Subscriber access provided by Universitaetsbibliothek | Johann Christian Senckenberg
Article
Templated dealloying: designing novel ultrastructures by memory effect Marius Kamp, Anna Tymoczko, Ulrich Schürmann, Jurij Jakobi, Christoph Rehbock, Stephan Barcikowski, and Lorenz Kienle Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00175 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Crystal Growth & Design
Templated dealloying: designing novel ultrastructures by memory effect Marius Kamp†, Anna Tymoczko‡, Ulrich Schürmann†, Jurij Jakobi‡, Christoph Rehbock‡, 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, Universitätsstrasse 7, 45141 Essen, Germany
nanoporous Au, yolk-shell nanoparticle, dealloying, laser ablation, memory effect
Abstract
Tailoring the morphology of nanoporous structures widens the scope of applications in catalysis and sensing. The synthesis of versatile nanoporous morphologies with the spatial distribution of porosity is permitted by dealloying of unique, metastable Au-Fe alloy template nanoparticles generated by laser ablation in liquid. This approach opens the door to a novel process, which 1 ACS Paragon Plus Environment
Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 24
involves a special transformation mechanism, including oxidation and Kirkendall effect, which is decisive for the stabilization of hollow structures with the spatial distribution of porosity and represents a memory effect of morphology. Within this work, nanoporous Au particles, hollow nanoporous Au shells with the spatial distribution of porosity, and yolk-shell-like Au nanoparticles encapsulated in ultrathin Au shells are synthesized. A distinct variation of crystallinity and an increased lattice strain is observed, which implies an improved catalytic activity for oxidation reactions.
Introduction The synthesis of nanoporous Au and hollow Au nanoparticles (NPs) with high catalytic and sensing activity has been frequently studied in recent years 1–4. In contrast to metal NPs, the open nanoporous structure is characterized by a high surface to volume ratio with increased density of active sites, which results in a high catalytic activity of these materials
5–7
. The reason for
advanced surface activity is based on the low coordination of Au surface sites and significant lattice strain, induced by the enhanced defect concentration of curved surfaces
2,7,8
. In addition,
ultrathin 2D metal films are easy to create, however, the generation of ultrathin 3D shells is far more difficult, particularly when nanoporous structures aspired. This is due to the instability of these shells, which demands advanced template strategies. For sensing applications, nanoporosity enables an enhanced sensitivity, due to its modified surface properties 9. The ultrastructure of NPs, which describes the internal phase segregation within NPs, is decisive for their properties. Hollow NPs show tunable optical properties like cancer theranostic
13
10
and are suitable for biomedical applications
11,12
,
(due to their mass efficient heat conversion) and anticancer drug 2 ACS Paragon Plus Environment
Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Crystal Growth & Design
delivery 14. Plasmonic applications benefit from thicker gold shells that naturally bear low defect densities, whereas ultrathin gold shells may show high defect densities, which are potentially beneficial in catalysis. Also, very thin hollow shells and porous structures show significantly reduced diffusion barriers
15
, a prerequisite for chemical reactions or cargo release. Nanoshells
prepared from templates are primarily synthesized by the Kirkendall effect, which describes an unbalanced inter-diffusion of two species. It is frequently applied to the oxidation of NPs, in which the inward diffusion of oxygen is slower than the outward diffusion of the metal ions in the NPs. Thus, morphological changes, including void formation, are observed, e.g. formation of NiO and CoO nanoshells from metallic NPs
16–18
. This process is limited to metal compound
structures, therefore, galvanic replacement reactions are applied for the synthesis of Ag-Au hollow NPs, but the process often results in nanocages and needs decisive control of the reaction parameters to prevent particle collapse
16,19,20
ultrathin shells (yolk-shell NPs), e.g. carbon
. The synthesis of metal NPs encapsulated in 21,22
and zirconia
23
, needs multiple involved
processing steps. Besides, Rabkin et al. showed an alternative approach in which Au-Ag particles are transformed into hollow particles by curvature-driven surface diffusion, supported by a silica substrate
24
. A synthesis example for nanoporous Au is based on a dealloying process from a
bimetallic alloy. By a wet chemical process based on galvanic dealloying 16, the less noble metal (lower reduction potential) is dissolved and a nanoporous material remains 11,16. In literature, AuAg alloys are frequently used in which vacancy coalescence stabilizes the nanoporous structure 3,16,25–27
. In addition, a residual concentration of undissolved metal is sufficient for the enhanced
catalytic activity in oxidation reactions
28–33
. The dealloying process is not limited to Ag-Au
alloys, but it can be applied to other alloy systems such as Au-Ge34 to synthesize nanoporous structures or particles, which are frequently used in catalysis 35. For instance, nanoporous Pt 36–38, 3 ACS Paragon Plus Environment
Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Ni
39
and Cu
40
Page 4 of 24
have been synthesized from Pt-Cu, Ni-Cu, and Cu-Mg alloys, respectively, by
dealloying. In contrast to fully miscible systems, a novel kind of educt material, namely Au-Fe alloys, offer the possibility to investigate the dealloying behavior of different ultrastructures (homogeneous NPs and phase segregated NPs). This alloy system was frequently investigated in the literature. For instance, Rabkin and co-workers showed that the formation of core-shell (CS) NPs with a Fe core and an Au shell can be synthesized from dewetting of thin films based on the miscibility gap of Fe and Au elements and the large difference in surface energy 41–43. An alternative approach for the generation of Au-Fe NPs from bulk alloy targets is laser ablation in liquids (LAL). In this context, it was shown that Au-rich alloy NPs can be obtained 44,45
. Next to CS, this technique allows the generation of Fe-rich metastable alloy and phase
segregated NPs 46. Wagener et al. 47 showed that CS and alloy NPs are produced in parallel, while the ultrastructure is controlled by the solvent. The CS ratio can be tuned and is increased when the NPs diameter is raised and Fe-rich targets are chosen 48. LAL is the only method that allows the generation of alloy and phase segregated colloidal, surfactant-free NPs, where both the core and shell are made from Au-Fe alloy, which may yield unequaled flexibility when it comes to the availability of educt materials for dealloying. The use of these complex alloyed template materials for dealloying could lead to novel nanoporous structures with versatile applications
49
.
Furthermore, an alloy system may result in sophisticated reaction mechanisms and the replacement of Ag by cheap and abundant Fe for the synthesis of colloidal nanoporous Au catalyst materials 50. In this work, galvanic dealloying is applied to the Au-Fe alloy system, including an alloy film and complex metastable phase segregated ultrastructures, resulting in nanoporous Au and
4 ACS Paragon Plus Environment
Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Crystal Growth & Design
versatile kind of hollow NPs with the spatial distribution of porosity. Moreover, a novel formation mechanism within the stabilization of ultrathin Au shells is depicted. Experimental Sections NPs are synthesized by one step, Pulsed Laser Ablation in Liquids (LAL) from a bulk alloy target in an organic solvent. Two samples of Au-Fe NPs, with a nominal composition of Au50Fe50 and Au20Fe80, were generated in acetone and 3-pentanone, respectively. An 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 3.1mJ/cm2 were used. For both systems, a lens with a focal length of 100 nm was used. The experimental details of the ablation process can be found elsewhere in a previous work 46. The thin film was synthesized by co-sputtering of a 99.99 % 2” Fe target (EVOCHEM GmbH) and a 99.99 % 2” Au (EVOCHEM GmbH) target with 50 W and 35 W, respectively. The argon gas flow was set to 30 sccm (standard cubic centimeters per minute) resulting in a pressure of 3.1*10-3 mbar. The alloy thin film, with fcc-type structure, has a film thickness of 450 nm (measured by Stylus Profilometer Bruker DektakXT) and is synthesized with deposition rates of 45 nm per minute on a (001) oriented NaCl single crystal (5 mm*5 mm), mounted on a rotating sample holder. The substrate was dissolved in methanol to produce a free-standing thin film, which is used as a reference sample for the etching experiments. All samples were etched with concentrated HNO3 (15.6 M) for 48 hours, to secure a completed etching process. Preliminary experiments (Figure S 1) showed that shorter reaction times of 24 hours lead to incomplete dealloying. The same trend was observed for lower nitric acid concentrations. When nitric acid at molarities of 0.1 M and 8 M (48 h) were used, no complete dealloying of Fe was observed (Figure S 2). For the TEM investigations, the samples were 5 ACS Paragon Plus Environment
Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 24
dispersed on platinum coated copper grids with a lacey carbon film (Plano GmbH) and a Tecnai F30 STWIN G2, with 300 kV acceleration voltage, was used. Z-contrast images were recorded with a high angle annular dark-field STEM detector. Energy dispersive X-ray (EDX) spectra were measured with a Si/Li detector (EDAX System). Scanning electron microscopy images are recorded with an InLense detector, with 5 kV acceleration voltage, at a Carl Zeiss Ultra Plus. Results and Discussion Before dealloying, all samples were characterized by transmission electron microscopy (TEM). The NPs with the nominal and average composition of Au50Fe50 and Au20Fe80 show three different ultrastructures of phase segregated NPs: core-shell (CS), multicore (MC) and multicoreshell (MCS) NPs. Representative scanning TEM images of these NPs are depicted in Figure 1 a), b) and c). MCS NPs have a thin Au-rich shell and a second (and thicker) Fe-rich inner shell around a single Au-rich core (Figure 1 e). All types of ultrastructures are present independent of the Au:Fe ratio (Figure S 3). In our previous works, we emphasized how the prevalence of CS over solid solution (SS) NPs emerges in correspondence with the particle size and the target composition
46,48
. There we usually characterized all structures discussed in this work as CS,
however recent results allow better differentiation between the different structures. In addition, a thin film with nanocrystalline microstructure (typical grain sizes: 10-20 nm) is investigated (Figure 1 d) as a reference sample with a solid solution structure. The EDX results (Figure S 4) show an average composition of Au56Fe44 (ûf = 1 at%) with homogeneous and uniform elemental distribution within the grains. The oxidation of the film can be excluded due to the absence of an oxygen signal in all EDX spectra (Figure S 4 b). From electron diffraction, an fcc structure and texture of the film along [112] is shown, cf. concentrations of diffuse intensity
6 ACS Paragon Plus Environment
Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Crystal Growth & Design
ACS Paragon Plus Environment
Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Paragon Plus Environment
Page 8 of 24
Page 9 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Crystal Growth & Design
single NPs (and aggregates) with residual Fe (
