Selective thermal reduction of single-layer MoO3 nanostructures on Au(1 1 1)

MoO3 is an interesting oxide prototype because its catalytic activity is sensitive to the presence and nature of defects. In this work, we demonstrate that we can control the number of defects in single-layer MoO3 nanostructures grown on Au(1 1 1) by a thermal reduction treatment. X-ray photoelectron spectroscopy demonstrates the formation of Mo5+ species and oxygen vacancies during annealing at 650 K. The percentage of Mo5+ increases with the duration of annealing, until a stable composition containing 50% Mo6+ and 50% Mo5+ is obtained. Surprisingly, the formation of lower oxidation states such as Mo4+ was not observed. The reduced MoOx islands remain one layer high, based on scanning tunneling microscope (STM) images. The two-dimensional nature of the reduced oxide nanocrystals may be due to a large barrier for structural reorganization and, thus, may account for the absence of Mo oxidation states lower than +5. Based on scanning tunneling microscopy images and density functional calculations, we propose that the formation of Mo5+ ions during annealing is not associated with formation of oxygen point defects, but can be attributed to the formation of extended one-dimensional shear defects. These reduced structures are useful for studying the dependence of reactivity on defect type, and present exciting possibilities for chemical sensors and other applications.