*2.5. Photoelectric Properties of Bare and Metal-Modified TiO2*

It is documented that the energies of valence and conduction bands of metal-modified semiconductors are shifted upward with a value depending on the height of the Shottky barrier, forcing the electrons and holes to move in different directions [14,28]. The quick charge carrier recombination, the time scale varying from μs to ns [29], is hindered, allowing the time for a charge transfer to occur at the interface. The space separation of photogenerated charge carriers inherently leads to the appearance of a certain surface photovoltage (SPV), the measurement of which can provide valuable information concerning the transfer dynamic of such carriers [30–32]. Thus, it was previously demonstrated that, for an n-type semiconductor, photoinduced electrons migrate towards the illuminated side of the material, giving rise to a negative SPV signal. Conversely, a positive SPV signal corresponds to a p-type semiconductor, in which case holes are directed from the surface to the bulk [33,34]. However, in both cases the surface photovoltage is wavelength dependent, being affected by the particular features of the semiconducting material, in terms of light absorption and transport of excess carriers [35,36].

The surface photovoltage was measured for each sample at several wavelengths and, as expected, SPV spectra (Figure 11) revealed in all the cases an n-type semiconducting character. Obviously, SPV signals measured under the actual experimental conditions correspond, in fact, to the potential difference between the Fermi level of ITO (indium tin oxide, + 0.35 V vs. NHE [34]) and that of the irradiated sample. Since the conduction band of non-stoichiometric TiO2 is located above its Fermi level with an average value of ca. 0.5 V [37], and by taking into account a value of around −0.33 V for the O2/O2 − level [1], it appears that the formation of O2 − species at the surface of the irradiated samples requires an SPV value higher than ca. −0.18 V. As the results from Figure 11 indicate, this condition is not fulfilled in the case of pristine TiO2, whereas at noble metal-modified samples O2 − formation is possible, at least in principle, for irradiation wavelengths lower than 380 nm. Nevertheless, the probability for this process increases with SPV signal, in the order Pt/TiO2 > Au/TiO2 > Ag/TiO2, as schematically illustrated in the inset in Figure 11. These findings are in excellent agreement with the activity for O2 − formation deduced from radical quenching experiments (see Figure 7).

**Figure 11.** Surface photovoltage (SPV) spectra of pristine TiO2 (1), Ag/TiO2 (2), Au/TiO2 (3), and Pt/TiO2 (4). Inset: corresponding energy diagram at 300 nm; the dashed lines indicating the Fermi levels of samples.

To emphasize the effect of noble metal modification of the titanium oxide on the O2 − generation process, chronoamperometric experiments were performed in dark, at an applied voltage of −1V. Figure 12 shows the time-variation of the oxygen reduction current, estimated as the difference between the current recorded in O2 atmosphere and that observed under Ar conditions. For easier comparison, the currents were expressed in terms of mass activity (oxygen reduction current normalized to the amount of the investigated powder sample). Pristine TiO2 exhibited negligible response (see curve 1 from Figure 12), which clearly demonstrates that the presence of noble metal particles is a prerequisite for O2 reduction. It was interesting to observe that, up to ca. 50 s, the current recorded at Au/TiO2 is higher than that at Pt/TiO2, although during further polarization the decrease in the current tends to become much slower for the latter (compare curves 3 and 4 from Figure 12). To better put into perspective the role of the noble metal nature, inset (a) in Figure 12 illustrates the decay of the oxygen reduction current on a log–log scale. Linear dependences were found in all cases, which could indicate a Langmuir adsorption kinetic control of O2 on the overall reduction process [38]. However, Pt/TiO2 exhibited the slowest current decrease, whereas for Au/TiO2 a change in slope was observed, the decline of the current becoming much steeper after only ca. 10 s, probably as result of a more sluggish adsorption of oxygen reactant species. Consequently, after about 200 s of continuous polarization, oxygen reduction current at Pt/TiO2 is more than twice as high as that observed with Au/TiO2. These results are important because they can provide

an explanation for the fact that, compared to the case of Au/TiO2, the total amount of O2 − produced at Pt/TiO2 is much higher (see inset in Figure 12) than would have been expected for rather small difference in terms of SPV signals between the two materials. Integration of the current responses from Figure 13 over the entire polarization time, yielded oxygen reduction charges of ca. 0.78, ca. 1.10, and ca. 1.46 mC g−<sup>1</sup> for Ag/TiO2, Au/TiO2, and Pt/TiO2, respectively. As illustrated by the inset (b) in Figure 12, based upon these values, corresponding amounts of O2 <sup>−</sup> species of 7.8, 11.4, and 15.2 nmol g−<sup>1</sup> were estimated as being formed at the investigated active samples. The maximum amount of oxygenated compounds (HQ + BQ + 1,2 DHBz) formed over Ag/TiO2 (4.7 μmoles), Au/TiO2 (5.1 μmoles), and Pt/TiO2 (9.5 μmoles) after 1 h of reaction time (0.5 h in case of Ag/TiO2), follows closely the tendency observed in polarization measurements of O2 reduction (Figure 12). The precise correlation between quantitative polarization and photocatalytic data concerning oxygenated compounds is difficult because the formation and depletion of O2 − by reaction with organic substrate(s) is a dynamic process compared to O2 adsorption on polarized surface. To build up a reliable kinetic, it is necessary to find out the rate of O2 − formation in reaction conditions. However, a close relationship between formation of O2 − species and mild oxidation of Ph is demonstrated by two independent experimental techniques (selective radical trapping and chronoamperometric experiments).

**Figure 12.** Time-variation of the mass activity for oxygen reduction at pristine TiO2 (1), Ag/TiO2 (2), Au/TiO2 (3), and Pt/TiO2 (4), at an applied voltage of −1 V. Insets: (**a**), log–log plots for the oxygen reduction current decay at noble metal-modified TiO2; (**b**), estimated O2 − amounts formed during 10 min of continuous polarization.

**Figure 13.** Distinct mechanisms of Phenol (Ph) photocatalytic oxidative conversion over bare and noble metal-modified TiO2.

The corroboration of entire experimental evidences collected in this research lead to the reaction scheme presented below.

We assumed, based on experimental facts, that Ph is oxidized non-selectively by ·OH radicals directly to CO2, apparently without producing in our experimental conditions detectable long-lived intermediates. Supported noble metals are responsible for O2 reduction to O2 −, by mediating the transfer of photoelectron from TiO2 to adsorbed O2. The subsequent O2 − reaction with Ph leads to formation of oxygenated products (HQ, BQ, 1,2 DHBz). The literature focuses mostly on reactivity O2 − in organic protic and aprotic solvents and less on the reactivity in aqueous solutions [21]. However, it is recognized that, O2 − disproportionates spontaneously in water, forming O2 and hydroperoxide anion (HO2 −). One possibility is that the reaction follows the superoxide dismutase (SOD) pathway (Equation (1)), proposed to explain the biological function of superoxide ion [39]:

$$\text{2O}\_2\text{O}\_2^- + \text{2H}^+ \rightarrow \text{O}\_2 + \text{H}\_2\text{O}\_2 \tag{1}$$

$$\text{R-H} + \text{H}\_2\text{O}\_2 \rightarrow \text{R-OH} + \text{H}\_2\text{O} \tag{2}$$

Equation (2) suggests that the insertion of oxygen in ortho and para position takes place by the reaction of organic substrate with H2O2 in vicinity or on supported noble metal(s). Alternatively, the oxidation mechanism may occur through activation of Ph by transfer of photogenerated hole followed by nucleophil attack of O2 − to generate organic peroxyl radicals which react further with H<sup>+</sup> and e<sup>−</sup> to form finally the hydroxylated organic compound (see Equation (3)) [40].

$$\text{R-H} + \text{h}^+ \rightarrow \text{(R-H)}^+ + \text{O}\_2^- \rightarrow \text{(R-H)-O-O}^\cdot + 2\text{ H}^+ + 2\text{ e}^- \rightarrow \text{R-OH} + \text{H}\_2\text{O} \tag{3}$$

Actually, there is a limited knowledge on the reaction mechanism concerning the interaction in aqueous media between the adsorbed O2 − and organic substrate in the presence of a catalytic metal. The published literature gives no information on the eventual role played an active metal in the above proposed reaction mechanisms.
