Exploring the Imaging of Perovskite Catalysts Bearing Exsolved Active Nanoparticles

The latest developments in new energy and heterogenous catalytic materials have led to novel ceramics being produced for an extensive range of applications such as autocatalysis, fuel cells, and chemical feedstock production.^2,3,4

One such material is non-stoichiometric A-site deficient perovskite, which exhibits a catalytically active metal doped on the B site. It is feasible via controlled synthesis and reduction conditions to tailor the size and morphology of nanoparticles via the advent of active metal cations.

Cautious scanning electron microscopy (SEM) must be deployed to examine the shape, distribution, size, and morphology. The perovskite ceramic sensitivity, non-conductive nature, and nanoparticle distribution on the decreased area results in a hard-to-image surface. This can often lead to nano decoration being missed entirely due to the lack of material comprehension and microscopic methods.

Figure 1. Distribution of platinum decoration on reduced areas of perovskite. Image Credit: Carl Zeiss Raw Materials

In industrial testing for autocatalysis, a sieved and pelletized sample is often tested and observed. This can result in considerable imaging problems due partly to debris, carbonaceous deposits from catalytic testing, and fine static particulate matter that can considerably decrease micrograph sharpness.

A typical solution provided for non-conductive materials is a sputter coat with Platinum (Pt), Gold (Au), Carbon (C), or a combination of coats with varying thicknesses of 20 nm to 5 µm for ceramic-type materials. This is often counterproductive as coatings can mask nanoparticles and falsely identify carbonaceous deposits and nanoparticles.

One more factor in precisely determining nanoparticles and distribution is the potential of coating substrate ceramics to catalytically react with normal desorption or adsorption-based methods to describe active metal sites. This makes such techniques impractical. Therefore, SEM image analysis of nanoparticles is a significant area of interest in identifying stability, activity, dispersion, and morphological features.

Electron beam-sensitive materials often need extra expertise and knowledge of the SEM best practice imaging method called sweet spot imaging.⁵ This article will illustrate analytical micrographs of nanoparticles developing from decreased La_0.4Ca₀.₃₉₂₅Ba_0.0075 Pt _0.005 Ti _0.995O3-δ¹ noted as Pt+LCT.

Results and Discussion

A-site deficient perovskite Pt+LCT was synthesized using solid-state synthesis and further decreased under 5% H₂/Ar leading to 0.5 wt% Pt. Emerged nanoparticles on the surface were studied with the help of ZEISS Sigma 300, ZEISS Sigma 500, ZEISS GeminiSEM 360, and ZEISS GeminiSEM 560.

For determining the imaging conditions of Pt+LCT, a range of apertures, working distance from the beam pole, and detector choice were chosen to assist the best imaging practice (Table 1). A scope of detectors on ZEISS field emission scanning electron microscopes (FE-SEM) was selected, all utilizing the ZEISS Gemini electron optical column.

Table 1. Range of conditions used to determine the sweet spot of the perovskite catalysts. Source: Carl Zeiss Raw Materials

Non-conductive and ceramic-type materials should be imaged at low keV (<5 keV) or variable pressure with a low working distance concerning the pole piece. This differs from separate microscopes, particularly if the detectors and beam stability are changeable.

A sweet spot analysis is needed to determine the best practice imaging for a few materials. Several parameters are run step-by-step to determine the best possible imaging conditions. The test matrix in Table 1 revealed the perfect conditions for the ZEISS GeminiSEM family columns.

Imaging ceramics at a greater keV (above 5 keV) damages the surface and sub-surface, leading to material degradation inside the chamber. An area of non-interest is chosen for use with higher keV (5-20 keV).

As soon as an optimal keV is selected for the solution, customizing the working distance for improved resolution is initiated as the next step. An approach aperture is needed between 5 µm of the chosen aperture, for example, 10 µm-25 µm must be experimented with to determine the optimal conditions and increase imaging radiance.

Imaging snapshots can be taken with a quick line scan to obtain the right imaging settings in combination with drift correction. Additional image corrections could be made post-scanning with software applications such as ZEISS SmartSEM Touch and ZEISS SmartSEM. When imaging conditions have been chosen, it is possible to note regions of interest without damaging the selected area.

For perovskite, an aperture of 20 µm was sufficient for imaging and a working distance of below 3 mm (Figure 2) throughout the Sigma family was needed to see nanometer-scaled features on the materials’ surface.

Figure 2. Working distance (2.5 mm) needed for imaging nanoparticles on ceramic. Image Credit: Carl Zeiss Raw Materials

Smaller nanoparticles measuring less than 8 nm finely dispersed on the surface of the perovskite were not fully visible with the secondary electron detector (ETSE). This is often deceptive for microscopists who have the potential to miss nanoparticle decoration by utilizing the wrong detector.

By making use of the Inlens SE detector, complete nanoparticle decoration was evident. Images were taken with the help of a quick <4-second line scan. Any longer would lead to visible beam damage to the material, as can be displayed in Figure 3.

Figure 3. (L) Standard SE detector with no decoration visible; (R) use of Inlens SE technology allows nanoparticle decoration to be observed. Image Credit: Carl Zeiss Raw Materials

Additional image correction and processing can be customized in an automatic manner for the Sigma family. SmartSEM Touch could be utilized together with imaging (Figure 4) to considerably decrease time spent rectifying image sharpness as well as brightness and contrast.

Figure 4. SmartSEM touch allows for rapid image improvements with automated drift correction and auto brightness contrast sharpening. Image Credit: Carl Zeiss Raw Materials

For GeminiSEM 560 and GeminiSEM 360 microscopes, a magnetic-clamp Kline stage blocking the eucentric stage in place was utilized to stabilize the image as well as the pendulum-based motion stabilization for the microscope’s core. This allows a reduced working distance from the pole piece and enables imaging of unstable or non-anchored particles to the stub (Figure 5).

Figure 5. Non-anchored particles can be easily observed due to the stability provided by the GeminiSEM family magnetic clamp, rendering stage movement minimal and improving image quality. Image Credit: Carl Zeiss Raw Materials

Additional analytical imaging could be obtained with the help of energy selective backscattered detector (Inlens EsB). Subsurface nanoscale composition is evident with clear compositional contrast. The second annular in-column detector is situated above the Inlens SE detector, enabling more accurate material information and contrast, as seen in Figure 6.

Figure 6. Inlens BSD allows for greater contrast and sub surface morphology identification of LCCNT. Image Credit: Carl Zeiss Raw Materials

Pt nanoparticles are visible in NanoVP variable pressure modes with a backscatter detector (BSD) (Figure 7), which can offer structurally considerable information via contrast and subsurface morphology.

Figure 7. The use of variable pressure mode allows for delicate imaging in both Inlens SE, SE, and BSE modes with the Sigma series. Image Credit: Carl Zeiss Raw Materials

Additional improvement could be obtained at variable pressures together with a low working distance compared to the pole piece ranging (between 3 to 5 mm). Nanoparticles are visible with the help of VP mode.

Inlens detectors could be utilized in VP mode to improve imaging and offer a contrast alongside BSD images.

Conclusions

It is possible to determine sweet spot imaging conditions for the ZEISS FE-SEM range (Sigma and GeminiSEM). It plays a methodical and practical role in determining ideal conditions for sample imaging.

For ceramics, such as the ones discussed in this study, catalytically active nanoparticles decorating an A-site deficient perovskite, cautious imaging, and analysis are significant tools for the catalytic activity to be determined.

Sweet spot imaging conditions were the same between the ZEISS Sigma and GeminiSEM families, with subtle discrepancies for every microscope type. The balance and stage of the ZEISS GeminiSEM family increased image stabilization and brilliance, leading to high-resolution imaging at low keV.

Inlens SE technology was the most significant in noting nanoparticle decoration for both Sigma and GeminiSEM. Additional detectors could be utilized in parallel, such as the Inlens EsB, to offer more subsurface and morphologically diverse regions of interest.

References

1. Kothari, M. et al. Platinum incorporation into titanate perovskites to deliver emergent active and stable platinum nanoparticles. Nature Chemicals, 13, 677–682 (2021).
2. Cassidy, M., Gamble, S. & Irvine, J. Application of Exsolved Structures as a Route to More Robust Anodes for Improved Biogas Utilisation in SOFCs. ECS Trans. (2015).
3. Papargyriou, D., Irvine, J.T.S. Nanocatalyst exsolution from (La, Sr) (Cr, M, Ni) O 3 (M Mn, Fe) perovskites for the fuel oxidation layer of Oxygen Transport Membranes. Solid State Ionics (2015).
4. Neagu, D., Oh, T., Miller, D., Ménard, H. & Bukhari, S. Nano-socketed nickel particles with enhanced coking resistance grown in situ by redox exsolution. Nature Communications. (2015).
5. Tandokoro, K., Nagoshi, M., Kawano, T., Sato, K. & Tsuno, K. Low-voltage SEM contrasts of steel surface studied by observations and electron trajectory simulations for GEMINI lens system. Microscopy, 67, 274–279 (2018)

This information has been sourced, reviewed and adapted from materials provided by Carl Zeiss Raw Materials.

For more information on this source, please visit Carl Zeiss Raw Materials.