|
Synthesis of nanoporous gold structures via dealloying of electrodeposited Au-Ni thin films
E. Rouya1, H. Bart-Smith2, M.R. Begley2, R. Kelly1, M. Reed3, G. Zangari1 1 Department of Materials Science and Engineering and CESE 2 Department of Mechanical and Aerospace Engineering 3Department of Electrical Engineering University of Virginia, 116 Engineer's Way, Charlottesville, VA 22904 USA E-mail: gz3e@virginia.edu |
|
Au-Ni alloy films were electrodeposited from an electrolyte containing Au sulfite, Ni sulfate and ethylendiamine. Complexation of the two metals leads to their reduction occurring at very similar potentials, forming a homogeneous alloy. Ni content in the alloy increased by applying a larger cathodic potential, as confirmed by Energy Dispersive Spectroscopy (EDS) analysis. X-ray Diffraction (XRD) results indicate that 250 – 500nm thick Au-Ni films deposited on Si(100)/Cr/Au electrodes develop an amorphous structure. Films could be crystallized into a metastable solid solution of Au and Ni by suitable annealing procedures.
Dealloying of the resulting alloy films in sulfuric acid was performed. The dealloyed morphology and level of porosity could be controlled effectively by employing the following methods: 1) dealloying as-deposited amorphous Au-Ni alloy films with different compositions, 2) dealloying a given alloy at various anodic potentials, and 3) dealloying as-deposited amorphous or partly crystallized Au-Ni films.
The length-scale of the porosity is found to increase with an increase of Ni content in the alloy (Figure 1), and also to decrease upon increase in the applied anodic potential; this potential is however limited to a certain value, above which defects become so dense that they assist in the initiation of surface crack growth. In addition, dealloying Au-Ni films with compositions exceeding an estimated critical composition of Au-Ni 80 at% results in large tensile stresses, which also leads to surface cracks. Finally, preliminary results show that dealloying crystallized Au-Ni films strongly impacts the dissolution rate of Ni atoms as they diffuse out of the alloy bulk and become oxidized; this shows that initial stress relaxation occurring during alloy crystallization strongly affects the dealloying behavior.
Figure 1 - Morphology of nanoporous gold obtained by dealloying Au-Ni alloys with different composition. The dealloying electrolyte is 10 mM H2SO4, dealloying potential is 0.75 V vs a calomel reference electrode.