Au–Pd Alloy Gradients Prepared by Laterally Controlled Template Synthesis

Abstract

Lateral control of template synthesis in nanoporous alumina membranes (NAMs) was previously shown by us to enable preparation of graded composite materials. Formation of thickness gradients of Cu was demonstrated using electrodeposition (or electrodissolution) of Cu in the NAM template under a lateral voltage drop applied to the working electrode. This approach is extended here to the formation of compositional gradients. The latter are achieved by electrochemical co‐deposition of two metals (Au and Pd) in the membrane pores from a mixed metal‐ion solution under a lateral potential drop, to form an alloy that shows a continuous lateral change of the Au/Pd ratio. Environmental scanning electron microscopy images of cross sections along the line of the applied voltage gradient show that the deposit height changes gradually, while local elemental analysis by energy dispersive spectrometry and X‐ray diffraction measurements confirm a continuous change of the alloy composition along the membrane matrix.

Laterally Controlled Template Electrodeposition of Polyaniline

“…The membranes were then used as templates for electrodeposition in the pores, such that the open side of the pores is exposed to the solution. Filling of the pores with solution occurs spontaneously after dipping, and, as shown previously, almost all the pores are filled with the deposited material 16,17 …”

“…Bohn and coworkers showed that an iR drop created in thin‐film electrodes by injection of in‐plane currents provides means for laterally‐controlled electrochemical manipulation 6 . This approach was applied to the preparation of various laterally inhomogeneous systems and graded materials, such as SAMs displaying spatially‐resolved adsorption of proteins, polymeric coatings showing graded composition and morphology 10–12 including 2D patterning, 13 as well as laterally variable alloy compositions 5,14–16 …”

“…Generation of geometrical 17 and compositional 16 gradients was demonstrated by electrochemical deposition of metal and alloy nanowires, respectively, in NAMs. Electrochemical deposition and dissolution of Cu nanowires in NAMs under a lateral potential gradient allowed preparation of lateral gradients of tens of micrometers in nanowire length, 17 while application of a lateral potential gradient during electrodeposition of an Au–Pd alloy in NAMs resulted in a gradient of alloy composition in the template 16 . Each nanowire formed in the NAM comprised a homogeneous Au–Pd alloy of a certain composition; under the lateral gradient of applied deposition potential a continuous change in the ratio of the two metals was observed on a macroscopic scale.…”

“…6 This approach was applied to the preparation of various laterally inhomogeneous systems and graded materials, such as SAMs displaying spatially-resolved adsorption of proteins, polymeric coatings showing graded composition and morphology [10][11][12] including 2D patterning, 13 as well as laterally variable alloy compositions. 5,[14][15][16] Our group has focused on template electrochemical synthesis of nanostructured graded materials in nanoporous alumina membranes (NAMs), by means of lateral potential gradients generated in thin Au film electrodes on the membrane and sustained by the insulating NAM matrix. Generation of geometrical 17 and compositional 16 gradients was demonstrated by electrochemical deposition of metal and alloy nanowires, respectively, in NAMs.…”

Surface gradients under electrochemical control

“…73 Using the same setup, compositional gradients were fabricated, by electrochemical co-deposition of Au and Pd in the membrane, to form an alloy that showed a continuous gradient in Au/Pd ratio. 74 Also hybrid polymer/metal NWs have been fabricated, with a gradient in length of polymer, by first electrodepositing polyaniline while employing an in-plane potential gradient, followed by the electrodeposition of Ag or Cu. 75 Another dynamic display of electrochemical gradients by an in-plane potential gradient was shown by Tada and co-workers.…”