Structure

4.4. Structure

4.4.1. Effect of Synthesis Temperature on the Crystal Structure of Nanoparticles

To investigate the influence of the synthesis temperature on the crystal structure of the nanoparticles, LSC and NiO-GDC20 powders are synthesized by SASP method at various temperatures, while the synthesis pressure, the total concentration of cations, and the NaCl concentration are kept constant at 900 mbar, 0.05 M, and 1.0 M, respectively. The X-ray diffraction (XRD) patterns of LSC powders synthesized at temperatures between 700 and 1000 °C are shown in Figure 4-8. At room temperature the samples exhibit rhombohedrally distorted perovskite structure with the space group R3̅c (no. 167). The lattice parameters obtained from the Rietveld refinement of the sample synthesized at 775 °C, 𝑎 = 5.4095(8)Å and 𝛼_𝑖𝑛𝑡 = 60.31(4)°, are in good agreement with literature data obtained for powders of the same elemental composition (𝑎 = 5.4048 Å and 𝛼_𝑖𝑛𝑡 = 60.33° [205]). According to Petrov et. al. [83], the change from cubic to rhombohedral structure at room temperature is caused by the La/Sr ratio being less smaller 1. The absence of NaCl reflections in XRD patterns confirms the complete removal of salt phase by the water rinsing procedure. Yet, a minor secondary phase of strontium carbonate (SrCO3, ICSD #62) is observed in powders synthesized at 700 °C. The formation of SrCO3 has been also reported for

(a) (b) (c) (d) (e)

25 nm

Figure 4-7: STEM image of the LSC sample (a) and high-resolution STEM-EDS elemental maps showing the distribution of (b) La, (c) Sr, (d) Co, and (e) O in nanoscale range.

Figure 4-8: X-ray diffraction patterns of LSC samples synthesized at various pyrolysis temperatures between 700 C and 1000 °C (after water rinsing).

the La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes obtained by USP method, where the precursor chemistry is modified by an organic complexing agent to obtain nanoparticles with high surface areas. Even though the precursor solution and reaction gas are carbon free in SASP method, the formation of SrCO3 can be explained by the dissolution of CO2 within the precursor solution or the intrusion of CO2 into the reaction zone through a gas leak during the synthesis. As the synthesis temperature is increased to 775 °C and above, the formation of the secondary SrCO3 phase is avoided and single-phase LSC powder is obtained at all synthesis temperatures. The increase of the synthesis temperature also leads to narrowing of the XRD reflections indicating the formation of larger crystallites, which is confirmed by Rietveld analysis. Figure 4-9 shows the XRD pattern of the LSC powder synthesized at 775 °C and its representative Rietveld analysis. The calculated lattice parameters and crystallite sizes are summarized in Table 4-4. The results reveal an increasing growth pattern of crystallite size of LSC nanoparticles as the synthesis temperature increases, which is most likely caused by the faster grain growth processes at high synthesis temperatures.

On the other hand, the lattice parameters are independent of the synthesis temperature.

Table 4-4: Crystallite size and lattice parameter of LSC powders synthesized at 700 °C, 775 °C, 900 °C, and 1000 °C calculated by Rietveld refinement.

Synthesis temperature (°C) 775 900 1000

Crystallite size (nm) 11(1) 12.0(9) 13.9(6) Lattice parameter, 𝒂 (Å) 5.4095(8) 5.4091(6) 5.410(4) Interaxial angle, 𝜶_𝒊𝒏𝒕 (°) 60.31(4) 60.29(3) 60.30(3)

Figure 4-9: Rietveld refinement of LSC sample synthesized at 775 °C

Figure 4-10 shows the XRD patterns of NiO-GDC20 (60:40 wt.%) composite powders synthesized at temperatures between 700 °C and 1000 °C. The synthesis at 700 °C results in a completely amorphous powder, since no reflections are detected in the XRD pattern. As the synthesis temperature is increased to 775 °C, the broad reflections belonging to NiO and GDC phases start to appear. Further increase of the synthesis temperature leads to narrowing of the XRD reflections indicating the formation of larger crystallites at higher synthesis temperatures. The crystalline

Figure 4-10: X-ray diffraction patterns of NiO-GDC20 samples synthesized at various pyrolysis temperatures between 700

°C and 1000 °C (after washing).

Figure 4-11: Rietveld refinement of NiO-GDC20 sample synthesized at 775 °C

powders consist of NiO and GDC phases with cubic rock salt and fluorite type structures (space groups of Fm3̅m (no. 225)), respectively. The lattice parameters of NiO and GDC phases obtained from the Rietveld analyses are 0.4176(7) nm and 0.5423(8) nm, respectively, which are in good agreement with reported values (𝑎_𝐺𝐷𝐶=0.5426 nm [206] and 𝑎_𝑁𝑖𝑂=0.41763 nm [207]). The absence of Gd2O3 reflections in XRD patterns and a relatively larger lattice parameter of GDC (𝑎_𝐺𝐷𝐶=0.5421(4) nm) compared to the undoped ceria (𝑎_{𝐶𝑒𝑂2}=0.5413 nm [208]) confirms the complete dissolution of Gd in the ceria host lattice. Any impurity phase evidencing a reaction between NiO and GDC phases is not detected by XRD experiments at all synthesis temperatures.

The effects of the synthesis temperature on the crystallite size, lattice parameter, and the phase composition of the samples are studied by the Rietveld analyses. Figure 4-11 shows measured XRD pattern of NiO-GDC20 composite powder synthesized at 775 C and its representative Rietveld analysis. The obtained crystallite sizes, lattice parameters and phase compositions are summarized in Table 4-5. At all synthesis temperatures, the desired weight fraction between NiO and GDC20 phases (60:40 wt.%) is achieved. Similar to the synthesis of LSC nanoparticles, the formation of larger crystallites is observed in NiO-GDC20 composite powder, as the synthesis temperature is increased. The lattice parameters are found to be independent of the synthesis temperature.

Table 4-5: Crystallite sizes and lattice parameters of NiO and GDC phases, and their weight fraction within the NiO-GDC20 composite powders synthesized at 775 C, 900 C, and 1000 C.

Synthesis

4.4.2. Effect of NaCl on the Crystal Structure of Nanoparticles

To investigate the effect of NaCl on the crystal structure of the nanoparticles, LSC and NiO-GDC powders obtained from USP and SASP methods are compared. The pyrolysis temperature and pressure, and the total concentrations of the cations leading to the desired powders are set to 775

C, 900 mbar, and 0.05 M, respectively, for each synthesis. The precursor solutions used in the USP method do not contain NaCl, while 1 M of NaCl concentration is chosen for the precursor solutions for SASP method.

The XRD patterns of NiO-GDC20 and LSC powders synthesized by USP and SASP methods (Figure 4-12) indicate that the desired phases are acquired independent of the synthesis method with high phase purities. NiO-GDC20 powders exhibit nanocrystalline nature consisting of NiO and GDC20 phases with cubic rock salt and fluorite type structures, respectively. In case of LSC, both synthesis methods result in the formation of single-phase nanocrystalline perovskite phase. The crystallite sizes, lattice parameters, and phase compositions (only for GDC20) of the NiO-GDC20 and LSC powders are calculated by the Rietveld analyses and summarized in Table 4-6 and Table 4-7, respectively.

Table 4-6: Crystallite sizes and lattice parameters of NiO and GDC phases and their weight fraction within the composite powders synthesized at 775 C using various NaCl concentrations calculated by Rietveld refinement.

NaCl concentration (M) 0 1

Phase NiO GDC NiO GDC

Crystallite size (nm) 9(1) 5(1) 8(1) 4(1)

Lattice parameter, a (Å) 4.179(5) 5.420(5) 4.179(4) 5.423(5)

Weight (%) 59 41 58 42

Table 4-7: Crystallite sizes and lattice parameters of LSC powders synthesized at 775 C using various NaCl concentrations calculated by Rietveld refinement.

NaCl Concentration (M) 0 1

Crystallite size (nm) 10(1) 11(1) Lattice parameter, 𝒂 (Å) 5.4089(7) 5.4095(8) Interaxial angle, 𝜶_𝒊𝒏𝒕 (°) 60.29(5) 60.31(4)

Under the specified synthesis conditions, no distinct difference is observed in the crystallite sizes and lattice parameters of the powders derived by USP and SASP methods. However, in literature there are discrepancies about the effect of salt phase on the product crystallinity. The common observation states that the SASP method facilitates the crystallization process and eventually leads to powders with larger crystallite sizes than powders obtained from USP method.

The different findings can be ascribed to the variation of the salt phase that is employed during the synthesis. Typically, the use of single or eutectic mixtures Na, K, and Li nitrates as an inert salt phase in SASP method leads to powders with larger crystallite sizes compared to USP derived powders. The reason lies in the fact that, such salts and their eutectic mixtures form a liquid-state media during the pyrolysis as their melting points are typically below 250 °C. This molten salt

Figure 4-12: X-ray diffraction patterns of NiO-GDC20 and LSC samples synthesized at NaCl concentrations of 0 M and 1 M at 775 °C (after washing).

phase can facilitate mass transport and ultimately lead to bigger crystallites. However, the use of inert salts such as NaCl with substantially higher melting temperatures and adjusting the pyrolysis temperatures below their melting points would not be expected to result in any enhancement of the crystallization processes.