- Email: [email protected]
CuO catalysts
Materials Science and Engineering C 33 (2013) 4725–4731
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Visible light induced degradation of methylene blue using CeO2/V2O5 and CeO2/CuO catalysts R. Saravanan a,b, S. Joicy a, V.K. Gupta c,d, V. Narayanan e, A. Stephen a,⁎ a
Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India Physics Research Centre, Dhanalakshmi College of Engineering, Dr. V.P.R. Nagar, Manimangalam, Chennai 601301, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India d Vice Chancellor, Dr. R M L Avadh University Faizabad, UP 224001, India 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 13 April 2013 Received in revised form 10 July 2013 Accepted 22 July 2013 Available online 30 July 2013 Keywords: Nanocomposite Photocatalyst Visible light Thermal decomposition method
a b s t r a c t In the present study, the nanocatalysts CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO were synthesized by thermal decomposition method. This method is simple, fast and cost effective compared with other preparation methods. The synthesized catalysts were characterized by different techniques. The XRD and XPS results confirmed the structure and the oxidization states of the nanocomposite materials. DRS results suggested that the prepared CeO2/V2O5 and CeO2/CuO nanocomposites can generate more electrons and holes under visible light irradiation. The photocatalytic activities of prepared catalysts were evaluated using the degradation of aqueous methylene blue solution as a model compound under visible light irradiation. In addition, the nanocomposite (CeO2/V2O5 and CeO2/CuO) materials were employed to degrade the textile effluent under visible light condition. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis is a rapidly emerging and promising technology coming up with clean, green, and sustainable innovation in environmental applications. The use of semiconducting compounds as photocatalysts has become increasingly important on providing alternative sources of energy and addressing problems associated with environmental pollution. In the past decades, semiconductor based photocatalysts such as TiO2 and ZnO have been most commonly used in the environmental applications due to their low cost, chemical stability against photo corrosion and chemical corrosion [1–16]. Apart from this, cerium oxide (CeO2) is a good semiconducting photocatalyst which is quite transparent to visible light and has an excellent UV absorption property due to its wider optical bandgap value. It can also degrade the organic pollutants. Even though CeO2 acts as an efficient photocatalyst, its wide bandgap energy limits its applications in visible region [17]. This has consequential implications for the use of CeO2 as solar or room light activated catalysts, because a major part of sunlight consists of visible light (~45%) and only 3–5% UV light. Research in the field of photocatalyst aims to increase the degradation efficiency in the visible light. Therefore, many efforts have been explored to extend the absorption wavelength of CeO2 in the visible region by using metal doping, semiconductor coupling and so on [18,19]. The coupled semiconductor photocatalysts ⁎ Corresponding author at: Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India. Tel.: +91 44 2220 2802; fax: +91 44 22353309. E-mail address: [email protected] (A. Stephen). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.07.034
improve the charge separation and enhance the photocatalytic activity due to high efficiency of the interfacial charge transfer. The coupled semiconductor materials have two different energy-level systems which play an important role in achieving charge separation. Coupling of different semiconductor oxides can reduce the bandgap, extending the absorbance range to visible light region, leading to electron–hole pair separation under irradiation and consequently achieving a higher photocatalytic activity [20]. In the narrow bandgap semiconductor, CuO has been coupled with some wide bandgap semiconductors to develop the photocatalytic efficiency [21,22]. Also vanadium pentoxide (V2O5) has relatively smaller bandgap (2.8 eV) and is expected to be an efficient photocatalyst under visible light [23]. Hence, for the effective utilization of visible light the bandgap of CeO2 can be reduced by coupling with CuO and V2O5 separately. In this present study, CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO nanocatalysts were synthesized by thermal decomposition method. All the prepared samples were characterized by various techniques and used for the photodegradation of methylene blue under visible light irradiation. 2. Experimental section 2.1. Materials Cerium acetate (Aldrich), copper acetate (Aldrich) and ammonium metavanadate (Aldrich) used in this present study were of analytical reagent grade. Methylene blue (MB) dye was purchased from Aldrich
4726
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
chemicals. All the chemicals were received and used without further purification. Throughout the experiment, all aqueous solution was prepared using double distilled water. 2.2. Preparation of nanosemiconductors (CeO2, V2O5 and CuO) Semiconductors of CeO2, V2O5 and CuO were synthesized via thermal decomposition method as follows: 3 g of cerium acetate, copper acetate and ammonium metavanadate was taken and each was weighed separately and grounded in a mortar for 3 h at room temperature and then each was annealed in an alumina crucible at 400 °C for 30 min separately. During this process, the temperature was raised at the rate of 4 °C/min. The samples were cooled at room temperature after the heat treatment. In comparison with other methods, this preparation method is simple, fast, cost effective, environmentally friendly and no special equipments were required. 2.3. Preparation of nanocomposites (CeO2/V2O5 and CeO2/CuO) A composite of CeO2/V2O5 was prepared by simple thermal decomposition method. Amount of cerium acetate and ammonium metavanadate (90:10 weight ratio) was mixed and grounded well for 3 h and then annealed in an alumina crucible at 400 °C for 30 min in a muffle furnace. Composite of CeO2/CuO was also prepared by the same method. Amount of cerium acetate and copper acetate (90:10 weight ratio) was weighed separately, then mixed and grounded well for 3 h and then annealed in an alumina crucible at 400 °C for 30 min in a muffle furnace. 2.4. Photocatalytic experiment 2.4.1. Measurement of photocatalytic degradation under UV light The photocatalytic procedure by our previous report is followed [24]. The schematic diagram of the UV-light photocatalytic experimental set-up is represented in Supporting Fig. 1. The photocatalytic activity of CeO2 catalyst was measured in terms of methylene blue (MB) decoloration. This photo-decolorization experiment was executed in a photocatalytic chamber integrated with an 8 W mercury vapor lamp (365 nm) positioned at the vessel axis and magnetic stirrer arrangement within the enclosure. In order to avoid direct contact of mercury lamp with the aqueous solution, it was roofed by quartz tube. 500 mg of CeO2 catalyst was suspended in 500 ml aqueous solution of MB with an initial concentration of 3 × 10−5 mol/l. The adsorption equilibrium between the dye and surface of catalyst was maintained by stirring in dark condition for 30 min before exposed to UV light. During irradiation, suspension was syringed at regular intervals of time and centrifuged to wipe out the catalysts. The absorption of MB was recorded on UV–Visible spectra at room temperature. 2.4.2. Measurement of photocatalytic degradation under visible light The photocatalytic activity under visible light in the literatures is followed [25,26]. The photograph and schematic diagram of the visible light photocatalytic chamber are represented in Supporting Fig. 2. The photocatalytic activity of CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO was investigated by the decomposition of MB in an aqueous solution under visible light irradiation. The visible light irradiation was carried out using a projection lamp (7748XHP 250 W, Philips, 532 nm) in a photoreactor. A 0.5% aqueous K2Cr2O7 solution circulating in the glass jacket was used for the purpose of UV cut-off. Reaction suspensions were prepared by adding 500 mg of the catalysts in 500 ml of MB solution taken within the initial concentration of 3 × 10−5 mol/l. The aqueous suspensions were magnetically stirred in the dark condition for 30 min to ensure adsorption equilibrium of MB on the catalyst surface before visible light illumination. Then, the suspension was irradiated under visible light under continuous stirring at room temperature, to obtain full suspension of the particles throughout the experiment. During irradiation, the suspensions were sampled at every 30 min and
immediately centrifuged to remove the catalyst particles and then, the absorption of MB aqueous solutions was measured by a UV–Vis spectrophotometer at a wavelength of 664 nm. Degradation efficiency can be calculated using the following equation: η ¼ ð1−C=C0 Þ 100
ð1Þ
where, C0 and C are the concentrations of the solution before illumination (t = 0) and after illumination for t minutes respectively. 2.4.3. Textile effluent The textile effluent was collected from dye industries (Tirupur, Tamilnadu, India). Before photocatalytic process, 50 ml of the textile effluent was diluted with 450 ml of distilled water (1:9 ratio) in order to reduce its opaque property. Therefore, the light can pass through the entire suspension without any loss. Initially, the diluted effluent was irradiated with visible light in the absence of catalyst. The result showed that there is no decoloration in the diluted effluent. It was concluded that the diluted effluent did not undergo any self decomposition. During the photocatalytic experiment, 500 mg of the catalyst was mixed with 500 ml of diluted dye. The reaction mixtures were stirred for 60 min under dark condition to establish adsorption–desorption equilibrium condition. The samples from the suspension were collected at regular intervals of time, centrifuged and filtered. Further, the irradiated sample was analyzed using a UV–Visible spectrophotometer. 2.5. Characterization details The structure of the prepared sample was analyzed using X-ray diffraction (XRD) analysis. The XRD was carried out by a Rich Siefert 3000 diffractometer using Cu Kα1 radiation (λ = 1.5406 Å). High resolution scanning electron microscopy (HR-SEM) and energy dispersive X-ray spectroscopy (EDX) analysis were carried out using FEI quanta FEG 200. The surface oxidation state and the presence of element in the sample were studied using XPS. The XPS spectra were carried out using an omicron ESCA spectrometer with monochromatized Al Kα radiation. The optical absorption spectrum was obtained on a CARY 5E UV–VIS– NIR spectrophotometer. The photocatalytic activity was measured by a Perkin–Elmer UV–Visible spectrometer RX1. 3. Results and discussions 3.1. Structural analysis The CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO samples prepared by thermal decomposition method were characterized by powder X-ray diffraction (XRD) technique to study their crystal structure, phase purity and crystallite size. The XRD patterns of the CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO samples were shown in Fig. 1. Fig. 1 (a) shows the XRD pattern of pure CeO2 nanoparticles. Diffraction peaks correspond to (111), (200), (220), (311), (222) and (400) lattice planes and exhibit face centered cubic structure of pure CeO2, which agrees well with standard JCPDS card no: 43-1002 with lattice parameters a = b = c = 5.411 Å. The XRD pattern of V2O5 is shown in Fig. 1 (b). All of the diffraction peaks correspond to (200), (010), (110), (210), (101), (400), (011), (111), (301), (501), and (002) lattice planes in this pattern and are assigned to orthorhombic structure of pure V2O5, which are well comparable with JCPDS card no: 89-0611. The lattice parameter values are a = 11.54 Å, b = 4.383 Å, and c = 3.571 Å. The Xray diffraction peaks given in Fig. 1 (c) assigned as (110), (111), (111), (111), (202), (020), (202), (113), (311), and (113) are found to match well with the monoclinic structure of CuO (JCPDS card no: 89-5899) with lattice parameters a = 4.689 Å, b = 3.420 Å, and c = 5.130 Å. Two types of phases were detected in Fig. 1 (d). One of the phases is well indexed to face centered cubic CeO2 (JCPDS card no: 43-1002) and the other one is identified to orthorhombic phase of V2O5 (JCPDS
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
4727
Table 2 Comparison of the d spacing value for all prepared semiconductors (CeO2, V2O5, and CuO). Sample code
Crystal structure
CeO2
Cubic
V2O5 CuO
Orthorhombic Monoclinic
dhkl formula dhkl ¼ dhkl ¼ dhkl ¼
−1 ðh2 þk2 þl2 Þ 2
Hkl
Calculated dspacing (Å)
JCPDS dspacing (Å)
111
3.125
3.124
010
4.355
4.383
111
2.522
2.522
a2
h
2
2
h2 a2
2
þ k2 þ cl 2
h a2
i−12
b
2
cosβ þ l 2 −2hlac c
sin2 β
2
þ k2
−12
b
XRD pattern indicates the formation of nanosized CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO materials.
3.2. Surface oxidation state analysis
Fig. 1. XRD pattern of all prepared samples.
no: 89-0611). However, the XRD pattern of CeO2/V2O5 is weak due to the low content of V2O5. From Fig. 1 (e), it was found that the cubic structure of CeO2 has co-existed with the monoclinic phase of CuO. In this XRD pattern, all the diffraction peaks are assigned to CeO2 and CuO as reported in JCPDS card nos: 43-1002 and 89-5899 respectively. Also this XRD pattern seems quite similar to that of pure CeO2 due to low content of CuO. No other characteristic peaks such as Ce2O3 and Cu2O were observed which suggests that the prepared nanocomposites are composed of CeO2 and CuO only. The lattice parameter values are listed in Table 1. The peaks of the nanocomposite (CeO2/V2O5 and CeO2/CuO) systems are slightly shifted compared with pure systems (CeO2, V2O5 and CuO). The deviation of nanocomposites from that of individual system suggests intra-granular coupling of the composites. Intra-granular coupling provides close contact between both phases [27]. The d spacing value for all pure semiconductors was theoretically calculated using their respective d spacing formula and the values are given in Table 2. The crystallite size of the prepared samples was calculated based on Scherrer's formula. The broadening of peaks in XRD pattern indicates the nanocrystalline nature of synthesized samples. Crystallite size for the maximum diffracted peaks of all synthesized samples was calculated and the values are given in Table 3. Hence, the
XRD result did not confirm the oxidation state of the present element. So, the surface oxidation state was analyzed through XPS measurement. Fig. 2 (a) and (b) represents the XPS spectrum of CeO2/ V2O5 and CeO2/CuO respectively. Fig. 2 (a) indicates that the CeO2/ V2O5 nanocomposite sample was composed of Ce4+, V5+, O and C. Fig. 2 (b) reveals that the CeO2/CuO nanocomposite sample consists of Ce4+, Cu2+, O and C. In both the samples, small amount of carbon (C1s) was present. The carbon peak arises itself by pumping oil in the vacuum system of XPS instrument. Hence, the XPS result confirmed that the CeO2/V2O5 and CeO2/CuO nanocomposites were presented with respective oxidation states without any impurities. This is in good agreement with the XRD result.
3.3. Morphology and elemental analysis High resolution scanning electron microscopic (HR-SEM) images of CeO2, CeO2/V2O5 and CeO2/CuO were shown in Fig. 3 (a, b & c). All the images were taken in 500 nm scale with 60,000 magnifications. From Fig. 3 (a), it was observed that CeO2 nanoparticles are spherical in shape and the diameter was in the range of 60–80 nm. HR-SEM image of CeO2/V2O5 and CeO2/CuO (Fig. 3 (b & c)) reveals irregular shaped particles which are due to the agglomeration of tiny spherical shaped particles. The elemental analyses of CeO2, CeO2/V2O5 and CeO2/CuO samples were examined by energy dispersive X-ray spectroscopy measurement and are illustrated in Supporting Fig. 3 (a, b & c). Supporting Fig. 3 (a) shows that the CeO2 nanoparticles were composed of Ce and O elements only and no other impurities were detected. The EDX spectrum of CeO2/V2O5 coupled catalysts (Supporting Fig. 3 (b)) indicated the existence of Ce, V and O elements in the sample. The EDX spectrum of CeO2/CuO coupled semiconductor (Supporting Fig. 3 (c)) also confirmed the existence of Ce, Cu and O elements in the sample.
Table 1 Lattice parameter values for all prepared samples. Sample code
CeO2 V2O5 CuO CeO2/V2O5 CeO2/CuO
Cerium oxide (CeO2)
Vanadium pentoxide (V2O5)
Copper oxide(CuO)
43-1002
89-0611
89-5899
Table 3 Crystallite size (D) and bandgap values for all synthesized samples. Sample code
Cubic Orthorhombic structure structure
Monoclinic structure
a (Å)
a (Å)
b (Å)
c (Å)
a (Å)
b (Å)
c (Å)
5.41 (5) – – 5.40 (3) 5.40 (4)
– 11.40 (1) – 11.56 (3) –
– 4.35 (4) – 4.35 (8) –
– 3.49 (5) – 3.54 (1) –
– – 4.70 (1) – 4.53 (8)
– – 3.43 (3) – 3.41 (2)
– – 5.12 (5) – 5.14 (1)
CeO2 V2O5 CuO CeO2/V2O5 CeO2/CuO
Cerium oxide (CeO2)
Vanadium pentoxide (V2O5)
Copper oxide (CuO)
Crystallite size (nm)
Crystallite size (nm)
Crystallite size (nm)
~13 – – ~11 ~9
– ~34 – ~12 –
– – ~36 – ~18
Bandgap value (eV)
3.28 2.66 1.83 2.62 2.59
4728
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
Fig. 2. XPS spectrum of (a) CeO2/V2O5 and (b) CeO2/CuO.
3.4. Bandgap analysis The bandgap of all prepared samples was investigated by UV–Vis diffuse reflectance spectra. The comparison of diffuse reflectance spectrum of CeO2/V2O5 and CeO2/CuO nanocomposites with those spectra of pure CeO2, V2O5 and CuO semiconductors is represented in Fig. 4 (a, b, c, d & e). Pure CeO2 has strong absorption in UV region of spectrum at around 378 nm, while pure V2O5 and CuO catalysts have strong absorption onset in visible region at 466 nm and 676 nm respectively. Compared to pure CeO2, the absorption edge of CeO2/V2O5 and CeO2/CuO nanocomposites has shifted towards the visible region due to the presence of lower bandgap semiconductors. The absorption edge at higher wavelengths revealed that the coupled catalysts can efficiently utilize the visible light for photocatalytic purpose. Further, the optical bandgap energy of all prepared samples was calculated using the following relation [28] and listed in Table 3. Eg ¼ 1240=λðeVÞ
ð2Þ
where, Eg represents bandgap energy (eV), and λ represents lower cutoff wavelength in nanometer. This result suggested that the prepared CeO2/V2O5 and CeO2/CuO nanocomposites can be photoexcited to generate more electron–hole pairs under visible light irradiation, which could result in higher photocatalytic performance. 4. Photocatalytic activity 4.1. MB degradation using CeO2 catalysts under UV light The photocatalytic degradation of MB using CeO2 catalyst was studied under UV light irradiation. The disappearance of the band at 664 nm indicates that MB has been photodegraded. The photocatalytic degradation efficiency of MB was found to be 84.5% (using Eq. (1). The photocatalytic mechanism is explained by the following equation based on earlier reports [29,30]. The schematic diagram (Supporting Fig. 4) represents the photocatalytic mechanism of CeO2 under UV light irradiation. When the presence of energy in UV light (photon) is equal to or greater than the bandgap of CeO2, the electrons receive the energy and transfer of electrons takes place from valence band (VB) to conduction band (CB) which results in the formation of a hole (h+) in the VB and an electron (e−) in the CB. The holes react with water and generate OH• radical, which can oxidize the organic pollutants. In the conduction band the electrons
react with oxygen and undergo reduction process and produce OH• radical. These radicals reduce the organic pollutants. This oxidation and reduction processes were capable of degrading the (MB) organic pollutants under UV light irradiation. þ
−
CeO2 þ hv→CeO2 ðhVB þ eCB Þ þ
þ
hVB þ H2 O→H þ OH −
•
ð4Þ
−•
eCB þ O2 →O2 −•
þ
•
þ
ð5Þ •
O2 þ H →HO2
ð6Þ
−
HO2 þ H þ eCB →H2 O2 −
•
H2 O2 þ eCB →OH þ OH þ
−
ð3Þ
hVB þ OH →OH
ð7Þ −
•
•
OH þ organic pollutants→degradation products
ð8Þ ð9Þ ð10Þ
4.2. MB degradation using CeO2, V2O5 and CuO catalysts under visible light The pure CeO2, V2O5 and CuO catalysts were used in the degradation of aqueous MB solution under visible light irradiation. There is no decoloration in the presence of pure CeO2 under visible light because the bandgap of CeO2 is 3.28 eV (from DRS). Thus, there is insufficient energy for the electron excitation between valence band and conduction band and no electron–hole pairs are generated under visible light irradiation. The absorption spectrum of MB has decreased gradually with increase in irradiation time using V2O5 and CuO catalysts. The photocatalytic degradation efficiency for all prepared samples was determined using Eq. (1) and the results are listed in Table 4.
4.3. MB degradation using nanocomposites under visible light The photocatalytic activities of CeO2/V2O5 and CeO2/CuO nanocomposites were examined by the decomposition of MB under visible light. The photocatalytic degradation efficiency for all prepared samples was determined using Eq. (1) and the values are listed in Table 4. This result suggested that the nanocomposites show higher photocatalytic
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
4729
Fig. 4. Diffuse reflectance spectrum (DRS) of all prepared samples.
4.5. Photocatalytic mechanism for coupled semiconductors 4.5.1. CeO2/V2O5 composites The photocatalytic mechanism of CeO2/V2O5 composite semiconductors is shown in Fig. 6. In order to determine the mechanism of the enhanced photocatalytic activity of the CeO2/V2O5 composite, the relative band positions of the two semiconductors were investigated, since the band-edge potential levels play a crucial role in determining the flowchart of photoexcited charge carriers in coupled semiconductors. The valence band (VB) edge and the conduction band (CB) edge positions of CeO2/V2O5 can be calculated from empirical formulae [31]. The electro negativity (X) values of the semiconductors (CeO2 [19] and V2O5 [32]) are 5.56 eV and 2.88 eV respectively. The calculated CB and VB edges of CeO2 were −0.58 eV and 2.7 eV, and of V2O5 were 0.27 eV and 2.93 eV respectively. The photocatalytic mechanism of the composite (CeO2/V2O5) is explained by the following equations [33–35]: CeO2 =V2 O5
visible light
→
−
− þ CeO2 =V2 O5 eCB þ hVB
þ
þ
ð11Þ −
CeO2 =V2 O5 ðeCB þ hVB Þ→V2 O5 ðhVB Þ þ CeO2 ðeCB Þ þ
−
hVB þ OH →OH −
•
ð13Þ
−•
eCB þ O2 →O2
ð14Þ
−•
•
H2 O þ O2 →OOH þ OH
−
•
Fig. 3. HR-SEM images of a) CeO2 and b) CeO2/V2O5 and c) CeO2/CuO.
2OOH →O2 þ H2 O2 −•
degradation efficiency than those of pure semiconductors within 210 min under visible light irradiation.
4.4. Textile effluent degradation using nanocomposites under visible light Fig. 5 (a & b) illustrates the time-dependent absorption spectra of textile effluent during visible light irradiation in the presence of CeO2/ V2O5 and CeO2/CuO catalysts with uniform irradiation of time. The figures show that the absorption peak gradually decreases with increase in exposure time. The nanocomposite (CeO2/V2O5 and CeO2/CuO) materials were found to effectively degrade the industrial effluent with an efficiency of 76.9% and 85.7% respectively.
•
−•
þ
ð15Þ ð16Þ
−
H2 O2 þ O2 →OH þ OH þ O2 •
ð12Þ
OH þ O2 þ hVB þ pollutants→degraded pollutants→CO2 ↑ þ H2 O
ð17Þ ð18Þ
Photocatalytic process is based on electron–hole recombination by means of bandgap excitation. As shown in Fig. 6, V2O5 with narrow bandgap energy (2.66 eV) could be easily excited by visible light and induce the generation of photoelectrons and holes. However, these photoelectrons and holes might recombine rapidly. That is why V2O5 shows poor photocatalytic activity. In case of CeO2, it could not be excited by visible light irradiation due to its wide bandgap of about 3.28 eV in this present work. Therefore, MB could not be degraded on CeO2 as well. When the CeO2/V2O5 nanocomposite is irradiated under visible
4730
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
Table 4 The degradation efficiency for all prepared samples (in percentage) with respective irradiation time. Sample code
CeO2 V2O5 CuO CeO2/V2O5 CeO2/CuO
Methylene blue (MB) (in percentage) 30 min
60 min
90 min
120 min
150 min
180 min
210 min
0.9 7.5 3.1 3.9 11.9
1.4 10.1 6.2 16.6 17.8
3.1 17.6 8.7 22.9 21.5
3.9 21.2 12.6 30.1 34.3
4.2 22.9 22.5 40.4 49.3
5.1 24.7 29.3 53.5 56.5
6.1 27.5 33.4 64.2 70.1
light, V2O5 could act as a sensitizer to absorb the visible light. Under visible light illumination, the electrons in the valence band of V2O5 are excited to the conduction band with simultaneous generation of the same amount of holes in the valence band. The photoactivated electrons in the conduction band of V2O5 are injected into the conduction band of CeO2. The injection of electrons from the conduction band of V2O5 into CeO2 particles was expected to retard the back reaction between the photogenerated charge carriers. This process of the electron–hole trapping which is facilitated in the mixed semiconductor system would increase the efficiency. Thus, the efficiency of photogenerated electron–hole in CeO2/V2O5 could be higher than those of pure semiconductors. Also CeO2/V2O5 photocatalyst shows red shift in the absorption
Fig. 6. Schematic diagram represents the charge transfer pathway during MB degradation process over CeO2/V2O5 nanocomposites under visible light irradiation.
wavelength range compared to CeO2, which would improve its photocatalytic activity in the visible range. 4.5.2. CeO2/CuO composites The photocatalytic mechanism of CeO2/CuO nanocomposites under visible light irradiation is shown in Fig. 7. The calculated CB and VB edges of CeO2 were −0.58 eV and 2.7 eV, and of CuO were 0.39 eV and 2.22 eV respectively. The electronegativity of CeO2 and CuO is 5.56 eV and 5.81 eV [36]. The photocatalytic mechanism of coupled semiconductor is explained by the following equations: visible light − þ CeO2 =CuO → CeO2 =CuO eCB þ hVB −
þ
þ
ð19Þ −
CeO2 =CuOðeCB þ hVB Þ→CeO2 ðhVB Þ þ CuOðeCB Þ þ
−
hVB þ OH →OH −
•
ð21Þ
−•
eCB þ O2 →O2
ð22Þ
−•
•
H2 O þ O2 →OOH þ OH
−
•
2OOH →O2 þ H2 O2 −•
•
−•
þ
ð23Þ ð24Þ
−
H2 O2 þ O2 →OH þ OH þ O2 •
ð20Þ
OH þ O2 þ hVB þ pollutants→degraded pollutants→CO2 þ H2 O
ð25Þ ð26Þ
When CeO2/CuO composite semiconductor is irradiated under visible light, only CuO catalysts can be activated since the observed bandgap energy of CuO in this study is 1.83 eV, whereas the electrons in the valence band of CeO2 can move to the valence band of V2O5. The holes in valence band of CeO2 and electrons in conduction band of CuO undergo oxidation process and reduction process respectively. In such a way, photoinduced electron hole pairs are efficiently separated and reduced the electron–hole recombination. Therefore, CeO2/CuO composites exhibit enhanced photocatalytic performance. 5. Conclusion
Fig. 5. Change in UV–Visible absorption spectra of textile effluent using (a) CeO2/V2O5 and (b) CeO2/CuO nanocatalysts under visible light irradiation.
The photocatalytic degradation test reveals that the prepared nanocomposite systems (CeO2/V2O5 and CeO2/CuO) exhibit high efficient visible light photocatalytic activities as compared to that for the pure systems (CeO2, V2O5, CuO) for the degradation of model methylene blue dye and textile effluent. The higher photocatalytic efficiencies are
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
Fig. 7. Schematic diagram represents the charge transfer pathway during MB degradation process over CeO2/CuO nanocomposites under visible light irradiation.
attributed to the superior charge separation by the nanocomposite system, thereby permitting the photogenerated electrons and holes to have enough time to contribute to the overall photocatalytic reactions. The photocatalytic activity of nanocomposite systems can have significant impact on the future improvement of highly capable visible-light catalysts for textile pollutant degradation. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2013.07.034. Acknowledgments We acknowledge the National Centre for Nanoscience and Nanotechnology, University of Madras, India for XPS analysis. References [1] X. Chen, Samuel S. Mao, Chem. Rev. 107 (2007) 2891–2959. [2] V.K. Gupta, A. Rastogi, A. Nayak, J. Colloid Interface Sci. 342 (2010) 533–539. [3] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, J. Colloid Interface Sci. 344 (2010) 497–507.
4731
[4] A.K. Jain, V.K. Gupta, A. Bhatnagar, Suhas, Sep. Sci. Technol. 38 (2003) 463–481. [5] V.K. Gupta, S.K. Srivastava, D. Mohan, S. Sharma, Waste Manage. 17 (1998) 517–522. [6] A.K. Jain, V.K. Gupta, A. Bhatnagar, Shubhi Jain, Suhas, J. Indian Chem. Soc. 80 (2003) 267–270. [7] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, J. Colloid Interface Sci. 342 (2010) 518–527. [8] V.K. Gupta, A. Mittal, A. Malviya, J. Mittal, J. Colloid Interface Sci. 335 (2009) 24–33. [9] A. Mittal, D. Kaur, A. Malviya, J. Mittal, V.K. Gupta, J. Colloid Interface Sci. 337 (2009) 345–354. [10] V.K. Gupta, B. Gupta, A. Rastogi, S. Agarwal, A. Nayak, J. Hazard. Mater. 186 (2011) 891–901. [11] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, J. Colloid Interface Sci. 340 (2009) 16–26. [12] V.K. Gupta, S. Agarwal, T.A. Saleh, Water Res. 45 (2011) 2207–2212. [13] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, J. Colloid Interface Sci. 343 (2010) 463–473. [14] V.K. Gupta, S. Agarwal, T.A. Saleh, J. Hazard. Mater. 185 (2011) 17–23. [15] A. Mittal, V.K. Gupta, A. Malviya, J. Mittal, J. Hazard. Mater. 151 (2008) 821–832. [16] J. Liqianga, S. Xiaojun, S. Jing, C. Weimin, X. Zili, D. Yaoguo, F. Honggang, Sol. Energy Mater. Sol. Cells 79 (2003) 133–151. [17] F. Chen, Y. Cao, D. Jia, Appl. Surf. Sci. 257 (2011) 9226–9231. [18] A. Aranda, et al., Chem. Commun. 48 (2012) 4704–4706. [19] N. Wetchakun, et al., Appl. Mater. Interfaces 4 (2012) 3718–3723. [20] C. Xu, L. Cao, G. Su, W. Liu, Y. Yu, X. Qu, J. Hazard. Mater. 176 (2010) 807–813. [21] Z.L. Liu, J.C. Deng, J.J. Deng, F.F. Li, Mater. Sci. Eng. B 150 (2008) 99–104. [22] G. Li, N.M. Dimitrijevic, L. Chen, T. Rajh, K.A. Gray, J. Phys. Chem. C 112 (2008) 19040–19044. [23] H.Y. Jang, T.A. Quand-Thao, H.O. Xuan-Huong, M.H.O. Sun-il, J. Korean Phys. Soc. 55 (2009) 2447–2450. [24] R. Saravanan, H. Shankar, G. Rajasudha, V. Narayanan, A. Stephen, Int. J. Nanosci. 10 (1 & 2) (2011) 253–257. [25] R. Saravanan, H. Shankar, T. Prakash, V. Narayanan, A. Stephen, Mater. Chem. Phys. 125 (2011) 277–280. [26] R. Saravanan, S. Karthikeyan, V.K. Gupta, G. Sekaran, V. Narayanan, A. Stephen, Mater. Sci. Eng. C 33 (2013) 91–98. [27] D. Zhang, Transit. Met. Chem. 35 (2010) 689–694. [28] Dhananjay S. Bhatkhande, Vishwas G. Pangarkar, Anthony ACM Beenackers, J. Chem. Tech. Biotech. 77 (2011) 102–116. [29] I.K. Konstantinou, T.A. Albanis, Appl. Catal. B Environ. 49 (2004) 1–14. [30] G. Li Puma, A. Bono, D. Krishnaiah, Joseph G. Collin, J. Hazard. Mater. 157 (2008) 209–219. [31] Z. Zhang, W. Wang, L. Wang, S. Sun, Appl. Mater. Interfaces 4 (2012) 593–597. [32] X. Yong, M.A.A. Schoonen, Am. Mineral. 85 (2000) 543–556. [33] N. Daneshvar, D. Salari, A.R. Khataee, J. Photochem. Photobiol. A Chem. 162 (2004) 317–322. [34] A. Akyol, H.C. Yatmaz, M. Bayramoglu, Appl. Catal. B: Environ. 54 (2004) 19–24. [35] R. Qiu, D. Zhang, Y. Mo, L. Song, E. Brewer, X. Huang, Y. Xiong, J. Hazard. Mater. 156 (2008) 80–85. [36] Y. Xu, M.A.A. Schoonen, Am. Mineral. 85 (2000) 543–556.
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Visible light induced degradation of methylene blue using CeO2/V2O5 and CeO2/CuO catalysts R. Saravanan a,b, S. Joicy a, V.K. Gupta c,d, V. Narayanan e, A. Stephen a,⁎ a
Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India Physics Research Centre, Dhanalakshmi College of Engineering, Dr. V.P.R. Nagar, Manimangalam, Chennai 601301, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India d Vice Chancellor, Dr. R M L Avadh University Faizabad, UP 224001, India 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 13 April 2013 Received in revised form 10 July 2013 Accepted 22 July 2013 Available online 30 July 2013 Keywords: Nanocomposite Photocatalyst Visible light Thermal decomposition method
a b s t r a c t In the present study, the nanocatalysts CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO were synthesized by thermal decomposition method. This method is simple, fast and cost effective compared with other preparation methods. The synthesized catalysts were characterized by different techniques. The XRD and XPS results confirmed the structure and the oxidization states of the nanocomposite materials. DRS results suggested that the prepared CeO2/V2O5 and CeO2/CuO nanocomposites can generate more electrons and holes under visible light irradiation. The photocatalytic activities of prepared catalysts were evaluated using the degradation of aqueous methylene blue solution as a model compound under visible light irradiation. In addition, the nanocomposite (CeO2/V2O5 and CeO2/CuO) materials were employed to degrade the textile effluent under visible light condition. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis is a rapidly emerging and promising technology coming up with clean, green, and sustainable innovation in environmental applications. The use of semiconducting compounds as photocatalysts has become increasingly important on providing alternative sources of energy and addressing problems associated with environmental pollution. In the past decades, semiconductor based photocatalysts such as TiO2 and ZnO have been most commonly used in the environmental applications due to their low cost, chemical stability against photo corrosion and chemical corrosion [1–16]. Apart from this, cerium oxide (CeO2) is a good semiconducting photocatalyst which is quite transparent to visible light and has an excellent UV absorption property due to its wider optical bandgap value. It can also degrade the organic pollutants. Even though CeO2 acts as an efficient photocatalyst, its wide bandgap energy limits its applications in visible region [17]. This has consequential implications for the use of CeO2 as solar or room light activated catalysts, because a major part of sunlight consists of visible light (~45%) and only 3–5% UV light. Research in the field of photocatalyst aims to increase the degradation efficiency in the visible light. Therefore, many efforts have been explored to extend the absorption wavelength of CeO2 in the visible region by using metal doping, semiconductor coupling and so on [18,19]. The coupled semiconductor photocatalysts ⁎ Corresponding author at: Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India. Tel.: +91 44 2220 2802; fax: +91 44 22353309. E-mail address: [email protected] (A. Stephen). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.07.034
improve the charge separation and enhance the photocatalytic activity due to high efficiency of the interfacial charge transfer. The coupled semiconductor materials have two different energy-level systems which play an important role in achieving charge separation. Coupling of different semiconductor oxides can reduce the bandgap, extending the absorbance range to visible light region, leading to electron–hole pair separation under irradiation and consequently achieving a higher photocatalytic activity [20]. In the narrow bandgap semiconductor, CuO has been coupled with some wide bandgap semiconductors to develop the photocatalytic efficiency [21,22]. Also vanadium pentoxide (V2O5) has relatively smaller bandgap (2.8 eV) and is expected to be an efficient photocatalyst under visible light [23]. Hence, for the effective utilization of visible light the bandgap of CeO2 can be reduced by coupling with CuO and V2O5 separately. In this present study, CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO nanocatalysts were synthesized by thermal decomposition method. All the prepared samples were characterized by various techniques and used for the photodegradation of methylene blue under visible light irradiation. 2. Experimental section 2.1. Materials Cerium acetate (Aldrich), copper acetate (Aldrich) and ammonium metavanadate (Aldrich) used in this present study were of analytical reagent grade. Methylene blue (MB) dye was purchased from Aldrich
4726
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
chemicals. All the chemicals were received and used without further purification. Throughout the experiment, all aqueous solution was prepared using double distilled water. 2.2. Preparation of nanosemiconductors (CeO2, V2O5 and CuO) Semiconductors of CeO2, V2O5 and CuO were synthesized via thermal decomposition method as follows: 3 g of cerium acetate, copper acetate and ammonium metavanadate was taken and each was weighed separately and grounded in a mortar for 3 h at room temperature and then each was annealed in an alumina crucible at 400 °C for 30 min separately. During this process, the temperature was raised at the rate of 4 °C/min. The samples were cooled at room temperature after the heat treatment. In comparison with other methods, this preparation method is simple, fast, cost effective, environmentally friendly and no special equipments were required. 2.3. Preparation of nanocomposites (CeO2/V2O5 and CeO2/CuO) A composite of CeO2/V2O5 was prepared by simple thermal decomposition method. Amount of cerium acetate and ammonium metavanadate (90:10 weight ratio) was mixed and grounded well for 3 h and then annealed in an alumina crucible at 400 °C for 30 min in a muffle furnace. Composite of CeO2/CuO was also prepared by the same method. Amount of cerium acetate and copper acetate (90:10 weight ratio) was weighed separately, then mixed and grounded well for 3 h and then annealed in an alumina crucible at 400 °C for 30 min in a muffle furnace. 2.4. Photocatalytic experiment 2.4.1. Measurement of photocatalytic degradation under UV light The photocatalytic procedure by our previous report is followed [24]. The schematic diagram of the UV-light photocatalytic experimental set-up is represented in Supporting Fig. 1. The photocatalytic activity of CeO2 catalyst was measured in terms of methylene blue (MB) decoloration. This photo-decolorization experiment was executed in a photocatalytic chamber integrated with an 8 W mercury vapor lamp (365 nm) positioned at the vessel axis and magnetic stirrer arrangement within the enclosure. In order to avoid direct contact of mercury lamp with the aqueous solution, it was roofed by quartz tube. 500 mg of CeO2 catalyst was suspended in 500 ml aqueous solution of MB with an initial concentration of 3 × 10−5 mol/l. The adsorption equilibrium between the dye and surface of catalyst was maintained by stirring in dark condition for 30 min before exposed to UV light. During irradiation, suspension was syringed at regular intervals of time and centrifuged to wipe out the catalysts. The absorption of MB was recorded on UV–Visible spectra at room temperature. 2.4.2. Measurement of photocatalytic degradation under visible light The photocatalytic activity under visible light in the literatures is followed [25,26]. The photograph and schematic diagram of the visible light photocatalytic chamber are represented in Supporting Fig. 2. The photocatalytic activity of CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO was investigated by the decomposition of MB in an aqueous solution under visible light irradiation. The visible light irradiation was carried out using a projection lamp (7748XHP 250 W, Philips, 532 nm) in a photoreactor. A 0.5% aqueous K2Cr2O7 solution circulating in the glass jacket was used for the purpose of UV cut-off. Reaction suspensions were prepared by adding 500 mg of the catalysts in 500 ml of MB solution taken within the initial concentration of 3 × 10−5 mol/l. The aqueous suspensions were magnetically stirred in the dark condition for 30 min to ensure adsorption equilibrium of MB on the catalyst surface before visible light illumination. Then, the suspension was irradiated under visible light under continuous stirring at room temperature, to obtain full suspension of the particles throughout the experiment. During irradiation, the suspensions were sampled at every 30 min and
immediately centrifuged to remove the catalyst particles and then, the absorption of MB aqueous solutions was measured by a UV–Vis spectrophotometer at a wavelength of 664 nm. Degradation efficiency can be calculated using the following equation: η ¼ ð1−C=C0 Þ 100
ð1Þ
where, C0 and C are the concentrations of the solution before illumination (t = 0) and after illumination for t minutes respectively. 2.4.3. Textile effluent The textile effluent was collected from dye industries (Tirupur, Tamilnadu, India). Before photocatalytic process, 50 ml of the textile effluent was diluted with 450 ml of distilled water (1:9 ratio) in order to reduce its opaque property. Therefore, the light can pass through the entire suspension without any loss. Initially, the diluted effluent was irradiated with visible light in the absence of catalyst. The result showed that there is no decoloration in the diluted effluent. It was concluded that the diluted effluent did not undergo any self decomposition. During the photocatalytic experiment, 500 mg of the catalyst was mixed with 500 ml of diluted dye. The reaction mixtures were stirred for 60 min under dark condition to establish adsorption–desorption equilibrium condition. The samples from the suspension were collected at regular intervals of time, centrifuged and filtered. Further, the irradiated sample was analyzed using a UV–Visible spectrophotometer. 2.5. Characterization details The structure of the prepared sample was analyzed using X-ray diffraction (XRD) analysis. The XRD was carried out by a Rich Siefert 3000 diffractometer using Cu Kα1 radiation (λ = 1.5406 Å). High resolution scanning electron microscopy (HR-SEM) and energy dispersive X-ray spectroscopy (EDX) analysis were carried out using FEI quanta FEG 200. The surface oxidation state and the presence of element in the sample were studied using XPS. The XPS spectra were carried out using an omicron ESCA spectrometer with monochromatized Al Kα radiation. The optical absorption spectrum was obtained on a CARY 5E UV–VIS– NIR spectrophotometer. The photocatalytic activity was measured by a Perkin–Elmer UV–Visible spectrometer RX1. 3. Results and discussions 3.1. Structural analysis The CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO samples prepared by thermal decomposition method were characterized by powder X-ray diffraction (XRD) technique to study their crystal structure, phase purity and crystallite size. The XRD patterns of the CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO samples were shown in Fig. 1. Fig. 1 (a) shows the XRD pattern of pure CeO2 nanoparticles. Diffraction peaks correspond to (111), (200), (220), (311), (222) and (400) lattice planes and exhibit face centered cubic structure of pure CeO2, which agrees well with standard JCPDS card no: 43-1002 with lattice parameters a = b = c = 5.411 Å. The XRD pattern of V2O5 is shown in Fig. 1 (b). All of the diffraction peaks correspond to (200), (010), (110), (210), (101), (400), (011), (111), (301), (501), and (002) lattice planes in this pattern and are assigned to orthorhombic structure of pure V2O5, which are well comparable with JCPDS card no: 89-0611. The lattice parameter values are a = 11.54 Å, b = 4.383 Å, and c = 3.571 Å. The Xray diffraction peaks given in Fig. 1 (c) assigned as (110), (111), (111), (111), (202), (020), (202), (113), (311), and (113) are found to match well with the monoclinic structure of CuO (JCPDS card no: 89-5899) with lattice parameters a = 4.689 Å, b = 3.420 Å, and c = 5.130 Å. Two types of phases were detected in Fig. 1 (d). One of the phases is well indexed to face centered cubic CeO2 (JCPDS card no: 43-1002) and the other one is identified to orthorhombic phase of V2O5 (JCPDS
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
4727
Table 2 Comparison of the d spacing value for all prepared semiconductors (CeO2, V2O5, and CuO). Sample code
Crystal structure
CeO2
Cubic
V2O5 CuO
Orthorhombic Monoclinic
dhkl formula dhkl ¼ dhkl ¼ dhkl ¼
−1 ðh2 þk2 þl2 Þ 2
Hkl
Calculated dspacing (Å)
JCPDS dspacing (Å)
111
3.125
3.124
010
4.355
4.383
111
2.522
2.522
a2
h
2
2
h2 a2
2
þ k2 þ cl 2
h a2
i−12
b
2
cosβ þ l 2 −2hlac c
sin2 β
2
þ k2
−12
b
XRD pattern indicates the formation of nanosized CeO2, V2O5, CuO, CeO2/V2O5 and CeO2/CuO materials.
3.2. Surface oxidation state analysis
Fig. 1. XRD pattern of all prepared samples.
no: 89-0611). However, the XRD pattern of CeO2/V2O5 is weak due to the low content of V2O5. From Fig. 1 (e), it was found that the cubic structure of CeO2 has co-existed with the monoclinic phase of CuO. In this XRD pattern, all the diffraction peaks are assigned to CeO2 and CuO as reported in JCPDS card nos: 43-1002 and 89-5899 respectively. Also this XRD pattern seems quite similar to that of pure CeO2 due to low content of CuO. No other characteristic peaks such as Ce2O3 and Cu2O were observed which suggests that the prepared nanocomposites are composed of CeO2 and CuO only. The lattice parameter values are listed in Table 1. The peaks of the nanocomposite (CeO2/V2O5 and CeO2/CuO) systems are slightly shifted compared with pure systems (CeO2, V2O5 and CuO). The deviation of nanocomposites from that of individual system suggests intra-granular coupling of the composites. Intra-granular coupling provides close contact between both phases [27]. The d spacing value for all pure semiconductors was theoretically calculated using their respective d spacing formula and the values are given in Table 2. The crystallite size of the prepared samples was calculated based on Scherrer's formula. The broadening of peaks in XRD pattern indicates the nanocrystalline nature of synthesized samples. Crystallite size for the maximum diffracted peaks of all synthesized samples was calculated and the values are given in Table 3. Hence, the
XRD result did not confirm the oxidation state of the present element. So, the surface oxidation state was analyzed through XPS measurement. Fig. 2 (a) and (b) represents the XPS spectrum of CeO2/ V2O5 and CeO2/CuO respectively. Fig. 2 (a) indicates that the CeO2/ V2O5 nanocomposite sample was composed of Ce4+, V5+, O and C. Fig. 2 (b) reveals that the CeO2/CuO nanocomposite sample consists of Ce4+, Cu2+, O and C. In both the samples, small amount of carbon (C1s) was present. The carbon peak arises itself by pumping oil in the vacuum system of XPS instrument. Hence, the XPS result confirmed that the CeO2/V2O5 and CeO2/CuO nanocomposites were presented with respective oxidation states without any impurities. This is in good agreement with the XRD result.
3.3. Morphology and elemental analysis High resolution scanning electron microscopic (HR-SEM) images of CeO2, CeO2/V2O5 and CeO2/CuO were shown in Fig. 3 (a, b & c). All the images were taken in 500 nm scale with 60,000 magnifications. From Fig. 3 (a), it was observed that CeO2 nanoparticles are spherical in shape and the diameter was in the range of 60–80 nm. HR-SEM image of CeO2/V2O5 and CeO2/CuO (Fig. 3 (b & c)) reveals irregular shaped particles which are due to the agglomeration of tiny spherical shaped particles. The elemental analyses of CeO2, CeO2/V2O5 and CeO2/CuO samples were examined by energy dispersive X-ray spectroscopy measurement and are illustrated in Supporting Fig. 3 (a, b & c). Supporting Fig. 3 (a) shows that the CeO2 nanoparticles were composed of Ce and O elements only and no other impurities were detected. The EDX spectrum of CeO2/V2O5 coupled catalysts (Supporting Fig. 3 (b)) indicated the existence of Ce, V and O elements in the sample. The EDX spectrum of CeO2/CuO coupled semiconductor (Supporting Fig. 3 (c)) also confirmed the existence of Ce, Cu and O elements in the sample.
Table 1 Lattice parameter values for all prepared samples. Sample code
CeO2 V2O5 CuO CeO2/V2O5 CeO2/CuO
Cerium oxide (CeO2)
Vanadium pentoxide (V2O5)
Copper oxide(CuO)
43-1002
89-0611
89-5899
Table 3 Crystallite size (D) and bandgap values for all synthesized samples. Sample code
Cubic Orthorhombic structure structure
Monoclinic structure
a (Å)
a (Å)
b (Å)
c (Å)
a (Å)
b (Å)
c (Å)
5.41 (5) – – 5.40 (3) 5.40 (4)
– 11.40 (1) – 11.56 (3) –
– 4.35 (4) – 4.35 (8) –
– 3.49 (5) – 3.54 (1) –
– – 4.70 (1) – 4.53 (8)
– – 3.43 (3) – 3.41 (2)
– – 5.12 (5) – 5.14 (1)
CeO2 V2O5 CuO CeO2/V2O5 CeO2/CuO
Cerium oxide (CeO2)
Vanadium pentoxide (V2O5)
Copper oxide (CuO)
Crystallite size (nm)
Crystallite size (nm)
Crystallite size (nm)
~13 – – ~11 ~9
– ~34 – ~12 –
– – ~36 – ~18
Bandgap value (eV)
3.28 2.66 1.83 2.62 2.59
4728
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
Fig. 2. XPS spectrum of (a) CeO2/V2O5 and (b) CeO2/CuO.
3.4. Bandgap analysis The bandgap of all prepared samples was investigated by UV–Vis diffuse reflectance spectra. The comparison of diffuse reflectance spectrum of CeO2/V2O5 and CeO2/CuO nanocomposites with those spectra of pure CeO2, V2O5 and CuO semiconductors is represented in Fig. 4 (a, b, c, d & e). Pure CeO2 has strong absorption in UV region of spectrum at around 378 nm, while pure V2O5 and CuO catalysts have strong absorption onset in visible region at 466 nm and 676 nm respectively. Compared to pure CeO2, the absorption edge of CeO2/V2O5 and CeO2/CuO nanocomposites has shifted towards the visible region due to the presence of lower bandgap semiconductors. The absorption edge at higher wavelengths revealed that the coupled catalysts can efficiently utilize the visible light for photocatalytic purpose. Further, the optical bandgap energy of all prepared samples was calculated using the following relation [28] and listed in Table 3. Eg ¼ 1240=λðeVÞ
ð2Þ
where, Eg represents bandgap energy (eV), and λ represents lower cutoff wavelength in nanometer. This result suggested that the prepared CeO2/V2O5 and CeO2/CuO nanocomposites can be photoexcited to generate more electron–hole pairs under visible light irradiation, which could result in higher photocatalytic performance. 4. Photocatalytic activity 4.1. MB degradation using CeO2 catalysts under UV light The photocatalytic degradation of MB using CeO2 catalyst was studied under UV light irradiation. The disappearance of the band at 664 nm indicates that MB has been photodegraded. The photocatalytic degradation efficiency of MB was found to be 84.5% (using Eq. (1). The photocatalytic mechanism is explained by the following equation based on earlier reports [29,30]. The schematic diagram (Supporting Fig. 4) represents the photocatalytic mechanism of CeO2 under UV light irradiation. When the presence of energy in UV light (photon) is equal to or greater than the bandgap of CeO2, the electrons receive the energy and transfer of electrons takes place from valence band (VB) to conduction band (CB) which results in the formation of a hole (h+) in the VB and an electron (e−) in the CB. The holes react with water and generate OH• radical, which can oxidize the organic pollutants. In the conduction band the electrons
react with oxygen and undergo reduction process and produce OH• radical. These radicals reduce the organic pollutants. This oxidation and reduction processes were capable of degrading the (MB) organic pollutants under UV light irradiation. þ
−
CeO2 þ hv→CeO2 ðhVB þ eCB Þ þ
þ
hVB þ H2 O→H þ OH −
•
ð4Þ
−•
eCB þ O2 →O2 −•
þ
•
þ
ð5Þ •
O2 þ H →HO2
ð6Þ
−
HO2 þ H þ eCB →H2 O2 −
•
H2 O2 þ eCB →OH þ OH þ
−
ð3Þ
hVB þ OH →OH
ð7Þ −
•
•
OH þ organic pollutants→degradation products
ð8Þ ð9Þ ð10Þ
4.2. MB degradation using CeO2, V2O5 and CuO catalysts under visible light The pure CeO2, V2O5 and CuO catalysts were used in the degradation of aqueous MB solution under visible light irradiation. There is no decoloration in the presence of pure CeO2 under visible light because the bandgap of CeO2 is 3.28 eV (from DRS). Thus, there is insufficient energy for the electron excitation between valence band and conduction band and no electron–hole pairs are generated under visible light irradiation. The absorption spectrum of MB has decreased gradually with increase in irradiation time using V2O5 and CuO catalysts. The photocatalytic degradation efficiency for all prepared samples was determined using Eq. (1) and the results are listed in Table 4.
4.3. MB degradation using nanocomposites under visible light The photocatalytic activities of CeO2/V2O5 and CeO2/CuO nanocomposites were examined by the decomposition of MB under visible light. The photocatalytic degradation efficiency for all prepared samples was determined using Eq. (1) and the values are listed in Table 4. This result suggested that the nanocomposites show higher photocatalytic
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
4729
Fig. 4. Diffuse reflectance spectrum (DRS) of all prepared samples.
4.5. Photocatalytic mechanism for coupled semiconductors 4.5.1. CeO2/V2O5 composites The photocatalytic mechanism of CeO2/V2O5 composite semiconductors is shown in Fig. 6. In order to determine the mechanism of the enhanced photocatalytic activity of the CeO2/V2O5 composite, the relative band positions of the two semiconductors were investigated, since the band-edge potential levels play a crucial role in determining the flowchart of photoexcited charge carriers in coupled semiconductors. The valence band (VB) edge and the conduction band (CB) edge positions of CeO2/V2O5 can be calculated from empirical formulae [31]. The electro negativity (X) values of the semiconductors (CeO2 [19] and V2O5 [32]) are 5.56 eV and 2.88 eV respectively. The calculated CB and VB edges of CeO2 were −0.58 eV and 2.7 eV, and of V2O5 were 0.27 eV and 2.93 eV respectively. The photocatalytic mechanism of the composite (CeO2/V2O5) is explained by the following equations [33–35]: CeO2 =V2 O5
visible light
→
−
− þ CeO2 =V2 O5 eCB þ hVB
þ
þ
ð11Þ −
CeO2 =V2 O5 ðeCB þ hVB Þ→V2 O5 ðhVB Þ þ CeO2 ðeCB Þ þ
−
hVB þ OH →OH −
•
ð13Þ
−•
eCB þ O2 →O2
ð14Þ
−•
•
H2 O þ O2 →OOH þ OH
−
•
Fig. 3. HR-SEM images of a) CeO2 and b) CeO2/V2O5 and c) CeO2/CuO.
2OOH →O2 þ H2 O2 −•
degradation efficiency than those of pure semiconductors within 210 min under visible light irradiation.
4.4. Textile effluent degradation using nanocomposites under visible light Fig. 5 (a & b) illustrates the time-dependent absorption spectra of textile effluent during visible light irradiation in the presence of CeO2/ V2O5 and CeO2/CuO catalysts with uniform irradiation of time. The figures show that the absorption peak gradually decreases with increase in exposure time. The nanocomposite (CeO2/V2O5 and CeO2/CuO) materials were found to effectively degrade the industrial effluent with an efficiency of 76.9% and 85.7% respectively.
•
−•
þ
ð15Þ ð16Þ
−
H2 O2 þ O2 →OH þ OH þ O2 •
ð12Þ
OH þ O2 þ hVB þ pollutants→degraded pollutants→CO2 ↑ þ H2 O
ð17Þ ð18Þ
Photocatalytic process is based on electron–hole recombination by means of bandgap excitation. As shown in Fig. 6, V2O5 with narrow bandgap energy (2.66 eV) could be easily excited by visible light and induce the generation of photoelectrons and holes. However, these photoelectrons and holes might recombine rapidly. That is why V2O5 shows poor photocatalytic activity. In case of CeO2, it could not be excited by visible light irradiation due to its wide bandgap of about 3.28 eV in this present work. Therefore, MB could not be degraded on CeO2 as well. When the CeO2/V2O5 nanocomposite is irradiated under visible
4730
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
Table 4 The degradation efficiency for all prepared samples (in percentage) with respective irradiation time. Sample code
CeO2 V2O5 CuO CeO2/V2O5 CeO2/CuO
Methylene blue (MB) (in percentage) 30 min
60 min
90 min
120 min
150 min
180 min
210 min
0.9 7.5 3.1 3.9 11.9
1.4 10.1 6.2 16.6 17.8
3.1 17.6 8.7 22.9 21.5
3.9 21.2 12.6 30.1 34.3
4.2 22.9 22.5 40.4 49.3
5.1 24.7 29.3 53.5 56.5
6.1 27.5 33.4 64.2 70.1
light, V2O5 could act as a sensitizer to absorb the visible light. Under visible light illumination, the electrons in the valence band of V2O5 are excited to the conduction band with simultaneous generation of the same amount of holes in the valence band. The photoactivated electrons in the conduction band of V2O5 are injected into the conduction band of CeO2. The injection of electrons from the conduction band of V2O5 into CeO2 particles was expected to retard the back reaction between the photogenerated charge carriers. This process of the electron–hole trapping which is facilitated in the mixed semiconductor system would increase the efficiency. Thus, the efficiency of photogenerated electron–hole in CeO2/V2O5 could be higher than those of pure semiconductors. Also CeO2/V2O5 photocatalyst shows red shift in the absorption
Fig. 6. Schematic diagram represents the charge transfer pathway during MB degradation process over CeO2/V2O5 nanocomposites under visible light irradiation.
wavelength range compared to CeO2, which would improve its photocatalytic activity in the visible range. 4.5.2. CeO2/CuO composites The photocatalytic mechanism of CeO2/CuO nanocomposites under visible light irradiation is shown in Fig. 7. The calculated CB and VB edges of CeO2 were −0.58 eV and 2.7 eV, and of CuO were 0.39 eV and 2.22 eV respectively. The electronegativity of CeO2 and CuO is 5.56 eV and 5.81 eV [36]. The photocatalytic mechanism of coupled semiconductor is explained by the following equations: visible light − þ CeO2 =CuO → CeO2 =CuO eCB þ hVB −
þ
þ
ð19Þ −
CeO2 =CuOðeCB þ hVB Þ→CeO2 ðhVB Þ þ CuOðeCB Þ þ
−
hVB þ OH →OH −
•
ð21Þ
−•
eCB þ O2 →O2
ð22Þ
−•
•
H2 O þ O2 →OOH þ OH
−
•
2OOH →O2 þ H2 O2 −•
•
−•
þ
ð23Þ ð24Þ
−
H2 O2 þ O2 →OH þ OH þ O2 •
ð20Þ
OH þ O2 þ hVB þ pollutants→degraded pollutants→CO2 þ H2 O
ð25Þ ð26Þ
When CeO2/CuO composite semiconductor is irradiated under visible light, only CuO catalysts can be activated since the observed bandgap energy of CuO in this study is 1.83 eV, whereas the electrons in the valence band of CeO2 can move to the valence band of V2O5. The holes in valence band of CeO2 and electrons in conduction band of CuO undergo oxidation process and reduction process respectively. In such a way, photoinduced electron hole pairs are efficiently separated and reduced the electron–hole recombination. Therefore, CeO2/CuO composites exhibit enhanced photocatalytic performance. 5. Conclusion
Fig. 5. Change in UV–Visible absorption spectra of textile effluent using (a) CeO2/V2O5 and (b) CeO2/CuO nanocatalysts under visible light irradiation.
The photocatalytic degradation test reveals that the prepared nanocomposite systems (CeO2/V2O5 and CeO2/CuO) exhibit high efficient visible light photocatalytic activities as compared to that for the pure systems (CeO2, V2O5, CuO) for the degradation of model methylene blue dye and textile effluent. The higher photocatalytic efficiencies are
R. Saravanan et al. / Materials Science and Engineering C 33 (2013) 4725–4731
Fig. 7. Schematic diagram represents the charge transfer pathway during MB degradation process over CeO2/CuO nanocomposites under visible light irradiation.
attributed to the superior charge separation by the nanocomposite system, thereby permitting the photogenerated electrons and holes to have enough time to contribute to the overall photocatalytic reactions. The photocatalytic activity of nanocomposite systems can have significant impact on the future improvement of highly capable visible-light catalysts for textile pollutant degradation. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2013.07.034. Acknowledgments We acknowledge the National Centre for Nanoscience and Nanotechnology, University of Madras, India for XPS analysis. References [1] X. Chen, Samuel S. Mao, Chem. Rev. 107 (2007) 2891–2959. [2] V.K. Gupta, A. Rastogi, A. Nayak, J. Colloid Interface Sci. 342 (2010) 533–539. [3] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, J. Colloid Interface Sci. 344 (2010) 497–507.
4731
[4] A.K. Jain, V.K. Gupta, A. Bhatnagar, Suhas, Sep. Sci. Technol. 38 (2003) 463–481. [5] V.K. Gupta, S.K. Srivastava, D. Mohan, S. Sharma, Waste Manage. 17 (1998) 517–522. [6] A.K. Jain, V.K. Gupta, A. Bhatnagar, Shubhi Jain, Suhas, J. Indian Chem. Soc. 80 (2003) 267–270. [7] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, J. Colloid Interface Sci. 342 (2010) 518–527. [8] V.K. Gupta, A. Mittal, A. Malviya, J. Mittal, J. Colloid Interface Sci. 335 (2009) 24–33. [9] A. Mittal, D. Kaur, A. Malviya, J. Mittal, V.K. Gupta, J. Colloid Interface Sci. 337 (2009) 345–354. [10] V.K. Gupta, B. Gupta, A. Rastogi, S. Agarwal, A. Nayak, J. Hazard. Mater. 186 (2011) 891–901. [11] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, J. Colloid Interface Sci. 340 (2009) 16–26. [12] V.K. Gupta, S. Agarwal, T.A. Saleh, Water Res. 45 (2011) 2207–2212. [13] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, J. Colloid Interface Sci. 343 (2010) 463–473. [14] V.K. Gupta, S. Agarwal, T.A. Saleh, J. Hazard. Mater. 185 (2011) 17–23. [15] A. Mittal, V.K. Gupta, A. Malviya, J. Mittal, J. Hazard. Mater. 151 (2008) 821–832. [16] J. Liqianga, S. Xiaojun, S. Jing, C. Weimin, X. Zili, D. Yaoguo, F. Honggang, Sol. Energy Mater. Sol. Cells 79 (2003) 133–151. [17] F. Chen, Y. Cao, D. Jia, Appl. Surf. Sci. 257 (2011) 9226–9231. [18] A. Aranda, et al., Chem. Commun. 48 (2012) 4704–4706. [19] N. Wetchakun, et al., Appl. Mater. Interfaces 4 (2012) 3718–3723. [20] C. Xu, L. Cao, G. Su, W. Liu, Y. Yu, X. Qu, J. Hazard. Mater. 176 (2010) 807–813. [21] Z.L. Liu, J.C. Deng, J.J. Deng, F.F. Li, Mater. Sci. Eng. B 150 (2008) 99–104. [22] G. Li, N.M. Dimitrijevic, L. Chen, T. Rajh, K.A. Gray, J. Phys. Chem. C 112 (2008) 19040–19044. [23] H.Y. Jang, T.A. Quand-Thao, H.O. Xuan-Huong, M.H.O. Sun-il, J. Korean Phys. Soc. 55 (2009) 2447–2450. [24] R. Saravanan, H. Shankar, G. Rajasudha, V. Narayanan, A. Stephen, Int. J. Nanosci. 10 (1 & 2) (2011) 253–257. [25] R. Saravanan, H. Shankar, T. Prakash, V. Narayanan, A. Stephen, Mater. Chem. Phys. 125 (2011) 277–280. [26] R. Saravanan, S. Karthikeyan, V.K. Gupta, G. Sekaran, V. Narayanan, A. Stephen, Mater. Sci. Eng. C 33 (2013) 91–98. [27] D. Zhang, Transit. Met. Chem. 35 (2010) 689–694. [28] Dhananjay S. Bhatkhande, Vishwas G. Pangarkar, Anthony ACM Beenackers, J. Chem. Tech. Biotech. 77 (2011) 102–116. [29] I.K. Konstantinou, T.A. Albanis, Appl. Catal. B Environ. 49 (2004) 1–14. [30] G. Li Puma, A. Bono, D. Krishnaiah, Joseph G. Collin, J. Hazard. Mater. 157 (2008) 209–219. [31] Z. Zhang, W. Wang, L. Wang, S. Sun, Appl. Mater. Interfaces 4 (2012) 593–597. [32] X. Yong, M.A.A. Schoonen, Am. Mineral. 85 (2000) 543–556. [33] N. Daneshvar, D. Salari, A.R. Khataee, J. Photochem. Photobiol. A Chem. 162 (2004) 317–322. [34] A. Akyol, H.C. Yatmaz, M. Bayramoglu, Appl. Catal. B: Environ. 54 (2004) 19–24. [35] R. Qiu, D. Zhang, Y. Mo, L. Song, E. Brewer, X. Huang, Y. Xiong, J. Hazard. Mater. 156 (2008) 80–85. [36] Y. Xu, M.A.A. Schoonen, Am. Mineral. 85 (2000) 543–556.