Photochemistry & Photocatalysis

Light Source

Wavelength 365 nm

Light intensity 1.0x

Catalyst (TiO2)

Nanoparticle density 14

Recombination rate 0.35

Pollutant load 18

Photons absorbed/s

–

Quantum yield

–

e⁻/h⁺ pairs live

–

Pollutant degraded

–

How it works

Photons (small bright particles) rain down from the light source. Each photon's energy depends only on its wavelength — the Grotthuss–Draper law says only absorbed light can drive a reaction, and only light energetic enough to be absorbed in the first place. TiO2 nanoparticles (grey spheres) have a bandgap around 3.2 eV (anatase, ≈387 nm); a photon that reaches a nanoparticle is absorbed only if its energy clears that gap, promoting an electron to the conduction band and leaving a hole behind (the excited state).

E_photon = 1240 / λ(nm)  eV
absorbed  ⇔  E_photon ≥ E_gap (≈3.2 eV)
e⁻/h⁺ pair → recombine (wasted, rate r)  or  → react with adsorbed pollutant (oxidize/reduce)

Wavelength — shift the light toward the UV (left) to clear the TiO2 bandgap more often, or into the visible (right) where most photons simply pass through unabsorbed.
Light intensity — how many photons arrive per second; more photons mean more absorption events, but only among those energetic enough.
Nanoparticle density — more catalyst surface area gives photons more chances to be captured before they exit the reaction volume.
Recombination rate — the fraction of excited electron–hole pairs that simply recombine and release the energy as heat instead of doing chemistry; a lower rate means a higher effective quantum yield.
Pollutant load — dye/pollutant molecules (small colored spheres) drifting near the catalyst; a surviving electron–hole pair that finds one degrades it a little at a time until it vanishes, mimicking advanced oxidation by hydroxyl radicals.

Real-world relevance: this is the mechanism behind self-cleaning coatings, TiO2 water-purification reactors and photocatalytic air scrubbers — sunlight or UV lamps drive the same excitation/recombination/redox competition at industrial scale.

Photon TiO2 nanoparticle Excited electron Hole Pollutant molecule

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