2.1 Instrumentation and materials

Time-resolved transient transmission measurements were performed using the third harmonic (355 nm, 5 ns full width at half-maximum) from a Q-switched Nd:YAG laser (Quanta-Ray, GCR-150). The transient signals were recorded with LKS 60 laser photolysis equipment. Steady-state UV–vis absorption was measured with a UV–vis spectrophotometer (Shimadzu, UV-1700). TiO₂ powder suspension was sonicated in an ultrasonic cleanser (Branson, 3510).

TiO₂ powder (P25, Degussa), pyrogallol (C₆H₃(OH)₃, HPLC grade) and high-purity nitrogen were used during the experiments.

2.2 Sample preparation

TiO₂ powder was suspended in 500 mL deionized water by sonicating for 10 min. The pH value of this suspension was 5.5. Through steady-state absorption spectra scanning, a TiO₂ concentration with an ideal extinction value at 355 nm was selected as the subsequent experimental TiO₂ concentration.

Concentrated pyrogallol solution was added to the TiO₂ suspension and stirred to form a uniform mixture. The desired volume of the mixture and concentrations of pyrogallol and TiO₂ were realized by adding suitable quantities of deionized water. The mixture was poured into a quartz cuvet, which was transparent on all sides, and the cuvet was placed inside the flash photolysis apparatus to produce a transmission system for the subsequent experiment. All procedures for the sample preparation were performed with shielding from UV light.

3.1 Steady-state absorption spectra

Fig. 1 shows the UV–vis scanning spectra of 10, 20 and 25 mg/L TiO₂ suspension. It was found that the absorbance over wavelengths of 200–600 nm had a very good linear correlation with the TiO₂ concentration. For example, at a wavelength of 355 nm, the linear regression equation for the absorbance and concentration of TiO₂ suspension was: A = 0.0327C + 0.0376 (R² = 0.9981). The TiO₂ concentration of 20 mg/L was selected as the subsequent experimental TiO₂ concentration as it had the most ideal extinction value at 355 nm, namely the absorbance equaled 0.680, which was neither very high nor very low. After the suspension was left for 1 h, the absorption spectra of TiO₂ suspension was measured again and showed scarcely any difference compared to the original spectra, which indicated that the TiO₂ suspension was homogeneous and stable.

Different amounts of pyrogallol concentrated solution were added to 20 mg/L TiO₂ suspension to give different concentrations of pyrogallol and TiO₂ mixed solutions. An absorption spectra measurement (Fig. 2) showed no influence on the absorbance at 355 nm compared to that of pure TiO₂ suspension. That is, pyrogallol had no absorbance at 355 nm and it had no direct photolysis at this wavelength. This guaranteed that the function of TiO₂ would be recognized if there was any photolysis of pyrogallol during the following experiment.

3.2 Electron–hole generation and recombination

Fig. 3 depicts the transient absorption spectra observed for the 50 mg/L pyrogallol + 20 mg/L TiO₂ mixture after 80 ns, 240 ns and 15 μs of laser excitation. The spectra recorded immediately after laser excitation (Δt = 80 ns) shows peak absorptions around 325 nm and 600 nm. Both the electron and the hole absorb at λ = 600 nm [11]. Electrons and holes then recombined rapidly and the peak absorbance dropped to zero after 15 μs of excitation. The peak absorption at 325 nm may indicate some new transient species formed, which could exist for a relatively longer time than the electrons and holes. To determine the characteristic of this transient species, 310 nm was selected as the subsequent detecting wavelength because the absorbance at 310 nm is more stable than that at 325 nm in the spectral range of the same peak.

3.3 Transient absorption with and without pyrogallol

The transient absorption of the 50 mg/L pyrogallol + 20 mg/L TiO₂ mixture and 20 mg/L TiO₂ pure solution after excitation detected at 310 nm is presented in Fig. 4. The recombination reaction of holes and electrons finished within 800 ns if no pyrogallol was added to the TiO₂ suspension. However, when pyrogallol was added, the transient absorbance did not drop to zero after hole and electron recombination finished, but a low obvious absorbance lasted for a relatively long period of time. This must be attributable to some newly generated transient species with an absorption at 310 nm.

From the time-resolved absorption curves in Fig. 4, it is also found that the recombination of holes and electrons finished in about 800 ns for pure TiO₂. Some literature has reported that the average lifetime of holes and electrons is only a few tens of picoseconds [12]. This does not conflict with our observation because the recombination competes with the trapping of free charges by coordination defects at the surface and by lattice defects in the nanoparticle bulk. After photoexcitation, some of the electron–hole pairs recombine rapidly and some migrate to the TiO₂ surface. Among the carriers on the surface of TiO₂, 45% of the carriers descend into surface traps and react with substances adsorbed on the surface slowly thereafter (after about 200 ns), and 10% of the carriers enter deep inside the TiO₂ nanoparticles and do not react at all [11,13,14]. The variation of transient absorption observed in this experiment, from its peak to zero, was due to the complicated generation and recombination of electrons and holes.

3.4 Transient absorption for different concentrations of pyrogallol

Fig. 5 shows the transient absorption results at 310 nm obtained as a function of pyrogallol concentration in the TiO₂ solution. The absorbance signal increased with increasing pyrogallol concentration. This is due to more transient species being generated when the amount of pyrogallol absorbed on the surface of TiO₂ was higher. There are two possibilities for the transient species. One is pyrogallol cations oxidized by holes and the other is the OH adducts formed from OH radicals.

3.5 Transient species determination

In order to determine the transient species present after laser photolysis of pyrogallol + TiO₂ solution, the influence of dissolved oxygen was studied. A mixture of 50 mg/L pyrogallol + 20 mg/L TiO₂ in a quartz cuvet was purged with high-purity nitrogen for 20 min to drive away dissolved oxygen. A flash photolysis experiment was conducted immediately after deoxygenation. The transient absorption spectra of both deoxy and air-equilibrated samples were similar, which suggested that the presence of oxygen had little effect on the transient absorption. If the transient species was an OH adduct, it should be easily oxidized by oxygen and the absorption signal of the oxygen-containing sample should attenuate obviously compared to the N₂-saturated sample. This lack of sensitivity to oxygen suggests that the transient species was not any OH adduct, but pyrogallol cations that could not be oxidized by oxygen.

3.6 Primary mechanism of the photooxidation of pyrogallol by titanium dioxide

Based on the previous results, it is deduced that the initial TiO₂ photocatalytic reaction mechanism for pyrogallol is the generation of photoexcited electrons–holes followed by the holes, instead of OH radicals, oxidizing pyrogallol into pyrogallol cations. The reactions are described as