## 3 Results and Discussion
In Figure 5, it is evident how, after 45 min, the DCF main peak at 276 nm almost disappeared in favor of a new peak located at 235 nm and a broad shoulder peak between 310 and 350 nm, both shifts typical of the carbazole family degradation products [49]. Figure 4: UV-vis absorbance spectra of a 10.0 mg/L DCF solution maintained in the dark at t = 0, 30, 60, 90, 120, and 150 min. Figure 3: Further Cu\({}_{2}\)ZnSnS\({}_{4}\) NPs STEM and elemental content with SEM-EDX characterization. (**a**) Selected area electronic diffraction (SAED) pattern, (**b**) SEM-EDX, (**c**) high-resolution (HR)TEM image, and (**d**) Fast Fourier Transform of the HRTEM image indicating lattice fringes. Moreover, during photolysis, the solution's appearance also changed gradually, changing from transparent and colorless to slightly turbid and yellow (a color associated with carbazoles), as noted via a simple visual inspection over 120 min (see Figure S5). The same experiment was repeated by filtering the Xe lamp to verify whether any other degradation path could occur under only visible light irradiation. Figure 6 shows a slight decrease in the DCF peak at 276 nm, indicating that very modest DCF degradation could occur under those conditions. However, no carbazoles were formed, demonstrating that only the UV portion of the light is responsible for that detrimental degradation path. Given the preliminary results regarding DCF photolysis through UV-vis or only visible light, the photocatalytic activity of the CZTS NPs was tested in both the configurations of the Xe lamp, UV-filtered and unfiltered. Regarding the first case, a successful CZTS-mediated photocatalysis method could open up new industrial avenues and facilitate strategies to degrade dangerous water micropollutants such as DCF indoors or directly in dedicated pipelines. For the second case, we wanted to verify whether the photocatalytic path prompted by CZTS NPs could, in any case, prevail over the already fast and natural photolysis of DCF shown previously in Figure 5.  
Figure 5: UV-vis absorbance spectra of a 10.0 mg/L DCF solution under 1 sun (1000 W/m\({}^{2}\)) irradiance through the use of a 300 W Xenon unfiltered lamp (UV-vis light); t = 0, 1, 3, 5, 15, 30, 60, and 120 min. Figure 6: UV-vis absorbance spectra of a 10.0 mg/L DCF solution under 1 sun irradiance through the use of a 300 W Xenon UV-filtered lamp (visible light only); t = 0, 1, 3, 5, 15, 30, 60, and 120 min. The UV-vis absorbance spectra of two photocatalysis reactions involving a 10.0 mg/L DCF solution, mediated by CZTS NPs (1.0 g/L) under 1 sun irradiance at the reactor surface (where 1 sun = 1000 W/m\({}^{2}\) at AM1.5) through the use of a 300W Xenon unfiltered lamp (UV-vis light, Figure 7a) and UV-filtered lamp (visible light only, Figure 7b) over 120 min, are shown in Figure 7.  
First, we can observe how the photocatalysis was successful in both cases. Based on Figure 7a, despite the use of the entire light spectrum, including the UV portion, it seems that by the end of the 120 min experiment, DCF was almost completely degraded, with no formation of carbazoles, previously identified in Figure 5 with peaks at 230 nm and a shoulder peak between 310 and 360 nm. It must be pointed out that, during the first few minutes of the photocatalysis (t = 1, 3, 5, and 15 min), traces of by-products were probably formed, as suggested by the shoulders at around 235 nm. However, no other peaks can be detected by the end of the 120 min, indicating that CZTS NPs prevail and degrade the remaining DCF or even the corresponding by-products. Moving to Figure 7b, showing results derived from using visible light only, it is evident how the spectra are more precise, and no other signals seem to be formed during the reaction, suggesting that photocatalysis only involving visible light for DCF degradation could be easily performed indoors or in dedicated industrial setups.