Toward a Unified Description of LuminescenceâLocal Structure

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Towards a Unified Description of Luminescence – Local Structure Correlation in Ln Doped CeO Nanoparticles: Roles of Ln Ionic Radius, Ln Concentration and Oxygen Vacancies 2

Daniel Avram, Margarita Sanchez-Dominguez, Bogdan E. Cojocaru, Mihaela Florea, Vasile I Parvulescu, and Carmen Tiseanu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02240 • Publication Date (Web): 13 Jun 2015 Downloaded from http://pubs.acs.org on June 18, 2015

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The Journal of Physical Chemistry

Towards a Unified Description of Luminescence – Local Structure Correlation in Ln doped CeO2 Nanoparticles: Roles of Ln Ionic Radius, Ln Concentration and Oxygen Vacancies Daniel Avram,a Margarita Sanchez-Dominguez,b Bogdan Cojocaru,c Mihaela Florea,c Vasile Parvulescu,c Carmen Tiseanu*a a

National Institute for Laser, Plasma and Radiation Physics, P.O.Box MG-36, RO 76900, Bucharest-Magurele, Romania

b

Centro de Investigacion en Materiales Avanzados (CIMAV, S.C.), Unidad Monterrey, Alianza Norte 202, 66600 Apodaca, Nuevo Leon, Mexico c

University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Biochemistry and Catalysis, 4–12 Regina Elisabeta Bvd., Bucharest, Romania

*

Corresponding author. Tel./fax: +40214574610.

E-mail address: [email protected]; [email protected];

ACS Paragon Plus Environment

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Abstract: We propose a physical model for luminescence properties of trivalent lanthanide (Ln) doped into CeO2 by use of low temperature, site selective, time-gated luminescence spectroscopy seconded by X-ray diffraction, Raman and Fourier transform infrared spectroscopy and transmission electron microscopy. The main findings can be summarized as follows: i) Ln situated to both left and right side to Gd in the Ln series, exhibit a two center distribution. Both Ln centers substitute for the tetravalent Ce fluorite sites being differentiated by the local symmetry: cubic, as a result of zero vacancy in the nearest-neighbor oxygen shell (cubic Ln center) and low symmetry, likely due to one vacancy in the nearest-neighbor oxygen shell (Ln - defect associate center); ii) A first example of Dy emission in an inversion (cubic) symmetry, characterized by strong lines at 679 and 764 nm is reported. This results is expected to challenge the way this lanthanide is currently used as a luminescence probe; iii) the relative contribution of the Ln centers to the overall emission depends on the Ln ionic radius: Sm exists predominantly as a cubic center while Er is found mostly as a vacancy associate; iv) Er, La codoped CeO2 can be used as an effective model system to separate the effects of Ln concentration and subsequently induced oxygen vacancies on the efficiency of CeO2 sensitization of Ln emission and v) Zr co - doping of CeO2 obstructs the formation of Ln – defect associates. The implications of our findings for the interpretation of data already present in the literature are also discussed.

Keywords: time-gated luminescence spectroscopy; lanthanide ions; nanoparticles; local symmetry; structural probe; emission decays; emission sensitization; charge - compensation; defects; CeO2; ZrO2


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1. Introduction Ceria (CeO2) has unique properties such as high mechanical strength, oxygen ion conductivity and oxygen storage capacity 1-3. It is established that, in all these applications, the generation and migration of oxygen vacancies plays a key role. Undoped ceria has relatively small oxygen vacancy formation energy, due to the capability for cerium to change oxidation state from (formally) +4 to (formally) +3 valence state. Nevertheless, a significant increase of CeO2 ionic conductivity is obtained by doping with aliovalent ions such as trivalent lanthanide (Ln) ions. Doping CeO2 with trivalent cations introduces one charge-compensating oxygen vacancy per two trivalent dopants and leads to a coupled intra - and inter - sublattice interactions between Ce4+, Ln3+, O2- and oxygen vacancy (Vo or VÖ double positively charged). To date, it is clear that the ionic conductivity of Ln doped ceria is determined by short range atomic interactions; however, a comprehensive description of the local structure around the trivalent dopant is still missing. Classical atomistic simulations of the energetics of defect cluster formation in trivalent doped CeO2 demonstrate that a combination of electrostatic and elastic energy controls the atomic geometry of defect clusters 4-8. The electrostatic part is mediated by redistribution of the electronic structure and the elastic part is mediated by lattice deformations, the latter being dependent on the dopant ion size. The binding energies between dopant cation and oxygen vacancies also have been shown to be a strong function of the dopant ion size: the lanthanides with ionic radii greater than Gd favor the interaction with the oxygen vacancy in next nearest-neighbor (NNN) mode while those lighter than Gd favor interaction in nearest-neighbor (NN) mode 5-7. Following doping CeO2 with trivalent Ln ions with different ionic radii, both the oxygen vacancy and Ce3+ concentrations can be tuned, which is important for many catalytic applications, including biocatalysis 9. Besides being an excellent promoter, the lanthanide dopant can also impart luminescence property to ceria host, extending thus the traditional applications of ceria materials to novel fields such as phosphors, bioimaging and therapy 10. Luminescence spectroscopy in combination with particular trivalent Ln used as structural probes, such as Eu or Sm, is expected to reveal the presence of small amount of defects, though indirectly, via huge luminescence sensitivity of Ln to their nearest oxygen environment. Indeed, the way the lanthanides interact with defects in CeO2 should strongly affect the luminescence properties and mechanisms. The bulkier lanthanides that repel the oxygen vacancy from the NN position preserve the cubic symmetry, 8-fold coordination of the substituted Ce4+ cation. Because the local symmetry at the dopant site presents an inversion center, only the magnetic dipole (MD) transitions are allowed, while the electric dipole (ED) transitions are forbidden. Smaller lanthanides interact with the


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oxygen vacancy in the NN mode which removes the inversion center and distorts strongly the local symmetry. In this case, for the 7-fold coordinated lanthanide, both magnetic and electric dipole transitions are allowed. Luminescence studies show that the Eu/Sm dopants occupy mainly the cubic Ce4+ sites; however depending on Ln concentration, annealing temperature, excitation conditions and synthesis approach, the presence of so - called distorted or asymmetric emission is also detected and assigned to one or several Eu/Sm in low symmetry, non – identified sites.

11-19

Since the luminescence approach typically used in literature is based on room – temperature

measurements, limited excitation range, steady-state prevalence over time - resolved measurements, the correlation between the emission properties on one hand and the distribution and nature of Ln centers in CeO2 on the other hand, could not be established. Therefore, fundamental questions - how are the defects distributed around Ln and what is the role of Ln ionic radius in determining the interactions between Ln and defects remained unanswered. Recently, some of us proposed a physical model of Eu luminescence in CeO2 based on two main centers that substitute for the fluorite Ce4+ positions. The two centers, with strongly distinct emission properties have been assigned to cubic/isolated Eu and Eu – vacancy associate.20 However, the suitability of this model for lanthanides with smaller or greater size than that of Eu is not validated. Another issue with relevance for controlling and optimizing the ceria functionality is related to Zr doping. Though both reducibility and oxygen storage capacity of CeO2 are reported to be enhanced upon doping with (tetravalent) Zr elements,21 – 22 the effects of Zr on ceria local structure are not well understood. Recently, by comparing the luminescence of Eu, used as a spectral probe, in the mixed (CeO2 - ZrO2) and parent oxides (CeO2 and ZrO2) it was shown that Zr doping inhibit the formation of Eu vacancy associates

23

. Again, whether the above mentioned effect is lanthanide specific or it can be regarded as

general across the lanthanide series is not clarified. Herein, we present a first unified description of luminescence – structure – relationships in lanthanide doped CeO2 nanoparticles. To achieve this goal, four lanthanides located to the left (Sm and Eu) and right side of Gd (Dy and Er) are investigated by use of an extensive set of luminescence measurements at room temperature down to 10K, seconded by X-ray diffraction, Raman and Fourier transform infrared spectroscopy and transmission electron microscopy. Based on the experimental observations presented in this paper, we propose a physical model of trivalent Ln doped CeO2 based on two main Ln centers with distinct mode of interaction with the oxygen vacancy. The effects of annealing temperature, defect and Ln concentration on the local oxygen environment surrounding each Ln center are investigated. The first example of Dy emission in inversion symmetry is reported. We also show


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that in Zr - doped CeO2 the analysis of Ln luminescence is able to disclose the presence of structural and compositional inhomogeneity and most notably, it reveals the absence of Ln – vacancy associates. Finally, the impact of our findings on published literature is highlighted.

2. Experimental 2.1. Materials The doped CeO2 powders containing different Eu, Sm, Dy, Er and La loadings were synthesized by using the citrate complexation method, as already reported

20

amounts

DyCl3·6H2O,

of

Eu(NO3)3·6H2O,

Sm(NO3)3·6H2O,

. To prepare the doped CeO2 nanoparticles, the calculated Er(NO3)3·5H2O

or

La(NO3)3·6H2O

and

Ce(NO3)3·6H2O were dissolved each in 25 mL hot distilled water (60°C). Then the corresponding solutions were mixed together and solid citric acid was added to obtain a molar ratio of metal ion:citric acid =1:1.2. The mixture was stirred on a hot plate at 60°C for 1h. The resulting mixture was evaporated slowly in a vacuum rotavapor at 60°C until turned to a transparent gel. The gel was dried in an electrical oven at 60°C under vacuum for 5 h and at 120°C overnight, without vacuum. All samples were calcined at 1000°C (heating rate of 10ºC/min) for 4 hours. Ceria based materials containing different concentration of La (5, 10 and 20%), Er (1 %), Eu (1 and 5%), Sm (0.1, 1 and 5%) and Dy (1 and 5%) were prepared. CeO2, ZrO2 and CeO2 - ZrO2, nanoparticles were synthesized under soft conditions by using the oil-in-water microemulsion method

23-24

. 1 g of CeO2, ZrO2 and CeO2 - ZrO2 were

impregnated with the required amount of a 0.004 M aqueous solution of Sm or Dy chloride and left stirring overnight at 60 °C. After that, the water was evaporated at 80 oC and the obtained powder was dried at 80 °C in a drying oven for several hours. All samples were calcined under air in ex situ conditions at 500, 750 and 1000 °C at a heating rate of 4 ºC/ min. 2.2. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Schimadzu XRD - 7000 diffractometer using Cu Kα radiation (λ = 1.5418 Å, 40 kV, 40 mA) at a scanning speed of 0.10 degrees min-1 in the 5 – 90 degrees 2Θ range. Raman analysis was carried out with a Horiba Jobin Yvon - Labram HR UV-Visible-NIR Raman Microscope Spectrometer at 633, 514 and 488 nm. The spectra were typically acquired under the following conditions: beam diameter of 1.0 ± 5% mm, spot diameter of 0.5 µm, spatial resolution of 0.35 µm and spectral resolution of 2 cm-1. For transmission electron microscopy (TEM), the particles aggregates were dispersed in water and deposited on


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lacey carbon Cu greed. The TEM images were realized using a Jeol ARM 200F electron microscope working at 200kV. TEM analysis was also carried out using a Field Emission Transmission Electron Microscope, JEM-2200FS, 200 kV, with 0.19 nm resolution in TEM mode, 0.1 nm resolution in Scanning Transmission Electron Microscopy (STEM) mode, spherical aberration correction in STEM mode and Energy Dispersive Spectroscopy detector (EDS, Inca). All TEM observations were performed at low magnification and low density of current to prevent the structure degradation of CeO2 - ZrO2 particles. Fourier transform infrared (FTIR) spectra were measured with a Thermo Electron Nicolet 4700 FTIR spectrometer with a Smart Accessory for diffuse reflectance measurements. The IR spectra were scanned in the region of 4000–400 cm-1 at the resolution of 4 cm-1. The final spectra resulted from accumulation of 200 scans. The characterization of the porous texture of the as-synthesized and calcined samples was performed by N2 adsorption at -196°C using a Micrometrics instrument (ASAP 2010). The BrunauerEmmett-Teller (BET) method was used to calculate the surface area from the data obtained at P/P0 between 0.01 and 0.995. Prior to surface area determination, the samples were outgassed at 150°C for 5 h. The pore size distribution of each sample was determined from the desorption branch of the N2 isotherm. DR-UV-Vis spectra were recorded with a Shimadzu UV-3600 UV-VIS-NIR spectrophotometer equipped with an integrating sphere. The baseline measurement was taken with BaSO4 and the spectra were recorded in the range of 220 – 700 nm. 2.3. Photoluminescence measurements. The luminescence measurements were carried out using a Fluoromax 4 spectrofluorometer (Horiba). The luminescence decays were measured by using the “decay by delay” feature of the phosphorescence mode. The average decay time, was calculated as integrated area of normalized decay.

∞

∞

0

0

τ = ∫ tI (t )dt / ∫ I (t )dt , where I (t )

is the normalized decay law. Time resolved emission spectra were recorded using a wavelength tunable (from 210 to 2300 nm) NT340 Series EKSPLA OPO (Optical Parametric Oscillator) operated at 10 Hz as excitation light source. The tunable wavelength laser has a narrow linewidth < 4 cm-1 with scanning step varying from 0.05 to 0.1 nm. The output pulse energy varied in the range of 1.9 – 22 mJ. As detection system, an intensified CCD (iCCD) camera (Andor Technology) coupled to a spectrograph (Shamrock 303i, Andor) was used. The time-resolved emission spectra were collected in the spectral range of 400 nm < λem < 900 nm using the box car technique. The gain of the micro-channel plate (MCP gain) was set at 100. Photoluminescence (PL) was detected with a spectral resolution of 0.05 nm and the input slit of the spectrograph was set to 10 µm. The temperature of the iCCD was lowered to -20 °C


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to improve its signal to noise ratio. The luminescence measurements were recorded at low – temperature (10 and 80 K) by use of exchange gas cryostats and at 300 K. For room-temperature emission measurements in the range of 1100 to 1700 nm, a fiber optic spectrometer (AvaSpec-NIR256-1.7TEC) with TE cooled InGaAs detector was used as a detection system (~6 nm spectral resolution).

3. Results and Discussion 3.1. Evidence for a two - centre model of Ln emission in CeO2 Ln (Ln= Eu, Sm, Dy and Er, concentration of 1 and 5 %) doped CeO2 synthesized by citric acid method and calcined in air at 1000 °C were investigated by use of low temperature (10 and 80 K), site selective, time-resolved excitation in the UV to Vis range. According to X-ray diffraction data and Raman spectra, the crystal structure was determined as single phase fluorite lattice with crystallite size varying from 34 to 46 nm (Figure S1). The emission of all samples has been excited by use of site selective time-gated excitation approach in the range of ~ 350 to ~ 480 nm. Based on the spectral deconvolution procedure described in Ref. 20 applied to the measured spectra, along with the analysis of the excitation spectra and emission decays, the co-existence of two distinct Ln centres was clearly established (see also Table S1 for details on the excitation and emission wavelengths used in experiments). Figure 1a, b, c illustrates the 10 K emission spectra, decays and excitation spectra of Eu doped CeO2. The data confirm definitely the two – centre distribution suggested by our previous room – temperature, twowavelength excited luminescence investigations

20

. The spectrum I is obtained by excitation into ceria O2- - Ce4+

charge-transfer (CT) band of CeO2 and assigned to Eu substituting for the cubic Ce4+ lattice sites (the emission is dominated by the allowed magnetic dipole (MD) transition around 591 nm; the MD and ED forbidden 5D0 - 7F0 emission around 579 nm is absent). The spectrum II excited in to f - f absorption of Eu is assigned to Eu in a distorted inversion less symmetry (emission is dominated by the relative intense ED 5D0 - 7F2 emission lines at ~ 611 and 632 nm). Since Eu and Sm have close ionic radii in 8-fold coordination (1.066 and 1.079 Å, respectively) it is expected that Sm display a similar two - centre type emission in CeO2. To date, no study succeeded to elucidate the number and nature of luminescent Sm centers 11 - 13, 19 which is likely due to the room-temperature, non-selective approach used in these investigations. As illustrated in Figure 1d, e, f the emission of Sm could be decomposed as a sum of two distinct spectra as done for Eu. Spectrum I obtained under excitation into the CT band of CeO2 shows a relative


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intense MD 4G5/2 - 6H5/2 transition at 573 nm and a negligible intensity for the ED 4G5/2 - 6H9/2 transition at 670 nm and is readily assigned to Sm in cubic Ce lattice sites.

Figure 1. The fingerprint emission and excitation spectra and emission decays of Eu, Sm and Dy in CeO2 (I and II refer to cubic and Ln - vacancy associate centers, respectively). Measurement temperature was 10 K (Eu) and 80 K (Sm and Dy). λex/ λem= 350 nm/591 nm and 468.3 nm/632 nm for Eu I and Eu II centers; λex/ λem= 350 nm/573 nm and 479.8 nm/607 nm for Sm I and Sm II centers and λex/ λem= 350 nm/679 nm and 428.3 nm/577 nm for Dy I and Dy II centers. Further details on the emission/excitation wavelengths and experimental parameters are listed in Table S1. Insets in (a) and (d) shows a zoomed view in the range of “forbidden” emission of cubic Eu and Sm centers. Spectrum II obtained with excitation into f-f absorption of Sm shows comparable intensities for the MD 4G5/2 -


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6

H5/2 and mixed25

- 28

MD + ED 4G5/2 - 6H7/2 transitions at 570 and 618 nm, respectively, and a non-negligible

intensity for the ED 4G5/2- 6H9/2 transition at 670 nm, and therefore is assigned to Sm in a low symmetry site. Dy probably represents the most interesting selection of small dopant of CeO2 since it usually displays a relative strong emission in the Vis range being also used as structural probe in a wide range of hosts 29. To date, the lack of Dy emission in CeO2 nanoparticles was assigned to the close location of Dy emitting level (around 480 nm) to ceria absorption edge is the main cause 30. Here, the emission of Dy was observed only at low temperature and could be subsequently decomposed in two spectra (Figure 1g, h, i) as found for Eu and Sm. The shape of spectrum I which is obtained by excitation into the CT band of CeO2 is dominated by strong emission lines around 679 and 764 nm. The emission is assigned to the MD 4F9/2 – 6H11//2 and 4F9/2 – 6H9/2/6F11/2 transitions, respectively. To the best of our knowledge, this is first example of Dy emission in an inversion site, irrespective of the host type. The spectrum II is characterized by a strong ED 4F9/2 – 6H13/2 transition around 574 nm followed by the 4F9/2 – 6H15/2 transition at 483 nm and the very weak MD 4F9/2 – 6H11//2 and 4F9/2 – 6H9/2/6F11/2 transitions. The spectrum is assigned to Dy in the low symmetry site. The nature of the perturbed Ln center is suggested by the main findings of the defects chemistry of trivalent doped CeO2. According to these, on substitution of trivalent Ln for Ce4+, the most probable defect formed is the oxygen vacancy1.

The

charge-compensation

mechanism,

described

in

the

Kröger-Vink

notation

as:

Ln 2 O 3 CeO 2 → 2Ln ′Ce + VO&& + 3O OX shows that the oxygen vacancies (VÖ, double positively charged) are induced to compensate for the introduced negative charge of Ln'Ce cations. Both experimental and computational works 4 – 8 established that the site preference for the vacancies depends on the ionic radius of the trivalent Ln: Ln dopants smaller than Gd tend to trap the oxygen vacancy at nearest – neighbor (NN) site strongly while Ln dopants larger than Gd repel the oxygen vacancy from NN sites. Based on the luminescence results presented in Figure 1 and the above mentioned principles of defects chemistry of trivalent doped CeO2 we have arrived at the physical model based on a two - Ln centre illustrated in Scheme 1. The ultra-narrow emission lines around 0.2 - 0.4 nm and the decays in the ms range indicate that both Ln centres occupy Ce4+ lattice sites (Figure 1). The Ln centres differentiate strongly by the local oxygen environment: an eight –fold LnO8 cube for the cubic Ln centre and an inversion less LnO7VÖ polyhedron around the Ln – vacancy associate. All decays are non-exponential which likely due to the incomplete separation of emission corresponding to centers I and II (Figure 1c, f, i).


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Scheme 1. Illustration of the two main Ln centers in Ln doped CeO2. Both centers substitute for the Ce4+ lattice sites but have distinct oxygen environments: 8-fold coordinated polyhedron, LnO8 with cubic symmetry (cubic Ln center) and 7- fold coordinated polyhedron, LnO7VÖ with inversionless, low symmetry (Ln - vacancy associate).

This assumption is confirmed by the decay of centre II which at long delays contains a long decay component which parallels the decay of centre I. The average decay times corresponding to cubic centre exceed several times those of Ln - vacancy associate which is consistent with the MD nature of the cubic type emission (Table 1).

Table 1. The average decay times and asymmetry ratio corresponding to cubic and Ln - vacancy associate centres

Average decay time (ms)

Asymmetry ratio

Ln (%)

1

Cubic Ln

Ln – VÖ associate

3.72 ± 0.055

0.71 ± 0.005

Cubic Ln

Ln – VÖ associate

0.30 ± 0.05

2.6 ± 0.05

0.05 ± 0.03 (R1)

4.60 ± 0.05 (R1)

0.84 ± 0.05 (R2)

2.37 ± 0.05 (R2)

1.89 ± 0.05 (R1)

6.98 ± 0.05 (R1)

0.95 ± 0.05 (R2)

6.15 ± 0.05 (R2)

Eu 5

3.77 ± 0.050

0.69 ± 0.005

1

4.5 ± 0.050

0.82 ± 0.005

5

1.5 ± 0.050

0.50 ± 0.005

1

1.2 ± 0.085

0.32 ± 0.020

5

0.78 ± 0.085

0.15 ± 0.015

Sm

Dy

The strongly dissimilar crystal-field around cubic and vacancy- associate is confirmed also by the splitting values of


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Sm ground state, 6H5/2 (46 and 371 cm-1) as inferred from the energy levels scheme proposed in Scheme S1. Besides the emission spectra, the excitation modes are also centre dependent: the CT band of CeO2 preferentially sensitizes the cubic Ln centres (most likely via Ce4+ - O2- - Ln bonds) 15 whilst the direct excitation into f-f absorption favours the Ln - vacancy associate

31

(Figure 1b, e, h). However, as it will be shown later, the efficiency of antenna –like

sensitization of Ln emission is strongly affected by the Ln induced oxygen vacancies. The luminescence properties of smaller size (1.066 Å) Er lanthanide doped CeO2 suggest the existence of predominant Er - vacancy associate center. Figure S2 and the associated text illustrates a detailed analysis at room – temperature and 80 K of green emission at ~ 540 nm corresponding to 4S3/2 - 4I15/2 but also to NIR emission at ~ 1500 nm corresponding to 4I13/2 - 4I15/2 transition which has a pronounced MD character25. Our findings agree with earlier X-ray absorption near edge structure and Extended X-Ray Absorption Fine Structure (EXAFS) studies on Er doped CeO2 results that evidenced the predominance of Er - vacancy associate over cubic center.32 Nevertheless, the absence of cubic Er center in CeO2 needs further spectroscopic confirmation based on a time-gated, high spectral resolution emission measurement in the range of 4I13/2 - 4I15/2 transition. 3.1.1. Local lattice distortion. The ratio of integrated intensities corresponding to 5D0 - 7F2 and 5D0 - 7F1 transitions of Eu, known as asymmetry ratio is a widely used indicator for analyzing the local symmetry, approaching zero value for the inversion symmetry33. Yet, the R value of the cubic Eu center is around 0.3 which suggests that following substitution of the smaller Ce4+, Eu distorts locally the lattice, promoting some emission in the forbidden ED spectral range (Figure 1a). For the cubic Sm center, the asymmetry ratio, defined as R1 = I(4G5/2 - 6H9/2)/I(4G5/2 6

H5/2), is very close to zero theoretical value, suggesting a negligible distortion (Figure 1d). A second asymmetry

ratio, defined as R2 = I(4G5/2 - 6H7/2)/I(4G5/2 - 6H5/2) can also be used as a measure of local asymmetry due to partial ED character of the 4G5/2 - 6H7/2 transition 19. Since the emission of Dy in cubic symmetry is here reported for the first time, we re-evaluate the use of Dy as local probe is needed. In literature a greater emission intensity ratio of 4F9/2 - 6H15/2 transition at 483 nm (claimed to be of magnetic dipole nature) to ED 4F9/2 - 6H13/2 transition at 577 nm is generally considered as an evidence for higher local symmetry probed by Dy.34 In light of our findings, we suggest that the analysis of local symmetry should consider the intensity ratios R1 = I(ED)4F9/2 - 6H13/2 /I(MD) 4F9/2 – 6H11//2 and R2 = I(ED) 4F9/2 - 6H13/2 / I(MD)


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F9/2 – 6H9/2/6F11/2 transitions (Figure 1g). Interestingly, by comparing the R(Eu) and R1 (Sm, Dy) values listed in

Table 1 (also Figure 1a, d, g) it results that the least local lattice distortion around the cubic center is measured for Sm which is most departed from Ce4+ cation in terms of ionic radii (1.079 to 0.97 Å). 3.1.2. Effect of annealing temperature. To clarify the effect of annealing on the two – Ln center distribution, we have compared the structural properties and emission properties of Ln doped CeO2 before calcination (as synthetized, AS) and after calcination at 1000 °C (Figure 2).

Figure 2. Typical XRD patterns (a), Raman spectra (b) FT-IR spectra (c) of Ln – CeO2 before and after annealing at 1000 °C. The emission decays and spectra of cubic Sm (d), Eu (e) and Dy (f) centers before and after annealing. λex= 350 nm. AS refers to as - synthesized sample. Upon annealing, the XRD patterns narrow substantially with crystallite size increasing from 8 nm (AS) to about 42 - 46 nm, which suggest a markedly improved crystallinity. The Raman spectra (for the AS samples) show a broad asymmetric band around 461 cm−1 assigned to the F2g phonon mode of fluorite lattice34 that is narrowed, become more symmetrical and shifted to 464 cm-1 after annealing. The decrease in the asymmetry is attributed to the improved phonon lifetime due to the growth in crystallite size. Along with the improved crystallization of CeO2


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nanoparticles, the reduction of structural defects, such as oxygen vacancies and also Ce3+ is expected

18, 36

. Further,

the surface area per volume (or the density of surface defects) decreases from 36 to 5,6 m2.g-1 and a good fraction of surface defects (i.e. OH stretching and bending related bands around 3435 cm-1 and 1640 cm-1 and citrate precursor related bands around 1300 - 1750 cm-1) are strongly reduced or removed (according to FT-IR spectra, Figure 2c). The role of these surface defects as non-radiative traps which reduce the emission intensity and shorten the emission decays is well known37. Despite the significant reduction of structural and surface defects on calcination, it found that the emission shape and decay of the cubic Ln (Eu, Sm and Dy) center remain basically unchanged (Figure 2d, e, f). Based on the structural and emission data, it can be suggested that structural (oxygen vacancy, presumably Ce3+) and surface defects do not influence the emission of cubic Ln centers, at least for the investigated doping range of 1 - 5%. A tempting explanation is that the oxygen vacancies are not randomly distributed inside ceria lattice. In this scenario, the cubic Ln centers locate into the more inner sites of CeO2 making the non-radiative quenching induced by the more distant surface defects ineffective. Such hypothesis agrees with earlier UV and Vis Raman investigations that demonstrate a preferential agglomeration of the Ln induced vacancies near the surface of CeO2 nanoparticles38. 3.2. Effects of Ln concentration versus subsequently introduced charge compensating vacancies. As reported in literature, the increase of Ln concentration induces three main effects: (i) the emission intensity increases up to a concentration level which is Ln and synthesis dependent. For example, for Eu doped CeO2, no concentration induced quenching of the emission intensity was observed up to 10% 17, 15%

18

or 19%

39

while for Sm a much lower concentration thresholds of 0.5 - 1 % 40or 1.5 % were found41 (ii) the emission intensity corresponding to so called perturbed or low symmetry Ln center is observed to increase over that of the cubic center and (iii) the contribution of CT band of CeO2 to sensitization of Ln emission become less effective compared to f-f absorption 42. To date, the nature of quenching mechanisms and the differentiation between the roles of Ln doping level on one hand, and the subsequently induced oxygen vacancies, on the other hand, still remain for study. To better understand the quenching mechanisms of Ln emission in CeO2, we analyze the emission dynamics that enables a


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deeper understanding of quenching mechanisms as compared to emission intensity. There is only few data on concentration dependency of luminescence decays of Sm or Eu doped CeO2

19, 20

. Previously, we have determined

that the emission decay of Eu cubic center was concentration independent up to 10% 20. The improved selectivity obtained with 10K measurements allow us to conclude here that not only the emission decay of cubic centers but also the emission decay of Eu - vacancy associates is concentration independent (Table 1, Figure S4). On the contrary, the emission decays of both Sm and Dy (cubic and vacancy associate) show strong dependency on concentration, (Figure S5, Table 1). Concentration dependent emission decays are generally related to crossrelaxation processes on intermediary levels in Ln - Ln pairs. Such processes are significant for Sm and Dy but less effective for Eu due to their specific energy level configuration

27, 43

. The comparison of the concentration-

independent emission decays of Eu on one hand, and concentration-dependent decays of Sm and Dy, on the other hand, suggest that the Ln – Ln interactions via cross - relaxation represent the main route for the concentration quenching, and the role of subsequently induced oxygen vacancies and possibly Ce3+ 18, 44 as potential non-radiative traps can be neglected. Next, to distinguish more clearly between the effects of Ln doping and subsequently introduced oxygen vacancies we examine the La, Er co - doped CeO2 selected as a model system. The concentration of luminescent Er was fixed at 1% whereas the La concentration was set at four values: 0, 5, 10 and 20%. The trivalent La co - dopant was selected since, on one hand, it has no f electrons and therefore does not interfere with the absorption and emission properties of luminescent Er and on the other hand, it generates oxygen vacancies due to charge compensation mechanism. Figure 3 illustrates the excitation spectra monitoring the green emission of Er around 543 nm (corresponding to 4S3/2 - 4I15/2 transition) together with XRD patterns and Raman spectra of the four Er, La co doped CeO2 samples. The XRD patterns (a) confirm that all samples have single, cubic fluorite structure with crystallite size decreasing from 46 nm (La = 0%) to 37 nm (La =20%). The increase in the lattice constant with La concentration (from 5.42 Å (La=0%) to 5.48 Å for La=20%) suggests that La doping into ceria lattice is effective. The lattice constant evolution with concentration is linear, being directly correlated with the ionic size of the La (1.22 Å), which is greater than that of Ce4+ (0.97 Å), both in 8- fold coordination. The fact that La co - doping introduces significant amount of oxygen vacancies is demonstrated by Raman spectra in (b). Besides the F2g band around 464 cm-1 characteristic of the cubic fluorite lattice, an additional defect band D1 around 550 cm-1 , assigned in literature to dopant induced oxygen vacancies (VÖ) is progressively enhanced with La concentration.


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Concurrently, a second defect band D2 around 595 cm-1 is observed which relate to Ce3+ 45, or more generically, to the loosening of selection rules due to doping.35 With increase of La concentration, the Raman spectra are progressively broaden, shifted to lower energy (from 464 (La=0%) to 458 cm-1 (La=20%)) in line with evolution of XRD patterns, sustaining thus formation of homogenous La, Er – co-doped CeO2 solid solutions. According to the excitation spectra in Figure 3c, with increase of La concentration from 0 to 20%, a significant, 20-fold decrease of the CT band intensity is measured. In fact, for 20% La, sensitization of green emission of Er by the CT band of CeO2 is no longer effective. Intuitively, it can be said that the presence of VÖ reduce the number of available OCe4 units that transfer the excitation energy to neighboring Ln ions. Besides intensity drop, the CT band is also red shifted from 360 nm (La=0%) to 372 nm (La=20%). The Inset in Figure 3b, c illustrates the correlation between the reduction of CT band intensity with the increase of defect (VÖ) mode intensity. It is observed that there is a clear correspondence between the increase of oxygen vacancies on one hand, and the reduction of CT band intensity and red shifting of peak value, on the other hand.

Figure 3. XRD patterns (a), Raman spectra (b) and excitation spectra (c) measured around 543 nm emission of Er (1%) La ( 0,5,10 and 20%) co - doped CeO2. Inset in (b, c) represents the evolution with La concentration of the integrated intensities corresponding to defect mode D1 (VO) obtained via Lorentzian deconvolution of the Raman profiles (black squares) and CT band of CeO2 (blue squares). Lines are drawn only for eye guide. In conclusions, our findings indicate that the band gap reduction of Ln doped CeO2 is due mostly to the oxygen vacancies induced by doping and the role of Ln 4f states may be less significant as suggested in literature 46.


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3. 3. Effects of Zr doping on local structure of CeO2 structure. Fate of Ln – oxygen vacancy associates It is well established that Zr doping of CeO2 increases the ceria reducibility and hence oxygen storage capacity; however the elucidation of doping effects at the atomic scale is still not well understood. Since Zr is isovalent with Ce, according to defect chemistry principles, Zr should not induce the generation of oxygen deficiency based on charge compensation considerations47-49. To get more in depth into effects of Zr doping on the atomic scale properties of CeO2, the Sm and Dy are used here for the first time as luminescence probes for the catalytically relevant CeO2 - ZrO2 composition (Ce/Zr atomic ratio =1) grown by a microemulsion method24. The samples have been calcined at 750 and 1000 °C and labeled as Sm/Dy - CeO2 - ZrO2 – 750 (1000). First, the phase composition, size, homogeneity and morphology were assessed by X –ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (Figure 3ad, Figure S5, S6 and S7).

Figure 4. XRD patterns and Raman spectra of Sm - CeO2 - ZrO2 (a, b) and Dy - CeO2 - ZrO2 (c, d) calcined at 750 and 1000 °C. c/t" refers to pseudo – cubic phase. The XRD patterns of Sm/Dy - CeO2 - ZrO2 - 750) are consistent with single cubic/ pseudo - cubic phase (chart number 00-054-0017), with a crystallite size and lattice constant estimated around 5 - 6 nm and 5.27 Å,


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respectively. The Raman spectra of Sm/Dy - CeO2 - ZrO2 - 750 show an intense band at 456 cm-1 followed by weaker and larger bands around 293 and 615 cm-1 are consistent with the pseudo - cubic phase. This phase is cubic in the cation sub – lattice but has internal tetragonal symmetry, due to oxygen displacement from the ideal fluorite lattice sites

50

. Note that, at 1000 °C, the phase segregation into ceria and zirconia enriched phases is more

pronounced for Sm - CeO2 - ZrO2 – 1000 than Dy - CeO2 - ZrO2 – 1000. Moreover, the Raman spectra get significantly narrower for Sm - CeO2 - ZrO2 – 1000 than Dy - CeO2 ZrO2 – 1000 that confirm a deeper phase separation for Sm dopant. TEM analysis of Sm - CeO2 - ZrO2 -750 and Sm - CeO2 - ZrO2 – 1000 (Figure S7) give particle sizes of 5 - 10 and 30 - 60 nm, respectively. A predominant cubic phase was determined for Sm - CeO2 - ZrO2 - 750 whilst a mixture of cubic and tetragonal structures was observed for Sm - CeO2 - ZrO2 -1000. TEM analysis of Dy - CeO2 - ZrO2 - 750 (Figure S5) shows only a cubic/ pseudo cubic phase with both small (5 – 15 nm) and large aggregates (20 – 40 nm). The Dy - CeO2 - ZrO2 - 1000 (Figure S6) presents a single aggregate morphology with some variation of the Ce/Zr atomic ratio of crystallites within the same aggregate. According to our approach described earlier for Eu doped CeO2 - ZrO223 for a unique oxygen environment around Ce/Zr cations, Eu emission should remain invariant to changes of the excitation wavelength and delay (δt) after the laser pulse (time-resolved mode). For the opposite case of a phase mixture, the optical signatures of Eu in cubic ceria and tetragonal zirconia are likely to co-exist, and the relative contribution of these to the overall emission depended on both the excitation wavelength and delay. Our approach based on multi-wavelength time-resolved excitation, is different to those used in Ref. 51 and 52 which are based on two-wavelength steady-state excitation (at 488 or 532 nm) or time-resolved selective excitation into 5D0 - 7F0 absorption (around 579 nm) of Eu. We only note that in these reports, since all these f-f absorptions are forbidden in cubic symmetry, the existence of Eu in a cerialike phase may be largely ignored. The emission of Sm - CeO2 - ZrO2 – 750 and Sm - CeO2 - ZrO2 – 1000 display a strong excitation wavelength dependency (Figures S8a, b and S9a, b) which signals the presence of multiple oxygen environments around Sm substituting for the Ce/Zr cations. Thus, when excitation is confined across the UV region, a relative strong emission observed with a similar shape to that of the cubic center in single CeO2 oxide (Figure 5a, b). A predominant ceria environment of Sm in the mixed oxides is further confirmed by the presence of an intense CT band of CeO2 in the


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excitation spectra of Sm (Figure S9a). It is noteworthy to observe the both XRD and Raman data suggest a rather homogenous pseudo – cubic phase (Figure 2a, b). This demonstrates that standard structural characterization techniques such X-ray diffraction but also the more sensitive Raman spectroscopy fails to reveal nanoscale inhomogeneity of these small sized solid solutions 51. When the excitation wavelength is switched from the CT band of CeO2 to Sm f - f absorptions, the emission shape evolves into one that is very close to Sm emission in tetragonal ZrO2. For comparison purpose, in Figure 5a, b, the characteristic emission spectrum of Sm in tetragonal ZrO2 (local symmetry is D2d, non-centrosymmetric) is included as a reference.

Figure 5. Characteristic emission spectra assigned to distinct Sm (a, b) and Dy (c, d) centers in CeO2 - ZrO2 calcined at 750 (a, c) and 1000 ºC (b, d). The Ce content decreases relative to Zr with the increase of spectra label. For comparison, also included is the fingerprint emission spectrum of Sm and Dy in tetragonal ZrO2. Schematic representations of homogeneity as probed by Sm or Dy luminescence are included between the spectra. For Sm, the schemes show increased segregation into ceria and zirconia-like phases with increasing calcination temperature form 750 to 1000 ºC. For Dy, the schemes show a perfect homogenous system (750 ºC) and the presence of mixed pseudo-cubic/tetragonal phases (1000 ºC). For each excitation wavelength, we have measured the time evolution of the emission spectra at delays after the


18

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laser pulse which increased from few µs up to few ms. For delay greater than ~ 1 - 2 ms, basically no changes of the (peak normalized) emission spectra occurred. This way, of over 20 analyzed spectra, at least two (Sm - CeO2 - ZrO2 - 750) or three distinct spectra (Sm - CeO2 - ZrO2 - 1000) are separated (Figure 5a, b) and assigned to Sm located in specific oxygen environments of the mixed oxides.It is established that when Zr4+ substitute for Ce4+ ions the oxygen displacement along the c axis from the cubic fluorite regular position, increase with Zr content determining a progressive change of the symmetry of solid solutions from cubic CeO2 to tetragonal ZrO2

53, 54

. We can therefore

assign the emission spectra of Sm - CeO2 - ZrO2 -750 (spectra labeled with (2) - (3)) and Sm - CeO2 - ZrO2 – 1000 (spectra labeled with (2) - (4)) to nanodomaines with Ce/ Zr atomic ratios slightly differing from the nominal unitary ratio. The Ce content decreases relative to Zr (and hence local symmetry) with the increase of spectra label in Figure 5a, b. In conclusion, depending on the excitation wavelength, the emission of Sm - CeO2 - ZrO2 resembles either that of cubic Sm center in CeO2 or that of tetragonal ZrO2. For Dy case, we first remark the very weak contribution of CT band of CeO2 to the excitation spectra of Dy in both CeO2 - ZrO2 -750 and CeO2 - ZrO2 -1000 (Figure S10). The observance of Dy emission under UV excitation is due mainly to f-f excitation into Dy absorption (around 352 nm). When the excitation is varied across the Dy f-f absorptions, the emission shape of Dy in CeO2 - ZrO2 -750 remains basically constant (Figure 5c, see also Figure S8b). This indicates a unique, homogenous oxygen environment around the Ce/Zr cations, which is also consistent with the XRD and Raman data. The Dy emission can be considered as a rare example of a lanthanide emission in a pseudo-cubic symmetry. The emission of Dy in a pseudo-cubic symmetry is definitely closer to that in tetragonal ZrO2 (also represented in Figure 5c, d as a reference) than CeO2. Nonetheless, the Dy emission in pseudo-cubic CeO2 - ZrO2 is broader and suggests a slightly higher tetragonal symmetry than in tetragonal ZrO2. A distinct oxygen environment of Ce/Zr cations in the pseudo-cubic CeO2 - ZrO2 compared to that in parent oxides was earlier determined by X-ray Absorption Near Edge Structure data 21. For sample calcined at higher temperature (Dy - CeO2 - ZrO2 – 1000), the Dy emission spectra show dependency on the excitation wavelength. Of ~ 15 analyzed spectra, at least three distinct spectra could be separated (Figure 5d, see also Figure S9c). Similar to Sm, the Dy spectra are assigned to nanodomaines with Ce/ Zr atomic ratios slightly differing from the nominal unitary ratio. The absence of ceria like environment observed from the emission and excitation spectra confirms that the inhomogeneity probed by Dy is predominantly of compositional nature. Nonetheless, the key observation from all spectra gathered in Figure 5 is the absence of emission attributed to Sm


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or Dy - vacancy associate in CeO2 - ZrO2. This observation is clearly confirmed by the comparison made in Figure S11. A similar effect (that is, the absence of the emission attributed to Eu – vacancy associate) was recently found with Eu doped CeO2 - ZrO2.23 In view of the different ionic radii of Eu, Sm and Dy, these findings suggest that the role of Zr as a scavenger50 for the Ln induced oxygen vacancies can be considered as general across the Ln series.

4. Summary and Conclusions The luminescence properties of Ln (Eu, Sm, Dy and Er) doped CeO2 nanoparticles have been in-depth investigated by low temperature, site selective, time-resolved luminescence spectroscopy. The measurements sustain a physical model based on two distinct Ln centers which substitute for Ce fluorite sites but differ by the presence of zero vacancy (cubic center ) and one vacancy (Ln - vacancy associate) in the first oxygen coordination shell. Based on the first reported example of Dy emission in an inversion site, the use of Dy as a structural probe is re-evaluated. The emission shape and decay of the cubic Ln centers measured before and after high- temperature calcination are highly similar, that is tentatively assigned to a non-random distribution of the oxygen vacancies into ceria lattice. The dependence of the luminescence decays on Ln concentration indicates that the cross-relaxation in Ln –Ln pairs prevail over the subsequently induced oxygen vacancies as non-radiative quenching paths. The effects of Ln concentration and subsequently induced oxygen vacancies on sensitization of Ln emission are well separated by selecting the Er, La co-doped CeO2 as a case study. Finally, it is shown that the Ln luminescence effectively reveals the effects of Zr doping on CeO2 at the atomic scale, a major effect concerning the scavenging property of Zr for the Ln induced oxygen vacancies.

5. Acknowledgements DA, BC and CT acknowledge the Romanian National Authority for Scientific Research (CNCS-UEFISCDI) (project number PN-II-ID-PCE-2011-3-0534) for the financial support. MF acknowledges UEFISCDI for financial support through the project JRP 13/2013. MF and CT also acknowledge COST Action CM1104 “Reducible oxide chemistry, structure and functions”. MSD is grateful to CONACYT (CB project with grant number CB2011/166649) and to NaNoTeCh, the National Nanotechnology Laboratory of Mexico. The authors also thank Dr. V. Teodorescu and Dr. Cesar Leyva Porras for the TEM measurements and analysis.


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Supporting Information Available Electronic Supplementary Information (ESI) available: XRD and Raman spectra, energy levels scheme for Sm in CeO2, experimental details, emission and excitation spectra, emission decays, TEM images and analysis. This information is available free of charge via the Internet at http://pubs.acs.org.

References 1.

Mogensen, M.; Sammes, N. M.; Tompsett, G. A. Physical, chemical and electrochemical properties of pure and doped ceria. Solid State Ionics, 2000, 129, 63-94

2.

Steele, B. C. H. Appraisal of Ce1− yGdyO2− y/2 electrolytes for IT-SOFC operation at 500 C. Solid State Ionics 2000, 129, 95-110

3.

Jacobson, A. J. Materials for solid oxide fuel cells. Chem. Mater. 2010, 22, 660-674

4.

Minervini, L.; Zacate, M. O.; Grimes, R. W. Defect cluster formation in M2O3-doped CeO2. Solid State Ionics, 1999, 116, 339-349

5.

Nakayama, M.; Martin, M. First-principles study on defect chemistry and migration of oxide ions in ceria doped with rare-earth cations. Phys. Chem. Chem. Phys., 2009, 11, 3241-3249

6.

Nolan, M. Charge compensation and Ce3+ formation in trivalent doping of the CeO2 (110) surface: the key role of dopant ionic radius. J. Phys. Chem. C, 2011, 115, 6671-6681

7.

Gerhardt-Anderson, R.; Nowick, A. S. Ionic conductivity of CeO2 with trivalent dopants of different ionic radii. Solid State Ionics, 1981, 5, 547-550

8.

Bogicevic, A.; Wolverton, C. Nature and strength of defect interactions in cubic stabilized zirconia. Phys. Rev. B, 2003, 67, 024106


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

9.

Page 22 of 33

Babu, S.; Thanneeru, R.; Inerbaev, T.; Day, R.; Masunov, A. E.; Schulte, A.; Seal, S. Dopant-mediated oxygen vacancy tuning in ceria nanoparticles. Nanotechnology. 2009, 20, 085713

10. Babu S.; Cho, J. H.; Dowding, J. M.; Heckert, E.; Komanski, C.; Das, S.; Colon, J.; Baker, C. H.; Bass, M.; Self, W. T.; Seal, S. Multicolored redox active upconverter cerium oxide nanoparticle for bio-imaging and therapeutics. Chem. Commun. 2010, 46, 6915-6917

11. Ursaki, V. V.; Lair, V.; Żivković, L.; Cassir, M.; Ringuedé, A.; Lupan, O. Optical properties of Sm-doped ceria nanostructured films grown by electrodeposition at low temperature. Opt. Mater. 2012, 34, 1897-1901

12. Souza, E. C. C.; Brito, H. F.; Muccillo, E. N. S. Optical and electrical characterization of samaria-doped ceria. Muccillo J. Alloys Compd. 2010, 491, 460-464 13. Fujihara, S.; Oikawa, M. Structure and luminescent properties of CeO2: rare earth (RE= Eu3+ and Sm3+) thin films. J. Appl. Phys., 2004, 95, 8002-8006

14. Wang, Z.; Quan, Z.; Lin, J. Remarkable changes in the optical properties of CeO2 nanocrystals induced by lanthanide ions doping. Inorg. Chem. 2007, 46, 5237-5242

15. Babu, S.; Schulte, A.; Seal, S. Defects and symmetry influence on visible emission of Eu doped nanoceria. Appl. Phys. Lett. 2008, 92, 123112 16. Guo, H.; Qiao, Y. Preparation, structural and photoluminescent properties of CeO2: Eu3+ films derived by Pechini sol–gel process. Appl. Surf. Sci. 2008, 254, 1961-1965 17. Liu, X.; Chen, S.; Wang, X. Synthesis and photoluminescence of CeO2: Eu3+ phosphor powders. J. Lumin. 2007, 127, 650 - 654

18. Kumar, A.; Babu, S.; Karakoti, A. S.; Schulte, A.; Seal, S. Luminescence properties of europium-doped cerium oxide nanoparticles: role of vacancy and oxidation states. Langmuir 2009, 25,10998-11007


22

Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19. Tiseanu, C.; Cojocaru, B.; Avram, D.; Parvulescu, V. I.; Vela-Gonzalez, A. V.; Sanchez-Dominguez, M. Isolated centres versus defect associates in Sm3+-doped CeO2: a spectroscopic investigation. J. Phys. D:Appl. Phys., 2013, 46, 275302

20. Avram, D.; Rotaru, C.; Cojocaru, B.; Sanchez-Dominiguez, M.; Florea, M.; Tiseanu, C. Heavily impregnated ceria nanoparticles with europium oxide: spectroscopic evidences for homogenous solid solutions and intrinsic structure of Eu3+-oxygen environments. J. Mater. Sci. 2014, 49, 2117-2126

21. Rodriguez, J. A.; Hanson, J. C.; Kim, J. Y.; Liu, G.; Iglesias-Juez, A.; Fernández-García, M. Properties of CeO2 and Ce1-x ZrxO2 nanoparticles: X-ray absorption near-edge spectroscopy, density functional, and time-resolved X-ray diffraction studies. J. Phys. Chem. B 2003, 107, 3535-3543

22. Nagai, Y.; Yamamoto, T.; Tanaka, T.; Yoshida, S.; Nonaka, T.; Okamoto, T.; Suda, A.; Sugiura, M. X-ray absorption fine structure analysis of local structure of CeO2–ZrO2 mixed oxides with the same composition ratio (Ce/Zr= 1). Catal. Today, 2002, 74, 225-234

23. Tiseanu, C.; Parvulescu, V.; Avram, D.; Cojocaru, B.; Boutonnet, M.; Sanchez-Dominguez, M. Local structure and nanoscale homogeneity of CeO2–ZrO2: differences and similarities to parent oxides revealed by luminescence with temporal and spectral resolution. Phys. Chem. Chem. Phys. 2014, 16, 703-710

24. Sanchez-Dominguez, M.; Boutonnet, M.; Solans, C. A novel approach to metal and metal oxide nanoparticle synthesis: the oil-in-water microemulsion reaction method. J. Nanopart. Res. 2009, 111, 18231829 25. Dodson, C.M.; Zia, R. Magnetic dipole and electric quadrupole transitions in the trivalent lanthanide series:Calculated emission rates and oscillator strengths. Phys. Rev. B, 2012, 86, 15102 26. Lupei, A.; Tiseanu, C.; Gheorghe, C.; Voicu, F. Optical spectroscopy of Sm3+ in C2 and C3i sites of Y2O3 ceramics. Appl Phys B, 2012, 108, 909-918

27. Yu, M.; Lin, J.; Wang, Z.; Fu, J.; Wang, S.; Zhang, H. J.; Han, Y. C. Fabrication, patterning, and optical properties of nanocrystalline YVO4: A (A= Eu3+, Dy3+, Sm3+, Er3+) phosphor films via sol-gel soft lithography. Chem. Mater. 2002, 14, 2224-2231


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

28. Blasse, G.; Bril, A.; Nieuwpoort, W. C. On the Eu3+ fluorescence in mixed metal oxides: Part I—The crystal structure sensitivity of thr intensity ratio of electric and magnetic dipole emission. J. Phys. Chem. Solids, 1966, 27, 1587-1592

29. Oomen, E. W. J. L.; Van Dongen, A. M. A. Europium (III) in oxide glasses: dependence of the emission spectrum upon glass composition. J. Non-Cryst. Solids 1989, 111, 205-213

30. Linares, R. C. Fluorescent properties of trivalent rare earths in fluorite structure oxides." JOSA 1966, 56, 1700-1702

31. The still great contribution of CeO2 charge – transfer (CT) band in the excitation spectra of Ln – vacancy associate is likely due to much stronger intensity of CT band of CeO2 (allowed transition) compared to Ln f-f absorptions (forbidden transitions).

32. Giannici, F.; Gregori, G.; Aliotta, C.; Longo, A.; Maier, J.; Martorana, A. Structure and oxide ion conductivity: local order, defect interactions and grain boundary effects in acceptor-doped ceria. Chemistry of Materials, 2014, 26, 5994-6006 33. Blasse, G.; Brill, A.; Nieuwpoort, W.C. On the Eu3+ fluorescence in mixed metal oxides: Part I—The crystal structure sensitivity of the intensity ratio of electric and magnetic dipole emission J. Phys. Chem. Solids, 1966, 27, 1587-1592

34. Gu, F.; Wang, S. F.; Lü, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. Structure evaluation and highly enhanced luminescence of Dy3+-doped ZnO nanocrystals by Li+ doping via combustion method. Langmuir, 2004, 20, 3528-3531

35. McBride, J. R.; Hass, K. C.; Poindexter, B. D.; Weber, W. H. Raman and x‐ray studies of Ce1− xRExO2− y, where RE= La, Pr, Nd, Eu, Gd, and Tb. J. Appl. Phys. 1994, 76, 2435-2441

36. Sakthivel, T. S.; Reid, D. L.; Bhatta, U. M.; Möbus, G.; Sayle, D. C.; Seal, S. Engineering of nanoscale defect patterns in CeO2 nanorods via ex situ and in situ annealing. Nanoscale, 2015, 7, 5169-5177


24

Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


37. Mialon, G.; Gohin, M.; Gacoin, T.; Boilot, J. P. High temperature strategy for oxide nanoparticle synthesis. Acs Nano, 2008, 2, 2505-2512

38. Guo, M.; Lu, J.; Wu, Y.; Wang, Y.; Luo, M. UV and visible Raman studies of oxygen vacancies in rareearth-doped ceria. Langmuir, 2011, 27, 3872-3877

39. Thorat, A. V.; Ghoshal, T.; Carolan, P.; Holmes, J. D.; Morris, M. A. Defect chemistry and vacancy concentration of luminescent europium doped ceria nanoparticles by the solvothermal method. J. Phys. Chem. C, 2014, 118, 10700-10710 40. Yoshida, Y.; Fujihara, S. Shape‐Controlled Synthesis and Luminescent Properties of CeO2: Sm3+ Nanophosphors. Eur. J. Inorg. Chem., 2011, 10, 1577-1583

41. Wu, J.; Shi, S.; Wang, X.; Li, J.; Zong, R.; Chen, W. Controlled synthesis and optimum luminescence of Sm3+-activated nano/submicroscale ceria particles by a facile approach. J. Mater. Chem. C, 2014, 2, 27862792

42. Shi, S.; Hossu, M.; Hall, R.; Chen, W.; Solution combustion synthesis, photoluminescence and X-ray luminescence of Eu-doped nanoceria CeO2: Eu. J. Mater. Chem. 2012, 22, 23461-23467

43. Van Uitert, L. G.; Johnson, L. F. Energy transfer between rare‐earth ions. J. Chem. Phys., 1966, 44, 35143522

44. Balducci, G.; Islam, M. S.; Kašpar, J.; Fornasiero, P.; Graziani, M. Reduction process in CeO2-MO and CeO2-M2O3 mixed oxides: A computer simulation study. Chem. Mater., 2003, 15, 3781-3785

45. Hernández, W. Y.; Laguna, O. H.; Centeno, M. A.; Odriozola, J. A. Structural and catalytic properties of lanthanide (La, Eu, Gd) doped ceria. J. Solid State Chem. 2011, 184, 3014-3020

46. Choudhury, B.; Choudhury, A. Lattice distortion and corresponding changes in optical properties of CeO2 nanoparticles on Nd doping. Curr. Appl Phys. 2013, 13, 217-223


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47. Chen, D.; Cao, Y.; Weng, D.; Tuller, H. L. Defect and transport model of ceria–zirconia solid solutions: Ce0.8Zr0.2O2− δ - An electrical conductivity study. Chem. Mater. 2014, 26, 5143-5150

48. Yang, Z.; Woo, T. K.; Hermansson, K.; Effects of Zr doping on stoichiometric and reduced ceria: A firstprinciples study. J. Chem. Phys., 2006, 124, 224704

49. Dutta, G.; Waghmare, U. V.; Baidya, T.; Hegde, M. S.; Priolkar, K. R.; Sarode, P. R. Origin of enhanced reducibility/oxygen storage capacity of Ce1-xTixO2 compared to CeO2 or TiO2. Chem. Mater., 2006, 18, 3249-3256

50. Maekawa, H.; Kawata, K.; Xiong, Y. P.; Sakai, N.; Yokokawa, H. Quantification of local oxygen defects around Yttrium ions for yttria-doped ceria–zirconia ternary system. Solid State Ionics 2009, 180, 314–319

51. Montini, T.; Speghini, A., Rogatis; L. D.; Lorenzut, B.; Bettinelli, M.; Graziani, M.; Fornasiero, P. Identification of the structural phases of CexZr1− xO2 by Eu (III) luminescence studies. J. Am. Chem. Soc. 2009, 131, 13155-13160

52. Primus, PA.; Menski, A.; Yeste, MP.; Cauqui, MA.; Kumke, MU. Fluorescence line narrowing spectroscopy as a tool to monitor phase transitions and phase separation in efficient nanocrystalline CexZr1xO 2:

Eu3+ catalyst materials. J. Phys. Chem. C 2015, 119, 10682-10692

53. Wang, H. F.; Gong, X. Q.; Guo, Y. L.; Guo, Y.; Lu; G. Z.; Hu, P. A model to understand the oxygen vacancy formation in Zr-doped CeO2: electrostatic interaction and structural relaxation. J. Phys. Chem. C 2009, 113, 10229-10232

54. Tang, Y.; Zhang, H.; Cui, L.; Ouyang, C.; Shi, S.; Tang, W.; Li, H.; Lee, J.-S.; Chen, L. First-principles investigation on redox properties of M-doped CeO 2 (M= Mn, Pr, Sn, Zr). Phys. Rev. B 2010, 82, 125104


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Figure 1. The fingerprint emission and excitation spectra and emission decays of Eu, Sm and Dy in CeO2 (I and II refer to cubic and Ln - vacancy associate centers, respectively). Measurement temperature was 10 K (Eu) and 80 K (Sm and Dy). λex/ λem= 350 nm/591 nm and 468.3 nm/632 nm for Eu I and Eu II centers; λex/ λem= 350 nm/573 nm and 479.8 nm/607 nm for Sm I and Sm II centers and λex/ λem= 350 nm/679 nm and 428.3 nm/577 nm for Dy I and Dy II centers. Further details on the emission/excitation wavelengths and experimental parameters are listed in Table S1. Insets in (a) and (d) shows a zoomed view in the range of “forbidden” emission of cubic Eu and Sm centers. 260x221mm (300 x 300 DPI)


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Figure 2. Typical XRD patterns (a), Raman spectra (b) FT-IR spectra (c) of Ln – CeO2 before and after annealing at 1000 °C. The emission decays and spectra of cubic Sm (d), Eu (e) and Dy (f) centers before and after annealing. λex= 350 nm. AS refers to as - synthesized sample. 245x131mm (300 x 300 DPI)



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Figure 3. XRD patterns (a), Raman spectra (b) and excitation spectra (c) measured around 543 nm emission of Er (1%) La ( 0,5,10 and 20%) co - doped CeO2. Inset in (b, c) represents the evolution with La concentration of the integrated intensities corresponding to defect mode D1 (VO) obtained via Lorentzian deconvolution of the Raman profiles (black squares) and CT band of CeO2 (blue squares). Lines are drawn only for eye guide. 290x82mm (268 x 268 DPI)


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Figure 4. XRD patterns and Raman spectra of Sm - CeO2 - ZrO2 (a, b) and Dy - CeO2 - ZrO2 (c, d) calcined at 750 and 1000 °C. c/t" refers to pseudo – cubic phase. 248x180mm (300 x 300 DPI)



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Figure 5. Characteristic emission spectra assigned to distinct Sm (a, b) and Dy (c, d) centers in CeO2 - ZrO2 calcined at 750 (a, c) and 1000 ºC (b, d). The Ce content decreases relative to Zr with the increase of spectra label. For comparison, also included is the fingerprint emission spectrum of Sm and Dy in tetragonal ZrO2. Schematic representations of homogeneity as probed by Sm or Dy luminescence are included between the spectra. For Sm, the schemes show increased segregation into ceria and zirconia-like phases with increasing calcination temperature form 750 to 1000 ºC. For Dy, the schemes show a perfect homogenous system (750 ºC) and the presence of mixed pseudo-cubic/tetragonal phases (1000 ºC). 284x162mm (274 x 274 DPI)


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Toward a Unified Description of LuminescenceâLocal Structure

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