Tracking the Mn diffusion in the carbon-supported nanoparticles through the collaborative analysis of atom probe and evaporation simulation

Carbon-supported nanoparticles have been used widely as efficient catalysts due to their enhanced surface-to-volume ratio. To investigate their structure-property relationships, acquiring 3D elemental distribution is highly required. Here, the carbon-supported Pt, PtMn alloy, and ordered Pt3Mn nanoparticles are synthesized and analyzed with atom probe tomography as model systems. The significant difference of Mn distribution after the heat-treatment was found. Finally, the field evaporation behavior of the carbon support was discussed and each acquired reconstruction was compared with computational results from the evaporation simulation. This paper provides a guideline for studies using atom probe tomography on the heterogeneous carbon-nanoparticle system that leads to insights toward to a wide application in carbon-supported nano-catalysts.


Introduction
Atom probe tomography (APT) is a powerful analytical method for mapping the three dimensional (3D) atomic distribution and determining the local chemical composition of materials with sub-nanometer spatial resolution and ppm-level elemental sensitivity (Kelly & Larson, 2012;Miller, 2012;Kelly & Miller, 2007;Gault et al., 2012). Due to its unique capability, this technique has been adopted to reveal grain boundary segregation (Hojin Lee et al., 2019;Kim et al., 2019), nano-precipitation (Kürnsteiner et al., 2017, and chemical compositions of nano-structures for various bulk materials (Gault et al., 2018;Jung et al., 2019).
Since investigation of chemical composition and information of 3D elemental distribution is highly important to understand property-structure correlation in nanoparticles, there have been many attempts to analyze using APT (Barroo et al., 2019;Changsoo Lee et al., 2019). Tedsree et al. analyzed Ag-Pd core-shell metal nanoparticles to reveal the relationship between formic acid decomposition and Pd shell thickness (Tedsree et al., 2011). Li et al. performed APT measurements on the carbon-supported Pt, Pt/Co alloy, and Ir-Pt core-shell nanoparticles by depositing the nanoparticles to pre-sharpened tips using electrophoresis (Li et al., 2014). Felfer et al. used PVD-Cr sputtering on the Au-Ag core-shell nanoparticles dispersed on the Si substrate to embed the nanoparticles (Felfer et al., 2014). More recently, Kim et al. developed the co-electrodeposition of a metal to embed various nanomaterials including freestanding hollow TiO2 nanowire (Lim et al., 2020), MoS2 nanosheet (Se-Ho Kim et al., 2020), Li4Ti5O12 (Kim et al., 2022), Pd nanoparticles (Kim et al., 2018;Jang et al., 2021), and Pd@Au core@shell nanoparticles (Se-Ho Kim et al., 2020).
While successful APT measurements of the nanoparticles have been performed using abovementioned methods, the discussion on the analyzed data are mainly focused on nanoparticles, and the effect of carbon support has often been neglected. The community has recently started to expand the analysis capabilities to carbon-based materials such as carbon fiber (Johansen & Liu, 2021;Johansen et al., 2021), carbon nanotube (Raghuwanshi et al., 2020), diamond (Schirhagl et al., 2015), graphene (Exertier et al., 2021) and self-assembled decanethiol molecules (Gault et al., 2010;Stoffers et al., 2012;Solodenko et al., 2021) not limited to metals or intermetallic compounds. However, since elemental carbon has the highest intrinsic evaporation field compared to any metallic element, the accurate data interpretation of carbon materials i.e. the carbon black support is challenging; the evaporation field value of covalent-bonded carbon calculated to be 103 V nm -1 whereas the most metals shows theoretical evaporation fields in the range of 15 to 60 V nm -1 (Larson et al., 2013;Gault et al., 2012). As a result, the presence of a carbon support adjacent to a metallic nanoparticle will generate inhomogeneous evaporation behavior at the specimen's surface and eventually result a distorted 3D atom map from the actual specimen, which is known as local magnification effect (LME). LME is critical to the data reconstruction algorithm since it influences atomic density fluctuation, interface shape distortion, and apparent intermixing of phases in acquired data that may provide mis-interpretation. The area of the LME affected region could reach up to scale of few nanometers (Madaan et al., 2015) depending on both composition and structure, thus, it is crucial to study the artifacts caused by carbon support on the nanoparticles and validate the reliability of the carbon-nanoparticle APT data.
Hence, for demonstration, we have chosen a series of Mn/Pt nanoparticles that are known as effective oxygen reduction electro-catalysts (Peng et al., 2020) as our model systems for investigation of the carbon-nanoparticle field evaporation. We compared the acquired APT data of each nanoparticle sample with numerical simulation using TAPSim software developed by Oberdorfer et al. (Oberdorfer et al., 2013)

Preparation of carbon-supported nanoparticles
To synthesize PtMn/C catalysts, 12 mg of manganese (III) acetylacetonate (Mn(acac)3), 250 mg of 2-hexadecanediol (HDD), and 50 mL of diphenyl ether were mixed in a 200 mL three-neck flask and sonicated for 5 min. Next, 50 mg of commercial Pt/C (TKK, 20 wt. % Pt) dispersed in 5 mL of hexane was injected into the prepared solution. The mixture was heated to 259 o C for 30 min with stirring of 100 rpm. After the reaction, PtMn/C catalysts were synthesized and obtained from the resultant mixture solution by adding acetone and washed three times with a 40 mL of mixture of acetone and hexane (3:1). After the washing procedure, the PtMn/C catalysts were dried in a vacuum for 12 h. Atomically ordered Pt3Mn/C catalysts were obtained by applying heat-treatment on the synthesized PtMn/C catalysts, at 700 o C under the reductive atmosphere (4 % H2 and 96 % Ar) for 4 h.

Preparation of atom probe specimens
In order to embed the carbon-supported nanoparticles in a metal film, we performed co-deposition of the nano-catalysts with the Co film. Co was selected as a matrix material since Co has a single isotope in nature. Co electrodeposition bath was prepared by dissolving 48 g of CoSO4·7H2O, 12 g of CoCl2·6H2O and 9 g of H3BO3 in 250 mL of distilled (DI) water.
Dried nano-catalysts were weighed and dispersed in the Co bath solution in 0.1 g L -1 ratio.
After the mixture was sonicated for 30 min at room temperature, 1 mL of colloidal solution was transferred immediately after sonication, to the vertical cell comprising of the Cu foil (0.2 cm 2 surface area) as a cathode and the Pt wire as an anode. Pulsed electrodeposition with 10 % duty cycle and 0.1 Hz pulse frequency was done to enhance the number density of the embedded nanoparticles and to reduce the formation of H2 bubbles (Jun et al., 2021). Each specimen was electrodeposited for 8000 s.
In order to facilitate the site-specific lift-out and increase the chance of capturing the nanoparticles within the topmost region of the resulting atom probe specimen, a new approach using cross-sectional sampling was developed (see Scheme 1). In previous sample preparation method (upper part in Scheme 1), electrodeposited composite film of nano-catalysts and matrix on substrate was directly moved to dual-beam focused ion beam (FIB, Helios NanoLab 450 F1) chamber. By contrast, the composite film was vertically mounted in a conductive resin, and polished to reveal the cross-section of electrodeposited matrix in the modified method (lower part in Scheme 1). A wedge of composite material is extracted using standard lift-out procedure, and fabricated into an APT specimen by annular milling as described in the previous report (Thompson et al., 2007). We described the detailed observation of APT specimens in Supplementary information. Scheme 1. Illustration of site-specific APT specimen fabrication procedure.

Characterization
The crystal structures of the synthesized PtMn/C, Pt3Mn/C and commercial Pt/C (TKK, 20 wt. % Pt) catalysts were determined by X-ray powder diffraction (XRD, RIGAKU SmartLab operated at 45 kV and 200 mA using Cu-K radiation). Bright field-transmission electron microscopy (BF-TEM) images were obtained using Tecnai TF30 ST operated at 300 kV. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) associated with energy dispersive X-ray spectroscopy (EDX) and spherical aberration correction were performed at 300 kV (FEI Titan cubed G2 60-300). APT analyses were carried out using a local electrode atom probe (CAMECA Instruments LEAP 4000X HR TM ) at the base temperature of 50 K. The laser pulse energy and frequency were 50 pJ and 125 kHz, respectively. Data reconstruction and analyses were done with the IVAS 3.8.2 software provided by CAMECA Instruments.

Simulations of field evaporation
On purpose of investigating the evaporation field induced artifact during the atom probe measurement, the computational simulation using the TAPSim software, developed by Oberdorfer et al. (Oberdorfer et al., 2013) was performed. Virtual APT specimens consisting of Co, with radius of 6 nm, shank angle of 20 degree, total length of 15 nm and lattice parameter of 0.36 nm was constructed using the custom Python 3.8 script. The each carbon-supported nanoparticle (Pt/C, PtMn/C, and Pt3Mn/C) was included in the virtual APT specimens, respectively, based on the detected number of atoms from APT results. PtMn nanoparticle was set to have Pt core with radius of 1.320 nm and Mn shell with thickness of 0.145 nm, yielding 3:1 ratio of Pt and Mn. Pt3Mn nanoparticle was set to have homogeneous ordered L12 structure.
Additionally, carbon sphere with radius of 15 nm adjacent to the metallic nanoparticle was included in the specimen as the representation of carbon support. The center of the carbon sphere had the same height as the metallic nanoparticle.
Evaporation field of each metallic element in the nanoparticles were set at 37, 30, and 45 V nm -1 each for Co, Mn and Pt, respectively, referring the theoretically calculated evaporation field values (Gault et al., 2012;Larson et al., 2013). In addition to the predicted value of 103 V nm -1 , the evaporation field of a carbon support was varied at 50, 75 and 103 V nm -1 for systematic study. Resulting detector hit map data from TAPSim evaporation simulation was reconstructed using standard shank angle reconstruction protocol (Larson et al., 2013), and re-visualized using IVAS 3.8.2 software.

Results and discussions
Characterization of carbon-supported nanoparticles Figure 1 shows XRD patterns of the Pt/C, PtMn/C, and Pt3Mn/C. The background peak at 2θ of 20 to 30 degree representing amorphous carbon (Ungar et al., 2002) is observed in all samples. In both Pt/C and PtMn/C samples, the relatively board peaks are originated from the disordered FCC Pt. No Mn-related peak is found. After the reductive heat-treatment of Pt3Mn/C, the ordered FCC peaks were observed indicating Mn ions were first reduced during the reaction in the solution and heat-treatment induced the chemical ordering of Pt3Mn.
The average diameters of Pt/C, PtMn/C, and Pt3Mn/C were measured to be 2.6 ± 0.4, 3.3 ± 0.5, and 4.2 ± 0.8 nm, respectively, (see Figure S2a- Figure 2a). In addition, Pt and C atoms were detected with high density at the border between low density and normal region. A two-dimensional (2D) contour plot, a graphical technique for representing an atom-density surface of overall collected atoms by plotting along the measurement direction, illustrates a clear low-density region in the reconstructed specimen (see Figure 2b and c). Figure 2d and 2e show the one-dimensional (1D) concentration and detected atom profiles across the Pt particles located on the interface between nanoparticle and matrix. Measured diameters of Pt particles were 2-3 nm based on a full width half maximum of the Pt concentration in 1D concentration profile (Figure 2e). More importantly, peaks for detected atoms were located at the border of low density and normal density region implying strong LME.

Simulated results of carbon-supported Pt nanoparticles
In APT analysis data of carbon-supported metallic nanoparticles, two major artifacts, namely formation of low-density area and overlapping of consisting elements were observed. density region is observed. The increase of the density rises more sharply as the evaporation field of C support increases. Additionally, the peak of Pt concentration is located at the border of low density and normal density region, which is in good qualitative agreement with the APT analysis result (Figure 2). The result indicates that the large evaporation field difference between metal and carbon atom could cause the severe density fluctuation within the atom map after APT analysis of carbon-supported metallic nanoparticles.

APT results of carbon-supported PtMn and Pt3Mn nanoparticles
In order to reveal the difference in the distribution of Mn atoms and the concentration of the as-synthesized PtMn/C and Pt3Mn/C catalysts, APT measurements were performed.   At initial state before evaporation, the core of the nanoparticle is enriched with Pt (red spheres), and Mn atoms (green spheres) are distributed on the surface of the nanoparticle as shown in Figure 5a. As a result, 1D concentration profile across the nanoparticle (Figure 5e) shows a single concentration peak of Pt, with two separated peaks of Mn at each end of the Pt concentration peak. After evaporation simulation, distortions resulted from LME are observed in the reconstruction maps (Figure 5b-d). Although the discrepancy from the initial state gets larger as the field of C increases, the concentration profile across each nanoparticle system (Figure 5f-h) shows the Mn atoms are enriched at the surfaces of the particle.   Mn diffusion in the nanoparticles during the synthetic process Scheme 2. Fabrication process of atomically ordered Pt3Mn/C Synthesis of atomically ordered Pt3Mn/C requires two-step process: reduction of Mn ions in solution and heat-treatment in reductive atmosphere (see Scheme 2). We observe the Mn enrichment at the surface of the particles in PtMn/C and homogeneous 3:1 Pt/Mn distribution in Pt3Mn/C from APT experiment (see Figure 4). In addition, simulated results of carbon-supported PtMn and Pt3Mn nanoparticles support the different tendency (see Figure 5 and 6). Thus, we suggest that Mn ions were reduced and only attached to the surface of Pt nanoparticles after chemical reaction in solution. Subsequently, during the reductive heattreatment, surface-reduced Mn atoms diffused into the core of the particles and resulted in the ordered L12 structure. Chi et al. reported the surface segregation of Pt in Pt3Co nanoparticle starts at 350 o C while L12 ordering in Pt3Co nanoparticles appears at 600 o C (Chi et al., 2015), implying the applied heat (259 o C) may not provide enough thermal driving force to initiate diffusion toward the particle core. Another possible reason could be Mn ions were not fully reduced instead binding with oxygen at the particle surface that may hinder the movement of Mn atoms. The MnxOy molecular ion peaks were detected in the acquired mass spectra and the intensity in PtMn/C was higher than that of Pt3Mn/C (see Figure S3).
conditions of the carbon are used to study the LME. It has shown that despite the artifacts in atomic density and the shape, the atomic movement within the nanoparticle during the heattreatment could have been tracked with APT. We envision that the presented study can pave the road for the application of APT analysis for various carbon supported nanomaterials. Figure S1a shows SEM image of the electrodeposited composite film of the nanocatalysts and the Co matrix. As backscattered electron (BSE) mode has a moderately high sensitivity in terms of atomic number, distinguished brightness indicates the different materials of the carbon containing nano-catalysts (dark area) and the matrix (bright area). Co matrix containing a relatively high density of the nano-catalysts ( Figure S1b) is selected and sliced using FIB for lift-out. Figure S1c shows a composite lamella before lift-out process with the completely embedded nano-catalysts. After annular milling, sharpened APT specimen containing nano-catalysts was obtained, as shown in Figure S1d.

Fabrication of APT specimen including carbon-supported nanoparticles
The final APT specimens were measured with bright-field TEM to first confirm possible existence of pores that result abruptly fracture during high-field evaporation process.
No noticeable crack or voids were observed in the specimen (see Figure S1e). In bright-field mode, elements with lower atomic mass show brighter contrast than elements with higher atomic mass. Thus, bright features showing sphere shape are the supporting carbon, while darker region is the Co matrix. Figure S1f shows HAADF-STEM image of sharpened APT specimen showing different contrast. In addition, even brighter spheres in size of few nanometers representing PtMn nanoparticles were observed without shape change, indicating the electrodeposition process has successfully captured the PtMn nanoparticles within matrix, without affecting the morphology of nano-catalysts (see Figure S1g).

Calculation of average particle size from XRD results
The 2-theta peaks from FCC (111) were centered at 39.71˚, 39.77˚, and 40.00˚ for Pt/C, PtMn/C, and Pt3Mn/C, respectively. No shift between Pt/C and PtMn/C indicates that enough mixing between Pt and Mn did not occur during the synthesis of PtMn/C. By contrast, 2-theta of Pt3Mn/C shows larger than PtMn/C indicating a reduction of the lattice constant. The average particle size was estimated by using the Scherrer equation as below (Scherrer, 1918).

D = 
Here, Dhkl is the average crystallite size in direction of hkl plane, K is crystallite shape factor which we used 0.9 (sphere shape) in this study (Holzwarth & Gibson, 2011), λ is the wavelength of the X-ray, Bhkl is the full-width at half-maxium of the X-ray diffraction peak from hkl and θ is the Bragg angle. The estimated average particle size of Pt/C, PtMn/C, and Pt3Mn/C were 2.1, 2.5, and 6.7 nm, respectively. Figure S2. BF-TEM images of (a) Pt/C, (b) PtMn/C, and (c) Pt3Mn/C. STEM-EDX maps of (d) Highresolution STEM images and EDX maps of Pt3Mn/C in atomic resolution.