Effects of Eu3+ ions doping on physicochemical properties of spinel-structured lithium-titanium oxide (Li4Ti5O12) as an efficient photoluminescent material

Materials Research Bulletin 134 (2021) 111084

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Effects of Eu3+ ions doping on physicochemical properties of spinel-structured lithium-titanium oxide (Li4Ti5O12) as an efficient photoluminescent material e, f ˙ ´ ski c, M. Ptak c, A. Roguska d, O. Chernyayeva d, P. Zurek M. Michalska a, b, *, K. Leman , A. Sikora g, P. Gołębiewski a, h, A. Szysiak a, A. Malinowska a, i, M. Małecka c a

Łukasiewicz Research Network - Institute of Electronic Materials Technology, W´ olczy´ nska 133, 01-919, Warsaw, Poland ˇ Department of Chemistry, VSB-Technical University of Ostrava, 17. listopadu 15/2172, 708 00, Ostrava-Poruba, Czech Republic c Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Ok´ olna 2, 50-950, Wrocław, Poland d Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224, Warsaw, Poland e Łukasiewicz Research Network - Electrotechnical Institute, Division of Electrotechnology and Materials Science, M. Skłodowskiej-Curie 55/61, 50-369, Wroclaw, Poland f Active Students Association, SEP Branch No. 1 in Wrocław, M. Skłodowskiej-Curie 55/61, 50-369, Wrocław, Poland g Wroclaw University of Science and Technology, Faculty of Microsystem Electronics and Photonics, Janiszewskiego 11/17, 50-372, Wrocław, Poland h Faculty of Physics, University of Warsaw, Pasteura 5, 02-093, Warsaw, Poland i Department of Biophysics and Human Physiology, Medical University of Warsaw, Chałubinskiego 5, 02-004, Warsaw, Poland b

A R T I C L E I N F O

A B S T R A C T

Keywords: Lithium titanium oxide Li4Ti5O12 Europium Solid-state synthesis Photoluminescent material Spinel

In this work we report our recent efforts on the investigation of the compositional and structural features of series of nanocrystalline Li4Ti5O12 (LTO) doped with Eu3+ ions powders, which were synthesized in a high-energy ballmilling (HEBM) process. Lithium carbonate, titanium and europium oxides were used as starting reagents. The ball-milled materials were turned into nanocrystalline powders due to the heating that was carried out in the air in the temperature range from 500 to 800 ◦ C. The synthesized LTO doped with Eu3+ powders were examined by a number of physicochemical techniques: X-ray powder diffraction (XRD), Raman spectroscopy, X-ray photo­ electron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), absorption and emission spec­ troscopy (photoluminescence and excitation spectra were recorded), diffuse reflection spectroscopy (UV-vis DRS), scanning electron (SEM), transmission (TEM) and atomic force (AFM) microscopy. Structural, morphological and photoluminescent properties were correlated and examined for the first time in this work. The obtained results suggest that LTO material doped with rare earth ions like europium could serve as a material for applications in optoelectronic devices such as white light emitting diodes (WLEDs).

1. Introduction Since 1991, when Sony announced the first generation of commer­ cially available secondary lithium-ion batteries (LIBs), they are still considered the most promising systems in the field of energy storage. LIBs are used in nearly every electronic device and electric vehicle all over the world. They owe their position to their properties such as high power capacity, good cyclability, flat charge/discharge profiles and high operating voltage [1–5]. Nevertheless, high cost of the electrode mate­ rials, low current densities acquired from the cells and safety issues force scientists and engineers to continuously improve LIBs. [1–5].

One of the objectives is to replace typically used graphite anode with a new type of anodic material. One of the most promising substances is lithium titanium oxide spinel (Li4Ti5O12, LTO) [6–10]. Due to the high potential of LTO electrode (1.55 V vs Li/Li+), application of this sub­ stance prevents formation of metallic lithium plating on a negative electrode during overcharge [6–10]. Thus, it results in significant safety improvement. Moreover, it does not exhibit structural changes during lithiation/delithiation processes (often called „zero-strain” electrode) and as a result it shows excellent cyclability [6–10]. However, compared to graphite, Li4Ti5O12 suffers from a lower theoretical specific capacity (175 mAhg− 1) and its insulating nature, which prevents from using it in

* Corresponding author at: Łukasiewicz Research Network - Institute of Electronic Materials Technology, W´ olczy´ nska 133, 01-919, Warsaw, Poland. E-mail address: [email protected] (M. Michalska). https://doi.org/10.1016/j.materresbull.2020.111084 Received 16 February 2020; Received in revised form 31 August 2020; Accepted 17 September 2020 Available online 25 September 2020 0025-5408/© 2020 Elsevier Ltd. All rights reserved.

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high current applications [6–10]. LTO’s energy band gap has been re­ ported to be about 2 eV according to first principles calculations based on density functional theory [11]. The experimental result of Li4Ti5O12 energy band gap has been appointed to be about 3.8 eV from the registered absorption spectra (estimated from UV–vis diffuse-reflectance spectroscopy) [12]. The conductivity of lithium-titanium oxide can be greatly improved by various modifications. First group contains surface modifications and coatings like: metallic Ag nanoparticles [8,7–10] and carbon nanostructures [13–15]. Another possibility is to introduce different dopants like i.e.: Mg2+ [16,17], Al3+ [18,19], Cr3+ [20], Zr4+ [21,22], V5+ [23,24], Mo4+ [25], Nb5+ [26]. To overcome difficulties connected to low conductivity LTO can be also prepared in the nano­ crystalline form. Y. Cai et al. have reported that by using the co-precipitation method, the carbon-coated hierarchical mesoporous microspheres of europiumdoped lithium titanium oxide of chemical formula Li4-x/2Ti5-x/ 2EuxO12+x@C (x = 0.004) could be obtained. Moreover, reported sam­ ples exhibited improved structural stability, high reversible capacity and enhanced electrochemical performances [27]. They have demonstrated: the highest initial discharge capacity of 198.7 mAhg− 1 at 1 C, after 1000 cycles at 5 C rate the discharge capacity of 173.4 mAhg− 1, and 92.1 mAhg− 1 at 100 C rate [27]. Yang et al. have utilized the sol-gel synthesis of trivalent europium-doped LTO (Eu3+: Li4Ti5O12) with different Eu3+ concentrations (0.1 mol%, 0.3 mol%, 1.0 mol%, 3.0 mol%) and have analyzed photoluminescence properties [28]. The authors have explained that Eu3+: Li4Ti5O12 powders synthesized by sol-gel method revealed strong red emission at 612 nm, corresponding to the 5D0–7F2 transition, indicating that Eu3+ ions occupy non-inversion symmetry positions. The strongest excitation peak, has been observed in the blue light region at 464 nm [28]. They have also reported that color co­ ordinates of all synthesized Eu3+: Li4Ti5O12 powders are almost identical and all of them can be potential used as a red-emitting phosphor for optical applications [28]. These interesting studies have opened a new way for applications of spinel-structured LTO doped with europium ions, not only in the energy area, but also as a component of optoelec­ tronic devices. In the present work, we demonstrate a new solid state synthesis approach to obtain a series of nanosized Li4Ti5O12 powders doped with Eu ions, which involves the use of a two-stage thermal treatment pre­ ceded by high-energy ball-milling (HEBM). The Eu3+ doping ions possess a larger ionic radius than the ions in the crystal structure and different valence. This charge mismatch may create defects in the crystal structure. The main focus of the current study is the influence of pres­ ence and concentration of Eu ions incorporated into LTO spinel structure on its structural and spectroscopic properties. In order to establish this correlation various techniques were used: X-ray powder diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) absorption and emission spectroscopy (photoluminescence and excitation spectra were recor­ ded), diffuse reflection spectroscopy (UV-vis DRS), scanning electron (SEM), transmission (TEM) and atomic force (AFM) microscopy. The structural, morphological, spectroscopic and luminescent studies, pre­ sented in this article, were carried out in terms of the verification of the usefulness of the lithium titanium oxide, typically used as the negative electrode (anode) in lithium-ion batteries, as a potential material for optoelectronic devices. Additionally, obtained results provided useful data concerning luminescent activities of investigated materials.

(99.99 % trace metals basis, Sigma-Aldrich). The samples labeled: LTO18-PT-A, LTO19-PT-A, LTO20-PT-A, LTO21-PT-A, LTO22-PT-A, LTO23-PT-A, LTO24-PT-A, and LTO25-PT-A corresponded to nominal Li:Eu:Ti molar composition ratio of: 3.99:0.01:5, 3.98:0.02:5, 3.97:0.03:5, 3:95:0.05:5, 3.9:0.1:5, 3.8:0.2:5, 3.7:0.3:5, and 3.5:0.5:5, respectively. The synthesis was carried out involving the high-energy ball-milling (HEBM) process, using ethanol (96 %, Avantor Perfor­ mance Materials Poland S.A.) as a medium. Milling was conducted in zirconia container with zirconia balls using a Planetary Mono Mill PULVERISETTE 6 (Fritsch, Germany). The HEBM process was conducted at a constant speed of 300 rpm and 2 h of processing time, respectively. In the following step the remaining alcohol medium was evaporated and the obtained powder was subsequently dried at 60 ◦ C in an air atmo­ sphere overnight. Finally, all the powders were ground in an agate mortar and annealed in the air in two-stage thermal treatment, con­ sisting of 6 h of annealing at 500 ◦ C followed by 20 h of annealing at 800 ◦ C. 2.2. Materials characterization Qualitative and quantitative phase analysis of synthesized Eu-doped LTO materials was performed by means of powder X-ray diffraction (XRD) method. The experiments were conducted using the universal Rigaku SmartLab 3 kW X-ray diffractometer equipped with a Cu X-ray tube (λ = 1.542 Å) and a 1D high-speed silicon semiconductor strip detector (D/teX Ultra 250). The powder diffraction patterns were recorded in the reflection Bragg-Brentano geometry (θ/2θ scan) using the continuous scanning mode. Qualitative and quantitative phase analysis as well as the refinement of structural parameters by Rietveld method were performed using a PDF4 + 2019 database and PDXL2 Software supplied by Rigaku. Raman spectra were collected using a Renishaw InVia Raman mi­ croscope equipped with confocal DM 2500 Leica optical microscope, a thermoelectrically cooled CCD as a detector and an argon laser oper­ ating at 488 nm. IR (Infrared) spectra in the mid-IR (4000− 400 cm− 1) and far-IR (400− 50 cm− 1) range were measured in KBr pellets and Nujol suspension on the polyethylene plate, respectively, using a Nicolet iS50 FT-IR spectrometer. The spectral resolution was set to 2 cm− 1. To confirm the chemical composition and chemical states of the el­ ements in the Eu-doped LTO powders XPS measurements were per­ formed by an ESCALAB-210 photoelectron spectrometer (VG Scientific) using AlKα (hν = 1486.6 eV) radiation as X-ray source. Survey scans were collected from 1350 to 0 eV with a pass energy of 75 eV. The high resolution spectra were recorded using 25 eV pass energy. A Shirley background subtraction was used to obtain the XPS signal intensity. The peaks were fitted using an asymmetric Gaussian/Lorentzian mixed function. The measured binding energies were corrected regarding the energy of C 1s at 284.5 eV. Advantage software (Version 4.75) was used for data processing. The absorption spectra were recorded using Cary 5000 UV–vis-NIR Spectrophotometer. The emission spectra were recorded on a Jobin Yvon THR1000 monochromator with CCD camera. Fourth harmonic (266 nm) of Q-switched Nd:YAG pulsed laser was used as an excitation source. The excitation spectra were registred using the FLS980 Fluorescence Spectrometer (Edinburgh Instruments) equipped with 450 W Xenon lamp and with standard photo-multiplier Hamamatsu R928 P detector. The decay profiles were recorded on a Lecroy digital oscilloscope. The Ultraviolet–visible diffuse reflectance spectra (UV–vis DRS) of series of LTO18-PT-A to LTO25-PT-A powders were recorded at wave­ lengths range from 220 to 800 nm at room temperature on a Shimadzu UV-2600 Series (Shimadzu Ltd). Barium sulphate (BaSO4) powder as a reference sample and an external 2D detector were used. The reflectance data were transformed using Kubelka-Munk (K.-M.) function and Tauc’s plot was used to determine the values of the indirect bandgap energies (Eg) [12].

2. Experimental part 2.1. Synthesis of LTO doped Eu powders Series of Eu-doped LTO powders were obtained by the solid state synthesis method. The starting reagents used for the synthesis were: ti­ tanium dioxide TiO2 (99 %, Sigma-Aldrich), lithium carbonate Li2CO3 (Purum, ≥99 %, Honeywell Fluka) and europium (III) oxide Eu2O3 2

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Fig. 1. XRD patterns of Li4Ti5O12 powders doped with various molar concentration of Eu3+ ions. The patterns are plotted with square root scale for better imaging of diffraction lines with the smallest intensities.

The morphology and particle size of all the synthesized powders were determined using scanning electron microscopy (SEM, Cross Beam Auriga, Carl Zeiss) and atomic force microscopy (AFM). Additionally, the morphology and microstructure for samples (LTO18-PT-A and LTO25-PT-A) with two extreme concentrations of the Eu3+-ions in the LTO matrix were investigated by TEM (Philips CM-20 SuperTwin operating at 160 kV). High-resolution transmission electron microscopy (HRTEM) images were examined with a DigitalMicrograph program. The data were acquired using an AFM instrument Innova (Bruker former Veeco) equipped with a 100μm × 100μm scanner. One should provide minimal tip-sample interaction in order to avoid surface modification, TappingMode (intermittent mode) was utilized. The samples were prepared by placing the powder on the glass substrate covered with the cyanoacrylates – based glue. After the glue was hard­ ened, it was possible to locate the small groups of particles using the optical microscope integrated with AFM and perform scanning proced­ ure. The data was acquired under ambient conditions (temperature approximately 23 ◦ C and relative humidity 35 %). The NSG10 probes from NTMDT were used (nominal tip radius rtip =6 nm, resonance fre­ quency range fres = 140–390 kHz, and spring constant k = 3.1–37 Nm− 1). The data was processed using the SPIP software (Image

Metrology Company, Denmark) [29]. 3. Results and discussion 3.1. XRD results Qualitative phase analysis of all synthesized Eu-doped LTO powders indicated dominant presence of spinel-type Fd3m cubic structure of lithium titanium oxide Li4Ti5O12 (LTO, ICDD-49− 0207). In addition, some oxides, such as Eu2Ti2O7 (ICDD-23− 1072), Eu2O3 (ICDD34− 0392), rutile form of TiO2 (ICDD-21− 1276) as well as a trace of an unidentified phases were also observed. The experimental diffraction patterns with marked matching phases are presented in Fig. 1. It is worth noting, that with the increase of europium content in the samples, the relative content of the Eu2Ti2O7 phase increases from the value at the limit of detection (tenths of a percent) for LTO18-PT-A and LTO19-PT-A samples to about 12 and 18 wt.% for LTO24-PT-A and LTO25-PT-A samples, respectively. A similar, but weaker increasing trend was observed for the Eu2O3 phase as well. The relative content of this phase reached only up to about 3 wt.% for the sample with the highest Eu content (LTO25-PT-A). Furthermore, the Rietveld refinement indicated

Fig. 2. Mid-IR (a) and far-IR (b) spectra of the Li4/3Ti5/3O4 spinel with different molar concentration of Eu3+ ions. 3

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Table 1 Chemical composition of LTO doped Eu powders. Sample

Li, at.%

Ti, at.%

O, at.%

Eu, at.%

C, at.%

LTO18-PT-A LTO19-PT-A LTO20-PT-A LTO21-PT-A LTO22-PT-A LTO23-PT-A LTO24-PT-A LTO25-PT-A

22.33 23.41 22.42 23.74 23.40 20.92 21.36 20.05

15.46 15.58 16.16 16.06 15.83 16.39 15.71 16.78

47.32 48.94 49.57 48.94 48.27 49.41 49.54 51.33

0.05 0.07 0.08 0.12 0.20 0.39 0.52 0.81

14.84 12.00 11.77 11.14 12.30 12.89 12.87 11.03

noting that vibrational spectra of spinel-type materials show a presence of bands that are forbidden by selection rules very often [30–35]. They usually originate from the disorder in the structure including a partial inversion [36]. In the studied here case the presence of additional bands is associated with the partial substitution of Li+/Ti4+ ions sharing the same 16d positions. The broadening of IR bands with increasing con­ centration of Eu3+ ions results from the even higher substitutional dis­ order (and/or created oxygen vacancies) induced by the presence of third metal ion with different valence. The contents of Eu3+ ions above 2.5 mol.% (sample LTO22-PT-A) induce appearing of a new band at 140 cm− 1 and about 382 cm− 1. Our XRD data showed that the increasing concentration of Eu3+ ions leads to the formation of other phases, i.e. Eu2O3, TiO2 oxides and Eu2Ti2O7 pyrochlore. The literature data strongly suggests that these bands should be assigned to pyrochlore [37] and rutile [38] phases, respectively. Raman spectra of studied samples are presented in Fig. 3. In general, the spectrum of the sample with the lowest concentration of Eu3+ ions is very similar to the literature data and shows bands at 750, 674, 516, 429, 345, 298, 265, 235, 143, and 101 cm− 1. The band at 674 cm− 1 was previously assigned to A1g stretching vibration of the TiO6 units and bands at 429 (Eg) and 345 (F2g), 265 (F2g) and 235 (F2g) cm− 1 to bending vibrations of the TiO6 octahedra with the contribution originating from the Li-O vibrations for band at 345 cm-1 [31]. Thus, the presence of bands at 750, 608 (visible only for the highest concentration of Eu3+ ions) 516, 298, 143 and 101 cm− 1 has different origin than predicted by the selection rules. The majority of them (750, 608, 525 and 101 cm− 1) was previously observed for pure Li4/3Ti5/3O4 and assigned to the dis­ order in the spinel structure. The literature Raman data of three minor phases show that their strongest bands are expected at 140 and 610 cm− 1 for rutile [39], at 336 and 459 cm− 1 for Eu2O3 [40] and at about 300 and 519 cm− 1 for Eu2Ti2O7 [41]. As one can see, a small increase in intensity observed at 298 cm− 1 and at 515 cm− 1 is associated with the increasing concentration of pyrochlore phase. The presence of narrow bands around 143 cm− 1 is due to the TiO2 impurity, however, their intensity cannot be correlated with the concentration of Eu3+ ions since Raman spectra were measured using a confocal microscope. Our spectra do not show the presence of Eu2O3 phase.

Fig. 3. Raman spectra of the Li4/3Ti5/3O4 as a function of Eu3+ ions molar concentration.

no significant differences in crystal lattice parameters of the Li4Ti5O12 phase as a function of Eu concentration. The determined crystal lattice parameters and the unit cell volumes of identified LTO phase are consistent with the standard values (a0 = 8.359 Å, V0 = 584.03 Å3) of Li4Ti5O12, ICDD-49− 0207 PDF card. The results suggest that the euro­ pium ions do not incorporate into the LTO structure, but form separate oxide phases. On the other hand, a presence of a small amount of Eu3+ ions in the LTO cannot be unambiguously excluded. The average sizes of LTO crystallites, estimated with Rietveld method, are in a narrow range from 61 to 65 nm. 3.2. Raman and IR studies X-ray diffraction studies showed that the Li4/3Ti5/3O4 spinel crys­ tallizes in a Fd3m (O7h ) space group with 8 (2) formula units per unit (primitive) cell. Li+ ions occupy the 8a (Td ) special positions but Li2+ ions together with Ti4+ ions statistically share the 16d (D3d ) special positions with site occupancy factors equal to 0.1667 and 0.8333, respectively [30]. The oxygen ions are located in the 32e (C3v ) general positions. The factor group analysis, therefore, predicts A1g+2A2u+2Eu+Eg+2F2u+3F2g+5F1u+F1g modes that can be sub­ divided into A1g+2A2u+2Eu+Eg+2F2u+3F2g+4F1u+F1g optical phonons and one triply degenerated acoustic mode F1u. The A1g, Eg and F2g modes are Raman-active and the F1u modes are IR-active, therefore, the selec­ tion rules predict that Raman and IR spectra should have only 5 (A1g+Eg+3F2g) and 4 (4F1u) bands, respectively. Fig. 2 presents mid- and far-IR spectra measured for the Li4/3Ti5/3O4 spinel doped with Eu3+ ions in a broad concentration range from 0.25 mol.% (LTO18-PT-A) to 12.5 mol.% (LTO25-PT-A). The spectrum of the Li4/3Ti5/3O4:0.25 mol.% Eu3+ (LTO18-PT-A) sample shows four strong IR bands located at 663, 468, 344, and 231 cm− 1. The positions of bands in the mid-IR region correlate well with previously reported data showing the presence of IR bands in the 641–650 and 457− 488 cm− 1 assigned to stretching vibrations of metal-oxygen bonds in the octahe­ dral site [31–33]. The two lowest bands at 344 and 231 cm− 1 correspond to translational motions in octahedral and tetrahedral sites, respectively [31,34,35]. The IR spectra of the Li4/3Ti5/3O4:0.25 mol.% Eu3+ (LTO18-PT-A) sample shows additional weaker bands at 596, 516, 300, and 267 cm− 1. Their intensity does not change with the increasing concentration of Eu3+ ions. Therefore, its origin cannot correlate with the presence of concentration-dependent amount of impurities found from X-ray diffraction data, i.e. Eu2O3, TiO2, and Eu2Ti2O7. It is worth

3.3. XPS analysis The XPS analysis revealed the presence of Li, Ti, O, Eu and C in Eudoped LTO powders with the calculated atomic composition listed in Table 1. An increase in the Eu concentration with increasing dopant addition is observed. The presence of C can be assigned to adventitious carbon, a thin layer of carbonaceous material usually formed on the surface of air exposed samples. Fig. 4 shows the high resolution Li 1s, Ti 2p, O 1s and Eu 4d XPS spectra for LTO24-PT-A sample. Li 1s core level XPS spectrum (Fig. 4a) in the low binding energy region revealed a Li 1 s peak at 54.2 eV originating from the Li-O bond in the LTO spinel structure. The Ti 2p core level XPS spectrum (Fig. 4b) consists of a clear set of doublet peaks (Ti 2p1/2 and 2p3/2 lines at approximately 464.1 eV and 458.5 eV, respectively) corresponding to the Ti4+ state [42,43]. The asymmetric O 1s peak (Fig. 4c) displays two dominant components corresponding to 4

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Fig. 4. High resolution XPS spectra for LTO24-PT-A: Li 1s (a), Ti 2p (b), O 1s (c) and Eu 4d (d).

Fig. 5. Absorption spectra of the Eu-doped LTO nanocrystalline powders.

Fig. 6. Eu3+ absorption in the Eu-doped LTO nanocrystalline powders in the visible range.

metal-oxide bonding (e.g. Ti-O bonding of the LTO spinel) at 529.9 eV – O (carbonyl and carboxyl groups) at 532.0 eV originating from and C– adventitious carbon layer, respectively [42,43]. The high resolution XPS Eu 4d spectrum (Fig. 4d) demonstrates a set of doublet peaks: Eu 4d3/2

and 4d5/2 with binding energies at 141.2 eV and 135.6 eV, respectively, corresponding to Eu3+ oxidation state [44–46]. Although the Eu 4d signal is less intense than the Eu 3d one, in this study the Eu 4d peak was 5

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Fig. 9. The luminescent decay profiles of the Eu-doped LTO nanocrystal­ line powders.

Fig. 7. Luminescence spectra of the Eu-doped LTO nanocrystalline powders.

highest emission intensity was noted for the LTO24-PT-A. What is worth noticing, the highest intensity was observed for the 5D0→7F1 transition. The most common emission exhibited by the Eu-doped materials is connected to the hypersensitive 5D0→7F2 transition of the electric dipole. This phenomenon may be explained by the fact of high cubic symmetry of investigated compound. According to the selection rules, the electric dipole transitions are forbidden for the centrosymmetric environment. These transitions can occur, e.g., due to the crystal-field splitting, however, their intensity is low. In turn, the 5D0→7F1 transi­ tion is a magnetic dipole (MD) transition, which is nearly independent of the environment of the Eu3+ ions, and thus its intensity can be higher in the investigated case. The observed emission of the Eu3+ ions is different than results reported in [28], where the Eu3+ ions were located in the low symmetry sites without the inversion center. The excitation spectrum is presented in Fig. 8. The broad excitation band in the UV region originates from the Eu3+ → O2− charge transfer (CT) transition. The charge transfer band depends on the crystal field and is unique for each crystal host. In this case, this band is observed in the range of about 250,300 nm, with the maximum at 279 nm. The charge transfer band is one of the most intensive, however, the 4f-4f transitions possess also strong intensity, especially 7F0 → 5D2 Fig. 8. The excitation spectrum of the LTO22-PT-A sample.

used for qualitative and quantitative analysis. Since the Eu 3d5/2 peak is strongly overlapped by Ti LMM Auger line, extracting the Eu component may be not reliable (see Supplementary, Fig. S1). Similar results were obtained for all the samples under the study. 3.4. Absorption and luminescence, UV–vis DRS analysis The intensive absorption bands in the UV region appear are due to the host crystal absorption (Figs. 5,6). The Eu3+ absorption peaks, registered in the visible area between 450 and 600 nm, have been assigned to the 5D2, 5D1 and 5D0 energy levels of the Eu3+ ions (see Fig. 6). The intensity of the absorption peaks was growing with the increasing amount of the Eu3+ ions. The highest emission intensity was observed for the highest concentration of Eu3+, while for the smallest concentrations of Eu3+ ions, absorption bands were not observed. The luminescence spectra of the Eu-doped LTO nanocrystalline powders are presented in Fig. 7. The emission occurs from the 5D0 to the 7 F0, 7F1, 7F2 and 7F3 energy levels of the Eu3+ ions. With the increase in the concentration of europium, the intensity of luminescence increased to a certain point, and then it decreased. The

Fig. 10. The emission lifetime values for each Eu-doped LTO sample. 6

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Fig. 11. The UV–vis DRS spectra (a) and the evaluation of band gap energies (b) of all synthesized of LTO doped with Eu nanocrystalline powders.

transition with the maximum at 465 nm, which has even higher intensity than the CT band (see Fig. 8). This feature makes the investigated samples suitable for application in White Light Emitting Diodes (WLEDs) systems, because strong 4f-4f blue absorption in these materials is very close to the blue light of the InGaN LED, which together with the YAG: Ce3+ phosphors are commonly used for white LED lighting. Emission decay profiles from the 5D0 multiplet of the Eu3+ ions are presented in Fig. 9. In order to perform the measurement powders were placed in small quartz tubes. Luminescence decay curves seem to have a twodimensional character, but according to the measurements made sepa­ rately for the quartz tube itself, the first, short decay curve comes from the light scattered from the laser on the quartz tube. In the case of samples that showed higher emission intensity, the short component of

the decay curve was lower in relation to the decay curve derived from europium ion. The lifetime (t) values of the most intensive emission peak, were determined using one exponential fitting according to the equation: y = y0 + A*exp(-(x-x0)/t)

(1)

The luminescence decay times were monitored for the peak intensity at 589.5 nm, which originated from the magnetic dipole transition 5D0 → 7F1. The determined values were presented in the form of a graph in Fig. 10. The results obtained are very similar to each other and are within the range of 1.55–1.75 ms. The slight tendency to shorten the lifetime of luminescence is evident with increasing concentration of dopant ions (due to the concentration quenching). However, the dif­ ference between the longest and the shortest result obtained is about 10

Fig. 12. SEM images of the Eu-doped LTO powders. 7

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Fig. 13. TEM and HRTEM images with DDP obtained for LTO18-PT-A and LTO25-PT-A samples.

%. Thus, it can be concluded that the observed luminescence decay times are practically independent of the concentration of the europium content. Fig. 11 a, b shows DRS and Tauc plot curves of all synthesized Eudoped LTO nanocrystalline powders. Fig. S2 (a–h) presents Tauc’s plots for all analyzed from LTO18-PT-A to LTO25-PT-A samples. Table S1 summarizes indirect band gap energy values (Kubelka-Munk func­ tion, Tauc’s spectra) derived from DRS spectra. All the measured sam­ ples exhibit two changes from the DRS spectra at about 300 and 375 nm, which is consistent with the results given by [12,47,48].The band gap of about 3.75 eV is a typical value expected for Li4Ti5O12 [12]. TiO2 has indirect Eg value of 2.97 eV [47] and was estimated only for the sample LTO18-PT-A, Eu2Ti2O7 has Eg of 2.4 eV [48].

differences in the width of the lattice fringes with the increasing con­ centration of europium ions in the LTO matrix were observed. It could be due to very low level of doping (in the range 0.05 – 0.81 at.%) as shown in Table 1. 3.6. AFM studies Obtained morphological (AFM (Fig. 14)) data provided information coherent with SEM investigation results. One can notice the presence of agglomerates, and these consist of complex shape grains of sizes in the range from 200 nm to 1000 nm. As the edges of grains are clearly defined, the steep features with certain angles at the corners, the internal structural ordering can be concluded. In order to obtain better visibility of fine structures, Sobel transform was successfully applied as in previ­ ous works [50–52]. Examples of selected images reveal morphological details showing a cylindrical shape of the particles with specific diam­ eter reduction in the middle of the particle. No significant differences in the average size or any particular properties of the samples can be found. It should be emphasized, that due to limitations of complex structures imaging caused by the scanning tip shape, some morphological infor­ mation can be lost or distorted. Therefore, careful image analysis is necessary to draw the conclusions. One should be aware, that the abovementioned limitation is common for powder materials.

3.5. SEM and TEM results Fig. 12 shows the micrographs of obtained Eu-doped LTO samples recorded using a scanning electron microscope. One can observe that doping LTO material with Eu3+ ions does not affect powder morphology. In all cases, grains are spherical and tend to agglomerate. Additionally, the HRTEM images were obtained for samples (LTO18PT-A and LTO25-PT-A) with two extreme concentrations of the Eu3+ions in the LTO matrix. A careful analysis of the high resolution images shows that the most common visible fringes could be assigned to (111) Li4Ti5O12 (d =0.48 nm [49]). As shown in Fig. 13, the d values, calcu­ lated from DDP (digital diffraction pattern) images, for LTO-18-PT-A and LTO-25-PT-A samples were very close to each other. No

4. Conclusions In this work we demonstrated two-stage solid state synthesis method 8

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Materials Research Bulletin 134 (2021) 111084

Fig. 14. AFM images of the Eu-doped LTO nanocrystalline powders.

of Eu-doped LTO powders using high-energy ball-milling followed by high temperature annealing in air. The use of various investigation methods allowed us to determine structural and physicochemical properties of fabricated LTO with different concentrations of Eu3+ ma­ terials. Qualitative and quantitative phase analysis of synthesized Eudoped LTO powders showed that Eu ions are present as separate oxide phases, such as Eu2Ti2O7 and Eu2O3, and probably in a small amount are doped in the structure of the LTO spinel. The observed emission of the Eu3+ ions is different than results reported in [28], where the Eu3+ ions were located in the low symmetry sites without the inversion center. Phonon properties and assignment of observed bands at Raman and IR spectra were presented. These results allowed to confirm the presence of impurities and the substitutional disorder of metal ions in the studied spinels. The correlation between SEM and AFM techniques was showed. The grains were in the range of 200–1000 nm and tend to agglomerate. Additionally, doping the LTO with Eu3+ ions does not affect powder morphology significantly. The intensity of the absorption peaks was growing, with increasing amount of the doped Eu3+ ions. The highest emission intensity was observed for the highest concentration of Eu3+, while for the lowest concentrations of Eu3+ ions, their absorption was not observed. The Eu3+ ions are located in a high symmetry site with the inversion center. The highest emission intensity at 589.5 nm corresponding to 5D0→7F1 magnetic dipole transition was observed for LTO24-PT-A sample. The Eu3+ → O2− charge transfer (CT) transition is one of the most intensive band and was observed in the range of about 250,300 nm (UV region), with the maximum at 279 nm. The 4f-4f transitions possess also strong intensity, especially 7F0 → 5D2 transition with the maximum at 465 nm, which is close to the InGaN/GaN emission. The results reported in this paper demonstrates that LTO doped with rare earth ions like europium may be used as a luminescent material in optoelectronic de­ vices such as white light emitting diodes.

Investigation, Writing - original draft, Writing - review & editing, Re­ sources, Data curation, Visualization, Supervision, Project administra­ ´ ski: Writing - original draft, Writing - review & editing, tion. K. Leman Investigation, Formal analysis, Conceptualization. M. Ptak: Writing original draft, Investigation, Formal analysis. A. Roguska: Writing original draft, Investigation, Formal analysis. O. Chernyayeva: Inves­ ˙ tigation, Formal analysis. P. Zurek: Investigation, Formal analysis. A. Sikora: Writing - original draft, Writing - review & editing, Investiga­ tion, Formal analysis. P. Gołębiewski: Writing - original draft, Writing review & editing, Investigation, Formal analysis. A. Szysiak: Formal analysis. A. Malinowska: Writing - review & editing, Investigation, Formal analysis. M. Małecka: Writing - review & editing, Investigation, Formal analysis. Declaration of Competing Interest 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. Acknowledgements This work was financially supported by The National Centre for Research and Development (NCBR) Poland through the research project cooperation between National Centre for Research and Development (NCBR) and the Ministry of Science and Technology of Taiwan (MOST). (contract no. PL-TW/IV/6/2017). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.materresbull.2020 .111084.

CRediT authorship contribution statement M. Michalska: Conceptualization, Methodology, Formal analysis, 9

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Materials Research Bulletin 134 (2021) 111084

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