*2.2. Morphology and Composition Analyses*

The morphology and crystalline features of the as-prepared catalysts were investigated by HRTEM and HAADF STEM images, as shown in Figure 3a–k. The results indicate that Ag\_MgGaAl and Au\_MgGaAl were composed by small nanoparticles, with a relatively narrow size, that are randomly distributed on the larger nanoparticles of the LDH, as shown by TEM images in Figure 3a,c. For both Ag\_MgGaAl and Au\_MgGaAl, almost spherical dark spots defined by an average size less than 20 nm are clearly seen on the larger nanoparticles of MgGaAl (av. size ~130 nm).

**Figure 3.** (**a**) TEM image of Au\_MgGaAl; (**b**) Higher magnification of TEM image of Au\_MgGaAl; (**c**) low magnification TEM image of Ag\_MgGaAl; (**d,e**) SAED pattern of Au\_MgGaAl; (**f**) SAED pattern of Ag\_MgGaAl; (**g**) HRTEM image of Ag\_MgGaAl; (**h**) HRTEM image of a single nanoparticle on Au\_MgGaAl with identification of the crystal plane of gold; (**k**) HAADF STEM image of Au\_MgGaAl and the corresponding STEM-EDS element maps.

Notably, HRTEM and SAED images in Figure 3b,d–k clearly identified Au and Ag nanoparticles that are well crystalized with lattice fringes defined by an interplanar spacing of 0.24 and 0.27 nm, respectively. This corresponds to the face-centered cubic (fcc) structure of Au (111) planes and Ag (111), being consistent with the SAED patterns [38]. The typical selected area electron diffraction (SAED) patterns display bright circular rings, featuring a complex pattern in which the diffraction rings of gold (see Figure 3d,e) and silver (Figure 3f), where (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of Au and Ag, respectively are combined with the set of the diffraction pattern derived from the LDH [34]. These results demonstrate that in the heterostructured catalysts nanoparticles of Au and Ag, with an average size lower than 20 nm are in close conjunction [37] with the MgGaAl matrix. In addition, highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was employed to analyze the element distribution in Ag\_MgGaAl and Au\_MgGaAl heterostructures. The elemental mapping by energy-dispersive spectroscopy (EDS) was performed under STEM and we present the EDS map of Au\_MgGaAl in Figure 3k. Results confirm that Mg, Ga, and Al are uniformly distributed over the entire catalysts, while Au is dispersed on the catalysts surface. In addition, the EDS analysis (see Supplementary Figure S1) indicates that Au and Ag content of the catalysts are 3.8 wt% for Au in Au\_MgGaAl and 3.5 wt% for Ag in Ag\_MgGaAl, which is close to the calculated content for the synthesis.

### *2.3. Optical Characteristics and Plasmonic Response*

The formation of the plasmonic metals-LDH heterostructures was further tracked by analyzing the optical behavior of Ag\_MgGaAl and Au\_MgGaAl by UVDR analysis and the results are shown in Figure 4. The absorption edge of MgGaAl lies in the low UV region while the heterostructured catalysts reveal enhanced absorption in the visible range due to the LSPR response of plasmonic silver and gold. For Au\_MgGaAl, the peak with a maximum at 550 nm is attributed to the SPR band of well dispersed nanoparticles of Au. It originates from the intraband excitation of electrons in the outer orbital (6sp) of the Au species and confirms the production of energized electrons [38,39]. For Ag\_MgGaAl, the broad peak at 360–600 nm is assigned to the plasmonic response of silver, covering all the visible range. Moreover, the absorption bands of the calcined catalysts displayed higher intensity in comparison to the as-synthesized ones. This can be a consequence of the increase of the nanoparticle size during the calcination process [40–42].

**Figure 4.** UVDR spectra of the studied photocatalysts.
