Hierarchical γ-BaB2O4 hollow microspheres: surfactant-assisted hydrothermal formation, phase conversion, optical properties and application as adsorbents

Three-dimensional (3D) hierarchical metal borate nanoarchitectures have attracted extensive interest for their wide use owing to their versatile compositions and unique properties, and their rational design and facile synthesis with high uniformity and complex interiors have long remained a great challenge. In this contribution, hierarchical γ-BaB2O4 hollow microspheres assembled from one-dimensional (1D) nanorods were successfully synthesized by a facile ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) assisted hydrothermal method (180 °C, 12 h), using Ba(OH)2·8H2O and H3BO3 as the raw materials. The effects of process parameters were investigated in detail, based on which a feasible formation mechanism was proposed. The hierarchical γ-BaB2O4 hollow microspheres exhibited a specific surface area of 46.25 m2 g−1. The subsequent mild phase conversion (700 °C, 3.0 h) led to β-BaB2O4 hollow microspheres with a well-preserved spherical morphology and high crystallinity, indicating hierarchical γ-BaB2O4 and β-BaB2O4 hollow microspheres of excellent thermostability. Both the hierarchical γ-BaB2O4 and β-BaB2O4 hollow microspheres exhibited a transparent characteristic from the ultraviolet to the visible region. Moreover, when employed as adsorbents, the hierarchical γ-BaB2O4 hollow microspheres presented satisfactory adsorption properties for the removal of Pb2+ from mimic waste water, suggesting the great potential applications of the 3D hierarchical porous or hollow nanoarchitectures of metal borates in the future.