## 3 Results and Discussion
### Photocatalytic Mechanism  
The mechanism for the photocatalytic degradation of MB with a ZnO photocatalyst is shown in Figure 9. The effectiveness of ZnO NPs in MB decomposition can be explained on the basis of the \(\bullet\)OH radicals and holes present on the surface of ZnO NPs, which break down MB dye in aqueous solutions.  
Figure 8: Cycling tests for photocatalytic degradation of MB, using Sample B, under UV-light irradiation.  
Figure 9: Schematic of ZnO NP photocatalytic processes.  
When a semiconductor particle absorbs a photon with an energy (\(\it{h}\it{v}\)) matching or exceeding its bandgap energy (E\({}_{\rm{g}}\)), an electron (e\({}_{\rm{cb}}\)\({}^{-}\)) is excited from the valence band (V\({}_{\rm{B}}\)) to the conduction band (C\({}_{\rm{B}}\)), leaving a hole (h\({}_{\rm{vb}}\)\({}^{+}\)) behind. The photo-generated electrons and holes can recombine and dissipate the input energy as heat, get trapped in metastable surface states, or react with electron donors and acceptors adsorbed on the semiconductor surface or within the surrounding double electrical layer of charged particles [14]. However, surface defect states, related to oxygen vacancies, are available to trap electrons or holes. Thus, the probability of recombination is reduced and subsequent redox reactions may occur.  
As mentioned above, oxygen vacancies play an important role in the degradation of dye under UV-light irradiation. They create an impurity level between the C\({}_{\rm{B}}\) and V\({}_{\rm{B}}\) of the vacuum-treated ZnO, which acts as a deep donor defect level, near the top of V\({}_{\rm{B}}\)[45]. The V\({}_{\rm{o}}\) defect energy levels used in Figure 9 are obtained by calculating the emission energy (2.64 eV) corresponding to the peak around 470 nm (PL spectrum shown in Figure 6). The calculated bandgap energy of 3.23 eV corresponds to the band-edge emission peak at around 384 nm, as shown in Figure 6. Indeed, an \"empty\" oxygen site (V\({}_{\rm{o}}\)\({}^{2+}\)) can trap either one (V\({}_{\rm{o}}\)\({}^{+}\)) or two (V\({}_{\rm{o}}\)\({}^{0}\)) electrons [41], thereby effectively reducing the recombination rate of the e\({}_{\rm{cb}}\)\({}^{-}\)-h\({}_{\rm{vb}}\)\({}^{+}\) pairs. More e\({}_{\rm{cb}}\)\({}^{-}\) and h\({}_{\rm{vb}}\)\({}^{+}\) migrate to the surface of the vacuum-treated ZnO and promote the photocatalytic degradation reactions.  
Moreover, particle size and photocatalytic activity are related. In particular, semiconductor NPs exhibit quantum size effects as the particle size decreases. This size reduction may improve the photocatalytic efficiency by increasing the effective bandgap [46]. In addition, reducing the particle size leads to a larger specific surface area. The latter is defined as the total surface area used in the redox reaction, with a corresponding increase in the number of active sites exposed. A decrease in particle size should also lead to higher photonic efficiency, due to a higher interfacial charge carrier transfer rate [47].  
For wastewater treatment, dispersibility of the catalyst is another performance factor. As more catalyst particles are dispersed in the liquid, the total reaction surface area increases and this leads to a higher light harvesting efficiency. This will promote the adsorption of molecules on the surface of the catalytic particles. Consequently, the overall photocatalytic efficiency of the pristine ZnO NPs increases after vacuum treatment.