Shape Evolution of MgO Nanocubes via Rapid pH Self-Adjustment at Oxide-Water Interfaces

Magnesium oxide (MgO) is a prototypical model for oxide-water interfaces and a technologically relevant material in catalysis, environmental remediation, and corrosion-resistant coatings. Here, the dissolution behavior and morphology evolution of MgO smoke nanocubes are investigated in aqueous media while simultaneously monitoring the self-adjusting solution pH under three initial conditions: distilled water (pH ≈ 6.4), mildly acidic HCl solution (pH = 6), and alkaline NaOH solution (pH = 11). Time-resolved TEM reveals that dissolution initiates preferentially at (110) edges, drives a progressive transition from cubic to truncated cubes and finally octahedral morphologies exposing (111) facets, and is accompanied by brucite [Mg-(OH)₂] formation whose extent depends on particle size and pH. Upon immersion in distilled water or a mildly acidic solution, the suspension pH rapidly rises to ≈11-12 within minutes, whereas this abrupt pH change is not observed when MgO is introduced directly into an already alkaline medium (pH 11). A subsequent quasi-stationary regime with pH maintained at 11-12 persists for days, showing that the vast majority of the observed morphological transformations in these systems actually proceed under strongly alkaline rather than nominally neutral or mildly acidic conditions. To rationalize these observations, density functional theory calculations are performed for ideal and defect-containing low-index MgO surfaces under H⁺-rich conditions, including protonated Mg-vacancy (V_MgH₂) defects relevant to nonoxidative dissolution. The calculations reveal a strong pH dependence of defective surface energies, pronounced anisotropy in the thermodynamic driving force for dissolution, and higher Mg²⁺ extraction barriers for compact (100) than for more open (110) and (111) terminations, consistent with the experimentally observed facet-dependent kinetics. These results present a unified, pH-dependent picture of MgO dissolution and morphology evolution in water, clarify that earlier "neutral" dissolution studies actually probed alkaline conditions, and offer guidelines for controlling MgO stability and Mg-(OH)₂ formation in aqueous media. They highlight how self-adjusted local pH and defect chemistry regulate dissolution, shape transformation, and passivation of oxide nanomaterials at solid-liquid interfaces relevant to environmental and energy-related (electro)-chemical systems.