Ultrasound and Microwave Coassisted Synthesis and Luminescent Properties of (Ca_1−𝑥,Ln_𝑥)MoO₄:Eu³⁺ (Ln = La, Gd; 0<𝑥<0.5) Phosphors

Abstract

In order to control reaction temperature and reduce processing time, a new method of ultrasound irradiation with microwave heating was used to synthesize (Ca_1−𝑥,Ln_𝑥)MoO₄:Eu³⁺ (Ln = La, Gd; 0<𝑥<0.5) phosphors at only 80°C in 30 minutes. Their crystal structures and morphologies which have been verified by X-ray diffraction (XRD) and scanning electron microscopy (SEM) exhibited gradual changes due to the insertion of lanthanide ions (La or Gd) in calcium molybdates. Codoping of lanthanum ions (𝑥=0.2) would enhance the emission intensities that were supported by fluorescent spectrophotometry (FL).

1. Introduction

Currently, rare earth molybdates have exhibited potential and widespread applications in the fields of luminescent materials with superior physical and chemical properties which have been attracting much attention [1–4]. Numerous methods of the synthesis processes have been developed and improved, such as the traditional solid-state method [5–7], sol-gel process [8, 9], or hydrothermal method [10–12]. However, most of the above treatments are complicated and have several technical problems, like relatively long reaction times, high temperatures, and massive energy supplies. Hence, the exploration of simple and convenient synthesis routes for energy saving and has been continued.

A promising method by combining the ultrasound and microwave irradiation together has been paid attention [13]. Ultrasonic forces during the precipitation reaction play a significant role on increasing chemical reactivity and improving chemical yields through the acoustic cavitations generated from the expansion of the insulation foams. The acoustic cavitations can produce the extremely high pressure and temperature in the reaction, thus providing a unique environment for the formation of chemical compounds with novel structures [14–17]. Microwave heating method is different from normal heating mode. It includes the following characteristics: heating uniformly and rapidly, low heat loss, and easy operation. Moreover, microwave heating technology not only can improve effectively the reaction conversion and selectivity, but also is one of the means to realize the green technology [18–20]. Therefore, we can combine the above two kinds of technology together to improve the synthetic scheme of the rare-earth molybdate compounds. Herein, in this paper, we synthesized a group of (Ca_1−𝑥, Ln_𝑥)MoO₄:Eu³⁺ (Ln=La, Gd; 0<𝑥<0.5) phosphors via the supersonic and microwave coassistance method without any surfactant or template. Their microstructures and photoluminescence properties were carefully studied.

2. Experimental Section

2.1. Materials and Methods

La₂O₃ (99.9%), Gd₂O₃ (99.9%), and Eu₂O₃ (99.9%) were purchased from Alfa Aesar company. Ca(NO₃)₂·4H₂O (99%), (NH₄)₆Mo₇O₂₄·4H₂O (99%), and ammonia water were provided by Aldrich company. Lanthanide nitrates were synthesized by dissolving the corresponding oxides in nitric acid solution. The supersonic assisted microwave reactor was manufactured by Nanjing Xianou Technology Company, China (XO-SM50). All the detailed experimental conditions were simply described here. Microwave power was set as 600 W, and reaction temperature was maintained at 80°C. Supersonic power remained to be 300 W with the reverse duty cycle of 2 s. All the operation time was set as 30 minutes. The picture of the machine in our lab was given in Scheme 1. The X-ray powder diffraction was measured on Bruker D8 diffractometer with Cu K𝛼 radiation (𝜆=0.1541nm) in the range of 2𝜃=20--80^∘. Scanning electronic microscope (SEM) was obtained with JSM-6360LV. Fluorescence spectra were obtained on an Edinburgh FLS920 spectrometer.

Scheme 1 

Photo of the supersonic assisted microwave reactor.

2.2. Synthesis of (Ca_1−𝑥, Ln_𝑥)MoO₄:0.05Eu³⁺ (Ln = La, Gd)

For the synthesis of (Ca_0.8, La_0.2)MoO₄:Eu³⁺ (5%) phosphors, La(NO₃)₃ (0.2 mmol), Ca(NO₃)₃  (0.8 mmol), and Eu(NO₃)₃ (0.05 mmol) were dissolved in water. Then the appropriate amount of ammonium heptamolybdate tetrahydrate, (NH₄)₆Mo₇O₂₄·4H₂O, was dissolved in deionized water and added in the above solution. After the pH value of the above mixed solution was adjusted to about 7, the mixture was transferred to a three-neck flask which adapted to the reactor. Microwave power was set as 600 W, and reaction temperature was maintained at 80°C. Supersonic power remained to be 300 W with the reverse duty cycle of 2 s. All the operation time was set as 30 minutes. Finally, the precipitates were retrieved by centrifugation, washed by deionized water several times, and then dried at 60°C in vacuum for 24 h. Meanwhile, a series of (Ca_1−𝑥, Ln_𝑥)MoO₄:0.05Eu³⁺ (Ln=La, Gd; 0<𝑥<0.5) phosphors was prepared by a similar procedure.

3. Results and Discussion

3.1. Structural Analysis

Because the crystal structure, morphologies, and sizes of samples have an enormous impact on the photophysical properties, the crystal structure, morphologies, and sizes of (Ca_1−𝑥, Ln_𝑥)MoO₄:Eu³⁺ (5 mol%) (Ln=La, Gd; 0<𝑥<0.5) (𝑥=0, 0.1, 0.2, 0.3, 0.4, and 0.5) phosphors obtained by ultrasonic and microwave assisted method were investigated by X-ray diffraction (XRD) and scanning electronic microscope (SEM), respectively. Figure 1 is the crystalline phases of (Ca_1−𝑥, La_𝑥)MoO₄:Eu³⁺ (5 mol%) (𝑥=0, 0.1, 0.2, 0.3, 0.4, and 0.5) samples. It can be seen that the Bragg reflection peaks of CaMoO₄:Eu³⁺ (5 mol%) correspond to the structure similar to the Joint Committee on Powder Diffraction Standards (JCPDS) file 29-0351 of CaMoO₄ with a tetragonal phase and space group I41/a (𝑎=𝑏=5.226Å,𝑐=11.430Å), which demonstrates that the doped europium ion cannot change the crystal structure. The patterns of XRD indicate that the structure of (Ca_1−𝑥, La_𝑥)MoO₄:Eu³⁺ (5 mol%) matches well with calcium molybdates when x was less than 0.2. The further increasing amounts of lanthanum still maintain the primary crystal structure, and the new peak at 23° grows very fast, suggesting that replacement of calcium ions by agglomeration of lanthanum ions may trigger the lattice distortion and partial phase changes. This stepwise difference also induced microstructure transformation and luminescence changes as indicated by the following measurements. It is worthy to mention that such a short reaction time (30 min) will be very efficient and superior to other synthetic processes such as hydrothermal method that needs more than 10 hours. A similar investigation was performed for (Ca_1−𝑥, Gd_𝑥)MoO₄:0.05Eu³⁺ with 𝑥=0, 0.1, 0.2, 0.3, 0.4, and 0.5. Figure 2 is the XRD patterns of those phosphors. As the gadolinium contents continually increase up to 0.5, the structures of samples have some changes and the relative intensity of diffraction peaks becomes weaker. Compared with (Ca_1−𝑥, La_𝑥)MoO₄:Eu³⁺ (5 mol%), the changes triggered by the insertion of gadolinium were more obvious because the ionic radius of Gd³⁺ (93.8 pm) was much smaller than lanthanum (106.1 pm) when they replaced large calcium ions (114 pm).

Figure 1 

XRD patterns for (Ca1−𝑥, La𝑥)MoO4:Eu3+ (5 mol%) (𝑥=0, 0.1, 0.2, 0.3, 0.4, and 0.5) phosphors.

Figure 2 

XRD patterns for (Ca1−𝑥, Gd𝑥)MoO4:0.05Eu3+ (𝑥=0, 0.1, 0.2, 0.3, 0.4, and 0.5) phosphors.

In order to investigate the influence of La³⁺ and Gd³⁺ content on the morphologies and sizes of samples prepared by ultrasound and microwave coassisted method, the SEM images are given in Figure 3. Figures 3(a)–3(f) are the SEM images of (Ca_1−𝑥, Ln_𝑥)MoO₄:Eu³⁺ (5 mol%) (Ln=La, Gd) phosphors with 𝑥=0, 0.2, and 0.5, respectively. Figures 3(a) and 3(b) revealed that the shape formed shows a relatively orderly alignment of rod-like structures with the length at about 1 μm. A sheet-like structure was observed next in Figure 3(c) when lanthanum ions were doped into the materials. The shape evolution due to the continuous addition of lanthanum elements was provided in Figure 3(d), and the size for the sheets grew into large aggregates. The reproducibility of the formation of this structure was also confirmed in the analogous samples of (Ca_1−𝑥, Gd_𝑥)MoO₄:Eu³⁺ (5 mol%) (Figures 3(e) and 3(f)).

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3 

SEM images of (Ca1−𝑥, Ln𝑥)MoO4:0.05Eu3+ (Ln=La and Gd) phosphors: (a) and (b) CaMoO4:0.05Eu3+, (c) (Ca0.8, La0.2)MoO4:0.05Eu3+, (d) (Ca0.5, La05)MoO4:0.05Eu3+, (e) (Ca0.8, Gd0.2)MoO4:0.05Eu3+, and (f) (Ca0.5, Gd0.5)MoO4:0.05Eu3+.

3.2. Photophysical Properties

For the sake of studying the effect of concentration of La³⁺ and Gd³⁺ on the photophysical properties, we have carefully carried out a series of experiments with the formula of (Ca_1−𝑥, Ln_𝑥)MoO₄:Eu³⁺ (5 mol%) (Ln=La, Gd) (𝑥=0.1, 0.2, 0.3, 0.4, and 0.5) prepared by ultrasonic and microwave coassisted method at a low temperature of 80°C in 30 minutes.

The excitation spectrum of (Ca_1−𝑥, La_𝑥)MoO₄:Eu³⁺ (5 mol%) (𝑥=0.2) phosphors was obtained by monitoring the emission wavelength at 616 nm under room temperature (Figure 4). Other samples show similar excitation feature except for difference intensity. A broad luminescence band ranging from 220 to 350 nm with the maximum at about 276 nm in the spectrum is attributed to charge transfer from oxygen to molybdenum atom. Other narrow and sharp peaks centered at 364, 383, 395, 417, 465 nm were assigned to intra-configurational f-f transitions (⁷F₀ → ⁵D₄, ⁷F₀ → ⁵G₂, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, and ⁷F₀ → ⁵D₂, resp.) of Eu³⁺ in the host of molybdate.

Figure 4 

Emission spectra of (Ca1−𝑥, La𝑥)MoO4:0.05Eu3+ (𝑥=0.1, 0.2, 0.3, 0.4, and 0.5) phosphors (𝜆ex=276nm) with excitation spectrum of (Ca0.8, La0.2)MoO4:0.05Eu3+ phosphor 𝜆em=616nm (inset).

The emission spectra of red phosphors (Ca_1−𝑥, La_𝑥)MoO₄:Eu³⁺ (5 mol%) with various concentration of La³⁺ (the monitoring wavelength is about 276 nm) are shown in Figure 4. All sharp emission lines in the emission spectra correspond to the electronic transitions as ⁵D₀ → ⁷F₁ (593 nm), ⁵D₀ → ⁷F₂ (615 nm), ⁵D₀ → ⁷F₃ (656 nm), and ⁵D₀ → ⁷F₄ (707 nm), respectively, and the most intense and sensitive emission lines at about 616 nm for electric-dipole transition (⁵D₀ → ⁷F₂) indicate that europium ion does not occupy the center of symmetry [21, 22]. The influence of mole percent of La³⁺ on the intensity of phosphors can be found from Figures 4 and 5. Along with the increasing amounts of La³⁺, the luminescent intensity reaches maximum at 0.2, then becomes weakened from 0.3 to 0.5. It is known that the ionic radii of trivalent lanthanide ions are similar to Ca²⁺; therefore, the lower doping contents of La³⁺ replacing lattice position of calcium will contribute to the europium(III) emissions. However, the association constants of lanthanide complexes are significantly higher than calcium complex. The lanthanide ions have a larger hydration sphere than Ca²⁺, and the ability of La³⁺ to accommodate ligands will be stronger than calcium. Accordingly, the concentrated lanthanum ions will easily cause particle agglomeration which could be seen in SEM images, and the europium luminescence might be quenched by aggregation effects.

Figure 5 

Emission spectra of (Ca1−𝑥, Gd𝑥)MoO4:0.05Eu3+ (𝑥=0.1, 0.2, 0.3, 0.4, and 0.5) phosphors (𝜆ex=276nm) with excitation spectrum of (Ca0.1, Gd0.9)MoO4:Eu3+ (5 mol%) phosphors (𝜆em=615nm, inset).

As shown in Figure 5, the excitation spectrum of (Ca_1−𝑥, Gd_𝑥)MoO₄:0.05Eu³⁺ (𝑥=0.1) phosphors was measured at about 616 nm, and the emission spectra of (Ca_1−𝑥, Gd_𝑥)MoO₄:Eu³⁺ (5 mol%) (𝑥=0.1, 0.2, 0.3, 0.4, and 0.5) phosphors were obtained at about 276 nm. As shown in Figure 5, the excitation spectrum consists of a broad and strong band of O²⁻-Mo⁶⁺ charge transfer transition ranging from 220 nm to 350 nm and several narrow peaks for internal energy levels transition of Eu³⁺ within wavelength range of 350 nm to 500 nm. Upon excitation at 276 nm, it can be seen that the strong emission is at about 616 nm due to ⁵D₀ → ⁷F₂ indicating that europium ion is located in a low symmetric site, and other relatively weak peaks in the wavelength range of 550–750 nm correspond to other transitions from ⁵D₀ to ⁷F_J (𝐽=1, 2, 3, 4). From Figure 6, it can be obviously seen that the correlation between the intensity and the contents of Gd³⁺ followed a linear equation 𝑌=−2084.6𝑋+1237.8 (𝑅²=0.9812).

(a)

(b)

Figure 6 

Effect of the different contents of La3+ (a) and Gd3+ (b) on the luminescent properties of the red-emitting phosphors.

4. Conclusions

In brief, (Ca_1−𝑥, Ln_𝑥)MoO₄:Eu³⁺ (5 mol%) (Ln=La, Gd; 0<𝑥<0.5) phosphors have been successfully synthesized by an ultrasound and microwave coassisted method at low temperature of 80°C in 30 min. The XRD and SEM results show that the La, Gd doped phosphors can gradually change the crystalline structures, and the sizes of those powders grew larger. The as-prepared materials were red emissive, and the doped concentrations of La³⁺ are optimized as 0.2 and 0.1, respectively. The above mentioned materials may be useful as luminescent probes in biomedical science and in light- emitting diodes applications.

Conflict of Interests

All the authors confirm that there is no conflict of interests with any financial organization regarding the materials or equipments mentioned in the paper.

Acknowledgments

Q. M. appreciates the National Natural Science Foundation of China (21002035).