Well-monocrystallized LaCrO₃ particles from a LaCrO₄ precursor by supercritical hydrothermal technique

Abstract

LaCrO₃ based materials are currently utilized in a variety of electrical applications at high temperature. However, the morphology of LaCrO₃ powder is much less understood as compared to its bulk counterpart. In this work, LaCrO₃ powder was synthesized from LaCrO₄ precursor by a supercritical hydrothermal technique (SHT). The majority exhibited a pure orthorhombic phase with well-monocrystallized grains of 3–9 μm. This work confirms that dominating faces in LaCrO₃ grains are {0 1 0} and {0 1 1} surfaces, which have equivalent surface-energy due to the similarity of their surface structure.

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1. Introduction

LaCrO₃ and related materials have drawn great attention recently as high-temperature electrodes in magnetohydrodynamic power generators, oxygen sensors, oxygen transport membranes for air separation and oxy-fuel combustion. They are also used as heating elements for electric appliances and interconnects in solid oxide fuel cells.^1–4 Many methods, through modifying raw materials, preparing temperature (T = 180–1600 °C), time (5 min–168 h or longer) and pressure (P = 1 atm–10.3 GPa), have been used to investigate LaCrO₃. Among them the mild hydrothermal technique (MHT) is applied at 240–260 °C for 96–168 h ^5,6 in an autoclave filled with aqueous solution under autogenous pressure (e.g. 260 °C/4.7 MPa). The obtained LaCrO₃ powder has a morphology with a cubic habit (1–13 μm). The supercritical hydrothermal technique (SHT) has advantages over MHT to obtain LaCrO₃ powder with high purity and homogeneity, fine crystallinity, narrow size distribution and controlled particle morphology. Beyond the supercritical point (373.946 °C/22.064 MPa)⁷ in the liquid–vapor space, water exists as small but liquid-like (associated) hydrogen-bonded clusters dispersed within a gas-like (dissociated) phase. The physical properties of the solvent such as dielectric constant (ε), density, ionic product, viscosity, self-diffusion, compressibility, etc. vary with T and/or P. The chemical properties are also greatly changed due to the changes in dissociation, solubility, diffusivity and reactivity aroused by hydrogen-bonding decreasing.^8,9

Yoshimura et al.^10,11 prepared LaCrO₃ powder via SHT using La₂O₃ and Cr(OH)₃·xH₂O as starting materials. Pure orthorhombic LaCrO₃ with irregular grains (0.7 ± 0.2 μm) was obtained (700 °C/3 h/100 MPa). Their work is innovative but has obvious imperfections. The oxide of La₂O₃ reacts heterogeneously with the hydrate of Cr(OH)₃·xH₂O. The obtained grains are irregular. The hydrate can easily absorb CO₂ from atmosphere, thus introducing impure LaCO₃OH phase. Another work by Rivas-Vázquez et al.^12,13 may be considered as a quasi-SHT using LaCl₃·7H₂O, Cr(NO₃)₃·9H₂O and NaOH as starting materials, because its temperature (350–425 °C) is in the vicinity of the supercritical point. Pure orthorhombic LaCrO₃ with irregular grains (∼0.3 μm) was obtained. The reaction temperature and autogenous pressure are not high enough and the time is too short (1–2 h). The precursor from the strong alkaline solution contains Na⁺ which is difficult to be eliminated.

Considering the supercritical water is a poor solvent for polar molecules but excellent for non-polar ones due to its low dielectric constant and poor H-bonding, many non-polar compounds could be completely miscible.^8,9,14 Higher pressure than the supercritical point is beneficial to enhance the reaction rate and supersaturation during nucleation and crystallization, but it would arouse the dielectric constant increasing. Otherwise, the Cr–O bond with [CrO₄] tetrahedra in LaCrO₄ has a stronger covalent character than that with [CrO₆] octahedra in LaCrO₃, and the La–O bond in LaCrO₄ is more ionic than that in LaCrO₃.¹⁵ Therefore, this work is to synthesize LaCrO₃ powder via SHT under the condition (700 °C/200 MPa) which is different from those just beyond the supercritical point, and using a non-hydroxide of LaCrO₄ precursor as the starting material. The phase purity, crystallization, particle size distribution and morphology are highly concerned.

2. Experimental procedure

LaCrO₄ precursor was synthesized by co-precipitation and subsequent calcination at 600 °C.^16,17 Raw materials were analysis-graded La₂O₃, Cr₂O₃, HNO₃ and ammonium solution (LOBA Chemie Co. Ltd.), and double-distilled water. Then the LaCrO₄ precursor (0.19 g) was encapsulated with water (0.24 g) in a gold capsule (4 mm external dia., 40 mm long, 0.1 mm thick, ∼0.45 mL) and vacuum-sealed by arc welding. The capsule was placed in the autoclave after seal-checking by hot water. Finally, LaCrO₃ powder was obtained via SHT in an externally-heated Tuttle–Roy vessel at 700 °C for 96 h under 200 MPa (H₂O). Experiments were repeated (e.g. using LaCrO₄ ∼0.2 g and water 0.24–0.29 g) for several times to verify the reproducibility of products.

The reaction process was studied by a differential scanning calorimetric and thermo-gravimetric instrument (DSC/TG, Netzsch STA 449C, Germany) at 10 °C min⁻¹ in N₂ (20 mL min⁻¹). Phase determination was carried out by X-ray diffraction (XRD) at room temperature (RT), using a D8 Advance diffractometer (Bruker, Germany) at 0.02° s⁻¹ (Cu K_α, 40 kV, 40 mA). The microstructure and composition of samples were examined by a field-emission scanning electron microscope (FESEM, HITACHI S-4800, Japan) and a transmission electron microscope (TEM, JEM-2100, JEOL, Japan) with selection area electron diffraction (SAED), equipped severally with an X-ray spectrometer for energy dispersive spectroscopy (EDS) analyses.

3. Results and discussion

3.1. Reaction process from LaCrO₄ precursor to LaCrO₃ powder

DSC/TG profiles of the precursor and powder via SHT, coupled with differential thermal gravity (DTG) curve, are shown in Fig. 1.

Fig. 1 DSC/TG profiles of the precursor (a) and powder via SHT (b) tested in N₂.

For the precursor, the endothermic peak at 814.7 °C, mass loss of −7.11 wt% (theoretical value −6.28 wt%) and DTG valley ranging 650–850 °C correspond to the decomposition:

This is in agreement with the reported (663–840 °C).^15,18,19 The endothermic peak at 1083.1 °C corresponds to the transition of LaCrO₃ from rhombohedral phase to cubic one, in agreement with the reported by Gupta et al. (1000–1300 °C)²⁰ but not with that by Hofer et al. (1550–1600 °C).²¹

For the powder via SHT, the endothermic peak at 240.1 °C corresponds to the transition of LaCrO₃ from orthorhombic phase to rhombohedral one, in agreement with the reported (236–290 °C).^{5,6,12,13,19,20} The exothermic peak at 710.5 °C, mass loss of −0.35 wt% and DTG valley correspond to the crystallization of trace La-rich compounds aroused by SHT (Fig. 2 and 3, Table 1, eqn (4) and (5)). The endothermic peak corresponding to the transition of LaCrO₃ from rhombohedral phase to cubic one has been merged in the wide valley. The exothermic peak at 1152.7 °C, mass increase of 1.45 wt% and DTG peak correspond to La₂Cr₃O₁₂ formation from the relieved O₂ and La₂O₃/Cr₂O₃ impurities which induced by the unexpected LaCrO₄ decomposition:¹⁵

Fig. 2 XRD patterns of the precursor (a) and powder via SHT (b) determined at RT. The inset is the enlargement from 30° to 80° for the powder via SHT.

Fig. 3 SEM and EDS results of the LaCrO₃ powder. (a) Grain morphology with different crystal surfaces: facetted morphology with {0 1 0} and {0 1 1}, and rectangular prismatic morphology with {0 0 1̄}, {1 1̄ 0} and {1̄ 1̄ 0}. (b–f) Particles are classified through EDS mapping in the different microarea: 0.1–0.4 μm (No. 1); 0.5–1 μm (No. 2); 1–3 μm (No. 3); 3–9 μm (No. 4); >9 μm (No. 5).

Table 1 EDS mapping microarea, grain size, La/Cr ratio and particles quantity of the LaCrO₃ powder via SHT

EDS mapping microarea	Grain size (μm)	La/Cr (mol%)	Particles quantity
No. 1	0.1–0.4	∼1	Trace
No. 2	0.5–1	1.4–1.7	Trace
No. 3	1–3	6–8	Trace
No. 4	3–9	∼1	Majority
No. 5	>9	∼1	Trace

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3.2. Phase determination of LaCrO₄ precursor and LaCrO₃ powder

XRD patterns of the precursor and powder via SHT are shown in Fig. 2.

The precursor resultant exhibits a pure monoclinic LaCrO₄ (pdf 89-0448, CSD 81938).^18,19 For the powder via SHT, the identified double peaks can tell that the powder is nearly a pure orthorhombic LaCrO₃ (pdf 89-4504, CSD 50755)²² with trace impurities. Here the peaks ranging 90–140° for LaCrO₃ powder were identified according to the refinement based on CSD 50755, and the refined lattice parameters are a = 5.5162(3) Å, b = 5.4802(6) Å, c = 7.7583(3) Å. The reaction can be expressed as:

Where ΔG = ΔG⁰ + 1/2RT ln P_O2 and ΔG⁰ = 85731 − 85.096T.¹⁵ When ΔG = 0, the LaCrO₄ starts to decompose: ln P_O2 = −20 623/T + 20.471. Initially these equations satisfied an open system, while this work involved a closed one. Considering that supercritical water is completely miscible with non-polar oxygen,^14,23 the above equations could then be imported in our system. At the supercritical point of 373.946 °C, P_O2 is equal to 10⁻⁵ atm. At the process temperature of 700 °C, P_O2 is 0.48 atm. These indicate the SHT is a favorably thermodynamic system. If it reacted completely, the oxygen partial pressure built up inside the capsule (0.45 mL) would arrive at ∼6.7 MPa by the decomposition of precursor (0.19 g) at 700 °C. So the water pressure would be under ∼193.3 MPa, which remains all the same at the supercritical state. Otherwise, the formation of tiny impurity phases of La₂CrO₆ and Cr₂O₃ etc. (Fig. 2 and 3, Table 1, eqn (4) and (5)) would relieve the oxygen pressure. During the decomposition of LaCrO₄ to LaCrO₃, oxygen coordination around La atom(s) changes from 9 to 12 and around Cr atom(s) from 4 to 6. The major structural rearrangement during decomposition is thus mainly linked to high activation energy (∼18 kJ mol⁻¹)²⁴ and slow kinetics.¹⁵

3.3. Microstructure and chemical composition of LaCrO₃ powder via SHT

Fig. 3 and Table 1 list the SEM/EDS results of the LaCrO₃ powder. Five kinds of particles are classified based on grain sizes through EDS mapping in the microarea No. 1 to No. 5, respectively. The majority of particles are pure (La/Cr ≈ 1) and well-crystallized grains (3–9 μm, No. 4). The others have only a trace quantity. Most grains have facetted morphology with {0 1 0} and {0 1 1} as dominating faces. A few of them have rectangular prismatic morphology with {0 0 1̄}, {1 1̄ 0} and {1̄ 1̄ 0} (Fig. 3a). This is different with the reported^10–13 in which irregular particles attained by SHT. Through ordinary MHT, LaCrO₃ particles are with a cubic habit.^5,6 Our work confirms first that dominating faces in LaCrO₃ grains are {0 1 0} and {0 1 1} which have equivalent surface-energy due to the similarity of their surface structure.²⁵ Such morphology could previously only be found in sintered samples through high temperature (e.g. 1450 °C/10 h/air),²⁰ indicating a drastically-increasing sinterability through SHT process.

Particles with small (0.1–0.4 μm, No. 1) or very large (>9 μm, No. 5) sizes are also pure LaCrO₃ (La/Cr ≈ 1). Two kinds of La-rich particles were found. One has a grain size of 0.5–1 μm with a La/Cr ratio of 1.4–1.7 (No. 2). The other is 1–3 μm with a La/Cr ratio of 6–8 (No. 3). The crystallite formation, growth and crystallization, from the LaCrO₄ precursor to LaCrO₃ grains, are initially as:¹⁵

Being more amphoteric, the Cr³⁺-hydroxide condenses less readily than the La³⁺-hydroxide at a given alkalinity.⁶ The oxidation product of tiny CrO₄²⁻ ions could be dissolved in the supercritical water and washed out later. This explains the large La/Cr ratio of 1.4–1.7 in La-rich particles.

Next, La₂CrO₆ decomposes to a mixture of LaCrO₃ and La₂O₃:¹⁵

Same analysis in eqn (4) can be applied in eqn (5). The large La/Cr ratio of 6–8 is due to the La₂O₃ resultant. The crystallization of one oxide in ordinary MHT proceeds by a dissolution–crystallization mechanism. However, the crystallization of LaCrO₃ in SHT proceeds simultaneously with the decomposition of LaCrO₄.^12,13 The supercritical water gives a favorable reaction field for particle formation, owing to the enhancement of reaction rate and large supersaturation during nucleation and crystallization through lowering the solubility of LaCrO₃ resultant. The process lasted 96 h and the reaction equilibrium is sufficient.

TEM images and SAED patterns of two typical LaCrO₃ grains (a and b) are shown in Fig. 4. The dark contrast is aroused by large size of grains (∼5 μm for Grain-a in Fig. 4a-I, and ∼9 μm for Grain-b in Fig. 4b-I). Both grains have a La/Cr ratio close to 1 by EDS (not show here). From the SAED pattern of Grain-a (Fig. 4a-II), it can only be indexed as [0 0 1] zone axis of the orthorhombic LaCrO₃. By tilting 18.85°, the [1̄ 1̄ 3] zone axis was obtained (Fig. 4a-III). For Grain-b, the [1̄ 1̄ 2] (Fig. 4b-II) and [0 2̄ 3] (Fig. 4b-III) zone axes with tilting degree of 19.7° can also be indexed to the orthorhombic phase. These SAED analyses have unambiguously confirmed the crystal symmetry of orthorhombic structure in LaCrO₃ samples by SHT. No cubic LaCrO₃ particles were observed. LaCrO₃ particles with single crystallites give a promising prospect for better properties comparing to those with polycrystallites.

Fig. 4 TEM bright field images and SAED patterns of two orthorhombic LaCrO₃ grains. (a-I) Grain-a (∼5 μm). (a-II) [0 0 1] zone axis. (a-III) [1̄ 1̄ 3] zone axis. The tilting degree of [0 0 1] and [1̄ 1̄ 3] is 18.85°. (b-I) Grain-b (∼9 μm). (b-II) [1̄ 1̄ 2] zone axis. (b-III) [0 2̄ 3] zone axis. The tilting degree of [1̄ 1̄ 2] and [0 2̄ 3] is 19.7°.

4. Conclusions

In this work, nearly pure orthorhombic LaCrO₃ particles, with well-monocrystallized grains of 3–9 μm, were synthesized at 700 °C for 96 h under 200 MPa by supercritical hydrothermal technique. The results confirm first that most of the obtained LaCrO₃ grains have facetted morphology with {0 1 0} and {0 1 1} as dominating faces. Such morphology could previously only be found in the sintered samples through high temperature, indicating a drastically-increasing sinterability through this process.

Acknowledgements

Authors gratefully thank Mr J.-H. Liu, Mr Z.-S. Gao, Mr D. Chen, Dr J. Li, Ms G.-G. Zhao and Ms H.-F. Zhang in WUT, Prof. B.-L. Wu in GUT, and Prof. R. Huang in ECNU for his TEM analysis. This work was supported by the Postdoctoral Foundation of WUT (Grant No. G2014-2016), and the Research Foundation of SKLWUT, China (Grant No. 2014-KF-6, 2015-KF-4 & 2016-KF-4).

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