Synthesis and luminescence properties of ZnGa2O4 spinel doped with Co2+ and Eu3+ ions

Synthesis and luminescence properties of ZnGa2O4 spinel doped with Co2+ and Eu3+ ions

Applied Surface Science 261 (2012) 830–834 Contents lists available at SciVerse ScienceDirect Applied Surface Science journal homepage: www.elsevier...

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Applied Surface Science 261 (2012) 830–834

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Synthesis and luminescence properties of ZnGa2 O4 spinel doped with Co2+ and Eu3+ ions Xiulan Duan ∗ , Fapeng Yu, Yuanchun Wu State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, PR China

a r t i c l e

i n f o

Article history: Received 8 June 2012 Received in revised form 28 July 2012 Accepted 26 August 2012 Available online 31 August 2012 Keywords: ZnGa2 O4 :Co, Eu Citrate sol–gel method Nanopowders Luminescence

a b s t r a c t ZnGa2 O4 nanopowders doped with Co2+ and Eu3+ ions, including dual doping, have been synthesized by citrate sol–gel method, and characterized by X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Results show that ZnGa2 O4 spinels were produced by calcining the gel above 500 ◦ C, with the crystallite size of 16–30 nm in the temperature range of 500–900 ◦ C. Co2+ ions are located at the tetrahedral sites of ZnGa2 O4 spinel by replacing Zn2+ , and Eu3+ ions are incorporated in the defect regions at the grain boundaries. The emission spectra of Eu-doped ZnGa2 O4 nanopowders display an intense emission at 615 nm belonging to 5 D0 –7 F2 transition of Eu3+ ions. With the introduction of Co into Eu-doped ZnGa2 O4 , the emission intensity at 615 nm decreases, while the luminescence at 680 nm due to tetrahedral Co2+ increases. The result indicates that the energy transfer occurred from Eu3+ to Co2+ ions. The energy transfer was also studied by the luminescence decay behavior. The emission of Co and Eu Co-doped ZnGa2 O4 also changed with annealing temperature. The luminescence properties of the doped ZnGa2 O4 nanopowders can be controlled by the variation of Co and Eu doping concentration and annealing temperature. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

2. Materials and methods

ZnGa2 O4 has attracted much attention as an important phosphor host material for application in field emission displays (FEDs), electroluminescence display, and vacuum florescent displays (VFDs) because they exhibit higher chemical stability than sulfide phosphors and can endure a high electron beam current [1–4]. With a bandgap energy of 4.4 eV, ZnGa2 O4 exhibits a strong blue emission due to the transition via a self-activation center of Ga O groups under excitation by both ultraviolet light and lowvoltage electrons [5–8]. Their luminescence properties can be tuned by doping transition metal or rare earth ions [9–12]. The luminescence properties of dual doped ZnGa2 O4 with Co and Eu have not been studied so far. Nanosized ZnGa2 O4 spinel powders have been synthesized by hydrothermal method, nonaqueous sol–gel methods and citrate sol–gel methods [13,14]. The citrate sol–gel method allows preparation of highly dispersed mixed oxide at low temperature. In this work, we report the synthesis of single and double doped ZnGa2 O4 nanopowders with Co2+ and Eu3+ ions by citrate sol–gel method and their luminescence properties.

2.1. Powder preparation

∗ Corresponding author. E-mail address: [email protected] (X. Duan). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.112

ZnGa2 O4 nanopowders doped with Co and Eu were synthesized by sol–gel method using citric acid as a chelating agent. Firstly, Zn(NO3 )2 ·6H2 O and Al(NO3 )3 ·9H2 O were dissolved in deionized water. A stoichiometric amount of Co(NO3 )2 ·6H2 O and Eu2 O3 , dissolved in HNO3 , was added. Then citric acid was added to the above solution with stirring. The molar ratio of metal ions to citric acid was 1:2. The mixed solution was stirred for 1 h and then heated in an 80 ◦ C water bath until a highly viscous gel was formed. The gels were dried in oven at 110 ◦ C and then fired to the desired temperatures (450–900 ◦ C) for 2 h. We have prepared ZnGa2 O4 :4%Co2+ , ZnGa2 O4 :4%Co2+ :4%Eu3+ , ZnGa2 O4 : 4%Co2+ :8%Eu3+ , ZnGa2 O4 :2%Co2+ :8%Eu3+ and ZnGa2 O4 :8%Eu3+ samples.

2.2. Characterization Powder X-ray diffraction (XRD) patterns of the samples were carried out on a Japan Rigaku D/Max-rA diffractometer using a Cutarget tube ( = 0.15418 nm) and a graphite monochromator. X-ray photoelectron spectra (XPS) were measured using a Thermofisher ESCALAB 250 X-ray photoelectron spectrometer with monochromatized Al K␣ X-ray radiation in ultrahigh vacuum (<10−7 Pa). The

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binding energies were calibrated by using C1s peak (284.6 eV) of carbon impurities as a reference. The peaks were deconvoluted after background subtraction, using a mixed Gaussian–Lorentzian function. The emission spectra and decay curves were measured using FLS900 fluorescence spectrophotometer. 3. Results and discussion 3.1. XRD analysis Fig. 1 shows the XRD patterns of ZnGa2 O4 :Co2+ (2%), Eu3+ (8%) nanopowders annealed at different temperatures. It is noted that the sample annealed at 450 ◦ C is amorphous. When the sample was heated to 500 ◦ C, the XRD pattern exhibits several distinct broad diffraction peaks indicating the nanocrystalline nature. The intensity of the peaks increases with increasing annealing temperature, indicating crystallite growth. All peaks indexed as (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0) crystal planes in the XRD patterns are assigned to cubic ZnGa2 O4 spinel phase (JCPDS No. 710843). The average grain size of the samples annealed at different temperatures was calculated by means of the Scherrer formula: D = 0.9/(ˇcos ), where  is the X-ray wavelength (0.15418 nm), ˇ is the full width at half-maximum (FWHM) intensity of the diffraction line, and  is the diffraction angle. The size of crystal grain is about 16–27 nm. The XRD patterns of the ZnGa2 O4 powders doped with different Co and Eu concentrations annealed at 700 ◦ C are shown in Fig. 2. At this temperature for all the samples, the characteristic peaks corresponding to the ZnGa2 O4 spinel phase are present (JCPDS No. 71-0843). The crystallite size was calculated to be 18–30 nm.

Fig. 2. XRD patterns of ZnGa2 O4 nanopowders doped with Co and Eu ions annealed at 700 ◦ C: (a) 8% Eu; (b) 2% Co, 8% Eu; (c) 4% Co, 8% Eu; (d) 4% Co, 4% Eu; (e) 4% Co.

3.2. XPS analysis The chemical composition and surface electronic state of the ZnGa2 O4 :Co2+ , Eu3+ nanopowders were analyzed by XPS. The survey XPS spectra are shown in Fig. 3. The fine spectra of the O 1s, Zn 2p and Ga 2p core levels were measured and displayed in Figs. 4–6. The survey XPS spectra indicate that no other elements were detected except for the original components and contaminated carbon. The C 1s peak at 284.6 eV of carbon contamination was used as reference. The O 1s XPS spectra for all the doped ZnGa2 O4 nanopowders show the main peak at about 530 eV (Fig. 4). The spectra

Fig. 1. XRD patterns of ZnGa2 O4 nanopowders doped with 2% Co and 8% Eu annealed at different temperatures.

Fig. 3. Survey XPS spectra of ZnGa2 O4 nanopowders doped with (a) 4% Co; (b) 4% Co, 4% Eu; (c) 4% Co, 8% Eu; (d) 2% Co, 8% Eu; (e) 8% Eu.

Fig. 4. O 1s XPS spectra of ZnGa2 O4 nanopowders doped with different concentrations of Eu and Co.

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Fig. 5. Zn 2p XPS spectra of ZnGa2 O4 nanopowders doped with (a) 4% Co; (b) 4% Co, 4% Eu; (c) 4% Co, 8% Eu; (d) 2% Co, 8% Eu; (e) 8% Eu.

are relatively broad and asymmetric, indicating more than one environment are available for O in the nanopowders. After deconvolution, O 1s spectra are composed of two peaks. The fitted O 1s spectrum of ZnGa2 O4 doped with 4%Co2+ and 4%Eu3+ is shown in the inset of Fig. 4. The peak at 530.2 eV is due to the lattice O in ZnGa2 O4 , while the peak at 532.3 eV can be assigned to chemisorbed water or oxygen molecules from environmental moisture [15]. The fine Zn 2p XPS spectra show that the Zn 2p3/2 and Zn 2p1/2 peaks are at about 1021.8 and 1044.9 eV, respectively (Fig. 5), and the gap between the two peaks is about 23 eV, which is consistent with the reference value of 22.97 eV [16]. The fine XPS spectra reveal Ga 2p3/2 and Ga 2p1/2 peaks at about 1118.2 and 1145.2 eV, respectively (Fig. 6). The gap between Ga 2p3/2 and Ga 2p1/2 is about 27 eV, which agrees with the reference value of 26.84 eV [16]. In general, the energy separation between Zn 2p3/2 and Ga 2p3/2 peaks (E) can be used as a sensitive tool to judge whether the obtained product is a complete formation of ZnGa2 O4 spinel with low E value or a physical mixture of metal oxide powders [17]. The E in the as-prepared samples is calculated to be about 96 eV, which agrees well with the reference for the formation of spinel [18]. The peak at about 1135.7 eV in Fig. 6 is due to Eu 3d5/2 . The value 1135.7 eV of Eu 3d5/2 is consistent with that of Eu3+ [19,20]. So Eu exists in the form of Eu3+ ions.

Fig. 6. Ga 2p and Eu 3d5/2 XPS spectra of ZnGa2 O4 nanopowders doped with (a) 4% Co; (b) 4% Co, 4% Eu; (c) 4% Co, 8% Eu; (d) 2% Co, 8% Eu; (e) 8% Eu.

Fig. 7. PL spectra of ZnGa2 O4 nanopowders doped with 2% Co and 8% Eu as a function of annealing temperature.

3.3. Emission analysis Fig. 7 shows the emission spectra of ZnGa2 O4 nanopowders doped with 2%Co2+ and 8%Eu3+ annealed at 700–900 ◦ C. The emission spectrum of the sample annealed at 700 ◦ C is composed of several emission peaks in the range 400–700 nm. The luminescence at 400–500 nm is due to the host lattice ZnGa2 O4 , while the emission peaks at 592 and 615 nm in the red region arise from the f–f transition of Eu3+ . Furthermore, the spectrum exhibits a broad weak band at 680 nm, which is attributed to the 4 T1 (4 P)–4 A2 (4 F) transition of tetrahedral Co2+ . The result indicates that Co2+ ions enter the tetrahedral sites of ZnGa2 O4 by replacing Zn2+ . As we know that Co2+ exhibits emission in the IR region, which lowers the visible emission efficiency. Therefore, the emission intensity at 680 nm is relatively weak [21]. With the increase of annealing temperature, the PL intensity corresponding to Eu3+ decreases, while the intensity related to Co2+ increases. This may be due to the energy transfer from Eu3+ ions to Co2+ ions. The PL spectra of ZnGa2 O4 nanopowders doped with different Eu and Co concentrations are displayed in Fig. 8. The sample

Fig. 8. PL spectra of ZnGa2 O4 nanopowders doped with (a) 4% Co; (b) 4% Co, 8% Eu; (c) 2% Co, 8% Eu; (d) 8% Eu.

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Fig. 9. PL decay curves of Eu3+ in ZnGa2 O4 nanopowders doped with (a) 4% Co, 8% Eu; (b) 2% Co, 8% Eu; (c) 8% Eu.

doped with 8% Eu shows strong emission at 615 nm accompanied by several weak emission peaks in the red region besides the luminescence at 400–500 nm. The peaks at 592, 615, 657 and 702 nm are assigned to 5 D0 –7 F1 , 5 D0 –7 F2 , 5 D0 –7 F3 , 5 D0 –7 F4 of Eu3+ , respectively. Note that the Eu3+ ion radius is 0.95 A˚ and it is obviously ˚ Therefore, the possibilgreater than the radius of Ga3+ ion (0.62 A). ity that Eu3+ ions enter the lattice of ZnGa2 O4 by substituting Ga3+ is very little. We think that Eu3+ ions penetrate into amorphous regions on the grain boundaries of the powders. The analysis is reasonable because the emission spectrum is similar to Eu3+ in other amorphous hosts [22]. The emission spectra of the sample doped with Eu3+ and Co2+ show not only a peak representing Eu3+ ions, but also a peak for Co2+ ions. With increasing Co2+ content, the emission intensity of the Co2+ ions at 680 nm increases, whereas the emission intensity of Eu3+ ions at 615 nm decreases. These results indicate the presence of an energy transfer from Eu3+ ions to Co2+ ions. It can be seen that the relative intensity of the two PL peaks at 615 and 680 nm changes with the doping content of Eu and Co in the samples. Based on this result, one can control the color of luminescence by changing the relative concentration of Eu and Co in ZnGa2 O4 nanopowders. To further investigate whether an energy transfer exists between Eu3+ and Co2+ , we have studied the luminescence decay

behavior. From Fig. 8, we know that the emission of Co2+ at 680 nm is relatively weak. Therefore, only the photoluminescence decay curves of Eu3+ ions have been measured at the emission peak of 615 nm. Fig. 9 shows the PL decay curves of Eu3+ ions in ZnGa2 O4 nanopowders doped with 8% Eu and 0–4% Co. The decay curves were fitted using the following double exponential function: I = A1 exp

 −t  t1

+ A2 exp

 −t 

(1)

t2

where I represents the emission intensity, A1 and A2 are constants, t1 and t2 are the decay constants. The average lifetimes were calculated using the following equation: t=

A1 t12 + A2 t22

(2)

A1 t1 + A2 t2

The fitting parameters and the calculated lifetime values are listed in Table 1. The average lifetime of Eu3+ for the 8%Eu3+ -doped ZnGa2 O4 sample is 0.856 ms, and the value decreases to 0.181 ms for the 2%Co2+ /8%Eu3+ co-doped sample. There is obviously a decreased lifetime of Eu3+ with increasing cobalt content, which indicates the occurrence of an energy transfer from Eu3+ to Co2+ in ZnGa2 O4 samples.

Table 1 The fitting parameters and the calculated lifetimes from the PL decay curves of Eu3+ in ZnGa2 O4 nanopowders doped with Eu3+ and Co2+ . Sample name

ZnGa2 O4 :8%Eu3+ ZnGa2 O4 :2%Co2+ , 8%Eu3+ ZnGa2 O4 :4%Co2+ , 8%Eu3+

Fitting parameters

Lifetime (ms)

A1

t1 (ms)

A2

t2 (ms)

567,604.21 1038.04 10,830.02

0.00222 0.02896 0.00478

11,767.15 3828.24 27,411.14

0.01956 0.20523 0.85800

0.856 0.181 0.005

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4. Conclusions The dual doping ZnGa2 O4 nanopowders with Co2+ and Eu3+ have been prepared by sol–gel method using citric acid as a chelating agent. The crystallite size of the as-prepared samples is 16–30 nm. Co2+ ions enter the lattice of ZnGa2 O4 spinel occupying the tetrahedral sites, while Eu3+ ions are located at the boundaries of spinel powders because of the large radius. The luminescence properties and the Eu3+ –Co2+ energy transfer of the ZnGa2 O4 :Eu3+ , Co2+ nanopowders were investigated. The Eu3+ singlely doped sample exhibits an intense emission at 615 nm. After Co2+ co-doping, the emission intensity of the Eu3+ decreases significantly, whereas the emission intensity of the Co2+ at 680 nm increases gradually. The fluorescence lifetime of Eu3+ is also greatly decreased. These results provide indirect evidence of the occurrence of the energy transfer from Eu3+ to Co2+ . The emission spectra of ZnGa2 O4 nanopowders codoped with Co and Eu also changed with the annealing temperature. The dual doping ZnGa2 O4 with Co and Eu may allow one to control the color of the emitted light by changing the relative concentration of the two ions and annealing temperature. Acknowledgement This work was supported by grants from the National Natural Science Foundation of China (50902089 and 51172130) and the Natural Science Foundation of Shandong Province (ZR2010EQ003). References [1] S. Itoh, H. Toki, Y. Sato, K. Morimoto, T. Kishino, Journal of the Electrochemical Society 138 (1991) 1509.

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