Cu Nanoparticles on TiO2: Growth, Oxidation and Interaction with Amino Acids

Abstract

This doctoral thesis investigates the growth and oxidation of Cu nanoparticles (NPs) on rutile TiO2(110) and anatase TiO2(101) single crystals, as well as their interaction with the amino acid cysteine. This work was initiated within the CORAERO project (Grant KA1-Co-06), which investigated airborne transmission routes of the SARS-CoV-2 to explore photocatalytic strategies for viral inactivation. Cu NPs on TiO2 have been chosen because their effectiveness in degrading organic substances, including viruses and bacteria, has already been proven. In contrast to pristine TiO2, which has a bandgap in the UV-light region and thus is not active under visible light, the research for Cu/TiO2 indicates visible light activity. Nevertheless, several open questions remain regarding the optimization of the NPs size, coverage, and oxidation state, as well as the molecular dynamics of the interaction with organic molecules. The model system of Cu NPs on TiO2 single crystals enables precise control over the surfaces and interfaces and allows for the use of surface-science techniques for investigation. The anoparticles were grown by molecular beam epitaxy (MBE) and the epitaxy was investigated with grazing-incidence X-ray diffraction (GIXRD), high-energy X-ray diffraction (HEXRD), grazing-incidence small angle scattering (GISAXS) and scanning electron microscopy (SEM). The oxidation was additionally investigated with X-ray photoelectron spectroscopy (XPS) and ultra-violet-visible spectroscopy (UV-Vis). Finally, the interaction with cysteine (an amino acid) was studied using infrared reflection absorption spectroscopy (IRRAS) and XPS. For the growth of Cu on rutile TiO2(110), the main epitaxy was confirmed to be Cu[110] || TiO2[001] with a [111]-orientation of Cu. In contrast to previous reports, a dependency of epitaxy on growth temperature could be established. An increase of the growth temperature from 100 °C to 400 °C led to an increase in the signal of a Cu[110]||TiO2[112] epitaxy (tilted 12° in-plane with respect to the main epitaxy Cu[110] || TiO2[001]). The oxidation kinetics at room temperature and ambient O2 pressure followed an inverse logarithmic oxide growth rate after an initial linear phase, in line with the oxidation theory of Cabrera and Mott. Though it was found that metallic Cu NPs were still present after one week of oxidation, epitaxial oxide was found only for very large NPs and high coverage. For the growth of Cu NPs on the anatase TiO2(101) surface, the main epitaxy was found to be Cu[001]||TiO2[010] with a Cu[470]-orientation, independent of the growth temperature. Similar to the oxidation of Cu on rutile, epitaxial Cu2O was found only for relatively large NPs (diameters of about 20 nm). The long-term stability of Cu/oxide core-shell particles with diameters of about 20 nm was confirmed by a strong plasmonic resonance (originating from metallic Cu) found after several months in air. The adsorption of cysteine on the Cu/TiO2 system depends on the Cu coverage as well as the oxidation state. For Cu on anatase TiO2(101), the dominant adsorption mechanism is Cu-S bonding and interaction of copper and the deprotonated carboxyl group. Within the limits of experimental resolution, adsorption of sulfur on titanium can not be ruled out. In the presence of Cu-Oxide, parts of the thiol group of cysteine are oxidized directly upon adsorption in UHV. In addition, the presence of nitrile signals suggests that parts of the thiol groups in cysteine are decomposed upon adsorption. On Cu NPs on rutile TiO2(110), the dominant adsorption mechanisms are different. The amino group appeared mostly unprotonated and was assumed to interact with the surface, whereas the thiol group was found both in molecular state (S-H) as well as adsorbed on Cu (S-Cu). In contrast to the anatase substrates, no sign of sulfur adsorption on Ti was found. As for the anatase substrates, parts of the thiol groups were oxidized upon adsorption and some samples showed nitrile signals, corresponding to the decomposition of cysteine. In addition, a new adsorption mode with protonated carboxyl group was observed for several samples, indicating limited surface interaction of the carboxyl group. The light-induced (photo-)oxidation of cysteine was studied for Cu NPs on both anatase and rutile, following oxidation of the thiol group of cysteine. The amount of oxidized sulfur was found to depend both on the Cu coverage and the oxidation state of the copper, revealing competition between the formation of strong Cu-S bonds and sulfur oxidation. Moreover, the oxidation of the thiol group progressed linearly with the time of exposure to air, independent of the light conditions, for all but one sample. For this high-Cu-coverage sample on rutile, featuring mostly of CuO in XPS, the oxidation of cysteine was found to only appear under white light, indicating photoactivity. Overall, these findings enhance our understanding of the Cu/TiO2 system regarding NP growth and oxidation as well as cysteine adsorption and degradation.