Materials Letters 63 (2009) 1023–1026

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Materials Letters

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Microwave hydrothermal synthesis and upconversion properties of NaYF4:Yb3+, Tm3+

with microtube morphology

Xun Chen a, Wanjun Wang a, Xueyuan Chen b, Jinhong Bi a, Ling Wu a,b,⁎, Zhaohui Li a, Xianzhi Fu a

a State Key Laboratory breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, PR Chinab State Key Laboratory of Structural Chemistry ,Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, PR China

⁎ Corresponding author. State Key Laboratory breResearch Institute of Photocatalysis, Fuzhou UniversitTel./fax: +86 591 83738608.

E-mail address: wuling@fzu.edu.cn (L. Wu).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.01.075

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 October 2008Accepted 27 January 2009Available online 5 February 2009

Keywords:LuminescenceOptical materials and propertiesMicrowaveUpconversion

Efficient upconversion Yb3+ and Tm3+ codoped β-NaYF4 is firstly synthesized via a novel and rapidmicrowave hydrothermal process. The as-prepared sample is characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The sample shows amicrotube morphology, which may be formed by the curliness of flakes. It is found that NaYF4:Yb

3+, Tm3+

microtubes can be synthesized using microwave hydrothermal method in a much shorter reaction timecompared with conventional hydrothermal method, and the upconversion fluorescent intensity is alsogreatly enhanced under 976 nm laser excitation. The energy transfer upconversion mechanisms and thepossible reason for the enhancement of the fluorescent intensity are also proposed.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, considerable attention has been paid to the studyof rare earth ions doped upconversion phosphors (UCPS) that emithigher energy photons via absorbance of lower energy excitingphotons [1]. They have potential applications such as color displays[2], solid-state laser and solar cells [3]. Among all these upconversionphosphors, NaYF4 has been proven to be an efficient host material forits low phonon energy and excellent chemical stability. At ambienttemperature and pressure, the NaYF4 exists in two polymorphs: α-NaYF4 and β-NaYF4, depending on the synthesis method. The β-NaYF4has been reported as the most efficient host material for green andblue UCPs [4]. There have been many methods to synthesize NaYF4nano- and microcrystals. Especially, hydrothermal treatment as atypical solution approach has been proven to be effective andconvenient in preparing various inorganic materials with diversecontrollable morphologies and architectures [5]. However, to synthe-size β-NaYF4, it usually needs rigorous synthesis conditions, compli-cated apparatus and long reaction time [6,7].

Microwave heating has been found to increase the kinetics ofcrystallization during the hydrothermal synthesis [8]. The mainadvantage of the introduction of microwaves into a reaction systemis the extremely rapid kinetics for synthesis. Dramatic increase in thereaction kinetics, up to two orders of magnitude, can be achieved by

eding Base of Photocatalysis,y, Fuzhou 350002, PR China.

ll rights reserved.

the microwave heating under hydrothermal conditions, due to thelocalized super heating of the solution [9]. Microwave hydrothermalis generally faster, eco-friendly and very energy efficient. In thispaper, we firstly report the microwave hydrothermal (MH) synthesisof β-NaYF4 and the upconversion property of β-NaYF4 co-doped withYb3+, Tm3+. It was found that β-NaYF4 can be synthesized success-fully in a much shorter time using microwave hydrothermal. Itsupconversion performances and transition mechanism were alsodiscussed.

Fig. 1. X-ray diffraction patterns of the NaYF4:Yb3+, Tm3+: (a) MH 180 °C for 4 h; (b) CH180 °C for 18 h; (c) CH 180 °C for 4 h.

1024 X. Chen et al. / Materials Letters 63 (2009) 1023–1026

2. Experimental

The starting materials were NaF, NH4HF2, Y(NO3)3·4H2O, Yb(NO3)3·4H2O, Tm(NO3)3·4H2O, which were all of analytical gradeand used as received without further purification. The dopingconcentration of Yb3+ and Tm3+ was fixed to 1% and 0.2%,respectively. The reactant mixture with a molar ratio of 3.0NaF:1.0Ln(NO3)3:6.0 NH4HF2 (Ln3+=Y3+, Yb3+, Tm3+) were put into the100 mL Teflon lined digestion vessel. Then 70mL deionized water wasadded. Under stirring, dilute HF solution was dripped to adjust the pHto 3–4. The reaction mixture was sealed in the vessel by the vesselcover acting as an overpressure release valve. Then the vessel wassurrounded by a safety shield and heated by a microwave synthesizer(ETHOS TC from Milestone Inc.) at 180 °C for 4 h. White suspensionswere formed after the microwave treatment. The products werewashedwith deionized water for several times and finally dried undervacuum at 80 °C. As a comparison, conventional hydrothermal (CH)

Fig. 2. Typical SEM and TEM image of the N

process was also used to synthesize NaYF4:Yb3+, Tm3+. In thisprocess, the reaction time was fixed to 4 h and 18 h, respectively.

The as-prepared samples were characterized by powder X-raydiffraction on a Bruker D8 Advance X-ray diffractometer at 40 kV and40 mA with Ni filtered Cu Kα radiation. Field scanning electronmicroscopy (SEM) was conducted on Hitachi S-3000N. The transmis-sion electron microscopy (TEM) and the high resolution transmissionelectron microscopy (HRTEM) images were taken by JEOL model JEM2010 EX instrument at the accelerating voltage of 200 kV. Theupconversion emission spectra at room temperature were detectedusing the Hamamatsu R943-02 photomultiplier tube and the Spex1000 M monochromator under 976 nm Ti: sapphire laser excitation.

3. Results and discussion

The XRD pattern of the MH sample, as shown in Fig. 1a, can bereadily indexed to a pure phase of β-NaYF4 (JCPDS-160334). However,

aYF4:Yb3+, Tm3+ synthesized by MH.

Fig. 4. Schematic diagram illustrating the mechanisms of NaYF4:Yb3+, Tm3+ upconversion.

1025X. Chen et al. / Materials Letters 63 (2009) 1023–1026

when using CH under the same temperature and time, there is anextra reflection peak at about 28.2° which could be identified as the(111) crystal plane of α-NaYF4 (JCPDS-060342) (Fig. 1c). In addition,only when the hydrothermal time is extended to 18 h could the purephase of β-NaYF4 be obtained (Fig. 1b).The reflection peaks in Fig. 1aare much shaper than that in Fig. 1b. These results indicate that β-NaYF4 can be synthesized in a much shorter reaction time and with amuch higher crystallinity when using microwave heating.

A typical SEM micrograph in Fig. 2a shows that the as-preparedNaYF4:Yb3+, Tm3+ exhibits tube-like morphology with smooth wall.It is found that the tubes are hollow and some of them have a slit onthe wall. Therefore, in the formation process of these tubes, someflakes are supposed to form firstly. Then, in the following stage, thetubes come to form by the curliness of the flakes, which may leave aslit on the tubewall. Fig. 2b showsmore clearly the hollow structure ofthe tubes. TEM images of a single tube in Fig. 2c further confirm themicrotubes morphology with an edge length of 2–3 μm and diameterof about 0.5 μm. The HRTEM image in Fig. 2d shows clearlattice fringes. The fringes of d=0.297 nm match that of the (110)crystallographic plane of β-NaYF4. The inset of Fig. 2d is thecorresponding SAED pattern. The bright and order diffraction dotssuggest a high crystallinity and single-crystal nature of the resultantNaYF4:Yb3+, Tm3+ microtube with growth direction along [110]. Thecorresponding EDS spectrum of the sample in Fig. 2e shows that it ismainly composed of Na, Y, and F atoms, and no other impurity isdetected except small amounts of Yb and Tm, indicating the Yb andTm elements are successfully doped in the NaYF4 crystals. In thisspectrum, signals corresponding to Cu arise from the TEM grid.

Room temperature upconversion fluorescence spectra of the as-prepared NaYF4:Yb3+, Tm3+ in the wavelength region of 250–750 nmare shown in Fig. 3. Several distinct emission bands centered around289, 346, 362, 450, 475, 647, 690 and 723 nm are observed with thepump power of 60 mW. It can be seen that a much higher overallfluorescent intensity is achieved on the MH sample. The upconversionmechanisms are presented in Fig. 4. Under the 976 nm excitation,electron of Yb3+ is exited from 2F7/2 to 2F5/2 level. The energy can betransferred to Tm3+ ion nonradiatively to excite it up to thecorresponding excited level. Multiple step energy-transfers may beresponsible for the up-converted emissions. Three successiveenergy transfers from Yb3+ to Tm3+ to populate the 3H5, 3F3, and 1G4

levels of Tm3+ are as follows: (Yb3+:2F5/2+Tm3+:3H6→Yb3+:2F7/2+Tm3+:3H5); (Yb3+:2F5/2+Tm3+:3F4→Yb3+:2F7/2+Tm3+:3F3); (Yb3+:2F5/2+Tm3+:3H4→Yb3+:2F7/2+Tm3+:1G4). As a result, the red(1G4→

3H5, 1G4→3F4, and 3F3→3H6) and blue (1G4→

3H6) emissionsare realized. For the population of the 1D2 level, the cross relaxationprocess of (3F2+3H4→

3H6+1D2) between Tm3+ ion may be the most

Fig. 3. Upconversion emission spectrum of NaYF4:Yb3+, Tm3+ under 976 nm excitation.

important, because of the large energy mismatch (about 3500 cm−1) forthe energy transfer progress of (Yb3+:2F5/2+Tm3+:1G4→Yb3+:2F7/2+Tm3+:1D2) [10]. In this case, the blue (1D2→

3F4) and near ultraviolet(1D2→

3H6) emissions are obtained. Furthermore, the 3P2 level can beexcited through the energy transfer from Yb3+ to Tm3+ (Yb3+:2F5/2+Tm3+:1D2→Yb3+:2F7/2+Tm3+:3P2). Consequently, after nonradiativelydecaying to the 0P3 level, theultraviolet bands centered at 289 and346nmoccurs with the transition of 0P3→3H6 and 0P3→3F4, respectively.

Thepossible reason for the enhancementoffluorescent intensity is themicrowave heating work in a totally different waywhen compared to theconventional heating [11]. Depending on the penetration of the micro-waves in the synthesis medium, microwave energy can heat the entireobject to the crystallization temperature rapidly and uniformly. Thisresults in homogeneous nucleation with high crystallinity. Usually, CHmethod utilizes convective heating due to the need for high-temperatureinitiated nucleation followed by controlled precursor addition to thereaction. But the convective heating can cause sharp thermal gradientsthroughout the bulk solution and inefficient, nonuniform reactionconditions which can lead more surface defects and oxygen impuritiesthan that of the microwave heating process. As known, these surfacedefects and oxygen impurities can act as ion concentration quencherswhichwould result in the decrease of the fluorescent intensity. Therefore,microwave heating can make the crystal lattice more perfect and hencegreatly increase the upconversion efficiency. Further researches are underway to make a full understanding of this microwave effect.

4. Conclusions

Compared with the CH method, β-NaYF4 codoped with Yb3+ andTm3+ could be synthesized by a MH process in much shorter reactiontime. SEM and TEM images presented themorphologywithmicrotubewhichmay be formed by the curliness of flakes. In addition, the overallupconversion fluorescent intensity is greatly enhanced under the976 nm excitation. For the first time, microwave heating wasintroduced to synthesize NaYF4 upconversion phosphors, whichwould serve as another promising way to prepare phosphors withenhanced upconversion efficiency.

Acknowledgements

Theworkwas supported by National Natural Science Foundation ofChina (20571015, 20777011), National Key Basic Research Program ofChina (973 Program: 2007CB613306 and 2008CB617507), andScientific Project of Fujian Province (2005HZ1008), China.

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References

[1] Auzel F. Chem Rev 2004;104:139–74.[2] Lee MH, Oh SG, Yi SC, Seo DS, Hong JP, Kim CO, et al. J Electrochem Soc 2000;147:

3139–42.[3] Liu JF, Yao QH, Li YD. Appl Phys Lett 2006;88:173119.[4] Kramer KW, Biner D, Frei G, Gudel HU, Hehlen MP, Luthi SR. Chem Mater 2004;16:

1244–51.[5] Liang JH, Peng Q, Wang X, Zheng X, Wang RJ, Qiu XP, et al. Inorg Chem 2005;44:

9405–15.

[6] Liang LF, Wu H, Hua HL, Wu MM, Su Q. J Alloys Compd 2004;368:94–100.[7] Wang DW, Huang SH, You FT, Qi SQ, Fu YB, Zhang GB, et al. J Lumin 2007;122:

450–2.[8] Komarneni S, Katsuki H. Pure Appl Chem 2002;74:1537–43.[9] Sreeja V, Joy PA. Mater Res Bull 2007;42:1570–6.[10] De G, Qin W, Zhang J, Zhang J, Wang Y, Cao C, et al. J Lumin 2007;122:128–30.[11] Rao KJ, Vaidhyanathan B, Ganguli M, Ramakrishnan PA. Chem Mater 1999;11:

882–95.