Metal-Oxide Nanoparticle Photocatalysis: UV Band-Gap Excitation

Light Source

Wavelength λ 350 nm

Photon energy 3.54 eV

Light intensity 70%

Pollutant molecules 18

Live Readouts

Band gap

3.20 eV

Cutoff λ

387 nm

Photons absorbed

e⁻/h⁺ pairs

Recombined

•OH radicals

Pollutant left

Degraded

TiO₂ nanoparticle

Electron (e⁻ → CB)

Hole (h⁺ → VB)

Hydroxyl radical •OH

Pollutant molecule

How it works

A photon only excites an electron across a semiconductor's band gap if its energy meets or exceeds that gap. For anatase TiO₂ the gap is ≈3.2 eV, which by E = hc/λ (1240 eV·nm / λ) corresponds to a hard cutoff at λ ≈ 387 nm — visible light (violet and longer) simply passes through unabsorbed, only UV does the work. That threshold is exactly what the wavelength slider crosses: push λ above 387 nm and the top band diagram keeps showing photons streaming past the nanoparticle with nothing happening.

E_photon = 1240 / λ(nm)  eV
absorbed ⇔ E_photon ≥ E_gap (3.20 eV)
h⁺ + OH⁻(surface) → •OH
e⁻ + O₂(surface)  → O₂•⁻
•OH + pollutant → CO₂ + H₂O (mineralized)

Wavelength — sets photon energy; only sub-387 nm (UV) photons are absorbed and create carriers.
Light intensity — how many photons per second arrive at the nanoparticle.
Pollutant molecules — organic molecules dissolved around the particle that hydroxyl radicals attack and mineralize into CO₂ + H₂O.
Electrons (cyan) and holes (red) are generated in pairs inside the particle and random-walk toward the surface; if they meet first, they recombine and do no chemistry (a real loss pathway that limits quantum yield). A hole that reaches the surface first can generate a hydroxyl radical, which then drifts out to attack a pollutant molecule.

Nanoparticle e⁻ h⁺ •OH pollutant