Template-free synthesis of hierarchical γ-Al2O3 nanostructures and their adsorption affinity toward phenol and CO2

Hierarchical γ-Al2O3 nanostructures with tuneable morphologies including irregular nanoflake assemblies, melon-like nanoflake assemblies, flower-like ellipsoids, hollow core/shell and hollow microspheres were successfully synthesized for the first time via a facile template-free hydrothermal method using aluminium sulfate, aluminium chloride and aluminium nitrate as aluminium sources, respectively, and thiourea as precipitating agent. Their phase structures, morphologies, textural and basic properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), N2 adsorption–desorption and CO2 temperature programmed desorption (CO2-TPD). The results indicate that the thiourea, type of anion in the aluminium source and the molar ratio of thiourea to Al3+ play an essential role in the formation of the aforementioned hierarchical γ-Al2O3. A growth mechanism of chemically induced self-transformation followed by cooperative self-assembly to form hierarchical nanostructures was proposed. In contrast, the γ-Al2O3 hollow core/shell microspheres with average pore size of 14.3 nm obtained from aluminium sulfate show the highest adsorption capacity of 28 mg g−1 towards phenol at 25 °C. However, the hierarchical γ-Al2O3 obtained from aluminium chloride and aluminium nitrate with smaller average pore size of 5.2 nm and 5.4 nm, respectively, is more effective for CO2 capture. This study provides new insights into the design and synthesis of hierarchical nanostructures for environmentally relevant applications.