Unusual Dealloying Effect in Gold/Copper Alloy Thin Films: The Role of Defects and Column Boundaries in the Formation of Nanoporous Gold

Abstract

Understanding the dealloying mechanisms of gold-based alloy thin films resulting in the formation of nanoporous gold with a sponge-like structure is essential for the future design and integration of this novel class of material in practical devices. Here we report on the synthesis of nanoporous gold thin films using a free-corrosion approach in nitric acid applied to cosputtered Au-Cu thin films. A relationship is established between the as-grown Au-Cu film characteristics (i.e., composition, morphology, and structure) and the porosity of the sponge-like gold thin films. We further demonstrate that the dealloying approach can be applied to nonhomogenous Au-Cu alloy thin films consisting of periodic and alternate Au-rich/Au-poor nanolayers. In such a case, however, the dealloying process is found to be altered and unusual etching stages arise. Thanks to defects and column boundaries playing the role of channels, the nitric acid is found to quickly penetrate within the films and then laterally (i.e., parallel to the film surface) attacks the nanolayers rather than perpendicularly. As a consequence to this anisotropic etching, the Au-poor layers are etched preferentially and transform into Au pillars holding the Au-rich layers and preventing them against collapsing. A further exposure to nitric acid results in the collapsing of the Au-rich layers accompanied by a transition from a multilayered to a sponge-like structure. A scenario, supported by experimental observations, is further proposed to provide a detailed explanation of the fundamental mechanisms occurring during the dealloying process of films with a multilayered structure.

Study of the Coarsening of Nanoporous Gold Nanowires by In Situ Scanning Transmission Electron Microscopy During Annealing

“…This length scale is directly related to the surface area properties and therefore has a profound influence on many critical functional properties . A fine control of the structure can be obtained by tuning the chemical or electrochemical dealloying parameters (i.e., temperature, time, acid concentration, or dealloying voltage) as well as post‐treatment (i.e., annealing) . Moreover, a critical issue limiting the applications of NPG is the thermodynamically unstable structure, which induces coarsening at elevated temperatures .…”

Gold-rich ligament nanostructure by dealloying Au-based metallic glass ribbon for surface-enhanced Raman scattering

“…The untreated Au-based metallic glass ribbon showed a broad peak for the amorphous structure. The broad peak located at 2 θ value of about 42° could be the combination of the information of Au x Cu 1−x 30 . The four characteristic peaks of the dealloyed ligament structure located at 2 θ values of 38.2°, 44.3°, 64.6°, and 77.6° were assigned to Au(111), Au(200), Au(220), and Au(311), respectively, with an ordered face-centered cubic (FCC) structure.…”

Dealloying of gold–copper alloy nanowires: From hillocks to ring-shaped nanopores

“…The transmission electron microscopy (TEM) cross-section images of the Au–Cu alloy nanowires show that the hillocks exhibit the same multilayered structure observed for the main body of the nanowire (Supporting Information File 1, Figure S2). This multilayered structure, not present in case of elemental metals, is related to the substrate rotation during the co-sputter deposition [23–24]. …”