| Photoinduced reactivity of TiO2 has been applied in water and air purification and remediation, photoelectrochemical solar energy conversion, sterilization, antifogging and self-cleaning, etc. Despite the contributions from a number of research groups, detailed mechanisms of the photocatalytic processes at the TiO2 surface remain elusive particularly regarding the initial steps, due to the inherent complexity of this minute photoelectrochemical system.There are still open questions concerning: (a) how about the role of surface hydroxyl groups, (b) which is the primary oxidants (photogenerated hole, hydroxyl radical, superoxide radical, or hydrogen peroxide), (c) whether the adsorption is prerequired and (d) whether the reaction takes place on the surface or in solution bulk in TiO2-assisted photodegradation of organic pollants. Another problem commonly found in photocatalytic degradation of organic contaminations is the deactivation of TiO2. Further studies are still essential.In this paper, we had focused our attention on the role of primary active species and the effect of TiO2 (Degussa P-25) surface modification on the photodegradation of modern organic pollutants.(1) Direct hole mechanism and the mechanism alterabilityIn the current paper, experiments were developed to testify on the kinetics ofphotoreaction of AO7 on UV-illuminated TiO2 through the use of charge-trappingspecies (hVb+ and -OH scavengers) as diagnostic tools. Orange II (OII) moleculesfirstly adsorbed strongly on the TiO2 surface and the following degradation reactionwas mostly initiated by the direct electron transfer reaction between a positive holeand a surface-bound OII molecule, other than by OH, while O2- and H2O2 played anegligible role.The relative percentage of hole mechanism (ph) was in the order of OI > OII > OGHowever, with additives (SC>42\ HCO3", or F'), which would change the surface chemistry of semiconductor, the initial steps could shift partly from direct hole oxidation to radical-induced mechanism, and the reaction sites probably could move from the particle surface to the solution. The potential mechanism alterability was in the order of 01 < Oil < OG Adsorption isotherm experiments showed there was no correlation between the adsorption ability of dyes and the photodegradation mechanism.(2) T1O2 surface modification by anionsThe surface characteristics and surface species of T1O2 nanoparticles were essential factors for the photocatalytic degradation of organic substrates. In the presence of (C2O42', SO42*, HCO3', or F"), the surface charges of T1O2 nanoparticles could be changed because of their strong adsorption ability. The surface occupation by these anions could competitive with adsorption of organic molecules and affect the degradation pathway.Effects of TiO2 surface fluorination on the active oxygen species formed at the oxidation site and the reduction site were investigated. Alcohols such as methanol and isopropanol, are usually used as a diagnostic tools of radicals mediated mechanism. Superoxide radical (O2") was determined by colorimetry of nitroblue tetrazolium (NBT), a prominent 02*' scavenger. Hydrogen peroxide (H2O2) was estimated by iodide-starch method. When TiO2 surface was fluorinated, althouth the adsorption of Oil was inhibited significantly, more OH at the oxidation site and more 02'" and H2O2 at the reduction site were produced, which indicated that the modification could greatly reduce the recombination of photogenerated electrons and holes, thus enhance the PCO rate. The sulfation effect on PCO is much more complicated. The inhibition may be limited by the ability of the sulfate radical itself to oxidize organics or by slowing down further recombination of electron and hole pair. On the other hand, in the presence of fluoride the T1O2 deactivation was significantly limited, due to the shielding effect on the surface sites, since fluoride shows very strong adsorption on T1O2 and it is so stable that it could not be oxidized by...
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