J. Cent. South Univ. (2017) 24: 2209-2214

DOI: https://doi.org/10.1007/s11771-017-3629-z

Fuel combustion synthesis and upconversion properties of Yb³⁺ and Er³⁺ dual-doped ZrO₂ nanocrystals

LIU Xiao-lin(刘小林)¹, ZHANG Ning(张宁)², LI Dan(李丹)¹, LI Zhi-cheng(李志成)²,

YOU Wei-xiong(游维雄)¹, ZHANG Qian(张骞)¹, XIA Li-bin(夏李斌)¹, YANG Bin(杨斌)¹

1. School of Materials Science and Engineering, Jiangxi University of Science and Technology,Ganzhou 341000, China;

2. School of Materials Science and Engineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany 2017

Abstract: Ytterbia and erbia dual-doped zirconia (ZrO₂: Yb³⁺, Er³⁺) nanophosphors were successfully synthesized by high-temperature fuel combustion at 1000 °C for 2 h. The effects of dopant concentration on the structure and upconversion properties were investigated by X-ray diffraction, transmission electron microscopy and photoluminescence, respectively. XRD patterns indicate that the main phase of products belongs to cubic ZrO₂ fluorite-type structure. TEM results show that different fuels have great influence on the morphologies of dual-doped ZrO₂ samples. Under 980 nm excitation, the glycine-calcined nanophosphors show high stimulated luminescence and doped-ion concentration-depended intensities. The intensely red upconversion emissions are attributed to the fact that the dual-doped Yb³⁺ and Er³⁺ ions result in the non-radiative relaxation, energy migration, and cross relaxation.

Key words: nanostructure; luminescence; upconversion; zirconia

1 Introduction

In recent years, rare-earth (RE) compounds and RE ions doped compounds have shown remarkable superiority in field of modern solid state lighting, such as light emitting diodes (LED), fluorescent lamps, field emission displays, and plasma display panels [1–4]. Especially, the use of up-converting RE ions doped nanophosphors has been carried out and sparked considerable interest. For example, intensive attention has been given to Er³⁺ ion doped nanophosphors emitting in the visible range by the upconversion process, because its metastable levels ⁴I_11/2 can be conveniently populated by near infrared (NIR) excitation (wavelength of 980 nm) [5–10]. In addition, green and red upconversion emissions can be simultaneously realized in Er³⁺ doped materials due to multi energy level structure of Er³⁺ ion. In order to improve the pumping efficiency of upconversion in lighting system, another ion is usually introduced into the host materials as a sensitizer. In Er³⁺ doped materials, the Yb³⁺ ion is an ideal candidate for sensitizer because it has a broad absorption band around the pump wavelength around 980 nm and a much higher absorption cross-section than the ⁴I_11/2 excited state of Er³⁺ [11, 12]. In the Yb³⁺ and Er³⁺ co-doped systems, the pumping energy can be absorbed by Yb³⁺ efficiently and transferred to the Er³⁺ ion to enhance the luminous intensity. Furthermore, the Yb³⁺ ion has only two energy levels, which excludes its other excited state absorption or upconversion losses in principle [13, 14]. Red and green emission on Er³⁺ doped and Yb³⁺—Er³⁺ dual-doped oxide nanocrystals has been widely reported, such as on gadolinium gallium garnet (GGG), yttrium aluminum garnet (YAG), Lu₂O₃, ZnO and Y₂O₃ [15–18], and some works informed strong emission on ZrO₂ [19–22]. However, the developed nanocrystalline upconverting phosphors still suffer from the chemical stability or need rigorous synthesis conditions. Therefore, it is very necessary to develop the low-cost and environmental-friendly oxide nanophosphors to meet with the requirements for upconversion photoluminescence.

Ytterbia and erbia dual-doped ZrO₂-matrix phosphor is an excellent candidate for use in upconversion luminescence due to its low phonon energy and stable chemical properties. The stretching frequency of the Zr—O bond is about 470 cm^–1, which is much lower than that of Al—O (870 cm^–1) or Si—O (1100 cm^–1) but higher than that of Y—O (300–380 cm^–1) [20, 21]. The low phonon energy of such nanocrystal opens up the possibility of higher efficient luminescence of active ions incorporate into the ZrO₂ host. Normally, ZrO₂ has three kinds of stable structural isomers: the monoclinic phase below 1100 °C, the intermediate tetragonal phase between 1100 and 2370 °C, and the cubic phase above 2370 °C. However, the Yb³⁺ and Er³⁺ doped ZrO₂ phosphor crystallizes in a rather tetragonal or cubic system than a monoclinic one. Oxygen vacancies, the cluster of earth ion or other impurities in ZrO₂: Yb³⁺, Er³⁺ systems are easily generated due to charge difference and doped-ion content change, which may deteriorate the luminescence intensity due to non-radiation transition. Therefore, it is desirable to study the role of Yb³⁺ and Er³⁺ concentration on the tunability of green-red upconversion emission.

Herein, we report the preparation of ZrO₂: Yb³⁺, Er³⁺ nanoparticle phosphors with different Yb³⁺ and Er³⁺ concentration by combustion synthesis using different fuels. The upconversion luminescence properties of ZrO₂: Yb³⁺, Er³⁺ phosphors were studied in detail by changing doping concentration of Yb³⁺ and Er³⁺ ions in the host. It is found that the ZrO₂: Yb³⁺, Er³⁺ nanocrystals with required controllable upconverting emission can be obtained at 1000 ^oC for a short sintering period of 2 h.

2 Experimental

A typical experimental procedure for the ZrO₂: 4% Er, 4% Yb sample synthesis as an example is described as follows: 2.459 g ZrO(NO₃)₂·2H₂O, 0.185 g Er(NO₃)₃·6H₂O and 0.180 g Yb(NO₃)₃·5H₂O were successively dissolved in 120 mL deionized water under magnetic stirring at 80 °C, and the solution was continuously stirred until a clear solution was obtained. The co-precipitation reaction immediately started when 1 mol/L ammonia solution was slowly added to the above mixed solution. Meanwhile, the pH value of the mixed solution was adjusted to 10 (pH=10) with slow dropwise of ammonia solution under magnetic stirring at 80 °C. The resultant solution was kept under stirring for about 30 min at 80 °C, and a white precursor product formed. After cooling to room temperature, the white precursor was transferred into 200 mL Teflon-lined autoclave which was sealed and aged at 100 °C for 12 h, after which, the precursor product was separated from the solution by centrifugation and washed with absolute ethanol once and dried at 60 °C. Then, the dried precursor was fully grounded with 1 g of one fuel from glycine, semicarbazlde hydrochloride (SH) or urea, and followed by calcination at 1000 °C for 2 h in air atmosphere to yield the Yb³⁺ and Er³⁺ dual-doped ZrO₂ powders. Other samples with different Yb³⁺ and Er³⁺ concentration were also synthesized by the same procedure with the corresponding starting materials.

X-ray powder diffraction (XRD) patterns of the as-prepared samples were recorded using a Bruker D8-Focus X-ray diffractometer with high-intensity Cu K_α radiation (λ=1.54178). Transmission electron microscopy (TEM) micrographs were taken with a Tecnai F20 field-emission transmission electron microscope. Upconversion luminescence and the fluorescence decays at the wavelength of 679 nm were recorded on an Edinburgh instruments FL920 spectrophotometer equipped with laser diode (LD) as the pump source and the excitation wavelength was 980 nm. The signal was detected with a NIR photomultiplier tube (PMT) (R5509, Hamamatsu) and all the spectra have been corrected according to the response curve of the instrument due to the different responses of the instrument at different wavelengths.

3 Results and discussion

Figure 1 shows the XRD patterns of ZrO₂: 4% Er, 4% Yb samples under calcination with different fuels (glycine, SH or urea). In the case of 4% Er and 4% Yb dual-doped ZrO₂, the XRD patterns only exhibit characteristic diffraction peaks of single cubic phase (JCPDS No. 49–1642). No diffraction peaks from impurities were observed in those XRD patterns, indicating that the main-phase cubic ZrO₂ can be obtained by the fuel combustion method at 1000 °C for 2 h using Zr(OH)₄ as precursor in the presence of different fuels. For different doped ion concentration, the XRD patterns for characteristic samples are compared in Figs. 2 and 3. It can be seen that a mixture of tetragonal and cubic phase is observed when the Er³⁺ concentration is lower than 3%. Notice that dominant peaks in both tetragonal and cubic are coincident and located at 30.12°. Tetragonal phase can be identified by the characteristic double peak (2,0,0)_t and (0,1,1)_t at 34.92° and 35.40°, and single peak (2,0,0)_c at 35.13° for cubic crystalline structure [19]. From XRD patterns of Fig. 2, these results suggest that Er³⁺ ion concentration has important influence on the stability of crystalline phase. Comparatively, different Yb³⁺ ion and constant Er³⁺ concentration (4%) were introduced into host to investigate the phase evolution of ZrO₂. Figure 3 shows that the increment of Yb³⁺ ion concentration and keeping Er³⁺ concentration consistent (4%) has no significant influence on the crystalline phase composition and peak intensities. All these results also confirm that the dual-addition of more than 3% Er and 1% Yb (molar fraction) to ZrO₂ can stabilize the cubic phase.

Fig. 1 XRD patterns of ZrO₂: 4% Yb³⁺, 4% Er³⁺ prepared by combustion synthesis:

Fig. 2 XRD patterns of dual-doped ZrO₂ for 4% Yb³⁺ and different concentrations of Er³⁺

Fig. 3 XRD patterns of dual-doped ZrO₂ for 4% Er³⁺ and different concentration of Yb³⁺

Figure 4 shows the TEM images of ZrO₂: 4% Yb³⁺, 4% Er³⁺ prepared by the combustion method at 1000 ^oC for 2 h in the absence or presence of urea, SH and glycine, respectively. One can see that different morphologies of dual-doped ZrO₂ were obtained under high-temperature calcination with different fuels (from Figs. 4(a)–(d)), indicating that the types of fuel have great influence on the morphologies of dual-doped ZrO₂ samples. In addition, uniform nanoparticles with size of 50 nm are the primary product of the combustion synthesis at 1000 °C when the chosen fuel is glycine (Fig. 4(d)). However, dual-doped ZrO₂ samples synthesized by decomposition reaction with the help of other fuels (urea, SH) have large particle size or agglomerate together seriously (>1 μm), which is unfavorable for improving luminescent efficiency [23].

The excitation process for the upconversion emission of Er³⁺ and Yb³⁺ ions under excitation wavelength of 980 nm can be illustrated in Fig. 5. The Yb³⁺ ion can be excited to the sole exited F_5/2 level by absorbing the first photon of NIR and then transferred the energy to Er³⁺ ion. The Er³⁺ ion is first promoted to the ⁴I_11/2 level, and further to ⁴F_7/2 due to absorption and energy transfer of another NIR photon. Direct excitation is also available, but the energy transfer process is dominant because of the long radiative life time of the excited ²F_5/2 level (typically 1 ms) and resonance between ²F_5/2→²F_7/2 transition of Yb³⁺ and ⁴I_15/2→⁴I_11/2 transition of Er³⁺. After being excited to ⁴F_7/2, Er³⁺ decays rapidly and non-radiatively to the ²H_11/2, ⁴S_3/2, ⁴F_9/2 or ⁴I_13/2 levels. As a result, the red emission located at 630–710 nm region is surely assigned to ⁴F_9/2→⁴I_15/2 transition of Er³⁺ ion. And the green emission located at 510–570 nm region corresponds to ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions of Er³⁺ ion. Figure 6 shows the upconversion photoluminescence spectra of the 4% Yb³⁺ and 4% Er³⁺ dual-doped ZrO₂ samples sintered by different fuels under excitation wavelength 980 nm. One can see that the 4% Yb³⁺, 4% Er³⁺ dual-doped ZrO₂ exhibits very high intensity of the red luminescence and low intensity of the green luminescence (I_red/I_green>10), perceptible to the naked eyes as a red emission. This quenching of the green luminescence may be due to the cross-relaxation process between Yb³⁺ ion and Er³⁺ ion, ²H_11/2/⁴S_3/2(Er)+²F_7/2(Yb)→⁴I_13/2(Er)+²F_5/2(Yb) [12]. In Fig. 6, the highest luminescence intensity is obtained from the 4% Yb³⁺, 4% Er³⁺ dual-doped ZrO₂ synthesized with glycine used as fuels. This result may be attributed to the small particle size of ZrO₂ nanopowders (about 50 nm) by using glycine, whereas the ZrO₂ materials prepared with other fuels have large particle size or agglomerate heavily.

The emission spectra as a function of doped-ion concentration are shown in Figs. 7 and 8. As observed in both figures, the upconversion emission is obviously influenced by the concentration of both Yb³⁺ and Er³⁺ ions. In Fig. 7, the increment of Er³⁺ with constant 4% Yb³⁺ firstly enhances the red band but always quenches the green band. This behavior may be due to the increase in the non-radiative relaxation of ⁴I_11/2→⁴I_13/2 with the Er³⁺ concentration increment. The non-radiative relaxation Er³⁺ (⁴I_11/2→⁴I_13/2) increases the population at the ⁴I_13/2 lever, which on time allows an increasing population at the ⁴F_9/2 level via the direct Yb³⁺ to Er³⁺ direct energy transfer process Yb³⁺ (²F_5/2, ²F_7/2)→Er³⁺(⁴I_13/2, ⁴F_9/2). Then the emission plots are followed by quenching the red band when the concentration of Er³⁺ ion is more than 3%. This quenching result may be the association with energy migration, which is caused by the high concentration of doped rare-earth ions. In addition, the increasing Yb³⁺ has relatively small influence on both bands when the Yb³⁺ ion increases to a critical concentration of 2% (shown in Fig. 8). Such phenomena shows that 2% Yb³⁺ in ZrO₂ host can provide enough excitation photon for the energy transfer from Yb³⁺ (donor) to Er³⁺ (acceptor).

Fig. 4 TEM images of ZrO₂: 4% Yb³⁺, 4% Er³⁺ prepared by combustion synthesis:

Fig. 5 Schematic for process of Yb³⁺ sensitized Er³⁺ upconversion luminescence

Fig. 6 Upconversion emission spectra of ZrO₂: 4% Yb³⁺, 4% Er³⁺ for different fuels

Fig. 7 Upconversion emission spectra of dual-doped ZrO₂ prepared by glycine combustion for 4% Yb³⁺ and different concentrations of Er³⁺

Fig. 8 Upconversion emission spectra of dual-doped ZrO₂ prepared by glycine combustion for 4% Er³⁺ and different concentrations of Yb³⁺

To investigate the influence of ions concentration in the dynamics of upconversion process, Fig. 9 shows the decay curves of Yb³⁺ and Er³⁺ dual-doped ZrO₂ for red (679 nm) emission. Notice that the fitting of the experiment date is a double exponential function and composed of the first linear and followed slow decay components, which are probably attributed to the cross relaxation described above, non-radiative processes, promotion to higher level, or the multisite nature of the ZrO₂ lattice. The decay curves also show that there is significant difference in the fluorescence decay rate when the different Yb³⁺ or Er³⁺ ion content is introduced into ZrO₂ nanoparticle host. The effective fluorescence lifetime is listed in Table 1 and calculated according to the equation . It can be seen that the decay time decreases with the increase of total doped ion concentration. Such diminish behavior is related to the presence of fluorescence quenching produced by the cluster formation of Er³⁺ ion due to the increment of concentration which is also responsible of the increase of the upconversion signal intensity.

Fig. 9 Typical decay time curves of dual-doped ZrO₂:

Table 1 Lifetime of ⁴F_9/2 level in different Yb³⁺ and Er³⁺ concentration doped samples

4 Conclusions

Yb³⁺ and Er³⁺ co-doped ZrO₂ phosphors have been successfully synthesized by the combustion synthesis at 1000 °C using ZrO(NO₃)₂·2H₂O, Er(NO₃)₃·6H₂O and Yb(NO₃)₃·5H₂O as starting raw materials. And the nanosized Yb³⁺ and Er³⁺ co-doped ZrO₂ particles can be easily and successfully manipulated using different fuels. Luminescence results show that the as-formed Yb³⁺ and Er³⁺ dual-doped ZrO₂ nanoparticles exhibit highly and controllable luminescent intensity by adjusting the appropriate concentration of doped Yb³⁺ and Er³⁺ ions. Cross-relaxation processes may be the main reason of weak green and strong red luminescence at the same time. Moreover, the luminescence decay characteristics of lifetime decrease with the increment of total doped ion concentration. Those results show that in Yb³⁺ and Er³⁺ dual-coped ZrO₂ system, doping ion concentration plays an important role in the high-intensity red upconversion emissions.

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(Edited by FANG Jing-hua)

Cite this article as: LIU Xiao-lin, ZHANG Ning, LI Dan, LI Zhi-cheng, YOU Wei-xiong, ZHANG Qian, XIA Li-bin, YANG Bin. Fuel combustion synthesis and upconversion properties of Yb³⁺ and Er³⁺ dual-doped ZrO₂ nanocrystals [J]. Journal of Central South University, 2017, 24(10): 2209–2214. DOI:https://doi.org/10.1007/s11771-017-3629-z.

Foundation item: Projects(51062004, 11464017) supported by the National Natural Science Foundation of China; Project(GJJ150671) supported by the Scientific Research Foundation for Universities from Education Bureau of Jiangxi Province, China; Project(20161BAB216123) supported by the Natural Science Foundation of Jiangxi Province, China; Projects(NSFJ2014-G13, Jxxjbs12005) supported by the Jiangxi University of Science and Technology, China

Received date: 2016-04-12; Accepted date: 2016-12-01

Corresponding author: LIU Xiao-lin, Lecturer, PhD; Tel: +86–797–8312078; E-mail: xlliu@jxust.edu.cn