Morphology transition engineering on MgO for enhanced dye adsorption without using surfactants as sacrifice templates

Hierarchical porous MgO is a promising adsorbent for dye removal because of its large Brunauer–Emmett–Teller specific surface area (S_BET) and abundant low-coordinated oxygen anions (LCO) sites. As hierarchical porous MgO particles with large S_BET values are typically prepared by complicated procedures under harsh conditions, such as high temperatures and high pressures, their large-scale production is impractical. Consequently, the preparation of hierarchical porous MgO with a large S_BET value under mild conditions is highly desirable. In this study, a morphology transition engineering strategy is introduced to change the morphology of simple MgO microspheres to an embroidered ball-shaped with a larger S_BET value via hydrolysis and calcination without using surfactants as sacrificial templates. During hydrolysis, numerous Mg(OH)₂ sheets form and attach to the MgO surface, thus increasing the S_BET value of the newly obtained MgO that forms by calcination (denoted as NM-x, where x is the hydrolysis time in hours). The sizes of the crystalline sheets were tuned by controlling the hydrolysis time. NM-12 exhibited the highest density of small-sized sheets on its surface and the largest S_BET value of 180.17 m² g⁻¹, which was 3.51 times that of the MgO precursor (51.89 m² g⁻¹). However, NM-24 (134.07 m² g⁻¹) had a higher adsorption efficiency for Congo red (CR) than NM-12, despite having a smaller S_BET value, which indicates that other factors are involved. NM-24 exhibited a lower probability of exposed (200) and (220) facets which were verified to repulse CR molecules by molecular dynamics simulations, and a greater number of LCO sites, which contributed to adsorption. Thus, this study introduces a facile method for preparing hierarchical porous MgO and examines the effects of LCO sites and exposed facet probabilities on its adsorption properties.