Preparation and luminescence of transparent silica glass-ceramics containing LaF3:Eu3+ nanocrystals

Materials Letters 271 (2020) 127764

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Preparation and luminescence of transparent silica glass-ceramics containing LaF3:Eu3+ nanocrystals Ping Huang a,1, Peili Luo a,1, Beiying Zhou c,⇑, Lianjun Wang a,b,⇑, Wan Jiang a,c a

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials & College of Materials Science and Engineering, Donghua University, Shanghai 201620, PR China Engineering Research Center of Advanced Glasses Manufacturing Technology, Ministry of Education, Donghua University, Shanghai 201620, PR China c Institute of Functional Materials, Donghua University, Shanghai 201620, China b

a r t i c l e

i n f o

Article history: Received 10 February 2020 Received in revised form 23 March 2020 Accepted 31 March 2020 Available online 1 April 2020 Keywords: LaF3:Eu3+ nanocrystals Transparent glass-ceramics Reddish emission LEDs

a b s t r a c t Silica glass-ceramics containing highly crystalline LaF3:Eu3+ nanocrystals (NCs) were successfully prepared at a relatively low temperature of 950 °C within a short time of 10 min by Spark Plasma Sintering. The LaF3:Eu3+ NCs were not only well retained without interfacial reaction occurring between LaF3:Eu3+ NCs and silica matrix phases, but also homogeneously dispersed in glass matrix. The luminescence peaks located within reddish-orange spectral scope were registered and identified as the intraconfigurational 4f6-4f6 transitions originated from Eu3+ optically active ions (5D0 ? 7FJ, J = 0–4). By adopting the LaF3:Eu3+ glass-ceramic as color convertor, the fabricated LED shows excellent reddish emission. Ó 2020 Elsevier B.V. All rights reserved.

1. Introduction Transparent glass–ceramic containing Eu3+ ions doped fluoride nanocrystals (NCs) have drawn massive attentions in the potential applications of color display, laser and optical amplifier for communication due to the excellent chemical stability, thermal stability and less lumen loss inherited from glass matrix [1,2]. Among kinds of fluoride, LaF3 is a suitable host for Eu3+ ions due to the low phonon energy (350 cm 1), which can reduce the nonradiative de-excitation probability caused by the multiphoton relaxation, and the similar size and electronic configuration can facilitate the Eu3+ ions to be doped in NCs [3–5]. A variety of methods such as melt-quenching and sol–gel have been adopted to prepare the LaF3:Eu3+ glass-ceramics (GCs). However, high melting temperature (~1450 ℃) and long processing time (>20 h) are always required in the traditional meltquenching method, which is not only time consuming but also will induce the ion overflow, crystal deconstruction and opaqueness ascribed to the abnormal grain growth of the NCs [4,6]. In addition, the NCs GCs obtained by sol–gel are also suffered from the problems of low chemical stability and poor mechanical properties. Moreover, the hydroxyl and carbon residues in the material pose negative impacts on the luminescence of the glass [7]. Therefore,

⇑ Corresponding authors. 1

E-mail addresses: [email protected] (B. Zhou), [email protected] (L. Wang). These authors contributed equally to this work.

https://doi.org/10.1016/j.matlet.2020.127764 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.

it is still a great challenge to controllably prepare LaF3:Eu3+ GCs with high luminescence properties. In this paper, by adopting the mesoporous silica FDU-12 and asprepared LaF3:Eu3+ NCs as the starting materials, LaF3:Eu3+ GCs were prepared at 950 °C within 10 min by Spark Plasma Sintering (SPS). The crystal phase, morphologies and luminescence properties of NCs and composite glasses were discussed. It was shown that highly crystalline LaF3:Eu3+ NCs were homogeneously dispersed into the glass matrix with well protection.

2. Experimental section The LaF3:Eu3+ NCs were synthesized by hydrothermal method. The deionized water solution of Eu(NO3)3
6H2O (Aladdin chemical) was mixed with La(NO3)3
6H2O (Aladdin chemical) at a concentration of 5 mol%, then added NHF4 aqueous solution and keep stirring at 75 °C for 2 h. After hydrothermal reaction at 140 °C for 18 h, the precipitation was washed and air dried at 60 °C. At last, the NCs was annealed at 500 °C for 2 h. Mesoporous silica FDU-12 were prepared according to the reported works [8,9]. Then, the NCs and FDU-12 were mixed at a certain mass percentage by ultrasonic dispersion. Mixed powders (0.5 g) were loaded into a cylindrical graphite die (d = 10 mm). Finally, the samples were sintered at 950 ℃ for 1 min under heating rate of 130 °C/min and uniaxial pressure of 50 MPa by SPS. After cooling down, all the samples were milled and surface-polished into 1 mm in thickness. The crystal phase analysis was identified via X-ray diffractometer (XRD).

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The morphologies were studied using a field emission scanning electron microscope (SEM) and transmission electron microscopy (TEM). The photoluminescence excitation spectra, emission spectra and lifetime were characterized by a fluorescence spectrophotometer. The color coordinate of as-fabricated LED was measured by the HCPOO spectrograph. 3. Results and discussion 3.1. LaF3:Eu3+ NCs From the TEM image in Fig. 1a, the LaF3:Eu3+ NCs were hexagonal nanoplate with a size less than 50 nm. The sharp diffraction peaks shown in the XRD patterns (Fig. 1b) can be well matched to the PDF card of LaF3, indicating the highly crystalline LaF3 based NCs were successfully synthesized. Notably, the (1 1 1) peak exhibits a slight shift towards larger 2h degree, which means the cells of the LaF3 NCs decrease to some extent. As the size of Eu3+ is smaller than La3+, it could be inferred that the Eu3+ ions entered the host lattice [10]. When monitored at 590 nm, the photoluminescence excitation (PLE) spectrum of LaF3:Eu3+ NCs presents a strongest peak at 396 nm (Fig. 1c). Depending on the energy transfer of Eu3+, it is belonging to the 7F0 ? 5L6 transition of Eu3+[11]. Moreover, when excited at 396 nm, the photoluminescence (PL) peaks of NCs are shown in the range of 500 nm–750 nm, which are corresponding to the transitions of 5D0 ? 7F0, 5D0 ? 7F1, 5D0 ? 7F2, 5 D0 ? 7F3 and 5D0 ? 7F4 respectively, and consistent to the reported LaF3:Eu3+ system [12,13]. 3.2. LaF3:Eu3+ GCs The photographs and XRD patterns of LaF3:Eu3+ GCs are shown in Fig. 2a. With the concentration increase, the transparency of the glasses decreases slightly and the sharp peaks of the LaF3:Eu3+ NCs

become more obvious. Benefitting from the low sintering temperature, the characteristic hump of the amorphous silica glass at ~ 23° could be observed and no silica crystallization peaks. The SEM in Fig. 2b presents that the NCs are dispersed homogeneously in glassy matrix. The well-formed ordered lattice fringes of LaF3:Eu3+ NCs and a clear boundary between the NCs and amorphous silica matrix could be observed in HRTEM image (Fig. 2c). The results demonstrate that LaF3:Eu3+ NCs have been successfully incorporated into the glass without interfacial reactions. Fig. 3a and b show the PLE and PL spectra of the 3 wgt% LaF3: Eu3+ GCs, which are similar to those of the corresponding LaF3: Eu3+ NCs. The sharp peaks and upper-lying 5D2 and 5D1 levels in PL spectrum indicate that majority of Eu3+ ions are still occupied in La3+ site of LaF3 crystal with low phonon energy. The R/O-ratio (the emission intensity of I(5D0 ? 7F2)/I(5D0 ? 7F1)) values calculated for prepared LaF3:Eu3+ NCs and LaF3:Eu3+ GC in PL spectrum are 0.5657 and 0.8525, respectively. Although the slightly higher R/ O value revealing that some Eu3+ ions diffused into the silica glass matrix that is more asymmetrical than LaF3 NCs hosts, most the Eu3+ ions were well retained in the LaF3 NCs during the sintering process [14]. The fitting lifetime further proves the above inference in Fig. 3c. The luminescence lifetime of LaF3:Eu3+ GC and LaF3:Eu3+ NC are well-fitted to first-order exponential function with s = 6.479 ms and s = 10.891 ms, respectively. Generally, the interaction between optically active ions and sited environment determines the types of photon transition as radiation and non-radiation, which further affects the decay of fluoresce lifetime. Herein, the similar fluorescence lifetime between LaF3:Eu3+ NC and LaF3:Eu3+ GC indicates that the amount of Eu ions diffused into the matrix is extremely small, which is not enough to cause a significant change in lifetime [15,16]. This result is accordance to the R/O values calculated from PL spectra, and further confirms the surmise that the Eu3+ ions were well preserved in LaF3 NCs.

Fig. 1. (a) TEM image of LaF3:Eu3+ NCs; (b) XRD patterns of LaF3 and LaF3:Eu3+ NCs, (inset: the magnification of the (1 1 1) peaks); (c) PLE and PL spectra of LaF3:Eu3+ NCs.

Fig. 2. (a) Photographs and XRD patterns of the as-sintered glasses with different NCs concentrations; (b) SEM and HRTEM images of 3 wt% LaF3:Eu3+ GC.

P. Huang et al. / Materials Letters 271 (2020) 127764

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Fig. 3. (a) PLE spectra at 590 nm and 612 nm and (b) PL spectra at 396 nm and 393 nm of 3 wt% LaF3:Eu3+ GC, LaF3:Eu3+ NCs and Eu3+ doped silica glass; (c) The lifetime and (d) the relative PL intensity at different temperatures of the NCs and 3 wt% LaF3:Eu3+ GC; (e) Color coordinate diagram of LED equipped with 3 wt% LaF3:Eu3+ GC, (inset: The real and lighting photographs of LED device).

The relative PL intensity of the LaF3:Eu3+ NCs and LaF3:Eu3+ GC at different temperatures from 298 K to 448 K are shown in Fig. 3d. Although the emission intensity declines with the increase of the temperature due to non-radiative transitions caused by thermal quenching, benefitting from the high thermal conductivity and stability of the glass matrix, the PL intensity of LaF3:Eu3+ GC retains 50% at 448 K compared with that at room temperature, which is significantly higher than that of LaF3:Eu3+ NCs (18.4%). In other words, the LaF3:Eu3+ GCs present better heat resistance than LaF3:Eu3+ NCs, which is sought after high-power WLEDs [17]. By encapsulating near-UV LED chips (395 nm) with 3 wgt% LaF3: Eu3+ GC, the LED emits the standard red light with a chromaticity coordinate of (0.6351, 0.3645) as the ‘‘B” point shown in Fig. 3e. Compared to the color coordinates ‘‘A” (0.5839, 0.4129) of NC, the color of GC is biased towards deep red, which is caused by the larger R/O ratio of GC emission spectrum. 4. Conclusions The red silica glasses containing highly crystalline LaF3:Eu3+ NCs were successfully achieved at 950 °C within 10 min by SPS. The as-prepared glasses appeared characteristic spectra in reddish-orange region. As expected, the LaF3:Eu3+ GC sample possesses better thermostability, whose luminescence intensity retained 50% at 450 K compared with that at room temperature, than the NCs with a small reservation of 18.4%. The LED equipped with the LaF3:Eu3+ GC shows bright red-emitting light. Signifi-

cantly, this research designs an effective way to fabricate the high-quality luminescent glasses by dispersing the rare earth ion doped nanocrystals.

CRediT authorship contribution statement Ping Huang: Methodology, Visualization, Writing - original draft. Peili Luo: Data curation, Investigation. Beiying Zhou: Supervision, Writing - review & editing. Lianjun Wang: Conceptualization, Validation, Writing - review & editing. Wan Jiang: Project administration, Validation.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51672041, 91963204), the Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-03-E00025), the Fundamental Research Funds for the Central Universities (2232019G-07).

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