Photon Energy and Photon Intermittence Effects on the Quantum Efficiency of Photoinduced Oxidations in Crystalline and Metastable TiO₂Colloidal Nanoparticles

We report quantum yields φ - NP for the photocatalytic oxidation of 3-nitrophenol (NP) in clear, aerated, aqueous colloids of crystalline or metastable TiO 2 nanoparticles as a function of photon wavelength (254 ≤ λ/nm ≤ 366) and photon absorption intermittence I ap (0.002 ≤ I ap /photons particle -1 s -1 ≤ 2). φ - NP 's vary as φ - NP ∝ I ap -0.21±0.05 at all λ's in metastable TiO 2 sols and are ∼20 times smaller than the I ap -independent φ - NP 's determined in crystalline suspensions. Higher energy photons are always more efficient. We infer that (1) hyperthermal holes are able to capture electrons from NP while being deactivated in both types of particles, (2) thermalized electrons and holes are trapped in metastable particles within nanoseconds and persist as such for minutes, and (3) shallower traps become populated at larger I ap 's. The similar action spectra of φ - NP and φ - S for nonchelating NP and bidentate salicylate (S) [ E 0 (NP/NP +• ) ≈ E 0 (S - /S • ) ≈ 2.8 V vs NHE], in the presence or absence of phosphate as a competing ligand, are evidence that hot carrier effects are indeed associated with outer-sphere interfacial redox reactions. Our data support k SC,max ≥ 6 × 10 5 cm s -1 for h + NP → NP .+, which is close to the adiabatic coupling limit.