Dealloying is widely utilized but is a dangerous corrosion process as well. Here we report an atomistic picture of the initial stages of electrochemical dealloying of the model system Cu(3)Au (111). We illuminate the structural and chemical changes during the early stages of dissolution up to the critical potential, using a unique combination of advanced surface-analytical tools. Scanning tunneling microscopy images indicate an interlayer exchange of topmost surface atoms during initial dealloying, while scanning Auger-electron microscopy data clearly reveal that the surface is fully covered by a continuous Au-rich layer at an early stage. Initiating below this first layer a transformation from stacking-reversed toward substrate-oriented Au surface structures is observed close to the critical potential. We further use the observed structural transitions as a reference process to evaluate the mechanistic changes induced by a thiol-based model-inhibition layer applied to suppress surface diffusion. The initial ultrathin Au layer is stabilized with the intermediate island morphology completely suppressed, along an anodic shift of the breakdown potential. Thiol-modification induces a peculiar surface microstructure in the form of microcracks exhibiting a nanoporous core. On the basis of the presented atomic-scale observations, an interlayer exchange mechanism next to pure surface diffusion becomes obvious which may be controlling the layer thickness and its later change in orientation.
We report the onset of electrochemical doping and subsequent visible light emission at 5V and 360K from a planar light-emitting electrochemical cell with a 1mm interelectrode gap containing poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene] (MEH-PPV), poly(ethylene oxide) (PEO), and XCF3SO3 (X=K,Li) as the active material. We rationalize the unprecedented low turn-on voltage of such wide-gap light-emitting electrochemical cells by demonstrating that the active material contains a mixture of crystalline PEO+XCF3SO3 domains and amorphous MEH-PPV domains at room temperature, but that the crystalline domains have melted at 360K resulting in a significant increase in the ionic conductivity.
We report on the influence of chemical surface modification on the selective dissolution and dealloying of Cu 3 Au (111) in 0.1 M H 2 SO 4 . Cu-Au alloys serve as longstanding model alloys for dealloying and stress corrosion cracking. Employing well-defined hexadecanethiol, mixed-aminobenzenethiol, and plasmapolymerized hexamethyldisiloxane surface layers we obtain detailed atomic-scale insight in the stability of the surfaces. The initial structural evolution of the modified surfaces is tracked by in situ X-ray diffraction using synchrotron radiation. In comparison to the usual sequence of surface states on unmodified Cu 3 Au (111) the modified surfaces develop a thicker and more stable passive-like Au-rich film below the critical potential. In this regime we observe anodic shifts in the potentials of the surface structural transitions and the suppression of an otherwise observed island morphology. This extreme stability of the modified passive-like surfaces is further confirmed by ICP-MS coupled to a scanning micro-electrochemical flow cell and ex situ SEM. Above the critical potential, the presence of the inhibiting protective layers leads to a localized dealloying mechanism with an altered microstructure of the forming nanoporous film in the form of micro-cracks. Understanding the stability of alloy surfaces is a prerequisite for further applications, e.g. for lithography or sensors.
We report the design of an improved electrochemical cell for atomic force microscope measurements in corrosive electrochemical environments. Our design improvements are guided by experimental requirements for studying corrosive reactions such as selective dissolution, dealloying, pitting corrosion, and∕or surface and interface forces at electrified interfaces. Our aim is to examine some of the limitations of typical electrochemical scanning probe microscopy (SPM) experiments and in particular to outline precautions and cell-design elements, which must necessarily be taken into account in order to obtain reliable experimental results. In particular, we discuss electrochemical requirements for typical electrochemical SPM experiments and introduce novel design features to avoid common issues such as crevice formations; we discuss the choice of electrodes and contaminations from ions of reference electrodes. We optimize the cell geometry and introduce standard samples for electrochemical AFM experiments. We have tested the novel design by performing force-distance spectroscopy as a function of the applied electrochemical potential between a bare gold electrode surface and a SAM-coated AFM tip. Topography imaging was tested by studying the well-known dealloying process of a Cu(3)Au(111) surface up to the critical potential. Our design improvements should be equally applicable to in situ electrochemical scanning tunneling microscope cells.
On self-assembled monolayer-covered Cu-Au substrates, localized volume shrinkage at initial dealloying sites leads to cracks within the attacked regions. It is started from well-controlled surface structures to gain fundamental insights in the driving mechanisms of localized corrosion and crack formation. Both the crack density and the crack morphology are critically dependent on surface orientation, crystallography, and inhibitor molecule species.
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