# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Catalysts as Unsupported Nanoparticles
#### 3.3.3 Imaging Other Encapsulated Nanomaterials  
Several other nanomaterials were analyzed using different synthesis/encapsulation approaches. One example of analyzed unsupported catalysts by APT involves Ag nanoparticles produced by a gas condensation process [127], the terminated gas condensation method. A similar process was used in the past to deposit Au clusters on FIM tips [128; 129; 130]. This methodology allows the deposition of nanoparticles with a narrow size distribution without co-deposition of carbon-based support materials or surfactant that complicate the APT analysis. In addition, the preparation of specimens suitable for APT does not require a FIB lift-out step, reducing the preparation time. A uniform layer of Ag nanoparticles is deposited on a Si micro-post (Figure 9a). The nanoparticles are then encapsulated with a Pt layer with e-beam deposition (appearing as protrusions on Figure 9b), and subsequently milled by FIB to obtain a tip-sample suitable for APT experiments (Figure 9c). The 3D reconstruction of the specimen reveals the presence of isolated Ag nanoparticles (using the isosurface method) with a size matching the mass-selected clusters during the synthesis and the SEM analysis. Surprisingly, a significant amount of Ag is spread throughout in the analyzed volume rather than being confined in the nanoparticles. This could be due to trajectory aberrations during APT analysis, to surface mobility of Ag under electric field, or to the presence of individual Ag atoms at the surface of the Si micro-post. In addition, Pt, Ga and O were detected, but the oxygen layer is relatively thin, indicating that the Pt capping layer successfully embedded and protected the nanoparticles from oxidation. Even though some improvements in the synthesis and analysis parameters are still needed, this approach allows for the analysis of narrowly distributed particles, without a time-consuming lift-out step, and with improved reconstruction as compared to electrophoresis-based depositions.  
Alternative encapsulation methods have been used to analyze nanomaterials but could be extended to catalytic materials. The first one is the atomic layer deposition (ALD) method providing a high surface coverage, conformality and a void-free structure, which has been successfully applied to the investigation of Si nanowires embedded in ZnO [131], and Pt nanoparticles embedded in an ZnO:Al matrix [132]. In another approach, a fusible bismuth-indium-tin alloy was used as embedding medium: Fe nanoparticles were dispersed on the fusible alloy and in situ melting was  
Figure 9: Study of Ag nanoparticles prepared by gas condensation process. (**a**) High density and clearly separated Ag nanoparticles on Si coupon; (**b**) e-beam deposition of Pt as embedding matrix; (**c**) final tip after FIB milling with nanoparticles embedded; and (**d**) 3D reconstruction showing the presence of isolated Ag nanoparticles and Ag atoms distributed throughout the tip-sample. Reprinted from [12777] under the terms of the Creative Commons Attribution License.  
done in a FIB system [133]. This study revealed boron contamination from the synthesis. Finally, Pd and C-supported Pt nanoparticles were analyzed by APT using a methodology consisting of the electrophoretic deposition of nanoparticles on a flat Cu substrate followed by electrodeposition of a Ni film acting as an embedding matrix for the nanoparticles. Reliable APT data could be obtained for Pd nanoparticles, but the carbon-based support leads to artifacts [134]. All these alternate methods require a further FIB lift-out step to obtain a needle-shaped specimen. The next section aims at investigating supported catalysts, which is the next level of complexity.