• journal_title：American Mineralogist
• Contributor：Hyun Na Kim ; Sung Keun Lee
• Publisher：Mineralogical Society of America
• Date：2013-07-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4364
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：7
• firstpage：1198
• section：Articles

Nanoparticles are known to have physical properties different from their bulk analogs, a characteristic that bears wide technological potential. Detailed knowledge of the atomic structures of diverse metastable/stable polymorphs in alumina nanoparticles with varying particle size is essential to understand their macroscopic properties and the behavior of their phase transitions. In this study, we report high-resolution solid-state ²⁷Al 2D triple-quantum (3Q) magic-angle spinning (MAS) and 1D MAS nuclear magnetic resonance (NMR) spectra for alumina nanoparticles with varying temperature and particle size with an aim to explore the nature of phase transitions in alumina nanoparticles. Although the ²⁷Al MAS NMR spectra of alumina nanoparticles cannot fully resolve all the crystallographically distinct Al sites for metastable aluminas such as γ-, δ-, and θ-Al₂O₃, the simulation of ²⁷Al MAS NMR spectra collected at different magnetic fields following the Czjzek model allows us to obtain the quantitative fractions of alumina polymorphs in nanoparticles and the NMR characteristics of their Al sites. The ²⁷Al 3QMAS NMR spectra resolved crystallographically distinct ^[6]Al and ^[4]Al sites in (γ,δ)- and θ-Al₂O₃ in the isotropic dimension for the first time. The fraction of θ-Al₂O₃ gradually increases up to ~1473 K at the expense of a decrease in (γ,δ)-Al₂O₃. The onset of formation of α-Al₂O₃ from metastable aluminas is observed above ~1493 K. Several phase transitions in alumina nanoparticles observed in the current study include, γ,δ → θ-Al₂O₃, γ,δ → α-Al₂O₃, and θ → α-Al₂O₃. Although the phase transition γ,δ → θ-Al₂O₃ occurs gradually with increasing annealing temperature from 873 to 1473 K, the phase transitions γ,δ → α-Al₂O₃ and θ → α-Al₂O₃ occur dramatically within a narrow temperature range between 1473 and 1573 K. The observed difference in temperature range (gradual vs. dramatic) for phase transition γ,δ → θ-Al₂O₃ and γ,δ,θ → α-Al₂O₃ originates from the different structural disorder in the metastable aluminas (i.e., γ-, δ-, θ-) and α-Al₂O₃. The effect of particle size on the phase transition (γ,δ) → θ-Al₂O₃ between 298 and ~1473 K is not observed significantly. On the other hand, the transition temperature for γ,δ,θ → α-Al₂O₃, where the 50% for alumina is α-Al₂O₃, apparently increases as the particle size increases (as evidenced by TEM observation), indicating a larger energy penalty for phase transitions into α-Al₂O₃ in larger alumina nanoparticles. This could be due to higher surface energy of θ-Al₂O₃ than that of α-Al₂O₃ and/or the fact that the transition from θ-Al₂O₃ to α-Al₂O₃ is kinetically favored for smaller nanoparticles. The mechanistic details of phase transitions among alumina polymorphs provided in the current study yield insights into the nature of the phase transition mechanisms for other oxide nanoparticles ubiquitous in the Earth’s surface environment.