Pulsed laser ablation in water under a high peak power density of 1.8 × 1011 W/cm2 using Q-switch mode and 1064 nm excitation was used to fabricate (H+,Al2+)-codoped Al2O3 nanocondensates having γ- and its derivative θ-type structure as characterized by electron microscopy and spectroscopy. The as-formed γ- and θ-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 γ-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 γ→θ transformation following the crystallographic relationship (3̅11̅)θ//(02̅2)γ; (02̅4̅)θ//(3̅11)γ. The formation of dense and (H+,Al2+)-codoped γ/θ-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 α-Al2O3 for potential optocatalytic applications.

Pulsed laser ablation on an Al target in water spiked with 0.05 and 1 M NaOH was employed to fabricate epitaxial β-NaAlO 2 and γ-Al 2 O 3 nanoparticles for X-ray diffraction and electron microscopic and spectroscopic characterizations. A higher NaOH concentration in the pulsed laser ablation in liquid (PLAL) process caused a higher content of NaAlO 2 3 4/5H 2 O and β-NaAlO 2 besides the predominant γ-Al 2 O 3 nanoparticles. Upon settling on the silica substrate at room temperature, the NaAlO 2 3 4/5H 2 O tended to develop as micrometer sized plates for the sample fabricated under a relatively low (i.e., 0.05 M) NaOH concentration. However, the crystal structures, d-spacings, mismatch strain, and morphology of the coexisting phases are almost the same for the samples fabricated under different NaOH concentrations. The β-NaAlO 2 phase (denoted as N), presumably derived from NaAlO 2 3 5/4H 2 O, was found to form intimate intergrowth with the defective γ-Al 2 O 3 following the crystallographic relationship (011) γ //(110) N ; [111] γ //[001] N and alternatively (101) γ //(110) N ; [141] γ // [001] N . The epitaxial composite phases have a significant internal compressive stress, (OH -, Na þ ) cosignature, and a mixed charge state of Al þ , Al 2þ , and Al 3þ and hence a smaller band gap (ca. 5.3 eV) for potential applications in the UV region.