Figure 2a demonstrates the XRD pattern of different samples including g-C3N4, ZnO, NiO, CZ, and CNZ photocatalyst. The g-C3N4 sample exhibits a single diffraction peak at 27.60° well matching to (002) crystal face resulting from graphite structure (JCPDS No. 87-1526) affirming the existence of nanolayers of g-C3N4. Similarly, the XRD pattern of ZnO exhibited 2θ peaks at 31.81°, 34.62°, 36.27°, 47.70°, 56.68°, 63.11°, 66.44°, and 68.18° well correspond to (100), (002), (101), (102), (110), (103), (112), and (201) crystal planes of hexagonal wurtzite ZnO (JCPDS No. 36-1451), respectively, confirming the formation of ZnO nanoparticles [37]. The XRD pattern of NiO exhibited three diffraction peaks at 2θ, 37.26°, 43.31°, and 62.73° matches with (111), (200) and (200) crystal planes of NiO (JCPDS No. 04-0835) [38]. The XRD pattern of CZ was noted to be similar to that of ZnO with no peak observed for g-C3N4 might be due to the higher concentration of ZnO. However, no extra peak was observed due to the integration of g-C3N4 in ZnO corroborating the crystallinity of the nanocomposite. A slight change in peak position with a minor blue shift was observed for the target CNZ ternary nanocomposite. A new peak at 11.19° was observed in the CNZ sample corresponding to interlayer stacking of g-C3N4 nanosheets. Overall, the XRD pattern of CNZ ternary nanocomposite exhibited the successful incorporation of metal oxide semiconductors (ZnO and NiO) into g-C3N4.
Figure 2b displays the Raman spectra including g-C3N4, ZnO, NiO, and CNZ photocatalyst. Typically, graphitic carbon nitride has two noticeable bands that are slightly displaced. The existence of sp3-hybridized carbon is suggested by the D band (1343 cm− 1), which can be explained by the material defects in structure and disordered regions. On the other hand, sp2-hybridized carbon is linked to the G band (1596 cm− 1), which is in good agreement with previous reports [39]. The Raman spectrum ZnO nanoparticles (NPs) demonstrated different peaks at 432 cm− 1, 577 cm− 1, 1041 cm− 1, and 1146 cm− 1 corresponding to E2H, E1(LO), E1(TO) + E1(LO), 2P vibration modes of ZnO NPs [40]. The wurtzite structure of ZnO NPs is confirmed by a strong E2H peak as corroborated by XRD measurements. NiO Raman spectrum exhibits characteristics peak of vibrational modes 1P TO mode (544 cm− 1), 1P LO (656 cm− 1), 2P LO + TO (1040 cm− 1), 2P LO (1261 cm− 1), and 2M (1731 cm− 1) [41]. The Raman spectrum of CNZ ternary nanocomposite demonstrated slight displacement in peak positions attributed to the formation of an integrated structure produced by the incorporation of two metal oxide semiconductors (ZnO and NiO) into g-C3N4. However, the incorporation did not cause any noticeable change in the structure of ternary nanocomposite.
The surface morphology of the pre-synthesized samples was examined by SEM. Figure 3 displays the SEM images of pure g-C3N4, ZnO, and NiO along CNZ ternary nanocomposite. The pure g-C3N4 (Fig. 3a) resembles a stacked plate structure. This finding suggests that there is interconnectivity between the g-C3N4 sheets. ZnO exhibits morphology like spherical nanoparticles (Fig. 3b) with a diameter ̴ 50 nm and NiO also reveals nanoparticle morphology (Fig. 3c) but due to agglomeration the exact size of NPs is crucial to estimate. The surface morphology of g-C3N4/NiO/ZnO ternary nanocomposite-based photocatalyst shows a random morphology with NiO and ZnO NPs anchored onto g-C3N4 nanosheets. The ternary nanocomposite generated a heterojunction structure that can effectively separate photogenerated charge carriers, increasing charge transfer efficiency and performance all around. The aggregation in Fig. 3d also affirms the high surface area of the ternary nanocomposite, a prerequisite for effective degradations of pollutants.
UV-Vis absorption spectra were recorded at room temperature in order to evaluate the optical characteristics of pristine g-C3N4, NiO, ZnO, CZ, and CNZ photocatalysts. A narrow absorbance in the ultraviolet (UV) region was noticed for the majority of the synthesized samples (Fig. 4a). However, CNZ revealed better light absorption and a noticeable redshift in the absorption edge towards the visible range. The better light absorption and redshift in the absorption edge of CNZ ternary nanocomposite-based photocatalyst is consistent with previous studies on nanocomposites designed particularly for visible light absorption [42]. The enhanced light absorption in the visible range and noticeable redshift indicate that absorption characteristics of CNZ ternary nanocomposite-based photocatalyst are successfully improved by integrating ZnO and NiO into g-C3N4 nanosheets. As a result, CNZ ternary nanocomposite is beneficial for photocatalytic processes as it can absorb more photons of visible light, encouraging the formation of electron-hole pairs and reactive species.
The PL spectra of ZnO, NiO, NZ, and CNZ are illustrated in Fig. 4b. The emission spectra of ZnO NPs and NiO NPs exhibited highly intense but narrower peaks, suggesting an increased rate of charge carrier recombination. Nevertheless, a drop in PL intensity for NZ indicates that combining ZnO with NiO considerably lowers the recombination rate of electron-hole pairs. The CNZ ternary nanocomposite-based photocatalyst demonstrated a suppressed or lower peak intensity as compared to the NZ binary nanocomposite as well as NiO NPs and ZnO NPs. The results corroborate that the formation of heterostructures using g-C3N4, NiO, and ZnO may efficiently inhibit the recombination of electron-hole pairs generated during photocatalysis. Additionally, by inhibiting the photoinduced charge carriers, the synergistic impact improves charge separation and reduces the rate at which photogenerated charge carriers recombine. Further evidence of the increased electron-hole separation in the CNZ ternary nanocomposite was provided by electrochemical impedance spectroscopy (EIS) analysis. The EIS spectra of g-C3N4, NiO, ZnO, and CNZ are shown in Fig. 4c. The spectra demonstrate that CNZ has a lower arc radius in the Nyquist plot than the other samples, suggesting more effective transfer and separation of photogenerated charge carriers. The enhanced separation of photoinduced electron-hole pairs in CNZ is highlighted by this outcome.
Due to its numerous potential applications in the field of solar energy conversion, photocatalysis has become a highly promising technology that might alleviate global energy constraints and environmental pollutants. Only 4% of ultraviolet (UV) radiation can be recognized in solar light, an endless natural energy source; visible light makes up a far bigger share (46%). Thus, boosting the photocatalytic efficiency of semiconductors in visible and ultraviolet light has emerged as a crucial field of investigation for optimizing solar energy utilization. To investigate the synergistic impact of the CNZ ternary nanocomposite-based photocatalyst, the photocatalytic activities of different photocatalysts were examined for the degradation of MB solution, a model organic pollutant, under visible light irradiation. The photodegradation curves of MB to time using several samples of the photocatalysts under UV light irradiation are displayed in Fig. 5a. With a photodegradation efficiency (PE) of 92% after 60 minutes of exposure, CNZ demonstrated the best performance among all the evaluated samples. In the same testing conditions, it outperformed other photocatalysts such as NZ (79% PE), CZ (82% PE), CN (60% PE), ZnO (77% PE), NiO (63% PE), and g-C3N4 (38% PE).
The ternary nanocomposite CNZ exhibited the highest PE, followed by the CZ and NZ photocatalysts. These results suggest that the photocatalytic activity for pollutant degradation under irradiation is significantly enhanced by the formation of CNZ ternary nanocomposite. In order to provide a quantifiable comparison of photocatalytic performance, a pseudo-first-order kinetic reaction model (ln(c/co) = kt) was employed to analyze the collected data [43]. The MB concentration at a given irradiation time is denoted by c in this equation, while the rate constant is denoted by k and the irradiation period is indicated by t. With a correlation coefficient (R2) larger than 0.9, Fig. 5b shows a strong linear connection, suggesting that the photocatalytic reaction follows first-order kinetics. Table 1 displays the k and R2 values for each photocatalyst that was utilized in the photodegradation of MB. The rate constant k for MB degradation using CNZ ternary nanocomposite-based photocatalyst is estimated to be around 0.0427 min− 1 quicker than the rate constants for CZ (0.0293 min− 1), CN (0.0155 min− 1), NZ (0.0261 min− 1), ZnO (0.0251 min− 1), NiO (0.0161 min− 1), and g-C3N4 (0.0080 min− 1). These findings imply that the CNZ photocatalyst discloses a much better MB degrading efficiency as compared to all other samples. Therefore, it can be presumed that CNZ can be utilized as an optimized photocatalyst for enhanced photocatalytic activity.
Table 1
k and R2 values using different photocatalysts.
Photocatalysts | k (min− 1) | R2 |
CNZ | 0.0427 | 0.9955 |
CZ | 0.0293 | 0.9844 |
CN | 0.0155 | 0.9841 |
NZ | 0.0261 | 0.9807 |
ZnO | 0.0251 | 0.9959 |
NiO | 0.0161 | 0.9811 |
g-C3N4 | 0.0080 | 0.9973 |
We also measured the effect of CZN photocatalyst concentration and initial dye (MB) concentration by varying their masses to gain further insights into the optimization of the photocatalyst quantity and dye concentration. CZN photocatalyst concentration varied from 0.25 g/L to 1 g/L. The PE dramatically increased up to 0.75 g/L but then declined, as seen in Fig. 6a. It has been established that increasing the photocatalyst concentration can result in the generation of further charge carriers and active species, both of which are essential for the degradation of MB. Agglomeration in the dye solution, on the other hand, can also result from using excessive photocatalyst which can reduce the quantity of light entering the solution and consequently decrease photodegradation efficiency. The optimal photocatalyst concentration was thus determined to be 0.75 g/L of CZN, which would enable effective light absorption and prevent the demand for additional catalysts. The effects of initial dye concentration were also evaluated across a range of 5 to 20 mg/L on the photodegradation of MB using 0.75 g/L of CZN. As shown in Fig. 6b, the highest photocatalytic performance was noted for 15 mg/L MB concentrations.
There was a noticeable drop in the PE above this concentration. A higher concentration of MB can reduce the photoactivation of the catalyst for photodegradation since it can obstruct light from entering the reaction system. In addition, the PE decreases with increasing dye concentrations due to excess MB molecules which may block active sites and reduce their availability for photodegradation. In order to investigate the function of the energetic species in the photodegradation process of the CNZ ternary nanocomposite-based photocatalyst, trapping experiments were conducted and displayed in Fig. 6c. It was probed out how different reactive species, such as hydroxyl radicals (•OH), holes (h⁺), and superoxide radicals (•O2−), contributed to photocatalytic degradation. To specifically quench these species, three types of trapping agents were used: AgNO3 was used to trap O2⁻ radicals, C3H8O for hydroxyl radicals, and C2H5OH was used to capture h⁺. The results reveal that adding 0.05 mM AgNO3 (electron scavenger) resulted in a slight decrease in the photocatalytic degradation of methylene blue (MB) over the CNZ ternary nanocomposite. This indicates that the main active species involved in the degradation process are not photogenerated electrons. By contrast, the addition of C3H8O and C2H5OH (hydroxyl radicals and hole capture) greatly inhibited the photocatalytic activity. These results confirm that the primary oxidative species responsible for the photocatalytic degradation of MB are photogenerated holes and •OH radicals. Reusability is another crucial component in evaluating the practical performance of photocatalysts [44]. The synthesized CZN photocatalyst exhibited remarkable stability as shown by Fig. 6d by retaining 86% degradation efficiency for MB despite five cycles of reuse. The results confirm that CZN demonstrates long-term durability in photocatalytic applications. On the contrary, the degrading efficiency of MB using commercial ZnO decreased significantly with each cycle, reaching just over 62% after five cycles. The subsequent decrease in photocatalytic activity originates due to small particle size and high dispersion of ZnO, making it difficult to completely remove from the reaction system resulting in an enormous loss of ZnO powder. In contrast, the better surface morphology and improved absorption of CZN ternary nanocomposite-based photocatalyst make solid-liquid separation simpler. These results highlight the stability and reliability of the CZN ternary nanocomposite-based photocatalyst for MB degradation in the presence of sunlight.