Optical temperature sensing properties of SnO2: Eu3+ microspheres prepared via the microwave assisted solvothermal process

Accepted Manuscript Title: Optical temperature sensing properties of SnO2 : Eu3+ microspheres prepared via the microwave assisted solvothermal process Authors: Subrata Das, Sudipta Som, Che-Yuan Yang, Chung-Hsin Lu PII: DOI: Reference:

S0025-5408(17)32167-0 http://dx.doi.org/10.1016/j.materresbull.2017.08.057 MRB 9536

To appear in:

MRB

Received date: Revised date: Accepted date:

1-6-2017 7-8-2017 29-8-2017

Please cite this article as: Subrata Das, Sudipta Som, Che-Yuan Yang, ChungHsin Lu, Optical temperature sensing properties of SnO2: Eu3+ microspheres prepared via the microwave assisted solvothermal process, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.08.057 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.

Optical temperature sensing properties of SnO2: Eu3+ microspheres prepared via the microwave assisted solvothermal process

Subrata Das, Sudipta Som, Che-Yuan Yang, Chung-Hsin Lu * Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C

*Corresponding author Email: [email protected], Tel: 886-2-23651428, Fax: 886-2-23623040

Graphical abstract

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Highlights  SnO2:Eu3+ hollow spheres were prepared via microwave-assisted solvothermal route  Crystalline SnO2:Eu3+ with tetragonal rutile phase was obtained within 10 min  The Eu3+ ion/ host emission ratio were measured with temperatures for sensing  The maximum relative sensitivity was calculated as 1.83% K-1 at 523 K  This research indicated the suitability of this phosphor for thermometry application

Abstract 2

Eu3+-activated SnO2 hollow microspheres were synthesized via a rapid microwaveassisted-solvothermal route within brief reaction duration of 10 min. The synthesis process was optimized via regulating the microwave power which accelerated the nucleation process along with the elevation of crystal growth. Microscopic analysis revealed the formation of hollow microspheres consisting of tiny SnO2 nanoparticles on the surface. The photoluminescence of SnO2:Eu3+ exhibited a broad band from 400 to 575 nm due to the host and dopant emissions. The emission intensity of SnO2:Eu3+ increased gradually with the increase of the Eu3+ concentration up to 0.75 mol%, and then diminished by the formation of Eu2Sn2O7 phase. The fluorescence intensity ratio (FIR) between the emissions of Eu3+ ions and host were measured by varying temperatures to obtain the sensing performance of materials and the maximum relative-sensitivity was calculated as 1.83% K-1. This research indicated the suitability of SnO2:Eu3+ hollow microspheres for phosphor thermometry. 1.

Introduction Recently, luminescence thermometry has been established as an accurate

technique with high detection sensitivity, spatial resolution and short acquisition times [1- 3]. The temperature sensors using luminescent materials are based on the change in luminescence behavior as a function of temperatures. The luminescence

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properties include the emission intensity, peak position, full width at half maxima of the emission spectrum and the characteristic lifetime of the excited state [4].

One of the interesting methods for sensing temperature involves measurement of the fluorescent intensity ratio (FIR) between the host emission and the emission related to the doped rare earth ions. Based on the above method, luminescent thermometers can be fabricated using different thermal probes, such as organic dyes, quantum dots (QDs) and rare earth ions. A number of researches have been carried out on thermometers based on rare earth doped hosts with wide band-gap semiconductor [1- 4].

Tin oxide (SnO2) with a broad band gap of 3.6 eV is known to be an outstanding n-type semiconductor and hence it has been widely used for versatile applications including gas sensors, solar cells, rechargeable batteries, supercapacitors, and transparent conducting electrodes owing to the exceptional electronic and optical properties [5- 9]. The rare earth doped SnO2 phosphors have not been widely used for temperature sensing. Recently, Eu3+-ions activated SnO2 [6] phosphors draw the research attention as a consequence of the efficient reddishorange emission and host emission. These materials are considered to be efficient candidates for thermometry applications. 4

Several chemical methods including the sol-gel process, hydrothermal method, radio-frequency sputtering technique and the precipitation route have been applied to synthesize SnO2 [10- 13]. These methods provide different approaches to adjust the optical and electrical properties and to tailor the size and morphology of the particles obtained. However, the conventional processes are difficult to control the sizes and morphology of the prepared particles well. Therefore, a rapid microwave-assisted solvothermal route has been developed to prepare SnO2: Eu3+ materials with spherical morphology for the optical usage [6]. Previously, Lee et al. reported the synthesis of SnO2 spherical particles via the microwave-assisted solvothermal route [10]. Recently, Som and co-workers reported the synthesis of NaYF4 prismatic micro rods via the microwave-assisted hydrothermal route [14]. Owing to the simultaneous heating and molecular homogeneity, the reaction duration reduces from 6 h to 30 min with the significant enhancement of the production via this route [6, 13] in comparison with conventional hydro/solvothermal route [10, 14].

Various reports indicate that the optical properties of SnO2 significantly depend on the morphology and structural characteristics [15]. Therefore, substantial efforts have been dedicated for synthesizing SnO2 nanostructures and 5

microstructures with different shapes such as microspheres [6], nano wires [16], nano rods [17], micro flowers [18], and hollow spheres [19]. Recently, the hollow structure with a porous surface has attracted considerable attention attributed to their unique features including low density, large interior spaces, high specific surface area and well penetration properties. Such features of a hollow structure have potential applications in catalysis, energy storage, ionic intercalation, surface functionalization, lightweight fillers, battery electrodes, acoustic insulation and photonic crystals [20- 22]. Therefore, many efforts have been devoted to synthesize SnO2 microspheres with hollow and porous structures via various methods. Recently, Xiao et al. [19] have prepared SnO2 hollow microspheres and studied the potential photocatalytic performance of the above. Li and coworkers [20] have reported porous SnO2 nanospheres for the gas sensing applications. Zhou et al. [21] have studied graphene enwrapped SnO2 hollow nanospheres for a robust high-capacity anode material for lithium-ion batteries. Recently, Liu et al. [22] fabricated SnO2 hollow spheres for the applications in dye-sensitized solar cells. Although various applications of SnO2 hollow spheres are reported in the literature, there is a lack of enough literature on the luminescence thermometric behavior of SnO2 hollow spheres. Furthermore, the underlying mechanism for the formation of hollow spherical morphology has not been well understood yet [12].

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Therefore, the present work is focused on the synthesis of Eu3+ doped SnO2 hollow microspheres using the microwave assisted solvothermal technique for the luminescence thermometric applications. The influence of the microwave irradiation power on the crystallinity, particle sizes and luminescence properties of the prepared samples were investigated. The formation mechanism of SnO2 hollow microspheres was discussed. The doping amount of Eu3+ ions was optimized in order to enhance the photoluminescence emission intensity. The trap emission properties of SnO2 host and the characteristic emission of Eu3+ ions were used to measure the FIR values in order to achieve the temperature sensing behavior. The sensor sensitivity and the temperature resolution of Eu3+-doped SnO2 hollow spheres were calculated.

2. Experimental Procedure Sn1-xO2: xEu3+ (x = 0.25–1.0) samples were synthesized via the microwave assisted solvothermal route using the starting raw ingredients in stoichiometric amount. First, the appropriate amounts of SnCl4·5H2O and urea were mixed in ethanol. The molar ratio of SnCl4·5H2O to urea was kept as 1:10. The metal nitrate solution was prepared via dissolving the stoichiometric amount of Eu2O3 in diluted 7

HNO3 via vigorous stirring. The metal nitrate solution was then dissolved with the previous mixtures and continuously stirred for 30 min. The as-obtained mixture solution was then transferred into a 75 mL Teflon-lined stainless steel autoclave and kept in the microwave digestion unit (ETHOS One MA 133) at frequency 2.45 GHz with various powers (200–600 W). The solution was irradiated via microwaves for 10 min at 200°C. The resultant white precipitates were washed with distilled water and ethanol in turn and dried at 80°C for 24 h. The precipitates were then calcined at 1000°C for 2 h in air.

The formed phases in the prepared powders were identified via X-ray diffraction (XRD, Philips X'Pert/MPD, Eindhoven, Netherlands) with CuKα radiation at 40 kV and 40 mA. The surface morphology of the powders was observed using a scanning electron microscope (SEM, FEI, SL30 SFEG Sirion, Eindhoven, Netherlands) and high-resolution transmission electron microscopy (TEM; JEM-3010, JEOL, Tokyo, Japan). The photoluminescence spectra of the powders were measured at room temperature using a fluorescence spectrometer (F4500, Hitachi, Tokyo, Japan) with a xenon lamp.

3. Results and discussions 3.1 Microscopic analysis 8

In order to investigate the influence of microwave power on the microstructures and luminescent intensity of the present sample, SnO2:0.25 mol% Eu3+ was prepared at various microwave powers. The SEM images of SnO2:0.25 mol% Eu3+, prepared via using different microwave irradiation powers and post annealed at1000°C, are shown in Fig. 1 (a- d). The insets represent the corresponding SEM images with high magnifications. When applying 200 W of microwave irradiation power, the as-prepared sample exhibited spherical shapes having porous surfaces with various diameters ranging from 50 to 500 nm (Fig. 1 (a)). Very close examination indicated that the as-prepared SnO2 microspheres were hollow in nature. Interestingly, the surface of microspheres consisted of close-packed small SnO2 nanoparticles, as shown in the inset of Fig. 1 (a). The average size of SnO2 nanoparticles was estimated to be 7 nm. The heat treatment on the samples synthesized by applying 200 W microwave irradiation resulted in an expansion of the spherical grains, and the diameters were measured to be in the range of 0.4–1.0 μm (Fig. 1 (b)). The magnified image of a single sphere indicated the aggregation of SnO2 nanoparticles owing to the heat treatment (inset of Fig. 1 (b)). The average size of the nanoparticles was measured to be 100 nm.

The sample synthesized with the microwave power of 400 and 600 W and annealed at 1000ºC retained the spherical shape (Fig. 1 (c)- (d)). However, with the 9

increase in microwave irradiation power from 200 to 600 W, the average grain size was reduced apparently from 1.0 to 0.7 μm. The detailed examination of large spherical grains revealed that the average size of SnO2 nanoparticles was also decreased from 100 to 12 nm owing to the increase in microwave irradiation power from 200 to 600 W, as observed in the inset of Fig. 1 (c)–(d). Such phenomenon is considered to be related to the nucleation process during microwave irradiation treatment. The decomposition of urea resulted in the generation of hydroxyl groups, which reacted with tin ions to form nuclei. Once the microwave power increased the urea hydrolysis rate was accelerated, and the number of nuclei increased, thereby causing the particle size to decrease [6, 13].

Further morphological examination of the prepared SnO2 microspheres by TEM clearly revealed that the spheres were hollow (Fig. 2). When applying 200 W of microwave irradiation power, the as-prepared SnO2 sample displayed hollow spheres with the measured diameters ranging from 200 to 500 nm (Fig. 2 (a)). Furthermore, small SnO2 nanoparticles with a crystallite size around 5 nm could be visible on the surface (inset of Fig. 3 (a)). However, the post annealing resulted in a morphological collapse accompanied by the enhancement in the average crystallite size from 5 to 100 nm, as shown in Fig. 2 (b). The average diameter of the spherical micro grains was also increased to around 1 μm because of the heat 10

treatment (Fig. 2 (b)). The crystal structure of the 1000°C- annealed microspheres synthesized at the microwave irradiation of 200 W was also examined via HRTEM. The as-received HRTEM image (Fig. 2 (c)) showed well-defined lattice fringes, indicating that the prepared particles are highly crystalline in nature. The distance between the lattice fringes of the prepared sample synthesized was measured to be 0.356 and 0.240 nm corresponding to the (1 1 0) and (1 0 1) facets of the tetragonal rutile phase of SnO2 (JCPDS 88-0287).

From the microscopic analysis, SnO2 hollow microspheres assembled with many nanoparticles were clearly observed. The similar pattern was also observed for SnO2 synthesized via the solvothermal reaction by Z.P. Li et al. [20] and several other researchers [23- 24]. According to the previous literature, SnO2 particles were formed from the precursor solution consisted of SnCl4 .5H2O under microwave assisted solvothermal reaction. At the primary stage of the reaction, numerous nanocrystallites with high energy aggregated to form small nanospheres. These small nanospheres might bunch up to assemble large nanospheres owing to the reduction of the overall surface energy [23]. The nanocrystallites present in the inner regions have more surface energy compared to the nanocrystallites present in the outer surface regions [20]. It is well known that the nanocrystallites with large surface energies have a strong propensity to dissolve in the abundant surrounding 11

solvent during solvothermal treatment [23]. Therefore, during the reaction, the inner region of the formed nanospheres commenced to dissolve progressively and finally led to the hollow spherical structure. The formation of SnO2 hollow spherical structure required at least 18h reaction duration as reported in the literature [20, 23, 24]. However, in the present case, the hollow spheres were formed within 10 min duration. It is owing to the inclusion of microwave in the conventional solvothermal treatment.

3.2

X-ray diffraction analysis The XRD pattern of the sample obtained using microwave power of 200 W

is presented in Fig. 3 (a). No impurity peaks were observed in the XRD patterns, indicating that the obtained powders became monophasic in nature. However, the sharpness of the XRD peaks was enhanced due to the post annealing at 1000ºC, as shown in Fig. 3 (b). Figure 3 (c)–(d) represents the XRD of the as-synthesized phosphors obtained using microwave powers of 400 and 600 W and annealed at 1000ºC. With the change in microwave power from 200 to 600 W, no significant change in XRD pattern was observed. It indicated that the microwave power did not affect the basic crystal structure.

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The precursors of SnO2 doped with various Eu3+ concentrations were obtained after microwave irradiation at 200°C for 10 min and the microwave power was fixed to 400 W. After annealing at 1000°C, well-crystallized powders were obtained. The XRD patterns of Sn1-xO2: x Eu3+ (x = 0.25–1.0) are illustrated in Fig. 4. The obtained diffraction patterns were fully consistent with the tetragonal rutile phase of SnO2 (JCPDS 88-0287). When the Eu3+ concentration was increased to 1.0 mol%, an impurity phase-Eu2Sn2O7 was formed that coexisted with SnO2, as shown in Fig. 4 (e). The ionic charge mismatch between Sn4+ and Eu3+ ions led to the formation of cubic Eu2Sn2O7 phase, as described earlier by Wang et al. [25].

3.3

Photoluminescence analysis Figure 5 depicted the excitation and emission spectra of SnO2: 0.25 mol%

Eu3+ prepared via using different microwave irradiation power and calcined at 1000°C. The powders prepared via various microwave irradiation powers exhibited similar spectra. Under the UV excitation of 250 nm, the emission spectra of the obtained powders were mainly associated with the trap emission of the SnO2 host (400–575 nm), and the characteristics emission peaks of Eu3+ ions (575–750 nm). The strong orange-red emission lines at about 587 and 613 nm were generated from the transitions 5D0 → 7FJ (J =1, 2) of Eu3+ ions, respectively (Fig. 5) [26]. The emission intensity of SnO2: 0.25 mol% Eu3+ prepared via the microwave 13

irradiation power of 400 W was observed as maximum. The increased reaction kinetics with the increase in microwave power up to 400 W was the main cause for the increment of PL intensity. However, with the further increment of the microwave power above 400 W, the luminescence intensity was observed to decrease.

It was considered that the particle sizes play an important role in the luminescence intensity. Samples with small particle sizes tend to have lots of surface defects owing to the large surface-to-volume ratio. With the increment of the microwave power, the particle size decreases and these defects concentration increases. The increment in defects provides a nonradiative relaxation path, thereby reducing the luminescence intensity [27-30]. Furthermore, small particles could induce relatively larger light scattering than bigger particles results in a decrease in luminescence [31]. According to the above outcomes, the samples were synthesized via fixing the microwave irradiation power at 400 W for the further studies.

The broad band emission 400–575 nm can be attributed to electron transition interceded by the oxygen vacancies in the band gap as reported by kumar et al. [32]. The energy level diagram has been anticipated on the basis of emission 14

spectra of the present phosphor, and the corresponding emission levels are displayed in Figure 6.

Figure 7 represents the emission spectra of Sn1-xO2: xEu3+ (x = 0.25– 1.0) monitored at the UV excitation wavelength of 250 nm. The emission spectra consisted of the traditional orange-red emission lines at about 587 and 613 nm generated from the transitions 5D0 → 7FJ (J =1, 2) of Eu3+ ions, respectively [29]. The characteristic emission lines of Eu3+ were accompanied by the broad emission of the host lattice located in the region of 400- 575 nm. The electric dipole transition 5D0 → 7F2 with ∆J = 2 is hypersensitive, and the intensity can vary by orders of magnitude, depending on the local environment. However, the magnetic dipole transitions (5D0 → 7F1) are insensitive to the site symmetry because they are parity-allowed. Hence, the (5D0 → 7F1)/(5D0 → 7F2) emission ratio can be used as a measure of the site symmetry of Eu3+. In Fig. 7, the strongest emission peak situated at 587 nm showed prominent and bright orange light due to the 5D0 → 7F1 magnetic dipole transition indicating that Eu3+ site has inversion symmetry [33]. Luminescence from the high excited states, such as 5D1, was not detected, indicating a very efficient nonradiative relaxation to the 5D0 level.

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In order to confirm the possible energy transfer (ET) between host and activators, the emission (450 nm) of SnO2 was also examined, as shown in Fig. 7. According to the above results, the ET process from the host to Eu3+ ions was confirmed to be negligible, as the blue emission of SnO2 was not changed much after Eu3+ doping into SnO2. As the Eu3+ concentration was increased from 0.25 to 0.75 mol%, the excitation and emission intensity enhanced gradually. However, the excitation and emission intensity began to diminish when Eu3+concentration reached to 1.0 mol%. As the Eu3+ concentration was raised to 1.0 mol%, the excitation and emission intensity decreased significantly owing to the existence of an impure Eu2Sn2O7 phase as indicated in XRD analysis (Fig. 4). 3.4 Temperature dependent photoluminescence analysis for thermometric applications It is well-known that the total number of atoms (population) in a given excited state can determine the intensity of any optical transition. Therefore, the ratio of the fluorescence intensities of two transitions can be written as [34, 35]: I1 g A h E E  1 1 1 exp(  12 )  C  exp(  12 ) I 2 g 2 A2 h 2 k BT k BT

(1)

where g1 and g2 are the degeneracy of the respective states, h is the Planck constant, υ is the frequency, A is the spontaneous emission rate, E is the energy of the level, T denotes to temperature and C is a constant. Eq. (1) indicates that the

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ratio of intensities of the transitions is directly related to temperature, and hence this equation can be used to measure the temperatures. Thus, the thermometric behavior of a luminescent material can be revealed via the knowledge of the relative intensity of two transitions, i.e., fluorescent intensity ratio (FIR) [33, 34].

Figure 8 displays the PL emission spectra of microwave assisted solvothermal derived SnO2: Eu3+ phosphor under excitation at 250 nm as a function of various temperatures (298–623 K). Two spectral regions are clearly visible in all the spectra. The first region is associated with the trap emission of the SnO2 host ranging from 400–575 nm. According to V. Kumar et al. [32], oxygen vacancies are recognized to be the most frequent defects and act as the radiative centers. They explained in their research that the broad trap emission band from 400–575 nm in the SnO2 host is mainly due to a singly occupied oxygen vacancy and doubly occupied oxygen vacancy states [32]. The second region is composed of the characteristics emission peaks of Eu3+ ions ranging from 575- 750 nm as explained before.

Based on the results of Fig. 8, it is clear that the trap emission and the characteristic PL emission intensity decreased with the increase in temperature. It is noteworthy that the characteristic transitions of Eu3+ ions decreased quickly with 17

temperature owing to the enhancement of nonradiative relaxation with temperature. However, slight changes in SnO2 host trap emission with the increment in temperature were observed. Therefore, the trap emission from the SnO2 host can act as the reference intensity (IR) and be measured as the area under the emission curve in the range 400–575 nm. The intensity of the characteristic Eu3+ transition (IT), which is mainly dependent on temperature, was achieved via measuring the spectral area in the range 575–750 nm. The ratio between IT and IR yielded the value of the FIR. In the present technique, the FIR value can be obtained by dividing the whole spectrum into two regions rather than by measuring two particular transitions. It may make the thermometric measurement considerably simple.

Figure 9(a) presents the temperature-dependence of the FIR data and it reveals that the FIR values decrease by approximately an order of magnitude with the variation of temperature in the range 298–623 K. The variation of FIR with temperature indicates the suitability of the present material for temperature sensor application.

The performance of a luminescence-based temperature sensor depends on the PL efficiency of the phosphor materials. The performance mainly includes 18

absolute sensitivity (Sa), relative sensitivity (Sr) and resolution of the temperature sensor. The absolute sensitivity is defined as the variation of the FIR value with temperature, and is given by [36, 37]: Sa 

( FIR ) T

(2)

According to the present analysis, the as-calculated absolute sensor sensitivity was varied in the range 0.00032 to 0.0159 K-1 with the temperature range from 298 to 623 K. Figure 9(b) presents the variation in absolute sensitivity with temperature for the purposes of FIR measurement. The relative sensor sensitivity is the absolute sensor sensitivity normalized with respect to the measured value. The relative sensitivity can be calculated as follows [36, 37]: Sr 

1 ( FIR )  100% FIR T

(3)

Figure 9(c) shows the variation of relative sensor sensitivities with temperature. It reveals that the relative sensor sensitivity increased with temperature up to 523 K and then decreases. The above finding indicated the suitability of Eu3+-doped SnO2 spheres for sensing the temperature in various electronic devices. The maximum relative sensor sensitivity was 1.83% K−1 at 523 K.

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An important characteristic of temperature sensor devices is the resolution of the temperature sensors. It can be defined as the minimal detectable change in signal [38, 39]. Lee et al. [40] describe a very general way to estimate the resolution. Figure 8 presents the estimated resolution of FIR temperature sensing. The resolutions were lower than 1K at a temperature ranging from 427 to 593K. The maximal resolution was obtained at 523 K and was estimated to be 0.53 K. Table 1shows the comparison of SnO2: Eu3+ system with the recently reported phosphors in terms of sensitivity and resolution. From Table 1 it is clear that the present phosphor exhibited very high sensitivity with a wide temperature range 298 K- 623 K. This discussion revealed the suitability of the material for temperature sensing applications.

4. Conclusions A microwave assisted solvothermal route was demonstrated in this study for the rapid synthesis (10 min) of hollow spherical SnO2: Eu3+ hollow microspheres. Well controlled microwave irradiation and the corresponding hydrothermal treatment yielded spherical particles having porous surfaces consisted of nanosized particles. The PL emission spectrum of SnO2: Eu3+ hollow spheres exhibited two spectral zones. The spectral zone at the low wavelength region was attributed to 20

trap emissions from the host. The sharp orange emission at the high wavelength region was attributed to the emission of Eu3+ ions. The emission intensity of SnO2: Eu3+ increased gradually with the increase in Eu3+ concentration up to 0.75 mol% and then decrease owing to the formation of an impurity phase. The fluorescent intensity ratio of the emissions of Eu3+ ions to SnO2 trap emissions depended strongly on temperatures. Therefore, the present materials were further studied for the sensing of temperature. The maximum sensitivity was estimated to be 1.83%K−1 at 523 K with the resolution of 0.53 K, indicating the suitability of the material for temperature sensing applications.

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Figure captions Figure 1: (a) SEM image of the as-synthesized SnO2: Eu3+ sample prepared via the microwave-solvothermal route with the microwave power of 200W. (b-d) SEM images of SnO2: Eu3+ sample prepared via the microwave-solvothermal route with the various microwave powers ((b) 200W, (c) 400W and (d) 600W) and postannealed at 1000ºC.

Figure 2: (a) TEM image and the magnifying TEM image (inset) of the assynthesized SnO2: Eu3+ sample prepared via the microwave-solvothermal route with the microwave power of 200W. (b) TEM and (c) HRTEM image of SnO2: Eu3+ sample prepared via the microwave-solvothermal route with the microwave power of 200W and post-annealed at 1000ºC.

Figure 3: (a) XRD of the as-synthesized SnO2: Eu3+ sample prepared via the microwave-solvothermal route with the microwave power of 200W. (b-d) XRD of SnO2: Eu3+ sample prepared via the microwave-solvothermal route with the various microwave powers ((b) 200W, (c) 400W and (d) 600W) and post-annealed at 1000ºC.

29

Figure 4: XRD of Sn1-xO2: x Eu3+ prepared via the microwave solvothemal route with the microwave power of 400 W and post annealed at 1000ºC. (x = (a) 0, (b) 0.25, (c) 0.5, (d) 0.75 and (e) 1.0).

Figure 5: PL emission spectra of SnO2: Eu3+ prepared via the microwavesolvothermal route with various microwave powers and post-annealed at 1000ºC.

Figure 6: Energy level diagram of Eu3+ ion in SnO2 host material and the host emission.

Figure 7: PL emission spectra of Sn1-xO2: x Eu3+ (x = 0-1.0) prepared via the microwave-solvothermal route with the microwave power of 400 W and postannealed at 1000ºC.

Figure 8: Variation of photoluminescence emission spectra with temperature ranging from 298- 623K.

Figure 9: Variation of (a) FIR value of 5D0 state of Eu3+ ions, (b) absolute sensor sensitivity, (c) relative sensor sensitivity and resolution with temperatures of Eu3+ ions in the SnO2 host. 30

1μm

200 nm

1μm

200 nm

1μm

200 nm

1μm

200 nm 31

Fig. 1

Fig. 2

32

Fig. 3

33

Fig. 4

34

Fig. 5

35

CB

2.48

5F

2

5L

6

5D

3

5D

2

5D

V0

..

j= 6 j= 4 j= 2 j= 0

0.00

36

5D

1 0

j= 5 7 j= 3 Fj j= 1

Emission

Fig. 6

717 nm

.

657 nm

V0

613 nm

1.24

587 nm

Energy (eV)

3.72

VB

Fig. 7

37

Fig. 8

38

(b)

(a)

(c)

Fig. 9

39

Table 1 Comparative analysis of relative sensitivity (Sr), peak temperature (Tm) and the corresponding temperature range (ΔT) of luminescence sensor materials.

Serial Phosphor

Sr at

No.

Tm (in

ΔT (Tm) in (K)

Ref.

%K-1) 1.

Gd2O3: Er3+/ Yb3+

0.2

295- 1000 (600) 31

2.

ZnO: Er3+/ Yb3+

0.6

273- 473 (273)

33

3.

GdVO4: Er3+/Yb3+

1.11

307-473 (307)

34

4.

YAG: Ce3+

0.2

315- 350 (350)

35

5.

Y2O3: Eu3+

2.6

473- 973 (973)

36

6.

TiO2: Eu3+

2.43

307- 533 (533)

5

7.

SnO2: Eu3+

1.83

298- 623 (523)

Present work

40