Influence of concentration and sintering temperature on luminescence properties of Eu3+:SnO2 nanocrystallites

JOURNAL OF RARE EARTHS, Vol. 30, No. 7, July 2012, P. 627

Influence of concentration and sintering temperature on luminescence properties of Eu3+:SnO2 nanocrystallites P. Psuja, W. Strek (Institute of Low Temperature and Structure Research of Polish Academy of Sciences, Okolna 2, 50-422 Wroclaw, Poland) Received 9 November 2011; revised 29 December 2011

Abstract: The nanopowders of SnO2 doped with different Eu3+ concentrations were synthesized using the modified Pechini method. The Eu3+ concentrations were high above solubility limit. The average size of crystallites was controlled by the sintering temperatures. The structure and the morphology of obtained powders were examined using the XRD (X-ray diffraction) and TEM (transmission electron microscopy) analyses. The Eu2Sn2O7 phase separation was observed at relatively high concentration of Eu3+ ions. The ZnS:Ag micropowders were mixed with the Eu3+:SnO2 powders and their normalized emission was used to measure a relative efficiency of Eu3+:SnO2. The photoluminescence spectra of mixed powders were measured in function of Eu3+ concentration and average size of nanocrystallites. The reference peak method was used for comparison of intensities of the samples and selection of optimal one. The influence of the average grain size and Eu3+ concentration on the phosphor's efficiency was discussed. The presented results confirmed the rightness of synthesis of the Eu3+:SnO2 in form of nanocrystalites with relatively high Eu3+ concentration. Keywords: SnO2; nanocrystallites; Eu3+; luminescence; phosphors; rare earths

The semiconductive luminescent materials are very interesting class of phosphors for the electroluminescence devices (ELD) and the field emission devices (FED). Recently besides, well known CdS and ZnS[1–6], ZnO[6] or TiO2[7,8], also rare earth doped semiconducting materials[7,8] focus the great attention. One of examples of that type of phosphors is SnO2 doped with trivalent Eu3+ ions. The SnO2 is a wide band gap, n-type semiconductor (3.6 eV)[9]. Due to an appropriate energetic configuration, the ground state (7F0) and, some of the metastable excited levels of the Eu3+ ion (5D0, 5 D1) are situated in the band gap of SnO2. It allows to observe a typical efficient red-orange emission from the excited 5 D0 to 7FJ (J=0–4) levels. In the last few years many authors have examined luminescent properties of SnO2 doped also with the other rare earth ions[9–15]. The main advantages of semiconducting phosphors are their interesting electrical properties. The most of applied nowadays the RE-doped cathodoluminescent phosphors are isolators with the energy gap above 5 eV[1–3,6,16]. The negative charges accumulated at the surfaces of phosphor grains can be a barrier for incoming primary electrons, and the same influence on the huge loss of the final brightness. It is known that to avoid the accumulation of charges (electrons) in the phosphor layer the materials must be both highly conductive and have a good luminescence efficiency at low voltages[6]. One way to solve this problem is to use the more conductive phosphors. Nevertheless, the most of them contain sulfur in the composition[1–6,17] or the harmful elements, like cadmium or lead[1,5]. For this

reason, it is worth to consider the possible application of the Eu3+:SnO2 as luminescent material for thin film electroluminescent devices (TFELD), vacuum fluorescent displays (VFD), or field emission light emitting devices (FED). However, because of the difference between ionic radius of Sn4+ (0.069 nm) and Eu3+ (0.095 nm)[18] and different valence, the substitution of Sn4+ by Eu3+ ions into the SnO2 lattice is very limited and produces defects. Assuming that, the solubility of Eu3+ in SnO2 is about 0.05 at.%–0.06 at.%[15,19], the preparation of homogenous samples with higher concentration (0.2 at.%–5 at.%), requested for intensification of total luminescence is a challenge. It was found, that at relatively high concentration of Eu3+ ions there may be formed Eu2Sn2O7[20]. The solution of this problem could be a fabrication of the Eu3+:SnO2 in the form of nanocrystallites. In such case the Sn4+ ions could be replaced by Eu3+ ions in the more flexible (comparing to a bulk material) SnO2 lattice as it was observed in other nanomaterials[21–24]. Some authors propose another solution of this problem[25,26]. They reported that stresses in the lattice caused by the Eu3+ ions may be compensated by the replacement of some Sn4+ ions by ions with a similar to tetravalent tin ionic radius, like Li+ (0.076 nm) or Mg2+ (0.072 nm)[18]. In this work the synthesis of Eu3+:SnO2 crystallites is presented. Their luminescence properties were measured. The influence of average grain size of nanocrystals and dopant concentration on the structure and luminescence properties were investigated by using the reference peak method pro-

Foundation item: Project supported by Polish Ministry of Science and Higher Education (N507 076 32/2186, RO 02 015 02 and N507 421236) Corresponding author: P. Psuja (E-mail: P. [email protected]; Tel.: +48-713435021) DOI: 10.1016/S1002-0721(12)60102-1

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posed earlier by Psuja et al.[16]. This method was based on simultaneous luminescence measurements of homogenously mixed composition of nanocrystalline phosphor with the micron size (3.5 μm) reference standard phosphor ZnS:Ag taking into account the well established observation that the luminescence yield of the micro grain phosphor do not exhibit the size dependence. The possible application of the Eu3+:SnO2 nanocrystallites in ELD and FED are discussed.

1 Experimental The Eu3+:SnO2 nanocrystalline particles were synthesized by the modified Pechini method[27]. The europium oxide (99.99 %, Alfa Aesar) was dissolved in the nitric acid. In the next step, the water solution of europium nitride was added to deionized water with stoichiometric amounts of the hydrated tin chloride (99.99%, Alfa Aesar), a citric acid (99.5%, Alfa Aesar) and ethylene glycol and stirred ultrasonically until a clear, transparent solutions was obtained. After that, solution was hold in the temperature of 80 ºC until the resin was formed. In the end the resins contained different Eu3+ concentrations (0.2 mol.%, 0.5 mol.%, 1 mol.%, 2 mol.%, 5 mol.%) were sintered at the different temperatures (950, 1000, 1100, 1200, 1300 ºC) in order to obtain different sizes of the SnO2:Eu3+ nanocrystallites. The structure and average grains size of obtained materials were determined by the X-ray diffraction (XRD—a Stoe Powder Sensitive Detector; filtered Cu Kα1 radiation). The TEM images were carried out for determination of morphology of obtained samples. To determine the influence of dopant concentration and sintering temperature on photoluminescent properties of obtained materials the reference peak method[16] was used. The obtained powders were mechanically mixed with the micrograins of ZnS:Ag in the same mass ratio. The silver doped zinc sulphide with 3.5 μm average grains size was chosen as a reference light emitting material due to the study of the influence of size of nanograins on the output light intensity. The mass ratio of ZnS:Ag to Eu:SnO2 was taken as 1:4. The aim of this experiment was to investigate the influence of the average size of nanocrystallites on the luminescence intensity by the reference to the wide, blue peak (450 nm) connected with the radiative transitions in the ZnS:Ag. The PL spectra were registered at the room temperature using a CCD spectropohotometer Avantes (350–1000 nm spectral range, ~0.35 nm resolution). The samples were excited using λex=266 nm of Nd:YAG laser system. The luminescent decay times were measured for the 5 D0→7F1 transition (588 nm) under λex=532 nm excitation (the second harmonic of Nd:YAG laser, 10 ns in pulse, 50 Hz) using a LeCroy WaveSurfer 400 oscilloscope.

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especially in the samples sintered at higher temperatures (>1100 ºC) (Fig. 1) and with higher Eu3+ concentration (>2%) (Fig. 2) some weak peaks corresponding to the Eu2Sn2O7 (JCPDS #130182) phase are also observed. The intensity of the peaks connected with the Eu2Sn2O7 crystalline phase increase with sintering temperature, and the Eu3+ concentration. Moreover, the peaks connected with the Eu2O3 (JCPDS #340392) crystalline phase appear for samples with 2% and higher Eu3+ concentration, and (even in samples with 5% Eu3+ concentration) only for the ones sintered at 1100 °C or less. This could suggest that some part of the Eu3+ ions did not substituted the Sn4+ ions, but separated in form of the Eu2Sn2O7 as well as the Eu2O3 crystalline phases. The average size of SnO2 crystallites, calculated using the Scherrer’s formula, distinctly increased with sintering temperature (Fig. 1) and decreased with Eu3+ concentration (Fig. 2). The influence of Eu3+ concentration on average grain size of obtained nanocrystallites is presented in Fig. 4. This dependence was approximated using a function depicted in Fig. 4. The morphology of the samples sintered at different temperatures is shown in TEM images (Fig. 3). The size of ob-

Fig. 1 XRD patterns of Eu3+:SnO2 samples sintered at different temperatures with 5 mol.% Eu3+ concentration

2 Results and discussion The analysis of XRD patterns confirms that the structure of the obtained materials is a tetragonal rutile crystalline phase of the cassiterite SnO2 (JCPDS #411445). However,

Fig. 2 XRD patterns of Eu3+:SnO2 samples with different Eu3+ concentrations sintered at 1000 ºC

P. Psuja et al., Influence of concentration and sintering temperature on luminescence properties of Eu3+:SnO2 …

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Fig. 3 TEM images of Eu3+:SnO2 powders sintered at 1000 (a), 1100 (b), 1200 (c), 1300 ºC (d)

Fig. 4 Influence of Eu3+ concentration on average grain size of Eu: SnO2 nanocrystallites sintered at 1000 °C

tained crystallites estimated on the base of TEM images is presented in Table 1. Analyzing the XRD patterns and TEM images it can be seen that the intensive peaks related with separation of the Eu2Sn2O7 phase appeared for the samples with the highest Eu3+ concentrations and, for the samples where the average sizes of crystallites were above 100 nm. It confirms the hypothesis about rightness of synthesis of Eu3+:SnO2 in form of nanocrystallites. The solubility limit of the Eu3+ in SnO2 is less than 0.1 at.%[15,19]. This could suggest that in all samples the most of Eu3+ ions should exist in the Eu2Sn2O7 or Eu2O3 phases. However, the XRD analysis (Fig. 2), and the luminescence features suggest that in the case of the nanocrystalline Eu3+:SnO2 powders the situation is quite different. The comparison of the photoluminescence spectra (λex= 266 nm) of the ZnS:Ag/Eu3+:SnO2 blends with Eu3+:SnO2 powders sintered at different temperatures (Fig. 5) and with different Eu3+ concentrations (Fig. 6) are presented. It is seen that the luminescence intensity of Eu3+:SnO2 significantly decreased with a sintering temperature, and increased with the Eu3+ concentration. In all spectra an orange emission (5D0→7F1) corresponding to magnetic dipole transition strongly dominates over red emission (5D0→7F2) corresponding to electric dipole transition. For all samples, the Table 1 Dependence of size of crystallites (estimated on the base of the TEM images) versus the sintering temperature Sinter temperature/ºC

1000

1100

1200

1300

Size of crystallites/nm

40–60

80–150

100–200

300

Fig. 5 Comparison of photoluminescence spectra (λex=266 nm) of ZnS:Ag/Eu3+:SnO2 blends with Eu3+:SnO2 crystallites sintered at different temperatures

Fig. 6 Comparison of photoluminescence spectra (λex=266 nm) of ZnS:Ag/Eu3+:SnO2 blends with Eu3+:SnO2 crystallites with different Eu3+ concentrations

calculated asymmetric ratios (the ratios of integrated intensities of the peaks attributed to the 5D0→7F2 transition to the integrated intensities of the peaks attributed to the 5D0→7F1 transition) tend to zero. For the Eu0,05Sn0,95O2 sintered at the temperature range 1000–1200 ºC the asymmetric ratio is around 0.097, and for the sample sintered at 1300 ºC is a little higher ~0.119. The mentioned above ratio insignificantly decrease with Eu3+ concentration to minimal value 0.089 for Eu0,005Sn0,995O2 sample. Those results suggest that excited Eu3+ ions were located at the high symmetry sites. Thus, in the case of the SnO2 lattice the Eu3+ ions replaced the Sn4+ ions, and that was indeed in synthesized and presented samples. The presented results suggest that the great majority of

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Eu3+ ions substitute Sn4+ ions in the SnO2 lattice, even in samples, where Eu3+ concentrations were high-above the solubility limit. The increasing of the asymmetric ratio for the sample sintered at 1300 ºC confirms the thesis of this work, and morphological and structural analysis. The luminescence decay times, fitted in all samples by the first order exponential decay (Fig. 7) determined for the 5 D0→7F1 transition (588 nm line) on c.a. 6.5 ms suggest that Eu3+ ions are placed in the SnO2 lattice. The shortest decay time of 5.94 ms was registered for sample with 5% concentration sintered at 1000 ºC. Those results also suggest that only a small part of the Eu3+ ions tend to separate in the form of the Eu2Sn2O7 or the Eu2O3. The comparisons of normalized intensities of photoluminescence of 5D0→7F1 transition are presented in Figs. 8 and 9. It is seen that in measured range the intensity of 5D0→7F1 transition decrease directly proportional with sintering temperature (Fig. 8). Simultaneously, the intensity of 5D0→7F1 transition is directly proportional to Eu3+ concentration (Fig. 9). It is worth to notice that luminescent intensity of the sample sintered at 1000 °C (40–60 nm) is more than 4 times stronger than the intensity of the sample sintered at 1300 °C (300 nm). Even more distinctly is the concentration dependence, where the intensity of samples with 5% Eu3+ concentration is respectively c.a. 76 times stronger than intensity of samples with 0.2% Eu3+ concentration. The greatest intensity was registered for the sample sintered at 1000 °C and with 5% Eu3+ concentration. This phenomenon (considering the Eu3+ solubility limit in the SnO2 and the sizes of the crystallites in the examined samples) indicates that in the nanocrystalline range the solubility of Eu3+ ions in the SnO2 strongly

JOURNAL OF RARE EARTHS, Vol. 30, No. 7, July 2012

Fig. 8 Comparison of 5D0→7F1 transition normalized intensity of photoluminescence of Eu3+:SnO2 powders sintered at different temperature

Fig. 9 Comparison of 5D0→7F1 transition normalized intensity of photoluminescence of Eu3+:SnO2 powders with different Eu3+ concentrations

exceed the limit for the bulk material. The similar situation was observed earlier by Zhang et al.[22] for Eu3+ ions in Y2SiO5 nanocrystalline powders. Wei et al.[23] described analogue example for Eu3+ ions in YBO3 nanocrystals, and Yu et al.[24] for Nd3+ ions in NdF3 and NdF3/SiO2 core-shell nanoparticles. The CIE chromatic coordinates x=0.607 and y=0.391[6,16] combined with the satisfactory luminescence properties allowed for the application of SnO2 semiconductive material as a phosphor for different types of displays or light emitting devices. In spite of some misfit of the CIE chromatic coordinates of the Eu3+:SnO2 with the red color standard, the highest comparing to the other phosphors conductivity place them for many applications - for example as monochromatic device’s communicative displays, signal lamps or an energy-saving field emission light sources[28].

3 Conclusions

Fig. 7 Luminescence decay times of Eu3+:SnO2 sintered at different temperatures (a) and with different Eu3+ concentrations (b)

In this work the synthesis of Eu3+:SnO2 nano- and submicron crystalline powders with different concentrations of dopant were presented. Following the reference peak method[16] we had been in position to perform the intensity analysis of Eu:SnO2 nanocrystalline powders with respect of

P. Psuja et al., Influence of concentration and sintering temperature on luminescence properties of Eu3+:SnO2 …

their grain sizes and dopant concentration. The presented results substantiate fabrication of Eu:SnO2 nanocrystallites with a relatively high Eu3+ concentration. However, the examinations showed that in samples sintered at the highest temperatures >1100 ºC with the average grain size above 100 nm and the concentration larger than 2% a part of Eu3+ ions tends to separate in the form of Eu2Sn2O7 and Eu2O3 crystalline phases. In summary, the materials characterize themselves by well luminescent properties, which also confirmed the rightness of synthesis of the Eu3+:SnO2 in form of the nanocrystallites with Eu3+ concentration high-above the solubility limit. The most efficient Eu3+:SnO2 phosphors with the optimal Eu3+ concentration and sintering temperature were established for 5% and 1000 °C, respectively. In spite of some misfit of the CIE chromatic coordinates of Eu3+:SnO2 with red color standard, highest, comparing to other phosphors conductivity, place them as worth of applications like a monochromatic device’s communicative displays, a signal lamps or energy-saving field emission light sources[28]. Acknowledgements: The authors would like to thank Mrs. Ewa Bukowska for the XRD measurements, Mrs. Ludwina Krajczyk for TEM images and Mr. Pawel Gluchowski for help in luminescence decay measurements. The special thanks are dedicated to Dr. Dariusz Hreniak for contribution to discussion over luminescent properties of examined materials.

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