Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Tailoring the Strength of Nanoporous Gold by Self-Assembled Monolayers of Alkanethiols Nadiia Mameka,*,† Lukas Lührs,‡ Stefan Heissler,§ Hartmut Gliemann,§ and Christof Wöll§ †
Institute of Materials Research, Materials Mechanics, Helmholtz-Zentrum Geesthacht, 21502 Geesthacht, Germany Institute of Materials Physics and Technology, Hamburg University of Technology, 21073 Hamburg, Germany § Institute of Functional Interfaces, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany ‡
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ABSTRACT: Because of the large specific surface area, the properties of nanoporous metals and in particular their mechanical properties are sensitive to chemical modifications of their surfaces. Here, we exploit self-assembled monolayers (SAMs) to modify a surface of nanoporous gold and study their effect on plastic behavior. The SAMs investigated here (i) are made from alkanethiols, which consist of a sulfur headgroup that strongly binds to metal substrates, a hydrocarbon chain, and an end group, and (ii) are known to spontaneously self-organize into well-ordered, dense twodimensional molecular films on the surface of coinage metals. Alkanethiols with various chain lengths and terminal groups were used to prepare SAMs on bulk nanoporous gold, and compression tests were performed on the SAM-modified and nonmodified macroscopic samples. Our experiments reveal a substantial, up to 50%, increase of the flow stress due to thiol adsorption. We attribute the strengthening to the adsorption locking of dislocation end points at the surface, which is mediated by the fairly strong metal−sulfur interaction. KEYWORDS: nanoporous, dealloying, self-assembled monolayers (SAMs), alkanethiols, surface tension, surface stress
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strength,10 an 8% increase of the stiffness,11 hinder the creep deformation,12 promote faster crack propagation,13 and enlarge the plastic Poisson’s ratio by >40%.9 Besides rendering novel functionalities to metals, the electrochemical control of the interface properties during mechanical testing has significantly advanced the overall understanding of how surface energy, surface stress, and surface excess elasticity affect the mechanics of high surface area materials. In the past decades, a spontaneous chemisorption of appropriately functionalized organic molecules has emerged as a flexible and efficient way to control and modify the surface properties of flat substrates,14−18 nanoparticles,19,20 and nanoporous structures.21−27 Presently, the most important class of SAM-forming molecules are organothiols,16 among which alkanethiols on gold are, by far, the most explored. Alkanethiols denoted as HS-R (where R = (CH2)nX is an organic tail) are composed of an −SH group that forms a strong thiolate bond with coinage metals or coinage-metalcoated substrates, an alkyl chain of variable length −(CH2)n−, and a terminal group X.28 As the chemical character of the
INTRODUCTION Dealloying-derived nanoporous gold (NPG) has proved to be a versatile model material to explore how surface modifications can affect bulk mechanics in small-scale systems. The size of the NPG microstructural constituentsligamentsusually reach dimensions of a few tenths of nanometers, affording large surface-to-volume ratios and, thus, maximizing the net surface area. The elastic and plastic properties of NPG mimic that of single Au nanowires or micrometer-scaled pillars, showing a noticeable size-dependent stiffness 1,2 and strength1,3−5 as well as tension−compression asymmetry of yield strength.6−9 Macroscopic nanoporous samples are readily available via dealloying, and conventional mechanical testing can be applied to study the interface-controlled behavior. The open porosity of the nanoporous structure enables effective manipulation of the surface states and modification of the large surface area in different environments.3 In this respect, infiltration of the nanoporous metallic “skeleton” with an aqueous electrolyte and controlling the metal−fluid interface by applying electric potentials has resulted in a number of interesting phenomena, in particular as regards the NPG mechanical behavior. It has been shown that formation of surface oxide layers via polarization of NPG wetted with electrolyte can induce a 2-fold increase of the © XXXX American Chemical Society
Received: August 8, 2018 Accepted: November 26, 2018 Published: November 26, 2018 A
DOI: 10.1021/acsanm.8b01368 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials
Figure 1. NPG sample preparation. (a) Ag75Au25 cylindrical specimens before and after dealloying. (b) Scanning electron micrographs of the NPG microstructure (fracture surfaces) after electrochemical voltammetric cycling of as-dealloyed specimen in 1 M HClO4 and (c) incubation in 10 mM ethanolic solution of MHO for several days. No change in microstructure of the SAM-modified NPG is apparent.
compression experiments. We used millimeter-sized NPG specimens and immersed them for different times in ethanolic solutions of alkanethiols with different concentrations and different hydrocarbon chain lengths and terminal groups before investigating their mechanical response. For the SAMs studied in this work we choose alkanethiols as the corresponding monomers since alkanethiol-based SAMs have been studied in great detail in terms of the reaction with gold surfaces and self-assembly process and due to their relatively high stability at ambient conditions.16 In all cases, we observed a substantial strengthening effect, with the magnitude of the strength enhancement depending on the particular alkanethiol and the immersion time. This observation is in contrast to previous works, which reported the opposite behavior.25,38 Generally, the strengthening was more pronounced for longer immersion times. We explain the difference to the previous reports by the fact that in the earlier investigations25,38 small material volumes were tested, while in our study bulk macroscopic samples were investigated, which allow the application of conventional, robust mechanical testing schemes. We feel that the approach used here allows minimizing any possible impact of local surface contaminations as well as of uncertainties in data analysis and interpretation.
organic surfaces formed by adsorption of the SAM molecules is determined by these X-groups, the choice of X allows tailoring the specific chemical functionality of the exposed outer surface, e.g., hydrophilicity, hydrophobicity, and the ability to anchor metal ions or biomolecules. The formation of SAMs from alkanethiols proceeds spontaneously (from a gas or liquid phase), and self-assembly yields ordered, dense two-dimensional molecular films on solid surfaces. van der Waals forces acting between neighboring alkyl chains assist in stabilizing the crystalline structure of the resulting self-assembled monolayers. The thicknesses of the SAMs depend on the length of thiol molecules and their spatial arrangement on the surface (their tilt angle) and are typically in the range 1−3 nm.15 It is interesting to note that thiol monolayers assembled on nanostructures,29,30 in particular on nanoporous Au substrates,21,22,27 have been shown in terms of well-ordered and dense organic films and exhibit an enhanced electrochemical stability compared to that of planar Au(111) surfaces. This observation has been associated with the presence of many low-coordinated atoms at the nanostructured surfaces, such as atomic step edges, vacancies, and adatoms, along with the presence of residual Ag atoms, which are typically present in NPG in small fractions and might segregate on the ligament surfaces,31 affording the greater thiolate packing density.32 For such irregular surfaces, density functional theory (DFT) calculations predict larger S−Au binding energies22,30 as compared to perfect Au(111), suggesting a stronger adsorption of the sulfur head on the defective or Ag-rich gold surfaces. Moreover, binding of thiols generates substantial stresses in the underlying Au as observed in cantilever bending33−36 and Xray diffraction experiments.37 The enhanced thiolate binding energies on defective surface sites and the SAM-induced stresses in the bulk of metallic substrates used for SAM growth are expected to affect dislocation behavior and, as a result, plastic flow, especially of high surface area solids. To explore this issue, in previous studies, indentation tests have been carried out, and a lower hardness of Au films38 and nanoporous Au specimens25 was found after they were coated with alkanethiol-based SAMs. The SAM-induced weakening of the material was attributed either to lowering the surface energy or to movement of dislocation end points at the surface, both of which are facilitated by Au−S bonding. In this study, the effect of thiol-based self-assembled monolayers on plastic deformation of macroscopic bulk NPG samples was directly probed by performing macroscopic
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MATERIALS AND METHODS
Nanoporous Au. NPG bulk cylindrical specimens ∼1 mm in diameter and ∼1.5−1.7 mm in length were prepared by electrochemical dealloying of Au25Ag75 alloy as reported previously.39 Briefly, the precursor alloy ingot was produced by arc melting, subjected to homogenizing annealing, machined into a wire, sectioned by a diamond wire saw, and annealed to relieve any residual stresses. Au25Ag75 cylindrical samples were subsequently dealloyed at a constant potential of 1.26 V versus standard hydrogen electrode (SHE) in 1 M HClO4 (60%, ACS grade, Merck) until the current decayed below 10 μA. Before SAM formation, the NPG specimens were electrochemically reduced in 1 M HClO4 (70%, Suprapur, Merck) by potential cycling between 0.01 and 1.51 V versus SHE at 5 mV s−1 to remove surface oxides and residual Ag. Finally, the samples were rinsed with ultrapure water and dried in air. As evidenced by energy-dispersive X-ray spectroscopy (EDX), this treatment yielded NPG samples with residual Ag contents
