*2.3. Effect of the Organic Compound on the CV*

Specific CV runs performed on the different samples in solutions containing different concentration of BPA, did not show a clear indication of the voltammetric peaks related to the direct oxidation of the organic molecule, at the electrode surface. Nevertheless, the photocurrents recorded in the presence of the organic were higher than those measured in the supporting electrolyte, under almost all the investigated potentials. This may be an indication that oxidation of BPA may occur by means of OH radicals originated from H2O, by the action of the photogenerated holes. The BPA molecule, acting as OH scavenger, accelerates the separation process of the charges, this in turn gives the increase in the photocurrent.

However, even in the absence of a direct reaction at the electrode surface, the effects of BPA are visible in the CV: the presence of BPA seems to interfere on the redox behavior of the M. For this kind of analysis, we investigated the Au/TiO2/Au and TiO2/Au samples, where the redox behavior of the M was more evident: also, they were the most performing systems in terms of photocurrent.

Figure 10a compares the CV obtained in solution with two concentrations of BPA, while Figure 10b compares the trend of CV for the differently M loaded samples in solution with the same concentration of BPA.

**Figure 10.** Comparison between the CV obtained at TiO2/Au sample: (**a**) in solution with two concentrations of BPA and (**b**) at samples differently M loaded, in solution with the same concentration of BPA (50 ppm).

The increase in the BPA concentration has two main effects: on one hand, a decrease in the current of the redox peaks (P3 and P4) it is observed, on the other hand, the peaks are more distant one from another, in terms of potential, this indicating a worst and more irreversible charge transfer process with the substrate.

This behavior may indicate that during the CV, in absence of the organic, the photogenerated charges can be available to activate the redox process of the NPs, under the imposed potentials. In particular, during the oxidative scan, the holes may: (i) generate OH radicals from water, or (ii) oxidise Au NPs. In the presence of the organic, BPA molecules act as OH radical scavengers, thus accelerating the path (i) and subtracting holes to the redox process of Au, which is less evident in the CV.

#### *2.4. Analysis of the Working Mechanism of the Structures*

From the results presented up to now, it is clear that the behavior of the investigated structures is rather complex: addition of Au NPs interferes not only with the charge transfer to and from the electrolyte, but also with the equilibrium and non-equilibrium interface energetics.

In order to better understand the working mechanism of the electrodes, it can be useful to recall some of the fundamental concepts on SC, how they behave under illumination, and, finally, how they interact with the electrolyte and the solutes in it contained.

From the structural point of view, we are dealing with coupling of SC and M. In the presence of bulk materials, the SC/M interface is expected to behave according to the well-developed theory of SC/metal Schottky contacts [48]. In the specific case, by considering the electronic affinity of TiO2 equal to 4,5 eV, and a work function (WF) for Au equal to about 5.3 eV [49], coupling of the two materials should lead to a Schottky barrier of 0.8 eV. However, when we deal with nanostructures, the position of the energetic levels may be different: depending on the conditions, and in particular on the morphology of the coupled phases, different values can be calculated for both the Fermi level of the SC, and the WF of the M, which may be affected also by the excess of charge on the NPs [49].

In our specific case, information on the location of the conduction band-edge (CB) can be derived from CV measurements, and, as suggested by the literature [50], we considered the potential of the peak as representative of the energetic level of the CB edge. Thus, a value of 4.4 eV has been calculated from the value of −0.78 V, at which the main peak of TiO2 appears in the CV of TiO2 sample.

Regarding the WF of M, we may consider that the OCV values of the samples with Au, were more positive than that of the reference TiO2 sample (see Figure 2): this may be an indication that the WF of the M was greater than the Fermi level of the SC. When the two materials are contacted, a spontaneous charge transfer occurred from TiO2 to Au, up to equilibrium leading to the formation of the Schottky barrier. Thus, the presence of Au can be seen as a reservoir of electrons, which are displaced from the CB of the SC: the Fermi level of the SC is lowered, and its potential made more positive by the presence of metal NPs.

When the effect of the irradiation is considered, we cannot neglect that M is present as NPs. In the specific case, optical analyses indicated that for the different samples, depending on the NP dimensions, and on the deep penetration of the Au deposition, plasmonic or scattering effects are originated. In fact, as pointed out in the Introduction, the presence of NPs, more or less distributed in the bulk of the SC or at the interfaces, may strongly enhance the effectiveness of light absorption.

Finally, also the potential of the donor/acceptor redox couples present in the solution must be considered: the Fermi level of the electrons in the CB should remain higher than the energetic level of the acceptor, otherwise it cannot receive electrons at the cathode. At the same time, the level of the holes (h+) in the VB of the SC should remain lower than that of the donor to which h+ will be transferred.

Accordingly, the scheme shown in Figure 11 illustrates the possible working mechanisms of the examined structures, when irradiated with the whole solar simulated light. Depending on the wavelength of the incident light, different effects could be achieved. The final mechanism should be a combination of the response of the TiO2 nanostructure, which is sensitive only to a fraction of the wavelengths (UV), and of the Au NP effects (both plasmonic and in terms of charge separation).

Under the fraction of UV radiation, effective on TiO2:

e− are excited in CB and, assisted by the applied potential, they may:


holes that do not recombine are displaced on the surface and they may:


holes arriving at the M may:


**Figure 11.** Schematic representation of the energetic levels involved in the charge activation and transfer processes.

The main phenomena related to the illumination of the M nanoparticle can be summarized as:

	- (7) be injected into the CB of the SC (then paths 1, 2)
	- (8) decay and thermalize through electron–electron and electron–phonon scattering.
