From nanodots to nanowires: simple ion-controlled morphology engineering of manganese oxide nanostructures

This study presents a simple and cost-effective approach for morphology control of manganese oxide (MnOx) nanomaterials through manipulation of reaction stoichiometry and post-synthesis processing. By systematically varying the molar concentration of hydrochloric acid (HCl) and applying controlled rinsing steps, a transition from nanoparticle-dominated (0D) to nanorod-rich (1D) morphologies was achieved without the use of templates or advanced fabrication techniques. Six samples synthesized under different conditions were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDX). The results suggest that morphology evolution is governed by a growth regime defined by the balance between Mn²⁺ generation, Mn²⁺ stabilization, and the resulting MnO₂ supersaturation. Within the studied range, increased acidity enhances Mn²⁺ generation and promotes MnO₂ formation, leading to higher effective supersaturation and the onset of anisotropic growth. In this framework, residual potassium (K⁺)-containing species are interpreted as secondary, growth-modifying factors that may influence the extent of anisotropy, rather than as primary determinants of phase formation or morphology. EDX analysis indicates a qualitative reduction in residual K content following rinsing, although these measurements remain semi-quantitative. The relationship between K content and morphology is therefore interpreted cautiously, as the effective K⁺–to–MnOx ratio is not directly measured and cannot be decoupled from concurrent changes in ionic strength and chloride concentration. Overall, these findings highlight the coupled roles of reaction stoichiometry and ion-mediated growth in shaping MnOx nanostructures, and demonstrate a simple, scalable strategy for morphology engineering in manganese oxide systems.