Laser-induced reactions on 2.4-nm colloidal TiO2 particles in aqueous solution: A study by time-resolved optoacoustic calorimetry

Laser-pulse induced optoacoustic calorimetry (LIOAC) was applied to study d issipation as heat and temporary storage of light energy absorbed by TiO2 p articles (2.4 nm mean diameter) in aerated aqueous solution at pH 1-2.6 (HC lO4). The TiO2 particles (3 g L-1, 47 mu M particle concentration) were exc ited by 6-ns laser pulses at 349 nm which produced ca. one e(-)/h(+)-pair p er particle during each pulse. Light scattering was weak and was taken into account in the determination of absorbed photon energy (50-60 mu J per pul se). The LIOAC signal of the sample measured in a time window of 2 mu s was analyzed with respect to that of a Na2CrO4 solution used as a calorimetric reference. Deconvolution of the sample signal disclosed only one prompt he at dissipation process. From LIOAC at 8-18 degrees C the fraction of prompt heat was (88 +/- 2)%, while (12 +/- 2)% of the al,sorbed molar photon ener gy was stored for at least 2 mu s. The finding is attributed to fast format ion of energy-storing species in competition with radiationless recombinati on of e(-)/h(+)-pairs. The energy balance is discussed in detail. Formation of trapped charge carriers (P1), oxidation of residual chloride (P2) and f ormation of quasi-free OH(aq). and HO2(aq). radicals (P3) all persisting in to the mu s-regime are considered as energy-storing processes. Energy balan ce analysis shows that P3 is the main energy-storing reaction in the system studied here. The instantaneous quantum yield of OH(aq). is estimated as ( 14 +/- 3)%. Strictu sensu it cannot be excluded that processes different fr om, however energetically similar to P3 effect temporary energy storage und er the present conditions, since the nature of the species cannot be inferr ed from LIOAC. Implications of the findings are discussed.