gallium trioxide, gallium sesquioxide
|Molar mass||187.444 g/mol|
|Appearance||white crystalline powder|
|Density||6.44 g/cm3, alpha
5.88 g/cm3, beta
|Melting point||1,900 °C (3,450 °F; 2,170 K) alpha
1725 °C, beta 
|Solubility||soluble in most acids|
|EU classification||not listed|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is: / ?)(|
Gallium oxide is precipitated in hydrated form upon neutralization of acidic or basic solution of gallium salt. Also, it is formed on heating gallium in air or by thermally decomposing gallium nitrate at 200-250˚C. It can occur in five different modifications, α,β,δ,γ and ε. Of these modifications β-Ga2O3 is the most stable form.
Preparation Methods for the Five Modifications
- β-Ga2O3 is prepared by heating nitrate, acetate, oxalate or other organic derivatives above 1000˚C.
- α-Ga2O3 can be obtained by heating β-Ga2O3 at 65kbars and 1100˚C for 1 hour giving a crystalline structure. The hydrated form can be prepared by decomposing precipitated and "aged" gallium hydroxide at 500˚C.
- γ-Ga2O3 is prepared by rapidly heating the hydroxide gel at 400˚C-500˚C. A more crystalline form of this polymorph can be prepared directly from gallium metal by a solvothermal synthesis.
- δ-Ga2O3 is obtained by heating Ga(NO3)3 at 250˚C.
- ε-Ga2O3 is prepared by briefly heating δ-Ga2O3 at 550˚C for 30 minutes.
Gallium(III) oxide is amphoteric. reacting with alkali metal oxides at high temperature to form e.g. NaGaO2, and with Mg, Zn, Co, Ni, Cu oxides to form spinels e.g. MgGa2O4. and dissolving in strong alkali to form a solution of the gallate ion, Ga(OH)−
- Ga2O3 + 6 HCl → GaCl3 + H2O
- Ga2O3 + H2 → Ga2O + H2O
- Ga2O3 + 4 Ga → 3 Ga2O
β-Ga2O3, with a melting point of 1740˚C, is the most stable crystalline modification. The oxide ions are in a distorted cubic closest packing arrangement, and the gallium (III) ions are in distorted tetrahedral and octahedral sites, with Ga-O bond distances of 1.83 and 2.00 Å respectively. These distortions are in fact the reasons for the great level of stability of β-Ga2O3.
Applications and Uses
Gallium(III) oxide is an important functional material. It has been studied in the use of lasers, phosphors and luminescent materials, has been shown to demonstrate catalytic properties and has also been used as an insulating barrier in tight junctions. The stable oxide of gallium, monoclinic β-Ga2O3, has current applications in gas sensors and luminescent phosphors and can be applied to dielectric coatings for solar cells. This stable oxide has also shown potential for deep-ultraviolet transparent conductive oxides.
Nanoribbons and nanosheets of Ga2O3 can be synthesized either by high temperature reaction of Ga0 with water or by evaporation of GaN at high temperature in the presence of oxygen. Analysis of the products of the thermal evaporation reaction is done using X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). Such analysis showed that the resulting structures were wool-like.
SEM reveals that the products consist of wire-like nanostructures and sheet-type structures. The TEM image shows the ribbon-like structure of Ga2O3. The EDS confirms that the nanostructures obtained are in fact Ga2O3.
The nanoribbon and nanosheets structures of Ga2O3 are pure, structurally uniform, single crystalline and free from dislocation. The structure of nanoribbons and nanosheets, that is, their wave-like and sheet-like shape, also indicates that their growth is as a result of growth kinetics, vapor-liquid-solid (VLS) method and vapor-solid method (VS). VLS and VS are two common growth mechanisms for nanowires. The VLS process, catalytic-assisted in nature, is one in which the metal particle is located at the growth of the wire and acts as the catalytic active site. For the VS process, oxide vapor, which is evaporated from the starting oxide at a higher temperature xone, directly deposits on a substrate at a lower temperature region and grows into ribbon-like nanostructures.
It is important to accurately determine the optical functions as these are essential for device simulations and improvement in material preparation. The thin Ga2O3 films are of commercial interest as gas sensitive material and Ga2O3 based glasses are among the best optical materials used in advanced technologies. Ellipsometry is a procedure that can be used to determine optical functions of the β-Ga2O3.
β-Gallium(III) oxide is also very important in the production of catalysts. It is needed for the preparation of Ga2O3-Al2O3 catalyst. The preparation of this catalyst involves reacting Al2O3 with aqueous solutions of gallium nitrate, followed by evaporation to dryness at 393K, and calcining in air (i.e.thermal decomposition of the compound) for 4 hours at 823K.
- Vacuum deposition
- Scanning electron microscopy
- Transmission electron microscopy
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