1. Introduction

Over the past several decade, lanthanide doped luminescence materials have attracted enormous attention because of their potential applications in many fields such as fluorescent probes, solar cell, photocatalysis, communications, solid laser devices [1–8]. Due to the unique electron configuration of lanthanide ions, they have abundant energy levels structure (known as the Dieke diagram) in the near-infrared, visible and ultraviolet spectral range [9–13]. This provides a huge space for the photon management. Common photon management process mainly includes UC (transforms two or more low-energy photons into one higher-energy photon) and DC (transforms one high-energy photon into one or more low-energy photons) [14–18]. These emissions from the trivalent lanthanide dopants are mainly due to electric and magnetic dipole optical transitions based on their unique intra 4f transitions, which are shielded by the outer 5s and 5p orbitals and, consequently, lead to sharp emissions and narrow bands [19,20].

Among these luminescent host materials, Gadolinium oxide (Gd₂O₃) is not only a kind of promising luminescent host material because of its good chemical durability, thermal stability, and low phonon energy (phonon cutoff = 600 cm⁻¹) and the ability of being easily doped with rare earth ions, but also an excellent paramagnetic material [21–24]. However, as a kind of luminescence material, their luminous efficiency is still one of the most important properties. Unfortunately, the best UC efficiency does not exceed 10% [9]. For DC luminescent materials, although they have high quantum efficiency, their absorption is restricted by some aspects. Therefore, their luminous efficiency is still too low to apply in many fields and has much space for improvement. For many years, a lot of researchers are devoted to improve the luminescence efficiency of the lanthanide doped luminescence materials, and many methods have been affirmed to be effective to enhance the luminescence emission intensity of them. Wherein, ion doping is considered to be an extremely effective and facile method to improve the luminescence efficiency. For example, Vineet Kumarr’s group reported a UC emission enhancement of La₂O₃: Yb/Tm nanoparticles by co-doping Zn²⁺-Mg²⁺ inert ions [25]. Song’s group and Chen’s group respectively reported the UC emission enhancement of Gd₂O₃: Yb/Ho (Tm) by doping Li⁺ ion [26,27]. I. Kaminska’s group reported a UC emission enhancement of Gd₂O₃: Yb/Er nanoparticles by introducing Zn²⁺ ions [28]. Hye-Kyung Moon’s group reported a DC emission enhancement of Gd₂O₃: Eu by doping Li⁺ ion [29]. In addition, many other inert ions such as K⁺, Ca²⁺, Sc³⁺ ions also have been affirmed to be effective to improve the luminescence efficiency of lanthanide doped matrix [30–34].

Unfortunately, most people are only concerned about the effect of single inert ion doping on the luminescence properties and few groups report dual inert ions co-doping method to further enhance the luminescent properties of Gd₂O₃ materials [35]. Herein, we prepared a series of Zn²⁺, Li⁺ inert ions single/co- doped Gd₂O₃: Ln³⁺ (Ln = Yb/Er, Eu) NCs by a simple precipitate method which followed by annealing at a high temperature. Compared with single doped Zn²⁺ or Li⁺ ion, co-doping Li⁺ and Zn²⁺ ions into Gd₂O₃: Yb/Er and Gd₂O₃: Eu NCs, they show better luminescence properties. Zn²⁺-Li⁺ co-doping is a better strategy to enhancing the luminous efficiency of Gd₂O₃: Yb/Er and Gd₂O₃: Eu NCs than Zn²⁺ or Li⁺ single doping strategy.

2. Experimental

2.1 Materials

All chemicals were of analytical grade and used without further purification. Gd(NO₃)₃·6H₂O (99.99%), Yb(NO₃)₃·6H₂O (99.99%), Er(NO₃)₃·6H₂O (99.99%) and Eu(NO₃)₃·6H₂O (99.99%) were supplied by Yutai Qingda Chemical Technology Co., Ltd. China. Zn(NO₃)₂·6H₂O (AR), LiNO₃ (AR) and Urea ((NH₂)₂CO, AR) were supplied by Beijing Fine Chemical Company.

2.2 Synthesis of the Gd₂O₃:Yb/Er/Zn/Li(10/2/2/1.5 mol%)

In a typical synthesis, 0.845 mmol Gd(NO₃)₃·6H₂O, 0.1 mmol Yb(NO₃)₃·6H₂O, 0.02 mmol Er(NO₃)₃·6H₂O, 0.02 mmol Zn(NO₃)₂·6H₂O and 0.015 mmol LiNO₃ were added into 50 mL deionized water to form a clear solution in a reaction bottle under vigorously stirring. Then 20 ml aqueous solution of 50 mmol urea was added into the stirring solution under magnetic stirring. Subsequently, the above solution was heated at 85 °C for 4 h in order to decompose the urea. The resulting suspension was separated by centrifugation, washed several times with deionized water and dried at 80 °C. Subsequently, the as-prepared precursor samples were annealed at 1100 °C for 4 h in air to obtain the Gd₂O₃: Yb/Er/Zn/Li (10/2/2/1.5 mol%) NCs. The other samples can be obtained by simply replacing the corresponding components and contents.

2.3 Characterization

The crystal structure and phase purity were analyzed by a Rigaku RU-200b X-ray powder diffractometer (XRD) using nickel-filtered Cu-Kα radiation (λ = 1.5406 Å). The size and morphology of the samples were investigated by transmission electron microscopy (TEM, Hitachi H-600). UC luminescence spectra and DC luminescence spectra were recorded using a Hitachi F-4500 fluorescence spectrophotometer with a power-controllable 980 nm CW diode laser (Maximum power 2W) and a xenon lamp as the excitation light sources.

3. Results and discussion

A series of Zn²⁺ ion, Li⁺ ion single doped or Zn²⁺-Li⁺ ions co-doped Gd₂O₃: Yb/Er NCs were synthesized by a simple precipitation method which followed by annealing at a high temperature. The TEM images of Gd₂O₃ precursor were shown in Fig. 1 and the XRD patterns of these annealed samples were shown in Fig. 2. As shown in Fig. 1, the Gd₂O₃ NCs doped with Zn²⁺ ion, Li⁺ ion or Zn²⁺-Li⁺ ion pairs are all uniform size, regular spherical particles. The size and morphology of Gd₂O₃ NCs remain the same after Zn²⁺ and Li⁺ ions doping, their mean size retain at 202 nm. As shown in Fig. 2, all the diffraction peaks of these samples can be well indexed to Gd₂O₃ (JCPDS No. 43-1014). No other impurity peak can be detected from these XRD patterns, implying that single/codoping Zn²⁺ and Li⁺ inert ions didn’t change the crystal phase of Gd₂O₃ samples. While, when zooming in on the pattern, the positions of the diffraction peaks are found to shift slightly with the increasing of single/codoping Zn²⁺ and Li⁺ inert ion concentrations. As shown in right part of Fig. 2(a) and 2(b), the diffraction peaks at 28.55° shift towards a larger angle as Zn²⁺/Li⁺ ion single doping concentrations increasing (Zn²⁺: 0 mol% – 4 mol%, Li⁺: 0 mol% – 6 mol%).

 

Fig. 1 TEM images of (a) Gd₂O₃: Yb/Er (10/2 mol%) NCs; (b) Gd₂O₃: Yb/Er/Zn (10/2/10 mol%) NCs; (c) Gd₂O₃: Yb/Er/Li (10/2/10 mol%) NCs; (d) Gd₂O₃: Yb/Er/Zn/Li (10/2/2.5/10 mol%) NCs; (e) Gd₂O₃: Yb/Er/Zn/Li (10/2/10/2.5 mol%) NCs; (f) Crystal structure of Gd₂O₃.

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Fig. 2 XRD patterns and main diffraction peaks of (a) Gd₂O₃: Yb/Er/Zn (10/2/x mol%, x = 0, 2, 4, 6, 8, 10) NCs; (b) Gd₂O₃: Yb/Er/Li (10/2/x mol%, x = 0, 2, 4, 6, 8, 10) NCs; (c) Gd₂O₃: Yb/Er/Zn/Li (10/2/4/x mol%, x = 0, 0.5, 1.0, 1.5, 2.0, 2.5) NCs; (d) Gd₂O₃: Yb/Er/Li/Zn (10/2/4/x mol%, x = 0, 0.5, 1.0, 1.5, 2.0, 2.5) NCs.

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Subsequently as the doping concentration continues increasing, this diffraction peaks shift towards to a smaller angle. It is known that a larger lattice constant is related to a smaller diffraction angle, and vice versa. Some explication is demonstrated as follow. When a small amount of Zn²⁺/Li⁺ ions was doped into Gd₂O₃ NCs, Zn²⁺/Li⁺ ion mainly occupies the Gd³⁺ crystal lattice site. Because the radius of the Zn²⁺/Li⁺ ion (Zn²⁺: 0.088 nm, Li⁺: 0.09 nm, all for six coordination) is smaller than the radius of Gd³⁺ ion (Gd³⁺: 0.1078 nm), substituting the Gd³⁺ ion with this smaller Zn²⁺/Li⁺ ion can induce the shrinking of the host lattice. But with higher doping concentrations, Zn²⁺/Li⁺ ions begin to take interstitial sites, and this can lead to the host lattice expanding. Besides, Li⁺ ion has lower valence than Zn²⁺ ion, when it occupies the Gd³⁺ crystal lattice site, more oxygen vacancies were produced, therefore Gd₂O₃:Yb/Er/Li (10/2/4 mol%) NCs has more oxygen vacancies than Gd₂O₃:Yb/Er/Zn (10/2/4 mol%) NCs. Similarly, when Zn²⁺ and Li⁺ ions were doped in simultaneously, Zn²⁺ ions fill the oxygen vacancies more effectively. Therefore, when the Zn²⁺ ion was codoped into Gd₂O₃:Yb/Er/Li (10/2/4 mol%) NCs, some oxygen vacancies were filled by Zn²⁺ ions, the host lattice expanded and the diffraction peaks shift towards to a smaller angle, as shown in Fig. 2(c) and 2(d) [36,37]. The schematic illustration of lattice change caused by Zn²⁺-Li⁺ dual inert ion doping is shown in Fig. 3.

 

Fig. 3 Schematic illustration of lattice change induced by Zn²⁺ and Li⁺ inert ions single/co-doping.

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In order to find the optimum doping concentration, we doped different concentrations of Zn²⁺ or Li⁺ ion into Gd₂O₃: Yb/Er (10/2mol%) NCs and their UC luminescence spectra were measured under the same conditions. As shown in Fig. 4, when these samples were excited by 980 nm laser, we can observed two obvious emission peaks, 561 nm and 658 nm, they are attributed to ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 respectively. With the increase of the inert ion doping concentration, the UC luminescence intensities increase first, then decrease. Among them, the best single doping concentration of Zn²⁺ ion is 4 mol%. Compared with the Zn²⁺-free sample, it shows 3 times enhancement in the UC luminescence intensity. The optimum single doping concentration of Li⁺ ion is also 4 mol%, the luminescence intensity exhibits 2 times enhancement.

 

Fig. 4 (a) UC luminescence spectra of Gd₂O₃: Yb/Er/Zn (10/2/x mol%, x = 0, 2, 4, 6, 8, 10) NCs excited by 30 W/cm² 980 nm laser; (b) UC luminescence spectra of Gd₂O₃: Yb/Er/Li (10/2/x mol%, x = 0, 2, 4, 6, 8, 10) NCs excited by 30 W/cm² 980 nm laser; (c) The ratios of UC luminescence enhancement by doping different Zn²⁺ ion concentration in Gd₂O₃: Yb/Er (10/2 mol%) NCs; (d) The ratios of UC luminescence enhancement by doping different Li⁺ ion concentration in Gd₂O₃: Yb/Er (10/2 mol%) NCs.

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Further we co-doped Zn²⁺ and Li⁺ ions in the Gd₂O₃: Yb/Er (10/2 mol%) NCs to study the effect of dual inert ion doping on UC luminescence performance. On the basis of best single doping concentration of Zn²⁺ or Li⁺ ion, we adjust concentration of another Li⁺ or Zn²⁺ ion in Gd₂O₃: Yb/Er (10/2 mol%) NCs and measured their UC luminescence spectra under the same condition for further comparison. The result spectra were shown in Fig. 5. When Li⁺ and Zn²⁺ ions were codoped into the Gd₂O₃: Yb/Er (10/2 mol%) NCs, the UC emission intensities show similar trend, increase first, then decrease. For the Gd₂O₃: Yb/Er/Li (10/2/4 mol%) NCs, increasing doping concentration of Zn²⁺ ion will make its UC luminescence intensity further improved. When the doping concentration of Zn²⁺ ion is 1.5 mol%, the luminescence enhancement ratio reaches the maximum, 12 times than that of Gd₂O₃: Yb/Er/Li (10/2/4 mol%) NCs. As for the Gd₂O₃: Yb/Er/Zn (10/2/4 mol%) NCs, continue increasing doping concentration of Li⁺ ion also can get a further luminescence enhancement. When the concentration of Li⁺ ion is 2 mol%, the UC luminescence intensity of the sample will get a slightly enhancement of 1.6 times than that of Gd₂O₃: Yb/Er/Zn (10/2/4 mol%) NCs.

 

Fig. 5 (a) UC luminescence spectra of Gd₂O₃: Yb/Er/Zn/Li (10/2/4/x mol%, x = 0, 0.5, 1, 1.5, 2, 2.5) NCs excited by 30 W/cm² 980 nm laser; (b) UC luminescence spectra of Gd₂O₃: Yb/Er/Li/Zn (10/2/4/x mol%, x = 0, 0.5, 1, 1.5, 2, 2.5) NCs excited by 30 W/cm² 980 nm laser; (c) The ratios of UC luminescence enhancement by doping different concentration of Zn²⁺ ion in Gd₂O₃: Yb/Er/Zn (10/2/4 mol%) NCs; (d) The ratios of UC luminescence enhancement by doping different concentration of Li⁺ ion in Gd₂O₃: Yb/Er/Li (10/2/4 mol%) NCs.

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Besides we studied the influence of Zn²⁺-Li⁺ co-doping on DC luminescence of Gd₂O₃: 5 mol% Eu NCs. Zn²⁺ or Li⁺ ion with different concentration was single doped or co-doped into Gd₂O₃: Eu NCs. The DC spectra of Gd₂O₃: Eu NCs excited by 269 nm were shown in Fig. 6 and Fig. 7 respectively, with single doped Zn²⁺ or Li⁺ ion and co-doped Zn²⁺-Li⁺ ions pair. As shown in Fig. 6(e), the excitation spectrum of Eu³⁺ ion monitored at 609 nm emission is composed of a strong broad band centered at 269 nm, which originates from the excitation of the O^2--Eu³⁺ charge transfer band (CTB). When the Gd₂O₃: Eu NCs were excited by 269 nm laser, we can observe three obvious emission peaks: 590 nm, 609 nm and 627 nm, they are attributed to ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₃ transitions respectively. Among them, 609 nm emission peak is the strongest. When Zn²⁺ ion or Li⁺ ion was doped into Gd₂O₃: Eu NCs, the intensities of DC luminescence show similar trend, increase first and then decrease. When the doping concentration of Zn²⁺ ion is 2 mol%, the DC luminescence intensity of the sample will get a maximum enhancement of 2.5 times than that of Zn²⁺-free sample. For the Li⁺ ion single doping, the optimum doping concentration is 4 mol%. Compared with the Li⁺-free Gd₂O₃: Eu NCs, the DC intensity of the samples doped with 4 mol% Li⁺ ion shows a 2.3 times enhancement.

 

Fig. 6 (a) DC luminescence spectra of Gd₂O₃: Eu/Zn (5/x mol%, x = 0, 2, 4, 6, 8, 10) NCs excited by 269 nm laser; (b) DC luminescence spectra of Gd₂O₃: Eu/Li (5/x mol%, x = 0, 2, 4, 6, 8, 10) NCs excited by 269 nm laser; (c) The ratios of DC luminescence enhancement by doping different Zn²⁺ ion concentration in Gd₂O₃: Eu (10/2 mol%) NCs; (d) The ratios of DC luminescence enhancement by doping different Li⁺ ion concentration in Gd₂O₃: Eu (10/2 mol%) NCs; (e) DC luminescence excitation spectrum of Eu³⁺ ion monitored at 609 nm.

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Fig. 7 (a) DC luminescence spectra of Gd₂O₃: Eu/Zn/Li (5/2/x mol%, x = 0, 0.5, 1, 1.5, 2, 2.5) NCs excited by 269 nm; (b) DC luminescence spectra of Gd₂O₃: Eu/Li/Zn (5/4/x mol%, x = 0, 0.5, 1, 1.5, 2, 2.5) NCs excited by 269 nm; (c) The ratios of DC luminescence enhancement by doping different Li⁺ ion concentrations in Gd₂O₃: Eu/Zn (5/2 mol%) NCs; (d) The ratios of DC luminescence enhancement by doping different Zn²⁺ ion concentrations in Gd₂O₃: Eu/Li (5/4 mol%) NCs.

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Then on the basis of optimum single doping concentration of Zn²⁺ or Li⁺ ion, we codoped another ion (Li⁺ or Zn²⁺) into the Gd₂O₃ NCs. When the Zn²⁺-Li⁺ ions were co-doped into the NCs, the DC luminescence intensity of Gd₂O₃: Eu NCs shows a further enhancement than that of single inert ion doping NCs. Among them, compared with 2 mol% Zn²⁺ or 4 mol% Li⁺ single doped Gd₂O₃: Eu NCs, the DC luminescence intensities of 2 mol% Zn/1.5 mol% Li and 4 mol% Li/1.5 mol% Zn co-doped Gd₂O₃: Eu NCs show further 2.2 and 1.4 times enhancement, respectively.

This luminescence enhancement phenomenon can be explained by a classic theory. When heterogeneous valence inert ion, Zn²⁺ or Li⁺ ion was doped into Gd₂O₃ NCs, it substitutes the Gd³⁺ crystal lattice site or takes interstitial sites, the unit cell of Gd₂O₃ NCs shrinks due to the smaller radius of Zn²⁺/Li⁺ than Gd³⁺ ion. On the other hand, oxygen vacancies are generated simultaneously in order to maintain the charge balance. These factors could lead to a change of the electron distribution density. The environment change around the rare earth ion could decrease the crystal symmetry around Er³⁺/Eu³⁺ ions, and lead to hypersensitive electron transition, as well as increasing the luminescence intensity of Er³⁺/Eu³⁺ ions. During this process, compared to the single doping Zn²⁺ or Li⁺ ion, dual ions doping can complement each other and destroy the symmetry of the crystal field around the luminescent ions more effectively. As a result, the UC and DC luminescence intensity can be further improved on the basis of single ion doping strategy [36–39].

The possible UC luminescence processes of the Gd₂O₃: Yb/Er NCs excited by 980 nm laser is shown in Fig. 8(a) and the possible DC processes of the Gd₂O₃: Eu NCs excited by 269 nm laser is shown in Fig. 8(b). As pictured in Fig. 8(a), when the Gd₂O₃: Yb/Er NCs were excited by 980 nm laser, compared with Er³⁺ ion, Yb³⁺ ion has the larger doping concentration and a much larger absorption cross section around 980 nm, therefore, the main pathway to populate the excited states of Er³⁺ ion is the energy transfer (ET) from Yb³⁺ to Er³⁺ ion, two ET processes from Yb³⁺ to Er³⁺ ion excite the ⁴I_15/2 level to ⁴I_11/2 and ⁴F_7/2 levels. Subsequent nonradiative relaxations of ⁴F_7/2 → ²H_11/2, ⁴F_7/2 → ⁴S_3/2 and ⁴S_3/2 → ⁴F_9/2 populate the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels respectively. And then back to the ground level ⁴I_15/2 and emit the green light (540 nm, ⁴S_3/2, ²H_11/2 → ⁴I_15/2,) and red light (660 nm, ⁴F_9/2 → ⁴I_15/2). As shown in Fig. 8(b), when the Gd₂O₃: Eu NCs was excited by 269 nm laser, the electrons were populated from ground level to Eu-O charge transfer band (CTB), subsequent a series of nonradiative relaxations to ⁵D₀ level, then back to ⁷F₁, ⁷F₂ or ⁷F₃ level, and emit 590 nm, 609 nm and 627 nm light.

 

Fig. 8 (a) Energy-level diagrams of Yb³⁺ and Er³⁺ ions, and possible UC processes excited by 980 nm laser; (b) Energy-level diagram of Eu³⁺ ion, and possible DC processes excited by 269 nm.

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4. Conclusions

In conclusion, Zn²⁺ and Li⁺ inert ions single/co-doping Gd₂O₃: Ln³⁺ (Ln = Yb/Er, Eu) NCs were synthesized by a simple precipitation method which followed by annealing at a high temperature. The Gd₂O₃: Ln³⁺ (Ln = Yb/Er, Eu) NCs with co-doped Zn²⁺ and Li⁺ ions will has a better performance both on the UC and DC luminescence than Gd₂O₃: Ln³⁺ (Ln = Yb/Er, Eu) NCs with single doped Zn²⁺ or Li⁺ ion. As well as the intensities of luminescence emission at different wavelengths all show obviously further enhancement. Zn²⁺-Li⁺ dual inert ion doping is a better strategy to enhancing the fluorescence emission efficiency of Ln³⁺ ions in Gd₂O₃ NCs, it is also applicable to other host materials.