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n-CdO heterojunction nanocomposite for enhanced photodegradation of organic pollutants under visible light irradiation
Journal of Rare Earths 37 (2019) 853e860
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A novel n-CeO2/n-CdO heterojunction nanocomposite for enhanced photodegradation of organic pollutants under visible light irradiation Karunamoorthy Saravanakumar a, b, Subramani Muthupoongodi c, Velluchamy Muthuraj a, * a b c
Department of Chemistry, V.H.N.S.N College, Virudhunagar, 626 001, Tamilnadu, India Department of Chemistry, Sri Kaliswari College, Sivakasi, 626 130, Tamilnadu, India Department of Chemistry, Thiagarajar College, Madurai, 625 009, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2018 Received in revised form 8 December 2018 Accepted 10 December 2018 Available online 3 April 2019
In this study, a series of novel visible light driven n-CeO2/n-CdO heterojunction (CeO2/CdO) nanocomposites were successfully fabricated by simple ultrasonication method. Several characterization tools including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-vis diffuse reflectance spectroscopy (UV-DRS), etc., were utilized to investigate the physicochemical properties of the catalyst and confirm the formation of heterojunction. Under visible light irradiations, the photocatalytic activities of the as-prepared CeO2/CdO nanocomposites were evaluated by degrading of Congo red (CR) and Rhodamine B (RhB) solutions. As a result, the CeO2/CdO (mass percentage ratio 1:3) nanocomposite displays remarkable performance for CR and RhB degradation. The enhancement in the photocatalytic performance of CeO2/CdO (1:3) nanocomposite can be attributed not only to the strong visible-light absorption region, separating the photogenerated electronhole pairs but also to the formation of n-n type heterojunction. The results also indicate that the CeO2/ CdO (1:3) nanocomposite has good stabilization and high reusability. In addition, the mechanism is proposed for the coupled semiconductors and possible reasons for the enhancement of visible-light photocatalytic efficiency are also discussed. This work can provide a new gateway to fabricate visible photocatalysts and promising candidate catalysts for poisonous wastewater treatment in the near future. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: n-CeO2/n-CdO Heterojunction Visible light Photocatalysis Photodegradation Rare earths
1. Introduction Over the past few years, the growing number of contaminants generated by human activities presents one of the formidable challenges to the sustainable development of modern human society.1e4 Various kinds of organic contaminants enter into water sources through industrial wastewater discharge. Wastewater generally contains large amount of organic pollutants (such as organic dyes, pesticides and antibiotics) which have caused serious hazard to human and aquatic ecosystems. Among them, organic dyes generated from textiles, leather, paper-making, dye houses and printing industries are considered as the primary pollutants in wastewater.5,6 Thus their elimination of organic pollution is a vital environmental issue. Traditional treatment processes including
* Corresponding author. E-mail addresses: [email protected], [email protected] (V. Muthuraj).
adsorption, reverse osmosis, chlorination, chemical oxidation, coagulation and flocculation, biological degradation and ozonolysis are often inadequate in treating many of the new, emerging micropollutants.7e9 Unfortunately, most of these physical, chemical and biological techniques can form secondary pollutants easily during degradation processes.10e12 Therefore, more effective wastewater treatment technologies are highly welcome. Advanced oxidation processes (AOPs) semiconductor photocatalysts have been widely studied for the degradation of toxic contaminants in environment remediation applications.13e15 The degradation and mineralization of dyes using nanostructured semiconductor under visible light has attracted great interest in recent years.16 The mechanism of photocatalytic degradation of dye molecules under visible light excitation is different from the traditional mechanism under UV irradiation. In these systems, the dye, rather than the semiconductor is subject to the visible light excitation. The excited dye molecule then transfers an electron into the conduction band of semiconductor, leading to the formation of
https://doi.org/10.1016/j.jre.2018.12.009 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
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a cationic radical of the dye. The injected electron then reacts with the oxygen adsorbed on the surface of photocatalyst nanoparticles and generates a series of reactive oxygen species (ROS) such as O2, OH, OH and H O . The subsequent radical chain reactions 2 2 involving photocatalytic process and the dye molecule lead to the degradation and mineralization of the dye pollutants.17 Cerium oxide (CeO2), an eco-friendly photocatalytic material, has stimulated great interest of the researchers due to its many fascinating properties, such as abundant mobile oxygen vacancies, high oxygen storage capability, excellent chemical stability and biocompatibility.18e20 Unfortunately pristine CeO2 can only be excited by little visible light absorption and has weak separation efficiency of photoinduced carriers because the wide bandgap energy (3.2 eV) hinders its widespread application in photocatalysis. Thus it is expected to extend light absorption of CeO2 into visible light region either by doping of metals/nonmetals or by forming heterojunctions between CeO2 and other narrow band gap semiconductors in order to generate visible light-driven catalysts.21,22 Coupled semiconductor photocatalysts may enhance the photocatalytic efficiency by improved charge separation and increasing the charge carrier lifetime. In particular, cadmium oxide (CdO) is a known n-type semiconductor with a direct band gap of 2.3e2.5 eV and an indirect band gap of 1.36e1.98 eV.23 In recent years, CdO nanomaterials have been used as an interconnection in a wide variety of commercial applications such as exhaust catalyst, UV absorbers, gas sensors, catalysis, solar cells, fuel cells, photodiodes, phototransistors and oxygen storage materials. Also, CdO is used as a visible-light-driven photocatalyst for water oxidation and organic pollutant degradation.24e27 Recently, CeO2/CdO nanocomposite has been reported by Magdalane et al. for the application of photocatalytic activity and antimicrobial activity. However, these results showed a slightly poor photocatalytic activity and catalyst preparation methods were too long.28,29 Herein, we reported a novel n-CeO2/n-CdO heterojunction photocatalyst which were successfully fabricated by ultrasonication method. The as-synthesized samples were characterized by various analytical techniques, XRD, FT-IR, SEM, TEM and UV-DRS were employed to study the crystal phase structures, morphologies and optical properties. Moreover, photocatalytic activity test was carried out and operational parameters such as catalyst loading, initial pH and dye solution concentration were examined. Ultimately, a plausible mechanism and photostability of the catalyst are also proposed in detail.
CeO2, the conditions are the same, except for the absence of CdO in the system. 2.3. Characterization techniques The crystal structure and phase of the synthesized nanocomposite was estimated by X-ray diffraction (XRD; PANalytical X'pert Pro.) in the 2q range from 10ºe80 with using mono chromatized Cu Ka (l ¼ 0.154178 nm) radiation. The surface morphology and microstructure of the catalysts were analyzed by scanning electron microscopy (SEM, VEGA3 TESCAN model) and transmission electron microscopy (PHILIPS CM 200 model). An energy dispersive spectrometer (EDX - Bruker Nano GmbH, X 50 flash Detector (model-5010)) was used to evaluate the elemental compositions of the nanocomposites. The structural information was carried out by the Fourier transform infrared spectroscopy (Shimadzu FT-IR 3000) using KBr as diluents in the range of 4000e400 cm1. A UV-vis diffused reflectance spectrometer (DRSmode) (UV-2450, Shimadzu) was used to determine the absorbance ranges of the samples and BaSO4 was used as a reflectance reference material. 2.4. Description about photocatalytic activity
2. Experimental
The photocatalytic activities of CeO2, CdO and CeO2/CdO catalysts were evaluated for the photocatalytic degradation of Rhodamine B dye (RhB) and Congo red (CR) dye solutions. In the photocatalytic reactions, 50 mg of catalyst was dispersed in dye solution with concentration of 10 mg/L in 100 mL reaction vessel under visible light irradiation. The characteristic absorption peaks of CR and RhB were chosen to monitor the photocatalytic degradation process. The lambda maximum for absorption wavelength of CR dye solution was observed at l ¼ 490 and 348 nm and RhB at l ¼ 553 nm. The organic dyes were subjected to radiation (l > 400 nm) for a definite period of time under tungsten incandescent lamp and intensity of the visible light is 150 mW/cm2. UV spectrum was recorded periodically with samples withdrawn from reaction vessel. The radiation was continued till the complete degradation is achieved. At given time intervals of 10 min irradiation, 3 mL of aliquots were collected. The degraded solutions were analyzed using the absorption peaks of corresponding dye. After centrifugation, the concentration of dye solution was measured with a Shimadzu 2600 UV-vis spectrometer. Finally during the degradation process, photocatalyst was separated from the reaction mixture and dried to carry out the reusability tests.
2.1. Materials
3. Results and discussion
All chemicals and regents used were of analytical regent grade and purchased from the Merck Company and were used without further purification.
3.1. XRD analysis
2.2. Preparation of CeO2/CdO photocatalyst CeO2/CdO catalyst was prepared by ultrasound assisted in situ chemical precipitation technique. 0.5 g of cerium nitrate hexahydrate was dissolved in 50 mL of deionized water, and then 1.5 g of cadmium nitrate was added into a reaction mixture and stirred for 30 min at room temperature. Under ultrasonic irradiation (300 W) at room temperature, 0.1 mol/L NaOH solution was added dropwise to raise the suspension pH to around 9 and the irradiation was continued for 30 min. Finally, the as-synthesized product was collected through centrifuged at 1000 r/min and repeatedly washed with deionized water/ethanol, and then dried in an oven at 80 C for 24 h and calcined at 500 C for 3 h. For the synthesis pure
To gain information about the crystalline structure and phase purity of the as-synthesized samples, XRD analyses were carried out. As is shown in Fig. 1, the peaks at 28.29 , 32.86 , 47.48 , 56.41, 59.05 , 69.21 and 76.71 are indexed to the (111), (200), (220), (311), (222), (400) and (311) crystal planes, which was in good agreement with the cubic phase structure of CeO2 (JCPDS card No: 89-8436). There are a number of intensive peaks observed at 33.07, 38.28 , 55.34 , 66.01 and 69.43 , which could be indexed to the (111), (200), (220), (311) and (222) crystal planes for cubic structure of CdO (JCPDS Card No. 05-0640). In the nanocomposite, both the diffraction peaks of the cubic phase structure were observed for CeO2 and CdO which clearly indicate the phase pure formation of CeO2/CdO nanocomposite. The strong and sharp diffraction peaks suggested that the CeO2/CdO nanocomposite was well crystalline in nature. No characteristic peaks from other
K. Saravanakumar et al. / Journal of Rare Earths 37 (2019) 853e860
Fig. 1. XRD patterns of as-synthesized CeO2, CdO and CeO2/CdO nanocomposite.
crystalline forms were noticed, which demonstrates that the CeO2/ CdO has high purity. The average crystallite size (D) of pure CeO2, CdO and CeO2/CdO nanocomposite was calculated using the Scherrer's formula,30
D¼
0:9l b cos q
(1)
where D is average crystallite size, l X-ray wavelength, b full width at half maximum (FWHM), and q Bragg's angle. The average crystallite sizes for the pure CeO2, CdO and CeO2/CdO nanocomposite was found to be 17, 29 and 13 nm, respectively. 3.2. FTIR analysis FT-IR spectrum helps in determining the appropriate functional group present in the as-synthesized CeO2, CdO and CeO2/CdO nanocomposites. FT-IR spectra of pure CeO2, CdO and CeO2/CdO nanocomposite were carried out in the wavenumber range between 4000 and 400 cm1 and are shown in Fig. S1. A broad band in the range of 3500e3700 cm1 is due to the OH stretching of surface adsorbed water molecules.31 The bending vibration band of physically adsorbed water molecules are also observed at 1600 cm1. The presence of sharp dominated absorption peaks at 450e650 cm1 is related to the MO (M ¼ Ce and Cd) bonds and confirmed that the prepared nanocomposite is CeO2/CdO.32 Generally, in spinel oxide metaleoxygen stretching frequencies are observed in the range of 700e900 cm1. 3.3. Morphological analysis The microstructure and morphological of the as-synthesized samples were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM micro images of pure CeO2, CdO and CeO2/CdO nanocomposite are shown in Fig. 2. The pristine CeO2 shows granular-like morphology and the particles are closely attached with each other which are observed in Fig. 2(a). The particles are of spongy-like structure morphology and without any specific shape of CdO particles which are shown in Fig. 2(b). Interestingly, the CdO micro particles are attached onto the surface of the CeO2 and image is shown in Fig. 2(c). Obviously as the number of micro particles attached on the surface increases, the
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agglomeration also increases. Moreover, the surface has many irregular small granules with spongy shape. The hetero-structures of as-synthesized CeO2/CdO nanocomposite were further confirmed by HR-TEM analysis. CeO2/CdO nanocomposite is taken as the representative for TEM, and it is given in Fig. S2. The fine particle is the aggregate of nano hybrid and shape is more or less spherical and it reveals the polycrystalline cubic crystal structure. Fig. S2 (a, b) display that the particles are more agglomerated with irregular shape of morphology. From the HRTEM image, the particles are in the range of 100 to 5 nm, which is consistent with XRD results. As shown in the HRTEM image (Fig. S2(c)), the interplanar lattice fringes have spacing of d ¼ 0.31 nm and d ¼ 0.27 nm corresponding to the (111) crystal plane of CeO2 and (111) crystal plane of CdO, respectively. Additionally, the selected area electron diffraction (SAED) pattern which shows bright rings reveals that the nanocomposite is high crystalline nature with cubic structures as shown in Fig. S2(d). It is also well supported by the XRD results. The chemical elemental composition analysis of CeO2/CdO nanocomposite was carried out by energy dispersive X-ray spectroscopy (EDS) measurements. The EDS spectra, as shown in Fig. 2(d), confirm that the synthesized nanocomposite is impurity free nature. In the spectra, only three intense peaks were observed. These peaks belong to constituent elements of CeO2/CdO nanocomposite like cerium (Ce), cadmium (Cd) and oxygen (O) atoms. Furthermore, the EDS elemental mapping showed that green, red and blue represent the distribution of Ce, Cd and O, respectively as depicted in Fig. 3. All these experiments authenticate explicitly that CeO2/CdO heterojunction catalyst has been successfully prepared.
3.4. Optical properties The optical properties and energy band gap of the assynthesized samples were studied by UV-vis diffuse reflectance spectra (UV-vis DRS) as displayed in Fig. 4. The pure CeO2 and pristine CdO show photo absorption threshold at approximately 415 and 490 nm, respectively, which fitted well with previous literature.33,25 The CeO2/CdO heterojunction shifts the absorption edge value at a higher wavelength (red shift). The red shift may be due to the development of some localized band gap because of oxygen vacancies in the nanocomposites and also Ce3þ. The optical property of the absorbance of CeO2/CdO heterojunction in the visible region suggests that it may be used as a good candidate for visible absorbing semiconductor materials. CdO combined CeO2 could reduce the band gap energy of CdO/CeO2 heterostructure nanocomposite and increases the amount of Ce4þ states, resulting in the formation of localized energy states that are closer to the conduction band.34 To evaluate the band gap of the as-synthesized pristine CeO2, CdO and CeO2/CdO heterojunction, diffused reflectance spectroscopy was carried out in the wavelength range of 200e800 nm. The band gap energy was determined by fitting the absorption data to the direct transition equation, (ahn) ¼ A (hn Eg)n
(2)
where n is the transition frequency, h the Plank's constant, A a constant quantity, Eg the band gap energy of the material, a the absorption coefficient and n can have values 1/2, 3/2, 2 and 3 depending on the mode of inter-band transition, i.e., direct allowed and direct forbidden, indirect allowed and indirect forbidden transition, respectively. The band gap of the CeO2, CdO, and CeO2/ CdO heterojunction has been deduced from the Kubelka-Munk (KM) plots as shown in Fig. 4(bed). The band gap energy optically
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Fig. 2. SEM micrographs of pure CeO2 (a), pure CdO (b) and CeO2/CdO (c) and EDS spectra of CeO2/CdO nanocomposite (d).
Fig. 3. EDS elemental mapping analysis of CeO2/CdO nanocomposite cadmium (red), cerium (green) and oxygen (blue).
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Fig. 4. UV-visible absorption (DRS) spectra (a) and Tauc's plots of pure CdO (b), CeO2 (c) and CeO2/CdO nanocomposite (d).
obtained in such a way was that approximately CeO2, CdO and CeO2/CdO heterojunction are 2.86, 2.17 and 2.69 eV, respectively. 3.5. Photocatalytic activity The photocatalytic activities of the CeO2 with different loading ratios of CdO were estimated by the degradation of CR dye solution under visible light irradiation which is displayed in Fig. 5(a). The photocatalytic efficiency of pristine CdO and CeO2 for CR degradation was 42% and 58% within 60 min of irradiation, respectively. The photocatalytic performance was enhanced after composition of CeO2 with CdO. Obviously, (1:3) CeO2/CdO heterostructure exhibits the maximum photocatalytic efficiency among all CeO2/CdO nanocomposites. The higher photocatalytic activity here can be attributed to synergetic effect of the two metal oxides, this leads to the decrease in the band gap energy. It can be observed that the photocatalytic efficiency increases with decreasing the band gap energy values of the nanocomposite due to the decrease in the energy required for the production of conduction band electrons and valence band holes. In generally, the coupled semiconductors both recombination (negative role) and production (positive role) of photogenerated charge carriers (eehþ) play a vital role in the photocatalytic activity of the systems. Note that the photocatalytic activity enhanced when the loading content of CeO2 increased from 0.5 to 1, but further increment causes decrement in photocatalytic performance. When the loading amount of CeO2 was over 1.5 wt%, the separation efficiency of photogenerated charge carriers decreased and further restrained the photocatalytic reaction.35,36 In these systems, the suitable amount of CeO2 is an important factor for photocatalytic degradation process. In order to perform a blank experiment, there is no degradation observed in the absence of light as well as in the absence of catalyst. The loading dosage of CeO2 in CeO2/CdO heterostructure nanocomposites played a critical role in their photocatalytic activities. Finally, it could be concluded
that the photocatalytic performance of (1:3) CeO2/CdO heterostructure was better compared with another photocatalyst, so this catalyst was chosen as an optimized catalyst for the characterization and further photocatalytic reactions. The reaction kinetics curve of photocatalytic degradation of dye solutions were evaluated by Langmuir-Hinshelwood model, it follows the pseudo-first order kinetics. The rate constant was determined by the slope ln (C0/C) vs time (t), which is illustrated in Fig. 5(b). The estimated rate constant k values are 0.0018, 0.01058, 0.01657, 0.02523, 0.0652 and 0.03276 for without catalyst, pure CdO, pure CeO, CeO/CdO (0.5:3), CeO/CdO (1:3) and CeO/CdO (1.5:3), respectively. The k value increased in the order without catalyst < pure CdO < pure CeO < CeO/CdO (0.5:3) < CeO/CdO (1.5:3) < CeO/CdO (1:3). It concluded that CeO/CdO (1:3) has higher rate constant and it has 6.2 and 3.9 times higher than that of pure CdO and pure CeO, respectively. The CeO/CdO (1:3) nanocomposite possesses significantly high photocatalytic degradation of dyes as compared with other nanocomposition and undoped CeO and CdO.37e39 The photocatalytic performance of as-synthesized pure CeO2, CdO and CeO2/CdO heterojunction was investigated by the photodegradation of CR and RhB under visible light irradiation. As depicted in Fig. 6, for CeO2/CdO catalyst, the intensity of this characteristic peak value of organic dye gradually decreases with the increase of time which indicates the photodegradation of dye solutions. The initial absorbance of the peak disappeared completely after 60 min of visible light irradiation which indicates the cleavage of conjugated chromophore structure of dyes and conversion into small aromatic intermediates.40,41 The CeO2/CdO heterojunction exhibits much higher photocatalytic capability compared with pure CeO2 and CdO. The enhancement in the photocatalytic performance of CeO2/CdO can be attributed to the n-n heterojunction and efficient separation of photogenerated electron-hole pair recombination. The control experiment, without photocatalyst, no significant degradation of CR and RhB were
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Fig. 5. Photodegradation of CR dye solution in the presence of different catalysts (a) and the corresponding plots of lnC/C0 vs time (b) under visible light irradiation in the following conditions. CR dye concentration ¼ 10 mg/L, amount of catalyst ¼ 50 mg, pH ¼ ~9, reaction temperature ¼ room temperature and irradiation time ¼ 60 min.
observed as the time was prolonged, which indicates that the photolysis of CR and RhB could be neglected. The photocatalyst amount added will affect the photocatalytic activity on degradation of organic dye solutions. A series of experiments were conducted with the catalyst dosage varied from 10 to 75 mg at constant organic dye pollutant concentration and other parameters were kept same which are illuminated in Fig. S3. The photocatalytic activity was increased with increasing the amount of photocatalyst dosage at 10e50 mg. In this system, 50 mg of the photocatalyst had the highest photocatalytic activity compared with the other amount of catalyst dosages. But the increase in the catalyst amounts beyond 50 mg, the photocatalytic efficiency was decreased which may be due to the light blocking by excessive amount of catalyst and prevents the light penetrations.42e44 Finally in this reaction medium, 50 mg of the photocatalyst was optimum amount for the photocatalytic degradation of organic pollutants. Fig. S3(c, d) show the kinetics profile for the degradation of dyes at different catalyst dosages and the rate constant values are given in Table S1. It was noted that the photodegradation of dye solutions followed pseudo-first order kinetics. The influence of the initial organic dye solutions (CR and RhB) concentration on the photodegradation under visible light illumination was also evaluated by varying the concentrations from 10,
Fig. 6. The temporal UV absorption spectral changes of dye solutions in the presence of CeO2/CdO (1:3) nanocomposite CR (a) and RhB (b) at irradiation time interval of 60 min. Amount of catalyst ¼ 50 mg, dye concentration ¼ 10 mg/L, pH ¼ ~9 and reaction temperature ¼ room temperature.
20 and 30 mg/L and other reaction parameters are the same. In this case Fig. S4 shows that the photodegradation rate decreases with the increases of the dye concentration which may be due to the screening of visible light by the dye itself. At high dye concentration, the colored dye solution may hinder the light during the photocatalytic process and also can generate filter effect which decreases the production of reactive oxygen species (ROS).45e47 In other words, the greater amount of dye molecules competing for the degradation process and the photo generation of charge carries separation was significantly reduced. The reaction kinetics curves of dyes degradation at different dye concentrations are displayed in Fig. S4(c, d). The corresponding rate constant (k) values are summarized in Table S1. In addition, the pH of the dye solution is also an important parameter for the photodegradation process because the waste water may be in acidic or basic condition. The initial pH of the dye solution was controlled by the addition of either HCl or NaOH (0.1 mol/L). In order to investigate the effect of initial pH on photocatalytic decolorization of CR and RhB dye, four experiments were conducted at different pH values of 3, 5, 7 and 9, at the initial dye concentration of 10 mg/L and the catalyst amount of 50 mg which are illustrated in Fig. S5. Increase in the pH of CR and RhB dye
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solution from 3 to 5 led to mildly decreased efficiency in the decolorization of organic dyes. Further increase in pH from 5 to 9 had apparent increase in dye decolorization efficiency. Enhancement of photodegradation efficiency may be because the number of $OH radical produced high basic condition and lower pH condition, the Hþ ion screens the active adsorption sites on the surface of the catalyst.48e50 Furthermore, the corresponding reaction kinetics curves are illustrated in Fig. S5(c, d), and the rate constant results are tabulated in Table S1. To obtain better understanding on the CeO2/CdO heterojunction, their valence band (VB) and conduction band (CB) edge potential were calculated via Mulliken electronegativity theory as follows, EVB ¼ X E0 þ 0.5Eg
(3)
ECB ¼ EVB Eg
(4)
where X is the absolute electronegativity of the constituent atoms in the semiconductor, E0 the energy of free electrons measured on the hydrogen scale (4.5 eV), Eg band gap energy of the semiconductor materials. According to the above equations, the Evb values of CeO2 and CdO are calculated to be 2.62 and 2.06 eV, respectively and Ecb values are estimated to be 0.234 and 0.11 eV, respectively. The CeO2/CdO heterojunction photocatalyst improves the photocatalytic activity for the degradation of CR and RhB dye solutions under visible light irradiation. The CeO2/CdO heterojunction irradiated by visible light leads to the creation of photogenerated electron in conduction band and holes in the valence band. The possible photocatalytic mechanism involves the excitation of an electron in conduction band of CeO2 and thereby the valence band (VB) of CdO acts as sink for hole. The electron in the conduction band uptake an O2 molecule to form an O2¡ free radical and then O2¡ reacts with surface water molecules to produce the OH radical. Finally the active radicals react with CR dye molecules to form the mineralized compounds.51e53
þ CeO2 =CdO þ hv/CeO2 =CdO e cb þhvb
(5)
þ þ CeO2 =CdO e cb þ hvb /CeO2 ecb þ CdO hvb
(6)
$ O2 þ e cb /O2
(7)
þ $ 2O$ 2 þ 2H /2 OH
(8)
$ O$ 2 ; OH þ dye solutions/Mineralized products
(9)
The stability and recyclability of photocatalyst is a deciding factor for practical applications.54,55 The stability of the CeO2/CdO nanocomposite was tested by photocatalytic degradation of CR and RhB dye solution cycled for five times under the same conditions and the experimental results are shown in Fig. S6. For each cycle, the CeO2/CdO photocatalyst was recycled by centrifuging, washing and drying for the next run. There is no obvious decrease during the photodegradation process of CR and RhB organic dyes after five degradation-regeneration cycles. About 80% of CR and RhB dye pollutants can be degraded over CeO2/CdO catalyst after five cycles, which indicates the catalyst has superior photostability and excellent reusability. This CeO2/CdO heterojunction could promote to be a new candidate of reusable and active photocatalyst in environmental applications.
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4. Conclusions In summary, n-CeO2/n-CdO heterojunction photocatalyst were synthesized by facile one-pot ultrasonication method. The n-CeO2/ n-CdO nanocomposite was characterized by a range of techniques to study its morphology, crystal structure, optical properties and chemical composition. The CeO2/CdO heterojunction is found to be the best catalyst for the photocatalytic degradation of CR and RhB. In particular, CeO2/CdO (1:3) nanocomposite exhibits the highest photocatalytic degradation efficiency which is higher than those of pristine CeO2 and CdO. The maximum 97% and 99% degradation of CR and RhB are (within 60 min under visible light irradiation) achieved, respectively. The reasons for photocatalytic activity enhancements are mainly the result of formation of nen heterojunctions between CeO2 and CdO and also increased light absorption properties. The CeO2 doping enhanced the photocatalytic activity of CdO by trapping the photoexcited electron and thereby suppressing the photogenerated charge carrier's recombination. The results obtained in the study can be useful and helpful in designing an up scalable, practical process for organic dye wastewater treatments. Acknowledgments We gratefully acknowledge to the College Managing Board, The Principal and Head of the Department (Chemistry), VHNSN College for providing necessary research facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jre.2018.12.009. References 1. Gaya UI, Abdullah AH. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems. J Photochem Photobiol C. 2008;9:1. 2. Ortega LMC, Sanchez LE, Hidalgo CJ, Marinas A, Marinas JM, Urbano F. A comparative study of photocatalytic degradation of 3-chloropyridine under UV and solar light by homogeneous (photo-Fenton) and heterogeneous (TiO2) photocatalysis. J Appl Catal B. 2012;127:316. 3. Mittal A, Malviya A, Kaur D, Mittal J, Kurup L. Studies on the adsorption kinetics and isotherms for the removal and recovery of methyl orange from wastewaters using waste materials. J Hazard Mater. 2007;148:229. 4. Wang CC, Li JR, Lv XL, Zhang YQ, Guo GS. Photocatalytic organic pollutants degradation in metaleorganic frameworks. Energy Environ Sci. 2014;7:2831. 5. Wang P, Huang BB, Dai Y, Whangbo MH. Plasmonic photocatalysts: harvesting visible light with noble metal nanoparticles. Phys Chem Chem Phys. 2012;14:9813. 6. Wang QZ, Li JJ, Bai Y, Lu XL, Ding YM, Yin S, et al. Photodegradation of textile dye rhodamine B over a novel biopolymer-metal complex wool-Pd/CdS photocatalysts under visible light irradiation. J Photchem Photobiol B Biol. 2013;126:47. 7. Won SW, Choi SB, Chung BW, Park D, Park JM, Yun YS. Biosorptive decolorization of reactive orange 16 using the waste biomass of corynebacterium glutamicum. Ind Eng Chem Res. 2004;43:7865. 8. Chatterjee S, Chatterjee S, Chatterjee BP, Das AR, Guha AK. Adsorption of a model anionic dye, eosin Y, from aqueous solution by chitosan hydrobeads. J Colloid Interface Sci. 2005;288:30. 9. Messina PV, Schulz PC. Adsorption of reactive dyes on titaniaesilica mesoporous materials. J Colloid Interface Sci. 2006;299:305. 10. Staszak W, Zawadzki M, Okal J. Solvothermal synthesis and characterization of nanosized zinc aluminate spinel used in iso-butane combustion. J Alloys Compd. 2010;492:500. 11. Kumar M, Tamilarasan R, Sivakumar V. Adsorption of Victoria blue by carbon/ Ba/alginate beads: kinetics, thermodynamics and isotherm studies. Carbohydr Polym. 2013;98:505. 12. Muthupoongodi S, Linda T, Sahaya SX, Liviu M, Balakumar S. Polymer-supported catalyst for effective degradation of organic dyes: 100% recovery of catalyst stability and reusability. Polym Bull. 2018;75:1867. 13. Hoffman MR, Martin ST, Choi W, Bahnemann DW. Environmental applications of semiconductor photocatalysis. Chem Rev. 1995;95:69.
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14. Bin AK, Madej SS. Comparison of the advanced oxidation processes (UV, UV/ H2O2 and O3) for the removal of antibiotic substances during wastewater treatment. Ozone Sci Eng. 2012;34:136. 15. Ganesh R, Boardman GD, Michelson D. Fate of azo dyes in sludges. Water Res. 1994;28:1367. 16. Weber EJ, Adams RL. Chemical- and sediment-mediated reductions of the azo dye disperse blue 79. Environ Sci Technol. 1995;29:1163. 17. Vinodgopal K, Wynkoop DE, Kamat PV. Environmental photochemistry on semiconductor surfaces: photosensitized degradation of a textile azo dye, acid orange 7, on TiO2 particles using visible light. Environ Sci Technol. 1996;30:1660. 18. Fisher TJ, Wang M, Ibrahim Y, Steffensmeier B, Cheung CL. Effect of sodium nitrate on microwave-assisted synthesis of ceria nanocubes. Mater Lett. 2016;178:71. 19. Primo A, Marino T, Corma A, Molinari R, Garcia H. Efficient visible-light photocatalytic water splitting by minute amounts of gold supported on nanoparticulate CeO2 obtained by a biopolymer templating method. J Am Chem Soc. 2011;133:6930. 20. Paier J, Penschke C, Sauer J. Oxygen defects and surface chemistry of ceria: quantum chemical studies compared to experiment. Chem Rev. 2013;113:3949. 21. Hu SC, Zhou F, Wang LZ, Zhang JL. Preparation of Cu2O/CeO2 heterojunction photocatalyst for the degradation of acid orange 7 under visible light irradiation. Catal Commun. 2011;12:794. 22. Li LZ, Yan B. CeO2eBi2O3 nanocomposite: two step synthesis, microstructure and photocatalytic activity. J Non-Cryst Solids. 2009;355:776. 23. Kaviyarasu K, Manikandan E, Paulraj P, Mohamed SB, Kennedy J. One dimensional well-aligned CdO nanocrystal by solvothermal method. J Alloys Compd. 2014;593:67. 24. Margan P, Haghighi M. Sono-coprecipitation synthesis and physicochemical characterization of CdOeZnO nanophotocatalyst for removal of acid orange 7 from wastewater. Ultrason Sonochem. 2018;40:323. 25. Saravanakumar K, Muthuraj V, Jeyaraj M. The design of novel visible light driven Ag/CdO as smart nanocomposite for photodegradation of different dye contaminants. Spectrochim Acta A. 2018;188:291. 26. Gülce H, Eskizeybek V, Haspulat B, Sarı F, Gulce A, Avcı A. Preparation of a new Polyaniline/CdO nanocomposite and investigation of its photocatalytic activity: comparative study under UV light and natural sunlight irradiation. Ind Eng Chem Res. 2013;52:10924. 27. Kumar PS, Selvakumar M, Babu SG, Karuthapandian S, Chattopadhyay S. CdO nanospheres: facile synthesis and bandgap modification for the superior photocatalytic activity. Mater Lett. 2015;151:45. 28. Magdalane CM, Kaviyarasu K, Judith VJ, Siddhardha Busi, Jeyaraj B. Photocatalytic activity of binary metal oxide nanocomposites of CeO2/CdO nanospheres: investigation of optical and antimicrobial activity. J Photchem Photobiol B Biol. 2016;163:77. 29. Magdalane CM, Kaviyarasu K, Judith Vijaya J, Jayakumar C, Maaza M, Jeyaraj B. Photocatalytic degradation effect of malachite green and catalytic hydrogenation by UVeilluminated CeO2/CdO multilayered nanoplatelet arrays: investigation of antifungal and antimicrobial activities. J Photchem Photobiol B Biol. 2017;169:110. 30. Anitha M, Saravanakumar K, Anitha N, Amalraj L. Influence of a novel codoping (Zn þ F) on the physical properties of nanostructured (1 1 1) oriented CdO thin films applicable for window layer of solar cell. Appl Surf Sci. 2018;443:55. 31. Saravanakumar K, Karthik R, Chen SM, Kumar JV, Prakash K, Muthuraj V. Construction of novel Pd/CeO2/g-C3N4 nanocomposites as efficient visible-light photocatalysts for hexavalent chromium detoxification. J Colloid Interface Sci. 2017;504:514. 32. Saravanakumar K, Muthuraj V. Fabrication of sphere like plasmonic Ag/SnO2 photocatalyst for the degradation of phenol. Optik. 2017;131:754. 33. Saravanakumar K, Mymoon Ramjan M, Suresh P, Muthuraj V. Fabrication of highly efficient visible light driven Ag/CeO2 photocatalyst for degradation of organic pollutants. J Alloys Compd. 2016;664:149. 34. Khan MM, Ansari SA, Pradhan D, Han DH, Lee J, Cho MH. Defect-induced band gap narrowed CeO2 nanostructures for visible light activities. Ind Eng Chem Res. 2014;53:9754.
35. Al-Hamdi AM, Mika S, Joydeep D. Gadolinium doped tin dioxide nanoparticles: an efficient visible light active photocatalyst. J Rare Earths. 2015;33:1275. 36. Lara-Lopez Y, Garcia-Rosales G, Jimenez-Becerril J. Synthesis and characterization of carbon-TiO2-CeO2 composites and their applications in phenol degradation. J Rare Earths. 2017;35:551. 37. Mohammadyari Parisa, Nezamzadeh-Ejhieh A. Supporting of mixed ZnSeNiS semiconductors onto clinoptilolite nano-particles to improve its activity in photodegradation of 2-nitrotoluene. RSC Adv. 2015;5:75300. 38. Prabhakar Vattikuti SV, Chan B, Venkata Reddy Ch, Ravikumar RVSSN. Improved photocatalytic activity of MoS2 nanosheets decorated with SnO2 nanoparticles. RSC Adv. 2015;5:86675. 39. You JH, Wang RC, Liu CB, Shi XJ, Han F, Guo R, et al. Facile synthesis and highly efficient selective adsorption properties of Y2Mo4O15 for methylene blue: kinetics, thermodynamics and mechanical analyses. J Rare Earths. 2018;36:844. 40. Gao L, Li ZH, Liu JW. Facile synthesis of Ag3VO4/b-AgVO3 nanowires with efficient visible-light photocatalytic activity. RSC Adv. 2017;7:27515. 41. Meng J, Pei JY, He ZF, Wu SY, Lin QY, Wei X, et al. Facile synthesis of g-C3N4 nanosheets loaded with WO3 nanoparticles with enhanced photocatalytic performance under visible light irradiation. RSC Adv. 2017;7:24097. 42. Saravanakumar K, Kumar PS, Kumar JV, Karuthapandian S, Robert P, Muthuraj V. Controlled synthesis of plate like structured MoO3 and visible light induced degradation of rhodamine B dye solution. Energy Environ Focus. 2016;5:50. 43. Nezamzadeh-Ejhieh A, Setareh K. Photocatalytic degradation of 4-nitrophenol with ZnO supported nano-clinoptilolite zeolite. J Ind Eng Chem. 2014;20:937. 44. Despotovic VN, Abramovic BF, Sojic DV, Kler SJ, Dalmacija MB, Bjelica LJ, et al. Photocatalytic degradation of herbicide quinmerac in various types of natural water. Water Air Soil Pollut. 2012;223:3009. 45. Vranjes M, Saponjic ZV, Zivkovic LS, Despotovic VN, Sojic DV, Abramovic BF, et al. Elongated titania nanostructures as efficient photocatalysts for degradation of selected herbicides. Appl Catal B. 2014;160e161:589. 46. Laouedj N, Elaziouti A, Benhadria N, Bekka A. CeO2 nanoscale particles: synthesis, characterization and photocatalytic activity under UVA light irradiation. J Rare Earths. 2018;36:575. 47. Dhanalakshmi M, Saravanakumar K, Lakshmi Prabavathi S, Abinaya M, Muthuraj V. Fabrication of novel surface plasmon resonance induced visible light driven iridium decorated SnO2 nanorods for degradation of organic contaminants. J Alloys Compd. 2018;763:512. 48. Fincur NL, Krstic JB, Sibul FS, Sojic DV, Despotovic VN, Banic ND, et al. Removal of alprazolam from aqueous solutions by heterogeneous photocatalysis: influencing factors, intermediates, and products. Chem Eng J. 2017;307:1105. 49. Saravanakumar K, Muthuraj V, Vadivel S. Constructing novel Ag nanoparticles anchored on MnO2 nanowires as an efficient visible light driven photocatalyst. RSC Adv. 2016;6:61357. 50. Armakovic SJ, Armakovic S, Cetojevic-Simin DD, Sibul F, Abramovic BF. Photocatalytic degradation of 4-amino-6-chlorobenzene-1,3-disulfonamide stable hydrolysis product of hydrochlorothiazide: detection of intermediates and their toxicity. Environ Pollut. 2018;233:916. 51. Ejhieh AN, Khorsandi M. Photodecolorization of Eriochrome Black T using NiSP zeolite as a heterogeneous catalyst. J Hazard Mater. 2010;176:629. 52. Karunamoorthy S, Velluchamy M. Design and synthesis of bandgap tailored porous Ag/NiO nanocomposite: an effective visible light active photocatalyst for degradation of organic pollutants. J Mater Sci Mater Electron. 2018;29: 20367. 53. Zahra S, Zinatloo-Ajabshir S, Salavati-Niasari M. Dysprosium cerate nanostructures: facile synthesis, characterization, optical and photocatalytic properties. J Rare Earths. 2017;35:805. 54. Sun DF, Wang K, Xu ZJ, Li RX. Synthesis and photocatalytic activity of sulfate modified Nd-doped TiO2 under visible light irradiation. J Rare Earths. 2015;33: 491. 55. Wang YB, Pan ZH, Qin DZ, Bai SZ, Peng QL. Preparation of Ce-TiO2/carbon aerogel electrode and its performance in degradation of 4-chlorophenol. J Rare Earths. 2018;36:374.
Contents lists available at ScienceDirect
Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths
A novel n-CeO2/n-CdO heterojunction nanocomposite for enhanced photodegradation of organic pollutants under visible light irradiation Karunamoorthy Saravanakumar a, b, Subramani Muthupoongodi c, Velluchamy Muthuraj a, * a b c
Department of Chemistry, V.H.N.S.N College, Virudhunagar, 626 001, Tamilnadu, India Department of Chemistry, Sri Kaliswari College, Sivakasi, 626 130, Tamilnadu, India Department of Chemistry, Thiagarajar College, Madurai, 625 009, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2018 Received in revised form 8 December 2018 Accepted 10 December 2018 Available online 3 April 2019
In this study, a series of novel visible light driven n-CeO2/n-CdO heterojunction (CeO2/CdO) nanocomposites were successfully fabricated by simple ultrasonication method. Several characterization tools including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-vis diffuse reflectance spectroscopy (UV-DRS), etc., were utilized to investigate the physicochemical properties of the catalyst and confirm the formation of heterojunction. Under visible light irradiations, the photocatalytic activities of the as-prepared CeO2/CdO nanocomposites were evaluated by degrading of Congo red (CR) and Rhodamine B (RhB) solutions. As a result, the CeO2/CdO (mass percentage ratio 1:3) nanocomposite displays remarkable performance for CR and RhB degradation. The enhancement in the photocatalytic performance of CeO2/CdO (1:3) nanocomposite can be attributed not only to the strong visible-light absorption region, separating the photogenerated electronhole pairs but also to the formation of n-n type heterojunction. The results also indicate that the CeO2/ CdO (1:3) nanocomposite has good stabilization and high reusability. In addition, the mechanism is proposed for the coupled semiconductors and possible reasons for the enhancement of visible-light photocatalytic efficiency are also discussed. This work can provide a new gateway to fabricate visible photocatalysts and promising candidate catalysts for poisonous wastewater treatment in the near future. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: n-CeO2/n-CdO Heterojunction Visible light Photocatalysis Photodegradation Rare earths
1. Introduction Over the past few years, the growing number of contaminants generated by human activities presents one of the formidable challenges to the sustainable development of modern human society.1e4 Various kinds of organic contaminants enter into water sources through industrial wastewater discharge. Wastewater generally contains large amount of organic pollutants (such as organic dyes, pesticides and antibiotics) which have caused serious hazard to human and aquatic ecosystems. Among them, organic dyes generated from textiles, leather, paper-making, dye houses and printing industries are considered as the primary pollutants in wastewater.5,6 Thus their elimination of organic pollution is a vital environmental issue. Traditional treatment processes including
* Corresponding author. E-mail addresses: [email protected], [email protected] (V. Muthuraj).
adsorption, reverse osmosis, chlorination, chemical oxidation, coagulation and flocculation, biological degradation and ozonolysis are often inadequate in treating many of the new, emerging micropollutants.7e9 Unfortunately, most of these physical, chemical and biological techniques can form secondary pollutants easily during degradation processes.10e12 Therefore, more effective wastewater treatment technologies are highly welcome. Advanced oxidation processes (AOPs) semiconductor photocatalysts have been widely studied for the degradation of toxic contaminants in environment remediation applications.13e15 The degradation and mineralization of dyes using nanostructured semiconductor under visible light has attracted great interest in recent years.16 The mechanism of photocatalytic degradation of dye molecules under visible light excitation is different from the traditional mechanism under UV irradiation. In these systems, the dye, rather than the semiconductor is subject to the visible light excitation. The excited dye molecule then transfers an electron into the conduction band of semiconductor, leading to the formation of
https://doi.org/10.1016/j.jre.2018.12.009 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
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a cationic radical of the dye. The injected electron then reacts with the oxygen adsorbed on the surface of photocatalyst nanoparticles and generates a series of reactive oxygen species (ROS) such as O2, OH, OH and H O . The subsequent radical chain reactions 2 2 involving photocatalytic process and the dye molecule lead to the degradation and mineralization of the dye pollutants.17 Cerium oxide (CeO2), an eco-friendly photocatalytic material, has stimulated great interest of the researchers due to its many fascinating properties, such as abundant mobile oxygen vacancies, high oxygen storage capability, excellent chemical stability and biocompatibility.18e20 Unfortunately pristine CeO2 can only be excited by little visible light absorption and has weak separation efficiency of photoinduced carriers because the wide bandgap energy (3.2 eV) hinders its widespread application in photocatalysis. Thus it is expected to extend light absorption of CeO2 into visible light region either by doping of metals/nonmetals or by forming heterojunctions between CeO2 and other narrow band gap semiconductors in order to generate visible light-driven catalysts.21,22 Coupled semiconductor photocatalysts may enhance the photocatalytic efficiency by improved charge separation and increasing the charge carrier lifetime. In particular, cadmium oxide (CdO) is a known n-type semiconductor with a direct band gap of 2.3e2.5 eV and an indirect band gap of 1.36e1.98 eV.23 In recent years, CdO nanomaterials have been used as an interconnection in a wide variety of commercial applications such as exhaust catalyst, UV absorbers, gas sensors, catalysis, solar cells, fuel cells, photodiodes, phototransistors and oxygen storage materials. Also, CdO is used as a visible-light-driven photocatalyst for water oxidation and organic pollutant degradation.24e27 Recently, CeO2/CdO nanocomposite has been reported by Magdalane et al. for the application of photocatalytic activity and antimicrobial activity. However, these results showed a slightly poor photocatalytic activity and catalyst preparation methods were too long.28,29 Herein, we reported a novel n-CeO2/n-CdO heterojunction photocatalyst which were successfully fabricated by ultrasonication method. The as-synthesized samples were characterized by various analytical techniques, XRD, FT-IR, SEM, TEM and UV-DRS were employed to study the crystal phase structures, morphologies and optical properties. Moreover, photocatalytic activity test was carried out and operational parameters such as catalyst loading, initial pH and dye solution concentration were examined. Ultimately, a plausible mechanism and photostability of the catalyst are also proposed in detail.
CeO2, the conditions are the same, except for the absence of CdO in the system. 2.3. Characterization techniques The crystal structure and phase of the synthesized nanocomposite was estimated by X-ray diffraction (XRD; PANalytical X'pert Pro.) in the 2q range from 10ºe80 with using mono chromatized Cu Ka (l ¼ 0.154178 nm) radiation. The surface morphology and microstructure of the catalysts were analyzed by scanning electron microscopy (SEM, VEGA3 TESCAN model) and transmission electron microscopy (PHILIPS CM 200 model). An energy dispersive spectrometer (EDX - Bruker Nano GmbH, X 50 flash Detector (model-5010)) was used to evaluate the elemental compositions of the nanocomposites. The structural information was carried out by the Fourier transform infrared spectroscopy (Shimadzu FT-IR 3000) using KBr as diluents in the range of 4000e400 cm1. A UV-vis diffused reflectance spectrometer (DRSmode) (UV-2450, Shimadzu) was used to determine the absorbance ranges of the samples and BaSO4 was used as a reflectance reference material. 2.4. Description about photocatalytic activity
2. Experimental
The photocatalytic activities of CeO2, CdO and CeO2/CdO catalysts were evaluated for the photocatalytic degradation of Rhodamine B dye (RhB) and Congo red (CR) dye solutions. In the photocatalytic reactions, 50 mg of catalyst was dispersed in dye solution with concentration of 10 mg/L in 100 mL reaction vessel under visible light irradiation. The characteristic absorption peaks of CR and RhB were chosen to monitor the photocatalytic degradation process. The lambda maximum for absorption wavelength of CR dye solution was observed at l ¼ 490 and 348 nm and RhB at l ¼ 553 nm. The organic dyes were subjected to radiation (l > 400 nm) for a definite period of time under tungsten incandescent lamp and intensity of the visible light is 150 mW/cm2. UV spectrum was recorded periodically with samples withdrawn from reaction vessel. The radiation was continued till the complete degradation is achieved. At given time intervals of 10 min irradiation, 3 mL of aliquots were collected. The degraded solutions were analyzed using the absorption peaks of corresponding dye. After centrifugation, the concentration of dye solution was measured with a Shimadzu 2600 UV-vis spectrometer. Finally during the degradation process, photocatalyst was separated from the reaction mixture and dried to carry out the reusability tests.
2.1. Materials
3. Results and discussion
All chemicals and regents used were of analytical regent grade and purchased from the Merck Company and were used without further purification.
3.1. XRD analysis
2.2. Preparation of CeO2/CdO photocatalyst CeO2/CdO catalyst was prepared by ultrasound assisted in situ chemical precipitation technique. 0.5 g of cerium nitrate hexahydrate was dissolved in 50 mL of deionized water, and then 1.5 g of cadmium nitrate was added into a reaction mixture and stirred for 30 min at room temperature. Under ultrasonic irradiation (300 W) at room temperature, 0.1 mol/L NaOH solution was added dropwise to raise the suspension pH to around 9 and the irradiation was continued for 30 min. Finally, the as-synthesized product was collected through centrifuged at 1000 r/min and repeatedly washed with deionized water/ethanol, and then dried in an oven at 80 C for 24 h and calcined at 500 C for 3 h. For the synthesis pure
To gain information about the crystalline structure and phase purity of the as-synthesized samples, XRD analyses were carried out. As is shown in Fig. 1, the peaks at 28.29 , 32.86 , 47.48 , 56.41, 59.05 , 69.21 and 76.71 are indexed to the (111), (200), (220), (311), (222), (400) and (311) crystal planes, which was in good agreement with the cubic phase structure of CeO2 (JCPDS card No: 89-8436). There are a number of intensive peaks observed at 33.07, 38.28 , 55.34 , 66.01 and 69.43 , which could be indexed to the (111), (200), (220), (311) and (222) crystal planes for cubic structure of CdO (JCPDS Card No. 05-0640). In the nanocomposite, both the diffraction peaks of the cubic phase structure were observed for CeO2 and CdO which clearly indicate the phase pure formation of CeO2/CdO nanocomposite. The strong and sharp diffraction peaks suggested that the CeO2/CdO nanocomposite was well crystalline in nature. No characteristic peaks from other
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Fig. 1. XRD patterns of as-synthesized CeO2, CdO and CeO2/CdO nanocomposite.
crystalline forms were noticed, which demonstrates that the CeO2/ CdO has high purity. The average crystallite size (D) of pure CeO2, CdO and CeO2/CdO nanocomposite was calculated using the Scherrer's formula,30
D¼
0:9l b cos q
(1)
where D is average crystallite size, l X-ray wavelength, b full width at half maximum (FWHM), and q Bragg's angle. The average crystallite sizes for the pure CeO2, CdO and CeO2/CdO nanocomposite was found to be 17, 29 and 13 nm, respectively. 3.2. FTIR analysis FT-IR spectrum helps in determining the appropriate functional group present in the as-synthesized CeO2, CdO and CeO2/CdO nanocomposites. FT-IR spectra of pure CeO2, CdO and CeO2/CdO nanocomposite were carried out in the wavenumber range between 4000 and 400 cm1 and are shown in Fig. S1. A broad band in the range of 3500e3700 cm1 is due to the OH stretching of surface adsorbed water molecules.31 The bending vibration band of physically adsorbed water molecules are also observed at 1600 cm1. The presence of sharp dominated absorption peaks at 450e650 cm1 is related to the MO (M ¼ Ce and Cd) bonds and confirmed that the prepared nanocomposite is CeO2/CdO.32 Generally, in spinel oxide metaleoxygen stretching frequencies are observed in the range of 700e900 cm1. 3.3. Morphological analysis The microstructure and morphological of the as-synthesized samples were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM micro images of pure CeO2, CdO and CeO2/CdO nanocomposite are shown in Fig. 2. The pristine CeO2 shows granular-like morphology and the particles are closely attached with each other which are observed in Fig. 2(a). The particles are of spongy-like structure morphology and without any specific shape of CdO particles which are shown in Fig. 2(b). Interestingly, the CdO micro particles are attached onto the surface of the CeO2 and image is shown in Fig. 2(c). Obviously as the number of micro particles attached on the surface increases, the
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agglomeration also increases. Moreover, the surface has many irregular small granules with spongy shape. The hetero-structures of as-synthesized CeO2/CdO nanocomposite were further confirmed by HR-TEM analysis. CeO2/CdO nanocomposite is taken as the representative for TEM, and it is given in Fig. S2. The fine particle is the aggregate of nano hybrid and shape is more or less spherical and it reveals the polycrystalline cubic crystal structure. Fig. S2 (a, b) display that the particles are more agglomerated with irregular shape of morphology. From the HRTEM image, the particles are in the range of 100 to 5 nm, which is consistent with XRD results. As shown in the HRTEM image (Fig. S2(c)), the interplanar lattice fringes have spacing of d ¼ 0.31 nm and d ¼ 0.27 nm corresponding to the (111) crystal plane of CeO2 and (111) crystal plane of CdO, respectively. Additionally, the selected area electron diffraction (SAED) pattern which shows bright rings reveals that the nanocomposite is high crystalline nature with cubic structures as shown in Fig. S2(d). It is also well supported by the XRD results. The chemical elemental composition analysis of CeO2/CdO nanocomposite was carried out by energy dispersive X-ray spectroscopy (EDS) measurements. The EDS spectra, as shown in Fig. 2(d), confirm that the synthesized nanocomposite is impurity free nature. In the spectra, only three intense peaks were observed. These peaks belong to constituent elements of CeO2/CdO nanocomposite like cerium (Ce), cadmium (Cd) and oxygen (O) atoms. Furthermore, the EDS elemental mapping showed that green, red and blue represent the distribution of Ce, Cd and O, respectively as depicted in Fig. 3. All these experiments authenticate explicitly that CeO2/CdO heterojunction catalyst has been successfully prepared.
3.4. Optical properties The optical properties and energy band gap of the assynthesized samples were studied by UV-vis diffuse reflectance spectra (UV-vis DRS) as displayed in Fig. 4. The pure CeO2 and pristine CdO show photo absorption threshold at approximately 415 and 490 nm, respectively, which fitted well with previous literature.33,25 The CeO2/CdO heterojunction shifts the absorption edge value at a higher wavelength (red shift). The red shift may be due to the development of some localized band gap because of oxygen vacancies in the nanocomposites and also Ce3þ. The optical property of the absorbance of CeO2/CdO heterojunction in the visible region suggests that it may be used as a good candidate for visible absorbing semiconductor materials. CdO combined CeO2 could reduce the band gap energy of CdO/CeO2 heterostructure nanocomposite and increases the amount of Ce4þ states, resulting in the formation of localized energy states that are closer to the conduction band.34 To evaluate the band gap of the as-synthesized pristine CeO2, CdO and CeO2/CdO heterojunction, diffused reflectance spectroscopy was carried out in the wavelength range of 200e800 nm. The band gap energy was determined by fitting the absorption data to the direct transition equation, (ahn) ¼ A (hn Eg)n
(2)
where n is the transition frequency, h the Plank's constant, A a constant quantity, Eg the band gap energy of the material, a the absorption coefficient and n can have values 1/2, 3/2, 2 and 3 depending on the mode of inter-band transition, i.e., direct allowed and direct forbidden, indirect allowed and indirect forbidden transition, respectively. The band gap of the CeO2, CdO, and CeO2/ CdO heterojunction has been deduced from the Kubelka-Munk (KM) plots as shown in Fig. 4(bed). The band gap energy optically
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Fig. 2. SEM micrographs of pure CeO2 (a), pure CdO (b) and CeO2/CdO (c) and EDS spectra of CeO2/CdO nanocomposite (d).
Fig. 3. EDS elemental mapping analysis of CeO2/CdO nanocomposite cadmium (red), cerium (green) and oxygen (blue).
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Fig. 4. UV-visible absorption (DRS) spectra (a) and Tauc's plots of pure CdO (b), CeO2 (c) and CeO2/CdO nanocomposite (d).
obtained in such a way was that approximately CeO2, CdO and CeO2/CdO heterojunction are 2.86, 2.17 and 2.69 eV, respectively. 3.5. Photocatalytic activity The photocatalytic activities of the CeO2 with different loading ratios of CdO were estimated by the degradation of CR dye solution under visible light irradiation which is displayed in Fig. 5(a). The photocatalytic efficiency of pristine CdO and CeO2 for CR degradation was 42% and 58% within 60 min of irradiation, respectively. The photocatalytic performance was enhanced after composition of CeO2 with CdO. Obviously, (1:3) CeO2/CdO heterostructure exhibits the maximum photocatalytic efficiency among all CeO2/CdO nanocomposites. The higher photocatalytic activity here can be attributed to synergetic effect of the two metal oxides, this leads to the decrease in the band gap energy. It can be observed that the photocatalytic efficiency increases with decreasing the band gap energy values of the nanocomposite due to the decrease in the energy required for the production of conduction band electrons and valence band holes. In generally, the coupled semiconductors both recombination (negative role) and production (positive role) of photogenerated charge carriers (eehþ) play a vital role in the photocatalytic activity of the systems. Note that the photocatalytic activity enhanced when the loading content of CeO2 increased from 0.5 to 1, but further increment causes decrement in photocatalytic performance. When the loading amount of CeO2 was over 1.5 wt%, the separation efficiency of photogenerated charge carriers decreased and further restrained the photocatalytic reaction.35,36 In these systems, the suitable amount of CeO2 is an important factor for photocatalytic degradation process. In order to perform a blank experiment, there is no degradation observed in the absence of light as well as in the absence of catalyst. The loading dosage of CeO2 in CeO2/CdO heterostructure nanocomposites played a critical role in their photocatalytic activities. Finally, it could be concluded
that the photocatalytic performance of (1:3) CeO2/CdO heterostructure was better compared with another photocatalyst, so this catalyst was chosen as an optimized catalyst for the characterization and further photocatalytic reactions. The reaction kinetics curve of photocatalytic degradation of dye solutions were evaluated by Langmuir-Hinshelwood model, it follows the pseudo-first order kinetics. The rate constant was determined by the slope ln (C0/C) vs time (t), which is illustrated in Fig. 5(b). The estimated rate constant k values are 0.0018, 0.01058, 0.01657, 0.02523, 0.0652 and 0.03276 for without catalyst, pure CdO, pure CeO, CeO/CdO (0.5:3), CeO/CdO (1:3) and CeO/CdO (1.5:3), respectively. The k value increased in the order without catalyst < pure CdO < pure CeO < CeO/CdO (0.5:3) < CeO/CdO (1.5:3) < CeO/CdO (1:3). It concluded that CeO/CdO (1:3) has higher rate constant and it has 6.2 and 3.9 times higher than that of pure CdO and pure CeO, respectively. The CeO/CdO (1:3) nanocomposite possesses significantly high photocatalytic degradation of dyes as compared with other nanocomposition and undoped CeO and CdO.37e39 The photocatalytic performance of as-synthesized pure CeO2, CdO and CeO2/CdO heterojunction was investigated by the photodegradation of CR and RhB under visible light irradiation. As depicted in Fig. 6, for CeO2/CdO catalyst, the intensity of this characteristic peak value of organic dye gradually decreases with the increase of time which indicates the photodegradation of dye solutions. The initial absorbance of the peak disappeared completely after 60 min of visible light irradiation which indicates the cleavage of conjugated chromophore structure of dyes and conversion into small aromatic intermediates.40,41 The CeO2/CdO heterojunction exhibits much higher photocatalytic capability compared with pure CeO2 and CdO. The enhancement in the photocatalytic performance of CeO2/CdO can be attributed to the n-n heterojunction and efficient separation of photogenerated electron-hole pair recombination. The control experiment, without photocatalyst, no significant degradation of CR and RhB were
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Fig. 5. Photodegradation of CR dye solution in the presence of different catalysts (a) and the corresponding plots of lnC/C0 vs time (b) under visible light irradiation in the following conditions. CR dye concentration ¼ 10 mg/L, amount of catalyst ¼ 50 mg, pH ¼ ~9, reaction temperature ¼ room temperature and irradiation time ¼ 60 min.
observed as the time was prolonged, which indicates that the photolysis of CR and RhB could be neglected. The photocatalyst amount added will affect the photocatalytic activity on degradation of organic dye solutions. A series of experiments were conducted with the catalyst dosage varied from 10 to 75 mg at constant organic dye pollutant concentration and other parameters were kept same which are illuminated in Fig. S3. The photocatalytic activity was increased with increasing the amount of photocatalyst dosage at 10e50 mg. In this system, 50 mg of the photocatalyst had the highest photocatalytic activity compared with the other amount of catalyst dosages. But the increase in the catalyst amounts beyond 50 mg, the photocatalytic efficiency was decreased which may be due to the light blocking by excessive amount of catalyst and prevents the light penetrations.42e44 Finally in this reaction medium, 50 mg of the photocatalyst was optimum amount for the photocatalytic degradation of organic pollutants. Fig. S3(c, d) show the kinetics profile for the degradation of dyes at different catalyst dosages and the rate constant values are given in Table S1. It was noted that the photodegradation of dye solutions followed pseudo-first order kinetics. The influence of the initial organic dye solutions (CR and RhB) concentration on the photodegradation under visible light illumination was also evaluated by varying the concentrations from 10,
Fig. 6. The temporal UV absorption spectral changes of dye solutions in the presence of CeO2/CdO (1:3) nanocomposite CR (a) and RhB (b) at irradiation time interval of 60 min. Amount of catalyst ¼ 50 mg, dye concentration ¼ 10 mg/L, pH ¼ ~9 and reaction temperature ¼ room temperature.
20 and 30 mg/L and other reaction parameters are the same. In this case Fig. S4 shows that the photodegradation rate decreases with the increases of the dye concentration which may be due to the screening of visible light by the dye itself. At high dye concentration, the colored dye solution may hinder the light during the photocatalytic process and also can generate filter effect which decreases the production of reactive oxygen species (ROS).45e47 In other words, the greater amount of dye molecules competing for the degradation process and the photo generation of charge carries separation was significantly reduced. The reaction kinetics curves of dyes degradation at different dye concentrations are displayed in Fig. S4(c, d). The corresponding rate constant (k) values are summarized in Table S1. In addition, the pH of the dye solution is also an important parameter for the photodegradation process because the waste water may be in acidic or basic condition. The initial pH of the dye solution was controlled by the addition of either HCl or NaOH (0.1 mol/L). In order to investigate the effect of initial pH on photocatalytic decolorization of CR and RhB dye, four experiments were conducted at different pH values of 3, 5, 7 and 9, at the initial dye concentration of 10 mg/L and the catalyst amount of 50 mg which are illustrated in Fig. S5. Increase in the pH of CR and RhB dye
K. Saravanakumar et al. / Journal of Rare Earths 37 (2019) 853e860
solution from 3 to 5 led to mildly decreased efficiency in the decolorization of organic dyes. Further increase in pH from 5 to 9 had apparent increase in dye decolorization efficiency. Enhancement of photodegradation efficiency may be because the number of $OH radical produced high basic condition and lower pH condition, the Hþ ion screens the active adsorption sites on the surface of the catalyst.48e50 Furthermore, the corresponding reaction kinetics curves are illustrated in Fig. S5(c, d), and the rate constant results are tabulated in Table S1. To obtain better understanding on the CeO2/CdO heterojunction, their valence band (VB) and conduction band (CB) edge potential were calculated via Mulliken electronegativity theory as follows, EVB ¼ X E0 þ 0.5Eg
(3)
ECB ¼ EVB Eg
(4)
where X is the absolute electronegativity of the constituent atoms in the semiconductor, E0 the energy of free electrons measured on the hydrogen scale (4.5 eV), Eg band gap energy of the semiconductor materials. According to the above equations, the Evb values of CeO2 and CdO are calculated to be 2.62 and 2.06 eV, respectively and Ecb values are estimated to be 0.234 and 0.11 eV, respectively. The CeO2/CdO heterojunction photocatalyst improves the photocatalytic activity for the degradation of CR and RhB dye solutions under visible light irradiation. The CeO2/CdO heterojunction irradiated by visible light leads to the creation of photogenerated electron in conduction band and holes in the valence band. The possible photocatalytic mechanism involves the excitation of an electron in conduction band of CeO2 and thereby the valence band (VB) of CdO acts as sink for hole. The electron in the conduction band uptake an O2 molecule to form an O2¡ free radical and then O2¡ reacts with surface water molecules to produce the OH radical. Finally the active radicals react with CR dye molecules to form the mineralized compounds.51e53
þ CeO2 =CdO þ hv/CeO2 =CdO e cb þhvb
(5)
þ þ CeO2 =CdO e cb þ hvb /CeO2 ecb þ CdO hvb
(6)
$ O2 þ e cb /O2
(7)
þ $ 2O$ 2 þ 2H /2 OH
(8)
$ O$ 2 ; OH þ dye solutions/Mineralized products
(9)
The stability and recyclability of photocatalyst is a deciding factor for practical applications.54,55 The stability of the CeO2/CdO nanocomposite was tested by photocatalytic degradation of CR and RhB dye solution cycled for five times under the same conditions and the experimental results are shown in Fig. S6. For each cycle, the CeO2/CdO photocatalyst was recycled by centrifuging, washing and drying for the next run. There is no obvious decrease during the photodegradation process of CR and RhB organic dyes after five degradation-regeneration cycles. About 80% of CR and RhB dye pollutants can be degraded over CeO2/CdO catalyst after five cycles, which indicates the catalyst has superior photostability and excellent reusability. This CeO2/CdO heterojunction could promote to be a new candidate of reusable and active photocatalyst in environmental applications.
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4. Conclusions In summary, n-CeO2/n-CdO heterojunction photocatalyst were synthesized by facile one-pot ultrasonication method. The n-CeO2/ n-CdO nanocomposite was characterized by a range of techniques to study its morphology, crystal structure, optical properties and chemical composition. The CeO2/CdO heterojunction is found to be the best catalyst for the photocatalytic degradation of CR and RhB. In particular, CeO2/CdO (1:3) nanocomposite exhibits the highest photocatalytic degradation efficiency which is higher than those of pristine CeO2 and CdO. The maximum 97% and 99% degradation of CR and RhB are (within 60 min under visible light irradiation) achieved, respectively. The reasons for photocatalytic activity enhancements are mainly the result of formation of nen heterojunctions between CeO2 and CdO and also increased light absorption properties. The CeO2 doping enhanced the photocatalytic activity of CdO by trapping the photoexcited electron and thereby suppressing the photogenerated charge carrier's recombination. The results obtained in the study can be useful and helpful in designing an up scalable, practical process for organic dye wastewater treatments. Acknowledgments We gratefully acknowledge to the College Managing Board, The Principal and Head of the Department (Chemistry), VHNSN College for providing necessary research facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jre.2018.12.009. References 1. Gaya UI, Abdullah AH. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems. J Photochem Photobiol C. 2008;9:1. 2. Ortega LMC, Sanchez LE, Hidalgo CJ, Marinas A, Marinas JM, Urbano F. A comparative study of photocatalytic degradation of 3-chloropyridine under UV and solar light by homogeneous (photo-Fenton) and heterogeneous (TiO2) photocatalysis. J Appl Catal B. 2012;127:316. 3. Mittal A, Malviya A, Kaur D, Mittal J, Kurup L. Studies on the adsorption kinetics and isotherms for the removal and recovery of methyl orange from wastewaters using waste materials. J Hazard Mater. 2007;148:229. 4. Wang CC, Li JR, Lv XL, Zhang YQ, Guo GS. Photocatalytic organic pollutants degradation in metaleorganic frameworks. Energy Environ Sci. 2014;7:2831. 5. Wang P, Huang BB, Dai Y, Whangbo MH. Plasmonic photocatalysts: harvesting visible light with noble metal nanoparticles. Phys Chem Chem Phys. 2012;14:9813. 6. Wang QZ, Li JJ, Bai Y, Lu XL, Ding YM, Yin S, et al. Photodegradation of textile dye rhodamine B over a novel biopolymer-metal complex wool-Pd/CdS photocatalysts under visible light irradiation. J Photchem Photobiol B Biol. 2013;126:47. 7. Won SW, Choi SB, Chung BW, Park D, Park JM, Yun YS. Biosorptive decolorization of reactive orange 16 using the waste biomass of corynebacterium glutamicum. Ind Eng Chem Res. 2004;43:7865. 8. Chatterjee S, Chatterjee S, Chatterjee BP, Das AR, Guha AK. Adsorption of a model anionic dye, eosin Y, from aqueous solution by chitosan hydrobeads. J Colloid Interface Sci. 2005;288:30. 9. Messina PV, Schulz PC. Adsorption of reactive dyes on titaniaesilica mesoporous materials. J Colloid Interface Sci. 2006;299:305. 10. Staszak W, Zawadzki M, Okal J. Solvothermal synthesis and characterization of nanosized zinc aluminate spinel used in iso-butane combustion. J Alloys Compd. 2010;492:500. 11. Kumar M, Tamilarasan R, Sivakumar V. Adsorption of Victoria blue by carbon/ Ba/alginate beads: kinetics, thermodynamics and isotherm studies. Carbohydr Polym. 2013;98:505. 12. Muthupoongodi S, Linda T, Sahaya SX, Liviu M, Balakumar S. Polymer-supported catalyst for effective degradation of organic dyes: 100% recovery of catalyst stability and reusability. Polym Bull. 2018;75:1867. 13. Hoffman MR, Martin ST, Choi W, Bahnemann DW. Environmental applications of semiconductor photocatalysis. Chem Rev. 1995;95:69.
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