*2.1. Characterization of the CuFeS<sup>2</sup> Samples*

The morphology and composition of the prepared H-CuFeS<sup>2</sup> and C-CuFeS<sup>2</sup> samples were analyzed through SEM and EDS-mapping (Figure 1). As shown in Figure 1, H-CuFeS<sup>2</sup> and C-CuFeS<sup>2</sup> samples appear as sphere-like structures, with the average diameter ranging 25–40 nm and 95–125 nm, respectively. The smaller particle size of the H-CuFeS<sup>2</sup> samples can be the result of hydrothermal treatment which hindered the particle growth. On the other hand, high concentration of N2H4·H2O was used as reducing agent to prepare

C-CuFeS<sup>2</sup> samples, resulting in particle agglomeration. In addition, Ostwald ripening may occur during heating procedure. Therefore, small C-CuFeS<sup>2</sup> samples dissolved and redeposited onto larger C-CuFeS<sup>2</sup> samples. From the results of energy dispersive spectrometer (EDS)-mapping (green color, S elements; blue color, Fe elements; and red color, Cu elements), the presence of Cu, Fe, and S elements in both CuFeS<sup>2</sup> samples were confirmed and dispersed well in their crystals. C-CuFeS2 samples, resulting in particle agglomeration. In addition, Ostwald ripening may occur during heating procedure. Therefore, small C-CuFeS2 samples dissolved and redeposited onto larger C-CuFeS2 samples. From the results of energy dispersive spectrometer (EDS)-mapping (green color, S elements; blue color, Fe elements; and red color, Cu elements), the presence of Cu, Fe, and S elements in both CuFeS2 samples were confirmed and dispersed well in their crystals.

CuFeS2 and C-CuFeS2 samples appear as sphere-like structures, with the average diameter ranging 25–40 nm and 95–125 nm, respectively. The smaller particle size of the H-CuFeS2 samples can be the result of hydrothermal treatment which hindered the particle growth. On the other hand, high concentration of N2H4·H2O was used as reducing agent to prepare

*Catalysts* **2022**, *11*, x FOR PEER REVIEW 3 of 21

**Figure 1.** SEM images and EDS-mapping of (**A**) H-CuFeS2 and (**B**) C-CuFeS2 samples. **Figure 1.** SEM images and EDS-mapping of (**A**) H-CuFeS<sup>2</sup> and (**B**) C-CuFeS<sup>2</sup> samples.

Figures 2 and 3 showed the TEM images and EDS spectra of the CuFeS2 samples. The diameters of both CuFeS2 samples from TEM images were consistent with the SEM results. We also found both CuFeS2 samples possessed 0.31 nm and 0.23 nm of lattice lines, corresponding to the crystal planes of (112) and (204). The EDS spectra of the prepared CuFeS2 samples confirm the presence of Cu, Fe, and S elements in their crystals, accordingly. The atomic ratios (Cu:Fe:S) for the H-CuFeS2 and C-CuFeS2 samples were determined to be 1.1:1:1.8 and 1.4:1:2.0, respectively. High content of Cu elements in the C-CuFeS2 in the sample is consistent with lower solubility predicted from smaller Ksp value of Cu2S when comparing with Fe2S3 (Ksp of Fe2S3: 3.7 × 10−19, Ksp of Cu2S: 2.0 × 10−47). Figures 2 and 3 showed the TEM images and EDS spectra of the CuFeS<sup>2</sup> samples. The diameters of both CuFeS<sup>2</sup> samples from TEM images were consistent with the SEM results. We also found both CuFeS<sup>2</sup> samples possessed 0.31 nm and 0.23 nm of lattice lines, corresponding to the crystal planes of (112) and (204). The EDS spectra of the prepared CuFeS<sup>2</sup> samples confirm the presence of Cu, Fe, and S elements in their crystals, accordingly. The atomic ratios (Cu:Fe:S) for the H-CuFeS<sup>2</sup> and C-CuFeS<sup>2</sup> samples were determined to be 1.1:1:1.8 and 1.4:1:2.0, respectively. High content of Cu elements in the C-CuFeS<sup>2</sup> in the sample is consistent with lower solubility predicted from smaller Ksp value of Cu2S when comparing with Fe2S<sup>3</sup> (Ksp of Fe2S3: 3.7 <sup>×</sup> <sup>10</sup>−19, Ksp of Cu2S: 2.0 <sup>×</sup> <sup>10</sup>−47).

XRD was used to investigate the crystal structure of the prepared CuFeS<sup>2</sup> samples. The XRD patterns of the prepared CuFeS<sup>2</sup> samples are shown in Figure 4A. The diffraction peaks at 29.5◦ , 49.1◦ , and 58.6◦ were identified and assigned to the (112), (204), (312), (204), and (312) faces of the tetragonal chalcopyrite CuFeS2, respectively (PDF 83-0983). Through the Scherrer equation, the average crystal size of H-CuFeS<sup>2</sup> and C-CuFeS<sup>2</sup> was 20.36 and 11.2 nm, respectively. The Raman spectra of the prepared CuFeS<sup>2</sup> samples are shown in Figure 4B. The Raman shifts at 212 cm−<sup>1</sup> , 276 cm−<sup>1</sup> , and 379 cm−<sup>1</sup> correspond to the S element, Cu(I)-S, and Fe(III)-S stretching vibration, respectively.

*Catalysts* **2022**, *11*, x FOR PEER REVIEW 4 of 21

**Figure 2.** TEM images and EDS spectra of H-CuFeS **Figure 2.** TEM images and EDS spectra of H-CuFeS 2 2 samples. samples. *Catalysts* **2022**, *11*, x FOR PEER REVIEW 5 of 21

XRD was used to investigate the crystal structure of the prepared CuFeS2 samples.

and (312) faces of the tetragonal chalcopyrite CuFeS2, respectively (PDF 83-0983). Through the Scherrer equation, the average crystal size of H-CuFeS2 and C-CuFeS2 was 20.36 and 11.2 nm, respectively. The Raman spectra of the prepared CuFeS2 samples are shown in Figure 4B. The Raman shifts at 212 cm−1, 276 cm−1, and 379 cm−1 correspond to the S ele-

**Figure 3.** TEM images and EDS spectra of C-CuFeS2 samples. **Figure 3.** TEM images and EDS spectra of C-CuFeS<sup>2</sup> samples.

ment, Cu(I)-S, and Fe(III)-S stretching vibration, respectively.

**Figure 4.** (**A**) XRD and (**B**) Raman spectra of H-CuFeS2 (black), and C-CuFeS2 (red) samples. **Figure 4.** (**A**) XRD and (**B**) Raman spectra of H-CuFeS<sup>2</sup> (black), and C-CuFeS<sup>2</sup> (red) samples.

As another quality assurance method, XPS analysis of the prepared CuFeS2 samples (Figures 5 and 6) revealed that it contains three elements: Cu, Fe, and S [48,49]. Highresolution XPS revealed Cu2p, Fe2p, and S2p in the H-CuFeS2 samples as shown in Figures 5B, 5C, and 5D, respectively. In Figure 5B, the peaks at 931.9 and 951.7 eV correspond to Cu+ 2p3/2 and Cu+ 2p1/2, respectively, whereas those at 933.2 and 953.0 eV correspond to Cu2+ 2p3/2 and Cu2+ 2p1/2, respectively. The peaks at 711.7 and 724.9 eV correspond to Fe2+ 2p3/2 and Fe2+ 2p1/2, respectively, whereas those at 715.2 and 734.3 eV correspond to Fe3+ 2p3/2 and Fe3+ 2p1/2, respectively (Figure 5C). The peaks at 162.5 and 167.8 eV correspond to S2− 2p and S6+ 2p, respectively (Figure 5D). For C-CuFeS2 samples, the peaks at 931.9 and 951.6 eV correspond to Cu+ 2p3/2 and Cu+ 2p1/2, respectively, whereas those at 934.1 and 952.7 eV correspond to Cu2+ 2p3/2 and Cu2+ 2p1/2, respectively (Figure 6B). The peaks at 711.4 As another quality assurance method, XPS analysis of the prepared CuFeS<sup>2</sup> samples (Figures 5 and 6) revealed that it contains three elements: Cu, Fe, and S [48,49]. Highresolution XPS revealed Cu2p, Fe2p, and S2p in the H-CuFeS<sup>2</sup> samples as shown in Figure 5B–D, respectively. In Figure 5B, the peaks at 931.9 and 951.7 eV correspond to Cu<sup>+</sup> 2p3/2 and Cu<sup>+</sup> 2p1/2, respectively, whereas those at 933.2 and 953.0 eV correspond to Cu2+ 2p3/2 and Cu2+ 2p1/2, respectively. The peaks at 711.7 and 724.9 eV correspond to Fe2+ 2p3/2 and Fe2+ 2p1/2, respectively, whereas those at 715.2 and 734.3 eV correspond to Fe3+ 2p3/2 and Fe3+ 2p1/2, respectively (Figure 5C). The peaks at 162.5 and 167.8 eV correspond to S2<sup>−</sup> 2p and S6+ 2p, respectively (Figure 5D). For C-CuFeS<sup>2</sup> samples, the peaks at 931.9 and 951.6 eV correspond to Cu<sup>+</sup> 2p3/2 and Cu<sup>+</sup> 2p1/2, respectively, whereas those at 934.1 and 952.7 eV correspond to Cu2+ 2p3/2 and Cu2+ 2p1/2, respectively (Figure 6B). The peaks at 711.4 and 724.8 eV correspond to Fe2+ 2p3/2 and Fe2+ 2p1/2, respectively, whereas those at 714.6 and 734.2 eV correspond to Fe3+ 2p3/2 and Fe3+ 2p1/2, respectively (Figure 6C). The peaks at 162.5 and 168.9 eV correspond to S2<sup>−</sup> 2p and S6+ 2p, respectively (Figure 6D).

and 724.8 eV correspond to Fe2+ 2p3/2 and Fe2+ 2p1/2, respectively, whereas those at 714.6 and 734.2 eV correspond to Fe3+ 2p3/2 and Fe3+ 2p1/2, respectively (Figure 6C). The peaks at 162.5 and 168.9 eV correspond to S2− 2p and S6+ 2p, respectively (Figure 6D). According to its peak area, the percentage of different oxidation states of each element in the prepared CuFeS2 samples can be estimated. In H-CuFeS2 samples, elemental According to its peak area, the percentage of different oxidation states of each element in the prepared CuFeS<sup>2</sup> samples can be estimated. In H-CuFeS<sup>2</sup> samples, elemental compositions were found 82.3% Cu<sup>+</sup> and 17.6% Cu2+ from Cu analysis, 66.9% Fe2+ and 33.1% Fe 3+ from Fe analysis, and 74.2% S2<sup>−</sup> and 25.7% S6+ from sulfur analysis. In C-CuFeS<sup>2</sup> samples, elemental composition was found to be 90.6% Cu<sup>+</sup> vs. 9.3% Cu2+ for Cu, 60.8% Fe2+ vs. 39.2% Fe 3+ for Fe, and 62.6% S2<sup>−</sup> vs. 37.3%. S6+ for S.

compositions were found 82.3% Cu+ and 17.6% Cu2+ from Cu analysis, 66.9% Fe2+ and 33.1% Fe 3+ from Fe analysis, and 74.2% S2− and 25.7% S6+ from sulfur analysis. In C-CuFeS2 samples, elemental composition was found to be 90.6% Cu+ vs. 9.3% Cu2+ for Cu, 60.8%

Fe2+ vs. 39.2% Fe 3+ for Fe, and 62.6% S2− vs. 37.3%. S6+ for S.

**Figure 5.** XPS spectra of H-CuFeS2 samples: (**A**) full scan, (**B**) Cu2p, (**C**) Fe2p, and (**D**) S2p. **Figure 5.** XPS spectra of H-CuFeS<sup>2</sup> samples: (**A**) full scan, (**B**) Cu2p, (**C**) Fe2p, and (**D**) S2p. **Figure 5.** XPS spectra of H-CuFeS2 samples: (**A**) full scan, (**B**) Cu2p, (**C**) Fe2p, and (**D**) S2p.

**Figure 6.** XPS spectra of C-CuFeS2 samples: (**A**) full scan, (**B**) Cu2p, (**C**) Fe2p, and (**D**) S2p. **Figure 6. Figure 6.** XPS spectra of C-CuFeS XPS spectra of C-CuFeS<sup>2</sup> samples: ( 2 samples: (**AA**) full scan, ( ) full scan, (**BB**) Cu) Cu2p2p, (, (**CC**) Fe ) Fe2p2p, and ( , and (**DD**) S) S2p2p. .

## *2.2. Degradation Performance of the CuFeS<sup>2</sup> Samples Catalysts* **2022**, *11*, x FOR PEER REVIEW 8 of 21

The degradation activity of the prepared CuFeS<sup>2</sup> samples was evaluated with RhB (20 ppm) first. According to our previous experience, the degradation efficiency decreased with an increasing dye concentration. This is because the excessive coverage of dye on the active surface of catalysts leads to a decrease in the catalytic activity. Thus, 20 ppm RhB was selected for the experiment. The variations in the RhB concentration (C/C0), where C<sup>0</sup> is the initial RhB concentration, and C is the RhB concentration at time t, with the reaction time for the prepared CuFeS<sup>2</sup> samples in the presence of H2O<sup>2</sup> (Fenton reaction) and Na2S2O<sup>8</sup> (Fenton-like reaction), were found in Figure 7. Prior to the addition of the oxidant, each catalyst (0.20 g) was introduced to the 20 ppm RhB solution for 30 min in the dark (indicated as "−30 min" in Figure 7) to reach equilibrium. The RhB concentration for H-CuFeS<sup>2</sup> samples after this equilibration time is lower than that of C-CuFeS<sup>2</sup> samples, reflecting RhB adsorption on H-CuFeS<sup>2</sup> samples. This is because smaller size of the H-CuFeS<sup>2</sup> samples had higher specific surface area than C-CuFeS<sup>2</sup> samples. Through the Fenton reaction, the degradation efficiency within 30 min was 32.3% and 26.4% for the H-CuFeS<sup>2</sup> and C-CuFeS<sup>2</sup> samples, respectively (black and blue curve). This suggests that the degradation performance of H-CuFeS<sup>2</sup> is better than that of C-CuFeS2, attributable to adsorption ability of high specific surface area for the H-CuFeS<sup>2</sup> samples. The results of RhB degradation through a Fenton-like reaction by the H-CuFeS<sup>2</sup> and C-CuFeS<sup>2</sup> samples were shown in the red and pink curve. The degradation efficiency within 30 min reaction time follows this order: H-CuFeS<sup>2</sup> (93.7%) > C-CuFeS<sup>2</sup> (66.3%), indicating H-CuFeS<sup>2</sup> having higher catalytic activity to produce •SO<sup>4</sup> − radicals. Furthermore, we found that degradation performance of •SO<sup>4</sup> <sup>−</sup> radicals is higher than that of •OH radicals for both CuFeS<sup>2</sup> samples. This is because of the different lifetimes of radicals (•SO<sup>4</sup> − radicals: 4 s, •OH radicals: 1 µs). Thus, the degradation system of H-CuFeS<sup>2</sup> through a Fenton-like reaction was selected for the further study. *2.2. Degradation Performance of the CuFeS2 Samples* The degradation activity of the prepared CuFeS2 samples was evaluated with RhB (20 ppm) first. According to our previous experience, the degradation efficiency decreased with an increasing dye concentration. This is because the excessive coverage of dye on the active surface of catalysts leads to a decrease in the catalytic activity. Thus, 20 ppm RhB was selected for the experiment. The variations in the RhB concentration (C/C0), where C0 is the initial RhB concentration, and C is the RhB concentration at time t, with the reaction time for the prepared CuFeS2 samples in the presence of H2O2 (Fenton reaction) and Na2S2O8 (Fenton-like reaction), were found in Figure 7. Prior to the addition of the oxidant, each catalyst (0.20 g) was introduced to the 20 ppm RhB solution for 30 min in the dark (indicated as "−30 min" in Figure 7) to reach equilibrium. The RhB concentration for H-CuFeS2 samples after this equilibration time is lower than that of C-CuFeS2 samples, reflecting RhB adsorption on H-CuFeS2 samples. This is because smaller size of the H-CuFeS2 samples had higher specific surface area than C-CuFeS2 samples. Through the Fenton reaction, the degradation efficiency within 30 min was 32.3% and 26.4% for the H-CuFeS2 and C-CuFeS2 samples, respectively (black and blue curve). This suggests that the degradation performance of H-CuFeS2 is better than that of C-CuFeS2, attributable to adsorption ability of high specific surface area for the H-CuFeS2 samples. The results of RhB degradation through a Fenton-like reaction by the H-CuFeS2 and C-CuFeS2 samples were shown in the red and pink curve. The degradation efficiency within 30 min reaction time follows this order: H-CuFeS2 (93.7%) > C-CuFeS2 (66.3%), indicating H-CuFeS2 having higher catalytic activity to produce •SO4− radicals. Furthermore, we found that degradation performance of •SO4− radicals is higher than that of •OH radicals for both CuFeS2 samples. This is because of the different lifetimes of radicals (•SO4− radicals: 4 s, •OH radicals: 1 μs). Thus, the degradation system of H-CuFeS2 through a Fenton-like reaction was selected for the further study.

**Figure 7.** Fenton and Fenton-like reactions for RhB degradation at different conditions: H-CuFeS2 in the presence of H2O2 (black), H-CuFeS2 in the presence of Na2S2O8 (red), C-CuFeS2 in the presence of H2O2 (blue), and C-CuFeS2 in the presence of Na2S2O8 (pink). **Figure 7.** Fenton and Fenton-like reactions for RhB degradation at different conditions: H-CuFeS<sup>2</sup> in the presence of H2O<sup>2</sup> (black), H-CuFeS<sup>2</sup> in the presence of Na2S2O<sup>8</sup> (red), C-CuFeS<sup>2</sup> in the presence of H2O<sup>2</sup> (blue), and C-CuFeS<sup>2</sup> in the presence of Na2S2O<sup>8</sup> (pink).

To maximize the degradation performance of H-CuFeS2, the effect from various concentrations of Na2S2O<sup>8</sup> was studied. As shown in Figure 8, the degradation efficiency increased with increasing Na2S2O<sup>8</sup> concentration. Due to low solubility of Na2S2O8, we selected 4.0 mM of Na2S2O<sup>8</sup> as the optimum required concentration of Na2S2O8. Dye adsorption on H-CuFeS<sup>2</sup> was observed in the absence of Na2S2O<sup>8</sup> (black cure in Figure 9). Although direct degradation of RhB by Na2S2O<sup>8</sup> without H-CuFeS<sup>2</sup> was noticed from the

experiment due to the high oxidizing strength of Na2S2O<sup>8</sup> (red curve in Figure 9), its rate of degradation cannot compete with H-CuFeS<sup>2</sup> samples in the presence of Na2S2O8, which achieved an impressive 98.8% within 10 min (blue cure in Figure 9). In addition, we also analyzed the degradation performances of Cu2S and FeS<sup>2</sup> nanoparticles to investigate which element is important for a Fenton-like reaction. As shown in Figure 10, the RhB degradation efficiency within 15 min reaches 64.1% and 89.0% for Cu2S and FeS nanoparticles, respectively. These results suggested that the FeS<sup>2</sup> nanoparticles catalyze Na2S2O<sup>8</sup> to produce •SO<sup>4</sup> − radicals better than Cu2S nanoparticles, indicating Fe component is important than Cu component for the Fenton-like reaction. hough direct degradation of RhB by Na2S2O8 without H-CuFeS2 was noticed from the experiment due to the high oxidizing strength of Na2S2O8 (red curve in Figure 9), its rate of degradation cannot compete with H-CuFeS2 samples in the presence of Na2S2O8, which achieved an impressive 98.8% within 10 min (blue cure in Figure 9). In addition, we also analyzed the degradation performances of Cu2S and FeS2 nanoparticles to investigate which element is important for a Fenton-like reaction. As shown in Figure 10, the RhB degradation efficiency within 15 min reaches 64.1% and 89.0% for Cu2S and FeS nanoparticles, respectively. These results suggested that the FeS2 nanoparticles catalyze Na2S2O8 to produce •SO4− radicals better than Cu2S nanoparticles, indicating Fe component is important than Cu component for the Fenton-like reaction.

To maximize the degradation performance of H-CuFeS2, the effect from various concentrations of Na2S2O8 was studied. As shown in Figure 8, the degradation efficiency increased with increasing Na2S2O8 concentration. Due to low solubility of Na2S2O8, we selected 4.0 mM of Na2S2O8 as the optimum required concentration of Na2S2O8. Dye adsorption on H-CuFeS2 was observed in the absence of Na2S2O8 (black cure in Figure 9). Alt-

*Catalysts* **2022**, *11*, x FOR PEER REVIEW 9 of 21

**Figure 8.** Fenton-like reaction for RhB degradation by H-CuFeS2 samples at different concentration of Na2S2O8. **Figure 8.** Fenton-like reaction for RhB degradation by H-CuFeS<sup>2</sup> samples at different concentration of Na2S2O<sup>8</sup> . *Catalysts* **2022**, *11*, x FOR PEER REVIEW 10 of 21

**Figure 9.** Fenton-like reaction for RhB degradation under different conditions: H-CuFeS2 samples only (black), Na2S2O8 only (red), and H-CuFeS2 in the presence of Na2S2O8 (blue). Top image: photographs of the RhB solution under the Fenton reaction at different reaction time. **Figure 9.** Fenton-like reaction for RhB degradation under different conditions: H-CuFeS<sup>2</sup> samples only (black), Na2S2O<sup>8</sup> only (red), and H-CuFeS<sup>2</sup> in the presence of Na2S2O<sup>8</sup> (blue). Top image: photographs of the RhB solution under the Fenton reaction at different reaction time.

**Figure 10.** Fenton-like reaction for RhB degradation in the presence of Na2S2O8 by using different

catalysts: Cu2S (black), FeS2 (red), and H-CuFeS2 (blue).

**Figure 9.** Fenton-like reaction for RhB degradation under different conditions: H-CuFeS2 samples only (black), Na2S2O8 only (red), and H-CuFeS2 in the presence of Na2S2O8 (blue). Top image: pho-

tographs of the RhB solution under the Fenton reaction at different reaction time.

**Figure 10.** Fenton-like reaction for RhB degradation in the presence of Na2S2O8 by using different catalysts: Cu2S (black), FeS2 (red), and H-CuFeS2 (blue). **Figure 10.** Fenton-like reaction for RhB degradation in the presence of Na2S2O<sup>8</sup> by using different catalysts: Cu2S (black), FeS<sup>2</sup> (red), and H-CuFeS<sup>2</sup> (blue).
