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Luminescence studies on undoped and lanthanide doped Zinc Tin Hydroxide
Journal of Luminescence 194 (2018) 675–681
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence studies on undoped and lanthanide doped Zinc Tin Hydroxide a,b,⁎
Dinesh K. Patel
, V. Sudarsan
a,⁎⁎
, S.K. Kulshreshtha
MARK
a
a
Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Casali Center for Applied Chemistry, Institute of Chemistry, The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Luminescence Europium ions Exciton recombination
Undoped and rare earth ions (Eu3+ and Dy3+) doped ZnSn(OH)6 particles were prepared at room temperature in aqueous medium. XRD studies confirmed that unit cell parameters increases with increase in concentration of lanthanide ions in the sample, suggesting its incorporation in the lattice. The particles are having cube like shape with size in the range of 200 nm, as revealed by TEM measurements. Luminescence studies confirm that host emission from the samples arises due to self-trapped exciton recombination with an activation energy for thermal quenching ~ 16.7 meV. Further it is inferred from the studies that lanthanide ions get incorporated up to 2% in the lattice. Energy transfer from host matrix to lanthanide ions has been observed in these samples.
1. Introduction Zinc hydroxystannate, ZnSn(OH)6 belongs to the category of perovskite structured hydroxides and has got potential application as nontoxic flame retardants, smoke suppressant and photo-catalysts [1–5]. In addition to this, it is also a precursor material for making composite oxides of tin and zinc which can be used for applications like gas sensing and energy storage [6,7]. ZnSn(OH)6 is also used as affinity probes (APs) for phosphopeptide enrichment and has shown selectivity towards multiple-phosphopeptides [8]. Addu et al. [9] have designed facile preparation method for a family of meso-cubes which include ZnSn(OH)6, Zn2SnO4 and SnO2, hollow cubes of SnO2 nanoparticles, nanoparticles of Zn2SnO4 and Sn@C, etc. It is also a potential anode material in lithium ion batteries with high packing densities. Number of reports are available on the synthesis and characterization of ZnSn (OH)6 [10–14] and they include methods like ion exchange, solid state metathesis, hydro/solvo thermal reaction, wet sono-chemical reactions, etc. [10–15]. It has been observed that above synthesis methods generally leads to formation of solid cube shaped or spherical ZnSn(OH)6 microcrystals. One of the important property which has been investigated in detail for ZnSn(OH)6 is their photo-catalytic activity. This is because ZnSn(OH)6 contains number of hydroxyl groups which can react with photo-generated holes leading to the formation of OH radicals. Presence of OH radical is known to improve the photo-catalytic activity of the material [4,5]. Even though there are number of studies on photo-catalytic properties of ZnSn(OH)6, there are only a very few studies on optical properties of this material, particularly absorption
⁎
and emission characteristics, nature of luminescent centers, effects of dopants, etc. Based on UV–visible optical absorption studies, bang gap of solid ZnSn(OH)6 cubes has been evaluated and found to be 4.1 eV [4,5]. The value changes to 3.8 eV, when the solid cubes become hollow. Information regarding the nature of luminescent centers, their lifetimes and their type of interaction with dopant luminescent ions is worth investigating as it will be helpful for understanding the optical and photocatalytic properties of the material. This is because as luminescence intensity from the sample increases, extent of radiative recombination increases and hence charge carriers are no longer available for oxidation and reduction reactions, thereby reducing photocatalytic activity. Keeping this in mind we have investigated optical properties of Zinc hydroxystannate both in presence and absence of dopant ions. For the first time, lanthanide ions doped Zinc hydroxystannate cubes have been synthesized at room temperature in aqueous medium. Energy transfer from host to lanthanide ions has been observed from these samples. Authors feel that inferences derived from the present study will be quite relevant while employing this material for photo-catalytic applications. 2. Preparation of lanthanide ion doped and undoped ZnSn(OH)6 samples 2.1. Reagents Zinc sulphate heptahydrate (ZnSO4·7H2O), sodium stannate (Na2SnO3), europium nitrate hydrate (Eu(NO3)3·xH2O), dysprosium
Corresponding author at: Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India. Corresponding author. E-mail addresses: [email protected] (D.K. Patel), [email protected] (V. Sudarsan).
⁎⁎
http://dx.doi.org/10.1016/j.jlumin.2017.09.034 Received 12 January 2017; Received in revised form 12 September 2017; Accepted 14 September 2017 Available online 19 September 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 194 (2018) 675–681
50 0
(a) (440)
(b )
Intensity
$
#
# # # #$ # #
(e) # #
# $$
50 25 0
Intensity
(422)
(024)
(400)
(311) (222)
(013)
(220)
(111)
(200)
D.K. Patel et al.
(d)
(c)
20 0
(b) 10 0
(c)
150 75 0
(d )
(a) 10
20
30
40
50
60
70
2θ /° Fig. 3. XRD patterns of ZnSn(OH)6 samples: (a) as prepared and heated at (b) 500 °C, (c) 600 °C, (d) 700 °C and (e) 900 °C. The symbols # and $ represent SnO2 and Zn2SnO4 phases respectively.
(e )
10
20
30
40
50
60
70 Weight Loss (%)
Fig. 1. XRD patterns of ZnSn(OH)6:Eu3+ samples containing (a) 0, (b) 0.5, (c) 1, (d) 2 and (e) 4 at% Eu3+ ions.
Table 1 Average crystallite size, unit cell parameters and cell volume for Eu3+ doped ZnSn(OH)6 samples. ZnSn(OH)6:Eu
Average crystallite size (nm)
a (Å)
Cell volume (Å)3
0% 0.5% 1.0% 2.0% 4.0% 6.0%
42 nm 42 nm 41 nm 43 nm 55 nm 37 nm
7.748 (3) 7.755 (2) 7.759 (3) 7.763(3) 7.767 (3) 7.766 (3)
465.23 466.42 467.12 467.73 468.70 468.40
TG DTA 688°C 237°C
2 0 -2 -4 -6 -8 -10
200
-12 400 600 800 1000 1200 Temperature (°C)
Fig. 4. DTA and TG curves corresponding to ZnSn(OH)6 sample.
Fig. 2. TEM images (a and b) and SAED pattern (c) of ZnSn(OH)6 sample.
676
Heat Flow (μW)
2 θ /°
104 102 100 98 96 94 92 90 88
Journal of Luminescence 194 (2018) 675–681
7
8
6
7
5
Intensity (arb. units)
Intensity (arb. units)
D.K. Patel et al.
77K 100K 150K 200K 250K 300K
4 3 2 1 0 400
450
500
550
600
650
(a)
Wavelength (nm)
77K 100K 150K 200K 250K 300K
253 nm
6
280 nm
5 4 3 2 1 0 200
250
300
Wavelength (nm)
350
(b)
Fig. 5. Emission spectra (a) from undoped ZnSn(OH)6 cubes at different temperatures. The excitation wavelength was 253 nm. Corresponding excitation spectra monitored at host emission (430 nm) are shown in (b).
Intensity
1
Samples were dispersed in methanol and a drop of this solution was put on carbon coated copper grid and dried properly prior to loading in TEM machine. All steady state luminescence and lifetime measurements were carried out at room temperature by using an Edinburgh Instrument FLSP 920 system, having a 450 W Xe lamp, a microsecond and a nanosecond flash lamp as the excitation sources. Around 20 mg of sample was mixed with 1 ml of methanol, made into slurry and spread over a quartz plate and dried under ambient conditions. The quartz slides with sample are kept at an angle of 45° with the incident beam using a solid state sample holder having facility for 3600 rotation. Emitted light was collected by using a red sensitive photomultiplier tube. Both emission and excitation spectra were corrected for the detector response and lamp profile respectively. Slit widths for the emission and excitation monochromators were maintained at 5 nm for all the measurements. It should be noted that pulse duration of microsecond flash lamp is around 1–2 µs.
Experimental Instrument response Fitted curve
0.1
0.01 20
40
60 Time (ns)
80
100
Fig. 6. Decay curve of undoped ZnSn(OH)6 upon 280 nm excitation and 430 nm emission at 77 K.
3. Results and discussion Fig. 1 shows XRD patterns of ZnSn(OH)6 samples containing different amounts of Eu3+ ions. All the patterns are characteristic of cubic structure of ZnSn(OH)6 phase. Average crystallite size calculated from line width of the XRD patterns using Scherrer equation is found to be around 42 nm as can be seen from Table 1. Average crystallite size remains same for samples containing up to 2% Eu3+. However for samples with 4 and 6 at% Eu3+ incorporation, average crystallite size values are higher or slightly lower. The exact reason for this could not be identified. Probably within error limits, they are same, as there cannot be any significant change in nucleation and growth mechanisms with the presence of small amounts of Eu3+ ions in the reaction medium. With increase in Eu3+ concentration, XRD peak at 2θ value of 22.94° shifts towards lower values as can be seen Fig. S1 of Supporting information. Lattice parameters evaluated from the diffraction patterns are given in Table 1. The value of unit cell parameter “a” is found to increase systematically up to 2% of Eu3+doping and beyond which it remains nearly same. This confirms the incorporation of Eu3+ ions in the lattice. In ZnSn(OH)6 both Zn2+ and Sn4+ are octahedrally coordinated [4] by oxygen ions. Ionic radii of Zn2+ and Sn4+ under octahedral configurations are 0.74 Å and 0.69 Å respectively whereas that of Eu3+ under the same coordination environment is 0.947 Å [16]. So it is likely that Eu3+ occupy Zn2+ site and create cation vacancies in the lattice. Up to certain extent of Eu3+ ions (up to 2 at% in the present case) cation vacancies are generally stabilized in the lattice. Beyond 2 at% Eu3+, due to increase in cation vacancies and associated poor stability, lattice do not accommodate Eu3+ ions, leading to identical lattice parameter for higher concentrations of Eu3+ ions. There is a
carbonate hydrate (Dy2(CO3)3.xH2O) were used as received without any further purification. 2.2. Preparation of undoped ZnSn(OH)6 sample In a typical procedure, 0.26 g of ZnSO4·7H2O was dissolved in 10 ml distilled water and to this solution 5 ml aqueous solution containing 0.3 g of sodium stannate was added dropwise at room temperature while stirring. The stirring was continued for 4 h. The precipitate obtained was washed twice with methanol followed by acetone and dried under ambient conditions to obtain crystalline ZnSn(OH)6. Similar procedure was used for preparing europium/dysprosium doped ZnSn (OH)6 samples except that the required amount ‘x’ at% of RE3+ ions where RE = Eu, Dy, etc. were added to the solution containing ‘100 − x’ at% of Zn2+ ions prior to the addition of Na2SnO3. 2.3. Characterization X-ray diffraction (XRD) studies were carried out by using Philips powder X-ray diffractometer (model PW 1071) with Ni filtered Cu-Kα radiation. Lattice parameters were calculated from least square fitting of diffraction peaks. Average crystallite size was calculated based on Scherrer relation, D = 0.9λ/βcosθ, where D is average particles size, λ is wavelength of X-rays and β is full width at half maximum (FWHM). Transmission electron microscopic (TEM) measurements were performed using 200 keV electrons in JEOL 2010 UHR TEM microscope. 677
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Fig. 7. Emission spectrum (a) and excitation spectrum (b) at room temperature for ZnSn(OH)6 cubes doped with different amounts of Eu3+ ions. Samples were excited at 394 nm and emission was monitored at 615 nm.
by following reactions.
possibility that protons present with surface hydroxyl groups of ZnSn (OH)6 can undergo ion exchange with Eu3+ ions (each Eu3+ ion can exchange with three protons) thereby leading to lattice expansion. A similar situation is observed in our earlier studies with GaOOH lattice [17]. However at this stage authors do not have an experimental proof to confirm this aspect in the present study. Fig. 2(a) and (b) show the TEM images of as prepared ZnSn(OH)6 sample. The images are characterized by cube shaped particles having average size in the range of around 200 nm (with a deviation of around ± 10 nm). There are also small cubes with size in the range of 40–100 nm. It is interesting see that there exists a large difference in the values of average crystallite size obtained from XRD line width and the particle size obtained from TEM images. This is because XRD line widths are measure of coherent regions present in the sample whereas TEM gives the actual particle size. In a single particle there can be several coherent regions and this can lead to lower average crystallite size when compared to particle size (measured from TEM mages). Selected area electron diffraction (SAED) pattern (Fig. 2(c)) consists of mainly dots, revealing single crystalline nature of sample with orientation in a particular direction. In order to understand the thermal stability of ZnSn(OH)6 phase, as prepared sample was heated at different temperatures and corresponding XRD patterns are shown in Fig. 3. It is observed that upon heating the as prepared sample to around 500 °C (Fig. 3(b)), crystalline ZnSn(OH)6 gets converted to an amorphous product and this agrees well with that reported earlier [18]. However, upon further heating to 600 °C, amorphous phase undergoes decomposition to form Zn2SnO4 and SnO2 phases as can be seen from Fig. 3(c). Decomposition of ZnSn(OH)6 can be represented in two steps
2ZnSn (OH)6 → 2ZnSnO3 + 6H2 O (Amorphous)
2ZnSnO3 (Amorphous)
→ Zn2 SnO4 + SnO2
(1) (2)
In order to further substantiate the fact that as prepared ZnSn(OH)6 cubes decompose upon heating, differential thermal analysis (DTA) along with thermo gravimetric analysis (TGA) were performed on the sample and both DTA and TGA curves are shown in Fig. 4. The exothermic peak around 237 °C (in DTA) and a weight loss of 16.4% (in TGA) is characteristic of the removal of three molecules of water leading to the formation of amorphous ZnSnO3 phase (step 1). On further heating, the ZnSnO3 phase decomposes to Zn2SnO4 and SnO2 phases around 688 °C (step 2). In an earlier study, we have reported blue emission from SrSn(OH)6 and confirmed that the emission arises due to recombination of selftrapped excitons [19]. To confirm whether such type of emission occurs in ZnSn(OH)6 also, detailed photoluminescence studies were carried out. Fig. 5(a) shows emission spectrum from undoped ZnSn(OH)6 sample recorded at different temperatures obtained after 253 nm excitation. At room temperature a very weak emission is observed around 425 nm. However up on cooling the sample up to 77 K, emission intensity increases and below 100 K, the intensity remains more or less same as can be seen from Fig. 5(a). A similar trend is also observed in the excitation spectrum shown in Fig. 5(b). At low temperature, the excitation spectrum consists of a broad asymmetric peak around 250 nm. The peak can be de-convoluted into two Gaussian peaks having peak maximum around 248 and 263 nm as can be seen from the 678
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1000 100
instrument response and sample response. By carrying out a re-convolution fit (removing the instrument response), the life time value is found to be ~ 6 ns. It is to be noted that self-trapped excitonic emission and corresponding decay depend on number of factors like nature of the host, extent of singlet and triplet character of traps (levels), interaction of trap levels with other levels etc. Lifetime values can vary from few nanoseconds to hundreds of nanoseconds depending up on the host and nature of trap levels [20]. Activation energy for thermal quenching of self-trapped excitonic emissions in ZnSn(OH)6 has been calculated from the straight line plot of variation of logarithm of emission intensity as a function of inverse of temperature. The integrated emission intensity normally decreases with increasing temperature and can be expressed by Eq. (3).
0.5% Eu
10 1 1000 100 10 1 10000 1000 100 10 1 10000 1000 100 10 1 10000 1000 100 10 1
Counts
1% Eu
2% Eu
I=
6% Eu
2
3
4
5
Time (ms) Fig. 8. Decay curves corresponding to 5D0 level of Eu3+ from ZnSn(OH)6 cubes containing different amounts of Eu3+ ions at room temperature.
Table 2 Lifetime values corresponding to the 5D0 level of Eu3+ from ZnSn(OH)6:Eu cubes. Numbers in brackets give relative percentage of different lifetime components. Excitation and emission wavelengths are 394 and 615 nm respectively. ZnSn(OH)6:Eu (%)
τ1 (µs)
τ2 (µs)
τ3 (µs)
τavg. (µs)
0.5% Eu
8.15 (5.7%) 130 (6.2%) 9.21 (2.9%) 7.25 (1.9%) 7.73 (0.4%)
343 (94.3%) 384 (93.8%) 398.6 (97.1%) 177.42 (7.8%) 187 (7.7%)
–
323.8
–
368.2
–
387.2
369.3 (90.3%) 349 (91.9%)
347.2
2.0% Eu 4.0% Eu 6.0% Eu
Ea
(3)
where I0 is the peak intensity at T = 0 K, c is a constant, Ea is the activation energy for thermal quenching process, and kB is Boltzmann constant [21–23]. From Eq. (3), Ea was found to be 16.7 meV (The numerical value of “c” is 237.8 and “I0” is 1.54 × 106 cps K−1). In order to further modify luminescence properties of ZnSn(OH)6 cubes, lanthanide ions, Eu3+ and Dy3+ were doped in the matrix and results are discussed in the following section. Fig. 7(a) shows room temperature emission spectrum from ZnSn (OH)6 cubes containing different amounts of europium ions obtained after 394 nm excitation. The emission spectrum is mainly characterized by peaks around 590 and 615 nm, which are due to magnetic and electric dipole allowed 5D0 → 7F1 and 5D0 → 7F2 transitions respectively, of Eu3+ ions. The relative intensity ratio of 5D0 → 7F2 (615 nm) to 5D0 → 7F1 (590 nm) transition, known as the asymmetric ratio of luminescence [24–27], is found to increase from 3.5 to 4.4 with increase in concentration of Eu3+ ions up to 2 at% and beyond 2 at%, it remains constant. For 4 and 6 at% Eu containing samples, the asymmetric ratio is found to be around 3.2 indicating that beyond 2 at% Eu3+ ions, europium forms separate phase. Further higher values of asymmetric ratio suggest that Eu3+ environment does not have a center of symmetry. Excitation spectra corresponding to 615 nm emission from ZnSn (OH)6 cubes containing different amounts of europium ions are shown in Fig. 7(b). The patterns consist of sharp peaks characteristic of intra 4 f transitions of Europium along with broad asymmetric peak over the region of 230–332 nm. The broad peak is considered to be arising due to overlapping of peaks characteristic of Eu-O charge transfer (~ 265 nm) and host excitation (~ 280 nm). Observation of host excitation peak while monitoring Eu3+ emission from the sample suggests that there is energy transfer between Eu3+ and host ZnSn(OH)6 lattice. As concentration of Eu3+ ions increases, Eu-O charge transfer process (265 nm peak) predominates over the host excitation thereby leading to shift of overall excitation peak maxima towards lower wavelength values (blue shift). At room temperature, the host absorption (280 nm) and emission peak are broad and less intense due to relatively large extent of non-radiative processes and scattering back ground arising from powder samples. Unlike this, Eu3+ absorption (along with O-Eu CTB) and emission are not much affected by temperature variation and this lead to predominance of Eu-O charge transfer peak (around 265 nm) over the peak characteristic of host in the excitation spectra. To further understand the nature of environment around Eu3+ ions in the sample, decay curve corresponding to 5D0 level of Eu3+ ions were recorded and are shown in Fig. 8. The corresponding lifetime values are given in Table 2. Decay curves are found to be nearly bi-exponential for samples doped with Eu3+ ions up to 2 at%. Shorter lifetime corresponds to Eu3+ ions present on the surface and longer lifetime corresponds to Eu3+ ions present in the bulk of ZnSn(OH)6 cubes [24,25]. For the sample containing 1 at% Eu3+, the lifetime components are found to be 130 µs (6.2%) and 384 µs (93.8%) with an average an average value of
4% Eu
1
1.0% Eu
I0 1 + ce (− kB T )
335.2
representative de-convoluted patterns corresponding to 150 and 100 K, shown in Fig. S2 of the Supporting information. The line shapes are same until 250 K. As the temperature reaches 300 K, the line shape changes slightly and two peaks (less intense) are observed at 247 and 280 nm. The possible reason for this could be that, as the temperatures increases some of the defects/traps which are close to conduction or valence band are not able to contribute in emission and these levels no longer trap charge carriers. Significant Stoke shift associated with emission and excitation spectra as well as increase in intensity of emission with decrease in temperature suggest that emission from the sample is arising due to recombination of self-trapped excitons. At room temperature, lifetime values corresponding to this emission could not be measured accurately as corresponding excited state decays with a very short lifetime which is beyond the measurable limit of our experimental system. But on cooling the sample to 77 K, we could measure lifetime as can be seen from the decay curve shown in Fig. 6. The decay is found to be multi-exponential and it is a combination of 679
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Fig. 9. Emission spectrum (a) and excitation spectrum (b) at room temperature from ZnSn(OH)6:Dy3+. Excitation wavelength was 280 nm and emission wavelength was 574 nm.
excited state. This leads to poor energy transfer from host to lanthanide ions as can be seen from Fig. 9(b). Our study reveals that ZnSn(OH)6 is not a good host for luminescent application, but can act as a good photo-catalyst as it prevents radiative recombination of electrons and holes.
around 368 µs. The shorter life time component for this sample is higher when compared to other samples. This is possibly arising from shift in the initial time selected while carrying out tail fit of corresponding decay curve. Also the error associated with this value must be higher when compared with major lifetime component and its relative percentage. For samples containing more than 2 at% Eu3+ ions, decay curve becomes tri-exponential. As discussed above, first shorter component corresponds to Eu3+ ions on the surface of particle, second component corresponds to Eu3+ ions present in the bulk and third component corresponds to separate europium containing phase. Such separate Eu3+ containing phase, upon finely mixing with ZnSn(OH)6 phase, is expected to show longer Eu3+ lifetime values compared to Eu3+ ions occupying a distorted environment in ZnSn(OH)6 lattice with nearby OH groups. Average lifetime value also increases upto 2 at% of Eu3+ ions and beyond which it remains almost constant. XRD and luminescence results indicate that upto 2 at%, Eu3+ ions get incorporated in the lattice. This is also in agreement with the asymmetric ratio values of luminescence obtained from emission spectra (Fig. 7(a)). To confirm the energy transfer from host to lanthanide ions, ZnSn (OH)6 was also doped with Dy3+ions. Unlike Eu3+ ion, Dy3+, do not have a charge transfer peak in the region corresponding to host absorption and hence excitation spectrum from Dy3+ doped sample can be used to confirm energy transfer from host to lanthanide ions as well as incorporation of lanthanide ions in the lattice. Fig. 9(a) shows the emission spectrum form 0.5 at% Dy3+ doped ZnSn(OH)6 cubes obtained after 280 nm excitation. The spectrum consists of host emission around 430 nm, over which sharp peaks corresponding to Dy3+ transitions at 480 and 575 nm, are superimposed. Observation of both host and Dy3+ emission obtained by host excitation at 280 nm suggests that Dy3+ ions are getting incorporated in the ZnSn(OH)6 host and there exists energy transfer from host to Dy3+ ions. The excitation spectrum corresponding to 575 nm emission from the sample is shown in Fig. 9(b). The spectrum consists of a broad peak around 280 nm which is characteristic of the host excitation. Observation of host excitation peak in the excitation spectrum obtained by monitoring Dy3+ emission further confirms the existence energy transfer from host to Dy3+/lanthanide ions and incorporation of Dy3+ ions in ZnSn(OH)6 lattice. Lifetime corresponding to host emission from the samples with and without Dy3+ doping will give an idea regarding the extent of energy transfer from host to Dy3+. However this could not be done as host lifetime from the sample doped with Dy3+ ions is very small (much less than 1 ns) and the same could not be measured accurately with the nanosecond flash lamp (output pulse width around 1 ns) existing with the machine. The excitation spectrum is noisy probably due to the lower concentration of Dy3+ ions combined with its poor luminescent intensity. Further OH groups in the host facilitate non-radiative decay of excited charge carriers as well as lanthanide ion
4. Conclusions Undoped and lanthanide ions doped cube shaped ZnSn(OH)6 crystals were prepared at room temperature in aqueous medium. Luminescence studies carried out at different temperature confirmed that host emission from the sample arises due to self-trapped exciton recombination with an activation energy for thermal quenching of emission ~ 16.7 meV. From emission and excitation measurements carried out using Eu3+ and Dy3+ as representative dopant lanthanide ions, occurrence of energy transfer from host to lanthanide ions has been established. Further XRD and luminescence studies also confirmed lanthanide ion incorporation in ZnSn(OH)6 lattice. Acknowledgements Authors would like to acknowledge Dr. V. K. Jain, Former Head, Chemistry Division and Dr. B. N. Jagatap, Former Director, Chemistry Group, Bhabha Atomic Research Centre for their encouragements during the course of this work. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.09.034. References [1] P.A. Cusack, A.W. Monk, J.A. Pearce, S.J. Reynolds, Fire Mater. 14 (1989) 23. [2] P.A. Cusack, Fire Mater. 17 (1993) 1. [3] L. Wang, K. Tang, Z. Liu, D. Wang, J. Sheng, W. Cheng, J. Mater. Chem. 21 (2011) 4352. [4] X. Fu, X. Wang, Z. Ding, D.Y.C. Leung, Z. Zhang, J. Long, W. Zhang, Z. Li, X. Fu, Appl. Catal. B 91 (2009) 67. [5] X. Fu, D.Y.C. Leung, X. Wang, W. Xue, X. Fu, Int. J. Hydrog. Energy 36 (2011) 1524. [6] W.J. Moon, J.H. Yu, G.M. Choi, Sens. Actuators B 80 (2001) 21. [7] T. Lana-Villarreal, G. Boschloo, A. Hagfeldt, J. Phys. Chem. C 111 (2007) 5549. [9] S.K. Addu, Facile Fabrication of Mesostructured Zn2SnO4 Based Anode Materials for Reversible Lithium Ion Storage (Theses), 332 Wayne State University, 2014, http:// digitalcommons.wayne.edu/oa_theses/332. [8] L.-P. Li, T. Zheng, L.-N. Xu, Z. Li, L.-D. Sun, Z.-X. Nie, Y. Bai, H.-W. Liu, Chem. Commun. 49 (2013) 1762. [10] S.K. Addu, J. Zhu, K.Y. Simon Ng, D. Deng, Chem. Mater. 26 (2014) 4472. [11] J.W. Kramer, S.A. Isaacs, V. Manivannan, J. Mater. Sci. 44 (2009) 3387. [12] J.W. Kramer, B. Kelly, V. Manivannan, Cent. Eur. J. Chem. 8 (2010) 65. [13] G. Wrobel, M. Piech, S. Dardona, Y. Ding, P.-X. Gao, Cryst. Growth Des. 9 (2009)
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[21] S.A. Arrhenius, Z. Physik, Chem 4 (1889) 96. [22] S.-J. Park, Y.-H. Hwang, H.-K. Kim, J. Korean Phys. Soc. 60 (3) (2012) 466. [23] T. Li, A.M. Fischer, Q.Y. Wei, F.A. Ponce, T. Detchprohm, C. Wetzel, Appl. Phys. Lett. 96 (2010) 0319061. [24] D.K. Patel, A. Sengupta, B. Vishwanadh, V. Sudarsan, R.K. Vatsa, R.M. Kadam, S.K. Kulshreshtha, Eur. J. Inorg. Chem. 2012 (10) (2012) 1609. [25] D.K. Patel, B. Vishwanadh, V. Sudarsan, R.K. Vatsa, S.K. Kulshreshtha, J. Am. Ceram. Soc. 94 (2) (2011) 482. [26] D.K. Patel, B. Rajeswari, V. Sudarsan, R.K. Vatsa, R.M. Kadam, S.K. Kulshreshtha, Dalton Trans. 41 (39) (2012) 12023. [27] D.K. Patel, B. Vishwanadh, V. Sudarsan, S.K. Kulshreshtha, J. Am. Ceram. Soc. 96 (12) (2013) 3857.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence studies on undoped and lanthanide doped Zinc Tin Hydroxide a,b,⁎
Dinesh K. Patel
, V. Sudarsan
a,⁎⁎
, S.K. Kulshreshtha
MARK
a
a
Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Casali Center for Applied Chemistry, Institute of Chemistry, The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Luminescence Europium ions Exciton recombination
Undoped and rare earth ions (Eu3+ and Dy3+) doped ZnSn(OH)6 particles were prepared at room temperature in aqueous medium. XRD studies confirmed that unit cell parameters increases with increase in concentration of lanthanide ions in the sample, suggesting its incorporation in the lattice. The particles are having cube like shape with size in the range of 200 nm, as revealed by TEM measurements. Luminescence studies confirm that host emission from the samples arises due to self-trapped exciton recombination with an activation energy for thermal quenching ~ 16.7 meV. Further it is inferred from the studies that lanthanide ions get incorporated up to 2% in the lattice. Energy transfer from host matrix to lanthanide ions has been observed in these samples.
1. Introduction Zinc hydroxystannate, ZnSn(OH)6 belongs to the category of perovskite structured hydroxides and has got potential application as nontoxic flame retardants, smoke suppressant and photo-catalysts [1–5]. In addition to this, it is also a precursor material for making composite oxides of tin and zinc which can be used for applications like gas sensing and energy storage [6,7]. ZnSn(OH)6 is also used as affinity probes (APs) for phosphopeptide enrichment and has shown selectivity towards multiple-phosphopeptides [8]. Addu et al. [9] have designed facile preparation method for a family of meso-cubes which include ZnSn(OH)6, Zn2SnO4 and SnO2, hollow cubes of SnO2 nanoparticles, nanoparticles of Zn2SnO4 and Sn@C, etc. It is also a potential anode material in lithium ion batteries with high packing densities. Number of reports are available on the synthesis and characterization of ZnSn (OH)6 [10–14] and they include methods like ion exchange, solid state metathesis, hydro/solvo thermal reaction, wet sono-chemical reactions, etc. [10–15]. It has been observed that above synthesis methods generally leads to formation of solid cube shaped or spherical ZnSn(OH)6 microcrystals. One of the important property which has been investigated in detail for ZnSn(OH)6 is their photo-catalytic activity. This is because ZnSn(OH)6 contains number of hydroxyl groups which can react with photo-generated holes leading to the formation of OH radicals. Presence of OH radical is known to improve the photo-catalytic activity of the material [4,5]. Even though there are number of studies on photo-catalytic properties of ZnSn(OH)6, there are only a very few studies on optical properties of this material, particularly absorption
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and emission characteristics, nature of luminescent centers, effects of dopants, etc. Based on UV–visible optical absorption studies, bang gap of solid ZnSn(OH)6 cubes has been evaluated and found to be 4.1 eV [4,5]. The value changes to 3.8 eV, when the solid cubes become hollow. Information regarding the nature of luminescent centers, their lifetimes and their type of interaction with dopant luminescent ions is worth investigating as it will be helpful for understanding the optical and photocatalytic properties of the material. This is because as luminescence intensity from the sample increases, extent of radiative recombination increases and hence charge carriers are no longer available for oxidation and reduction reactions, thereby reducing photocatalytic activity. Keeping this in mind we have investigated optical properties of Zinc hydroxystannate both in presence and absence of dopant ions. For the first time, lanthanide ions doped Zinc hydroxystannate cubes have been synthesized at room temperature in aqueous medium. Energy transfer from host to lanthanide ions has been observed from these samples. Authors feel that inferences derived from the present study will be quite relevant while employing this material for photo-catalytic applications. 2. Preparation of lanthanide ion doped and undoped ZnSn(OH)6 samples 2.1. Reagents Zinc sulphate heptahydrate (ZnSO4·7H2O), sodium stannate (Na2SnO3), europium nitrate hydrate (Eu(NO3)3·xH2O), dysprosium
Corresponding author at: Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India. Corresponding author. E-mail addresses: [email protected] (D.K. Patel), [email protected] (V. Sudarsan).
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http://dx.doi.org/10.1016/j.jlumin.2017.09.034 Received 12 January 2017; Received in revised form 12 September 2017; Accepted 14 September 2017 Available online 19 September 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
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2θ /° Fig. 3. XRD patterns of ZnSn(OH)6 samples: (a) as prepared and heated at (b) 500 °C, (c) 600 °C, (d) 700 °C and (e) 900 °C. The symbols # and $ represent SnO2 and Zn2SnO4 phases respectively.
(e )
10
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40
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70 Weight Loss (%)
Fig. 1. XRD patterns of ZnSn(OH)6:Eu3+ samples containing (a) 0, (b) 0.5, (c) 1, (d) 2 and (e) 4 at% Eu3+ ions.
Table 1 Average crystallite size, unit cell parameters and cell volume for Eu3+ doped ZnSn(OH)6 samples. ZnSn(OH)6:Eu
Average crystallite size (nm)
a (Å)
Cell volume (Å)3
0% 0.5% 1.0% 2.0% 4.0% 6.0%
42 nm 42 nm 41 nm 43 nm 55 nm 37 nm
7.748 (3) 7.755 (2) 7.759 (3) 7.763(3) 7.767 (3) 7.766 (3)
465.23 466.42 467.12 467.73 468.70 468.40
TG DTA 688°C 237°C
2 0 -2 -4 -6 -8 -10
200
-12 400 600 800 1000 1200 Temperature (°C)
Fig. 4. DTA and TG curves corresponding to ZnSn(OH)6 sample.
Fig. 2. TEM images (a and b) and SAED pattern (c) of ZnSn(OH)6 sample.
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Heat Flow (μW)
2 θ /°
104 102 100 98 96 94 92 90 88
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600
650
(a)
Wavelength (nm)
77K 100K 150K 200K 250K 300K
253 nm
6
280 nm
5 4 3 2 1 0 200
250
300
Wavelength (nm)
350
(b)
Fig. 5. Emission spectra (a) from undoped ZnSn(OH)6 cubes at different temperatures. The excitation wavelength was 253 nm. Corresponding excitation spectra monitored at host emission (430 nm) are shown in (b).
Intensity
1
Samples were dispersed in methanol and a drop of this solution was put on carbon coated copper grid and dried properly prior to loading in TEM machine. All steady state luminescence and lifetime measurements were carried out at room temperature by using an Edinburgh Instrument FLSP 920 system, having a 450 W Xe lamp, a microsecond and a nanosecond flash lamp as the excitation sources. Around 20 mg of sample was mixed with 1 ml of methanol, made into slurry and spread over a quartz plate and dried under ambient conditions. The quartz slides with sample are kept at an angle of 45° with the incident beam using a solid state sample holder having facility for 3600 rotation. Emitted light was collected by using a red sensitive photomultiplier tube. Both emission and excitation spectra were corrected for the detector response and lamp profile respectively. Slit widths for the emission and excitation monochromators were maintained at 5 nm for all the measurements. It should be noted that pulse duration of microsecond flash lamp is around 1–2 µs.
Experimental Instrument response Fitted curve
0.1
0.01 20
40
60 Time (ns)
80
100
Fig. 6. Decay curve of undoped ZnSn(OH)6 upon 280 nm excitation and 430 nm emission at 77 K.
3. Results and discussion Fig. 1 shows XRD patterns of ZnSn(OH)6 samples containing different amounts of Eu3+ ions. All the patterns are characteristic of cubic structure of ZnSn(OH)6 phase. Average crystallite size calculated from line width of the XRD patterns using Scherrer equation is found to be around 42 nm as can be seen from Table 1. Average crystallite size remains same for samples containing up to 2% Eu3+. However for samples with 4 and 6 at% Eu3+ incorporation, average crystallite size values are higher or slightly lower. The exact reason for this could not be identified. Probably within error limits, they are same, as there cannot be any significant change in nucleation and growth mechanisms with the presence of small amounts of Eu3+ ions in the reaction medium. With increase in Eu3+ concentration, XRD peak at 2θ value of 22.94° shifts towards lower values as can be seen Fig. S1 of Supporting information. Lattice parameters evaluated from the diffraction patterns are given in Table 1. The value of unit cell parameter “a” is found to increase systematically up to 2% of Eu3+doping and beyond which it remains nearly same. This confirms the incorporation of Eu3+ ions in the lattice. In ZnSn(OH)6 both Zn2+ and Sn4+ are octahedrally coordinated [4] by oxygen ions. Ionic radii of Zn2+ and Sn4+ under octahedral configurations are 0.74 Å and 0.69 Å respectively whereas that of Eu3+ under the same coordination environment is 0.947 Å [16]. So it is likely that Eu3+ occupy Zn2+ site and create cation vacancies in the lattice. Up to certain extent of Eu3+ ions (up to 2 at% in the present case) cation vacancies are generally stabilized in the lattice. Beyond 2 at% Eu3+, due to increase in cation vacancies and associated poor stability, lattice do not accommodate Eu3+ ions, leading to identical lattice parameter for higher concentrations of Eu3+ ions. There is a
carbonate hydrate (Dy2(CO3)3.xH2O) were used as received without any further purification. 2.2. Preparation of undoped ZnSn(OH)6 sample In a typical procedure, 0.26 g of ZnSO4·7H2O was dissolved in 10 ml distilled water and to this solution 5 ml aqueous solution containing 0.3 g of sodium stannate was added dropwise at room temperature while stirring. The stirring was continued for 4 h. The precipitate obtained was washed twice with methanol followed by acetone and dried under ambient conditions to obtain crystalline ZnSn(OH)6. Similar procedure was used for preparing europium/dysprosium doped ZnSn (OH)6 samples except that the required amount ‘x’ at% of RE3+ ions where RE = Eu, Dy, etc. were added to the solution containing ‘100 − x’ at% of Zn2+ ions prior to the addition of Na2SnO3. 2.3. Characterization X-ray diffraction (XRD) studies were carried out by using Philips powder X-ray diffractometer (model PW 1071) with Ni filtered Cu-Kα radiation. Lattice parameters were calculated from least square fitting of diffraction peaks. Average crystallite size was calculated based on Scherrer relation, D = 0.9λ/βcosθ, where D is average particles size, λ is wavelength of X-rays and β is full width at half maximum (FWHM). Transmission electron microscopic (TEM) measurements were performed using 200 keV electrons in JEOL 2010 UHR TEM microscope. 677
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Fig. 7. Emission spectrum (a) and excitation spectrum (b) at room temperature for ZnSn(OH)6 cubes doped with different amounts of Eu3+ ions. Samples were excited at 394 nm and emission was monitored at 615 nm.
by following reactions.
possibility that protons present with surface hydroxyl groups of ZnSn (OH)6 can undergo ion exchange with Eu3+ ions (each Eu3+ ion can exchange with three protons) thereby leading to lattice expansion. A similar situation is observed in our earlier studies with GaOOH lattice [17]. However at this stage authors do not have an experimental proof to confirm this aspect in the present study. Fig. 2(a) and (b) show the TEM images of as prepared ZnSn(OH)6 sample. The images are characterized by cube shaped particles having average size in the range of around 200 nm (with a deviation of around ± 10 nm). There are also small cubes with size in the range of 40–100 nm. It is interesting see that there exists a large difference in the values of average crystallite size obtained from XRD line width and the particle size obtained from TEM images. This is because XRD line widths are measure of coherent regions present in the sample whereas TEM gives the actual particle size. In a single particle there can be several coherent regions and this can lead to lower average crystallite size when compared to particle size (measured from TEM mages). Selected area electron diffraction (SAED) pattern (Fig. 2(c)) consists of mainly dots, revealing single crystalline nature of sample with orientation in a particular direction. In order to understand the thermal stability of ZnSn(OH)6 phase, as prepared sample was heated at different temperatures and corresponding XRD patterns are shown in Fig. 3. It is observed that upon heating the as prepared sample to around 500 °C (Fig. 3(b)), crystalline ZnSn(OH)6 gets converted to an amorphous product and this agrees well with that reported earlier [18]. However, upon further heating to 600 °C, amorphous phase undergoes decomposition to form Zn2SnO4 and SnO2 phases as can be seen from Fig. 3(c). Decomposition of ZnSn(OH)6 can be represented in two steps
2ZnSn (OH)6 → 2ZnSnO3 + 6H2 O (Amorphous)
2ZnSnO3 (Amorphous)
→ Zn2 SnO4 + SnO2
(1) (2)
In order to further substantiate the fact that as prepared ZnSn(OH)6 cubes decompose upon heating, differential thermal analysis (DTA) along with thermo gravimetric analysis (TGA) were performed on the sample and both DTA and TGA curves are shown in Fig. 4. The exothermic peak around 237 °C (in DTA) and a weight loss of 16.4% (in TGA) is characteristic of the removal of three molecules of water leading to the formation of amorphous ZnSnO3 phase (step 1). On further heating, the ZnSnO3 phase decomposes to Zn2SnO4 and SnO2 phases around 688 °C (step 2). In an earlier study, we have reported blue emission from SrSn(OH)6 and confirmed that the emission arises due to recombination of selftrapped excitons [19]. To confirm whether such type of emission occurs in ZnSn(OH)6 also, detailed photoluminescence studies were carried out. Fig. 5(a) shows emission spectrum from undoped ZnSn(OH)6 sample recorded at different temperatures obtained after 253 nm excitation. At room temperature a very weak emission is observed around 425 nm. However up on cooling the sample up to 77 K, emission intensity increases and below 100 K, the intensity remains more or less same as can be seen from Fig. 5(a). A similar trend is also observed in the excitation spectrum shown in Fig. 5(b). At low temperature, the excitation spectrum consists of a broad asymmetric peak around 250 nm. The peak can be de-convoluted into two Gaussian peaks having peak maximum around 248 and 263 nm as can be seen from the 678
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1000 100
instrument response and sample response. By carrying out a re-convolution fit (removing the instrument response), the life time value is found to be ~ 6 ns. It is to be noted that self-trapped excitonic emission and corresponding decay depend on number of factors like nature of the host, extent of singlet and triplet character of traps (levels), interaction of trap levels with other levels etc. Lifetime values can vary from few nanoseconds to hundreds of nanoseconds depending up on the host and nature of trap levels [20]. Activation energy for thermal quenching of self-trapped excitonic emissions in ZnSn(OH)6 has been calculated from the straight line plot of variation of logarithm of emission intensity as a function of inverse of temperature. The integrated emission intensity normally decreases with increasing temperature and can be expressed by Eq. (3).
0.5% Eu
10 1 1000 100 10 1 10000 1000 100 10 1 10000 1000 100 10 1 10000 1000 100 10 1
Counts
1% Eu
2% Eu
I=
6% Eu
2
3
4
5
Time (ms) Fig. 8. Decay curves corresponding to 5D0 level of Eu3+ from ZnSn(OH)6 cubes containing different amounts of Eu3+ ions at room temperature.
Table 2 Lifetime values corresponding to the 5D0 level of Eu3+ from ZnSn(OH)6:Eu cubes. Numbers in brackets give relative percentage of different lifetime components. Excitation and emission wavelengths are 394 and 615 nm respectively. ZnSn(OH)6:Eu (%)
τ1 (µs)
τ2 (µs)
τ3 (µs)
τavg. (µs)
0.5% Eu
8.15 (5.7%) 130 (6.2%) 9.21 (2.9%) 7.25 (1.9%) 7.73 (0.4%)
343 (94.3%) 384 (93.8%) 398.6 (97.1%) 177.42 (7.8%) 187 (7.7%)
–
323.8
–
368.2
–
387.2
369.3 (90.3%) 349 (91.9%)
347.2
2.0% Eu 4.0% Eu 6.0% Eu
Ea
(3)
where I0 is the peak intensity at T = 0 K, c is a constant, Ea is the activation energy for thermal quenching process, and kB is Boltzmann constant [21–23]. From Eq. (3), Ea was found to be 16.7 meV (The numerical value of “c” is 237.8 and “I0” is 1.54 × 106 cps K−1). In order to further modify luminescence properties of ZnSn(OH)6 cubes, lanthanide ions, Eu3+ and Dy3+ were doped in the matrix and results are discussed in the following section. Fig. 7(a) shows room temperature emission spectrum from ZnSn (OH)6 cubes containing different amounts of europium ions obtained after 394 nm excitation. The emission spectrum is mainly characterized by peaks around 590 and 615 nm, which are due to magnetic and electric dipole allowed 5D0 → 7F1 and 5D0 → 7F2 transitions respectively, of Eu3+ ions. The relative intensity ratio of 5D0 → 7F2 (615 nm) to 5D0 → 7F1 (590 nm) transition, known as the asymmetric ratio of luminescence [24–27], is found to increase from 3.5 to 4.4 with increase in concentration of Eu3+ ions up to 2 at% and beyond 2 at%, it remains constant. For 4 and 6 at% Eu containing samples, the asymmetric ratio is found to be around 3.2 indicating that beyond 2 at% Eu3+ ions, europium forms separate phase. Further higher values of asymmetric ratio suggest that Eu3+ environment does not have a center of symmetry. Excitation spectra corresponding to 615 nm emission from ZnSn (OH)6 cubes containing different amounts of europium ions are shown in Fig. 7(b). The patterns consist of sharp peaks characteristic of intra 4 f transitions of Europium along with broad asymmetric peak over the region of 230–332 nm. The broad peak is considered to be arising due to overlapping of peaks characteristic of Eu-O charge transfer (~ 265 nm) and host excitation (~ 280 nm). Observation of host excitation peak while monitoring Eu3+ emission from the sample suggests that there is energy transfer between Eu3+ and host ZnSn(OH)6 lattice. As concentration of Eu3+ ions increases, Eu-O charge transfer process (265 nm peak) predominates over the host excitation thereby leading to shift of overall excitation peak maxima towards lower wavelength values (blue shift). At room temperature, the host absorption (280 nm) and emission peak are broad and less intense due to relatively large extent of non-radiative processes and scattering back ground arising from powder samples. Unlike this, Eu3+ absorption (along with O-Eu CTB) and emission are not much affected by temperature variation and this lead to predominance of Eu-O charge transfer peak (around 265 nm) over the peak characteristic of host in the excitation spectra. To further understand the nature of environment around Eu3+ ions in the sample, decay curve corresponding to 5D0 level of Eu3+ ions were recorded and are shown in Fig. 8. The corresponding lifetime values are given in Table 2. Decay curves are found to be nearly bi-exponential for samples doped with Eu3+ ions up to 2 at%. Shorter lifetime corresponds to Eu3+ ions present on the surface and longer lifetime corresponds to Eu3+ ions present in the bulk of ZnSn(OH)6 cubes [24,25]. For the sample containing 1 at% Eu3+, the lifetime components are found to be 130 µs (6.2%) and 384 µs (93.8%) with an average an average value of
4% Eu
1
1.0% Eu
I0 1 + ce (− kB T )
335.2
representative de-convoluted patterns corresponding to 150 and 100 K, shown in Fig. S2 of the Supporting information. The line shapes are same until 250 K. As the temperature reaches 300 K, the line shape changes slightly and two peaks (less intense) are observed at 247 and 280 nm. The possible reason for this could be that, as the temperatures increases some of the defects/traps which are close to conduction or valence band are not able to contribute in emission and these levels no longer trap charge carriers. Significant Stoke shift associated with emission and excitation spectra as well as increase in intensity of emission with decrease in temperature suggest that emission from the sample is arising due to recombination of self-trapped excitons. At room temperature, lifetime values corresponding to this emission could not be measured accurately as corresponding excited state decays with a very short lifetime which is beyond the measurable limit of our experimental system. But on cooling the sample to 77 K, we could measure lifetime as can be seen from the decay curve shown in Fig. 6. The decay is found to be multi-exponential and it is a combination of 679
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Fig. 9. Emission spectrum (a) and excitation spectrum (b) at room temperature from ZnSn(OH)6:Dy3+. Excitation wavelength was 280 nm and emission wavelength was 574 nm.
excited state. This leads to poor energy transfer from host to lanthanide ions as can be seen from Fig. 9(b). Our study reveals that ZnSn(OH)6 is not a good host for luminescent application, but can act as a good photo-catalyst as it prevents radiative recombination of electrons and holes.
around 368 µs. The shorter life time component for this sample is higher when compared to other samples. This is possibly arising from shift in the initial time selected while carrying out tail fit of corresponding decay curve. Also the error associated with this value must be higher when compared with major lifetime component and its relative percentage. For samples containing more than 2 at% Eu3+ ions, decay curve becomes tri-exponential. As discussed above, first shorter component corresponds to Eu3+ ions on the surface of particle, second component corresponds to Eu3+ ions present in the bulk and third component corresponds to separate europium containing phase. Such separate Eu3+ containing phase, upon finely mixing with ZnSn(OH)6 phase, is expected to show longer Eu3+ lifetime values compared to Eu3+ ions occupying a distorted environment in ZnSn(OH)6 lattice with nearby OH groups. Average lifetime value also increases upto 2 at% of Eu3+ ions and beyond which it remains almost constant. XRD and luminescence results indicate that upto 2 at%, Eu3+ ions get incorporated in the lattice. This is also in agreement with the asymmetric ratio values of luminescence obtained from emission spectra (Fig. 7(a)). To confirm the energy transfer from host to lanthanide ions, ZnSn (OH)6 was also doped with Dy3+ions. Unlike Eu3+ ion, Dy3+, do not have a charge transfer peak in the region corresponding to host absorption and hence excitation spectrum from Dy3+ doped sample can be used to confirm energy transfer from host to lanthanide ions as well as incorporation of lanthanide ions in the lattice. Fig. 9(a) shows the emission spectrum form 0.5 at% Dy3+ doped ZnSn(OH)6 cubes obtained after 280 nm excitation. The spectrum consists of host emission around 430 nm, over which sharp peaks corresponding to Dy3+ transitions at 480 and 575 nm, are superimposed. Observation of both host and Dy3+ emission obtained by host excitation at 280 nm suggests that Dy3+ ions are getting incorporated in the ZnSn(OH)6 host and there exists energy transfer from host to Dy3+ ions. The excitation spectrum corresponding to 575 nm emission from the sample is shown in Fig. 9(b). The spectrum consists of a broad peak around 280 nm which is characteristic of the host excitation. Observation of host excitation peak in the excitation spectrum obtained by monitoring Dy3+ emission further confirms the existence energy transfer from host to Dy3+/lanthanide ions and incorporation of Dy3+ ions in ZnSn(OH)6 lattice. Lifetime corresponding to host emission from the samples with and without Dy3+ doping will give an idea regarding the extent of energy transfer from host to Dy3+. However this could not be done as host lifetime from the sample doped with Dy3+ ions is very small (much less than 1 ns) and the same could not be measured accurately with the nanosecond flash lamp (output pulse width around 1 ns) existing with the machine. The excitation spectrum is noisy probably due to the lower concentration of Dy3+ ions combined with its poor luminescent intensity. Further OH groups in the host facilitate non-radiative decay of excited charge carriers as well as lanthanide ion
4. Conclusions Undoped and lanthanide ions doped cube shaped ZnSn(OH)6 crystals were prepared at room temperature in aqueous medium. Luminescence studies carried out at different temperature confirmed that host emission from the sample arises due to self-trapped exciton recombination with an activation energy for thermal quenching of emission ~ 16.7 meV. From emission and excitation measurements carried out using Eu3+ and Dy3+ as representative dopant lanthanide ions, occurrence of energy transfer from host to lanthanide ions has been established. Further XRD and luminescence studies also confirmed lanthanide ion incorporation in ZnSn(OH)6 lattice. Acknowledgements Authors would like to acknowledge Dr. V. K. Jain, Former Head, Chemistry Division and Dr. B. N. Jagatap, Former Director, Chemistry Group, Bhabha Atomic Research Centre for their encouragements during the course of this work. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.09.034. References [1] P.A. Cusack, A.W. Monk, J.A. Pearce, S.J. Reynolds, Fire Mater. 14 (1989) 23. [2] P.A. Cusack, Fire Mater. 17 (1993) 1. [3] L. Wang, K. Tang, Z. Liu, D. Wang, J. Sheng, W. Cheng, J. Mater. Chem. 21 (2011) 4352. [4] X. Fu, X. Wang, Z. Ding, D.Y.C. Leung, Z. Zhang, J. Long, W. Zhang, Z. Li, X. Fu, Appl. Catal. B 91 (2009) 67. [5] X. Fu, D.Y.C. Leung, X. Wang, W. Xue, X. Fu, Int. J. Hydrog. Energy 36 (2011) 1524. [6] W.J. Moon, J.H. Yu, G.M. Choi, Sens. Actuators B 80 (2001) 21. [7] T. Lana-Villarreal, G. Boschloo, A. Hagfeldt, J. Phys. Chem. C 111 (2007) 5549. [9] S.K. Addu, Facile Fabrication of Mesostructured Zn2SnO4 Based Anode Materials for Reversible Lithium Ion Storage (Theses), 332 Wayne State University, 2014, http:// digitalcommons.wayne.edu/oa_theses/332. [8] L.-P. Li, T. Zheng, L.-N. Xu, Z. Li, L.-D. Sun, Z.-X. Nie, Y. Bai, H.-W. Liu, Chem. Commun. 49 (2013) 1762. [10] S.K. Addu, J. Zhu, K.Y. Simon Ng, D. Deng, Chem. Mater. 26 (2014) 4472. [11] J.W. Kramer, S.A. Isaacs, V. Manivannan, J. Mater. Sci. 44 (2009) 3387. [12] J.W. Kramer, B. Kelly, V. Manivannan, Cent. Eur. J. Chem. 8 (2010) 65. [13] G. Wrobel, M. Piech, S. Dardona, Y. Ding, P.-X. Gao, Cryst. Growth Des. 9 (2009)
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