Role of surface defects in catalytic properties of CeO2 nanoparticles towards oxygen reduction reaction

Accepted Manuscript Role of Surface Defects in Catalytic Properties of CeO2 Nanoparticles towards Oxygen Reduction Reaction

K. Sudarshan, S.K. Sharma, Ruma Gupta, Santosh K. Gupta, F.N. Sayed, P.K. Pujari PII:

S0254-0584(17)30582-5

DOI:

10.1016/j.matchemphys.2017.07.064

Reference:

MAC 19871

To appear in:

Materials Chemistry and Physics

Received Date:

02 December 2016

Revised Date:

21 June 2017

Accepted Date:

15 July 2017

Please cite this article as: K. Sudarshan, S.K. Sharma, Ruma Gupta, Santosh K. Gupta, F.N. Sayed, P.K. Pujari, Role of Surface Defects in Catalytic Properties of CeO2 Nanoparticles towards Oxygen Reduction Reaction, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys. 2017.07.064

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ACCEPTED MANUSCRIPT Role of Surface Defects in Catalytic Properties of CeO2 Nanoparticles towards Oxygen Reduction Reaction K. Sudarshan1*, S.K. Sharma1, Ruma Gupta2, Santosh K. Gupta1, F.N. Sayed3 and P.K. Pujari1 1Radiochemistry

Division, Bhabha Atomic Research Centre, Mumbai-400085, India Division, Bhabha Atomic Research Centre, Mumbai-400085, India 3Chemistry Division, Bhabha Atomic Research Centre, Mumbai-400085, India 2Fuelchemistry

*Corresponding author: [email protected] Abstract CeO2 nanoparticles have been prepared by gel combustion method. The as-prepared nanoparticles were calcined at 500, 550 and 600 C. The crystallite size of the nanoparticles has been determined using X-ray diffraction. Catalytic activity has been studied by measuring the Tafel slope in oxygen reduction reaction in cyclic voltammetery. The nanoparticles samples showed higher catalytic activity than bulk ceria sample. Surprisingly, smaller size nanoparticles with large surface area showed less catalytic activity than larger size nanoparticles. Positron annihilation, X-ray photo electron spectroscopy and photoluminescence studies indicated the presence of oxygen vacancies as well as larger surface defects. It has been found that surface defect concentration increased with the increase in calcination temperature and the catalytic activity of the nanoparticles is directly correlated to the surface defect concentrations. Keywords: nano ceria, electro catalysis, positron annihilation spectroscopy, oxygen vacancies

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ACCEPTED MANUSCRIPT Introduction Redox reactions are of great importance in different life processes as well as energy conversion systems like fuel cell technology [1]. Different metal oxides especially transition metal oxides are studied for their catalytic activity in different redox reactions [2, 3]. Ceria has been of interest in catalysis of reactions in the treatment of exhaust gases [4-6]. Ceria based catalysts have also been developed for different organic reactions [7]. CeO2 has been used as a catalytic promoter with Pd/C and Pt/C catalysts in oxygen reduction and methanol oxidation reactions etc [8-11]. Enhancement in catalytic activity of copper doped CeO2 towards total oxidation of naphthalene [12] and CO oxidation to CO2 [13] is attributed to oxygen vacancy defects induced by copper doping. Ceria itself or with dopants is also used as a catalyst in redox reactions. [14-16] Graciani et al have reported that special reaction pathways/adsorption sites can be created at the metal-metal oxide interface by tuning the interface of catalysts for conversion of carbon dioxide to methanol [17]. Possible role of various types of defects/surface chemistry in the catalytic properties has been reported in ceria and other oxide catalysts [18-20]. Oxygen vacancies assisting the transition of Ce4+ to Ce3+ on the surface of mesoporous ceria nanotubes has shown to be aiding the conversion of CO to CO2 [21] and total oxidation of toluene [22]. Oxide nanocrystals showed different catalytic properties based on the crystal planes exposed and is attributed to the morphology-dependent surface chemistry of the exposed planes [23, 24]. Defects induced by exposure to light were shown to enhance the photocatalytic activity of CeO2 for degradation of phenol and its derivatives [25]. Hence, in order to ascertain the role of different types of defects on the catalytic activity of CeO2 nanoparticles, it is essential to indentify the atomic defects present on the surface of nanoparticles. Positron annihilation spectroscopy is considered one of the most suitable techniques for characterization of open volume defects due to the propensity of positrons to get localized in the low density areas in the materials [26]. When the energetic positrons are implanted into matrix, they quickly thermalise and get trapped preferentially in defects where the repulsion from the nuclei core is minimum. Depending on the electron density around the positron, the positron lifetime varies and accordingly the lifetime of the positrons annihilating from the defects is larger than the lifetime in the defect free materials. Positron lifetime values are different for different kinds of defects and it is possible to identify the possible defects based on the lifetime components extracted. Positron annihilation lifetime spectroscopy (PALS) has been used to understand the defect evolution in the nanoparticles. [27]. Annihilation gamma quanta are Doppler broadened due to the momentum of the annihilating electrons. Doppler broadening spectroscopy is used to identify the defect surroundings. Two detector Doppler broadening, commonly known as coincidence Doppler broadening (CDB) gives superior information about the elements around the positron annihilation site or defect [28]. In the present study, CeO2 nanoparticles have been prepared and ‘as prepared’ ceria was calcined at different temperatures (500, 550 and 600 C) to vary the crystallite size. The nanoparticles have been characterized by X-ray diffraction (XRD) measurements. Cyclic voltammetry measurements were performed to investigate the role of ceria nanoparticles in promoting oxygen reduction reaction (ORR). For comparison these measurements have also been performed on bulk ceria. PALS, CDB of annihilation gamma-rays, X-ray photo-electron 2

ACCEPTED MANUSCRIPT spectroscopy and photoluminescence measurements have been carried out to investigate the type, size as well as density of the defects to understand their role in the catalysis of ORR. The studies would help in tuning the catalytic activity of nanoceria via defect engineering. Experimental The cerium oxide nanoparticles were synthesized using facile gel combustion method. AR grade Ce(NO3)3.6H2O as metal source and glycine as fuel were used. The net oxidizing valency of cerium nitrate and reducing valency of the glycine (fuel), were calculated using the valencies of the individual elements. The ratio was found to be 1.66 for present system. However, a bit of deficient ratio is known to give soft agglomerated powders, which further helps to achieve higher sinterability and lower particle sizes. Hence, a fuel deficient ratio (60%) compared to the stoichiometry ratio was taken for the synthesis. The required amount of cerium nitrate was weighed and dissolved in the minimum volume of dilute nitric acid and appropriate amount of fuel (glycine) was added to it. Highly viscous liquid, resulted after thermal dehydration of thise solution around 90 oC. The temperature of the hot plate was raised to 250 oC as soon as the viscous liquid was formed. The precursor swelled and auto ignited, with rapid evolution of gases to produce voluminous powder. This voluminous powder is as-prepared product. To remove the carbonaceous products, as-prepared sample was heated at 500 oC for 1 hour. To get different size of the particles, samples were also calcined at 550 and 600 oC for 1 hour. The samples are called as ceria-500, ceria-550 and ceria-600 in further discussion. Ceria of microcrystallites is commercial ceria sample of 99.999% purity obtained from Spex Industries Inc. USA and is referred to as ‘ceria-bulk’. XRD patterns of the samples were recorded using RIGAKU Miniflex-600 diffractometer operating in the Bragg-Brentano focusing geometry. Cu-Kα radiation source (λ= 1.5406Å) has been used as X-ray source. The samples were also characterized by X-ray photo-electron spectroscopy (XPS), using an Mg K source (make: RIBER system). For electrochemical measurements of oxygen reduction in presence of ceria, electrode was prepared by adding 10 wt% of graphite carbon to 1 mg of sample followed by addition of 5 wt% nafion solution as the binder. Sufficient amount of isopropyl alcohol (IPA) was then added to make an ink out of which 10 L (optimized value) was drop casted over a glassy carbon electrode. Cyclic Voltammetry was performed using CHI 760D electrochemical workstation with a voltammetric cell having three electrodes viz. glassy carbon disk (GC) working electrode (area, A = 0.071 cm2), platinum wire counter electrode and Ag/AgCl reference electrode. The potentials were quoted with respect to Ag/AgCl reference electrode. The measurements were carried out at scan rate ranging from 0.01- 0.2V/s in 1M KOH solution saturated with oxygen. There were iR compensations and no stirring were used for the experiments. All the measurements were carried out at room temperature (25±1∘C). PALS measurements were carried out to investigate the defects in ceria nanoparticles. PALS measurements were carried out with a lifetime spectrometer consisting of two plastic scintillation detectors and the time resolution of the spectrometer was 265 ps. Carrier free 22Na, (15 Ci) deposited and dried between two 8 micron kapton films was used as positron 3

ACCEPTED MANUSCRIPT source. The positron source was completely immersed in adequate amount of CeO2 powder samples to stop all positrons in the sample. Approximately one million counts were recorded in each spectrum. Silicon was used as a reference for correcting the fraction of positrons annihilating in the source. The computer program, PALSfit [29] was used for the analysis of the spectra. CDB spectra were measured using two identical HPGe detectors placed at 180 to each other and having energy resolution of 2 keV at 1332 keV of 60Co. The difference in energies of the two annihilation  rays is expressed as cPL, where PL is the longitudinal component of the annihilating electron momentum. Coincidence events were selected where the sum of energy of the annihilating gamma-rays (ET) doesn’t differ from 1022 keV by more than 2.1 keV. Total of about 7-8 million counts were acquired under the full energy of 511 keV in each spectrum. CDB spectra were area normalized and ratios of the spectra with respect to silicon are compared to investigate if the positron annihilation sites or defects are changing with calcinations temperature. Further details of the data analysis can be found elsewhere [30]. The shape (S) and wing (W) parameters were evaluated from the Doppler broadened annihilation radiation spectra. The S-parameter is calculated as fractional area in the energy range 511±0.84 keV and the W-parameter is evaluated as fractional wing area in the region of 4.2≤|Eγ-511|≤6.3 keV, where Eγ is the energy of gamma radiation. Photoluminescene (PL) measurements were carried out using an Edinburgh CD-920 unit equipped with M 300 grating monochromators. The data acquisition and analysis were done by F-900 software. A 150 W Xenon flash lamp having variable frequency range of 10– 100 Hz was used as excitation source and was operated at 100 Hz. Approximately 40 mg of powder sample was mixed with few drops of 4 % collodion solution and the resulting slurry was pasted over a glass plate using spatula. This was dried at room temperature and used for the PL measurements.

Results and Discussion The powder X-ray diffraction patterns of as prepared and calcined ceria nanoparticles at different temperatures are given in Fig.1A along with XRD pattern of bulk ceria powder. XRD patterns of all the samples match well with the cubic lattice of pure CeO2 and are in agreement with the literature (ICSD # 072155). It is also clearly seen from the Fig. 1A that the diffraction peaks become narrower with increasing in calcination temperature. The broadening of the diffraction peaks would have contributions from lattice strain, crystallite size and intrinsic instrumental broadening. Williamson-Hall approach of uniform deformation can be used to decipher these contributions and estimate the crystallite size [31]. After correcting for the instrumental broadening, the peak broadening () can be given by Equation 1 Equation 1 having contributions from crystallite size and strain, 𝐾𝜆

𝛽 = 𝐷 𝑐𝑜𝑠𝜃 + 4𝜀 𝑡𝑎𝑛𝜃

(1)

In equation 1, first and second terms represent the size and strain broadening, respectively. D is the crystallite size,  is the wavelength of X-ray, 2 is the scattering angle and  is the 4

ACCEPTED MANUSCRIPT strain. Crystallite sizes in the nanoparticle samples were estimated from intercept of cos vs 4sin plots as shown in Fig.1B taking the value of K = 0.90 usually used for cubic crystals. The crystallite sizes estimated from the broadening are given in Table 1. The uncertainties quoted on the crystallite size and strain are the fitting errors from Williamson-Hall plot shown in Fig. 1B. It is seen from the Table 1 that the crystallite size increased from 16 nm to 27 nm due to calcination at higher temperature. The crystallite size in the bulk ceria samples was more than 0.2 μm.

(311) (222)

(220)

(111) (200)

(A)

Intensity (a.u.)

Ceria-500

Ceria-550

Ceria-600

Ceria-bulk

ICSD # 072155

30

40

50

60

70

2 (deg)

Fig. 1A. Powder XRD patterns of CeO2 samples.

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0.014

Ceria-500 Ceria-550 Ceria-600

Equation Adj. R-Square B B

0.012

B B B

 cos()

B

0.010

0.008 (B) 1.0

1.2

1.4

1.6

1.8

2.0

4 sin()

Fig. 1B. Plot of cos vs 4sin from XRD patterns of CeO2 samples for determining crystallite size using Williamson-Hall approach.

Table 1. Crystallite sizes of CeO2 samples as determined from Fig. 1B. Sample

Fitting parameters (Figure 1B) Intercept Slope (strain) -3 Ceria-500 (8.73 ± 0.74)10 (2.41 ± 0.53)10-3 Ceria-550 (6.32 ± 0.56)10-3 (2.61 ± 0.40)10-3 Ceria-600 (5.02 ± 0.39)10-3 (2.34 ± 0.28)10-3

Crystallite size (nm) 16 ± 2 21 ± 2 27 ± 2

To understand efficacy of ceria in promoting ORR, cyclic voltammetry (CV) was performed. Figure 2 shows the cyclic voltammogram of bare GC electrode and different ceria samples in oxygen saturated 1 M KOH at a scan rate of 0.01Vs-1. The cyclic voltammograms are overlaid in the same scale for the sake of comparison. It shows two reduction peaks. The first peak (1) at around -0.4 V shown in Figure 2 can be attributed to a two electron reduction process described by equation (1) [9, 32, 33] 𝑂2 + 𝐻2𝑂 + 2𝑒 - → 𝐻𝑂2 - + 𝑂𝐻 -

(1)

The second broad reduction peak (1*) at around -0.9 V can be assigned to a further two electron reduction process described by equation (2): 𝐻𝑂2 - + 𝐻2𝑂 + 2𝑒 - → 3𝑂𝐻 -

(2)

6

Current Density,J / mAcm

-2

ACCEPTED MANUSCRIPT

0.00

-0.05

-0.10

1*

Bare GC Ceria-bulk Ceria-500 Ceria-550 Ceria-600

-0.15

-0.20 -1.2

1 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

E / V vs Ag/AgCl Figure 2: Comparison of cyclic voltammograms obtained at GC electrode, GC/ ceria-bulk, GC/ceria-500, GC/ceria-550, GC/ceria-600 respectively in oxygen saturated 1 M KOH at a scan rate of 0.01Vs-1. It can be clearly seen from the Figure 2 that from ceria-500 to ceria-600, the peak positions are slightly shifted but the intensity of current is increased significantly. Figure 3 shows the comparison of polarization curves for ORR over the bulk ceria and ceria-600 at a scan rate of 10mV/s. The enhanced ORR kinetics due to ceria was checked by estimating a Tafel slope in the low current density range. The higher the Tafel slope, faster the overpotential increases with the current density. Thus, for an electrochemical reaction to obtain a high current at low overpotential, the reaction should exhibit a lower Tafel slope. The values of Tafel slope of the ceria samples are lower than that of bare GC (Table 2), which indicates the enhanced ORR kinetics in the presence of ceria.

-5.0 -5.5

-2

Log ( J / Acm )

-6.0 -6.5 -7.0 -7.5 -8.0 -8.5 -9.0 -9.5 -10.0

Ceria-600 Ceria-bulk

-10.5 -11.0 -11.5 -12.0 -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

7 E / V vs Ag/AgCl

0.0

0.2

ACCEPTED MANUSCRIPT

Figure 3: Polarization curves for the oxygen reduction reactions over the bulk ceria and ceria-600 catalysts at a scan rate of 10mV/s.

Table. 2. Tafel slope obtained at different electrodes S.No 1. 2. 3. 4. 5.

Sample GC GC/ceria-bulk GC/ceria-500 GC/ceria-550 GC/ceria-600

Tafel Slope / mV dec-1 152 140 129 125 115

The efficacy of promoting ORR varies as ceria-600 >ceria-550 > ceria-500 > ceriabulk. It is reasonable that the nanoparticles of ceria are better promoters of ORR as the surface area offered by nanoparticles is much higher than the bulk. However, among the nanoparticles samples, the order of promoting the reaction is in inverse order to that expected based on surface area of nanoparticles. As seen from table 1, nanoparticles size is in the order ceria-500 < ceria-550 < ceria-600 and surface to volume ratio and ORR promotion are expected to be in the reverse order to the size of nanoparticles. To understand this difference and the factors contributing to this, positron annihilation spectroscopy and photoluminescence studies have been carried out. As discussed earlier, it has been reported that defects on oxide surfaces aid catalytic activity and positron annihilation spectroscopic and photoluminescence are known to be good techniques to characterize the various kinds of defects. The positron annihilation lifetime spectra of all the ceria samples are shown in Fig. 4. All the spectra are normalised to unity at the peak position. It is clearly seen from the Figure that the average lifetime of positron in bulk ceria is much shorter compared to the ceria nanoparticles. The average positron lifetime in the ceria nanoparticles are observed to decrease with an increase in the calcination temperature. The positron lifetime spectra were fitted to a multi exponential function convoluted with resolution using the program PALSfit. All the spectra could be fitted well to three lifetime components. The longest component was of 2 ns and the corresponding intensity in all the cases was approximately 1%. Such a long lived component of low intensity is usually reported in powder samples due to positron annihilations in inter granular spaces [34]. Hence, this component is not discussed further. The shorter lifetime components denoted as 1 and 2 in increasing order, which accounted for about 99% intensity are listed in Table 3. In the case of nanoparticles, 1 is attributed to positron annihilations in the bulk of nanoparticles while 2 is shown to be the result of positron annihilations on the nanoparticles’ surface [35, 36]. The average positron lifetime is the signature of the overall electron density in the material. The average lifetime is calculated from the individual lifetimes (1 and 2) and the corresponding intensities (I1 and I2) as 8

ACCEPTED MANUSCRIPT  ave 

 1 I1   2 I 2 I1  I 2

; and is also given in Table 3. The errors quoted on individual lifetimes and

Counts (normalised at zero time)

intensities are from fitting of the experimental spectra while the errors on the average lifetimes are propagated from the individual fitting errors on lifetimes and intensities.

Ceria-500 Ceria-550 Ceria-600 Ceria-bulk

1

0.1

0.01

1E-3 6

7

8

9

10

11

12

Time (ns)

Fig.4. Positron annihilation lifetime spectra of CeO2 samples. The counts at peak position is normalised to unity.

Table 3. Positron annihilation lifetimes and the corresponding intensities in CeO2 samples. Sample Ceria-500 Ceria-550 Ceria-600 Ceria-bulk

1 (ps) 269 ± 12 225 ± 8 194 ± 6 171 ± 7

I1(%) 59.4 ± 1.5 48.3 ± 0.9 47.2 ± 0.7 91.9 ± 0.5

2 (ps) 396 ± 16 379 ± 11 357 ± 8 342 ± 23

I2(%) 39.9 ± 1.5 50.5 ± 0.9 51.1 ± 0.7 7.8 ± 0.5

ave (ps) 320 ± 24 304 ± 16 279 ± 12 184 ± 18

I2/I1 0.67 1.05 1.08 0.085

The bulk positron lifetime in CeO2 crystals is reported as 185-190 ps [37,38]. The average positron lifetime in ceria sample with micro crystallites (ceria-bulk) in the present experiment is 184±18 and is in agreement with the previous studies. 1 in the present case in nano ceria samples varied from 269 to 194 ps while 2 in all the samples varied from 396 to 342 ps. 1 in the present experiment is close to 262 ps reported by Chang et al. [38] but higher than 187.9-211 ps reported by Shi et al. [39] in nanoceria. In both these cases the shorter positron lifetime component has been attributed to smaller vacancies of the type Ce3+vacancy combination. 2 in the range of 342-396 ps is attributed to larger vacancy clusters. These values are in the close range of lifetime values reported in literature; 375.1-431.9 ps by 9

ACCEPTED MANUSCRIPT Shi et al. [39], 393 ps by Thorat et al. [40] and ~350-400 ps by chang et al. [38]. 1 in the ceria nanoparticles in the present experiment is close to 253±4 ps reported by Sato [41] in Gd doped CeO2 which was attributed to the positron annihilations in the open spaces of the size of few atoms. The average positron diffusion length in single crystals of oxides samples is expected to higher than the crystallite sizes used in the present experiment [42]. Hence fraction of positrons diffuses to the surface of the nanoparticles and annihilate exclusively from near surface regions. The studies on TiO2 nanoparticles with modified surfaces indicate that 2 is principally from positron annihilations in the defects near the surfaces [35, 36]. It has been reported that defect densities in nanoparticles are reduced with the increase in calcination temperature or nanoparticles growth [43]. Decrease in the average positron lifetime with the increase in calcination temperature suggests increase in average electron density. The values of 1 and 2 show the presence of smaller point defects in the bulk phase and larger vacancy clusters near the surface in all the ceria samples, their corresponding concentrations vary significantly. I2 increased from 40% in ceria-500 to 50% in ceria-550, ceria-600 while this intensity is only 7.8% in the bulk sample. The intensity shows that larger vacancy cluster concentration is higher in ceria-550 and ceria-600 samples. With the increase in the calcination temperature, smaller vacancies from bulk phase migrate towards the surface, agglomerate to form larger vacancies and then finally anneal out. By the same mechanism, ceria-550 and ceria-600 samples show larger concentration of vacancy clusters near the surfaces than ceria-500 due to migration and agglomeration of smaller vacancies from the bulk phase. Ceria-bulk sample shows very small concentration of larger vacancy clusters indicating that vacancies have annealed out due to particle growth.

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3.5

Ceria-500 Ceria-550 Ceria-600 Ceria-bulk

Ratio with respect silicon

3.0

2.5

2.0

1.5

1.0

0

5

10

15

20

25

30

-3

PL (10 m0c)

Fig.5A. Ratio curves of coincidence Doppler broadening spectra of ceria samples with respect to silicon. The spectra have been area normalised before taking the ratio. CDB measurements provide information about the annihilation electron momentum. The Doppler broadened spectra are convoluted with the gamma-ray detector resolution. To amplify the differences between the different Doppler broadened spectra, the ratio curves of area normalised Doppler broadened spectra are taken with a suitable reference. This makes the differences between spectra at higher momentum region more evident. Fig.5A shows the ratio curves of annihilation gamma-ray Doppler broadening spectra of ceria samples with respect to the spectrum of silicon. The peak around 10-12 E-3 m0c in the ratio curves is typical of the positron annihilations with the core electrons of the oxygen probably surrounding the cation vacancies or other larger vacancy clusters. The features are similar in all the samples but the magnitudes of the peaks differ. The height of this peak is also influenced by trapping of the positrons in different defect sites. The trapping of positrons in defects enhances the lower momentum component and reduce the higher momentum components/height of the discussed peak. Bulk ceria samples with lowest concentration of defects shows larger peak in the ratio spectra. Ceria-500 and ceria-600 samples also show similar trend with the lifetime data, ceria-550 shows more positron trapping (lower peak heights). S- and W-parameters are evaluated from Doppler broadening spectra and are shown in Fig.5B. The higher S-parameters indicate more localisation of positrons at the defect sites. S-W data from the ceria samples fall on a straight line indicating that the nature of the defects in all these samples are similar but concentration varies. Higher S- and lower W-parameters

11

ACCEPTED MANUSCRIPT in ceria-550 when compared to both ceria-500 and ceria-600 might be due larger lattice strain as indicated in Table 1, leading to positron localisation.

0.052

C er ia -6 C er 00 ia -5 00

W-Parameter

0.056

0.048

0.044

0.040 0.50

0.51

0.52

0.53

0.54

C er ia -5 50

0.060

C er ia -b ul k

0.064

0.55

0.56

0.57

S-Parameter

Fig.5B. S-W correlation plot from Doppler broadening of annihilation radiation in ceria samples. The line is an eye guide only. Photoluminescence properties are also greatly influenced by defects. Figure 6 shows the emission spectra of the ceria samples at excitation wavelength of 230nm. Inset of Fig. 6 shows the intensity normalised emission spectra to decipher the changes in the emission peaks. The emission spectra of ceria nanoparticles have bands around 435, 454, 471, 488, 514, 542 and 558 nm. The emission spectra of CeO2 nanoparticles in the present experiment are similar to those reported by Wang et al [44] and Aslam et al [25] but differ from those reported by Malleshappa et al. [45] and Jhamsidhi et al [46]. It is to be noted that the PL spectra are very sensitive to preparation and processing conditions. The emission bands in all the nanoparticle samples are similar but their relative intensities differ. The emission in the range of 400 to 550 nm (<3 eV) is mostly associated with oxygen vacancies with trapped electrons which lie in the range of Ce 4f to O 2p band [47] in CeO2. The defect emission has been shown to either increase or decrease due to the presence of oxygen vacancies depending if the vacancies are of radiative and non-radiative nature [48-50]. In general, it is accepted that bulk defects act as charge carrier recombination centres enhancing photoluminescence while surface defects act as non-radiative traps and reduce photoluminescence [51-53]. The relative emission intensity is in the order of ceria-500 > ceria-600 > ceria-550 > ceria-bulk as seen from Fig. 6. PALS studies also indicate that the concentration of the smaller size defects present in the bulk decreases with the increase in particle size while the concentration of the surface defects increases. Both the factors contribute to reduction of the photoluminescence with increase in the particle size. Lower emission intensity in ceria-550 might have also contribution from higher strain which is shown to reduce emission [54]. Bulk ceria shows 12

ACCEPTED MANUSCRIPT lower fraction of emission intensity in >500 nm region than nano ceria (inset of Figure 6) and the average emission is shifted towards lower wavelength due less defect states.

2.06 450

514

471

435

Intensity (area normalised) 400

450

500

550

600

Wavelength (nm)

542

6000

488

2.25

2.48

2.75 471

514

8000

Intensity (rel)

488

10000

Ceria-500 Ceria-550 Ceria-600 Ceria-bulk

454

12000

3.10

emmision energy (eV)

558

4000

2000

400

500

600

700

Wavelenth (nm)

Fig.6. Photoluminescence spectra of ceria samples with ex=230 nm. Inset shows the intensity normalised spectra to decipher the changes in the emission profiles. XPS is also a well known technique to study the oxygen vacancies in ceria [55, 56]. The core level XPS spectrum of O 1s is shown in Figure 7. The XPS spectra could be deconvoluted into two peaks. The lower binding energy peak (OI) at 529.4 eV is due to O2attached to Ce4+ while the higher binding energy peak (OII) appearing as shoulder in the XPS spectra is due to oxygen bound to Ce3+ site. The 1s electrons in oxygen are more tightly bound to Ce3+ than Ce4+ [57]. The oxygen vacancy concentrations are proportional to the area of peak OII. Peak areas from Figure 7 show least oxygen vacancies in bulk ceria where the OII was not decipherable. In the other nano ceria samples, the area under OII peak increases as 6.4%, 8% and 11% in samples calcined at 500, 550 and 600 0C, respectively. These concentrations are also consistent with PALS on defects.

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Ceria-bulk

Ceria-500

Intensity (arb. units)

OI

OI

OII

Ceria-550

Ceria-600

OI

OII

524 526 528 530 532 534

OI

OII

524 526 528 530 532 534 536

Binding energy (eV)

Fig. 7. X-ray Photo-electron spectra of O 1s in the ceria samples. From the positron annihilation lifetime studies, I2/I1 is taken as the ratio of surface to bulk defects in the samples. It has been suggested that the catalytic properties of oxide catalyst can be tuned by tuning the ratio of surface to bulk defects [20, 58]. In the present experiment, I2/I1 is in the order ceria-bulk << ceria-500 << ceria-550  ceria-600. As seen from reduction in the Tafel slope, catalytic activity follows the general order of I2/I1 or surface to bulk defects. The oxygen concentration in the ceria samples also follows the same order as I2/I1 from PALS measurements. From the above studies, it can be concluded that the surface defects which are primarily oxygen vacancies with different trapped electrons. The oxygen vacancies also result in the formation of Ce3+ in the ceria samples. The vacancies associated with Ce3+ make electrons readily available for reduction of adsorbed oxygen.

Conclusion Nanoparticles of ceria were prepared by gel combustion method and were tested using cyclic voltammetry for their ability to promote oxygen reduction reaction. Nanoparticles of ceria were found to be better promoters of ORR reaction than bulk ceria. However, among the nanoparticles, the catalytic ability didn’t follow the expected trend based on their surface area. Further characterizations using positron annihilation spectroscopy and photoluminescence measurements showed that the nanoparticles showing better catalytic properties have large concentration of vacancy clusters associated with their surfaces. The XPS studies showed higher oxygen vacancy concentration in the samples with better catalytic properties. These vacancies aid the adsorption of oxygen and Ce3+ associated with these vacancies helps in the reduction reaction. The observations are consistent with literature where large vacancy clusters on oxide catalysts are proposed to aid the catalysis.

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ACCEPTED MANUSCRIPT Acknowledgements Authors thank Dr. A.K. Tyagi, Chemistry Division, Bhabha Atomic Research Centre (BARC), for helpful discussions and Shri. B.G. Vats, Fuel Chemistry Division, BARC for XRD measurements. Authors also thank Dr. C.L. Prajapat and Dr. DeepakTyagi for their help in XPS measurements.

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ACCEPTED MANUSCRIPT Highlights 

Nanoparticles of ceria are prepared using gel combustion method.

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Catalytic activity of the nano ceria towards electrochemical ORR has been studied.

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Higher concentration of surface defects in samples with better catalytic activity

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Higher oxygen vacancy concentrations in samples with better catalytic activity

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Oxygen vacancies/surface defects are aiding oxygen adsorption and reduction