The integration of photocatalysts and sorbents into a single photoelectrochemical system opens up fresh opportunities in improving the efficiency of photodegradation of aquatic pollutants. The photoelectrodes of this type can be obtained via the immobilization of nanostructurated TiO₂ onto the carbon-fibres which are known to posses high conductivity and pronounced sorption ability. The photoelectrochemical characterization of TiO₂ modified carbon fibre electrodes and the application of this system in the electrochemically-assisted degradation of model organic compounds are presented here.
The fibres of graphitized carbon were modified with TiO₂ with the use of the sol-gel technique and then sintered at 450 ⁰C in argon. The comparison of the cyclic voltammograms obtained in Fe(CN)₆^4- / Fe(CN)₆^3- couple for carbon fibres before and after modification has revealed that TiO₂ coating occupies ca. 30 % of the surface of carbon-fibre electrode. The electrode assemblies employed for the photoelectrochemical investigations have consisted of ~ 5000 TiO₂ - modified fibres ca. 10 mm in diameter. The experiments were carried out under the potentiostatic conditions.
The analysis of photoelectrochemical behaviour of TiO₂ - modified carbon fibres evidences that these systems most likely operate in the 91mixed 92 photovoltaic-photogalvanic regime, the photocurrent generation efficiency being dependent on the potential drop along the fibres as well as on the ratio of the exchange current of carbon-fibre support in the dark and that of the nanostructurated TiO₂ under actinic illumination. The performed photoelectrochemical measurements have also intimated that the semiconductor photocatalyst immobilized on the carbon fibres provides a convenient way of manipulating the photoctalytic reactions by applying external biases thus accelerating the injection of photogenerated holes or electrons into the solution. It is seen from photocurrent versus voltage dependence depicted in Fig. 1 that both the anodic and cathodic photocurrents can be generated under UV illumination depending on the electrode potential, the cathodic photoelectrochemical process is consistent with the reduction of molecular oxygen.

Fig. 1 Polarization curve of TiO₂-modified carbon-fibre electrode under UV illumination. Electrolyte: 0.25 M Na₂SO₄.
Potential is referred to Ag/AgCl,Cl-(sat.) electrode.

The efficiency of photocurrent generation in case of TiO₂ - modified carbon fibre electrodes (i.e., the photocurrent normalized to the illuminated area) was found to be close to the photocurrent generation efficiency at conventional thin-film TiO₂ electrode prepared with the use of sol-gel technique similar to that employed for the modification of carbon fibres. In the case of the indifferent electrolyte containing no redox additives, the photocurrent at the anodically-biased TiO₂ - modified carbon - fibre electrode exhibits initial decrease by ca. 20% and then remains unchanged during the long-term photoelectrochemical oxidation of water. The TiO₂ coverage measurements performed with the use of electrochemical technique have evidenced that the observed decrease in the photoactivity of the TiO₂ - modified carbon-fibre electrodes is not due to the photocatalyst degradation and presumably results from the partial oxidation of TiO₂/carbon fibre junction. The TiO₂ - modified carbon fibres of different diameter and resistivity can be also assembled together as a fabric forming a fractal photoelectrodic system which combines high photocatalytic efficiency with good mechanical and collector properties. The macro-, micro-, and mesoporosity of the carbon-fibre support as well as its electrocatalytic activity can be effectively regulated by varying the nature of precursors used for preparation of carbon fibres (cellulose, polyacrylonitrile, etc.) and employing different procedures of surface modification of the resulting fibres. The high electrode surface area (more than 300 square meters per gram) inherent in these fractal photoelectrodes makes them candidates for the profound photocatalytic purification of water. These systems integrate the major advantages of conventional photocatalytic systems (e.g., high surface-to-volume ratio) with high sorption capacity and the possibility of applying the external biases. Moreover, by contrast to suspensions of semiconductor photoctalysts, these photoelectrodic assemblies can be readily removed from solution purified and then regenerated electrochemically for successive use.
The photoelectrochemical system comprising the semiconductor photocatalyst immobilized on the carbon fabric support can be also used for removal of organic pollutants absorbed by the soil: the fabric applied over the contaminated spot on the ground is heated by passing the electric current that results in the transfer of organics from the soil to the TiO₂ - modified carbon-fibre electrode assembly; the collected contaminant can be then mineralized during the course of electrochemically - assisted photocatalytic degradation.

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