H+- and Al2+-Codoped Al2O3 Nanoparticles with Spinel-Type Related Structures by Pulsed Laser Ablation in Water

Pulsed laser ablation in water under a high peak power density of 1.8 x 10(11) W/cm(2) using Q-switch mode and 1064 nm excitation was used to fabricate (H+,Al2+)-codoped Al2O3 nanocondensates having gamma- and its derivative theta-type structure as characterized by electron microscopy and spectroscopy. The as-formed gamma- and theta-Al2O3 nanocondensates are mainly 10 to 100 nm in size and have a significant internal compressive stress (>10 GPa) according to cell parameters and vibrational spectroscopy, due to a significant shock loading effect in water. The gamma-Al2O3 nanocondensates are nearly spherical in shape but become cuboctahedra when grown to ca. 100 nm to exhibit more facets as a result of martensitic gamma ->theta transformation following the crystallographic relationship ((3) over bar1 (1) over bar)(theta)//(0 (2) over bar2)(gamma); (0 (2) over bar(4) over bar)(theta)//((3) over bar 11)gamma. The formation of dense and (H+,Al2+)-codoped gamma/theta-Al2O3 rather than aluminum hydrates sheds light on the favored phases of the Al2O3-H2O binary at high temperature and pressure conditions in natural dynamic settings. The nanocondensates thus formed have a much lower minimum band gap (5.2 eV) than bulk alpha-Al2O3 for potential optocatalytic applications.