*2.3. Proposed Photodegradation Mechanism for CFX and LFX*

As previously shown, gold nanoparticles improve catalytic efficiency, which increases appreciably when rGO and gC3N<sup>4</sup> are incorporated. The most efficient catalyst for the photodegradation of CFX and LFX (10%Au@ZnONPs-3%rGO-3%gC3N4) incorporates different characteristics that act synergistically on its behavior, that is, increased surface area and smaller bandgap. The considerable increase in the specific area of the catalyst can provide more active sites during the photocatalytic reaction, thus producing more photogenerated electrons, which can also lead to less recombination of photogenerated charge carriers. This decrease in recombination is also favored by the presence of Au nanoparticles, which act as electron sinks. The displacement towards the visible region of the bandgap, with respect to other catalysts studied, is another of the decisive factors for the observed behavior.

In this context, and to evaluate the photodegradation mechanism, some scavengers were added to the reaction. In this photocatalytic study, benzoquinone (BQ), ethylenediaminetetraacetic acid disodium salt (EDTA), and methanol (MetOH), were employed to capture superoxide radicals (·O2−), holes (h<sup>+</sup> ), and hydroxyl radicals (·OH), respectively [55,56], determining the percentage of degradation of CFX and LFX after 3 h of reaction (see Figure S6). As can be seen, BQ hindered photoactivity noticeably, suggesting the main role of the O2<sup>−</sup> reactive species in the photodegradation process. EDTA and MetOH hindered the reaction to a lesser extent, which supports the fact that h<sup>+</sup> and ·OH do not play a prominent role in the degradation process. This effect is similar for both antibiotics (see Figure S6a,b), although in the case of LFX, the effects of all scavengers were certainly greater.

Taking into account these results, together with the determination of the bandgaps (see Figure 5), a photodegradation mechanism of CFX and LFX using 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> (Figure 9) has been proposed. For this, the Mulliken electronegativity theory [57,58] has been used, which allows establishing the band edge position of the

different components of the catalyst and, in this way, determining the migration direction of the photogenerated charge carriers in the composite (Equations (1) and (2)).

$$\rm{E\_{CB}} = \rm{X} - \rm{E\_C} - 0.5 \rm{E\_g} \tag{1}$$

$$\mathbf{E\_{VB}} = \mathbf{E\_{CB}} + \mathbf{E\_{g}} \tag{2}$$

where ECB and EVB are the edge potentials of the valence band (VB) and conduction band (CB), respectively, X is the absolute electronegativity, E<sup>C</sup> is the energy of free electrons on the hydrogen scale (4.50 eV) [59,60], and E<sup>g</sup> is the bandgap. X values for ZnO and gC3N<sup>4</sup> are 5.75 [61] and 4.73 eV [62], respectively. The calculated ECB and EVB edge positions for Au@ZnONPs are −0.355 and 2.855 eV, respectively, while for gC3N<sup>4</sup> the calculated values were −1.165 and 1.625 eV, respectively, being in agreement with values previously determined in other investigations [62,63].

**Figure 9.** Schematic diagram of the proposed photodegradation mechanism of CFX and LFX under visible radiation.

Under visible radiation (Figure 9), the electrons of the VB of gC3N<sup>4</sup> are excited towards the conduction band (CB), giving rise to h<sup>+</sup> in the VB. Due to the potential difference with respect to ZnONPs, the electrons of the conduction band of gC3N<sup>4</sup> move towards the CB of Au@ZnONPs. Au acts as a sink for electrons [64,65], which is why they move towards Au and it is there that they react with the adsorbed molecular oxygen to generate superoxide anions, which in turn can react with water molecules to form hydroxyl radicals. According to studies carried out with different scavengers, the O2<sup>−</sup> anion is the one that preferentially participates in the photodegradation process of antibiotics, giving rise to small molecules as a by-product of the reaction, CO<sup>2</sup> and water. The holes formed in the VB of gC3N<sup>4</sup> and, to a lesser extent in ZnO, will promote the oxidation of CFX and LFX also leading to degradation. However, this second pathway, as evidenced above, is not the most favored pathway. AuNPs, in addition to acting as electron sinks and thus reducing recombination processes, also contribute to the system through the surface plasmon resonance mechanism, reacting with molecular oxygen and generating superoxide anions. Photogenerated electrons are also transferred to rGO that, as in the case of gold, acts as an electron acceptor and transport medium in the photocatalytic system, suppressing the recombination of e−-h<sup>+</sup> pairs. Photodegradation reactions therefore also occur on rGO sheets, increasing the specific surface area and active reaction sites [65].
