Self-activated emission and spectral temperature-dependence of Gd8V2O17 phosphor

Author’s Accepted Manuscript Self-activated emission and spectral temperaturedependence of Gd8V2O17 phosphor Suyin Zhang, Pengyue Zhang, Yanlin Huang, Hyo Jin Seo www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(18)31918-5 https://doi.org/10.1016/j.jlumin.2018.11.054 LUMIN16124

To appear in: Journal of Luminescence Received date: 18 October 2018 Revised date: 13 November 2018 Accepted date: 28 November 2018 Cite this article as: Suyin Zhang, Pengyue Zhang, Yanlin Huang and Hyo Jin Seo, Self-activated emission and spectral temperature-dependence of Gd8V2O17 p h o s p h o r , Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.11.054 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 galley proof before it is published in its final citable 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.

Self-activated emission and spectral temperature-dependence of Gd8V2O17 phosphor Suyin Zhang,1 Pengyue Zhang,1 Yanlin Huang,2 Hyo Jin Seo 3*c 1

2

College of Standardization, China Jiliang University, Hangzhou 310018, China

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China 3

Department of Physics and Interdisciplinary Program of Biomedical Engineering, Pukyong National University, Busan 608-737, Republic of Korea

Abstract Gd8V2O17 was prepared via the facile solid-state reaction. The optical absorption, temperature-dependent luminescence, and decay lifetimes were investigated. Gd8V2O17 has a direct allowed electronic transition with band gap energy of 3.18 eV. Under the excitation of UV light, the phosphor shows the typical self-activated luminescence from the charge transfer (CT) transitions in VO43− groups. The emission intensity has a nearly constant value from 10 to 150 K, while it increases above 150 K. And the emission has an abnormal blue-shift with the increase of temperature from 10 to 300 K, so the luminescence color varies from green to blue. Over the whole temperature ranges, the maximum emission wavelength showed a linear dependence on temperature; consequently, the temperature-dependent properties have a potential advantage in luminescence thermometry. The luminescence mechanism was discussed on the proposed two

*

Corresponding authors: [email protected] (Hyo Jin Seo); Tel.: +82-51-629 5568; fax:+82-51-6295549.

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VO4 emission centers and the energy transfer between them.

Graphical abstract

Keywords: Vanadate; Luminescence; Optical materials and properties; Optical thermometry; Phosphorescence.

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1 Introduction Rare-earth (RE)-activated phosphors have been widely investigated due to the engineering applications and basic science. RE-activated phosphors have been widely applied in so many fields, such as white light-emitting diodes (W-LEDs), lasers devices, biological labels, and components in a variety of display technologies [1-6]. Compared with RE-activated materials, self-activated phosphors present some advantages such as RE free, low cost, wide emission wavelength, low heat temperature for preparation, etc. Especially, vanadates usually show bright self-activated emission with a wide wavelength from 400 to 750 nm due to charge transfer (CT) transitions of an electron from O2p to V3d in the VO4 groups [7,8]. The excitation in vanadates is very efficient because it is a spin-allowed CT transition, which locates in UV- and near-UV wavelength regions making it more suitable for white light-emitting diodes (W-LEDs). Although the self-activated emission has been widely investigated, few reports have been concerned on thermometry of self-activated vanadates. The luminescence of a phosphor could act as non-contact optical thermometry for temperature remote detection. This is based on the fact that temperature has great and subtle influences on the luminescence characteristics such as quantum efficiency, color (wavelength), decay lifetime and full width at half maximum (FWHM) etc. It is well-known that the luminescence and spectral characteristics of a phosphor usually show a great dependence on the surrounding temperature [9-13]. In recent years, the thermometry detections have been widely developed on the base of spectral measurements. Compared with the conventional invasive thermometers, this method has some advantages such as simplicity, high-spatial resolution,

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accurate, rapid response and noninvasive, coverage of a large temperature range (from 10 to 2000 K) [14]. Luminescence thermometry has been developed according to the temperature-dependent luminescence of RE-activated phosphors such as the peak wavelength and bandwidth [15], emission intensity [14], luminescence intensity ratio [16-19], fluorescence lifetime [20,21], up-conversion [22,23], and decay rise time [24]. This work reports a novel luminescence thermometry of the self-activated Gd8V2O17. The temperature-dependent luminescence was investigated. The luminescence wavelength shows a linear relation with the temperature in the region of 10-300 K. Meanwhile, the color (CIE values) changes according to temperature. The mechanism was discussed on the proposed model.

2 Experimental Gd8V2O17 powders were synthesized via the facile solid-state reactions. The raw reactants are NH4VO3, and Gd2O3. Firstly, NH4VO3, and Gd2O3 with the stoichiometric weight were ground by ball-milling in alcohol medium for 2h. The slurries were dried in a drying oven and then were calcined at 950 °C for 6h. Secondly, the powders were thoroughly ground again and then sintered at 1500 C for 6h to obtain the final products. X-ray powder diffraction (XRD) patterns were taken on a Rigaku D/Max diffractometer (40 KV, 30 mA) with Bragg-Brentano geometry using Cu-Kα1 radiation (λ=1.5405 Å). The XRD data in 2θ ranging from 10 to 70° were collected at a scanning mode with a step size of 0.02° and a rate of 4.0° min-1. The SEM morphologies of the samples were observed by means of a field emission scanning electron microscopy (FE-SEM, Japan, Hitachi, S-4800). Diffuse reflection spectrum (DRS) was taken on a Cary 5000 UV–Vis-NIR spectrophotometer by using BaSO4 powder as a standard reference. The photoluminescence (PL) excitation and emission spectra of the phosphor were

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measured on a luminescence spectrometer (Perkin-Elmer LS-50B). The luminescence and decay curves were investigated in the temperature region from 10-300K. The samples were fixed in the vacuum specimen chamber with liquid helium flow. The excitation source is 266 nm created from the fourth harmonics of pulsed Nd-YAG laser. A digital storage oscilloscope (500 MHz, Tektronics TDS754A) was used to record the dynamic data.

Intensity (a. u.)

3 Results and discussion

sample

PDF#22-0297 20

30

40

50

60

2 Theta (degree)

Fig. 1 XRD pattern of Gd8V2O17 compared with standard PDF#2 standard card (No. 22-0297).

Fig. 2 SEM photos (a, b), EDX spectrum with the elemental components (c), and the digital 5

picture (d) of Gd8V2O17 powders. Figure 1 shows XRD pattern of the Gd8V2O17 phosphor, which completely agrees with the PDF2 standard card No. 22-0297 (Gd8V2O17) in the International Centre for Diffraction Data (ICDD) database. The results confirm that all the samples are the targeted single phase of Gd8V2O17. The typical SEM images are shown in Fig. 2 (a, b). The powders contain regular ball-like particles (5-10 μm) with loose aggregation. The particles present smooth surface on the view of the picture. EDX measurements (Fig. 2 c) confirmed the elements of V, Gd, and O in the lattices. The ratio values were detected via elemental face scanning in the EDS measurements, which is in agreement with the formula of Gd8V2O17. The appearance of the powder sample presents faint yellow color (Fig. 2 d).

Fig. 3 UV-vis reflective spectrum of Gd8V2O17; Inset is estimation for band gap energy (Eg).

Gd8V2O17 phosphor has an optical absorption from UV to near-UV wavelength (Fig. 3), which is assigned to the intrinsic band transitions, i.e., the well-known ligand metal charge transfer (CT) in VO4 groups (oxygen to metal ions). The band transitions were explained in Tauc formula, which is related to the determination of band gap energy (Eg): αhυ = A(hυ-Eg)n/2, where ν, α, and h are

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the frequency of incident light, optical absorption coefficient and Planck’s constant, respectively. And A is a characteristic constant. The nature of the optical transitions can be determined by n, which is the direct and indirect for n=1 and n=4, respectively. Fig. 3 presents the relation plot between (αhυ)2 and (hυ) (n=1). It is reasonable that the band transition has a direct transition nature. The gap energy (Eg) was determined to be 3.18eV.

Fig. 4 Photoluminescence excitation (a) and emission (b) of Gd8V2O17. Inset in (b) is the schematic emission model in VO4.

Figure 4 exhibits the photoluminescence excitation and emission of Gd8V2O17. The excitation presents a broad band (3.51-5.64 eV) (Fig. 4a). The profile characteristic of the luminescence spectra is similar to those reported in self-activated vanadate phosphors [25], i.e., electronic charge transfer from O-2p to V-3d orbits in [VO4]3− groups [26]. The excitation spectrum in Fig. 4(b) is asymmetric, which was decomposed into two Gaussian sub-bands of Ex1 (4.428eV, 1

A1→1T2) and Ex2 (3.72eV, 1A1→1T1). Accordingly, the phosphor has the emission processes

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T2→1A1 (Em1) and 3T1→1A1 (Em1) (Fig. 4b). Figure 5 (a) shows the temperature-dependent luminescence. It can be seen that not only

the emission intensities but also the wavelength varies with the temperature. Fig. 5 (b) is 7

integrated intensity at different temperatures, which has a nearly constant value from 10 to 150 K, while it increases above 150 K. It can be noted that the emission has different profiles, and shows an obvious blue-shift with the increase of temperature. This is abnormal by taking the mechanism in a luminescence system.

Fig. 5 Temperature-dependent (10-300K) luminescence (a) and integrated intensities (b) of Gd8V2O17, the Gaussian decomposition of the spectrum at 200 K (c); and the decay curves for the proposed emission bands of V-1 and V-2.

It is well-known that thermally active phonon could contribute to the spectrum via the interactions with luminescent center. The high-temperature induces vigorous electron-phonon interactions because of the heavy population phonon density. Accordingly, a red-shift of spectrum could be expressed by Varshini equation [27]: E (T )  E0 

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aT 2 , here E(T) is the T b

energy difference between ground-state and excited-state at a temperature (T) , and a and b are the parameters, E0 is energy difference at 0K. According to this equation, the transition energy should decrease with the increase of temperature, i.e., the spectral wavelength has a red-shift.

Fig. 6 The proposed configurational coordinate diagram with two different VO4 centers (V-1, V-2) showing the energy barriers of E1 and E2.

The break point at 150 K could be induced by the possible change of phase formation at this temperature; however, this suggestion should be further confirmed by experiments. The unusual properties suggest some structural effects exert on the temperature-dependent emission. By now the detailed crystal structure of Gd8V2O17 is not clear. We proposed a tentative luminescence mechanism as the follows. There could be two different VO4 centers of V-1 (higher energy) and V-2 (lower energy) in Gd8V2O17 lattices in Fig. 5(c), and there are energy transfers between them. Two emission centers could be confirmed by the different decay lifetimes for the two emission wavelength as shown in Fig. 5(d). The broad band at 200 K was suggested from V-1 and V-2 with the different lifetimes. Figure 6 shows the configuration coordinate diagram concerning two VO4 (V-1 and V-2) with

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different energy barriers E1 (low temperature) and E2 (higher temperature). In this model, the thermally active phonon-assisted tunneling from the excited states of V-2 (excited states of low-energy) to V-1 (high-energy) was proposed [28]. In a low temperature system, E1 can be easily conquered resulting in dominant low-energy emission (V-2). While, the thermal back-transfer over the barrier E2 is possible at higher temperature, resulting in the higher-energy emission (V-1). And thus, the blue-shift is observed with increasing temperature.

Fig. 7 temperature-dependent color coordinates (a) and maximum emission wavelength of Gd8V2O17 (b).

Fig. 7 (a) displays color coordinates of Gd8V2O17. The luminescence changes from green to deep-blue from 10 to 300K. Fig. 7 (b) presents the maximum emission wavelength of Gd8V2O17 at the selected temperature. Over the whole temperature ranges, the maximum linearly decreases, i.e, W=471-0.2291*T. It is well-known that luminescence characteristics of a phosphor usually have

a

strong

dependence

on

surrounding

temperature.

According

to

the

temperature-dependent maximum wavelength (Fig. 7 b), the surrounding temperature in a system could be easily estimated with the assist of the emission of Gd8V2O17. Especially, as a noninvasive thermometer, temperature remote detection in a rigorous environment could be 10

applied by Gd8V2O17 phosphor thermography. This method is superior to the reported thermometry phosphors relying on the detection of luminescence intensity or lifetimes [14,19,20,23].

4. Conclusions The well-crystallized Gd8V2O17 was synthesized via the facile solid-state reactions. The phosphor has a direct allowed electronic transition with band energy of 3.18eV. Under UV-light, the phosphor shows typical self-activated luminescence from the allowed charge transfer transitions in VO43−. The temperature-dependent (10-300K) luminescence of Gd8V2O17 shows some abnormal properties. The emission has a nearly constant value from 10 to 150 K, while it increases above 150 K. The spectra have an abnormal blue-shift from 10 to 300 K. Over the whole temperature ranges, the maximum emission linearly decreases (W=471-0.2291*T); accordingly, the color coordinates change from green to deep-blue when the temperature alters from 10 to 300K. There could be two VO4 emission centers and the energy transfers between them. The thermally active phonon-assisted tunneling from the low-energy excited states (V-2) to high-energy excited states emission (V-1) was proposed. The result indicates that the remote temperature evaluation could be realized via monitoring the emission color or the wavelength; this is superior to the reported thermometry phosphors with the detection of the luminescence intensity or lifetimes.

Acknowledgements This work was supported by Education Department Scientific Research Program of Zhejiang Province (Y201329502), Natural Science Foundation of Zhejiang Province (LY16C200006) Soft

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Science Research Program of Zhejiang Province (no.2016C25G2080022)

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Figures captions Fig. 1 XRD experimental pattern of Gd8V2O17 compared with standard PDF#2 standard card (No. 22-0297). Fig. 2 SEM photos (a, b), EDX spectrum with the elemental components (c), and the digital picture (d) of Gd8V2O17 powders. Fig. 3 UV-vis reflective spectrum of Gd8V2O17; Inset is estimation for band gap energy (Eg). Fig. 4 Photoluminescence excitation (a) and emission (b) of Gd8V2O17. Inset in (b) is the schematic emission model in VO4. Fig. 5 Temperature-dependent (10-300K) luminescence (a) and integrated intensities (b) of Gd8V2O17, the Gaussian decomposition of the spectrum at 200 K (c); and the decay curves for the proposed emission bands of V-1 and V-2. Fig. 6 The proposed configurational coordinate diagram with two different VO4 centers (V-1, V-2) showing the energy barriers of E1 and E2. Fig. 7 temperature-dependent color coordinates (a) and maximum emission wavelength of Gd8V2O17 (b).

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