Semiconductor nanoparticles (NPs), such as those of nickel oxide (NiO), exhibit catalytic activity toward the urea oxidation reaction (UOR), making it an attractive alternative to the kinetically slow oxygen evolution reaction (OER). Such nanomaterials have demonstrated improved electrochemical performance but experience a transformation in their electronic (defect) structure compared to bulk materials. Thus, we combine optoelectronic transient absorption (TA) with electrochemical characterization to understand the relations of electronic defect structures and morphology to the electrochemical performance of nanosized catalysts. Two distinct syntheses were performed: (I) a sol−gel Ni(OH)2 approach with subsequent calcination, forming mesoporous NiO particles, and (II) an organometallic colloidal NiO NP route. Successful formation of NiO was confirmed for all syntheses by X-ray diffraction, and morphology was explored by scanning (transmission) electron microscopy. When increasing the calcination temperature (from 250 to 700 and 850 °C), NiO particles from synthesis (I) grow larger and experience a removal of intra-bandgap states upon more complete formation to NiO. NiO NPs from synthesis (II) present a defined electronic structure with low defect density within the TA ranges probed, whereas an annealing step introduces delocalized defects. During electrocatalytic measurements (KOH or KOH + urea electrolyte), we distinguished surface area effects by morphology from intrinsic catalytic activities governed by defect states and correlated less defective materials with higher intrinsic activities, as well as lower charge transfer resistances. This work advances material characterization by a holistic combination of a comparably rare optoelectronic spectroscopic strategy with electrochemical characterization to understand fundamental material properties and their influence on catalytic performance, demonstrating niche investigation strategies of NiO in UOR catalysis.
Kern et al. (Tue,) studied this question.