Morphology and doping concentration effect on the luminescence properties of SnO2:Eu3+ nanoparticles

Journal Pre-proof Morphology and doping concentration effect on the luminescence properties of 3+ SnO2:Eu nanoparticles I.E. Kolesnikov, D.S. Kolokolov, M.A. Kurochkin, M.A. Voznesenskiy, M.G. Osmolowsky, E. Lähderanta, O.M. Osmolovskaya PII:

S0925-8388(20)30003-7

DOI:

https://doi.org/10.1016/j.jallcom.2020.153640

Reference:

JALCOM 153640

To appear in:

Journal of Alloys and Compounds

Received Date: 23 October 2019 Revised Date:

30 December 2019

Accepted Date: 1 January 2020

Please cite this article as: I.E. Kolesnikov, D.S. Kolokolov, M.A. Kurochkin, M.A. Voznesenskiy, M.G. Osmolowsky, E. Lähderanta, O.M. Osmolovskaya, Morphology and doping concentration effect on the 3+ luminescence properties of SnO2:Eu nanoparticles, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153640. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Credit Author Statement I.E. Kolesnikov — Conceptualization, Investigation, Writing - original draft, Writing - review & editing D.S. Kolokolov — Investigation, Data curation M.A. Kurochkin — Investigation, Visualization M.A. Voznesenskiy — Software, Visualization M.G. Osmolowsky — Formal analysis, Validation E. Lähderanta — Supervision, Writing - review & editing O.M. Osmolovskaya — Supervision, Writing - review & editing

Morphology and doping concentration effect on the luminescence properties of SnO2:Eu3+ nanoparticles I.E. Kolesnikov*,a,b, D.S. Kolokolova, M.A. Kurochkina, M.A. Voznesenskiya, M.G. Osmolowskya, E. Lähderantab, O.M. Osmolovskayaa a b

St. Petersburg State University, St. Petersburg 199034, Russia LUT University, Lappeenranta 53850, Finland

Contact information e-mail address: [email protected] (I. Kolesnikov) postal address: Ulianovskaya, 5, St.Petersburg, 198504, Russia

Abstract Morphology and Eu3+ doping effect on structural and photoluminescence properties of tin dioxide nanoparticles obtained by co-precipitation and hydrothermal methods are reported and analyzed for the first time. The samples were characterized by means of transmission electron microscopy (TEM), powder X-ray diffraction (XRD), specific surface area (SSA) estimation. TEM, XRD and SSA analyses showed that in the case of co-precipitation method the nanoparticles were spherical. Hydrothermal treatment leads to formation of cubic nanoparticles. An average particle size increased from 3 to 5 nm and from 6 to 11 nm along with increase of Eu3+ concentration for spherical and cubic nanoparticles, respectively. Steady-state and kinetics photoluminescence properties of nanophophors with different morphology were studied and compared. Radiative and nonradiative decay rates and Judd-Ofelt parameters were calculated using the model of 4f–4f intensity theory. It was shown, that preferred positions of substitution in SnO2 host differ depending on Eu3+ doping concentration and particle morphology. Keywords: Eu3+, Luminescence, Lifetime, Concentration quenching, DFT calculations 1. Introduction Tin oxide (SnO2), an n-type semiconductor with a wide band gap (Eg = 3.6 eV at 300 K), have a wide range of applications including the transparent conducting media, gas sensors and photocatalysts. [1–4] In recent years, considerable efforts have been focused on the synthesis of SnO2 nanoparticles (NPs) with different size and shape and the study of their novel properties. [3,4] To obtain spherical NPs the fast and simple precipitation method is used, and the size variation is achieved by calcination at different temperature. [1–4] Other particle shapes, such as nanorods, can be synthesized under hydrothermal conditions. [5,6] So, depending on synthesis conditions SnO2 NPs with different size and shape can be prepared. Additional benefit of tin dioxide is the rutile crystal structure with D4h (or P42/mnm) symmetry, which is highly tolerant to the substitution.[7] This is why SnO2 can be regarded as a promising host material. This statement is confirmed by the large number of works dedicated to the doping of tin dioxide by 3d [7], and 4f [8–10] elements. The luminescence properties of rare earth doped SnO2 have been the subject of numerous investigations.[11–13] A significant number of works have been dedicated to the synthesis and 1

photoluminescence of SnO2:Eu3+ NPs that exhibit orange-red emission.[14–16] Using the methods described above nanospheres, nanorods and nanofibers of SnO2:Eu3+ can be obtained. [17] Experimental works available in the literature focused in general on the impact of annealing temperature of the SnO2:Eu3+ particle obtained by different synthetic procedure on luminescence properties.[13,18,19] We are deeply convinced that the distribution of doping ions is significantly affected by the synthesis conditions, and sintering process can lead to the formation of dopant enrichment and depletion regions. However, such an important parameter as concentration quenching is practically not investigated for SnO2:Eu3+ phosphors, only a limited number of papers was found.[20,21] In addition, one knows that particle size and shape affect the luminescence properties. [22–25] To the best of our knowledge, there is no such work for SnO2:Eu3+ particles. So, the detailed consideration of the morphology and doping concentration impact on photoluminescence performance is still missing. It is well known that luminescence properties are highly dependent on doping ions positions in a host crystal lattice.[26–28] So, the determination of preferred Eu3+ site locations in SnO2 becomes extremely important. In accordance to [20], Eu3+ ions occupy the sites of Sn4+ ions in the SnO2 crystal. In the case of bulk material, the position of substitution could be determined based on crystallographic data using Rietveld method.[20,29] For nanoobject, especially below 10 nm size, the applicability of this method is limited to definition of cell parameters. In the last decade, computational studies of tin dioxide have been actively developed. [29,30] Firstprinciples electronic structure calculations on SnO2 have been carried out with different methods, most of them in the framework of density functional theory (DFT).[31] DFT has proven to be accurate for the description of structural properties of SnO2. Only few computational studies on the structural and optical properties of SnO2:Eu3+ have been reported. [30,32] Moreover, the influence of structural properties on luminescence properties has not been discussed yet. To overcome the above mentioned drawbacks, we have carried out a systematic experimental and theoretical study of structural and photoluminescence properties of SnO2:Eu3+ NPs series with two morphological types. 2. Experimental Materials Tin (IV) chloride (SnCl4, 98%), Eu2O3 (98%) and NH4OH (25%) were purchased from Vekton (Russia), and used as raw materials for SnO2:Eu3+ synthesis. All the reagents were commercially available, analytical grade and used as received without further purifications. Preparation of SnO2:Eu3+ NPs with different morphology SnO2:Eu3+ NPs were prepared using co-precipitation method (1st series) and hydrothermal treatment of freshly obtained suspension (2nd series) as presented in Figure 1.

2

Figure 1. Schematic illustration of synthesis of SnO2:Eu3+ NPs with different morphology. In both cases, first 0.5 g of Eu2O3 was dissolved in 5 mL of concentrated nitric acid at 50°C into 25 cm3 volumetric flask. The obtained solution was filled to mark by distilled water and used for further synthesis. Typical synthesis procedure in the case of co-precipitation method is the following. 1.5 mL of 1 M SnCl4, the required amount of Eu ions solution corresponding to 1– 25 at.% in relation to the Sn content, and 6 M NH4OH were slowly dropwise added into 50 ml of deionized water inside a round-bottom flask under vigorous stirring. The reaction mixture was kept 15 minutes at pH 3. In order to induce the morphology transformation, the suspension of 5, 15 and 25 at.% Eu3+ doped samples were transferred into a 200 mL Teflon lined state of the art autoclave, sealed and heated at 260°C for 5 h. For all the samples, the resulting phosphor powders were separated from the solution by centrifugation using Sigma 2-16P and washed on a Vibramax shaker several times with distilled water and freeze dried. The concentration of Eu(III) cations in supernatant and washing solutions are quantified by means of Arsenazo III reagent in accordance with [33]. At first the solutions were evaporated up to 25 mL, that the appropriate amount of the samples was placed into 25 cm3 volumetric flask, 1 cm3 of Arsenazo III 0.1 wt.% solution was added; then deionized water was filled to mark and the solution was mixed thoroughly. The solutions were kept during 10 minutes and then the absorbance was measured against the reagent blank at the wavelength of 651 nm in a 10 mm cuvette. The Eu3+ quantity was obtained from a calibration curve, which has been prepared with known amounts (0-20 µg per mL) of europium. The synthesized samples (5 mg) and potassium bromide (300 mg) were pressed into pellets (diameter 13 mm) to carry out photoluminescence studies. Characterization XRD patterns were obtained using a D2 Phaser diffractometer (Bruker, Germany), with the following conditions: Cu-Kα radiation (λ = 0.15406 nm) to 30 kV and 10 mA, steps of 0.02°. The patterns were recorded within the range between 10 and 100º. The refinement of the lattice parameters was performed by the Rietveld method using Topas 4.2 software (Bruker AXS, Germany). The average crystallites sizes were evaluated from the XRD results according to Scherrer formula [1–4]: (1) where λ is the X-ray wavelength of 1.5406 Å, θ is the Bragg's angle and β is the full width at half maximum (FWHM). In accordance to refined lattices parameters, the cell distortion were 3

calculated ∆с



using

с

с с

the

following

equations:

∆





∗ 100%

and

∗ 100%. The nanoparticles sizes and shapes were determined by

transmission electron microscopy (TEM) using a JEM 107 microscope (JEOL, Japan). Specific surface area (SSA) estimation was performed by the Brunauer-Emmett-Teller method on a surface area analyzer Micromeritics ASAP-2020MP with accuracy of 4%. Steady-state luminescence spectra were recorded with a modular fluorescence spectrometer Fluorolog-3 equipped with a Xe-arc lamp (450 W power). Luminescence decay curves were obtained using Xe-flash lamp (150 W power, 3 µs pulse width) as an excitation source. Computational procedure The dopant positions in SnO2:Eu3+ structure are computed within density functional theory (DFT), using pseudopotentials and a plane wave basis in ABINIT software package.[34] The conducted structural optimization was performed using the Broyden-Fletcher-Goldfarb-Shanno minimization algorithm. Cell parameters were fixed and taken equal to those obtained from crystallography data. In order to study the effects of Eu3+ dopant in the SnO2 crystals, the 6-atom primitive unit cell was expanded eight times (2 × 2 × 2 extension), which resulted in 48-atom supercell. Europium doping was done by replacing one, two or four of the supercell Sn atoms by the Eu. In case of two and four Eu atoms per supercell their positions were chosen based on uniformity. 1, 3 and 5 positions were considered. Drawing were obtained in Vesta software package.[31] Also, Vesta was used to generate coordinates of the atoms in supercell. 3. Results and discussion Structure and morphology of SnO2:Eu3+ NPs In order to study the impact of morphology on luminescence properties, two concentration series of SnO2:Eu3+ NPs with different shape were prepared by using two common wet chemistry methods. The samples of the 1st series were synthesized with wide Eu3+ doping concentration range (1, 2, 5, 7, 10, 15, 25, 50 at.%) at room temperature by co-precipitation method to provide an uniform distribution of dopant in crystal lattice and obtain the spherical NPs. To get the NPs with the other morphology (2nd series) freshly prepared suspensions of 5, 15 and 25 at.% SnO2:Eu3+ were held at the sealed autoclave (e.g. hydrothermal treatment) to initiate the particle growth and shape change. For all the samples the spectrophotometric determination of Eu3+ ions in supernatant and washing solutions were conducted (see Experimental section for details). In any of the cases Eu3+ ions were not detected, which indicated that all ions are incorporated into the structure of resulting product. Figure 2 shows the TEM image and size distribution of as-prepared SnO2:Eu3+ samples with different morphology. The average diameters of NPs are presented in Table 1.

4

Figure 2. TEM images of as-prepared SnO2:Eu3+ NPs: (a) spherical 5 at.%, (b) spherical 10 at.%, (c) spherical 25 at.%, (d-f) size distribution spherical, (g) cubic 5 at.%, (h) cubic 15 at.%, (i) cubic 25 at.%, (j-l) size distribution of cubic. All the samples obtained by co-precipitation method consist of uniform spherical shape NPs with average size up to 5 nm. The hydrothermal treatment leads to the formation of cubic NPs with narrow size distribution. The obtained results are in a good agreement with SSA values (Table 1). Eu3+-doped SnO2 nanoparticles with similar size were earlier synthesized via microwave [14], chemical co-precipitation process with the assistance of CTAB [20], and simple hydrothermal [35] techniques. The synthesized concentration series were characterized using XRD (Figure3 and Figure S1, S2). All the diffraction peaks can be indexed to tetragonal SnO2 (ISDD 00-041-1445), no additional 5

phases were found. It should be noted that, in general, no significant change of the diffraction peaks intensity for spherical NPs with the increase of doping concentration, indicating a minimal lattice distortion. Due to the hydrothermal treatment, the relative intensity of cubic NPs diffraction peaks is gradually increased compared with spherical ones.

Figure 3. XRD patterns of SnO2:Eu3+ NPs with different doping concentration and morphology: (a) spherical, (b) cubic. The Rietveld refinement yields lattice parameters, which are listed in Table 1 along with the errors (in brackets). Table 1. Morphological parameters of as-prepared spherical (S) and cubic (C) SnO2:Eu3+ samples: crystallite size (dXRD), cell distortion (∆a, ∆c), TEM size (dTEM), specific surface area (SSA). C(Eu3+) (at.%)

dXRD (nm)

Refined lattice parameters a(Å)

c(Å)

∆a*, %

∆c*, %

dTEM (nm)

SSA (m2·g-1)

0

2.282(73)

4.7750(20)

3.1860(16)

0

0

3.0±0.1

270

5S

2.331(90)

4.7583(17)

3.1924(15)

-0.35

0.20

3.3±0.2

215

15S

2.024(94)

4.7662(59)

3.2164(51)

-0.18

0.95

4.6±0.1

230

25S

1.00(15)

4.7656(57)

3.2220(39)

-0.20

1.13

4.8±0.4

215

5C

4.065(65)

4.7427(12)

3.18699(96)

-0.68

0.03

6.6±0.3

150

15C

5.752(62)

4.7382(71)

3.18484(55)

-0.77

-0.04

7.9±0.2

110

25C

6.399(63)

4.7392(62)

3.18605(48)

-0.75

0.00

10.9±0.4

95

For spherical SnO2:Eu3+ NPs (first series, co-precipitation method) the unit cell expanded along c axis and diminished along a axis due to the replacement of the smaller Sn4+ (rSn4+ = 0.083 nm) by 6

larger Eu3+ (rEu3+ = 0.095 nm). In addition, compared with the undoped SnO2, the crystallite size of Eu3+-doped SnO2 NPs decreased along with increase of doping concentration, suggesting that the incorporation of Eu3+ ions suppresses the growth of SnO2 nanocrystals due to the formation of cationic defects. According to TEM and SSA results, the particles sizes slightly increased with Eu3+ doping concentration growth. So, the particle becomes more polydisperse at the higher doping concentrations. For cubic SnO2:Eu3+ NPs c parameter almost does not change, whereas the cell diminishes along a axis. Thus, the changes in the atoms positions are less pronounced than in the case of spherical nanoparticles. Regarding the crystallite sizes, the opposite trend compared with spherical NPs is observed. The dXRD values as well as the TEM size grow monotonically with Eu3+ doping concentration increasing. The observed difference between spherical and cubic NPs was probably due to the transition from mild (room temperature, 15 min) to harsh (260°C, 5 hours) synthetic conditions, which led to the formation of more crystalline and more defect-free structure close to undoped sample. Taking into account that pH 3 is near the zero charge point for tin dioxide [36], we can assume that under hydrothermal condition the spherical NPs are grouped and coalesced; these processes are accompanied by recrystallization, which is possible only under harsh conditions. Photoluminescence properties of spherical SnO2:Eu3+ NPs The excitation spectra of spherical SnO2:Eu3+ NPs concentration series in range from 260 to 590 nm are presented in Figure 4a. These spectra were monitored for the most prominent transition 5D0–7F2 at emission wavelength λem=615 nm. One can see that excitation spectrum consists of broad band in UV region below 350 nm and series of narrow characteristics lines from 295 to 550 nm. The broad UV band can be ascribed to SnO2 host absorption with subsequent energy transfer to Eu3+ ion. [37] Sharp peaks are attributed to the typical intraconfigurational transitions inside 4f shell from ground level 7F0 to excited levels 5FJ+5IJ (295 nm), 5L8 (316 nm), 5D4 (360 nm), 5L7 (376 and 380 nm), 5L6 (392 nm), 5D3 (413 nm), 5D2 (462 nm), 5D1 (524 and 532 nm) and 5D0 (580 nm). It should be noted that ratio between intensity of host band and intensity of any intra-configurational Eu3+ transition monotonically decreases along with increase of doping concentration. So, intensity of host band and 7F0–5L6 line is comparable for SnO2:Eu3+ 5 at.% NPs, whereas for higher doped samples the excitation spectrum is dominated by intra-configurational transition. Figure 4b shows emission spectra of spherical SnO2:Eu3+ NPs concentration series. These measurements were carried out in the spectral range 500–750 nm upon 392 nm excitation. The emission spectra consist of characteristic sharp lines attributed to the intraconfigurational f-f transitions. The luminescence intensity of such transitions strongly depends on the site symmetry of rare earth ions. It is well known that the Sn4+ ions in the SnO2 host have a D4h symmetry. Taking into account that Eu3+ ions substitute Sn4+ ions in the sites of SnO2, Eu3+ ions should have the same symmetry. [38] As there is not inversion symmetry at the Eu3+ lattice site, the forced electric-dipole 5D0–7F2 (615 nm) and 5D0–7F4 (699 nm) transitions are stronger than the magnetic dipole 5D0–7F1 (592 nm) one. Noteworthy, treatment temperature during synthesis strongly affects emission spectrum shape. It was found that the forced electric-dipole 5D0–7F2 transition prevails at synthesis temperatures up to 800 oC [14,20], whereas the magnetic dipole one, 5D0–7F1, is the most prominent for SnO2:Eu3+ samples treated at temperatures higher than 1000 oC [16,19,39]. The lower intensity bands situated at 536, 556 and 652 nm can be assigned to the 5D1–7F1, 5D1–7F2 and 5D0–7F3 transitions, respectively. 7

Figure 4. (a) Excitation and (b) emission spectra of spherical SnO2:Eu3+ NPs with different doping concentration. Figure 5 shows concentration dependence of the integrated luminescence intensity of the most intensive transition 5D0−7F2 upon 392 nm excitation. Increase of doping concentration leads to two competitive processes: on the one hand, to the increase of the amount of luminescence centers and, thus, radiative recombination. On the other hand, to the increase of the probability of energy transfer between europium ions, which enhances the efficiency of the nonradiative processes.[40] Typically, there is an optimal doping concentration in the host. After that adding extra dopant ions leads to the reduction of luminescence intensity, which is caused by concentration quenching effect. [41–44] However, in our case the luminescence intensity of spherical SnO2:Eu3+ NPs monotonically increased along with number of Eu3+ ions, and concentration quenching was not observed up to very high doping concentration (50 at.%). Such behavior can be explained by presence of high amount of doping ions on the surface.[45] It is well-known that surface to volume ratio in NPs has a significant effect on their properties. This ratio increases with the decrease of radius of spherical NPs, and, as a result, a greater portion of the atoms resides on the particle surface. According to TEM images, average size of synthesized spherical NPs is about 4 nm. Simple calculations confirm that 60% of atoms in such NPs are situated on the surface. Assuming the fact that surface and volume ions are substituted by europium ions with equal probability, about 60% of doping ions are located at the surface. When such high amount of dopant ions resides on the surface, the surface quenching of emission is considerably stronger than the concentration quenching. Therefore, one is able to observe only increase in emission intensity due to larger number of emission centers upon the increase in dopant concentration.[45]

8

Figure 5. Integral intensity of 5D0−7F2 transition of spherical SnO2:Eu3+ NPs as a function of doping concentration. Besides steady-state measurements, important information about luminescence properties of synthesized nanophosphors could be extracted from fluorescence kinetics. The luminescence decays of spherical SnO2:Eu3+ NPs concentration series were measured monitoring intensity of the most prominent forced electric dipole transition at λem = 615 nm upon λex = 392 nm excitation (Figure 6a). The experimental curves of all synthesized nanophosphors were fitted by single exponential function:

! ∙ (2) 5 where τf is the observed lifetime of D0 level. Figure 6b shows the observed lifetimes with error bars as a function of Eu3+ doping concentration. One can see that the obtained lifetimes decrease from 0.24 ms to 0.21 ms with increase of Eu3+ ions number.

Figure 6. (a) Decay curves of spherical SnO2:Eu3+ NPs (λex = 392 nm; λem = 615 nm), (b) 5D0 level lifetime of Eu3+ ions as a function of doping concentration. The lifetime of excited state is determined by set of radiative and nonradiative processes taking part in decay. Radiative decay rate is determined by dipole transition strength and local-field correction. Nonradiative processes contain multiphonon relaxation, quenching on impurities (e.g. OH– group) and cooperative processes (cross-relaxation, energy migration). These processes were discussed in detail in our earlier works [26,46]. It should be noted that metastable 5D0 level 9

of Eu3+ ions can be nonradiatively quenched via OH– groups or diffusion processes. Multiphonon relaxation has extremely low probability because of the energy gap (12000 cm-1) and the wavenumber of the most intense vibration of usual inorganic host (<1000 cm-1). To study the radiative and nonradiative decay rates of Eu3+-doped phosphors, 4f–4f intensity theory is usually applied. In this theory 5D0–7F1 transition is taken as an internal standard due to its magnetic dipole character (A0-1 = AMD,0·n03, where AMD,0 = 14.65 s-1 is the spontaneous emission probability for the 5D0–7F1 transition in vacuo).[47] Dealing with NPs, usual refractive index n0 should be substituted by effective refractive index neff, which contains information about surrounding medium refractive index nmed (in our case surrounding medium is potassium bromide, 1.56) and material refractive index (nSnO2 = 2.01) .[48,49] "#$$ % ∙ "&'() + (1 − %) ∙ ".#/ (3) where x is the “filling factor” showing what fraction of space is occupied by the nanoparticles. For effective refractive index calculation, we used filling factor of 0.23 taking into account size of synthesized SnO2 NPs.[48] The magnetic dipole 5D0–7F1 transition probability was determined to be 67 s-1. The radiative emission probabilities A0–λ (λ = 2, 4) can be obtained from following equation: 0

0

1

3 4 5 74 6 23 4 6 74 5

(4)

where ν0–λ and I0–λ are, respectively, the frequency and integral intensity of the corresponding 5 D0−7Fλ transition in the emission spectrum. The total radiative emission probability, Ar, is sum of all the A0–λ values (λ = 1, 2, 4). Using observed lifetime and calculated total radiative decay rate we can calculate nonradiative probability Anr as: 2

0'8

9!

− 08

(5)

and quantum efficiency: ;<

:

(6)

;< =; <

The Judd–Ofelt intensity parameters (Ω2 and Ω4) provide information about the luminescence behavior of Eu3+ ions. The radiative transitions probabilities are expressed as: 0

1



>?@A B C #

2

DE C

?@F4

G∑

J),? I

L< ON ||Q || TRS >L

)

(7)

where χ = neff(neff2+2)2/9 is Lorenz local field correction and neff = 1.66 is the effective index of refraction of the medium. The square reduced matrix LV ON LLQ ( ) LL TR) WL

)

0.0032 and

)

LV ON LLQ ( ) LL TR? WL 0.0023 were taken from [50]. The calculated values of radiative and nonradiative decay rates, quantum efficiencies, and Judd– Ofelt intensity parameters are listed in Table 2. Table 2. Radiative (Ar), nonradiative (Anr) and total (Atotal) decay rates of the 5D0 level, quantum efficiencies (η), and Judd–Ofelt intensity parameters (Ω2 and Ω4) as a function of Eu3+ doping concentration in spherical SnO2 host. C(Eu3+) (at.%) 1 2 5

Ar (s-1)

Anr (s-1)

Atotal (s-1)

η (%)

380 340 320

3750 4030 4180

4130 4370 4500

9 8 7

10

Ω2 (10-19 cm2) 3.8 3.4 3.0

Ω4 (10-19 cm2) 7.5 6.5 6.2

7 10 15 25 50

330 330 330 340 320

4170 4270 4370 4140 4390

4500 4600 4700 4480 4710

7 7 7 8 7

3.2 3.2 3.2 3.6 3.1

6.5 6.5 6.1 5.7 6.0

As can be seen from Table, the radiative decay rate almost did not change in concentration series. Therefore, we can conclude that Eu3+ introduction in spherical SnO2 host only slightly affects strength of the dipole transition and local-field correction. The nonradiative decay rate increased with doping concentration. The observed growth can be explained by increase of spatial energy migration with subsequent quenching on impurities. [51–53] Ω2 parameter reflects environment around the rare earth ion and covalency of RE–O bond, whereas Ω4 parameter is affected by electron density on the surrounded O2– ions. Both parameters demonstrated tendency to decline along with growth of doping concetration. Photoluminescence properties of cubic SnO2:Eu3+ NPs The excitation and emission spectra of cubic SnO2:Eu3+ NPs concentration series are shown in Figure 7. Spectral line positions of cubic SnO2:Eu3+ NPs in excitation spectra are similar to the spherical ones, however, the luminescence intensity of 4f-4f transitions became much lower comparing with host band. To compare emission intensities of spherical and cubic NPs with the same doping concentrations, we carried out photoluminescence measurements under the same conditions (Figure 7b). As can be seen, spherical NPs are much brighter than cubic ones. Moreover, in case of cubic NPs the optimum doping concentration was found to be 15 at.%. Further increase of Eu3+ amount led to the concentration quenching. Different emission dependence on Eu3+ doping concentration for spherical and cubic NPs is most probably explained by different size of NPs. According to TEM data (Figure 2), cubic NPs are twice bigger than spherical NPs. Therefore, cubic SnO2:Eu3+ NPs are less affected by the surface quenching of emission.

Figure 7. a) Excitation and b) emission spectra of cubic SnO2:Eu3+ NPs with different doping concentration. The luminescence decays of cubic SnO2:Eu3+ concentration series were measured monitoring intensity of the most prominent forced electric dipole transition at λem = 615 nm upon λex = 392 nm excitation (Figure 8a). Bi-exponential function (

11





+ ) ) was required to fit experimental decay curves of all cubic SnO2:Eu3+ NPs. The average lifetime was calculated from the obtained fitting parameters [54]. ) ) 2 [2 + ) [) [ \ (1) 2 [2 + ) [) where I1 and I2 are pre-exponential factors; τ1 and τ2 are lifetimes. The average lifetimes of cubic SnO2:Eu3+ NPs versus Eu3+ doping concentration are presented in Figure 8b. The radiative and nonradiative decay rates, quantum efficiencies, and Judd–Ofelt intensity parameters were calculated also for cubic SnO2:Eu3+ NPs. The obtained values are listed in Table 3. 2

5

Figure 8. (a) Decay curves of cubic SnO2:Eu3+ NPs (λex = 392 nm; λem = 615 nm), (b) 5D0 level lifetime of Eu3+ ions as a function of doping concentration. Table 3. Radiative (Ar), nonradiative (Anr) and total (Atotal) decay rates of the 5D0 level, quantum efficiencies (η), and Judd–Ofelt intensity parameters (Ω2 and Ω4) as a function of Eu3+ doping concentration in cubic SnO2 host. C(Eu3+) (at.%) 5 10 15

Ar (s-1)

Anr (s-1)

Atotal (s-1)

η (%)

320 340 290

2120 2720 2490

2440 3060 2780

13 11 10

Ω2 (10-19 cm2) 3.0 3.5 2.8

Ω4 (10-19 cm2) 5.2 5.1 4.3

Analyzing results from Table 2 and 3, we can see that radiative decay rate does not depend on morphology and size of synthesized NPs, whereas nonradiative decay rate is almost two times lower for cubic NPs. Ω2 parameters are comparable in spherical and cubic SnO2:Eu3+ NPs, while Ω4 parameters of cubic NPs are significantly lower. DFT calculations DFT calculations were carried out to investigate doping ions distribution in host lattice of SnO2 NPs with different morphology and, as a result, concentration quenching phenomenon from the quantum chemistry point of view. To determine the structure of as-prepared samples based on the experimental data of refined cell parameters, total energies were calculated for all reasonable doping atoms distributions in the crystal supercell. Preferred positions of europium atoms in crystal cell were obtained by the lowest value of total energy. As we can see from Figure 9, the spherical and cubic samples containing 5 and 15 at.% of Eu3+ ions demonstrated the same position for substitution. Thus, the hydrothermal treatment 12

leads to the enlargement of particle and crystallite sizes but does not affect the Eu3+ ions location. At higher Eu3+ concentration preferred positions for substitution differ for spherical and cubic NPs. As one can see from the Figure, Eu3+ ions form planes, that alternate with Sn-planes. For spheres the Eu-planes are parallel to 001 planes, while for cubes they are parallel to 100 plane. The distance between Eu-planes is 2c for spheres that is smaller than 2a for cubes, but the distance between the nearest Eu3+ ions for cubes is equal to c, which is smaller than a, the distance between the nearest Eu3+ ions for spheres.

Figure 9. Position of Eu3+ substitution in SnO2 lattice at different Eu3+ doping concentration and morphology (gray are tin atoms, red are oxygen atoms, light-violet are europium atoms), obtained by calculation in ABINIT software package[34], drawings of crystal supercells are made in Vesta[31]. 4. Conclusion In summary, we reported on the first demonstration of both experimental and theoretical approaches to study of Eu3+ concentration quenching effect in SnO2 crystal host. The spherical and cubic Eu3+-doped SnO2 NPs were successfully synthesized using coprecipitation and hydrothermal methods, respectively. XRD study defined that both spherical and cubic NPs had pure tetragonal phase without any impurities. TEM and SSA results displayed that increase of Eu3+ doping concentration leads to growth of synthesized SnO2:Eu3+ NPs size irrespective to morphology: from 3.0 to 4.8 nm (spherical NPs) and from 6.6 to 10.9 nm (cubic NPs). Emission and excitation spectra consisted of characteristic narrow bands corresponding to the intra-configurational 4f-4f transitions inside Eu3+ ions. The most intensive luminescence line of SnO2:Eu3+ NPs attributed to the forced electric dipole transition 5D0−7F2 transition. Luminescence kinetics measurements showed 5D0 lifetime decrease with Eu3+ doping concentration growth. Radiative and nonradiative decay rates as well as Judd-Ofelt parameters were obtained using the 4f–4f intensity theory for SnO2:Eu3+ NPs with different doping concentration and morphology. Radiative decay rate did not depend on morphology, whereas nonradiative decay rate was almost two times lower for cubic NPs. Evidence of luminescence concentration quenching of spherical SnO2:Eu3+ NPs was not observed up to 50 at.% substitution, whereas cubic NPs starts to be quenched from 15 at.%. This difference was 13

explained in terms of preferred doping positions for substitution, which were computed by DFT method based on the experimental crystallography data for SnO2:Eu3+ concentration series. It was shown that in case of spherical NPs, Eu3+ ions occupy positions providing maximum distance between dopants, while for highly doped cubic NPs the Eu3+ ions are placed close to each other. Supporting information The Supporting Information is available free of charge at DOI: Structural parameters of spherical Eu3+-doped SnO2 NPs, XRD pattern of undoped SnO2, and XRD patterns of spherical SnO2:Eu3+ with different doping concentration. Acknowledgments Experimental studies were performed using the equipment of the Research Park of St. Petersburg State University (Centre for X-ray Diffraction Studies, Chemical Analysis and Materials Research Centre, Centre for Innovative Technologies of Composite Nanomaterials, Centre for Optical and Laser Materials Research). References [1] H. Köse, Ş. Karaal, A.O. Aydin, H. Akbulut, Structural properties of size-controlled SnO2 nanopowders produced by sol–gel method, Mater. Sci. Semicond. Process. 38 (2015) 404–412. [2] N.C. Horti, M.D. Kamatagi, N.R. Patil, M.N. Wari, S.R. Inamdar, Photoluminescence properties of SnO2 nanoparticles: Effect of solvents, Optik (Stuttg). 169 (2018) 314–320. [3] S.K. Tammina, B.K. Mandal, N.K. Kadiyala, Photocatalytic degradation of methylene blue dye by nonconventional synthesized SnO2 nanoparticles, Environ. Nanotechnology, Monit. Manag. 10 (2018) 339–350. [4] A.S. Ahmed, A. Azam, M.M. Shafeeq, M. Chaman, S. Tabassum, Temperature dependent structural and optical properties of tin oxide nanoparticles, J. Phys. Chem. Solids. 73 (2012) 943–947. [5] G. Cheng, J. Chen, H. Ke, J. Shang, R. Chu, Synthesis, characterization and photocatalysis of SnO2 nanorods with large aspect ratios, Mater. Lett. 65 (2011) 3327– 3329. [6] M.A.Z.G. Sial, M. Iqbal, Z. Siddique, M.A. Nadeem, M. Ishaq, A. Iqbal, Synthesis and time-resolved photoluminescence of SnO2 nanorods, J. Mol. Struct. 1144 (2017) 355– 359. [7] Y. Feng, W.-X. Ji, B.-J. Huang, X. Chen, F. Li, P. Li, et al., The magnetic and optical properties of 3d transition metal doped SnO 2 nanosheets, RSC Adv. 5 (2015) 24306– 24312. [8] S.X. Yu, L.W. Yang, Y.C. Li, X. Qi, X.L. Wei, J.X. Zhong, Preferred orientation growth and size tuning of colloidal SnO2 nanocrystals through Gd3+ doping, J. Cryst. Growth. 367 (2013) 62–67. [9] H.S. Arif, G. Murtaza, H. Hanif, H.S. Ali, M. Yaseen, N.R. Khalid, Effect of La on structural and photocatalytic activity of SnO2 nanoparticles under UV irradiation, J. Environ. Chem. Eng. 5 (2017) 3844–3851. [10] D. Liu, T. Liu, H. Zhang, C. Lv, W. Zeng, J. Zhang, Gas sensing mechanism and properties of Ce-doped SnO2 sensors for volatile organic compounds, Mater. Sci. Semicond. Process. 15 (2012) 438–444. [11] A. Ahmed, M.N. Siddique, T. Ali, P. Tripathi, Defects assisted improved room temperature ferromagnetism in Ce doped SnO2 nanoparticles, Appl. Surf. Sci. 483 (2019) 463–471. 14

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Highlights • • • • •

Spherical and cubic Eu3+-doped SnO2 NPs were synthesized via co-precipitation and hydrothermal methods. Emission and excitation spectra consisted of narrow bands assigned to 4f-4f transitions. Concentration quenching in SnO2:Eu3+ phosphors depended on morphology and nanoparticle size. Radiative and nonradiative decay rates and Judd-Ofelt parameters were calculated. DFT method showed different preferred Eu3+ positions for highly-doped spherical and cubic NPs.

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: