Journal of Nanomaterials
Volume 2015 (2015), Article ID 573253, 7 pages
http://dx.doi.org/10.1155/2015/573253

Research Article

Upconversion Luminescence Properties of Y₂O₃:Yb, Er and Y₂O₂S:Yb, Er Nanoparticles Prepared by Complex Precipitation

Ying Tian,1,2 Yao Fu,¹ Mingming Xing,¹ and Xixian Luo¹

¹Optoelectronic Technology Institute, Dalian Maritime University, Dalian, Liaoning 116026, China
²NanoMaterials Group, Department of Applied Physics and Center for New Materials, Aalto University, 00076 Aalto, Finland

Received 8 December 2014; Revised 22 February 2015; Accepted 23 February 2015

Academic Editor: Gong-Ru Lin

Copyright © 2015 Ying Tian et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The Yb³⁺, Er³⁺ doped Y₂O₃ and Y₂O₂S upconversion nanophosphors were prepared by the direct complex precipitation method with the mixed solution of NH₄HCO₃ and NH₃·H₂O as the complex precipitant. The precipitate of calcined at 900°C in air presents the pure Y₂O₃ with cubic structure, and the calculated crystalline size is about 26 nm, while the Y₂O₂S:Yb, Er nanocrystals were obtained by annealing the same precipitate at 900°C but in the atmosphere of N₂ gas containing sulfur vapor. The obtained sample presents the pure hexagonal structure of Y₂O₂S with calculated crystalline size of 29 nm. According to the transmission electronic microscopy (TEM), the nanophosphors exhibit uniform quasispherical shape and size about 30 nm. By using the 980 nm excitation laser, the properties of upconversion luminescence and energy transfer processes were studied in detail for the different concentration of Yb³⁺ in Er³⁺ doped Y₂O₃ as well as the Yb³⁺, Er³⁺ codoped Y₂O₂S nanocrystals. The high-efficient red and yellow upconversion emissions were both observed by naked eyes in day time corresponding to the Y₂O₃:Yb, Er and Y₂O₂S:Yb, Er phosphors, respectively. Thus the upconversion nanoparticles combining its high efficient emission would pave the way for ideal fluorescence probes in biological applications.

1. Introduction

An upconversion process is one that takes multiple photons of lower energy and converts them to one photon of higher energy. There has been considerable research on the upconversion phosphor since it was found by Professor Auzel [1] for the first time and put forward being used in infrared detection and short-wave laser. More recently, people have found that if the upconversion phosphors can realize the nanocrystalline, combining its unique characteristics of luminescence, such as lack of background phosphorescence, no photobleaching during assay, and narrow emission bands [2, 3], it will have enormous potential applications in the areas of biomedical diagnosis [4], the anticounterfeiter [5], and display screen and X-ray imaging [6]. In particular, the Y₂O₃ and Y₂O₂S materials have attracted much interest as host materials for its excellent chemical stability, insoluble in water, high melting point, and low phonon energy [7–11]. Therefore, the high luminescence efficiency can be achieved in those systems via suitable selection of rare earth doping ions and excitation routes.

Many synthesis techniques of rare earth oxide and oxysulfide nanomaterials have been studied, such as precipitation method [12, 13], combustion synthesis [14], and emulsion liquid membrane system [15]. Among them, the precipitation method has become a promising chemical preparation rout owing to its advantages of simple process without complex equipment, easy doping, and low production cost. In particular, the homogeneous precipitation method has drawn a great attention in the fields of nanomaterial synthesis. Because the precipitants sedimentate out homogeneously and slowly in the solution, which accordingly conduce to the homogeneous formation of precipitates. And the obtained nanoparticles are uniform and fine in size [13, 16]. The urea (CO(NH₂)₂) as the precipitant has been most commonly used in the homogeneous precipitation method. However, the hydrolysis of urea is very slow process and need certain temperature. Moreover, the low concentration of urea is a necessity in order to obtain uniform nanoparticles; thus the big amount of reaction solution causes lots of difficulties in the posterior processes, such as filtration.

In the current work, the nanocrystalline upconversion phosphors were prepared by the direct complex precipitation method using the ammonium water and ammonium bicarbonate as the complex precipitants. This novel method makes use of the principal of homogeneous precipitation method but successfully overcomes the disadvantages of the urea homogeneous precipitation route. The Yb³⁺, Er³⁺ codoped Y₂O₃ and Y₂O₂S upconversion nanophosphors are obtained with homogeneous morphology and meanwhile exhibit high efficient upconversion luminescence. The luminescence properties and mechanisms are discussed in detail for different host materials and doping concentrations.

2. Materials and Methods

2.1. Experiment Principles

In the complex precipitant solution of NH₄HCO₃ and NH₃·H₂O, the equilibrium balances in the solution are shown as follows:

When the rare earth ions join in the reaction,

Here , , and are the equilibrium constants of the three relevant reactions, respectively. Since the ammonium bicarbonate is strong electrolyte, the first reaction is a complete reaction. While the ammonium water and both possess the properties of weak electrolytes, so that the quantity of the OH⁻ and ions is little in the solution. When the complex precipitant solution of NH₄HCO₃ and NH₃·H₂O was dropped into the mixed solution of Y³⁺, Yb³⁺, and Er³⁺ ions, in the small area around the complex precipitants, the precipitants ionize out OH⁻ and ions slowly according to the reversible reactions listed above. Then there is always little change of the OH⁻ and concentrations within the small area, and the reversible reaction goes towards the positive direction with the formation of the precipitates. Namely, the concentration of the OH⁻ and within the small area is correlative with the formation of precipitates. Therefore, the precipitation process is a local homogeneous process. When introducing the rare earth ions, the precipitates of carbonate (, where the Re represents Y, Yb, and Er) were formed by reacting with the OH⁻, ions simultaneously.

2.2. Materials Preparation

The certain amount of NH₄HCO₃ powders was dissolved into the NH₃·H₂O solution and then stirred vigorously to obtain the complex precipitant solution of 70 mL, and concentrations of the NH₄HCO₃ and NH₃·H₂O were 3 and 1 mol/L, respectively. The nitrate solutions of rare earth (Y₂O₃, Yb₂O₃, and Er₂O₃ powers (99.99%)) were weighed on the stoichiometry and mixed thoroughly, where the doped concentrations of the activator Er³⁺ were 0.8 mol%, and concentrations of the sensitizer Yb³⁺ were changed from 0 to 11 mol%. Then the complex precipitant solution was slowly dropped into the nitrate solution of rare earth that was vigorously stirred. The mixed solution was continuously stirred for 30 mins and then stayed for 2 hs. The resultant white precipitates of were centrifuged and washed by water, ethanol, and acetone, respectively. The precursor was finally dried at 60°C.

Y₂O₃:Yb, Er nanophosphors were obtained by annealing the precursor in air at 900°C for 1 h. While Y₂O₂S:Yb, Er nanophosphor was obtained by calcining the same sample at 900°C for 1 h in the atmosphere of N₂ gas containing sulphur vapour, the sulphur vapour was generated by heating a sulfur powder at 400°C and then flowed into the tube by the N₂ gas.

2.3. Characterizations

X-ray diffraction (XRD) patterns of the samples were recorded on Hitachi DMAX-3A diffractometer equipped with ( nm) radiation. The morphology of the sample was characterized by TEM (Tecnai G² 20). The Hitachi F-4500 fluorescence spectrophotometer was utilized to measure the upconversion luminescence spectra. The excitation source was 980 nm laser diode (LD). The power of the LD was measured by the laser power meter (LPE-1 type).

3. Results and Discussions

3.1. Structure and Morphology

Figure 1 shows the XRD patterns of the samples that were calcined at 900°C in air and in N₂ gas containing sulfur vapor, respectively. The doping concentrations of Yb³⁺ and Er³⁺ ions of the two samples were both 8 and 0.8%. The results show that the sample calcined in air presents the pure Y₂O₃ phases with cubic structure, while the sample calcined in N₂ gas containing sulfur vapor exhibits the hexagonal structure of Y₂O₂S. Both of the patterns present pure Y₂O₃ and Y₂O₂S crystal structures without other phase. This result implies that the ions of Yb³⁺ and Er³⁺ have no influence on the crystal structure. And the as-prepared powder obtained by the complex precipitation can be sulfurized thoroughly using the simple method of flowing N₂ gas containing the sulfur vapor. According to the Scherrer formula [17], , the calculated crystallite size of theY₂O₃:Yb, Er and Y₂O₂S:Yb, Er particles was 26 and 29 nm, respectively.

Figure 1: XRD patterns of (a) Y₂O₃:Yb, Er and (b) Y₂O₂S:Yb, Er nanocrystals calcined at 900°C ( and denote the characteristic peaks corresponding to Y₂O₃ and Y₂O₂S, resp.).

For the observation of nanoparticles morphology, we loaded the Y₂O₂S:Yb, Er sample dispersed with ethanol on a TEM grid mesh. As shown in the TEM image (Figure 2), the obtained particles are in the quasispherical shape. Although some nanoparticles aggregate and form large particles, there are many well-dispersed nanoparticles around the agglomerates. Thus, it is feasible to observe the homogeneity of the particles as well as to evaluate the mean particle size. The assessed mean particle size from TEM image is about 30 nm, which exhibits a fairly good agreement with the calculated crystalline particle size on the basis of XRD measurement. In addition, the uniformity of the nanoparticles in both shape and size is achieved as shown in Figure 2, thereby indicating the promising advantages of the simple complex precipitation method.

Figure 2: TEM image of the Y₂O₂S:Yb, Er phosphor annealed at 900°C.

3.2. Upconversion Luminescence Properties

Despite the ground bulk upconversion phosphors exhibit high luminescent efficiency, the big size and uneven morphology prevent their utilizations in the fields of biological applications as well as high-resolution displays. As suggested in the TEM measurements, the upconversion phosphors synthesized by the complex precipitation method reach to the high requirements of nanosize and homogeneity in both shape and size distribution. These features are ideal for the biological applications. Meanwhile, the very bright upconversion luminescence of the nanophosphors is observed by naked eyes when excited by LD with power as low as 5 mW. Figure 3 presents the upconversion emission spectra of the Er³⁺ (0.8 mol%), Yb³⁺ (0–11 mol%) codoped Y₂O₃ nanophorsphors by the excitation of 980 nm LD. The green and red emission bands at about 563 and 660 nm are assigned to the ²H11/2/⁴S3/2⁴I15/2 and ⁴F9/2⁴I15/2 intra-4f transitions of Er³⁺, respectively. Upconversion luminescence is generally composed of ground state absorption (GSA), excited state absorption (ESA), energy transfer (ET), multiphonon relaxation, cross relaxation, and radiative transition [18]. Figure 3   (i) shows the upconversion luminescence spectrum of Y₂O₃:Er³⁺ (Er: 0.8 mol%) sample. The excitation wavelength matches the absorption transition between the ground state ⁴I15/2 and excited level ⁴I11/2. After the first-level excitation, two mechanisms are considered as follows [19]. One route is the simple ESA, where the same energy photons are absorbed by the excited state Er³⁺ ion from the ⁴I11/2 to the ⁴F7/2 level. The other route is ETU (energy transfer upconversion), where a mutual interaction between two excited Er³⁺ (⁴I11/2) may occur, with the results of one ion deexcited to ⁴I15/2 while the other ion is excited to ⁴F7/2 level. The above two upconversion mechanisms can both populate the ⁴F7/2 state which then decays nonradiatively to the ²H11/2/⁴S3/2 and ⁴F9/2 levels, subsequently emitting green and red luminescence when returning to the ground state ⁴I15/2, respectively. Due to the low doping concentration of Er³⁺ ion in the Y₂O₃:Er³⁺ sample, it was considered that the probability of occurring the first ESA mechanism is much more than the ETU mechanism between two Er³⁺ ions.

Figure 3: Upconversion spectra of the Er³⁺ (0.8 mol%), Yb³⁺ (0–11 mol%) codoped Y₂O₃ samples calcined at 900°C when excited by 980 nm LD.

Figure 3 (ii)–(v) show the upconversion spectra of different concentration of Yb³⁺ in Er³⁺ doped Y₂O₃ nanocrystals under 980 nm LD excitation. It can be seen from the spectra that the relative intensity ratio ( and represent the red and green emission, resp.) rapidly increases with increasing the Yb³⁺ concentration, where the Yb³⁺ ions obviously play an important role. Since the Yb³⁺ ion has a much higher absorption cross-section than that of Er³⁺ ion at 980 nm [20], the roles that the Er³⁺ ion plays in the population of the excitation levels decrease with the increasing of the Yb³⁺ concentration.

As shown in the energy level diagram (Figure 4), the upconversion green and red emission mechanisms of the Y₂O₃:Yb, Er samples are analyzed as follows.

Figure 4: Schematic energy level diagram of Yb³⁺ and Er³⁺ ions under excitation of 980 nm LD.

The upconversion channel of green luminescence (1) is as follows:

(1);  (  denotes the process of multiphonons relaxation).

The ⁴F7/2 state of Er³⁺ is populated via the twice energy transfer of channel (1), after nonradiative relaxation to the ⁴S3/2/⁴H11/2 levels and then returns to the ground state emitting the upconversion green luminescence via radiation transition.

The upconversion channels of red luminescence (2), (3), and (4) are as follows:

(2)after the population of 4S3/2 level by channel (1), occur;(3);(4); .

One can see that, three channels are involved in populating the excited state ⁴F9/2 of Er³⁺ ion, that is, multiphonons relaxation from ⁴S3/2 level, cross-relaxation process between Er³⁺ ions, and the ⁴I13/2 level (Er³⁺) reabsorbing the energy from excited state ²F5/2 (Yb³⁺) to further populate the ⁴F9/2 level via the process of ET1, as shown in Figure 4.

It is well known that the increase of concentration of Yb³⁺ in Er³⁺ doped nanocrystals should greatly promote the ET from Yb³⁺ (²F5/2) to Er³⁺ (⁴I11/2) [21]. Thus more Er³⁺ will be excited to the ⁴I11/2 level by the ET0, and correspondingly more Er³⁺ will be excited to the ⁴F7/2 state with the increase of the population of ⁴I11/2 through ET2 as shown in Figure 4. According to (see [22]),

where is the cross-relaxation probability, , are the population densities of the two states which are involved in the cross-relaxation process. Therefore, the cross-relaxation probability between the ⁴I11/2 and ⁴F7/2 levels increased greatly with the increase of Yb³⁺ concentration. This directly contributes to the population of the ⁴F9/2 level to a large extent. Additionally, the population increase of the ⁴I11/2 state will also result in the decay of Er³⁺ ions from ⁴I11/2 to ⁴I13/2 level; thus the process of ET1 is enhanced due to the increasing of population densities of Er³⁺ (⁴I13/2) and Yb³⁺ (²F5/2) ions. This also contributes to the population of ⁴F9/2 level. Therefore, the occurrence probabilities of channels (3) and (4) both increase with the increase of Yb³⁺ concentration, which consequently result in the enhancement of population of ⁴F9/2 level. As observed in Figure 3, the relative intensity ratio increases gradually with the Yb³⁺ concentration increasing from 0 to 11%.

The pump mechanism can be studied by the relationship between the upconversion emission intensity and pump power; that is,

where is the intensity of the upconversion emission, is pump power, and is the number of pump-photons required to populate the emitting state [23]. In order to confirm the upconversion mechanism discussed above, the intensity of green (563 nm) and red (660 nm) emission is measured as a function of pump power in three samples of Y₂O₃:0.8% Er, Y₂O₃:1% Yb, 0.8% Er, and Y₂O₃:11% Yb, 0.8% Er, as plotted in Figure 5. For the sample of Y₂O₃:Er, the slope corresponding to the emission intensities of the transitions ²H11/2/⁴S3/2⁴I15/2 and ⁴F9/2⁴I15/2 is both approximately 2. This indicates that generation of these transitions is predominantly due to two-photon absorption by Er³⁺. For the sample of Y₂O₃:1% Yb, 0.8% Er, the slope values are slightly smaller than 2, which shows the upconversion mechanism is two-photon process but including some cross-relaxation and the ET processes from Yb³⁺ to Er³⁺ [23]. For the sample of Y₂O₃:11% Yb, 0.8% Er, the slope values are even smaller than that in the Y₂O₃:1% Yb, 0.8% Er sample, which implies that the upconversion mechanism must include more cross-relaxation and the ET processes from Yb³⁺ to Er³⁺. These results further confirm the upconversion mechanism stated above.

(a)

(b)

(c)

Figure 5: The upconversion emission intensity as a function of pump power for the three samples: (a) Y₂O₃:0.8% Er, (b) Y₂O₃:1% Yb, 0.8% Er, and (c) Y₂O₃:11% Yb, 0.8% Er.

Figure 6 shows the upconversion spectra of Y₂O₃:Yb, Er and Y₂O₂S:Yb, Er nanocrystal samples by the 980 nm LD excitation, and the doping concentrations of Yb³⁺ and Er³⁺ in the two samples are the same about 8 and 0.8%, respectively. Although the main peaks in the spectra are both red emission arising ⁴F9/2⁴I15/2 transition of Er³⁺, there are still some differences between them. The maximum of red emission peak of the Y₂O₃:Yb, Er sample is located at 660 nm; moreover, the green luminescence is almost quenched comparing with the intensity of red emission. When excited by LD only with the power of 5 mW, the upconversion red luminescence can be seen by naked eyes in the daytime. While in the spectrum of Y₂O₂S:Yb, Er sample, the maximum of red emission peak (668 nm) shifts to longer wavelength about 8nm when compared with that of yttrium oxide sample, and the green emission is enhanced to a certain extent. Similarly, under the excitation of 980 nm LD with power of 5 mW, the upconversion luminescence also can be seen in the daytime but appearing yellow to the naked eyes.

Figure 6: The upconversion spectra of Er³⁺ (0.8 mol%), Yb³⁺ (8 mol%) codoped Y₂O₂S and Y₂O₃ samples under the 980 nm excitation.

The phosphors of Y₂O₃ and Y₂O₂S come from the same precursor sample, and the heat treatment, doping concentration, and measure conditions are all the same, so it is believed that the obvious differences between the two spectra must be related to the different host. As the intrinsic phonon energies of Y₂O₃ and Y₂O₂S are 597 and 520 cm⁻¹, respectively [24, 25], Y₂O₃ obviously possesses much higher phonon energy. So the probability of phonon-assisted nonradiative relaxation of ⁴S3/2⁴F9/2 and ⁴I11/2⁴I13/2 is much larger in Y₂O₃ host, which lead to the more effectively bypass or quench the green-light emitting states, as shown in Figure 6.

As compared with the Y₂O₃:Yb, Er samples, in the spectrum of Y₂O₂S:Yb, Er nanocrystals, the red shift of the main peak may be caused by the nephelauxetic effect in the system composed by the sulphured compounds [16]. Since the Er³⁺ ion is surrounded by the sulfur in the oxysulfide, this increases the covalent interaction in the system and shifts the transitions to smaller energies compared with the yttrium oxide. Pires has observed similar phenomenon in the Eu³⁺ doped oxysulfide system, that is, the emission peak arising from the ⁵D₀⁷F₂ transition of Eu³⁺ red shifts 14 nm in the oxysulfide compared with the oxide. The differences in the peak shape may be correlative to the crystal structure of the host crystal. When changing the crystal structure from cubic into hexagonal structure (from oxide to oxysulfide), the variation of the crystal-field surrounding the Er³⁺ ion may lead to the differences in the peak width and splitting mode.

4. Conclusions

The Y₂O₃:Yb, Er and Y₂O₂S:Yb, Er upconversion phosphor nanoparticles with crystallite size of 26 and 29 nm have been successfully prepared by the complex precipitation method by using the mixed solution of NH₄HCO₃ and NH₃·H₂O as the complex precipitant, and the two samples present the pure cubic and hexagonal structure, respectively. For the Y₂O₃:Yb, Er sample, when keeping the Er³⁺ concentration a constant (0.8 mol%), the ratio has increased greatly with the increase of Yb³⁺ concentration from 0 to 11 mol%. This could be attributed to the ET between rare earth ions and the cross-relaxation processes. In addition, the upconversion emission spectra of the two samples before and after sulphuration are different from each other. This may be correlated to the crystal structure and the intrinsic properties of different host crystals. It is worthy pointing out that the high efficient upconversion luminescence from the Y₂O₃:8% Yb, 0.8% Er and Y₂O₂S:Yb 8%, 0.8% Er phosphors can be seen by naked eyes in daytime when excited by the 980 nm LD with power as low as 5 mW. Therefore, these up-converting phosphor nanoparticles, with excellent luminescence properties, may become ideal fluorescence probes in the biochip technology and may be utilized as the anticounterfeiter and display screen materials.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors thank the New Century Educational Talent Plan of Chinese Education Ministry (NCET-10- 0171), Fundamental Research Funds for the Central Universities (3132015139, 3132015150, 3132015153, and 3132014327), Foundation of Liaoning Educational Committee (L2013201, L2014208, and L2014212), and Liaoning Excellent Talents in University (LR2013020).

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