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Published April 8, 1999 | public
Journal Article

Photon Energy and Photon Intermittence Effects on the Quantum Efficiency of Photoinduced Oxidations in Crystalline and Metastable TiO_2 Colloidal Nanoparticles


We report quantum yields φ-NP for the photocatalytic oxidation of 3-nitrophenol (NP) in clear, aerated, aqueous colloids of crystalline or metastable TiO2 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.

Additional Information

© 1999 American Chemical Society. Received: July 10, 1998; In Final Form: January 30, 1999. This work was financially supported by the National Research Council of Argentina (CONICET). Ignacio Gil Torró (University of Valencia, Spain) collaborated with this project as a fellow of the Instituto de Cooperación Iberoamericano.

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