Fundamental reactions in illuminated titanium dioxide nanocrystallite layers studied by pulsed laser

Titanium dioxide layers, composed of 5 nm diameter closely packed nanocrystallites prepared by spin coating of concentrated TiO2 sols (titanium isopropoxide hydrolysis), were exposed to pulsed laser photolysis, in the presence as well as in the absence of added reactants. Time profiles in the range 390-700 nm have been studied in the nanosecond time range. TiO2 layers immersed in liquids (acidic or alkaline water, CCl4, CCl4/CBr4 mixture, cyclohexane) show the same absorption vs time profiles as the dry layers. Iodide ions (0.5-7.6 M in water) convert the holes to I-2(-) within less than 10 ns (quantum yield approaching unity is observed at the highest concentration). The absorption of I-2(-) (peaking at 390 nm) is relatively stable during the first 4 mu s, in contrast to the decay of the electron absorption which is only slightly different than in iodide-free solutions. This result is unexpected if the decay of the electron absorption is because of electron-hole recombination. Alcohols (methanol and 2-propanol) at high concentrations unexpectedly reduce the initially observed electron absorption (time resolution 10 ns) by up to 4-fold, without affecting the shape of the nanosecond time profile. The alcohol effect is assigned to formation of an alcoholic positive ion radical which is more reactive in recombination with conduction band electrons than the original hole. The electron scavenger H2O2 reduces the initial electron absorption without affecting the shape of the nanosecond time profile. It is concluded that (a) the decay of the visible absorption in the nanosecond time range is largely because of gradual electron trapping, with only a partial contribution of electron-hole recombination; (b) reactions with scavengers are important in the femtosecond-picosecond time range (reactions of h(vb)(+) and e(cb)(-)) and in the microseconds or longer time (reactions of the respective trapped species), but the absorbance changes in the nanosecond time range are not affected by scavengers; (c) even in the absence of hole scavengers, trapping of the electron competes successfully with recombination when no more than one electron-hole pair is produced in a nanocrystallite. Most electrons still exist after several microseconds.