Photoluminescence of Eu3+-doped CaZrO3 red-emitting phosphors synthesized via microwave-assisted hydrothermal method

Journal Pre-proof Photoluminescence of Eu-doped CaZrO red-emitting phosphors synthesized via microwave-assisted hydrothermal method Wagner Costa Macedo (Conceptualization) (Investigation) (Writing original draft) (Validation), Airton Germano Bispo Junior (Conceptualization) (Investigation) (Writing - original draft) (Visualization), Kleper de Oliveira Rocha (Writing - review and editing), Agda Eunice de Souza Albas (Writing - review and editing), Ana Maria Pires (Writing - review and editing) (Supervision), Silvio Rainho Teixeira (Resources) (Supervision), Elson Longo (Resources) (Funding acquisition)

PII:

S2352-4928(19)31725-8

DOI:

https://doi.org/10.1016/j.mtcomm.2020.100966

Reference:

MTCOMM 100966

To appear in:

Materials Today Communications

Received Date:

16 December 2019

Revised Date:

27 January 2020

Accepted Date:

28 January 2020

Please cite this article as: Costa Macedo W, Germano Bispo Junior A, de Oliveira Rocha K, de Souza Albas AE, Pires AM, Rainho Teixeira S, Longo E, Photoluminescence of Eu-doped CaZrO red-emitting phosphors synthesized via microwave-assisted hydrothermal method, Materials Today Communications (2020), doi: https://doi.org/10.1016/j.mtcomm.2020.100966

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Photoluminescence of Eu-doped CaZrO red-emitting phosphors synthesized via microwave-assisted hydrothermal method

Wagner Costa Macedo, Airton Germano Bispo Junior, Kleper de Oliveira Rocha, Agda Eunice de Souza Albas, Ana Maria Pires, Silvio Rainho Teixeira, Elson LongoSão Paulo State University (UNESP), Dept. of Physics and Dept. of Chemistry, Presidente Prudente, São Paulo, 19060-900, Brazil

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[email protected], [email protected], [email protected], [email protected], [email protected] São Paulo State University (UNESP), Dept. of Chemistry, Bauru, São Paulo, 17033360, Brazil

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[email protected]

Federal University of São Carlos (UFSCar), CDMF, São Carlos, São Paulo, 13565-

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905, Brazil

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[email protected]

Microwave-assisted hydrothermal method is appropriate to obtain zirconate

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

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Highlights

phosphors

Luminescent quenching by concentration takes place at concentrations higher

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

than 3%

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High-resolution selective emission spectra can be used as a spectroscopic probe

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Eu replaces Ca and Zr sites in both stoichiometric and non-stoichiometric phases

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Calcium zirconate/europium phosphors excited at 250 nm exhibit bright luminescence

*Corresponding Author E-mail: [email protected] Tel./Fax: +55 18 3229 5741 Roberto Simonsen, 305, Presidente Prudente, São Paulo,19060-900, Brazil

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Abstract Eu-doped (1, 3 and 5 %) CaZrO red-emitting phosphors were synthesized for the first time using the microwave-assisted hydrothermal method followed by heat treatment at 1,200 °C. After the crystallization, the orthorhombic CaZrO and the non-stoichiometric cubic CaZrO phases were identified for all samples, and the portion of the nonstoichiometric phase increases as the Eu concentration increases. The excitation spectra of the phosphors display Eu f-f excitation bands (namely at 395 nm and 464 nm) and the

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Eu→O charge transfer band at 250 nm, confirming that the CaZrO host acts as

sensitizer for the Eu luminescence. Likewise, the emission spectra feature the typical Eu emission within the red spectral region, displaying CIE color coordinates of (0.642; 0.332), (0.663; 0.330) and (0.668; 0.327) for the 1, 3 and 5 % doped samples,

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respectively, indicating high emission color purity. By using Eu as spectroscopic probe,

the high-resolution selective emission spectra (14 K) confirm that Eu replaces Ca and Zr

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local sites in both CaZrO and the CaZrO phases. Finally, the high absolute emission quantum yield of the 3 %-doped phosphor found at 393 nm (0.29±0.03) confirms its potential application as red-emitting phosphor, especially in rapid identification of

WLED applications.

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latent fingerprints, security ink or as red-emitting coating of near-UV LEDs for

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Keywords: Solid-state lighting, luminescence, microwave-assisted hydrothermal

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method, red-emitting phosphors.

1. Introduction Nowadays, UV-to-visible downshifting converter phosphors based on

Rare Earth (RE) doped oxide perovskites (ABO) are receiving extensive attention due to their alluring features such as chemical, thermal and structural stabilities, intense photoluminescence, transparency to UV radiation and environmentally friendly properties [1,2,3,4, ]. Among the several perovskite-like phosphors,

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CaZrO:Eu(CZO:Eu) stands out due to its bright red-light emission, low amount of heavy metals and absence of sulfur in the composition, which are some of the drawbacks currently found in the composition of the main commerciallyavailable red-emitting phosphors such as ZnCdS:Ag or YOS:Eu [5]. Among the different RE ions, Eu has been playing an important role as luminescent activator in inorganic materials due to its sharp emission bands in the red spectral region that result of electronic transitions within the 4f electronic configuration [6]. As some of those transitions are sensitive to the local environment of Eu, the ion may be used not only as an optical center but also as a

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spectroscopic probe to understand the impacts of the doping on the crystalline lattice [7].

To date, CZO:Eu has been investigated as rapid identification of latent

fingerprints, security ink and as red-emitting component of white-light-emitting

diodes (WLEDs) [8]. In this last case, the use of the CZO:Eu phosphor as coating

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of near-UV-emitting LEDs is an alluring alternative to come up with WLEDs

featuring low correlated color temperature (< 4,000 K) and high color rendering

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index (> 80) for warm WLEDs [8]. However, although there are several papers so far on the synthesis of CZO:Eu [8,9,10], some issues such as the optimization of

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the synthesis and luminescence, quenching of Eu luminescence due to the charge difference between Eu and Ca, and impacts of the Eu doping on the CaZrO lattice still need to be addressed.

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To overcome such challenges, in this study, we have chosen an adapted Microwave-Assisted Hydrothermal (MAH) synthesis of the CZO:Eu phosphor, followed by heat treatment, as reported in a previous paper for the undoped CZO

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matrix [4]. The rationales of such selection lie on the possibility of getting nanoshaped particles with good homogeneity control and the decrease of the

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calcination temperature to get the crystalline phase. Besides that, MAH and similar methods are, in general, environmentally friendly (use of a non-organic medium of synthesis) and energetically viable (short synthesis time and temperature) [11,12,13,14,15]. It is worth pointing out that several methods such as solid-state reaction or sol-gel synthesis are used to obtain CZO:Eu, but no MAH synthesis is reported so far, as represented in the state-of-the-art of the CZO:Eu synthesis (supplementary material, Table S1). Most of those routes

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involve calcination temperatures higher than 1,300 ºC or else, lower temperatures, but getting materials with low crystallinity degree. Therefore, inspired by improving the synthesis and luminescent features of CZO:Eu, the aim of this study is to synthesize red-emitting phosphors based on CaZrO:Euby the MAH route. The amount of dopant was isoeletronically changed from 1 up to 5 % in order to optimize the Eu red-light emission and discuss on the impacts of the doping on the crystalline structure, taking advantage of the Eu spectroscopic probe property by using high-resolution selective excitation and

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emission spectroscopy (14 K).

2. Materials and Methods

Synthesis and calcination conditions. The samples were synthesized in a

similar way to that reported in a previous paper, where undoped CaZrO nanoparticles were obtained [4]. Initially, for the undoped sample (CZO), the base-solution for the

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MAH synthesis was prepared in distilled water by adding 0.01 mol of CaCl.2HO (99%, Synth), 0.01 mol of ZrOCl.8HO (99.5%, Alphatec) and 50 mL of NaOH (6 mol/L)

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(98%, Synth) under constant stirring. The solution was kept under ultrasonic bath for 5 min; after that, it was transferred to a Teflon cup (100 mL) and subjected to the MAH

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process at 140 ºC. The conditions for the Eudoped sample syntheses were the same, however, during the preparation of the solutions, predetermined amounts of EuCl (0.045 mol/L) were added for the 1, 3 and 5 % isoelectronic doping (CaEuZrO – CZO1Eu,

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CaEuZrO – CZO3Eu and CaEuZrO – CZO5Eu, respectively). After the MAH synthesis, the samples were washed several times with distilled water (pH = 7) and placed in a kiln (110 ºC) for 12 hours. Finally, all samples were subjected to heat

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treatment up to 1,200 ºCover 1 hour, with 10 ºC/min heating rate upon static air atmosphere.

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X-Ray Diffraction analysis (XRD). The crystalline phases of all synthesized

samples were determined by means of an X-ray diffractometer (XRD-6000, Shimadzu) at 298 K using Cu K (λ = 1.5406 Å) and Cu K (λ = 1.5444 Å) radiation, divergence and reception slits of 1 º in continuous scan mode, 40 kV voltage, 30 mA current, 2 º/min scan speed and 2θ angular range from 10 º to 80 º. The diffractograms were identified using the crystallographic records of the Joint Committee on Powder Diffraction Standards – International Center for Diffraction Data (JCPDS-ICDD) database. An

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estimation of the average crystallite sizes of the analyzed samples was performed using the Scherrer equation [16] and the XPowderX program (version 2016.01.15), where the backgrounds were subtracted, the Full Width at Half Maximum (FWHM) for the most intense peak for each phase values were taken into account and a form factor of 0.9 were used. Energy Dispersive X-Ray Fluorescence Spectroscopy (EDX-FS). Approximate values for the atomic percentages of the synthesized samples were obtained using an energy dispersive X-ray fluorescence spectrometer (EDX7000, Shimadzu), with Rh atoms as the primary radiation. Scanning was analyzed from Na to

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U, in the qualitative-quantitative mode, under normal temperature and pressure conditions over an area of approximately 80 mm where biaxially-oriented polyethylene terephthalate (boPET, Mylar) substrates were used. A sample standard of 92.054 %Al/3.754 %-Sn/2.826 %-Si/0.812 %-Cu/0.293 %-Ni/0.261 %-Fe (Shimadzu Corp.

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01045) were used for the equipment calibration. The method is accurate to within 0.1 %.

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Scanning Electron Microscopy (SEM). The morphologies of the synthesized samples were observed using a scanning electron microscope (EVO LS 15, Zeiss)

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where the samples were previously metallized with Au.

Photoluminescence Spectroscopy. Photoluminescence spectra were carried out at 14 and 298 K with a modular double-grating excitation

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spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Horiba Scientific) coupled to a R928 Hamamatsu photomultiplier. The reciprocal Linear Dispersion is 2.66 nm.mm, the excitation and emission sleets were placed at 2

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and 0.03 mm, respectively, which results in a bandpass of Δλ = 0.079 nm. The emission decay curves were acquired with the same instrumentation coupled to a TBX-

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04 photomultiplier tube module (950 V). The exciting source was a Horiba Scientific pulsed diode light source (SpectraLED-355, peak at 356 nm). Absolute emission quantum yield (q). The absolute emission quantum

yields (q) were measured at 298 K using a system (Quantaurus-QY Plus C13534, Hamamatsu) with a 150 W xenon lamp coupled to a monochromator for wavelength discrimination, an integrating sphere as the sample chamber, and a

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multichannel analyzer for signal detection. The method is accurate to within 10 %.

3. Results and Discussion The powder XRD diffractograms of the samples calcinated at 1,200 ºC are shown in Figure 1 (a). Two phases are observed in the XRD profile of all samples and they were indexed to the CaZrO (lakargite, JCPDS-35-790, orthorhombic crystalline structure) and the non-stoichiometric CaZrO(NaCl type, JCPDS-26-341, cubic crystalline structure) phases. It is worth pointing out that the formation of this non-

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stoichiometric phase usually follows the formation of the main phase and a previous report concluded that the percentage relation between the CaZrO/CaZrO phases in the CZO sample obtained by MAH is 65/35 wt% [4].

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Figure 1. (a) Powder XRD of CZO:Eu samples compared to the undoped CZO. Assignments: ♦ = CaZrO phase (JCPDS-35-790), * = CaZrOphase (JCPDS-26-341). (b) Crystalline structure of orthorhombic CaZrO phase, Pcmn (62) space group. (c) Crystalline structure of cubic CaZrOphase, Fm3̅m (225) space group. The crystalline structures are based on the CIF file available on Inorganic Crystal Structure Database (ICSD), ICSD number 97463 (CaZrO phase) and ICSD number 202847 (CaZrOphase).

In both phases, the diffraction peaks are well-defined and narrow, confirming

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that the synthesis conditions were enough to get phases with desirable degree of crystallinity. As the Eu content increases in the samples, the intensity of the diffraction peak at 30 º assigned to the CaZrOphase starts to increase compared to the peak at 31 º

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(CaZrO phase), leading us to conclude that Eu favors the formation of the CaZrOphase. An characterization that allows us to indirectly verify this higher percentage of the nonstoichiometric phase in samples with a greater amount of europium, is thermal analysis

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(Figure S1). Even after the initial heat treatment, the recarbonation process takes place, where CaCO is formed. This carbonate is formed mainly on the surface of the particles

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and in the grain boundaries, more energetic regions that are susceptible to the adsorption of atmospheric CO. It is possible to observe that the weight losses are greater for the CZO5Eu sample, the same sample that presents a greater portion of the nonstoichiometric phase, as indicated by the diffractograms. Yet, no other peak assigned to spurious phases were observed as the Eu molar fraction increases in the system. In the CaZrO phase, Ca is inserted in a site with coordination number (CN) 8 without inversion center and Zr occupies sites with CN 6, with inversion center, as represented in Figure 1 (b). However, after the doping with Eu, S. Sakaida and Y.

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Shimokawa et al. have shown that the ZrO octahedron does not have an inversion center anymore due to oxyanions lacking in the lattice of CaZrO [17,18]. Already the nonstoichiometric CaZrOphase is formed from the replacement of Zr by Ca in ZrO, and both Ca and Zr are inserted in equivalent cubic sites with CN 8 featuring inversion center [17,18], Figure 1 (c). In both phases, Eu replaces Zr and Ca since the difference of ionic radius between the dopant and the network-forming cations is lower than 30 % [19]. The ionic radii of Ca, Zr and Eu ions into the CaO, ZrO, EuO and EuO polyhedra are listed in Table 1 and the ionic radii difference (D) is calculated according to Equation 1, in

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which R (CN) is the radius of the host cation and R (CN) is the radius of Eu. Therefore, from then on, we will assume that Eu may occupy 3 non-equivalent local sites: EuO-as

(Eu replacing Ca in the CaZrOphase, without inversion center), EuO-as(Eu replacing Zr in the CaZrOphase, without inversion center) and EuO-i(Eu replacing both Ca and Zr in the CaZrOphase, with inversion center). The photoluminescence technique will be

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helpful to understand the site occupancy of Eu in both phases.

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Table 1. Ionic radii of Ca, Zr and Eu ions into the CaO, ZrO, EuO and EuO polyhedra. Polyhedra Ionic radius / Å D/ % CaO 1.12 ZrO 0.72 EuO 1.07 4 EuO 0.95 24

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𝐷𝑟 = (𝑅ℎ (𝐶𝑁) − 𝑅𝑑 (𝐶𝑁))⁄𝑅ℎ (𝐶𝑁) (𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 1) The average crystallite size values of the CZO samples are shown in Table 2. As

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the Eu content in the lattice increases, the average crystallite size values decrease. This tendency is correlated to the amount of both CaZrO and CaZrOphases in the phosphor

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composition that is dependent on the Eu content, to which controls the boundaries under synthesis, determining the mass transfer through the grain boundaries as well as the segregation of Eu ions on the grain boundaries. [20,21,22] Table 2. Average crystallite size of the CZO samples. Sample Crystallite size / nm CaZrO phase CaZrOphase CZO 70±2 89±2 CZO1Eu 33±1 32±1 CZO3Eu 28±1 26±1 CZO5Eu 25±1 26±1

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All the CZO-based samples were obtained as highly aggregate particles without a defined shape (Figure 2), to which agrees with other previously-reported synthetic routes [3,23,24,25], even that the undoped CZO displays a tendency to form spherically and oval-shaped particles highly agglomerated, leading to a large surface area. This particle agglomeration tendency must be correlated with the high annealing temperature, to which leads to the agglomeration and sintering of the precursor particles. Additionally, the insertion of Eu results in an apparent and gradual loss of the oval shape of the agglomerated particles since the samples with higher percentages of europium (CZO3Eu and CZO5Eu) present smoother regions, probably due to the larger

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portion of the non-stoichiometric phase present in these samples, as indicated by the diffractograms in Figure 1 (a).

Figure 2. SEM images of the CZO samples. Magnification of 1.00 KX and EHT of 30.00 KV.

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To verify the doping concentration accuracy of the MAH method in the

synthesis of CZO:Eu, all samples were subjected to EDX-FS characterization, and the

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results are summarized in Table 3. It is important to note that in this kind of characterization using the spectrometer described in the section 2, only atoms ranging

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from Na to U are accurately detected (ppm to ppb concentrations, specifically for Ca and Eu the instrument detection limit is ~1 ppm and ~0.1 ppm for Zr). In this regard, only Ca, Zr and Eu are identified for CZO and CZO:Eu, as expected. The peaks related

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to the characteristic decay after excitation and identified to determine the percentages were: K and K for Ca (3.7 and 4 keV), K for Zr (15.7 keV) and L, L, L and L for Eu

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(5.8, 6.5, 6.8 and 7.5 keV).

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Table 3. EDX-FS of the CZO and CZO:Eu samples. Amostras Atomic Percentage (%) Ca Zr CZO 41.9±0.1 58.1±0.1 CZO1Eu 32.1±0.1 66.9±0.1 CZO3Eu 24.5±0.1 72.7±0.1 CZO5Eu 21.0±0.1 74.1±0.1

Eu 0.9±0.1 2.8±0.1 4.9±0.1

There is a clear deviation in the stoichiometric relationship between Ca and Zr atoms in the undoped sample, where a Ca/Zr ratio of 50/50 is expected, but values of 41.881/58.119 are observed. This is mainly related to the aforementioned nonstoichiometric phase formation (cubic CaZrO), which has less Ca in its structure. The

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formation of this secondary phase is closely linked to variations in the available ions in the hydrothermal solution at the beginning of the synthesis (possible stoichiometric deviations). Keeping this in mind, for the samples synthesized here, two are the main factors that cause the non-stoichiometric phase formation and (1) the stoichiometric deviations adopted for the isoelectronic doping and (2) the formation of water-soluble species during the precursor synthesis by the MAH processes, to which were removed during the washing steep, leading to deviations of the phosphor stoichiometry.[26,27,28] As the percentage of Eu increases, the percentage of Ca decreases as a result of

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the larger formation of the non-stoichiometric phase, as observed in the X-ray diffractograms. Additionally, the observed percentages of Eu (0.957, 2.783 and 4.892 %) are very close to the proposed values (1, 3 and 5 %), suggesting that the MAH method is a good candidate for this kind of doping.

To infer on the luminescent features of the Eu-doped CZO samples, excitation

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and emission spectra were carried out (Figure 3). In the excitation spectra (298 K)

shown in Figure 3 (a), a broad excitation band in the higher energy spectral region is

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assigned to the Eu→O charge transfer band (CTB) confirming that the host acts as sensitizer for the UV radiation absorption by Eu. Lower intensity excitation lines are

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also noticed in the 320-450 nm spectral range and they are ascribed to the Eu f-f transitions from the F ground state to the excited ones, forbidden by Laporte’s selection

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rule.

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Figure 3. (a) Excitation spectra (298 K) monitored at 613 nm, (b) emission spectra (298 K) excited at 260 nm, (c) 1931 Commission Internationale d´Eclairage (CIE) chromaticity diagram [29] and (d) absolute emission quantum yield (q) of the phosphors.

The emission spectra of the phosphors excited at 260 nm (298 K), Figure 3 (b), reveal the characteristic set of Eu transitions from the D emitting state to the F (J = 0-4)

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states, being the D→F transition in the red spectral region, the most intense one. The high number of components for the Eutransitions, namely more than one component for the non-degenerated D→F transition and more than three for the D→F transition promptly indicates that Eu is inserted in more than one non-equivalent local coordination site. The red-light emission of the samples was quantified by the calculation of the Commission Internationale d´Eclairage (CIE) color coordinates, Figure 3 (c), and the

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(x;y) coordinates lie within the red spectral region, with values of (0.642; 0.332), (0.663;0.330) and (0.668; 0.327) for the CZO1Eu, CZO3Eu and CZO5Eu samples, respectively, revealing high emission color purity. To further investigate the emission features and take advantage of the spectroscopic probe property of Eu, high-resolution emission spectra (14 K) were carried out for the CZO3Eu sample (Figure 4). For the CZO3Eu sample, 2 well-defined components for the D→F transition are noticed in the emission spectrum (14 K), Figure 4 (a), and as the non-degendered D→F transition is forbidden according to the selection rules but it is relaxed in cases where Eu is inserted in local sites without inversion center

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(C, C or C point groups) [6], the two components confirm that Eu is inserted in at least two non-equivalent local sites without inversion center. It is feasible to associate those

Eu local sites to the replacement of Ca and Zr by Eu in the CaZrO phase since the EuOas and EuO-as local sites do not have inversion center.

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Figure 4. High-resolution emission spectra (14 K) of the CZO3Eu sample excited at 260 nm in the (a) D→F, (b) D→F, and (c) D→F transition region. The * highlights the components of each transition.

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Focusing our attention in the D→Ftransition, Figure 4 (b), it occurs by a magnetic dipole mechanism strongly favored in Eu local sites with inversion center, and

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a maximum contribution of three components by each non-equivalent Eu local sites is expected [6]. Seven components for the D→F transition is noticed in the emission spectrum of the CZO3Eu sample. In this set of components, EuO-as and EuO-as local

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sites with C, C or Cpoint group may contribute with 3 component each one [7], i.e., six components of the D→F transition come from EuO-as and EuO-as local sites. This observation promptly indicates that the other component comes from a Eu local site

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with an inversion center. Among the set of possible Eu local sites highlighted by XRD, the other site available and not yet assigned is the CaO/ZrOpolyhedron (EuO-i) in the

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CaZrOphase with O point group and inversion center. Therefore, the third Eu local site is assigned to the EuO-i local site, confirming that Eu replaces Ca and Zr in both CaZrO and CaZrOphases. Already the D→F transition of Eu is a pseudo-quadrupole transition, which

means that its intensity is more influenced by the Eu local asymmetry than the intensities of the other Eu transitions.[6] Moreover, since the degeneracy of an L energy level is 2J + 1, for each D→F transition, a maximum of 2J + 1 lines is expected, leading to a maximum of 5 components for the D→F transition. [Error! Bookmark not

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defined.]. Eight well-defined componentsfor theD→F transition are noticed in the emission spectrum, Figure 4 (c); however, as at least five components for sites with C, C or C point groups and two components for the O group are expected [6Error! Bookmark not defined.], the high number of components leads to an overlap of the bands, making the assignments difficult. The energy of all D→Ftransition components is shown in Table 4. Table 4. Energy values of the D→F, D→F and D→F transition components excited at 260 nm for the CZO3Eu sample. Assignment EuO-as EuO-as EuO-as EuO-as EuO-as EuO-i EuO-as EuO-as EuO-as

Transition D→F

Energy / ±3 cm 16,471 16,281 16,217 16,139 15,979 15,933 15,855 15,802

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D→F

Energy / ±3 cm 17,268 17,202 17,108 17,088 17,067 16,989 16,972 16,891 16,728

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Transition D→F

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Trying to assign the components of the D→Fand D→Ftransitions, first, it was considered the nephelauxetic effect which correlates the shift of the energy (cm) of the D→F transition with the number of donor atoms bounded to Eu. [6] The nephelauxetic

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effect, also named as ‘‘cloud-expanding’’ effect, is related to the red-shift of the energy of the D→F transitions in Eu coordination systems compared to the free ion, to which

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has been assigned to the reduction of the 4f electron repulsion interactions due to interpenetration of the electron charge distribution coming from the ligands bonded to Eu.[6] According to the solvent-independent nephelauxetic model of Eu developed by

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Choppin and Wang[30] and represented by Equation 2 (CN is the coordination number of the Eu local site and 𝛥𝑣̃ is the shift relative to the energy of the D→F transition

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compared to the energy of the same transition in [Eu(HO)], 17,276 cm), the shift relative to the energy of the transition increases as the CN of the Eu local site increases. Therefore, it is feasible to assign the higher-energy (lower 𝛥𝑣̃ compared to [Eu(HO)]) D→F component at 579.5 nm (17,268 cm) to the EuO-as local site and the other one at 581.3 nm (17.202 cm) featuring lower energy (higher 𝛥𝑣̃ compared to [Eu(HO)]) to the EuO-as site. 𝐶𝑁 = 0.237𝛥𝑣̃ + 0.628 (𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 2)

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Considering the D→F transition, selective excitation spectra (14 K) measured by fixing the emission wavelength in each component was carried out and compared to the excitation spectra of the D→F transition components (Figure 5). For the D→F transition component coming from the EuO-as local site (579.5 nm), two components for the F→L transition at 392.0 and 395.0 nm and two components for the F→D transition at 462.0 and 466.0 nm are noticed in the excitation spectrum. Already for the D→F transition component of the EuO-aslocal site (581.3 nm), two components for the F→L transition at 394.9 and 396.9 nm and a single component for the F→D transition at 465.5 are noticed.

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Interestingly, the D→F transition components at 584.0, 585.0 and 586.0 nm display the same excitation spectra of the EuO-as local site, confirming that they arise from the EuO-as local site. On the other hand, the D→F components at 589.0, 592.0

and 597.0 nm have the same excitation spectra of the EuO-assite. Finally, the excitation spectrum of D→F transition component at 587.0 nm displays only one component at

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393.4 nm for the F→L transition and two components at 464.1 and 466.4 nm for the

F→D transition, different from both EuO-as and EuO-as local sites, leading us to assign

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it to the EuO-i local site. Due to the overlap of the D→F transition components, it was not possible to assign them to the Eu local sites. The assignments of all D→Ftransition

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components are also shown in Table 4.

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Figure 5. Selective excitation spectra (14 K) of the D→Fand D→Ftransition components of the CZO3Eu sample.

To further examine the nature of each non-equivalent Eu local sites, selective emission decay curves were monitored at around the two D→F components of EuO-as

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and EuO-as local sites, and the decay-curves are shown in the supplementary material, Figure S2. As it was not possible to separate the D→F components coming from the

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EuO-i local site due to the overlap with the other components, the emission decay curve of the EuO-i site was not carried out. All the decay curves reveal a single exponential behavior in agreement with the experimental site-selective conditions and from the data best fit, the D state lifetime () values were calculated for EuO-as and EuO-as local sites (Table 5). Table 5. Dstate lifetime (τ) values of each D→F component excited at 260 nm (298 K) of all CZO:Eu samples. Sample

λ/ nm

τ / ms

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CZO1Eu CZO3Eu CZO5Eu

578 581 578 581 578 581

2.5±0.1 2.7±0.1 3.0±0.1 3.5±0.1 2.6±0.1 2.7±0.1

The D state lifetime values found for both D→F transition components lie within the 2.5 – 3.5 ms range and they are in accordance with other data found for Eubased zirconate phosphors [31,32]. Attending to the fact that the experimental transition probability may be expressed as  =+, in which  and  represent the non-radiative and radiative D state lifetimes, an increase in the lifetime may be assigned, among other

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effects, to a decrease of the non-radiative losses or/and enhancement of the radiative processes. The D state lifetime values for both D→F transition components also increase from the doping concentration of 1 % to 3 % and then, they decrease,

suggesting that the CZO3Eu sample displays the best radiative decay rate from the D

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excited state – this same best-limit concentration was reported by I. P. Sahu et al. on the synthesis of CZO:Eu (1, 2, 3, 4 and 5 %) by the solid state reaction [33]. To make sure

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that this proposition is correct, the absolute emission quantum yield values of the samples will be helpful.

The absolute emission quantum yield (q) of all Eu-based samples was measured

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monitoring different excitation wavelengths, namely at around 260 nm and within the f-f levels (L) at 395 nm and (D) at 465 nm, Figure 3 (d). In all the measured excitation

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wavelengths, the CZO3Eu sample displays the best emission quantum yield values, suggesting that at higher doping concentrations, luminescent quenching by concentration takes place. As the Eu concentration increases, the distance between the

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doping ions decreases within the crystalline lattice, enhancing the probability of energy transfer between Eu. Thus, the chances of a defect in the network be close to an Eu

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center increases, leading to deactivation through non-radiative pathways. Finally, by using the absolute emission quantum yield value monitored at 395

nm of the CZO3Eu sample as figure of merit, Table 6, the value reported by us is among the best reported within the state-of-the-art of red-emitting phosphors, especially under excitation at 395 nm. It is worth pointing out that there is no paper so far that reports any emission quantum yield value of the CZO:Eu phosphor.

Table 6. Figure of merit of the absolute quantum yield (q) for the Eu-based phosphors. The excitation wavelength (λ) is also indicated.

14 Sample CZO1Eu

CZO3Eu

CZO5Eu

λ / nm 260 395 465 260 395 465 260 395 465 310 275 271 394 254 254 254 254

q 0.062±0.006 0.073±0.007 0.044±0.004 0.16±0.01 0.29±0.03 0.19±0.02 0.12±0.01 0.081±0.008 0.053±0.005 0.07 0.10 0.145 0.016 0.20-0.50 0.25-0.30 0.90 0.50-1

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YVO:Eu Y(MoO):Eu BiSiO:Eu BiSiO:Eu YOF:Eu MgYBO:Eu YO:Eu YOS:Eu

Reference This study This study This study This study This study This study This study This study This study [34] [35] [36] [36] [37] [37] [37] [37]

Therefore, the finds reported here confirm that the MAH process was successful to synthesize high-crystalline RE-doped oxide phosphors, as shown in another papers [38,39,40] but more specifically in this case, for the Eu-doped CaZrO with relatively

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high emission quantum yield, qualifying the CZO:Eu as an efficient red-emitting

phosphor for application as coating of near-UV LEDs, in rapid identification of latent

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fingerprints as well as in security ink.[8]

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4. Conclusions

Herein, an adapted microwave-assisted hydrothermal synthesis of CaZrO:Eu(1 – 5 %) phosphors followed by heat treatment was introduced.

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Agglomerated particles with high structural ordering were obtained at 1,200 ºC as a mixture of CaZrO and CaZrO phases, and Eu replaces Ca and Zr local sites in both phases, i.e., occupying three non-equivalent local sites, two with CN 8 or 6 without

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inversion center, and another with CN 6 and inversion center. All phosphors excited at 250 nm exhibit bright luminescence in the red spectral region as a result of Eu f-f

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transitions, and by high-resolution selective excitation and emission spectra, it was possible to assign all the D→F and D→F transition components to each Eu nonequivalent local site. Finally, among the series of phosphors, the 3 %-doped sample displayed the best absolute emission quantum yield at 395 nm (0.29±0.03), qualifying it as a potential red-emitting phosphor applied in rapid identification of latent fingerprints, in security ink or as coating of near-UV LEDs for WLEDs.

CRediT author statment

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Wagner Costa Macedo: Conceptualization, Investigation, Writing – Original Draft, Validation. Airton Bispo Germano Junior: Conceptualization, Investigation, Writing – Original Draft, Visualization. Kleper de Oliveira Rocha: Writing – Review and Editing. Agda Eunice de Souza Albas: Writing – Review and Editing.

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Ana Maria Pires: Writing – Review and Editing, Supervision. Silvio Rainho Teixeira: Resources, Supervision. Elson Longo: Resources, Funding Acquisition.

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Declaration of interests

5. Acknowledgements

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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This work was supported by FAPESP/CEPID [2013/07296-2], CNPq [573636/2008-7] and CAPES through a scholarship (PhD – POSMAT) awarded to W. C. Macedo. A. G.

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Bispo-Jr also thanks FAPESP for the scholarship (2016/20421-9 and 2017/21995-1). Laboratório de Microscopia Eletrônica de Varredura (FCT-UNESP) is acknowledged for the SEM measurements, and L. D. Carlos from CICECO-Aveiro Institute of

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Materials (University of Aveiro - Portugal) is acknowledged for the photoluminescence and quantum yield measurements.

Data Availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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Figure Captions

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Figure 1. (a) Powder XRD of CZO:Eu samples compared to the undoped CZO. Assignments: ♦ = CaZrO phase (JCPDS-35-790), * = CaZrOphase (JCPDS-26-341). (b) Crystalline structure of orthorhombic CaZrO phase, Pcmn (62) space group. (c) Crystalline structure of cubic CaZrOphase, Fm3̅m (225) space group. The crystalline structures are based on the CIF file available on Inorganic Crystal Structure Database

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(ICSD), ICSD number 97463 (CaZrO phase) and ICSD number 202847 (CaZrOphase).

Figure 2. SEM images of the CZO samples. Magnification of 1.00 KX and EHT of 30.00

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Figure 3. (a) Excitation spectra (298 K) excited at 613 nm, (b) emission spectra (298 K) excited at 260 nm, (c) 1931 Commission Internationale d´Eclairage (CIE) chromaticity

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diagram [29] and (d) absolute emission quantum yield (q) of the phosphors.

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Figure 4. High-resolution emission spectra (14 K) of the CZO3Eu sample excited at 260 nm in the (a) D→F, (b) D→F, and (c) D→F transition region. The * highlights the

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components of each transition.

Figure 5. Selective excitation spectra (14 K) of the D→Fand D→Ftransition components of the CZO3Eu sample.

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