In heterogeneous photocatalysis, titanium dioxide (TiO₂) is used as a photocatalyst to degrade organic pollutants under ultraviolet light. When exposed to UV-A light (wavelength ≈ 365 nm), TiO₂ absorbs photons, exciting electrons from the valence band to the conduction band, generating electron-hole pairs. These charge carriers initiate redox reactions at the surface:

• The conduction band electrons (e–) reduce O₂ to superoxide radicals (O₂•⁻)
• The valence band holes (h⁺) oxidize water to hydroxyl radicals (•OH)

These reactive species attack organic pollutants, breaking them down into carbon dioxide, water, and harmless byproducts.

Experimental data from a 2021 study show that under constant UV illumination (180 W/m²) and excess oxygen, the degradation of methylene blue (MB) follows pseudo-first-order kinetics with respect to pollutant concentration:

$\ln\left(\frac{[MB]_0}{[MB]_t}\right) = kt$

Where (k) is the rate constant and (t) is time in minutes.

In three trials, the experimenters tested:

1. TiO₂ film on glass (low surface area)
2. TiO₂ nanoparticles suspended in solution (high surface area)
3. TiO₂ nanoparticles + 0.1 M NaCl

Question 1

Which of the following best explains why the TiO₂ nanoparticle suspension achieved a significantly faster degradation rate than the film?

A. Increased thermal conductivity enhanced UV photon absorption

B. Greater surface area promoted more active sites for redox reactions

C. Nanoparticles scattered UV photons more uniformly

D. Water’s higher refractive index enhanced reaction kinetics