Combinatorial Nanopowder Synthesis Along the ZnO–Al2O3Tie Line Using Liquid‐Feed Flame Spray Pyrolysis

Abstract

Liquid‐feed flame spray pyrolysis (LF‐FSP) of mixtures of alumatrane [Al(OCH2CH2)3N]/zinc acetate dihydrate [Zn(O2CCH3)2·2(H2O)] or zinc propionate [Zn(O2CCH2CH3)2]/aluminum acetylacetonate [Al(Acac)3] dissolved in EtOH in known molar ratios can be used to combinatorially generate nanopowders along the ZnO–Al2O3tie‐line. LF‐FSP was used to produce (ZnO)x(Al2O3)1−xpowders withx=0–1.0. Powders were characterized by X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, thermal gravimetric analysis, differential thermal analysis, and BET. The resulting powders had average particle sizes (APSs) <100 nm with the majority being <50 nm. Analytical data suggest that at concentrations of interest for transparent conducting oxides, <10 mol% Al2O3the particle morphologies are combinations of plates and rods that grow withc/aratios close to 1. The spinel phase dominates at (ZnO)x(Al2O3)1−x(x=0.5 and 0.3). In the latter case, the currently accepted phase diagram for the ZnO–Al2O3couple indicates that phase separation should occur to form zinc spinel (ZnAl2O4) and α‐alumina. It appears that the rapid quenching during LF‐FSP helps to preserve the spinel phase at ambient temperature giving rise to kinetic nanopowder products along the ZnO2–Al2O3tie‐line. Finally, the solubility of ZnO in Al2O3and vice versa in the materials produced by LF‐FSP suggest apparent flame temperatures reached before quenching are 1700°–1800°C. Efforts to re‐pass the spinel phase powders, (ZnO)x(Al2O3)1−x,x=0.5 and 0.3 through the LF‐FSP system were made with the hope of generating core shell materials. However, instead thex=0.5 material generated materials closer to thex=0.3 composition and pure ZnO nanoparticles that coat the former materials. These results suggest that at LF‐FSP flame temperatures ZnO remains in the vapor phase for sufficient times that Al3+oxy‐ions generated promote nucleation of finer particles leaving essentially phase pure ZnO still in the vapor phase to condense giving the two distinct particle morphologies observed.