Fluorescence Line-Narrowing Spectroscopy as a Tool to Monitor ...

Article pubs.acs.org/JPCC

Fluorescence Line-Narrowing Spectroscopy as a Tool to Monitor Phase Transitions and Phase Separation in Efficient Nanocrystalline CexZr1−xO2:Eu3+ Catalyst Materials Philipp-Alexander Primus,*,† Antonia Menski,† Maria P. Yeste,‡ Miguel A. Cauqui,‡ and Michael U. Kumke*,† †

Institute of Chemistry, Physical Chemistry, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany Department of Materials Science, Metallurgical Engineering and Inorganic Chemistry, University of Cádiz, 11510 Puerto Real (Cádiz), Spain

‡

S Supporting Information *

ABSTRACT: Despite the wide range of industrial applications for ceria-zirconia mixed oxides (CexZr1−xO2), the complex correlation between their atomic structure and catalytic performance is still under debate. Catalytically interesting CexZr1−xO2 nanomaterials can form homogeneous solid solutions and, depending on the composition, show phase separation under the formation of small domains. The characterization of homogeneity and atomic structure of these materials remains a major challenge. High-resolution emission spectroscopy recorded under cryogenic conditions using Eu3+ as a structural probe in doped CeZrO2 nanoparticles offers an effective way to identify the different atomic environments of the Eu3+ dopants and, subsequently, to monitor structural parameters of the ceria-zirconia mixed oxides. It is found that, in stoichiometric CeZrO2:Eu3+, phase separation occurs at elevated temperatures beginning with the gradual formation of (pseudo)cubic crystallites in the amorphous materials at 500 °C and a sudden phase separation into tetragonal, zirconia-rich and cubic, ceria-rich domains over 900 °C. The presented technique allows us to easily monitor subtle changes even in amorphous, high surface area samples, yielding structural information not accessible by conventional techniques such as X-ray diffraction (XRD) and Raman. Moreover, in reference experiments investigating the reducibility of largely unordered Ce0.2Zr0.8O2:Eu3+, the main reduction peak in temperature-programmed reduction measurements appeared at exceptionally low temperatures below 200 °C, thus suggesting the outstanding potential of this oxide to activate catalytic oxidation reactions. This effect was found to be dependent on the amount of Eu3+ dopant introduced into the CeZrO2 matrix as well as to be connected to the atomic structure of the catalyst material.

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INTRODUCTION Ceria (CeO2) and ceria-based mixed oxides have been extensively investigated due to their wide range of applications in industrial catalysis,1−3 as cathode material in solid oxide fuel cells, and in environmental and life-science applications.4−6 The outstanding catalytic properties of these materials are connected to ceria’s ability to easily release oxygen and to the high mobility of the lattice oxygen. Despite the large scale of technical applications for ceria-based oxides, many questions concerning the relationships between structure, redox properties, and catalytic activity remain unanswered, posing serious analytical challenges for the research community.7−12 This is especially true for doped materials and mixed oxides of ceria, the majority of ceria-based industrial catalysts.13−15 Mixed oxides often show improved thermal stability and excellent redox properties. They can exist in different stable and metastable phases depending on the composition and pretreatment conditions.16−21 The particle size, morphology, and © 2015 American Chemical Society

dopant distribution affect the formation of different phases and the catalytic properties.22 There are strong indications that unordered ceria catalysts are favorable for certain catalytic applications.15,23,24 To enable goal-oriented design of CeZr-based nanoscaled catalyst materials, an in-depth understanding of the relationship between structure and catalytic activity is indispensible. This is a challenging task due to limitations of the most common techniques for structural investigations, Raman spectroscopy and X-ray diffraction (XRD). The sensitivity of XRD is reduced when working with nanomaterials due to size-dependent broadening of the reflexes. Slight changes in the structure and additional phases are easily obscured when the size of the crystallites is in the low nanometer range. Furthermore, XRD Received: February 6, 2015 Revised: April 13, 2015 Published: April 22, 2015 10682

DOI: 10.1021/acs.jpcc.5b01271 J. Phys. Chem. C 2015, 119, 10682−10692

Article

The Journal of Physical Chemistry C

In order to further improve the speciation analysis the inhomogeneous broadening has to be overcome. Therefore, all measurements are performed at ultralow temperatures (T < 10 K) to avoid atomic motion. Using a narrow-bandwidth (∼2 pm), tunable dye laser in combination with high-resolution optical detection and the outstanding environmental sensitivity of the Eu3+ luminescence, site-selective spectroscopy yields precise information on the atomic structure of a given material. The general technique is often referred to as fluorescence line narrowing spectroscopy (FLNS).33,34 We adopt the terminology in this work although the Eu3+ emission is not referred to as fluorescence but as luminescence. In order to evaluate the potential of FLNS contributing to an understanding of the catalytic activity of CeZr-based materials, small europium-doped ceria zirconia (CZE) nanoparticles were synthesized via a microemulsion technique under mild conditions35 and characterized by XRD, TEM, and FLNS. The particles proved to be amorphous after synthesis, whereas annealing leads to pronounced crystalline character. Furthermore, FLNS offers a unique insight into the phase separation process in the metastable, stoichiometric mixed oxide. As reference experiments for catalysis the release of oxygen was investigated in temperature-programmed reduction (TPRH2) experiments. Compared to similar materials these codoped nanoparticles synthesized under mild conditions showed an outstanding catalytic performance with the Eu3+ ions acting as an orthogonal dopant.

and Raman spectroscopy fail in the investigation of materials with low crystallinity which, according to previous research and measurements presented in this work, show exceptional results in catalysis.15,22,23 The application of luminescent probes, such as Eu3+, is a sensitive, versatile tool to gain structural insight, helping to ultimately correlate structural properties with catalytic properties.26 The unique luminescence properties of Eu3+ allow structural insights into crystalline and amorphous nanomaterials on the atomic level.27 According to Judd−Ofelt theory, the spectra emitted by Eu3+ dopants are determined by their respective atomic surroundings, more precisely by the crystal field symmetry and strength.28,29 Recently, researchers demonstrated two different approaches using Eu3+ as a spectroscopic probe in ceria-zirconia-based mixed oxides: Montini et al. used laser excitation of Eu3+ at λex = 532 nm or λex = 488 nm to measure the transition between the two nondegenerated 5D0 and 7F0 levels (0−0 transition) in order to gain information about the number of different Eu3+ environments in the material.30 This approach is based on the nephelauxetic effect, which causes the 0−0 transition to shift to longer wavelengths with increasing crystal field strength. The number of Gaussian curves fitted under the 0−0 emission band represents the number of lattice sites present in the sample.30,31,32 Choosing a different approach, Tiseanu et al. showed that the Eu3+ emission spectra in CeZrO2 are very sensitive to the excitation wavelength (from λex = 240 nm to λex = 527.5 nm).31 They used a variety of different excitation wavelengths as well as time-gated detection to extract asymmetry ratios RA, an indicator for the degree of inversion symmetry of the Eu3+−O2− coordination polyhedron. Combined with Raman and XRD data, this approach yielded a detailed picture of different Eu3+ environments present in different samples. The drawback is rooted in the excitation into target levels which are split by the crystal field (J > 0; e.g., 7F0−5D2 ∼ 464 nm). The splitting of the excited levels differs for different lattice sites in the sample, usually leading to the excitation of multiple sites. Resulting spectra are a superposition of emission lines from different sites, making extensive and careful data analysis and a fundamental understanding of the photophysics of Eu3+ mandatory. In contrast, 0−0 excitation is intrinsically selective and leaves much less room for uncertainty. With the aim of exploiting the full potential of Eu3+ luminescence spectroscopy and allowing clear and intuitive data evaluation, we combined the principles of both of the referenced approaches. In this work we use the 0−0 transition to selectively excite Eu3+ species and record the emission of the 5D0 → 7Fj (j = 1− 3) transitions. By using the 0−0 transition for excitation, we combine the discrimination between different sites (crystal field strength) and combine it with the information gained from the longer-wavelength emissions affected by the crystal field symmetry.31 In a typical measurement the excitation wavelength scans the range of the 0−0 transition, selectively exciting only certain species at a certain wavelength. A threedimensional plot of the emission spectra against the excitation wavelength easily identifies different dopant environments. The logarithmic emission intensity is translated to a color scale representing the third dimension. Parameters like the asymmetry ratio, the spectral broadening, and, in the case of site-selective excitation, the Stark splitting patterns can then be investigated.

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EXPERIMENTAL SECTION Preparation of the Materials. Europium-doped ceria zirconia (CZE) nanoparticles were synthesized using an oil in water microemulsion technique under mild conditions first described by Sanchez-Dominguez et al.35 The molar fraction of europium was kept constant at 1%mol in relation to the total amount of host cations Me4+ (Me4+ = Ce4+ + Zr4+) in the mixed oxides. The ratio between the two host cations cerium and zirconium was varied. Three different samples were synthesized with the following compositions: 0.2 (20% Ce4+), 0.5 (50% Ce4+), and 0.8 (80% Ce4+). The numbers in the sample name give relative molar fraction of Ce4+ relative to Zr4+ in the oxide host. It is known that CZE0.2 and CZE0.8 form stable crystal lattices (tetragonal and cubic, respectively) and that CZE0.5 is a metastable system, in which both types can be present. Incorporation of 1%mol Eu3+ as a structural probe enables spectroscopic analysis of the sample. Chemicals. Cerium(III)-2-ethylhexanoate (Sigma-Aldrich), zirconium(IV)-2-ethylhexanoate (Alfa Aesar), europium(III)-2ethylhexanoate (Strem Chemicals), SYNPERONIC 13/6.5LQ-(TH), SYNPERONIC 13/6-LQ-(AP), and SYNPERONIC 13/5-LQ-(TH) were used (all SYNPERONIC surfactants are synthetic fatty alcohol ethoxylates provided by Croda). The prepared microemulsions consist of 60% water, 20% surfactant (mixture of SYNPERONIC surfactants), and 20% oil (organometallic salts dissolved in n-hexane with the varying molar ratios). After vigorous stirring at room temperature, addition of concentrated ammonia results in the formation of nanoparticles. Afterward, the created nanoparticles were separated from the solution through centrifugation, washed with an ethanol/chloroform mixture, and dried at 100 °C. Some of the synthesized powders have undergone different temperature treatments: annealing under air at temperatures of 500−900 °C for 4 h or annealing under air at 1000 °C for 5 h. 10683



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RESULTS AND DISCUSSION In the following, the names of all the samples investigated are structured according to the following scheme: CZEX-T, where CZE stands for ceria zirconia mixed oxide doped with 1%mol Eu3+, CexZr1−xO2:Eu X stands for “fraction of Ce4+” in synthesis, 0.2, 0.5, or 0.8 T stands for the “temperature”; the “as-synthesized” sample is denoted 25 according to the synthesis temperature. For the annealed samples, T is substituted by the annealing temperature in °C. If not further specified all samples contain 1 mol % of Eu3+ with regard to the sum of cerium and zirconium atoms used in synthesis. Morphological Characterization. An overview of the particle sizes and crystallite sizes measured using TEM and XRD is presented in Table 1 for comparison. In order to relate

For the catalytic experiments an additional cleaning procedure was carried out. In the reactor, the dry particles were heated to 500 °C with 3 °C/min for 1 h in a stream of He with 5% O2 in order to remove organic remnants from the particle surface. FLNS Measurements. FLNS measurements were carried out by cooling dry samples inside a copper sample holder attached to the cold plate of a closed-cycle liquid helium cryostat. The sample room was evacuated before cooling. Selective excitation was achieved using a narrow bandwidth (2 pm @ 500 nm) dye laser (Cobra Stretch, Sirah Laser- and Plasmatechnik). The laser dye Pyrromethene 597 (λem = 575− 610 nm) was purchased from Sirah Laser- and Plasmatechnik and dissolved in ethanol. It was chosen to enable excitation of the 5D0−7F0 transition of Eu3+ (λex = 575−582 nm depending on the Eu3+ species, denoted as a 0−0 transition in the text). The excitation light was coupled into one of the branches of a Y-shaped fiber bundle, while the other branch was connected to a spectrograph (Shamrock SR-303i, Andor Technology) equipped with optical grating (600 lines/mm). A two-lens system was used to direct the laser light at the sample and to collect the emission light for the spectrograph. The light was measured using an intensified CCD camera (iStar DH 720, Andor Technology). The laser repetition rate was set to 10 Hz with a pulse length of 8 ns. In a typical measurement of a total luminescence spectrum (TLS) for a good overview when using Eu3+ as a structural probe, the excitation wavelength is scanned between λex = 575 and 582 nm, and for every Δλex = 0.05 nm usually 100 spectra are accumulated. For higher resolution, Δλex is decreased, and for better S/N more spectra are accumulated per step. The asymmetry ratios RA give a good impression of the degree of inversion symmetry in the coordination polyhedron [RA]. RA is defined as the integrated intensity ratio between the hypersensitive electronic dipole transition 5 D0 → 7F2 and the magnetic dipole transition 5D0 → 7F1 (around λem = 610 nm)

Table 1. Results and Literature Data of the Morphological Characterization Using TEM and XRD sample CZE0.2

CZE0.5

CZE0.8

a

XRD Measurements. X-ray diffraction measurements were carried out using a powder diffractometer (D5005, Siemens AG) equipped with a Cu−Kα (1 and 2) radiation source at 1.54056 Å. Reflexes were recorded between 3° and 70°/2θ with a resolution of 0.02°/ 2θ and 4 s accumulation time per step. From the XRD measurements the crystallite sizes were calculated using the Scherrer eq 1, where K is a shape factor that is 1 for spherical particles. The wavelength of the used Xray-radiation is λ; the angle between the incoming radiation and the lattice plane is named as Bragg angle θ; and fwhm is the full width at half-maximum of the reflex. K ·λ ·180 fwhm·π ·cos θ

annealing temperature

crystallite size XRD

particle size TEM

25 °C 500 °C 1000 °C 25 °C 500 °C 750 °Cb 1000 °C 25 °C 500 °C 1000 °C

∼2 nm 9 nm 33 nm ∼2 nm 4 nm 5 nmb 27 nm ∼2 nm 9 nm 39 nm

7 nmb 600 °C for exactly the same sample composition but synthesized under much harsher conditions,22 suggesting a strong influence of the catalyst structure on the reducibility. Furthermore, it has been observed in the same study that despite incorporation into the bulk lattice impregnation and bulk doping lead to substantial differences in the redox behavior, highlighting the importance of techniques monitoring even very small structural modifications.22 The effect of the Eu3+ content on the reducibility of the samples has also been investigated by TPR-H2, and the results are shown in Figure 4. For comparative purposes, the TPR-H2 trace obtained for a commercial undoped CeZr oxide has also been included. A clear influence of Eu3+ content is deduced from results shown in Figure 4. In effect, a reduction of the Eu3+ content from 1% to 0.2% led to a slight shift of the reduction peak by 35 °C to higher temperature. Nevertheless, the reduction maximum still lays at outstanding 160 °C compared to previously found temperatures of T ∼ 600 °C.2,22 No structural differences were measurable between both samples (see Supporting Information, Figures SI1 and SI2). The results are underlined by the finding that predominantly amorphous, europium-doped, pure ceria nanoparticles also showed an improved response in TPR-H2 experiments as well as a greatly improved reactivity and selectivity toward CO in the selective oxidation of CO in the presence of H2 (PROX).23 As a reference undoped ceria nanoparticles synthesized following the same synthesis procedure developed by Sanchez-Dominguez et al. were used.35 In the attempt to understand the differences in the atomic structure of the CZE particles we measured the TLS of the samples after undergoing thermal treatment at different temperatures, monitoring small changes in the amorphous structure as well as crystalline domains. FLNS (Fluorescence Line Narrowing Spectroscopy). In the following, a homogeneous group of Eu3+ ions sharing the same general symmetry with respect to their coordination polyhedron is referred to as “Eu3+ species” (which can be broadened in the case of more unordered materials but also includes well-defined dopant sites). One species is always

Table 2. Comparison to Reference Data of the Characterization Results of the HRTEM Image of CZE0.8500 Shown in Figure 1 experimental distances

distances (ICCD: 04-014-4191)

plane

3.2 Å 2.8 Å

3.1 Å 2.7 Å

(111) (200)

(Figure 2, left). In order to clarify the results from XRD, a comparison with the ICDD-PDF database for the different compositions and a comparison with cubic and tetragonal reference were carried out. XRD spectra show a tetragonal crystallization for the zirconia-rich CZE0.2-1000 samples, while the ceria-rich oxide CZE0.8-1000 crystallizes in a cubic lattice after annealing at high temperatures. The assignments were also supported by Raman measurements of CZE0.2-500 and CZE0.8-500 (see Figure SI5, Supporting Information), respectively. For both samples the respective spectra were in excellent agreement with data reported in the literature.40,41 While the Raman spectrum of the CZE0.2-500 indicates a tetragonal lattice typically found for t-ZrO2, the corresponding spectrum of CZE0.8-500 is dominated by the F2g lattice vibration typically found for cubic CeO2. The XRD of the metastable CZE0.5-500 resembles the XRD of the cubic CZE0.8-500 sample, possibly indicating a pseudocubic structure.37 This is also supported by the Raman spectrum found for this sample, in which only broad bands were observed. Annealing of the CZE0.5 sample at 1000 °C leads to phase separation detectable by XRD (Figure 2, right). The XRD reflexes match the respective reflexes for the ceriarich oxides (cubic, c) and zirconia-rich oxides (tetragonal, t). Catalytic Characterization. The reducibility of the CZEX500 materials was investigated using TPR-H2 experiments, and the results were evaluated in comparison to literature data (Figure 3). The europium-doped CZE0.5 and CZE0.8 samples show typical TPR-H2 profiles with a main reduction peak at 690 and 590 °C, respectively. The results are similar to TPR-H2 profiles published by other researchers for similar materials.22,38 In contrast, the CZE0.2 sample showed an exceptional reducibility at very low temperatures with the main reduction 10685


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Figure 2. XRD reflexes for all samples annealed at different temperatures (left) and for all samples annealed at 1000 °C in comparison to reference data (right).

spectra are plotted by translating the logarithmic emission intensities (z-axis) to a color scale from the lowest intensities (black < blue) to the highest (green < brown < white). In the TLS the emission wavelength is shown on the x-axis, and the excitation wavelength is shown on the y-axis. Horizontal traces represent the emission spectra, whereas vertical traces represent excitation spectra. Using this approach, each Eu3+ species excited via its particular 0−0 absorption will show up as a horizontal pattern of bright spots in the TLS. The observations are summarized in Table 3 and later discussed in the context of their annealing temperature. Note that the 5D0−7F0 transition used for excitation is strictly forbidden for dopants located in centrosymmetric environ-

characterized by the same Stark splitting pattern and is usually easily identified in the total luminescence spectra (TLS). When the excitation band of the species is very broad, the splitting pattern and the excitation wavelength change with the crystal field strength, which is visible in the TLS. Please note that the term “site” is traditionally used to describe a defined lattice site showing very narrow excitation and emission bands and wellresolved Stark splitting patterns. The Stark splitting of the lower energy 5D0 → 7Fj (j = 1−6) transitions contains information on the symmetry of the individually probed Eu3+ site in site-selective spectra. TLS give a clear overview of the amount and homogeneity of different Eu3+ species present in one sample (Figure 5). The 10686


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shows very similar spectral broadening (see Table 3). The similarities indicate a largely unordered, amorphous structure for all the different compositions. The amorphous character is attributed to the mild synthesis conditions at room temperature, preventing the formation of a crystal lattice. It has to be noted that the TLS contributions from europium ions located in amorphous domains and on the nanoparticle surface cannot be discriminated at this point. Nevertheless, surface species of Eu3+ are expected to appear at shorter excitation wavelengths in the TLS due to weaker ligand fields. Annealed Samples CZE-500. Annealing at 500 °C leads to distinct changes in the TLS, in which brighter and sharper spots become visible at longer excitation wavelengths (λex > 580 nm) along with a broad emission contribution resembling the emission of the amorphous particles. The excitation band of the broad, amorphous-type emission is further broadened by almost one nanometer (fwhmex ∼ 2.4 nm) compared to the CZE-25 samples indicating greater variation in the atomic environments. Bright, well-defined spots in the TLS indicate that groups of Eu3+ ions with very similar coordination spheres are formed, typical for crystalline hosts. The longer excitation wavelengths of these, more ordered, crystalline europium species indicate stronger ligand fields compared to more amorphous environments or the surface of the nanoparticles. The sharp emission lines corresponding to the different Stark levels enable further characterization of the probed lattice sites. We attribute the sharp signals to small crystallites formed in the previously amorphous domains in the samples. This is in good agreement with the broad XRD reflexes. There is a distinct difference between the sharp emission signals of CZE0.2-500 and the other compositions CZE0.5-500 and CZE0.8-500, which is also reflected in the XRD patterns (Figure 2). CZE0.2-500 shows a tetragonal structure in XRD. Additional to the retained, broad amorphous type emission, a sharp emission at λex = 580.9 nm can be attributed to Eu3+ located in a tetragonal lattice. The asymmetry ratio RA (see Experimental Section) with a dominant 5D0−7F2 emission supports the identification of a tetragonal symmetry around the Eu3+ ions. In this work we avoid the use of the exact values for RA due to its lack of comparability to different data sets, caused by crosstalk contributions from different emissive sites depending on excitation wavelength, bandwidth, and other measurement parameters. CZE0.5-500 and CZE0.8-500 show strong similarities in their respective XRD (Figure 2) and luminescence spectra (Figure 5). The XRD data show the formation of a cubic (in the case of CZE0.8-500) or possibly pseudo cubic structures (in CZE0.5-500). The TLS show, along with the amorphous Eu3+ species, one dominant, sharper species. The emission intensity ratio (RA) suggests that these species are related to the cubic lattice due to the dominant 5D0−7F1 emission, typical for centrosymmetrical coordination polyhedrons. Annealed Samples CZE-1000. The zirconia-rich sample CZE0.2-1000 and ceria-rich sample CZE0.8-1000 show distinct spectra, each showing one dominant species in the TLS (Figure 6). The well-defined species appear at excitation wavelengths of λex = 580.9 nm (CZE0.2-1000) and λex = 580.2 nm (CZE0.81000). The spectra are representative for the species first appearing in the respective samples after annealing at 500 °C, excited at the same excitation wavelength. The sharpened XRD signals suggest a larger crystallite size which is supported by the increased particle size after annealing. Compared to a previous

Figure 3. TPR-H2 curves of the different CZEX-500 samples doped with 1% Eu3+ showing the water signal.

Figure 4. Comparison of the TPR-H2 curves of three ceria-zirconia samples with different Eu3+ content: CZE0.2 sample doped with 1% Eu3+, CZE0.2 doped with 0.2% Eu3+ (surface area of 23 m2/g), and a CZ0.2 reference sample (no Eu3+, surface area of 40 m2/g).

ments, thus reducing the transition probability toward 0 (in the case that no lattice distortion is lowering the symmetry). This may lead to undetected species or very low emission intensities for Eu3+ ions located in high symmetry sites due to inefficient excitation of the 0−0 transition. However, it can be expected that distortions in different Eu3+-doped CZEX samples are strong enough so that even sites with Oh symmetry show measurable 0−0 emission bands.30 The different species are further characterized by analyzing their respective splitting patterns. As-Synthesized Samples CZE-25. All as-synthesized samples show broadened emission signals excited via one broadened excitation band in the TLS (fwhmex ∼ 1.5 nm). The signal resembles emission signals found in amorphous hosts such as frozen water, different glasses, and amorphous ceria particles.23,34,39 It is remarkable that almost no differences in the TLS were found for the CZEX-25 (X = 0.2, 0.5, and 0.8, respectively) despite their differences in composition. The excitation maximum may shift slightly with higher cerium content but 10687


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Figure 5. TLS measured at 4 K of the different Eu3+-doped CZE mixed oxides after different thermal treatments (585 nm < λem < 635 and 577 nm < λex < 581.5 nm). The samples were left at room temperature (RT) or treated at 500 and 1000 °C, respectively. Please note that the logarithmic intensities are translated to a color scale, creating the impression of a background around otherwise sharp peaks.

Table 3. Overview of the Different Eu3+ Species Excited at Different Wavelengths Visible in the TLSa

although still sharing the same general symmetry with respect to the oxygen coordination polyhedron. The metastable compotition CZE0.5-1000 shows more emission signals when compared to the two stable compositions with X = 0.2 and 0.8, respectively. A phase separation is observed in the XRD data, and the TLS shows distinct peaks representative of the emission signals seen in the cubic and the tetragonal samples at the excitation wavelengths of λex (cubic) = 580.2 nm and λex (tetragonal) = 580.9 nm. Both results indicate phase separation at high temperatures. Other researchers observed a phase separation at temperatures above 1100 °C.30 The slightly lower temperature at which the cubic/ tetragonal phase separation occurs in the samples investigated here may be caused by differences in the particle size, surface area, and surface morphology. In order to monitor the phase separation in detail, a series of stoichiometric CZE0.5-T samples annealed at different temperatures between 500 °C < T < 1000 °C were measured (Figure 7). A beginning of the phase separation has been suggested for annealing temperatures as low as 750 °C.19 In the sample annealed at 500 °C we can identify the appearance of a single, ordered phase along with an unordered domain (Figure 5). Figure 7 shows the evolution of the emission signals with increasing annealing temperatures, indicating a steady rise in intensity for a (pseudo)cubic signal and a reduction of the emission signal of the amorphous domain. Compared to the information gained from XRD (see Supporting Information Figure SI3), the TLS allows us to monitor the phase separation in much more detail: The excitation maximum of the pseudocubic signal in the CZE0.5-600, CZE0.5-700, and

excitation maximum λex and [fwhmex] annealing

species

25 °C

amorphous

500 °C

amorphous cubic tetragonal

1000 °C

CZE0.2 579.5b [1.5] nm 578.8b [2.4] nm -

CZE0.5

579.7b [1.5] nm 579.0b [2.4] nm 580.2 [0.3] nm -

cubic

581.0 [0.5] nm 578.9b [3.3] nm -

cubic′

-

579.2b [3.0] nm 580.1 [0.8] nm -

tetragonal

580.9 [0.5] nm

580.8 [0.5] nm

amorphous

CZE0.8

579.6b [1.6] nm 579.1b [2.4] nm 580.1 [1.0] nm -

580.2 [0.3] nm 580.4 [1.0] nm -

The species are characterized by their excitation wavelengths λex and the full-width at half maximum (fwhmex) of their respective excitation bands determined using a Gaussian fit of the most intense local emission maxima of each species. bAmorphous and surface-type emission. a

study investigating pure ceria particles, the emission lines are generally broadened despite high crystallinity.23 This is probably caused by the variation in the cation environment for a given lattice site (Ce4+ and Zr4+ compared to just Ce4+), 10688


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Figure 6. TLS measured at 4 K of CZE0.2-1000 (left) showing one tetragonal species (λex = 580.9 nm) and one weak, amorphous species (λex = 578.9 nm). CZE0.8-1000 (right) showing one dominant cubic emission (λex = 580.2 nm) and a weaker contribution (λex = 580.5 nm) also assigned to the cubic lattice. Additional, broader emissions are too weak to be analyzed.

Figure 7. TLS measured at 4 K of the annealed CZE0.5 mixed oxides from 600 °C (top left) to 900 °C (bottom right).

depleted of Zr4+ during the crystallization of a tetragonal domain. The excitation maximum of the pseudocubic domain in CZE0.5-900 slightly shifts toward shorter wavelengths as a consequence. Figure 8 shows that annealing at 1000 °C ultimately leads to two sets of emission bands: tetragonal signals are easily identified at λex = 580.9 nm, and the cubic signals further shifted to the expected excitation wavelength of

CZE0.5-800 samples (λex = 580.4 nm) is slightly shifted toward longer wavelengths compared to the ceria-rich CZE0.8-500 and CZE0.8-1000 samples (λex = 580.2 nm). We attribute that to the altered cation environment due to the increased amount of Zr4+. Nevertheless, before annealing at 900 °C there is no indication for the formation of a zirconia-rich, tetragonal phase. This means that only with an annealing temperature of 900 °C a critical temperature is reached, and the pseudocubic domain is 10689


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Above 900 °C annealing temperature zirconium-rich, tetragonal structures form rapidly along with cerium-rich cubic structures. The results paint a much more detailed picture of the phase separation process, further supported by the XRD results. Seemingly contrary to published data in the literature, Ce0.2Zr0.8O2 doped with only 0.2% Eu3+ showed exceptional oxygen release properties with the main reduction peak at only 160 °C. Triggered by the exceptional catalytic activity of the CZE0.2 nanoparticles doped with only 0.2% Eu3+, corresponding measurements analyzing structure and catalytic activity are in progress in order to elucidate the relation between the Ce/Zr ratio, the amount of europium, and the catalytic performance.

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ASSOCIATED CONTENT

S Supporting Information *

TLS of CZE0.2-1% Eu3+ and CZE0.2-0.2% Eu3+, as synthesized and annealed at 500 °C XRD spectra of the samples CZE0.5600, CZE0.5-700, CZE0.5-800, and CZE0.5-900. Additional TEM and HRTEM images Raman spectra of CZE0.2-, CZE0.5-, and CZE0.8-500 and experimental details The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b01271.

Figure 8. TLS measured at 4 K of the annealed CZE0.5-1000 sample with emission and excitation traces for the cubic (λex = 580.2 nm) and tetragonal (λex = 580.9 nm) domains.

λex = 580.2 nm, both in accordance to the tetragonal CZE0.21000 and cubic CZE0.8-1000 references. Figure 7 also indicates that the excitation band of the pseudocubic domain broadens with annealing temperatures between 700 and 900 °C. It can be observed that the relative intensities of the three 5D0−7F1 transitions differ between the cubic and tetragonal site. The cubic site shows one main emission with shoulders on each side, while the tetragonal 5 D0−7F1 emission is comprised of three peaks with similar intensity. The pseudocubic signal shows an emission that lies in between the two described patterns with three peaks where the peak in the middle is the most intensive emission line.

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AUTHOR INFORMATION

Corresponding Authors

*Michael U. Kumke: [email protected] *Philipp-A. Primus: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

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The authors declare no competing financial interest.

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CONCLUSION Here, we described our first results on the spectroscopy characterization of CeZrO2 nanoparticles doped with Eu3+. It was found that the Eu3+ dopants show an orthogonal effect. While initially the Eu3+ dopants were intended to work as a structural probe in order to investigate the structural changes in the materials occurring upon thermal treatment (which would be part of a “normal” catalysis cycle), it was found that a minor amount of Eu3+ ions incorporated into the host matrix also had a drastic effect on the redox properties of the material. The luminescence spectra recorded at ultralow temperatures shed new light on the alteration of the host matrix of the europium ions. Data evaluation is greatly simplified by using the discrimination of the different lattice sites through 0−0 excitation and the emission patterns of the lower energy emission bands opposed to analysis of single bands or single spectra with uncertain contributions. The spectra indicate an increasing crystallinity after treatment at temperatures above 500 °C and changes in the emission band structure of the amorphous-type emission. Different lattice sites for the Eu3+ dopants in the crystalline domains were clearly identified for the different annealing temperatures. It was found that the addition of 0.2% or 1% of Eu3+ can drastically reduce the reduction temperature to below 200 °C in amorphous catalyst materials, showing a dependence on the Eu3+ content. The phase separation of the metastable CZE0.5 indicated by XRD was monitored in detail using FLNS. Measurements show a gradual reduction of the amorphous emission contribution and a gradual formation of a pseudocubic domain up to 900 °C.

ACKNOWLEDGMENTS The authors are grateful for the measurement of TEM images by Dr. Claudia Prietzel and Prof. Dr. Joachim Koetz from the Colloid Chemistry Department, University of Potsdam, and Prof. Juan C. Hernández-Garrido, Universidad de Cádiz. The authors also would like to thank Dr. Christina Günter from the Geoscience department, University of Potsdam for the collection of all XRD data. The authors thank Croda for the supply of the SYNPERONIC surfactant samples. We acknowledge financial support from MINECO/FEDER/Ref: MAT2013-40823-R.

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ABBREVIATIONS FLNS, fluorescence line-narrowing spectroscopy; TLS, total luminescence spectra; TPR, temperature-programmed reduction; XRD, X-ray diffraction; TEM, transmission electron microscopy

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