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

摘要

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 less than or equal λm less than or equal 366) and photon absorption intermittence I_(ap) (0.002 less than or equal Iap/photons particle~(-1) s~(-1) less than or equal 2). φ-NP's vary as φ_(-NP) varies directly as I_(ap)-0.21±0.05 at all λ's in metastable TiO_2 sols and are approximately 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 Iap's. The similar action spectra of φ_(-NP) and φ_(-S) for nonchelating NP and bidentate salicylate (S) [E_0(NP/NP~(+·)) approximately equals E_(0(S-/S·)) approximately equals 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 kSC,max greater than or equal 6×10~5 cm s~(-1) for h~+NP->NP~+, which is close to the adiabatic coupling limit.