Tungsten Trioxide
Properties
| State | Solid |
| Color | Lemon yellow (room temperature); changes with polymorph |
| Solubility | Insoluble in water; soluble in alkaline solutions (forming tungstates) |
| Melting Point | 1473 °C |
| Boiling Point | Approximately 1700 °C (sublimes) |
About Tungsten Trioxide
Tungsten trioxide is a lemon-yellow inorganic solid whose color, crystal structure, and electrochemistry all trace back to its d^0 W(VI) center. The bandgap of around 2.6 eV puts the absorption edge in the violet-blue, leaving the yellow we see, and makes WO3 a wide-gap n-type semiconductor whose conduction band can be tuned by oxygen vacancies and small-cation intercalation. Structurally it is a distorted ReO3 framework — corner-sharing WO6 octahedra in three dimensions — with a cascade of polymorph transitions: monoclinic gamma at room temperature, orthorhombic at 320 °C, tetragonal beta at 720 °C, and a low-temperature triclinic form below 17 °C. The same corner-sharing framework that defines the structure also provides the open channels that make WO3 the canonical electrochromic material. Sputter-deposit a 300 nm WO3 film between an ITO front electrode and a Li+-conducting electrolyte, apply -2 V, and electrons plus Li+ insert to form Li_x WO3 — the W(V) centers create an intervalence charge-transfer absorption that turns the film deep blue. SageGlass dynamic windows in airports and hospitals run on this exact mechanism. WO3 is also the chemistry feedstock for tungsten metal: H2 reduction at 800 to 1100 °C strips the oxygen and yields the tungsten powder that gets pressed and sintered into wire, plate, and heavy-alloy stock.
Where you'll encounter it
If you have ever specified an electrochromic dynamic window for a high-end office build or watched the windows on the new Boeing 787 transition from clear to dark blue at the press of a button, you have used WO3. The Dreamliner windows specifically use a 5-layer thin-film stack with WO3 as the cathodic electrochromic layer and NiO as the complementary anodic layer, switching in 60 to 90 seconds with a few volts of drive and holding the state with no current. In gas sensing, WO3 thin films heated to 200 to 400 °C show resistance changes of 10x or more in response to ppm-level NO2 or H2S, exploited in fixed-installation industrial gas detectors and portable air-quality monitors. On the photocatalysis side, WO3 absorbs visible light (unlike TiO2, which needs UV) and is being studied for solar water splitting and degradation of dye-house wastewater. Walk through a tungsten powder plant in Xiamen and the lemon-yellow WO3 piles upstream of the H2 reduction furnaces are unmistakable.
Common Uses
- Cathodic electrochromic layer in dynamic smart-window systems for buildings and aircraft
- H2-reduction feedstock for tungsten metal powder used in wire, plate, and heavy-alloy production
- Sensing layer in chemiresistive gas detectors for NO2, H2S, NH3, and ozone
- Visible-light photocatalyst for water splitting research and dye-bath wastewater treatment
- Yellow ceramic pigment for high-temperature glazes resistant to firing-cycle reduction
- Catalyst component in selective catalytic reduction of NOx in stationary diesel exhaust streams
- Charge-storage layer in research electrochemical capacitors and pseudocapacitor electrodes
- Optical coating for ultraviolet-blocking and selective-transmission filter stacks
Safety Information
GHS classification: Specific Target Organ Toxicity Repeated Exposure Category 2 for the respiratory tract. OSHA PEL for soluble tungsten compounds is 1 mg/m^3 as an 8-hour TWA, and 5 mg/m^3 for insoluble compounds; the ACGIH TLV is 1 mg/m^3. Chronic inhalation of WO3 dust is associated with reactive airways and pulmonary fibrosis in tungsten powder plant workers. Standard controls are local exhaust ventilation at all transfer and screening points, P100 respirators for cleanout and bag-off operations, and routine spirometry surveillance for chronic exposures above the action level. The compound is not a carcinogen by IARC, not skin-sensitizing, and not classified as flammable. Aqueous tungstate solutions formed when WO3 is dissolved in alkali are mildly basic and not significantly more hazardous than the parent solid.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.