Scholarly article on topic 'Optical, magnetic properties and visible light photocatalytic activity of CeO2/SnO2 nanocomposites'

Optical, magnetic properties and visible light photocatalytic activity of CeO2/SnO2 nanocomposites Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — S. Usharani, V. Rajendran

Abstract CeO2/SnO2 nanocomposites were synthesized by the wet chemical method, using various pH such as 4.5, 5.5 and 6.5. The prepared CeO2/SnO2 samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Transmission electron microscopy (TEM), UV-Diffuse Reflectance Spectroscopy (UV-DRS) and Photoluminescence (PL) spectroscopy. The XRD results confirmed the presence of both cubic fluorite (CeO2) and cassiterite (SnO2) structure. The FTIR results confirmed the formation of a Ce–O and Sn–O bond. The EDX studies showed the chemical compositions of the CeO2/SnO2 sample. The TEM analysis showed that the agglomeration of the particles decreases with the increase of the pH value. The VSM analysis confirms the ferromagnetic behavior of CeO2/SnO2 nanocomposites. The photoluminescence emission studies of CeO2/SnO2 nanocomposites showed strong peaks in the UV region, and several peaks in the visible region which are likely to have originated from the oxygen vacancies and are potential material for electronic device applications and catalytic activities. The photocatalytic activities of the prepared (pH-6.5) CeO2/SnO2 nanocomposites are evaluated using the degradation of aqueous methylene blue (MB) solution under visible light irradiation.

Academic research paper on topic "Optical, magnetic properties and visible light photocatalytic activity of CeO2/SnO2 nanocomposites"

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Optical, magnetic properties and visible light photocatalytic activity of CeO2/SnO2 nanocomposites

S. Usharani *, V. Rajendran

Department of Physics, Presidency College, Chennai 05, Tamilnadu, India

ARTICLE INFO

Article history: Received 17 August 2016 Revised 28 September 2016 Accepted 16 October 2016 Available online xxxx

Keywords: CeO2/SnO2 Structural Optical

Ferromagnetism Photocatalyst

ABSTRACT

CeO2/SnO2 nanocomposites were synthesized by the wet chemical method, using various pH such as 4.5, 5.5 and 6.5. The prepared CeO2/SnO2 samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Transmission electron microscopy (TEM), UV-Diffuse Reflectance Spectroscopy (UV-DRS) and Photoluminescence (PL) spectroscopy. The XRD results confirmed the presence of both cubic fluorite (CeO2) and cassiterite (SnO2) structure. The FTIR results confirmed the formation of a Ce-O and Sn-O bond. The EDX studies showed the chemical compositions of the CeO2/SnO2 sample. The TEM analysis showed that the agglomeration of the particles decreases with the increase of the pH value. The VSM analysis confirms the ferromagnetic behavior of CeO2/SnO2 nanocomposites. The photoluminescence emission studies of CeO2/SnO2 nanocomposites showed strong peaks in the UV region, and several peaks in the visible region which are likely to have originated from the oxygen vacancies and are potential material for electronic device applications and catalytic activities. The photocatalytic activities of the prepared (pH-6.5) CeO2/SnO2 nanocomposites are evaluated using the degradation of aqueous methylene blue (MB) solution under visible light irradiation.

© 2016 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC

BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4XI/).

1. Introduction

Nanoscience deals with small sized nanomaterials, and the physical and chemical properties are changed due to the synthesis of nanoparticles with small size, that enhances its applications compared to its bulk material [1,2]. The catalyst's size, structure, crystalline nature and various applications of nanoparticles are greatly influenced by their physical and chemical properties [3]. Various research groups reported the effect of solvents, surfactants and pH on metal oxide nanocomposites, which play an important role to change the structural, optical and magnetic properties [4-6]. In particular, the pH effect strongly influenced the shape, size and photocatalytic activity of metal oxide nanocomposites [7]. Photocatalysis is a rapidly emerging technology due to its ease of degrading harmful organic dyes in the presence of semiconducting materials, such as catalysts [8]. Mixed metal oxide (MMO) nanomaterials such as ZnO/SnO2, Bi2O3-ZrO2, ZrO2/CeO2, CeO2/ V2O5, CeO2/CuO and CeO2/ZnO etc. are studied by various research groups, and they revealed the enhanced visible light photocatalytic activity compared to their individual components, due to their

* Corresponding author. E-mail address: sushamoorthy15@gmail.com (S. Usharani). Peer review under responsibility of Karabuk University.

unique optical and chemical properties [8-12]. The MMOs of SnO2 and CeO2 have received much attention because of their excellent gas sensing and photocatalytic activity [13,14]. In addition, the room temperature ferromagnetic materials are widely accepted because of their applications in the rapidly growing fields of spintronics, nano electronics and magneto electronics [15,16]. In this paper, we reported the synthesis, structural, optical, morphological and magnetic properties of CeO2/SnO2 nanocomposites. In this investigation, the magnetic properties of CeO2/SnO2 nanocom-posites are primarily studied. The photocatalytic activity of CeO2/ SnO2 nanocomposite is carried out for pH-6.5 against MB dye under visible light irradiation. To the best of our knowledge, there is no report in the literature regarding the synthesis of CeO2/SnO2 nanocomposites with different pH synthesized by the wet chemical method.

2. Experimental

2.1. Materials

Analytical grade of Cerrous chloride (CeCl3, 99.99%), tin(II) chloride dihydrate (SnCl2 2H2O, 99.99%), ammonium hydroxide (NH4OH, p25%) and methylene blue (C16H18ClN3S H2O) were purchased from Aldrich and used without any further purification.

http://dx.doi.org/10.1016/j.jestch.2016.10.008

2215-0986/® 2016 Karabuk University. Publishing services by Elsevier B.V.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Double distilled water was used as a solvent to prepare the CeO2/ SnO2 nanocomposites.

2.2. Preparation of CeO2/SnO2 nanocomposites

0.1 M of cerrous chloride and tin(ll) chloride dihydrate solutions were prepared separately and stirred for 15 min. These aqueous solutions were mixed with constant stirring for 1 h at room temperature. After 1 h the aqueous ammonium hydroxide solution was added dropwise to attain the pH value of 4.5 under vigorous stirring. The stirring was continued overnight at room temperature and the resulting milky white colloidal gel product was obtained. The prepared sample was initially dried at 100 °C for 12 h, and later the dried sample was transferred into the alumina crucible and calcined in a muffle furnace at 600 °C for 8 h. The same procedure was repeated to prepare the CeO2/SnO2 nanocomposites of pH-5.5 and 6.5.

2.3. Photocatalytic experiment

2.3.1. Measurement of photocatalytic degradation under Visible light The photocatalytic degradation of MB dye was performed under visible light irradiation in the presence of CeO2/SnO2 nanocompos-ite of pH-6.5 as a catalyst. The visible light irradiation was carried out using a projection lamp (7748XHP 250W, Philips, 532 nm) in a photo reactor. A 0.5% aqueous K2Cr2O7 solution circulating in the glass jacket was used for the purpose of UV cut-off. 500 mg of CeO2/SnO2 catalyst was suspended in 500 ml aqueous solution of MB with an initial concentration of 3 x 10-5 mol/L. The adsorption equilibrium between the dye and surface of the catalyst was maintained by stirring in dark condition for 30 min before being exposed to visible light. During irradiation, the suspension was syringed at regular intervals of time and centrifuged to wipe out the catalysts. The absorption of MB was recorded on UV-visible spectra at room temperature. Degradation efficiency was calculated using the following equation [17].

g = (1 - C/Cc) x 100

3. Results and discussion

The XRD patterns of the CeO2/SnO2 nanocomposite prepared with different pH values are shown in Fig. 1(a-c). The diffractogram indicates the formation of cubic fluorite structure of CeO2 (JCPDS card No. 43-1002) at 26 values 28.5, 33.2, 47.4, 56.4 and 59.1, that correspond to (1 1 1), (2 0 0), (2 2 0), (311) and (2 2 2) crystal planes, respectively. Similarly, the diffraction peaks indicate that the formation of cassiterite type tetragonal structure of SnO2 (JCPDS card No.41-1445) at 26 values 26.5, 33.9, 37.7, 51.8, 64.9 and 65.9 correspond to (1 1 0), (1 0 1), (2 0 0), (2 1 1), (1 1 2), and (300) crystal planes, respectively [18]. From the indexed diffrac-togram, the peak intensity of CeO2 planes increased with increasing pH value, and it reveals that the prepared CeO2/SnO2 nanocomposites possess good crystallinity at pH-6.5. Referrable to the influence of pH, the plane intensities increase with increasing pH value, because the large amount of OH- ions leads to hydrolysis, which then releases chloride ions to form the CeO2/ SnO2 nanocomposite [19]. The crystallite sizes of the CeO2/SnO2 nanocomposite are calculated by using the Debye-Scherrer equation (Eq. (2)) from full width at half maximum (FWHM) values of the CeO2 and SnO2 planes.

D = 0.89k/b cos 8

where, C0 and C are the concentrations of the solution before illumination (t = 0) and after illumination for t minutes, respectively.

where k represents the wavelength of the X-ray, h indicates Bragg's angle, and b is the FWHM of the characteristic peaks. It was found to be 18, 15 and 12 nm for the CeO2/SnO2 nanocomposites at the pH values of 4.5, 5.5 and 6.5 respectively. The XRD results show that the size of the particles decreases with increasing the pH value. The crystallite size can be controlled by varying the pH, as reported by previous literatures [20-23].

The FTlR spectra of the prepared CeO2/SnO2 nanocomposites at three different pH values are shown in Fig. 2(a-c). From the spectra, the peaks observed at 3411 and 1631 cm-1 are ascribed to the presence of the -OH and H2O functional groups. The overlapped absorption band in the region 522-682 cm-1 is due to the Sn-O-Sn and Ce-O stretching vibrations and is in close agreement with the reported values [24,25].

Fig. 3(a-c) is the TEM image of the CeO2/SnO2 nanopowder for three different pH values. It shows the formation of the agglomerated spherical shaped nanoparticles. The particle agglomeration decreases with respect to increments of pH and there is no change

2.4. Characterization details

The structure of the prepared sample was analyzed using X-ray diffraction (XRD) analysis. The XRD pattern of the nanopowders was recorded using a powder X-ray diffractometer (Schimadzu model: XRD 6000 using CuKa (k = 1.5417 A) radiation. The crystallite size of the nanocomposites was calculated using Debye Scher-rer's formula D = 0.89k/pcos8, where k represents the wavelength of the X-ray, Q indicates Bragg's angle, and b is the FWHM of the characteristic peaks. The Fourier transform infrared (FTIR) spectra were taken using an FTIR model Bruker IFS 66 V spectrometer. The EDX studies were carried out by the Philips model CM 20. High-resolution images and selected area electron diffraction patterns were observed with a JEOLJEM-2200FS transmission electron microscope (TEM) operating at 200 kV. The magnetic properties were measured in a BHV-55 magnetometer, (Riken, Japan) at room temperature. Diffuse reflectance UV-vis spectra were recorded in Nujol mode on a CARY 5E UV-vis-NIR spectrophotometer. The photocatalytic activity was measured by a Perkin-Elmer UV-Visi-ble spectrometer RX1. The photoluminescence emission spectra were carried out on a Fluoromax-4 spectrofluorometer with a Xe lamp as the excitation light source.

Fig. 1. Powder X-ray diffraction patterns of different pH effect on CeO2/SnO2 nanocomposite: (a) 4.5, (b) 5.5 and (c) 6.5.

0 500 1000 1500 2000 2500 3000 3500 4000 Wavenumber (cm"1)

Fig. 2. FTIR spectra of different pH effect on CeO2/SnO2 nanocomposite: (a) 4.5, (b) 5.5 and (c) 6.5.

in the morphology of the samples. It is clear that as the pH value increases the size of the particles decreases and the shape becomes more spherical. According to the Ostwald ripening process, this difference can be attributed to the difference in the reduction rate of the precursor and poor balance between the nucleation and growth processes [26]. From the TEM analysis, the average size of the particles was found to be 20, 15 and 10 nm for the pH values of 4.5, 5.5 and 6.5 respectively. It agreed well with the XRD results. The SAED patterns of the samples are shown in an inset of Fig. 3(a-c) which reveals the polycrystalline nature of the samples. Fig. 3(d-f) shows the EDX spectra of the CeO2/SnO2

nanocomposites at the three different pH values of 4.5, 5.5 and 6.5. It confirms that Sn, Ce and O are the only elements present in the CeO2/SnO2 nanocomposite which demonstrated the purity of the prepared catalyst at pH 4.5, 5.5 and 6.5.

Fig. 4 shows the room temperature M-H loop of pH-6.5 CeO2/ SnO2 nanocomposites, analyzed using VSM. The VSM results exhibited the saturated hysteresis loop which indicates the ferromagnetic nature. The obtained ferromagnetic behavior of the nanocomposite is due to the oxygen vacancies [27]. It is noticed that the Ce3+ forms in CeO2 and this reduction in positive charge is compensated by a corresponding number of Vo. The variation of the particle size results in the formation of surface defects such as Vo, which endows it with the ability to exist in either the Ce3+ or Ce4+ state on the particle surface. These Vo are considered to be a possible origin to enhance ferromagnetism in nanomaterials [28,29]. The ferromagnetic behavior of the prepared nanostruc-tures may also be due to the smaller grain sizes [30]. Hence, the room temperature ferromagnetism is associated with particle size and oxygen vacancies [31]. The values of Coercivity (Hc) and Saturation of magnetization (Ms) of CeO2/SnO2 nanocomposites are 545.83 G and 237.22 E-6 emu. This ferromagnetic nanocomposite is a promising material for spintronic devices [15].

The DRS-UV absorption spectra of the prepared samples at three different pH values are shown in Fig. 5(a-c). The absorption edges are observed at 385, 375 and 368 nm at the pH values of 4.5, 5.5 and 6.5 respectively. As observed from the spectra, the absorption edges are shifted to a high energy region and the blue shifts occurred in all the samples. Considering the blue shift of the absorption positions of the bulk SnO2, the absorption onset of the present samples is assigned to the direct transition of the electrons in the SnO2 nano crystals [32]. The absorption spectra

Fig. 3. TEM images of different pH effect on CeO2/SnO2 nanocomposite: (a) 4.5, (b) 5.5 and (c) 6.5. (SAED patterns are inserted). EDX spectra of different pH effect on CeO2/ SnO2 nanocomposite: (d) 4.5, (e) 5.5 and (f) 6.5.

Fig. 4. Room nanocomposite.

0 5000 Applied field (Oe)

temperature magnetization curve of pH-6.5 CeO2/SnO2

Fig. 5. DRS-UV absorption spectra of different pH effect on CeO2/SnO2 nanocomposite: (a) 6.5, (b) 5.5 and (c) 4.5.

showed strong absorption below 400 nm. lt is noted that the absorption of ceria in the UV region originates from the charge transfer transition between the O2- (2p) and Ce4+ (4f) orbit in CeO2 [33]. Further, the optical band gap energy of all the prepared samples was calculated using the following equation.

(ahu) = C(hu - Eg)n

Here a is the absorption coefficient, hu is the photon energy, C is the constant, and n = 1/2 for a directly allowed transition. The optical absorption coefficient a was calculated according to the equation a = (2.303 x 103Ap)/lc, where A is the absorbance of the sample CeO2/SnO2, p is the real density of, l is the path length, and c is the concentration of the CeO2/SnO2 suspensions. For the indirect transitions, the plots of (ahu)2 versus photon energy for the different pH-4.5, 5.5 and 6.5 CeO2/SnO2 nanocomposites, are shown in Fig. S1. The band gap energies of the samples prepared for different pH values were obtained by the (ahu)2 versus (hu) curve (shown in Supplementary material) are 3.2, 3.3 and 3.4 eV respectively. The increase in the band gap energy suggests that

Table 1

The crystallite size (D) and band gap values of CeO2/SnO2 nanocomposite at different pH-4.5, 5.5 and 6.5.

pH Crystallite size (nm) Band gap value (eV)

4.5 20 3.2

5.5 15 3.3

6.5 10 3.4

300000

200000-

100000 -

-pH4.5

-pH5.5

■ -pH6.5

л (b) /

ytc) r 1 ' 1 1 1 1 1

Wavelength (nm)

Fig. 6. PL emission spectra of different pH effect on CeO2/SnO2 nanocomposite: (a) 4.5, (b) 5.5 and (c) 6.5.

the CeO2/SnO2 nanocomposite is a potential material for optoelectronic devices. The crystallite size and band gap values of different pH-4.5, 5.5 and 6.5 are presented in Table. 1.

Fig. 6(a-c) shows the room temperature PL emission spectra of the CeO2/SnO2 nanocomposites for different pH values. The strong UV emission peak at 329 nm is ascribed to the nearest band edge emission of the excitons. In addition to this, the strong blue, green emission peak at 484 and weak blue green emission peaks appearing in 451, 470, and 493 nm, are likely to have transition in the defect states [15,33]. Further, at room temperature, the PL emission spectra of the CeO2/SnO2 nanocomposites exhibit the intensity of luminescent emission which increases with the decrease of pH. These UV and visible emission peaks of the CeO2/SnO2 nanocom-posites are potentially useful for optoelectronic devices and photo-catalytic applications.

The photocatalytic activity of the as-prepared CeO2/SnO2 nanocomposite at pH-6.5 for degradation of methylene blue dye under visible light irradiation is shown in Fig. 7. The absorbance peak at 663.4 nm was observed from the UV-vis absorbance spectra of the initial solution with MB dye. Initially, the slow and steady degradation of MB dye has identified the irradiation time up to 90 min, as shown in Fig. 7. Due to the photo absorption of the CeO2/SnO2 in the presence of visible light irradiation, the electrons absorb energy and move from the valence band to the conduction band, which results in the electron-hole pair generation [18]. After 90 min irradiation time, it was observed that in the conduction band the electrons reacting with oxygen undergo reduction processes and produce superoxide radicals, and it further produces the OH" radical which degrades the organic pollutants of MB dye [34]. Furthermore, the holes react with water and generate the OH" radical, which can oxidize the organic pollutants of the MB dye [35]. The photocatalytic experiment was conducted up to 240 min. After 150 min there is no significant peak corresponds to degradation of MB dye; which implies that 150 min is adequate

respectively. This shows that the decreasing particle size is due to the increasing of pH, which controls the particle size. The TEM studies revealed the decrease in particle agglomeration with respect to increments of pH. The UV and photoluminescence property of the CeO2/SnO2 nanocomposites shows that it could be used for optoelectronic device applications. VSM measurements confirm the ferromagnetic nature of these materials, which is suitable for spintronic devices. The phtocatalytic activities of the CeO2/SnO2 nanocomposites confirmed that these types of catalysts may open up a new avenue for environmental remediation for the treatment of wastewater. These results demonstrate that the size of the nanoparticles can possibly be controlled by changing the various pH values. The size controlled nanocomposite materials, make them promising candidates for future optoelectronic devices, spintronic devices, gas sensing and visible light photocatalytic performance.

Fig. 7. Change in UV absorption spectra of MB degradation using (pH-6.5) CeO2/ SnO2 nanocomposite under visible light irradiation.

Time (min)

Fig. 8. Degradation of MB verse time under visible light.

to get complete degradation. The trend of degradation rate is in line with literature reports [36-38]. The oxidation and reduction processes were capable of degrading the organic pollutants of the MB dye under visible light [39]. In conclusion, the hydroxyl and super oxide radicals are the main active species in the degradation of the MB dye [34]. Furthermore, the percentage degradation curve showed the photocatalytic efficiency of the CeO2/SnO2 nanocomposites as seen in Fig. 8. It indicated that the CeO2/SnO2 nanocomposite possessed 80% degradation efficiency towards the MB dye.

4. Conclusion

CeO2/SnO2 nanocomposites were successfully synthesized by the wet chemical method with various pH values, such as 4.5, 5.5 and 6.5. The XRD patterns reveal the presence of a mixed phase of SnO2 and CeO2; the peak intensity of CeO2 planes increased with the increasing of pH-6.5 and possessed good crystallinity as the pH value is increased. The FTIR results confirmed the formation of the Ce-O and Sn-O bond. The EDX studies revealed the chemical compositions of the CeO2/SnO2 sample. From the TEM studies, the average size of the particles was found to be 20, 15 and 10 nm

Acknowledgement

The authors are thankful to the SAIF, Indian Institute of Technology, Chennai for providing the facilities.

This research did not receive any specific grant from funding agencies in the public, commercial, or- not- for profit sector.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jestch.2016.10.

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