Vertically aligned two-dimensional ZnO nanowall networks: Controllable catalyst-free growth and optical properties

•Two-dimensional ZnO nanowall networks were grown via a two-step growth method.•We detailedly investigated the growth mechanisms of ZnO nanowall networks.•The micro-hole size and density can be adjusted by changing the oxygen flow rate.•The micro-hole related contaminants adsorption behaviors cause the variation of DLE.•Temperature-dependent PL results confirm the excellent optical quality of ZnO. We report the fabrication of vertically aligned two-dimensional (2D) ZnO nanowall networks (NNWs) by a catalyst-free metal–organic chemical vapor deposition system via a two-step growth method. The growth mechanism of the resulting NNWs was investigated detailedly by monitoring the morphology evolution at different growth stages. The formation of rough and striped grains in the initial stage was the determinant factor, which facilitated and guided the subsequent growth of NNWs. In our case, the obtained ZnO NNWs were of single-crystal wurtzite structure with a small full width at half-maximum (481arcsec) of (0002) ω-rocking curve. We also demonstrated the feasibility of controlling the morphology, structural and optical features of ZnO NNWs by adjusting the oxygen flow rate. A controllable evolution of micro-hole size and density of ZnO NNWs was observed. In addition, possible microcavity-related light oscillation and contaminants adsorption behaviors were proposed to explain the shift of ultraviolet peak and deep-level emission variation for the produced ZnO NNWs. Temperature-dependent photoluminescence results behaving with strong exciton effects further confirmed the relatively excellent quality of the obtained ZnO NNWs. It was reasonably believed that the novel 2D nanostructures featuring high crystallinity and remarkable optical properties can be employed in reliable optoelectronic nanodevices.