**3. Materials and Methods**

### **3. Materials and Methods**  *3.1. Preparation of CuFeS<sup>2</sup>*

*3.1. Preparation of CuFeS2* All chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA) and were of analytical grade and used without further purification. In this study, hydrothermal (H) and co-precipitated method (C) were used to prepare CuFeS2 samples, representing as H-CuFeS2 and C-CuFeS2, respectively. For hydrothermal procedure, 0.989 g of CuCl, and 2.703 g of FeCl3·6H2O were added to 57 mL of deionized water, with stirring for 10 min. Then, 8 mL of Na2S ·9H2O (0.02 mol) was added dropwisely into the above green mixture. All chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA) and were of analytical grade and used without further purification. In this study, hydrothermal (H) and co-precipitated method (C) were used to prepare CuFeS<sup>2</sup> samples, representing as H-CuFeS<sup>2</sup> and C-CuFeS2, respectively. For hydrothermal procedure, 0.989 g of CuCl, and 2.703 g of FeCl3·6H2O were added to 57 mL of deionized water, with stirring for 10 min. Then, 8 mL of Na2S ·9H2O (0.02 mol) was added dropwisely into the above green mixture. After stirring for 30 min, the black mixture was transferred into a Teflon-lined stainless-steel

autoclave. The autoclave was sealed and heated in an electric oven at 200 ◦C for 10 h. After the autoclave naturally cooled to room temperature, the precipitates were centrifuged (5000 rpm, 15 min) and washed three times with ethanol and deionized water, and then dried in vacuum at 60 ◦C overnight. In addition, Cu2S and FeS<sup>2</sup> nanoparticles were prepared following similar method without adding FeCl3·6H2O and CuCl precursor, respectively.

For the co-precipitated method, 4.95 mg of CuCl, and 0.0135 g of FeCl3·6H2O were added to 20 mL of deionized water, with stirring at 70 ◦C for 10 min. Then, 1 mL of NH4OH (30%) and 1 mL of N2H4·H2O (64–65%) were added dropwise into the above mixture with stirring at 70 ◦C for 3 h. After that, 0.024 g of Na2S·9H2O was added into the above brown mixture with stirring at 70 ◦C for 3 h. Finally, the black precipitates were centrifuged (5000 rpm, 15 min) and washed three times with ethanol and deionized water, and then dried in vacuum at 60 ◦C overnight.
