Enhanced photochemical hydrogen generation by sensitization of TiO2 with Sn (IV)-porphyrins and noble metal nanoparticles

• Verbesserte photochemischen Wasserstofferzeugung durch Sensibilisierung von TiO2 mit Sn (IV) -Porphyrinen und Edelmetall-Nanopartikeln

Liu, Xiao; Offenhäusser, Andreas (Thesis advisor); Palkovits, Regina (Thesis advisor)

Aachen (2016)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

Hydrogen is an environmentally friendly carbon-free energy resource. The use of hydrogen can reduce the consumption of natural resources and solve many environmental problems produced by using fossil fuels. Recent years have seen a renewed interest in the possibility to generate hydrogen by harvesting solar energy based on the charge separation in semiconductors. Among various semiconductors, TiO2 is one of the most extensively studied compounds acted as UV-light photosensitizer or mediator (under visible and infrared light), due to its long term stability, low cost, and non-toxicity. However, the solar to hydrogen efficiency of TiO2 in photocatalytic water-splitting reaction is rather low because of its two drawbacks: wide band gap and poor ability to separate the photo-generated charges. In order to solve these problems, a number of techniques have been employed in strengthening hydrogen production, such as chemical additives, ion doping, noble metal loading, and dye sensitization. The aim of this PhD thesis is to study the photosensitizer structure-efficiency relationship in photocatalytic hydrogen production through dye sensitization and noble metal loading of TiO2 in order to enhance the quantum efficiency of hydrogen generation.Firstly, we established the relationships between the structure of a series of meso-substituted Sn(IV) porphyrins (Sn(IV)Ps) and their efficiency as photosensitizers for hydrogen production in the Sn(IV)Ps/Pt/TiO2 nanocomposite system. It was found that the time course and type of the photochemically reduced species of Sn(IV)Ps, which are essential intermediates, are important factors and depend on the electronegativity of the metal center, the character of meso-substituents of porphyrin ring, and pH, and are correlated with the redox potential sequence of the respective Sn(IV)Ps: SnTMPyP>SnTPyP>SnTPPS>SnTPPC. Moreover, the synergic effect of the excitation of TiO2 and mostly Q-bands of Sn(IV)Ps was proposed for the first time, which can enhance the efficiency of photocatalytic hydrogen generation in the system. Secondly, two phenomena of the Pt nanoparticles (NPs) catalyst prepared by photoreduction method were found and discussed: (1) the self-assembly of the Pt NPs into Pt dendrites on indium tin oxide thin film, which can be explained by diffusion-limited aggregation theory; (2) a coordination interaction between the amine ligand of metalloporphyrin and Pt nanostructures, leading to the nanospheres formation. These phenomena are the demonstrations of the easily accessible surface of the Pt nanostructures synthesized by photoreduction method compared with the citrate reduced Pt nanostructures.Thirdly, a number of methods were applied to assemble the dual properties of Au NPs regarding plasmonic absorption and Pt NPs related to effective electron trapping and catalysts, and the assembled products showed electronic interaction with TiO2. Both size-dependent shift in the Fermi level of Au/TiO2 composite and SPR intensity of Au NPs may influence the hydrogen yield, but in the opposite way. The link molecule of 3-mercaptopropionic acid between Au NPs and TiO2 is not favorable for the electron transfer compared with the direct loading Au NPs on the surface of TiO2. The synergic effect between Au SPR and TiO2 excitation was confirmed, which can promote the efficiency of the photocatalytic hydrogen production systems.Finally, the Ostwald ripening process in a nanocomposite comprising Au NPs and TiO2 semiconductor under electrochemical conditions was studied. The phenomenon was considered in relation to previous observations on the Ostwald ripening process in metallic nanostructures. Possible processes involved were discussed, and a mechanism was proposed based on the size dependence of the electrochemical parameters of Au nanostructures. Nanocomposites similar to those used here are expected to be widely used in solar energy conversion and catalysis, both of which are essentially electrochemically governed processes in solutions. The important consequences of this process could have direct implications for both catalytic and light-adsorbing properties of the transformed nanocomposite due to the size effect, and help to explain the behavior of semiconductor/metal nanocomposite materials and heterojunctions under electrochemical conditions. Furthermore, we anticipate that the same transformations could be observed for other material combinations.