## 3 Results and Discussion  
This study successfully used a simple and well-known hot-injection method at 280 \({}^{\circ}\)C in a N\({}_{2}\) atmosphere to prepare CZTS [44; 45]. In a typical experiment, two solutions were prepared for the synthesis process: the first solution (SOL-1) contained all metal precursors and was obtained by dissolving, in a 50 mL three-neck flask, 734 mg Cu(CH\({}_{3}\)COO)\({}_{2}\):H\({}_{2}\)O, 439 mg SmCl\({}_{2}\):2H\({}_{2}\)O, and 417 mg Zn(CH\({}_{3}\)COO)\({}_{2}\):2H\({}_{2}\)O in 20 mL of oleylamine (OLA) (in that order); the second solution (SOL-2) contained 128.70 mg of sulfur powder in 10 mL OLA. Both mixtures were connected to a vacuum/N\({}_{2}\) Schlenk line and vacuumed and degassed separately (SOL-2 was degassed for 1 h at 60 \({}^{\circ}\)C, while SOL-1 was degassed at 100 \({}^{\circ}\)C for 1 h). The SOL-1 flask was then filled with N\({}_{2}\), and the temperature was raised to 190 \({}^{\circ}\)C. The mixture was held for 10 min while the mixture color quickly turned from blue to yellow. The temperature was raised to 270 \({}^{\circ}\)C, and finally, SOL-2 was rapidly injected into the SOL-1 flask. After the injection process, the reaction mixture immediately changed color from pale yellow to dark brown, and the system was left to react at 270 \({}^{\circ}\)C for 30 min. The flask was finally removed from the heating mantle and cooled naturally to room temperature in a N\({}_{2}\) atmosphere. A mixture of chloroform and ethanol (in a ratio of 1:5 \(v/v\)) was added to the reaction flask, and the resulting suspension was centrifuged at 8000 rpm for 2 min. The supernatant was eliminated, and this washing step was repeated five times. Finally, the precipitated solid nanoparticles were dried in vacuum and stored under N\({}_{2}\). The so-obtained CZTS NPs were fully characterized through different analytical techniques to ensure their high quality. Based on XRD and Raman measurements, the good quality of the CZTS NPs was proven (Figure 1). Through XRD, only the peaks of CZTS were detectable (DB card number 00-026-0575), with no secondary phases being detectable. In the Raman spectrum, it is possible to see that the CZTS's lone peak slightly shifted to lower wavenumbers, typical of a nanometric crystalline structure [46]. No secondary phases were detected. Further characterization was performed using TEM and SEM-EDX measurements. The CZTS NPs had a spherical shape with an average diameter of 11 nm (Figure 2). The resulting elemental ratio perfectly respects the p-type semiconductor Cu\({}_{2}\)ZnSnS\({}_{4}\) with copper-poor and zinc-rich condition [47]. Cu/(Zn+Sn) = 0.86, and Zn/Sn = 1.14 (Figure 3). Figure 1: X-ray diffraction (XRD) pattern (**left**) and Raman Spectroscopy (RS) (**right**) of the synthesized Cu\({}_{2}\)ZnSnS\({}_{4}\) NPs. The CZTS NPs' photocatalytic activity, by the use of both UV-visible and only visible light, was tested in a water environment and in the presence of low concentrations of DCF, representing one of the most dangerous emerging water micropollutants. The UV-based degradation of DCF through direct photolysis is a well known process, and it has been reported in the literature [48]. This path leads to the formation of a plethora of degradation products. Among them, the derived carbazoles show toxicity and persistence in the environment, which is even more worrying than DCF itself. In this study, first, a possible DCF hydrolysis reaction was evaluated. For this purpose, a 10.0 mg/L DCF solution was kept in the dark and monitored by UV-vis spectroscopy for 120 min, showing no hydrolysis reactions, as depicted in Figure 4.  
As anticipated, DCF readily undergoes UV photolysis, leading to several toxic by-products, so in the first instance, it was necessary to verify whether this direct photolysis took place in our experimental conditions before checking the activity of the CZTS NP photocatalyst.