*2.1. Characterization of Catalysts*

Three types of catalysts were synthesized, based on Au@ZnONPs, Au@ZnONPs-3%rGO, and Au@ZnONPs-3%rGO-3%gC3N4. In these catalysts, the percentage of rGO and gC3N<sup>4</sup> was always maintained at 3%, although percentages of Au nanoparticles of 1%, 5% and 10% were used, thus, a total of nine catalysts were obtained. All these catalysts were used for the photodegradation of CFX and LFX, and the most efficient catalyst (10%Au@ZnONPs-3%rGO-3%gC3N4) was the one that was fully characterized by different techniques.

The BET surface area of the catalysts was analyzed (see Table 1). ZnO nanoparticles showed a relatively large surface area of 24 m2g −1 . Incorporation of Au on the surface increased the specific area of the material to 58 m2/g in the case of 1%Au@ZnONPs, and continued to increase, as a function of Au loading, up to a maximum value of 78 m2/g in 10%Au@ZnONPs. This behavior, which has been described previously with different metallic nanoparticles, is expected to contribute to the catalytic activity of the material. The incorporation of 3%rGO, which already has a very large surface area, considerably increases the surface area of the material, reaching values as high as 196 m2/g in the case of 10%Au@ZnONPs-3%rGO. The incorporation of gC3N<sup>4</sup> also increased the area of the material, although not as drastically as in the case of rGO, reaching area values of 229 m2/g in the case of 10%Au@ZnONPs-3%rGO-3%gC3N4. This last catalyst, with the highest specific area, is the most active in the processes studied, as will be seen later.

**Table 1.** BET surface area of the different as-synthesized materials.


The dispersion of 10%Au on the support of ZnONPs was followed by elemental mapping (see Figure 1). Figure 1b,c show the distribution of Zn and Au, respectively, corresponding to the SEM image of Figure 1a. As can be seen, Au presents some aggregates, although, in general, and considering the high percentage of the metal, a good dispersion is observed. Using this material as the starting catalyst, 3% rGO and 3% gC3N<sup>4</sup> were incorporated. Figure 2 shows the TEM and HR-TEM images obtained for the most active catalyst in the photodegradation reactions (10%Au@ZnONPs-3%rGO-3%gC3N4). Figure 2a shows the TEM image of the catalyst, in which the heterostructure can be observed, together with the highly dispersed gold nanoparticles on the surface. The inset image presents the selected area electron diffraction (SAED) pattern of the photocatalyst, demonstrating the crystalline nature of the sample. Figure 2b shows the HRTEM image of a ZnO nanoparticle showing the distinct lattice fringes with interplanar spacing of 0.28 nm, indexed to the (100) crystal plane of ZnO with a hexagonal wurtzite structure [38]. Figure 2c shows a

region of the catalyst gC3N4. A crystalline structure is observed, whose lattice distance is approximately 0.33 nm, corresponding to the (002) plane of gC3N<sup>4</sup> [39,40]. Figure 2d shows the distribution of gold nanoparticles on the heterostructure, together with the area of the sample that has been identified as rGO. One of these nanoparticles is the one that has been magnified in Figure 2e, whose interplanar spacing of ca. 0.23 nm has been indexed to (111) crystal plane of Au [41]. Figure 2f presents the ultra-high resolution detail of a gold nanoparticle just over 1 nm in diameter. This particle has been further enlarged to appreciate the details of the icosahedral structure. The white lines delineate the boundaries between five different crystal domains on the nanoparticle. One of the faces shows the interplanar spacing of 0.23 nm, assigned to Au (111).

**Figure 1.** (**a**) SEM image of 10%Au@ZnONPs, (**b**) the corresponding elemental mapping of Zn and (**c**) Au. HV (20 keV), magnification 2500×, and WD (15 mm).

**Figure 2.** (**a**) TEM and (**b**–**f**) HR-TEM images of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> . (**a**) Low magnification and SAED pattern of the heterostructure; (**b**) magnified image of a ZnO nanoparticle; (**c**) crystalline structure of gC3N<sup>4</sup> ; and (**d**–**f**) images at different magnification of AuNPs.

10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> was also characterized by X-ray diffraction (XRD). Figure 3 shows the diffraction pattern of the catalyst, along with that of rGO, gC3N4, and ZnONPs for comparison purposes. There is a broad peak at ca. 23.8◦ for rGO, assigned to the (002) crystal plane, indicating that most of the oxygen functional groups, characteristic of GO, have been removed from the surface [42]. Additionally, rGO shows a second peak at about 43◦ assigned to the (100) plane of the hexagonal carbon structure. The XRD pattern of gC3N<sup>4</sup> is shown in Figure 3b. Two broad peaks are observed at ca. 13◦ and 27.2◦ , which have been assigned to crystalline planes (100) and (002), respectively [39,40]. The peak shown at 27.2◦ , and which is also the most intense, corresponds to an interplanar spacing of 0.33 nm, the crystallographic planes which were observed in Figure 1c. The diffraction peaks of ZnONPs (see Figure 3c) can be unambiguously indexed to the ZnO phase of

hexagonal wurtzite [43], whose peaks are the dominant ones in the 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> catalyst, as observed in Figure 3d. Neither rGO nor gC3N<sup>4</sup> are detected in the XRD of the catalyst, possibly because the proportion of these components in the heterostructure is too small. Additionally, a low intensity peak is observed at ca. 38.1◦ , which has been assigned to Au (111) [41], and whose crystallographic planes were observed in Figure 2e.

**Figure 3.** (**a**) XRD patterns of rGO; (**b**) gC3N<sup>4</sup> ; (**c**) ZnONPs; and (**d**) 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> .

Figure 4 shows the Raman spectrum of 10%Au@ZnONPs-3%rGO-3%gC3N4, along with those of rGO, gC3N4, and ZnONPs. As can be seen, rGO is characterized by two broad bands at 1357 and 1600 cm−<sup>1</sup> (Figure 4a) that have been assigned to bands D (A1g mode), and G (E2g mode of sp<sup>2</sup> carbon atoms), respectively. The intensity ratio of these bands (IG/ID) is ca. 0.95, indicating that the GO reduction is not too high [43]. gC3N<sup>4</sup> (Figure 4b) shows two typical bands, similar to those seen in rGO, although slightly displaced. Band D is associated with the possible presence of sp<sup>3</sup> carbon, justified by structural defects and disarrangements, while band G is associated, in the same way as in rGO, with the presence of sp<sup>2</sup> carbon [44]. ZnONPs is characterized by showing two peaks at at ca. 566 cm−<sup>1</sup> and 1143 cm−<sup>1</sup> (see Figure 4c), which have been assigned to A1-LO and E1-LO vibration modes of ZnO, respectively [45,46]. Both peaks are indicative that ZnONPs has a wurtzite-like crystal structure, as established by XRD. The Raman spectrum of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> is characterized by showing a main peak at 1042 cm−<sup>1</sup> , which could come from the peak observed at 1143 cm−<sup>1</sup> in ZnONPs. Around ca. 630 cm−<sup>1</sup> an undefined peak is observed, which could also have its origin in the peak observed at 566 cm−<sup>1</sup> in ZnONPs. The displacements observed in the peaks of the catalyst support the fact that this material is an integrated heterostructure separate from its different components. Additionally, two small peaks are observed at ca. 1301 and 1588 cm−<sup>1</sup> that must necessarily have their origin in the contribution of rGO and gC3N4.

**Figure 4.** (**a**) Raman spectra of rGO; (**b**) gC3N<sup>4</sup> ; (**c**) ZnONPs; and (**d**) 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> .

The absorption of radiation by catalysts is crucial for their catalytic activity, so the different systems were analyzed using Tauc plots. As shown in Figure 5a, ZnONPs shows a bandgap in the UV region (3.23 eV). The incorporation of Au nanoparticles on the surface of the semiconductor (Figure 5b) produces a slight shift at 3.21 eV, still in the UV region. gC3N<sup>4</sup> (Figure 5c) and rGO (Figure 5d) show bandgaps at 2.79 and 2.33 eV, respectively, already in the visible range. The addition of these components will be able to displace the bandgap of the catalyst towards the visible region. In fact, Figure 5e shows how the bandgap moves towards 2.77 eV in the 10%Au-ZnONPs-3%rGO composite. The addition of gC3N<sup>4</sup> (Figure 5f) still causes a greater displacement, in this case up to 2.73 eV, falling squarely in the visible region. The results obtained justify the activity of the 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> heterostructure under irradiation with visible light, as will be described in the section corresponding to catalytic results.

10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> was also characterized by XPS. As shown in Figure 6a, two peaks at 1020.6 eV and 1044.1 eV have been assigned to the binding energies of Zn2p3/2 and Zn2p1/2, respectively, indicating the presence of Zn2+ [47]. Furthermore, the spin-orbit splitting of these two peaks at 23.5 eV also confirmed the presence of ZnO [48]. The transition corresponding to O1s (see Figure 6b) showed a major peak at ca. 530.2 eV, which was assigned to O2<sup>−</sup> species in the ZnO network, and a shoulder at ca. 532.1 eV, assigned to O2<sup>−</sup> in oxygen-deficient regions, respectively [49]. The Au4f peak (Figure 6c) was fitted to two peaks at 83.3 and 86.9 eV, attributed to Au4f7/2 and Au4f5/2 double peaks, respectively, in metallic gold (Au<sup>0</sup> ) [50]. The C1s transition (Figure 6d) showed a peak at 287.6 eV, which was assigned to C-N-C bonds, and a less intense one at 284.8 eV attributed to the aromatic C atom in the s-triazine ring, respectively [51,52]. Nitrogen from graphitic carbon nitride was evidenced by the N1s transition (see Figure 6e). This clearly asymmetric peak could be deconvolved into two components, showing an intense peak at 398.7 eV, which was assigned to C=N-C, indicating the presence of triazine rings, and a less intense peak at 400.1 eV that was assigned to the presence of tertiary N atoms (N-(C)3) [53,54]. As previously shown, the XPS analysis further confirmed the association of the different components (Au, ZnO, rGO, and gC3N4) in the heterostructure.

**Figure 5.** (**a**) Tauc plots of (αhν) <sup>2</sup> versus energy (eV), and determination of the bandgap energy of ZnONPs; (**b**) 10%Au@ZnONPs; (**c**) gC3N<sup>4</sup> ; (**d**) 10%Au@ZnONPs-3%rGO; (**e**) 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> and (**f**) rGO.

**Figure 6.** (**a**) XPS core level spectra for 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> : Zn2p; (**b**) O1s; (**c**) Au4f; (**d**) C1s; and (**e**) N1s.
