Functional Inversion of Trisodium Citrate from a [0001] Growth Suppressor to a Promoter in ZnO Thin Film Fabrication

Trisodium citrate is a well-established molecular modifier for ZnO thin films renowned for its role in suppressing [0001] growth by selectively passivating the (0002) facet in conventional, slow-growth fabrication methods. A paradoxical functional inversion is demonstrated herein: under fast-growth conditions using a mist-based fabrication, trisodium citrate acts as a powerful promoter, yielding well-aligned, high-aspect-ratio nanorods with a strong [0001] orientation. This functional switch is attributed to a fundamental change in the dominant growth unit, from the neutral Zn-(OH)₂ species prevalent under slow-growth conditions to the anionic Zn-(OH)₄ ^2- complex enabled by the fast-growth process. This change in growth unit chemistry activates the transformation of citrate from a static suppressor into a mobile promoter via surface diffusion and lowering the crystallization energy barrier. These findings establish a new framework where the synthesis kinetics dictate the dominant growth unit chemistry, which, in turn, governs the ultimate function of a molecular additive, offering a new strategy for the rational fabrication of anisotropic nanostructures.