Improved luminescence in YVO4:Eu3+@YVO4 core–shell nanoparticles through surface-confined thermal diffusion of Eu3+

Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Improved luminescence in YVO4:Eu3 þ @YVO4 core–shell nanoparticles through surface-confined thermal diffusion of Eu3 þ Dini Xie a,b, Hongshang Peng a,n, Shihua Huang a, Fangtian You a, Xiqing Zhang a, Gang Wang b a Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China b Oxide Technology Laboratory, BOE Technology Group Co. Ltd., Beijing 100176, China

art ic l e i nf o

a b s t r a c t

Article history: Received 6 April 2015 Received in revised form 26 May 2015 Accepted 29 May 2015

In this paper, we reported a type of highly luminescent YVO4:Eu3 þ @YVO4 core–shell nanoparticles (NPs) obtained by a surface-confined thermal diffusion method. YVO4:Eu3 þ particle cores with a doping ratio of 50% were firstly synthesized, followed by the regrowth of pure YVO4 shells. Thereafter, the raw YVO4:Eu3 þ @YVO4 core–shell NPs were embedded into a silica xerogel, which underwent annealing at 800 °C for different times. Thanks to the protection of silica matrix and thermal diffusion of Eu3 þ , the asobtained optimal NPs maintained the initial small size (19 nm), but exhibited considerably improved luminescent properties: the intensity was enhanced by 13 times and absolute quantum yield raised up to 14.4%. Such a good performance was interpreted by the corporation of three factors, i.e. improved crystallization, decreased concentration quenching and absent surface quenching. Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.

Keywords: YVO4:Eu3 þ Nanocomposites Diffusion Concentration quenching Surface effect Optical materials and properties

1. Introduction Luminescent nanomaterials have been studied for decades because of their unique optical properties [1,2]. Various luminescent nanomaterials such as quantum dots and rare-earth doped nanocrystals (NCs) have been widely used in a variety of applications such as color display, optical information storage, bioimaging and biosensor, biomedical studies, etc. [3,4]. Thus, there is an urgent demand of improving luminescent efficiency of nanomaterials, which may be affected by two fundamental factors: crystallinity and surface effect. Insufficient crystallinity means unordered structure and more defects which act as quenching centers consuming excited photoelectrons without radiation. And the large specific surface area of nanomaterials, unsaturated bonds on the surface and high surface energy will result in surface quenching and greatly decrease the lifetime and luminescent efficiency of nanomaterials [5,6]. Over the years, core–shell structure is utilized to weaken the surface effect, reduce surface quenching so as to enhance efficiency of light-emitting materials [7]. The shell provides a chemically-designed protection and prevents quenching by external environment [8]. A large number of research works are dedicated n

Corresponding author. E-mail address: [email protected] (H. Peng).

to selection of shell materials and optimization of shell thickness [9,10]. In the meantime, a phenomenon that cannot be overlooked was observed: ions' diffusion in core–shell NPs. DiMaio et al. [11] studied the diffusion of Eu3 þ in LaF3:Eu3 þ @LaF3 core–shell NPs under 650 °C. The diffusion of doping ions was significant enough to result in drastic color changes as emissions from higher-energy manifolds were no longer concentration quenched. In another work, Zheng et al. [12] observed Mn2 þ diffusion in MnS@ZnS core–shell NCs annealed at relatively lower temperatures (220– 300 °C). The diffusion of Mn2 þ in MnS@ZnS NCs from the MnS core to the NCs surface resulted in the PL surface quenching. If doped ions in particle core diffuse through the shell to surface, then surface effect cannot be prevented any more and surface quenching of luminescence still remains. For practical applications, it is important to know how elevated temperatures and time affect luminescence properties of doped core–shell NPs. In this work, surface-confined thermal diffusion of Eu3 þ in YVO4:Eu3 þ @YVO4 core–shell NPs and its influence on PL properties were investigated. The core–shell NPs were buried in silica xerogel to prevent aggregation, and diffusion of Eu3 þ was realized by thermal annealing at 800 °C. The as-treated core–shell NPs exhibited enhanced PL in both intensity and lifetime in comparison to the unannealed counterpart NPs. The improvements were not only attributed to ameliorated crystallinity but also to the surface-confined diffusion of Eu3 þ , which could decrease

http://dx.doi.org/10.1016/j.matlet.2015.05.148 0167-577X/Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.

Please cite this article as: D. Xie, et al., Improved luminescence in YVO4:Eu3 þ @YVO4 core–shell nanoparticles through surface-confined thermal diffusion of Eu3 þ , Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.148i

D. Xie et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Fig. 1. TEM images of (a) YVO4:Eu3 þ 50% NPs and (b) YVO4:Eu3 þ 50%@YVO4 core–shell NPs prepared by co-precipitation method under 60 °C. The insets show respective particle sizes' statistics. SEM images of YVO4:Eu3 þ 50%@YVO4 core–shell NPs dispersed in silica xerogel before (c) and after (d) annealing at 800 °C for 10 h.

concentration quenching while the diffusion distance was controlled to prevent the common surface quenching. Hence such diffusion strategy is very promising to be used to prepare ionsdoped luminescent nano-materials with high quantum yield.

2. Experimental procedures 2.1. Materials synthesis YVO4:Eu3 þ 50% NPs were prepared by a co-precipitation method at 60 °C [13]. A 0.1 M solution of (Y, Eu)(NO3)3 was mixed with a 0.1 M solution of sodium citrate. This led to the formation of a white precipitate of lanthanide citrate, which was completely dissolved by the addition of a 0.1 M solution of Na3VO4. The clear and colorless resulting mixture was subsequently aged at 60 °C for 30 min. Finally, the solution was cooled and dialyzed against deionized water to remove the excess ions. To prepare YVO4:Eu3 þ 50%@YVO4 core–shell NPs, a 0.1 M solution of Y(NO3)3 and a 0.1 M solution of sodium citrate were mixed together in stirring under room temperature, then a 0.1 M solution of Na3VO4 was added, and the resulting transparent oligomer solution was injected to the prepared YVO4:Eu3 þ 50% colloidal solution under 60 °C in stirring by a BT100-2J peristaltic pump, whose injection speed was set to 10 mL/h. After all the transparent colloidal solution of oligomer was pumped into, the

obtained mixture was aged for 30 min. The mixed solution was cooled and dialyzed against purified water for 2 days. The molar ratio of core and shell is 1:14. The core–shell NPs were buried in silica xerogel, in order to sustain an elevated temperature without aggregation. The silica xerogel was prepared by an acid–base two-step catalytic process [14]. The starting solutions were prepared by mixing tetraethyl orthosilicate (TEOS), ethanol and water under vigorous magnetic stirring at room temperature. The mole ratios of ethanol to TEOS and water to TEOS were both 5:1. An acid catalyst of 0.01 M hydrochloric acid ethanol solution and an alkaline catalyst of 1 M ammonia ethanol solutionwere successively injected at a 50 min interval.The mixed solution was stirred, allowed to stand for 10 min, and then prepared YVO4:Eu3 þ @YVO4 core–shell NPs colloid solution was added. The mixture was stirred until homogenous, then let stand for a few minutes until turned into gel, which was dried in an oven in a temperature range of 80– 120 °C during a period of 20 h. The obtained xerogel was annealed at 800 °C in a muffle furnace for different times. The molar ratio of silica to rare-earth vanadate in the xerogel was about 500:1. 2.2. Characterization Morphology of the prepared samples was examined by transmission electron microscopy (TEM, JEOL JEM 1400) and a fieldemission scanning electron microscope (FESEM, Hitachi S4800).

Please cite this article as: D. Xie, et al., Improved luminescence in YVO4:Eu3 þ @YVO4 core–shell nanoparticles through surface-confined thermal diffusion of Eu3 þ , Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.148i

D. Xie et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Fig. 2. XRD patterns of YVO4:Eu3 þ 50%@YVO4 core–shell NPs taken out from silica xerogel before and after annealed. (a) Unannealed, (b) heat to 800 °C and immediately stop heating and cool to room temperature, (c–e) heat to 800 °C and keep for 2, 6, 10 h respectively and cool to room temperature. The normalized diffraction peaks near 25° (2θ) are shown on the right.

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Fig. 3. (A) Excitation spectra of YVO4:Eu 50%@YVO4 core–shell NPs dispersed in silica xerogel annealed at 800 °C for 10 h. (B) Emission spectra of YVO4:Eu3 þ 50%@YVO4 core–shell NPs dispersed in silica xerogel and annealed at 800 °C for 0–10 h. The emission spectra were excited at 297 nm. Inset depicts the emission intensities at 621 nm for different annealing times.

The crystalline structure of samples was assessed from X-ray diffraction patterns (XRD, Bruker D85) employing Cu Kα radiation with a diffracted beam monochromator. The PL spectra were recorded in a Hitachi F4600 spectrophotometer. The luminescence dynamics were investigated by a FL920 spectrometer (Edinburgh Instruments) using a pulsed Xe lamp as the excitation source. Absolute PL quantum yields were evaluated at room temperature using a Hamamatsu Photonic Multi-Channel Analyzer, C 10027.

3. Results and discussion In order to research the thermal diffusion process of Eu3 þ in core–shell NPs, a thin shell is needed and thus a large core–shell molar ratio of 1:14 is chosen. Fig. 1(a) and (b) shows the TEM images of YVO4:Eu3 þ 50% NPs and YVO4:Eu3 þ 50%@YVO4 core– shell NPs synthesized by co-precipitation method, which show a good dispersion. The insetsdepict particle sizes' statistics, which are collected from more than 600 particles. Their average sizes are 9 nm and 19 nm respectively, indicating the successful coating of YVO4 shell. The as-prepared YVO4:Eu3 þ 50%@YVO4 core–shell NPs were dispersed in silica xerogel and characterized by SEM. In Fig. 1 (c) and (d), the silica NPs with an average of about 30 nm are closely arranged in xerogel, which can provide well protection to

YVO4:Eu3 þ 50%@YVO4 core–shell NPs under high annealing temperature. The YVO4:Eu3 þ 50%@YVO4 core–shell NPs dispersed in silica xerogel were collected by washing with sodium hydroxide solution. Fig. 2 shows XRD patterns of as-obtained NPs. It can be seen that all diffraction peaks are indexed to tetragonal structure of YVO4, which are in accordance with JCPDS card no. 72-0274. In addition, the intensities of diffraction peaks significantly increase with the extension of annealing time, but their widths hardly change. The results indicate that the crystallinity of NPs is ameliorated while their sizes are kept constantly, owing to the protection of silica xerogel. The PL spectra of YVO4:Eu3 þ 50%@YVO4 core–shell NPs embedded in silica xerogel and annealed at 800 °C for 10 h are depicted in Fig. 3. The excitation spectra exhibits a broad overlap excitation peak with a maximum 297 nm and a shoulder at about 265 nm in the range of 220–360 nm, which are assigned to charge transfer bands (CTB) of V–O and Eu–O [15]. In the emission spectra, the 5D0–7FJ transitions (J ¼0–4) of Eu3 þ can be clearly observed, and their origins in detail are labeled in the figure. The integrated emission intensity of the samples is increased by about 13 times after 10 h of annealing, as the absolute PL quantum yield is enhanced from less than 1% to 14.4%. Such an enhancement could be mainly attributed to the following two factors: (i) heat-treatment can enhance the crystallinity of

Please cite this article as: D. Xie, et al., Improved luminescence in YVO4:Eu3 þ @YVO4 core–shell nanoparticles through surface-confined thermal diffusion of Eu3 þ , Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.148i

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samples, which may effectively improve luminescence efficiency; (ii) it may also promote the diffusion of Eu3 þ within particles, which will reduce the local concentration and weaken concentration quenching effect [16,17] (the discussion about the diffusion of Eu3 þ is shown in Supporting information). It should be noted that the particle shell may fail to prevent surface quenching in ions-doped luminescent core–shell NPs, because the doped ions can diffuse through shell to surface under high annealing temperature or with long annealing time. Therefore, preparation and application of ion-doped luminescent NPs should be cautious of the diffusion of ions. On the other hand, thermal diffusion property can be exploited to obtain luminescent NPs with high efficiency if designed delicately, just as the attempts we have made in this work.

4. Conclusion In summary, small sized YVO4:Eu3 þ NPs with high luminescence efficiency were synthesized by using of the strategy of surface-confined thermal diffusion. YVO4:Eu3 þ 50%@YVO4 core–shell NPs were firstly prepared with a co-precipitation method, and then annealed at 800 °C to render the diffusion of Eu3 þ ions after being embedded into silica xerogel. Different annealing times gave rise to different luminescence efficiencies and NPs with 10 h of annealing turned out to be the optimal: luminescent intensity was enhanced by 13 times and absolute PL quantum yield was raised up to 14.4%. Further investigations in decay curves revealed that the diffusion of Eu3 þ ions reduced the concentration quenching in NPs, which accounted for the enhanced luminescence efficiency along with the improvement of crystallinity.

Acknowledgments This work was financially supported by National Natural Science Foundation of China (Grant nos. 61078069 and 10979009), Program for New Century Excellent Talents in Universities of

Ministry of Education of China (Grant no. 12-0771), Fundamental Research Funds for the Central Universities (Grant no. 2010JBZ006).

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015.05.148

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Please cite this article as: D. Xie, et al., Improved luminescence in YVO4:Eu3 þ @YVO4 core–shell nanoparticles through surface-confined thermal diffusion of Eu3 þ , Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.148i