Temperature-Dependent Ultrastructure Transformation of AuâFe

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Temperature dependent ultrastructure transformation of Au-Fe nanoparticles investigated by in situ STEM Marius Kamp, Anna Tymoczko, Ulrich Schürmann, Jurij Jakobi, Christoph Rehbock, Klaus Raetzke, Stephan Barcikowski, and Lorenz Kienle Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00809 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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Crystal Growth & Design

Temperature dependent ultrastructure transformation of Au-Fe nanoparticles investigated by in situ STEM Marius Kamp†, Anna Tymoczko‡, Ulrich Schürmann†, Jurij Jakobi‡, Christoph Rehbock‡, Klaus Rätzke §, Stephan Barcikowski‡, Lorenz Kienle†* †AG-Synthesis and Real Structure, Institute for Materials Science, Technical Faculty of the Christian-Albrechts-University of Kiel, Kaiserstrasse 2, 24143 Kiel, Germany ‡Technical Chemistry and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Universitaetsstrasse 7, 45141 Essen, Germany § AG-Multicomponent materials, Institute for Materials Science, Technical Faculty of the Christian-Albrechts-University of Kiel, Kaiserstrasse 2, 24143 Kiel, Germany

Nanoparticle, core shell, Au-Fe alloy, transmission electron microscopy, in situ heating, morphology

Three dimensional morphology changes of bimetallic nanoparticles (NP) with nominal composition Au50Fe50 and Au20Fe80, generated by pulsed laser ablation in liquid, are monitored in situ and ex situ via scanning transmission electron microscopy (STEM) and electron tomography. The samples are made up by a chemically segregated core shell (CS) nanoparticles (NP) structure, with an Au-rich shell and Fe-rich core, and solid solution (SS) NP in the pristine state. Further, the examinations reveal information about a sequence of characteristic changes

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from the pristine metastable and intermediate ultrastructures up to thermodynamically stable products. In the case of the Au20Fe80 sample, a metastable spherical CS morphology is transformed at equilibrium conditions into a cube-shaped Fe-rich core faceted by truncated Aurich pyramids. For the Au50Fe50 sample, the Au-rich shell is solved into the Fe-rich core and chemically homogeneous (SS) NP are formed. Interestingly, this transformation was proven to occur via an intermediate ultrastructure with lamellar segregation, not previously reported as a transient state during in situ heating. Based on these observations, a correlation between the composition and the morphology at equilibrium is suggested, in accordance to the bulk phase diagram of Au-Fe. At the same time, our examinations directly prove that laser ablation synthesis creates non-equilibrium nanoparticle morphologies, frozen in metastable, spherical core-shell particles.

Bimetallic NP are most suitable to combine decisive properties of their single constituents. Magneto-plasmonic core shell (CS) NP are used for medical applications like magnetic resonance imaging (MRI) or magnetic thermotherapy. This combined with plasmonic optical properties and ease of functionalization of the Au surface by thiolated biomolecules makes them ideal

1–4

. Another major advantage of such microstructures is the high magnetic moment of the

Fe-rich core compared to a FeOx core, which is often used instead

5,6

. Performing the synthesis

without partial oxidation of the Fe core is challenging, especially for wet chemical methods

7,8

.

With Laser Ablation in Liquid (LAL) the CS morphology form is dominant in the solvent based synthesis. This results in a Fe-rich core ultrastructure without oxygen contamination, so that the high magnetic moment of the Fe-rich core is completely accessible 9. The process is feasible for


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production in the range of gram per hour and free of any precursors or surfactants 10. The longterm stability in the liquid is just as important as the thermal stability at elevated temperatures. The thermal stability and interrelated morphology changes upon heating of NP were frequently observed in the past, and phase diagrams for binary nanoalloys and bulk alloys differ. Due to experimental limitations, the development of nanophase diagrams is complicated, and theoretical calculations have to be applied 11,12. Considering the diverse applications of Fe@Au NP, the Au-Fe system is frequently investigated

13–17

. The thermally induced transformation has been observed by Naitabdi et al.18,

and CS Fe@Au NP are synthesized by annealing at 300 °C with Au50Fe50 composition. Scanning tunneling microscope measurements at 700 °C reveal the formation of Au-Fe alloy by surface state analysis valid for size-selected NP with an average diameter of 5 nm, however, no observation of the internal morphology is possible. Langlois et al.19 synthesized Au-Fe NP by physical vapor deposition of Au and Fe at 800 °C on an amorphous alumina layer resulting in the equilibrium morphology of Fe cube, capped with truncated Au pyramids (so called tetrakis hexahedrons). The interface analysis by transmission electron microscopy (TEM) indicates the epitaxial growth of Au on the Fe cube along [001], which represents a configuration with minimized interfacial energy

20

. The experimental observation of the morphology captures a

detailed analysis of the equilibrium shape of NP with a mean diameter of 13 nm that is taken as a reference for this work. Special interest is given to the prediction of equilibrium ultrastructure at the nanoscale and the possible metastable morphologies for bimetallic NP. Two different methods are described, which allow a prediction of bimetallic nanocrystal morphology: The Wulff construction theorem and parallel tempering Monte Carlo simulations

21–23

. For bulk crystals, Wulff construction is an


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established method for the prediction of crystal morphology by minimization of surface energy, which is calculated by the respective energy contributions of crystal plane orientations

22

.

According to literature, it is possible to apply the theorem on nanostructures and for bimetallic NP

21

. Studies from Ringe et al.

21

indicate a size-dependent, thermodynamic equilibrium

morphology for separated (two phase) bimetallic NP, while homogeneous alloys have spherical equilibrium shape without size dependence. The large contribution of surface energy as well as the influence of composition on the equilibrium shape can be calculated by Wulff theorem on the nanoscale. The prediction of the equilibrium ultrastructure by Monte Carlo simulation (parallel tempering modeling) is based on semi empirical potentials derived from individual potentials for Fe and Au atoms by density functional theory. However, only one result possessing the desired representative number of atoms is available in the literature for the Au-Fe system 23. Simulations for up to 2000 atoms reveal an equilibrium ultrastructure of Fe cube with truncated Au pyramids based on a semi-empirical model developed by Calvo et al.

23

. The deformation of the

morphology is based on the large difference in surface energies of Fe and Au and the low interface energy along [100] between Fe and Au 20. The bulk phase diagram is considered as a reference for the investigated Au-Fe nanoalloy system because the expected derivations allow a qualitative comparison for NP with a diameter larger 50 nm

11,12,24

. The temperature dependent transformation can be modelled, but to the best

of our knowledge, no experimental proof for the coexistence or transformation from spherical CS Fe@Au to Fe cube with truncated Au pyramids is available in the literature. In situ TEM investigations of the temperature induced transformation are analyzed in collaboration with the nominal composition in this work. Conventional TEM yields 2D projections of 3D ultrastructures; however, by means of electron tomography, a 3D model of the NP is accessible.


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The 3D models can be applied to analyze the morphology and space distribution of respective components of the ultrastructures without superposition artifacts.

NP are generated by one step LAL 25, whereby, a pulsed laser beam ablates a bulk alloy target in a solvent, synthesizing NP by the creation of a cavitation bubble. Two lasers are used for synthesis: a 8 ns Nd:YAG laser (RofinSinar Technologies, Plymouth) at 1064 nm with a repetition rate of 15 kHz and a fluence of 3.85 mJ/cm2, and a 10 ps (Ekspla) Nd:YAG laser at 1064 nm with a repetition rate of 100 kHz and a fluence of 3.1 mJ/cm2. For both systems a lens with a focal length of 100 nm was used to focus the beam through a glass window into a batch reactor containing the Au-Fe alloy target emerged in 30 ml 99.8% acetone (Carl Roth GmbH, Karlsruhe). The morphological change is investigated in high angle annular dark field (HAADF)-STEM mode, permitting atomic-number dependent Z-contrast. The thermal stability is investigated by in situ STEM heating experiments with a Gatan 652 double tilt heating holder. The reliable and stable temperature measurement was confirmed in previous studies

26,27

. A

heating rate of 10 °C per minute is applied and the temperature is held for 10 min before recording images for each selected temperature, i.e. 100, 200, 250, 300, 350, 400,…700 °C to minimize the thermal drift and ensure thermal equilibrium. Thus, within experimental error, each temperature is correlated with a time (duration). Heating experiments are performed with two samples generated by targets with two compositions and solutions: one sample with the nominal composition of Au50Fe50 generated in acetone with the picosecond laser, and the second sample with a nominal composition of A20Fe80 generated in 3-pentanone with the nanosecond laser. All samples are dispersed on molybdenum TEM grids (Plano GmbH) with lacey carbon film for TEM investigation at a Tecnai F30 STwin G2 with 300 kV acceleration voltage. Energy


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dispersive X-ray (EDX) measurements are performed with a Si/Li detector (EDAX System). For tomography tilting series a high angle tilting holder was used with applied maximum tilt angles of 65° to reduce artifacts from missing wedge 28,29. A linear tilt scheme with a step size of 1° was chosen to record the tilting series. All images are used for reconstruction, which is based on two algorithms called filtered back projection real space reconstruction and simultaneous iterative reconstruction technique 30,31.

The NP generated exhibit two major ultrastructures, i.e. SS and CS (Figure 1). In this work, the focus is set on the thermal transformation of NP for two different compositions, the Au50Fe50 sample, and the Au20Fe80 sample, which contain a high CS fraction of 42 % and 62 %, respectively. Both samples were analyzed with respect to their particle size distributions given in the supporting information (Figure S 1). It shows bimodal size distributions and a high CS fraction for sample Au20Fe80, a more detailed discussion on size distribution and the influence of the composition is given in Tymoczko et al. 32. Besides the composition, the NP ultrastructure is analyzed to examine the respective relative stabilities and the transformation mechanisms. The CS morphology with an Au-rich shell and a Fe-rich core (Au@Fe) shows excellent stability against hydrochloric acid, demonstrating the closed morphology of the Au-rich shell 9. Example images of both ultrastructures are shown in Figure 1a) and 1c). A representative STEM-EDX elemental linescan of a CS NP is depicted in Figure 1b) to validate the Fe@Au CS morphology. The Fe signal is maximum at the center of the particle and decreases at the position of the shell, where the Au signal is at the maximum. In addition, it can be seen that oxidation of the Fe-rich core is excluded, because of the absence of an O-K signal.


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Figure 1. a) Representative ultrastructure of CS NP b) elemental linescan from STEM-EDX c) representative image of SS NP. Scale bars are 20 nm.

With CS diameter the ratio of core to shell diameter decreases (thicker shells), while the composition is kept constant. Experimentally it was not possible to verify the exact composition of single NP, due to the limitation of the microscope equipment and signal to noise ratio, but from the observations and EDX measurements on several NP assemblies, it can be concluded that there should be no large variance in the nominal composition of the NP. To ensure that the composition of the NP is the nominal target composition, EDX measurements in STEM mode were performed. STEM images and the respective EDX spectra are shown in the supporting information (Figure S 2-S 5), while the quantitative results are given in Table 1. Measurements are performed over areas of NP containing all representative ultrastructures.

Table 1. Chemical composition from STEM-EDX measurements (average of NP, σ2 is the variance).


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σ2

Au50Fe50 Sample

σ2

Element

Au20Fe80 Sample

Fe (K)

81.1 at%

4.7 at%

50.5 at%

1.8 at%

Au (L)

18.9 at%

4.7 at%

49.5 at%

1.8 at%

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The stability of the representative ultrastructure is evaluated qualitatively during in situ heating experiments in a temperature window between room temperature and 700 °C. In our observations we were able to analyze the changes of the same nanoparticles, so the identification of intermediate ultrastructures and potentially time resolved observations is possible. All ultrastructures are stable up to 250 °C, independent from the sample and type of ultrastructure. The, three different representative ultrastructures: SS, as well as thick-shell CS and thin-shell CS, were separately investigated during heating up to 700 °C. The observations at elevated temperatures are depicted in sketches for the Au20Fe80 sample and the Au50Fe50 sample in Figure 2 a) and b), respectively. The initial stage of each transformation is depicted and indicates the different thermal stability of the ultrastructures. The observations prove that the CS morphology with thin and closed Au-rich shell transforms into a bimetallic Janus-like morphology in order to minimize interface energy (Figure 2b) CS2). These Observations are described in literature before, with the appearance of a thin Au shell around the Fe, by Amram et al.

13,16

. Thermally

more stable ultrastructures like SS and thick-shell CS, start transforming at higher temperatures by building up faceted forms (Figure 2a) CS, 2b) CS1). The sketches are based on NP that are located next to each other and have the same diameter, to secure an equal local temperature (diffusion constant D(T)) and interparticle diffusion length. During controlled cooling to room temperature, the NP maintain their morphology, indicating the conservation of the equilibrium configuration (Figure S 6). The resulting quasi equilibrium ultrastructures, which are present at


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700 °C and at room temperature after annealing, show drastic differences from the initial NP. Heating of the Au20Fe80 sample maintains (for CS) and produces (for SS) chemical inhomogeneity with Fe-rich core and Au-rich shell (Figure 2b) CS1 and SS). These results underline a transformation behavior that is expected from the bulk phase diagram as segregated structures are most stable at room temperature as well as at 700 °C. Consequently, a preservation of SS NP and the transformation from CS to SS for the Au50Fe50 sample at elevated temperatures (Figure 2a)) CS and SS) is expected and observed. In the case of the Au50Fe50 sample a two phase region is present up to 550 °C (phase diagram and experimental results in Figure 2a) CS, respectively). However, the chemical segregation vanishes at elevated temperatures, resulting in miscibility of Fe and Au and the stability of the SS NP at 700 °C. Example Z-contrast images of the morphology transformations are shown in Figure 2a) and 2b), underlining the nominal composition dependent morphology of equilibrium ultrastructure at 700 °C. Therefore, in case of the Au50Fe50 sample, the formation of SS NP at 700 °C seems to be controlled by the nominal composition, independent of the pristine ultrastructure at room temperature. Based on this, it may be concluded that the heating experiments to 700°C generate equilibrium structures, which are retained at room temperature and, clearly verified by the SS ultrastructures found in the nanoparticles with a nominal composition of Au50Fe50. All findings observed in situ with nanometer resolution are in accordance with previously observed experimental results by Naitabdi et al. 18 and parallel tempering Monte Carlo simulation of Calvo et al 23.


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Figure 2. Representative Z-contrast images recorded upon in situ heating for selected temperatures on the left side and sketches of the temperature dependent morphology transformation (grey Fe; yellow Au) on the right side. a) The Au50Fe50 sample b) The Au20Fe80 sample; scale bars are 25 nm.

For further development of the theoretical models and understanding of nanostructure transformations, the morphology change is analyzed more closely in the following paragraphs with a concentration on the nanoparticles with the nominal composition of Au20Fe80. Therefore, single NP the with representative morphology and size of the Au20Fe80 sample were selected and


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analyzed. Concerning the exemplary transformation from SS to separated tetrakis hexahedrons (Figure 2b) SS) the theoretical expectations correlate with our experimental findings, indicating the equilibrium character of the Fe-rich cube with truncated Au-rich pyramids as described in literature

19

. Moreover, a transient lamellar intermediate stage depicted in Figure 2b) SS is

observed during in situ heating. With Z-contrast imaging it is concluded that the intermediate ultrastructure is based on alternating Au-rich and Fe-rich stripes, establishing large interface area. With the available TEM techniques, it is not possible to distinguish between a lamellar surface segregation and segregation throughout the complete volume of the NP. In literature, the lamellar segregation is observed in bulk alloys 33,34 and in microparticles

13,14

, where a complete

segregation throughout the respective volume is observed. Note that the ordered lamellar spacing we found is 2 nm within the 50 nm NP. This result seems to not follow the observations of Amram and Rabkin regarding their lamellar spacing (200 nm) created by Au-Fe bilayer heating, but it shows the strong dependency on NP size, composition, and experimental conditions. The reduction of the particular interface area of Au fcc a Fe bcc is energetically favored and can be minimized at the (001) interface. Therefore, in the truncated tetrakis hexahedron, the structures are rotated by 45° around [001] resulting in Bain orientation relationship {001}Au ǁ {001}Fe and Au ǁ Fe 35. The interface formation and orientation relationship is experimentally and theoretically investigated in literature

15,20

and is based on the large differences in surface

energies of the Au-Fe system. The results are proven by FFT images from high-resolution TEM (HRTEM) micrographs (Figure S 7). At higher temperatures the diffusion length is sufficient to form exactly this interface area, resulting in the observed tetrakis hexahedron reported by Langlois et al.

19

. Thus, the remarkable morphology of the ultrastructure recreated from SS can

be explained by the energy contributions of the Fe and Au interfaces.


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The predicted ultrastructure of a Fe-rich core with truncated Au-rich pyramids is frequently observed for the Au20Fe80 sample, however, for small shell thicknesses (

Temperature-Dependent Ultrastructure Transformation of AuâFe

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