Exploring the Local Optoelectronic Properties of ALD Grown TiO2 Photoelectrode-Coatings with Atomic Force Microscopy Methods

Abstract

Photoelectrochemical cells (PECs) hold great promise as an environmentally friendly method of converting sunlight into valuable fuels. However, the semiconductor components of PEC photoelectrodes are susceptible to corrosion under photoelectrochemical conditions.In order to protect the surface of these semiconductors from degradation, a thin overlayer of Titanium dioxide (TiO2) is often applied by atomic layer deposition (ALD). In addition to their protecting function, the physicochemical properties of these thin films significantly affect the overall performance of the device. The local nano-scale optoelectronic characteristics, which considerably influence their macroscopic properties, have rarely been reported in the literature.AFM methods constitute a powerful tool to investigate photo-induced charge separation, charge transport and recombination on photoelectrode surfaces with nanometer resolution.In this context, we have employed Tunneling Conductive AFM (TUNA) and Kelvin Probe Force Microscopy (KPFM) in the dark and under illumination to investigate the surface of ALD-grown TiO2. The synthesis conditions determine the morphology, which has a strong impact on the local optoelectronic properties. TUNA measurements reveal higher photogenerated currents on the grain and facet boundaries, showing that these regions can be favorable conduction pathways. Based on KPFM experiments we can quantify the more efficient photo-induced charge separation on the crystalline TiO2 inclusions, which generate a significant surface photopotential (~450 mV) upon UV-illumination. Categorization of different regions of surface topography based on morphology allows us to analyze the evolution of surface potential over time and extract localized time constants for carrier dynamic processes on the films. This spatially resolved information provides valuable insight to guide the development of macroscopically stable and efficient photoelectrodes through the engineering of microstructure at the nano scale.