Remote Control Electrodeposition: Principles for Bipolar Patterning of Substrates without an Electrical Connection

Abstract

We describe electrolyte design for bipolar electrochemical growth and patterning of a range of materials on an electrically floating substrate using the scanning bipolar cell (SBC). In the SBC, bipolar electrodeposition is driven by local potential variation generated beneath a rastering microjet anode connected to a far-field cathode. Metal reduction occurs beneath the microjet when the substrate is approached, provided the electrolyte possesses a suitable reducing agent that undergoes oxidation across the substrate far-field. We use a series of metal reduction reactions (Ni, Cu, Au, Ag) that cover a wide range of nobility, and couple them to the oxidation of ascorbic acid or ferrous ion, depending upon the metal used. The reversibility or irreversibility of the local metal reduction reaction dictates details of the required electrolyte thermodynamics. For irreversible deposits (Ni, Au), there is a wide thermodynamic operating window for the bipolar counter reaction. Reversible deposits that are easily etched (Cu, Ag) have tight thermodynamic windows; deposit stability requires the use of metastable electrolytes. We provide a simple scaling relationship that incorporates the electrolyte thermodynamics, interfacial charge transfer kinetics, and SBC operating conditions, then demonstrate its use through a 10X reduction in the spatial dimensions of local nickel reduction chemistry. Bipolar electrochemistry enables remote control of an electrochemical system without any direct electrical connection to the conducting substrate.1-13 Bipolar electrochemical systems involve spatially segregated, equal and opposite reduction and oxidation on an electrically floating substrate. The driving force for bipolar electrochemistry is the ohmic potential variation in an electrolyte that forms during the passage of current. When there is an appreciable potential drop through solution, and a conductor is in that potential gradient, the path of least resistance for current flow can sometimes be through the conductor via bipolar electrochemical reactions occurring on different regions of the conductor. To achieve remote control materials patterning, the electrolyte chemistry and electrochemical cell must be tailored to work together.In recent years, bipolar electrochemical systems have been used for new kinds of electrochemical applications ranging from electrodeposition to electroanalytical chemistry. For example, bipolar electrochemistry can be used to grow interconnects between electrically isolated conducting posts in an electrolyte.1,2 Bipolar electrochemistry has also been successfully used for screening electrocatalysts where a hard to detect reaction rate is readily visualized by an equal and opposite electrochemical indicator.3-5 Typical bipolar systems have used resistive electrolyte solutions and macroscopic electrochemical cells to produce the electrolyte potential gradients needed to drive bipolar reactions on small conductors. Here, we demonstrate and generalize a microscopic electrochemical cell configuration (termed ...

Exploring the Kinetic and Thermodynamic Relationship of Charge Transfer Reactions Used in Localized Electrodeposition and Patterning in a Scanning Bipolar Cell

“…Local bipolar electrodeposition in the SBC is experimentally demonstrated for two characteristic systems: kinetically irreversible (Ni) and kinetically reversible (Cu or Ag) electrodeposition chemistries. Prior work outlined the electrolyte design guidelines necessary to achieve both spatially and temporally stable deposits using the SBC for these bipolar systems (Braun and Schwartz, 2016c). Specifically, kinetically irreversible electrodeposition chemistries can be paired with any bipolar oxidation chemistry resulting in Δ E BC < 0, whereas kinetically reversible electrodeposition chemistries must be paired with a bipolar oxidation reaction that produces a marginally downhill thermodynamic relationship (i.e., Δ E BC > 0).…”

“…Previous work by our group demonstrated that a rastering microjet nozzle can be employed for localized bipolar electrodeposition and patterning on an electrically floating substrate, a system we called a scanning bipolar cell (SBC) (Braun and Schwartz, 2015, 2016a,b,c). Initial applications of the SBC on a copper bipolar electrode involved copper electrodeposition in the region beneath the nozzle (near-field) and copper dissolution of the substrate material in the region surrounding the nozzle (far-field) (Braun and Schwartz, 2015).…”

“…Controlling the initial quantity of copper charge on the substrate available for oxidation translated to self-limited patterning with the SBC, where local nickel deposition (electron acceptor) terminated upon complete etching of the copper “fuel” (electron donor). Rather than rely on metal dissolution as the bipolar counter reaction, an electrolyte-born redox couple was used to generalize electrodeposition in the SBC for a diverse range of metals (Braun and Schwartz, 2016c). Computational methods validated analytical scaling relationships approximating current flow and coupling between thermodynamics, kinetics, and transport in the SBC (Braun and Schwartz, 2016a).…”

Analytical and Computational Scaling Relationships for the Coupled Phenomena that Control Local Bipolar Electrochemical Behavior

“…8,9 Previous work by our group experimentally demonstrated localized bipolar electrochemistry using a rastering microjet anode, with a far-field cathode, that we called the scanning bipolar cell (SBC). [14][15][16] Initial applications of the SBC used an electrically-isolated copper metal substrate as the electron donating oxidation chemistry associated with the bipolar couple. Local electrodeposition of metal ions beneath the SBC microjet was the electron accepting reduction chemistry.…”

“…The fraction of current that passes through the conductive substrate and participates in bipolar electrochemistry is coupled to the ohmic drop through solution, charge transfer kinetics, and thermodynamic relationship of the bipolar couple in the near-field and far-field of the substrate. For bipolar electrochemical current to flow through the substrate when the near-field and far-field chemistry is thermodynamically uphill (as is often desirable 16 ), the potential drop through solution must exceed the thermodynamic potential difference of the bipolar couple, E BC .…”