Morphology of ZnOand CeO2 Nanoparticles
TEM images of nano-CeO2 show that ceria nanoparticles were fairly well dispersed in solution, while nano-ZnO appeared more agglomerated (Fig.1a-d). Fig. 1e-f shows the powder X-ray diffraction patterns of ZnO & CeO2nanoparticles. Nano-ZnO diffraction pattern corresponded well with hexagonal wurtzite structure with the diffraction peaks at (1 0 0), (1 0 1), (0 0 2), (1 0 2), (1 1 0), and (1 0 3) planes in good agreement with the standard hexagonal ZnO (JCPDS no. [75-0576]). XRD pattern for CeO2 NPs corresponded well to that of CeO2 standard JCPDS no. [65-5923], with peaks of (1 1 1), (2 0 0), (2 2 0), and (3 1 1) suggesting cubic-faced ceria nanoparticles.
The diffuse reflectance spectra of the ZnO and CeO2 and 1:1 CeO2: ZnO composite (1C1Z) is shown in Fig. 2 (a-c). The intercept of plot of photon energy (eV) versus ((𝑅) ∗hν)1/2 (Kubelka-Munk function) revealed that the bandgap energy for ZnO& CeO2 was 3.12 eV and 3.24 eV, respectively. The band gap energy obtained for CeO2in this study was higher than that reported in other studies (Sane et al. 2018; Veedu et al. 2020). Upon addition of the ceria to the ZnO nanoparticles, the bandgap energy reduced to 3.08 eV. It can be noticed that addition of CeO2 to ZnO helped decrease the band gap energy, which may result in an increased efficiency of the composite photocatalyst. Similar decrease in the band gap energy was observed by in other studies(Lang et al. 2016; Rodwihok et al. 2020; Veedu et al. 2020). Luo et al. (Luo et al. 2020) noted that with 3% doping of CeO2 on ZnO, the band gap energy reduced from 3.12 to 3.04 eV, and attributed the decrease to increased oxygen vacancies created upon addition on CeO2, a rare-earth metal oxide found to be abundant in oxygen vacancies.
Photocatalytic Degradation of Reactive Black Dye
Control experiments were performed to evaluate the auto-degradation of RB dye. As seen in Fig. 3(a), degradation was negligible under dark conditions. In the presence of UV light and aeration, but absent the catalyst, less than 5% degradation was observed, most likely due to photolytic degradation of the dye (Kuo and Ho 2001). Addition of CeO2 NPs to either an aerated or non-aerated dye solution showed insignificant dye degradation, suggesting no adsorption of RB on CeO2 (Fig. 3a). However, when ZnO was added to both an aerated and non-aerated dye solution, in dark conditions, up to 7% dye removal was observed, indicating sorption.
Table 1. First order rate constant and percent degradation for various ZnO:CeO2 composite
Catalyst Loading = 1 g/L
|
Photocatalyst
|
1000 mg/L
|
500 mg/L
|
250 mg/L
|
k1
(min-1)
|
R2
|
η (%)
|
k1
(min-1)
|
R2
|
η (%)
|
k1
(min-1)
|
R2
|
η (%)
|
ZnO
|
0.009
|
0.99
|
56.3
|
0.047
|
0.99
|
96.3
|
0.113
|
0.99
|
99.1
|
Ceria
|
0.021
|
1.00
|
84.9
|
0.031
|
0.99
|
91.5
|
0.071
|
0.99
|
95.8
|
2C1Z
|
0.021
|
0.98
|
85.4
|
0.039
|
0.98
|
95.1
|
0.089
|
0.99
|
97.7
|
1C1Z
|
0.021
|
0.98
|
87.7
|
0.061
|
0.98
|
99.5
|
0.136
|
0.92
|
99.9
|
0.5C1Z
|
0.017
|
0.99
|
80.4
|
0.050
|
0.98
|
98.4
|
0.128
|
0.98
|
100.0
|
0.33C1Z
|
0.012
|
0.97
|
66.1
|
0.033
|
0.99
|
91.5
|
0.092
|
0.99
|
98.6
|
0.25C1Z
|
0.012
|
0.99
|
68.2
|
0.041
|
0.97
|
96.5
|
0.114
|
0.98
|
99.6
|
Fig. 3 (b & c) and Fig. 4 show the photocatalytic ability of various ZnO: CeO2 wt. ratios on RB degradation at [RB] = 1000 mg/L and 1 g/L catalyst, and Table 1 shows the percent degradation. Within 90 minutes, > 60% degradation was observed with the use of different weight ratios of CeO2 and ZnO nanoparticles. The absorbance spectra of RB decreased significantly within the first 15 minutes upon use of CeO2-ZnO nanocomposites, with near complete degradation within 1 hour (Fig. 4).The degradation reached near equilibrium within 70 minutes and significantly reduced degradation rates was observed beyond reaction time of 70 minutes. The percent degradation was highest for 1C1Z (87.7%), while it was 80.4, 66.1, 68.2 % and 85.4% for 1C2Z, 0.33C1Z, 0.25C1Z, and 2C1Z respectively. Decreasing the amount of CeO2 in the ZnO-CeO2 heterojunction nanocomposite appeared to reduce RB degradation rates significantly (Table 1). Among the various wt. ratios of the composites investigated, IC!Z provided the maximum dye degradation. It is hypothesized that as the CeO2 content is increased, efficient separation of electron-hole pair occurs favoring redox reactions on the surface, and hence increased percent degradation was observed.
Typically, photocatalytic degradation kinetics can best be described using first-order rate kinetics. Fig. 3c shows the plot of ln (C/C0) vs. time for the various ZnO:CeO2 nanocomposites. The first order plot clearly indicated that ZnO: CeO2at 1:1 wt. ratio (1C1Z) performed the best among the various photocatalysts (Fig. 3c). The first order rate constants determined for 1C1Z was almost 3-fold higher than that for ZnO, and almost similar to pure CeO2 at 1000 mg/L RB concentration and 1 g/L catalyst loading (Table 1). Increasing ZnO content in the composite decreased the first order rate constant, while increasing CeO2 content increased it, largely due to the decrease in the band gap energy upon addition of CeO2. The data was also fitted to a second order rate expression (Figure 5). The data fitted well to the pseudo-second order rate expression as well (quation 3, Table 2) at 1000 mg/L RB concentration and 1 g/L catalyst loading (R2: 0.93 – 1.00). Sane et al. (Sane et al. 2018)reported better fit of a second order model to the kinetic data on the degradation of reactive dyes using CeO2. The rate constants reported in their study ranged from 0.0085 – 0.016 L/mg min for the CeO2 photocatalyst investigated for reactive dyes in the concentration range of 10-100 mg/L, slightly higher values of rate constant to the values obtained in our study.
Table 2. Second order rate constant for various ZnO: CeO2 composite
Catalyst Loading = 1 g/L
|
Photocatalyst
|
1000 mg/L
|
500 mg/L
|
250 mg/L
|
k2 x 10-5 (L- mg-1-min-1)
|
R2
|
k2 x 10-5 (L- mg-1-min-1)
|
R2
|
k2 x 10-5 (L- mg-1-min-1)
|
R2
|
ZnO
|
1.40
|
0.98
|
64.39
|
0.85
|
1040.76
|
0.81
|
Ceria
|
5.60
|
0.93
|
23.02
|
0.86
|
168.94
|
0.80
|
2C1Z
|
6.36
|
0.98
|
46.32
|
0.89
|
333.68
|
0.76
|
1C1Z
|
6.97
|
0.94
|
274.68
|
0.51
|
5204.00
|
0.33
|
0.5C1Z
|
4.26
|
0.98
|
97.81
|
0.57
|
766.77
|
0.61
|
0.33C1Z
|
2.24
|
1.00
|
26.73
|
0.88
|
525.50
|
0.72
|
0.25 C1Z
|
2.20
|
0.98
|
52.04
|
0.66
|
1414.63
|
0.55
|
Effect of Initial Dye Concentration
The effect of initial dye concentration on degradation rates was examined. Fig. 6 and 7 show the kinetic data and the respective first-order kinetic plots for RB concentration of 500 mg/L (6a & b) and 250 mg/L (7 a & b), at 1 g/L catalyst loading, respectively. In about 75 minutes, greater than 90% degradation was observed for various weight ratios of CeO2 and ZnO nanoparticles. Similar to that observed for 1000 mg/L RB concentration, decreasing CeO2 content in the nanocomposite decreased percent degradation, albeit marginally for an initial concentration of 500 mg/L (Fig. 6). The degradation kinetics clearly fitted to first order rate expression with high R2 values, but appeared to be poorer fits for second-order rate expressions (Table 2). As the initial RB concentration is lowered to 250 mg//L, percent removal increased to near 100% (Fig. 7a). Similar to the previous observation, the data fitted better to first-order rate expressions, rather than second-order rate expressions (Fig. 7b). Similarly, increasing CeO2 fraction on ZnO did not significantly improve the percent degradation since near complete degradation was observed for all photocatalysts used within 45 minutes (Fig. 7a). Equilibrium was also obtained within 45 minutes with negligible change in rate of degradation after 30 minutes. Although degradation rate was slower for CeO2 and 1C1Z initially, the degradation rate of 1C1Z after 45 minutes was comparable with that of ZnO photocatalysts. Decrease in the initial RB concentration resulted in faster degradation, as observed by a 2-5 fold increase in the first order rate constant (Table 2). For 1C1Z, the rate constant amplified almost 6 times from 0.021 to 0.136 min-1 as RB concentration decreased from 1000 to 250 mg/L. It was observed generally that as initial RB concentration decrease, the rate constant and therefore the initial rate (kCo) increased. For the same number of reactive sites on the surface, decrease in [RB] moieties due to decreasing initial concentration results in increased adsorption on the surface and increased degradation. At higher concentration, there is competition for the reactive sites, hence the initial rate is lowered. In all the experiments with varying RB concentrations and ZnO: CeO2 weight ratios, 1C1Z exhibited the highest degradation rate, with the first order rate constant almost twice as high as that of pure ZnO.
Effect of Catalyst Loading
To further evaluate the photocatalytic process, kinetic experiments were performed at varying catalytic loadings using the best performing photocatalyst, 1C1Z, and its efficacy compared to that of pristine nano-ZnO and nano-CeO2. Fig.8 (a-c) shows the kinetic data for catalyst loading of 0.5 g/L (RB concentration = 250 – 1000 mg/L) and Fig.8 (d-f) shows the kinetic data for catalyst loading of 1.5 g/L (RB concentration = 250 – 1000 mg/L). Table 3 presents the percent degradation and constants to the first – order data fit. For 250 and 500 gm/L RB concentration (Fig. 8), there was no appreciable difference in degradation rate among the three photocatalysts investigated, ZnO and CeO2 and 1C1Z. Near complete degradation was observed within 60 minutes, particularly at low concentrations. At low catalyst loading (0.5 g/L) and high RB concentration, the degradation rates were lower. Equilibrium was not attained within 50 minutes for 1000 mg/L and 500 mg/L RB concentration, while it was observed with 250 mg/L RB concentration. The percent degradation was 35-40% for 1000 mg/L, 60-65% for 500 mg/L and almost 100 % for 250 mg/L, at 0.5 g/L. The percent degradation was 55-60% for 1000 mg/L, 85-95% for 500 mg/L and almost 100 % for 250 mg/L, at 1.5 g/L. As the catalyst loading was increased, degradation rates were faster as evidenced by the near complete completion in lesser reaction times. Increasing catalyst loading increased percent removals and degradation rates as there is an expected rise in the number of reactive sites.
The data was fitted to first-order rate expression, and the rate constant deduced. The first order rate constant for degradation of dye by ZnO was observed to be 0.008 min-1 (R2 =0.953), while it was 0.01 min-1 (R2= 0.983) and 0.007 min-1 (R2 =0.988) using CeO2 and 1C1Z respectively for RB concentration of 1000 mg/L. Similarly, for 500 mg/L RB dye concertation the first order rate constants were found to be 0.027 min-1 (R2 = 0.993), 0.027 min-1 (R2 = 0.992) and 0.019 (R2 = 0.984) for ZnO, CeO2 and 1C1Z respectively. Correspondingly, the first order rate constants for 250 mg/L RB dye concentration provided the values of 0.106 min-1 (R2 = 0.933), 0.07 min-1 (R2 = 0.978) and 0.04 (R2 = 0.99) for ZnO, CeO2 and 1C1Z respectively.
Table 3. First order rate constant and percent degradation for various catalyst loadings
Catalyst Loading = 0.5 g/L
|
|
Photocatalyst
|
1000 mg/L
|
500 mg/L
|
250 mg/L
|
k1
(min-1)
|
R2
|
η (%)
|
k1
(min-1)
|
R2
|
η (%)
|
k1
(min-1)
|
R2
|
η (%)
|
ZnO
|
0.008
|
0.95
|
47.5
|
0.027
|
0.99
|
85.8
|
0.104
|
0.95
|
99.1
|
Ceria
|
0.010
|
0.98
|
60.9
|
0.027
|
0.99
|
89.0
|
0.070
|
0.98
|
97.5
|
1C1Z
|
0.007
|
0.99
|
42.1
|
0.019
|
0.98
|
79.2
|
0.040
|
0.99
|
82.9
|
Catalyst Loading = 1.5 g/L
|
|
Photocatalyst
|
1000 mg/L
|
500 mg/L
|
250 mg/L
|
|
k1
(min-1)
|
R2
|
η (%)
|
k1
(min-1)
|
R2
|
η (%)
|
k1
(min-1)
|
R2
|
η (%)
|
ZnO
|
0.018
|
0.99
|
78.4
|
0.050
|
0.99
|
98.1
|
0.143
|
0.95
|
99.6
|
Ceria
|
0.026
|
0.98
|
91.6
|
0.066
|
0.99
|
99.2
|
0.073
|
0.88
|
98.5
|
1C1Z
|
0.017
|
0.98
|
75.8
|
0.037
|
0.99
|
96.1
|
0.138
|
0.96
|
99.7
|
Table 3 presents the rate constant for a catalyst loading of 1.5 g/L. It can be seen that increasing loading three times increased the rate constant also 3 fold, for 1000 mg/L. However, when the initial RB concentration was 250 mg/L, the rate constant remained constant or marginally increased. This suggested that RB concentration was the rate controlling factor when the number of reactive sites was in abundance.The results from this study were compared to results from studies wherein binary metal oxide photocatalysts were used for dye degradation. As seen in Table 4, when ZnO: CeO2was used in a ratio of 1:5 for Rhodamine degradation, the efficiency was 98%, albeit at low dye concentration (24 mg/L). Similar efficiencies were reported when the base material ZnO was doped with other metal oxides such as Bi2O3, CdO etc. It has also been noted that doping a second metal oxide on ZnO significantly improved degradation efficiencies, as is the case here.
Table 4. Comparison of results to reported studies
Photocatalyst
|
Experimental condition
|
Degradation efficiency %
|
Reference
|
ZnO
|
Binary
|
CuO/ZnO
|
CuO/ZnO = 2 g/L
[MO] = 20 mg/L;
Irradiation time = 60 min
|
50.0
|
90.0
|
(Liu et al. 2008)
|
CdS/ZnO
(1:3)
|
CdS/ZnO = 0.5 g/L
[RhB] = 24 mg/L
Irradiation time = 90 min
|
~ 45.0
|
100
|
(Li et al. 2011)
|
CeO2/ZnO
(1:5)
|
CeO2/ZnO = 0.5 g/L
[RhB] = 24 mg/L
Irradiation time = 180 min
|
82.3
|
98
|
(Li et al. 2011)
|
Bi2O3/ZnO
(1:23)
|
Bi2O3/ZnO = 1 g/L
[RhB] = 4.8 mg/L
Irradiation time = 180 min
|
~ 15.0
|
85.0
|
(Yang et al. 2014)
|
CdO/ZnO
(10 wt.%
CdO)
|
CdO/ZnO = N/A
[MB] = NA mg/L
Irradiation time = 360 min
|
4.9
|
49.7
|
(Saravanan et al. 2015)
|
CeO2/ZnO
(10:90)
|
CeO2/ZnO = NA
[RhB] = 1000 mg/L
Irradiation time = 150 min
|
4.2
|
95.9
|
(Rajendran et al. 2016)
|
Mg/ZnO
(7.5 wt% Mg)
|
Mg/ZnO = 50 mg/L
[RhB] = 20 mg/L
Irradiation time = 120 min
|
15.0
|
75.0
|
(Pradeev raj et al. 2018)
|
CeO2/ZnO
(1:1)
|
CeO2/ZnO = 1 g/L
[RB] = 1000 mg/L
Irradiation time = 90min
|
56.3
|
87.7
|
Our work
|
Effect of pH on RB degradation
The effect of pH on RB degradation was evaluated on the best performing photocatalyst, 1C1Z at various solution pH and the its efficacy compared to that of pristine nano-ZnO and nano-CeO2. Here, the effect of pH was performed at 1000 mg/L dye concentration and 1 g/L catalyst loading (Fig. 9). In the acidic pH region, degradation efficiencies were very low (< 30%) for all the photocatalysts investigated. However, as pH was increased to 11, higher degradation of 99%,98% and 47% was observed for 1CIZ, ZnO and CeO2respectively. Previous research has shown that photocatalytic reactions are pH-dependent, primarily caused by the surface charge of the catalyst and the molecular structure of the organic contaminant. The increase in percent degradation at higher pH can be attributed to the increase in the number of hydroxyl ions. The pKa value for reactive black dye is 3.8 and 6.9. The pH at point of zero charge (pHPZC) for CeO2 has been reported to be 6.9 while that for ZnO has been 8.7-9.7 (Meshram et al. 2017). Since RB is an anionic dye, electrostatic attraction between the dye molecule and catalyst surface at pH <pHPZC results in the dye removal at acidic pH. Since the pHPZC shifts towards more alkaline pH in a ZnO: CeO2 mixture, higher percent removals are expected. Sane et al. (Sane et al. 2018) reported that activity of CeO2 for dye degradation was higher at neutral pH, and followed the order: 7 > 9.2 > 4. However, when ZnO based catalysts were used for degradation of Rhodamine – B dye, higher pH provided the best degradation (Saffari et al. 2020). Highest percentage removal of dye was observed at a pH of 11. Among all the photocatalyst studied, 1C1Z performed the best, clearly indicating that the addition of CeO2 to ZnO significantly improved the photocatalytic activity of the composite.