Splitting water with light: TiO₂ photocatalysis
Illuminate a TiO₂ film in water with UV and watch electrons and holes make H₂ and O₂ — artificial photosynthesis.
Goal
Trace the photon → e⁻–h⁺ pair → H₂/O₂ chain and evaluate why TiO₂ needs UV rather than visible light.
Apparatus and reagents
TiO₂ (rutile or anatase) film or powder, water with a little NaOH or electrolyte, UV lamp (~365 nm), gas-collection tubes for H₂ and O₂, protective eyewear rated for UV.
Procedure
- Start with the lamp off and confirm that no gas evolves on TiO₂ in the dark.
- Switch on the UV lamp and follow the e⁻–h⁺ pairs toward the reduction (H₂) and oxidation (O₂) sites.
- Compare the bubble volumes collected: expect roughly twice as much H₂ as O₂.
- Discuss which photons work: TiO₂ anatase has Eg ≈ 3.2 eV, so only λ ≲ 390 nm is absorbed.
What to observe
- Gas only evolves under UV: in the dark the semiconductor stays inert because no e⁻–h⁺ pairs exist.
- The H₂ volume is about twice the O₂ volume, matching the 2:1 stoichiometry of 2 H₂O → 2 H₂ + O₂.
Explanation
A photon with hν ≥ Eg promotes an electron to the conduction band, leaving a hole in the valence band. Conduction-band electrons reduce H⁺ to H₂ (2 H⁺ + 2 e⁻ → H₂) while valence-band holes oxidise water to O₂ (4 h⁺ + 2 H₂O → O₂ + 4 H⁺). Water splitting needs ΔG = +237 kJ/mol (≈1.23 V per electron), so the catalyst’s band edges must straddle the H⁺/H₂ and O₂/H₂O redox levels — TiO₂ does, but only under UV, which is why visible-light photocatalysts remain an active research frontier.
History of the experiment
Chemists behind it
Related topics
Virtual experiment: a simplified model to build intuition. It does not replace real lab work or safety training; never repeat chemistry at home without supervision.