## 3 Results
### Photocatalytic Properties of ZnO Powder
Figure 3: Photocatalytic performance of the synthesized ZnO powder: absorption spectra of DNP (**a**) and CIP (**b**) photodegradation under simulated solar light; kinetic linear fitting curves for DNP (**c**) and CIP (**d**) and corresponding apparent rate constants. The mass photoactivity of commercial nanopowder photocatalysts is evidently higher than the one of the synthesized microparticles due to its lower particle size, higher surface area, and more active sites exposed for photoreaction [34]. However, on the one hand, the rate of collision and agglomeration is considerably higher for the lower particle size, leading to the formation of much larger agglomerates during the cycling test of the photocatalytic degradation. Particle aggregation and agglomeration influence the optical properties of materials and, therefore, their ability to absorb and scatter the incoming radiation, also affecting their photocatalytic activity. Moreover, the highly active nanoparticles are more vulnerable to photocorrosion when compared to microparticles [35]. On the other hand, nanoparticle synthesis requires more time and cost. Furthermore, it is not always possible to scale the nanoparticle synthesis process. In this work, the processing rate of Zn microparticles was 20 g/min and this can be increased in the future. There may be several reasons for the high photoactivity of ZnO microstructures. First of all, it may be due to a structural factor. It was found [36] that nanorods have the greatest photocatalytic activity and this is associated with a high proportion of (100) texture. According to [37; 38], in the plane (100) is the lowest energy of formation of defects, in particular, oxygen vacancies. According to XRD (Figure 2b), the ratio is \(\mathrm{I}_{100}/\mathrm{I}_{001}\)-1.27. Taking into account the large number of rod-like (001) structures of ZnO (Figure 2a), the value of the ratio 1.27 indicates the presence of a high fraction of (100) texture in the ZnO powder. The next important factor is the adhesion levels of charge carriers in the ZnO band gap, which are associated with defects. Adhesion levels inhibit recombination of charge carriers and enhance photocatalytic activity. To determine the energy zone structure of the ZnO powder, XPS valence band spectra were obtained and the band gap was determined (Figure 4). The presence of energy state density near Fermi energy can be seen. These levels expand the photosensitivity of ZnO to the visible area. The VB\({}_{\mathrm{max}}\) energy was estimated to be 2.3 eV. Based on this, CB\({}_{\mathrm{min}}\) was estimated to be \(-\)0.9 eV. The levels in the band gap are mainly associated with oxygen vacancies and interstitial zinc [39]. For oxygen vacancies in the neutral V\({}_{\mathrm{O}}\) and 2+ charge states V\({}_{\mathrm{O}}\)\({}^{2+}\), a localized occupied state is recognized in the band gap at 2.5-2.6 eV and 0.9-1 eV below the conduction-band minimum [40], respectively, and this suggests the activity of ZnO tetrapods in the visible region of the spectrum. Figure 4: _Cont._  
This correlates with the structural features of ZnO powder and their effect on photocatalytic properties. As can be seen, the presence of nitrogen in the volume of ZnO structures does not lead to a narrowing of the band gap; however, energy levels associated with the nitrogen mixture in the band gap are present. For example, the energy state (0.3-0.4) eV below the bottom of the conduction zone is associated with the V\({}_{\text{O}}\)-N defect complex [41]. There are also data on the acceptor level N2p above the valence zone by 0.76 eV [18].