Synthesis and hydrogen absorption of high-specific-surface ultrafine theta-Al2O3 nanowires

• The θ-Al2O3 generated from the slow transformation of γ-Al2O3.
• The ultrafine nanowire structure formed from the recrystallization of γ-Al2O3 nanowire.
• The nanowires possess average diameters of sub-10 nm and lengths up to several micrometers.
• Their Brunauer–Emmett–Teller (BET) specific surface area was measured to be~120 m2/g.
• Hydrogen uptake capacity is up to 5.57 wt% at 77 K, 1.51 wt% at 298 K and 0.81 wt% at 557 K, respectively.

Ultrafine theta-Al2O3 (θ-Al2O3) nanowires with average diameters of sub-10 nm and lengths up to several micrometers have been successfully synthesized via a novel two-stage method. With temperature gradually elevated, θ-Al2O3 was generated from the slow transformation of gamma-phase Al2O3 (γ-Al2O3), and its ultrafine nanowire structure was formed from the recrystallization of γ-Al2O3 nanowire. By means of nitrogen adsorption–desorption isotherm at 77 K and the multipoint Brunauer–Emmett–Teller (BET) method, the specific surface area of the ultrafine θ-Al2O3 nanowires is found to be up to~120 m2/g. With the high surface area, the material displays excellent gas absorption ability. Hydrogen absorption capacities could be measured to be~5.57 wt% at 77 K (liquid nitrogen), 1.51 wt% at 298 K (room temperature) and 0.81 wt% at 557 K, when a pressure of 30 atm (~3 MPa) is applied. The crystal or chemical nature of metastable θ-Al2O3 phase may play a vital role in the observed excellent gas storage capacity.