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Narrowband ultraviolet B emission from gadolinium activated Y3Ga5O12 nano-garnets
Accepted Manuscript Title: Narrowband ultraviolet B emission from gadolinium activated Y3 Ga5 O12 nano-garnets Authors: M.S. Pathak, N. Singh, Vijay Singh, S. Watanabe, T.K.Gundu Rao, Jung-Kul Lee PII: DOI: Reference:
S0025-5408(16)32589-2 http://dx.doi.org/10.1016/j.materresbull.2017.09.002 MRB 9542
To appear in:
MRB
Received date: Revised date: Accepted date:
21-12-2016 30-8-2017 2-9-2017
Please cite this article as: M.S.Pathak, N.Singh, Vijay Singh, S.Watanabe, T.K.Gundu Rao, Jung-Kul Lee, Narrowband ultraviolet B emission from gadolinium activated Y3Ga5O12 nano-garnets, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Narrowband ultraviolet B emission from gadolinium activated Y3Ga5O12 nano-garnets M. S. Pathak a, 1, N. Singh a, 1, Vijay Singh a, *, S. Watanabe b, T. K. Gundu Rao b, Jung-Kul Lee a, * a
1 Both
Department of Chemical Engineering, Konkuk University, Seoul 143-701, Korea b Institute of Physics, University of Sao Paulo, SP, 05508-090, Brazil
authors equally contributed to the paper.
*Corresponding authors: Email addresses: [email protected] (V. Singh) : [email protected] (J.-K. Lee)
Graphical abstract:
Highlights:
A narrowband ultraviolet B emitting nano-garnet was synthesized.
1
Narrow band centered at 311.9 nm, good for phototherapy application.
EPR results indicate the possible location of the Gd3+ ion.
Gd3+ ion is likely to be located at the distorted dodecahedral site.
Abstract Gadolinium (Gd3+) doped Y3Ga5O12 nanocrystalline garnet was prepared by nitrate-fuel combustion technique that involves organic fuel glycine. Sample characterization was performed by powder X-ray diffraction and scanning electron microscope. Upon ultraviolet light excitation at 273nm, Y3Ga5O12:Gd3+ sample exhibits a sharp narrowband ultraviolet B emission at 311.9 nm. The electron spin resonance spectrum of gadolinium doped Y3Ga5O12 samples exhibit resonance signals with the effective g values at g ≈ 2.0, 2.2 and 5.8. These signals are attributed to Gd3+ ion located in distorted surroundings experiencing a relatively strong crystal field.
Keywords: Combustion; nanocrystalline Garnet; Gd3+ ions; Y3Ga5O12; ESR; Photoluminescence 1. Introduction: Ultraviolet radiation is a part of electromagnetic spectrum that reaches the earth from the Sun. It is well established that the light in the wavelength range 400-200 nm is normally referred to as ultraviolet (UV) radiation. The UV band (400-200 nm) is sub-divided into UVA (400-320 nm), UVB (320-280 nm) and UVC (280-200 nm). UVA and UVB are used in treating several skin diseases. UVC is generally used in germicidal applications such as killing bacteria in drinking water. Phototherapy device uses ultraviolet radiation for treatment of skin disorders. The specific ultraviolet radiation used in the phototherapy lamp is UVB [1-3]. UVB radiation is subdivided into broadband (300-280 nm) and narrowband (320-300 nm) emissions. During phototherapy
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investigations it was found that UV in the narrow region showed remarkable therapeutic effects, while that in the broadband region was not very effective [4]. Investigations on narrowband ultraviolet-emitting phosphors are important because these materials have great potential to be applied in the phototherapy lamp industry. It is usually considered that Gd3+ 6P → 8S transitions are ideal for narrowband UVB emissions. Gd3+ ion is isoelectronic with Eu2+ and likewise has very long electron spin relaxation times, as a consequence, the luminescence of Gd3+ ion consists of sharp line transitions (6P → 8S), mainly at 311/312/313 nm. Gd3+ is of particular interest because of its U-spectrum. It is to be noted that various methods have been used to probe the local structural environment of gadolinium (Gd3+) ion in crystals and glasses. Among these, optical absorption, electron spin resonance (ESR) and Mössbauer spectroscopy are used extensively. In recent times, there has been an increase in the use of ESR spectroscopy as a method to understand the site symmetry of rare earth/transition metal ions when incorporated into host matrix. The usual methods of determining rare earth/transition metal ions concentrations in host matrices are chemical analysis and atomic absorption spectroscopy. Although they are accurate, both methods are destructive and relatively slow. However, ESR is considered as a nondestructive and comparatively easy method. In this context, we have studied the Gd-doped Y3Ga5O12 (YGG) phosphor using ESR spectroscopic method. There is a growing interest in the studies of garnets doped with transition-metal/rare-earth ions because of the wide practical applications in laser and luminescence materials [5-8]. Summaries of their spectroscopic investigations appear in detail in several review articles [9-12]. Garnet crystals doped with transition-metal/rare-earth ions are widely investigated and have found surprising applications in areas such as optical high-pressure sensor [13], luminescence 3
[14], solid state lasers [15, 16], scintillation [17, 18], medical procedures [19], imaging [20], displays [21], flow cytometry [22], phosphors [23-25]. Keeping in view of several potential applications and due to great demand of narrowband ultraviolet-emitting phosphors, in the present work, we have prepared gadolinium doped YGG phosphor. To the best of our knowledge, there are no reports about combustion synthesized Gd3+ doped YGG phosphor. Solution combustion method is well known and is widely used for synthesis of powders. Currently, there has been a significant interest in the solution combustion synthesis of complex oxide phosphors because it is innovative, simple, low toxicity of raw material, accurate stoichiometric ratio, low process temperature, considerable energy and time saving technique as compared to conventional methods. In this work, we successfully synthesized YGG:Gd sample by using solution combustion technique. In this paper we present photoluminescence (PL) and electron spin resonance (ESR) experiments on gadolinium doped YGG phosphor. Both PL and ESR techniques are well known as powerful experimental tools to study electronic and magnetic properties of transition metals and rare-earth ions in semiconductors and they have not been exploited up to now for gadolinium doped YGG phosphor. In addition, prepared sample was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. 2. Materials preparation and analysis In the present investigation, Y3Ga5O12:Gd(0.05) sample was synthesized using a combustion method. In a typical synthesis, Y(NO3)3∙6H2O, Ga(NO3)3∙xH2O, Gd(NO3)3∙6H2O and C2H5NO2 as starting materials. Glycine (C2H5NO2) was dissolved in a minimum quantity of deionized water in a porcelain dish with 100 ml capacity to obtain a homogeneous solution. Glycine (C2H5NO2) was added as a fuel for combustion and its amount was calculated using total 4
oxidizing (O) and reducing valences (F) based on the concept of propellant chemistry. To perform the combustion reaction the solution was transferred into a furnace preheated to 550oC. The water quickly evaporated and the mixture formed foam, within which a vigorous reaction between the nitrates and glycine soon initiated and ended. The entire combustion process was completed in about 3 to 5 minutes. The resultant fluffy white masses were crushed into a fine powder. The powder was then placed in alumina crucibles to be given an annealing treatment at 1000C for 4h in air to remove the thermal stress and impurities during its preparation. This prepared powder was utilized without any further treatments. Powder XRD pattern was recorded in the 2 range from 05o to 80o on a Philips X’Pert Xray diffractometer with graphite monochromatized CuK radiation. Powder morphology was studied using scanning electron microscopy (SEM, Hitachi, Japan). Emission and excitation spectra were recorded using a Fluorolog 3-22 spectrometer (Jobin Yvon) with a 450W xenon lamp as an excitation source. A powdered sample of 100 mg was taken in a quartz tube for the ESR measurements. ESR spectra of the sample were recorded on a JEOL FE1X ESR Spectrometer operating in the X-band frequencies with a field modulation of 100 kHz. 3. Results and discussion 3.1 X-ray diffraction Fig. 1 shows the X-ray diffraction pattern for the Gd3+ doped YGG at room temperature. The results of the combustion-derived powder were in good agreement with those reported in the PDF for YGG (JCPDS No. 83-1036, a=b=c =12.27Å), and the peaks correspond to a cubic structure with no additional peaks of other impurity phases. The cubic phase is not modified by the addition of gadolinium in YGG matrix. Compared with other reported methods as the sol-gel
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[26, 27] and Czochralski [18] methods, the nanocrystalline YGG:Gd3+powder synthesized here by combustion method is at rather low calcining temperatures and the process is much simpler. The average crystallite size, D, can be estimated from the broadened peaks by using DebyeScherrer equation, D=0.9λ/βcosθ, where β is the full width at half maximum of a diffraction line located at an angle θ, and λ is the X-ray diffraction wavelength. The strongest peaks used to calculate sizes were 2θ = 29.01o, 32.41o, 35.60o, 53.62o, and 55.91o. The estimated average crystallite size of the sample was found to be ~26.18 nm.
3.2 SEM studies Fig. 2 shows the low and high magnification SEM images of YGG:Gd3+ powder sample. It can be observed from low resolution SEM image (Fig. 2A) that the powder shows agglomerates. Synthesized powders exhibit a high degree of porosity due to the large amount of gases that escape during combustion reaction. It is also noticed that pores are interconnected with a large number of voids and it is a typical nature of combustion derived powders. The release of gases generates many pores producing the synthesized powders friable and loosely agglomerated. Fig. 2B depicts the higher magnification SEM image that shows the presence of several small elongated shape particles. The presence of small particles and porosity are an inherent feature of combustion-derived powders. These types of porous powders are highly friable which facilitates easy grinding to obtain finer particles. From higher magnification SEM image it is clear that crystallites are fused together to form many nanoparticles with dumbbell shape. 3.3 Photoluminescence studies Gd3+ is one of the rare earth ions that have been widely used as an activator for different host materials to prepare phosphors. Figure 3 shows the photoluminescence (a) excitation and (b)
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emission spectra of YGG:Gd3+ sample. The excitation spectrum was recorded when monitoring the Gd3+ emission at 312 nm. The sharp peaks shown in the figure can be assigned to the 4f→4f intraconfiguration forbidden transitions of Gd3+ [28]. Figure 3(a) shows the excitation bands at 244.4 nm (40916 cm-1), 246.5 nm (40567 cm-1), 252.9 nm (39541 cm-1), 273.2 nm (36603 cm-1), 276.0 nm (36231 cm-1), and 279.4 nm (35790 cm-1) which are attributed to the transitions of 8S7/2 → 6D5/2, 8S7/2 → 6D7/2, 8S7/2 → 6D9/2, 8S7/2 → 6I11/2, 8S7/2 → 6I9/2, 6I17/2, and 8S7/2 → 6I7/2, respectively. The observed 6D and 6I absorption bands are listed in Table 1. These excitation bands are in agreement with those reported by Alonso et al. [29] and Binnemans et al. [30] for Gd3+ ions in fluorozirconate glass matrix. Binnemans et al. [30] calculated the Judd-Ofelt parameters (Ω2, and Ω6) for Gd3+ in aqueous and crystalline environment. They reported that the Judd-Ofelt parameters are dependent on the local symmetry, differing from aqueous to amorphous and crystalline environments. Figure 3b shows the emission spectrum of YGG:Gd3+ sample. The emission spectrum was recorded when monitoring the Gd3+ excitation at 273 nm. The emission spectrum has two bands at 306.2 nm (32658 cm-1) and 311.9 nm (32061 cm-1) which were assigned to 6P5/2 → 8S7/2, 6P7/2 → 8S7/2 transitions, respectively. It is speculated that in the present system of YGG:Gd3+ phosphor Gd3+ ions occupy Y3+ ion sites as the ionic radii of Y3+ (1.02 Å) and Gd3+ (1.05 Å) are comparable. From several aforesaid reports [1-4], emission at 311.9 nm (6P7/2 → 8S7/2, Gd3+) could be classified as UVB radiation. Further, this phosphor can be investigated for possible use in phototherapy lamps to treat skin diseases like vitiligo, psoriasis, eczema and other photoresponsive skin disorders. In the next section, the existence of Gd3+ in YGG matrix was also confirmed by ESR experiments. 3.4 Electron Spin Resonance studies
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The room temperature electron spin resonance (ESR) spectrum of YGG:Gd3+ sample is shown in Fig. 4(a). Some of the main signals are at g ≈ 2.0, 2.2 and 5.8. Low temperature ESR spectrum of YGG:Gd3+ sample at 123 K is shown in Fig. 4(b). As observed in the room temperature ESR spectrum, the low temperature ESR spectrum shows the same features as the room temperature spectrum. In accordance with the Boltzmann law, the intensity of the resonance signals increase with decreasing temperature. The g-values are found to be independent of temperature in the measured temperature range indicating that the structural environment do not change for the Gd3+ ion in the YGG lattice. Rare-earth ions Gd3+ and Eu2+ have half filled 4f shell with the electronic configuration [Xe]4f7. The spin quantum number S is 7/2 and the ground state is 8S7/2. With a pure S-state having L=0, the ions have a completely spherical charge cloud. The crystal field arising from the immediate neighboring ions will not affect the ion due to this spherical charge distribution and the crystal field interaction is negligibly small. However, higher order perturbations involving spin-orbit coupling give rise to crystal field effects and consequently there will be splittings. These perturbations admix higher states like 6P7/2 and 6D7/2 having L 0 with the pure S-state. The ion interacts with the crystal field through the mediation by spin-orbit admixtures and the ion does not couple directly with the crystal field. Ground state splittings originate from these admixtures and thus result in crystal field effects in the ESR spectrum. The local symmetry of the ion determines the crystal field interaction as the symmetry changes the nearest number of neighboring ions, their relative distances with respect to Gd3+ ion and their relative locations change. Magnitude of the crystal field interaction will change leading to a change in the observed ESR spectrum.
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Gd3+ ion in host lattices like glass systems, disordered lattices and certain polycrystalline powder systems exhibit somewhat similar spectra [31-34] as in the present sample. Such spectrum, in Brodbeck and Iton [35] description, is termed as a ‘U-spectrum’. There have been several explanations for the observation of this kind of spectrum in Gd doped glass system or in disordered systems and the reasons are not satisfactory. Gd3+ ion located at sites with high coordination number are considered to give rise to signals at g = 5.6, 2.6 and 1.96 while low coordination number sites with strong crystal field is expected to lead to the weak shoulder at about g = 4.2. However, Broadbeck and Iton [35] in a critical analysis offer a more complete and clear explanation of the U-type spectra. The observed spectrum has been classified by them based on the magnitude of crystal field interaction (HCF) in relation to the Zeeman splitting h (where is the microwave frequency). Three regions have been identified which depend on the ratio HCF /h and the regions are weak, intermediate and strong. Weak crystal field (CF) region corresponds to the case where HCF /h ¼. In this region, ESR spectra are mainly concentrated in the g=2.0 region and higher order transitions are forbidden. Intermediate region is considered in two parts viz., (a) lower intermediate and (b) higher intermediate CF regions. ESR spectrum is spread over a wide range of g-values with values varying in the region 2.0 < g < and ¼ HCF/ h 1 in the lower intermediate region. On the other hand, for the case of higher intermediated CF regions, ESR lines with g > 2.0 are observed. Strong crystal field is characterized by H CF/ h≥4 and the ESR lines are due to transitions controlled by resonance within the Kramer’s levels. Y3Ga5O12 belongs to the A3B5O12 family of garnets and has a cubic symmetry structure with space group Ia3d [36]. The structure consists of a network of GaO6 octahedra and GaO4
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tetrahedra connected by shared oxygen ions which are at the corners of a polyhedra. These polyhedra form chains along the three crystallographic directions and form dodecahedral cavities. Y3+ ions occupy these cavities. Hence, YGG lattice contain three crystallographically different cation sites. The Ga3+ ions are located in 24 tetrahedral S4 sites and 16 octahedral S6 sites with four fold and six fold coordination. On the other hand, Y3+ cations are located at a site coordinated to eight oxygens in a dodecahedral arrangement. The ionic radius of Y3+ ion is 1.02 Å in a 8-fold coordination. Ga3+ ion has 0.62 Å radius in an octahedral coordination (6-fold) and an ionic radius of 0.47 Å in a 4-fold coordination [37]. On the other hand, the ionic radii of Gd3+ ion are 0.94 Å and 1.05 Å in six-fold and eight-fold coordination, respectively. Due to similar ionic radii and chemical properties, lanthanide ions are expected to mainly enter into the distorted dodecahedral sites by replacing the Y3+ ions and to be coordinated to eight O2− ions [38, 39]. Therefore, it is very likely that Gd3+ ion in YGG occupies the Y3+ sites in the lattice. It was mentioned earlier that Gd3+ ion located in a crystalline lattices and at sites with minimal distortion experiencing weak crystal field gives rise to spectrum concentrated mainly in the g = 2.0 region. The observed spectrum, however, is characteristic of spectrum expected from a disordered system or from an ion experiencing a relatively strong crystalline field. Y3Al5O12 and Y3Ga5O12 [40] have the same crystalline structure. There will be a change in the lattice parameter in the two garnets as Ga ionic radius (0.62 Å) is larger than the Al ionic radius (0.53 Å) [41]. Nakatsuka et al. [42] have determined the proportion of covalent bonding in the Al-O and Ga-O bonds utilizing the equations given by Brown and Shanon [43]. The proportion of covalent bonding (q) is directly proportional to the covalency of the cation-oxygen bond. The
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proportion of covalent bonding of Ga-O bond is estimated to be 43% (for octahedral site) whilst the value for Al-O bond is 33%. The values for tetrahedral site are 51.5% and 45%, respectively. These estimates show that covalency of Ga-O bond is higher than Al-O bonds and this introduces a distortion in the neighboring dodecahedra of Y3Ga5O12 system. As mentioned earlier, Gd3+ ion is located at the distorted dodecahedra. Due to the distortion, Gd3+ ion experiences a relatively stronger crystal field and one may expect lines in the g > 2.0 region. It is speculated that the ESR lines observed in YGG:Gd3+ is a reflection of stronger crystal field experienced by the Gd3+ ion.
4. Conclusions In this study we have demonstrated that using the solution combustions single-phase yttriumgallium garnet (Y3Ga5O12) can be produced at substantially lower temperatures than using the conventional solid-state method. X-ray diffraction study reveals that the nanocrystals are formed in a single phase of the garnet structure with cubic structure. By SEM analysis, it is observed that crystallites are fused together to form many nanoparticles with dumbbell shape. Upon ultraviolet excitation at 273 nm, narrowband emission with a peak wavelength at 311.9 nm was observed from the gadolinium 6P7/2 → 8S7/2 transition. Low field ESR lines as well as lines near g = 2.0 are observed in the ESR spectrum of Y3Ga5O12:Gd3+ sample. The observation of this kind spectrum seems to originate from the distortions of dodecahedral site resulting from the higher covalency of the Ga-O bond in Y3Ga5O12 as compared to the Al-O bond in Y3Al5O12 garnet. Further the present results, though preliminary in nature, suggest that the narrowband UVB emission peaks at 311.9 nm of the Gd3+, making it a possible phosphor for application in phototherapy lamps.
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Acknowledgements This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030091450, 20153010092130).
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Figure captions:
Fig. 1. Powder XRD pattern of Y3Ga5O12:Gd3+
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(A)
(B)
Fig. 2. SEM images of Y3Ga5O12:Gd3+ (a)
1.0
273.2
0.012
252.9
0.8 0.6
PL Intensity (a.u.)
PL Intensity (a.u.)
0.010
276.0
246.5
0.008 244.4 0.006 0.004 0.002 240
0.4
250 Wavelength (nm)
260
252.9
0.2
279.4
246.5 244.4
0.0 240
250
260
270
280
290
Wavelength (nm)
(b)
17
311.9
PL intensity (a.u.)
1.0 0.8 0.6 0.4 0.2 0.0 290
306.2
300
310
320
330
340
350
Wavelength (nm)
Fig. 3. Photoluminescence spectra of Y3Ga5O12:Gd3+ (a) Excitation spectrum of Y3Ga5O12:Gd3+ (λem= 312 nm) and (b) Emission spectrum of Y3Ga5O12:Gd3+ (λex= 273 nm)
18
Fig. 4. ESR spectra of Y3Ga5O12:Gd3+ at room temperature (black) and at 123 K (blue).
19
Table 1: The observed band positions for Y3Ga5O12:Gd3+ along with their assignments
Transition
Observed band positions
S7/2 →
This work
8
Wavelength (nm)
Ref. [30]
Ref. [29]
Wavenumber (cm-1)
6
D5/2
244.4
40916
41070
41084
6
D7/2
246.5
40567
40770
40750
6
D9/2
252.9
39541
39730
39740
I11/2
273.2
36603
36540
36550
I9/2, 6I17/2
276.0
36231
36310
36337
279.4
35790
35910
35945
6
6
Wavenumber (cm-1)
Reported
6
I7/2
20
S0025-5408(16)32589-2 http://dx.doi.org/10.1016/j.materresbull.2017.09.002 MRB 9542
To appear in:
MRB
Received date: Revised date: Accepted date:
21-12-2016 30-8-2017 2-9-2017
Please cite this article as: M.S.Pathak, N.Singh, Vijay Singh, S.Watanabe, T.K.Gundu Rao, Jung-Kul Lee, Narrowband ultraviolet B emission from gadolinium activated Y3Ga5O12 nano-garnets, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Narrowband ultraviolet B emission from gadolinium activated Y3Ga5O12 nano-garnets M. S. Pathak a, 1, N. Singh a, 1, Vijay Singh a, *, S. Watanabe b, T. K. Gundu Rao b, Jung-Kul Lee a, * a
1 Both
Department of Chemical Engineering, Konkuk University, Seoul 143-701, Korea b Institute of Physics, University of Sao Paulo, SP, 05508-090, Brazil
authors equally contributed to the paper.
*Corresponding authors: Email addresses: [email protected] (V. Singh) : [email protected] (J.-K. Lee)
Graphical abstract:
Highlights:
A narrowband ultraviolet B emitting nano-garnet was synthesized.
1
Narrow band centered at 311.9 nm, good for phototherapy application.
EPR results indicate the possible location of the Gd3+ ion.
Gd3+ ion is likely to be located at the distorted dodecahedral site.
Abstract Gadolinium (Gd3+) doped Y3Ga5O12 nanocrystalline garnet was prepared by nitrate-fuel combustion technique that involves organic fuel glycine. Sample characterization was performed by powder X-ray diffraction and scanning electron microscope. Upon ultraviolet light excitation at 273nm, Y3Ga5O12:Gd3+ sample exhibits a sharp narrowband ultraviolet B emission at 311.9 nm. The electron spin resonance spectrum of gadolinium doped Y3Ga5O12 samples exhibit resonance signals with the effective g values at g ≈ 2.0, 2.2 and 5.8. These signals are attributed to Gd3+ ion located in distorted surroundings experiencing a relatively strong crystal field.
Keywords: Combustion; nanocrystalline Garnet; Gd3+ ions; Y3Ga5O12; ESR; Photoluminescence 1. Introduction: Ultraviolet radiation is a part of electromagnetic spectrum that reaches the earth from the Sun. It is well established that the light in the wavelength range 400-200 nm is normally referred to as ultraviolet (UV) radiation. The UV band (400-200 nm) is sub-divided into UVA (400-320 nm), UVB (320-280 nm) and UVC (280-200 nm). UVA and UVB are used in treating several skin diseases. UVC is generally used in germicidal applications such as killing bacteria in drinking water. Phototherapy device uses ultraviolet radiation for treatment of skin disorders. The specific ultraviolet radiation used in the phototherapy lamp is UVB [1-3]. UVB radiation is subdivided into broadband (300-280 nm) and narrowband (320-300 nm) emissions. During phototherapy
2
investigations it was found that UV in the narrow region showed remarkable therapeutic effects, while that in the broadband region was not very effective [4]. Investigations on narrowband ultraviolet-emitting phosphors are important because these materials have great potential to be applied in the phototherapy lamp industry. It is usually considered that Gd3+ 6P → 8S transitions are ideal for narrowband UVB emissions. Gd3+ ion is isoelectronic with Eu2+ and likewise has very long electron spin relaxation times, as a consequence, the luminescence of Gd3+ ion consists of sharp line transitions (6P → 8S), mainly at 311/312/313 nm. Gd3+ is of particular interest because of its U-spectrum. It is to be noted that various methods have been used to probe the local structural environment of gadolinium (Gd3+) ion in crystals and glasses. Among these, optical absorption, electron spin resonance (ESR) and Mössbauer spectroscopy are used extensively. In recent times, there has been an increase in the use of ESR spectroscopy as a method to understand the site symmetry of rare earth/transition metal ions when incorporated into host matrix. The usual methods of determining rare earth/transition metal ions concentrations in host matrices are chemical analysis and atomic absorption spectroscopy. Although they are accurate, both methods are destructive and relatively slow. However, ESR is considered as a nondestructive and comparatively easy method. In this context, we have studied the Gd-doped Y3Ga5O12 (YGG) phosphor using ESR spectroscopic method. There is a growing interest in the studies of garnets doped with transition-metal/rare-earth ions because of the wide practical applications in laser and luminescence materials [5-8]. Summaries of their spectroscopic investigations appear in detail in several review articles [9-12]. Garnet crystals doped with transition-metal/rare-earth ions are widely investigated and have found surprising applications in areas such as optical high-pressure sensor [13], luminescence 3
[14], solid state lasers [15, 16], scintillation [17, 18], medical procedures [19], imaging [20], displays [21], flow cytometry [22], phosphors [23-25]. Keeping in view of several potential applications and due to great demand of narrowband ultraviolet-emitting phosphors, in the present work, we have prepared gadolinium doped YGG phosphor. To the best of our knowledge, there are no reports about combustion synthesized Gd3+ doped YGG phosphor. Solution combustion method is well known and is widely used for synthesis of powders. Currently, there has been a significant interest in the solution combustion synthesis of complex oxide phosphors because it is innovative, simple, low toxicity of raw material, accurate stoichiometric ratio, low process temperature, considerable energy and time saving technique as compared to conventional methods. In this work, we successfully synthesized YGG:Gd sample by using solution combustion technique. In this paper we present photoluminescence (PL) and electron spin resonance (ESR) experiments on gadolinium doped YGG phosphor. Both PL and ESR techniques are well known as powerful experimental tools to study electronic and magnetic properties of transition metals and rare-earth ions in semiconductors and they have not been exploited up to now for gadolinium doped YGG phosphor. In addition, prepared sample was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. 2. Materials preparation and analysis In the present investigation, Y3Ga5O12:Gd(0.05) sample was synthesized using a combustion method. In a typical synthesis, Y(NO3)3∙6H2O, Ga(NO3)3∙xH2O, Gd(NO3)3∙6H2O and C2H5NO2 as starting materials. Glycine (C2H5NO2) was dissolved in a minimum quantity of deionized water in a porcelain dish with 100 ml capacity to obtain a homogeneous solution. Glycine (C2H5NO2) was added as a fuel for combustion and its amount was calculated using total 4
oxidizing (O) and reducing valences (F) based on the concept of propellant chemistry. To perform the combustion reaction the solution was transferred into a furnace preheated to 550oC. The water quickly evaporated and the mixture formed foam, within which a vigorous reaction between the nitrates and glycine soon initiated and ended. The entire combustion process was completed in about 3 to 5 minutes. The resultant fluffy white masses were crushed into a fine powder. The powder was then placed in alumina crucibles to be given an annealing treatment at 1000C for 4h in air to remove the thermal stress and impurities during its preparation. This prepared powder was utilized without any further treatments. Powder XRD pattern was recorded in the 2 range from 05o to 80o on a Philips X’Pert Xray diffractometer with graphite monochromatized CuK radiation. Powder morphology was studied using scanning electron microscopy (SEM, Hitachi, Japan). Emission and excitation spectra were recorded using a Fluorolog 3-22 spectrometer (Jobin Yvon) with a 450W xenon lamp as an excitation source. A powdered sample of 100 mg was taken in a quartz tube for the ESR measurements. ESR spectra of the sample were recorded on a JEOL FE1X ESR Spectrometer operating in the X-band frequencies with a field modulation of 100 kHz. 3. Results and discussion 3.1 X-ray diffraction Fig. 1 shows the X-ray diffraction pattern for the Gd3+ doped YGG at room temperature. The results of the combustion-derived powder were in good agreement with those reported in the PDF for YGG (JCPDS No. 83-1036, a=b=c =12.27Å), and the peaks correspond to a cubic structure with no additional peaks of other impurity phases. The cubic phase is not modified by the addition of gadolinium in YGG matrix. Compared with other reported methods as the sol-gel
5
[26, 27] and Czochralski [18] methods, the nanocrystalline YGG:Gd3+powder synthesized here by combustion method is at rather low calcining temperatures and the process is much simpler. The average crystallite size, D, can be estimated from the broadened peaks by using DebyeScherrer equation, D=0.9λ/βcosθ, where β is the full width at half maximum of a diffraction line located at an angle θ, and λ is the X-ray diffraction wavelength. The strongest peaks used to calculate sizes were 2θ = 29.01o, 32.41o, 35.60o, 53.62o, and 55.91o. The estimated average crystallite size of the sample was found to be ~26.18 nm.
3.2 SEM studies Fig. 2 shows the low and high magnification SEM images of YGG:Gd3+ powder sample. It can be observed from low resolution SEM image (Fig. 2A) that the powder shows agglomerates. Synthesized powders exhibit a high degree of porosity due to the large amount of gases that escape during combustion reaction. It is also noticed that pores are interconnected with a large number of voids and it is a typical nature of combustion derived powders. The release of gases generates many pores producing the synthesized powders friable and loosely agglomerated. Fig. 2B depicts the higher magnification SEM image that shows the presence of several small elongated shape particles. The presence of small particles and porosity are an inherent feature of combustion-derived powders. These types of porous powders are highly friable which facilitates easy grinding to obtain finer particles. From higher magnification SEM image it is clear that crystallites are fused together to form many nanoparticles with dumbbell shape. 3.3 Photoluminescence studies Gd3+ is one of the rare earth ions that have been widely used as an activator for different host materials to prepare phosphors. Figure 3 shows the photoluminescence (a) excitation and (b)
6
emission spectra of YGG:Gd3+ sample. The excitation spectrum was recorded when monitoring the Gd3+ emission at 312 nm. The sharp peaks shown in the figure can be assigned to the 4f→4f intraconfiguration forbidden transitions of Gd3+ [28]. Figure 3(a) shows the excitation bands at 244.4 nm (40916 cm-1), 246.5 nm (40567 cm-1), 252.9 nm (39541 cm-1), 273.2 nm (36603 cm-1), 276.0 nm (36231 cm-1), and 279.4 nm (35790 cm-1) which are attributed to the transitions of 8S7/2 → 6D5/2, 8S7/2 → 6D7/2, 8S7/2 → 6D9/2, 8S7/2 → 6I11/2, 8S7/2 → 6I9/2, 6I17/2, and 8S7/2 → 6I7/2, respectively. The observed 6D and 6I absorption bands are listed in Table 1. These excitation bands are in agreement with those reported by Alonso et al. [29] and Binnemans et al. [30] for Gd3+ ions in fluorozirconate glass matrix. Binnemans et al. [30] calculated the Judd-Ofelt parameters (Ω2, and Ω6) for Gd3+ in aqueous and crystalline environment. They reported that the Judd-Ofelt parameters are dependent on the local symmetry, differing from aqueous to amorphous and crystalline environments. Figure 3b shows the emission spectrum of YGG:Gd3+ sample. The emission spectrum was recorded when monitoring the Gd3+ excitation at 273 nm. The emission spectrum has two bands at 306.2 nm (32658 cm-1) and 311.9 nm (32061 cm-1) which were assigned to 6P5/2 → 8S7/2, 6P7/2 → 8S7/2 transitions, respectively. It is speculated that in the present system of YGG:Gd3+ phosphor Gd3+ ions occupy Y3+ ion sites as the ionic radii of Y3+ (1.02 Å) and Gd3+ (1.05 Å) are comparable. From several aforesaid reports [1-4], emission at 311.9 nm (6P7/2 → 8S7/2, Gd3+) could be classified as UVB radiation. Further, this phosphor can be investigated for possible use in phototherapy lamps to treat skin diseases like vitiligo, psoriasis, eczema and other photoresponsive skin disorders. In the next section, the existence of Gd3+ in YGG matrix was also confirmed by ESR experiments. 3.4 Electron Spin Resonance studies
7
The room temperature electron spin resonance (ESR) spectrum of YGG:Gd3+ sample is shown in Fig. 4(a). Some of the main signals are at g ≈ 2.0, 2.2 and 5.8. Low temperature ESR spectrum of YGG:Gd3+ sample at 123 K is shown in Fig. 4(b). As observed in the room temperature ESR spectrum, the low temperature ESR spectrum shows the same features as the room temperature spectrum. In accordance with the Boltzmann law, the intensity of the resonance signals increase with decreasing temperature. The g-values are found to be independent of temperature in the measured temperature range indicating that the structural environment do not change for the Gd3+ ion in the YGG lattice. Rare-earth ions Gd3+ and Eu2+ have half filled 4f shell with the electronic configuration [Xe]4f7. The spin quantum number S is 7/2 and the ground state is 8S7/2. With a pure S-state having L=0, the ions have a completely spherical charge cloud. The crystal field arising from the immediate neighboring ions will not affect the ion due to this spherical charge distribution and the crystal field interaction is negligibly small. However, higher order perturbations involving spin-orbit coupling give rise to crystal field effects and consequently there will be splittings. These perturbations admix higher states like 6P7/2 and 6D7/2 having L 0 with the pure S-state. The ion interacts with the crystal field through the mediation by spin-orbit admixtures and the ion does not couple directly with the crystal field. Ground state splittings originate from these admixtures and thus result in crystal field effects in the ESR spectrum. The local symmetry of the ion determines the crystal field interaction as the symmetry changes the nearest number of neighboring ions, their relative distances with respect to Gd3+ ion and their relative locations change. Magnitude of the crystal field interaction will change leading to a change in the observed ESR spectrum.
8
Gd3+ ion in host lattices like glass systems, disordered lattices and certain polycrystalline powder systems exhibit somewhat similar spectra [31-34] as in the present sample. Such spectrum, in Brodbeck and Iton [35] description, is termed as a ‘U-spectrum’. There have been several explanations for the observation of this kind of spectrum in Gd doped glass system or in disordered systems and the reasons are not satisfactory. Gd3+ ion located at sites with high coordination number are considered to give rise to signals at g = 5.6, 2.6 and 1.96 while low coordination number sites with strong crystal field is expected to lead to the weak shoulder at about g = 4.2. However, Broadbeck and Iton [35] in a critical analysis offer a more complete and clear explanation of the U-type spectra. The observed spectrum has been classified by them based on the magnitude of crystal field interaction (HCF) in relation to the Zeeman splitting h (where is the microwave frequency). Three regions have been identified which depend on the ratio HCF /h and the regions are weak, intermediate and strong. Weak crystal field (CF) region corresponds to the case where HCF /h ¼. In this region, ESR spectra are mainly concentrated in the g=2.0 region and higher order transitions are forbidden. Intermediate region is considered in two parts viz., (a) lower intermediate and (b) higher intermediate CF regions. ESR spectrum is spread over a wide range of g-values with values varying in the region 2.0 < g < and ¼ HCF/ h 1 in the lower intermediate region. On the other hand, for the case of higher intermediated CF regions, ESR lines with g > 2.0 are observed. Strong crystal field is characterized by H CF/ h≥4 and the ESR lines are due to transitions controlled by resonance within the Kramer’s levels. Y3Ga5O12 belongs to the A3B5O12 family of garnets and has a cubic symmetry structure with space group Ia3d [36]. The structure consists of a network of GaO6 octahedra and GaO4
9
tetrahedra connected by shared oxygen ions which are at the corners of a polyhedra. These polyhedra form chains along the three crystallographic directions and form dodecahedral cavities. Y3+ ions occupy these cavities. Hence, YGG lattice contain three crystallographically different cation sites. The Ga3+ ions are located in 24 tetrahedral S4 sites and 16 octahedral S6 sites with four fold and six fold coordination. On the other hand, Y3+ cations are located at a site coordinated to eight oxygens in a dodecahedral arrangement. The ionic radius of Y3+ ion is 1.02 Å in a 8-fold coordination. Ga3+ ion has 0.62 Å radius in an octahedral coordination (6-fold) and an ionic radius of 0.47 Å in a 4-fold coordination [37]. On the other hand, the ionic radii of Gd3+ ion are 0.94 Å and 1.05 Å in six-fold and eight-fold coordination, respectively. Due to similar ionic radii and chemical properties, lanthanide ions are expected to mainly enter into the distorted dodecahedral sites by replacing the Y3+ ions and to be coordinated to eight O2− ions [38, 39]. Therefore, it is very likely that Gd3+ ion in YGG occupies the Y3+ sites in the lattice. It was mentioned earlier that Gd3+ ion located in a crystalline lattices and at sites with minimal distortion experiencing weak crystal field gives rise to spectrum concentrated mainly in the g = 2.0 region. The observed spectrum, however, is characteristic of spectrum expected from a disordered system or from an ion experiencing a relatively strong crystalline field. Y3Al5O12 and Y3Ga5O12 [40] have the same crystalline structure. There will be a change in the lattice parameter in the two garnets as Ga ionic radius (0.62 Å) is larger than the Al ionic radius (0.53 Å) [41]. Nakatsuka et al. [42] have determined the proportion of covalent bonding in the Al-O and Ga-O bonds utilizing the equations given by Brown and Shanon [43]. The proportion of covalent bonding (q) is directly proportional to the covalency of the cation-oxygen bond. The
10
proportion of covalent bonding of Ga-O bond is estimated to be 43% (for octahedral site) whilst the value for Al-O bond is 33%. The values for tetrahedral site are 51.5% and 45%, respectively. These estimates show that covalency of Ga-O bond is higher than Al-O bonds and this introduces a distortion in the neighboring dodecahedra of Y3Ga5O12 system. As mentioned earlier, Gd3+ ion is located at the distorted dodecahedra. Due to the distortion, Gd3+ ion experiences a relatively stronger crystal field and one may expect lines in the g > 2.0 region. It is speculated that the ESR lines observed in YGG:Gd3+ is a reflection of stronger crystal field experienced by the Gd3+ ion.
4. Conclusions In this study we have demonstrated that using the solution combustions single-phase yttriumgallium garnet (Y3Ga5O12) can be produced at substantially lower temperatures than using the conventional solid-state method. X-ray diffraction study reveals that the nanocrystals are formed in a single phase of the garnet structure with cubic structure. By SEM analysis, it is observed that crystallites are fused together to form many nanoparticles with dumbbell shape. Upon ultraviolet excitation at 273 nm, narrowband emission with a peak wavelength at 311.9 nm was observed from the gadolinium 6P7/2 → 8S7/2 transition. Low field ESR lines as well as lines near g = 2.0 are observed in the ESR spectrum of Y3Ga5O12:Gd3+ sample. The observation of this kind spectrum seems to originate from the distortions of dodecahedral site resulting from the higher covalency of the Ga-O bond in Y3Ga5O12 as compared to the Al-O bond in Y3Al5O12 garnet. Further the present results, though preliminary in nature, suggest that the narrowband UVB emission peaks at 311.9 nm of the Gd3+, making it a possible phosphor for application in phototherapy lamps.
11
Acknowledgements This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030091450, 20153010092130).
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Acta Cryst. A29 (1973) 266-282.
Figure captions:
Fig. 1. Powder XRD pattern of Y3Ga5O12:Gd3+
16
(A)
(B)
Fig. 2. SEM images of Y3Ga5O12:Gd3+ (a)
1.0
273.2
0.012
252.9
0.8 0.6
PL Intensity (a.u.)
PL Intensity (a.u.)
0.010
276.0
246.5
0.008 244.4 0.006 0.004 0.002 240
0.4
250 Wavelength (nm)
260
252.9
0.2
279.4
246.5 244.4
0.0 240
250
260
270
280
290
Wavelength (nm)
(b)
17
311.9
PL intensity (a.u.)
1.0 0.8 0.6 0.4 0.2 0.0 290
306.2
300
310
320
330
340
350
Wavelength (nm)
Fig. 3. Photoluminescence spectra of Y3Ga5O12:Gd3+ (a) Excitation spectrum of Y3Ga5O12:Gd3+ (λem= 312 nm) and (b) Emission spectrum of Y3Ga5O12:Gd3+ (λex= 273 nm)
18
Fig. 4. ESR spectra of Y3Ga5O12:Gd3+ at room temperature (black) and at 123 K (blue).
19
Table 1: The observed band positions for Y3Ga5O12:Gd3+ along with their assignments
Transition
Observed band positions
S7/2 →
This work
8
Wavelength (nm)
Ref. [30]
Ref. [29]
Wavenumber (cm-1)
6
D5/2
244.4
40916
41070
41084
6
D7/2
246.5
40567
40770
40750
6
D9/2
252.9
39541
39730
39740
I11/2
273.2
36603
36540
36550
I9/2, 6I17/2
276.0
36231
36310
36337
279.4
35790
35910
35945
6
6
Wavenumber (cm-1)
Reported
6
I7/2
20