Angewandte Werkstofftechnik

In the transition from fossil fuels to renewable energy, efficient energy conversion and storage systems are required. One of the promising technologies that can support this transition is photoelectrochemical (PEC) water-splitting, in which solar energy is directly used to split water into oxygen and hydrogen. One of the main challenges in the design of efficient PEC systems is the optimization of the semiconductor material systems on the electrodes, which convert sunlight into electrochemical energy. The photoelectrochemical performance of these materials is significantly affected by the microstructure at the interface to the electrolyte, including the distribution of grains, grain boundaries and surface defects. Scanning probe microscopy (SPM) allows to simultaneously analyze and correlate morphological, mechanical, electrical and electrochemical properties at the nanometer scale. In this work, a series SPM methods were employed to investigate, with high spatial resolution, the effect of film morphology on the electrical and electrochemical properties of semiconductor photoelectrodes. Thin TiO2 films synthesized by atomic layer deposition (ALD) were used as model systems. Conductive atomic force microscopy (CAFM) measurements revealed anisotropy in the photocurrent activity, which was correlated with underlaying crystalline orientations with the help of high-resolution topography and complementary microscopy techniques. Kelvin probe force microscopy (KPFM) studies revealed a slow charge redistribution upon and after illumination due to the trapping and detrapping of photogenerated charges. Current-voltage (I-V) characterization was employed to investigate charge carrier transport mechanisms occurring at the metal tip and crystalline TiO2 nanojuction. Finally, electrochemical AFM (EC-AFM) was used to study the photocorrosion of partially crystalline TiO2 in-situ under the working conditions of the photoanode. The results thus provide new insights into the fundamental microscopic processes in photoelectrodes, from which strategies for further efficiency improvements can be derived.