterials, have at least one dimension measured in nanometers, 10-9 meters. Nanoparticles are of interest to chemists due to their distinct properties, which differ significantly from the bulk material. This unique behaviour of nanoparticle structures arises from a large surface area to bulk ratio, and is related to the shape of the nanomaterial (particles or rods). A special class of nanomaterials are nanocrystals, which can simply be characterized as nanosized crystalline materials, or materials created by repeating units connected in space. Titanium dioxide (TiO2) is one of the mostly widely used nanocrystalline materials and is utilized in many technological applications, such as self-cleaning glasses, solar cells, gas-solid sensors, and memory devices. Understanding its different properties is crucial for increasing efficiency of these devices that have the possibility to change the way we produce, consume and store energy. In addition, fundamental understanding of a versatile material like TiO2 will also lead to discovering new applications for its use. Like many crystalline materials, titanium dioxide can form more than one possible structure. TiO2 has three common forms, rutile, anatase, and brookite. While rutile is the most common form found in nature, anatase and brookite structures often occur in nanoscale materials. In this thesis, the structural, chemical and electrical properties of all three forms of TiO2 are studied using theoretical methods. Theoretical methods allow chemists to examine molecules and nanomaterials in ways that are not possible by experiments. Here, the theoretical approach applied considers the TiO2 structure at the level of atoms, creating a detailed description of interactions between atoms and providing an opportunity to study the influence of different structural features on the nanosized TiO2 properties. For example, this thesis studies how TiO2 behaves after contact with water, since TiO2 nanocrystals often are used in water. TiO2 nanoparticles can also interact with each other, which was studied here by modelling two nanoparticles interacting with each other. In experiments it is difficult to separate all the processes that are occurring at the same time. Using models can help to understand these different processes, which take place on the TiO2 surfaces and are the key to modern technological applications. This type of theoretical investigation is important to understand the complex processes occurring in complicated systems. Only by first understanding what is occurring, and therefore what is important, can new systems be rationally designed. As an example, the developed TiO2 model has been applied to study how a device could be used to convert the sunlight into electric power and most importantly how it could be done effectively and efficiently. Nanomaterial-based technologies are rapidly growing in economic and technological importance. Computational projects, such as those presented in this thesis, could potentially accelerate the progress of nanoparticle research, reducing the costs required for each discovery.