Conversion of [0001] Textured ZnO Nanofilm into [011̅0] Directed Nanowires Driven by CO Adsorption: In Situ Carbothermal Synthesis and Complementary First Principles Thermodynamics Simulations

Abstract

An in situ carbothermal process has been utilized to successfully convert sputtered (0001) oriented ZnO nanofilm into large scale [011̅0] directed nanowires at 800−900 °C in a tubular reaction chamber. To understand the conversion process and unravel the driving force behind this growth phenomenon, a comparative study has been carried out on the thermal annealing processes of ZnO nanofilms under different gas flows (with or without Ar flow) and atmosphere (with or without graphite source input) controls. It is found that the graphite source and Ar flow are both necessary to induce the conversion of ZnO (0001) oriented nanofilm into ZnO nanowires grown along [011̅0]. By heating the graphite source, a C- or CO-rich reducing atmosphere was generated and carried downstream by Ar flow during the conversion process, which proved to be the key. Complementary first principles computations suggest that changes in the ordering of surface energies in the presence of CO occur. As compared with the clean facets with an order of {101̅0}, {112̅0}, and {0001} from most to least stable, upon CO adsorption at high temperatures (900 °C), the {112̅0} and {0001} surfaces become more stable as compared to {101̅0}. This provides a strong thermodynamic driving force for the growth of nanowires along ⟨101̅0⟩ directions.