## 3 Results and Discussion
### Finding a Potential Technological Application of the ZnO/Clay Composite  
To study the relevant role as a preservative of nanoparticulated ZnO, market available raw materials are used. The selected matrix was modified bentonite clay with the presence of another inorganic preservative as silver. The choice of modified bentonite clay with Ag cations, hereafter Clay, is due to the well-known antibacterial properties of Ag [31; 32]. Even, previous studies show the good antibacterial activity of this modified clay against different types of bacteria [30; 33]. Therefore, it is mandatory to complement the antibacterial activity of Ag cation with a preservative against fungi such as ZnO. For a better integration of ZnO obtained with modified bentonite clay, we pose a dry dispersion of them at room temperature to avoid alteration of the materials [34]. Recently, the dry dispersion method was proposed to obtain hierarchical nanoparticle-microparticle systems with unusual properties by mixing oxides of dissimilar materials [34; 35; 36; 37; 38]. These hierarchically-dispersed materials raise additional environmental value since their synthesis is particularly clean. They are prepared by mixing oxides of different materials using a residue-free and solvent-free nanodispersion method that leads to hierarchical nano-microparticle systems [34; 35; 36; 37; 38]. More importantly, it has been proven that, after the partial reaction of two oxides, the creation of interfaces at the nano-scale range endows these materials with new properties that range from magnetic or optic to catalytic [34; 35; 37; 38], due to proximity and diffusion phenomena. Consequently, here, the dry dispersion method is taking advantage of the surface energy differences between dissimilar oxide that it attained by shaking the modified clay with ZnO obtained, and 1 mm ZrO\({}_{2}\) balls in a 60 cm\({}^{3}\) nylon container for 5 min at 50 rpm using a tubular-type mixer. Different composites ZnO/Clay are obtained according to the concentration of nanoparticulated ZnO, from 2 to 60 wt.%. The structural characterization of the composites is carried out by XRD (Figure 1). As discussed above, the XRD pattern of ZnO obtained matches the hexagonal wurtzite structure ZnO (JCPDS Card No. 36-1451). The XRD pattern of bentonite (Figure S3 (Supplementary Materials)) shows the typical diffraction peaks (JCPDS Card No. 003-0019) and the presence of cristobalite as an impurity (JCPDS Card No. 039-1425). As expected, the XRD pattern of ZnO/Clay composite displays an increase in the intensity of the diffraction peaks relative to ZnO while the diffraction peaks of clay decrease their intensity accordingly. In order to clarify the addition of ZnO and the dispersion state of ZnO/Clay composites, a FE-SEM characterization is performed. For this, it is necessary to separately characterize the morphology of each ZnO/Clay composite components (that is, the bentonite clay and the nanoparticulated ZnO). So, the bentonite clay is composed of layers or compacted plates (Figure S4a (Supplementary  
Figure 1: Structural characterization of ZnO obtained and ZnO/Clay composites. XRD diffractograms of ZnO, bentonite clay and modified clay with different compositions of ZnO (from 2% to 60%). Materials)). While the morphology of nanoparticulated ZnO (Figure S4b (Supplementary Materials)) are nanoparticles with sizes of ca. 56 nm. Note that the expected behavior about the dispersion of nanoparticulated ZnO is a partial coating of clay aggregates, according to the dry dispersion procedure [34; 36; 37]. Figure 2 displays the changes in clay morphology corresponding to the addition of nanoparticulated ZnO. The 2 wt.% of dispersed ZnO on clay (Figure 2a) is practically not observed in the micrograph. From 6 wt.% of ZnO (Figure 2b), nanoparticles of ZnO are deposited onto the clay surface. For concentration between 10 wt.% (Figure 2c) and 20 wt.% (Figure 2d) of dispersed ZnO, the presence of nanoparticle onto clay surface increases, reaching a coated clay.