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Gold(III) oxide
Gold(III) oxide
Gold(III) oxide
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Gold(III) oxide
Names
IUPAC name
Gold(III) oxide
Other names
Gold trioxide, Gold sesquioxide, Auric oxide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.013.748 Edit this at Wikidata
EC Number
  • 215-122-1
UNII
  • InChI=1S/2Au.3O/q2*+3;3*-2
    Key: DDYSHSNGZNCTKB-UHFFFAOYSA-N
  • [O-2].[O-2].[O-2].[Au+3].[Au+3]
Properties
Au2O3
Molar mass 441.93
Appearance red-brown solid
Density 11.34 g/cm3 at 20 °C[1]
Melting point 298 °C (568 °F; 571 K)[2]
insoluble in water, soluble in hydrochloric and nitric acid
Structure
Orthorhombic, oF40
= Fdd2, No. 43[1]
Hazards
GHS labelling:[1]
GHS07: Exclamation mark
Warning
H315, H319
P264, P264+P265, P280, P302+P352, P305+P351+P338, P321, P332+P317, P337+P317, P362+P364
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
0
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Gold(III) oxide (Au2O3) is an inorganic compound of gold and oxygen with the formula Au2O3. It is a red-brown solid that decomposes at 298 °C.[3]

According to X-ray crystallography, Au2O3 features square planar gold centers with both 2- and 3-coordinated oxides. The four Au-O bond distances range from 193 to 207 picometers.[1] The crystals can be prepared by heating amorphous hydrated gold(III) oxide with perchloric acid and an alkali metal perchlorate in a sealed quartz tube at a temperature of around 250 °C and a pressure of around 30 MPa.[4]

References

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Gold(III) oxide

View on Grokipedia
from Grokipedia
Gold(III) oxide is an inorganic compound with the chemical formula Au₂O₃, appearing as a red-brown solid that is insoluble in water and decomposes upon heating to yield metallic gold and oxygen gas. This compound exhibits an orthorhombic crystal structure in the space group Fdd2, featuring gold atoms in a square planar coordination geometry with oxygen, and a density of 11.34 g/cm³. Its molar mass is 441.93 g/mol, and it is a semiconductor with a band gap of approximately 0.85 eV (from DFT calculations), more stable than lower oxides like Au₂O. Au₂O₃ is thermodynamically stable but decomposes at temperatures around 160–298 °C, depending on conditions, and can be formed under high-pressure hydrothermal environments or via oxidation processes. Anhydrous Au₂O₃ is typically synthesized by heating hydrated forms (Au₂O₃·xH₂O) in with perchlorates at 250 °C and 30 MPa in sealed vessels (see Synthesis section for details). In terms of applications, gold(III) oxide serves as a precursor for halogen-free synthesis of gold nanoparticles by reduction in solvents like , enabling non-toxic production routes. It is also used in and materials applications, such as creating gold/silica nanocomposites and thin films via techniques like deposition.

Properties

Physical properties

Gold(III) oxide appears as a red-brown or brownish-black solid powder. Its molar mass is 441.93 g/mol. The compound has a density of 11.34 g/cm³ at 20 °C. Gold(III) oxide does not melt but instead decomposes at temperatures around 250–298 °C, releasing oxygen gas and forming metallic gold. The compound is insoluble in water but dissolves in concentrated solutions of hydrochloric acid, nitric acid, and sodium cyanide. Gold(III) oxide adopts the orthorhombic crystal system with space group Fdd2 (No. 43).

Chemical properties

Gold(III) oxide, with the Au₂O₃, represents the of in which the metal exhibits the . This compound is recognized as the most stable of relative to other stoichiometries such as Au₂O, owing to its lower formation energy as determined by first-principles calculations. It decomposes upon heating into elemental and oxygen gas, highlighting its thermal stability under ambient conditions. The formation of Au₂O₃ from its elements is exothermic, with a (ΔH_f°) of -13.0 ± 2.4 kJ/mol, which renders the reverse decomposition process endothermic and contributes to the compound's inherent thermodynamic instability at elevated temperatures. Despite this, Au₂O₃ demonstrates amphoteric behavior, dissolving in strong acids such as concentrated nitric or to yield gold(III) salts like auric nitrate or sulfate. It also reacts with strong bases to form aurate complexes, underscoring its dual acid-base reactivity typical of oxides of post-transition metals. Hydrated forms of gold(III) oxide exhibit sensitivity to light and moisture, undergoing gradual decomposition over time in the presence of these factors, which can accelerate the release of oxygen and reversion to metallic . Furthermore, the compound is diamagnetic, attributable to the paired electrons in the low-spin d⁸ electronic configuration of Au(III) ions.

Structure

Crystal structure

Gold(III) oxide, Au₂O₃, crystallizes in the with Fdd2 (No. 43). The structure consists of a three-dimensional network of AuO₄ units, where each atom is coordinated to four oxygen atoms in a rectangular see-saw , a distorted form of square planar coordination typical for Au(III). This arrangement features two types of oxygen atoms: one in a water-like coordination to two Au atoms and the other in trigonal non-coplanar coordination to three Au atoms. Density functional theory calculations yield conventional unit cell parameters of a = 3.91 , b = 10.54 , and c = 12.88 , with Au–O bond lengths spanning 1.96–2.07 (196–207 pm). Experimental X-ray diffraction data confirm the orthorhombic Fdd2 symmetry and the AuO₄ coordination polyhedra with Au–O distances in the range of 193–207 pm, though precise lattice parameters vary slightly across studies due to . The contains 40 atoms (16 Au and 24 O), forming a face-centered orthorhombic lattice with Z = 8 formula units. The anhydrous form of Au₂O₃ represents the primary stable polymorphic phase under ambient conditions, with no other binary gold(III) oxide polymorphs reported as thermodynamically favored. In comparison to the lower oxide Au₂O, which adopts a cubic cuprite structure and is metastable with a positive heat of formation, Au₂O₃ exhibits greater thermodynamic stability, possessing a negative heat of formation of approximately -0.52 eV per formula unit. This enhanced stability arises from the higher oxidation state of gold and the more robust Au–O bonding network in the orthorhombic phase.

Bonding and electronic properties

Gold(III) oxide, Au₂O₃, exhibits a around the Au(III) centers that approximates square planar arrangement, consistent with the d⁸ electronic configuration of Au(III) and the stabilizing effects of , which favor low-spin square planar coordination for such systems to minimize electron-electron repulsion in the t₂g orbitals. This geometry arises from the where each Au atom is bonded to four oxygen atoms, though distortions lead to a rectangular see-saw-like form in the orthorhombic lattice. The Au-O bonds in Au₂O₃ possess significant covalent character, driven by strong hybridization between the Au 5d orbitals and O 2p orbitals, alongside a partial ionic contribution from the formal Au³⁺ and O²⁻ ions. This mixed bonding nature results in relatively short Au-O bond distances, ranging from 1.93 to 2.07 , which reflect the directional overlap of the filled Au 5d orbitals with oxygen lone pairs, enhancing orbital mixing and bond strength compared to purely ionic analogs. Electronic structure calculations using (DFT) within the generalized gradient approximation reveal that Au₂O₃ is a with a direct of approximately 0.85 eV, arising from the valence band dominated by Au 5d and O 2p states and the conduction band from Au 6s/6p contributions. These computations also demonstrate the thermodynamic stability of Au₂O₃ over the monoxide Au₂O, with a calculated of formation of -0.519 eV per for Au₂O₃ versus +0.228 eV for Au₂O, attributing the preference to greater 5d-2p hybridization in the trivalent oxide. Infrared spectroscopy provides insights into the vibrational properties, with the Au-O stretching mode appearing as a strong absorption band around 646 cm⁻¹, indicative of the covalent bonding and consistent with the predicted highest-frequency phonon mode at 581 cm⁻¹ from DFT.

Synthesis

Early preparation methods

Gold(III) oxide was first prepared in the mid-19th century through precipitation from solutions of gold(III) chloride using alkalis, yielding a brown solid that could be dried at around 100 °C to obtain the anhydrous compound. A standard early laboratory method involved the precipitation of auric hydroxide, Au(OH)₃, from solutions of gold(III) salts using bases, followed by careful drying at low temperatures to obtain Au₂O₃ as a dark powder, avoiding decomposition. Early literature highlighted significant challenges in these preparations, including frequent contamination with lower oxides such as Au₂O, which arose from incomplete oxidation or reduction during heating and handling. A key historical contribution came from Gerhard Krüss in 1887, who through detailed investigations confirmed Au₂O₃ as the stable higher oxide of gold, distinguishing it from previously misidentified forms and establishing its composition via analytical methods. Decomposition of Au₂O₃ was noted early on as a reliable route to pure gold metal, with heating above 250 °C leading to quantitative reduction to elemental and oxygen gas release.

Contemporary synthesis routes

Contemporary synthesis routes for gold(III) oxide (Au₂O₃) focus on achieving high purity and controlled morphology through advanced pressure-assisted and oxidative techniques, often yielding materials suitable for catalytic and materials applications. These methods contrast with earlier approaches by incorporating modern equipment like sealed high-pressure vessels and plasma systems to minimize impurities and enhance scalability. A key laboratory method involves the hydrothermal dehydration of amorphous hydrated Au₂O₃ in the presence of and an . The mixture is heated at 250 °C under 30 MPa pressure in sealed tubes, promoting the formation of anhydrous, crystalline Au₂O₃ with improved structural integrity. Oxidative routes using atomic oxygen or plasma have gained prominence for preparing thin films or nanoparticle-supported Au₂O₃. Exposure of metal surfaces or AuCl₃ to atomic oxygen at low temperatures, such as 150 K, facilitates the formation of stable surface and subsurface Au₂O₃ layers without requiring high thermal input. Similarly, oxygen-dc plasma treatment of films generates Au₂O₃ through direct oxidation, allowing precise control over oxide thickness and composition. Recent plasma-assisted, solvent-free processes further enable ultra-fast synthesis of Au₂O₃ nanoparticles, demonstrating scalability for sensing platforms. Solvothermal approaches in aqueous media under , such as 300 atm, also yield pure Au₂O₃ powders from precursors, emphasizing the role of extreme conditions in stabilizing the phase.

Reactivity

Gold(III) undergoes according to the balanced equation 2Au2O3→4Au+3O22 \mathrm{Au_2O_3} \rightarrow 4 \mathrm{Au} + 3 \mathrm{O_2} This reaction initiates at temperatures between 160 °C and 298 °C, with complete to metallic and oxygen gas achieved by approximately 300 °C. The process reflects the compound's limited thermal stability, consistent with observations in its chemical properties. The decomposition mechanism proceeds via stepwise oxygen loss, where gold(III) oxide first forms the intermediate gold(I) oxide, Au₂O, before further reduction to elemental . Kinetic analyses of the step yield an activation energy of approximately 150 kJ/mol, varying slightly with the material's crystallinity (e.g., 165 kJ/mol for amorphous forms). A related gas-phase transport variant employs as a reductant at around 400 °C, generating short-lived volatile gold species that enable the deposition of thin films. Particle size significantly influences the decomposition onset, with smaller oxide nanoparticles exhibiting reduced stability and decomposing at lower temperatures, such as ~100 °C, due to higher . Historically, controlled of (III) oxide has served in purification, allowing selective removal of oxygen while recovering high-purity metal.

Acid-base reactions

Gold(III) oxide displays amphoteric properties, dissolving in strong acids to form corresponding gold(III) complexes while remaining stable in milder conditions. It reacts with concentrated to produce , as represented by the equation: Au2O3+8HCl→2HAuCl4+3H2O\mathrm{Au_2O_3 + 8 HCl \rightarrow 2 HAuCl_4 + 3 H_2O} This reaction highlights the oxide's ability to undergo and coordination with chloride ligands. Similarly, gold(III) oxide dissolves in concentrated , forming gold(III) nitrate via: Au2O3+6HNO3→2Au(NO3)3+3H2O\mathrm{Au_2O_3 + 6 HNO_3 \rightarrow 2 Au(NO_3)_3 + 3 H_2O} However, the oxide shows no reaction with dilute acids at room temperature, owing to its limited solubility under these conditions. In basic media, gold(III) oxide reacts with to form sodium aurate, demonstrating its basic character: Au2O3+2NaOH+H2O→2NaAuO2+2H2O\mathrm{Au_2O_3 + 2 NaOH + H_2O \rightarrow 2 NaAuO_2 + 2 H_2O} This process involves and formation of the aurate , [AuO₂]⁻ or equivalently [Au(OH)₄]⁻ in hydrated form. The of gold(III) oxide is -dependent, dissolving readily in acidic media at pH < 2 where aquahydroxogold(III) complexes like Au(OH)₂(H₂O)₂⁺ predominate, but remaining stable and precipitating in neutral conditions around pH 7. These acid-base interactions are leveraged in hydrometallurgical processes for recovery. For instance, after dissolution in HCl, gold(III) species can be selectively extracted from aqueous solutions using phosphine oxides such as Cyanex 925, facilitating efficient separation and purification.

Reduction processes

, Au₂O₃, undergoes reduction to metallic via reaction with according to the equation Au₂O₃ + 3CO → 2Au + 3CO₂, typically at temperatures around 400 °C. This process facilitates gas-phase transport of , proceeding through a short-lived volatile gold(I) oxide intermediate, Au₂O, which enables deposition of downstream. In organic media, serves as both reductant and stabilizer for Au₂O₃, yielding halogen-free gold nanoparticles under atmosphere at 130 °C for 24 hours or 180 °C for 15 minutes. The reaction follows the Au₂O₃ + 6RNH₂ → 2Au + byproducts (where R is the oleyl chain), producing uniform nanoparticles of 5–9 nm diameter with polydispersity index <1.5, as confirmed by UV–Vis spectroscopy showing a plasmonic peak at 526 nm. oxidizes to nitriles and amides, ensuring no contamination, which is advantageous for applications in bimetallic Au–Ag nanoparticles. Aqueous reductions employ agents like , NaBH₄, to convert Au₂O₃ to metallic , often in hydrated form (Au₂O₃·xH₂O). For instance, treatment with 0.1 M NaBH₄ in water at partially reduces Au₂O₃ encapsulated in silica, forming ultrasmall Au nanoparticles (1.5–2.3 nm) upon subsequent annealing at 200 °C. similarly reduces Au(III) species in aqueous media to Au(0), though direct studies on Au₂O₃ are limited; the process typically yields metallic precipitates or stabilized nanoparticles. Electrochemical reduction of Au₂O₃ to Au(0) occurs at potentials below 0.5 V versus the (SHE) in acidic media, following the half-reaction Au₂O₃ + 6H⁺ + 6e⁻ → 2Au + 3H₂O (E⁰ ≈ 1.36 V vs. SHE for the reverse oxidation). This process forms surface gold atoms or nanoparticles, with the oxide layer reducing selectively in scans. Partial reduction to Au(I) species is possible using mild reductants like ascorbic acid, which selectively lowers the without full conversion to Au(0), often forming intermediates like AuCl₂⁻ in chloride-containing solutions adaptable to oxide precursors. This stepwise approach controls morphology in synthesis. Many reductions of Au₂O₃ exhibit kinetics with respect to reductant concentration, as seen in initial steps of Au(III) to Au(0) conversions monitored by . These kinetics facilitate precise control over reaction rates and product sizes in production.

Applications

Catalytic uses

Gold(III) oxide, particularly when supported on metal oxides such as γ-Al₂O₃, promotes the oxidation of CO to CO₂ at low temperatures below 100 °C. In these systems, the initial Au(III) species contribute to the catalyst's activity, with activation occurring through partial reduction under reaction conditions involving CO, O₂, and trace water, leading to high CO conversion rates even at . Supported forms of Au₂O₃ on TiO₂ or Al₂O₃ enhance catalytic activity for CO oxidation due to the perimeter interfaces between the gold oxide and the support, which facilitate O₂ activation and CO adsorption at the boundary sites. Deactivation mechanisms include of gold species at high temperatures, resulting in larger particles and diminished availability. Au₂O₃ functions as a catalyst with high selectivity in the water-gas shift process, where it facilitates CO insertion into Au–OH bonds. It is also used in oxygen activation for electrochemical .

Materials and nanotechnology applications

Gold(III) oxide serves as a valuable precursor in the synthesis of gold nanoparticles, where its reduction in neat produces monodisperse, spherical Au nanoparticles with narrow bands, enabling applications in plasmonics and chemical sensing. This halogen-free method avoids contaminants, resulting in high-purity nanoparticles approximately 5-10 nm in diameter, which exhibit strong resonance for optical detection of biomolecules. In processes, gold(III) oxide is dissolved in acidic solutions and reacted with to form stable gold(III) complexes, such as cyanoauric acid, which are employed in acid baths for depositing thin, adherent layers on electronic components like connectors and circuit boards. These baths operate under controlled conditions below 6 to ensure efficient complexation and uniform deposition, enhancing conductivity and corrosion resistance in . Gold(III) oxide is suitable for applications in ceramics and due to its stability. Thin films of metallic are fabricated using via the Au₂O₃-CO transport reaction, where gold(III) oxide reacts with at around 400 °C to form transient gold(I) carbonyl species that deposit as coherent on substrates for optical coatings. These provide high reflectivity and are applied in mirrors and anti-reflective layers due to gold's plasmonic properties.

Safety and toxicology

Health and environmental hazards

Gold(III) oxide is classified under the Globally Harmonized System (GHS) as a skin irritant (H315) and a serious eye irritant (H319), with potential for respiratory irritation (H335) from dust inhalation. These classifications stem from its ability to cause local irritation upon contact or exposure, though it exhibits low systemic , with an oral LD50 exceeding 2000 mg/kg in rats. While the compound generally shows low overall toxicity, individuals sensitive to gold may experience from exposure. Chronic health effects from gold(III) oxide are rare, but inhalation of its dust over extended periods may contribute to -like respiratory conditions due to particulate accumulation in the lungs. Environmentally, gold(III) oxide demonstrates low mobility in attributable to its insolubility, limiting widespread dispersion. In aquatic systems, the released Au³⁺ ions can bioaccumulate in organisms, posing risks through trophic transfer. Ecotoxicity assessments indicate an LC50 greater than 100 mg/L for , with minimal additional impact from oxygen liberation during . Overall, its environmental persistence is low due to insolubility, but careful management is advised to prevent localized ion release.

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