*3.4. GO Transformation*

Under UV illumination, the color of the GO suspension changed from light yellow to dark brown (Figure S5), indicating that some oxygen-containing functional groups attached to the GO surface might be removed [19]. The variation of the solution absorbance with time was further determined by UV–vis spectrophotometry (Figure S6). The peak at 225 nm was attributed to the π -π\* transition of unsaturated C-C bonds of GO. After 6 h of UV irradiation, the absorbance at 225 nm increased, indicating that the sp<sup>3</sup> structure of GO was reduced and the sp<sup>2</sup> structure had been recovered [21]. It should be noted that our previous study demonstrated that UV light intensity greatly affected the absorbance of GO, and the absorption peak at 225 nm could be redshifted to 255 nm at a high light intensity of 54 mW cm<sup>−</sup><sup>2</sup> in 4 h [23]. In the present study, the light intensity was only 10 mW cm<sup>−</sup>2; thus, the absorption peak did not shift significantly. Raman spectra were further used to analyze the GO samples before and after UV illumination. The D band at approximately 1350 cm<sup>−</sup><sup>1</sup> and the G band at approximately 1580 cm<sup>−</sup><sup>1</sup> are the two characteristic peaks of GO. Peak D represents the vibration of sp<sup>3</sup> carbon atoms, and peak G is the characteristic peak of carbon sp2. The ratio of *I*D/*I*G is usually used as a qualitative measurement of the disorder degree caused by nonaromatic sp<sup>3</sup> carbon defects. After 6 h of illumination, *I*D/*I*G decreased only from 0.8481 to 0.8438 (Figure S7), indicating that the graphitization structure of GO was somewhat improved and that the sp<sup>2</sup> region was expanded. However, the insignificant decrease in *I*D/*I*G suggested that the UV light intensity was not high enough in the current study, which was in accordance with the changes in UV–vis absorbance.

**Figure 6.** Effects of HA on SMZ degradation kinetics (**a**) and 1O2 production (**b**).

Based on the above results, we proposed the possible cotransformation pathways of GO and SMZ (Figure 7). Similar to semiconductors, GO generated electrons and holes under illumination. Electrons could be captured by O2 to generate O2· −, which was further converted into 1O2 and ·OH. Meanwhile, GO could form excited-state GO\* under illumination, and then O2 accepted excess energy and generated 1O2. Therefore, ROS generated in the above ways promoted SMZ degradation. At the same time, GO could capture electrons to reduce its surface oxygen-containing functional groups.

**Figure 7.** Proposed pathways for ROS generation and transformation of GO and SMZ under UV light.
