Chemical Preparation of GaO(OH) Crystals or Zeppelins

A. C. Tas, P. J. Majewski, and F. Aldinger, "Synthesis of Gallium Oxide Hydroxide Crystals in Aqueous Solutions with or without Urea and Their Calcination Behavior," Journal of the American Ceramic Society, Vol. 85(6), pp. 1421-1429 (2002).

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Abstract
Gallium oxide hydroxide (GaOOH.xH2O) single crystals were synthesized in aqueous solutions by using two different precipitation techniques; a) homogeneous decomposition of urea and b) forced hydrolysis in pure water.

Precipitation of crystals started at exactly the same pH value (i.e., 2.05 at 85C) in both cases. The morphology of crystals turned out to be quite different (zeppelin-like with urea, rod-like without urea) in each of the above methods. Calcination of these gallium oxide hydroxide crystals in air at temperatures >500C transformed them into Ga2O3.
(For further details, please contact Dr. A. Cuneyt Tas at the e-mail address "c_tas@hotmail.com")

Characterization of the samples were performed by X-ray diffractometry, scanning electron microscopy, thermogravimetry/differential thermal analysis, Fourier transform infrared spectroscopy, and ICP-AES, carbon and nitrogen analyses.

GaOOH zeppelins prepared in water in the presence of urea (low magnification)
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GaOOH zeppelins prepared in water in the presence of urea (high magnification)
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TEM analysis of GaOOH zeppelins

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GaOOH rod-like crystals prepared in pure water
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Conclusions
The following conclusions can be made from the results presented:

(1) Upon heating a dilute (0.026 M) aqueous solution of gallium nitrate at about 90C for 95 minutes (i.e., forced hydrolysis), rod-like, monodispersed, 1 to 3 m-long orthorhombic GaOOH.xH2O single crystals can be formed (starting at the pH value of 2.05 at 85C), where the value of x is in the vicinity of 0.25.

(2) Upon heating a dilute (0.026 M) aqueous solution of gallium nitrate, which also contains 0.304 M urea (i.e., homogeneous decomposition of urea), to about 85C, zeppelin-shaped, monodispersed, 200-500 nm-long, orthorhombic GaOOH.xH2O single crystals can be formed, again at the exact pH value of 2.05. The value of x in the formula is approximately 0.11.

(3) Both types of gallium oxide hydroxide single crystals transform first into rhombohedral and then monoclinic polymorphs of Ga2O3, upon calcination in an air atmosphere for 6 h at temperatures >500C.

(4) The initial zeppelin-like morphology (which is also observed for alpha-FeOOH by previous researchers) of crystals, formed in solutions containing urea, starts slowly disappearing (changing into a nodular one) upon calcination at temperatures  >750C, but the rod-like morphology of crystals formed by forced hydrolysis persisted even after calcination at 1200C. The rod-like crystals of GaOOH.xH2O became nanoporous on their visible surfaces with an increase in calcination temperature.

(5) Carbon and nitrogen analyses, as well as the FTIR results showed that the formed crystals (following drying at 90C) did not contain any structural CO32- and NO3- ions.

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