# Photocatalytic Activity of Monosized AuZnO Composite Nanoparticles
## 3 Results and Discussion
### Optical Properties of AuZnO Composite Nanoparticles  
As mentioned above, the PEO-PPO-PEO molecules are placed on the surfaces of AuZnO composite nanoparticles. Therefore, the composite nanoparticles are both hydrophobic and hydrophilic, and their transfer between the non-polar and the polar solvents does not require further surface modification. The UV-Vis absorption and fluorescence spectra of the AuZnO composite nanoparticles dispersed in water for the former and in hexane for the latter were recorded in Figures 5 and 6, respectively. The UV-Vis absorption spectra of AuZnO composite nanoparticles are compared with Au and ZnO nanoparticles of similar size dispersed in water, as shown in Figure 5. The spectra clearly show band edges at 364 nm, 358 nm, and 362 nm for S1, S2, and S3 respectively, with similar shapes but blue shifted bands compared to the pure ZnO (366 nm), which is the most characteristic absorption of the ZnO semiconductor. The shift and band shape change are related to the SPR of the Au. For S3, there are two kinds of absorption bands. One is centered at 362 nm from the band edge of ZnO, and the other is centered with a maximum at 569 nm from the SPR of the nanostructured Au. Compared to pure Au nanoparticles (542 nm), the SPR band shifts to higher wavelengths. Similar SPR band red shift phenomena were reported for Au-ZnO, AuFe, Ag-ZnO, and Ag-Fe\({}_{3}\)O\({}_{4}\) nanocomposites [35; 36; 37; 38]. These shifts may be due to the charge transfer between the Au nanoparticles and the ZnO nanoparticles in the heterodimeric structure [39]. The AuZnO heterodimeric nanostructures that regulate SPR band red shift can open the door to advanced materials for construction. Figure 4: Fourier transform infrared spectroscopy (FTIR) spectra of the AuZnO(S1–S3) composite nanoparticles (**a**–**c**) and the PEO-PPO-PEO (**d**). The photoluminescence spectrum of AuZnO composite nanoparticles in \(n\)-hexane was excited at a wavelength of \(\lambda\) = 360 nm, as shown in Figure 6. Emissions come from ZnO and Au nanostructures. In the case of sample S1, a unique emission of 400 nm appears, as shown by the inverted triangle, with a distinct peak at about 474 nm and a relatively strong emission at about 576 nm. In the spectrum, the blue band around 400 nm is most likely to undergo acceptor levels from the donor level of the Zn gap to the Zn vacancies [40]. The blue band at about 474 nm is due to the electronic transition of the donor level produced by valence band oxygen vacancies [41], and emission at about 580 nm is generally attributed to singly ionized oxygen vacancies in ZnO, due to recombination between electrons in the deep or superficial defect levels and holes in the valence band [42]. In sample S2, the intensity of the band gap emission is enhanced, and the position is slightly shifted to 402 nm. The same is true for the distinct plateau at 474 nm, which also has a strong emission at about 578 nm. Sample S3 also exhibited a strong emission at about 403 nm, with a firm plateau at about 476 nm and another emission at about 580 nm. It is of interest to emphasize that the photoluminescence spectra of the polymer-coated AuZnO composite nanoparticles (S1-S3) dispersed in hexane is gradually red shifted with the increase of concentration of Au. This is because that when nanosized Au combines with ZnO, the interface interaction causes the influence of the charge change on the Au surface [43]. Figure 5: Ultraviolet-visible light absorbance spectrometer (UV-Vis) spectra of Au, ZnO, and AuZnO nanoparticles (S1–S3) dispersed in water. Figure 6: Photoluminescence spectra of AuZnO(S1–S3) in \(n\)-hexane under excitation of the wavelength \(\lambda\) = 360 nm. Figure S7 shows the thermogravimetric and differential thermal analysis (TG-DTA) curves of PEO-PPO-PEO-terminated AuZnO(S2) composite nanoparticles (Figure S7, shown in supplementary materials). It can be seen from the TG curve in the figure that the weight loss of PEO-PPO-PEO-capped AuZnO composite nanoparticles can be divided into three stages.