Evidence of Low-Dimensional Surface Structures for Oxide Materials: Impact on Energy Conversion
Knowledge of surface properties of energy materials at elevated temperatures is essential for understanding their reactivity and performance in energy conversion. Here we show that single-phase oxide materials, such as CoO, at elevated temperatures corresponding to thermodynamic equilibrium, consist of homogeneous bulk phase and a low-dimensional surface structure, LDSS, which differs from the bulk phase in terms of crystalline structure, chemical composition, defect disorder, and semiconducting properties. It has been documented that the CoO/O 2 system in gas/solid equilibrium exhibits three types of LDSSs that are formed in (a) reducing conditions, (b) oxidizing conditions, and (c) highly oxidizing conditions corresponding to the vicinity of the CoO/Co 3 O 4 phase boundary. Oxidation of these LDSSs leads to (a) decrease of the Fermi level, (b) nil change of the Fermi level, and (c) increase of the Fermi level, respectively. The effects b and c, which are not observed for the bulk phase, indicate that the LDSS is quasi-isolated. This discovery has been revealed by in situ surface monitoring of CoO at elevated temperatures using work function, WF, measurements. The results reported in the present work are reflective of the local properties of the LDSSs that are formed on the surface of CoO single crystal including defect disorder and the related semiconducting properties. This finding paves the way for the development of novel approaches in processing of energy conversion and refractory systems with enhanced performance through defect engineering of the surface layer.