3.1. Characterization of Green ZnO Particles
In this work, optical analysis was performed to confirm the formation of green ZnO particles synthesized using
Chrysanthemum spp. flower extract.
Figure 1a shows the UV–visible absorption spectra of Zn
2+,
Chrysanthemum spp. flower extract, and ZnO particles. Accordingly, a maximum absorption peak at 324 nm was observed for the ZnO particles.
Figure 1b also indicates a color change from light yellow to brown when Zn
2+ was added to the
Chrysanthemum spp. flower extract under ultrasound irradiation at 60 °C. This phenomenon may be explained by the fact that
Chrysanthemum spp. flower extract contains a number of natural compounds such as folic acid, polyphenols, niacin, quercetin, reducing sugars, and flavonoids [
26]. These substrates have functional groups that can chelate with Zn
2+ ions to generate Zn
2+–natural compound complexes. Under heating and microwave irradiation conditions, the complexes can be converted into ZnO particles. The same maximum absorption value was reported in several recent works that used
C. fistula and
M. azedarach [
27] and
Cayratia pedata [
28] plant extracts for the bio-mediated synthesis of ZnO particles.
The optical analysis of the green ZnO particles could be better elucidated by UV–visible diffuse reflectance spectroscopy. We observed an absorption edge located between 400 and 420 nm. To determine the bandgap energy of the green ZnO particles, the Tauc equation (Equation (2)) could be used.
where
Eg denotes the bandgap (eV) of the ZnO particles,
h denotes Planck’s constant (6.63 × 10
−34 J·s),
λ (nm) denotes the maximum absorption wavelength, and
c denotes the velocity of light [
29]. The Tauc plot of (αhν)
2 versus (hν) was established to calculate the bandgap of the ZnO particles.
Figure 2 exhibits a bandgap energy of 3.0 eV for fresh ZnO particles synthesized using
Chrysanthemum spp. flower extract. This value was well commensurate with the bandgap energy values of both chemically synthesized and bio-mediated ZnO published in the literature [
20,
30,
31]. Therefore, the green ZnO particles could serve as a photocatalyst under solar light illumination, including both ultraviolet and visible light.
Herein, the chemical bonding properties of green ZnO particles synthesized using
Chrysanthemum spp. flower extract were determined using FT-IR spectroscopy (
Table 2). According to
Figure 3, a range of peaks at 530–715 cm
−1 was assigned to hexagonal-phase Zn–O bonding, confirming the formation of ZnO particles. The location of these characteristic peaks was in good accordance with previous works [
20,
27,
32]. The existence of primary amines, with N–H stretching vibrations, on the surface of the green ZnO particles could be demonstrated by the typical peak at 860.1 cm
−1 [
33]. Meanwhile, C–O stretching, diagnosed by a peak at 1124 cm
−1, was attributable to secondary or tertiary alcohols. Strong C–H bending footprints, belonging to aldehydes or reducing sugars, were observed at 1390 cm
−1 [
33]. While alkene compounds could be identified at 1632 cm
−1, there was a narrow peak at 1746 cm
−1, ascribed to the C=O stretching of carbonyl groups of esters, δ-lactone, or ketones [
34]. The strong absorbance intensity of the peak at 2300 cm
−1 (C–N/C=N) may have been due to the presence of tertiary amines or imines. At 2385 cm
−1, the sharp band of C=C bending may have belonged to aromatic compounds, e.g., quinones, coumarins, tannins, or terpenes (such as carotenoids) [
35]. The very typical signals at 2858–2923 cm
−1 were due to the absorbance of symmetric methylene groups (CH
2), aliphatic groups, or aldehydes from natural compounds. We observed N–H vibrations (broad band) at 3450 cm
−1, which could have signaled amide groups, primary amines or proteins from alkaloids, or betalains in the
Chrysanthemum spp. flower extract. Finally, a medium band of O–H vibrations at 3730 cm
−1 could be assigned to either hydroxyl groups of alcohols, polyphenols, polysaccharides, quercetin, genin, or gallic acid in the
Chrysanthemum spp. flower extract, or surface-chemisorbed H
2O [
36]. Several works have reported the same chemical bonds in hexagonal-phase ZnO particles synthesized by bio-based methods using the extracts of various plants, such as
Phlomis leaves [
32],
Cassia fistula and
Melia azedarach leaves [
27],
Canna indica flowers [
20], and
Myristica fragrans fruit [
37]. Therefore, the surface of the hexagonal-phase green ZnO nanoparticles synthesized using
Chrysanthemum spp. flower extract contained a variety of functional groups, which were expected to have good photocatalytic activity.
To determine the crystalline structure of the green ZnO particles synthesized using
Chrysanthemum spp. flower extract, the X-ray powder diffraction pattern was observed. As shown in
Figure 4, the green ZnO particles showed a typical XRD pattern with summits (2θ, degrees) located at 31.92° (100), 34.62° (002), 36.52° (101), 47.72° (102), 56.84° (110), 63.18° (103), and 68.36° (112). Moreover, the pattern profile of the as-synthesized ZnO in this work was matched well with the standard reference for ZnO particles (JCPDS #36–1451), as shown in
Figure 4. It was also in line with the pattern profile of the hexagonal wurtzite-structured ZnO reported in previous works [
20,
27,
32]. The pattern of the green ZnO particles showed very-high-intensity peaks, and no strange peaks were observed. This indicated that the green ZnO particles synthesized using
Chrysanthemum spp. flower extract had a high degree of purity. To estimate the average size of the green ZnO particles, Scherer’s equation could be used (Equation (3)).
where
K is a constant equal to 0.90,
β is the full-width at half-maximum height of the diffraction peaks at Bragg’s angle
θ, and
λ is the wavelength of the X-ray radiation. According to Equation (3), the average size of the green ZnO particles synthesized using
Chrysanthemum spp. flower extract was estimated to be 30.24 nm. The particle size of the green ZnO particles in this work was in excellent agreement with results from previous studies using extracts such as
Canna indica flowers (29.85 nm) [
20],
Capparis zeylanica leaves (32 nm) [
28],
Solanum nigrum leaves (29.79 nm) [
39],
Costusigneus leaves (31 nm) [
40], and
Uncaria gambir leaves [
41] for the bio-mediated synthesis of ZnO particles.
Figure 5 presents the scanning electron microphotographs of the green ZnO particles synthesized using
Chrysanthemum spp. flower extract. The morphology of the green ZnO particles had a level of agglomeration. Due to the calcination during synthesis, crystals grow under the control of many factors, such as van der Waals forces, polarities, and electrostatic attractions [
16]. As a result, ZnO particles can form many defects on their surfaces, which results in a large surface area. This phenomenon is explained by the thermal decomposition of natural compounds in
Chrysanthemum spp. flower extract, e.g., folic acid, polyphenols, niacin, quercetin, reducing sugars, and flavonoids [
26]. The influence of the calcination temperature on the morphology of green ZnO particles was reported by Thi et al. [
42]. However, a calcination temperature of 600 °C could be to minimize the self-clustering and agglomeration of green ZnO particles.
The chemical composition of the green ZnO particles synthesized using
Chrysanthemum spp. flower extract could be analyzed by energy-dispersive X-ray spectroscopy. According to
Figure 6, the ZnO particles contained O and Zn as the main components (>94%) in their structure. Our findings revealed that the ZnO particles had a high degree of purity. A minute amount of other elements such as N (4.23%), Cl (0.29%), P (0.32%), and K (0.96%) was observed in the ZnO nanomaterial. These traces could have come from the
Chrysanthemum spp flower extract. The calcination of the sample released most of the volatile components, leaving a number of elements in the structure of the ZnO. Several works have reported similar high-purity ZnO particles synthesized using various plant extracts, such as
Hibiscus sabdariffa,
Cannabis Jatropha,
Tinosporacordifolia,
Thymbra spicata, and
Cinnamomum camphora [
43,
44,
45,
46]. As such, ZnO particles could exhibit sufficient photocatalytic activity against organic dyes. To better elucidate this, the green ZnO particles synthesized using
Chrysanthemum spp. flower extract were further investigated.
3.2. Taguchi Design and Model Optimization
In this study, the Taguchi design was used to create an experimental space comprising four factors, including dye concentration (A, 10–50 mg/L); ZnO dosage (B, 0.33–1.0 g/L); contact time (C, 30–120 min); and pH (D, 4–10). As shown in
Table 3, there were nine experiments in total (entry 1–9) for this design, and the average degradation efficiency value (11.1–96.9%) was 38.66%. To better elucidate the effect of the main parameters, the analysis of variance (ANOVA) conducted based on the Taguchi design (3
4) is presented in
Table 4.
The
p value of the regression was found to be 0.009, corresponding to an F value of 17.38. This value was far lower than 0.05, indicating that the regression model was statistically significant. Moreover, the coefficient of determination R
2 and adjusted R
2 were very high at 0.946 and 0.891, respectively. Therefore, the experimental data in this study offered high reliability. The probability values of terms A, B, C, D could be used to clarify whether they were statistically significant. The results revealed that the initial concentration, contact time, and pH were statistically significant at
p < 0.05, while the ZnO dose was statistically insignificant. From the ANOVA table, a regression equation was established to show the correlation between the variables and responses, as follows (Equation (4)):
To determine the contribution of the parameters, the value of the sequential sums of squares was calculated. The contribution of each parameter was equal to the ratio between the sequential sums of squares of that parameter and the total sequential sums of squares. According to
Figure 7, the most important contributor was contact time, and pH was the least important contributor. The contributions were ranked as follows: contact time (rank 1: 43.29%) > initial concentration (rank 2: 39.83%) > pH of the solution (rank 3: 13.25%) > ZnO dosage (rank 4: 3.63%). Importantly, the contact time and initial concentration contributed to 83% of the total main effect.
Figure 8a,b show the effect of these parameters on the degradation efficiency; the optimum conditions were obtained at a low concentration, medium dosage, high reaction time, and high pH.
Certain diagnostic plots can be useful for model validation and the assessment of model reliability.
Figure 8c provides a visual representation of the data distribution, with red points tending to gather around the blue line. This normal probability plot indicates a high level of compatibility, and there were no abnormal points observed in the dataset.
Figure 8d presents the standardized residuals, with all values in the range of −2 to +2. No specific patterns, outliers, or large residuals were observed in the plot. The fitted values were sufficient to ignore the errors caused by the data noise. There was no need to rerun the experiments for adjustment. Therefore, the model was adequate to predict the optimal results, and the assumptions of the regression model were satisfied with high suitability. Model confirmation was conducted to verify the predictions.