2.3. Morphological Characterization
The morphology and structure of pure MoS
2 and CoO@meso–CN@MoS
2 were performed by SEM (
Figure 3a–c). The SEM image of CoO@meso–CN@MoS
2 photocatalyst indicates the proof of sheets with length of 200 nm and diameter about 50 nm, respectively (
Figure 3b,c). With increasing CoO@meso–CN content, the surface density of the CoO@meso–CN@MoS
2 nanoplates increases in MoS
2 plates. At the same time, no apparent alteration in length and diameter of CoO@meso–CN@MoS
2 nanosheets was seen. It was believed that the doped nanostructured CoO@meso–CN expanded the change in Gibbs free energy (∆G) and at the same time decreased the surface energy of CoO@meso–CN@MoS
2 photocatalyst [
15]. The corresponding energy-dispersive X-ray spectroscopy (EDS) elemental analysis indicated that most of the elements were Mo (65.6%), S (23.3%), and Co (1.79%), respectively, which are characteristic elements corresponding to the 3 wt% CoO@meso–CN@MoS
2 heterostructure materials (
Figure 3d). The molar ratio of the elements Mo to S was identified to be approximately 2:1 for heterostructure materials.
Transmission electron microscopy (TEM) was implemented on the CoO@meso–CN@MoS
2 photocatalyst. The TEM images of CoO@meso–CN@MoS
2 hybrids further established the presence of CoO@meso–CN@MoS
2 nanocomposites and pure MoS
2 materials in the sample (
Figure 4). The single 3 wt% CoO@meso–CN@MoS
2 nanocomposites is shown in
Figure 4a. The conjunction of two sets of lattice fringes corresponding to the (002) and (111) crystal planes of MoS
2 and CoO@meso–CN, respectively, was obviously observed for the CoO@meso–CN@MoS
2 heterostructure material (
Figure 4b). The measured interplanar spacings of 0.28 nm and 0.36 nm can be applied to the (111) plane of CoO@meso–CN photocatalyst and the (002) plane of hexagonal MoS
2, respectively. The close contact among the heterostructure promotes powerful drift of charge carriers at the interfacial zone, increasing their separation. According to the TEM measurement results, it can be observed that the nanomaterials CoO@ meso-CN firmly attach to the surface of MoS
2 photocatalyst. The structured deepness that reached between the CoO@meso–CN and MoS
2 photocatalyst was partially attributed to Van der Walls interactions [
16]. The lattice fringes suggest the high quality of crystallinity of the MoS
2 and CoO@meso–CN photocatalyst. Elemental mapping images (
Figure 3c–f) suggest that Mo, S, Co, and O elements were uniformly distributed in nanocomposites. Therefore, the above results indicate the successive growth of 3 wt% CoO@meso–CN@MoS
2.
2.4. Structural and Surface Properties Analysis
The FTIR spectrum of the CoO@meso–CN@MoS
2 heterostructure nanomaterial with different concentrations is shown in
Figure 5a. Absorption bands were raised at 534, 820, 1140, 1384, 1695, and 2395 cm
−1 of MoS
2 nanomaterials (dotted line) [
17]. The band at 534 cm
−1 was due to the Mo–S bonds, and the band at 820 cm
−1 was due to the construction of S–S bonds for the synthesis of the MoS
2 heterostructure nanomaterial. The broad absorbance bands of 3430 cm
−1 can be attributed to the adsorbed hydroxyl groups of the surface of MoS
2. The crystal structure of the hybrids could be further supported by Raman spectroscopy. As shown in
Figure 5b, the Raman spectrum of the hybrids was comparable to that of the bulk component. The frequencies of 380 and 406 cm
−1 can be referred to the MoS
2 modes [
17,
18]. Comparing the two component Raman peaks of the as-prepared hybrid, it was apparent that the Raman bands shift towards higher energy (
Figure 3b inset), and their intensities relatively decline as the grain size diminishes. Therefore, the recognized shift was due to the effect of diminishing grain size on alternative properties of the nanoparticles. When the grain size reduces, the vibrational properties of these components may change. Mostly, contraction appears within the nanoparticles that is due to the size-induced radial pressure, which serves to increase the drive constants following declines in interatomic distances. We infer that the shift in the Raman spectra of the nanohybrid is due to the narrower grain size and its impact on the drive constants and vibrational amplitudes of the nearest neighbor bonds. X-ray photoelectron spectroscopy (XPS) was carried out to verify the chemical states and bonding architecture of hybrids (
Figure 5c–e). The survey XPS spectra imply that the prepared hybrids consist of Mo, S, and Co. These clear peaks of the heterostructure can be apparently recognized in the hybrid sample. The high-resolution XPS of the Mo spectrum demonstrated two peaks at 229.5 and 395.7 eV, which could be attributed to Mo 3d5/2 and Mo 3p3/2, respectively. The famed feature peaks at 162.4 eV can be attributed to S, which reveal S 2p3/2, verifying the chemical structure of MoS
2 [
18]. In addition, measurement of the thermal gravimetric analysis (TGA) data (
Figure 5f), and the amount of CoO@meso–CN nanoparticles in MoS
2 hybrids was only 14.48 wt%, suggesting that the CoO@meso–CN on the MoS
2 hybrid was quite less. The uniform distribution of less CoO@meso–CN nanoparticles on the MoS
2 hybrid was critical to the construction of the CoO@meso–CN@MoS
2 heterostructure materials. BET analysis showed that the composite possesses high surface area (10.43 m
2/g) shown in
Figure 5g,h. The Barrett–Joyner–Halenda method was applied for determining the pore size, i.e., diameter ~24 nm.
2.5. Photocatalytic Degradation Performance
The photocatalytic action of the nanocomposites has been explored to understand the degradation of organic chemicals using visible light for essential energy. The optimization of the photocatalytic degradation response is achieved by applying aqueous solution to RhB as a classical pollutant.
Figure 6 indicates the photocatalytic performance of 60 mg of nanohybrid photocatalysts with different ratios via the degradation of 20 ppm aqueous solution of RhB under visible light. Over 60 min of degradation time, from 73% to 92% degradation of RhB was observed using a nanohybrid photocatalyst with varied concentration, respectively (
Figure 6a,b). In addition, the reaction rate constant of the 3 wt% nanohybrid was around two times (2.5 and 11 times) higher than that of pure MoS
2 (CoO@meso–CN and TiO
2) for the degradation of RhB. The pure MoS
2 photocatalysts declined by only approximately 73% of the RhB, which is rather lower than the degradation observed for the nanohybrid photocatalyst alone. These results suggest that the nanohybrid is a capable photocatalyst and that the hybrid formation is favorable for response improvement. However, the entire 3 wt% nanocomposites exhibited relatively enhanced photocatalytic capability compared with the pure essentials (73%) and pure CoO@meso–CN nanoparticles (61%). The probable pseudo-first-order rate constant (k
app) for RhB degradation is revealed in
Figure 6c. All of the nanocomposites showed greater degradation rates for RhB compared with the pure MoS
2 and CoO@meso–CN components. The apparent rate constant for the 3 wt% nanohybrid was approximately 2 and 2.5 times higher than that of the pure MoS
2 and pure CoO@ meso-CN nanoparticles, respectively. The effect of the catalyst amount of RhB degradation has been investigated by using different masses of nanohybrid photocatalysts (20–80 mg), and the rate of RhB degradation enhanced considerably, up to 60 mg. Also, the catalyst amount had a small effect on the degradation rate (
Figure 6d). The initial RhB amount was also assorted between 20–80 ppm using 60 mg of the nanohybrid photocatalyst. Using up to 20 ppm for initial RhB amounts, the degradation rate achieved >93% degradation in 60 min of irradiation time (
Figure 6e). With 40 ppm used as initial RhB amounts, 80% degradation was reached after 60 min of irradiation. The degradation efficiency decayed gradually with increased RhB amount. The gradual reduction in the RhB degradation rate at higher initial amounts was due to the saturation of the action section over the photocatalyst surface. The optimization investigation showed that 60 mg of CoO@meso–CN@MoS
2 hybrid was able to reach 93% degradation of a 20 ppm RhB solution during 60 min of response time. On the other hand, we constructed cycling studies of the photocatalytic degradation progression by repeatedly presenting the same catalyst five times.
Figure 6f shows that after five continual runs, no significant deactivation of nanohybrid was recognized, and the degradation potential of RhB reduced lower than 1%, suggesting that the hybrid had positive cycling stability. Only ≈3% of the catalytic performance declined after it was irradiated under visible light for a long time (10 h). The recycling efficiency and durability of photocatalysts are two essential points that should be verified in future functional applications. The nanostructured CoO@meso–CN firmly attached to the surface of the MoS
2 and acts as a valuable electron transfer medium, building charge separation due to the electron–hole recombination, then enhancing the photocatalytic capability. Although the photocatalytic abilities of the nanohybrid varied based on different ratios, they were all superior to that of the MoS
2 photocatalysts. With increases in concentrations of CoO@meso–CN nanoparticles, the photocatalysis of the developed heterojunction nanocomposites reached a maximum for the hybrid with 60 min, and it declined when the process was further prolonged. Similar studies were also conducted for the commercially available MoS
2 in which the hybrid preceded by 60 min exhibited the best photocatalytic performance. Obviously, the nanohybrid displayed outstanding visible light photocatalytic efficiency compared with that of commercially available MoS
2 powder. This result demonstrates that this nanohybrid decline organic compound has improved performance compared with pure MoS
2 and CoO@meso–CN nanoparticles.
The effect of the initial pH of the solution and the nanohybrid dosage from various organic compounds has been studied in this work.
Figure 7a shows that the pH of the initial solution significantly impacts the photodegradation of RhB, CR, and MB. When the solution was neutral or alkaline, the removal performance of RhB, CR, and MB was lower than in the acidic solution (removal was raised from 42% to 92%, 32% to 95%, and 38% to 96%). However, the inter-attraction was weak when the pH value of solution was raised (pH > 7). Therefore, RhB, CR, and MB has distinguished remove performance in acidic solution because of the chromophore molecules. The percentage of RhB, CR, and MB removal was raised from 28% to 88%, 46% to 92% and 52% to 94% (pH = 7) when the nanohybrid dosage extended from 1 to 80 mg per 500 mL, as presented in
Figure 7b. That is because the developed nanohybrid will contribute further oxidizing species to proceed with RhB, CR, and MB molecules. However, when the nanohybrid dosage is expanded to 40 mg per 500 mL, the percentage of dye removal slightly decreased due to the extra nanohybrid that increased the turbidity of the solution and covered part of the visible light. The degradation efficiency gradually decreases with increasing concentration of the three organic compound molecules. In order to assess the degradation rate of the organic pollutants (RhB, CR, and MB) examined in this work, total organic carbon (TOC) values of the pollutants were determined before and after 60 min of halogen lamps irradiation, and the results are presented in
Figure 7c simultaneously with the degradation performance measured under the same conditions. The degradation rate of RhB, CR, and MB was verified by measuring the total organic carbon (TOC) degeneration. The TOC removal performance of RhB, CR, and MB reached 70.3%, 56.9%, and 83.2%, respectively. Note that all of the studied target pollutants can be significantly degraded with a halogen lamp as the driving light source. Therefore, using halogen lamps irradiation as the driving energy of a nanohybrid photocatalyst can be a potential tool for wastewater management. In order to recognize the stability and reusability of the as-prepared catalyst, nanocomposites were involved to reuse in the recycling test. The result is further achieved by XRD analysis of CoO@meso–CN@MoS
2 nanohybrid before and after the photocatalytic degradation pathway of RhB. As shown in
Figure 7d, it is remarkable that the major peaks of XRD pattern has nearly no changes after three continuing cycles, which serve to demonstrate that it has great photostability and reusability.
To further understand the photocatalysis process, trapping analyses of the active species were carried out during photocatalytic degradation of RhB. As indicated in
Figure 8a, for nanocomposites, the photocatalytic degradation of RhB presents no noticeable change with isopropanol (IPA, 1 mmol/L, quencher of •OH) compared with no quencher, which suggests that •OH is not the primary active specie. When ethylenediaminetetraacetic acid disodium (EDTA-2Na, 1 mmol/L, quencher of h
+) was included in the solution, the degradation of RhB slowed down slightly but was achieved at about 80 min, which implies that the photogenerated holes play a small part in the photocatalytic process. In comparison, a supplement of 1, 4-benzoquinone (BQ, 0.1 mmol/L, quencher of •O
2−) led to complete quenching of photodegradation, suggesting that •O
2− was the primary active species and plays a major role in the photocatalytic process. Thus, it is apparent that •O
2− from the reaction of photogenerated electrons and O
2 are the essential oxidative species, while the photogenerated h
+ also support the absolute term for the degradation of RhB in the nanohybrid composite photocatalyst system. In addition, to demonstrate the active species generated in the scheme, an electron paramagnetic resonance (EPR) system was implemented to notice the generated radicals in the photocatalytic process under visible light irradiation using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the radical trapping agent (
Figure 8b), which can trap the •O
2− form DMPO•O
2− complexes. It is established that DMPO is commonly employed as a radical trapping agent forming DMPO•O
2− and DMPO•OH
− [
19,
20]. Briefly, 4 mg of the nanohybrid sample was dispersed in methanol (for •O
2−) and, thus, DMPO (20 mM) was included with ultrasonic dispersion for 5 min. The signal intensity is continuously raised with increasing irradiation time. The component peaks of DMPO•O
2—and DMPO•OH
− can be detected in the methanol and aqueous dispersion of nanohybrid nanocomposite after visible light irradiation, implying that the •O
2− radicals were developed from the reaction of photogenerated electrons and O
2 molecules in the photocatalytic processes. Associated with the trapping analyze (
Figure 8a), it can be assumed that •O
2− played a crucial role in the photocatalytic process. These results suggest that •O
2− is the primary active species and the principal mechanism for the great photocatalytic action, supported by h
+, whereas •OH showed only a rather trivial role in the whole photocatalytic process.
Photocatalytic response was formed when the photoexcited electron was benefit of the packed valence band of composite photocatalyst to the vacant conduction band as the absorbed photon energy, hν, equals or exceeds the band gap between the composite photocatalyst moving rear a hole in the valence band. Therefore, electron and hole pair (e
−–h
+) were formed. The bands of motive in photocatalysis are the occupied valence band (VB) and it is quite vacant conduction band (CB), which is commonly identified by band gap energy (E
bg). The heterostructure may be photoexcited by electron-donor zone (reducing locations) and electron-acceptor zone (oxidizing locations), serving superiority range of redox process. When the composite is irradiated with light (hν) of the largest power than that of the band gap, an electron is generated from the VB to the CB moving a positive hole from the valence band and a negative electron from the conduction bands as described in
Figure 9. As visible light irradiation, negative electrons (e
−) from the VB of MoS
2 was excited to the CB, with equal amount of positive holes (h
+) moved from VB. Transferred via the reduced potential energy, the photogenerated electrons from CB of MoS
2 tended to move to that of CoO@meso–CN. As a result, the electron-hole separation from MoS
2 interface would be two ways, namely, the photogenerated electrons from CB of CoO@meso–CN prone to move to CB of MoS
2 with reducing potential, although the holes from VB of MoS
2 shift to VB of CoO@meso–CN with higher potential. The photogenerated electrons and holes were separated from the interfaces of CoO@meso–CN@MoS
2, which decreased their recombination opportunity and promoted them to emigrate productively to the interface of CoO@meso–CN and MoS
2, respectively. Besides, another point suppressing the catalysis may be the apparent scale growth of the MoS
2 aggregates which increased the migration distance among the interface-separated electrons from the surfaces of MoS
2 and raised their recombination probability.