Materials Science and Engineering C 33 (2013) 91–98
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Enhanced photocatalytic activity of ZnO/CuO nanocomposite for the degradation of textile dye on visible light illumination R. Saravanan a, S. Karthikeyan b, V.K. Gupta c, d, G. Sekaran b, V. Narayanan e, A. Stephen a,⁎ a
Materials science centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India Environmental technology division, Central Leather Research Institute, Adyar, Chennai 600 020, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India d King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia e Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India b c
a r t i c l e
i n f o
Article history: Received 3 May 2012 Received in revised form 21 July 2012 Accepted 7 August 2012 Available online 18 August 2012 Keywords: Textile dye Nano composite ZnO/CuO Thermal decomposition method Photocatalytic activity
a b s t r a c t The photocatalytic degradation of organic dyes such as methylene blue and methyl orange in the presence of various percentages of composite catalyst under visible light irradiation was carried out. The catalyst ZnO nanorods and ZnO/CuO nanocomposites of different weight ratios were prepared by new thermal decomposition method, which is simple and cost effective. The prepared catalysts were characterized by different techniques such as X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, Fourier transform infrared spectroscopy and UV–visible absorption spectroscopy. Further, the most photocatalytically active composite material was used for degradation of real textile waste water under visible light illumination. The irradiated samples were analysed by total organic carbon and chemical oxygen demand. The efficiency of the catalyst and their photocatalytic mechanism has been discussed in detail. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the population growth with its increased needs has prompted many industrial developments and this has led to an increase in pollution. For example textile industries use coloured dyes such as methylene blue, acid red 14, remazol red RR, reactive blue 19, methyl orange etc. Usage of these coloured dyes becomes a major source of environmental contamination. This leads to surface and ground water contamination [1–9]. One of the best ways to reduce contamination of water is by photocatalytic treatment. Titanium dioxide (TiO2) and zinc oxide (ZnO) are widely used in the degradation process. The energy levels of the conduction and valence bands and the electron affinity of ZnO are similar to TiO2. Compared to TiO2, ZnO is easily available and the added advantage of ZnO is that it absorbs a larger fraction of the solar spectrum than TiO2 and has a high photocatalytic activity [10–13]. ZnO is a good semiconducting material for photocatalytic applications, due to its environmental stability and low cost when compared with other nanosized metal oxides. But the disadvantage of ZnO is that it absorbs only in the UV region because of its large band width of 3.2 eV (λ = 380 nm). The major problem is that only about 4 to 5% of solar spectrum falls in the UV range [14–16]. Therefore, the effective use of solar energy still remains a challenge in photocatalytic application. ⁎ Corresponding author. Tel.: +91 44 2220 2802; fax: +91 44 2235 3309. E-mail address: [email protected] (A. Stephen). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.011
The recombination process is considered as one of the most important factors that controls photocatalytic activity. Fast recombination rate of the photogenerated electron–hole pairs should be hindered for enhanced photocatalytic activity. The photocatalytic activity of ZnO can be improved by various techniques such as modification of ZnO by non-metal doping [17], addition of transition metals [18] as well as use of coupled semiconductors. The coupled semiconductor materials have two types of energy-level systems which play an important role in achieving charge separation. Coupling of different semiconductor oxides can reduce the band gap, extending the absorbance range to visible region leading to electron–hole pair separation under irradiation and consequently, achieving a higher photocatalytic activity [19–22]. These systems also exhibit higher degradation of organic pollutants. The CuO–TiO2 [23], WO3–TiO2 [24], ZnO/TiO2 [25], ZnO/SnO2 [19], TiO2/MgO [26], and SnO2/ZnO [27] are the types of coupled semiconductors that were successfully synthesized. Composite materials are not only used for catalytic activity but also for various applications such as gas sensor, electric conductivity and so on. Further, copper oxide (CuO) is a semiconductor material with a narrow band gap (1.7 eV). It is non-toxic and its constituents are available in abundance. CuO has received widespread attention because of its various applications in electronic and optoelectronic devices such as lithium ion electrode materials, heterogeneous catalysts, gas sensors and solar cells [28–31]. ZnO coupled with CuO has already been proposed for various applications such as conductivity studies, photocatalytic activity,
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magnetic properties and gas sensors [32–35] by researchers. Compared to all the other methods of preparation, the present method is simple and cost effective. The ZnO nanorods and ZnO coupled with CuO (ZnO/CuO) were prepared by thermal decomposition method. The prepared samples were characterized by various techniques; the results are discussed in detail. The catalysts were used for the photocatalytic degradation of methylene blue and methyl orange under visible light irradiation. Further the sample exhibiting the best photocatalytic activity was employed for the degradation of textile waste water. 2. Experimental 2.1. Materials Zinc acetate dihydrate (Rankem) and copper acetate (Rankem) used in the present study were of analytical reagent grade. Methylene blue (MB) and methyl orange (MO) were purchased from Aldrich chemicals. All aqueous solutions were prepared using double distilled water. 2.2. Synthesis of ZnO and ZnO/CuO composites ZnO nanorods were synthesized by grinding 3.0 g of zinc acetate dihydrate in a mortar for one hour and then annealed in an alumina crucible at 350 °C which is similar to the synthesis procedure followed in one of our previous works [36]. The TGA curve of zinc acetate dihydrate is shown in Fig. 1(a). Initial weight loss of 16% at 170 °C is due to dehydration, yielding anhydrous zinc acetate. A weight loss of 52%, in the range of 171–312 °C indicates the decomposition of acetate and the formation of zinc oxide. This helped to determine the residual zinc oxide value of 32% which coincides with the calculated theoretical residual value of 37.5% when ZnO is the only residue. The 5.5% weight difference is due to the sublimation of zinc acetate species or zinc organic composition such as Zn4O (CH3CO2)6 [36–38]. There was no further decomposition beyond 312 °C. The process resulted in the synthesis of ZnO after a total weight loss of 68%. In our method, the annealing temperature was kept at 350 °C for 3 h. To prepare ZnO/CuO, stoichiometric amount of zinc acetate dihydrate and copper acetate (50:50, 90:10, 95:5, 97:3 and 99:1 weight ratios) were mixed and ground for 1 h. This was annealed in an alumina crucible at 350 °C for 3 h using a muffle furnace in air atmosphere. Fig. 1(b) shows the TGA graph of zinc acetate dihydrate
Fig. 2. The X-ray diffraction patterns of (a) ZnO, (b) CuO, (c) ZnO/CuO (99:1), (d) ZnO/CuO (97:3), (e) ZnO/CuO (95:5), (f) ZnO/CuO (90:10) and (g) ZnO/CuO (50:50).
mixed with copper acetate, composite (95:5) system in the same temperature range. A weight loss of 51.5%, in the range of 160– 320 °C indicates the decomposition of acetates and the formation of oxides. 2.3. Measurement of photocatalytic activity The photocatalytic activity was estimated by measuring the decomposition rate of MB and MO aqueous solutions in visible light irradiation. The visible light irradiation was carried out using a projection lamp (7748XHP 250 W, Philips, 532 nm) in a photoreactor and an acetone jacket was used to cut-off UV radiation. Reaction suspensions were prepared by adding the 500 mg of catalyst in 500 mL of aqueous MB and aqueous MO solution taken with an initial concentration of 3 × 10 −5 moles/L. The aqueous suspension containing MB/ MO and the photocatalyst was irradiated while being continuously stirred. The analytical samples from the suspension were collected at regular intervals of time, centrifuged and filtered. The concentration of MB and MO in each sample was analysed using UV–visible spectrophotometer at a wavelength of 664 and 464 nm respectively. The photocatalytic efficiency was calculated using the expression η ¼ ð1−C=C0 Þ x100
ð1Þ
where C0 is the concentration of MB or MO before illumination and C is the concentration after irradiation time. 2.4. Textile waste water The textile waste water was collected from one of the textile industries. Compared to the concentration of methylene blue and methyl orange, the colour (intensity) of the real sample is very high. The textile waste water was diluted to 1:9 ratio using distilled water. The diluted industrial effluent was mixed with the 500 mg of the best catalytically active sample. The textile waste water sample containing the catalyst was irradiated under visible light under constant stirring. The irradiated samples were collected at regular intervals of time, centrifuged and filtered. These samples were analysed by total organic carbon (TOC) and chemical oxygen demand (COD) using standard methods. 2.5. Characterization details
Fig. 1. TGA curve of (a) Zinc acetate dihydrate and (b) Zinc acetate dihydrate mixed with copper acetate (95:5).
The thermo gravimetric analysis (TGA) was performed at a heating rate of 10 °C per minute in air atmosphere. The thermal analysis was carried out by a NETZSCH STA 409 simultaneous thermal analyzer. The TEM images were taken using TECNAI, PHILIPS, Netherlands. The
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Table 1 Lattice parameter and surface area values for prepared samples. Samples
ZnO
CuO
Hexagonal (79–0208)
Monoclinic (89–5899)
Lattice parameters (Å)
ZnO CuO ZnO(99)/CuO (1) ZnO(97)/CuO (3) ZnO(95)/CuO (5) ZnO(90)/CuO (10) ZnO(50)/CuO (50)
BET surface area (m2/g)
Lattice parameters (Å)
a
c
a
b
c
0.326 – 0.327 0.326 0.325 0.326 0.326
0.520 – 0.523 0.521 0.520 0.520 0.520
– 0.470 – – – 0.465 0.466
– 0.342 – – – 0.344 0.344
– 0.512 – – – 0.515 0.515
UV–visible absorption spectra were obtained using a CARY 5E UV– VIS-NIR Spectrophotometer. X-ray diffraction (XRD) analysis was done using a Rich Siefert 3000 diffractometer under Cu Kα1 radiation (λ =0.15405 nm). The Brunauer–Emmett–Teller (BET) equation was
8.6 – – – 15.4 – 9.7
used to calculate the specific surface area using Micromeritics ASAP 2030. Field Emission‐Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-ray Spectroscopy analysis were carried out using HITACHI-SU6600. FT-IR spectra were observed using a Bruker–Tensor
Fig. 3. FE-SEM images of (a) ZnO, (b) ZnO/CuO (99:1), (c) ZnO/CuO (97:3), (d) ZnO/CuO (95:5), (e) ZnO/CuO (90:10) and (f) ZnO/CuO (50:50).
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27 Fourier Transform Infra Red spectrophotometer .The photocatalytic activity was measured using Perkin‐Elmer UV–visible spectrometer RX1, Total carbon content (TOC) was carried out using a TOC DIGESTER TR 420 MERCK and Chemical oxygen demand was measured using a SPECTRA LAB COD DIGESTER 2015M. 3. Result and discussion 3.1. Structure analysis The Powder X-ray diffraction patterns of ZnO, CuO and ZnO/CuO samples are shown in Fig. 2. The XRD pattern of ZnO is shown in Fig. 2 (a), which exhibits (100), (002), (101), (102), (110), (103), (200), (112) and (201) characteristic peaks of ZnO with hexagonal structure. The lattice parameter values are shown in Table. 1. The lattice parameters of pure ZnO sample match well with the JCPDS file no: 79–0208. For the comparison purpose we have prepared CuO by the same method at the same temperature and the XRD result is shown in Fig. 2 (b). The characteristic peaks of the prepared CuO were indexed to monoclinic structure and the lattice constant values are similar to JCPDS file no: 89–5899. Fig. 2 (c, d and e) represents samples in which CuO (1%, 3% and 5%) is coupled with ZnO. The CuO content ≤ 5 wt.% shows that the reflection peaks (CuO) have very low intensity. So we cannot find out the lattice parameters in the XRD pattern (Fig. 2c, d and e). The pattern of samples containing 10% and 50% CuO coupled with ZnO shown in Fig. 2 (f) and (g) clearly indicates the formation of composite ZnO/CuO. No other segregation of phases was detected in the XRD pattern and lattice parameter values of all prepared samples are shown in Table 1. Hence the XRD results indicate the formation of ZnO/CuO composite. 3.2. Surface analysis The Field Emission Scanning Electron Microscopy (FE-SEM) images of ZnO and 1 wt.% CuO are shown in Fig. 3(a) and (b). The image indicates randomly distributed nanorods with an average diameter of ~ 35 nm and a length of 500 nm. Fig. 3(c) and (d) shows 3 wt.%. and 5 wt.% CuO doped samples. The length of the rod decreases by 200 nm. For the samples 10 wt.% and 50 wt.% of CuO are shown in Fig. 3(e) and (f) and the length is found to be around 50–100 nm. Thus, addition of CuO may influence the size and morphology of ZnO by its involvement in the nucleation and growth [39]. It is evident from the observed FE-SEM images that the size of the nanorods decreases with an increase in the percentage of CuO. The BET specific surface area values are shown in Table 1. The BET surface area of 5 wt.% of CuO sample is comparatively higher than Fig. 5. FTIR Spectra of (a) ZnO, (b) ZnO/CuO (99:1) and (c) ZnO/CuO (97:3). Fig. 5.1. FTIR spectra of (d) ZnO/CuO (95:5), (e) ZnO/CuO (90:10) and (f) ZnO/CuO (50:50).
Fig. 4. TEM image of ZnO/CuO (95:5).
ZnO due to the lower particle size. The surface area of 50 wt.% CuO decreases might be due to the deposition of excess crystalline CuO on the surface of ZnO. A similar observation has been recently reported by Leghari et al. [40]. The EDXS analysis confirms the composition of the samples. As shown in Fig. 4 the TEM image of ZnO/ CuO (95:5) symbolizes nanorod‐shaped particle. The characteristic functional groups of the particles were investigated using FTIR spectra. In Fig. 5 the broad absorption band observed at ~ 3428 cm −1 corresponds to the O–H stretching vibrations of water present in ZnO and CuO. The absorbance band found at ~ 2923 cm −1 is assigned to the residual organic component. The band at ~ 1645 cm −1 can be associated with the bending vibrations of H2O molecules. The absorption bands at ~ 1570 cm −1 and ~ 1412 cm −1 in both the samples are due to the carbonyl groups of the carboxylate ions which might remain adsorbed on the surface of ZnO and CuO.
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95
Fig. 6. UV–vis absorption spectra of all prepared samples.
The peaks appearing between 400 cm−1 and 600 cm−1 are assigned to the metal–oxygen (M–O) stretching mode [41]. In Fig. 5 (a), the stretching mode of ZnO nanorod appears at 546 cm−1. Fig. 5 (b, c and Fig. 5.1 d) indicates that there is not much variation up to 5% CuO incorporation, new peaks or shift in the peak at 546 cm−1 is observed for 10 wt.% and 50 wt.% incorporation as shown in Fig. 5.1 (e and f). This shift is due to the formation of separate CuO crystals as proved by the XRD. The broad peak observed in the range 460–560 cm−1 is the combination of Cu–O and Zn–O vibrations. 3.3. Optical property The room temperature DRS (UV–Vis) absorption spectrums for ZnO and ZnO/CuO nanorods are shown in Fig. 6. The absorption intensity of UV and visible region is higher for ZnO/CuO when compared to pure ZnO. The intensity is the highest at about 5 wt.% in our case. As the percentage of CuO is increased (10 wt.% and 50%) the absorption decreases. There is a red shift in the absorption with increase in the amount of CuO incorporation. These results suggest that the photocatalytic efficiency of the ZnO/CuO composites will be higher than that of the ZnO nanorods. 3.4. Photocatalytic degradation of textile dyes under visible light illumination The change in optical absorption spectra of MB and MO by ZnO/CuO (95:5) catalyst under visible light irradiation for different time intervals is shown in Fig. 7(a) and (b). The disappearance of the band at 664 nm indicates that MB has been photodegraded as shown in Fig. 7(a). In the case of methyl orange the disappearance of the band at 464 nm is shown in Fig. 7(b). The change in concentration of MB and MO under visible light irradiation for different time intervals is shown in Fig. 8(a) and (b) respectively. The degradation efficiency determined for all the prepared samples are shown in Table 2. The maximum photocatalytic activity is obtained for five percentage of CuO coupled with ZnO sample as compared with all other prepared samples due to large surface area as shown in Table 1. It is known that the photocatalytic redox reaction mainly takes place on the surface of the photocatalysts, so the surface properties significantly influence the efficiency of photocatalysts. Few research groups have used ZnO/CuO for the degradation of various dyes (RhB, Acid red88, Cr(VI), Acid Orange 7 and Methyl Orange) in the recent past under UV and visible light irradiation [15,33,42–45]. Among them, two research groups have explained the
Fig. 7. Photodegradation of (a) MB and (b) MO for different exposure times under visible light illumination.
degradation of methyl orange under visible light. ZnO/CuO (different weight percentages) synthesized by thermal decomposition method provided an effective way for cost-effective, simple and fast process compared with all other previous reports. These ZnO/CuO (95:5 weight ratio) samples show higher degradation efficiency when compared with the data reported in the literature [42–44]. This variation in degradation time is due to the difference in synthesis method, the particle size, crystallinity of the catalyst and the dopant concentration. The degradation of colour from waste water is often more significant than the degradation of other organic colourless chemicals because the waste water contains a lot of colour and has a toxic odour [46]. Change in the colour of the textile dye by ZnO/CuO (95:5) catalyst under visible light irradiation for different time intervals is shown in Fig. 9. It indicates that with increase in irradiation time the colour disappears steadily. Decolourization efficiency is inversely related to the dye concentration. The colour degradation is closely related to TOC data [5,47]. Fig. 10 shows TOC and COD results for degradation of textile dye for different intervals of time and the data are shown in Table 3. The TOC and COD results exhibit that the concentration of textile dye decreases significantly with increasing visible light irradiation. The results demonstrate that ZnO/CuO (99:5) degrade the textile water under visible light. The catalyst exhibits high photocatalytic activity not only in the decolourization but also in mineralization of colourless organic pollutants. The TOC and COD results represent more than 90% of the initial waste water degradation using coupled semiconductor ZnO/CuO (99:5) as a catalyst.
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There is no decolouration of the sample in the presence of pure ZnO nanorods under visible light irradiation, since the band gap of ZnO is 3.2 eV. The band gap corresponds to the UV region of the electromagnetic radiation and the absorbance spectrum of ZnO is flat in the visible region. The same behaviour was reported by earlier researchers [50]. The composite photocatalyst mechanism is explained by the following equation based on earlier reports [1,5,14,16,44,50,51]. −
þ
þ visible light
−
ZnOðe þ h Þ=CuOðe þ h Þ
þ
→
−
−
þ
þ
ZnOðe þ e Þ=CuOðh þ h Þ ð3Þ
−
h þ OH →OH
−
ð4Þ
−
e þ O2 →•O2
ð5Þ
−
−
H2 O þ •O2 →OOH• þ OH
ð6Þ
2OOH•→O2 þ H2 O2
ð7Þ
−
−
ð8Þ
OH• þ •O2 þ hvb þ pollutants→degrade pollutant
H2 O2 þ •O2 →OH• þ OH þ O2
−
þ
ð9Þ
−
þ
ð10Þ
OH• þ •O2 þ hvb þ degrade pollutant→CO2 ↑ þ H2 O
Fig. 8. Effect of catalyst loading on the photocatalytic degradation of (a) MB and (b) MO under visible light.
3.5. Photocatalytic mechanism The conduction and valence band positions of the two semiconductors at the point of zero charge can be calculated by the following formula [21,48] Evb ¼ X−Ee þ 0:5Eg
ð2Þ
where Evb is the valence band potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. The electronegativity values for CuO and ZnO are 5.81 eV and 5.79 eV respectively [49]. Ee is the energy of free electrons on the hydrogen scale (~ 4.5 eV) and Eg is band gap energy of the semiconductor.
When the visible light is irradiated, the electron transfer may occur from the valence band of CuO (p-type) to the valence band of ZnO (n-type). This is possible due to the work function of CuO being similar to that of ZnO (5.3 eV). On the other hand, transfer of holes may occur from the more anodic valence band of ZnO to the cathodic valence band of CuO [44]. At the same time, the ZnO/CuO materials exhibit red shift in the absorption wavelength range compared with that of ZnO, which might also benefit the improvement of photocatalytic performance. The UV-absorbance data suggests that ZnO/CuO nanocomposite has improved the photocatalytic activity. At higher percentages (10% and 50%) of CuO, the photocatalytic activity decreases. This is due to the presence of CuO that improves charge recombination rate. The same behaviour has already been reported for CuO–TiO2 [23] and TiO2/WO3 [52]. At the same time, the absorption value at higher percentages (10% and 50%) decreases compared to lower percentages (1%, 3%, 5%) of CuO samples. Hence the photocatalytic activity efficiency has improved for composite material (ZnO/ CuO) to direct or indirect decomposition of organic dyes under visible light.
Table 2 Photocatalytic degradation efficiency (in percentage). Samples
ZnO ZnO/CuO ZnO/CuO ZnO/CuO ZnO/CuO ZnO/CuO
(99:1) (97:3) (95:5) (90:10) (50:50)
Methylene blue (MB)
Methyl orange (MO)
Irradiation time
Irradiation time
30 min
60 min
90 min
120 min
30 min
60 min
90 min
120 min
0.2 20.3 48.5 63.2 17.5 7.7
0.3 54.1 77.1 91.2 28.2 17.1
0.6 77.6 92.1 96.6 50.33 29.9
0.9 96.3 96.9 97.2 71.28 38.5
2.4 12.6 18.1 40.6 13.9 12.4
2.6 25.7 42.6 54.3 18.9 19.3
2.7 33.4 54.3 75.1 26.7 23.7
3.1 48.5 69.2 87.7 28.3 27.7
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Fig. 9. Photography image represents change in colour of textile dyes using ZnO/CuO (95:5) catalyst for different exposure time under visible light irradiation.
colourless organic pollutants. The preparation method of catalyst is new, simple, fast and cost effective. Thus, it can be used for the synthesis of future composite photocatalysts. Acknowledgements We acknowledge the National Centre for Nanoscience and Nanotechology, University of Madras, India for FE-SEM and EDXS characterizations. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.msec.2012.08.011. References
Fig. 10. TOC and COD results for catalyst (5 wt.% CuO) loading on the photocatalytic degradation of textile waste water under visible light irradiation.
4. Conclusion The coupled semiconductor ZnO/CuO possesses higher photocatalytic degradation of MB and MO when compared to ZnO under visible light since coupling of ZnO/CuO reduces the band gap, extending the wavelength range to visible light region leading to electron–hole pair separation under visible light irradiation and consequently, achieving a higher photocatalytic activity. The maximum efficiency is observed for 5% CuO loaded on ZnO. This environmental friendly composite material was used for the degradation of real textile dye effluent under visible light illumination. The catalyst exhibits high photocatalytic activity not only in decolourization but also in mineralization of
Table 3 TOC and COD data for degradation of textile pollutant. Time (min)
COD mg/L
COD/COD0
TOC mg/L
TOC/TOC0
0 30 60 90 120 150 180 210 240 270 300
462 320 210 185 158 123 100 88 77 50 38
1 0.69 0.45 0.40 0.34 0.26 0.22 0.19 0.16 0.11 0.08
224 153 105 97 84 53 41 33 29 25 20
1 0.68 0.47 0.43 0.37 0.24 0.18 0.14 0.12 0.11 0.09
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Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Enhanced photocatalytic activity of ZnO/CuO nanocomposite for the degradation of textile dye on visible light illumination R. Saravanan a, S. Karthikeyan b, V.K. Gupta c, d, G. Sekaran b, V. Narayanan e, A. Stephen a,⁎ a
Materials science centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India Environmental technology division, Central Leather Research Institute, Adyar, Chennai 600 020, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India d King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia e Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India b c
a r t i c l e
i n f o
Article history: Received 3 May 2012 Received in revised form 21 July 2012 Accepted 7 August 2012 Available online 18 August 2012 Keywords: Textile dye Nano composite ZnO/CuO Thermal decomposition method Photocatalytic activity
a b s t r a c t The photocatalytic degradation of organic dyes such as methylene blue and methyl orange in the presence of various percentages of composite catalyst under visible light irradiation was carried out. The catalyst ZnO nanorods and ZnO/CuO nanocomposites of different weight ratios were prepared by new thermal decomposition method, which is simple and cost effective. The prepared catalysts were characterized by different techniques such as X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, Fourier transform infrared spectroscopy and UV–visible absorption spectroscopy. Further, the most photocatalytically active composite material was used for degradation of real textile waste water under visible light illumination. The irradiated samples were analysed by total organic carbon and chemical oxygen demand. The efficiency of the catalyst and their photocatalytic mechanism has been discussed in detail. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the population growth with its increased needs has prompted many industrial developments and this has led to an increase in pollution. For example textile industries use coloured dyes such as methylene blue, acid red 14, remazol red RR, reactive blue 19, methyl orange etc. Usage of these coloured dyes becomes a major source of environmental contamination. This leads to surface and ground water contamination [1–9]. One of the best ways to reduce contamination of water is by photocatalytic treatment. Titanium dioxide (TiO2) and zinc oxide (ZnO) are widely used in the degradation process. The energy levels of the conduction and valence bands and the electron affinity of ZnO are similar to TiO2. Compared to TiO2, ZnO is easily available and the added advantage of ZnO is that it absorbs a larger fraction of the solar spectrum than TiO2 and has a high photocatalytic activity [10–13]. ZnO is a good semiconducting material for photocatalytic applications, due to its environmental stability and low cost when compared with other nanosized metal oxides. But the disadvantage of ZnO is that it absorbs only in the UV region because of its large band width of 3.2 eV (λ = 380 nm). The major problem is that only about 4 to 5% of solar spectrum falls in the UV range [14–16]. Therefore, the effective use of solar energy still remains a challenge in photocatalytic application. ⁎ Corresponding author. Tel.: +91 44 2220 2802; fax: +91 44 2235 3309. E-mail address: [email protected] (A. Stephen). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.011
The recombination process is considered as one of the most important factors that controls photocatalytic activity. Fast recombination rate of the photogenerated electron–hole pairs should be hindered for enhanced photocatalytic activity. The photocatalytic activity of ZnO can be improved by various techniques such as modification of ZnO by non-metal doping [17], addition of transition metals [18] as well as use of coupled semiconductors. The coupled semiconductor materials have two types of energy-level systems which play an important role in achieving charge separation. Coupling of different semiconductor oxides can reduce the band gap, extending the absorbance range to visible region leading to electron–hole pair separation under irradiation and consequently, achieving a higher photocatalytic activity [19–22]. These systems also exhibit higher degradation of organic pollutants. The CuO–TiO2 [23], WO3–TiO2 [24], ZnO/TiO2 [25], ZnO/SnO2 [19], TiO2/MgO [26], and SnO2/ZnO [27] are the types of coupled semiconductors that were successfully synthesized. Composite materials are not only used for catalytic activity but also for various applications such as gas sensor, electric conductivity and so on. Further, copper oxide (CuO) is a semiconductor material with a narrow band gap (1.7 eV). It is non-toxic and its constituents are available in abundance. CuO has received widespread attention because of its various applications in electronic and optoelectronic devices such as lithium ion electrode materials, heterogeneous catalysts, gas sensors and solar cells [28–31]. ZnO coupled with CuO has already been proposed for various applications such as conductivity studies, photocatalytic activity,
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magnetic properties and gas sensors [32–35] by researchers. Compared to all the other methods of preparation, the present method is simple and cost effective. The ZnO nanorods and ZnO coupled with CuO (ZnO/CuO) were prepared by thermal decomposition method. The prepared samples were characterized by various techniques; the results are discussed in detail. The catalysts were used for the photocatalytic degradation of methylene blue and methyl orange under visible light irradiation. Further the sample exhibiting the best photocatalytic activity was employed for the degradation of textile waste water. 2. Experimental 2.1. Materials Zinc acetate dihydrate (Rankem) and copper acetate (Rankem) used in the present study were of analytical reagent grade. Methylene blue (MB) and methyl orange (MO) were purchased from Aldrich chemicals. All aqueous solutions were prepared using double distilled water. 2.2. Synthesis of ZnO and ZnO/CuO composites ZnO nanorods were synthesized by grinding 3.0 g of zinc acetate dihydrate in a mortar for one hour and then annealed in an alumina crucible at 350 °C which is similar to the synthesis procedure followed in one of our previous works [36]. The TGA curve of zinc acetate dihydrate is shown in Fig. 1(a). Initial weight loss of 16% at 170 °C is due to dehydration, yielding anhydrous zinc acetate. A weight loss of 52%, in the range of 171–312 °C indicates the decomposition of acetate and the formation of zinc oxide. This helped to determine the residual zinc oxide value of 32% which coincides with the calculated theoretical residual value of 37.5% when ZnO is the only residue. The 5.5% weight difference is due to the sublimation of zinc acetate species or zinc organic composition such as Zn4O (CH3CO2)6 [36–38]. There was no further decomposition beyond 312 °C. The process resulted in the synthesis of ZnO after a total weight loss of 68%. In our method, the annealing temperature was kept at 350 °C for 3 h. To prepare ZnO/CuO, stoichiometric amount of zinc acetate dihydrate and copper acetate (50:50, 90:10, 95:5, 97:3 and 99:1 weight ratios) were mixed and ground for 1 h. This was annealed in an alumina crucible at 350 °C for 3 h using a muffle furnace in air atmosphere. Fig. 1(b) shows the TGA graph of zinc acetate dihydrate
Fig. 2. The X-ray diffraction patterns of (a) ZnO, (b) CuO, (c) ZnO/CuO (99:1), (d) ZnO/CuO (97:3), (e) ZnO/CuO (95:5), (f) ZnO/CuO (90:10) and (g) ZnO/CuO (50:50).
mixed with copper acetate, composite (95:5) system in the same temperature range. A weight loss of 51.5%, in the range of 160– 320 °C indicates the decomposition of acetates and the formation of oxides. 2.3. Measurement of photocatalytic activity The photocatalytic activity was estimated by measuring the decomposition rate of MB and MO aqueous solutions in visible light irradiation. The visible light irradiation was carried out using a projection lamp (7748XHP 250 W, Philips, 532 nm) in a photoreactor and an acetone jacket was used to cut-off UV radiation. Reaction suspensions were prepared by adding the 500 mg of catalyst in 500 mL of aqueous MB and aqueous MO solution taken with an initial concentration of 3 × 10 −5 moles/L. The aqueous suspension containing MB/ MO and the photocatalyst was irradiated while being continuously stirred. The analytical samples from the suspension were collected at regular intervals of time, centrifuged and filtered. The concentration of MB and MO in each sample was analysed using UV–visible spectrophotometer at a wavelength of 664 and 464 nm respectively. The photocatalytic efficiency was calculated using the expression η ¼ ð1−C=C0 Þ x100
ð1Þ
where C0 is the concentration of MB or MO before illumination and C is the concentration after irradiation time. 2.4. Textile waste water The textile waste water was collected from one of the textile industries. Compared to the concentration of methylene blue and methyl orange, the colour (intensity) of the real sample is very high. The textile waste water was diluted to 1:9 ratio using distilled water. The diluted industrial effluent was mixed with the 500 mg of the best catalytically active sample. The textile waste water sample containing the catalyst was irradiated under visible light under constant stirring. The irradiated samples were collected at regular intervals of time, centrifuged and filtered. These samples were analysed by total organic carbon (TOC) and chemical oxygen demand (COD) using standard methods. 2.5. Characterization details
Fig. 1. TGA curve of (a) Zinc acetate dihydrate and (b) Zinc acetate dihydrate mixed with copper acetate (95:5).
The thermo gravimetric analysis (TGA) was performed at a heating rate of 10 °C per minute in air atmosphere. The thermal analysis was carried out by a NETZSCH STA 409 simultaneous thermal analyzer. The TEM images were taken using TECNAI, PHILIPS, Netherlands. The
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Table 1 Lattice parameter and surface area values for prepared samples. Samples
ZnO
CuO
Hexagonal (79–0208)
Monoclinic (89–5899)
Lattice parameters (Å)
ZnO CuO ZnO(99)/CuO (1) ZnO(97)/CuO (3) ZnO(95)/CuO (5) ZnO(90)/CuO (10) ZnO(50)/CuO (50)
BET surface area (m2/g)
Lattice parameters (Å)
a
c
a
b
c
0.326 – 0.327 0.326 0.325 0.326 0.326
0.520 – 0.523 0.521 0.520 0.520 0.520
– 0.470 – – – 0.465 0.466
– 0.342 – – – 0.344 0.344
– 0.512 – – – 0.515 0.515
UV–visible absorption spectra were obtained using a CARY 5E UV– VIS-NIR Spectrophotometer. X-ray diffraction (XRD) analysis was done using a Rich Siefert 3000 diffractometer under Cu Kα1 radiation (λ =0.15405 nm). The Brunauer–Emmett–Teller (BET) equation was
8.6 – – – 15.4 – 9.7
used to calculate the specific surface area using Micromeritics ASAP 2030. Field Emission‐Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-ray Spectroscopy analysis were carried out using HITACHI-SU6600. FT-IR spectra were observed using a Bruker–Tensor
Fig. 3. FE-SEM images of (a) ZnO, (b) ZnO/CuO (99:1), (c) ZnO/CuO (97:3), (d) ZnO/CuO (95:5), (e) ZnO/CuO (90:10) and (f) ZnO/CuO (50:50).
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27 Fourier Transform Infra Red spectrophotometer .The photocatalytic activity was measured using Perkin‐Elmer UV–visible spectrometer RX1, Total carbon content (TOC) was carried out using a TOC DIGESTER TR 420 MERCK and Chemical oxygen demand was measured using a SPECTRA LAB COD DIGESTER 2015M. 3. Result and discussion 3.1. Structure analysis The Powder X-ray diffraction patterns of ZnO, CuO and ZnO/CuO samples are shown in Fig. 2. The XRD pattern of ZnO is shown in Fig. 2 (a), which exhibits (100), (002), (101), (102), (110), (103), (200), (112) and (201) characteristic peaks of ZnO with hexagonal structure. The lattice parameter values are shown in Table. 1. The lattice parameters of pure ZnO sample match well with the JCPDS file no: 79–0208. For the comparison purpose we have prepared CuO by the same method at the same temperature and the XRD result is shown in Fig. 2 (b). The characteristic peaks of the prepared CuO were indexed to monoclinic structure and the lattice constant values are similar to JCPDS file no: 89–5899. Fig. 2 (c, d and e) represents samples in which CuO (1%, 3% and 5%) is coupled with ZnO. The CuO content ≤ 5 wt.% shows that the reflection peaks (CuO) have very low intensity. So we cannot find out the lattice parameters in the XRD pattern (Fig. 2c, d and e). The pattern of samples containing 10% and 50% CuO coupled with ZnO shown in Fig. 2 (f) and (g) clearly indicates the formation of composite ZnO/CuO. No other segregation of phases was detected in the XRD pattern and lattice parameter values of all prepared samples are shown in Table 1. Hence the XRD results indicate the formation of ZnO/CuO composite. 3.2. Surface analysis The Field Emission Scanning Electron Microscopy (FE-SEM) images of ZnO and 1 wt.% CuO are shown in Fig. 3(a) and (b). The image indicates randomly distributed nanorods with an average diameter of ~ 35 nm and a length of 500 nm. Fig. 3(c) and (d) shows 3 wt.%. and 5 wt.% CuO doped samples. The length of the rod decreases by 200 nm. For the samples 10 wt.% and 50 wt.% of CuO are shown in Fig. 3(e) and (f) and the length is found to be around 50–100 nm. Thus, addition of CuO may influence the size and morphology of ZnO by its involvement in the nucleation and growth [39]. It is evident from the observed FE-SEM images that the size of the nanorods decreases with an increase in the percentage of CuO. The BET specific surface area values are shown in Table 1. The BET surface area of 5 wt.% of CuO sample is comparatively higher than Fig. 5. FTIR Spectra of (a) ZnO, (b) ZnO/CuO (99:1) and (c) ZnO/CuO (97:3). Fig. 5.1. FTIR spectra of (d) ZnO/CuO (95:5), (e) ZnO/CuO (90:10) and (f) ZnO/CuO (50:50).
Fig. 4. TEM image of ZnO/CuO (95:5).
ZnO due to the lower particle size. The surface area of 50 wt.% CuO decreases might be due to the deposition of excess crystalline CuO on the surface of ZnO. A similar observation has been recently reported by Leghari et al. [40]. The EDXS analysis confirms the composition of the samples. As shown in Fig. 4 the TEM image of ZnO/ CuO (95:5) symbolizes nanorod‐shaped particle. The characteristic functional groups of the particles were investigated using FTIR spectra. In Fig. 5 the broad absorption band observed at ~ 3428 cm −1 corresponds to the O–H stretching vibrations of water present in ZnO and CuO. The absorbance band found at ~ 2923 cm −1 is assigned to the residual organic component. The band at ~ 1645 cm −1 can be associated with the bending vibrations of H2O molecules. The absorption bands at ~ 1570 cm −1 and ~ 1412 cm −1 in both the samples are due to the carbonyl groups of the carboxylate ions which might remain adsorbed on the surface of ZnO and CuO.
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95
Fig. 6. UV–vis absorption spectra of all prepared samples.
The peaks appearing between 400 cm−1 and 600 cm−1 are assigned to the metal–oxygen (M–O) stretching mode [41]. In Fig. 5 (a), the stretching mode of ZnO nanorod appears at 546 cm−1. Fig. 5 (b, c and Fig. 5.1 d) indicates that there is not much variation up to 5% CuO incorporation, new peaks or shift in the peak at 546 cm−1 is observed for 10 wt.% and 50 wt.% incorporation as shown in Fig. 5.1 (e and f). This shift is due to the formation of separate CuO crystals as proved by the XRD. The broad peak observed in the range 460–560 cm−1 is the combination of Cu–O and Zn–O vibrations. 3.3. Optical property The room temperature DRS (UV–Vis) absorption spectrums for ZnO and ZnO/CuO nanorods are shown in Fig. 6. The absorption intensity of UV and visible region is higher for ZnO/CuO when compared to pure ZnO. The intensity is the highest at about 5 wt.% in our case. As the percentage of CuO is increased (10 wt.% and 50%) the absorption decreases. There is a red shift in the absorption with increase in the amount of CuO incorporation. These results suggest that the photocatalytic efficiency of the ZnO/CuO composites will be higher than that of the ZnO nanorods. 3.4. Photocatalytic degradation of textile dyes under visible light illumination The change in optical absorption spectra of MB and MO by ZnO/CuO (95:5) catalyst under visible light irradiation for different time intervals is shown in Fig. 7(a) and (b). The disappearance of the band at 664 nm indicates that MB has been photodegraded as shown in Fig. 7(a). In the case of methyl orange the disappearance of the band at 464 nm is shown in Fig. 7(b). The change in concentration of MB and MO under visible light irradiation for different time intervals is shown in Fig. 8(a) and (b) respectively. The degradation efficiency determined for all the prepared samples are shown in Table 2. The maximum photocatalytic activity is obtained for five percentage of CuO coupled with ZnO sample as compared with all other prepared samples due to large surface area as shown in Table 1. It is known that the photocatalytic redox reaction mainly takes place on the surface of the photocatalysts, so the surface properties significantly influence the efficiency of photocatalysts. Few research groups have used ZnO/CuO for the degradation of various dyes (RhB, Acid red88, Cr(VI), Acid Orange 7 and Methyl Orange) in the recent past under UV and visible light irradiation [15,33,42–45]. Among them, two research groups have explained the
Fig. 7. Photodegradation of (a) MB and (b) MO for different exposure times under visible light illumination.
degradation of methyl orange under visible light. ZnO/CuO (different weight percentages) synthesized by thermal decomposition method provided an effective way for cost-effective, simple and fast process compared with all other previous reports. These ZnO/CuO (95:5 weight ratio) samples show higher degradation efficiency when compared with the data reported in the literature [42–44]. This variation in degradation time is due to the difference in synthesis method, the particle size, crystallinity of the catalyst and the dopant concentration. The degradation of colour from waste water is often more significant than the degradation of other organic colourless chemicals because the waste water contains a lot of colour and has a toxic odour [46]. Change in the colour of the textile dye by ZnO/CuO (95:5) catalyst under visible light irradiation for different time intervals is shown in Fig. 9. It indicates that with increase in irradiation time the colour disappears steadily. Decolourization efficiency is inversely related to the dye concentration. The colour degradation is closely related to TOC data [5,47]. Fig. 10 shows TOC and COD results for degradation of textile dye for different intervals of time and the data are shown in Table 3. The TOC and COD results exhibit that the concentration of textile dye decreases significantly with increasing visible light irradiation. The results demonstrate that ZnO/CuO (99:5) degrade the textile water under visible light. The catalyst exhibits high photocatalytic activity not only in the decolourization but also in mineralization of colourless organic pollutants. The TOC and COD results represent more than 90% of the initial waste water degradation using coupled semiconductor ZnO/CuO (99:5) as a catalyst.
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There is no decolouration of the sample in the presence of pure ZnO nanorods under visible light irradiation, since the band gap of ZnO is 3.2 eV. The band gap corresponds to the UV region of the electromagnetic radiation and the absorbance spectrum of ZnO is flat in the visible region. The same behaviour was reported by earlier researchers [50]. The composite photocatalyst mechanism is explained by the following equation based on earlier reports [1,5,14,16,44,50,51]. −
þ
þ visible light
−
ZnOðe þ h Þ=CuOðe þ h Þ
þ
→
−
−
þ
þ
ZnOðe þ e Þ=CuOðh þ h Þ ð3Þ
−
h þ OH →OH
−
ð4Þ
−
e þ O2 →•O2
ð5Þ
−
−
H2 O þ •O2 →OOH• þ OH
ð6Þ
2OOH•→O2 þ H2 O2
ð7Þ
−
−
ð8Þ
OH• þ •O2 þ hvb þ pollutants→degrade pollutant
H2 O2 þ •O2 →OH• þ OH þ O2
−
þ
ð9Þ
−
þ
ð10Þ
OH• þ •O2 þ hvb þ degrade pollutant→CO2 ↑ þ H2 O
Fig. 8. Effect of catalyst loading on the photocatalytic degradation of (a) MB and (b) MO under visible light.
3.5. Photocatalytic mechanism The conduction and valence band positions of the two semiconductors at the point of zero charge can be calculated by the following formula [21,48] Evb ¼ X−Ee þ 0:5Eg
ð2Þ
where Evb is the valence band potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. The electronegativity values for CuO and ZnO are 5.81 eV and 5.79 eV respectively [49]. Ee is the energy of free electrons on the hydrogen scale (~ 4.5 eV) and Eg is band gap energy of the semiconductor.
When the visible light is irradiated, the electron transfer may occur from the valence band of CuO (p-type) to the valence band of ZnO (n-type). This is possible due to the work function of CuO being similar to that of ZnO (5.3 eV). On the other hand, transfer of holes may occur from the more anodic valence band of ZnO to the cathodic valence band of CuO [44]. At the same time, the ZnO/CuO materials exhibit red shift in the absorption wavelength range compared with that of ZnO, which might also benefit the improvement of photocatalytic performance. The UV-absorbance data suggests that ZnO/CuO nanocomposite has improved the photocatalytic activity. At higher percentages (10% and 50%) of CuO, the photocatalytic activity decreases. This is due to the presence of CuO that improves charge recombination rate. The same behaviour has already been reported for CuO–TiO2 [23] and TiO2/WO3 [52]. At the same time, the absorption value at higher percentages (10% and 50%) decreases compared to lower percentages (1%, 3%, 5%) of CuO samples. Hence the photocatalytic activity efficiency has improved for composite material (ZnO/ CuO) to direct or indirect decomposition of organic dyes under visible light.
Table 2 Photocatalytic degradation efficiency (in percentage). Samples
ZnO ZnO/CuO ZnO/CuO ZnO/CuO ZnO/CuO ZnO/CuO
(99:1) (97:3) (95:5) (90:10) (50:50)
Methylene blue (MB)
Methyl orange (MO)
Irradiation time
Irradiation time
30 min
60 min
90 min
120 min
30 min
60 min
90 min
120 min
0.2 20.3 48.5 63.2 17.5 7.7
0.3 54.1 77.1 91.2 28.2 17.1
0.6 77.6 92.1 96.6 50.33 29.9
0.9 96.3 96.9 97.2 71.28 38.5
2.4 12.6 18.1 40.6 13.9 12.4
2.6 25.7 42.6 54.3 18.9 19.3
2.7 33.4 54.3 75.1 26.7 23.7
3.1 48.5 69.2 87.7 28.3 27.7
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Fig. 9. Photography image represents change in colour of textile dyes using ZnO/CuO (95:5) catalyst for different exposure time under visible light irradiation.
colourless organic pollutants. The preparation method of catalyst is new, simple, fast and cost effective. Thus, it can be used for the synthesis of future composite photocatalysts. Acknowledgements We acknowledge the National Centre for Nanoscience and Nanotechology, University of Madras, India for FE-SEM and EDXS characterizations. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.msec.2012.08.011. References
Fig. 10. TOC and COD results for catalyst (5 wt.% CuO) loading on the photocatalytic degradation of textile waste water under visible light irradiation.
4. Conclusion The coupled semiconductor ZnO/CuO possesses higher photocatalytic degradation of MB and MO when compared to ZnO under visible light since coupling of ZnO/CuO reduces the band gap, extending the wavelength range to visible light region leading to electron–hole pair separation under visible light irradiation and consequently, achieving a higher photocatalytic activity. The maximum efficiency is observed for 5% CuO loaded on ZnO. This environmental friendly composite material was used for the degradation of real textile dye effluent under visible light illumination. The catalyst exhibits high photocatalytic activity not only in decolourization but also in mineralization of
Table 3 TOC and COD data for degradation of textile pollutant. Time (min)
COD mg/L
COD/COD0
TOC mg/L
TOC/TOC0
0 30 60 90 120 150 180 210 240 270 300
462 320 210 185 158 123 100 88 77 50 38
1 0.69 0.45 0.40 0.34 0.26 0.22 0.19 0.16 0.11 0.08
224 153 105 97 84 53 41 33 29 25 20
1 0.68 0.47 0.43 0.37 0.24 0.18 0.14 0.12 0.11 0.09
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