Fuel-oxidizer ratio tuned luminescence properties of combustion synthesized Europium doped cerium oxide nanoparticles and its effect on antioxidant properties

Author’s Accepted Manuscript Fuel-oxidizer ratio tuned luminescence properties of combustion synthesized Europium doped Cerium Oxide Nanoparticles and its effect on antioxidant properties G. Vinothkumar, R. Amalraj, K. Suresh Babu www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)30065-2 http://dx.doi.org/10.1016/j.ceramint.2017.01.053 CERI14515

To appear in: Ceramics International Received date: 25 November 2016 Revised date: 9 January 2017 Accepted date: 10 January 2017 Cite this article as: G. Vinothkumar, R. Amalraj and K. Suresh Babu, Fueloxidizer ratio tuned luminescence properties of combustion synthesized Europium doped Cerium Oxide Nanoparticles and its effect on antioxidant p r o p e r t i e s , Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.01.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fuel-oxidizer ratio tuned luminescence properties of combustion synthesized Europium doped Cerium Oxide Nanoparticles and its effect on antioxidant properties G. Vinothkumar, R. Amalraj, K. Suresh Babu* Centre for Nanoscience & Technology, Madanjeet School of Green Energy Technologies, Pondicherry University (A Central University), Puducherry-605014, India. *

Corresponding author: Phone: +91-413-2654976 email: [email protected]

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Graphical Abstract:

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Abstract: Influential role of fuel to oxidizer ratio (F/O) as the controlling parameter in combustion synthesis of europium doped cerium oxide was studied in terms of defect chemistry, optical property and antioxidant capacity. Europium (5 mol%) doped cerium oxide nanoparticles synthesized by solution combustion in fuel deficient (F/O = 0.6, 1.1) and stoichiometric (F/O = 1.6) conditions resulted in size ranging from 6 to 25 nm while excess fuel (F/O = 2.1) lead to the lower size of 17 nm. Raman spectroscopic analysis showed the formation of intrinsic and europium ion induced extrinsic oxygen vacancies and the defect concentration was found to be decreasing with F/O ratio. Photoluminescence emission was dominated by magnetic dipole transition in F/O=0.6, 1.1 and electrical dipole in F/O=1.6, 2.1 which resulted in a persistent luminescence. Fenton reaction generated hydroxyl radical scavenging activity was influenced by the surface oxygen vacancy concentration and crystallites size. In addition to size and defect, morphology of the nanoparticle plays a significant role in determining the antioxidant efficacy.

Keywords: nano; ceria; defect; antioxidant; bio

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1. Introduction Recent advances in nanotechnology offer formulation of novel nanoparticles in which different functionalities like imaging, sensing, drug delivery and therapy etc., are multiplexed together on a single platform. Cerium oxide (ceria, CeO2) is one of the rare earth oxide, currently being investigated in biomedical applications such as in biosensing [1], antibacterial activity [2], protection of healthy cells in radiotherapy [3], ocular [4] as well as neuro protection [5] and as an antioxidant due to its superior catalytic performance and biocompatibility. The reactivity of ceria, primarily arise from its ability to switch reversibly between +3 and +4 states mediated by oxygen vacancies in the lattice [6]. When two Ce3+ ions replace two Ce4+ ions, the resultant net positive charge is compensated by the generation of one oxygen vacancy (O2-) in the ceria lattice. Bulk, stoichiometric ceria exists predominantly as Ce4+ and the ratio between the relative concentration of Ce3+and Ce4+ varies with various parameters such as size, addition of divalent or trivalent dopants, exposing high energy crystal planes by morphology tuning [7,8]. However, in the nano regime, Ce3+ concentration increases as a function of decreasing size which makes ceria as an important therapeutic agent unlike the micron sized bulk counterpart [6,9]. Many diseases such as Alzheimer and cancer are reported to be occurring through free radical mediation. Free radicals have unpaired valence electron which makes it highly reactive. Oxygen based free radicals known as reactive oxygen species (ROS) like hydroxyl radicals (•OH) are by-products of normal metabolic processes. The ROS are extremely reactive with biomolecules which leads to oxidative stress, lipid peroxidation and permanent DNA damage [10]. The ability of ceria nanoparticle to scavenge harmful reactive oxygen and nitrogen species such as hydroxyl (•OH), superoxide anion (•O2-), nitric oxide (•NO), hydrogen peroxide (H2O2), peroxynitrite (ONOO-) anion etc., were shown to mimic the 4

natural antioxidant enzymes like superoxide dismutase and catalase [11]. Recently, ceria and trivalent lanthanum doped ceria nanoparticles showed the influence of concentration of Ce3+ active sites in enhancing the hydroxyl radical scavenging activity [7,8]. Thus, enhancing the oxygen vacancy in ceria is crucial for tuning Ce3+/Ce4+ ratio for efficient radical scavenging activity. In spite of the therapeutic potential of cerium oxide, poor luminescent property in the visible region limits the scope for imaging. Several reports are available on improving the visible light emitting character of cerium oxide in the presence of luminescent rare-earth ions such as europium etc., to combine therapeutic and imaging platforms together [12,13]. The advantage of doping rare earth europium ions include biocompatibility, reduced auto 

fluorescence and low signal to noise ratio [14]. The formation of oxygen vacancies ( Vo ) upon doping trivalent europium ions can be represented using Kröger-Vink notation as,

1 x ' ' Eu 2O3  2CeCe  2CeCe  2 EuCe  2Vo  2Oox  O2 2

(1)

 x where the CeCe is neutral Ce in the lattice, Vo is the positively charged oxygen vacancy

respectively. Substitutional doping of Ce4+ with Eu3+ results in charge imbalance which is neutralized by the creation of oxygen vacancy (O2-) and Ce3+ [13]. Though increase in oxygen vacancy concentration in ceria can be beneficial as an antioxidant, the associated defects can adversely affect luminescent properties. The oxygen vacancies present at room temperature in the nanoparticle acts as luminescent quenching centers [15]. Many reports are available on improving the luminescent properties of doped ceria nanoparticles [12,13]. Our earlier report indicated an enhancement in emission by the minimization of lattice defects upon high temperature treatment albeit with an increase in crystallite size and loss of Ce3+ concentration [16]. Recently, it has been reported on the 5

multifunctional ceria doped with europium and iron nanostructures exhibiting excellent luminescent properties on annealing at high temperature [17]. Though higher annealing temperature resulted in an enhancement of luminescent properties, a corresponding reduction in oxygen vacancy and Ce3+ concentration makes the ceria nanoparticles less active towards therapeutic application. Hence, the role of oxygen vacancies/surface defects in improving radical scavenging activity as well as quenching the emission property on a multi-functional model based system need to be critically understood for practical application. Generation of vacancy defects in the lattice can be severely affected by the method of preparation like precipitation, hydrothermal, sol-gel, nature of precursor etc., [2,18,19]. Solution combustion method is a rapid, self-propagating, one pot high temperature synthesis method, offering highly crystalline and homogeneous oxides structures with the flexibility for tailoring the size which in turn affects the associated surface area and defects. In solution combustion method, the reaction between fuel such as urea or glycine with oxidizer (metal nitrates) triggers an exothermic reaction to reach a temperature of 1000°C in a short span of time. Any deviation from stoichiometry creates either fuel rich or lean environment which influence the resultant temperature and duration of the combustion reaction. Shi et al., investigated the phosphor properties of 60 nm ceria particles with respect to europium concentration (0 to 16 mol%) using the mixture of nitrate:urea (1:5) and reported a multi fold increase in emission through combustion synthesis than that of conventional solid state reaction which were carried out at 1000°C for 2 hours [20]. However, the relation between fuel-to-oxidizer (F/O) ratio on tuning the size, defects, emission and potential of the material for the radical scavenging efficiency still remains to be explored. In the present work, a solution combustion method was used to synthesize 5 mol% europium doped ceria nanoparticle with variation in F/O ratio in order to tune the size and oxygen vacancy concentration. We investigated the possibility of achieving enhanced Ce3+ 6

concentration by inducing oxygen defects in the lattice through solution combustion method by varying the F/O ratio as the controlling parameter for efficient •OH radical scavenging activity. In addition, the role of contradicting behaviour of oxygen defects in improving antioxidant capacity, while quenching luminescent property and excited state lifetime analysis of Eu3+ doped cerium oxide nanoparticles were explored. 2. Experimental Details 2.1. Material Preparation Analytical grade metal nitrates Ce (NO3)3.6H2O (Himedia, 99.0%), Eu (NO3)3. 5H2O (Alfa Aesar, 99.5%) and glycine (CH2NH2CO2H, (Fisher Scientific, 98.5%) were used as oxidizer and fuel, respectively, for the combustion synthesis. The molar ratio between fuel and oxidizer known as fuel-to-oxidizer ratio (F/O) was calculated using the equation given below [21].



 (Coefficien t of oxidizing elements in specific formula)  (valency )  F / O ( 1) Coefficien t of reducing elements in specific formula )  (valency )

The F/O ratio was varied from fuel lean (0.6 and 1.1), stoichiometric (1.6) and fuel

(2)

excess

(2.1) condition in order to evaluate the effect of fuel content on the structural, morphological and surface properties while maintaining the europium concentration at 5 mol% . In a typical synthesis, calculated oxidizer (2.5217 g of cerium nitrate and 0.1309 g of europium nitrate and fuel (0.2755 g of glycine) for F/O of 0.6, were dissolved in 100 ml of doubly distilled water and sonicated for 5 minutes. Further, the solution was heated at 80  C in an oven until the solution becomes a transparent gel. The gel was then transferred to a hot plate maintained at 400  C to initiate the combustion reaction. Exothermic reaction between fuel and oxidizer lead to the formation of pale yellowish powder. The resultant powder was washed thrice with

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double distilled water and centrifuged at 10,000 rpm and subsequently dried at 80°C. Similar procedure was adopted to prepare samples with varying F/O ratio of 1.1, 1.6 and 2.1.

2.2. Characterization Crystal structure of the nanoparticle was analysed by X-ray diffraction (XRD) using Rigaku Ultima IV X-ray diffractometer with monochromatic Cu-Kα radiation (λ=0.154056 nm) at a scanning rate of 2°/min. The mean crystallite size was calculated by Scherrer formula

t

0.9  cos 

(3)

where  represents full width at half maximum,  is the Bragg’s angle. Surface morphological details of the sample was observed with the help of scanning electron microscopy (SEM, Hitachi S-3400N) operated at 30keV. High resolution transmission electron microscopic (HRTEM) images were obtained by using FEI-Tecnai G2 20 S-TWIN operated at 200 keV attached with Bruker XFlash 6T130 detector. Room temperature Raman spectra of the samples were recorded using confocal Raman spectrometer (Renishaw, RM 2000) with an excitation wavelength of 514.5 nm from an Ar+ ion laser source. Optical absorption spectra were recorded using Perkin Elmer UV Lambda S650 spectrophotometer. A Hitachi-7000 Spectrofluorimeter equipped with a 150 W Xe lamp was used to study the photoluminescence properties at room temperature. Luminescence lifetime measurements were carried out with Horiba Jobin Yvon Fluorolog-FL3 equipped with a 450 W Xenon lamp having pulsed nano LED source and a spectral resolution of approximately 2 nm. 2.3. Hydroxyl Radical Scavenging Activity Methyl violet (MV) dye (SR Laboratories), tris-HCl buffer (SR Laboratories, 99.0%), iron sulfate (FeSO4.7H2O) and hydrogen peroxide (35%H2O2,HIMEDIA) were used to 8

elucidate the hydroxyl radical scavenging activity. A Fenton type reaction was used to generate hydroxyl radicals which can directly degrade the methyl violet dye in the absence of nanoparticles. The change in degradation of methyl violet in the presence and absence of nanoparticle was monitored using optical absorption spectra. First the stock solutions of MV (2 mM), Tris-HCl (0.5 M), FeSO4 (0.75 mM), H2O2 (5.0 M) and 1mM of ceria nanoparticles were prepared. From the stock, a 4 ml solution containing the final concentration of 1.2 x 10-5 M MV, 0.15 mM FeSO4, 1.0 M of H2O2, 0.1 M tris-HCl buffer and10 μM of CeO2 was made and thoroughly mixed for 5 minutes prior to absorbance measurement. 3. Results and Discussion 3.1. Structural analysis XRD patterns of europium doped ceria particle prepared through combustion synthesis with various F/O ratio are shown in Fig. 1. Appearance of diffraction peaks at 2θ values of 28.61°, 33.14°, 47.57°, 56.35° and 59.13° corresponds to (111), (200), (220), (311) and (222) planes of cubic fluorite structured ceria (ICDD card #01-073-6328). Absence of additional peaks in XRD indicates the phase purity and homogeneous presence of Eu3+ ions in host lattice due to the comparable ionic radii of Eu3+ (0.1066 nm) and Ce4+ (0.97 nm) [22,23]. XRD pattern shows a broader peak at lower F/O ratio which became narrower in stoichiometric and fuel rich conditions [24,25]. The mean crystallites size calculated using Scherrer’s formula is shown in Table 1. Mean crystallite size was found to be increasing from 6.4 to 10.6 nm on varying F/O ratio from 0.6 to 1.1, respectively. A sudden increase in crystallite size to 25.2 nm was observed at stoichiometric condition (F/O =1.6) but a further increase in F/O resulted in a reduction of crystallite size to 17.8 nm. The increase in crystallite size with F/O ratio can be attributed to the higher flame temperature generated with the increase in fuel content. The

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flame temperature in the combustion process can be influenced by the choice of fuel as well as F/O ratio [26,27]. At stoichiometric condition (F/O=1.6), the reaction between the nitrates and glycine resulted in the generation of large amount of heat due to the evolution of higher temperature during the complete combustion. Unlike stoichiometric condition, the formation of lower flame temperature in the case of fuel lean (F/O = 0.6 and 1.1) and rich (F/O = 2.1) conditions lead to a slower growth of crystallites. A similar result was observed by Hwang et al., in the combustion synthesis of CeO2 where the adiabatic flame temperature was experimentally found to higher at stoichiometric F/O ratio than that of fuel-lean and fuel-rich conditions [28].

Fig. 1. XRD patterns of europium doped nanoceria for F/O ratio of 0.6, 1.1, 1.6 and 2.1

Lattice parameter of the samples calculated using Bragg equation through the least square fitting of XRD peaks is given in Table. 1. Bulk ceria shows a lattice parameter of 0.5411 nm and reduction in size, generally results in an increase of lattice parameter due to the 10

preferential formation of Ce3+ at nanoscale which has higher ionic radii (0.1143 nm) than Ce4+ (0.097 nm) [29]. In the present work, independent of F/O ratio, lattice expansion was observed, in comparison to bulk ceria. However, a reduction in lattice parameter was observed with an increase in F/O ratio from 0.6 to 1.1 which remained similar for F/O ratio of 1.6 and 2.1. Presence of Eu3+ as dopant (ionic radii = 0.1066 nm) as well as the smaller size may contribute towards the lattice expansion. Since the dopant concentration was kept constant (5 mol %) for all the samples, the observed lattice expansion could have been primarily caused by the changes in size. From the observed changes in the lattice parameter, it can be conjectured that higher concentration of Ce3+ and defect in the form of oxygen vacancies (to compensate the charges, discussed in Raman analysis) present at smaller sizes. Table 1. Structural parameters, band gap and oxygen vacancy concentration of the europium doped ceria nanoparticles synthesized with varying F/O ratio

Fuel-tooxidizer (F/O) ratio

Mean Crystallite size (nm)

Lattice parameter (nm)

Lattice Strain (x 10-3)

Band Gap (eV)

0.6

6.4

0.5426

7.24

2.32

Oxygen Vacancy Concentration N (x 1021 cm-3) 2.12

1.1

10.9

0.5418

3.49

2.38

1.63

1.6

25.2

0.5414

0.565

2.63

1.25

2.1

17.8

0.5414

0.627

2.58

1.33

Smaller crystallite size as well as presence of dopants may introduce lattice strain which can shift the 2θ peak position in XRD. With the increase in crystallite size and ionic radii difference between Eu3+ and Ce4+, the peak position shifted towards lower angle which can be attributed to the strain present in the ceria lattice [30]. Table 1 shows the strain values calculated by the following Williamson-Hall method relation,

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 cos  

0.9  4 sin  D

(4)

where  is the full width at half maxima,  is the lattice strain and D is the crystallite size. The observed lattice strain reduced with F/O ratio, reached a minima at stoichiometric condition and further increase in F/O values resulted in lowering of strain which confirms the role of crystallite size on strain.

Fig. 2. SEM images of the europium doped ceria synthesized with different F/O ratio of (a) 0.6, (b) 1.1, (c) 1.6 and (d) 2.1 with the inset photograph showing the color of the respective sample. Fig. 2 shows the SEM micrographs of the 5 mol% europium doped ceria sample prepared by varying F/O ratio. The deficiency of fuel raise the temperature to a smaller extend which is insufficient to generate large volume of gases in order to reduce the agglomeration. However, generation of higher temperature at higher F/O (1.1, 1.6 and 2.1) 12

resulted in the formation and escape of large volume of gases, thereby breaking the agglomeration of the particles.

Nanocrystalline nature of the europium doped ceria particle was further examined by transmission electron microscopy. Fig. 3 shows the representative TEM images under low (Fig. 3a) and high magnifications (Fig. 3b) along with SAED pattern (Fig. 3c) of doped cerium oxide for F/O=1.6. The crystallites exhibit irregular shapes and average crystallite size estimated using IMAGE J software was found to be 20-25 nm which is in good agreement with the XRD analysis of the corresponding sample. The observed lattice fringes with an interplanar spacing of 0.313 nm correspond to (111) plane of CeO2. Ring pattern corresponding to (111), (200), (220) and (311) planes in selected area diffraction pattern indicates the presence of polycrystalline nature of ceria.

Fig. 3. (a) Low and (b) high resolution TEM images of europium doped ceria with F/O=1.6; inset in (b) shows the interplanar spacing and (c) represents the selected area diffraction pattern.

Fig. 4 shows the Raman spectra of europium doped ceria nanoparticle obtained with an excitation wavelength of 514.5 nm. Bulk, pure ceria exhibits Raman active F2g mode of cubic fluorite structured ceria (space group of

̅ m) at 465 cm-1 which arise from the

symmetric stretching vibrations (v1) of oxygen around cerium atoms (Ce-O bonds) [31]. In

13

the present work, though F2g peak was noticed at 464 cm-1, a prominent shift towards low frequency was observed at low F/O ratio (Fig. 4 inset). A shift from asymmetric to symmetric

Fig. 4. Raman spectra of europium doped ceria with F/O ratio, inset shows the F2g peak position shift nature of the F2g peak was observed with an increase in F/O ratio. Asymmetric nature and shift of the Raman F2g peak arise from size/strain induced phenomenon associated with lattice expansion and increase in oxygen vacancy creation [22,32]. In addition to F2g peak, two weak peaks were observed around 548 and 601 cm-1 that can be attributed to the second-order non-degenerate LO vibrational modes [31]. The peak around 601 cm-1 arise from intrinsic defects present in the ceria while the extrinsic defects around 548 cm-1 corresponds to dopant (Eu3+ substituting Ce4+) induced effect in the ceria lattice. In order to understand the contribution of the intrinsic and extrinsic effects mediated formation of oxygen vacancy in the lattice, the peaks centred around 548 and 601 cm-1 were deconvoluted and area under the curves were plotted (Fig. 5). A similar feature was observed

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by Nakajima et al., in gadolinium doped cerium oxide using UV-Raman spectroscopy where the author claims the band centred around 548 cm-1 include oxygen vacancies with distorted

Fig. 5. Raman spectra of europium doped ceria showing selective region (a) extrinsic (I548) and intrinsic (I601) oxygen vacancy contribution and I548/I601 ratio (b) octahedral symmetry [33]. Another band centred around 601 cm-1 emerge from the defect space containing dopant ion without any oxygen vacancy. Therefore, the intensity ratio of I548/I601 indirectly gives the information about the Ce3+ concentration associated with the formation of oxygen vacancies. Despite the higher oxygen vacancy concentration calculated for F/O=0.6 using asymmetric F2g band, I548/I601 ratio specifies a lower Ce3+ concentration for F/O=0.6 sample in Fig. (5b). This can be attributed to the fact that at a smaller crystallite size around 5 nm, the asymmetric nature of F2g band arising from lattice expansion is largely due to the strain effect as given in Table 1 with no increase in the Ce3+ concentration [34,35].

The oxygen vacancies concentration present in the europium doped ceria lattice can be calculated from Phonon Confinement model [16] using the relation between the half width at half maxima (HWHM) of F2g band obtained from Raman spectra and grain size (dg) as given below

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(cm 1 )  5  51.8 / d g (nm)

(5)

The correlation length (L) which is the average distance between two lattice defects is related to the grain size and α is the radius of the CeO2 units (0.34 nm) by

  L(nm)  3   2d  g

2

  (d g  2 ) 3  4d g2  





(6)

The defect concentration (N) can be calculated using the following relation between correlation length (L) using the following relation,

N (cm 3 )  3 / 4L3

(7)

The calculated oxygen vacancy concentration using the above equation is given in Table 1. Higher oxygen vacancy concentration was observed at low F/O ratio which decreases with the increase in F/O ratio due to the generation of higher temperatures which annihilates the defects. The calculated oxygen vacancy concentration from Raman spectra was found to be higher than that of various methods reported indicating the potential of the technique to engineer oxygen vacancy concentration [16,36,37].

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Fig. 6 shows the FTIR spectra of the europium doped ceria synthesized with different F/O ratio in the frequency range between 4000 cm-1 and 400 cm-1. A broad peak centred around 3420 cm-1 and 1636 cm-1 can be attributed to O-H stretching and H-O-H bending vibrations, respectively, from the presence of water molecules. A sharp vibrational band at 1384 cm-1 arise from the Ce-O-Ce vibrational mode of cerium oxide [38]. The M-O type characteristic vibrational band corresponding to Ce-O symmetric vibrational around 723 and 565 cm-1 confirms the formation of cerium oxide. Moreover asymmetric stretching vibrations and

Fig. 6. FTIR spectra of europium doped ceria at different F/O ratio stretching vibrations of C-H molecule at 2926 and 2852 cm-1 that were present on the ceria surface [39,40]. 3.2. Optical properties

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In order to understand the effect of F/O ratio on optical properties, optical absorption and photoluminescence spectra were recorded. The optical absorption spectra of the samples

Fig. 7. Optical absorption spectra of europium doped ceria for various F/O ratio are shown in Fig. 7.

All the samples exhibited a maximum absorption in the UV region with two broad peaks centred around 295 and 345 nm which can be attributed to the overlapped charge transfer bands of O2- to Ce4+ (295 nm) and O2- to Ce3+ (255 nm) and interband transitions, respectively [41]. Band gap energy calculated from optical absorption spectra is given in in Table. 1. Lowering of F/O ratio, progressively red shifted the absorption band edge due to the modification in defects/oxygen vacancies in the ceria lattice with the introduction of intermediate energy levels. Reduction of particles size lead to the formation of dangling surface bonds with poor coordination with trivalent europium doping creating large number of surface defects/oxygen vacancies. Intermediate defect energy levels in the lattice avert the condition of being confined spatially resulting in the observed red shift. Red shift in the absorption spectra arise from the electron-phonon coupling effect that increases with the 18

decrease in particle size. However, the coupling energy is high enough to overcome the confinement effects of smaller particles leading to red shift in the spectra [42]. Pure ceria has a weak luminescent emission in the UV region centred around 350 nm due to its 5d energy levels with no emission characteristics in the visible light region [43]. The recorded luminescent excitation spectra at 591 and 611 nm obtained for the europium doped ceria samples with different F/O ratio are shown in Fig. 8. Excitation spectra exhibit an intense and broad O2- (2p) to Ce4+ (4f) charge transfer (CT) absorption centred around 350 nm which arise from the transfer of electrons from oxygen to cerium and another band at 466 nm corresponding to europium f-f transitions [44]. Though the CT band was not active for all the samples, the absorption lines around 466 nm 532 nm were found to be the strongest transitions in exciting the europium ions in the lattice directly. The CT band, in particular, was not observed at less than stoichiometric conditions of F/O (0.6 and 1.1). In general, exciting at the CT absorption band is known to sensitize the energy transfer process between the host and dopants. Therefore, the negligible intensity of the CT band indicates the possibility of poor interaction or energy transfer between the host and dopant Eu3+ ions which in turn influences the nature of emission spectrum. The CT band may be influenced by electronegativity of ligand atoms, electron affinity of metal ions and ligand to metal ion distance [45]. Our XRD analysis showed a lattice expansion for F/O=0.6 and 1.1 due to the increase in the distance between the ligand O2- and Ce4+ thereby limiting the efficiency in electron transfer than that of high fuel ratio. A similar decrease in the intensity of CT band observed in fluoride system was ascribed to reduced electron transfer between oxygen and metal ion [46]. Our XRD, Raman and UVVis analysis clearly indicated the presence of higher oxygen vacancy concentration in F/O=0.6 and 1.1 in comparison to other samples. Altogether, the lattice expansion, oxygen vacancy and reduction of Ce4+ to Ce3+ play a crucial role on the intensity of CT band. 19

Fig. 9. Emission spectra (excited at 350 and 466 nm) of europium doped ceria synthesized with F/O ratio (a) 0.6, (b) 1.1, (c) 1.6 and (d) 2.1 (dotted line curves indicate samples with MD transition dominating spectrum) along with corresponding luminescence image. Fig. 8. Excitation spectra of europium doped ceria monitored with 591 nm and 611 nm

The emission spectra were recorded with two excitation wavelength i.e., at 350 nm for indirect excitation that belongs to CT band of the host and 466 nm for direct excitation of luminescent europium ions. It is well known that europium in free form has series of sharp emission lines in the orange-red region, but when doped (in solid state), its emission is largely influenced by crystal field splitting depending of the host lattice. Differences in the intensity profile of the bands located at 591 nm and 611 nm were observed for different 20

excitation wavelengths (Fig 9a and b). Generally, differences in the intensity and shape profile may arise due to the size, morphology, crystallinity, presence of defects such as oxygen vacancies and site symmetry of luminescent ions in the host lattice.

Emission transitions observed under direct and indirect excitation wavelengths of 578, 591, 610 and 653 nm correspond to 5D0→7F0, 5D0→7F1, 5D0→7F2 and 5D0→7F3 transitions, respectively. The two prominent transitions from 5D0→7F1 and 5D0→7F2 observed at 591 and 611 nm arise from the magnetic dipole (MD) and electric dipole (ED) transitions, respectively. The weak 5D0→7F0 transition at 578 nm is forbidden by both ED and MD selection rules J  1; S  0 , but appears in the spectrum due to the low symmetry environment [47]. The Ce4+ ions occupy centrosymmetric site (Oh) surrounded by eight oxygen atoms in cubic fluorite structure. Eu3+ ions having slight difference in ionic charge and higher ionic radii, occupy Ce4+ centrosymmetric site in ceria leading to the distortion in the local symmetry around europium. The ED transitions or forced ED transitions are usually parity forbidden by the selection rules but it is not always strictly maintained. The ED transition is hypersensitive to the local crystal environment and arises when the Eu3+ are located in host lattice without inversion symmetry as a result of distorted crystal field environment. The transitions 5D0→7F1 and 5D0→7F3 are MD allowed transitions but not sensitive to the local crystal environment according to the selection rules. The MD transition at 591 nm dominates the emission spectrum particularly for low F/O ratio of 0.6 and 1.1 (indicated as dotted lines in Fig. 9).

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However, ED transition observed at 611 nm dominates the emission spectrum of F/O=1.6 and 2.1 (solid line in Fig. 8) irrespective of the excitation wavelengths. Normally intense emission is expected for the ED transition due to the higher strength of the oscillator (0.01-1) when compared to the MD oscillator strength (10-6) [48]. MD transition at 591 nm dominates the emission spectrum when the Eu3+ ions are located with inversion centre in the ceria lattice and it is not generally influenced by the local crystalline environment while the dominant ED transition indicates Eu3+ ions are located in a site without inversion symmetry [49].

Fig. 10. Asymmetric Ratio specifying local environment around Eu3+ ions, R(I590/I611) The asymmetric ratio, R, is the ratio of intensity of ED to MD transition (I611/I591) can be used as a measure to characterize the local crystalline environment around Eu3+ in the ceria lattice (R<1: highly symmetric and R>1: distorted symmetry). Fig. 10 shows the

22

asymmetric ratio of the samples against F/O ratio. It can be observed that the samples F/O=0.6 and 1.1 have highly symmetric local environment around Eu3+ ions in the ceria lattice but samples F/O=1.6 and 2.1 lattice provide distorted symmetry around Eu3+ ions. The distorted crystalline environment around Eu3+ in ceria lattice may be resulted from the aggressive and rapid combustion process at stoichiometric and higher F/O ratio samples

Fig. 11. Lifetime measurement of MD-591 nm (left) and ED-611 nm transition (right) excited using 460 nm F/O=1.6 and 2.1 when compared to the lower F/O=0.6 and F/O=1.1. Further the lifetime measurements of the samples excited at 460 nm and decay processes at 591 nm (5D0→7F1) and 611 nm of (5D0→7F2) carried out for in-depth understanding of the various trap sites and local environment around europium ions in the host lattice and the spectra are shown in Fig. 11.The lifetime of the 591 and 611 nm transitions were calculated by fitting the data with the following equation,

I  I o exp( t /  )

(8)

where I is the intensity of radiation at time t, Io is the maximum intensity and  is the lifetime of the excited state. All the curves were fitted well with mono-exponential decay indicating the homogenous crystal field environment around Eu3+ ion in the single lattice thus,

23

excluding the possibilities of multiple trap sites from the lattice environment. The lifetimes calculated by fitting the mono-exponential decay curve are given in Table 2. The lifetime of the 591 and 611 nm emission for fuel lean ratio was found to be very low but increased at stoichiometric conditions and saturated under fuel rich condition. The radiative and non-radiative routes of decay processes are crucial for the lifetime of any excited states. It is well known that the defect related energy states could influence the luminescence and it function as luminescence quenching sites. Table 2. Lifetime of excited state luminescence (excitation wavelength = 460 nm) Lifetime of 591 nm emission

Lifetime of 611 nm emission

(ms)

(ms)

F/O=0.6

0.67

0.82

F/O=1.1

1.11

1.20

F/O=1.6

1.98

1.65

F/O=2.1

1.90

1.68

Sample

In the present case, luminescent lifetime values reduced with decreasing size or increased oxygen vacancy concentration which can be attributed to an increase in nonradiative decay process. The presence of higher level of defects/oxygen vacancies in the ceria lattice did not quench the luminescence instead a change in the excited state lifetime was observed. Due to the smaller crystallite size of the samples prepared with F/O=0.6 and 1.1, large extent of europium ions may be exposed to the surface of the nanoparticles [13,24] as indicated in the broad shape of XRD peaks. This may be the cause for the reduced excited state lifetime resulted from PL spectra. A similar result was obtained when Mn2+ ions lie close to the surface has reduced lifetime when compared to the Mn2+ ions protected by a thick overcoating of ZnS [50].

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Fig. 12. CIE colour coordinates diagram of Eu3+ doped cerium oxide excited with 350 nm and 466 nm 3.2.4. CIE 1931 representation

The luminescent light emission from the luminescent materials can be represented in terms of CIE chromaticity colour space coordinates. The colour coordinates obtained for the emission of different samples under excitation wavelength 350 and 466 nm using CIE 1931 colour diagram are shown in Fig. 12. The emission peaks of the samples showed dominant peaks at 591 nm in smaller sized samples with higher oxygen vacancies (F/O=0.6 and 1.1) and 611 nm in F/O=1.6 and 2.1. As seen from the Fig. 12, almost all the samples excited with 466 nm fall in the same region of CIE colour space but when excited with CT band of 350 nm, the coordinates occupy orange-red and deep red region. 3.3. Hydroxyl radical Scavenging activity

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The hydroxyl radical scavenging activity was assessed by a qualitative calorimetric method as reported earlier [8] using Fenton reaction to generate hydroxyl radicals using methyl violet, Fe2+, hydrogen peroxide and Tris-HCl buffer to maintain pH around 4.5. Generation of hydroxyl radical by Fenton reaction can be written as, Fe2+ + H2O2 → Fe3+ + OH• + OH‾

(9)

Fig. 13 shows the absorption spectrum of the 2.0 mM methyl violet with the absorbance maximum observed at 590 nm. No change in the absorbance was observed when methyl violet was incubated separately with any of H2O2, Fe2+ or CeO2. However, when methyl violet incubated with Fe2+ and H2O2, absorbance values reduced drastically. The reduced absorbance indicates the generation of hydroxyl radicals by the interaction of H2O2 and Fe2+ through Fenton reaction which degrades the methyl violet, thereby lowering the absorbance. The change in absorbance value can be qualitatively taken as the amount of radicals generated in the Fenton process. Cerium oxide nanoparticles at 10 micro molar concentrations were incubated with the assayed solution for 5 minutes and its change in absorbance was measured at an interval of 5 minutes for a period of 50 minutes (10 cycles). The change in absorbance (MV to MV+H2O2+Fe2+) gradually reduced as seen from the Fig. 13 after incubation of methyl violet with 10 micro molar concentration of nanoparticles. The incubated nanoparticles scavenge the hydroxyl radicals and partially prevented methyl violet from degradation. The hydroxyl radical scavenging mechanism of cerium oxide can be written as follows [51], Ce3+ + OH• + H+ → Ce4+ + H2O

(10)

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When scavenging the hydroxyl radicals, the available reactive Ce3+ sites at the surface

Fig. 13. Absorbance changes for methyl violet in the presence of hydroxyl radicals and europium doped ceria nanoparticles (a) F/O=0.6, (b) F/O=1.1, (c) F/O=1.6 and (d) F/O=2.1. are oxidized and converted into Ce4+ sites. Various proof are available in the literature for the regeneration of ceria surface from Ce4+ to Ce3+ sites [51–53]. The oxidation-reduction cycle of Ce4+ and Ce3+ in cerium oxide nanoparticles offers continuous protection for the dye from degradation by scavenging hydroxyl radicals. From the Eq. (8), it is very clear that the role of Ce3+ ions is essential for tuning the radical scavenging activity of ceria. For this factor, the ratio Ce3+/Ce4+ on the surface was enhanced through size reduction and europium ion doping with different F/O in the synthesis process confirmed with XRD and Raman analysis.

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Our XRD and Raman analysis revealed that smaller crystallite size showed higher

Fig. 14. Change in absorbance peak of methyl violet incubated with EDC samples at 10 micro molar concentration. (ΔA = Absorbance of bare MV – Absorbance of EDC incubated MV at different time) oxygen vacancy (manifestation of stabilizing Ce3+ sites on the surface) concentration prepared with the F/O ratio of 0.6 than others as shown in Table 1. Hence, a better radical scavenging activity was expected at F/O of 0.6 than other samples. The changes in the absorbance of the various nanoparticles incubated MV peak vs time plot is shown in Fig. 14. Only a marginal change in absorbance was observed for all the samples indicating their potential towards hydroxyl radical scavenging. But comparatively a higher change in absorbance can be seen for F/O=0.6 which was negligible for F/O=1.1. In fact nanoparticle with F/O=0.6 could not offer sustained protection to methyl violet over time irrespective of its higher oxygen vacancy concentration. On the contrary, sample with F/O=1.1 showed

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better radical scavenging activity due to the abundant presence of highly reactive Ce3+ site which prevent the methyl violet from radical facilitated degradation. The higher catalytic activity of F/O=1.1 sample can be understood in terms of the defect concentration induced by europium dopant as observed from the Raman analysis shown in Fig. 5. The trivalent europium doping has significantly favoured the reduction of Ce4+ ions to Ce3+ ions in order to balance the charge imbalance in F/O=1.1 and F/O=2.1. However, due to the poor concentration of host oxygen vacancy exist in F/O=2.1 from the Raman spectra analysis, F/O=1.1 show superior hydroxyl radical scavenging activity amongst the EDC prepared under different fuel concentrations. Highly agglomerated nature of F/O=0.6 reduce the available reactive Ce3+ sites at the surface of ceria which substantially quench the antioxidant activity. 4. Conclusion In this work, using different F/O ratio in solution combustion method europium doped ceria nanoparticles were prepared in different size. Raman spectroscopic analysis revealed that the variation in F/O ratio and europium ion doping enabled the tuning of size and oxygen vacancies on the surface that stabilizes highly active Ce3+ sites on the surface. The effect of Eu3+ ions incorporation on luminescence emission and radical scavenging activity were investigated. Europium ions preferentially occupied in sites with inversion symmetry for samples prepared with fuel lean ratio and distorted symmetry in stoichiometric and fuel excess ratio. As a result, MD transition at 590 nm dominates for F/O=0.6, F/O=1.1 and ED transition at 610 nm dominated the PL spectrum for F/O=1.6 and F/O=2.1. In all samples, persistent luminescence emission was observed in terms of intensity profile irrespective of size, oxygen vacancy concentration or different excitations used in the PL study. Catalytic measurements showed that EDC nanoparticles synthesized with fuel lean condition (F/O =

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1.1) offered continuous protection to methyl violet by scavenging hydroxyl radicals. This high performance can be correlated to the smaller size and highly reactive Ce3+ sites on the surface of the ceria. Moreover, it is speculated that the agglomeration of the nanoparticles at lower fuel ratio (F/O=0.6) considerably affect the antioxidant capacity of ceria. The contribution of oxygen vacancy generation selectively from trivalent RE ion dopant to the host ceria enhanced the antioxidant performance and luminescence property of the material. Acknowledgements Authors are grateful for the financial support provided through Start-up grant (PU/PC/Start-Up Grant/2011-12/312) of Pondicherry University. Authors also thank Central Instrumentation Facility (CIF), Pondicherry University for the characterization of the samples. References [1]

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