Electronic structure and photocatalytic activity of samarium doped cerium oxide nanoparticles for hazardous rose bengal dye degradation

Journal Pre-proof Electronic Structure and Photocatalytic Activity of Samarium Doped Cerium Oxide Nanoparticles for Hazardous Rose Bengal Dye Degradation Surjeet Chahal, Neha Rani, Ashok Kumar, Parmod Kumar PII:

S0042-207X(19)32329-2

DOI:

https://doi.org/10.1016/j.vacuum.2019.109075

Reference:

VAC 109075

To appear in:

Vacuum

Received Date: 29 August 2019 Revised Date:

10 November 2019

Accepted Date: 13 November 2019

Please cite this article as: Chahal S, Rani N, Kumar A, Kumar P, Electronic Structure and Photocatalytic Activity of Samarium Doped Cerium Oxide Nanoparticles for Hazardous Rose Bengal Dye Degradation, Vacuum, https://doi.org/10.1016/j.vacuum.2019.109075. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

Electronic Structure and Photocatalytic Activity of Samarium Doped Cerium Oxide Nanoparticles for Hazardous Rose Bengal Dye Degradation Surjeet Chahal, Neha Rani, Ashok Kumar and Parmod Kumar* Department of Physics, Deenbandhu Chhotu Ram University of Science and Technology, Murthal-131039, Haryana, India

Abstract: Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) samples have been synthesized by sol-gel method for spintronics and photodegradation applications. Structural properties were studied using X-ray diffraction that showed single-phase crystalline nature with some structural distortions. Raman spectroscopy was employed to study the defects where longitudinal optical mode confirmed the presence of defects in the form of oxygen vacancies. Magnetic studies infer that saturation magnetization (MS) increased with Sm doping from 0.36×10-3 emu/g for pure CeO2 to 15.8×10-3 emu/g for Sm 6 % and then, decreased for Sm 8 %. Reported results also demonstrated the excellent photodegradation of UV-irradiated rose bengal dye. X-ray photoelectron spectroscopy (XPS) technique was used to illustrate the electronic structure, including the contribution of each ionic state, i.e. Ce3+ and Ce4+. Oxygen vacancies and formation of new networks led to the origin of room temperature ferromagnetic behaviour and excellent photocatalytic degradation of harmful rose bengal dye. Keywords: CeO2; Raman spectroscopy; XPS; oxygen vacancies; room temperature ferromagnetism; photocatalysis. Corresponding author: [email protected]

1. Introduction: A lot of research has been concentrated on cerium oxide nanoparticles (NPs) in past decade due to wide range of applications in diverse fields such as catalysis due to its exceptional catalytic activity, solid oxide fuel cells, solar cells, gas sensing, supercapacitor devices, spintronic devices and many other biomedical applications [1-9]. Its unique property to reduce from Ce4+ oxidation state to Ce3+ on the doping of metal ions is considered to be the deciding factor for a wide range of applications. The reduction of the oxidation state of cerium ions induces defects in the form of oxygen vacancies (VO) in cubic fluorite lattice structure[10]. Rare earth (RE) doped ceria NPs and nanocomposites have appreciable photodecomposition of pollutant dyes in the presence of ultra-violet (UV) light or sunlight [11]. Doping of RE metal ions promotes stability and high surface to volume ratio, which makes it a suitable dopant to improve electrical, magnetic, optical and catalytic properties of ceria. Apart from superior catalytic applications, at nanoscale it displays ferromagnetism at room temperature (RTFM), and the reason behind this behaviour is still a dispute. Some investigations reported the origin of RTFM on the basis of formation of oxygen vacancies (VO) and F-center exchange (FCE) mechanism [12-13]. It has been found that oxygen vacancies are favourable for FM behaviour in cerium oxide nanoparticles [14]. There are a lot of other factors such as the nature of dopant, size, enhanced surface to volume ratio, morphology, synthesis route, band gap and type of defects induced on metal ion doping responsible for enhancing these applications. Moreover, the performance of these materials is very much sensitive to their structural and electronic properties. Magnetic character in such semiconductors requires some extent of trivalent/ divalent metal ion as a dopant having partially filled d or f orbital to mediate ferromagnetic behaviour. Such dilute magnetic semiconductors (DMS) display considerable optical properties and RTFM behaviour

simultaneously making it a promising material for spintronic devices, magnetic recording, and magneto-optic devices. This letter comprising of Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) nanoparticles prepared via sol-gel route. Examination of structural properties has been carried out using XRD, and vacancy defects were studied by Raman and XPS analysis. Evolution of RTFM has been discussed on the basis of VO and FCE mechanism. Promising results of rose bengal (RB) dye degradation have been obtained under UV light irradiation by using assynthesized samples as a catalyst.

2. Experimental details: 2.1. Sample preparation: Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) samples were synthesized by solgel technique. Cerium nitrate and samarium nitrate (Alfa-Aesar) were used as precursors and dissolved in the optimum amount of distilled water. The clear solutions of both the precursors were mixed and heated at 80 °C with constant stirring to obtain a viscous gel solution. Then, the gel was placed on a hot plate at 120 °C for 2 hours to dry and then, annealed at 500 °C for 2 hours. 2.2. Photocatalytic activity: 5 ppm solution of rose bengal (RB) dye was prepared in distilled water. The synthesized samples act as a photocatalyst to degrade the pollutant dye concentration. A 300 W UV-source of light was used for photodegradation. 0.05 gm of catalyst was suspended in 50 ml of dye solution and stirred in the dark for 30 minutes to reach the equilibrium condition. Then suspended solution was placed under UV illumination for 90 minutes. The

samples were extracted after every 30 minutes and absorption spectra obtained for further analysis from UV-Vis. spectrophotometer. 2.3. Characterizations: Synthesized samples were characterized on X-ray diffractometer (Rigaku X-Ray Diffractometer with CuKα X-ray source having wavelength λ=1.54 Å) for phase analysis. Morphological studies were explored using Cryo-Transmission Electron Microscopy (TEM). Raman spectra were recorded using STR 500 Confocal Micro Raman spectrometer having DPSS laser (wavelength 532 nm at 12.5 mW source power). The magnetic properties were studied through the M-H hysteresis loop using Vibrating Sample Magnetometer (Microsense EZ9 VSM) by search coil method. Photocatalytic absorption measurements were conducted using LABINDIA UV 3092 UV-VIS Spectrophotometer. Mixed valance state properties were explored using X-ray photoelectron spectroscopy (XPS) PHI 5000 VersaProbe II with AES, FEI Inc. having Al Kα(1486.6 eV, monochromatic) X-ray source and vacuum pressure of ~ 10-10 Torr. The electron emission angle was 45º, analysed area 1 mm2 and pass energy 376 eV for XPS measurement.

3. Results and discussion: 3.1. XRD analysis: XRD pattern of all the synthesized samples exhibits single-phase crystalline fluorite structure, as shown in Fig. 1. Absence of any peak from other impurities confirmed the incorporation of Sm ions in the host lattice. Crystallite size was calculated from Scherrer’s equation (

=

/

) and found to be ~ 8.5 nm for pure CeO2. The lattice parameter was

calculated using the standard formula

= √ℎ +

+

for cubic structures. Peaks up to

Sm 6% shifted towards the lower angle and then, higher side for Sm 8% whose impact can be

clearly seen on the lattice parameter (given in Table 1). The variation in average crystallite size, lattice constant, strain and dislocation density (δ) with Sm concentration are shown in Table 1. It is noted that lattice parameter for pure CeO2 (~ 5.417 Å) is higher than bulk CeO2 (~ 5.411 Å) which may be due to the reduction of Ce4+ ions to Ce3+ ions and in case of Sm doped samples, the ionic radii of Sm3+ ions (103 pm) is larger than that of Ce4+ (87 pm) that results in the lattice expansion [15-17]. An increase in lattice parameter with Sm doping concentration up to 6% and then, decrement for 8% can be a result of saturation of oxygen vacancies at 6%. On further doping, Sm ions start to occupy interstitial sites rather than Ce4+ sites, since, Sm3+ ions exceed the threshold for the substitution on Ce4+ sites [18]. The dislocation density is the total length of dislocation lines per unit volume calculated using δ = 1/D2. It is observed that gradual increment in dislocation density in the lattice system is due to increasing disorder up to 6% Sm ion concentration. Further, dislocations in the system cause stress in the host matrix, where the positive sign of stress indicates the presence of tensile stress and the negative sign represents compressive stress [19]. The stress in the system can be calculated using the following relations: = =

( −

!

) ×

where a is the lattice parameter of synthesized samples, ao is the lattice parameter of standard CeO2 (5.411 Å from JCPDS no. - 34-0394) and Young’s modulus is 180 GPa for cerium oxide. In pure ceria, tensile stress is found to be ~ 0.22 GPa that goes on increasing up to 6% Sm ion concentration and then decreases as shown in Table 1. This behaviour can be explained as Sm ions are unable to substitute Ce4+ site after 6% doping, which allows relaxation in tensile stress [19].

Fig. 1. XRD pattern of Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) synthesized samples.

Table 1 Various parameters calculated from XRD, TEM and UV-Vis. spectroscopy for synthesized Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) nanoparticles. Sample Lattice Crystallite Lattice Stress Dislocation Particle Band Parameter Size (nm) strain (GPa) density Size gap (nm-2)×10-3 (Å) (nm) (eV) Pure CeO2 5.418 8. 5 0.00123 0.22 13.7 9.1 3.23 Sm 2% 5.422 8. 4 0.00195 0.35 14.1 8.9 3.20 Sm 4% 5.423 8. 3 0.00220 0.39 14.4 8.9 3.13 Sm 6% 5.425 8.1 0.00257 0.46 15.2 8.6 3.10 Sm 8% 5.423 8.3 0.00227 0.40 14.6 8.7 3.17

3.2. TEM: TEM images of synthesized pure and Sm doped CeO2 samples is shown in Fig. 2. It can be clearly observed from Fig. 2 (a) that most of the particles have spherical morphology and the average particle size is found to be 9.1 nm for pure CeO2. Further, the TEM images of Sm doped samples shows that the particle size decreases with doping concentration

(tabulated in Table 1) and seems to be more agglomerated. The particle size is in well agreement with crystallite size from XRD results. The high resolution TEM images for Sm doped samples are given in Fig. 2 (f-i). The planes are clearly visible in HRTEM images which show that these nanoparticles are highly crystalline in nature.

Fig. 2. TEM images of (a) pure CeO2, (b) Sm 2%, (c) Sm 4%, (d) Sm 6% and (e) Sm 8% samples respectively; and HRTEM images of (f) Sm 2%, (g) Sm 4%, (h) Sm 6% and (i) Sm 8% nanoparticles respectively.

3.3. Raman spectroscopic analysis: Raman spectroscopy is a non-destructive technique which is helpful in analyzing defects in the lattice structure. Raman spectra of Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) samples in Fig. 3 indicate a strong peak at ~ 461 cm-1 and two broad peaks at ~ 255 cm-1 and ~ 600 cm-1. In a perfect fluorite cubic structure of CeO2, only the most intense peak

appears; as only q = 0 phonons are Raman active. But, the introduction of disorder/defects makes q ≠ 0 phonons Raman active and starts to contribute to Raman spectra. Therefore, phonons related to other than zone boundary also contribute with increase in asymmetry [2022]. The peak at ~ 461 cm-1 indicates triply degenerate F2g mode for cubic fluorite structure of CeO2 that shows symmetric stretching modes of O ions around the Ce ions [2326]. Raman spectra is very sensitive to disorder in the lattice structure (here disorder is driven by VO). The shift on the lower wavenumber side appears upon doping of Sm ions, which is a consequence of the increase in Ce3+ ion concentration. This increase in shifting with doping concentration is due to the reduction in the energy of Ce-O vibrations as a result of lattice strain and lattice defects [27-28]. Any type of disorder is represented by longitudinal optical (LO) mode that represents the vibrations due to electron-phonon interactions. The area under broad LO mode is a tool to compare VO and conversion of Ce4+ to Ce3+. Comparative analysis of the concentration of VO with doping is intended by the ratio of area under LO (ALO), and F2g (AF2g) peaks are shown in Table 2 [28-29]. Further, the concentration of VO is found to be increasing with Sm content from 1.9 % for pure CeO2 to 10.5 % for Sm 6% as shown in Table 2 and Fig. 4. This can be understood as; at Sm 6%, the system achieved a saturation level and hence, a decrease in VO (9.1 %) appears for Sm 8% as Sm ions started to occupy interstitial sites or the surface than Ce4+ sites. On the other hand, forbidden transverse optical (TO) mode appears at ~ 255 cm-1 due to intrinsic defects caused on doping of Sm trivalent ions in the host lattice. Results of Raman spectroscopy related to structural disorder are in good agreement with XRD results. Table 2 Parameters calculated from Raman spectroscopy and VSM for synthesized Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) samples. Oxygen MS Samples Area under Area under ALO/AF2g

Pure CeO2 Sm 2% Sm 4% Sm 6% Sm 8%

LO peak (ALO) 0.57 1.41 1.77 3.06 2.53

F2g peak (AF2g) 29.99 29.30 26.42 29.02 27.92

0.019 0.048 0.067 0.105 0.0906

vacancies VO (%) 1.9 4.8 6.7 10.5 9.1

(emu/g)×10-3 0.36 6.77 11.34 15.80 4.74

Fig. 3. Raman spectra of Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) samples (a) Inset shows pattern for red shift on increasing doping concentration. (b) Inset shows magnified image of the pattern for area under LO mode.

Fig. 4. Variation of oxygen vacancies with Sm content in synthesized Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) samples.

3.4. X-ray Photoelectron Spectroscopy: X-ray photoelectron spectroscopy is a surface-sensitive technique carried out to explore the electronic properties of the elements present in a material [30-31]. XPS setup was calibrated using standard silver and gold samples. Then, survey scan for pure CeO2 and Sm 4% samples and high-resolution elemental scan of Ce 3d and O 1s have been carried out at room temperature. Ce 3d spectra of both samples in Fig. 5 (a and b) indicate 3d3/2 and 3d5/2 spin-orbit doublets. It is clear from Ce 3d spectra for pure CeO2 and Sm 4% that mixedvalence states (Ce3+ and Ce4+) are present, which leads to non stoichiometric behaviour. The peaks u''', u'', u' and u refer to Ce 3d3/2 and v''', v'', v', and v peaks correspond to Ce 3d5/2 levels. Out of these peaks, u' and v' peaks are indicative of Ce3+ oxidation state and presence of Ce3+ state that indicates defects in the form of oxygen vacancies [32]. Contribution of both oxidation states Ce3+ and Ce4+ can be calculated using the following semi-quantitative formula [33]:

#



%$=

#

)%

=

&' + &( + &' + &' + &( + &( + &( + &(

&'

+ &'

&'

&' + &' + &' + &( + &( + &( + &' + &' + &' + &( + &( + &( + &(

where, Ax is the integrated area under peak x. The integrated area under all the corresponding peaks is listed in Table 3. The atomic fraction of Ce3+ has been calculated and found to be ~ 26.6 % & ~ 42.8 % and Ce4+ is ~ 73.4 % & ~ 57.2 % for pure CeO2 and Sm 4% samples respectively. It is evident from the calculations that the contribution of Ce3+ has been increased for Sm 4% than the pristine sample. Higher the content of Ce3+, higher will be the oxygen vacancies to maintain the charge neutrality in the lattice system. Ce3+ introduces new networks in the form of Ce3+-O-Ce3+ and Ce3+-O-Ce4+ along with Ce3+-O-Sm3+ and Ce4+-OSm3+ due to doping of Sm ions [34]. Also, the possibilities of association of the trivalent Ce3+ ions to form Ce2O3 phase are feasible since some of the Ce3+ ions would not be balanced by oxygen vacancies [35]. O 1s spectra have been fitted in two peaks Olat and Oads for both pure and 4 % Sm doped samples are shown in Fig. 5 (b and d respectively). Olat peak corresponds to lattice oxygen ions and Oads to the surface adsorbed oxygen ions [35]. Intensity and area of peak Oads are associated with the oxygen vacancies in the crystal structure [36]. It can be observed from Table 3 that area and intensity both have been increased for Oads in 4 % Sm doped cerium oxide compared to the pristine ceria. Hence, it has been concluded that oxygen vacancies are increased on doping of trivalent Sm3+ ions. The concept of defects/oxygen vacancies comes out from XPS corroborates well with Raman and XRD results.

Fig. 5. XPS peak fitting of pure CeO2 and Sm 4% samples; (a) shows peak fitting of Ce 3d and (b) O 1s for pure CeO2; (c) peak fitting of Ce 3d and (d) O 1s for Sm 4% nanoparticles.

Table 3 Peak position and area of all the peaks fitted for Ce 3d and O 1s in XPS spectra for synthesized pure CeO2 and Sm 4% samples. Pure CeO2 Sm 4% Peaks Peak Position Peak area Peak Position Peak area (eV) (eV) u''' 916.1 1446 915.4 844 Ce 3d3/2 u'' 906.4 1269 906.1 547 u' 901.3 1262 899.6 1964 u 900.0 873 899.5 292 v''' 897.6 1985 896.9 900 Ce 3d5/2 v'' 888.0 1997 887.2 1796 v' 883.2 1953 882.1 1724 v 881.5 1282 880.9 546 O 1s Olat 528.4 1532 527.5 1032 Oads 530.7 1353 530.4 1050

On the basis above discussion, oxygen vacancies and new network formation results in the modifications of various properties of Sm doped CeO2 samples. These defects and new

networks are helpful in enhancing the magnetic character as well as photocatalytic efficiency of the synthesized catalysts.

3.5. Magnetic measurement: The focus of the study is to observe the magnetic behaviour of as-synthesized Sm doped CeO2 samples that make them suitable for spintronic devices. Fig. 6 presents magnetization loops at room temperature after subtracting diamagnetic contributions that originate from the sample holder. Synthesized samples are showing weak RTFM behaviour with ~ (200-300) Oe coercivity. The saturation magnetization (MS) is strictly dependent on Sm3+ ion doping concentration. It is noted that MS values increase gradually from 0.36×10-3 emu/g for pure CeO2 (as shown in the upper inset of Fig. 6) to 15.8×10-3 emu/g for Sm 6% sample and then, decreases for Sm 8%. These values of MS are higher than the ones reported by X. Ma et al. [12, 18], Gao-Ren Li et al. [14] and Kumar et al. [37]. The origin of RTFM may be due to defects in the form of VO reason being the exchange interaction of 4f electron spin and magnetic moments resulting due to VO in the lattice [38]. It can also be explained on the basis of FCE mechanism where the overlapping of d orbital and F-centre (here, Fcentres are basically VO that has trapped electrons) occurs. The trapped electrons have a spin parallel to neighbouring two Sm ions which are involved in the formation of VO. Hence, the pattern observed for MS corroborates well with the results of VO in Raman spectroscopy in Table 2 [37]. New networks in the form of Ce3+-O-Ce4+, Ce3+-O-Ce3+ and Ce4+-O-Sm3+ introduced magnetic moments which are also responsible for the enhancement in net magnetization [28]. Therefore, doping concentration, nature of dopant and defects induced in host lattice are the deciding factor for the magnetic character in these oxide semiconductors.

Fig. 6. M-H loop of Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) samples.

3.6. UV-Vis. spectroscopic analysis: UV-Vis. spectra for synthesized samples are employed in Fig. 7 to study the optical properties. A continuous shift towards higher wavenumber has been observed with Sm content. Tauc plot among (αhν)2 versus hν has been plotted to depict the band gap energy. Band gap values are found to be decreased with Sm content from 3.23 eV for pure CeO2 to 3.10 eV for Sm 6% and then increases for Sm 8% (as shown in Table 1). This type of variation mainly depends on the defects in the form of oxygen vacancies introduced with Sm doping in the host matrix [12]. Hence, these results corroborate well with Raman spectroscopic results.

Fig. 7. (a) UV vis. spectra and (b) Tauc plot of Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) nanoparticles.

3.7. Photocatalytic activity: The experiments were performed to study the photocatalytic activity of synthesized Sm doped CeO2 (Sm = 0%, 4%, 6% and 8%) catalysts for degradation of rose bengal (RB) dye. Fig. 8 (a, b, c and d) shows absorption spectra of RB dye concentration with catalyst in the presence of a UV source of light for different time intervals. The peak intensity of absorption spectra decreases with time from 0 to 90 min. The results revealed that percentage degradation for pure CeO2 was ~ 72% in 90 min. of exposure under UV-light. Percentage degradation then gradually increases for Sm 4% and Sm 6%, which comes out to be ~ 84 % and ~ 89 % respectively. A decrease in percentage degradation is observed for 8% Sm concentration, which is ~ 81 %. This pattern obtained for degradation of RB dye can be

explained on the basis of band gap and oxygen vacancies. Since band gap was decreased, and VO were increased respectively, up to 6% of Sm ion concentration then followed a reverse trend for Sm 8% due to saturation of Sm ions at Ce4+ site. Hence, photocatalytic degradation also followed the same pattern. The terms related to degradation have been calculated using the following formulae: %

= − ln / 1/

#−# × 100 #

# 0= #

=

2/

where Co and C are the initial and final concentration of RB dye, respectively, k-rate constant and t-irradiation time. The half-life time (t1/2) is the time taken to degrade 50% of RB dye concentration [39]. A plot between -ln(C/Co) and time of UV irradiation is shown in Fig. 8 (f) that follows pseudo first order degradation equation as shown in equation 2 [40]. The rate constant (k) was estimated from linearly fitted slope and found to be 0.01314, 0.02015, 0.02384 and 0.01774 min-1 (as shown in Fig. 8 f) for Sm doped CeO2 (Sm = 0%, 4%, 6% and 8%) catalysts. The linear correlation coefficient (R2) was found to be 0.970, 0.986, 0.987 and 0.978 for pure CeO2, Sm 4%, Sm 6% and Sm 8% samples. The values of R2 for first order kinetics are close to unity which confirms that the degradation process follows first order kinetic equation [41].

Fig. 8. Absorption spectra for photocatalytic degradation of RB dye with (a) pure CeO2 (b) Sm 4% (c) Sm 6% and (d) Sm 8% catalysts for different time intervals; (e) Relative concentration of RB dye with UV illumination time for of Sm doped CeO2 (Sm = 0%, 4%, 6% and 8%) samples versus degradation time under UV light illumination and (f) experimental and linear fitted plots of –ln(C/Co) versus degradation time

Electrical energy per order: Electrical energy per order is the amount of electricity used to degrade the dye up to 90% of its efficiency. Time taken to degrade the RB dye up to 90 % of its initial concentration has been calculated using the relation

3

= ln(10) /

[42]. Furthermore,

electricity cost (Ec) to degrade this much concentration of dye was estimated using equation given below. The cost comes out to be merely INR 2.26 (given in Table 3) for Sm 4% which is quite effective as shown in the pie chart in Fig. 9 (c). Hence, low-cost consumption is a deciding factor in any application.

45 =

6 × 3 × 4.68 1000 × 60

where t90 is the time (in minute) taken to degrade any pollutant up to its 90 % efficiency, k is rate constant, Ec is electricity cost, P is power (in watt) of UV source of light, 4.68 is the electricity cost (in Indian Rupees) per unit in our locality if one is consuming at least 500 units per month.

Fig. 9. (a) Percentage degradation of RB dye with time; (b) Variation of rate constant k with Sm concentration; (c) Electricity cost (in Indian Rupees) to degrade 90 % of the RB dye and (d) Mechanism structure and proposed photocatalytic degradation mechanism for rose bengal dye

A photodegradation phenomena, as shown in Fig. 9 (d) is based on the ease of migration of electrons (ejected on UV illumination) from valance band to conduction band. In conduction band, O2 molecules act as a scavenger for these electrons and yield oxygen radicals (O2•-). Furthermore, the holes left behind in the valance band oxidize hydroxyl species and H2O• to generate OH• radicals. These super-oxide anions (O2•-) and OH• radicals are responsible for the degradation of organic dyes [43]. The formation of radicals and

degradation is given in Fig. 9 (d) and can be easily understood with the help of the following equations: = ;<

+ > → >•=

% ℎA< + >B = → >B •

>•= + >B • + CD E → B > + #> (

F CD E )

Table 4 Parameters calculated from photodegradation of RB dye using synthesized Sm doped CeO2 (Sm = 0%, 4%, 6% and 8%) catalysts Sample Degradation k R2 t1/2 t90 Electricity (% in 90 (min-1) (min) (min) cost min) (INR) Pure CeO2 72.10 0.01314 0.970 52.8 175.2 4.10 Sm 4% 84.08 0.02015 0.986 34.4 114.3 2.67 Sm 6% 89.04 0.02384 0.987 29.1 96.6 2.26 Sm 8% 80.89 0.01774 0.978 39.1 129.8 3.04

Conclusion: Sm doped CeO2 (Sm = 0%, 2%, 4%, 6% and 8%) samples have been synthesized using sol-gel method. XRD was performed for crystalline structural confirmation, and structural defects in the form of stress and strain were determined. Further, defect study has been carried out using Raman spectroscopy that revealed the defects are present in the form of oxygen vacancies, which ascended with the doping concentration of Sm ions up to 6 %. Sm ions were replacing Ce4+ ions up to 6 % doping concentration and attained a saturation level. After further doping, Sm ions started to occupy the interstitial sites or the surface rather than Ce4+ sites. Moreover, the saturation magnetization obtained for pure ceria was 0.36×10-3 emu/g that goes on increasing to a value of 15.8×10-3 emu/g for 6% Sm concentration and then decreases for 8% Sm concentration. This increment in saturation magnetization was due to exchange interactions of 4f electron spin and magnetic moments resulting due to VO in the

lattice. FCE mechanism also explained the origin of RTFM in synthesized samples. Experiment for photodegradation demonstrated the excellent results for the degradation of RB dye under UV illumination. XPS analyzed the origin of new networks in support of oxygen vacancies that explained the variation in saturation magnetization and photocatalytic results with Sm concentration. The results showed that the synthesized samples with enhanced UV driven photocatalytic performance and RTFM behaviour could be a potential material to use in environmental and spintronic applications.

Acknowledgement: This work was financially supported by Department of Science and Technology for the

research

grant

under

DST-INSPIRE Faculty scheme

(No.

DST/INSPIRE/04/2015/003149).

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Highlights: • • • •

Presence of oxygen vacancies confirmed by Raman spectroscopy and XPS Photocatalytic degradation of rose bengal dye under UV illumination Origin of ferromagnetism due to defects in the lattice structure XPS showed the impact of new networks (Ce3+-O-Ce4+, Sm3+-O-Ce4+ and Ce3+-O-Ce3+) on photocatalytic and ferromagnetic properties

Conflict of Interest Authors declare that there is no conflict of interest.