Synthesis and optical properties of La₂O₂CO₃:Eu³⁺ hollow sphere phosphors by hydrothermal synthesis assisting with double-crucible carbonization method

Abstract

Lanthanum oxide, europium oxide, hydrochloric acid, ammonia water, and carbon powder were used as the starting materials. Detailed characterizations were attained by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry/differential thermal analysis (TG-DTA), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. The La(OH)₃ precursor was synthesized by an hydrothermal synthesis method at 120 °C for 24 h, which was then converted into La₂O₂CO₃ phase through calcining the precursor at 800 °C for 2 h in a carbon atmosphere. It was demonstrated that the as-obtained La₂O₂CO₃ is a hollow sphere with uniform size about 200 nm. The formation mechanism of the La₂O₂CO₃ phase has also been proposed. Photoluminescence analysis showed that the La₂O₂CO₃:Eu³⁺ phosphors have the strongest red emissions at 614 nm under 281 nm ultraviolet (UV) light excitation, which correspond to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The La₂O₂CO₃:Eu³⁺ phosphors have double-exponential decay behavior and the calculated lifetime is determined to be t₁ = 0.205 μs and t₂ = 1.177 μs.

Introduction

Lanthanide compounds are an attractive class of materials and have unique optical, catalytic, and magnetic properties, resulting from the unique configuration of unpaired 4f electrons and the rich structures of their energy levels [1,2,3]. Among them, lanthanum compounds have been widely studied in its hydroxide [4, 5], oxide [6], oxysulfide [7], oxysulfate [8], fluoride [9], oxyfluoride [10], oxychloride [11], carbonate [12], or oxycarbonate forms [13, 14], and these substances have been extensively applied in optoelectronic devices [15], phosphors [16, 17], solid electrolyte [8], catalytic [18], sorbent [19], gas sensor [20], and so on. Lanthanum oxycarbonate (La₂O₂CO₃), which is emerging as a kind of novel optical material, has been prepared into different microstructures and morphological assemblies [21]. It is known that many inorganic materials with different microstructures have various properties, even if they have the same chemical composition. For example, Ding et al. [22] presented a simple method to fabricate size-controlled La₂O₂CO₃ nanoparticles by annealing La(OH)₃ nanocrystallines in air atmosphere, which is printed on an interdigital electrode as a sensing material for CO₂ detection. Moreover, Chen et al. [23] prepared La₂O₂CO₃ nanorods with diameters of 13–15 nm and lengths of 100–150 nm by annealing the La(OH)₃ nanorods in a furnace at 400 °C for 2 h. It is shown that the sensor exhibits high gas response for CO₂ gas at an operating temperature of 325 °C, making it a potential candidate for practical detectors for CO₂ gas. Furthermore, Hou et al. [24] reported that they synthesized La₂O₂CO₃ samples with different morphologies (nanorod-shapes and nanoplate-shapes) at 500 °C for 2 h by an hydrothermal method and a precipitation method, respectively. They observed that one of the nanorod-shaped samples exhibited the best catalytic properties among the investigated samples in the oxidative coupling of methane (OCM) at low temperatures (420–500 °C). Therefore, the investigation of lanthanum oxycarbonate with controlled special structures and sizes would offer an opportunity to design these compounds as materials for specific applications.

As is known to us all, the hollow spherical structures possess low density, large surface area, stability, surface permeability, and well-aligned nanoporous structures, all of which make them attractive for scientific study. For instance, they might be used for nanoreactors, environmental applications, drug delivery, catalysts, batteries, supercapacitors, gas sensors, photoacoustic imaging, etc. Meanwhile, the hollow phosphors might radiate light more effectively than its counterpart due to their interior hollow structure. In many cases, however, the examples mentioned above are currently limited to mesoscale hollow spheres, that is, with sphere diameters exceeding 100 nm [25]. Various synthetic strategies have been developed for the preparation of such hollow structures including template and template-free methods, hydrothermal and solvothermal approaches, ultrasound, and microwave-assisted synthesis, ion-exchange synthesis, aerosol route, chemical vapor deposition, ostwald ripening, phase inversion, excimer laser ablation, and ultra-high voltage hard anodization [26,27,28]. However, to the best of our knowledge, there are few reports about La₂O₂CO₃:Eu³⁺ hollow sphere phosphors by hydrothermal synthesis assisting with the double-crucible carbonization method.

In this regard, we focus on two aspects in this study. Firstly, hollow spherical La₂O₂CO₃:Eu³⁺ phosphors were synthesized by the double-crucible carbonization of the La(OH)₃:Eu³⁺ precursors which were prepared via a facile hydrothermal process. Secondly, phase structure, thermal conversion process, morphology, and optical properties of the products are discussed based on the results of XRD, FT-IR, TG-DTA, FE-SEM, TEM, UV-vis, and fluorescence spectroscopy analyses.

Experimental procedure

Sample preparation

Lanthanum oxide (La₂O_3, 99.99% purity), europium oxide (Eu₂O_3, 99.99% purity), hydrochloric acid (HCl, AR), ammonia water (NH₃·H₂O, AR), and carbon powder (C, AR) were used as the starting materials. Rare earth oxide powders (La₂O₃, Eu₂O₃) were purchased from Jining Tianyi New Material Co. Ltd. China. The other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd. Firstly, LaCl₃ solutions with La³⁺ ion concentration of 0.1 M was prepared by dissolving the corresponding La₂O₃ in dilute hydrochloric acid (HCl) solution. Secondly, LaCl₃ solution with volume of 60 mL was transferred to a 100-mL Teflon-lined stainless steel autoclave. Accompanied with stirring, ammonia water (NH₃·H₂O) was added to LaCl₃ solution until a pH value of 7 was attained. The autoclaves were sealed and heated in an electric blast drying oven for hydrothermal synthesis at 120 °C for 24 h. After being naturally cooled to room temperature, the product was collected by filtering and cleaning with deionized water and absolute ethanol several times and then dried at 80 °C for 12 h in order to obtain the precursor. Finally, the precursor was calcined at 800 °C for 2 h in a carbon atmosphere by a double-crucible carbonization method in order to obtain the final product. The Eu³⁺ ion-doped target product was also synthesized using the same procedure. The doping molar concentration of Eu³⁺ ions was 9% in this study. The specific flow chart is shown in Fig. 1.

Fig. 1

Schematic illustration for the fabrication process of La₂O₂CO₃

Sample characterization

Phase and structure analyses were conducted using X-ray powder diffraction (XRD) and Flourier transform infrared spectra (FT-IR) tools. The XRD pattern was operating at 40 kV and 30 mA with CuK_α = 0.15406 nm using a D8 Advance X-ray diffractometer. Fourier transform infrared spectra (FT-IR) were recorded in the region of 4000–400 cm⁻¹ using an Agilent Cary 660 FT-IR spectrophotometer by the KBr method. Thermogravimetry (TG) and differential thermal analysis (DTA) were performed using simultaneous differential thermal analysis and thermo-gravimetry (Q600 Simultaneous DSC-TGA). The absorption spectra were acquired on an Agilent Cary 5000 UV-visible spectrometer with barium sulphate as a reference. The particle morphologies of the products were observed by a Hitachi SU8010 field emission scanning electron microscope (FE-SEM) and a JEM-2100F transmission electron microscope (TEM). Photoluminescence (PL) spectra and decay times were measured using a Hitachi F-7000 fluorescence spectrophotometer equipped with IBH TemPro fluorescence lifetime measurement system.

Results and discussion

Phase and structure analysis

Figure 2 shows the XRD patterns of the precursor, the product calcined in a carbon atmosphere, as well as their corresponding standard JCPDS cards. It can be seen from Fig. 2a, that the precursor sample exhibits sharp diffraction peaks, which match the hexagonal lanthanum hydroxide phase (La(OH)₃, JCPDS-00-036-1481) without any impurity peaks, as shown in Fig. 2b. The chemical reactions in the formation of the precursor can be expressed as follows:

$$ {\mathrm{La}}_2{\mathrm{O}}_3+6\mathrm{HCl}\rightleftharpoons {\mathrm{La}\mathrm{Cl}}_3+{3\mathrm{H}}_2\mathrm{O} $$

(1)

$$ {\mathrm{La}\mathrm{Cl}}_3\rightleftharpoons {\mathrm{La}}^{3+}+{3\mathrm{Cl}}^{-} $$

(2)

$$ {\mathrm{NH}}_3\cdotp {\mathrm{H}}_2\mathrm{O}\rightleftharpoons {{\mathrm{NH}}_4}^{+}+{\mathrm{OH}}^{-} $$

(3)

$$ {\mathrm{La}}^{3+}+{3\mathrm{OH}}^{-}\rightleftharpoons \mathrm{La}{\left(\mathrm{OH}\right)}_3\downarrow $$

(4)

Fig. 2

XRD pattern of the La(OH)₃ and the La₂O₂CO₃

After the precursor was calcined at 800 °C for 2 h in a carbon atmosphere, the characteristic diffraction peaks of the sample (Fig. 2c) are basically indexed to the hexagonal phase of La₂O₂CO₃ (JCPDS-01-084-1963), as shown in Fig. 2d. No La(OH)₃ or La₂O₃ phases were identified, revealing that the as-synthesized product is mainly composed of La₂O₂CO₃ phase, corresponding to the following chemical reaction:

$$ 2\mathrm{La}{\left(\mathrm{OH}\right)}_3+2\mathrm{C}+{\mathrm{O}}_2\rightleftharpoons {\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{CO}}_3+{3\mathrm{H}}_2\mathrm{O} $$

(5)

Fourier transform infrared spectroscopy (FT-IR) was usually employed as an additional probe to evidence the presence of OH⁻ groups, CO₃²⁻ groups, as well as other inorganic species. The FT-IR spectra of the La(OH)₃ and the La₂O₂CO₃ in the range 4000–400 cm⁻¹ are shown in Fig. 3. As can be seen from Fig. 3a, the IR absorption bands at 3612 cm⁻¹, 3430 cm⁻¹, 1630 cm⁻¹, and 650 cm⁻¹ were associated with the OH stretching vibration and La-OH bending modes in lanthanum hydroxide. The bands at 1486 cm⁻¹ and 1382 cm⁻¹ are assigned to the C-O stretching vibration due to containing a small amount of CO₂. The result indicated that the La(OH)₃ has a high capacity for adsorbing CO₂. As can be seen from Fig. 3b, the absorption bands at 3630 cm⁻¹ and 1630 cm⁻¹ were associated with the water molecule vibration, indicating the existence of absorbed water in the La₂O₂CO₃. The bands between 1518 and 1362 cm⁻¹ in the spectra of the La₂O₂CO₃ reveal the existence of carbonate groups, originating from the reaction of the La(OH)₃ with carbon dioxide (CO₂) in the carbon atmosphere. In addition, the two main vibration peaks of the La₂O₂CO₃ sample at 1085 cm⁻¹ and 856 cm⁻¹ were attributed to the υ₂ and υ₁ modes of the La₂O₂CO₃ [29]. The peak at 500 cm⁻¹ was ascribed to the La-O stretching frequencies of the La₂O₂CO₃ [30]. These arguments are supported by the XRD results.

Fig. 3

FT-IR spectra of the La(OH)₃ samples and the calcined La₂O₂CO₃ samples

Thermal conversion process

To analyze the thermal conversion process of the precursor La(OH)₃ and the mixed sample (La(OH)₃ + C), TG and DTA measurements were carried out under a flowing air atmosphere. As shown in Fig. 4, the changes in the precursor (La(OH)₃) at elevated temperatures can be summarized as two steps: the first step occurred at the temperature region ranging from room temperature to ~ 350 °C. The weight loss of 11% at ~ 350 °C is likely due to the removal of hydration water in the chemical formula. According to the DTA trace, the endothermic peak centered at 340 °C indicates the process related with dehydration of the La(OH)₃, corresponding to the following reaction:

$$ \mathrm{La}{\left(\mathrm{OH}\right)}_3\rightleftharpoons {\mathrm{La}}_2{\mathrm{O}}_3+{3\mathrm{H}}_2\mathrm{O}\uparrow $$

(6)

Fig. 4

TG-DTA curves of the La(OH)₃ and the mixed sample of La(OH)₃ and carbon powder: the full and dotted curves represent TG and DTA curves, respectively

At the second step, the weight was almost unchanged at ~ 350 to 800 °C, indicating La₂O₃ is stable. The changes in the mixed sample at elevated temperature can be summarized as four steps: the first step occurred at a temperature region ranging from room temperature to ~ 70 °C; the second step occurred at ~ 70 to 280 °C; the third step occurred at ~ 280 to ~ 515 °C; and the last step occurred at ~ 515 to 800 °C. The first weight loss of 10% was attributed to the evaporation of water absorbed on to the surface of the mixed sample. Accordingly, this weight loss corresponds to a very weak endothermic peak at around 54 °C in the DTA curve. The second step, the weight was almost unchanged in the TG curve. The third step weight loss of 52.46% is likely due to two reasons. On the one hand, the process was related with the dehydration of the La(OH)₃, which is consistent with the thermal decomposition process of the La(OH)₃ analyzed above. On the other hand, the weight loss was attributed to the oxidation of carbon powder, and accompanied with carbon powder reacting with oxygen in the container to form CO₂. Furthermore, the La(OH)₃ was dehydrated to produce La₂O₃, which continues to react with CO₂ to form the La₂O₂CO₃ as evidenced, according to the DTA trace, by the broad exothermic peak centered at 480 °C. In the last step, the weight was almost unchanged, indicating La₂O₂CO₃ is stable at ~ 515 to 800 °C. These arguments are supported by the XRD and FT-IR results.

Morphology analysis

FE-SEM images of the La(OH)₃ and the La₂O₂CO₃ samples are shown in Fig. 5. The La(OH)₃ sample appears to be an aggregate on a large scale, quasi-spherical in shape, and the average size is about 1~ 2 μm in diameter (Fig. 5a). As can be seen from Fig. 5b, the morphology (shape, size distribution, and dispersity) of the La₂O₂CO₃ sample was similar to that of the La(OH)₃ sample. However, the La₂O₂CO₃ particle is more concentrated than the La(OH)₃ due to the high temperature carbonization process.

Fig. 5

FE-SEM images of a the precursor La(OH)₃ and b the La₂O₂CO₃ samples

To clarify the hollow structure of the two samples, the morphology was further characterized by a TEM test. In Fig. 6 are shown TEM images of the La(OH)₃ and the La₂O₂CO₃ samples. Interestingly, it can be seen from Fig. 6a that the La(OH)₃ sample possesses a hollow sphere structure with a diameter of ~ 200 nm and a shell thickness of about 80 nm. Figure 6b gives a TEM image of the La₂O₂CO₃ and exhibits structure information. Careful observation shows that the carbonized sample still remains hollow structure and inheriting the shape of its La(OH)₃ precursor. Such a fascinating morphology was seldom seen before in the La₂O₂CO₃ material. Based on the above experimental results, a possible mechanism for the formation of the hollow spheres was proposed as follows: Initially, numerous tiny nuclei were formed in the hydrothermal reaction system, and the growth of the nucleus lead to the formation of nanoparticles. Then, for the minimization of surface energy, a great deal of nanoparticles tends to aggregate and assemble into a quasi-spherical structure with increasing hydrothermal time. Finally, the carbonized samples also maintained their parents’ morphologies and the change in the size is very slight.

Fig. 6

TEM images of a the precursor La(OH)₃ and b the La₂O₂CO₃ samples

Optical properties

UV-vis spectroscopy was used to characterize the optical absorbance of the La(OH)₃ and La₂O₂CO₃ hollow spheres. The absorption spectra were carried out to resolve the interband (valence-conduction band) transition, which allows us to calculate the band gap. The UV-vis absorption band edges located at 324 nm and 385 nm (see the slope in Fig. 7), respectively. It is well known that the optical band gap and absorption wavelength of a semiconductor have a characteristic relation:

$$ {E}_{\mathrm{g}}=1240/\lambda\ \left(\mathrm{eV}\right) $$

(7)

in which E_g is the optical band gap and λ is the wave length of the photon (nm). Therefore, the E_g values of the La(OH)₃ and the La₂O₂CO₃ hollow spheres can be obtained by the extrapolation of the above relation to be 3.83 eV and 3.22 eV, respectively. This UV-vis spectroscopy shows characteristic band maxima at 260 nm and 280 nm for the La(OH)₃ and the La₂O₂CO₃ hollow spheres, respectively. The La₂O₂CO₃ has a wider and higher absorption band than the La(OH)₃, which is helpful for improving light absorption of the La₂O₂CO₃ hollow sphere.

Fig. 7

The absorption spectra of the La(OH)₃ and the La₂O₂CO₃ samples

Figure 8 presents the PL excitation and emission spectra of the La(OH)₃:Eu³⁺ and the La₂O₂CO₃:Eu³⁺ phosphors. As shown in Fig. 8a, the excitation spectra of the two phosphors are mainly composed of two excitation peaks located at 248 nm and 281 nm, respectively, which is due to the charge transfer transitions from the 2p orbital of the O²⁻ ions to the 4f orbital of the Eu³⁺ ions [31]. This result is consistent with the above UV-vis spectra. Moreover, two weak peaks centered at 394 nm and 464 nm are assigned to the ⁷F₀ → ⁵L₆ and the ⁷F₀ → ⁵D₂ transition of the Eu³⁺, respectively. Upon excitation with 248 nm UV light, the La(OH)₃:Eu³⁺ phosphor shows the corresponding emission spectrum consisting of the characteristic transitions of Eu³⁺, i.e., ⁵D₀ → ⁷F₁ (594 nm), ⁵D₀ → ⁷F₂ (619 nm) and ⁵D₀ → ⁷F₄ (692 nm), and the strongest one is located at 619 nm (⁵D₀ → ⁷F₂). Furthermore, upon excitation with 281 nm UV light, the La₂O₂CO₃:Eu³⁺ phosphor shows a strong bright-red luminescence and the corresponding emission spectrum consists of the characteristic transitions of Eu³⁺, i.e., ⁵D₀ → ⁷F₁ (594 nm), ⁵D₀ → ⁷F₂ (614 nm, 627 nm), ⁵D₀ → ⁷F₃ (655 nm), and ⁵D₀ → ⁷F₄ (706 nm). The strongest one is located at 614 nm, ascribed to the ⁵D₀ → ⁷F₂ of the Eu³⁺ ion. The ⁵D₀ → ⁷F₂ transition is symmetrically sensitive electric dipole transitions, and only the Eu³⁺ ion occurs when it is in a low symmetry position. On the basis of the above analyses, it can be clearly seen that there are two great differences in the PL spectra of the La(OH)₃:Eu³⁺ and the La₂O₂CO₃:Eu³⁺ phosphors, as follows: First, the excitation band of the La(OH)₃:Eu³⁺ phosphor shifts 33 nm to a shorter-wavelength direction than that of the La₂O₂CO₃:Eu³⁺ phosphor. The excitation spectra result reveals that the obvious redshift occurs from the La(OH)₃:Eu³⁺ to the La₂O₂CO₃:Eu³⁺ phosphors, and the position of the CTS bands depends on the different host lattice. In addition, the excitation band intensity of the La₂O₂CO₃:Eu³⁺ phosphor is greater than that of the La(OH)₃:Eu³⁺ phosphor, indicating that the La₂O₂CO₃:Eu³⁺ phosphor possesses higher light absorption ability. Second, the ⁵D₀ → ⁷F₂ transition intensity in the La₂O₂CO₃:Eu³⁺ phosphor is greatly enhanced in comparison with that in La(OH)₃:Eu³⁺ phosphor. These results reveal that the La₂O₂CO₃ is a better candidate host material than the La(OH)₃ for Eu³⁺ doping phosphor, as confirmed by the luminous photographs of the La(OH)₃:Eu³⁺ and the La₂O₂CO₃:Eu³⁺ phosphors under a 254 nm UV lamp (the inset of Fig. 8b). Based on the results of the luminescence spectrum, the following possible mechanism is proposed. The energy level diagram of Eu³⁺ ions in the La₂O₂CO₃ host lattice with all probable dipole transitions is shown in Fig. 9. As mentioned above, the La₂O₂CO₃:Eu³⁺ phosphors show intense luminescence in the visible light region. The intense luminescence of Eu³⁺ originates from an efficient energy transfer from the O²⁻ state to the Eu³⁺energy state and then the electrons are expected to excite to the ⁵L₆ state, which decays nonradiatively to the ⁵D₀ state followed by radiative decay to lower levels of ⁷F_J (J = 1–4) [32]. This is the origin of the luminescence processes of the Eu³⁺ ion in the visible light region.

Fig. 8

PL excitation and emission spectra of the La(OH)₃:Eu³⁺ and the La₂O₂CO₃:Eu³⁺ phosphors. The inset of b is the La(OH)₃:Eu³⁺ and the La₂O₂CO₃:Eu³⁺ luminous photographs under 254 nm UV lamp

Fig. 9

The proposed possible energy mechanism of the Eu³⁺ ion-doped La₂O₂CO₃ phosphor. The position of the ground ⁷F₀ states of ions is assumed to be zero

Figure 10 shows the decay curves of the La(OH)₃:Eu³⁺ and the La₂O₂CO₃:Eu³⁺ phosphors excited by ultraviolet light at 248 nm and 281 nm, respectively. The luminescence life times for the strongest ⁵D₀ → ⁷F₂ transitions of Eu³⁺ in the La(OH)₃ and the La₂O₂CO₃ hosts are shown in Fig. 10. The fluorescence decay curves can be well fitted by a double-exponential function as

$$ I={I}_0+\mathrm{Aexp}\left(t/{\tau}_1\right)+\mathrm{Bexp}\left(t/{\tau}_2\right) $$

(8)

where I represents the decaying luminescence intensity, I₀ is the initial intensity, t the time, τ₁ and τ₂ are the decay time for the exponential components, respectively. The fitting results are shown inside Fig. 10. Two lifetimes, τ₁ = 0.244 μs and τ₂ = 1.244 μs have been obtained for the Eu³⁺ transition in La(OH)₃ host and two lifetimes, τ₁ = 0.205 μs and τ₂ = 1.177 μs also have been obtained for Eu³⁺ transition in La₂O₂CO₃ host. It can be seen from Fig. 10 that the fluorescence lifetime of La₂O₂CO₃:Eu³⁺ is almost the same as that of the La(OH)₃:Eu³⁺. The result suggests that the La₂O₂CO₃:Eu³⁺ phosphor may have potential applications in the ultra-fast X-ray image field.

Fig. 10

The decay curves of the La(OH)₃:Eu³⁺ and the La₂O₂CO₃:Eu³⁺ phosphors

Conclusions

In summary, the La₂O₂CO₃:Eu³⁺ hollow sphere phosphors were successfully achieved by hydrothermal synthesis assisting with a double-crucible carbonization method. The present study shows that the precursor La(OH)₃ was synthesized by an hydrothermal synthesis method at 120 °C for 24 h, and La₂O₂CO₃ could be obtained by calcining the La(OH)₃ at 800 °C for 2 h in a carbon atmosphere. It was demonstrated that the obtained La₂O₂CO₃ has a hollow quasi-sphere structure and a typical size of about 200 nm in diameter. The strongest emission peak for the La₂O₂CO₃:Eu³⁺ bright-red phosphor was observed to be located at 614 nm under 281 nm UV light excitation, and this phosphor possesses double-exponential decay behavior, and the corresponding fluorescence lifetimes are τ₁ = 0.205 μs and τ₂ = 1.177 μs. These results suggest that the La₂O₂CO₃:Eu³⁺ hollow sphere may have potential applications in optoelectronic devices (fast X-ray imaging), catalytic, sorbent, and gas sensor fields.