Synthesis of Cerium Tetrafluoride and Cerium Trifluoride Nanoscale Polycrystals from Ammonium Hydrogen Difluoride.

This paper reported a dry synthesis and characterization of cerium tetrafluoride (CeF4) and cerium trifluoride (CeF3) nanoscale polycrystals (NPs). The CeF4 NPs were spherical or flaky and approximately 10 ± 2 nm in diameter. The CeF3 NPs were rod-shaped nanorods with a length of about 150 ± 5 nm and a diameter of about 20 ± 2 nm. The first step was to synthesize the intermediate product-(NH4)4CeF8 by mixing CeO2 and NH4HF2 at a molar ratio of 1:6 at 390 K. The structural characterization was analyzed by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Then, (NH4)4CeF8 was heated in an argon gas flow to synthesize the CeF3 and CeF4 NPs. The products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The properties of CeF3 and CeF4 NPs were further evaluated by transmission electron microscopy (TEM), selected area electron diffraction pattern (SAED), and high-resolution transmission electron microscopy (HRTEM). The findings provided an alternative strategy for the synthesis of nanometer fluorides, which could be a reference for high-performance research on other nanometer fluorides.

Introduction

Due to the special up-conversion luminescence performance and superior chemical and physiological properties, rare-earth fluoride up-conversion nanomaterials with a multicolor output have been widely used in the fields of multicolor displays, photonic devices, and biomedicine.[1−3] As a typical representative of rare-earth fluoride, cerium fluoride is a new type of up-conversion luminescent material, which has shown broad application prospects in many aspects such as displays, optical storage, biological analysis imaging, photodynamic therapy, etc.[4−9] Also, cerium fluoride has been abundantly applied in catalysis owing to the unique 4f electron, which is very sensitive for electron transfer and is easily involved in the hydrogen absorption/desorption of MgH2.[10] Few of the properties and reactions of CeF4 have been described. It belongs to an isostructural group that includes the tetrafluorides of zirconium, thorium, uranium, and plutonium.[11] Cerium fluoride (CeF3) has attracted increasing attention in virtue of its technological importance as an inorganic scintillating crystal. CeF3 is considered as one of the most promising scintillators for the next-generation experiments in high-energy physics because of its high density, fast response, and high-radiation resistance. At the same time, it is also an important fluorescent matrix material due to its low vibration energy; the quenching of the excited state of rare-earth ions is minimized.[12] Also, as we all know, even a small number of radioactive materials require lots of engineering and management controls. Therefore, non-radioactive elements with similar properties to radioactive counterparts, which are called substitutes, are more efficient at the beginning of work. Cerium can be used as an alternative element to uranium for radiation-free experimental exploration.[11]

The different application properties of nanocrystals are closely related to their morphology, structure, and size. The morphology, structure, and size of nanocrystals obtained by different preparation methods are also different. The routes for synthesizing rare-earth fluorides mainly include the wet process and dry process.[13] Many researchers have explored the synthesis of rare-earth fluoride by the wet route.[14−17] The wet process mainly includes the precipitation method, microemulsion method, hydrothermal method, solvothermal method, and sol–gel method. However, according to most reports so far, the uniformity and dispersion of synthesized nanomaterials are far from ideal, which will greatly limit their application in biomarkers.[18] In addition, toxic organic metal precursors and hazardous coordination solvents used in the wet route have a great impact on the environment, which limits its application on the industrial scale. Therefore, the development of effective and environmentally friendly synthetic routes for rare-earth fluorides is still a challenge. The dry process avoids most of the disadvantages of the wet process and has the advantages of a short reaction process, less impurity, low oxygen in fluoride, low reaction temperature, and safe operation.[19] It has been considered to be one of the preferred methods for preparing high-purity fluoride, which has been used in a variety of rare-earth fluorinations.[20,21]

In this paper, we demonstrated a dry synthesis route on CeF4 and CeF3 NPs that could be developed into a commercial fabrication process for cerium fluoride. CeO2 and NH4HF2 were mixed at a molar ratio of 1:6, ground, and then reacted at 390 K in vacuum for 4 h to obtain (NH4)4CeF8. After that, these ammonium cerium fluoride species were decomposed by heating to 570 and 1070 K for 10 h to get CeF4 and CeF3 nanoscale polycrystals. The effect of the temperature and ratio of raw materials on the fluorination rate was investigated with cerium oxide and ammonium fluoride as raw materials. Furthermore, the optimal experimental conditions for the preparation of CeF4 and CeF3 nanoscale polycrystals by dry fluorination were determined.

Method

Materials and Characterization

Cerium dioxide (99.9%, Aladdin) and ammonium bifluoride (98%, Aladdin) were used as received. Ar was dried with CaCl2(96%, Aladdin) to remove water.

For the sample for SEM observation, powders (1 mg) were pasted on a conductive adhesive, and after vacuum pumping to remove the excess powders, the powders were sprayed with gold and were then scanned.

For the sample for TEM observation, the specimen was prepared by the following steps: (i) the powder was dispersed in cyclohexane by ultrasonic agitation; (ii) then, the suspension was dropped onto the carbon film.

The X-ray powder diffraction pattern of the samples was recorded at ambient temperature using a Bruker D2 Phaser powder X-ray diffractometer with Cu Kα radiation.

Scanning electron microscopy—energy-dispersive X-ray spectroscopy (SEM—EDS, ZEISS EVO MA) with an acceleration voltage of 15 kV was used for sample morphological characterization.

Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on an FEI Tecnai F20 S-TWIN transmission electron microscope with a field emission gun operating at 200 kV.

Reaction of CeO2 and NH4HF2

The synthesis of (NH4)4CeF8 was explored by molar ratios of CeO2 and NH4HF2 of 1:4, 1:6, and 1:8 at temperatures ranging from room temperature to 430 K. All operations were carried out under anaerobic conditions. NH4HF2 used in the reaction easily absorbed water from the air. Before the experiment, NH4HF2 was dried in an oven at 350 K for 60 min. The powder mixtures were ground in an agate mortar for 10 min. The precursor powders were placed in a vessel made from polytetrafluoroethylene tubes (100 mm high with 50 mm in diameter and 5 mm thick), and the vessel was placed in a vacuum oven. First, the powders were treated in an inert glove box at room temperature for 2 months.

After that, we chose a molar ratio of 1:6 to study the influence of temperature on the synthesis process of ammonium cerium fluoride. According to the properties of NH4HF2, synthesis experiments were conducted at 380, 390, 400, 410, and 420 K. The composition and structure of the products were characterized by XRD, and the morphology was observed by SEM.

Synthesis of CF4 NPs

Cerium tetrafluoride (CeF4) NPs were synthesized by heating (NH4)4CeF8 from the reaction of CeO2 and NH4HF2. The powder of (NH4)4CeF8 was placed in an agate mortar, ground for 10 min, and then transferred to a platinum crucible, which was in a quartz tube closed at one end. At the beginning of the experiment, the argon gas was fed at a speed of 10 mL/min for 30 min to clean up the furnace tube. Then, the furnace was heated to 570 K at 5 K/min and kept for 10 h, during which the argon continued flowing into the furnace tube at a rate of 8 mL/min until the reaction was completed and the temperature was cooled down to room temperature. The morphology and structure of CeF4 NPs were determined by SEM, EDS, TEM, and HRTEM.

Synthesis of CF3 NPs

The powders of (NH4)4CeF8 synthesized in step 2.1 were put into a platinum crucible, which was in a quartz tube sealed at one end. After being cleaned with argon gas for 30 min, the furnace was heated to 1070 K by 5 K/min and kept for 10 h, during which the flow rate of argon was maintained at 8 mL/min until it was reduced to room temperature. The product was characterized by XRD, SEM, TEM, and HRTEM.

Results and Discussion

Synthesis of Ammonium Cerium Fluoride

The synthesis of ammonium cerium fluoride by the solid–solid reaction between CeO2 and NH4HF2 was explored in this paper. The reaction was based on the following stoichiometric equations

The product ammonium cerium fluoride was determined by XRD.

Reaction at Room Temperature

The mixtures of CeO2 and NH4HF2 at molar ratios of 1:4, 1:6, and 1:8 were milled for 10 min under anaerobic conditions and then reacted at room temperature for 2 months. The products were analyzed by XRD, as shown in Figure .

Figure 1

XRD patterns of the CeO2 and NH4HF2 mixture at molar ratios of 1:4, 1:6, and 1:8 at room temperature for 2 months.

Peaks of (NH4)4CeF8 were detected in the samples. The diffraction peaks of (NH4)4CeF8 were detected at angles of 15.3, 15.8, 19.0, and 24.0. At a stoichiometry molar ratio of 1:4, CeO2 (at angles of 28.5, 33.0, 47.4, and 56.3) remained in the product after 2 months, which was difficult to remove in the subsequent reaction. At a molar ratio of 1:6, there was a small amount of residual NH4HF2 and no CeO2 remained after 2 months. At a molar ratio of 1:8, CeO2 could not be detected in the product, but there was a large amount of remaining NH4HF2. Also, the intermediate product (NH4)2CeF6 appeared (at angles of 14.6, 28.1, 46.4) in the XRD images of molar ratios of 1:4 and 1:8.

In this reaction, one of the substances was NH4HF2, which melted at 401 K and dissociated into HF, H2, and N2 above 514.5 K.[22]

Therefore, to ensure the reaction, CeO2 should be converted to ammonium cerium fluoride completely. So, NH4HF2 should be above stoichiometry in the reaction. However, excessive NH4HF2 contained a large number of non-decomposed impurities, which affected the purity of the product. In summary, the product was relatively pure (NH4)4CeF8 with remaining NH4HF2 at a molar ratio of CeO2 and NH4HF2 of 1:6.

So, the following explorations were carried out with a molar ratio of 1:6. For the mixtures with a molar ratio of 1:6, the SEM images of the starting powders and products mixed at room temperature for 10 days, 30 days, and 2 months are shown in Figure .

Figure 2

Morphologies of starting powders and the products based on eq . (a) Starting powders; (b) mixed for 10 days; (c) mixed for 30 days; (d) mixed for 2 months.

Figure shows the morphologies of the starting powders and the powders with different reaction times based on eq . The starting powders were relatively large (Figure a). After mixing and reacting at room temperature for 10 days, the particles’ size of the mixture became smaller. It could be seen from Figure b and Figure c that the degree of powdering on the surface of the particles gradually deepens with time. After being mixed for 2 months, the mixtures turned into 2–5 μm and flocked together, which is shown in Figure d. It can be seen from the figures that the number of product particles per unit volume increased gradually with time. To compare with ammonium manganese trifluoride (NH4MnF3) nanoparticles with a size of 20–80 nm, which was synthesized by a reverse microemulsion method,[23] in our work, the particles in the reaction ranged from about 30 to 50 μm at the beginning to about 2–5 μm at the end, which laid a good foundation for the subsequent reaction.

Reaction at Different Temperatures

According to the thermal properties of raw material NH4HF2, the reaction temperature of the mixture was studied, which is shown in Figure .

Figure 3

XRD patterns of CeO2 and 6NH4HF2 mixture reaction at 380–430 K.

The solid phase reaction at room temperature was so slow that the reaction between CeO2 and NH4HF2 took about 2 months. Theoretically, some reactions are favored by raising the temperature in a specific range. At 380 K, the reaction between CeO2 and NH4HF2 was a solid–solid reaction. The reaction was relatively slow, but due to sufficient reaction time, the products were mainly (NH4)4CeF8. The diffraction peaks of (NH4)2CeF6 were detected in the sample. Further analysis of the product at 390 K showed the disappearance of (NH4)2CeF6. In addition to that, the XRD analysis indicated that (NH4)4CeF8 diffraction peaks were stronger. For the reaction between 400 and 514.5 K, the NH4HF2 became liquid for solid–liquid reaction and solid–gas reaction. So, there would be partial loss of NH4HF2. In addition, the heating rate played an important role in the loss of NH4HF2; a quick temperature rise may lead to the rapid volatilization of NH4HF2. The analysis has verified the formation of (NH4)2CeF6 of the product of 400–420 K where the peaks of (NH4)2CeF6 are reobserved and those of (NH4)4CeF8 are diminished. Intermediates such as (NH4)2CeF6 were produced due to the volatilization loss of NH4HF2 within this temperature range. When the temperature reached 514.5 K, NH4HF2 would be decomposed into HF, H2, and N2, just as shown in eq . The decomposed gas flowed out of the vessel with Ar quickly, causing the reactants to fail to react adequately. At this point, the peaks of (NH4)4CeF8 became weak, almost disappearing, while the peaks of (NH4)2CeF6 became stronger and even became the main products. At the higher temperature, amounts of intermediates might be generated in the product due to the rapid decomposition and escape of NH4HF2. Similar reactions were reported before.[24−26]

Rietveld refinements of the as-synthesized nanomaterials at 390 K were performed by using the general structure analysis system (TOPAS academic) program and are diagrammatically plotted in Figure . It can be seen from the figure that the product was a pure (NH4)4CeF8 phase at 390 K. The Rietveld refinement results manifest that the measured and calculated diffraction peaks are well matched. The reliable parameters fit well with a space group of C2/c. The refined lattice parameters were a = 13.05131 Å, b = 6.67305 Å, and c = 13.63411 Å.

Figure 4

Rietveld refinement plots of the products at 390 K.

Based on the theoretical and experimental results, the temperature was set at about 390 K, the molar ratio of CeO2 and NH4HF2 was 1:6, and the reaction time was 4 h in the synthesis of ammonium cerium fluoride.

Formation of CeF4 NPs

When the reaction temperature was between 470 and 620 K, the fluorine compound would be decomposed into tetrafluoride. The reaction for the formation of CeF4 is given in eq .

The ingredient characterization of the samples was performed with SEM, EDS, and SAED, which are shown in Figure .

Figure 5

(a) SEM image of an area of CeF4 NPs, (b) EDS image of CeF4 NPs, (c) TEM and HRTEM images of CeF4 NPs, and (d) SAED pattern of CeF4 NPs.

The SEM micrographs of CeF4 are shown in Figure a, which shows the almost uniform distribution of CeF4 NPs (marked by white arrows). The sizes of CeF4 NPs were about 10 ± 2 nm. The cobalt fluoride (CoF2) nanoparticles (NPs) were 20–70 nm and prepared by the reverse microemulsion method for the first time by Khan et al.[27] The EDS analysis shown in Figure b reveals the presence of Ce and F at atomic percentages of 80 and 20%, respectively. Figure c shows the TEM and HRTEM images of CeF4. It could be seen that CeF4 NPs were composed of flat-elongated nanoparticles with a much larger particle size. The obvious lattice fringes indicated that NPs were highly crystalline. The bright circular rings and spots in the SAED pattern (Figure d) showed the presence of CeF4 NPs (ICSD #089621[28]). However, CeF4 could not be detected after prolonged heating to 20 h when the temperature was lower than 470 K. As if the temperature was higher than 720 K, CeF3 was produced in the product. Moreover, the conditions under which CeF4 converted to CeF3 in different reaction vessels were not completely the same.[12] The decomposition proceeds with the elimination of molecular fluorine, according to the following equation (eq )[29]In this work, the platinum crucible reactor was used and the temperature should be controlled below 1114 K.

Formation of CeF3 NPs

The composition of the product was characterized by XRD. The XRD diffraction peaks for the synthesized CeF3 NPs are shown in Figure .

Figure 6

XRD pattern of CeF3 NPs by heating (NH4)4CeF8 at 1070 K.

The decomposition product of ammonium cerium fluoride at 1070 K was flaky and porous CeF3 NPs. Figure shows the XRD pattern of the CeF3 NPs prepared by heating the (NH4)4CeF8 at a temperature of 1070 K. All the peaks at 2θ values of 24.40, 24.95, 27.83, 43.96, 45.13, 50.91, 52.83, 64.83, 68.76, 69.65, 71.09, and 81.69 could be indexed to the cubic cell of the CeF3 phase (002), (110), (111), (300), (113), (302), (221), (214), (304), (115), (411), and (404), respectively, which were in good agreement with the reported value for pure CeF3. Also, the XRD measurement results were in agreement with a previous report.[30] The SEM, TEM, SAED, and HRTEM characterizations of the product are shown in Figure . The CeF3 NPs were rod-shaped nanoparticles with a length of about 150 ± 20 nm and a diameter of about 20 ± 2 nm, which are shown in Figure a. It could be observed from Figure a that the particles seem to be porous. Particles mostly contained voids of different sizes due to the evolution of large amounts of gases during the combustion. The particles with agglomeration are observed in Figure b. From the SAED pattern shown in Figure c, the bright circular rings and spots were entirely consistent with CeF3 NPs. The polycrystalline nature of the sample was further confirmed by SAED patterns. The HRTEM image as shown in Figure d further confirms the synthesis of CeF3 NPs. Tashi et al.[31] reported a facile hydrothermal synthesis and characterization of hexagonal Eu3+ doped and Eu3+/Ce3+ co-doped NaGdF4 nanophosphors and the influence of different radii of rare-earth ions on crystal morphology. Furthermore, on the basis of this study, they also demonstrated a Ser-CS (serine-functionalized NaYF4:Ce3+/Gd3+/Eu3+@NaGdF4:Tb3+ core shell) nanophosphor for the effective detection of nitroaromatic derivatives.[32]

Figure 7

(a) SEM image of an area of the CeF3 NP powder. (b) TEM image of CeF3 NPs. (c) SAED pattern of CeF3 NPs. (d) HRTEM image of CeF3 NPs.

Also, the heating rate and the maximum reaction temperature needed to be controlled. When the heating rate reached above 8 K/s, CeOF would appear in the product, which was difficult to remove. Similarly, when the reaction temperature reached 1170 K, a certain amount of CeOF would also appear in the product.

Conclusions

In summary, a dry process by mixing CeO2 and NH4HF2 and the decomposition of the intermediate were devised for the formation of CeF4 and CeF3 NPs. The reaction conditions were evaluated by adjusting the molar ratio, reaction temperature, and reaction time. After various characterization methods, the rod-shaped CeF3 NPs about 150 ± 5 nm long and 20 ± 2 nm in diameter and the spherical or flaky CeF4 with 10 ± 2 nm were obtained.

The exploration process of the route provided a certain reference for the following study of fluoride.