Characterization and photoluminescence studies of Eu2+-doped ...

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Nov 26, 2010 - BaSO4 phosphor prepared by the recrystallization method. J. Manam · Puja Kumari · S. Das. Received: 25 May 2010 / Accepted: 19 October ...
Appl Phys A (2011) 104:197–203 DOI 10.1007/s00339-010-6101-6

Characterization and photoluminescence studies of Eu2+ -doped BaSO4 phosphor prepared by the recrystallization method J. Manam · Puja Kumari · S. Das

Received: 25 May 2010 / Accepted: 19 October 2010 / Published online: 26 November 2010 © Springer-Verlag 2010

Abstract Eu doped BaSO4 was prepared by the recrystallization method and characterization of the material was done by using X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) techniques. From the XRD pattern of Eu doped BaSO4 compound, it was found that the prominent phase formed was BaSO4 and traces of other phases were very weak and the result of FTIR spectrum of BaSO4 :Eu shows that the sulfur-oxygen stretch was found at around 1100 cm−1 . The room-temperature PL spectra of the Eu doped BaSO4 sample showed one peak centered at 374 nm, which is the characteristic emission of Eu2+ ion. This emission band at 374 nm corresponds to the 4f6 5d → 4f7 (8 S7/2 ) transitions of Eu2+ ions. The excitation spectrum taken at the wavelength 374 nm extends over a wide range of wavelengths from 220–350 nm with a strong peak at around 260 nm. Furthermore, the present sample shows good crystal quality and high photoluminescence sensitivity. Hence our results suggest possible potential applications of Eu doped BaSO4 phosphor in optoelectronic devices.

1 Introduction The luminescence studies of BaSO4 compounds are of interest because of their high equivalent absorption coefficient (Zeff = 45), low cost and easy handling process and J. Manam · P. Kumari · S. Das () Department of Applied Physics, Indian School of Mines, Dhanbad 826004, Jharkhand, India e-mail: [email protected] Fax: +91-326-2296563

therefore it is particularly suited for applications in radiation dosimetry as well as for detecting very small exposures of low energy X-rays [1]. The first preparation of BaSO4 , doped by Dy, was reported in 1974 [1, 2]. Then, a second preparation was carried out in the years 1984 and 1986 and used to study hadronic and electromagnetic cascade showers [1]. Also BaSO4 doped with suitable impurities (e.g. Dy, Eu, P) can find an application as an X-ray storage phosphor due to its high luminescence sensitivity coupled with high effective atomic number [1–3]. It has been observed in our studies that the doping of different impurities in the BaSO4 host lattice plays an important role in the photoluminescence (PL) process and enhances the PL sensitivity. On the other hand, since the last decade, rare-earth ion doped phosphor materials have attracted growing interest as a consequence of the unique electronic properties of the 4f electrons of the dopants. Among rare-earth ions, Eu3+ and Eu2+ form red, green and yellow phosphors when they are doped in suitable matrices. Eu is a chemical element with atomic number 63 and atomic weight 151.97. Divalent Eu in small amounts happens to be the activator of the bright blue fluorescence of some mixed sulfates. Due to this intensive blue emission, the Eu doped compounds are extensively applied to lighting, field emission displays (FED), cathode ray tubes (CRT) and plasma display panels [4, 5]. On the other hand barium sulfate is getting more and more attention as useful luminescent host because of the stable crystal structure, and high physical and chemical stability [1, 2, 6–8]. Also the ionic radius of Eu2+ is almost the same as that of the barium ion due to which barium ions can easily be replaced by Eu ions during doping. These advantages make the Eu2+ doped BaSO4 compound a promising candidate for high efficient phosphor. So keeping these in view, photoluminescence studies of BaSO4 are carried out by incorporating Eu2+ impurity in the host lattice.

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2 Sample preparation Microcrystalline BaSO4 :Eu sample was prepared by the method of recrystallization [1, 2, 6]. The Eu doped BaSO4 sample has been prepared by mixing BaCO3 (99%, s.d. Fine-Chem LTD., Mumbai) and conc. H2 SO4 (90%, Universal Laboratories) in stoichiometric ratio, adding 1.0 wt% of europium chloride in the mixture. Then the mixture was heated up to 30 min at 1023 K and then rapidly cooled it to room temperature. Finally the prepared sample was ground and sieved to obtain fine mesh powders. Conc. H2 SO4 is highly toxic and through skin contact with the acid will lead to serious skin burns and its chronic exposure may cause lung damage and possibly cancer. Necessary precautions were taken by adding conc. H2 SO4 slowly and carefully and using lab coat, gloves, mask etc. The preparation of the sample was done in the Luminescence Laboratory of the Department of Applied Physics, Indian School of Mines, Dhanbad.

3 Instrumentation The formation of Eu doped BaSO4 compound was confirmed by XRD, SEM, EDS and FTIR studies. X-ray diffractogram of this compound was taken at room temperature in a wide range of Bragg angle 2θ (15◦ ≤ 2θ ≤ 100◦ ) using Panalytical High Resolution XRD-I, PW 3040/60 at a scanning rate of 1.00 degree per minute. The FTIR studies carried out on FTIR Spectrometer (Perkin Elmer, Spectrum RX1, USA) with KBr pellet technique from 4000 cm−1 to 400 cm−1 . The morphology of the powder phosphor was observed by using a Jeol, Japan; JSM-6390LV scanning electron microscope (SEM) equipped with an EDS Analyzer operating at 30 keV. The system enables to obtain the resolution of 2 nm. For SEM studies, the sample was gold coated using a sputter coater Polaron SC7610 system. The diffuse reflectance, PL emission and PL excitation spectra of the Eu doped BaSO4 sample were measured at room temperature. UV-VIS Reflectance spectrum was recorded in the range of 300–800 nm by using Eu2+ doped BaSO4 pellet, with the help of ‘UV-VIS-NIR spectrophotometer DH-2000 BAL’. In a typical experiment 10-mg Eu doped BaSO4 pellet was obtained by using hydraulic pressure (7–0 psi). Photoluminescence studies were made by using personal computer based fluorescence spectrophotometer (Hitachi, F-2500) with 150 W Xenon lamp (selfdeozonating lamp house) light source.

4 Results and discussions 4.1 XRD Results The X-ray diffraction of Eu doped BaSO4 sample was taken at room temperature and is shown in Fig. 1. The sharp and

Fig. 1 XRD pattern of Eu doped BaSO4 sample at room temperature

single peaks of the XRD pattern suggest the formation of single phase new BaSO4 :Eu compound. From the two theta values of diffraction lines, the interplanar spacing ‘d’ of the XRD peaks was calculated. From the analysis of the XRD pattern of Eu doped BaSO4 compound, it was found that the sample belongs to the orthorhombic structure at room temperature in correspondence with the JCPDS data base of card number 01-076-0213, and the corresponding lattice parameters were a = 8.85, b = 5.44, c = 7.13 with α = β = γ = 90° [9]. The first peak of XRD for the prepared sample arises at around 20°. The wide angle XRD pattern for the sample exhibits five intensive peaks in the range of 20–45 degrees, which are the main characteristic peaks of BaSO4 having orthorhombic crystal structure. The XRD peaks are fairly sharp and intensive, suggesting the fine crystalline nature of Eu doped BaSO4 compound. It is worth mentioning that the (h k l) values of most prominent peaks for Eu doped BaSO4 are (2 0 0), (2 1 0), (1 0 2), (2 1 1), (1 1 3) and the corresponding d values are 4.42, 3.43, 3.307, 3.09, 2.115 Å as shown in Fig. 1. The experimental d-values for the intensive peaks of the XRD pattern of Eu doped BaSO4 compound are compared along with the dvalues of the JCPDS data base having card number 01-0760213 [9] and the comparisons are given in Table 1. 4.2 Scanning Electron Microscopy (SEM) The microstructure of the as-prepared Eu doped BaSO4 powder sample is studied by SEM and is shown in Fig. 2. The SEM photograph shows single morphology with different particle size. The particles can be categorized into three types: one that shows particles with particle size 0.4– 1.0 μm. The second kind of particles has a size of about 1.2–1.5 μm and third one has a particle size 1.6–4.4 μm

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Table 1 Comparison of experimental and standard d-values for Eu doped BaSO4 compound Pos. [°2Th.]

FWHM [°2Th.]

d-Ref. [Å]

d-exp. [Å]

h

k

l

20.0759

0.1004

4.424

4.41937

2

0

0

22.8727

0.1004

3.886

3.88493

1

1

1

25.9226

0.1004

3.432

3.43434

2

1

0

26.9130

0.1004

3.307

3.31017

1

0

2

28.8297

0.1338

3.092

3.09430

2

1

1

31.6096

0.1338

2.826

2.82823

1

1

2

32.8718

0.2007

2.720

2.72246

3

0

1

40.8415

0.2007

2.203

2.20772

2

2

1

42.6349

0.1338

2.115

2.11891

1

1

3

44.0414

0.1338

2.049

2.05445

4

1

0

49.0384

0.1004

1.850

1.85615

3

0

3

Fig. 2 SEM photograph of Eu2+ doped BaSO4

and this non-uniform particle size is caused due to the nonuniform distribution of temperature and mass flow during the synthesis. Luminescence efficiencies are related to the phosphor crystallite size with the optimum size being in the 1.0–10 μm range. Smaller crystals are less efficient because of the lower bulk emission intensity. Also, tighter packing of the smaller particles will increase the probability that the emitted light will get lost within the screen. Larger crystals cause difficulties in coating the phosphor particles into smooth, thin screens. This powder sample can easily be termed an ‘ultra fine phosphor’ because the particle size is less than 10 μm [10]. Many coworkers have studied PL of rare earth doped alkaline earth sulfates and they found less PL efficiency in case of nanocrystalline samples than the microcrystalline samples. For example, Gong et al. [11] have observed such phenomena in case of rare earth doped BaSO4 nanocrystallites. They have attributed this to the spin–orbit coupling interaction. Salah et al. [12] have also observed same phenomena in Dy doped CaSO4 and according to them this phenomenon occurs because of the difference between the ionic

Fig. 3 EDS of Eu (1.0 wt%) doped BaSO4

radii of the impurity Dy3+ and the lattice cation Ca2+ which may lead to such spin–orbit coupling interaction. In view of these observations, we choose microcrystalline Eu doped BaSO4 for the present study. 4.3 Energy Dispersive Spectroscopy (EDS) The EDS (Energy Dispersive Spectroscopy) analysis of Eu doped BaSO4 is shown in Fig. 3. The results of EDS of the BaSO4 :Eu (1.0 wt% of Eu) powder sample shows that they are mainly composed of Ba, S and O with a small amount of Eu, which is determined by the period of reaction time and the ratio of the reaction source. The EDS pattern confirms the presence of Eu in the BaSO4 powder and weight percentage is very nearly equal to the doped value of Eu in BaSO4 . The peaks due to the presence of ‘C’ are also seen, which probably came from the carbon tape used to support the sample. Table 2 lists the weight percentage as well as atomic percentage of the elements in BaSO4 :Eu sample.

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4.4 Fourier Transform Infrared Spectroscopy (FTIR) The ‘FTIR’ structure of Eu doped BaSO4 as observed experimentally is shown in Fig. 4. Normally, sulfate contains two S=O and two S–O bonds. Actually, the four S–O bonds are equivalent. The sulfur-oxygen stretches of inorganic sulfates are found from 1140 to 1080 cm −1 [13, 14]. In our results of FTIR spectrum of prepared BaSO4 (Fig. 4) the sulfur-oxygen stretch is found at 1100 cm−1 . Like any other bonds, sulfate bonds can bend, giving rise to one or two bands normally in the 680 to 610 cm−1 range. These bands are seen in the spectrum of BaSO4 :Eu (1.0 wt%) (Fig. 4) near 610 and 637 cm−1 . It is worth noticing that the bending bands are sharper than the stretching bands. This is commonly observed in inorganic infrared spectra. The unmarked groups of peaks near 2000 cm−1 (Fig. 4) are overtones and characteristic bands around 3000 cm−1 and 1600 cm−1 are ascribed to atmospheric water vapor, since KBr readily absorbs moisture in the air, and these undesirable peaks in all spectra we obtained can be seen, and this indicates that the

prepared sample consists of a certain amount of moisture [13, 14]. Fortunately, these undesirable peaks do not affect the identification of the substances involved in this experiment due to different absorption positions of water and the possible product. 4.5 Reflectance study The optical reflectance spectrum of BaSO4 :Eu2+ (1.0 wt%) powder sample is shown in Fig. 5. This spectrum is reported in terms of ‘R’, where R is the reflectance of the material.

Table 2 The wt% and at% of the elements present in Eu (1.0 wt%) doped BaSO4 Elements

Atomic %

Weight %

C

3.91

1.20

O

62.84

34.17

S

16.24

14.13

Ba

14.22

49.53

Eu

2.79

0.97

Fig. 4 FTIR spectra of BaSO4 :Eu (1.0 wt%) sample at room temperature

Fig. 5 Optical reflectance spectrum of BaSO4 :Eu2+ (1.0 wt%) powder sample

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Thus the y-axis represents the fraction of incident light reflected at a given wavelength. The Eu2+ doped BaSO4 sample begins to absorb at about 230 nm, and reaches a maximum at around 270 nm. The absorption is observed as one band in the UV range peaked at around 270 nm corresponding to the transitions from the 8 S7/2 ground state of Eu2+ ions to the excited states of mixed 4f6 5d configuration. Similar ionic radius values of the elements (r Ba2+ = 1.35 Å, r Eu2+ = 1.31 Å) favor the substitution of Ba2+ by Eu2+ in the host lattice. Crystal field causes splitting of the d-states of Eu2+ into two levels, t2g and eg [15–19]. It is just the transitions to those levels that are observed in the absorption spectrum. 4.5.1 Band gap calculation For Eu+2 doped BaSO4 the reflectivity (R) spectra were recorded in the 250–500 nm wavelength (λ) range. The reflectivity R of the material having refractive index n and absorption coefficient α is given by [20, 21] R=

αλ 2 ) (n − 1)2 + ( 4π αλ 2 (n + 1)2 + ( 4π )

(1)

The refractive index of Eu doped BaSO4 taken in this work is 1.64 [22]. The value of R has been recorded for every value of the wavelength in the 250–500 nm wavelength (λ) range by using a UV-VIS-NIR spectrophotometer. Then for every R and λ, the value of absorption coefficient α was calculated in the 250–500 nm wavelength range by using (1). The optical band gap Eg is related to the absorption coefficient α by the Tauc relation [20, 21]. According to the Tauc (1994) relation, the absorption coefficients, α, for a direct band gap is given by (2). α(hν) ∼ C1 (hν − Eg )1/2

(2)

Here C1 is the proportionality constant, which is different for different transitions [21]. For an allowed direct transition an extrapolation of the linear region of a plot of (αhν)2 on the y-axis versus photon energy (hν) on the x-axis gives the value of the optical band gap, Eg . Since E = hν, when (αhν) = 0. The plots of (αhυ)2 versus hυ for the Eu doped BaSO4 is shown in Fig. 6. Taking the value of refractive index n (=1.64) the estimated band gap for the Eu doped BaSO4 is Eg = 4.66 eV. It is worth mentioning that the value of the band gap depends upon the method of preparation. For example, Numan Salah et al. [22] have done thermoluminescence studies of microcrystalline Eu doped BaSO4 prepared by the chemical co-precipitation method, and in their work, the value of optical energy gap for the same material was 3.48 eV, which was calculated from the absorption spectra of Eu doped BaSO4 .

Fig. 6 Plot for (αhν)2 as a function of the incident photon energy (hν) for the BaSO4 :Eu2+ (1.0 wt%) phosphor

In the present work, a totally different method has been adopted for the synthesis of a BaSO4 : Eu sample, in which a high temperature was required for the preparation. The difference in the values of optical energy gap of the same sample may be due to the different techniques of preparation or it may be due to a difference in temperature during synthesis. 4.6 Photoluminescence study 4.6.1 Photoluminescence emission spectra The room-temperature photoluminescence spectra of the Eu doped (1.0 wt%) BaSO4 sample showed one simple and intensive peak centered at 374 nm (excitation wavelength 250 nm), which is the characteristic emission of Eu2+ ions (Fig. 7). This emission band peaked at 374 nm, which corresponds to the 4f6 5d → 4f7 (8 S7/2 ) transitions of Eu2+ ions [15, 23, 24]. This strong violet-blue emission exhibited by the Eu2+ ions is very useful in lamp and display applications. Moreover, this kind of simple and intensive peak behavior has been observed in crystalline materials, and the XRD analyses confirmed the crystallinity of the Eu doped BaSO4 in this study. Furthermore, the present sample shows good crystal quality and high photoluminescence sensitivity. Hence our result suggests possible potential applications of Eu doped BaSO4 phosphors in optoelectronic devices. Figure 7 shows the influence of Eu contents on the PL intensities of BaSO4 : Eu2+ phosphor. The doping concentration of Eu has been varied from 0.25 to 1.5 wt%. The optimal doping concentration of Eu, which generated the maximum intensity, was 1.0 wt%. The concentration quenching phenomena [25–27] caused the PL emission intensity to decrease if the Eu concentration was greater than 1.0 wt%.

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Fig. 9 Energy level scheme of Eu2+ doped BaSO4 phosphor Fig. 7 Emission spectra of Eu2+ doped BaSO4 phosphor with different concentration of Eu

Fig. 8 Excitation spectra of Eu2+ doped BaSO4 phosphor with different concentration of Eu

4.6.2 Photoluminescence excitation spectra Figure 8 shows the excitation spectra of BaSO4 :Eu2+ with 1.0 wt% Eu2+ doping concentration. The excitation spectrum taken at the wavelength 374 nm extends over a wide range of wavelengths from 220–350 nm with a strong peak at 260 nm (Peak 1, Fig. 8) accompanied by a shoulder at around 270 nm (Peak 2, Fig. 8) and 320 nm (Peak 3, Fig. 8). The strong excitation peaked around 260 nm originate from host lattice excitation, which can be concluded from the reflectance studies. Most importantly, two excitation bands (2 and 3) are ascribed to the 4f7 (8 S7/2 ) → 4f6 5d(t2g ) and

4f7 (8 S7/2 ) → 4f6 5d(eg ) transitions in Eu2+ electronic levels [15, 23, 24]. It is well known that the wavelength position of the emission band of Eu2+ strongly depends on the host lattice. In the crystal structure of BaSO4 , it can be interpreted that each Ba2+ atom is coordinated with four oxygen atoms. Moreover, Eu2+ ions are expected to occupy the Ba2+ lattice sites. The energy level scheme and the main mechanisms involved in the generation of Eu+2 emissions in BaSO4 are shown in Fig. 9. The typical band emission of Eu2+ in AESO4 (AE = Ca, Ba, Sr & Mg) host lattice can be realized in two different ways. The first way is to excite the Eu2+ ion directly in its own excitation levels. The electron from the ground state [4f7 (8 S7/2 )] of Eu2+ is excited into the higher energy levels of Eu2+ [4f6 5d(t2g )]. The excited free electron then relaxes to the 4f6 5d(eg ) excited state of Eu2+ by a nonradiative process, followed by a radiative transition from the 4f 6 5d(eg ) excited state to the 4f7 (8 S7/2 ) ground state, giving rise the typical emission of Eu2+ in BaSO4 host lattice as shown in the Fig. 9. The second option is to excite the host lattice, followed by efficient energy transfer from the host lattice to the Eu2+ ion, which also results in the typical emission of Eu2+ .

5 Conclusions In summary, this work reports the correlation between the structural and optical characterization of Eu2+ doped BaSO4 phosphor. Violet-blue emitting BaSO4 :Eu2+ phosphor has successfully been prepared by using recrystallization method. X-ray diffraction (XRD) patterns revealed that the present sample exhibited pure orthorhombic crystal structure at room temperature and the ultrafine particle size of Eu doped BaSO4 was confirmed by SEM. The EDS pattern confirms the presence of Eu in the BaSO4 powder, and the weight percentage is very nearly equal to the doped value

Characterization and photoluminescence studies of Eu2+ -doped BaSO4 phosphor prepared

of Eu in BaSO4 . Based on the results of reflectance studies the photoluminescence emission of Eu2+ doped BaSO4 phosphor has been recorded at room temperature and it exhibits a broad emission band with maxima at about 374 nm in consistence with the excitation spectra with maxima at about 264 nm. This strong violet-blue emission exhibited by the Eu2+ ions is very useful in lamp and display applications.

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