- Structure of gold (III) oxide
- Electronic aspects
- Hydrates
- Properties
- Physical appearance
- Molecular mass
- Density
- Melting point
- Stability
- Solubility
- Nomenclature
- Applications
- Glass staining
- Synthesis of aurates and fulminant gold
- Handling self-assembled monolayers
- References
The gold oxide (III) is an inorganic compound whose chemical formula is Au 2 O 3. Theoretically, its nature could be expected to be of the covalent type. However, the presence of a certain ionic character in its solid cannot be completely ruled out; or what is the same, assume the absence of the Au 3+ cation together with the O 2- anion.
It may seem contradictory that gold, being a noble metal, can rust. Under normal conditions, gold pieces (like the stars in the image below) cannot be oxidized by contact with oxygen in the atmosphere; however, when they are irradiated with ultraviolet radiation in the presence of ozone, O 3, the picture is different.
Gold stars. Source: Pexels.
If gold stars were subjected to these conditions, they would turn a reddish-brown color, characteristic of Au 2 O 3.
Other methods to obtain this oxide would involve the chemical treatment of said stars; for example, by converting the mass of gold to its respective chloride, AuCl 3.
Then, to AuCl 3, and the rest of the possible gold salts formed, a strong basic medium is added; and with this, the hydrated oxide or hydroxide, Au (OH) 3, is obtained. Finally, this last compound is thermally dehydrated to obtain Au 2 O 3.
Structure of gold (III) oxide
Crystal structure of Au2O3. Source: Materialscientist
The upper image shows the crystal structure of gold (III) oxide. The arrangement of the gold and oxygen atoms in the solid is shown, either considering them neutral atoms (covalent solid), or ions (ionic solid). Indifferently, it is enough to remove or place the Au-O links in any case.
According to the image, it is assumed that the covalent character predominates (which would be logical). For that reason, atoms and bonds are shown represented by spheres and bars, respectively. The golden spheres correspond to the gold atoms (Au III -O), and the reddish ones to the oxygen atoms.
If you look closely, it will be seen that there are AuO 4 units, which are joined by oxygen atoms. Another way to visualize it would be to consider that each Au 3+ is surrounded by four O 2-; of course, from an ionic perspective.
This structure is crystalline because the atoms are arranged in the same long-range pattern. Thus, its unit cell corresponds to the rhombohedral crystalline system (the same one in the upper image). Therefore, all the Au 2 O 3 could be constructed if all those spheres of the unit cell were distributed in space.
Electronic aspects
Gold is a transition metal, and its 5d orbitals are expected to interact directly with the 2p orbitals of the oxygen atom. This overlapping of their orbitals should theoretically generate conduction bands, which would turn Au 2 O 3 into a solid semiconductor.
Therefore, the true structure of Au 2 O 3 is even more complex with this in mind.
Hydrates
Gold oxide can retain water molecules within its rhombohedral crystals, giving rise to hydrates. As such hydrates are formed, the structure becomes amorphous, that is, disordered.
The chemical formula for such hydrates can be any of the following, which in fact are not fully clarified: Au 2 O 3 ∙ zH 2 O (z = 1, 2, 3, etc.), Au (OH) 3, or Au x O y (OH) z.
The formula Au (OH) 3 represents an oversimplification of the true composition of said hydrates. This is because within gold (III) hydroxide, researchers have also found the presence of Au 2 O 3; and therefore it is meaningless to treat it in isolation as a "simple" transition metal hydroxide.
On the other hand, an amorphous structure could be expected from a solid with the formula Au x O y (OH) z; since it depends on the coefficients x, y and z, whose variations would give rise to all kinds of structures that could hardly exhibit a crystalline pattern.
Properties
Physical appearance
It is a reddish-brown solid.
Molecular mass
441.93 g / mol.
Density
11.34 g / mL.
Melting point
Melts and decomposes at 160ºC. Therefore, it lacks a boiling point, so this oxide never boils.
Stability
Au 2 O 3 is thermodynamically unstable because, as mentioned at the beginning, gold does not tend to oxidize under normal temperature conditions. So it is easily reduced to become the noble gold again.
The higher the temperature, the faster the reaction, which is known as thermal decomposition. Thus, Au 2 O 3 at 160ºC decomposes to produce metallic gold and release molecular oxygen:
2 Au 2 O 3 => 4 Au + 3 O 2
A very similar reaction can occur with other compounds that promote said reduction. Why reduction? Because gold regains the electrons that oxygen took from it; which is the same as saying that it loses bonds with oxygen.
Solubility
It is a solid insoluble in water. However, it is soluble in hydrochloric acid and nitric acid, due to the formation of gold chlorides and nitrates.
Nomenclature
Gold (III) oxide is the name governed by the stock nomenclature. Other ways to mention it are:
-Traditional nomenclature: auric oxide, because the 3+ valence is the highest for gold.
-Systematic nomenclature: dioro trioxide.
Applications
Glass staining
One of its most eminent uses is to give certain materials a reddish color, such as glass, as well as giving them certain properties inherent to gold atoms.
Synthesis of aurates and fulminant gold
If Au 2 O 3 is added to a medium where it is soluble, and in the presence of metals, aurates can precipitate after the addition of a strong base; which are made up of AuO 4 anions - in the company of metal cations.
Likewise, Au 2 O 3 reacts with ammonia to form the fulminant gold compound, Au 2 O 3 (NH 3) 4. Its name derives from the fact that it is highly explosive.
Handling self-assembled monolayers
Certain compounds, such as dialkyl disulfides, RSSR, are not adsorbed in the same way on gold and its oxide. When this adsorption occurs, an Au-S bond forms spontaneously, where the sulfur atom exhibits and defines the chemical characteristics of said surface depending on the functional group to which it is attached.
RSSRs cannot be adsorbed on Au 2 O 3, but they can on metallic gold. Therefore, if the surface of the gold and its degree of oxidation are modified, as well as the size of the particles or layers of Au 2 O 3, a more heterogeneous surface can be designed.
This Au 2 O 3 -AuSR surface interacts with the metal oxides of certain electronic devices, thus developing future smarter surfaces.
References
- Wikipedia. (2018). Gold (III) oxide. Recovered from: en.wikipedia.org
- Chemical Formulation. (2018). Gold (III) oxide. Recovered from: formulacionquimica.com
- D. Michaud. (2016, October 24). Gold rust. 911 Metallurgist. Recovered from: 911metallurgist.com
- Shi, R. Asahi, and C. Stampfl. (2007). Properties of the gold oxides Au 2 O 3 and Au 2 O: First-principles investigation. The American Physical Society.
- Cook, Kevin M. (2013). Gold Oxide as a Masking Layer for Regioselective Surface Chemistry. Theses and Dissertations. Paper 1460.