The photocatalytic decomposition of chlorine-substituted aromatic and aliphatic compounds using of metal oxide semiconductors either in the aqueous heterogeneous suspensions or in an immobilized form has received considerable attention due to the prospect of the environmental applications of these systems [1,2]. The semiconductor particulate films are of special interest for water detoxification since the photohole yield in their case can be drastically enhanced by applying an anodic bias [2]. The present study has been concerned with the effect of the external polarization on the mechanism and efficiency of the electrochemically-assisted photoctalytic degradation of o-chlorophenol in the alkaline aqueous solution.
The photocatalytic and photoelectrochemical experiments were performed with the use of 200 nm-thick nanostructurated TiO₂ films immobilized onto a conducting glass plates. The TiO₂ films were derived from titanium dioxide colloidal solutions prepared via hydrolysis of TiCl₄. The TiO₂-coated glass plates were sintered at 200øC and 400 øC that leads to the formation of amorphous and amorphous-crystalline oxide films, respectively.

Fig.1 Kinetics of the photoelectrochemical (a, d) and photocatalytic (b, c) degradation of aerated aqueous solution of o-chlorophenol on the surface of amorphous (a, b) and amorphous-crystalline (c, d) nanostructurated TiO2 films at applied potential of +0.8 V (a, d) and in open-circuit conditions (b, c). The kinetic curve is corrected for the o-chlorophenol losses arising from direct photolysis at λ > 312 nm.

The photoelectrochemical measurements have been carried out under potentiostatic conditions (three-electrode scheme of polarization with Ag/AgCl reference electrode). The photoelectrochemical (under anodic biases) and the photocatalytic (under open-circuit conditions) degradation of o-chlorophenol (o-CP) in the 10^{- 4} M o-CP + 0.05 M Na₂SO₄ + 0.02 M NaOH solution was studied with the use of UV-spectroscopy. The illumination was provided by low-pressure mercury lamp equipped with 312 nm cut-off filter eliminating the short-wavelength UV radiation to ensure the selective excitation of TiO₂ and to minimize the direct homogeneous photolysis of o-CP.
It is seen from the kinetic curves depicted in Figure 1 that the rates of photocatalytic degradation of o-CP are almost the same for amorphous and amorphous-crystalline TiO₂ films. Figure 1 also indicates that the efficiency of the electrochemically-assisted photodegradation of o-CP on the amorphous TiO₂ film appears to be much lower then that of the pure photocatalytic one while in the case of amorphous-crystalline films the opposite situation occurs: the o- CP conversion rate greatly increases when the TiO₂ film is maintained at an external anodic bias. The observed decrease on the rate of o-CP degradation under anodic polarization (i.e., under conditions beneficial for the charge separation) can be attributed to the fact that not only the photogenerated minority carriers but also non- equilibrium minority carriers (electrons) from the semiconductor conduction band are involved in the process of the photoctalytic destruction of chlorine-substituted phenols [1]. The role of the latter process becomes marginal under the anodic polarization when the photoelectrons are effectively extracted from the film, the modest increase in the photogeneration efficiency under anodic bias in the case of amorphous TiO₂ film (see Fig.2) being inadequate to compensate the repercussions arising from the interlocking of reaction pathways involving the conduction band electrons. By contrast, the drastic enhancement of the photogeneration efficiency observed for the anodically- biased amorphous-crystalline TiO₂ films (Fig.2) leads to the substantial increase in the rate of photoelectrochemical degradation o-CP by comparison with the photocatalytic one. It should be also noted that the photoelectrochemical oxidation of o-CP is accompanied with the formation of great amount of radical intermediates which are prone to condense at the semiconductor surface. As the result, the TiO₂ electrodes gradually loose their activity during the course of illumination (Fig. 2).

Fig.2 Photocurrent vs. potential plots for amorphous (a) and amorphous-crystalline (b) nanostructurated TiO2 films in 0.05 M Na2SO4 + 0.02 M NaOH. The insert shows the temporal variation of the photocurrent at amorphous-crystalline TiO2 electrode in the presence of 10-4 M o-chlorophenol (electrode potential is of +0.8 V).

The results obtained in the present study demonstrate that the mechanism of photoelectrochemical and photocatalytic destruction of aromatics in aqueous media can differ radically since in the latter case the photogenerated majority carriers are failed to be involved into the multistep photoinduced reactions occurring at the oxide surface. As the result, the mineralization of the aromatic pollutants in the photocatalytic systems may occur with higher quantum yield then in the photoelectrochemical ones even though a higher photogeneration efficiency is formally attained under the external polarization. Moreover, the favourable conditions are formed in the photoelectrochemical cells for the condensation of photoproduced intermediates that leads to the parasite fouling of the semiconductor surface and also affects the performance of the photoelectrochemical systems.
In the report these results will be compared with quantitative data obtained by the EPR technique on the concentration and location of paramagnetic centres on the TiO₂ surface for the samples prepared at different temperatures.

References:

1.	J. Theurich, M. Lindner, D. W. Bahnemann Langmuir12 (1996) 6368.
2.	K. Vinodgopal, S. Hotchandani, P. V. Kamat J. Phys. Chem. 97 (1993) 9040.

Acknowledgements: The authors are grateful to the INTAS (grant No. 94-0266) for financial support.

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