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bentonite nanocomposites for catalytic degradation of toxic organic wastes
Applied Clay Science 183 (2019) 105312
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
Clay semiconductor hetero-system of SnO2/bentonite nanocomposites for catalytic degradation of toxic organic wastes Avis Tresa Babu, Rosy Antony
T
⁎
Post graduate and Research Department of Chemistry, Nirmalagiri College, Kannur, Kerala 670701, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Bentonite SnO2 Clay semiconductor system Organic dyes Nanocomposites
SnO2/Bentonite (SBT) nanocomposites containing varying amounts of SnO2 (10, 20 & 30 wt%) were prepared by co-precipitation method and characterized. X-ray diffraction studies indicated the tetragonal rutile structure of SnO2 nanoparticles located on the surface bentonite layers. FTIR analysis data confirmed the presence of SneO bonds. TGA indicated higher thermal stability of the SBT nanocomposites. FESEM studies ascertained the incorporation of globular SnO2 crystallites into the numerous nano-flakes of clay particles. The tau plot from UV–Visible DRS studies revealed that the greater loading of SnO2 reduced the band gap energy and might be due to defects in the crystal system. The reduced intensity of PL peaks in SBT composites compared to SnO2 indicated the greater catalytic activity of the nanocomposites. Pristine SnO2 causes photodegradation on UV irradiation where as SBT nanocomposites degrade these dyes in visible light itself. It is interesting to note that the SBT nanocomposites show photocatalytic degradation of cationic and anionic dyes by 5 min whereas SnO2 nanoparticles take 3 h for complete discolouration. The results showed that modification of SnO2 nano metal oxide with bentonite increased the percentage discoloration of the dyes from 70 to 100%. Hence nano sized SnO2 supported bentonite acts as an efficient and environmentally benign photocatalyst.
1. Introduction Nanometal oxides find wide applications in the fields of gas sensors, biomedical devices, lithium batteries, solar cells, optoelectronic devices, coatings, transparent electrodes and photocatalyst (Kumar et al., 2016; Reddy et al., 2016). Of these TiO2, MgO, Fe2O3, SnO2 and ZnO are extensively used as catalysts. But they show tendency to agglomerate when exposed due to high surface energy which results in lower catalytic efficiency. To overcome these problems, scientists adopt many methods to incorporate semiconductor nanoparticle on stable substrates (Pouraboulghasem et al., 2016). Natural clays are used as solid support to get clay semiconductor nanocomposites (Patil et al., 2015.). The increase in basal space and cation exchange capacity result in increased photocatalytic reactivity (Meshram et al., 2011). The structural and morphological properties of nano metal oxides depend on the method of formation. The synthetic techniques include thermal decomposition, coprecipitation, hydrothermal, electrodeposition, sol–gel, solvothermal, solution combustion method etc. (Kang et al., 2007; Chiu and Yeh, 2010). In coprecipitation, inorganic salts are used as precursors. The homogenous solution of the precursor is prepared and the salts are precipitated as hydroxides or oxalates. This method is simple and low cost since water is used as the medium of ⁎
reaction. It has the advantage of following mild reaction conditions with control of particle size. Nano metal oxide of tin (SnO2) is an interesting semiconducting material because of its wide band gap (Eg = 3.6 eV, at 25 °C) (Vadivel and Rajarajan, 2015), nontoxicity, chemical inertness and low cost. The unique photocatalytic properties of SnO2 depend on its morphology, crystallinity, electronic structure, and surface active sites (Wang et al., 2015). Solid supported nano metal oxide catalysts show better performance and one such solid matrix is the natural clays. Using the clay supports semiconductor particles attain the colloidal nature, which can be sedimented easily. Clays are attractive because of their low cost, inert behaviour, high specific surface area, mechanical strength and structural properties (Garrido-ramírez et al., 2010; Herney-ramirez et al., 2010; Momba et al., 2013). Their open pores attract contaminants into them and hence increase the adsorption performance of the dispersed nanoparticles. Bentonite, a locally available and naturally occurring clay with large surface area is mainly composed of montmorillonite as clay mineral. Its unique structure comprises of silica tetrahedral and aluminum octahedral sheets (Darvishi et al., 2016b). It shows good thermal stability, high physical properties and chemical resistance and it has been extensively used as solid matrix for catalysts and sorbents (Papp et al.,
Corresponding author. E-mail address: [email protected] (R. Antony).
https://doi.org/10.1016/j.clay.2019.105312 Received 26 January 2019; Received in revised form 19 September 2019; Accepted 20 September 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 1. (a) XRD pattern and (b) FTIR spectra.
Fig. 2:. (a) PL spectra and (b) TGA curves of SBT.
(Shipway et al., 2000; Templeton et al., 2000). The ultra violet or visible irradiation of the photocatalyst effectively decomposes the dye molecules into non-toxic and inert compounds. Since nano sized particles can affect human health and the environment, they are made into composites with other substrates/fillers which result in synergistic properties. Thus heterogeneous semiconductor photocatalysts are very efficient to remove organic pollutants from aqueous media (Sun et al., 2002). The present work deals with the synthesis of the hetero-system SnO2/Bentonite nanocomposites. SnO2 was supported on the clay using Tin chloride as precursor. The photocatalytic performance of the SBT hetero-system has been investigated for the degradation of both cationic and anionic dyes in presence of Fenton reagent. Thus SnO2 incorporated bentonite nanocomposites function as an efficient photocatalyst for the degradation of hazardous industrial dye wastes. It is interesting to note that SBT nanocomposites loaded with 30% SnO2 gives instantaneous degradation of the dyes even in visible light.
2001; Momba et al., 2013; Toor et al., 2015). Thus bentonite is a potential material for geotechnical engineering and in daily life. Dyes present in textile, paper and industrial effluents are toxic and carcinogenic (Noorimotlagh et al., 2014). These coloured chemical compounds prevent sunlight penetration into water bodies and affect aquatic ecosystem (Hamdaoui and Chiha, 2014). The complex stable aromatic structure of dyes makes them nonbiodegradable. Methylene blue & malachite green are cationic dyes where as methyl orange is anionic which have adverse effects on living things and cause severe environmental issues. The effluent treatment methods include physical techniques such as adsorption, sedimentation, filtration, flocculation and chemical processes like chlorination, ozonization, photocatalysis (Santoyo et al., 2003; Rykowska et al., 2008; Rauf and Ashraf, 2009; Srivastava et al., 2011; Jana et al., 2013; Hanis et al., 2015). In these methods pollutants are transferred from one phase to another phase. But advanced oxidation processes (AOPs) cause destruction of the pollutants in an environment friendly manner (Rajamanickam and Shanthi, 2013). The unstable highly reactive radicals such as O2., OH% etc. attack the dye molecules and convert them into carbon dioxide, water, and mineral salts (Matthews, 1988; Mas, 1997; Grzechulska et al., 2000). Photocatalytic decomposition of industrial waste materials by metal oxide semiconductors is an emerging environmentally viable technique
2. Experimental SnCl4. 5H2O, NaOH, H2O2, bentonite, methylene blue, methyl orange and malachite green dye were purchased from Merck chemical reagent Co. Ltd. India. Stock solutions of MB, MO & MG were prepared 2
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 3. UV–Visible DRS spectrum (inset: Tauc plot) of (a) SnO2, (b) SBT-10, (c) SBT-20 & (d) SBT-30
spectrum was recorded with Fluorescence spectrophotometer (Agilent, Cary Eclipse). Thermogravimetric analysis (TGA) was done with the help of Netzsch-STA 449-F5 machine, in the temperature range 30–800 °C with heating rate 10 K /min under N2 environment. The UV–Vis absorbance of Methylene blue, malachite green and methyl orange solutions was recorded by using a Intech UV–Vis Spectrophotometer-Double beam-2800. The UV diffuse reflectance spectrum was recorded on a Jasco-v-550 UV/Vis spectrophotometer. Homemade photoreactor was used for photocatalytic degradation of dyes.
by dissolving 400 mg of dye in one liter of millipore water. 2.1. Synthesis of SnO2-bentonite (SBT) SBT nanocomposites were prepared by mixing aqueous suspension of bentonite with varying amounts of stannic chloride (10, 20, and 30 wt%) and the pH was maintained at 11–12 by the addition of sodium hydroxide at 60 °C. The mixture was stirred magnetically for 5 h. The precipitate obtained was washed, dried at 80 °C for 24 h and calcined at 600 °C for 2 h. Samples are denoted as SBT-10, SBT-20 and SBT-30, where the number indicates the percentage composition of SnO2 in the nanocomposites.
2.4. Photocatalytic degradation experiments The nanomaterials SnO2, SBT-10, SBT-20 & SBT-30 were examined for the photocatalytic decomposition of MB, MO and MG (10 mg/L). The aqueous dye solution (100 mL) containing 250 mg catalyst and 0.08 M of H2O2 was stirred for 30 min and kept in dark to attain equilibrium. In the photoreactor SnO2 NPs were subjected to irradiation by a UV light source (150 W) and cooled by water circulation, where as SBT nanocomposites degrade dyes even in visible light. 3 mL was taken off after regular intervals of time and the absorbance of the supernatant detected spectrophotometrically. The photodegradation efficiency is calculated as
2.2. Synthesis of SnO2 For comparative studies SnO2NPs were synthesized by the reaction between stannic chloride penta hydrate and sodium hydroxide. The aqueous solutions were mixed and vigorously stirred for 30 min at 60 °C. The resulting precipitate was filtered, washed and dried at 80 °C for 24 h. Finally it was calcined at 600 °C for 2 h. 2.3. Characterization The synthesized samples were characterized by X-ray diffraction analysis (Rigaku Miniflex-600:40 kV and 20 mA current). FTIR Spectroscopy was carried out using Agilent-Technologies-Cary 630 –ATR. Surface morphology was studied by using FESEM taken on a FEI Nova NanoSem 450 electron microscope. The photoluminescence
Degradation efficiency (%) =
Co − Ct × 100 C0
(1)
where, Co is the initial dye concentrations (mg/L) and Ct is the final concentration after time t (min). 3
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 4. (I): SEM images of (a) Bentonite, (b) SnO2 & (c) SBT. (II): (a) SEM images & (b) EDAX elemental analysis images of SBT - 30.
intensity (FWHM), and θ is the Bragg angle. The (110) plane was used in the Scherrer formula for calculation of cyrstal grain size. The calculated average crystallite sizes of the samples SnO2, SBT-10, SBT-20, SBT-30 and BT are 31.14, 39.38, 35.24, 34.22 and 45.72 nm respectively.
3. Results and discussion 3.1. X-ray diffraction analysis (XRD) Fig. 1 (a) shows the diffraction patterns of bentonite, SnO2 and the hetero-system (SBT). The characteristic peaks of bentonite are at 2θ values of 19.8°, 21.8°, 36.0° and 62.5°. The XRD peaks of SnO2 NPs at 2θ values of 26.8, 34.18, 38.22, 52.02, 54.9, 57.98, 62.16, 65.04, 72.04 & 78.81which attributes to (110), (101), (200), (211), (220), (002), (310), (301), (202), (321) & (222) reflecting planes respectively. This indicates the tetragonal rutile structure of pristine SnO2 (JCPDS No.41–1445). In alkaline medium, due to ion exchange reaction SnO2 is deposited on the bentonite clay surface. Hence XRD of SBT shows new peaks in the composite due to SnO2. This is a clear proof for incorporation of SnO2 in the bentonite matrix. The new peaks at 2θ values of 26.81, 34.27 & 51.93 are attributed to (110), (101) & (101) lattice planes respectively of SnO2 in SBT. The incorporation of SnO2 causes decrease of basal spacing of the bentonite which results in the reduction in intensity of the peaks (Djellabi et al., 2014). On comparing the XRD with pure bentonite, it can be seen that peaks of SBT becomes weaker. SBT-30 contains relatively lower amount of bentonite and hence the high proportion of SnO2 cause masking of the peak at 20°. The crystallite size of the NPs was calculated by using Scherrer formula as
D=
kλ βCosθ
3.2. Fourier transform infrared spectroscopy (FTIR) The FTIR spectra of the bentonite, SnO2 and SBT nanocomposites are shown in the Fig. 1(b). The peaks at 603 cm−1 and 459 cm−1 are due to Sn-O-Sn and O-Sn-O stretching respectively in pure SnO2 where as the peak at 511 cm−1 corresponds to Sn-O-Sn stretching in SBT nanocomposites. In bentonite, absorption frequencies at 1030 cm−1 and 534 cm−1 are attributed to SieO stretching and for Si-O-Al bending respectively (Benguella, 2009).The lower wave number of Sn-O-Sn peak as well as the higher wave number of Si-O-Al and SieO bonds in SBT nanocomposites might be due to the coexistence of bentonite in SnO2 (Peikertová, 2015). The broad band near 3452 cm−1 corresponds to the OeH bond. 3.3. Photoluminescence spectroscopy (PL) The PL spectra of SnO2 and SBT nanocomposites are shown in Fig. 2(a). The pristine SnO2 exhibits strong emission peaks, which indicates the high rate of electron– hole recombination suggesting low photocatalytic activity. The peaks observed at 364, 378 and 395 nm indicate that defects exist in SnO2 and the peaks observed around 345 and 390 nm denotes defects in SBT nanocomposites. The broad peaks may be due to the deep traps represented by tin and oxygen vacancies
(2)
where D is the crystallite size, λ is the X-ray wavelength, k is a dimensionless shape factor, β is the line broadening at half the maximum 4
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 5. The UV–Visible absorbance spectra for photodegradation of MB (a) SnO2, (b) SBT-10, (c) SBT-20 & (d) SBT-30.
which can be ascribed to crystallite size, crystallinity and defect associated with oxygen vacancies (Khan et al., 2013a). The spectrum of SnO2 shows that it absorbs mainly in the UV region because of its wide band gap, whereas the spectrum of the SBT nanocomposite showed a broad absorption peak upto 600 nm in the visible region which was assigned to the surface plasmon resonance absorption of the SBT (Khan et al., 2013b). This confirms the incorporation of SnO2 in bentonite matrix effectively. The Tauc plot helps us to calculate the band gap energy in a crystalline semiconductor. For direct transition, their optical absorption near the band edge follows the equation:
or interstitials. These crystal defects function as efficient traps for the photo excited charge carriers (Van Dijken et al., 2000; Peikertová, 2015) and cause shift in the visible region which enhances the photocatalytic activity of the nanocomposites (Pudukudy and Yaakob, 2013, 2014). The PL spectra of SBT nanocomposites exhibit lower intensity peaks as more excited electrons are trapped and transferred stably through the interface. The weak PL peaks for SBT reveals the effective separation of photogenerated electrons and holes. In general, the efficient charge separation and the inhibited electron-hole recombination by the SBT are favorable for enhancing the photo-activity of the nanocomposites. The PL spectra showed that incorporation of SnO2 into bentonite surface can effectively prevent electron-hole recombination during visible light irradiation. This explains why SBT nanocomposites cause photodegradation of organic dyes even in visible light.
(αhυ) = A(hυ − Eg )1/2
(3)
where α, h, ν, Eg, and A are the absorption coefficient, Plank constant, light frequency, band gap energy, and constant A respectively (Hajijafari-bidgoli et al., 2017). The band gap energy was obtained by extrapolating the linear portion of the (α h ν) n curve versus hν to zero. Fig. 3(a–d) illustrates the UV–vis diffuse spectra as well as the Tauc plot of the NPs. The estimated values of band gap energy for SnO2, SBT-10, SBT- 20 & SBT-30 nanocomposites were obtained as 3.41, 2.10, 1.80 and 1.51 eV respectively. Band gap enegy (Eg) influences the degradation efficiency of the nanocomposites. It can be seen that Eg decreases from 3.41 eV through 1.51 eV and this might be due to the formation of defects in the crystal system, which results in new energy level termed Fermi energy level. Now electronic transitions can occur from valence band to fermi energy level. The decrease in band gap value from 3.41 eV may be due to the defects in the crystal system which can be oxygen vacancies or crystal defects. This leads to the generation of photon induced charge carriers which can be trapped by oxygen vacancies resulting in the lower rate of recombination (Wang et al., 2007; Tsai et al., 2017). The charge
3.4. Thermogravimetric analysis (TGA) Fig. 2(b) compares the TGA curves of SnO2 with SBT which shows that the nanocomposites possess higher thermal stability. This might be due to the presence of bentonite matrix which is a well known thermal insulator. The slight decrease in weight below 100 °C can be due to loss of solvent molecules adsorbed on the nanocomposites. 3.5. UV–Visible diffuse reflectance spectroscopy (UV-DRS) Fig. 3 (a–d) shows the UV–vis diffuse absorption spectra of SnO2, SBT-10, SBT- 20 & SBT-30 nanocomposites. SnO2 shows broad peak of absorption maximum in UV region between 248 and 327 nm whereas SBT nanocomposites exhibit peaks of λmax at 270–530 nm. After the ion exchange process, the peak of the SnO2 shows red shift 5
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 6. The UV–Visible absorbance spectra for photodegradation of MG (a) SnO2, (b) SBT-10, (c) SBT-20 & (d) SBT-30.
et al., 2002). The progress of the photodegradation reaction as indicated from the colour change is accompanied by a decrease in absorption maximum. The introduction of H2O2 in the system favored the increase in the concentration of OH∙ radicals and leads to rapid decomposition of dye. Table 1 illustrates the degradation efficiency of SBT-10, SBT-20 and SBT-30 nanocomposites in comparison with that of SnO2. SBT-30 exhibits superior photodegradation efficiency. Discolouration was completed within 5 min for MG, 15 min for MB and 60 min for MO.
carriers can react with oxygen and water molecules at the surface to produce and these free radicals attack dye molecules. 3.6. Field emission scanning electron microscopy (FESEM) and energydispersive X-ray analysis spectroscopy (EDAX) Field emission scanning electron microscopic analysis gives information about the shape of nanoparticles. Fig. 4(I) (a–c) shows the SEM image of bentonite, SnO2 and SBT nanocomposites which indicate the incorporation of globular SnO2 crystallites into numerous nanoflakes of clay particles. Bentonite has a porous structure, suitable for SnO2 immobilization. It can be seen that the structure of the heterosystem has changed and SnO2 nano-particles are suitably placed in the pores (Darvishi et al., 2016a). Fig. 4(II) (a & b) gives the SEM image of SBT 30 along with EDAX mapping results of selected areas of SEM image. The mapping result proves the co existence of Sn, Al, Si, C, O, Na, Mg, Ca, Ti, Fe in the nanocomposites
3.7.2. Photocatalytic mechanism The proposed mechanism of photocatalysis is shown in Fig. 8. From the PL experiments, it can be suggested that electrons are photoexcited from the valence band to conduction band of SnO2 and repelled by negatively charged bentonite surface. Thus electrons can be easily transferred to the conduction band of SnO2 (Hou et al., 2007). These electrons combine with oxygen to form superoxide (O2−) radical. H2O2 which is a strong oxidizing agent present in the medium improves the generation of more (OH∙) radicals in the photocatalytic degradation (Li and Peng, 2018a, 2018b; Peng et al., 2019).H2O2 increases the rate of hydroxyl radical formation. The hydroxyl radicals would be available for attack on the eN]Ne bond of the dye. It may function as an alternative electron acceptor to oxygen [Eq. (4)], which consequently increase the rate of the photocatalytic process.
3.7. Photocatalytic degradation studies 3.7.1. Degradation efficiency In the present study SnO2 nanocomposite is compared with the SBT10, SBT-20 and SBT-30 nanocomposites for photodegradation of MB, MG and MO dyes and their UV absorption spectra are given in Fig. 5 (ad), Fig. 6 (a-d) & Fig. 7 (a-d). The characteristic absorption peaks of MB, MO and MG are at 664, 457 and 618 nm respectively. It can be seen that the eN]Ne bond of the dyes are the most active sites for oxidative attack and degradation indicates the breakup of the chromophore (Sun
O2 – + H2 O2 → OH– + •OH + O2
(4)
H2O2 may get reduced at the conductance band to produce hydroxyl radicals. [Eq. (5)]:
eCB− + H2 O2 → OH– + •OH 6
(5)
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 7. The UV–Visible absorbance spectra for photodegradation of MO (a) SnO2, (b) SBT-10, (c) SBT-20 & (d) SBT-30. Table 1 Degradation efficiency of SBT nanocomposites for MB, MG & MO. Sample
Type of dye
Irradiation time (min)
Photodegradation efficiency (%)
SnO2 10% SnO2/Bent 20% SnO2/Bent 30% SnO2/Bent SnO2 10% SnO2/Bent 20% SnO2/Bent 30% SnO2/Bent SnO2 10% SnO2/Bent 20% SnO2/Bent 30% SnO2/Bent
Methylene blue
150 60 45 15 150 15 15 5 135 75 75 60
65.78 98.20 98.49 99.82 98.08 98.12 99.35 99.57 97.99 97.63 97.89 98.77
Malachite green
Methyl orange
with the dyes. The degradation efficiency for all the dyes after three cycles is shown in Fig. 9(a). The degradation efficiency values were found to be 95%, 93% & 90% respectively for MG, MB & MO after three cycles. This indicates that these photocatalysts can be reused without much decrease in degradation efficiency. Fig. 9(b) compares the xrd of fresh and reused catalysts. It is evident that xrd pattern of regenerated catalyst is similar to that of the original nanocomposite which indicates its stability.
Photolysis of H2O2 would also produce hydroxyl radicals [Eq. (6)]:
H2 O2 + hv → 2•OH
(6)
The hydrogen peroxide, sorbed by the surface of the photocatalyst can scavenge the photo-generated holes (hCB +) also [Eq. (7)]:
hCB+ + H2 O2 → H+ + •HO2
(7)
These free radicals formed react with the dye molecule and degrade it to CO2 and H2O (Nie et al., 2017).
3.7.4. Comparison of degradation efficiency for MB, MG and MO dyes Table 2 shows the comparison of the as –synthesized nanocomposites with other metal oxide based clay catalysts for the
3.7.3. Stability of the photocatalyst In order to study the stability and reusability of the nancomposite (SBT-30) the photodegradation experiment was done for three cycles 7
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 8. Photodegradation mechanism of SBT.
Fig. 9. (a) Bar graph of degradation efficiency of dyes after three cycles & (b) XRD of SBT-30 before and after degradation of three cycles. Table 2 Comparison of degradation percentages of MB, MG and MO dyes. Type of catalyst
Dye
Degradation (%)
Time in minutes
References
TiO2/montmorillonite TiO2-Fe3O4-bentonite Bentonite-N/Fe-TiO2 ZnO/montmorillonite TiO2/Ag3PO4/bentonite ZnO–TiO2/clay ZnO/Tunisian clay ZnO/SiO2-clay TiO2/sepiolite SnO2/bentonite (SBT) SnO2/bentonite (SBT) SnO2/bentonite (SBT)
MB MB MB MB MB MG MG MB MO MB MG MO
93.2 90 100 40 100 100 100 100 90.7 99.82 99.57 98.77
350 90 180 150 130 130 60 180 70 15 5 60
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nanocomposites degrade these dyes in visible light itself. This is the first report of application of clay-semiconductor system of SnO2/Bentonite for the degradation of toxic organic dye wastes.
photodegradation of MB, MO and MG reported in the literature. Compared to conventional clay based metal oxide catalysts, SBT took only few minutes for photodegradation of both cationic and anionic dyes. Thus by simple co-precipitation method, naturally occurring and low cost clay could be modified into superior photocatalyst. Pristine SnO2 causes photodegradation on UV irradiation where as SBT 8
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
4. Conclusions
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Rauf, M.A., Ashraf, S.S., 2009. Fundamental principles and application of heterogeneous photocatalytic degradation of dyes in solution. J. Mater. Sci. Mater. Electron. 151, 10–18. Reddy, M.V., Linh, T.T., Hien, D.T., Venkateshwara, B., Chowdari, R., 2016. SnO2 based materials and their energy storage studies. ACS Sustain. Chem. Eng. 4, 6268–6276. Rykowska, I., Wasiak, W., Byra, J., 2008. Extraction of copper ions using silica gel with chemically modified. Chem. Pap. 62, 255–259. Santoyo, A., Carrasco, J.L., Gomez, E., Martin, F., Montesinos, A.M., 2003. Application of reverse osmosis to reduce pollutants present in industrial wastewater. Desalination. 155, 101–108. Shipway, A.N., Katz, E., Willner, I., 2000. Nanoparticle arrays on surfaces for electronic, optical and sensor applications. Chemphyschem. 1, 18–52. Srivastava, V., Weng, C.H., Singh, V.K., Sharma, Y.C., 2011. Adsorption of nickel ions from aqueous solutions by nano alumina : kinetic, mass transfer, and equilibrium
SnO2 incorporated bentonite nanocomposites were prepared by coprecipitation method. XRD, FTIR, UV-DRS, FESEM, TGA and PL studies confirmed that SnO2 was incorporated in the bentonite matrix. X-ray diffraction pattern revealed the tetragonal rutile structure of SnO2 nanoparticles supported on the clay. The peak at 511 cm−1 corresponds to Sn-O-Sn stretching of SBT nanocomposites in FTIR. The higher thermal stability of the SBT nanocomposites as evidenced by TGA might be due to the presence of natural clay, a thermal insulator. FESEM studies ascertained the presence of SnO2 crystallites in the bentonite surface. The UV-DRS spectrum of the SBT nanocomposites showed a broad absorption peak upto 600 nm in the visible region which was assigned to the surface plasmon resonance absorption of the SBT. The weak PL peaks in SBT composites compared to SnO2 point towards the higher catalytic activity of the SBT. It is interesting to note that SBT nanocomposites loaded with 30% SnO2 gives instantaneous degradation of the cationic and anionic dyes even in visible light. Thus it is concluded that the low cost and eco-friendly clay semiconductor nanocomposites of SnO2 with Bentonite (SBT) is an efficient photocatalyst. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors are thankful for the facilities provided by the DST-FIST of Nirmalagiri College, Kuthuparamba, Kerala, India. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References Akkari, M., Aranda, P., Ben, A., Amara, H., Ruiz-hitzky, E., 2016. Organoclay hybrid materials as precursors of porous ZnO / silica-clay heterostructures for photocatalytic applications. Beilstein J. Nanotechnol. 7, 1971–1982. Bel, H., Ben, M., Elena, M., Das, P., 2016. Photocatalytic degradation of methyl green dye in aqueous solution over natural clay-supported ZnO – TiO2 catalysts. J. Photochem. Photobiol. A. 315, 25–33. Benguella, B., 2009. Adsorption of bezanyl red and nylomine green from aqueous solutions by natural and acid-activated bentonite. Desalination. 235, 276–292. Cao, X., Liu, C., Hu, Y., Yang, W., Chen, J., 2016. Synthesis of N / Fe comodified TiO 2 loaded on bentonite for enhanced photocatalytic activity under UV-Vis light. J. Nano Mater. 1. Chen, W., Xiao, H., Xu, H., Ding, T., Gu, Y., 2015. Photodegradation of methylene blue by TiO 2 -Fe 3O4 -bentonite magnetic nanocomposite. Int. J. Photoenergy 2015, 1–7. Chiu, H., Yeh, C., 2010. Hydrothermal synthesis of SnO2 nanoparticles and their gassensing of alcohol. J. Phys. Chem. C 111, 7256–7259. Darvishi, R., Soltani, C., Haghighat, Z., 2016a. Visible light photocatalysis of a textile dye over ZnO nanostructures covered on natural diatomite. Turk. J. Chem. 40, 454–466. Darvishi, R., Soltani, C., Jor, S., Safari, M., 2016b. Enhanced sonocatalysis of textile wastewater using bentonite- supported ZnO nanoparticles: response surface methodological approach. J. Environ. Manag. 179, 47–57. Djellabi, R., Ghorab, M.F., Cerrato, G., Morandi, S., Gatto, S., Oldani, V., Di Michele, A., Bianchi, C.L., 2014. Photoactive TiO 2 – montmorillonite composite for degradation of organic dyes in water. J. Photochem. Photobiol. A. 295, 57–63. Fatimah, I., Wang, S., Wulandari, D., 2011. ZnO / montmorillonite for photocatalytic and photochemical degradation of methylene blue. Appl. Clay Sci. 53, 553–560. Garrido-ramírez, E.G., Theng, B.K.G., Mora, M.L., 2010. Clays and oxide minerals as catalysts and nanocatalysts in Fenton-like reactions — a review. Appl. Clay Sci. 47, 182–192. Grzechulska, J., Hamerski, M., Morawski, A.W., 2000. Photocatalytic decomposition of oil in water. Water Res. 34, 1638–1644. Hadjltaief, H., Ameur, S., Da Costa, P., Zina, M., Galvez, M.E., 2018. Photocatalytic decolorization of cationic and anionic dyes over ZnO nanoparticle immobilized on natural Tunisian clay. Appl. Clay Sci. 152, 148–157. Hajijafari-bidgoli, S., Sadeghzadeh-attar, A., Bafandeh, M.R., 2017. Structural and optical properties of Sr-modified bismuth silicate nanostructured films synthesized by sol gel method. J. Nanostruct. 7, 258–265. Hamdaoui, O., Chiha, M., 2014. Removal of methylene blue from aqueous solutions by
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Tsai, C., Liu, C., Fan, C., Hsi, H., Chang, T., 2017. Synthesis of a SnO 2 / TNT heterojunction nanocomposite as a high- performance photocatalyst. J. Phys. Chem. C 121, 6050–6059. Vadivel, S., Rajarajan, G., 2015. Effect of W doping on structural, optical and photocatalytic activity of SnO 2 nanostructure thin films. J. Mater. Sci. Mater. Electron. 26, 7127–7133. Van Dijken, A., Meulenkamp, E.A., Vanmaekelbergh, D., Meijerink, A., 2000. The luminescence of nanocrystalline ZnO particles: the mechanism of the ultraviolet and visible emission. J. Lumin. 89, 454–456. Wang, Y., Zhang, L., Deng, K., Chen, X., Zou, Z., 2007. Low temperature synthesis and photocatalytic activity of rutile TiO 2 nanorod. J. Phys. Chem. C 111, 2709–2714. Wang, J., Fan, H., Yu, H., 2015. Synthesis of hierarchical porous Zn-doped SnO 2 spheres and their photocatalytic properties. J. Mater. Eng. Perform. 24, 4260–4266.
studies. J. Chem. Eng. Data 56, 1414–1422. Sun, Z., Chen, Y., Ke, Q., Yang, Y., Yuan, J., 2002. Photocatalytic degradation of cationic azo dye by TiO 2 / bentonite nanocomposite. J. Photochem. Photobiol. A 149, 169–174. Templeton, A.C., Wuelfing, W.P., Murray, R.W., 2000. Monolayer-protected cluster molecules characterization of MPCs. Acc. Chem. Res. 33, 27–36. Thanh, T., Tran, T., Doan, V.D., Thai, T., Ho, T., Le, V.T., Nguyen, H.T., 2018. Novel of TiO2/Ag3PO4/bentonite composite photocatalysts: preparation, characterization and application for degradation of methylene blue in aqueous solution. Environ. Eng. Sci. 36, 1–10. Toor, M., Jin, B., Dai, S., Vimonses, V., 2015. Activating natural bentonite as a costeffective adsorbent for removal of Congo-red in wastewater. J. Ind. Eng. Chem. 21, 653–661.
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
Clay semiconductor hetero-system of SnO2/bentonite nanocomposites for catalytic degradation of toxic organic wastes Avis Tresa Babu, Rosy Antony
T
⁎
Post graduate and Research Department of Chemistry, Nirmalagiri College, Kannur, Kerala 670701, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Bentonite SnO2 Clay semiconductor system Organic dyes Nanocomposites
SnO2/Bentonite (SBT) nanocomposites containing varying amounts of SnO2 (10, 20 & 30 wt%) were prepared by co-precipitation method and characterized. X-ray diffraction studies indicated the tetragonal rutile structure of SnO2 nanoparticles located on the surface bentonite layers. FTIR analysis data confirmed the presence of SneO bonds. TGA indicated higher thermal stability of the SBT nanocomposites. FESEM studies ascertained the incorporation of globular SnO2 crystallites into the numerous nano-flakes of clay particles. The tau plot from UV–Visible DRS studies revealed that the greater loading of SnO2 reduced the band gap energy and might be due to defects in the crystal system. The reduced intensity of PL peaks in SBT composites compared to SnO2 indicated the greater catalytic activity of the nanocomposites. Pristine SnO2 causes photodegradation on UV irradiation where as SBT nanocomposites degrade these dyes in visible light itself. It is interesting to note that the SBT nanocomposites show photocatalytic degradation of cationic and anionic dyes by 5 min whereas SnO2 nanoparticles take 3 h for complete discolouration. The results showed that modification of SnO2 nano metal oxide with bentonite increased the percentage discoloration of the dyes from 70 to 100%. Hence nano sized SnO2 supported bentonite acts as an efficient and environmentally benign photocatalyst.
1. Introduction Nanometal oxides find wide applications in the fields of gas sensors, biomedical devices, lithium batteries, solar cells, optoelectronic devices, coatings, transparent electrodes and photocatalyst (Kumar et al., 2016; Reddy et al., 2016). Of these TiO2, MgO, Fe2O3, SnO2 and ZnO are extensively used as catalysts. But they show tendency to agglomerate when exposed due to high surface energy which results in lower catalytic efficiency. To overcome these problems, scientists adopt many methods to incorporate semiconductor nanoparticle on stable substrates (Pouraboulghasem et al., 2016). Natural clays are used as solid support to get clay semiconductor nanocomposites (Patil et al., 2015.). The increase in basal space and cation exchange capacity result in increased photocatalytic reactivity (Meshram et al., 2011). The structural and morphological properties of nano metal oxides depend on the method of formation. The synthetic techniques include thermal decomposition, coprecipitation, hydrothermal, electrodeposition, sol–gel, solvothermal, solution combustion method etc. (Kang et al., 2007; Chiu and Yeh, 2010). In coprecipitation, inorganic salts are used as precursors. The homogenous solution of the precursor is prepared and the salts are precipitated as hydroxides or oxalates. This method is simple and low cost since water is used as the medium of ⁎
reaction. It has the advantage of following mild reaction conditions with control of particle size. Nano metal oxide of tin (SnO2) is an interesting semiconducting material because of its wide band gap (Eg = 3.6 eV, at 25 °C) (Vadivel and Rajarajan, 2015), nontoxicity, chemical inertness and low cost. The unique photocatalytic properties of SnO2 depend on its morphology, crystallinity, electronic structure, and surface active sites (Wang et al., 2015). Solid supported nano metal oxide catalysts show better performance and one such solid matrix is the natural clays. Using the clay supports semiconductor particles attain the colloidal nature, which can be sedimented easily. Clays are attractive because of their low cost, inert behaviour, high specific surface area, mechanical strength and structural properties (Garrido-ramírez et al., 2010; Herney-ramirez et al., 2010; Momba et al., 2013). Their open pores attract contaminants into them and hence increase the adsorption performance of the dispersed nanoparticles. Bentonite, a locally available and naturally occurring clay with large surface area is mainly composed of montmorillonite as clay mineral. Its unique structure comprises of silica tetrahedral and aluminum octahedral sheets (Darvishi et al., 2016b). It shows good thermal stability, high physical properties and chemical resistance and it has been extensively used as solid matrix for catalysts and sorbents (Papp et al.,
Corresponding author. E-mail address: [email protected] (R. Antony).
https://doi.org/10.1016/j.clay.2019.105312 Received 26 January 2019; Received in revised form 19 September 2019; Accepted 20 September 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 1. (a) XRD pattern and (b) FTIR spectra.
Fig. 2:. (a) PL spectra and (b) TGA curves of SBT.
(Shipway et al., 2000; Templeton et al., 2000). The ultra violet or visible irradiation of the photocatalyst effectively decomposes the dye molecules into non-toxic and inert compounds. Since nano sized particles can affect human health and the environment, they are made into composites with other substrates/fillers which result in synergistic properties. Thus heterogeneous semiconductor photocatalysts are very efficient to remove organic pollutants from aqueous media (Sun et al., 2002). The present work deals with the synthesis of the hetero-system SnO2/Bentonite nanocomposites. SnO2 was supported on the clay using Tin chloride as precursor. The photocatalytic performance of the SBT hetero-system has been investigated for the degradation of both cationic and anionic dyes in presence of Fenton reagent. Thus SnO2 incorporated bentonite nanocomposites function as an efficient photocatalyst for the degradation of hazardous industrial dye wastes. It is interesting to note that SBT nanocomposites loaded with 30% SnO2 gives instantaneous degradation of the dyes even in visible light.
2001; Momba et al., 2013; Toor et al., 2015). Thus bentonite is a potential material for geotechnical engineering and in daily life. Dyes present in textile, paper and industrial effluents are toxic and carcinogenic (Noorimotlagh et al., 2014). These coloured chemical compounds prevent sunlight penetration into water bodies and affect aquatic ecosystem (Hamdaoui and Chiha, 2014). The complex stable aromatic structure of dyes makes them nonbiodegradable. Methylene blue & malachite green are cationic dyes where as methyl orange is anionic which have adverse effects on living things and cause severe environmental issues. The effluent treatment methods include physical techniques such as adsorption, sedimentation, filtration, flocculation and chemical processes like chlorination, ozonization, photocatalysis (Santoyo et al., 2003; Rykowska et al., 2008; Rauf and Ashraf, 2009; Srivastava et al., 2011; Jana et al., 2013; Hanis et al., 2015). In these methods pollutants are transferred from one phase to another phase. But advanced oxidation processes (AOPs) cause destruction of the pollutants in an environment friendly manner (Rajamanickam and Shanthi, 2013). The unstable highly reactive radicals such as O2., OH% etc. attack the dye molecules and convert them into carbon dioxide, water, and mineral salts (Matthews, 1988; Mas, 1997; Grzechulska et al., 2000). Photocatalytic decomposition of industrial waste materials by metal oxide semiconductors is an emerging environmentally viable technique
2. Experimental SnCl4. 5H2O, NaOH, H2O2, bentonite, methylene blue, methyl orange and malachite green dye were purchased from Merck chemical reagent Co. Ltd. India. Stock solutions of MB, MO & MG were prepared 2
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 3. UV–Visible DRS spectrum (inset: Tauc plot) of (a) SnO2, (b) SBT-10, (c) SBT-20 & (d) SBT-30
spectrum was recorded with Fluorescence spectrophotometer (Agilent, Cary Eclipse). Thermogravimetric analysis (TGA) was done with the help of Netzsch-STA 449-F5 machine, in the temperature range 30–800 °C with heating rate 10 K /min under N2 environment. The UV–Vis absorbance of Methylene blue, malachite green and methyl orange solutions was recorded by using a Intech UV–Vis Spectrophotometer-Double beam-2800. The UV diffuse reflectance spectrum was recorded on a Jasco-v-550 UV/Vis spectrophotometer. Homemade photoreactor was used for photocatalytic degradation of dyes.
by dissolving 400 mg of dye in one liter of millipore water. 2.1. Synthesis of SnO2-bentonite (SBT) SBT nanocomposites were prepared by mixing aqueous suspension of bentonite with varying amounts of stannic chloride (10, 20, and 30 wt%) and the pH was maintained at 11–12 by the addition of sodium hydroxide at 60 °C. The mixture was stirred magnetically for 5 h. The precipitate obtained was washed, dried at 80 °C for 24 h and calcined at 600 °C for 2 h. Samples are denoted as SBT-10, SBT-20 and SBT-30, where the number indicates the percentage composition of SnO2 in the nanocomposites.
2.4. Photocatalytic degradation experiments The nanomaterials SnO2, SBT-10, SBT-20 & SBT-30 were examined for the photocatalytic decomposition of MB, MO and MG (10 mg/L). The aqueous dye solution (100 mL) containing 250 mg catalyst and 0.08 M of H2O2 was stirred for 30 min and kept in dark to attain equilibrium. In the photoreactor SnO2 NPs were subjected to irradiation by a UV light source (150 W) and cooled by water circulation, where as SBT nanocomposites degrade dyes even in visible light. 3 mL was taken off after regular intervals of time and the absorbance of the supernatant detected spectrophotometrically. The photodegradation efficiency is calculated as
2.2. Synthesis of SnO2 For comparative studies SnO2NPs were synthesized by the reaction between stannic chloride penta hydrate and sodium hydroxide. The aqueous solutions were mixed and vigorously stirred for 30 min at 60 °C. The resulting precipitate was filtered, washed and dried at 80 °C for 24 h. Finally it was calcined at 600 °C for 2 h. 2.3. Characterization The synthesized samples were characterized by X-ray diffraction analysis (Rigaku Miniflex-600:40 kV and 20 mA current). FTIR Spectroscopy was carried out using Agilent-Technologies-Cary 630 –ATR. Surface morphology was studied by using FESEM taken on a FEI Nova NanoSem 450 electron microscope. The photoluminescence
Degradation efficiency (%) =
Co − Ct × 100 C0
(1)
where, Co is the initial dye concentrations (mg/L) and Ct is the final concentration after time t (min). 3
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 4. (I): SEM images of (a) Bentonite, (b) SnO2 & (c) SBT. (II): (a) SEM images & (b) EDAX elemental analysis images of SBT - 30.
intensity (FWHM), and θ is the Bragg angle. The (110) plane was used in the Scherrer formula for calculation of cyrstal grain size. The calculated average crystallite sizes of the samples SnO2, SBT-10, SBT-20, SBT-30 and BT are 31.14, 39.38, 35.24, 34.22 and 45.72 nm respectively.
3. Results and discussion 3.1. X-ray diffraction analysis (XRD) Fig. 1 (a) shows the diffraction patterns of bentonite, SnO2 and the hetero-system (SBT). The characteristic peaks of bentonite are at 2θ values of 19.8°, 21.8°, 36.0° and 62.5°. The XRD peaks of SnO2 NPs at 2θ values of 26.8, 34.18, 38.22, 52.02, 54.9, 57.98, 62.16, 65.04, 72.04 & 78.81which attributes to (110), (101), (200), (211), (220), (002), (310), (301), (202), (321) & (222) reflecting planes respectively. This indicates the tetragonal rutile structure of pristine SnO2 (JCPDS No.41–1445). In alkaline medium, due to ion exchange reaction SnO2 is deposited on the bentonite clay surface. Hence XRD of SBT shows new peaks in the composite due to SnO2. This is a clear proof for incorporation of SnO2 in the bentonite matrix. The new peaks at 2θ values of 26.81, 34.27 & 51.93 are attributed to (110), (101) & (101) lattice planes respectively of SnO2 in SBT. The incorporation of SnO2 causes decrease of basal spacing of the bentonite which results in the reduction in intensity of the peaks (Djellabi et al., 2014). On comparing the XRD with pure bentonite, it can be seen that peaks of SBT becomes weaker. SBT-30 contains relatively lower amount of bentonite and hence the high proportion of SnO2 cause masking of the peak at 20°. The crystallite size of the NPs was calculated by using Scherrer formula as
D=
kλ βCosθ
3.2. Fourier transform infrared spectroscopy (FTIR) The FTIR spectra of the bentonite, SnO2 and SBT nanocomposites are shown in the Fig. 1(b). The peaks at 603 cm−1 and 459 cm−1 are due to Sn-O-Sn and O-Sn-O stretching respectively in pure SnO2 where as the peak at 511 cm−1 corresponds to Sn-O-Sn stretching in SBT nanocomposites. In bentonite, absorption frequencies at 1030 cm−1 and 534 cm−1 are attributed to SieO stretching and for Si-O-Al bending respectively (Benguella, 2009).The lower wave number of Sn-O-Sn peak as well as the higher wave number of Si-O-Al and SieO bonds in SBT nanocomposites might be due to the coexistence of bentonite in SnO2 (Peikertová, 2015). The broad band near 3452 cm−1 corresponds to the OeH bond. 3.3. Photoluminescence spectroscopy (PL) The PL spectra of SnO2 and SBT nanocomposites are shown in Fig. 2(a). The pristine SnO2 exhibits strong emission peaks, which indicates the high rate of electron– hole recombination suggesting low photocatalytic activity. The peaks observed at 364, 378 and 395 nm indicate that defects exist in SnO2 and the peaks observed around 345 and 390 nm denotes defects in SBT nanocomposites. The broad peaks may be due to the deep traps represented by tin and oxygen vacancies
(2)
where D is the crystallite size, λ is the X-ray wavelength, k is a dimensionless shape factor, β is the line broadening at half the maximum 4
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 5. The UV–Visible absorbance spectra for photodegradation of MB (a) SnO2, (b) SBT-10, (c) SBT-20 & (d) SBT-30.
which can be ascribed to crystallite size, crystallinity and defect associated with oxygen vacancies (Khan et al., 2013a). The spectrum of SnO2 shows that it absorbs mainly in the UV region because of its wide band gap, whereas the spectrum of the SBT nanocomposite showed a broad absorption peak upto 600 nm in the visible region which was assigned to the surface plasmon resonance absorption of the SBT (Khan et al., 2013b). This confirms the incorporation of SnO2 in bentonite matrix effectively. The Tauc plot helps us to calculate the band gap energy in a crystalline semiconductor. For direct transition, their optical absorption near the band edge follows the equation:
or interstitials. These crystal defects function as efficient traps for the photo excited charge carriers (Van Dijken et al., 2000; Peikertová, 2015) and cause shift in the visible region which enhances the photocatalytic activity of the nanocomposites (Pudukudy and Yaakob, 2013, 2014). The PL spectra of SBT nanocomposites exhibit lower intensity peaks as more excited electrons are trapped and transferred stably through the interface. The weak PL peaks for SBT reveals the effective separation of photogenerated electrons and holes. In general, the efficient charge separation and the inhibited electron-hole recombination by the SBT are favorable for enhancing the photo-activity of the nanocomposites. The PL spectra showed that incorporation of SnO2 into bentonite surface can effectively prevent electron-hole recombination during visible light irradiation. This explains why SBT nanocomposites cause photodegradation of organic dyes even in visible light.
(αhυ) = A(hυ − Eg )1/2
(3)
where α, h, ν, Eg, and A are the absorption coefficient, Plank constant, light frequency, band gap energy, and constant A respectively (Hajijafari-bidgoli et al., 2017). The band gap energy was obtained by extrapolating the linear portion of the (α h ν) n curve versus hν to zero. Fig. 3(a–d) illustrates the UV–vis diffuse spectra as well as the Tauc plot of the NPs. The estimated values of band gap energy for SnO2, SBT-10, SBT- 20 & SBT-30 nanocomposites were obtained as 3.41, 2.10, 1.80 and 1.51 eV respectively. Band gap enegy (Eg) influences the degradation efficiency of the nanocomposites. It can be seen that Eg decreases from 3.41 eV through 1.51 eV and this might be due to the formation of defects in the crystal system, which results in new energy level termed Fermi energy level. Now electronic transitions can occur from valence band to fermi energy level. The decrease in band gap value from 3.41 eV may be due to the defects in the crystal system which can be oxygen vacancies or crystal defects. This leads to the generation of photon induced charge carriers which can be trapped by oxygen vacancies resulting in the lower rate of recombination (Wang et al., 2007; Tsai et al., 2017). The charge
3.4. Thermogravimetric analysis (TGA) Fig. 2(b) compares the TGA curves of SnO2 with SBT which shows that the nanocomposites possess higher thermal stability. This might be due to the presence of bentonite matrix which is a well known thermal insulator. The slight decrease in weight below 100 °C can be due to loss of solvent molecules adsorbed on the nanocomposites. 3.5. UV–Visible diffuse reflectance spectroscopy (UV-DRS) Fig. 3 (a–d) shows the UV–vis diffuse absorption spectra of SnO2, SBT-10, SBT- 20 & SBT-30 nanocomposites. SnO2 shows broad peak of absorption maximum in UV region between 248 and 327 nm whereas SBT nanocomposites exhibit peaks of λmax at 270–530 nm. After the ion exchange process, the peak of the SnO2 shows red shift 5
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 6. The UV–Visible absorbance spectra for photodegradation of MG (a) SnO2, (b) SBT-10, (c) SBT-20 & (d) SBT-30.
et al., 2002). The progress of the photodegradation reaction as indicated from the colour change is accompanied by a decrease in absorption maximum. The introduction of H2O2 in the system favored the increase in the concentration of OH∙ radicals and leads to rapid decomposition of dye. Table 1 illustrates the degradation efficiency of SBT-10, SBT-20 and SBT-30 nanocomposites in comparison with that of SnO2. SBT-30 exhibits superior photodegradation efficiency. Discolouration was completed within 5 min for MG, 15 min for MB and 60 min for MO.
carriers can react with oxygen and water molecules at the surface to produce and these free radicals attack dye molecules. 3.6. Field emission scanning electron microscopy (FESEM) and energydispersive X-ray analysis spectroscopy (EDAX) Field emission scanning electron microscopic analysis gives information about the shape of nanoparticles. Fig. 4(I) (a–c) shows the SEM image of bentonite, SnO2 and SBT nanocomposites which indicate the incorporation of globular SnO2 crystallites into numerous nanoflakes of clay particles. Bentonite has a porous structure, suitable for SnO2 immobilization. It can be seen that the structure of the heterosystem has changed and SnO2 nano-particles are suitably placed in the pores (Darvishi et al., 2016a). Fig. 4(II) (a & b) gives the SEM image of SBT 30 along with EDAX mapping results of selected areas of SEM image. The mapping result proves the co existence of Sn, Al, Si, C, O, Na, Mg, Ca, Ti, Fe in the nanocomposites
3.7.2. Photocatalytic mechanism The proposed mechanism of photocatalysis is shown in Fig. 8. From the PL experiments, it can be suggested that electrons are photoexcited from the valence band to conduction band of SnO2 and repelled by negatively charged bentonite surface. Thus electrons can be easily transferred to the conduction band of SnO2 (Hou et al., 2007). These electrons combine with oxygen to form superoxide (O2−) radical. H2O2 which is a strong oxidizing agent present in the medium improves the generation of more (OH∙) radicals in the photocatalytic degradation (Li and Peng, 2018a, 2018b; Peng et al., 2019).H2O2 increases the rate of hydroxyl radical formation. The hydroxyl radicals would be available for attack on the eN]Ne bond of the dye. It may function as an alternative electron acceptor to oxygen [Eq. (4)], which consequently increase the rate of the photocatalytic process.
3.7. Photocatalytic degradation studies 3.7.1. Degradation efficiency In the present study SnO2 nanocomposite is compared with the SBT10, SBT-20 and SBT-30 nanocomposites for photodegradation of MB, MG and MO dyes and their UV absorption spectra are given in Fig. 5 (ad), Fig. 6 (a-d) & Fig. 7 (a-d). The characteristic absorption peaks of MB, MO and MG are at 664, 457 and 618 nm respectively. It can be seen that the eN]Ne bond of the dyes are the most active sites for oxidative attack and degradation indicates the breakup of the chromophore (Sun
O2 – + H2 O2 → OH– + •OH + O2
(4)
H2O2 may get reduced at the conductance band to produce hydroxyl radicals. [Eq. (5)]:
eCB− + H2 O2 → OH– + •OH 6
(5)
Applied Clay Science 183 (2019) 105312
A.T. Babu and R. Antony
Fig. 7. The UV–Visible absorbance spectra for photodegradation of MO (a) SnO2, (b) SBT-10, (c) SBT-20 & (d) SBT-30. Table 1 Degradation efficiency of SBT nanocomposites for MB, MG & MO. Sample
Type of dye
Irradiation time (min)
Photodegradation efficiency (%)
SnO2 10% SnO2/Bent 20% SnO2/Bent 30% SnO2/Bent SnO2 10% SnO2/Bent 20% SnO2/Bent 30% SnO2/Bent SnO2 10% SnO2/Bent 20% SnO2/Bent 30% SnO2/Bent
Methylene blue
150 60 45 15 150 15 15 5 135 75 75 60
65.78 98.20 98.49 99.82 98.08 98.12 99.35 99.57 97.99 97.63 97.89 98.77
Malachite green
Methyl orange
with the dyes. The degradation efficiency for all the dyes after three cycles is shown in Fig. 9(a). The degradation efficiency values were found to be 95%, 93% & 90% respectively for MG, MB & MO after three cycles. This indicates that these photocatalysts can be reused without much decrease in degradation efficiency. Fig. 9(b) compares the xrd of fresh and reused catalysts. It is evident that xrd pattern of regenerated catalyst is similar to that of the original nanocomposite which indicates its stability.
Photolysis of H2O2 would also produce hydroxyl radicals [Eq. (6)]:
H2 O2 + hv → 2•OH
(6)
The hydrogen peroxide, sorbed by the surface of the photocatalyst can scavenge the photo-generated holes (hCB +) also [Eq. (7)]:
hCB+ + H2 O2 → H+ + •HO2
(7)
These free radicals formed react with the dye molecule and degrade it to CO2 and H2O (Nie et al., 2017).
3.7.4. Comparison of degradation efficiency for MB, MG and MO dyes Table 2 shows the comparison of the as –synthesized nanocomposites with other metal oxide based clay catalysts for the
3.7.3. Stability of the photocatalyst In order to study the stability and reusability of the nancomposite (SBT-30) the photodegradation experiment was done for three cycles 7
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A.T. Babu and R. Antony
Fig. 8. Photodegradation mechanism of SBT.
Fig. 9. (a) Bar graph of degradation efficiency of dyes after three cycles & (b) XRD of SBT-30 before and after degradation of three cycles. Table 2 Comparison of degradation percentages of MB, MG and MO dyes. Type of catalyst
Dye
Degradation (%)
Time in minutes
References
TiO2/montmorillonite TiO2-Fe3O4-bentonite Bentonite-N/Fe-TiO2 ZnO/montmorillonite TiO2/Ag3PO4/bentonite ZnO–TiO2/clay ZnO/Tunisian clay ZnO/SiO2-clay TiO2/sepiolite SnO2/bentonite (SBT) SnO2/bentonite (SBT) SnO2/bentonite (SBT)
MB MB MB MB MB MG MG MB MO MB MG MO
93.2 90 100 40 100 100 100 100 90.7 99.82 99.57 98.77
350 90 180 150 130 130 60 180 70 15 5 60
(Djellabi et al., 2014) (Chen et al., 2015) (Cao et al., 2016) (Fatimah et al., 2011) (Thanh et al., 2018) (Bel et al., 2016) (Hadjltaief et al., 2018) (Akkari et al., 2016) (Liu et al., 2018) Present work Present work Present work
nanocomposites degrade these dyes in visible light itself. This is the first report of application of clay-semiconductor system of SnO2/Bentonite for the degradation of toxic organic dye wastes.
photodegradation of MB, MO and MG reported in the literature. Compared to conventional clay based metal oxide catalysts, SBT took only few minutes for photodegradation of both cationic and anionic dyes. Thus by simple co-precipitation method, naturally occurring and low cost clay could be modified into superior photocatalyst. Pristine SnO2 causes photodegradation on UV irradiation where as SBT 8
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4. Conclusions
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SnO2 incorporated bentonite nanocomposites were prepared by coprecipitation method. XRD, FTIR, UV-DRS, FESEM, TGA and PL studies confirmed that SnO2 was incorporated in the bentonite matrix. X-ray diffraction pattern revealed the tetragonal rutile structure of SnO2 nanoparticles supported on the clay. The peak at 511 cm−1 corresponds to Sn-O-Sn stretching of SBT nanocomposites in FTIR. The higher thermal stability of the SBT nanocomposites as evidenced by TGA might be due to the presence of natural clay, a thermal insulator. FESEM studies ascertained the presence of SnO2 crystallites in the bentonite surface. The UV-DRS spectrum of the SBT nanocomposites showed a broad absorption peak upto 600 nm in the visible region which was assigned to the surface plasmon resonance absorption of the SBT. The weak PL peaks in SBT composites compared to SnO2 point towards the higher catalytic activity of the SBT. It is interesting to note that SBT nanocomposites loaded with 30% SnO2 gives instantaneous degradation of the cationic and anionic dyes even in visible light. Thus it is concluded that the low cost and eco-friendly clay semiconductor nanocomposites of SnO2 with Bentonite (SBT) is an efficient photocatalyst. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors are thankful for the facilities provided by the DST-FIST of Nirmalagiri College, Kuthuparamba, Kerala, India. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References Akkari, M., Aranda, P., Ben, A., Amara, H., Ruiz-hitzky, E., 2016. Organoclay hybrid materials as precursors of porous ZnO / silica-clay heterostructures for photocatalytic applications. Beilstein J. Nanotechnol. 7, 1971–1982. Bel, H., Ben, M., Elena, M., Das, P., 2016. Photocatalytic degradation of methyl green dye in aqueous solution over natural clay-supported ZnO – TiO2 catalysts. J. Photochem. Photobiol. A. 315, 25–33. Benguella, B., 2009. Adsorption of bezanyl red and nylomine green from aqueous solutions by natural and acid-activated bentonite. Desalination. 235, 276–292. Cao, X., Liu, C., Hu, Y., Yang, W., Chen, J., 2016. Synthesis of N / Fe comodified TiO 2 loaded on bentonite for enhanced photocatalytic activity under UV-Vis light. J. Nano Mater. 1. Chen, W., Xiao, H., Xu, H., Ding, T., Gu, Y., 2015. Photodegradation of methylene blue by TiO 2 -Fe 3O4 -bentonite magnetic nanocomposite. Int. J. Photoenergy 2015, 1–7. Chiu, H., Yeh, C., 2010. Hydrothermal synthesis of SnO2 nanoparticles and their gassensing of alcohol. J. Phys. Chem. C 111, 7256–7259. Darvishi, R., Soltani, C., Haghighat, Z., 2016a. Visible light photocatalysis of a textile dye over ZnO nanostructures covered on natural diatomite. Turk. J. Chem. 40, 454–466. Darvishi, R., Soltani, C., Jor, S., Safari, M., 2016b. Enhanced sonocatalysis of textile wastewater using bentonite- supported ZnO nanoparticles: response surface methodological approach. J. Environ. Manag. 179, 47–57. Djellabi, R., Ghorab, M.F., Cerrato, G., Morandi, S., Gatto, S., Oldani, V., Di Michele, A., Bianchi, C.L., 2014. Photoactive TiO 2 – montmorillonite composite for degradation of organic dyes in water. J. Photochem. Photobiol. A. 295, 57–63. Fatimah, I., Wang, S., Wulandari, D., 2011. ZnO / montmorillonite for photocatalytic and photochemical degradation of methylene blue. Appl. Clay Sci. 53, 553–560. Garrido-ramírez, E.G., Theng, B.K.G., Mora, M.L., 2010. Clays and oxide minerals as catalysts and nanocatalysts in Fenton-like reactions — a review. Appl. Clay Sci. 47, 182–192. Grzechulska, J., Hamerski, M., Morawski, A.W., 2000. Photocatalytic decomposition of oil in water. Water Res. 34, 1638–1644. Hadjltaief, H., Ameur, S., Da Costa, P., Zina, M., Galvez, M.E., 2018. Photocatalytic decolorization of cationic and anionic dyes over ZnO nanoparticle immobilized on natural Tunisian clay. Appl. Clay Sci. 152, 148–157. Hajijafari-bidgoli, S., Sadeghzadeh-attar, A., Bafandeh, M.R., 2017. Structural and optical properties of Sr-modified bismuth silicate nanostructured films synthesized by sol gel method. J. Nanostruct. 7, 258–265. Hamdaoui, O., Chiha, M., 2014. Removal of methylene blue from aqueous solutions by
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