Synthesis and investigation of hybrid nanomaterials for photocatalytic and spectroanalytical applications

Abstract

This thesis is divided into two parts where the first part of the thesis deals with the design and synthesis of heterostructures for efficient photocatalysis e.g. for water remediation while the second part deals with the design of effective substrates suitable for sensitive surface-enhanced Raman scattering (SERS) analysis. Investigating different hybrid nanomaterials which can serve as efficient photocatalysts, the wide band gap semiconductors ZnO and ZnS have been combined with vertically aligned carbon nanotubes (VACNTs) to form ZnO@CNT and ZnS@CNT nanocomposite heterostructures which are able to make use not only from the ultraviolet (UV) region of light but from most of the solar spectrum. Through atomic layer deposition (ALD) ZnO nanoparticles are directly deposited on VACNTs which could be converted in a gas phase sulfidation process to ZnS@CNT nanocomposites. Depending on the gas phase conversion temperature different ZnS/ZnO@CNT, sphalerite-based ZnS@CNT and wurtzite-based ZnS@CNT nanocomposites could be obtained where at high conversion temperatures defects could be even induced in the ZnS nanoparticles. This study revealed that wurtzite-based ZnS@CNT nanocomposites with induced defects show the highest photocatalytic activity towards the degradation of methyl orange which was used as model pollutant under simulated sunlight. Furthermore, the photocatalytic properties of two-dimensional titanium chalcogenides have been investigated. These materials possess in contrast to traditional titanium dioxide a small band gap and a layered structure which is advantageous for photocatalytic applications. The different titanium chalcogenides were prepared by a chemical vapor transport method whereby tuning of the ratio of the starting elements and the reaction temperature a series of different titanium chalcogenides (TiS3, TiS2, TiS, TiSe2 and TiTe) could be synthesized. The investigation of titanium sulfides, TiS3 and TiS which is defect-rich TiS2 showed promising results concerning their photocatalytic activity. Non-stoichiometric titanium disulfide shows a large number of defects which are responsible for a high photocatalytic and thermocatalytic activity. Excellent recyclability of these materials was also found and was attributed to the spontaneous formation of titanium sulfide/titanium oxide heterostructures due to the surface oxidation with time further increasing the photocatalytic activity of the material. New approaches for the preparation of efficient SERS substrates with high enhancement factors were investigated as they are crucial for trace analysis e.g. of bioactive compounds. First, a facile plasma-assisted approach for the preparation of SERS substrates has been developed. Different plasma treatments using different plasma gases and different parameters have been investigated on thin transparent silver films of 10 nm thickness and thick non-transparent silver films of 200 nm thickness. Hydrogen, nitrogen and argon plasma were found to increase the surface roughness of these sputtered silver films, thus increasing their SERS enhancement factors significantly. Subsequent oxidation-reduction plasma treatment of the 200 nm thick silver sputtered films through oxidation with oxygen plasma followed by reduction with either hydrogen, nitrogen or argon plasma enabled the formation of complex three-dimensional porous silver films showing high SERS enhancement factors. Finally, aluminum/anodic aluminum oxide/silver (Al/AAO/Ag) substrates have been investigated to elucidate the different factors which could increase the SERS response obtained from such substrates. It was found that a possible chemical enhancement from iodine species introduced in barrier-type anodic aluminum oxide by iodine oxoacid electrolytes could effectively increase the SERS enhancement factors of these substrates compared to other dense barrier-type anodic aluminum oxides obtained from citrate buffer and porous anodic aluminum oxide (PAOX) films. These findings could be valuable for the preparation of more effective SERS substrates based on Al/AAO/Ag thin layer compositions.