Controlled morphology and EDTA-induced pure green upconversion luminescence of Er³⁺/Ho³⁺-Yb³⁺ co-doped NaCe(MoO₄)₂ phosphor

Abstract

Uniform and well-crystallized NaCe(MoO₄)₂: Er³⁺/Ho³⁺, Yb³⁺ up-conversion (UC) phosphors have been successfully synthesized by a facile hydrothermal route at 180 °C for 12 h at pH = 9. The crystalline phase, size and morphology were systematically characterized using powder X-ray diffraction (XRD) and field emission-scanning electron microscopy (FE-SEM). The experimental results showed that the ethylene diamine tetraacetic acid (EDTA) was a key parameter which not only determined their spacial arrangement, but also affected the size distributions of the final products. Moreover, we found that EDTA could tune the band edge absorption of the molybdate system by changing the lattice parameters, so as to realize their bicolour tunable luminescence of Er³⁺/Ho³⁺-Yb³⁺ co-doped NaCe(MoO₄)₂ for the first time. Studies of the behavior as a function of dopant concentration were all described. The different UC mechanisms of the samples with and without EDTA were systematically depicted as well. It was found that an innovative route to increase the green UC emission and simultaneously suppress red UC emission in Er³⁺/Ho³⁺-Yb³⁺ co-doped NaCe(MoO₄)₂ prepared with the assistance of 0.1 g EDTA, which enhanced the efficiency of the single green color.

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1. Introduction

Since the UC luminescence of rare-earth (RE) ions was discovered by Auzel in the mid-1960s,^1,2 it has been the focus of much research. Potential applications include solar cells,^3–6 novel display technologies,^7–9 inks for secure printing¹⁰ and, more recently, biophysics.^11–13 Their advantages include near-infrared (NIR) excitation, low cytotoxicity, weak autofluorescence, high chemical stability, and low photobleaching, which make them more desirable than conventional organic dyes or quantum dots for bioimaging application.^14–17 The photoluminescence emission from rare earth doped materials arises due to the transitions within the 4fⁿ energy manifolds which are forbidden because of the electric dipole selection rule but these are allowed due to the mixing of configurations having opposite parity. One of the most exciting properties of these materials is frequency UC where two or more photons having relatively low energy are absorbed by the material and subsequently converted to a higher energy photon (this is an anti-Stokes process). However, the commonly investigated lanthanide activators such as Er³⁺ and Ho³⁺ ions contain abundant metastable excited states, and the dominant emission usually lies in the non-green region with relatively low green emission intensity. Hence, a strategy to boost the green emission intensity will be useful for UC applications.

It is well known that lattice parameter is the fundamental factor that affects the luminescence property.^18,19 During the hydrothermal process, the introduction of EDTA with functional groups into the reaction system can effectively change the band edge absorption of MoO₄²⁻ of the final product by changing the lattice parameters, which makes effort to build energy level match conditions between MoO₄²⁻ and Er³⁺ or Ho³⁺, as a result, the cross relaxation (CR) among Er³⁺ ions and Ho³⁺ ions can be efficiently decreased. Then, we can acquire the intense green UC luminescence by a novel energy transfer (ET) pathway which entails a strong ground state absorption of Yb³⁺ ions and the excited state absorption of Yb³⁺-MoO₄²⁻ dimers, followed by an effective energy transfer to the high energy state of Er³⁺ (Ho³⁺) ions.^20–22 This strategy based on EDTA effect is not only successful in achieving pure green 547 or 541 nm UC emission but can also act as an alternative approach for precise UC color tuning and provide further insight into the UC mechanism.

As a fascinating group of inorganic-functional materials, rare earth molybdate compounds, which share the scheelite-like (CaWO₄) iso-structure, exhibit high chemical durability, favorable physical properties, excellent thermal and hydrolytic stability and can be widely used in catalysts, high-performance phosphors, UC materials, negative thermal expansion materials and so forth.^23–26 In addition, they have a relatively low lattice phonon energy, which would increase the possibility of radiative transitions (RTs) and in turn result in a high quantum yield of UC process.

In the current study, we report an efficient method for the controlled fabrication of 3D hierarchical architectures of NaCe(MoO₄)₂ with a high yield and good uniformity via a facile and mild hydrothermal method in EDTA-mediated processes.²⁷ For the first time, we research the UC luminescence properties of Er³⁺/Ho³⁺-Yb³⁺ co-doped NaCe(MoO₄)₂ synthesized with further calcination. The effects of the contents of rare-earth ions and the EDTA introduced into the reaction systems on the UC luminescence intensity are discussed in detail. The possible UC luminescence mechanisms are elucidated through power dependence. We first extend the capability of the EDTA-mediated hydrothermal approach to achieve intense green UC luminescence in NaCe(MoO₄)₂ system by changing the lattice parameters. The relatively high infrared to visible upconversion efficiencies are obtained with values up to 2.08% (NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺) and 2.01% (NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺). These results indicate that this phosphor material shows great prospect in practical application in 3D display technology, green light laser and biological field.

2. Experimental procedure

2.1. Preparation of NaCe(MoO₄)₂ microcrystals

Materials. All the chemicals were commercially available and used without further purification. Erbium oxide, holmium oxide and ytterbium oxide (all 99.99%) were used as the starting raw materials. All other chemicals used were analytical grade without further purification. The cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) was used as the cerium source. Rare earth nitrate (Er(NO₃)₃, Ho(NO₃)₃ and Yb(NO₃)₃) stock solutions of 0.05 M, 0.05 M and 0.05 M were prepared by dissolving Er₂O₃, Ho₂O₃ and Yb₂O₃ in concentrated HNO₃ at elevated temperature. The ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) was used as the molybdenum source and sodium hydroxide (NaOH) as the sodium source. Meanwhile, EDTA was used as the “shape modifier”.

Synthesis of rare earth-doped NaCe(MoO₄)₂. In a typical synthesis procedure, stoichiometric amounts of Ce(NO₃)₃·6H₂O and rare earth nitrate (Er(NO₃)₃, Ho(NO₃)₃ and Yb(NO₃)₃) was dissolved in 20 mL deionized water. Subsequently, 0.1 g EDTA was added to form a Ce³⁺–EDTA complex. After vigorous stirring for 10 min, 16 mL aqueous solution containing 0.5714 mmol (NH₄)₆Mo₇O₂₄·4H₂O was added dropwise into the above solution under continuous strong magnetic stirring at room temperature. The pH of the yellow colloidal solution was adjusted to 9 by adding a desired amount of NaOH solution (5 M). NaOH immediately reacted with the resulting yellow suspension, and a slurry-like yellow precipitate was formed. After additional agitation for 1 h, the as-obtained mixing solution was poured into a Teflon bottle held in a stainless steel autoclave, sealed and maintained at 180 °C for 12 h. The systems were then allowed to air-cool to room temperature. The final products were separated by means of centrifugation, washed with ethanol and deionized water in sequence, and finally dried in vacuum at 60 °C for 6 h. Then, some samples were heated to 800 °C at a heating rate of 10 °C min⁻¹, kept at this temperature for 4 h, and naturally cooled to room temperature. The final product was a yellow powder and subsequently kept for further characterization.

2.2. Characterization

The powder X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer at a scanning rate of 15° min⁻¹ in the 2θ range from 10° to 65°, with graphite monochromatized Cu Kα radiation (λ = 0.15405 nm). The morphology and size of the obtained samples were examined by a field emission-scanning electron microscope (FE-SEM, XL30, Philips). The upconversion emission spectra were recorded by exciting the sample with 980 nm diode laser (LD) using a Zolix Omni-λ 500 spectrograph. The upconversion efficiency was obtained by exciting the sample with 971 nm LD using an Andor SR-500i spectrometer (Andor Technology Co., Belfast, UK). The diffuse reflectance spectra were recorded using a Hitachi U-4100 spectrophotometer. All the measurements were performed at room temperature.

3. Results and discussion

3.1. Crystal structure and morphology of the NaCe(MoO₄)₂

XRD. The composition and phase purity of the as-synthesized products are characterized by XRD. The representative powder X-ray diffraction patterns of the obtained compounds are displayed in Fig. 1. The peak positions of the as-prepared products match perfectly with those of the tetragonal phase of the oxide (JCPDS card no. 79-2242) with I4₁/a lattice symmetry, which demonstrates that the rare earth dopants are entirely incorporated in the NaCe(MoO₄)₂ host lattice at the present doping level by substituting for the Ce³⁺ and do not alter the host structure significantly. No other impurity peaks are observed, revealing the high purity of the as-prepared samples. The lattice parameters of all the rare earth-doped NaCe(MoO₄)₂ samples have been refined and the results are summarized in Table 1. It can be concluded that the well-crystallized NaCe(MoO₄)₂ phosphor can be obtained under the present synthetic conditions.

Fig. 1 XRD powder patterns of NaCe(MoO₄)₂ samples acquired at pH = 9 by hydrothermal method at 180 °C for 12 h: (a) 2%Er³⁺, 5%Yb³⁺ (with 0.1 g EDTA); (b) 2%Er³⁺, 5%Yb³⁺ (with 0.1 g EDTA annealed at 800 °C); (c) 3%Er³⁺, 5%Yb³⁺; (d) 3%Er³⁺, 5%Yb³⁺ (annealed at 800 °C); (e) 1%Ho³⁺, 10%Yb³⁺ (with 0.1 g EDTA annealed at 800 °C); (f) 1%Ho³⁺, 10%Yb³⁺ (annealed at 800 °C). The standard data for NaCe(MoO₄)₂ (JCPDS card 79-2242) is shown as reference.

Table 1 The lattice parameters and corresponding band edge absorption of doped NaCe(MoO₄)₂ samples prepared at sintered temperature for 800 °C

NaCe(MoO4)2	a = b (Å)	c (Å)	α = β = γ (Å)	Cell volume (Å^3)	Band edge absorption (eV)
2%Er, 5%Yb (with EDTA)	5.3087(11)	11.6515(26)	90	328.37	2.49
1%Ho, 10%Yb (without EDTA)	5.3085(16)	11.6551(31)	90	328.45	2.47
3%Er, 5%Yb (without EDTA)	5.3098(11)	11.6570(33)	90	328.66	2.40
1%Ho, 10%Yb (with EDTA)	5.3181(14)	11.6561(46)	90	329.66	2.27

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FE-SEM. In the synthesis of inorganic nano/microcrystals many organic additives have been employed for the modifications of certain crystallographic surfaces.^28–31 In the present work, it is found that the EDTA introduced to the reaction system has a crucial effect on the morphology and size distribution of the final products. Fig. 2 illustrates the representative FE-SEM images of the samples prepared hydrothermally at 180 °C for 12 h at pH = 9. As shown in Fig. 2a, the product obtained in the EDTA-absence process is almost entirely composed of tetragonal particles with uniform size of about 4 μm. However, the introduction of 0.1 g EDTA into the reaction system can bring on an intriguing morphology, as illustrated in Fig. 2b, which demonstrates that the product consists of well dispersed 3D microflower structures with an average diameter of 1.5 μm. The FE-SEM image observed with a higher magnification (Fig. 2c) shows the smooth and flat surfaces of the particles with a size of about 0.26 μm in thickness. The high-magnification FE-SEM image (Fig. 2d) reveals that these individual flower-like architectures are constructed from a large quantity of single-shuttle-like microcrystals, and then these single-shuttle-like flakes further self-assemble in a radial way to construct the 3D flower-like architectures. The FE-SEM images of NaCe(MoO₄)₂: Er³⁺, Yb³⁺ phosphor calcined at 800 °C for 4 h are displayed in Fig. 2e and f. The image exhibited in Fig. 2e shows tetragonal crystal without clear edges and corners. When 0.1 g EDTA is added, we obtain microstructures of nearly spherical and they constitute a network structure (Fig. 2e). The above-stated results indicate that the introduction of 0.1 g EDTA is undoubtedly prerequisite for the formation of such microcrystal textures in the present system since the additive can selectively adhere to some specific crystallographic facet and thus modify the crystal growth dynamically.^32–35

Fig. 2 The FE-SEM images of the NaCe(MoO₄)₂: Er³⁺, Yb³⁺ crystallites obtained in the EDTA-absence (left) and obtained in the presence of 0.1 g EDTA (right) (a) and (b) panoramic image; (c) and (d) magnified image; (e) and (f) image calcined at 800 °C for 4 h.

Here, one might have a question: what is the original driving force for the lattice parameters and morphology evolution in solution under mild hydrothermal conditions? To this end, on the basis of the previous research,²⁷ the NaCe(MoO₄)₂ microflowers can be formed by a means of a “nucleation-aggregation-dissolution-recrystallization” mechanism. An EDTA molecule has six binding sites, including four COOH(COO⁻) carboxylic groups and two single pairs of electrons on the nitrogen atom. When a single EDTA molecule chelates with a metal ion, all of its six binding sites participate in the reaction. The controlled release of Ln cations from the Ln–EDTA complex helps to separate the nucleation and growth stages. Furthermore, the decomplexed EDTA anions will attach to the surface of the nanocrystals and lead to formation of well-dispersible nanocrystals in water. In the crystal growth stage, a delicate balance between the thermodynamic growth and kinetic growth regimes can strongly control the final structure of the nanocrystals.^36,37 When the thermodynamic growth regime is driven by a sufficient supply of thermal energy (KT), the most stable crystal structure is preferred. The incipient nanoparticles have a tendency to selfaggregate into larger particles to minimize the surface energy.³⁸ The larger the surface-to-volume ratio is, the more Er³⁺/Ho³⁺ ions are located at/near the surface of the nanoparticles relative to the Er³⁺/Ho³⁺ ions in the core.^39,40 Therefore, the samples prepared with 0.1 g EDTA have more Er³⁺/Ho³⁺ ions at/near the surface. NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺ with the assistance of 0.1 g EDTA is slightly smaller than those prepared in the EDTA-absence, which is attributed to the smaller sizes of the Er³⁺ (0.0945 nm) compared to Ce³⁺ (0.107 nm). NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ with the assistance of 0.1 g EDTA increases the lattice parameters mainly because the radius of Ho³⁺ (0.0958 nm) is larger than that of Er³⁺ (0.0945 nm) and Yb³⁺ (0.0925 nm).⁴¹

3.2. Up-conversion photoluminescence of Er³⁺/Ho³⁺-Yb³⁺ co-doped NaCe(MoO₄)₂

Dependence of UC spectra on dopant concentration. NaCe(MoO₄)₂, which shares scheelite-like structures, shows excellent thermal and hydrolytic stability and is demonstrated to be an efficient host for other lanthanide ions to produce various luminescence properties.^42,43 The UC fluorescent emission spectra for all selected NaCe(MoO₄)₂ samples are measured at sintered temperature for 800 °C under the excitation of a 980 nm laser diode, and the results indicate that the UC emission intensity and spectral properties depend on dopant concentrations. The UC photoluminescence spectra for NaCe(MoO₄)₂: xEr³⁺, yYb³⁺ in the EDTA-absence are displayed in Fig. 3. The typical UC spectra show a weak emission corresponding to the transitions of ²H_11/2 to ⁴I_15/2 and ⁴S_3/2 to ⁴I_15/2 centered at around 526, 547 nm (green), and a stronger emission attributes to the intra 4f–4f ⁴F_9/2 to ⁴I_15/2 electronic transition of the Er³⁺ ions at about 678 nm (red). The emission bands are extended in a wide range of wavelength which represents large Stark splitting that arises due to high crystal field of NaCe(MoO₄)₂ matrix. In Fig. 3a, the room temperature UC emission intensity versus Er³⁺ ions concentration at fixed Yb³⁺ concentration has been plotted. The optimal doping concentration of Er³⁺ ions is 3 mol%. Fig. 3b presents the UC emission intensity versus the concentration of Yb³⁺ ions in the co-doped NaCe(MoO₄)₂ phosphor varied from 0.03 to 0.10 keeping Er³⁺ ions concentration fixed at 0.03. It can be observed that the fluorescent intensities increase first and then decrease sharply with the content of Yb³⁺ beyond 0.05. Fig. 4 shows the UC emission spectra of NaCe(MoO₄)₂: xEr³⁺, yYb³⁺ with the assistance of 0.1 g EDTA. The peak positions of the as-synthesized products do not change, but the green UC emission intensities increase remarkably and the red light emission intensities decrease sharply. The UC emission spectra of NaCe(MoO₄)₂: xHo³⁺, yYb³⁺ in the presence of 0.1 g EDTA are presented in Fig. 5, which shows the intensity as function of different dopant concentrations. The intense green UC emission peak centered at 541 nm arises from the intra-4f electronic transition ⁵F₄/⁵S₂ → ⁵I₈ of Ho³⁺ ions. The weak red emission peak located at 659 nm attributes to the ⁵F₅ → ⁵I₈ transitions of Ho³⁺ ions.^44–46 Concentration dependent studies reveal that the optimal composition is realized for a 1% Ho³⁺ and 10% Yb³⁺-doping concentration. The UC intensity decreases with excessive amounts of Er³⁺/Ho³⁺ ions due to the concentration quenching effect, which can also confirm the occurrence of energy transfer between the nearest Er³⁺/Ho³⁺ and Er³⁺/Ho³⁺/Yb³⁺ ions. With increasing Er³⁺/Ho³⁺ concentration, the distance between Er³⁺/Ho³⁺ and Er³⁺/Ho³⁺/Yb³⁺ ions will be reduced, which strengthens non-radiative relaxation and causes the decrease of UC luminescence intensity.⁴⁷ When appropriate amounts of Yb³⁺ ions are added, the UC emission intensities increase, this is attributed to the more populated green and red emitting levels of Er³⁺/Ho³⁺, which due to an efficient ET from the sensitizer Yb³⁺-MoO₄²⁻–dimer to Er³⁺/Ho³⁺ for the larger absorption cross-section of Yb³⁺-MoO₄²⁻–dimer around 980 nm and larger energy overlap between Yb³⁺-MoO₄²⁻ and Er³⁺/Ho³⁺.^20–22 However, the fluorescence intensity decreases sharply when the Yb³⁺ concentration reaches an optimum performance, which may be mainly caused by the energy back transfer (EBT).^48,49 The higher Yb³⁺ concentration, the higher is the ratio between EBT from Er³⁺/Ho³⁺ to Yb³⁺-MoO₄²⁻ and energy transfer from Yb³⁺-MoO₄²⁻ to Er³⁺/Ho³⁺.⁵⁰

Fig. 3 Typical UC luminescence spectra of NaCe(MoO₄)₂: xEr³⁺, yYb³⁺ phosphors sintered at 800 °C for 4 h in the EDTA-absence: (a) y = 0.05; x = 0.02, 0.03, 0.04, and (b) x = 0.03; y = 0.03, 0.05, 0.10.

Fig. 4 Typical UC luminescence spectra of NaCe(MoO₄)₂: xEr³⁺, yYb³⁺ phosphors sintered at 800 °C for 4 h with the assistance of 0.1 g EDTA: (a) y = 0.05; x = 0.01, 0.02, 0.03, and (b) x = 0.02; y = 0.03, 0.05, 0.10.

Fig. 5 Typical UC luminescence spectra of NaCe(MoO₄)₂: xHo³⁺, yYb³⁺ phosphors sintered at 800 °C for 4 h with the assistance of 0.1 g EDTA.

In addition, the EDTA is a key parameter which not only determines their spacial arrangement, but also affects the UC luminescence properties of the final products. Fig. 6 exhibits the comparison between the UC emission spectra of NaCe(MoO₄)₂: Er³⁺/Ho³⁺, Yb³⁺ prepared with the assistance of 0.1 g EDTA and the UC emission spectra obtained in the EDTA-absence. It can be visually observed that the red light emission intensity is stronger than the green when the samples are prepared in the EDTA-absence from Fig. 6. When 0.1 g EDTA is added, we can surprisingly obtain intense pure green emissions. This is mainly caused by the change of forbidden gap of MoO₄²⁻ after joining EDTA. For present NaCe(MoO₄)₂: Er³⁺/Ho³⁺, Yb³⁺, the UC luminescence can be easily observed by the naked eyes at room temperature. The insets of Fig. 6 present the typical photographs of the photoluminescence for the four samples. The luminous color of NaCe(MoO₄)₂: Er³⁺/Ho³⁺, Yb³⁺ changes from yellow to green when the EDTA is added. The extremum quantum efficiency η_UC (the ratio of the luminescence power emitted by sample over the power absorbed in the infrared (950–1000 nm range)), which obeys the relation:⁵¹

Fig. 6 Measured UC emission spectra for (a) NaCe(MoO₄)₂: Er³⁺, Yb³⁺ and (b) NaCe(MoO₄)₂: Ho³⁺, Yb³⁺ at room temperature pumped by the 980 nm NIR laser. The insets present the photographs of the photoluminescence for the four samples.

It has been reported that the maximum upconversion luminescence efficiency of 0.5152% is obtained with BaGd₂O₄: Yb³⁺ (4%), Er³⁺ (1%).⁵² In our experiments, the relatively high upconversion efficiency of 2.08% (NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺) and 2.01% (NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺) are obtained with the assistance of 0.1 g EDTA at room temperature under 0.01 W excitation. Especially, the samples have the high upconversion efficiency at low excitation power, which indicates that the phosphors can have potential application in biological field. The specific UC luminescence mechanisms and population processes in NaCe(MoO₄)₂: Er³⁺/Ho³⁺, Yb³⁺ system under 980 nm excitation will be elucidated through the electronic energy levels diagram and power dependence.

Mechanism of UC emission. The lattice parameters of four NaCe(MoO₄)₂: Er³⁺/Ho³⁺, Yb³⁺ samples are calculated attributed to an tetragonal structure with I4₁/a space group which is in good agreement with the standard JCPDS card no. 79-2242. The corresponding lattice parameters and band edge absorption of molybdate system are given in Table 1. With the increase of lattice parameters, the bond length becomes longer, crystal field becomes weaker that leads to the decrease of the band edge absorption,^53–56 which is consistent with the results of the diffuse reflectance spectra.

The diffuse reflectance spectra of NaCe(MoO₄)₂: Er³⁺/Ho³⁺, Yb³⁺ at room temperature are depicted in Fig. 7. The Kubelka–Munk function is defined in equation:

here K and S represent the absorption coefficient and scattering coefficient. R_∞ is the limit value of the reflection coefficient R in the infinite thick sample. According to the diffuse reflection spectra, we can get the absorption wavelength threshold of the four samples. It can be calculated that the band edge absorption are 2.40 eV (NaCe(MoO₄)₂: 3%Er³⁺, 5%Yb³⁺ without EDTA), 2.49 eV (NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺ with the assistance of 0.1 g EDTA), 2.47 eV (NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ without EDTA), 2.27 eV (NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ with the assistance of 0.1 g EDTA), respectively.

Fig. 7 The room-temperature diffuse reflectance spectra of doped NaCe(MoO₄)₂. All samples are prepared at sintered temperature for 800 °C.

An energy transfer mechanism is proposed for the current EDTA-induced UC pure green emission. Unlike the cases without MoO₄²⁻, here the sensitization through the Yb³⁺-MoO₄²⁻ dimer complex form, which entails both ground state absorption (GSA) and the excited state absorption (ESA). The Yb³⁺-MoO₄²⁻–dimer ground state is signed by |²F_7/2, ¹A₁〉, the intermediate excited state in the NIR by |²F_5/2, ¹A₁〉, and the correlative higher excited states by |²F_7/2, ³T₁〉, |²F_7/2, ³T₂〉, |²F_7/2, ¹T₁〉, and |²F_7/2, ¹T₂〉, respectively. Yb³⁺-MoO₄²⁻–dimer is excited to the |²F_7/2, ³T₂〉 level by GSA (|²F_7/2, ¹A₁〉 → |²F_5/2, ¹A₁〉), ESA (|²F_5/2, ¹A₁〉 → |²F_7/2, ¹T₁〉) and nonradiative relaxations (NR) from |²F_7/2, ¹T₁〉 level. As depicted in Fig. 8a and b, the population at the ⁴F_7/2 level in Er³⁺ can be achieved through a high excited state energy transfer (HESET) from |²F_7/2, ³T₂〉 state of the Yb³⁺-MoO₄²⁻–dimer to Er³⁺ process ⁴I_15/2 (Er³⁺) + |²F_7/2, ³T₂〉 (Yb³⁺-MoO₄²⁻–dimer) → ⁴F_7/2 (Er³⁺) + |²F_7/2, ¹A₁〉 (Yb³⁺-MoO₄²⁻–dimer) because the larger absorption cross section of Yb³⁺-MoO₄²⁻–dimer. Subsequent the two meta-stable levels of ²H_11/2 and ⁴S_3/2 are populated by the NR from ⁴F_7/2 to ²H_11/2 and ⁴S_3/2 levels. However, in the case of NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺, the energy transfer efficiency is very low, because of the energy mismatch and the instability of |²F_7/2, ¹T₁〉 level (Fig. 8a). When Er³⁺ content is high, the cross relaxation (CR) process ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2 may readily take place due to the small distance between Er³⁺. Thus, the CR greatly improves the population of the ⁴F_9/2 energy levels, resulting in the rise of 678 nm red emission. When 0.1 g EDTA is added, the band edge absorption is increased to 2.49 eV, making |²F_7/2, ³T₂〉 level and ⁴F_9/2 level match well (Fig. 8b). The HESET process significantly avoids the lattice phonon quenching of oxide matrix, which contributes to the extraordinary UC efficiency of green emissions in the present system. In the case of NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ (Fig. 8c), the band edge absorption is 2.47 eV, which has little energy mismatch with ⁵F₃ energy levels. The CR process ⁵F₃ + ⁵I₈ → ⁵F₅ + ⁵I₇ may readily take place due to a small energy mismatch, which improves the population of the ⁵F₅ energy level, resulting in the enhancement of red emission. The electronic energy levels diagram of NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ synthesized in the presence of 0.1 g EDTA is illuminated in Fig. 8d. The band edge absorption is 2.27 eV, which is a little higher than ⁵F₄/⁵S₂ energy level. Therefore, the population at the ⁵F₄/⁵S₂ level in Ho³⁺ can be achieved through HESET from |²F_7/2, ³T₂〉 state of the Yb³⁺-MoO₄²⁻–dimer to Ho³⁺ process ⁵I₈ (Ho³⁺) + |²F_7/2, ³T₂〉 (Yb³⁺-MoO₄²⁻–dimer) → ⁵F₄/⁵S₂ (Ho³⁺) + |²F_7/2, ¹A₁〉 (Yb³⁺-MoO₄²⁻–dimer) because the larger absorption cross section of Yb³⁺-MoO₄²⁻–dimer. Then, the ⁵F₅ levels can be populated by the NR from ⁵F₄/⁵S₂ to ⁵F₅ levels. Finally, when most of the Ho³⁺ ions in ⁵F₄/⁵S₂ level make transition to ⁵I₈ level, strong green UC luminescence (centered at 541 nm) emissions are observed.

Fig. 8 Schematic energy levels of NaCe(MoO₄)₂ and the proposed UC mechanisms under the excitation of 980 nm light. (a) NaCe(MoO₄)₂: 3%Er³⁺, 5%Yb³⁺ (without EDTA); (b) NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺ (with 0.1 g EDTA); (c) NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ (without EDTA); (d) NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ (with 0.1 g EDTA).

In order to determine the number of pump photons responsible for the UC mechanism, the green (²H_11/2/⁴S_3/2 → ⁴I_15/2/⁵F₄, ⁵S₂ → ⁵I₈) and red (⁴F_9/2 → ⁴I_15/2/⁵F₅ → ⁵I₈) emissions were measured as a function of excitation power have been represented in Fig. 9. Following the UC mechanism, the visible luminescence emission intensity of the UC phosphor and the infrared excitation power obey the relation

I_UC ∝ (P_pump)ⁿ,

where n is the number of pump photons used to populated the UC emission state.^57,58 It is observed that all the slopes of the three fitted lines corresponding to 526, 547 and 678 nm emissions of NaCe(MoO₄)₂: 3%Er³⁺, 5%Yb³⁺ are 1.41, 1.24, and 1.35 from Fig. 9a. For the sample of NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺ prepared with the assistance of 0.1 g EDTA, the values of n are determined to be 1.51, 1.47, and 1.35 (Fig. 9b). The slopes of the two fitted lines corresponding to 541 and 659 nm emissions of NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ are 0.997 and 1.35 (Fig. 9c). For the sample of NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ prepared with the assistance of 0.1 g EDTA, the values of n are determined to be 1.39 and 1.78 (Fig. 9d). The UC emission intensity decreases with the further increase in pump power due to saturation effect and the thermal equilibrium involved between two close levels. The density of phonons increases, which leads to the increase of nonradiative relaxation and luminescence intensity quenching because of the increment in pump power.⁵⁹ As is well known, for any UC mechanism, the value of n presents the number of near-infrared (NIR) excitation photons absorbed per visible photon emitted. For the two-photon processes, n should equal or be close to 2. However, in our experiment, the values of n are much smaller than the photon number for the transitions. This is mainly attributed to the competition between linear decay and UC processes for the depletion of the intermediate excited states, which is theoretically described by Pollnau et al.⁶⁰ It can be deduced that when the particle size decreases from bulk (obtained in the EDTA-absence) to small (obtained in the presence of 0.1 g EDTA), the value of n increases, which is attributed to the suppression of the saturation effect. The following two factors contribute to increasing the NR processes, which will increase the linear decay process: (1) as the particle size decreases, the NR processes will increase.⁶¹ (2) The introduction of EDTA ensures enhanced HESET from Yb³⁺-MoO₄²⁻–dimer to Er³⁺/Ho³⁺ and thus increasing the NR processes.

Fig. 9 Plot (log–log) of emission intensity versus excitation power in different samples. (a) NaCe(MoO₄)₂: 3%Er³⁺, 5%Yb³⁺; (b) NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺ with the assistance of 0.1 g EDTA; (c) NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺; (d) NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ with the assistance of 0.1 g EDTA.

4. Conclusion

In conclusion, the NaCe(MoO₄)₂: Er³⁺/Ho³⁺, Yb³⁺ microcrystals have been successfully prepared by a facile one-step hydrothermal synthesis method at 180 °C for 12 h at pH = 9. The results indicate that the EDTA introduced into the reaction systems is found to be crucial in determining the morphology and UC photoluminescence properties. The UC fluorescent intensity depends on the dopant concentrations, and the optimal UC emission can be obtained in NaCe(MoO₄)₂: 3%Er³⁺, 5%Yb³⁺ (without EDTA), NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺ (with EDTA), NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ (without EDTA), NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺ (with EDTA), respectively. The luminous color of NaCe(MoO₄)₂: Er³⁺/Ho³⁺, Yb³⁺ changes from yellow to green when the EDTA is added. For the first time, we find that the bicolour tunable luminescence can be realized by the change of the lattice parameters, which can tune the band edge absorption of molybdate system. The two-photon excitation UC mechanism of the four samples are systematically proposed to explain the observed up-conversion luminescence. The introduction of 0.1 g EDTA into the reaction system can increase the green UC luminescence intensity and simultaneously suppress red UC luminescence intensity. The relatively high upconversion efficiency of 2.08% (NaCe(MoO₄)₂: 2%Er³⁺, 5%Yb³⁺) and 2.01% (NaCe(MoO₄)₂: 1%Ho³⁺, 10%Yb³⁺) are obtained with the assistance of 0.1 g EDTA at room temperature under 0.01 W excitation. All these results suggest that Er³⁺/Ho³⁺-Yb³⁺ co-doped NaCe(MoO₄)₂ phosphor materials could be explored 3D display technology, green light laser, laser medical treatment, biological field and so on.

Acknowledgements

This work was supported by the Science and Technology Development Planning Project of Jilin Province (20130522173JH), partially sponsored by China Postdoctoral Science Foundation, supported by National Found for Fostering Talents of Basic Science (No. J1103202) and by Outstanding Young Teacher Cultivation Plan in Jilin University.

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