Enhancement on afterglow properties of Eu3+ by Ti4+, Mg2+ incorporation in CaWO4 matrix

Haoyi Wu; Yihua Hu; Fengwen Kang; Nana Li

doi:10.1557/jmr.2012.16

Enhancement on afterglow properties of Eu3+ by Ti4+, Mg2+ incorporation in CaWO4 matrix

Published online by Cambridge University Press: 07 February 2012

Fengwen Kang and

Haoyi Wu: Affiliation:
School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Higher Education Mega Center, Guangzhou, 510006, Guangdong Province, People’s Republic of China
Yihua Hu*: Affiliation:
School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Higher Education Mega Center, Guangzhou, 510006, Guangdong Province, People’s Republic of China
Fengwen Kang: Affiliation:
School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Higher Education Mega Center, Guangzhou, 510006, Guangdong Province, People’s Republic of China
Nana Li: Affiliation:
School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Higher Education Mega Center, Guangzhou, 510006, Guangdong Province, People’s Republic of China
*: a)Address all correspondence to this author. e-mail: huyh@gdut.edu.cn

Article contents

Abstract
INTRODUCTION
EXPERIMENTAL SECTION
RESULTS AND DISCUSSION
CONCLUSIONS
References

Abstract

The long afterglow phosphor, CaWO4: Eu3+, is synthesized and the intensity and duration of its afterglow can be enhanced by the Ti4+ and Mg2+ incorporation. The x-ray diffraction patterns depict pure tetragonal CaWO4 of all samples. The emission spectra show the Eu3+ emission and the charge transfer (CT) emission of WO42−. The intensity of CT increases with the Mg2+ incorporation. The excitation spectra monitoring 616 nm exhibit the strongest CT band with Ti4+ incorporation. These results indicate that Mg2+ enhances the efficiency of CT emission of WO42− while the Ti4+ enhances the energy transfer rate from CT to Eu3+. Since the thermoluminescence (TL) curves do not imply a new trap, the enhancement of the afterglow results from the coreinforcement of CT efficiency and energy transfer rate.

Keywords

Luminescence Optical properties Lanthanide

Information

Type: Articles
Information: Journal of Materials Research , Volume 27 , Issue 6 , 28 March 2012, pp. 959 - 964
DOI: https://doi.org/10.1557/jmr.2012.16 [Opens in a new window]
Copyright: Copyright © Materials Research Society 2012

I. INTRODUCTION

Phosphors have a broad application in our daily life, e.g., cathode ray tubes, television, fluorescent lamps, detectors, and light-emitting diodes.Reference Jüstel, Nikol and Ronda¹^, Reference Höppe² The luminescent lifetime of phosphors applied in above fields is usually short, while some phosphors possess a long lifetime, which is called long afterglow. It refers to a luminescence phenomenon that persists for a long time after excitation. Materials with long afterglow can store the absorbed energy from external irradiation and release the energy usually as visible light gradually. Due to this energy storage characteristic, visible light can be obtained from long afterglow materials in dark environment without electricity. It more or less assists to energy saving and sustainable development. Phosphors with long afterglow are commercially applied in emergency indications, road signs, displays, decoration, etc.Reference Lei, Sha, Zhang, Liu, Man and Yue³^, Reference Ju, Wei, Zheng, Gao, Zhang and Liu⁴ More recently, the application of long afterglow phosphors (LAPs) was extended to medical diagnostics.Reference de Chermont, Chanéac, Seguin, Pellé, Maîtrejean, Jolivet, Gourier, Bessodes and Scherman⁵^, Reference Wu, Wang, Chen and Yan⁶ The employment of LAPs substantially enhanced the signal-to-noise ratio because the autofluorescence of tissues was avoided during the detection. Hence, LAPs possess a considerable commercial prospect.

By now, tricolor LAPs have been achieved, i.e., Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ (blue, ∼470 nm),Reference Xiao and Xiao⁷^, Reference Lin, Tang, Zhang, Wang and Zhang⁸ SrAl₂O₄: Eu²⁺, Dy³⁺ (green, ∼510 nm),Reference Murayama, Takeuchi, Aoki and Matsuzawa⁹^, Reference Matsuzawa, Aoki, Takeuchi and Murayama¹⁰ and Y₂O₂S: Eu³⁺, Ti⁴⁺, Mg²⁺ (red, 625 nm).Reference Li, Liu and Ai¹¹^, Reference Ai, Liu, Li and Xiao¹² Among these, the afterglow properties of blue and green phosphors are much superior to the red one (the afterglow duration of Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ and SrAl₂O₄: Eu²⁺, Dy³⁺ lasts more than 10 h while the one of Y₂O₂S: Eu³⁺, Ti⁴⁺, Mg²⁺ maintains several minutes, which is far away from commercial application). To enhance the red long afterglow, some new materials are developed. Ca₂Si₅N₈: Eu²⁺Reference Miyamoto, Kato, Honna, Yamamoto and Ohmi¹³^, Reference Lei, Machida, Horikawa, Hanzawa, Kijima, Shimomura and Yamamoto¹⁴ and Ca₂SnO₄: Sm³⁺Reference Ju, Wei, Zheng, Gao, Zhang and Liu⁴^, Reference Xu, Wang, Zeng and Gong¹⁵ have been reported to exhibit red long afterglows. However, calcining temperatures for synthesis of these materials are high (>1300 °C), and it is not conducive to industrial manufacture. In 2004, Liu et al.Reference Liu, Liu, Yuan, Zhang, Rong and Huang¹⁶ observed the long afterglow of Eu³⁺ in CaWO₄ matrix. The afterglow duration also persisted several minutes. Since the afterglow of Y₂O₂S: Eu³⁺ can be induced by Ti⁴⁺, Mg²⁺ codoping,Reference Hong, Zhang, Fan and Wang¹⁷^, Reference Murazaki, Arak and Ichinomiya¹⁸ one may consider if such influence occurs in Eu³⁺ doped CaWO₄ matrix. In the present work, the influence of Ti⁴⁺ and Mg²⁺ codoping on the afterglow properties of CaWO₄: Eu³⁺ is investigated and the results are encouraging. In addition, the explanation on the afterglow enhancement is discussed.

II. EXPERIMENTAL SECTION

To study the enhanced afterglow properties, four samples, Ca_0.99WO₄: Eu³⁺_0.01 (denoted as CE), Ca_0.98WO₄: Eu³⁺_0.01, Ti⁴⁺_0.01 (denoted as CET), Ca_0.98WO₄: Eu³⁺_0.01, Mg²⁺_0.01 (denoted as CEM), and Ca_0.97WO₄: Eu³⁺_0.01, Ti⁴⁺_0.01, Mg²⁺_0.01 (denoted as CETM) are synthesized via high-temperature solid state reaction. The CaCO₃ (99%), WO₃ (99%), TiO₂ (99%), MgO (99%), and Eu₂O₃ (99.99%) are used as the raw materials. Initially, the raw materials were weighted according to the nominal compositions. Then the powders were mixed and ground thoroughly for 2 h. After that, the powder mixtures were calcined at 1000 °C for 2 h in an oven with about 5.6 °C min⁻¹ heating rate. After cooling to room temperature, the samples in the form of white powder are obtained.

The phase identification of all obtained samples was carried by x-ray diffraction (XRD) using a XD-2 diffractometer (Beijing PGENERAL) with Cu Kα irradiation (λ = 1.5406 Å) at 36 kV tube voltage and 20 mA tube current. The scan range was 2θ from 10 to 70° with 6° min⁻¹ speed. The photoluminescence (PL) was investigated by a F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) with a Xe lamp excitation. The afterglow spectrum was also measured using the same equipment. The samples are excited by a mercury lamp (254 nm, 60 W) for 1 min, and the measurement was carried out without external excitation (the Xe lamp was closed). The decay curves and thermoluminescence (TL) curves of the samples were measured using a FJ427A1 thermoluminescent dosimeter (CNNC Beijing Nuclear Instrument Factory, Beijing, China). 0.005 g powder for each sample was used. During detection of decay curves, the temperature was maintained at 30 °C. Prior to the detection, the powder was excited by a mercury lamp for 1 min. For the TL curves measurement, the heating rate was 1 °C Sec⁻¹. The range of the measurement is from room temperature to 300 °C. Prior to the measurement, the powder was also exposed to a mercury lamp for 1 min. The interval between the measurement and the removal of excitation was 10 min.

III. RESULTS AND DISCUSSION

A. Phase identification

The XRD patterns of all obtained samples are measured and the results are shown in Fig. 1. The samples exhibit the similar diffraction patterns with the approximate peaks locations. According to the JCPDF No. 85-0443 standard card, the structure of the samples can be regarded as tetragonal CaWO₄ with space group I41/a (No. 88). The crystal is body-centered, and the cell parameters of the samples in the present work are around a = b = 5.23 Å and c = 11.35 Å. No other structure is observed from the patterns. The incorporated Eu³⁺, Ti⁴⁺, and Mg²⁺ ions tend to substitute the Ca²⁺, W⁶⁺, and Ca²⁺ ions, respectively, due to the similar ionic sizes (Ca²⁺, eighth coordination, 112 p.m.; W⁶⁺, fourth coordination, 42 p.m.; Eu³⁺, eighth coordination, 106.6 p.m.; Ti⁴⁺, fourth coordination, 42 p.m.; and Mg²⁺, eighth coordination, 89 p.m.).Reference Shannon¹⁹ Small amounts of Eu³⁺, Ti⁴⁺, and Mg²⁺ incorporation has not had a significant effect on the crystal structure.

FIG. 1.

X-ray diffraction patterns of all obtained samples.

The mean crystal sizes of the obtained materials are estimated using Scherrer formulaReference Patterson²⁰:

(1)

$D = {{0.89\lambda } \over {\beta \cos (\theta )}},$

where D is the average crystal size for the particles. λ represents the wave length, which is 0.15406 nm in this work. β is the width at half intensity of the peak and θ is the degree of the maximum intensity of the peak. The (101) and (112) crystal planes are selected for estimation and the estimated results are listed in Table I. The mean crystal sizes of the samples are around 56–64 nm, indicating that incorporation of small amount of Eu³⁺, Ti⁴⁺, and Mg²⁺ is not regarded a significant influence on the crystal size.

TABLE I.

Estimated results of the mean crystal sizes of samples.

B. PL analysis

The emission spectra of the obtained samples are shown in Fig. 2. As can be seen, each spectrum consists of a broad emission band and several obvious emission sharp lines. The broad band located around 415 nm is ascribed to the charge transfer (CT) from W⁶⁺ 5d state to O²⁻ 2p state of the matrixReference Pang, Lin, Wang, Yu, Zhou and Han²¹ while the sharp lines can be attributed to the f–f transitions configuration of Eu³⁺ ions. These sharp lines locate around 590, 616, 650, and 700 nm, which can be designated as ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃, and ⁵D₀ → ⁷F₄, respectively. The intensity of the CT emission of the WO₄²⁻ group is enhanced by the incorporation of Ti⁴⁺ and further enhanced by the incorporation of Mg²⁺. It reaches a maximum value (more than five times stronger than the one of CE) when the phosphor is incorporated with Ti⁴⁺ and Mg²⁺ simultaneously. It means the incorporation of small amounts of Ti⁴⁺ and Mg²⁺ can improve the luminescence of CaWO₄ matrix.

FIG. 2.

Emission spectra of all samples.

The excitation spectra of the samples monitoring 616 nm emission are presented in Fig. 3. Similarly, a broad absorption band corresponding to the CT in WO₄²⁻ group can be observed. The maximum values of the bands locate around 268 nm, 264 nm, 262 nm, and 257 nm, corresponding to CE, CET, CEM, and CETM, respectively. Incorporation of Ti⁴⁺ and Mg²⁺ results in the blue shift of CT. The sharp lines can be ascribed to the absorption of the Eu³⁺ ions. The lines at 360, 380, 394, 412, 464, and 531 nm can be designated as ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵L₇, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, ⁷F₀ → ⁵D₂, and ⁷F₀ → ⁵D₁, respectively. As presented in the figure, the samples with Ti⁴⁺ incorporation present weaker intensities of ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵L₇, and ⁷F₀ → ⁵L₆ than the samples without Ti⁴⁺. Meanwhile, the CTs of the samples with Ti⁴⁺ incorporation are a bit stronger. This indicates that the Ti⁴⁺ ions in the crystal weaken the f–f absorption of Eu³⁺ but reinforce the energy transfer from CT to Eu³⁺ emission.

FIG. 3.

Excitation spectra of the samples.

The afterglow spectrum of CETM was measured, and the result is exhibited in Fig. 4. Two sharp lines corresponding to the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions of Eu³⁺ ions are observed. The other samples show the similar results. Other emissions of Eu³⁺ or the WO₄²⁻ group are not obvious, implying that ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ make the chief contribution to the afterglow emission. The calculated color coordinates (International Commission on Illumination standard) of the afterglow is (x, y:0.566, 0.346), meaning that a red afterglow can be observed.

FIG. 4.

Afterglow emission spectrum of CaWO₄: Eu³⁺, Ti⁴⁺, Mg²⁺.

C. Decay characterization

To demonstrate the afterglow properties of the samples, the decay curves were recorded and shown in Fig. 5. It clearly shows that the initial intensity of the afterglow is enhanced by Ti⁴⁺ incorporation and further enhanced by Mg²⁺ incorporation. The CETM, which is incorporated by Ti⁴⁺ and Mg²⁺ simultaneously, shows the strongest intensity. The sequence of the intensity from strong to weak maintains as CETM > CEM > CET > CE during the whole interval. This can be confirmed via Fig. 5(b), which shows the decay process in the last 200 s. From this, it can be deduced that the afterglow duration of CaWO₄: Eu³⁺ can be significantly prolonged by Ti⁴⁺ and Mg²⁺ incorporation.

FIG. 5.

(a) Decay curves of the afterglow of all samples. (b) Decay curves measured in the last 200 s.

In general, the decay process of LAPs can be evaluated using curve fitting technique. In many cases, the fitting curve is the double exponential equation since the decay process can be divided into a rapid-decaying process and a slow-decaying one. To some extent, the latter reflects the duration of the afterglow because it is determined by the tail of the decay. The form of the equation is as followsReference Lin, Dong, Tan, Liu, Zhang, Chen, Qiu, Zhao and Xu²²^, Reference Dorenbos, Krumpel, van der Kolk, Boutinaud, Bettinelli and Cavalli²³:

(2)

$I = I_0 + I_1 \exp \left( {{{ - t} \over {\tau _1 }}} \right) + I_2 \exp \left( {{{ - t} \over {\tau _2 }}} \right),$

where I ₀ is the original properties of the sample; I represents the phosphorescent intensity; I ₁ and I ₂ are two constants; t represents the time; and τ ₁ and τ ₂ are decay constants which decide the decay rate for the rapid and the slow exponentially decay components, respectively. In the present work, this equation with the identical I ₀ is also utilized to fit the experimental data and the fitting results are listed in Table II. The values of τ ₂ in the table imply that the sequence of afterglow duration from long to short confirms to CETM > CEM > CET > CE. Hence, it can be deduced the longest duration from CETM. In other words, incorporation of Ti⁴⁺ and Mg²⁺ enhances the afterglow properties of CaWO₄: Eu³⁺ significantly.

TABLE II.

Fitting results of the decay process of samples.

D. TL analysis

The long afterglow of phosphors is considered to be induced by the thermal liberation of trapped carriers. When phosphors are exposed under external illumination, charge carriers (electrons and/or holes) can be excited from ground state to excitation state. Then charge carriers are partially captured by the trap(s). The thermal liberation of carriers from trap(s) to luminescence centers accompanied by charge recombination results in a delay emission after the removal of external illumination. This is the afterglow of the phosphors. If the phosphors are heated, trapped carriers will be liberated rapidly in a suitable temperature, overcoming the activation energy. Thus, the higher temperature the rapid liberation of trapped carriers locates, the deeper the trap should be. A temperature dependence luminescence curve more or less reflects the status of the traps. This curve is called TL. The TL curves of the samples in the present work are exhibited in Fig. 6. The shape of the curves is similar, while the glow intensities are different. The incorporation of Ti⁴⁺ and Mg²⁺ increases the TL intensity. This means that the probability of the capture of carriers is lifted. The sequence of the TL intensity from strong to weak is CETM > CEM > CET > CE, which agrees well to the one of decay intensity. More carriers in the traps results in a larger number of liberated carriers, and it persists for a period of time, leading to a stronger intensity and a long duration of afterglow. That is why a stronger TL corresponds to a better property of the afterglow. In a word, the similarity of the TL curves with different intensities implies that the incorporation of Ti⁴⁺ or Mg²⁺ does not induce a new trap, but lifts the trapping probability of carriers.

FIG. 6.

(a) Thermoluminescence (TL) curves of all samples. (b) Deconvolution of the TL curve of CaWO₄: Eu³⁺, Ti⁴⁺, Mg²⁺.

Deconvolution of the TL presents five TL bands [shown in Fig. 5(b)], implying five traps of the phosphors. The cause of traps in phosphors is still an open question, but it is agreed that lattice defects play an important role on it. One possible defect is the Ca²⁺ vacancies (denoted as V_Ca). When two Eu³⁺ ions replace two Ca²⁺ ions in the matrix, one V_Ca is induced due to the charge compensation. Two different Ca²⁺ sites in CaWO₄Reference Xiao, Xiao, Liu, Ai, Li and Wang²⁴ lead to two different V_Ca. The V_Ca has two positive charges and they act as hole traps. Eu³⁺ ion in the Ca²⁺ site is another kind of defect, and oxygen vacancies may also be the defects. The cluster of defects makes the cause of traps more complicated than we expected. Further research should be carried out to reveal it.

E. Discussions

The afterglow of Eu³⁺ in CaWO₄ can be induced by the 254 nm excitation (mercury lamp) efficiently. Due to the overlap of the 254 nm excitation and the CT of WO₄²⁻ group, and the energy transfer from CT to Eu³⁺ is possible,Reference Gürmen, Daniels and King²⁵ it is reasonable to deduce that the afterglow of Eu³⁺ in CaWO₄ is originated from the energy transfer from CT to the activator. As shown in Fig. 7, the band gap of CaWO₄ matrix is around 5.6 eV and the lowest level of Eu³⁺ is around 2.3 eV below the top of valence band (VB),Reference Shi, Liu, Gao and Zhou²⁶ thus locations of other levels can be estimated. When the phosphors are excited by an excitation available to CT, electrons transfer from the 2p state O²⁻ to the 5d state of W⁶⁺ and then a large number of free carriers are generated because the 5d state of W⁶⁺ dominates the bottom of conduction band (CB) while the 2p state of O²⁻ dominates top of VB.Reference Zhang, Holzwarth and Williams²⁷ Then carriers migrate in CB and/or VB. Some of the carriers are trapped by traps without an immediate liberation. After the removal of excitation, trapped carriers can be thermally liberated. The reverse process leads to the CT accompanied by the energy transfer to Eu³⁺ ions. The afterglow of Eu³⁺ is generated.

FIG. 7.

Scheme of mechanism on afterglow generation on CaWO₄: Eu³⁺.

Since the Ti⁴⁺ and Mg²⁺ do not induce a new trap, two factors may be considered as influences on the afterglow. One is the efficiency of CT and the other is the energy transfer rate from CT to Eu³⁺. As shown in the emission spectra, the incorporation of Mg²⁺ significantly enhances the emission of CT of WO₄²⁻ group. The Mg²⁺ ions substitute the Ca²⁺ ions in the crystal and then a small amount of MgWO₄ structure is formed. Since the quantum efficiency of MgWO₄ approaches 100%,Reference Yen, Shionoya and Yamamoto²⁸ the small amount of MgWO₄ may assist to increase the efficiency of the matrix. That is why the incorporation of Mg²⁺ enhances the CT emission. On the other hand, the excitation spectra monitoring 616 nm emission illustrate that the Ti⁴⁺ weakens some f–f absorptions of Eu³⁺ (⁷F₀ → ⁵D₄, ⁷F₀ → ⁵L₇, and ⁷F₀ → ⁵L₆) but reinforces the energy transfer from CT of WO₄²⁻ to Eu³⁺ emission because of the largest value of CT absorption band. Then the energy transfer rate from CT to Eu³⁺ can be enhanced by Ti⁴⁺ incorporation. This result is in agreement with the work of Hong et al.Reference Hong, Zhang, Fan and Wang¹⁷ Consequently, Mg²⁺ and Ti⁴⁺ enhance the afterglow properties via the enhancement of efficiency of CT and the energy transfer rate from CT to Eu³⁺, respectively. On the other hand, in the case of incorporation of Ti⁴⁺ ions in W⁶⁺ sites, more oxygen vacancies will be created in the vicinity due to charge compensation, leading to increased trap concentration. In the case of Mg²⁺ substituting Ca²⁺, the probability of replacement of Eu³⁺ for Ca²⁺/Mg²⁺ is lowered because the difference of ionic size is enlarged, leading to a lower Eu³⁺ concentration in matrix. This is beneficial to the enhancement of Eu³⁺ phosphorescence due to the longer activator—vacancy distance for a given concentration of V_Ca.Reference Clabau, Rocquefelte, Le Mercier, Deniard, Jobis and Whangbo²⁹ However, this does not play a crucial role on the enhancement of afterglow because the Mg²⁺ incorporation is low and the substitution of Eu³⁺ for Ca²⁺ is not affected significantly.

IV. CONCLUSIONS

A red afterglow can be observed in CaWO₄: Eu³⁺ phosphors. It is caused by the energy transfer from CT to Eu³⁺ ions. The incorporation of Mg²⁺ enhances the efficiency of CT while the one of Ti⁴⁺ enhances the rate of the energy transfer from CT to Eu³⁺. The simultaneous incorporation of Ti⁴⁺ and Mg²⁺ significantly increases the intensity and duration of the afterglow emission. The CaWO₄: Eu³⁺, Ti⁴⁺, Mg²⁺ may not yet meet commercial applications, but the present work at least provides a possible way to reinforce the afterglow properties of Eu³⁺.

ACKNOWLEDGMENT

The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21071034 and 20871033).

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