Photon energy and photon intermittence effects on the quantum efficiency of photoinduced oxidations in crystalline and metastable TiO2 colloidal nanoparticles

We report quantum yields phi-(NP) for the photocatalytic oxidation of 3-nit rophenol (NP) in clear, aerated, aqueous colloids of crystalline or metasta ble TiO2 nanoparticles as a function of photon wavelength (254 less than or equal to lambda/nm less than or equal to 366) and photon absorption interm ittence I-ap (0.002 less than or equal to I-ap/photons particle(-1) s(-1) l ess than or equal to 2). phi-(NP)'s vary as phi-(NP) proportional to I-ap(- 0.21+/-0.05) at all lambda's in metastable TiO2 sols and are similar to 20 times smaller;than the I-ap-independent phi-(NP)'s determined in crystallin e suspensions. Higher energy photons are always more efficient. We infer th at (1) hyperthermal holes are able to capture electrons from NP while being deactivated in both types of particles, (2) thermalized electrons and hole s are trapped in metastable particles within nanoseconds and persist as suc h for minutes, and (3) shallower traps become populated at larger I-ap's. T he similar action spectra of phi-(NP) and phi(-s) for nonchelating NP and b identate salicylate (S) [E-0(NP/NP+.) approximate to E-0(S-/S-.) approximat e to 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 ou ter-sphere interfacial redox reactions. Our data support k(SC,max) greater than or equal to 6 x 10(5) cm s(-1) for h + NP --> NP+, which is close to t he adiabatic coupling limit.