*3.4. Photocatalytic Degradation of Orange II under UVA and White Light*

The photocatalytic activities of ZnO and ZnO–Ag were evaluated by exposure of samples in glass beakers to UVA (365 nm wavelength UVP pen Ray model 11SC-1L) or white irradiation (fluorescent lamp PL G23 11 W 6400 K, 400–730 nm). Photocatalytic tests were performed on 10 mL of aqueous samples containing 50–1000 mg L−<sup>1</sup> of photocatalyst and 10 mg L−<sup>1</sup> of OII, at pH 7 and room temperature. The fluorescent lamp is located externally on one side, at approximately 3 cm from the vial, and the UVA lamp is in the center of the samples, using a submerged quartz tube. Two types of control samples were performed in parallel: direct photolysis control samples of OII under the same irradiation conditions; and adsorption control samples containing photocatalysis and OII, under the same sample preparation but kept in darkness. The solutions were stirred for 30 min in dark to achieve adsorption equilibrium. At regular intervals, spectrophotometric measurements were performed to monitor OII concentration in a BioTek PowerWave XS2 micro-plate spectrophotometer (Winooski, VT, USA). The photodegradation yield (%) was determined using the following equation:

$$\text{Yield } (\%) = (\text{C}\_{\text{OII}, \text{I}} - \text{C}\_{\text{OII}, \text{t}}) \text{(C}\_{\text{OII}, \text{0})} \times 100\text{\%} \tag{1}$$

#### *3.5. Determination of Kinetic Parameters*

The determination of kinetic parameters was performed by adjusting a pseudo-first order kinetic model (Equation (2)) to each set of photocatalyst concentration used in the UVA and white light studies. The linearization of this equation (Equation (3)) and the expression used to calculate the half-life (Equation (4)) are shown below:

$$\mathbf{C}\_{\rm OII,t} = \mathbf{C}\_{\rm OII,i} \mathbf{e}^{-\rm kt} \tag{2}$$

$$\ln\left(\mathbf{C}\_{\text{OII},\text{i}}/\mathbf{C}\_{\text{OII},\text{t}}\right) = \text{kt},\tag{3}$$

$$\mathbf{t}\_{1/2} = \ln(2)/\mathbf{k}\_{\prime} \tag{4}$$

being k the kinetic constant; t the time of the experiment and t1/<sup>2</sup> the half-life of the compound under study.

#### **4. Conclusions**

ZnO nanoparticles were prepared by a simple polyol-mediated method and successfully decorated with Ag nanoclusters, obtaining a novel nanocomposite (ZnO–Ag) with different degrees of silver loadings (1.3–7.4% *w*/*w*). In addition, the final dispersions of nanoparticles received a photochemical treatment to remove the residual Ag, avoiding the interferences in the subsequent photodegradation step. The influence of the Ag content on ZnO regarding the removal of Orange II was studied, obtaining that the presence of this noble metal at 1.3% greatly enhanced the photocatalytic activity, which suggests the potential of this nanocomposite to be applied in prospective applications in the field of water treatment, both in drinking and wastewater treatment plants. Semiconductor photocatalysis represents a promising alternative to conventional technologies since the use of chemicals would be avoided and solar energy could be used as photon source. In addition, the unspecific oxidation mechanisms in AOPs allow degradation and mineralization of a wide range of pollutants.

In this work, photocatalytic studies were performed under UVA and white light, obtaining the optimum concentrations of catalyst and nanoclusters that achieved removal percentages up to 75% for visible light after 3 h and nearly complete removal for UVA after 1 h. Further research is needed to fully explore this photocatalysis in practical applications. One of the main drawbacks of this catalyst is its separation from the water matrix after treatment. Its immobilization on a suitable support that avoids additional steps such as centrifugation, which is the method used so far, will allow the reuse of the photocatalyst in different water treatment cycles, bringing this research closer to a real wastewater treatment plant. An alternative for this immobilization is the deposition of the NPs onto magnetic nanoparticles, which can be easily separated by applying a magnetic field. Moreover, immobilization over supports such as silica, zeolites or alumina would improve the recovery of the catalysts towards their industrial applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/1/31/s1. Figure S1: AFM topography image and line profiles of small Ag nanoclusters deposited on mica.

**Author Contributions:** Conceptualization and supervision, M.T.M., G.F., M.A.L.-Q. and C.V.-V.; investigation, J.G.-R.; resources, Y.B.B., D.B.; writing–original draft, L.F.; writing–review & editing, J.G.-R., M.T.M., C.V.-V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by two projects granted by Spanish Ministry of Science, Innovation and Universities: MODENA Project CTQ2016-79461-R and CLUSTERCAT Project MAT2015-67458-P, and Fundación Ramón Areces, Spain (Project CIVP18A3940).

**Acknowledgments:** J.G.-R. thanks the Xunta de Galicia Counseling of Education, Universities and Vocational Training for his predoctoral fellowship. M.T.M., G.F. and J.G.-R. belong to CRETUS Institute. The authors also thank Xunta de Galicia for the CRETUS (AGRUP2015/02) and AEMAT (ED431E-2018/08) Strategic Partnerships, and the use of RIAIDT-USC analytical facilities. The authors belong to the Galician Competitive Research Groups ED431C-2017/22 and ED431C-2017/29, co-funded by FEDER.

**Conflicts of Interest:** The authors declare no conflict of interest.
