From the solid-vacuum interface to the working electrode – Nature, reactivity and functionality of hydrogen-derived electron centers in semiconducting oxides

An experimental strategy relying on a combination of complementary electrochemical and spectroscopic in situ methods will be established to characterize the electronic properties of hydrogen-related trap states in the semiconductors at different levels of complexity ranging from nanoparticles in vacuum to working electrodes. Near-stoichiometric ZnO and TiO2 nanoparticles will be prepared by metal organic chemical vapor synthesis (MO-CVS). The semiconductor oxides will be submitted to different doping strategies (ex-situ post-synthesis doping, in-situ process-induced doping) in powder form or after immobilization on conducting substrates. Self-doped TiO2 and ZnO nanostructures will furthermore be synthesized by wet-chemical approaches and direct electrodeposition, respectively. Importantly, shallow donors may originate not only from materials’ synthesis and processing, but can also be generated – reversibly or irreversibly – during operation. This is particularly true for hydrogen impurities as hydrogen sources are ubiquitous along the whole process chain of a material. Special emphasis will therefore be put on the elucidation of the in-situ self-doping effect on SC electrodes upon photoexcitation. For this purpose a fundamental understanding of the underlying process steps both at the SC/electrolyte interface and in the SC bulk has to be gained. In this project we seek at identifying the interface-related semiconductor (SC) properties which determine the generation, the reactivity and the functionality of hydrogen-related electronic centers in ZnO and TiO2 nanoparticle systems. Hydrogen doping will be studied both under model conditions (high vacuum) as well as under application-relevant conditions by a combination of complementary electrochemical and spectroscopic in situ methods. While nanosized SCs in contact with an electrolyte are of utmost importance in many applications, they are characterized by a high level of complexity and a low level of definition, which render studies aiming at an understanding of the interface at a molecular level difficult. The proposed project envisages an approximation to the complex nature of a SC/electrolyte interface by complementing the study of working electrodes with the investigation of nanoparticulate systems at defined conditions upon a stepwise increase of complexity. SC oxides, once n-type H-doped at high vacuum conditions, will serve as a starting point for the consecutive modification of the interface aiming, ultimately, at the stepwise built-up of a charged SC/electrolyte interface.