Phase-controllable synthesis, shape evolution and optical performances of CePO₄ nanocrystals via a simple oil-bath route

Abstract

Rare-earth inorganic orthophosphates have been attracting considerable research interest because of their significant luminescent properties. In the present work, we employed a facile oil-bath route to successfully realize phase-controlled synthesis of CePO₄ nanocrystals. The reactions were carried out at 110 °C for 30 min. The phases of the as-obtained products were determined by X-ray powder diffraction (XRD) analyses. Experiments showed that a mixed phase of hexagonal and monoclinic CePO₄ was prepared at the EG/H₂O volume ratio of 14/5, keeping the introduced EG/H₂O total volume of 19 mL unchanged. Hexagonal CePO₄ nanocrystals were obtained at the EG/H₂O volume ratio below 14/5; while at a volume ratio above 14/5, monoclinic ones were produced. Interestingly, the morphology of the final product underwent a change from long nanorods, to short nanorods, to nanorod bundles, to shuttle-like nanocrystals, and finally to spheroid nanocrystals with the increase of the EG/H₂O volume ratio. Furthermore, the optical performances of the final products with different forms were also investigated.

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1. Introduction

Over the past two decades, the synthesis and luminescent properties of lanthanide orthophosphates (LnPO₄) and lanthanide(III)-doped lanthanide orthophosphates (LnPO₄:Ln³⁺) has always attracted extensive interest because of their potential applications in color displays,¹ field-effect transistors,² optoelectronics,³ medical and biological labels,⁴ solar cells,⁵ and light sources.⁶ For example, CePO₄ and doped CePO₄ can be used in luminescent lamps as highly efficient emitters of green light.⁷ Generally, CePO₄ bears two forms in nature: hexagonal structure with a chemical formula of CePO₄·xH₂O (x = 0.3–0.5) (known as Rhabdophane) and monoclinic one (monazite structure).⁸ The former is low-temperature stable and has wide applications in the tribology due to its layered structure.⁹ The latter is high temperature stable and can be obtained through calcining hexagonal CePO₄. Owing to the melting-point of ∼2000 °C, monoclinic CePO₄ is one of the most stable orthophosphate materials. It can be applied under the high temperature as heat-resistant and ceramic materials.¹⁰ Also, monoclinic CePO₄ has a promising application as a potential storage material for nuclear waste because it can form solid solutions with tetravalent actinides, such as U⁴⁺ and Th⁴⁺, and can exist for billions of years.¹¹ Moreover, it is found that CePO₄ presents a potential application in hydrogen fuel cells as proton conduction membrane.¹² Importantly, widened applications can be found in doped CePO₄ materials. For instance, CePO₄:Tb³⁺ nanocrystals have been extensively used in determination of vitamin C,¹³ formaldehyde,¹⁴ immunoassay,¹⁵ and sensing.¹⁶

Generally, CePO₄ can be prepared by the solid-phase reaction and the solution-phase route. The solid-phase reaction is often carried out under high temperature, so the final product is always monoclinic CePO₄. Now, however, the solution-phase route attracts much interest since it can conveniently control the morphology of the final product, which is difficult for the solid-phase preparation. For example, the microemulsion technology¹⁷ and the direct precipitation method¹⁸ have been developed for syntheses of CePO₄ nanowires. Among various solution-phase routes, however, the hydrothermal technology is always extensively used for preparation of pure CePO₄ and/or doped CePO₄ nanocrystals with various shapes and sizes.^19–22 Different from the high-temperature solid-phase reaction, hexagonal CePO₄ is often prepared by the solution-phase route due to the low reaction temperature. Thus, the post-treatment is necessary for the conversion of the hexagonal to the monoclinic form. In 2003, Yang and coworkers hydrothermally prepared hexagonal and monoclinic CePO₄ nanocrystals at 100 °C and 200 °C for 8 h, respectively,²² which is a useful attempt for the phase-controlled synthesis of CePO₄ under the mild condition. However, it still remains a huge challenge for realizing rapid synthesis of CePO₄ with controllable phase and shape through the hydrothermal method.

In the current paper, we successfully realized phase-controllable synthesis of nanosized CePO₄ particles in a simple system through a mild oil-bath route. Ce(NO₃)₃·6H₂O and KH₂PO₄ were selected as the reactants, ethylene glycol (EG) and water as the mixed media. No surfactant or structure-directing reagent was employed. The reaction was carried out at 110 °C for 30 min. It was found that CePO₄ nanocrystals gradually changed from hexagonal to monoclinic phase with the increase of the EG/H₂O volume ratio from 19/0 to 0/19. Simultaneously, the morphology of the final product evolved from long nanorods, to short nanorods, to nanorod bundles, to shuttle-like nanocrystals, and finally to spheroid nanocrystals. Furthermore, the optical performances of the final products with different phases were also investigated.

2. Experimental section

All reagents and chemicals are analytically pure, bought from Sinopharm Chemical Company and used without further purification.

2.1 Synthesis of CePO₄

In a typical experimental process, 1.0 mmol (0.4342 g) of Ce(NO₃)₃·6H₂O and 1.0 mmol (0.1361 g) KH₂PO₄ were firstly dissolved into 1 mL concentrated HNO₃ in a 50 mL three-necked flask. Then, 19 mL mixed solvents of ethylene glycol (EG) and H₂O with the volume ratio of 0/19, 2/17, 4/15, 6/13, 8/11, 10/9, 12/7, 14/5, 16/3, 18/1 and 19/0 were added into the above solution, respectively. After the as-obtained systems were heated in oil bath at 110 °C for 30 min, the systems were cooled down to room temperature naturally. The white precipitates were collected and washed with deionized water several times, and finally dried at 60 °C for 12 h in vacuum oven.

2.2 Characterization

X-ray diffraction (XRD) patterns of the products were carried out on Shimadzu XRD-6000 X-ray diffractometer (Cu Kα radiation, k = 0.154060 nm), employing a scanning rate of 0.02 s⁻¹ and 2θ ranges from 10° to 80°. Morphological and elemental analyses of the as-obtained products were accomplished on a Hitachi S-4800 field emission scanning electron microscope (SEM), employing an accelerating voltage of 5 kV and 15 kV (for energy dispersive spectrum analysis). High resolution transmission electron microscopy (HR/TEM) images were carried out on a JEOL 2010 transmission electron microscope, employing an accelerating voltage of 200 kV. The holey carbon film was used as the support. Photoluminescence (PL) spectra were recorded on a FLSP 920 with a Xe lamp at room temperature. The UV-vis diffuse reflectance spectra (DRS) were recorded by a Shimadzu UV2450 spectrometer using MgO as a standard at room temperature.

3. Results and discussion

3.1 Structure and morphology characterization

Fig. 1a depicts the XRD patterns of the products prepared by the present oil-bath route from the systems with various EG/H₂O volume ratios at 110 °C for 30 min. With the increase of the EG/H₂O volume ratio from 0/19 to 12/7, the products exhibited same XRD patterns, which were indexed as hexagonal CePO₄ phase by comparison with JCPDS card files no.74-1889. After increasing the EG/H₂O volume ratio to 14/5, the hexagonal and monoclinic phases were detected simultaneously, implying that the final product was a mixture composed of hexagonal and monoclinic CePO₄. Upon further increasing the EG/H₂O volume ratio, hexagonal CePO₄ gradually decreased and monoclinic one increased. After the EG/H₂O volume ratio were increased to 16/3, the final product could be arranged to monoclinic CePO₄ by comparison with JCPDS card files no.73-0478. Fig. 1b gives the EDS analyses of the products prepared from systems with the EG/H₂O volume ratio of 0/19 and 19/0, respectively. Two products presented the same EDS result although they owned different forms. The above experimental facts indicated that the phase-controlled synthesis of CePO₄ could be realized only via changing the original EG/H₂O volume ratio in the present simple system.

Fig. 1 (a) XRD patterns of the products prepared from systems with various EG/H₂O volume ratios by the present oil-bath route at 110 °C for 30 min, (b) the EDS analyses of the products prepared from systems with the EG/H₂O volume ratios of 0/19 and 19/0, respectively.

SEM observations showed that the morphology of the final product could also be tuned by the original EG/H₂O volume ratio. As shown in Fig. 2a, when the EG/H₂O volume ratio was 0/19, the product comprised of plentiful long nanorods. After increasing the EG/H₂O volume ratio to 6/13, the product was made of abundant short nanorods; and some short nanorods collocated with each other to form nanorod bundles (see Fig. 2b). Upon further enhancing the EG/H₂O volume ratio to 12/7, nanorod bundles with integrated midsections started to appear (Fig. 2c). According to the XRD result shown in Fig. 1a, the final product still belonged to the hexagonal phase. After the EG/H₂O volume ratio of 14/5 was used, the product presented dumbbell-like structures constructed by nanorod bundles with integrated midsections (Fig. 2d). Here, the product was a mixture of hexagonal and monoclinic form based on the XRD analysis. After increasing the volume ratio of EG/H₂O to 16/3, only monoclinic CePO₄ was obtained. Simultaneously, carrot-like products were formed due to the ceaseless growth of the midsections of nanorod bundles (see Fig. 2e). Upon further enhancing the volume ratio of EG/H₂O to 18/1, the final product was composed of shuttle-like grains (Fig. 2f). After only 19 mL EG was employed, the product displayed spheroid-like structures (Fig. 2g). The above experimental facts clearly disclosed the shape transformation of CePO₄ from long nanorods, to short nanorods, to dumbbell-like structures, to carrot-like structures, to shuttle-like grains, and finally to spheroid-like grains with the increase of the EG/H₂O volume ratio from 0/19 to 19/0.

Fig. 2 SEM images of the products obtained from systems with different ratios of EG/H₂O: (a) 0/19, (b) 6/13, (c) 12/7, (d) 14/5, (e) 16/3, (f) 18/1 and (g) 19/0.

Fig. 3 depicts representative TEM images of the products prepared from systems with the EG/H₂O volume ratio of 0/19, 12/7, 16/3 and 18/1, respectively. As seen from Fig. 3a, the final product prepared from the system with the EG/H₂O volume ratio of 0/19 was made of long nanorods with smooth surfaces. A HRTEM images is exhibited in the inset of Fig. 3a. The clear lattice fringes showed good crystallinity of nanorods. The distance between neighbouring planes was measured to be ∼0.31 nm, which is very close to 0.30936 nm of the (111) plane and 0.30549 nm of the (200) one. Furthermore, a SAED pattern is displayed in the inset of Fig. 3a, too. Regular and bright diffraction dots confirmed the single crystal nature of nanorods. Combining HRTEM image, SAED pattern and XRD pattern, one can find that nanorods should grow along the [001] direction of hexagonal CePO₄. This is consistent of the growth habit of hexagonal CePO₄.²¹ However, several weak diffraction dots also existed in the SAED pattern, which were attributed to the monoclinic CePO₄. For example, two spots could be indexed as the (012) plane in the SAED pattern. The upper spot belonged to hexagonal CePO₄ and the below one to monoclinic CePO₄. The above fact implied the final product was a mixture of hexagonal and monoclinic CePO₄ although monoclinic CePO₄ could not be detected by XRD analyses. Fig. 3b shows a typical TEM image of the product obtained from the system with the EG/H₂O volume ratio of 12/7. The product was composed of short nanorods and nanorod bundles with integrated midsection. The inset in Fig. 3b displayed the HRTEM image and SAED pattern of a short nanorod. The lattice fringes and diffraction dots could still be detected, indicating that the product still owned good crystallinity. However, the product had shown marked polycrystalline feature compared with long nanorods given in Fig. 3a. Fig. 3c displays two carrot-like grains, which were obtained from the system with the EG/H₂O volume ratio of 16/3. This was in consistent with the SEM result shown in Fig. 2e. The HRTEM image exhibited two intersecting planes, which corresponded the (−212) and (200) one of monoclinic CePO₄ form, respectively (see the inset in Fig. 3c). Fig. 3d depicts a representative TEM image of the product prepared from the system with the EG/H₂O volume ratio of 18/1. A shuttle-like particle was clearly visible. Also, a HRTEM image given in the inset in Fig. 3c exhibited clear stripes, indicating the product owned good crystallinity. Obviously, TEM observations likewise uncovered the shape conversion of the final product with the increase of the EG/H₂O volume ratio, which were consistent with the SEM results.

Fig. 3 TEM and HRTEM images of the final CePO₄ products prepared from systems with different ratios of EG/H₂O: (a) EG/H₂O = 0/19, (b) EG/H₂O = 12/7, (c) EG/H₂O = 16/3 and (d) EG/H₂O = 18/1.

3.2 Possible formation mechanism

Many studies have discovered that hexagonal CePO₄ nanowires/nanorods can be easily obtained from hydrothermal systems.^21,22 Fang et al. explained the above fact from the structural viewpoint of hexagonal CePO₄.²¹ It was considered that hexagonal CePO₄ comprised infinite linear chains extending along the c axis. Thus, from the thermodynamic perspective, the growth of hexagonal CePO₄ along the c axis direction needed the lowest activation energy. This caused a higher growth rate along the c axis. Namely, CePO₄ grew preferentially along the [001] direction and led to the abnormally strong intensity of the (200) peak in the XRD pattern of hexagonal CePO₄.²¹ In the present work, the system consisted of Ce(NO₃)₃, KH₂PO₄, H₂O and/or EG. No additive was employed. When only H₂O (the EG/H₂O volume ratio = 0/19) was used, hexagonal CePO₄ long nanorods were obtained. The XRD pattern indeed showed the abnormally strong intensity of the (200) peak (see Fig. 1), implying the preferential growth along the [001] direction. Also, the above result was proved by the SAED pattern and HRTEM image (see the inset in Fig. 3a). With the increase of EG amount in the system, the phase and morphology of CePO₄ gradually changed under the same oil-bath conditions. After only EG (the EG/H₂O volume ratio = 19/0) was employed, spheroid-like monoclinic CePO₄ particles were formed. Distinctly, EG played a crucial role in the phase- and morphology-controlled synthesis of CePO₄. In 1998, Fasol's research uncovered that faster ionic motion usually ensured a reversible pathway between the fluid phase and solid phase and allowed ions to adopt correct positions in developing crystal lattices.²³ It is well known that EG has a bigger viscosity than water. Therefore, the system viscosity increased with the increase of the EG/H₂O volume ratio, which gradually slowed down the motion rate of Ce³⁺ and PO₄³⁻ ions. This was unfavorable for the growth of hexagonal CePO₄ nanorods. On the other hand, two –OH groups exist in an EG molecule. It is possible that a chelate ion is formed between EG and Ce³⁺ ion owing to the strong affinity of cerium to oxygen. Namely, EG could still act as a structure-directing reagent in the formation of CePO₄ micro/nanostructures with certain phases and shapes. Thus, with the increase of the EG/H₂O volume ratio from 0/19 to 19/0, the phase of CePO₄ changed from hexagonal to monoclinic and the morphology evolved from long nanorods, to nanorod bundles, to dumbbell-like structures, to shuttle-like grains and finally to spheroid-like particles. Scheme 1 clearly illustrates the correlation between the original volume ratio of EG/H₂O and the phase and morphology of the final product.

Scheme 1 The correlation between the original volume ratio of EG/H₂O and the phase and morphology of the final product.

3.3 Optical properties

Fig. 4 depicts the PL spectra of CePO₄ nanocrystals with neat hexagonal phase, mixed phase and pure monoclinic form, respectively. All excitation spectra locate in the range from 240–320 nm, and all emission one in the range of 310–410 nm, which are in good agreement with ref. 24a. Nevertheless, some minor differences can still be found in PL spectra. In the excitation spectrum of hexagonal CePO₄ (see Fig. 4a), for instance, a strong peak at 291 nm and two weak shoulder peaks at 287 and 304 nm can be seen. They correspond to the transitions from the ground state of ²F_5/2 of Ce³⁺ to the different components of the excited Ce³⁺ 5d states split by the crystal field.²⁴ Two excitation peaks with the equal intensity appear at 295 and 303 nm in the mixed CePO₄ phase. However, the peak at 295 nm markedly increases in the excitation spectrum of neat monoclinic CePO₄. Namely, the phase change from hexagonal to monoclinic causes slight change of their excitation spectra. Similar phenomenon is also found in their emission spectra. As shown in Fig. 4b, two strong peaks centered at 330 and 342 nm can be readily found in three spectra under the excitation of 290 nm light. They should come from the two transitions of Ce³⁺ ion emission: from the lowest component of the ²D state to the spin–orbit components of the ground state, ²F_7/2 and ²F_5/2.²⁵ Different from the emission spectrum of hexagonal CePO₄, however, two weak peaks at 365 and 380 nm can be detected in those of CePO₄ with mixed phase and monoclinic form.

Fig. 4 The excitation (a) and emission (b) spectra of the products prepared from the systems with the EG/H₂O volume ratios of 0/19 (labeled as hexagonal), 14/5 (labeled as mixed phase) and 18/1 (labeled as monoclinic) at 110 °C for 30 min.

Moreover, many studies have shown that CePO₄ can strongly absorb the ultraviolet light.^21,25,26 Fig. 5 exhibits the UV-vis diffuse reflectance spectra of CePO₄ nanocrystals with neat hexagonal phase, mixed phase and pure monoclinic form, respectively. Four absorption peaks at 217, 237, 258, and 274 nm are visible in each curve, which correspond to the transitions from the ground state ²F_5/2 (4f¹) of Ce³⁺ to the crystal field split levels of the Ce^{3+ 2}D (5d¹) excited states.²¹ Furthermore, the strongest peak locates at 274 nm, which is in good agreement with the previous report.²¹

Fig. 5 UV/Vis absorption spectra of the products prepared from the systems with the EG/H₂O volume ratios of 0/19 (labeled as hexagonal), 14/5 (labeled as mixed phase) and 18/1 (labeled as monoclinic) at 110 °C for 30 min.

4. Conclusions

In summary, the phase-controlled synthesis of CePO₄ nanocrystals was successfully realized by a simple oil-bath route at 110 °C for 30 min, only through changing the original EG/H₂O volume ratio. Experiments showed that the mixed phase of hexagonal and monoclinic CePO₄ nanocrystals was produced at the EG/H₂O volume ratio of 14/5. Hexagonal CePO₄ was prepared at the EG/H₂O volume ratio below 14/5; and monoclinic CePO₄ was obtained at the volume ratio above 14/5. At the same time, with the increase of the EG/H₂O volume ratio the morphology of the final product gradually evolved from long nanorods, to short nanorods, to nanorod bundles, to shuttle-like nanocrystals, and finally to spheroid nanocrystals. Furthermore, experiments showed that all products owned similar PL and UV-vis absorption spectra, but some minor differences could still be found owing to the phase and morphology change of the final product. The present synthesis route is simple and rapid for the phase- and shape-control of CePO₄ nanocrystals, which paves a new path for the phase- and shape-controlled synthesis of other rare earth orthophosphates.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21171005), Key Foundation of Chinese Ministry of Education (210098) and Special and Excellent Research Fund of Anhui Normal University for the fund support.

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