# Photocatalytic Activity of Monosized AuZnO Composite Nanoparticles
## 3 Results and Discussion
### Photocatalytic Testing  
The photocatalytic performances in UV and visible light regions were investigated via the degradation of RhB, which is commonly used as a probe molecule to evaluate the photocatalytic activity of catalysts. Figure 7 shows the photocatalytic activity of prepared AuZnO(S1-S3) and ZnO for the degradation of RhB under (a) UV irradiation and (b) visible light irradiation. Experiments without a catalyst addition and with no light conditions were also shown in the figure for comparison purposes. Figure 7 presents a comparative study for the degradation of RhB under UV in the presence of different catalysts. RhB decomposes 85.7% (curve 1), 57.2% (curve 2), and 34.4% (curve 3), respectively, when AuZnO(S1-S3) acted as a photocatalyst for 90 min under UV irradiation, while 53.1% (curve 4), and 6.4% (curve 6) degradation were observed in the presence of ZnO under UV irradiation and in the dark. At the same time, 4.3% (curve 5) is barely exposed, and only with UV light (photolysis). Figure 7 shows the photocatalytic activity of the prepared AuZnO(S1-S3) and ZnO, to degrade RhB under visible light irradiation. It is observed that the degradation of RhB is 63.5% (curve 1), 31.8% (curve 2), 18.7% (curve 30), and 49.9% (curve 4) respectively. The photocatalytic activity order of AuZnO composite, both under UV and under visible light is S3 \(<\) S2 \(<\) S1. Samples S1 and S2 reveal better capability than pure ZnO under UV irradiation, while only S1 shows better capability than ZnO under visible light. Sample S3 shows the lowest photocatalytic activity. XRD analysis indicates that Au amount increased from S1, S2, to S3, revealing the surfaces of S3 could almost completely covered by Au, which is most likely the main reason for the poor performance of S3 owing to the reduced the surface contact area between RhB and ZnO. In this experimental range, the smaller amount of Au will make the better photocatalytic activity of AuZnO composite nanoparticles. We believe that the appropriate amount of Au on ZnO is the main factor for the enhancement of the photocatalytic activity.  
In order to explore the reusability of the AuZnO catalyst under UV and visible light conditions, RhB was degraded with the AuZnO catalyst and circulated under the same conditions for three cycles, and the results are shown in the Figure 8. It was noteworthy that the AuZnO(S1) photocatalyst still showed good photostability and photocatalytic activity under UV (Figure 8a) and visible light irradiation (Figure 8b), even after repeated use for three cycles. The recyclability of AuZnO is due to the resistance and stability to photo-corrosion, which plays a pivotal role for its efficient use, especially for a greener and environmentally-friendly approach. Similarly, the recyclability of AuZnO(S2-S3) composite nanoparticles was tested under UV irradiation (Figure S4). The photocatalytic activities of the S1 composite nanoparticles were 85.7%, 90.9%, and 86.2%, those of S2 were 57.2%, 62.9%, and 63.4%, and those of S3 were 34.4%, 36.3%, and 33.0%. The results show that the photocatalytic activity of AuZnO(S1-S3) remains unchanged after repeated use for three cycles. The photocatalytic activities order of AuZnO composite under UV irradiation is always S3 \(<\) S2 \(<\) S1. The AuZnO(S3) composite nanoparticles after cyclic experiments were characterized by XRD. As showed in Figure S5a, the structure of AuZnO(S3) composite nanoparticles did not change after cycling experiments, compared with the AuZnO(S3) composite nanoparticles of Figure S5b (Figure S5, shown in supplementary materials). This indicates that the AuZnO(S3) composite nanoparticles have good stability as a catalyst in the dye solution.