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Abstract
An efficient soft chemistry method to modify the phase, shape and optical thermometry of NaGdF4:2%Er3+ nano-phosphors through doping Ca2+ ion is reported. With the introduction of Ca2+, the phase changes from the GdF3:2%Er3+ to NaGdF4:2%Er3+ was achieved, and the shapes of NaGdF4:2%Er3+ were modified from irregular particles to pure hexagonal NaGdF4 microtubes. These modifications derive from the charge redistribution on the nucleus surface through internal electron charge transport between Gd3+ in a lattice and co-doped Ca2+ ion. An obvious enhancement of the total fluorescence intensity was observed after doping the Ca2+ ion. Moreover, an interesting phenomenon was observed that the fluorescence intensity of the mixed GdF3:2%Er3+ and NaGdF4:2%Er3+ was not be quenched at the high temperature more than 473 K. A maximum relative sensitivity of 0.00213/K (416 K) was obtained at 20%Ca2+ doping. These results indicate that NaGdF4:Er3+/Ca2+ can be applied in optical temperature sensor.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Rare earth ions doped NaMF4 (M = Y, Gd) phosphors have been extensively applied in color displayers, solid-state lasers, bioimages, optical sensors, due to their features of rich energy level structures, multicolor emissions, long luminescent lifetimes and independence of hosts [1–10]. The hexagonal NaGdF4 is regarded as a perfect up-conversion (UC) host due to its low phonon energy, high luminescent intensity, high Ln3+ (lanthanum) solubility and high chemical stability [11–14]. In order to improve the fluorescent efficiency of rare earth ions doped NaGdF4 for applications, many methods were explored, for example, adjusting the doping concentration of doping ions, modifying the size of particles, and changing the annealing temperature and so on [15,16]. Wu’s group modified the crystal phase and size of NaYF4:Yb3+/Er3+ nanocrystals by doping Gd3+ ions [17]. Wang doped Ce3+ ions into NaGdF4:Yb3+/Ho3+ and achieved the conversion from green to red color [18]. By adding Li+ ions, Kong’s group greatly improved the crystallinity and enhanced UC luminescence of NaYF4:Yb3+, Tm3+ nanocrystals [19]. Xu’s group obtained the perfect temperature sensing properties at high temperature by co-doping Nd3+/Yb3+ into NaYF4 nanoparticles [20]. Marciniak’s group obtained perfect relative sensitivity value at low temperature by adjusting the size of NaYF4:Yb3+/Er3+ [21]. Above published works show that the introduction of the hetero valent impurity ions into NaMF4 effectively modified the crystal phase, size, luminescence intensity, and optical temperature sensitivity properties [22–24].
It was reported that the material composition, dimensions, morphology and phase also had a great effect on the spectrum and optical temperature behaviors of rare earth ions doped phosphors [25–27]. Tian’s group reported the temperature sensing behavior was dependent on the sizes of the Gd2O3:Yb3+/Er3+ phosphors, and the sensitivity value increased with the decreasing the particle size [28]. Alencar’s group studied the optical thermometry performance of Y2O3:Er3+ with three distinct sizes and observed that the phosphors with the smallest average size were the most sensitive [29]. Chakradhar’s group also observed that the Eu3+ doped oxysulphides with the smaller particle size had the higher luminescent intensity [30]. Above researches focused on modifying the optical thermometry through controlling the sizes of phosphors. It lacks the study on other conditions, such as phase transfer, shape and so on. In this work, the NaGdF4:2%Er3+ with a wide range of Ca2+ dopant concentrations were synthesized by a hydrothermal method. By introducing Ca2+, the phase changes from the GdF3:2%Er3+ to NaGdF4:2%Er3+ was achieved, the shapes were modified from irregular particles to pure hexagonal NaGdF4 microtubes, and the temperature sensing properties were also improved. An obvious green and red UC enhancement were observed with 25%Ca2+ doping under 980 nm laser excitation. Interestingly, it is found that the fluorescence intensity of the mixed GdF3:2%Er3+ and NaGdF4:2%Er3+ was not be quenched at the high temperature more than 473 K.
2. Experimental
The raw materials are hydrochloric acid (AR), CTAB (AR), NaF (AR), ethanol (AR), Ca(NO3)2 (AR), Gd2O3 (99.99%) and Er2O3 (99.99%). Er3+ doped NaGdF4 microtubes were prepared by a hydrothermal method. The Gd2O3 and Er2O3 were dissolved in hydrochloric acid, and then the solution was heated to evaporate the water completely. The obtained rare earth metal trichloride was dissolved in deionized water to prepare the stock solutions of GdCl3 and ErCl3 (0.2 mol/L). In a representative synthesis process, 0.292g CTAB was add to 28 mL of distilled water thorough stirring, then an aqueous solution of 3.920 mL GdCl3 (0.2 mol/L), 0.08mL ErCl3 (0.2 mol/L) was mixed with CTAB solution. It was then vigorously stirred by a magnetic stirrer at room temperature for 30 min, while NaF solution was slowly added in drops. The colloidal solution was transferred into a teflon vessel at 466.5 K for 24 h. The final products were collected, washed several times with ethanol, and purified by centrifugation. Samples were then dried in an oven for 6 h at 373 K. Similarly, the same method was used for Er3+ and Ca2+ co-doped NaGdF4 microtubes through adding the Ca(NO3)2 with different concentrations.
Structures of the samples were investigated by X-ray diffraction (XRD) using a X'TRA (Switzerland ARL) equipment provided with Cu tube with Kα radiation at 1.54056 Å. The size and shape of the samples were observed by a JSM-IT300 scanning electron microscope (SEM) (JEOL Ltd., Tokyo, Japan). Luminescence spectra were obtained by the Acton SpectraPro Sp-2300 Spectrophotometer with a photomultiplier tube equipped with 980 nm laser as the excitation sources. Different temperature spectra were obtained by using an INSTEC HCS302 Hot and Cold System.
3. Results and discussion
The XRD patterns of NaGdF4:2%Er3+, x%Ca2+ (x = 0, 5, 10, 12.5, 15, 20, 25, 30, 35, 40) synthesized with a wide range of Ca2+ concentrations from 0 to 40 mol% are shown in Fig. 1. When the calcium ion concentration is very low, XRD patterns show that the samples are pure GdF3 structure (JCPDS Card no. 49-1804). With the increase of Ca2+ concentration, the diffraction peaks could be indexed to mixture of the GdF3 and hexagonal NaGdF4 (JCPDS Card no. 27-0699). When the Ca2+ concentration is more than 30%, pure hexagonal NaGdF4 microtubes appear. The corresponding XRD patterns suggest that the Ca2+ are completely embedded into the crystal lattice of NaGdF4 by replacing the sites of Gd3+ ions, without finding obvious shifting of diffraction peaks due to the F- vacancies and ionic radius match between Gd3+ (R = 1.193 Å) and Ca2+ (R = 1.260 Å) [31].
The SEM images of NaGdF4:2%Er3+ co-doped with Ca2+ are shown in Fig. 2. When the Ca2+ concentration changes from 0 mol% to 5 mol%, nano-particles are obtained as shown in Figs. 2(a) and 2(b). With the increase of Ca2+ concentration, the mixture of GdF3 and NaGdF4 appear, as shown in Figs. 2(c)-2(g). When the Ca2+ concentration exceeds 25%, the samples are composed of pure hexagonal hollow microtubes with diameters of about 0.7 μm, as shown in Figs. 2(h)-2(j). It was found that the morphology have no change with the further increase of Ca2+ concentration, and the size have no obvious change, as shown in Figs. 2(h)-2(j). It means that the shape and size of samples can be modified by doping Ca2+ ion. With the increase of Ca2+ ion, the phase change from the GdF3:2%Er3+ to NaGdF4:2%Er3+ is achieved, and the shape of NaGdF4:2%Er3+ is modified from irregular particles to pure hexagonal NaGdF4 microtubes.
Fig. 2 SEM images of NaGdF4:2%Er3+ doped with (a) 0 mol%Ca2+, (b) 5 mol%Ca2+, (c) 10 mol%Ca2+, (d) 12.5 mol%Ca2+, (e) 15 mol%Ca2+, (f) 20 mol%Ca2+, (g) 25 mol%Ca2+, (h) 30 mol%Ca2+, (i) 35 mol%Ca2+, (j) 40 mol%Ca2+.
Base on the results stated above, a possible growth mechanism for the NaGdF4:2%Er3+ controlled by the Ca2+ doping is proposed in Fig. 3. The mechanism involves the dynamic growth of Ca2+ doped NaGdF4:2%Er3+ microtubes. First, the growth process of hollow structure could be explained that surfactant off the water phase is dependent on the interaction of the hydrophobic groups in the organic molecules of its surfactant molecules in which the molecules form ordered aggregates to form micelles [32]. The hydrophilic group surrounds the rod like micelles composed of CTAB [33]. Due to its positive adsorption, the F- in aqueous solution is first attracted to the micelle surface. Then, when Ca2+ ions are added into the system, they occupy the Gd3+ sites in NaGdF4:2%Er3+. For each doped Ca2+ ion, an F- vacancy is introduced into the surface of grain coupled with Ca2+ ion to reach charge balance during the dynamic growth. At this moment, a positive outward-facing transient electric dipole is formed on the nano-crystals surface. It should be emphasized that the NaGdF4:2%Er3+ microtube is electrically neutral at this stage. The transient electric dipoles on the microtube surface expedite the F- ions diffusion from the solution to the microtube driven by charge attraction. Afterwards, the Gd3+ and Na+ cations in the solution react with F- anions on the microtube surface to maintain electrical neutrality of microtube, which results in the growth of microtube [34]. Since CTAB is dissolved in organic solvent and cooled to room temperature at the end of the reaction, CTAB was removed after washing, resulting in a final hollow structure. With the increase of Ca2+ concentration, Gd3+ was totally reacted to form NaGdF4:2%Er3+, then whether or not to increase the Ca2+ concentration, the morphology and size of NaGdF4:2%Er3+ did not change.
Fig. 3 Possible formation mechanism of NaGdF4:2%Er3+, x%Ca2+(x = 0, 5, 10, 12.5, 15, 20, 25, 30, 35, 40).
Figure 4 shows the visible up-conversion emission spectra of the NaGdF4:2%Er3+ doped with different Ca2+ concentrations, which are measured under 980 nm laser excitation with the pump power of 638 mW/mm2. It is observed that there are three emission bands centered at 525, 543, and 657 nm, which are ascribed to 2H11/2→4I15/2 (525 nm), 4S3/2→4I15/2 (543 nm), 4F9/2→4I15/2 (657 nm) transitions, respectively. Meanwhile, an obvious enhancement of the emission bands is observed with the increase of Ca2+ concentration. The enhancement of green emission, red emission and total emission intensity reach the maximum at 25mol%Ca2+, and then decrease with further increasing the Ca2+ concentration to 40%, as shown in Fig. 4(b).
Fig. 4 (a) The spectra and CIE(X,Y) chromaticity coordinates diagram, (b) the peak intensity of NaGdF4: 2%Er3+, x%Ca2+ samples with x = 0, 5, 10, 12.5, 15, 20, 25, 30, 35, 40.
In order to investigate the effect of Ca2+ doping on the UC emission, we determined the number of photons dependent on the red and green luminescence intensity, which obey the relationship with the pumping power as follow [35]:
where I is the intensity of UC emission, P is the pumping power of infrared laser, and n is the pumping photons number demanded for emitting one UC photon. From the double logarithmic plots of the emission intensity as a function of the excitation power (Fig. 5), it shows that two infrared photons are needed to emit green and red luminescence regardless of Ca2+, indicating that the Ca2+ doping does not change the UC process. The enhancement of UC luminescence caused by Ca2+ doping is attributed to two factors, including lowering the local crystal field symmetry around Er3+ ions after Ca2+ doping and improving the morphology of the NaGdF4 microtubes. However, when Ca2+ doping concentration exceed 25%, the UC emission intensity decreases due to the formation of more F- vacancies [36].Fig. 5 Log-log of the UC emission intensity against laser excitation power for NaGdF4: 2%Er3+, x%Ca2+ samples with x = 0, 10, 20, 30.
To explore the quality of optical temperature behavior, the temperature dependent photoluminescence spectra are measured in the temperature range from 298 K to 573 K through using a 980 nm laser as the excitation source, as shown in Fig. 6. It can be observed that the photoluminescence spectra contain green, red, and infrared emission bands, which are assigned to the 2H11/2→4I15/2 (525 nm), 4S3/2→4I15/2 (543 nm), 4F9/2→4I15/2 (657 nm), and 4I9/2→4I15/2 (808 nm) transitions of Er3+ ion, respectively. An interesting phenomenon is found that at high temperature serious fluorescence quenching occurs when the samples are pure GdF3 and pure NaGdF4 respectively, as shown in Figs. 6(a) and 6(d). However, when the sample is a mixture of GdF3 and NaGdF4, high temperature fluorescence quenching is inhibited. The obvious luminescence enhancement is observed when the Ca2+ concentration are 10 mol% and 20 mol% as shown in Figs. 6(b) and 6(c).
It is necessary to study the temperature dependent fluorescence intensity ratio (R) in the performance of optical thermometry. The relation between R and T is expressed as [37]:
where a is constant. The b is a correction term for the comprehensive population of thermally coupled energy levels induced by the thermal population and the nonradiative relaxation. The sensitivity dependent quality of optical thermometry is defined as [38]:where a and b are the constants from Eq. (2).Figure 7(a) shows temperature dependent emission intensity ratios of adjacent emission bands (525 nm and 543 nm) of NaGdF4:2%Er3+ co-doped with various Ca2+. The experimental points can match well with a line. The slope values of fitting lines are dependent on the Ca2+ concentration. By co-doping with Ca2+, the local crystal field in the host matrix around the Er3+ luminescent center can be modulated. As a result, the ratios of 525 nm/543 nm emissions can be changed. The luminescent thermometer originated from the thermally coupled 2H11/2 and 4S3/2 levels can be also modified [39,40]. Figure 7(b) shows all the sensitivity lines with different Ca2+ concentration. The highest sensitivity is achieved when the Ca2+ concentration is 20%. As can be seen from Fig. 2, it corresponds to a mixed phase of GdF3 and NaGdF4. All the sensitivity lines increase and then decrease with the temperature increase. The maximum value at (455 K, 2.0 × 10−3/K) is achieved when the Ca2+ concentration is 20 mol%. The NaGdF4:Er3+/Ca2+ phosphors show excellent sensitivity property in temperature ranging from room temperature to high temperature.
Fig. 7 Temperature dependent (a) arrhenius plots of emission intensity ratios of 524 nm/543 nm, (b) sensitivity of NaGdF4:2%Er3+,x%Ca2+(x = 0, 5, 10, 12.5, 15, 20, 25, 30, 35, 40).
It was reported that the population of excited states were affected by excitation powers [41]. Therefore, it is necessary to study the influence of excitation powers on the sensitivity values. Figure 8(a) shows the excitation power dependent intensity ratios of 525 nm and 543 nm emission bands of NaGdF4:2%Er3+,20%Ca2+. The slope values of fitting lines are dependent on the excitation powers. Figure 8(b) shows the sensitivity lines under 980 nm excitation with different excitation powers at 232.8 mW/mm2, 366 mW/mm2, 638 mW/mm2, 907.2 mW/mm2, 1166 mW/mm2, 1507.4 mW/mm2, respectively. The highest sensitivity 2.13 × 10−3/K (416 K) is achieved when the excitation power is 1507.4 mW/mm2. It is clearly that the sensitivity value increases with the increase of pump power density. At the higher excitation density, the sample temperature increases slowly. The phonon-assisted anti-Stokes excitation from 4S3/2 to 2H11/2 becomes obvious. The 2H11/2 population induced by phonon-assisted anti-Stokes excitation increases greatly [42,43]. Therefore, the sensitivity is enhanced when the sample temperature increases at the higher excitation density.
Fig. 8 (a) Temperature dependent arrhenius plots of intensity ratios of 525 nm/543 nm emission bands, (b) sensitivity of NaGdF4:2%Er3+, 20%Ca2+ at different excitation powers.
4. Conclusion
In summary, NaGdF4:2%Er3+ hexagonal microtubes were synthesized through the hydrothermal method. The Ca2+ doped concentration has a significant influence on the phase-transfer of the NaGdF4: Er3+ from the mixed phase to the pure hexagonal phase. The Ca2+ in NaGdF4 host by substituting Gd3+ ions induces the formation of F- vacancies in the solution to modify the shape and size. As a result, an obvious enhancement of visible UC was observed via 25%Ca2+ doping. In the mixed phase, the fluorescence intensity increase with the temperature increase at high temperature. A maximum relative sensitivity of 2.13 × 10−3/K (416 K) was obtained at 20%Ca2+ doping under 1507.4 mW/mm2 excitation. This work presents a novel strategy to improve the UC and optical thermometry performances of phosphors.
Funding
National Natural Science Foundation of China (NSFC) (11404171), Jiangsu Natural Science Foundation for Excellent Young Scholar (BK20170101), Scientific Research Foundation of Nanjing University of Posts and Telecommunications (NY215174, NY217037, NY218015), Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX17_0230), Opening Project of State Key Laboratory of Green Building Materials.
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