Deep Eutectic Solvent Synthesis of Perovskite Electrocatalysts for Water Oxidation.

Oxide perovskites have attracted great interest as materials for energy conversion due to their stability and structural tunability. La-based perovskites of 3d-transition metals have demonstrated excellent activities as electrocatalysts in water oxidation. Herein, we report the synthesis route to La-based perovskites using an environmentally friendly deep eutectic solvent (DES) consisting of choline chloride and malonic acid. The DES route affords phase-pure crystalline materials on a gram scale and results in perovskites with high electrocatalytic activity for oxygen evolution reaction. A convenient, fast, and scalable synthesis proceeds via assisted metathesis at a lower temperature as compared to traditional solid-state methods. Among LaCoO3, LaMn0.5Ni0.5O3, and LaMnO3 perovskites prepared via the DES route, LaCoO3 was established to be the best-performing electrocatalyst for water oxidation in alkaline medium at 0.25 mg cm-2 mass loading. LaCoO3 exhibits current densities of 10, 50, and 100 mA cm-2 at respective overpotentials of approximately 390, 430, and 470 mV, respectively, and features a Tafel slope of 55.8 mV dec-1. The high activity of LaCoO3 as compared to the other prepared perovskites is attributed to the high concentration of oxygen vacancies in the LaCoO3 lattice, as observed by high-resolution transmission electron microscopy. An intrinsically high concentration of O vacancies in the LaCoO3 synthesized via the DES route is ascribed to the reducing atmosphere attained upon thermal decomposition of the DES components. These findings will contribute to the preparation of highly active perovskites for various energy applications.

Introduction

Perovskites with a general formula of ABO3± have attracted great interest for energy-related applications due to their structural flexibility and stability. Particularly, La-based perovskites, LaTO3 (T = 3d late-transition metal), have demonstrated high electrocatalytic activity for oxygen evolution reaction (OER),[1,2] which is crucial for rechargeable metal–air batteries as well as water electrolysis. However, overcoming slow kinetics caused by multi-electron transfer remains a great challenge and requires a large overpotential.[3]

To design a cost-effective and high-performance electrocatalyst for OER, substantial efforts have been made to understand the reaction mechanism and ideally to recognize effective activity descriptors.[4−6] Shao-Horn and co-workers have identified several descriptors relevant to the OER activity of perovskites,[6] namely the number of d electrons, charge-transfer energy (covalency), and optimal eg orbital occupancy. Additionally, structural factors, such as metal–oxygen–metal bond angles, were found to be relevant. Oxygen vacancies, VO, have also been shown to play a key role in the OER activity of perovskite oxide catalysts.[7−11]

Deep eutectic solvents (DESs) are emerging green solvents that are inexpensive, non-toxic, and biodegradable,[12] rendering them interesting media for sustainable synthesis of functional materials. They can be prepared by mixing a hydrogen-bond donor and acceptor (typically quaternary ammonium salt choline chloride) in a desired molar ratio.[13] Due to the formation of a eutectic, the melting point of the mixture is lowered as compared to its individual components.

We have previously shown that the DES synthesis route facilitates the preparation of complex oxides with mixed-valent states of metal cations,[14,15] giving access to mixed-valent ternary Zn and Cu vanadates. The reduced metal centers V4+ and Cu+ were found to modify electronic structures and optical properties of the resultant vanadates. This DES route, giving access to complex oxides with VO, prompted us to target green synthesis of perovskite electrocatalysts for alkaline OER.

Herein, we present a convenient, scalable, and green DES route to La-based perovskite oxides, LaTO3 (T = Mn, Mn/Ni, or Co), comprising dissolution of metal salts and binary metal oxides in environmentally benign solvents followed by heat treatment under air. The resultant materials were tested as electrocatalysts for water oxidation in alkaline medium, and LaCoO3 was found to exhibit the best catalytic activity among the prepared perovskites. High-resolution transmission electron microscopy (HRTEM) studies were carried out to gain insights into the structures, and a plausible reaction mechanism is discussed.

Results and Discussion

We developed a green DES synthesis route for LaCoO3, LaMn0.5Ni0.5O3, and LaMnO3 perovskites due to the expected high electrocatalytic activity of these materials in alkaline OER.[5,16,17] The DES of choice was a eutectic mixture of malonic acid (mp 135–137 °C) as the hydrogen-bond donor and choline chloride (mp 302 °C) as the hydrogen-bond acceptor mixed in a 1:1 molar ratio. The resulting eutectic mixture has a melting point of 10 °C. The binary precursors manganese oxide Mn2O3 and metal salts (LaCl3, NiCl2, and CoCl2) were dissolved in this medium at 70 °C. The produced solutions were then calcined at moderate temperatures of 200–400 °C in an open crucible under air to remove NH3(g) and HCl(g) side products stemming from the decomposition of the DES resulting in semi-amorphous solids. Upon further calcination at 900–1000 °C, the leftover hydrocarbon was removed via combustion, resulting in the crystallization of the target perovskites.

As determined by powder X-ray diffraction (PXRD), our DES route provides phase-pure perovskites LaCoO3, LaMn0.5Ni0.5O3, and LaMnO3 (Figure ). Structural parameters from Rietveld refinement are summarized in Figure S1 and Table S1.

Figure 1

Collected PXRD and calculated reference patterns of DES-derived perovskites.

The PXRD patterns of LaTO3 (T = Mn, Co) indicate a rhombohedral crystal structure with the space group R3̅c. TO6 octahedra are slightly tilted, namely rotated about the threefold rotation axis as compared to a cubic perovskite (Figure S2). The tilt angles (4.697° for Mn and[18] 5.3565° for Co[19]) are small due to the large ionic radius of 12-coordinated trivalent La.[20] A significantly larger unit cell volume of LaMnO3 cf. LaCoO3 arises from the greater ionic radius of high-spin Mn3+ cf. Co3+ in the octahedral environment (Table S1).

The PXRD pattern of LaMn0.5Ni0.5O3 is indexed in a monoclinic unit cell (P21/c, Figure S3a,b), although the possibility of a rhombohedral polymorph (R3̅, Figure S3c,d) present as a minor phase cannot be ruled out due to the similarity of the PXRD patterns. Moreover, the structural transition from a low-temperature monoclinic phase to a high-temperature rhombohedral phase occurs at room temperature according to Blasco and co-workers.[21] Notably, there are two distinct T–O (T = Mn, Ni) distances: 1.902 Å, which is typical for Mn4+ in metal oxides and 2.025 Å, which is characteristic of Ni2+.[21] As a result, the different octahedral environments introduce significant distortion in the perovskite structure and create two crystallographically independent B-sites. Neutron diffraction studies have revealed that there is a site preference for Mn and Ni cations in both polymorphs, classifying LaMn0.5Ni0.5O3 as a double perovskite with a nominal formula La2MnNiO6.[21,22]

As compared to traditional solid-state synthesis methods,[23,24] the DES methodology reported here provides rapid access to perovskite materials without the need of intermediate regrinding/reheating efforts or reaction temperatures >1000 °C. The DES protocol not only facilitates homogeneous mixing of metal cations in the solution but also favors the formation of reactive intermediates. Specifically, a labile ternary intermediate, LaOCl, was found to form at ≥400 °C (Figure S4), which further reacts with binary metal oxides (Mn3O4, NiO, or Co3O4) at 900 °C or 1000 °C to form the respective perovskites. According to the literature, these findings suggest that our synthesis follows an assisted metathesis reaction pathway. In particular, Todd and co-workers reported synthesis of YMnO3 via assisted metathesis.[25,26] The reaction between Mn2O3, YCl3·6H2O, and Li2CO3 can proceed according toin which binary oxides react at 600 °C, or according towhich is an example of metathesis reaction.[25,26] The latter reaction is faster and stabilizes metastable orthorhombic YMnO3 at a lower temperature of 500 °C because of reactive ternary intermediates yttrium oxochloride YOCl and lithium manganese oxide LiMnO2.[25,26] Considering that our DES route does not require multiple grinding steps or reheating at elevated temperatures, we conclusively prove that LaOCl is a kinetically labile intermediate in the DES synthesis of La-based perovskite oxides (Figure S4), similar to the assisted metathesis reaction pathway in the reported synthesis of YMnO3.

Next, we investigated the electrocatalytic activity of the newly synthesized perovskites in alkaline OER. To facilitate the preparation of the electrocatalytic inks and to increase the surface area for improved OER activity, the materials were ball-milled into fine powders. Notably, the ball-milling process does not result in structural changes in the materials but slightly decreases the average crystallite size (Table S2) and induces strain. The ball-milled perovskites were then formulated as inks in ethanol-containing conductive Nafion ionomer as a binder,[27] and the ink was deposited onto the Ni foam current collector. More details on the anode fabrication and electrochemical testing can be found in the Supporting Information. Experimentally, the perovskites were found to demonstrate the best OER properties at 0.25 mg cm–2 mass loading (Figure S5). In addition, the perovskites required prolonged activation via cyclic voltammetry (CV) for about 700 cycles to achieve steady-state conditions (Figure S6), after which the materials exhibit constant OER performance.

Figure and Table S3 summarize the electrochemical water oxidation properties in 1.0 M NaOH of DES-synthesized, ball-milled LaCoO3, LaMn0.5Ni0.5O3, and LaMnO3 electrocatalysts supported on Ni foam, in comparison with the standard reference IrO2 electrocatalyst supported on Ni foam as well as pure Ni foam. High electrocatalytic performance was observed for all prepared electrocatalysts (Figure a). LaCoO3 affords the most favorable apparent OER properties based on the geometric area, with current densities j of 10, 50, and 100 mA cm–2 reached at overpotentials η of around 390 (η10), 430 (η50), and 470 (η100) mV, respectively (Table S3). Notably, both Mn-containing perovskites, LaMnO3 and LaMn0.5Ni0.5O3, have comparable OER properties, while exhibiting higher OER overpotentials as compared to the best-performing LaCoO3 electrocatalyst (Figure a, Table S3). The shallow Tafel slope b = 55.8 mV dec–1 was estimated for LaCoO3, while LaMnO3 and LaMn0.5Ni0.5O3 demonstrate slightly higher values of b = 65.8 mV dec–1 and b = 60.3 mV dec–1, respectively (Figure b, Table S3).

Figure 2

Anodic polarization curves (a), Tafel (b), Nyquist (c), and chronopotentiometric (d) plots for Ni foam-supported perovskites and reference IrO2, all recorded in 1 M NaOH at room temperature with a mass loading of 0.25 mg cm–2. The inset in (c) shows an equivalent electrical circuit model used to fit the Nyquist plots.

Notably, significant capacitive currents were observed for perovskites between approximately 1.40 and 1.55 V in Figure a, which could be indicative of a variation in the electrochemically active surface area (ECSA) of the electrocatalysts. To investigate how the intrinsic OER activities of LaCoO3, LaMn0.5Ni0.5O3, and LaMnO3 materials compare, the activity data in Figure a were normalized to the measured double-layer capacitance Cdl (Figure S7, Table S3). Assuming that all the perovskites have a similar specific capacitance, Cp, and neglecting the contribution of cation intercalation into the material, ECSA can be inferred to be proportional to Cdl. From the resultant normalized data shown in Figure S8, one can conclude that the intrinsic activity of LaCoO3 (per unit ECSA) is higher than that of LaMn0.5Ni0.5O3 and LaMnO3, and thus, its better performance is not simply related to its higher ECSA.

To gain insights into the OER kinetics assuming aqueous electrochemical assembly,[28] electrochemical impedance spectroscopy experiments were carried out on a stable electrocatalytic system at ≈η10, as illustrated by the Nyquist plots (Figure c). Experimental data were fitted using an equivalent circuit model (Figure c, inset) accounting for a resistor, which is in series with parallel combination of a resistor and a constant phase element. Specifically, the resistor Rs is the Ohmic resistance from the electrolyte and all contacts. The time constant R1–CPE1 is associated with the interfacial resistance from electron transport between the electrocatalyst and Ni foam current collector, while the time constant Rct–CPE2 is the charge-transfer resistance (Rct) at the catalyst–electrolyte interface.[29] Importantly, shallow Rct values typically reflect faster charge-transfer kinetics.[29] The analysis of the obtained semi-circles (Figure c) as well as derived parameters (Table S3) reveals fastest charge-transfer kinetics over the anode/electrolyte interface in the case of Ni foam-supported LaCoO3, as reflected by its smallest Rct = 2.1692 Ω, compared to Ni foam-supported LaMn0.5Ni0.5O3 (Rct = 2.6762 Ω) and LaMnO3 (Rct = 2.7260 Ω).

Interestingly, we observed that the CV curves of the perovskites feature pre-oxidation peaks at potentials lower than their respective OER onsets (Figures a, S5, and S6). The trend of the pre-oxidation peak potential, LaCoO3 (1.51 V) > LaMnO3 ≈ LaMn0.5Ni0.5O3 (1.48 V), agrees well with that of charge-transfer resistance Rct, LaCoO3 < LaMnO3 ≈ LaMn0.5Ni0.5O3 (Table S3). These observed pre-oxidation peaks are characteristic of Co3+/Co4+ and Mn3+/Mn4+ redox couples in the respective perovskites, indicating oxidation of the electrocatalysts during alkaline OER.

Having established that LaCoO3 demonstrates excellent OER properties, we further investigated if this electrocatalyst can generate a stable current during alkaline OER. To this end, the anode was subjected to continuous stability testing by means of chronopotentiometry, where LaCoO3 showed excellent long-term stability in 1 M NaOH electrolyte during the tested 100 h, affording reasonably steady current density j of 10 and 100 mA cm–2 at overpotentials η of only ≈390 and 450 mV, respectively (Figure d).

The activity and stability of the synthesized perovskite electrocatalysts were also compared to the standard platinum group metal reference IrO2 electrocatalyst at the same mass loading of 0.25 mg cm–2 (Figure ). Despite possible differences in the surface area between LaCoO3 and IrO2, the perovskite can be deduced to offer at least comparable OER properties to the significantly more expensive and less abundant reference electrocatalyst. Overall, the low OER overpotential, shallow Tafel slope, fast kinetics, and stable long-term performance of DES-synthesized LaCoO3 (Figure , Table S3) highlight the potential of this material in electrolysis applications.

To rationalize the observed excellent activity and stability of our LaCoO3 in alkaline water oxidation, we carried out detailed electron microscopy investigations of the material before and after electrocatalytic testing. Figure S9 shows a representative z-contrast high-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) image of the sample after ball milling but before OER, together with the corresponding energy-dispersive X-ray spectroscopy mapping in the STEM mode (STEM–EDX). These images indicate that La, Co, and O elements are uniformly distributed across the sub-μm-sized LaCoO3 particles. Notably, neither N nor Cl elements are present in the as-synthesized LaCoO3, confirming that DES constituents are eliminated during combustion reaction. This contrasts with the recent report about N doping into O sites in LaCoO3 prepared by annealing of the pristine oxide powder in the atmosphere of ammonia gas.[30]

Interestingly, high-resolution HAADF–STEM and selected area electron diffraction (SAED) analyses of LaCoO3 before OER evidence the existence of a large number of oxygen vacancies, VO, in the sample (Figure ) clearly manifested in the HAADF–STEM images by the presence of dark stripes in the Co–O layers. VO are also highlighted by the appearance of weak superstructure spots in the respective SAED patterns, which are attributed to the ordering of the dark stripes. The measurement of the interlayer distances revealed the expansion of the La–La distance from a typical 0.38 to 0.46 nm in the VO stripe. Both the appearance of dark contrast Co–O layers and the increase in La–La distances can be attributed to VO layers, as has been previously observed in perovskite structures.[31−33] Such layers of VO change Co coordination from CoO6 octahedral to CoO5 square pyramidal. The formation of VO in the DES-synthesized perovskites is consistent with our previous studies,[14,15] where we used a DES route to prepare mixed-valent ternary Zn and Cu vanadates (M2V2O7– and MV2O6–, M = Zn or Cu) and found annealing the reaction mixture to intrinsically introduce VO.

Figure 3

Top panel: representative [010] HAADF–STEM images of LaCoO3 after ball milling together with the corresponding SAED patterns (insets) evidencing dark stripes of VO. White arrows in the magnified selection of the SAED pattern (right top panel inset) indicate superstructure spots. Bottom panel: magnified HRTEM image with an overlaid structural model and intensity line scan profile showing periodic change of La–La distances from typical 0.38 nm to expanded 0.46 nm in VO stripes.

Remarkably, to the best of our knowledge, there are no studies reporting the formation of VO in bulk LaCoO3 prepared without doping of metal sites or any post-treatment under reducing conditions, and so far, such examples have only been reported in LaCoO3 films.[32,34] Even annealing of pristine LaCoO3 in the reducing atmosphere of ammonium gas does not induce the formation of VO, with HAADF–STEM of N-doped LaCoO3 indicating a regular perovskite structure.[30] Most notably, transmission electron microscopy (TEM), HAADF–STEM, and SAED investigations of our LaCoO3 electrocatalyst after stability testing at a constant current density of 10 mA cm–2 for 100 h (Figure d) clearly indicate that the VO are preserved in the structure of spent LaCoO3 (Figure ).

Figure 4

Representative low-magnification [110] TEM and [001] HAADF–STEM (left panels) as well as high-resolution [001] HAADF–STEM (right panel) images together with the corresponding SAED patterns (insets) for LaCoO3 after alkaline OER stability testing at a constant current density of 10 mA cm–2 for 100 h (Figure d), highlighting that VO and their ordering have been preserved in the structure.

Interestingly, the high-resolution HAADF–STEM and SAED study of the LaMnO3 electrocatalyst with the lower electrocatalytic activity for OER as compared to LaCoO3 showed no evidence of VO (Figure ). Typical La–La distances of 0.39 nm were observed in the structure. We hypothesize that such a difference in the VO formation between LaCoO3 and LaMnO3 can be attributed to the higher reduction potential of Co3+/Co as compared to Mn3+/Mn, affording greater stability of the +3 oxidation state for Mn in LaTO3 as compared to Co. Thus, partial reduction under the conditions of our DES route is more likely to occur in the case of LaCoO3. Similarly, no evidence of VO was found in the case of LaMn0.5Ni0.5O3 (data not shown).

Figure 5

Left panel: Representative HAADF–STEM images of LaMnO3 after ball milling, together with the corresponding SAED pattern (insets), evidencing the absence of VO in the material. White arrows indicate the typical La–La distances of 0.39 nm. Right panel: HAADF–STEM image with simultaneously collected STEM–EDX mappings of La, O, Mn, and their mixture, indicating a homogeneous distribution of all the elements within the sample.

For the LaCoO3 catalyst reported herein, a plausible electrocatalytic mode of action is consistent with lattice OER (LOER).[4,5] Although OER over perovskites or other oxides can proceed via a conventional four-consecutive proton-coupled electron transfer mechanism with metal cations as the catalytically active sites,[35] examples of alternative or simultaneously occurring LOER mechanisms are increasingly reported in the literature.[36] In LOER, the oxidation of the lattice oxygen of the electrocatalyst is accompanied by continuous dissolution–redeposition of metal cations, which leads to O2 evolution.[37] The resultant VO, formed by the evolution of the lattice O, are then compensated by OH– from the alkaline electrolyte,[38] affording stability to the electrocatalyst and preventing collapse due to depletion in structural O. Furthermore, the LOER mechanism has been demonstrated to be closely associated with the presence of VO in perovskites. For example, in La1–SrCoO3−δ, the existence of VO accelerates the dissolution of La/Sr via the LOER mechanism, thus facilitating the in situ reconstruction of the perovskite surface into a highly active CoO(OH) shell.[39] Accordingly, VO have proven to be one of the main factors governing the activity of perovskites in alkaline OER,[40] somewhat similar to the field of solid oxide fuel cells.[41]

In our study, dissolution of the surface La3+ occurs upon OER, resulting in the segregation of a La- and O-containing phase next to the well-isolated LaCoO3 particles as indicated by STEM–EDX analysis (Figure S10) of LaCoO3 after OER stability testing for 100 h (Figure d). This is most likely La(OH)3, formed by the precipitation of the as-leached La3+ in NaOH electrolyte solution under anodic oxidation conditions.[42] The surface of LaCoO3 could then be reconstructed in situ into a tiny Co-rich shell, perhaps CoO(OH).[39] In our previous studies, we clearly confirmed the in situ formation of active and stable shells of Fe, Co, or Ni oxide/oxyhydroxide nanoparticles during OER over various phosphide and boride pre-catalysts.[43−46] In sharp contrast, the formation of such a Co oxide/oxyhydroxide shell was not observed by TEM on the surface of our spent LaCoO3 (Figure ). As such, the process of the formation of a Co-rich shell over our LaCoO3 may be rather dynamic owing to the continuous dissolution–redeposition of Co.[39] Furthermore, the thus-formed Co-rich shell is suggested to deliver O2 through anodic oxidation of lattice O in LaCoO3. Most notably, the presence of a large number of VO in LaCoO3 is proposed to enhance its OER activity via high oxygen diffusivity,[7] thus affording efficient refilling of the evolved lattice oxygen.[40,47] The fact that VO remain after 100 h of alkaline OER over our LaCoO3 (Figure ) indicates that there is a dynamic equilibrium state between electrolyte–surface–bulk during OER.[39,40,48] Importantly, inductively coupled plasma optical emission spectroscopy chemical analysis of the electrolyte after alkaline OER testing did not reveal any traces of La, Ni, Co, or Mn but merely the presence of 0.022 ppm of Pt admixture, which, most likely, stems from leaching of the Pt wire counter electrode.

Perovskites featuring oxygen vacancies have been shown to be highly active electrocatalysts for alkaline water oxidation. For example, plasma treatment was used to introduce VO in PrBa0.5Sr0.5Co1.5Fe0.5O5+δ, resulting in highly active catalyst material with b = 94 mV dec–1.[8] In another study, the four-layered perovskite La5Ni3CoO13−δ, synthesized through glycine nitrate combustion, was found to be enriched with oxygen defects and showed exceptional electrocatalytic OER activity (b = 35 mV dec–1).[49] Ce-doping of LaCoO3 was found to significantly increase the concentration of VO as compared to pristine LaCoO3, with La0.96Ce0.04CoO3−δ synthesized using a microwave/ultrasound-assisted hydrothermal route (b = 80 mV dec–1), outperforming pure LaCoO3 (b = 124 mV dec–1) in alkaline OER.[9] The herein reported DES synthesis route provides a straightforward approach to introduce VO in LaCoO3, placing the material with η10 = 390 mV and b = 55.8 mV dec–1 among the best of the state-of-the-art perovskite electrocatalysts for alkaline water oxidation (Figure ).

Figure 6

Comparison between the Tafel slopes as well as overpotentials required for driving current density of 10 mA cm–2 for our best-performing electrocatalyst, LaCoO3, and reported state-of-the-art perovskite electrocatalysts for alkaline water oxidation.[8,9,49−54]

Conclusions

In summary, we have prepared a series of La-containing perovskites LaTO3 (T = Mn, Mn/Ni, or Co) using an environmentally friendly DES synthesis route. DES synthesis enables uniform mixing of metal precursors and the formation of labile intermediates, while the unique reaction environment achieved upon DES calcination facilitates formation of oxygen vacancies in the LaCoO3 structure. LaCoO3 was found to be a stable electrocatalyst with OER activity among the best reported for perovskites. The high activity was attributed to the presence of a large number of structural oxygen vacancies as evidenced by electron microscopy. Further studies that address a deeper understanding of the effect of oxygen vacancies on OER over DES-derived perovskites, as well as the optimization of their OER properties via nanostructuring, will be the subjects of our future research efforts.

Experimental Section

Catalyst Synthesis

The DES was prepared by mixing malonic acid and choline chloride in a 1:1 molar ratio. The mixture was heated in a beaker at 70 °C until it became liquid. Metal precursors were added to the solvent and dissolved under vigorous stirring at 70 °C (0.7158 g of LaCl3 and 0.2304 g Mn2O3 in 87 g of the solvent for LaMnO3 | 0.5368 g of LaCl3, 0.0865 g Mn2O3, and 0.1418 g NiCl2 in 72.5 g of the solvent for LaMn0.5Ni0.5O3 | 0.6135 g of LaCl3 and 0.3248 g CoCl2 in 80.0 g of the solvent for LaCoO3). Afterward, the metal-precursor-containing solution was transferred to a porcelain crucible for calcination (7 g of solution in a 30 mL crucible). The heat treatment was performed in a box-type muffle furnace for all the samples. For all heat treatments, a heating rate of 10 °C min–1 was used.

For the synthesis of LaMnO3 and LaMn0.5Ni0.5O3, the precursor solution was heated at 230 °C for 2 h and then calcined at 900 °C for LaMnO3 or 1000 °C for LaMn0.5Ni0.5O3 in the span of 8.5 h. LaCoO3 was prepared by heating the precursor solution at 300 °C for 2 h, followed by calcination at 900 °C for 8.5 h. Calcination was conducted in an open crucible under the air. Afterward, the samples were cooled to room temperature by switching off the furnace.