*2.4. XPS Analysis*

The surface state of gold and silver in the heterostructured catalysts was explored by XPS analysis and the corresponding spectra are shown in Figure 5.

**Figure 5.** High-resolution XPS spectra of (**a**) Ag3d in Ag\_MgGaAl; (**b**) Au4f in Au\_MgGaAl.

The peaks at 373.68 and 367.58 eV, associated to Ag 3d 3/2 and Ag 3d 5/2 transitions, respectively (see Figure 5a), indicate that on the surface of Ag\_MgGaAl silver is present as Ag0. This is supported by the fact that the formation of silver nanoparticles was achieved under light that promoted the reduction of Ag<sup>+</sup> to Ag<sup>0</sup> [43]. The state of gold nanoparticles is described by the peaks at 85.25 and 89.47 eV in Figure 5b, which come from Au4f 7/2 and Au 4f 5/2 transitions, respectively, revealing the presence of a mixed states of Auδ<sup>+</sup> (δ = 1, 1, 3) on the surface of Au\_MgGaAl [44–46]. In both heterostructures, the signals generated by gallium (Supplementary Figure S2) formed peaks at 1145.2 eV and 1118 eV, revealing the presence of Ga 2p 1/2 and Ga 2p 3/2 as Ga3+ cations in the LDH layer.

### *2.5. Photocatalytic Performance in 2-PNh and 2-DCB Photodegradation in Aqueous Solutions*

The photocatalytic activity of the plasmonic catalysts was evaluated in the photodegradation of 4-NPh and p-DCB. The characteristic absorptions of 4-NPh at 390 nm and p-DCB at 220 nm were used to monitor the photocatalytic degradation process [47]. Among the examined catalysts, Ag\_MgGaAl exhibited the best performance, completely decomposing the tested pollutants. As shown in Figure 6a,b for Ag\_MgGaAl, the main adsorption peaks of the pollutants decreased gradually as solar irradiation proceeded, such that the peak almost vanished after 90 min for p-DCB while for 4-NPh the corresponding peak vanished after 360 min.

Figure 7 presents the effective mineralization of 4-NPh (Figure 7i) and 2-DCB (Figure 7ii) evaluated by TOC measurements for the catalysts irradiated by solar and UV light. Firstly, it reveals that the studied catalysts have not been active when irradiated by UV light. This clearly reveals the role of plasmonic characteristics in harvesting the light energy to promote photocatalysis. Thus, under solar light the presence of silver in Ag\_MgGaAl highly increased the TOC removal up to 87% as compared to Au\_MgGaAl, which mineralized only 71% of 4-NPh in the same experimental conditions. Further, the degradation of 2-DCB Ag\_MgGaAl showed the strongest photocatalytic activity under solar irradiation while, almost no photodegradation was achieved under UV irradiation. Thus, the photocatalytic activity of the tested catalysts followed the orders Ag\_MgGaAl > Au\_MgGaAl > Ag\_MgGaAlcal > Au\_MgGaAlcal with degradation efficiencies of 2-DCB calculated to be 91%, 76%, 61% and 47% respectively, after 90 min of irradiation with solar light. MgGaAl was not active for the degradation of 2-DCB, while the degradation efficiency for 4-NPh reached only 11% on the LDH precursor. These results reveal the essential roles of the plasmonic responses on promoting the photons harvesting from the visible range of solar light and promoting pollutant degradation [48], but further disclose that

the silver-based photocatalyst led to the best performance. This can be due to the much wider plasmonic peak of nanosilver that points out that Ag\_MgGaAl is able to harvest light over a more extended visible light-responsive range in comparison to Au\_MgGaAl [49]. The excellent activity of the Ag\_MgGaAl sample was a good reason to test the performance of this photocatalyst using a mixture of 4-NPh and p-DCB (1/1 molar ratio).

**Figure 6.** UV−vis absorption spectra of (**a**) 4-NPh; and (**b**) p-DCB solutions in the presence of Ag\_MgGaAl and solar irradiation.

**Figure 7.** Photocatalytic performance evaluated by TOC measurements for (**i**) 4-NPh removal, after 360 min of irradiation, shown as: (a) Ag\_MgGaAl; (b) Au\_MgGaAl; (c) Ag\_MgGaAcal; (d) Au\_MgGaAlcal; (e) MgGaAl; (**ii**) p-DCB removal after 100 min of irradiation for: (f) Ag\_MgGaAl; (g) Au\_MgGaAl; (h) Ag\_MgGaAlcal (j) Au\_MgGaAl, by ( \_ ) solar and ( \_ ) UV light.

For 4-NPh (see Figure 8a) a delay of about 120 min was noticed for 4-NPh when found in the mixture of the pollutants. Thus, after 360 min the mixture exposed to the photocatalyst reveal a degradation yield of 75% of 4-NPh after 360 min of irradiation by solar light. This reveals a competition between the pollutants for the adsorption sites or for the HO· radicals involved in the photocatalytic process. On the other hand, p-DCB shows a different degradation profile (Figure 8b), with a fast degradation fitting a Langmuir type allure. After this, the degradation was quite slow, reaching almost 50% after 360 min of irradiation. The different behavior further points out that in their aqueous mixture p-DCB is highly preferred to 4-NPh in the adsorption process in the beginning of irradiation by solar light.

**Figure 8.** Pollutant % removal of: (**a**) 4-NPh; and (**b**) p-DCB from an aqueous solution containing both pollutants on Ag\_MgGaAl.

The reusability of the catalysts was investigated via cycle experiments using a Ag\_MgGaAl hybrid photocatalyst. For each test, the reusability of Ag\_MgGaAl was evaluated by successive addition of the pollutants such that, before each cycle, the concentration of 4-NPh and p-DCB was equal to 0.125 mmol/L for each. As exhibited in Figure 9, there was just a slight decrease after testing three times, indicating that Ag\_MgGaAl can be considered to be a stable and recyclable photocatalyst for the degradation of 4-NPh and p-DCB, respectively.

**Figure 9.** Recycling of Ag\_MgGaAl photocatalyst for 4-NPh (Δ) and p-DCB () degradation.

Information about the reactive species formed during the degradation of 4-NPh was obtained by using quenchers, such as isopropanol, that acts as efficient radical scavengers for •OH and benzoquinone, which is acting as an O2•<sup>−</sup> radical scavenger [37,48]. The results are given in Supplementary Figure S3 and show that in the presence of isopropanol the TOC removal (%) of 4-NPh decreased from 87% to 51%, pointing out that •OH radicals act as major reactive species in the photocatalytic system. Further, when benzoquinone was added to trap O2•−, an obvious inhibition of degradation process was observed, from 87% to 59%, revealing that the addition of benzoquinone decreased the amount of involved O2•<sup>−</sup> during the degradation process. Based on these results, the possible photocatalytic mechanism for the degradation of the studied pollutants is illustrated in Scheme 1 where the pollutants are denoted as Ar-X. It describes that, under solar irradiation, nanoparticles of silver have given rise to photogenerated electron–hole pairs and, some of the photogenerated electrons at the CB of the plasmonic silver could be transferred

to the CB of the LDH, due to the more negative CB position of the LDH than that of nanosilver [50]. Thus, the dissolved oxygen formed •O2 <sup>−</sup> radicals, and then further •OH by combining with H2O. These are preliminary results regarding the mechanism of the studied photocatalytic degradation and will be explored further in future work.

**Scheme 1.** Possible photocatalytic mechanism for the pollutant's degradation.

Next, we used the experimental data fitting to the pseudo-first, pseudo-second, Weber intraparticle diffusion and Elovich kinetic models, which were used to further investigate the kinetics of the photodegradation of 4-NPh, p-DCB and their mixture, under solar light. Each data set was fitted to the above-described models and the accuracy of the proposed models and the corresponding correlation coefficients are described in Figure 10 and Table 2. The kinetic analysis reveals that the pseudo-first order model fits relatively well when just one pollutant is present in the aqueous solution. As shown by the conversion data presented in Figure 8, the reaction rate increased almost six times for p-DCB degradation in comparison to that of 4-NPh degradation. Therefore, when the pollutants are mixed together, the pseudo-first order model is almost acceptable to fit the data concerning 4-NPh degradation (R<sup>2</sup> = 0.8245), while for p-DCB degradation, the pseudo-second order model fits the decomposition of p-DCB in the mixture with 4-NPh. However, to point out the above proposed kinetic models we present in Figure 10a,b the linear fit for all the systems describing the pseudo-first and pseudo-second order kinetics. Furthermore, the Weber intraparticle diffusion model [51] was chosen to highlight the influence of the intraparticle diffusion on the reaction rate, considering that the adsorption on the photocatalyst surface of the transformed species is a key step of the process, and results are shown in Figure 10c. If data fitted to a linear plot defined as qt versus t0.5 with the plot passing through the origin, the intraparticle diffusion is the only rate-controlling step [45]. In the case of our reactions, the plots contain two linear portions for the conversion of p-DCB, alone or mixed with 4-NPh. This reveals that there is more than one rate-controlling step; thus, the adsorption of p-DCB occurs by weaker binding on the photocatalyst surface, followed by the oxidation through the HO radicals, while the ionic structure of the 4-NPh could favor binding forces through electrostatic attraction with the catalysts. Moreover, the deactivating effect the nitro group exerted on the aromatic ring might contribute to a certain inertia of this molecule in the oxidation step. We further reveal that the Elovich model did not fit for any of the studied catalysts because there are no linear dependencies between ln t and qt, as shown by results in Figure 10d hence, the α and β parameters were not calculated for this model.

**Figure 10.** Kinetic analysis as fitted to: (**a**) pseudo-first order; (**b**) pseudo-second order; (**c**) Weber intraparticle diffusion; (**d**) Elovich kinetic models.


**Table 2.** Kinetic parameters and the correlation coefficients R2.

\* calculated for the first linear range.
