Cadmium Sulfide
Properties
| State | Solid (crystalline) |
| Color | Yellow to orange |
| Solubility | Insoluble in water; soluble in concentrated acids |
| Melting Point | 1750 °C (sublimes) |
| Boiling Point | Sublimes around 980 °C |
About Cadmium Sulfide
Cadmium sulfide is the canonical II–VI direct-bandgap semiconductor and the molecule that taught a generation of physical chemists how to think about quantum confinement. The bandgap is 2.42 eV at 300 K — sharp absorption edge near 515 nm — which means CdS reflects orange and red wavelengths and absorbs everything bluer, hence the saturated yellow-orange color that gave "cadmium yellow" its name in the 1840s. Two crystalline polymorphs matter: the hexagonal wurtzite form (the natural mineral greenockite, the thermodynamic product at high temperature) and the cubic zinc-blende form (hawleyite), both with tetrahedral coordination of Cd and S. When you grind CdS down to particles below ~6 nm — the bulk exciton Bohr radius — the bandgap blueshifts continuously as the electron and hole are squeezed into a smaller box, and the photoluminescence color tunes from red through green to violet with decreasing size. That size-tunable emission, first measured cleanly on CdS and CdSe colloidal nanocrystals by Bawendi, Brus, and Ekimov, is what the 2023 Nobel Prize in Chemistry recognized. Industrially, the most important use is now thin-film photovoltaics: a 50–100 nm CdS layer deposited by chemical bath deposition serves as the n-type window of CdTe solar cells (the absorber layer is CdTe; CdS sits on top, transparent to most of the solar spectrum the absorber needs). Historically, CdS was the principal yellow pigment in late-19th-century oil painting — Van Gogh's sunflowers, Monet's water lilies, Matisse's interiors — and conservators now track its slow oxidation to CdSO4 and CdCO3 as one of the great pigment-degradation puzzles in art history.
Where you'll encounter it
If you've run a hot-injection colloidal synthesis with cadmium oleate and TOP-S in octadecene, the orange-to-red dispersions that come out are CdS quantum dots, often used as a teaching reaction in nanocrystal labs because the size-tunable color is dramatically visible under a UV lamp. In a thin-film PV process, chemical bath deposition of CdS — thiourea + cadmium acetate + ammonia at 70 °C — is the standard way to put down the n-window layer on First Solar's CdTe modules. In a paintings-conservation studio, the slow whitening or fading of cadmium-yellow areas in 1880s–1920s oils is now imaged by µ-XRF and XANES to map where the original CdS has converted to CdSO4 hydrate.
Common Uses
- n-type window layer in CdTe thin-film photovoltaic modules
- Colloidal quantum dots for size-tunable luminescence in displays and bioimaging
- Historical artist pigment (cadmium yellow) in late-19th-century oil paintings
- CdS photoresistors and visible-light photodetectors
- Phosphors in cathode-ray and electroluminescent displays
Safety Information
Toxic, mutagenic, and carcinogenic. GHS H301+H331 (toxic if swallowed and if inhaled), H340 (mutagenic), H350 (IARC Group 1 carcinogen, cadmium and compounds), H361 (suspected reproductive toxicity), H410 (very toxic to aquatic life). OSHA PEL for cadmium dust 5 µg/m³ as Cd; ACGIH TLV 10 µg/m³ TWA. CdS is much less bioavailable than CdCl2 or CdSO4 (the Ksp of CdS is about 10⁻²⁸, so dissolution in physiological fluids is minimal), but inhaled fines and acid digestion in the stomach still release Cd²⁺. EU REACH SVHC; restricted under RoHS Directive 2011/65/EU. Handle in a fume hood; dispose as RCRA D006.
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.