Fatemeh Khodam,
Zolfaghar Rezvani* and
Ali Reza Amani-Ghadim
Department of Chemistry, Faculty of Basic Sciences, Azarbaijan Shahid Madani University, Tabriz, Iran. E-mail: zrezvani@azaruniv.ac.ir; z_rezvani@yahoo.com; Fax: +98 413 432 7541; Tel: +98 413 432 7541
First published on 5th February 2015
With the purpose of the enhancement of photocatalytic performance in the visible region and efficient electron–hole separation, we reported a facile method for the synthesis of a mixed metal oxide/ZnO/CNT (MMO/ZnO/CNT) nanohybrid derived from ZnO/Co-Ni-Al layered double hydroxide (LDH) precursor. The structural and morphological aspects of the synthesized products were characterized by X-ray diffraction, scanning electron microscopy, UV/vis diffuse reflectance spectra, and FT-IR spectroscopy. The photocatalytic activity of the synthesized ZnO/MMO/CNT nanohybrid was investigated by photocatalytic degradation of C.I. Acid Red 14, as a model pollutant, under visible light irradiation. The photocatalytic activity of ZnO/MMO/CNT was also compared with TiO2–P25, ZnO, and ZnO/Co-Ni-Al-LDH/CNT. The experimental results revealed that in comparison with other used photocatalysts, ZnO/MMO/CNT nanohybrid was an efficient photocatalyst under visible light irradiation. The effect of operational parameters including photocatalyst content, dye concentration, pH, and irradiation time on the photocatalytic removal efficiency of dye was investigated and optimized using response surface methodology approach. The photocatalyst dosage of 0.009 g, initial dye concentration of 20 mg L−1, pH of 4.12, and irradiation time of 150 min were obtained as the optimum condition. In the proposed optimum condition, the catalyst reusability tests were carried out for five runs. Negligible decrease in degradation efficiency confirmed high potential of stability and reusability for the ZnO/MMO/CNT photocatalyst.
Layered double hydroxides (LDHs) are a class of anionic (anion exchanging) clays or hydrotalcite-like compounds,21 consisting of brucite-like materials, in which a fraction of the divalent cations has been replaced isomorphosly by trivalent cations producing positively charged layers and interlayer charge-compensating anionic species or counter ions between the layers. Some hydrogen bonded water molecules may occupy the remaining free space of the interlayer space. These materials are described according to the standard formula:
[M1−xIIMxIII(OH)2]x+[Xx/mm−·nH2O]x−, abbreviated as [MII-MIII-X], where MII and MIII are divalent and trivalent metal ions, respectively, and Xm− the interlayer anions with x being defined as the MII/(MII + MIII) ratio. Due to the flexible ion-exchangeability and tunable composition, layered double hydroxides have emerged as one of the most promising materials for their unique and attractive properties and feasibility of applications in various fields such as catalysis, photocatalysis, catalyst support, adsorbents and drug delivery systems.22 LDHs are converted into mixed metal oxides (MMO) after calcination at the temperature range of 300 to 700 °C.23,24 It was reported that calcined LDHs could be used as photocatalysts for the photodegradation of organic pollutants, taking the form of highly dispersed metal oxides.13
In the present work, in order to combine the unique properties of carbon nanotubes with layered double hydroxides to obtain more excellent photocatalytic performance under visible light irradiation, we proposed a model for the synthesis of ZnO/mixed metal oxide/CNT (ZnO/MMO/CNT) nanohybrid that is derived from ZnO/Co-Ni-Al layered double hydroxide (LDH) precursor. The specific objectives of this work were: (1) synthesis and characterization of the ZnO/MMO/CNT mixed metal oxide nanohybrid, (2) investigation of photocatalytic activity of the synthesized ZnO/MMO/CNT mixed metal oxide nanohybrid for the removal of C.I Acid Red 14 (AR14), as a model organic pollutant, in aqueous solution, and (3) optimization and modeling of photocatalytic performance by response surface methodology (RSM) approach.
The synthesis of Co-Ni-Al-LDH was similar to that described above, but without using CNT (CNT-COONa) and Zn(CH3COO)2·2H2O. After aging of solution for 72 h in an oil bath at 60 °C, the resulting precipitate was centrifuged, thoroughly washed by distilled water, and dried in an oven at 40 °C.
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Variables (factors) | Symbol | Ranges and actual values of coded levels | ||||
---|---|---|---|---|---|---|
−α | −1 | 0 | +1 | +α | ||
ZnO/MMO/CNT (g) | x1 | 0.005 | 0.008 | 0.011 | 0.014 | 0.017 |
Time (min) | x2 | 30 | 60 | 90 | 120 | 150 |
pH | x3 | 4 | 5.5 | 7 | 8.50 | 10 |
Initial dye concentration (mg L−1) | x4 | 20 | 30 | 40 | 50 | 60 |
Run | Coded variables | DE, % | ||||
---|---|---|---|---|---|---|
ZnO/MMO/CNT (g) | Time (min) | pH | [Dye]0 (mg L−1) | Experimental | Predicted | |
1 | 0 | 2 | 0 | 0 | 94.87 | 94.1404 |
2 | −1 | −1 | −1 | 1 | 68.55 | 70.8108 |
3 | 0 | 0 | 0 | 0 | 96.01 | 96.1114 |
4 | 0 | 0 | 0 | 0 | 94.56 | 96.1114 |
5 | −1 | 1 | 1 | −1 | 87.89 | 88.4721 |
6 | 1 | −1 | −1 | 1 | 84.17 | 84.2971 |
7 | 1 | −1 | 1 | 1 | 78.37 | 79.6792 |
8 | 1 | −1 | −1 | −1 | 90.84 | 93.4992 |
9 | 2 | 0 | 0 | 0 | 88.66 | 85.1621 |
10 | −1 | 1 | −1 | −1 | 98.02 | 98.4125 |
11 | 0 | 0 | 0 | 2 | 86.21 | 84.0037 |
12 | −1 | 1 | −1 | 1 | 87.35 | 86.9754 |
13 | 0 | 0 | 0 | 0 | 95.11 | 96.1114 |
14 | −2 | 0 | 0 | 0 | 72.02 | 73.1071 |
15 | −1 | −1 | 1 | 1 | 68.45 | 68.6404 |
16 | 0 | 0 | −2 | 0 | 98.18 | 96.1587 |
17 | 0 | 0 | 2 | 0 | 81.99 | 81.6004 |
18 | −1 | −1 | 1 | −1 | 79.24 | 78.7275 |
19 | 0 | 0 | 0 | −2 | 98.71 | 98.5054 |
20 | 1 | −1 | 1 | −1 | 81.66 | 82.7438 |
21 | −1 | −1 | −1 | −1 | 88.38 | 87.0354 |
22 | 1 | 1 | −1 | −1 | 98.91 | 99.4288 |
23 | −1 | 1 | 1 | 1 | 84.13 | 83.1725 |
24 | 0 | 0 | 0 | 0 | 95.86 | 96.1114 |
25 | 0 | −2 | 0 | 0 | 75.36 | 73.6787 |
26 | 1 | 1 | 1 | 1 | 86.71 | 88.7638 |
27 | 1 | 1 | 1 | −1 | 87.6 | 87.0408 |
28 | 0 | 0 | 0 | 0 | 97.86 | 96.1114 |
29 | 0 | 0 | 0 | 0 | 98.5 | 96.1114 |
30 | 1 | 1 | −1 | 1 | 92.8 | 95.0142 |
31 | 0 | 0 | 0 | 0 | 94.88 | 96.1114 |
Source of variations | DFb | SSc | Adj-MSd | F-value | P-value | Critical F-value |
---|---|---|---|---|---|---|
a R2 = 97.22%, adjusted R2 = 94.78%.b Degree of freedom.c Sum of squares.d Adjusted mean square. | ||||||
Regression | 14 | 2378.90 | 169.22 | 39.90 | 0.000 | 2.373 |
Linear terms | 4 | 1479.37 | 369.842 | 86.85 | 0.000 | |
Square terms | 4 | 751.30 | 187.824 | 44.11 | 0.000 | |
Interaction terms | 6 | 148.24 | 24.706 | 5.80 | 0.002 | |
Residual error | 16 | 68.13 | 4.258 | — | — | — |
Lack-of-fit | 10 | 54.37 | 5.437 | 2.37 | 0.151 | 4.060 |
Pure error | 6 | 13.76 | 2.294 | — | — | — |
Total | 30 | 2447.03 | — | — | — | — |
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Fig. 1 FT-IR spectrum of MWCNTs, MWCNT-COOH, Co-Ni-Al-LDH, Co-Ni-Al-LDH/ZnO/CNT, and ZnO/MMO/CNT nanohybrid. |
The XRD patterns of MWCNT-COOH, Co-Ni-Al-LDH, Co-Ni-AlLDH/CNT, Co-Ni-Al-LDH/ZnO/CNT nanohybrid, and Co-Ni-Al-Zn/CNT mixed metal oxide (ZnO/MMO/CNT) nanohybrid are shown in Fig. 2. The XRD patterns of Co-Ni-Al-LDH, Co-Ni-Al-LDH/CNT, and Co-Ni-AlLDH/ZnO/CNT exhibit the characteristic reflections of the LDH structure. The peaks at 11.44°, 22.77°, 34.16°, 60.3°, and 61.5° are indexed as planes (003), (006), (110), and (113). In the XRD patterns of Co-Ni-Al-LDH, Co-Ni-Al-LDH/ZnO/CNT, these reflections are indexed to a typical hydrotalcite-like structure (JSPDC no. 15-0087).30 In case of Co-Ni-Al-LDH/ZnO/CNT, reflection peaks in the range of 2θ = 32–36° (Fig. 2c and d) are attributed to the zinc hydroxide and ZnO phase (JCPDS card no. 36-1451). The disappearance of (002) and (101) diffraction peaks of the CNT-COOH (graphite layers) in the Co-Ni-Al-LDH/CNT, Co-Ni-Al-LDH/ZnO/CNT, and ZnO/MMO/CNT nanohybrids confirm the formation of nanohybrids and it can be indicated that the MWCNT-COOH incorporates into the LDHs and ZnO/MMO networks. According to the results, the unit cell parameters and interlayer distances are very similar to the original LDH, confirming that the zinc hydroxide is adsorbed by the surface of the LDHs rather than intercalated between the interlayers. After the calcination of the Co-Ni-Al-LDH/ZnO/CNT nanohybrid at 300 °C, the layered structure of Co-Ni-Al-LDH is completely destroyed and ZnO/MMO/MWCNT nanohybrid is formed. In the case of ZnO/MMO/MWCNT nanohybrid most of the diffraction peaks may be indexed to the Wurtzite of ZnO (JCPDS card no. 36-1451). Because of low calcinations temperature (300 °C) of Co-Ni-Al-LDH/ZnO/CNT and consequently low crystallinity of metal oxides, as shown in XRD pattern, no peak assigned to the cobalt oxide, nickel oxide, and aluminum oxide were observed in the ZnO/MMO/MWCNT nanohybrid. These results are similar to those reported by Klemkaite et al.31
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Fig. 2 XRD patterns of MWCNT-COOH, Co-Ni-Al-LDH, Co-Ni-Al-LDH/CNTs, Co-Ni-Al-LDH/ZnO/CNT, and ZnO/MMO/CNT nanohybrid. |
Based on our results, the photocatalytic activity of ZnO/MMO/MWCNT sharply depends on the calcinations temperature of Co-Ni-Al-LDH/ZnO/CNT. The best photocatalytic activity of ZnO/MMO/MWCNT nanohybrid is achieved in calcination temperature of 300 °C. Increasing of calcination temperature of Co-Ni-Al-LDH/ZnO/CNT to higher than 300 °C results in decrease in photocatalytic activity of derived ZnO/MMO/MWCNT nanohybrid. This fact may be related to the increasing of grain size of metal oxides in the ZnO/MMO/MWCNT nanohybrid by increasing the calcinations temperature.
Fig. 3a and b shows SEM images of Co-Ni-Al-LDH and ZnO/MMO/CNT nanohybrid. Fig. 3a confirms the formation of crystals with perfect sheet shapes and LDH sheets, in which the predominantly smooth textures have been produced.
In the SEM image of the ZnO/MMO/CNT nanohybrid (Fig. 3b), ZnO nanorods can be easily distinguished (red circles). It is well known that the final morphology of ZnO crystals is related to both their intrinsic crystal structure and external factors. The catalytic activity of the ZnO nanostructures possesses a sequence of nanorods > nanoflowers > nanopyramids > nanoprisms.32
Fig. 4 shows the TGA and DTG curves of NiCoAl-LDH/CNT and ZnO/NiCoAl-LDH/CNT nanohybrid samples. Both samples have similar thermal decomposition behavior. For NiCoAl-LDH/CNT, three degradation stages are distinguishable in TGA. Below 180 °C, water releases from the surface and LDH interlayer. The temperature range of 180–270 °C corresponds to the second stage with the dehydration of the brucite-like layers, and the final stage is the decomposition of carbonate ions in the interlayer occurring in the range of 270–500 °C. The DTG curve shows two distinct peaks for both samples. The first peak around 110 °C is ascribed to the removal of water. The peak at 270 °C for both of the samples corresponds to the decomposition of carbonate ions in the interlayer of LDH. The small peak in the DTG curve of ZnO/NiCoAl-LDH/CNT nanohybrid sample arises from the dehydration of LDH layers. For the NiCoAl-LDH sample, the small peak was absent in the DTG curve, remarking the presence of an overlap between the dehydration and decomposition of carbonate ions in these samples.28
The photodegradation of AR14 was negligible in the direct photolysis (visible light only). The removal of dye was less than 10% in the absence of ZnO/MMO/CNT nanohybrid which indicates that light decomposition of AR14 in ZnO/MMO/CNT-visible light process is attributed to photocatalytic properties of ZnO/MMO/CNT nanohybrid. The photocatalytic activity of ZnO/MMO/CNT nanohybrid was also compared with ZnO and TiO2–P25 as typical reference photocatalysts. As seen in Fig. 5, the removal efficiency of ZnO/MMO/CNT nanohybrid (96.2%) is considerably greater than that of ZnO (18.57%) and TiO2–P25 (28.14%). These findings may be attributed to the fact that the coupling of semiconductors provides different band-gaps and energy levels which is an interesting way to increase the charge separation and expand the energy range of photoexcitation for photocatalytic process. To verify the obtained results, the UV/vis diffuse reflectance spectra (DRS) technique was used. The DRS spectra of NiCoAl-ZnO-LDH/CNT, ZnO/MMO/CNT nanohybrid, and ZnO nanoparticle are illustrated in Fig. 6. Both samples show one absorption edge around 350 nm that is related to the absorption edges of ZnO, but ZnO/MMO/CNT nanohybrid shows another peak emerged at 400–800 nm which represents the ability of the visible response. The phenomena imply good ability of ZnO/MMO/CNT nanohybrid for utilizing sunlight owing to its wide light-adsorption range. Also, the optical band gap energies (Eg) of the prepared NiCoAl-Zn-LDH/CNT, ZnO/MMO/CNT nanohybrid, and ZnO were determined by UV-vis DRS, and the obtained DRS results are reported according to the Kubelka–Munk function (3).33,34
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As shown in Fig. 7, the optical bandgap energies of the prepared NiCoAl-Zn-LDH/CNT, ZnO/MMO/CNT nanohybrid, and ZnO can be derived from UV-vis DRS data by plotting (αhv)2 against photon energy (hv), followed by extrapolation of the linear part of the spectra to the energy (hv) axis. The calculated values of the bandgap energies for NiCoAl-Zn-LDH/CNT and ZnO were 3.17 and 3.21 eV respectively, but for ZnO/MMO/CNT nanohybrid, two band gap energies in 2.15 and 2.75 eV were obtained which confirms a red shift in the absorbance spectra of ZnO/MMO/CNT nanohybrid.
Based on some literature data,35–38 during a photocatalytic process equipped with mixed metal oxides irradiated with ultra violet or visible light, degradation of an organic molecule is generally operated by the formation of the electron/hole pairs on the surface of the photocatalyst. ZnO/MMO/CNT has high adsorbing capacity as well as high efficiency for the photocatalytic degradation. The pre-adsorption of the substrate (dye) onto the photocatalyst and the photoexcitation of the semiconductor followed by the formation of the electron/hole pairs are two prerequisites for highly efficient degradation.24 The photogenerated valence band holes react with either water (H2O) or hydroxyl ions (OH−) adsorbed on the catalyst surface. Then, highly reactive hydroxyl radicals (OH˙) are produced, that promotes the degradation of target pollutants. According to some ref. 39, in the case of acid red 14, the bonds of NN and C–N are the major targets for hydroxyl radicals (˙OH) and photon electrons (e−) during the degradation of azo dye compounds. Besides, dissolved oxygen can capture photogenerated electrons from the conduction band to generate superoxide ions (˙O2−). Based on recently reported researches,40 ˙O2− has been proposed as the major photocatalytic oxidant in the photocatalytic oxidation of azo dyes under light irradiation. The superoxide ions can then react with water to produce hydrogen peroxide and hydroxyl ions. Cleavage of hydrogen peroxide by the conduction band electrons yields further hydroxyl radicals and hydroxyl ions. The hydroxyl ions can react with the valance band holes to from additional hydroxyl radicals. The tentative photodegradation mechanism of acid red 14 by MMO/ZnO/CNT nanohybrid under visible light irradiation is given by eqn (4)–(9).
MMO/ZnO/CNT nanohybrid + hν → (eCB− + hVB+) MMO/ZnO/CNT nanohybrid | (4) |
h+ + H2O → H+ + ˙OH | (5) |
h+ + OH− → ˙OH | (6) |
e− + O2˙→ ˙O2− | (7) |
˙O2− + H2O + H+ → H2O2 + OH− | (8) |
H2O2 + e− → ˙OH + OH− | (9) |
The resulting ˙OH radical, being a very strong oxidizing agent (standard redox potential +2.8 V), and the degradation of dye can be achieved by their reaction with hydroxyl radicals (˙OH) or by direct attack from the valence band holes. Scheme 1 shows the schematic diagram of a visible light photocatalytic tentative mechanism with ZnO/MMO/CNT nanohybrid.
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Scheme 1 Schematic representation of visible light photocatalytic process in the presence of ZnO/MMO/CNT nanohybrid. |
y = 96.1114 + 3.0137x1 + 5.1154 − 3.6396x3 − 3.6254x4 − 4.2442x12 − 3.0505x22 − 1.8080x32 − 1.2142x42 − 1.3619x1x2 − 0.6919x1x3 + 1.7556x1x4 − 0.4081x2x3 + 1.1969x2x4 + 1.5344x3x4 | (10) |
The predicted degradation efficiency obtained from eqn (10) is provided in Table 2. The significance and adequacy of the model was tested using the analysis of variance (ANOVA) calculation and the obtained results are summarized in Table 3. According to ANOVA analysis, the Fisher's F-value of regression (equal to 39.9) was considerably much higher than the critical F-value confirming the significant of the obtained second-order polynomial model.41 It was also revealed from ANOVA results that all model terms, including first and second order main effects and interaction effects, are significant because of small p-values (significant probability values). In statistics, higher F-value or lower p-value (<0.05) for one term is considered to be significant. Moreover, P-value greater than 0.05 for the lack of a fit test (LOF) demonstrates insignificant LOF for the obtained model. The comparison of experimental and predicted degradation efficiencies (R2 = 97.22%, adj-R2 = 94.78%) reveals that the model suitably describes the relation between the response and the variables.42 In addition to three mentioned criteria for evaluating the model adequacy, the residuals were utilized to survey the model significance. As shown in Fig. 8, a random pattern of residuals in Fig. 8a and straight line of residuals in Fig. 8b indicate the normal and independent distribution of residuals. It means that the model is a good predictor.
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Fig. 8 Residual plots for removal efficiency of AR14 (a) normal probability plots of residuals and (b) residuals versus fits plots. |
The Student's t distribution and Pareto analysis were utilized to evaluate the significance of the model terms (Table 4). The P-values were considered as the checking tools for significant term determination. P-values smaller than 0.05 indicate that the coefficients of this model are significant. In addition to Student's t distribution, the percentage effect of each term was determined with the Pareto analysis, which calculates the percentage effect of each term on the response.27,43 Based on the results presented in Table 4, all linear and square terms of the model are important, but the importance of irradiation time (25.41%) and ZnO/MMO/CNT × ZnO/MMO/CNT (17.49%) have the main effect on the degradation of AR14. The interaction terms are not of much importance due to higher p-values. Among the interaction terms, x1x4 and x3x4 interactions are relatively significant.
Coefficient | Coefficient estimate | T-value | P-value | Percentage effects of model terms, % | |
---|---|---|---|---|---|
b0 | Constant | 96.1114 | 123.228 | 0.000 | — |
b1 | x1 | 3.0137 | 7.155 | 0.000 | 8.82 |
b2 | x2 | 5.1154 | 12.144 | 0.000 | 25.41 |
b3 | x3 | −3.6396 | −8.641 | 0.000 | 12.86 |
b4 | x4 | −3.6254 | −8.607 | 0.000 | 12.76 |
b11 | x1x1 | −4.2442 | −10.999 | 0.000 | 17.49 |
b22 | x2x2 | −3.0505 | −7.905 | 0.000 | 9.04 |
b33 | x3x3 | −1.8080 | −4.685 | 0.000 | 3.17 |
b44 | x4x4 | −1.2142 | −3.147 | 0.006 | 1.43 |
b12 | x1x2 | −1.3619 | −2.640 | 0.018 | 1.80 |
b13 | x1x3 | −0.6119 | −1.186 | 0.253 | 0.36 |
b14 | x1x4 | 1.7556 | 3.403 | 0.004 | 2.99 |
b23 | x2x3 | −0.4081 | −0.791 | 0.440 | 0.16 |
b24 | x2x4 | 1.1969 | 2.320 | 0.034 | 1.39 |
b34 | x3x4 | 1.5344 | 2.974 | 0.009 | 2.29 |
The 3-dimensional response surface and 2-dimensional counter plots (Fig. 9) were utilized to survey the interaction and individual effects of catalyst dosage, pH, and contact time on removal efficiency. Fig. 9 illustrates the effect of the catalyst dosage and irradiation time on the degradation efficiency of AR14. The enhancement of AR14 degradation is observed with increasing the MMO/ZnO/CNT nanohybrid dosage. It can be concluded that the total active surface area increases with increasing catalyst dosage. As seen in Fig. 9, at the higher loading of optimum concentration of the catalyst, the degradation efficiency of AR14 is decreased. This observation can be explained on the basis of the total active sites on the catalyst surface and the penetration of light into suspension. Due to an increase in the turbidity of the suspension, decrease in light penetration occurs and hence, the photoactivated volume of suspension decreases.
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Fig. 9 Contour and surface plots showing the effect of the catalyst dosage and irradiation time on the degradation efficiency of AR14 by ZnO/MMO/CNT nanohybrid (T = 26 °C, pH = 7). |
Fig. 10 shows the effects of initial concentration of dye and irradiation time on the degradation efficiency of AR14. The degradation efficiency of AR14 decreased with the increase of initial concentration of dye. When initial concentration increases, more organic substances are adsorbed on the surface of the catalyst. Therefore, there are only a few active sites for the photodegradation process. Furthermore, when the concentration of dye solution increases, the photons get intercepted before they can reach the catalyst surface.
Fig. 11 reveals the effects of initial pH and contact time on the degradation efficiency of AR14. In general, pH plays an important role in the determination of the surface charge properties of the photocatalyst and the charge of the dye molecules, which is effective in the total active surface sites available for both the reactant and the photon absorptions. On the other hand, the zero charge point of the catalyst is important in the photocatalytic processes. Theoretically, at pH < point of zero charge (PZC), the surface gets positively charged, which enhances the adsorption of negatively charged dye anions through electrostatic forces of the attraction. At pH > (PZC), the surface of the catalyst gets negatively charged, which favors the adsorption of cationic dye. In this study, the PZC of MMO/ZnO/CNT nanohybrid was determined 5.50. In Fig. 10, it can be observed that the degradation efficiency of AR14 increases with decrease in initial pH value. At low pH values, the H+ ions with the excess concentration tend to easily interact with the azo group (–NN–) containing lone-pair electrons. On the other hand, in low pH value electrostatic attraction is formed between negatively charged AR14 and positively charged MMO/ZnO/CNT nanohybrids. Also, decrease is observed in the degradation efficiency at initial pH that is higher than the PZC. There may be a strong Columbic repulsion between the negatively charged surface of MMO/ZnO/CNT nanohybrids and the negatively charged AR14 dye molecules.
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Fig. 11 Contour and surface plots showing the effects of initial pH and contact time on the efficiency of AR14 by ZnO/MMO/CNT nanohybrid ([Dye]0 = 40 mg L−1, catalyst dosage = 0.011 g). |
According to the obtained polynomial model, optimum values of the photocatalyst dosage, pH, and irradiation time were 0.009 g, 4.12, and 150 min respectively. The predicted degradation efficiency value of AR14 at optimum conditions was ≈99.41% and the corresponding experimental value obtained was 98.08%.
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Fig. 12 Reusability of MMO/ZnO/CNT nanohybrid within five consecutive experimental runs. AR14 concentration of 20 mg L−1, photocatalyst dosage of 0.009 g L−1, and reaction time of 150 min. |
It has been reported that pure ZnO semiconductor suffers from the photoinduced dissolution (photocorrosion) when ZnO is irradiated by the UV light.44,45 Based on our previously reported results, ZnO/MMO exhibits better photostability under UV irradiation than pure ZnO.28 In addition, doped or decorated ZnO shows almost no photoactivity loss under visible light.46
In fact, based on the degradation efficiency of ZnO/MMO/CNT nanohybrid photocatalyst under visible light, we can conclude that the ZnO/MMO/CNT assembles prevent ZnO semiconductor from photocorrosion during the photocatalytic process.
To verify good ability of ZnO/MMO/CNT nanohybrid for utilizing the sunlight, an experiment was carried out under sunlight at optimized operational parameters (initial AR14 concentration = 20 mg L−1, irradiation time = 210 min, ZnO/MMO/CNT nanohybrid content = 0.009 g L−1, and initial pH = 4.12). The results indicate that after 210 min process under sunlight, more than 75.25% of dye was removed. Although removal efficiencies under sunlight is lower than visible light, this result is acceptable and indicates that the absorption edge of ZnO/NiCoAl-LDH/CNT nanohybrid shifts to the visible region in mixed metal oxide (ZnO/MMO/CNT) nanohybrid which may be beneficial in increasing capability of photocatalytic activity in sunlight.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra17001f |
This journal is © The Royal Society of Chemistry 2015 |