3.1. NPs surface characterization
Figure 2 displays the FTIR spectra where their results allowed to identify the stabilizing process utilizing thioglycolic acid molecules. The absorption band range of 2550–2670 cm− 1 indicated the presence of SH bonds in TGA ligands (Mili et al. 2023). The breaking of this bond and the corresponding loading of thiol molecules on NPs surface were associated with the changes observed for this band in the nanoparticle spectra. Zn–S stretching vibrations were identified with the absorption band at 563 cm− 1 (Ouni et al. 2022). The sulfonate groups on TGA-capped ZnS NPs was verified by the absorption bands at 780 cm− 1 (–C–S), 850 cm− 1 (–S-H), 1429 cm− 1 (–COO-), and 1508 cm− 1 (– C = O) (Mili et al. 2023). The O–H elongation vibration of TGA molecules was identified via the wide absorption band at 3340 cm− 1 (Ouni et al. 2023b). These findings showed that the TGA ligand was attached to the ZnS surface.
XRD pattern of the ZnS-TGA NPs is shown in Fig. 3. The large diffraction peaks demonstrated the NPs nanometric size. The diffraction standards of wurtzite (JCPDS card No. 80 − 0020) and zinc blende (JCPDS card No. 80 − 0007) phases were used for comparison and analysis of NPs results. The diffraction peaks located at 2θ = 28.95°, 33.76°, 47.58°, and 55.27° were associated with the diffraction planes (111), (200), (220), and (311), respectively, of the zinc blende phase. The (101) crystallographic wurtzite ZnS plane was identified via the weak peak at 31.08°. It was determined that even though the hexagonal phase's contribution to the XRD pattern was minimal, its existence cannot be ruled out. Therefore, symmetry circumstances favoring the most preferred (111) direction led to the nucleation of cubic crystal structure rather than hexagonal structure (Ouni et al. 2021). To provide a more precise measurement of the stabilizer's impact, the average size D and full width at half maximum (FWHM) of TGA-capped ZnS NPs were estimated by fitting the XRD pattern to a theoretical profile. The Debye-Scherrer equation fitted the Gaussian profile to the Bragg peaks obtaining FWHM. This parameter allowed to calculated the average crystallite size (D, nm) (Rahman et al. 2023):
$$D=\frac{K\lambda }{\beta \text{c}\text{o}\text{s}\left(\theta \right)}$$
7
where β is the complete width at half maximum of the diffraction peak in radians, K is the shape factor (0.9), and θ is the Bragg diffraction angle. Note that the X-ray wavelength was 1.54 Å. The computed average crystallite size was 7.15 ± 0.1 nm. The lattice constants of the hexagonal and cubic phases were obtained from Equations (8) and (9) (Vijaya Bharathi et al. 2023):
$${d}_{hkl}^{2}=\frac{{a}_{c}^{2}}{{h}^{2}+{k}^{2}+{l}^{2}}$$
8
$${d}_{hkl}^{2}= \frac{1}{\frac{4( {h}^{2}+hK+{K}^{2})}{3{a}_{h}^{2}} +\frac{l}{{c}_{h}^{2}}}$$
9
where (ac, ah, and ch) are the lattice constants of the cubic and hexagonal phases of nanocrystals, (hkl) are the Miller indices, and dhkl is the inter-reticular distance that is provided for both structures. The lattice parameters with the average calculated values were: a = 3.8°A, c = 6.9°A (hexagonal), and a = 5.4°A (cubic). The Scherrer formula provided a restricted value of the NCs size, and accounted for the size effects related to the diffraction data. Bhattacharjee and Chattopadhyay (2002) have indicated that this model ignores other parameters, such as lattice strain, dislocation density, and stacking fault that could be used to rectify NC sizes (Bhattacharjee and Chattopadhyay 2023). Therefore, Eq. (10) calculated these variables, and Table 1 lists the estimated values (Ouni et al. 2023b).
Table 1
Structural properties of TGA-capped ZnS nanocrystals.
Sample
|
Crystallite size D (nm)
|
Dominant Plans dhkl
|
Lattice constant (Å)
|
Structure
|
Strain (ε)
|
Dislocation density (δ) (lines/m2)x1015
|
Stacking fault (SF)
|
ZnS-TGA
|
7.15
|
C (111)
|
a = 5.4
|
ZB
|
0.00698
|
19.56
|
0.014
|
a = 3.8
c = 6.9
|
WZ
|
$$\epsilon =\frac{\beta cos\left(\theta \right)}{4} ;\delta =\frac{1}{{D}^{2}} ; SF=\frac{{2\pi }^{2}}{{45\left(tan\theta \right)}^{\frac{1}{2}}}{\beta }_{hkl}$$
10
The term lattice strain (ε) describes the regularity that is distorted or altered due to crystal flaws like lattice (Nikolić et al. 2023). Therefore, the amount of flaws in the nanocrystal is explained by the SF and δ (Li et al. 2023b). The value of dislocation density indicated that the semi-conductor nanocrystals were less ordered due to small size. Therefore, the highest possibility of dislocations was due to small NPs size since they tend to stabilize their higher surface energy. The structural strain is linked with the NCs surface stress from the TGA capped surface during the relaxation and growth of atomic positions at SF interface (Bel Haj Mohamed et al. 2021).
Figure 4a repots the HR-TEM measurements to analyze the particle size and morphology of ZnS-TGA NCs. These nanocrystals displayed a spherical shape and the estimated average ZnS-TGA diameter was 5.91 ± 0.5 nm (Fig. 4b). The interplanar distance measured for NCs was 0.36 nm and this value was close to the (111) plane of zinc blende ZnS (0.312 nm). Figure 4c displays the elemental nanocrystals composition where S and Zn were the major elemental components.
3.2. Optical characterization of TGA-capped ZnS NPs
Figure 5 shows the absorption spectrum of NPs. This spectrum contained broad absorption bands in the UV region that extended to the visible region. The first electronic transition 1Se-1Sh was associated to the absorption edges at 307 nm (Khawla et al. 2022). This spectrum showed a blue shift due to the size effect of the ZnS-TGA NPs, which differed from the response obtained for the bulk ZnS (344 nm). Eq. (11) calculated the NPs band gap energy (Eg) (Hiti et al. 2023):
αhν = A (hν-Eg)n (11)
where A is a constant, α is the absorption coefficient, and hυ is the incident photon energy. The parameter n is a function of the transition type, which is equal to ½ for direct semiconductors. The extrapolation of the tangent of the near edge band allowed the determination of the band gap energy, see Fig. 5b. The band gap was 3.75 eV with a blue shift due to the quantum confinement effect (Kapoor et al. 2023). This optical behavior was different than that obtained for bulk ZnS (Eg = 3.6 eV). ZnS-TGA NCs showed λ > 300 nm and this result suggested that they can be used in solar-irradiation-based applications.
The emission spectrum of TGA-capped ZnS QDs colloidal aqueous solution after helium-cadmium laser irradiation (325 nm) at 20°C is reported in Fig. 6a. At around 440 nm, the photoluminescence spectrum exhibited a broad and strong band with an FWHM of approximately 77 nm. The small NPs size and the large surface-to-volume ratio of the imperfect surface passivation resulted in a high size distribution where both were the causes of the high FWHM value. The primary cause of either extensive (aggregates, cavities, dislocations, etc.) or punctual (interstitial, substitutional, vacancies, etc.) defects was the synthetic precursor stoichiometric ratio and the development process (Khawla et al. 2022). A Gaussian function was applied to deconvolute ZnS-TGA QD emission spectrum by considering three primary bands in the visible range: 495 nm (2.50 eV), 440 nm (2.82 eV), and 408 nm (3.04 eV). The band at 408 nm was associated to the direct band-to-band recombination, which was related to blue emission. It was proposed that the blue luminescence was caused by electrons localized on sulfur vacancies (VSs) with holes in the valence band transitioning into the band at 440 nm (Ouni et al. 2022). The contribution from cadmium interstitial of (IZn) to the valence band (VB) was ascribed to the 495 nm band of green emission (Ouni et al. 2021). The significant emission was attributed to the zinc interstitial and sulfur vacancy generated in the ZnS-TGA NPs, which played an important role in the photodegradation process. Note that the reaction temperature, capping agent concentration, refluxing duration, and capping agent concentration affect the photoluminescence performance. Figure 6b reports the PL spectra of NPs in the aqueous solution that were recorded at 80–300 K. Figure 6c displays the PL peak position (in eV) versus temperature. The emission spectra of ZnS QDs showed a broad band at 416 nm (2.98 eV) (band edge "BE") besides two low-energy broad bands at 2.81 eV (441 nm) (defect 1) and 2.56 eV (484 nm) (defect 2) at tested temperatures. PL spectra of QDs obtained at low- and room-temperatures were similar. The ZnS-TGA QDs excitonic state induced considerable blue shifts as the temperature decreased and, consequently, the PL bands were narrower while increasing in their intensity. The emission of defects and FWHM of band edge increased with temperature but with a minor shift in their maximum location. Varshni model (Varshni 1967) was used to calculate the weak blue shift of PL peak location. The result was 0.4 meV as the temperature decreased from 300 to 80 K. This trend has been observed for different semiconductors in the same temperature range and represented the energy band gap's shrinkage caused by the increment of temperature because of the lattice's thermal expansion and exciton-phonon interaction (Bel Haj Mohamed et al. 2014). The band gap was widened by these processes, which in turn caused the excitonic emission to shift blue. However, the energy positions of D1 and D2 bands were slightly altered due to the temperature increment. This tendency was most likely caused by the QDs size distribution and the strong attachment of impurity levels (surface defects or trap state) to the ZnS-TGA lattice near the forbidden band gap (Wang et al. 2011). Conversely, the intensities of excitonic emission and trapping increased, particularly at low temperatures. The suppression of phonon-coupled thermal quenching was associated with this result and the different temperature sensitivity of the excitonic and trapping states in QDs (Nonoguchi et al. 2007). The phonon coupling strength also increased with temperature. Consequently, the non-radiative recombination probability of holes and electrons was linked to the phonon absorption and PL intensity reduction (Xu et al. 2011).
3.3 Impact of operating conditions on MB dye adsorption on NPs
Figure 7 reports the dependence of MB adsorption with respect to the aqueous solution pH. These studies were performed with 10 mg/L dye concentration, 1 g/L of NPs dosage and pH 5–9. Dye removal increased from 58.51 to 64.91% when solution pH changed from 5 to 7. This increment on the dye removal was caused by electrostatic interactions between the negatively charged NPs and the positively charged MB molecules (Ouni et al. 2023). In contrast, the dye removal decreased with further increments until pH 9. Consequently, the best adsorption condition for MB dye was pH 7.
Figure 8 reports the impact of NPs mass on MB adsorption. The mass of the nano-adsorbents varied from 0.1 to 2 g, while the other operating conditions were fixed, i.e.: 298 K, pH 7, 10 mg/L dye concentration, and 120 min contact time. The dye removal improved with the increment of NPs mass because the number of adsorption sites available increased with the specific surface area, thus favoring dye adsorption (Sen 2023). It was determined that MB removal rate was 74% when using 1 g of organic TGA-capped ZnS NPs. This may be explained by a greater number of adsorption sites as well as better NP dispersion in the aqueous solution. The adsorption capacity of NPs decreased by a further addition of adsorbent mass. For high NPs dosages, the active sites with higher energy become less available, resulting in the occupation of low energy active sites and the reduction of adsorption capacity (Arshadi et al. 2014). Hence, 1 g/L of NPs was selected for the dye removal studies.
Adsorption was studied over time to determine the adsorbed dye amount at different contact periods (Fig. 9). For the first 5 min, the removal of MB dye was fast followed by a slower dye adsorption rate from 10 to 120 min until reaching the equilibrium. This trend was associated with the high surface-to-volume ratio of the nanometer-sized adsorbents. Therefore, the maximum dye adsorption was obtained at 120 min before proceeding with the photocatalytic activity.
Figure 9 also reports the initial dye concentration effect on the NPs adsorption performance. The increase of dye content in the aqueous solution improved the NPs adsorption capacity because the mass transfer was enhanced (Mohd Ramli et al. 2023). The results showed that the MB adsorption capacities increased from 6.99 to 30.92 mg/g when adsorbate concentration changed from 10 to 50 mg/L. This trend was associated with the high mass transfer gradient that favored the diffusion of MB molecules on NPs surface, thus enhancing the adsorption interactions.
MB adsorption capacity versus solution temperature is shown in Fig. 10. NPs adsorption performance decreased with the temperature increment from 298 to 318 K indicating an exothermic process. This solution temperature increase reduced the binding interaction forces involved in dye removal (Xue et al. 2023). Therefore, a solution temperature of 298 K was used as the best condition for MB dye adsorption using these NPs.
3.4 Dye adsorption kinetics
Dye kinetic studies using NPs and their respective modeling are shown in Fig. 11. The adsorption capacity varied from 6.61 to 30.37 mg/g in these kinetic tests when the MB concentration changed from 10 to 50 mg/L. MB dye removal was favored by NPs particle size. ZnS-TGA specific surface area was 36.82 cm2/g. Several kinetic models were used to calculate essential information for the application of these nano-adsorbents in the dye removal process. Two kinetic models were utilized: the pseudo-second order (PSO) and the pseudo-first order (PFO). They fitted the MB adsorption data, and the results are given in Fig. 11. PFO model is commonly applied to analyze the adsorption of water pollutants and is defined as (Bai et al. 2020; Revellame et al. 2020):
Q t = Qe (1-exp(-K1t)) (12)
where K1 (min− 1) is the adsorption rate constant, Qe (mg/g) is the calculated equilibrium adsorption capacity (mg/g) and Qt (mg/g) is the experimental adsorption capacity quantified at time t. PSO model is a type of kinetic model that describes the rate at which an adsorption process occurs, which is given by the next expression (Arshadi et al. 2014; Afolabi et al. 2020):
Q t = \(\frac{{Q}_{e }^{2}{K}_{2}t}{1+{Q}_{e}{K}_{2}t}\) (13)
where K2 (mg/g⋅min) is the corresponding PSO rate adsorption constant. Table 2 provides the kinetic modeling results. The best equation for the correlation of adsorption kinetic data was chosen using the R2 value. The modeling results indicated that PFO was the best kinetic to fit the MB dye adsorption on NPs with R2 = 0.99. Calculated Qe values using this model were closer to the experimental ones. Note that the dye adsorption kinetic implied two stages: dye diffusion on NPs surface and the adsorbent saturation at equilibrium condition.
3.5 Dye adsorption isotherms
Figure 12 reports the experimental MB adsorption isotherm and the results obtained from the corresponding data correlation. Langmuir-Freundlich, Langmuir and Freundlich equations were used to fit the dye adsorption equilibrium. A monolayer adsorption process is assumed by the Langmuir isotherm where the adsorption occurs on a limited number of identical adsorption sites. It is mathematically using the next expression (Chen et al. 2022):
$${Q}_{e}={Q}_{max}\frac{{K}_{L}{C}_{e}}{1+{K}_{L}{C}_{e}}$$
14
where KL (L/mg) is the Langmuir adsorption energy and Qmax (mg/g) is the NPs monolayer adsorption capacity. The empirical Freundlich model accounts for heterogeneous surfaces and usually describes a multilayer adsorption (Ezzati 2020):
$${Q}_{e}={K}_{L}{C}_{e}^{{n}_{F}}$$
15
where nF is the adsorption intensity parameter, and KF (mg/g) is the Freundlich adsorption constant. The Langmuir -Freundlich (L-F) isotherm equation is based on the previous isotherms that can describe heterogeneous surfaces. The equation of this isotherm is written as (Bel Haj Mohamed et al. 2022a):
$${Q}_{e}=\frac{{Q}_{max}{(K}_{LF}{{C}_{e})}^{{n}_{LF}}}{1+{(K}_{LF}{{C}_{e})}^{{n}_{LF}}}$$
16
where nLF is the heterogeneity index and\({ \text{K}}_{\text{L}\text{F}}\) is the adsorption affinity constant. The results of isotherm modelling are reported in Table 3. These isotherm models fitted the experimental dye adsorption data of capped ZnS nano-adsorbents with R2 = 0.88–0.98. L-F equation showed the highest R2 and was the best model to describe these experimental results. This model predicted Qmax = 33.15 mg/g, which was consistent with the experimental value of 30.92 mg/g. The results from the Langmuir model suggested that the surface of synthesized NPs samples was homogeneous for MB adsorption. Calculated nF values from Freundlich model were > 1 (nF = 2.16) indicating that adsorption was favorable (Dawood and Sen 2012) and involved physical interaction forces (Duran et al. 2011; Cazetta et al. 2011). These results prove that these NPs are an alternative for treatment of wastewater polluted by dye molecules.
On the other hand, statistical physics modeling was useful to describe the interactions between dye molecules and NPs surface, and to correlate the experimental adsorption isotherms using parameters associated with these microscopic interactions. This modelling approach was performed to improve the interpretation of MB adsorption process via the estimation of the adsorption energies, the number of adsorbed dye molecules per NPs adsorption site (n) and NPs adsorption site density (DM). A monolayer advanced model fitted the experimental isotherms of the MB dye adsorption on NPs. It assumed that the cationic dye molecules formed a monolayer on the particle surface. The layer of adsorbed MB molecules was formed due to the interactions between MB dye and NPs surface. This model also considered that the main NPs adsorption site could interact with a variable number of adsorbate molecules (superior, inferior or equal to 1) (Sellaoui et al. 2015). It is defined as:
$${Q}_{e}=\frac{n \bullet {D}_{m}}{1+{\left(\frac{{C}_{1/2}}{{C}_{e}}\right)}^{n}}$$
17
![](https://myfiles.space/user_files/122228_c8a1650c59388082/122228_custom_files/img1707825654.png)
where C1/2 is the half-saturation dye concentration (mg/L). The adsorption energy (ΔEa1, kJ/mol) is (Pang et al. 2020):
where Cs = 40 mg/L is the MB dye solubility, R = 8.314 × 10− 3 kJ/K⋅mol is the universal ideal gas constant and T (K) is the adsorption temperature.
Table 4 and Fig. 12 provide the results of data modelling. The calculated DM values decreased with temperature from 26.45 to 10.64 mg/g. Note that if n values increased, the space in the ZnS-TGA nano-adsorbents surface reduced and, consequently, the number of adsorption sites available for dye removal became limited (Ouni et al. 2023b). Indeed, a high DM value implies a high effectiveness of the adsorbent since more adsorption sites are available for pollutant adsorption (Bel Haj Mohamed et al. 2022a). Calculated n values ranged from 1.22 to 2.40 indicating that MB dye molecules could be bound or adsorbed via a nonparallel adsorption orientation or inclined, which corresponds to a multimolecular adsorption (Bel Haj Mohamed et al. 2022a). MB dye molecules interacted with NPs surface involving an aggregation process to form trimers and dimers in the aqueous solution (i.e., n > 1). This multimolecular adsorption phenomenon usually takes place for dye molecules as reported in other studies (Bel Haj Mohamed et al. 2022a). The adsorption capacities at saturation (Qsat) decreased with the aqueous solution temperature from 32.24 mg/g at 298 K to 25.61 mg/g at 318 K. The calculated adsorption energies for this exothermic adsorption ranged from 25.92 to 23.31 kJ/mol, implying the existence of physical interactions between the ZnS-TGA nano-adsorbent surface and MB dye molecules. Hydrogen bond and electrostatic interactions could be implicated in the MB loading on NPs surface.
Table 4
Parameters of the Hill model for the MB adsorption on TGA-capped ZnS NPs
T (K)
|
n
|
DM (mg/g)
|
C1/2 (mg/L)
|
Qsat (mg/L)
|
Δ\({E}_{1}^{a}\) (kJ/mol)
|
298
|
1.22
|
26.45
|
1.14
|
32.24
|
25.92
|
308
|
1.91
|
13.87
|
2.35
|
26.43
|
24.94
|
318
|
2.40
|
10.64
|
5.91
|
25.61
|
23.31
|
Table 5 contains the MB adsorption capacities of other ZnS nanoparticles reported in different studies (Woolard et al. 2002; Makama et al. 2016; Mrunal et al. 2019; Hosny et al. 2021; Liu et al. 2022; Bel Haj Mohamed et al. 2022a; Abed et al. 2022). These results showed that TGA-capped ZnS nano-adsorbents exhibited a higher MB adsorption capacity than those of uncovered ZnS NPs (Liu et al. 2022), ZnS encapsulated with broccoli extract (Abed et al. 2022) and ZnS encapsulated with mercaptopropionic acid (Bel Haj Mohamed et al. 2022a).
Table 5
Comparison of MB adsorption capacities of TGA-capped ZnS NPs and other adsorbents reported in literature.
Sample
|
Dye
|
Time (min)
|
Qmax (mg/g)
|
Reference
|
ZnS-MPA
|
MB
|
120
|
25.18
|
(Bel Haj Mohamed et al. 2022)
|
ZnS
|
MB
|
120
|
15.65
|
(Liu et al. 2022)
|
ZnS-Broccoli
|
MB
|
60
|
9.00
|
(Abed et al. 2022)
|
Zeolite
|
MB
|
240
|
10.82
|
(Mrunal et al. 2019)
|
Cu2O
|
MB
|
1000
|
2.08
|
(Woolard et al. 2002)
|
TiO2
|
MB
|
120
|
13.10
|
(Makama et al. 2016)
|
ZnO
|
MB
|
180
|
9.59
|
(Hosny et al. 2021)
|
NiO
|
MB
|
180
|
17.10
|
(Hosny et al. 2021)
|
ZnS-TGA
|
MB
|
120
|
30.92
|
This work
|
3.6 Sunlight-based Photocatalytic Degradation of MB Dye using ZnSTGA NPs
3.6.1. Degradation Pathways
Initial tests using the MB solution were conducted without NPs, and a marginal degradation (about 10% in 180 min) was noted. This degradation could be caused by the dye molecules' self-sensitization light or by OH* radicals initiated from water (blank test) (Muthukumaran et al. 2019). This finding proved that removal of this cationic organic dye by direct photolysis was ineffective and showed that dye degradation was almost negligible in the absence of photocatalysts. Figure 13 reports the UV-vis absorption spectra of MB dye solutions containing TGA-capped ZnS NPs as a function of irradiation duration, whereas Table 6 shows dye degradation (%) obtained for pollutant concentrations of 10–25 mg/L. The degrading performance of ZnS-TGA nanocatalysts was evaluated using four different dye concentrations. The results showed that the absorbance band gradually diminished with increasing solar irradiation duration, demonstrating the disintegration of the MB chromophoric structure (Sharmin and Basith 2022). Consequently, the blue color disappeared due to the breakdown of the azo function (C-S + = C) source of the blue color and indicated that the hydroxyl radicals had attacked the aromatic compounds through the creation of radical intermediates (Sun et al. 2023 (Sun et al. 2023). Therefore, under solar radiation, the concentration of MB dye decreased in the presence of ZnS-TGA nanocatalysts until the initial dye solution fully discolored, signifying that MB molecules were entirely broken down The impact of the initial MB concentration on the decomposition efficacy is shown in Table 7. After 180 min under sunlight exposure, the maximum MB concentration produced the lowest photocatalytic effectiveness of 72.22%, while the best degradation of 91.10% was observed for 10 mg/L. The small size of TGA molecules utilized as stabilizer and the specific surface (i.e., 36.82 m2/g) of the nanocatalysts can be associated to these efficiencies. This high efficiency could be the result of ZnS broad band gap energy, which raised e-h pair redox potential and improved photocatalytic performance. (Ouni et al. 2021). As the initial MB concentration increased, the dye degradation decreased. As the dye molecules covered the active sites, the production of OH* and O2*− radicals on the photocatalyst surface were reduced at high dye concentrations, which was the main cause of this behavior. An additional plausible reason might be that the dye's ability to block sunlight resulted in a decrease in the nanocatalysts capacity to absorb light (Ouni et al. 2021). Dye molecules have a greater capacity to absorb sunlight than ZnS-TGA nanocatalysts, which lowered the catalytic reaction's efficiency as free radical concentrations dropped (Ouni et al. 2023b). MB dye degradation rate constant was calculated using the Langmuir-Hinshelwood (L-H) kinetic model. Figure 14(a) presents photodegradation plots of ln (A0/A) vs. time t for MB degradation. The lowest MB concentration (e.g., 10 mg/L) produced the best degradation constant K (i.e., 0.025 min− 1), as shown in Fig. 14(b). The half-time value (i.e., 47.46 min) of the MB dye demonstrated an increase in photodegradation under sunlight with the use of TGA-capped ZnS nanocatalysts. The degradation efficiency decreased as the dye concentration increased; the maximum initial dye concentration resulted in a degradation rate of 0.011 min− 1. The degradation rates were significantly influenced by the MB content in the aqueous solution. These findings verified the high photocatalytic activity of TGA-capped ZnS NPs for the removal of MB. The maximum performance values for the adsorption/photodegradation of MB utilizing other photocatalytic materials are shown in Table 8. (Jothibas et al. 2018; Senobari and Nezamzadeh-Ejhieh 2018; Delsouz Khaki et al. 2018; Ye et al. 2018; Chen et al. 2019; Sabouri et al. 2020; Kannan et al. 2020; Ramki et al. 2020).
Table 6
MB degradation efficiency of TGA-capped ZnS NPs under sunlight irradiation using 10 mg/L of initial dye concentration.
|
t (min)
|
|
0
|
10
|
20
|
30
|
45
|
60
|
120
|
180
|
MB Degradation (%)
|
0
|
21.55
|
28.36
|
41.18
|
43.55
|
59.36
|
67.72
|
91.10
|
Table 7
Calculated parameters for the sunlight irradiation-based photodegradation of MB in aqueous solution using TGA -capped ZnS NPs.
Dye concentration (mg/L)
|
Degradation rate constant K (min− 1)
|
Degradation efficiency η (%)
|
10
|
0.011
|
91.10
|
15
|
0.008
|
84.12
|
20
|
0.006
|
73.95
|
25
|
0.001
|
72.22
|
Table 8
Comparison of MB dye degradation efficiency of various materials reported in the literature.
Sample
|
Irradiation
|
Time (min)
|
Degradation efficiency (%)
|
Reference
|
ZnS/RGO
|
Visible light
|
240
|
89.43
|
(Chen et al. 2019)
|
ZnS-PVP
|
Sunlight
|
180
|
49.00
|
(Ramki et al. 2020)[81]
|
TiO2/ZnO
|
Visible light
|
120
|
73.20
|
(Delsouz Khaki et al. 2018)
|
CuO/CdS
|
UV light
|
120
|
84.10
|
(Senobari et al. 2018)
|
ZnS-A
|
UV light
|
180
|
93.00
|
(Kannan et al. 2020)
|
Ni-ZnS
|
Sunlight
|
180
|
87.38
|
(Jothibas et al. 2018)
|
ZnS
|
Visible light
|
120
|
78.41
|
(Ye et al. 2018)
|
Egg-NiO
|
UV light
|
240
|
79.00
|
(Sabouri et al. 2020)
|
ZnS-TGA
|
Sunlight
|
180
|
91.10
|
This work
|
The sunlight-based MB degradation with TGA-capped ZnS nanocatalysts implied several stages. The first step was related to the dye molecules adsorption phase on the nanocatalyst surface. ZnS-TGA photocatalyst absorbed photons from sunlight irradiation, typically ultraviolet (UV) or visible light. An electron was advanced by this absorption from the valence band to the conduction band, resulting in the formation of an electron-hole pair. Within the ZnS photocatalyst, the absorbed energy produced an excited state in which an electron was excited to the conduction band (becoming a negatively charged electron) and a hole (a positive charge) was left behind in the valence band. High levels of reactivity were observed in the holes in the valence band and the excited electrons in the conduction band. The ZnS photocatalyst surface was the site of interactions between these charged species and molecules of adsorbed reactant. By giving their extra energy to reactant molecules such adsorbed O2, the excited electrons in the conduction band reduced them. This created a superoxide reactive species (O2*-), which may take part in the chemical process. Conversely, water, hydroxide ions, or other molecules on the catalyst surface may be oxidized by the holes in the valence band, producing very reactive oxygen species (such hydroxyl radicals, or OH*) that can start a variety of chemical processes. MB dye molecules can be efficiently decomposed by radical-active OH* and O2*- species to produce intermediates such as propionic acid and malonic acid, as well as the end products CO2 and H2O. These free radicals served as potent oxidizers as well as active sites for the photocatalytic degradation of organic pollutants. By moving the electron from the conduction band back to the hole in the valence band, or closing the electron-hole pair, the photocatalyst can revert to its initial state following the degradation reaction. The photocatalyst might be utilized again because of this regeneration. Equations (19)–(25) summarize the primary responses that might be a part of this degradation process:
a) Adsorption phase
ZnS-TGA + MB → ZnS-TGA-MB (19)
b) (e-)-(h+) pair generation
ZnS-TGA + hν → e
𝐁𝐂− + h
𝐁𝐕+ (20)
c) Production of superoxide and hydroxyl radicals
e- + O2→ O2*- (21)
H2O + h+→OH* + H+ (22)
h+ + OH- → OH* (23)
d) Degradation phase
O2*- + MB → Degradation products + CO2 + H2O (24)
OH*+ MB → Degradation products + CO2 + H2O (25)
It should be highlighted that these free radicals are essential to the photocatalytic process that breaks down the MB dye. The hydroxyl radical initiated the degradation of the cationic dye molecules by attacking their C-S+=C bond and other bonds. According to these findings, TGA-capped ZnS NPs represent a potentially useful photocatalyst that may be made using a non-toxic feedstock and used to clean up organic contaminants from the environment when exposed to sunlight.
3.6.2. Recycling of the ZnSTGA Photocatalysts
The recycling of ZnS photocatalysts involves their recovery and reuse after driven the photocatalytic reactions. Reusability studies are required to evaluate the NPs long-term viability and help to make decisions about their use in different applications. NPs photocatalytic stability for MB dye degradation was tested for 5 continuous cycles, see the results reported in Fig. 15. NPs were collected after each cycle, rinsed with ethanol and deionized water, and dried for 24 h at 100°C before the next degradation cycle. Results showed that the photocatalytic efficiency remained nearly unchanged and the MB degradation was 88 ± 3% after 5 successive degradation cycles. These findings proved the high photocatalytic and physiochemical stability of NPs.