Solar light responsive Sm-Zn ferrite nanoparticle as efficient photocatalyst

Materials Science & Engineering B 225 (2017) 86–97

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Solar light responsive Sm-Zn ferrite nanoparticle as efficient photocatalyst a,b

b,⁎

a

b

MARK

b

S.K. Rashmi , H.S. Bhojya Naik , H. Jayadevappa , R. Viswanath , S.B. Patil , M. Madhukara Naikb a b

Department of Chemistry, Sahyadri Science College, Shimoga 577203, Karnataka, India Department of Studies and Research in Industrial Chemistry, School of Chemical Sciences, Kuvempu University, Shankaraghatta 577451, Karnataka, India

A R T I C L E I N F O

A B S T R A C T

Keywords: ZnSmxFe2−xO4 Photocatalytic activity Solar light driven Methyl Orange Lattice parameter

In this article, a series of Sm substituted zinc spinel ferrite nanoparticles (ZnSmxFe2-xO4) were fabricated by coprecipitation method. The effect of samarium substitution and annealing temperature on the crystal structure of zinc ferrite were explored in this study. The lattice parameter and crystallite size are increased with the increase in the Sm content. The visible light absorption ability extended as Sm content was increased up to (x = 1.5) and, which was the best optimal dosage for photocatalytic degradation of Methyl Orange (MO). This red shift in absorbance unequivocally indicates them as potential agent in solar light driven photocatalytic activity. Such enhancement can be ascribed to the diminution in the band gap of zinc ferrite upon samarium substitution (1.42 eV). Present study provides an excellent competency of rare earth substituted ferrite as a new class of photocatalyst over dye degradation under solar light irradiation.

1. Introduction Rare earth ion substituted spinel ferrite nanoparticles has received much more prominence due to its captivating extensive application. The prodigious change in the structural, magnetic and electrical properties of ferrite on rare earth ion substitution has been reported by many groups some of them are on La, Sm, Gd, Eu, Pr, Dy, Er, Tb, Ce and Y [1–8]. The result proclaims that the behaviour of substituted spinel ferrite varies for different rare earth ion and numerous studies shows the influence of samarium ion on Cd, Mg, Co, Mg-Cd, Li-Ni, Mg-Ni, and Mg-Zn ferrite which ensues in the phenomenal modification in their magnetic susceptibility [9], dielectric behaviour [10], photocatalytic property [11], electrical and structural properties [12–15]. The modification in the properties of ferrite induced by the structural distortion on substitution of larger rare earth ion in the place of smaller iron ion [16]. Fascinatingly, inner transition metal substituted ferrite has emerged as more efficient and new form of photocatalyst in heterogeneous catalysis process because of their substantial modification in optical properties with impressive visible light driven photocatalysis. Visible light responsive photocatalysis has become promising green technology for environmental remediation and energy conversion purpose. It is an economical and clean energy driven advanced oxidation process which makes it inexpensive and non-hazardous [17]. Especially, azo dyes the synthetic organic colorants which are largely used in the textile industries has become a serious environmental

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Corresponding author. E-mail address: hsb_naik@rediffmail.com (H.S. Bhojya Naik).

http://dx.doi.org/10.1016/j.mseb.2017.08.012 Received 15 May 2017; Received in revised form 5 August 2017; Accepted 16 August 2017 Available online 22 August 2017 0921-5107/ © 2017 Elsevier B.V. All rights reserved.

problem owing to its toxicity, non-biodegradability, recalcitrant and low visible light photocatalytic activity. To overcome this problem number of research groups have developed a series of nano-photocatalyst. Among them ferrite was considered as an efficient nano-photocatalyst because they exhibits an excellent optical absorption over low energy photon and makes it capable of absorbing visible light radiation due to its narrow band gap (hν ∼ 2 eV). Engrossingly, nanoferrites with spherical structure provide higher surface area and larger availability of catalytic sites for adsorption of dye molecule and improve its catalytic efficiency [18]. The properties of ferrites are sensitive to the site, nature and amount of metal incorporated in the structure [19]. The crystallisation of spinel structure involves elevated temperature with longer duration. Specifically, inner transition metal substituted ferrites have higher thermal stability than pure one and more energy is needed to complete grain crystallization and growth [3,20]. Among spinel ferrites, zinc ferrite has gained pronounced attention with normal spinel structure as magnetic and electric properties with semiconductor photocatalysis owing to its relatively small band gap (1.9 eV) [21]. Many efforts have been made to improve the capability of using a great extent of solar radiation by substituting divalent and trivalent transition metal in zinc ferrite [22]. Furthermore, to improve the quantum efficiency of ZnFe2O4 on photocatalysis by modifying it with metal, non-metal and inner transition metal has been reported [23–26]. However, only few results are reported on photocatalytic application of

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microscopy, FEI, Technai G2, F30 (accelerating potential 300 kV, line resolution 2.0 Å) was employed to probe the microstructure, particle size distribution and high resolution imaging of ZnFe0.5Sm1.5O4. The FTIR spectra were recorded using a Nicolet IR200FT-IR spectrometer by KBr pellet technique. The photoluminescence (PL) spectrum of the sample was recorded using a Jasco FP-8500 fluorescence spectrometer with a 450W Xenon lamp as light source. The photoabsorption measurement of ferrites were analysed by UV–Vis spectrophotometer (Shimadzu, UV-1650 PL model) dispersed in ethylene glycol after ultrasonication.

rare earth ion substituted ferrite. Nd substituted Ni ferrite was reported by our group [11], where the partial replacement of Fe3+ ions by Nd3+ ions resulted in the formation of metastable energy states originated by Nd 4f electrons with in the energy gap, which resulted in the reduce of optical band gap of nickel ferrite with enhanced photocatalytic activity. Besides this doping of rare earth ion (Sm, Dy and Nd) in magnesium ferrite was reported by Thankachan Smitha et al [27] with excellent catalytic activity in the degradation of 4-chlorophenol, these results were possible due to the doping of rare earth ions, which facilitates the large availability of charge carriers and mobility in case of doped nanoparticles. These literature reports indicate that rare earth ion as a dopant proves to be a new and more superior class of dopant in case of ferrite nanoparticles with supreme solar light driven photocatalyst for environmental issues. Several fabrication techniques have been used to produce nanosized ferrite. In that, chemical precipitation is a simple technique compared to other methods [28]. Albeit of several studies on zinc ferrite and its supremacy over photocatalytic properties, there is no such systematic report on influence of sm3+ ion on the structural, optical and photocatalytic properties of zinc ferrite. Towards this end, here we have synthesised samarium substituted zinc ferrite nanoparticle via co-precipitation method and has studied its photocatalytic activity under natural solar light illumination.

2.3. Solar light irradiation The photocatalytic performance of MO was evaluated by ZnSmxFe2−xO4 photocatalyst under solar light illumination. The experiment was executed with 100 ml of MO solution at different dye concentration (10, 15, 20, 25 and 30 mg L−1) in deionised water. The ZnSm1.5Fe0.5O4 photocatalyst of 0.1 g was added to the MO solution and kept in dark for 40 min under vigorous stirring to attain adsorptiondesorption equilibrium between dye and photocatalyst at room temperature, then the solution was irradiated under visible light on sunny day of March between 11.30 a.m. and 3 p.m. At regular interval of time, 10 ml of sample was taken out and was magnetically separated the solid sample. The photodegradation of MO was monitored by UV–Vis spectrophotometer, which showed the absorbance at 465 nm using distilled water as reference. Recycle ability was performed by collecting the ZnSm1.5Fe0.5O4 solid particles after each run, which was dried and used again as photocatalyst.

2. Experimental 2.1. Preparation of ZnSmxFe2−xO4 nanophotocatalyst Co-precipitation method was adopted for the fabrication of samarium substituted zinc ferrite nanoparticles with nominal formula ZnSmxFe2−xO4 (x = 0.0, 0.5, 1.0, 1.5 and 2.0) [29]. Zinc nitrate (Zn (NO3)2·6H2O), iron nitrate (Fe(NO3)3·9H2O), samarium chloride (SmCl3·6H2O), sodium hydroxide (NaOH) and Methyl Orange (MO) were of analytical grade and used directly as received. Zinc nitrate, ferric nitrate and samarium chloride as a source material were weighed in stoichiometric ratio and dissolved in de-ionised water separately which were then mixed together under stirring. Sodium hydroxide (a precipitating agent) of 2.0 M was added slowly under constant stirring until the pH reaches 12, which results in the formation of brown precipitate, then 2–3 drop of oleic acid was added as surfactant. The precipitate was brought to a temperature of about 353 K and left for an hour. The precipitate was washed repeatedly with de-ionised water and ethanol in order to remove excess of surfactant, if any. Thus, the obtained solid product was filtered and dried at 333 K for 12 h and then annealed at different temperature of 673 K and 873 K for 8 h.

3. Results and discussion Powder X-Ray diffraction pattern of pure and Sm substituted zinc ferrite nanoparticles (ZnSmxFe2−xO4, x = 0.0, 0.5, 1.0, 1.5, and 2.0) were annealed at 673 and 873 K and are manifested in Fig. 1. Annealing is an eminent technique for the formation of crystal structure and crystallite size of ferrite as a function of temperature. As annealing temperature raises intensity of the peak, crystallinity and crystallite size also increases. In divergence, additional broadening, defect concentration (grain boundaries, dislocation) and impurities drive on diminution. The broad peaks with weak crystalline phase indicating the difficulty in crystallization upon addition of Sm for the sample annealed at 673 K (Fig. 1(a)) and was quite difficult to index. On the other hand the diffraction peak of the sample annealed at 873 K can be easily indexed to the cubic spinel ferrite with Fd3m (227) space group, matches well with JCPDS card no.82-1048 and card no.39-0858 for zinc ferrite (ZnFe2O4) and zinc samarium oxide (ZnSm2O4) are shown in Fig. 1(b). In the case of x = 0, zinc ferrite exhibits cubic crystalline structure. For all the Smsubstituted ferrite, a slight shift of prominent peak towards the lower angle can be observed, which confirms the occupancy of samarium ions in zinc ferrite [30]. However, SmFeO3 of orthorhombic phase was formed with initial cubic phase, especially for x = 0.5 and x = 1.0 (ZnSm0.5Fe1.5O4 and ZnSm1Fe1O4) [10]. This can be explained on the basis of solubility limits of Sm3+ ions in the spinel lattice owing to the substantial difference between the ionic radii of Sm3+ (0.964 Å) and Fe3+ (0.645 Å) ions [31,32]. But sample with x = 2.0, resulted in the formation of cubic crystalline structure of zinc samarium oxide, where the Fe3+ ions were completely replaced by Sm3+ ions with certain amount of Sm2O3 phase formation. The crystalline size of zinc-samarium ferrites nanoparticle was computed by Debye-Scherrer’s formula [33].

2.2. Material characterisation The phase composition and crystalline structure of ferrite nanocrystals annealed at different temperature were studied by powder Xray diffraction (PANalytical X’pert Pro Multipurpose Diffractometer with graphite-filtered Cu-Kα radiation source (λ = 1.5406 Å). X-rays were generated from a Cu anode supplied with 45 kV and a current of 40 mA. The sample fixed in the reflection mode was scanned in the ambient atmosphere over 2θ range from 10 to 90°. The resolution of this instrument in terms of scanning angle is 0.001°. Fixed anti-scatter and divergence slits of 1/32° were used together with a beam mask of 10 mm and all scans were carried out in ‘continuous’ mode. The reflected beam was collected with a PW3011/20 (Miniprop. Large window) sealed proportional point detector positioned behind a 0.09° parallel plate collimator. The crystallite size was calculated by standard Scherer formula. The average crystallite size was calculated by standard Scherer formula. The surface morphology and chemical composition of ferrite samples were examined by field emission scanning electron microscopy (FESEM-Carl ZEISS, Supra 40VP) and energy dispersive Xray spectroscopy (EDS). The high resolution transmission electron

D = 0.94λ / β cosθ

(1)

where, D is the crystallite size (nm), λ is the wavelength of X-ray, β is the full-width at half maxima measured in radian and θ is the diffraction angle. The lattice parameter of the ZnFe2−xSmxO4 was determined using the following relationship [34]. 87

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Fig. 1. Powder X-ray diffraction pattern of ZnFe2−xSmxO4 ferrite (x = 0.0, 0.5, 1.0, 1.5, 2.0) nanoparticles annealed at (a) 673 K and (b) 873 K.

a= dhkl (h2 + k 2 + l 2)1/2

crystal system [29,35] and directs reduction in the lattice parameter. A contrast result was observed for x = 1.5 owing to the high stoichiometric ratio and larger ionic radii of Sm3+ ion in ZnSmxFe2−xO4 nanoparticles. The higher annealing temperature of (873 K) induces a rise in lattice parameter; this temperature is high enough to make Sm3+ ions enter into lattice site and to form the Sm3+–O2− bond due to larger ionic radii as compared to Fe3+ ions [36]. It was observed that the lattice parameter increases linearly with Sm3+ concentration obeying Vegard’s law. This can be explained on the basis of relative ionic radii of Fe3+ and Sm3+ ions. The partial replacement of larger ionic radii of Sm3+ (0.964 Å) ions with smaller ionic radii of Fe3+ (0.645 Å) in octahedral B-site of spinel structure, prompts the expansion of unit cell, which in turn endorses the occupancy of Sm3+ ion in zinc ferrite. The values of crystallite size were in good agreement with TEM data and further increased with an increase in the temperature, which participate in the substantial grain growth with inter granular pores. The analogous result have also been reported for rare-earth ion substituted Mg-ferrite [16], Ni- ferrite [11], Mn-Zn ferrite [37,38] and Co-ferrite [39]. The morphology and microstructure of the ZnFe2O4 and ZnSm0.5Fe1.5O4 annealed at 673 and 873 K were studied by FE-SEM micrographs. The surface morphology of zinc spinel ferrite shows compressed arrangement of pretended spherical shaped grains with good homogeneity are shown in Fig. 2(a) and (b). Meanwhile, the formation of SmFeO3 secondary phase with soft agglomerates of grains is more pronounced for Sm substituted system (Fig. 2(c) and (d)). The microstructure of ZnSm1.5Fe0.5O4 annealed at 673 K (Fig. 2(c)) was found to be nanosized spherical with intermingled rod like structure. At

(2)

where, ‘a’ is the lattice constant, dhkl is interplanar distance and hkl are miller indices. The average crystallite size and lattice parameter were tabulated in Table 1. Zinc ferrite own normal spinel structure, where divalent (Zn2+) ions occupy tetrahedral A site and trivalent (Fe3+) ions occupy octahedral B site in spinel lattice. Table 1 imputes the modification in the value of crystallite size and lattice parameter heated thermally at 673 and 873 K. On Sm3+ ion substitution, the crystallite size and lattice parameter were found to be oscillate with annealing temperature. The values of lattice parameter show a discrepancy from 8.432 to 8.192 Å and 8.442 to 9.228 Å for 673 K and 873 K, respectively. At lower annealing temperature, diffusion of Sm3+ ion can be seen at the grain boundaries rather than entering in to the lattice site, which exert pressure on the Table 1 Average crystallite size, lattice parameter, absorbance edges and energy band gap of ZnSmxFe2−xO4 nanoparticles with different samarium concentration. Samples

x = 0.0 x = 0.5 x = 1.0 x = 1.5 x = 2.0

D (nm)

a (Å)

673 K

873 K

673 K

873 K

Absorbance edge (nm) 873 K

20.71 28.48 31.87 33.12 26.66

41.54 43.48 48.01 50.31 47.79

8.432 8.214 8.193 8.208 8.192

8.442 9.021 9.032 9.064 9.228

375 381 439 534 374

Energy band gap (eV) 873 K 1.95 1.92 1.63 1.42 1.95

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Fig. 2. Comparison of ZnFe2O4 and ZnSm0.5Fe1.5O4 FE-SEM micrographs annealed at 673 K (a) & (c) and 873 K (c) & (d).

from histograms was 50 nm (Fig. 4(d)). The average crystallite size estimated from both PXRD and TEM micrographs were in good agreement with each other. The FTIR spectra of ZnSmxFe2−xO4 nanoparticles are taken in the range 400–4000 cm−1 are shown in Fig. 5. Ferrite shows two prominent metal-oxygen frequencies. The higher frequency band ν1 and lower frequency band ν2 were observed in the range of 600–500 cm−1 and 450–380 cm−1 and was assigned to tetrahedral and octahedral metal stretching, which are consider to be the typical bands of spinel structure [41]. The band corresponding to 3400 cm−1 and 1600 cm−1 represents the stretching and bending vibration of adsorbed H-O-H molecule on the surface of nanoparticles. zinc ferrite showed vibrational frequencies at 531 cm−1 (ν1) and 380–410 cm−1 (ν2) are due to tetrahedral M-O stretching vibration on A site and octahedral M-O stretching vibration on B site. Absorption frequency ν2 was slightly shifted towards higher frequency and ν1 towards lower frequency side with increasing samarium content. This distinctly indicates that the shifting of bands was more pronounced by the substitution of Fe3+ ions by Sm3+ ions (larger ionic radii) on octahedral B site which intends to increase the bond length of Sm3+-O2− on B site and enlarges in the unit cell dimension. The increase in site radius results in the reduction of fundamental frequency and central frequency which shifts towards the lower side [42].

873 K, samples show the consistent spherical structure with enlarged grains. The micrograph of all samples shows cohesion of grains to some extent affected by magnetic interaction. The average grain size increases with increase in annealing temperature and Sm content as shown in Fig. 2(b) and (d). Fig. 3(a)–(c) depicts EDS spectra of ZnSmxFe2−xO4 nanoparticles (x = 0.0, x = 0.5 and x = 1.5) annealed at 873 K. Fig. 3(a) was composed of Zn, Fe, and O elements and the atomic ratio of Zn:Fe was very close to 1:2 ratios. Fig. 3(b and c) indicates the presence of Zn, Sm, Fe, and O elements, where the atomic ratio of Zn:Sm:Fe for x = 1.5 was close to 1:1.5:0.5, which confirms the formation of ZnSm1.5Fe0.5O4 nanoparticle as shown in Fig. 3(c). Fig. 4(a)–(d) shows TEM, HRTEM images along with SAED and particle size histogram of ZnSm1.5Fe0.5O4 nanoparticles. The TEM micrographs at low magnification endorses that the most of particles appear to be spherical in shape and are agglomerated to some extent as shown in Fig. 4(a) and (b). The strong magnetic interactions and presence of water between ferrite particles may result in such type of agglomeration [40]. The selected area electron diffraction (SAED) pattern reveals the position of diffraction rings corresponding to crystalline nature of spinel phase and lattice fringes in HRTEM image with a interplanar spacing of 0.2733 nm corresponding to (311) crystal planes of spinel phase. The mean crystallite size of ZnSm1.5Fe0.5O4 nanoparticles

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Fig. 3. EDS spectra of ZnSmxFe2−xO4 nanoparticles annealed at 873 K; (a) x = 0, (b) x = 0.5 and (c) x = 1.5 samples.

M.A. Khan et al. [39] observed similar shift in frequency band ν1 from 572 to 583 cm−1 to lower frequency and band ν2 shift from 397 to 411 cm−1 to higher frequency side due to redistribution of cation on both tetrahedral and octahedral sites with substitution of larger ionic radius of terbium in CoFe2O4 absorption band. B P Jacob et al. [43] has also observed that the band ν1 and ν2 shift towards the lower frequency side with increasing gadolinium content in Ni-Cd ferrite. The photoabsorption properties of ZnSmxFe2−xO4 nanoparticles were investigated by UV–Vis spectrophotometer. It was clear that all the samples exhibited strong photoabsorption in visible light region (Fig. 6). Visually, the incorporation of Sm3+ ions in ZnFe2O4 spinel matrix was substantiated by the color change of the Sm-ZnFe2O4 nanoparticles. Thus ZnFe2O4 were brown, whereas the ZnSm1.5Fe0.5O4 was typically dark green, signifying an apparent change in electronic structure of substituted sample. The absorption of ZnFe2O4 in visible region was due to the transition of photo excited electrons from O-2p level to Fe-3d level [44]. The absorption edge had significant red shift for samarium substituted samples. As Sm content was increased, absorbance shifted towards the longer wavelength as summarised in Table 1. In precise, the ZnSm1.5Fe0.5O4 (534 nm) sample shows

prodigious shift in absorption band toward longer wavelength, which suggest a decrease in the band gap and makes them a potential applicant for photocatalytic studies under visible region. The shift in absorbance was due to the site-substitution mechanism, substitution of Sm3+ ion in ZnFe2O4, which directs the change in the octahedral lattice with expansion of unit cell. The ZnSm1.5Fe0.5O4 nanoparticle own abundant surface and interface defects in the agglomerated nanoparticle, which induce an additional sub energy level in the system as a dopant energy level [45]. In contrast to this, the absorbance related to ZnSm2O4 shows blue shift due to the disappearance of Fe+3 ions in ZnSmxFe2−xO4, such type of shift was also noted in our previous report [11]. The optical band gap of ZnSmxFe2−xO4 nanoparticles were studied by absorption spectra. The calculation of energy band gap involves the fundamental absorption, which corresponds to electron excitation. The incident photon energy (hν) and absorption coefficient ‘α’ of ZnSmxFe2−xO4 nanoparticle were evaluated from the fundamental relations [46,47]:

I = Io e− 90

(3)

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Fig. 4. TEM micrographs with different magnification (a) & (b) of ZnSm1.5Fe0.5O4 nanoparticles. HRTEM image with SAED pattern (c) and (d) shows particle size distribution.

A = log(Io/ I )

(4)

α = 2.303(A/ t )

(5)

the above results it can be concluded that, ZnSm2O4 samples has no additional sub energy level or donar level between valence band (O-2p) and conduction band (Sm-4f) due to the complete replacement of Fe3+ ions by Sm3+ ions. Consequently, ZnSm1.5Fe0.5O4 nanoparticle have greater band gap value (1.42 eV) than the theoretical energy which is required for water splitting (1.23 eV) which makes them an efficient photocatalyst with respect to visible light utilisation. The influence of samarium ion on photocatalytic activity of zinc ferrite nanoparticle was examined using MO as a targeted molecule and model pollutant for photodegradation under solar light illumination. The photocatalytic performance of ZnSmxFe2−xO4 nanoparticles as a photocatalyst over different Sm content was examined (Fig. 8). The suspension was exposed to sunlight irradiation at different interval of time, absorption results were recorded for another 80 min. The absorption edge of MO solution occurred at a wavelength of 463 nm and 270 nm. The absorption peak at 463 nm drops gradually with irradiation time in turns of intensity and directs the photodegradation of MO over ZnSm1.5Fe0.5O4 photocatalyst are shown in Fig. 8. With cumulative illumination time, degradation of MO reaches to 92.33% in 80 min. The higher photodegradation effeciency of ZnSm1.5Fe0.5O4 sample makes it efficient photocatalyst in the removal of dye. The percentage photodegradation was calculated by Eq. (6)

where, A is the absorbance and t is thickness of the ZnSmxFe2−xO4 sample. The band gap energy value was estimated by a extrapolating the linear portion curve obtained by plotting (αhν)1/n vs. photon energy (hν) to α = 0 for different n values (1/2, 2, 3/2 and 3) where, exponent value (n) denotes electronic transition depending on the type of transition [48]. The best linear fit observed for n = 1/2 assigned for direct allowed transition (Tauc plot) (Fig. 7) [49]. From Tauc plot the energy band gap of ZnSmxFe2−xO4 nanoparticles was calculated (Table 1). For zinc ferrite the energy band gap was 1.95 eV compares well with Xu et al. [50]. Electronic band structure related to zinc ferrite shows an excitation of electron from O-2p orbital to Fe-3d orbital. As samarium content increases the energy band gap value decreases, especially in case of x = 1.5 sample shows a red shift of about 1.42 eV contrast to other samples (x = 0.0, 0.5, 1.0, and 2.0). The red shift in absorbance is predominantly caused by partial replacement of Fe3+ ions by Sm3+ ions owing to the introduction of Sm 4f electrons in ZnFe2O4, which forms a donor energy level below and closer to the conduction band, which can be explained on the basis of surface phenomena [11]. For the sample with x = 2.0, the energy band gap increased to 1.95 eV, this increase may be due to the smaller bond length of Sm-O when compare to the normal spinel ferrite, it can be explained on the basis of atomic pair distribution function [51]. From

Photodegradation % = (C0−Ct )/Ct ∗100

(6)

Samarium substituted zinc ferrite sample prepared via co91

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Fig. 7. Band gap from plots of (αhν)2 vs. hν of ZnSmxFe2−xO4 photocatalyst annealed at 873 K.

Fig. 5. FTIR spectra of the ZnSmxFe2−xO4 annealed at 873 K nanoparticles.

Fig. 8. Absorption spectra of MO (10 mg L−1) with respect to time in presence of ZnSm1.5Fe0.5O4 catalyst (0.1 g L−1) under solar light irradiation.

were excited by single excitation wavelength of 400 nm with the excitation source of 150 W Xenon lamp. It was clear from Fig. 9 that the PL emission band was lopsided and broadened with multiple peaks indicating the participation of different luminescence centres in the radiative processes Sm substituted ZnFe2O4 samples. The characteristic transition emission at 430 nm in the violet region can be linked to shallow defects in the band gap and linked to a more ordered structure, which may arise from non-stoichiometric ZnFe2O4 and non-radiative recombination process, as a band-to-band transition in ZnFe2O4 having a band gap of 1.95 eV [53]. ZnFe2O4 has the intrinsic emission arising from a band edge to band edge transition only. For Sm substituted samples, the band related to the characteristic transitions derived from the Sm3+ ions is visible. The strong violet emission around 430 nm and weak secondary green-yellow emissions at around 534 and 597 nm were observed. Thus each color signifies a different type of electronic transition and is linked to a specific structural arrangement. The green, yellow and red emissions are linked to defects deeply inserted in the

Fig. 6. Optical absorption spectra of ZnSmxFe2−xO4 nanoparticle annealed at 873 K.

precipitation process showed much higher activity than zinc ferrite. The narrow band gap of ZnSm1.5Fe0.5O4 (1.42 eV) [11] indicates that the substitution of Sm3+ at x = 1.5 appear to be an optimal dosage of MO, where the highest activity was noted. Ensuing in significant utilization of visible light and enhanced separation of photoinduced charge carriers, reflects as potentially higher photocatalytic activity [52]. Above the optimal limit (Sm3+), recombination of photo generated charge carrier takes place owing to its wide band gap and shows the decrease in photocatalytic activity. Photoluminescence spectra of pure and Sm substituted ZnFe2O4 nanoparticles (x = 0.0 to 2.0) was recorded at room temperature to detect the optical properties and are shown in Fig. 9. All the samples 92

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electronic levels between the conduction and the valence band and might be due to the increase in intrinsic defects [53]. The lower intensity of PL emission proposes that the photocatalyst has less of an opportunity for electron–hole pairs recombination and facilitates the migration of charge carriers more effectively than the ZnFe2O4. A series of mechanism has been proposed by many research groups [58,59] for visible light driven photodegradation, on the basis of these reports and our result we have demonstrated a simple mechanism (Scheme 1). In comparison with ZnSmxFe2−xO4, pure ZnFe2O4 also excited by solar light but it has higher recombination of photoinduced electron-hole pair, which leads to the low visible light photocatalytic activity of the organic pollutants. Another reason could be attributed to the larger band gap of the ZnFe2O4 (1.95 eV). The energy band gap of photocatalyst determines the wavelength of light that can be absorbed and leading to the generation of electron-hole pairs. The substitution of the Sm3+ ion in ZnFe2O4 results enhanced optical absorption in visible light region imply the improved photodegradation efficiency due to the formation of metastable Sm-4f energy levels closer to the lower edge of the conduction band gap of ZnFe2O4, which points the reduction in the band gap. Another reason is defects derived from Sm substitution would acts as trapping sites and facilitate the separation of photogenerated electron-hole pairs and extend the life time of charge carrier [60]. As rare earth elements are known to be good acceptors [61]. When electron gets excited from valence band (VB) to new energy level (ZnSmxFe2−xO4) conduction band (CB) formed in the system under solar light irradiation, the photogenerated holes in valence band react with surface water or hydroxyl ion to produce OH% radical species, which is a potential oxidant in the degradation of MO and simultaneously, electrons in the conduction band reacts with adsorbed oxygen molecule to produce O2%−. It further, combines with H+ to produce HO2% [62], which react with trapped electron to generate OH% [63]. It can be concluded that OH%, HO2%, O2%− and h+ VB are active species involved in the photodegradation of MO. Based on the above analysis, the photochemical reaction for the degradation of MO under solar light irradiation over ZnSmxFe2−xO4 photocatalyst was expressed as follows.

Fig. 9. Photoluminescence emission spectra of ZnSmxFe2−xO4 photocatalyst.

band gap and to a greater disorder in the lattice [54]. The broad green and yellow emission, observed in all Sm substituted ZnFe2O4 may be attributed to the transition of Sm3+ between 4G5/2 and 6H5/6 states [55]. The green-yellow emission is also assigned to the interstitial oxygen defects occurred by distorted octahedral lattice with incorporation of Sm3+ ions in ZnFe2O4 host lattice. The photoluminescence emission observed in the Sm substituted ZnFe2O4 samples are due to the transitions between the energy levels in the 4f5 electron configuration. 4f electrons of rare-earth ion in solids are quite similar to free ions and are rather insensitive to the effects of the surrounding field. Makishima et al. [56] earlier investigated the luminescence of Sm3+ in BaTiO3 host lattice and found that some foreign ions can change the relative strength of the emissions owing to a charge compensation mechanism. On the basis of their results, they concluded that one series of the emissions is attributed to Sm3+ at the Ti4+ site, while the other series of emissions is related to the presence of Sm3+ in the Ba2+ site. The reduction in the intensity of emission band with increasing the Sm3+ concentration intends the increasing number of oxygen vacancies and interstitial metal defects created in tetrahedral and octahedral sites of the above ferrite compositions. The lower PL emission intensity indicates the decrease in recombination of photoinduced electron–hole pairs rate, thus the higher photocatalytic activity [57]. This behaviour can be attributed to the appearance of new

− + ZnSm x Fe2 − x O4 + hν → ZnSm x Fe2 − x O4 (eCB + hVB )

→ ZnSm x Fe2 − x O4 + energy

ZnSmx Fe2 ZnSmx Fe2 ZnSmx Fe2

− %− −xO4(e CB)O2→O2 −xO4(h

+ + % VB)+H2O→H +OH

−xO4(h

+ − % VB)+OH →OH

O2%−+H+→HO2%

Scheme 1. Possible reaction mechanism for the photocatalytic degradation of MO over ZnSm1.5Fe0.5O4 nanoparticle.

93

(7)

(8) (9) (10) (11)

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Fig. 10. Impact of initial dye concentration (10, 15, 20, 25 and 30 mg L−1) on (a) adsorption, (b) photocatalytic degradation, and (c) photodegradation kinetics of MO over ZnSm1.5Fe0.5O4 nanoparticle under solar light irradiation (catalyst dosage = 0.1 g L−1). + % − % 2e− CB+HO2 +H →OH +OH

HO2%,

%

OH ,

O2%−,

h

+ VB+MO

→degradation products

The adsorption of dye molecule on the surface of photocatalyst attains equilibrium at 40 min and was constant over time (dark reaction). The photocatalytic kinetics for ZnSm1.5Fe0.5O4 nanoparticle at various concentrations for the dye was explored. To comprehend the photodegradation kinetics of MO, Langmuir-Hinshelwood model was applied [64], this model describes rate constant of the photodegradation of MO and it was found that the degradation of MO approximate fits the first order kinetic equation:

(12) (13)

Fig. 10(a) and (b) shows the adsorption and photodegradation of MO on various initial dye concentrations over ZnSm1.5Fe0.5O4 nanoparticles. The photocatalytic activity of ZnSm1.5Fe0.5O4 photocatalyst on the various dye concentration are in the order as follows; 30 mg L−1 < 25 mg L−1 < 20 mg L−1 < 15 mg L−1 < 10 mg L−1. 94

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Fig. 11. Photocatalytic degradation of MO in five cycles using ZnSm1.5Fe0.5O4 nanoparticle.

ln(Ct / C0) = −kt

(14)

remove high variety of environmental disrupters in water.

where, Ct is concentration of MO in the solution at irradiation time t, C0 is the initial concentration at t = 0 and k is apparent first order rate constant. The plot of ln (Ct/C0) as a function of irradiation time, which is approximate linear and the value of k can be obtained via its slope (Fig. 10(c)). The obtained k values are 0.0107, 0.0086, 0.0062, 0.0047 and 0.0043 min−1 for initial dye concentration of 10, 15, 20, 25 and 30 mg L−1, respectively. Obviously, the initial dye concentration of MO influences on the photocatalytic behaviour of ZnSm1.5Fe0.5O4 nanoparticles and also on its rate constant. When initial concentration was higher, significant decrease in the photodegradation was observed, due to the absorbance of solar light by dye rather than the photocatalyst. Thus, the best photocatalytic degradation was achieved for 10 mg L−1 of dye concentration with k value of 0.0107 min−1. Moreover, excessive adsorption of dye molecule over the surface of photocatalyst might decrease the penetration of solar light. Thus, few photons reach the surface of photocatalyst and lower the radial species; as a result more time will be required for the degradation of MO [65]. This ensues that rate constant are higher when dye concentration are lower. The dye and catalyst loading is important parameters that affect photocatalytic activity. These results indicate that the excellent photocatalytic degradation was achieved at mg L−1 of dye concentration with 0.1 g/100 ml of catalyst. At higher concentration of dye, adsorption of dye molecule on the surface of catalyst was high and surface was also saturated, which leads to decrease the photonic efficiency followed by catalytic deactivation [66]. If catalyst loading is higher, then degradation efficiency decreases, due to an unfavourable light scattering and reduction in the light penetration is also observed [67]. To investigate the photostability and reusability of ZnSm1.5Fe0.5O4 photocatalyst under solar light irradiation, the degradation experiment was conducted for five runs with same sample (Fig. 11.). In each cycle, suspension was separated from solution by external magnetic field. After five cycles, there was no loss of activity and the degradation efficiency of MO was about 89%, which makes remarkable photostability. Ferrites as photocatalyst possess a good magnetic property [68] and easily recoverable from catalytic system in order to reuse them for longterm degradation process. This class of photocatalyst is expected to

4. Conclusion Samarium substituted zinc ferrite nanoparticles were successfully synthesised via co-precipitation method. The influence of rare earth (Sm3+) ions on crystal structure led to a formation of cubic spinel structure with a secondary samarium iron oxide (SmFeO3) phase. The insertion of Sm3+ ion in the ZnFe2O4 spinel matrix shows an increase in lattice parameter and crystallite size attributed to larger ionic radii of Sm3+ ion as compared to Fe3+ ion in the B sites. The spherical morphology of pure and substituted ZnFe2O4 were observed from FE-SEM and HRTEM images. The particle size distribution of ZnSm1.5Fe0.5O4 nanoparticle (50 nm) was much large than the zinc ferrite as confirmed from HRTEM analysis. The EDS analysis clearly reveals the expected stoichiometry of the samples. The band frequency ν2 in FTIR shifted to higher frequency with increase in samarium content, confirms the occupancy of rare earth ion in octahedral B site. The optical absorption edge showed significant red shift for samarium substituted samples, especially for composition ZnSm1.5Fe0.5O4 (534 nm) with narrow band gap of 1.42 eV, which proved to be optimal samarium dosage to achieve highest photocatalytic degradation of MO. The UV–visible spectra showed the quenching in the peak intensity of dye with direct sunlight illumination time in presence of ZnSm1.5Fe0.5O4 photocatalyst. The photoabsorption results in complete degradation of MO under direct sunlight irradiation in 80 min and the color removal proves the photomineralisation of MO. The excellent photocatalytic degradation of MO was achieved for 10 mg L−1 of dye concentration, with first order kinetic rate constant k of 0.0107 min−1. Moreover, ZnSmxFe2−xO4 photocatalyst are highly magnetic in nature, which greatly facilitate separation and repetitive reuse of photocatalyst, which exhibits photocatalytic efficiency of 89% to mineralize different organic pollutant. This article demonstrate that the rare earth ion (Sm3+) substituted ZnFe2O4 nanoparticles will acts as a new class of photoactive agent in solar light driven photocatalytic activity for environmental purification.

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Acknowledgements [26]

One of the authors, S. K. Rashmi expresses their gratitude for University Grant commission (UGC), New Delhi for providing RGNF (JRF-RGNF-2015-17-SC-KAR-12376) and SAIF, IIT Bombay for facilitating HRTEM images. The author also acknowledges IISc (Bangalore) for FE-SEM and other spectral data of the samples.

[27]

[28]

References

[29]

[1] L. Zhao, H. Yang, L. Yu, Y. Cui, X. Zhao, S. Feng, Study on magnetic properties of nanocrystalline La-, Nd-, or Gd-substituted Ni–Mn ferrite at low temperatures, J. Magn. Magn. Mater. 305 (2006) 91–94. [2] S.E. Jacobo, S. Duhalde, H.R. Bertorello, Rare earth influence on the structural and magnetic properties of Ni Zn ferrites, J. Magn. Magn. Mater. 272 (2004) 2253–2254. [3] Z. Peng, X. Fu, H. Ge, Z. Fu, C. Wang, L. Qi, H. Miao, Effect of Pr3+ doping on magnetic and dielectric properties of Ni–Zn ferrites by ‘‘one-step synthesis’’, J. Magn. Magn. Mater. 323 (2011) 2513–2518. [4] S.A.K. Sahib, M. Suganthi, V. Naidu, S. Pandian, M. Sivabharathy, Synthesis of double lanthanide doped nano ferrite meta material for microstrip patch antenna application, Int. J. ChemTech Res. 6 (2014) 4615–4624. [5] A.A. Kadam, S.S. Shinde, S.P. Yadav, P.S. Patil, K.Y. Rajpure, Structural, morphological, electrical and magnetic properties of Dy doped Ni–Co substitutional spinel ferrite, J. Magn. Magn. Mater. 329 (2013) 59–64. [6] A.B. Gadkari, T.J. Shinde, P.N. Vasambekar, Magnetic properties of rare-earth ion (Sm3+) added nano crystalline Mg–Cd ferrites, prepared by oxalate co-precipitation method, J. Magn. Magn. Mater. 322 (2010) 3823–3827. [7] A.B. Gadkari, T.J. Shinde, P.N. Vasambekar, Structural analysis of Y3+ doped Mg–Cd ferrites prepared by oxalate co-precipitation method, J. Mater. Chem. Phys. 114 (2009) 505–510. [8] F.X. Cheng, J.T. Jia, Z.G. Xu, B. Zhou, C.S. Liao, C.H. Yan, L.Y. Chen, H.B. Zhao, Microstructure, magnetic, and magneto-optical properties of chemical synthesized Co-RE (RE=Ho, Er, Tm, Yb, Lu) ferrite nanocrystalline films, J. Appl. Phys. 86 (1999) 2727–2732. [9] E. Melagiriyappa, H.S. Jayanna, Structural and magnetic susceptibility studies of Samarium substituted magnesium–zinc ferrites, J. Alloys Compd. 482 (2009) 147–150. [10] E. Melagiriyappa, M. Veena, A. Somashekarappa, G.J. Shankaramurthy, H.S. Jayanna, Dielectric behavior and ac electrical conductivity in samarium substituted Mg–Ni ferrites, Indian J. Phys. 88 (2014) 795–880. [11] K.N. Harish, H.S. Bhojya Naik, P.N. Prashanth kumar, R. Viswanath, Optical and photocatalytic properties of solar light active Nd-substituted Ni ferrite catalysts: for environmental protection, ACS Sustain. Chem. Eng. 1 (2013) 1143–1153. [12] A.B. Gadkari, T.J. Shinde, P.N. Vasambekar, Structural analysis of Sm3+ doped nanocrystalline Mg–Cd ferrites prepared by oxalate co-precipitation method, Mater. Charact. 60 (2009) 1328–1333. [13] A. Gadkari, T. Shinde, P. Vasambekar, Influence of rare-earth ions on structural and magnetic properties of CdFe2O4 ferrites, Rare Met. 29 (2010) 168–173. [14] S. Xavier, S. Thankachan, B.P. Jacob, E.M. Mohammed, Effect of samarium substitution on the structural and magnetic properties of nanocrystalline cobalt ferrite, J. Nanosci. (2013), http://dx.doi.org/10.1155/2013/524380. [15] M.F. Al-Hilli, S. Li, K.S. Kassim, Structural analysis, magnetic and electrical properties of samarium substituted lithium–nickel mixed ferrites, J. Magn. Magn. Mater. 324 (2012) 873–879. [16] S. Thankachan, B.P. Jacob, S. Xavier, E.M. Mohammed, Effect of samarium substitution on structural and magnetic properties of magnesium ferrite nanoparticles, J. Magn. Magn. Mater. 348 (2013) 140–145. [17] J.Z. Bloh, R. Dillert, D.W. Bahnemann, Transition metal-modified zinc oxides for UV and visible light photocatalysis, Environ. Sci. Pollut. Res 19 (2012) 3688–3695. [18] S.D. Jadhav, P.P. Hankare, R.P. Patil, R. Sasikala, Effect of sintering on photocatalytic degradation of methyl orange using zinc ferrite, Mater. Lett. 65 (2011) 371–373. [19] P. Li, Q. Chen, Y. Lin, G. Chang, Y. He, Effects of crystallite structure and interface band alignment on the photocatalytic property of bismuth ferrite/(N-doped) graphene composites, J. Alloys Compd. 672 (2016) 497–504. [20] Y.M.Z. Ahmed, E.M.M. Ewair, Z.I. Zaki, In situ synthesis of high density magnetic ferrite spinel (MgFe2O4) compacts using a mixture of conventional raw materials and waste iron oxide, J. Alloys Compund. 489 (2010) 269–274. [21] X. Li, Y. Hou, Q. Zhao, W. Teng, X. Hu, G. Chen, Capability of novel ZnFe2O4 nanotube arrays for visible-light induced degradation of 4-chlorophenol, Chemosphere 82 (2011) 581–586. [22] A.I. Borhan, P. Samoila, V. Huleac, A.R. Iordan, M.N. Palamaru, Effect of Al3+substituted zinc ferrite on photocatalytic degradation of Orange I azo dye, J. Photochem. Photobiol. A 279 (2014) 17–23. [23] A. Manikandan, J.J. Vijaya, M. Sundararajan, C. Meganathan, L. John Kennedy, M. Bououdina, Optical and magnetic properties of Mg-doped ZnFe2O4 nanoparticles prepared by rapid microwave combustion method, Superlattices Microstruct. 64 (2013) 118–131. [24] S. Singhal, R. Sharma, C. Singh, S. Bansal, Enhanced photocatalytic degradation of methylene blue using ZnFe2O4/MWCNT composite synthesized by hydrothermal method, Indian J. Mater. Sci. (2013), http://dx.doi.org/10.1155/2013/356025. [25] K.N. Harish, H.S. Bhojya Naik, P.N. Prashanth kumar, R. Viswanath, Synthesis,

[30]

[31] [32]

[33]

[34] [35] [36] [37]

[38]

[39]

[40]

[41] [42]

[43]

[44]

[45]

[46]

[47]

[48] [49]

[50]

[51]

[52]

[53]

[54]

[55]

96

enhanced optical and photocatalytic study of Cd–Zn ferrites under sunlight, Catal. Sci. Technol. 2 (2012) 1033–1039. A.I. Borhan, P. Samoila, V. Hulea, A.R. Iordan, M.N. Palamaru, Photocatalytic activity of spinel ZnFe2-xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution, J. Taiwan Inst. Chem. Eng. 45 (2014) 1655–1660. S. Thankachan, M. Kurian, D.S. Nair, S. Xavier, E.M. Mohammed, Effect of rare earth doping on structural, magnetic, electrical properties of magnesium ferrite and its catalytic activity, IJIRSET 3 (2014) 529–537. T.J. Shinde, A.B. Gadkari, P.N. Vasambekar, Effect of Nd 3+ substitution on structural and electrical properties of nanocrystalline zinc ferrite, J. Magn. Magn. Mater. 322 (2010) 2777–2781. K. Maaz, S. Karim, A. Mumtaz, S.K. Hasanian, J. Liu, J.L. Duan, Synthesis and magnetic characterization of nickel ferrite nanoparticles prepared by co-precipitation route, J. Magn. Magn. Mater. 321 (2009) 1838–1842. M.M. Rashad, R.M. Mohamed, H. El-Shall, Magnetic properties of nanocrystalline Sm-substituted CoFe2O4 synthesized by citrate precursor method, J. Mater. Process. Technol. 198 (2008) 139–146. W.V. Aulock, Handbook of microwaves ferrites Materials, Newyork, London, 1965. A.A. Sattar, A.M. Samy, R.S. El-Ezza, A.E. Eatch, Effect of rare earth substitution on magnetic and electrical properties of Mn–Zn ferrites, Phys. Status Solidi A 193 (2002) 86–93. J. Qu, C. Luo, Q. Cong, X. Yuan, A new insight into the recycling of hyperaccumulator: synthesis of the mixed Cu and Zn oxide nanoparticles using Brassica juncea L, Int. J. Phytorem. 14 (2012) 854–860. H.P. Klung, L.E. Alexander, X-ray diffraction Procedures, New York, EUA, 1962. L. Zhao, H. Yang, L. Yu, Y. Cui, X. Zhao, S. Feng, Magnetic properties of Re-substituted Ni–Mn ferrite nanocrystallites, J. Mater. Sci. 42 (2007) 686–691. L.J. Zhao, H. Yang, L.X. Yu, Y.M. Cui, Effects of Gd2O3 on structure and magnetic properties of Ni-Mn ferrite, J. Mater. Sci. 41 (2006) 3083–3087. K.K. Bharathi, G. Markandeyulu, C.V. Ramana, Electrical, and magnetoelectric properties of Sm- and Ho-substituted nickel ferrites, J. Phys. Chem. C. 115 (2010) 554–560. V.J. Angadi, B. Rudraswamy, E. Melagiriyappa, Y. Shivaraj, S. Matteppanavar, Y. Shivaraj, S. Matteppanavar, Effect of Sm3+ substitution on structural and magnetic investigation of nano sized Mn–Sm–Zn ferrites, Indian J. Phys. 90 (2016) 881–885. M.A. Khan, M.J. ur Rehman, K. Mahmood, I. Ali, M.N. Akhtar, G. Murtaza, I. Shakir, M.F. Warsi, Impacts of Tb substitution at cobalt site on structural, morphological and magnetic properties of cobalt ferrites synthesized via double sintering method, Ceram. Int. 41 (2015) 2286–2293. Z. Karimi, Y. Mohammadifar, H. Shokrollahi, Sh. Khameneh Asl, Gh. Yousefi, L. Karimi, Magnetic and structural properties of nanosized Dy-doped cobalt ferrite synthesized by co-precipitation, J. Magn. Magn. Mater. 361 (2014) 150–156. R.K. Selvan, C.O. Augustin, L.B. Berchmans, R. Sarawathi, Combustion synthesis of CuFe2O4, Mater. Res. Bull. 38 (2003) 41–45. K.B. Modi, M.K. Rangolia, M.C. Chhantbar, H.H. Joshi, Study of infrared spectroscopy and elastic properties of fine and coarse grained nickel-cadmium ferrites, J. Mater. Sci. 41 (2006) 7308–7318. H. Lv, L. Ma, P. Zeng, D. Ke, T. Peng, Synthesis of floriated ZnFe2O4 with porous nanorod structures and its photocatalytic hydrogen production under visible light, J. Mater. Chem. 20 (2010) 3665–3672. B.P. Jacob, S. Thankachan, S. Xavier, E.M. Mohammed, Effect of Gd3+ doping on the structural and magnetic properties of nanocrystalline Ni–Cd mixed ferrite, Phys. Scr. 84 (2011) 1–6. X. Li, Y. Hou, Q. Zhao, L. Wang, A general, one-step and template-free synthesis of sphere-like zinc ferrite nanostructures with enhanced photocatalytic activity for dye degradation, J. Colloid Interface Sci. 358 (2011) 102–108. H.M. Pathan, J.D. Desai, C.D. Lokhande, Modified chemical deposition and physicochemical properties of copper sulphide (Cu2S) thin films, Appl. Surf. Sci. 202 (2002) 47–56. M. Srivastava, A.K. Ojha, S. Chaubey, A. Materny, Synthesis and optical characterization of nanocrystalline NiFe2O4 structures, J. Alloys Compd. 481 (2009) 515–519. J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous germanium, Phys. Status Solidi B 15 (1966) 627–637. B.D. Viezbicke, S. Patel, B.E. Davis, D.P. Birnie, Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system, Phys. Status Solidi 252 (2015) 1700–1710. S. Xu, D. Feng, W. Shangguan, Preparations and photocatalytic properties of visiblelight-active zinc ferrite-doped TiO2 photocatalyst, J. Phys. Chem. C 113 (2009) 2463–2467. M. Ishimaru, Y.I. Hirotsu, I.V. Afanasyev-Charkin, K.E. Sickafus, Atomistic structures of metastable and amorphous phases in ion irradiated magnesium−aluminate spinel, J. Phys. Condens. Matter 14 (2002) 1237–1247. C.H. Liang, F.B. Li, C.S. Liu, J.L. Lu, X.G. Wang, The enhancement of adsorption and photocatalytic activity of rare earth ions doped TiO2 for the degradation of Orange I, Dyes Pigments 76 (2008) 477–484. A. Manikandan, J.J. Vijaya, L.J. Kennedy, M. Bououdina, Structural, optical and magnetic properties of Zn1−xCuxFe2O4 nanoparticles prepared by microwave combustion method, J. Mol. Struct. 1035 (2013) 332–340. V.M. Longo, L.S. Cavalcante, R. Erlo, V.R. Mastelaro, A.T. De Figueiredo, J.R. Sambrano, S. de Lazaro, A.Z. Freitas, L. Gomes, N.D. Vieira, J.A. Varela, E. Longo, Strong violet–blue light photoluminescence emission at room temperature in SrZrO3: joint experimental and theoretical study, Acta Mater. 56 (2008) 2191–2202. V.M. Longo, M.D.G.S. Costa, A.Z. Simoes, I.L.V. Rosa, C.O.P. Santos, J. Andres,

Materials Science & Engineering B 225 (2017) 86–97

S.K. Rashmi et al.

[56] [57]

[58]

[59]

[60]

[61] [62]

E. Longo, J.A. Varela, On the photoluminescence behavior of samarium-doped strontium titanate nanostructures under UV light. A structural and electronic understanding, Phys. Chem. Chem. Phys. 12 (2010) 7566–7579. S. Makishim, H. Yamamoto, T. Tomotsu, S. Shionoya, Luminescence spectra of Sm3+ in BaTiO3 host lattice, J. Phys. Soc. Jpn. 20 (1965) 2147–2151. K. Fujihara, S. Izumi, T. Ohno, M. Matsumura, Time-resolved photoluminescence of particulate TiO2 photocatalysts suspended in aqueous solutions, J. Photochem. Photobiol. A 132 (2000) 99–104. S. Horikoshi, A. Saitou, H. Hidaka, N. Serpone, Thermal and non-thermal effects of microwave radiation on the photocatalyst and on the photodegradation of rhodamine-B under UV/vis radiation, Environ. Sci. Technol. 37 (2003) 5813–5822. Z. Chen, D. Li, W. Zhang, Y. Shao, T. Chen, M. Sun, X. Fu, Photocatalytic degradation of dyes by ZnIn2S4 microspheres under visible light irradiation, J. Phys. Chem. C 113 (2009) 4433–4440. Z. Hu, D. Chen, S. Wang, N. Zhang, L. Qin, Y. Huang, Facile synthesis of Sm-doped BiFeO3 nanoparticles for enhanced visible light photocatalytic performance, Mater. Sci. Eng. B 220 (2017) 1–12. J.K. Reddy, B. Srinivas, V.D. Kumari, M. Subrahmanyam, Sm3+-doped Bi2O3 photocatalyst prepared by hydrothermal synthesis, ChemCatChem 1 (2009) 492–496. Y. Ma, N. Lu, Y. Lu, J.N. Guan, J. Qu, H.Y. Liu, Q. Cong, X. Yuan, Comparative study

[63]

[64]

[65] [66] [67]

[68]

97

of carbon materials synthesized “Greenly” for 2-CP removal, Sci. Rep. 9 (2016) 29167. S. Kaneco, M.A. Rahman, T. Suzuki, H. Katsumata, K. Ohta, Optimization of solar photocatalytic degradation conditions of bisphenol A in water using titanium dioxide, J. Photochem. Photobiol. A 163 (2004) 419–424. C. Zhao, M. Pelaez, X. Duan, H. Deng, K. O’Shea, D. Fatta-Kassinos, Role of pH on photolytic and photocatalytic degradation of antibiotic oxytetracycline in aqueous solution under visible/solar light: kinetics and mechanism studies, Appl. Catal. B 134 (2013) 83–92. A. Mills, R.H. Davies, D. Worsley, Water purification by semiconductor photocatalysis, Chem. Soc. Rev. 22 (1993) 417–425. H. Chun, W. Yizhong, T. Hongxiao, Destruction of phenol aqueous solution by photocatalysis or direct photolysis, Chemosphere 41 (2000) 1205–1209. J. Arana, J.M. Nieto, J.H. Melian, J.D. Rodriguez, O.G. Diaz, J.P. Pena, O. Bergasa, C. Alvarez, J. Mendez, Photocatalytic degradation of formaldehyde containing wastewater from veterinarian laboratories, Chemosphere 55 (2004) 893–904. A.I. Borhan, A.R. Iordan, M.N. Palamaru, Correlation between structural, magnetic and electrical properties of nanocrystalline Al3+ substituted zinc ferrite, Mater. Res. Bull. 48 (2013) 2549–2556.