Optical Materials 34 (2011) 274–277

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Optical Materials

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Preparation and upconversion property of TeO2:Er3+/Yb3+ nanoparticles

Hui Hu a, Yan Bai a,⇑, Minwen Huang b, Biyin Qin a, Jie Liu a, Wenjie Zheng a,⇑a Department of Chemistry, Jinan University, Guangzhou 510632, People’s Republic of Chinab School of Chemistry and Environment, Jiaying University, Meizhou, Guangdong 514000, People’s Republic of China

a r t i c l e i n f o

Article history:Received 15 January 2011Received in revised form 15 June 2011Accepted 19 August 2011Available online 23 September 2011

Keywords:Upconversion luminescenceTeO2 nanoparticlesEr3+/Yb3+ co-doped

0925-3467/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.optmat.2011.08.025

⇑ Corresponding authors. Tel.: +86 2088561095.E-mail addresses: tbaiyan@jnu.edu.cn (Y. Bai), tzhw

a b s t r a c t

Different crystal structure of TeO2 nanoparticles were used as the host materials to prepare the Er3+/Yb3+

ions co-doped upconversion luminescent materials. The TeO2 nanoparticles mainly kept the original mor-phology and phase after having been co-doped the Er3+/Yb3+ ions. All the as-prepared TeO2:Er3+/Yb3+ nano-particles showed the green emissions (525 nm, 545 nm) and red emission (667 nm) under 980 nmexcitation. The green emissions at 525 nm, 545 nm and red emission at 667 nm were attributed to the2H11/2 ?

4I15/2, 4S3/2 ?4I15/2 and 4F9/2 ?

4I15/2 transitions of the Er3+ ions, respectively. For thea-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles, three-photon process involved in the green (2H11/2 ?

4I15/2)emission, while two-photon process involved in the green (4S3/2?

4I15/2) and red (4F9/2 ?4I15/2) emissions.

For the b-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles, two-photon process involved in the green (2H11/2

? 4I15/2), green (4S3/2 ?4I15/2) and red (4F9/2 ?

4I15/2) emissions. It suggested that the crystal structureof TeO2 nanoparticles had an effect on transition processes of the Er3+/Yb3+ ions. The emission intensitiesof the a-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles and b-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles weremuch stronger than those of the (a + b)-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

The upconversion luminescent materials are excellent opticalmaterials with high photochemical stability, sharp emission band-widths, large anti-stokes shift, remarkable light penetration depthand the absence of background fluorescence [1,2]. Nowadays, Theyhave been applied in many fields [3–6]. Especially, upconversionluminescent materials are very promising for biological and medi-cal investigations [7–14]. A typical upconversion luminescentmaterial consists of two components, dopant ions that are capableof upconversion and the host material that can accommodate thesedopant ions. Selecting suitable host materials is a key factor forupconversion luminescent materials. Ongoing research is mainlyconcerned with the quest for host materials with increased conver-sion efficiencies. It is well known that TeO2 (optical fibers, crystalsand glassy) are suitable host materials [15–23]. The conventionalprocessing is using bulk TeO2 or TeO2 glasses as the host materialsfor preparing upconversion luminescent materials. Nanoscaleluminescent materials have attracted much interest because theoptical properties are different from those of the bulk samples,such as lifetime improvement and spectral stability [24]. However,no research has yet been conducted, to our knowledge, on usingTeO2 nanoparticles as the host materials for preparing upconver-sion luminescent materials.

ll rights reserved.

j@jnu.edu.cn (W. Zheng).

In our latest research, we reported the preparation of telluriumdioxide (a-TeO2, b-TeO2) nanoparticles in a mild condition [25]. Inthis study, TeO2 nanoparticles were used as the host materials toprepared the Er3+/Yb3+ ions co-doped upconversion luminescentmaterials. The as-prepared TeO2:Er3+/Yb3+ nanoparticles werecharacterized by transmission electron microscope (TEM)technique, X-ray diffraction (XRD) measurement, Ultraviolet–visible–near infrared spectra (UV–Vis–NIR) and upconversionemission spectra.

2. Experimental

2.1. Materials

Chemicals were all analytical grade. Ytterbium oxide (Yb2O3)and erbium oxide (Er2O3) with purity of 99.99% were purchasedfrom Minmetals (Beijing) Rare Earth Institute Company. Sodiumtellurite (Na2TeO3), gallic acid (GA), acetum (HAc) and nitric acid(HNO3) were purchased from Aladdin Chemical Corporation.Double-distilled water was used to prepare the solutions. All thematerials were used without further purification.

2.2. Preparation of TeO2:Er3+/Yb3+ nanoparticles

First, appropriate amounts of Er2O3 and Yb2O3 powders weredissolved in dilute HNO3 solution, respectively, to prepare 0.02 MEr(NO3)3 and 0.02 M Yb(NO3)3 solutions. Then, a volume of 10 mL

H. Hu et al. / Optical Materials 34 (2011) 274–277 275

of 0.1 M Na2TeO3 was added into 20 mL of 0.1 M HAc to preparea-TeO2 nanoparticles sol. A volume of 10 mL of 0.1 M Na2TeO3

was added into 20 mL of 0.1 M GA to prepare b-TeO2 nanoparticlessol [25]. A volume of 15 mL of a-TeO2 sol was mixed with 15 mL ofb-TeO2 sol to prepare (a + b)-TeO2 nanoparticles sol. In a typicalprocedure for the preparation of upconversion luminescenta-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles, the appropriateamounts of 0.02 M Er(NO3)3 and 0.02 M Yb(NO3)3 were added intothe a-TeO2 nanoparticles sol respectively. After stirring for 20 min,the as-obtained mixing solution was transferred into a reactionkettle and hydrothermal reacted at 180 �C for 18 h. As the reactionkettles cooled to room temperature naturally, the precipitate wasseparated by centrifugation, washed with deionized water, driedunder vacuum for 24 h and sintered at 600 �C for 2 h to obtainthe a-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles. A similar proce-dure was used for the preparation of b-TeO2:Er3+/Yb3+ nanoparti-cles and (a + b)-TeO2:Er3+/Yb3+ nanoparticles.

2.3. Characterization

Transmission electron microscopy (TEM; TECNAL-10, PHILIPS)technique was used to observe the morphology of the as-preparedTeO2:Er3+/Yb3+ nanoparticles. X-ray diffraction (XRD) measurementwas used to characterized the crystallization of the as-preparedTeO2:Er3+/Yb3+ nanoparticles. The source of radiation was Ka radi-ancy of copper at 36 kV and 20 mA in the range of 10–80� by stepscanning with a step size of 0.02�. (MSAL XD-2, Beijing University,China) Ultraviolet–visible–near infrared spectra (UV–Vis–NIR) andupconversion emission spectra were used to investigate the opticalproperty of the as-prepared TeO2:Er3+/Yb3+ nanoparticles. Theabsorption spectra were recorded in the region 200–1100 nm usingCray 5000 UV–Vis–NIR Spectrophotometer. Upconversion emissionspectra were measured under 980 nm excitation using HitachiF-4500 fluorescence spectrophotometer.

3. Results and discussion

3.1. Morphology and structure of the TeO2:Er3+/Yb3+ nanoparticles

As shown in Fig. 1, the a-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparti-cles were irregular flakes with the particle diameter ranging from40 to 400 nm (Fig. 1a), which were in agreement with those ofa-TeO2 nanoparticles [25]. The b-TeO2:Er3+/Yb3+ (3/10 mol%) nano-particles were ellipsoidal with the particle diameter ranging from30 to 200 nm (Fig. 1b), which were in agreement with those of

Fig. 1. TEM Images: (a) a-TeO2:Er3+/Yb3+ (3/10 mol%) nanop

b-TeO2 nanoparticles [25]. The results indicated that the TeO2

nanoparticles mainly kept the original morphology after havingbeen co-doped the Er3+/Yb3+ ions.

Fig. 2 showed the X-ray diffraction (XRD) patterns of theTeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles. For the a-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles (Fig. 2a), all the peaks could beindexed as the tetragonal phase a-TeO2 , which were in good agree-ment with the standard literature data (JCPDS file number 11-0693).For the b-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles (Fig. 2c), all thepeaks could be indexed as the orthorhombic phase b-TeO2 , whichwere in good agreement with the standard literature data (JCPDS filenumber 75-0882). The results indicated that the TeO2 nanoparticleskept the original phase after having been co-doped the Er3+/Yb3+

ions. The X-ray diffraction patterns of (a + b)-TeO2:Er3+/Yb3+

(3/10 mol%) nanoparticles (Fig. 2e) could be indexed as a mixtureof the tetragonal (JCPDS file number 11-0693) and orthorhombic(JCPDS file number 75-0882) phases of TeO2.

3.2. Absorption and upconversion luminescence properties of the TeO2

:Er3+/Yb3+ nanoparticles

Fig. 3 showed the absorption spectra of the a-TeO2:Er3+/Yb3+

nanoparticles. There was no absorption band between 350 and1100 nm in the a-TeO2 nanoparticles (Fig. 3d). For the a-TeO2:Er3+

nanoparticles (Fig. 3c), it showed obvious absorption bands around645, 545, 525, 488 and 381 nm, which were attributed to the4I15/2 ?

4F9/2, 4I15/2 ?4S3/2, 4I15/2 ?

2H11/2, 4I15/2 ?4F7/2 and

4I15/2 ?3P9/2 transitions of Er3+ ions, respectively [26]. For the

a-TeO2:Yb3+ nanoparticles (Fig. 3b), it showed an obvious absorp-tion band around 980 nm, which was attributed to the 2F7/2 ? 2F5/2

transition of Yb3+ ions [26]. For the a-TeO2:Er3+/Yb3+ nanoparticles(Fig. 3a), it showed obvious absorption bands around 645, 545,525, 488 and 381 nm of Er3+ ions, adding to the absorption bandaround 980 nm of Yb3+ ions. The absorption spectra of b-TeO2:Er3+/Yb3+ nanoparticles were similar to those of a-TeO2:Er3+/Yb3+ nano-particles. These showed that all the absorption bands were attrib-uted to the transitions of Er3+/Yb3+ ions. Also, as shown in Fig. 3,there was a large absorption cross-section around 980 nm of Yb3+

ions. It was well known that the Yb3+ ions could absorb the light at980 nm and sensitized Er3+ ions efficiently [27]. So the upconversionemission spectra were measured under 980 nm excitation.

Fig. 4 showed the upconversion emission spectra of theTeO2:Er3+/Yb3+ nanoparticles. For Er3+/Yb3+ ions co-doped in differ-ent crystal structure of TeO2 nanoparticles, all the as-preparedsamples showed green emissions (525 nm, 545 nm) and red

articles; (b) b-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles.

15 20 25 30 35 40 45 50 55 60 65 70

Inte

nsity

(a.u

.)

2θ (degree)

(d)

(e)

(c)

(a)

(b)

α−TeO2

β−TeO2

Fig. 2. XRD patterns: (a) a-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles; (b) a-TeO2 nanoparticles; (c) b-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles; (d) b-TeO2 nanoparticles; (e)(a + b)-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles.

300 400 500 600 700 800 900 1000

(d)

Wavelength (nm)

(c)

(b)

Abs

orba

nce

381 nm488 nm

525 nm 654 nm 980 nm(a)

Fig. 3. Absorption spectra: (a) a-TeO2:Er3+/Yb3+ nanoparticles; (b) a-TeO2:Yb3+

nanoparticles; (c) a-TeO2:Er3+ nanoparticles; (d) a-TeO2 nanoparticles.

500 550 600 650 7000

2500

5000

7500

10000

12500

15000

4F9/2 → 4I15/2

4S3/2 → 4I15/2

2H11/2 → 4I15/2

a b c

Upc

onve

rsio

n In

tens

ity (a

. u.)

Wavelength (nm)

Fig. 4. Upconversion emission spectra of TeO2:Er3+/Yb3+ nanoparticles under980 nm excitation: (a) a-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles; (b) b-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles; (c) (a + b)-TeO2:Er3+/Yb3+ (3/10 mol%)nanoparticles.

276 H. Hu et al. / Optical Materials 34 (2011) 274–277

emission (667 nm) under 980 nm excitation. The green emissionsat 525 nm, 545 nm and red emission at 667 nm were attributed tothe 2H11/2 ?

4I15/2, 4S3/2 ?4I15/2 and 4F9/2 ?

4I15/2 transitions of theEr3+ ions, respectively. The emission intensities of the a-TeO2:Er3+/

Yb3+ (3/10 mol%) nanoparticles (Fig. 4a) and b-TeO2:Er3+/Yb3+

(3/10 mol%) nanoparticles (Fig. 4b) were much stronger than thoseof the (a + b)-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles (Fig. 4c).

3.3. Dependence of the upconversion emission intensities on excitationintensities

In order to understand the upconversion mechanisms of theobserved emission bands, the upconversion emission intensitiesIup of these transitions were measured as a function of pumppower P. Fig. 5 showed the dependence of the upconversionemission intensities on excitation intensities from as-preparedsamples. The calculated results showed that slopes value froma-TeO2:Er3+/Yb3+ (3/10 mol%) were 2.8, 2.4 and 2.3 for green(2H11/2 ?

4I15/2), green (4S3/2 ?4I15/2) and red (4F9/2 ?

4I15/2) emis-sions, respectively. The results indicated that the upconversionmechanism corresponding to green (2H11/2 ?

4I15/2) emissionoccurred via a three-photon, while those corresponding to green(4S3/2 ?

4I15/2) and red (4F9/2 ? 4I15/2) emissions occurred via atwo-photon. The slopes value from b-TeO2:Er3+/Yb3+ (3/10 mol%)were 2.0, 1.8 and 1.7 for green (2H11/2 ?

4I15/2), green(4S3/2 ?

4I15/2) and red (4F9/2 ?4I15/2) emissions, respectively,

which indicated that the upconversion mechanism correspondingto green (2H11/2 ?

4I15/2), green (4S3/2 ?4I15/2) and red (4F9/2 ?

4I15/2) emissions occurred via a two-photon. (Fig. 5b)

3.4. Possible upconversion processes of TeO2:Er3+/Yb3+ nanoparticles

The upconversion mechanisms of TeO2:Er3+/Yb3+ nanoparticleshad been investigated. Fig. 6 was the schematic energy level dia-gram of Er3+ and Yb3+ ions. As indicated in Fig. 6, the Yb3+ ions wereexcited to the 2F5/2 state from the ground state 2F7/2, then trans-ferred the energy to Er3+ ions, while the Er3+ ions also absorbed980 nm photons. For the green (2H11/2 ?

4I15/2, 4S3/2 ? 4I15/2) andred (4F9/2 ?

4I15/2) emissions of Er3+ ions, an initial energy fromYb3+ ions (2F5/2) excited Er3+ ions (4I15/2) to the 4I11/2 states. Subse-quent nonradiative relaxations of Er3+ (4I11/2) ? Er3+ (4I13/2)occurred. Then, a second 980 nm photon from the excited Yb3+ ionsexcited Er3+ ions (4I13/2, 4I11/2) to Er3+ ions (4F9/2, 4F7/2). A third980 nm photon from the excited Yb3+ ions excited Er3+ ions(4F9/2) to Er3+ ions (2H9/2). The Er3+ ions in the 4F7/2 and 2H9/2 statesrelaxed nonradiatively by a fast multiphoton decay process to the

5.05.56.06.57.07.58.08.59.09.5

10.0 545 nm slop:2.4 667 nm slop:2.3 525 nm slop:2.8

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2

4.55.05.56.06.57.07.58.08.5 525 nm slop:2.0

545 nm slop:1.8 667 nm slop:1.7

LnI up

LnI up

Lnp (mW)Lnp (mW)

(a) (b)

Fig. 5. Dependence of the upconversion emission intensities on excitation intensity: (a) a-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles; (b) b-TeO2:Er3+/Yb3+ (3/10 mol%)nanoparticles.

Fig. 6. Energy level diagram of Er3+, Yb3+ ions and the proposed UC emissionmechanism.

H. Hu et al. / Optical Materials 34 (2011) 274–277 277

2H11/2 and 4S3/2 states, and the dominant green (2H11/2 ?4I15/2,

4S3/2 ? 4I15/2) emissions occurred. Alternatively, the Er3+ ions inthe 4F9/2 states mainly relaxed radiatively to the ground states4I15/2, which caused the red (4F9/2 ?

4I15/2) emission.

4. Conclusion

Different crystal structure of TeO2 nanoparticles were used asthe host materials to prepare the Er3+/Yb3+ ions co-doped upcon-version luminescent materials. The TeO2 nanoparticles mainly keptthe original morphology and phase after having been co-doped theEr3+/Yb3+ ions. All the as-prepared TeO2:Er3+/Yb3+ nanoparticlesshowed the green emissions at 525 nm and 545 nm and red emis-sion at 667 nm under 980 nm excitation, which were attributed tothe 2H11/2 ?

4I15/2, 4S3/2 ? 4I15/2 and 4F9/2 ?4I15/2 transitions of the

Er3+ ions, respectively. The emission intensities of the a-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles and b-TeO2:Er3+/Yb3+ (3/10 mol%)nanoparticles were much stronger than those of the(a + b)-TeO2:Er3+/Yb3+ (3/10 mol%) nanoparticles.

Acknowledgements

This work was supported by Natural Science Foundation ofChina (21075053), the Natural Science Foundation of Guangdong

Province (9251401501000001). We thank Professor Jianxin Mengat Chemistry Department of Jinan University for his assistance inupconversion luminescence spectra detection.

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