Hydrothermal Synthesis of LaCO₃OH and Ln³⁺-doped LaCO₃OH Powders under Ambient Pressure and Their Transformation to La₂O₂CO₃ and La₂O₃

Abstract

Orthorhombic and hexagonal lanthanum(III) hydroxycarbonate ($LaCO_3OH$) and $Ln^{3+}$-doped $LaCO_3OH$ ($LaCO_3OH:Ln^{3+}$, where Ln = Ce, Eu, Tb, and Ho) powders were prepared by a hydrothermal reaction under ambient pressure and characterized by thermogravimetry, powder X-ray diffraction, infrared and luminescence spectroscopy, and field-emission scanning electron microscopy. The polymorph of $LaCO_3OH$ depended on the reaction temperature, inorganic salt additive, species of $Ln^{3+}$ dopant, and solvent. The calcination of orthorhombic $LaCO_3OH:Ln^{3+}$ (2 mol %) powers at $600^{\circ}C$ yielded a mixture of hexagonal and monoclinic $La_2O_2CO_3:Ln^{3+}$ powders. The relative quantity of the latter increased with decreasing ionic radius of the $Ln^{3+}$ dopant ion and increasing doping concentrations. On the other hand, the calcination of hexagonal $LaCO_3OH:Ln^{3+}$ (2 mol %) powders at $600^{\circ}C$ resulted in a pure hexagonal $La_2O_2CO_3:Ln^{3+}$ powder, regardless of the species of $Ln^{3+}$ ions (Ln = Ce, Eu, and Tb). The luminescence spectra of $LaCO_3OH:Ln^{3+}$ and $La_2O_2CO_3:Ln^{3+}$ were measured to examine the effect of their polymorph on the spectra.

Introduction

Trivalent lanthanide (Ln3+) ions have high affinity to the carbonate (CO3 2−) ion, as evidenced by the occurrence of stable minerals such as (La,Ce, Nd)2(CO3)3·8H2O (lanthanite), (La,Ce)2(CO3)3·4H2O (calkinsite), and (La,Ce)(F,OH)CO3 (bastnaesite).1 Of the lanthanum carbonates, LaFCO3 and LaCO3OH are useful starting materials for their thermal decomposition products such as LaOF, La2O2CO3, and La2O3, which are attractive host materials of phosphors.2-6 LaFCO3 powders exist as a single polymorph (hexagonal one), whereas LaCO3OH powders exist in two polymorphs, i.e., orthorhombic (o-) and hexagonal (h-) LaCO3OH. To date, LaCO3OH powders have been synthesized using a variety of methods, including the hydrolysis of LaCl3-trichloroactic acid solution by ammonia,7 hydrolysis of La(III) carbonate under high8 and ambient9 pressure, reaction of LaBrOH with CO2,9 solvothermal reaction of La2O3 in a mixed solvent of ionic liquid and water,10 and solvent-free dissociation of La(III) acetate hydrate under autogenic pressure at 700 ℃.11 LaCO3OH powders have been also prepared by a hydrothermal reaction in an autoclave using the following reagents: LaCl3 and thiourea,12,13 La(NO3)3 and urea,4,14 La(NO3)3, glucose, and acrylamide,15 La2O3 and glycine,16 LaCl3 and gelatin,17 La(oleate)3 and aqueous tertbutylamine, 18 La3+-EDTA complex and urea,19 and La(NO3)3 and NH4HCO3.20 According to these reports, most LaCO3OH powders prepared by a hydrothermal reaction in an autoclave had the hexagonal polymorph. On the other hand, o- LaCO3OH powders sometimes formed in an autoclave.15,17 LaCO3OH powders can also be prepared by a hydrothermal reaction without the need for an autoclave,3,21 as for LaFCO3 powders.22 To date, there are still no reports on the factors determining the polymorph of LaCO3OH powders obtained by a hydrothermal reaction under ambient pressure.

In this study, LaCO3OH powders were prepared by a hydrothermal reaction from the solution containing La(NO3)3 and urea without the need for an autoclave. The effects of the reaction temperature, inorganic salt additive, species of Ln3+ dopant, and solvent on the polymorph of LaCO3OH powders were investigated. Urea is used widely as the precipitation reagent of metal ions because it thermally decomposes into NH3 and CO3 2− ions in neutral and basic aqueous solutions.23 The as-prepared LaCO3OH powders were characterized by powder X-ray diffraction (XRD), Fourier transformation infrared (IR) spectroscopy, thermogravimetric (TG) and differential thermal analysis (DTA), and fieldemission scanning electron microscopy (FE-SEM). Ln3+- doped LaCO3OH (LaCO3OH:Ln3+, where Ln = Ce, Eu, Tb, and Ho) powders were also synthesized to determine the effect of dopants on the polymorph of La2O2CO3 and La2O3. The luminescence spectra of the LaCO3OH:Eu3+ and La2O2CO3:Eu3+ powders were measured to examine the effect of their polymorph on the spectra.

 

Experimental Section

Synthesis of LaCO3OH and LaCO3OH:Ln3+ Powders. All starting materials, La(NO3)3·6H2O (99.9%), Ce(NO3)3·6H2O (99.99%), Eu(NO3)3·6H2O (99.9%), Tb(NO3)3·6H2O (99.99%), Ho(NO3)3·5H2O (99.9%), and urea (99.0%) were purchased from Sigma-Aldrich Co. and used as-received without further purification. In a typical synthesis of LaCO3OH powder, 8.66 g (2.00 × 10−2 mol) of La(NO3)3·6H2O and 6.01 g (1.00 × 10−1 mol) of urea were dissolved in 50 mL of a mixed solvent of ethylene glycol (EG) and water. EG to water volume ratios were 9:1, 8:2, 7:3, 6:4, 5:5, 2:8, and 0:10. The solution was boiled for 24 h under ambient pressure and cooled to room temperature. Sequentially, the white precipitate was separated by centrifugation, washed several times with water and ethyl alcohol, and dried in an oven at 70 ℃. The quantities of La(NO3)3·6H2O, urea, and solvent were kept constant through this study. LaCO3OH:Ln3+ powders were prepared in the same way as that for LaCO3OH powders except that Ln(NO3)3 salts (2 mol %) were added to the solution containing 8.49 g of La(NO3)3·6H2O.

Characterization. The LaCO3OH powders and their thermal decomposition products were characterized by powder XRD (PANalytical X’Pert PRO MPD X-ray diffractometer) using Cu-Kα radiation operating at 40 kV and 30 mA and IR spectroscopy (Nicolet 6700, Thermo Scientific). The TG and DTA curves of LaCO3OH powders were recorded on an SDT Q600 apparatus (TA Instruments) at a heating rate of 10 ℃/min. The morphology of the product powders was investigated by FE-SEM (Hitachi S-4200). The luminescence spectra of LaCO3OH:Eu3+ and La2O2CO3:Eu3+ were measured at ambient temperature on a JASCO FP-6500 spectrofluorometer with a 150 W xenon lamp.

 

Results and Discussion

Synthesis of LaCO3OH and LaCO3OH:Ln3+ Powders. The LaCO3OH powders were synthesized by a hydrothermal reaction in various mixed solvents such as water-EG and water-DMSO, without the use of an autoclave. EG is used widely as a solvent to control the morphology of many materials.452425 EG is mixed with water at any ratio. The boiling point (B.P.) of the water-EG solution can be controlled easily by EG to water volume ratio. Recently, Mao et al. prepared LaCO3OH powders by a hydrothermal reaction in water-EG solvents using an autoclave and reported that the volume ratio had remarkable effect on the species and morphology of the particles.4 Boiling (B. P. ≥ 118 ℃) in the mixed solvents containing more than 70 volume % EG gave a pure h-LaCO3OH powders (Figure 1(a)). In contrast, o-LaCO3OH powders mixed with a very small amount of h-LaCO3OH powders were obtained by boiling (B. P. ≤ 110 ℃) in mixed solvents containing less than 60 volume % EG (Figure 1(b)). These results are considerably different from those obtained by hydrothermal synthesis in an autoclave,4 where h-LaCO3OH powders were obtained in mixed solvents containing less than 50 volume % EG, whereas La2(CO3)3(H2O)8 and La2(CO3)3(OH)2 were obtained in mixed solvents containing more than 50 volume % EG.

To determine if the reaction temperature is a major factor influencing the polymorph of the LaCO3OH powders, the LaCO3OH powders were prepared in mixed solvents of water and DMSO (B. P. = 189 ℃). Boiling (B. P. = 125 ℃) the solution in a mixed solvent containing 80 volume % DMSO yielded a pure o-LaCO3OH phase, whereas boiling (B. P. = 143 ℃) the solution in a mixed solvent containing 90 volume % DMSO yielded an o-LaCO3OH powder mixed with a small amount of h-LaCO3OH powder, as shown in Figure 1(c). Therefore, even boiling at a higher B. P. in the water-DMSO system than in the water-EG system did not yield pure h-LaCO3OH powders. This suggests that the polymorph of LaCO3OH powders is determined not only by the reaction temperature but also by the solvent. Further evidence of the reaction temperature being not the main factor determining the polymorph of the LaCO3OH powders was provided by the formation of o-LaCO3OH powders in an autoclave at very high reaction temperatures.15,17 The reason why the phase of the LaCO3OH powders depends on the solvent remains unclear. EG appears to be a favorable solvent for the preparation of h-LaCO3OH powders at relatively low temperatures. In contrast to DMSO, EG has similar characteristics to water in that EG molecules can form hydrogen-bonded networks. These characteristics of EG may induce variations in the morphology and polymorph by changing the volume ratio of water-EG solvents.

Figure 1.XRD patterns of samples obtained by a hydrothermal reaction in water-EG solvents with (a) ≥ 70 and (b) ≤ 60 volume % of EG, (c) in water-DMSO containing 90 volume % DMSO, and (d) in water added with KNO3: ●, unidentified phase.

Low reaction temperatures in hydrothermal reactions in which La(NO3)3·6H2O and urea were used as reagents resulted in the formation of pure o-LaCO3OH powders,3 but caused a low yield. For example, the reaction temperature of 66 ℃ in water caused the formation of o-LaCO3OH powders in 24% yield. This yield increased with increasing reaction temperatures, but increasing the temperatures caused the formation of h-LaCO3OH powders. Therefore, it is difficult to synthesize pure o-LaCO3OH powders in high yield using La(NO3)3·6H2O as the La3+ ion source. Pure o-LaCO3OH powders were obtained in ca. 100% yield by boiling aqueous solution containing La(NO3)3·6H2O, urea, and KNO3, in which the KNO3 to La(NO3)3·6H2O mole ratio was two and higher (Figure 1(d)). The addition of 0.100 mol NaNO3 or NH4NO3, however, yielded a mixture of o- and h-LaCO3OH powders with almost equal quantity. Inorganic ions have been used to control the morphology of nanomaterials.26,27 The effect of salts on the phase has also been reported in the synthesis of nanostructured orthorhombic and hexagonal EuF3 from a reaction of Eu(NO3)3·6H2O with XF (X = H+, NH4 +, and alkali ions).27

In a mixed solvent containing 80 volume % EG, doping LaCO3OH with 2 mol% Ln3+ (Ln = Ce and Eu) into LaCO3OH yielded h-LaCO3OH:Ln3+, but the doping with 2 mol % Tb3+ and Ho3+ ions yielded h-LaCO3OH:Tb3+ mixed with a very small amount of o-LaCO3OH:Tb3+ and o-LaCO3OH:Ho3+ mixed with a smaller amount of h-LaCO3OH:Ho3+, respectively. This suggests that doping with Ln3+ ion having smaller ionic radius is more effective in inducing the formation of o- LaCO3OH. This doping effect on the polymorph can be explained in terms of the phase diagrams for La2O3-H2O-CO2 ternary systems for the lanthanide series, which were established by Kutty et al.28 Based on the phase diagrams, they showed that the stable polymorph of LaCO3OH is orthorhombic for elements with atomic number 64 and higher. On the other hand, doping LaCO3OH with 2 mol % Ln3+ ions in aqueous solution containing KNO3 resulted in pure o-LaCO3OH:Ln3+, regardless of the presence of Ln3+ ions.

Characterization of LaCO3OH Powders. The h- and o- LaCO3OH powders, which were obtained in a mixed solvent containing 80 volume % EG and aqueous solution containing KNO3, respectively, were characterized by TG, IR spectroscopy, and SEM. Figure 2 shows the TG and DTA curves of the LaCO3OH powders. The TG curves indicated the following decomposition processes:

The observed weight losses accompanied by the decomposition of h- and o-LaCO3OH·xH2O to La2O3 were 28.9 and 26.1%. Therefore, the values of x for h- and o- LaCO3OH·xH2O were estimated to be 0.73 and 0.25, respectively. The first weight loss in the temperature range from room temperature to ca. 400 ℃ was assigned to the loss of adsorbed water (xH2O), as expressed in equation (1). The abrupt weight losses above ca. 400 ℃ and 700 ℃ were attributed to reactions (2) and (3), respectively. The theoretical theoretical weight loss accompanied by reactions (2) and (3) is 15.0% and 11.9%, which were similar to 14.6% and 11.3% observed for o-LaCO3OH, respectively. A comparison of slopes in the temperature range from 500 to 700 ℃ indicated that the rate of the LaCO3OH to La2O2CO3 transformation is much slower for h- than o-LaCO3OH. DTA curves showed that reactions (2) and (3) were endothermic. In addition, the endothermic peak for reaction (2) appeared at higher temperature for o-LaCO3OH than for h-LaCO3OH, whereas the opposite was observed for reaction (3).

Figure 2.TG (solid line) and DTA (dotted line) curves of (a) h- and (b) o-LaCO3OH powders.

Figure 3.IR spectra of (a) o-LaCO3OH, (b) o-LaCO3OD, and (c) h-LaCO3OH powders.

Figure 3 shows IR spectra of the h- and o-LaCO3OH powders. The IR spectrum (Figure 3(a)) of the o-LaCO3OH powder was similar to those of o-LaCO3OH powders obtained by boiling aqueous solution containing La(NO3)3 and urea3 and by a reaction of LaBrOH with CO2 9 but different from that of an o-LaCO3OH powder prepared in an autoclave. 17 The OH-stretching band appeared at 3443 cm−1 for o-LaCO3OH and at 3617 and 3484 cm−1 for h-LaCO3OH. In many reports, the two bands for h-LaCO3OH were assigned to structural OH and adsorbed H2O,11,14,16,18 but they can both be assigned to the stretching bands of two nonequivalent structural OH groups.9 The very broad band centered at ca. 3300 cm−1 in Figure 3(c) was ascribed to the OHstretching of adsorbed H2O because the IR spectrum measured after drying at 200 ℃ for 2 days showed that the stretching bands of two structural OH groups were invariant and the very broad band centered at ca. 3300 cm−1 almost disappeared. The bands below 1500 cm−1 in Figure 3 were assigned to the vibrational modes of coordinated CO3 2− ion. Four normal modes of free CO3 2− ion appear at 1063 cm−1 (ν1), 879 cm−1 (ν2), 1415 cm−1 (ν3), and 680 cm−1 (ν4).29 Lowering of symmetry from D3h to C2v or Cs by coordination causes the presence of an IR-inactive ν1 mode and splitting of the degenerate ν3 and ν4 modes.30 As shown in Figure 3(a), ν3 and ν4 modes were split into two peaks (1478 and 1407 cm−1, 723 and 694 cm−1, respectively) and ν1 mode was observed at 1074 cm−1 for o-LaCO3OH. The peak at 856 cm−1 was attributed to the ν2 mode. The peak at 795 cm−1 was assigned to the deformation vibration of CO3 2− ion rather than some vibration of structural OH.11 This assignment is based on a comparison in the IR spectra (Figures 3(a) and 3(b)) between the o-LaCO3OH and o-LaCO3OD powders. The latter powder was prepared using D2O (99.9 atom% D, Sigma-Aldrich) instead of H2O as a solvent. The structural OD-stretching band appeared at 2542 cm−1 in Figure 3(b). Assuming that the force constant is the same in structural OH and OD groups, the structural OD-stretching frequency was estimated to be 2512 cm−1, which is close to that observed. Except the structural OD-stretching band, each peak in Figure 3(b) appeared at almost the same position as the corresponding peak in Figure 3(a), showing that the peak at 795 cm−1 is associated with deformation vibration of CO3 2− ion. The IR spectrum (Figure 3(c)) of the h-LaCO3OH powder was more complicated due to multiple splitting of all the modes. The multiple splitting can be interpreted in terms of nonequivalent carbonate groups.30

The morphology of the h- and o-LaCO3OH powders was observed by SEM. As shown in Figure 4, the h-LaCO3OH particles were almost spherical with mean particle size of ~600 nm, whereas the o-LaCO3OH particles were rhombic.3

Figure 4.SEM images of (a) h- and (b) o-LaCO3OH powders.

Transformation of LaCO3OH:Ln3+ to La2O2CO3:Ln3+ and La2O3:Ln3+. As shown in Figure 2, LaCO3OH decomposed to La2O3 through La2O2CO3. Figure 5 shows XRD patterns of the powders obtained by calcination of h- and o- LaCO3OH powders at 600 and 800 ℃ for 2 h. The calcination of both powders at 600 ℃ gave the hexagonal (h-) La2O2CO3 (JCPDS Card No. 37-0804), as in other studies.4,15 As shown in Figures 5(a) and 5(b), the h-La2O2CO3 powder derived from o-LaCO3OH exhibited narrower diffraction peaks than that from h-LaCO3OH, due to the larger particle size. The XRD pattern (Figure 5(c)) of the powder obtained by calcination of o-LaCO3OH at 800 ℃ was indexed to hexagonal (h-) La2O3 (JCPDS Card No. 05-0602), as for that of the powder obtained by calcination of h-LaCO3OH at 800 ℃.

Figure 5.XRD patterns of samples obtained by calcination of (a) o- and (b) h-LaCO3OH at 600 ℃ for 2 h and (c) by calcination of o-LaCO3OH at 800 ℃ for 2 h.

The effect of dopants on the polymorph of La2O2CO3 and La2O3 was examined with various dopants and concentrations. The calcination of h-LaCO3OH:Ln3+ (2 mol %, Ln = Ce, Eu, and Tb) at 600 ℃ for 2 h yielded h-La2O2CO3 powders. Their XRD patterns were the same as that (Figure 5(b)) of the powder obtained by calcination of h-LaCO3OH. On the other hand, the polymorph of the La2O2CO3 powders obtained by calcination of o-LaCO3OH:Ln3+ (2 mol %, Ln = Ce, Eu, Tb, and Ho) at 600 ℃ for 2 h depended on the dopants, as shown in Figure 6. Doping with 2 mol % Ce3+ ion resulted in pure h-La2O2CO3, but an increase in the doping concentrations of Ce3+ ion caused the formation of monoclinic (m-) La2O2CO3 (JCPDS Card No. 48-1113). Though not shown here, the calcination of o-LaCO3OH:Ce3+ (10 mol %) at 600 ℃ produced pure m-La2O2CO3. The pure m-La2O2CO3 was also obtained by calcination of La(OH)3 at 400 ℃ for 2 h.32 On the other hand, doping with Eu3+, Tb3+, and Ho3+ ions resulted in a mixture of h- and m-La2O2CO3. The relative quantity of the latter phase increased with decreasing ionic radius of the Ln3+ ion.

Figure 6.XRD patterns of samples obtained by calcination of o- LaCO3OH:Ln3+ (2 mol %, Ln = Ce, Eu, Tb, and Ho) at 600 ℃ for 2 h.

The calcination of o-LaCO3OH:Ln3+ (2 mol %, Ln = Ce, Eu, Tb, and Ho) at 800 ℃ for 2 h yielded h-La2O3 powders, irrespective of the species of Ln3+ ions. On the other hand, as shown in Figure 7, the calcination of h-LaCO3OH:Ln3+ (Ln = Eu and Tb) at 800 ℃ yielded an h-La2O3 powder with a very small amount of cubic La2O3 (ICCD–PDF # 98-005- 1138 and 98-006-9895).

Figure 7.XRD patterns of samples obtained by calcination of h- LaCO3OH:Ln3+ (2 mol %, Ln = Ce, Eu, and Tb) at 800 ℃ for 2 h.

Luminescence Properties of LaCO3OH:Eu3+ and La2O2- CO3:Eu3+ Powders. When acting as an emitting activator, the Eu3+ ion is an excellent structural probe because of high sensitivity of its luminescence spectra to changes in local symmetry. The effect of the polymorph on the luminescence spectra was examined for LaCO3OH:Eu3+ (2 mol %) and La2O2CO3:Eu3+ (2 mol %) powders. As shown in Figures 8(a-1) and 8(a-2), the emission spectra (at λex = 395.8 nm) of the h- and o-LaCO3OH powders were considerably different in shape, suggesting that the local symmetry of the Eu3+ sites occupied in h- and o-LaCO3OH are different. The spectrum (Figure 8(a-1)) of h-LaCO3OH:Eu3+ showed two unresolved bands at ca. 590 and 618 nm, which were attributed to the 5D0→7F1 and 5D0→7F2 transitions, respectively. The appearance of the two unresolved bands indicated the presence of two Eu3+ sites with similar local symmetry in h- LaCO3OH:Eu3+.

Figure 8.Emission spectra of (a-1) h- and (a-2) o-LaCO3OH:Eu3+ (2 mol %) powders (λex = 395.8 nm) and of samples obtained by calcination of (b-1) h- and (b-2) o-LaCO3OH:Eu3+ (2 mol %) powders (λex = 280.4 nm) at 600 ℃ for 2 h. The insets are their corresponding luminescence photographs at 254 nm UV lamp irradiation.

Figures 8(b-1) and (b-2) exhibit the emission spectra (at λex = 280.4 nm) of La2O2CO3:Eu3+ (2 mol%) powders obtained by calcination of h- and o-LaCO3OH:Eu3+ (2 mol %) at 600 ℃ for 2 h. The emission bands were more intense for o- LaCO3OH:Eu3+ than for h-LaCO3OH:Eu3+, but both emission spectra had a similar shape. The similarity can be explained in terms of the polymorph, respectively. As discussed above, the sample derived from h-LaCO3OH:Eu3+ was a pure h- La2O2CO3 powder, whereas the sample derived from o- LaCO3OH:Eu3+ was an h-La2O2CO3 powder with a small amount of m-La2O2CO3. As shown in the insets, the luminescence of four phosphors in Figures 8(a-1), 8(a-2), 8(b-1) and 8(b-2) were strong red, dark red, red, and vivid yellowred to the naked eye, respectively, at 254 nm UV lamp irradiation.

 

Conclusions

The polymorph of LaCO3OH prepared by a hydrothermal reaction depended on the reaction temperature, inorganic salt additive, species of Ln3+ dopant, and solvent. The calcination of o-LaCO3OH:Ln3+ (2 mol %) powders at 600 ℃ yielded a mixture of h- and m-La2O2CO3:Ln3+ powders. The relative quantity of the latter increased with decreasing ionic radius of the Ln3+ ion and increasing doping concentrations. On the other hand, the calcination of h-LaCO3OH:Ln3+ (2 mol %) powders at 600 ℃ gave a pure h-La2O2CO3:Ln3+ powder, irrespective of the species of Ln3+ ion (Ln = Ce, Eu, and Tb). The emission spectra of the h- and o-LaCO3OH: Eu3+ were considerably different in shape, suggesting that the local symmetry of the Eu3+ sites occupied in h- and o- LaCO3OH are different. The emission spectra of their thermal decomposition products at 600 ℃ had a similar shape due to the small difference in their polymorph.