Influence of Ag nanoparticles on luminescent ...

Journal of Non-Crystalline Solids 358 (2012) 2788–2792

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Influence of Ag nanoparticles on luminescent performance of SiO2:Tb 3 + nanomaterials Dekai Zhang a, b, Xiaoyun Hu a, b,⁎, Ruonan Ji a, Suchang Zhan a, Jianhua Gao a, b, Zhiyun Yan a, Enzhou Liu c, Jun Fan c, Xun Hou a, d a

Department of Physics, Northwest University, Xi'an, 710069, China National photoelectric technology and Functional Materials & Application of Science and Technology International Cooperation Base, Northwest University, Xi'an, 710069, China School of Chemical Engineering, Northwest University, Xi'an 710069, China d State Key Laboratory of Transient Optics and Technology, Xi'an Institute of Optics & Precision Mechanics, Chinese Academy of Sciences, Xi'an 710068, China b c

a r t i c l e

i n f o

Article history: Received 11 May 2012 Received in revised form 30 June 2012 Available online 23 July 2012 Keywords: Rare earth doping; Localized surface plasmon resonance; Radiative decay rates; Luminescence

a b s t r a c t Tb3+ single-doped SiO2 (SiO2:Tb3+) and Tb3+, Ag co-doped SiO2 (SiO2:Tb3+–Ag) nanostructured luminescent materials were prepared by a modified Stöber method. Their microstructure and optical property were investigated using scanning electron microscopy, ultraviolet visible absorption and photoluminescence spectrophotometry. Results show that the samples are composed of well-dispersed near-spherical particles with a diameter about 50 nm, the highest fluorescence intensity is obtained when the doping concentration of Tb3+ is 4.86 mol%, the corresponding internal quantum efficiency is 13.6% and the external quantum efficiency is 8.2%. The experimental results indicate that Ag nanoparticles can greatly enhance the light absorption at 226 nm and the light emission at 543 nm of SiO2:Tb 3+–Ag, and the fluorescence lifetime reduces with increasing Ag concentration in SiO2:Tb 3+–Ag. Additionally, the present results indicate that fluorescence enhancement is attributed to the local field enhancement and the increased radiative decay rates induced by Ag nanoparticles. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Rare earth luminescent materials have an attractive application perspective in sensor, display device, optical storage, LED, etc. [1–4], especially rare earth doped SiO2. SiO2 is a good host material for fluorescence materials because of its good transmittance and thermal stability, easy doping by rare-earth elements, low toxicity and cheapness [5–9]. Among the lanthanide ions, Tb 3+ doped nanomaterials have attracted great attention for their good excellent visible light emission from 400 nm to 600 nm and longer fluorescence life time. In the current study, SiO2:Tb 3 + was selected because of its favorable physicochemical and luminescence properties [8]. Noble-metals nanoparticles (NPs) have been widely used as an enhancer in luminescent materials because they can enhance the fluorescence intensity of rare earth ions based on their localized surface plasmon resonance (LSPR), which has received attention from researcher all over the world [10–13]. Selvan [11] reported that Ag NPs could effectively increase the fluorescence intensity of Eu 3 + in silica gel. Results show that the LSPR peak of Ag NPs is located in 390 nm and the intensity of Eu 3+ increases by 10 times after doping

⁎ Corresponding author at: Department of Physics, Northwest University, Xi'an, 710069, China. E-mail address: [email protected] (X. Hu). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.07.004

of Ag NPs, and the above optical enhancement is attributed to the localized field enhancement near Eu 3 +. In this paper, Tb 3+ single-doped SiO2 (SiO2:Tb 3+) and Tb 3+, Ag co-doped SiO2 (SiO2:Tb 3+–Ag) nanostructured luminescent materials were prepared by a modified Stöber method. The influence of Ag NPs on luminescence performance of SiO2:Tb 3+ was investigated in detail, and the mechanism of fluorescence enhancement was discussed as well.

2. Experiment 2.1. Preparation of SiO2:Tb 3+ SiO2:Tb 3+ was synthesized by a modified Stöber method [9]. Solution A was prepared by mixing 5 ml TEOS and 30 ml C2H5OH at room temperature for 30 min, then adding Tb (NO3)3 and stirring for a further 2 h. Solution B was prepared by mixing 9 ml ammonia (28 wt.%) and 50 ml C2H5OH. Subsequently, solution A was added dropwise to solution B, and the resulting solution was stirred for 12 h, then the precipitate was separated by centrifugation, washed with absolute alcohol and deionized water for three times respectively, followed by drying at 50 °C for 24 h in a vacuum oven with a pressure of 0.08 MPa. Finally, SiO2:Tb 3+ was obtained by annealing the particles at 600 °C for 2 h.

D. Zhang et al. / Journal of Non-Crystalline Solids 358 (2012) 2788–2792

2.2. Preparation of SiO2:Tb 3+–Ag First, Ag sol was prepared by the method used by Meisel [10]. Appropriate amounts of AgNO3 was dissolved in 500 mL deionized water and heated up to boil, then sodium citrate solution (1 mg/L, 10 mL) was added dropwise to the above solution and stirred vigorously. After stirring for 10 min, the mixture was cooled down to room temperature in a nature manner, and then the large particles in the solution were removed by centrifugation. Thereafter, SiO2: Tb 3 +–Ag was obtained following a similar experimental process described in Section 2.1, except for doping silver NPs, which was achieved by adding Ag sol into the solution B before adding solution A into solution B.

3. Characterizations The morphology of samples was observed by a field-emission scanning electron microscopy (SEM, FEI, Quanta 400 FEG). The emission spectra, fluorescence quantum yield and life time were analyzed using a fluorescence spectrophotometer (Hitachi, F-7000, Xe lamp 150 W, PMT voltage 400 V, slit width 5 nm). Ultraviolet visible (UV–vis) absorption spectra were obtained on a UV–vis spectrophotometer (Shimadzu, UV3600).

4. Results and discussion 4.1. Microstructure analysis SEM images of SiO2:Tb3+ and SiO2:Tb3+–Ag are presented in Fig. 1. It is apparent that SiO2:Tb3+ consists of a large number of nanoparticles which tend to adhere to each other due to its high surface energy during the thermal treatment (Fig. 1a), the diameter of each particle was about 50 nm. As shown in Fig. 1b, there is almost no difference between SiO2: Tb3+–Ag and SiO2:Tb3+ samples, suggesting that Ag NPs doping has little influence on the morphology SiO2:Tb3+. Ag NPs and Tb3+ are successfully doped in SiO2:Tb3+–Ag samples according to the energy dispersive spectroscopy (EDS) pattern of SiO2:Tb3+–Ag in Fig. 2. Fig. 3 is the X–ray element area profile of SiO2:Tb3+–Ag. It shows that the elements of Ag and Tb are well dispersed in the sample. Fig. 1. (a) SEM image of SiO2:Tb3+. (b) SEM image of SiO2:Tb3+–Ag.

4.2. Optimization of Tb 3+ concentration During the research we found that there was a better doping amounts for the Tb3+ in SiO2, which was 4.86 mol% based on SiO2. As can be seen from Fig. 4, at first, the fluorescence intensity for excitation spectra and emission spectra increase with the increasing of Tb3+ concentration in SiO2 matrix, however, the florescence quenching is observed when the Tb3+ concentration reaches 7.30 mol%, which is mainly caused by Tb3+ exchange interaction. With a higher concentration, the distance between ions is shorter, interactions enhance, and the exciting energy transfers to the defect state though the interaction between ions and loses in the form of non-radiative processes. Additionally, the fluorescence quantum yield of the sample was measured using a Hitachi F-7000 fluorescence spectrometer equipped with an integrating sphere (diameter 70 mm). The results show that the internal quantum yield is 13.6%, and external quantum yield is 8.2%. The calculation process is based on the Formulae (1) and (2). Measurement parameters are defined as follows: ABT: amount of absorbtion; FL: amount of fluorescence; AB: absorptance.

ηin ¼

FL 100% ABT

ð1Þ

Fig. 2. EDS pattern of SiO2:Tb3+–Ag.

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2790


Fig. 3. X‐ray element area profile of SiO2:Tb3+–Ag (1.0%).

ηex ¼

FL AB 100% ABT

ð2Þ

4.3. Influence of doping Ag NPs on luminescence performance of SiO2:Tb3+ 4.3.1. Absorption spectrum of Ag NPs Absorption spectrum of Ag sol is shown in Fig. 5. The absorption band at 421 nm is attributed to resonance absorption of Ag NPs,

suggesting that Ag + has been reduced by sodium citrate after the reaction, and it disperses in the sample with Ag 0 state.

4.3.2. Influence of Ag doping concentration Fig. 6(a) shows the excitation spectrum of SiO2:Tb 3+–Ag, the excitation peaks are also located at 226 nm. The emission spectrum of the samples excited at 226 nm is shown in Fig. 6(b). The emission peak at 543 nm results from the 5D4 → 7F5 transition of Tb 3 +. It can be seen clearly that Ag NPs can greatly enhance that fluorescence intensity


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of Tb 3+, the fluorescence intensity increases by 25% and 35% when the Ag concentration is 0.5 mol% and 1.0 mol% respectively. Fig. 7 shows the fluorescence life time of samples. It can be seen that the fluorescence life time at 543 nm are 2.768 ms, 2.737 ms and 1.890 ms for SiO2:Tb 3+, SiO2:Tb 3+–Ag (0.5 mol%) and SiO2: Tb 3 +–Ag (1.0 mol%) respectively, indicating that the fluorescence

life time obviously decreases with the increase of Ag concentration. The fluorescence life time decreases with quantum yield enhancing due to the radiative rate decays increasing. The fluorescence quantum yield Qmand life time τmof fluorescence material can be calculated by Eqs. (3) and (4) [12]. It can be concluded from above results that Ag NPs can increase Γm of Tb 3+ with the

Fig. 5. Absorption spectra of Ag sol.

Fig. 7. Fluorescence life time of SiO2:Tb3+–Ag.

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interaction between electromagnetic field and rare-earth ions due to the very high field gradients near a metallic particle. The local field enhancement may enhance absorption, emission, and energy transfer from SiO2 host to Tb 3+ ions [13,14]. It can be concluded from above results that the fluorescence enhancement is attributed to the local field enhancement and the increased radiative decay rates resulting from Ag NPs. 5. Conclusion

Fig. 8. Absorption spectra of SiO2:Tb3+–Ag.

fluorescence life time decrease from 2.768 ms to 1.890 ms, it means that a higher Qm and florescence intensity according to the Eqs. (3) and (4), which is consistent with the excitation and emission spectra in Fig. 4. Qm ¼

Γ þ Γm Γ þ Γ m þ knr

ð3Þ −1

τm ¼ ðΓ þ Γ m þ knr Þ

ð4Þ

where Qm knr Γ Γm τm

is the fluorescence quantum yield is the rate of non-radiative decay to the ground state is rate of radiative decays is the radiative rate increases is fluorescence life time

4.3.3. Absorption spectrum of SiO2:Tb 3+–Ag Absorption spectrum of SiO2:Tb3+–Ag samples are shown in Fig. 8. The resonant absorption band of Ag NPs at 421 nm are not observed in all the samples, which may be related to SiO2 matrix around Ag NPs. However, there is a strong absorption peak of silica at 226 nm, the absorption intensity increases with the increasing of Ag concentration in the SiO2:Tb3+–Ag, leading to fluorescence spectrum enhancement. The fluorescence enhancement of Tb3+ may be attributed to local field enhancement of metallic NPs. Because there is an additional

Tb3+ single-doped SiO2 and Tb3+, Ag co-doped SiO2 nanostructured luminescent materials were prepared by a modified Stöber method. The sample composed of 50 nm spherical particles, the highest fluorescence intensity is obtained when the doping concentration of Tb3+ is 4.86 mol%, the corresponding internal quantum efficiency is 13.6% and the external quantum efficiency is 8.2%. The absorption of silica at 226 nm is greatly enhanced by Ag NPs, fluorescence life time reduces with increasing Ag NPs doping concentration, and emission intensity of Tb3+ at 543 nm increases by 35%. The fluorescence enhancement is attributed to the local field enhancement and the increased radiative decay rates resulting from Ag nanoparticles. Acknowledgements This work has been supported by the National Natural Science Foundation of China (grant no. 21176199), the Research Foundation for the Doctoral Program of Higher Education of China (20096101110013), the Natural Science Foundation of Shaanxi Province (grant no. 2011JM1001), the Natural Science Foundation of Shaanxi Province (grant no. 2012JM1020), the Industrialization Cultivation Item of Shaanxi Province Educational Department (grant no. 2011JG05). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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