Hydrogen-derived electron centers in semiconducting oxides

The introduction of shallow donors in semiconductor (SC) oxides by hydrogen impurities and/or by oxygen deficiency (self-doping) has been identified as a promising strategy to optimize the materials performance in applications ranging from sensors, supercapacitors and batteries to solar fuel generation. Whereas recent years have witnessed an increasing research effort, which resulted in a plethora of synthesis strategies of self-doped oxides and in impressive performance increases, less is known about the physicochemical fundamentals governing the doping process and about the nature and functionality of the underlying electronic states. This is particularly true for nanoparticulate semiconductors under application- relevant conditions. Such a knowledge, however, is necessary to accurately tune the materials functional properties under working conditions. In this project we seek at identifying the interface-related SC properties which determine the generation, the reactivity and the functionality of hydrogen-related electronic centers in ZnO and TiO2 nanoparticle systems. 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.

The introduction of shallow donors in semiconductor (SC) oxides by hydrogen impurities and/or by oxygen deficiency (self-doping) has been identified as a promising strategy to optimize the materials' performance in applications ranging from sensors, supercapacitors and batteries to solar fuel generation. Whereas recent years have witnessed an increasing research effort, which resulted in a plethora of synthesis strategies of self-doped oxides and in impressive performance increases, less is known about the physicochemical fundamentals governing the doping process and about the nature and functionality of the underlying electronic states. Such a knowledge, however, is necessary to accurately tune the material's functional properties under working conditions. In this project we seeked at identifying the interface-related SC properties, which determine the generation, the reactivity and the functionality of hydrogen-related electronic centers in ZnO and TiO2 nanoparticle systems. For this purpose, a bottom-up approach based on a high-vacuum/high-pressure reactor system was developed, which allows for materials' processing and characterization under high vacuum conditions, at the SC/gas, the SC/liquid and the SC/electrolyte interface. By combining spectroscopic and electrochemical studies of model systems based on nanoparticles prepared by chemical vapor synthesis it was possible to contrast the generation and stability of hydrogen-derived electron centers for different interface conditions qualitatively and quantitatively. Hydrogen-related electron centers generated at the SC/electrolyte interface during material synthesis or operation were investigated with respect to their impact on the functional electrode properties in photocatalytic and photosynthetic applications. Special emphasis was put on the elucidation of processing-induced property changes, which are associated with the controlled formation of particle/particle interfaces. While particle/particle contacts are a prerequisite to impart electronic conductivity to nanoparticle electrodes, they also constitute regions of high trap density. We have shown that these traps slow down electron transport and act as recombination centers. However, our results clearly demonstrate that the branching ratio between efficient charge carrier separation and detrimental recombination in random nanoparticle networks can be tuned by particle consolidation. In addition, quantum-chemical calculations performed by our collaboration partners indicate that the accumulation of electron/proton centers modifies the trap distribution at these interfaces. The findings provide an explanation for the transient enhancement of the photoelectrocatalytic activity, which is observed following a chemical hydrogen doping (by atomic hydrogen) or an electrochemical hydrogen doping (upon electrode polarization). Importantly, our study highlights the importance of particle/particle interfaces in nanoparticle films and provides processing strategies to actively manipulate the density of electronic states and, as such, constitute elegant ways to improve charge separation within nanoparticle-based materials. By exploiting the reactivity of hydrogen-related electron centers for the deposition of electroactive nanostructures at the SC/electrolyte interface we developed furthermore an electro- and photochemical deposition route for the preparation of ternary metal oxide films following green chemistry principles.