Low dimensional Zinc- and Copper Oxides and their Electronic, Vibrational and Photocatalytic Properties

Abstract: Pollution of water resources is a growing problem in the world and this has drawn the attention to photocatalysis, which is an emerging technology for water purification. In this thesis, low dimensional zinc oxide and copper oxides, which are promising photocatalytic materials, have been studied. In the initial work, an approach for determining the crystal orientation in ZnO nanomaterials was developed based on polarized Raman spectroscopy. The approach was extended to non-polarized Raman spectroscopy for convenient crystal orientation determination. The results were corroborated by density functional theory (DFT) calculations providing a full vibrational mode analysis of ZnO, including higher-order Raman scattering. Photocatalyst materials based on both ZnO and copper oxides were synthesized, starting with visible light absorbing Cu2O prepared by low temperature thermal oxidation of flat and 3D structured Cu-foils. Defect induced Raman scattering revealed Raman activity in modes that are only IR active or optically silent in pristine Cu2O, with mode assignments supported by DFT calculations. Experiment with solar light illuminated Cu2O showed efficient degradation of organic water-soluble molecules and degradation rates could be further increased by 3D structuring into nanopillars. With the aim of creating a combined photocatalyst that use favourable properties from several materials, nanoparticles of ZnO were synthesized and deposited onto Cu2O, Cu4O3 and CuO. ZnO of sufficiently small size exhibit quantum confinement, which allowed for tuning of the electronic and optical properties of ZnO and this was utilized for energy level alignment in heterojunctions with copper oxides. The heterojunctions were shown to facilitate charge transfer which improved the photocatalytic properties of the dual catalysts compared to the single components. The quantum confinement effects in ZnO nanoparticles were further investigated by more detailed electrochemical measurements. The main finding was that quantum confinement results in a large decrease in the available electronic density of states which has clear implications on the capacitance and photon absorption in the material. Raman spectroscopy has been a central tool in all work, and the thesis ends with a study that goes through and explain spurious Raman signals. The contribution shows how to identify and avoid spectral artefacts and other light generating processes that compete with the Raman signal and guide the acquisition of good quality spectra.