Electrochemical deposition of platinum within nanopores on silicon: Drastic acceleration originating from surface-induced phase transition

Abstract

An electrochemical reaction within nanopores is remarkably decelerated once a diffusion-limited condition is reached due to the difficulty in supply of reactants from the bulk. Here, we report a powerful method of overcoming this problem for electrochemical deposition of platinum within nanopores formed on silicon. We made the pore wall surface of the silicon electrode hydrophobic by covering it with organic molecules and adopted platinum complex ions with sufficiently large sizes. Such ions, which are only weakly hydrated, are excluded from the bulk aqueous electrolyte solution to the surface and rather hydrophobic in this sense. When the ion concentration in the bulk was gradually increased, at a threshold the deposition behavior exhibited a sudden change, leading to drastic acceleration of the electrochemical deposition. Using our statistical-mechanical theory for confined molecular liquids, we show that this change originates from a surface-induced phase transition: The space within nanopores is abruptly filled with the second phase within which the ion concentration is orders of magnitude higher. When the affinity of the surface with water was gradually reduced with fixing the ion concentration, qualitatively the same transition phenomenon was observed, which can also be elucidated by our theory. The utilization of the surface-induced phase transition sheds new light on the design and control of a chemical reaction in nanospace.

Effect of cation species on surface-induced phase transition observed for platinum complex anions in platinum electrodeposition using nanoporous silicon

“…When the ion concentration in the bulk is gradually increased, at a threshold the deposition behavior exhibits a sudden change, leading to drastic acceleration of the electrochemical deposition. 26 Using our statistical-mechanical theory for confined molecular liquids, we have shown that this change originates from a surfaceinduced phase transition: The space within a nanopore is abruptly filled with the second phase in which the ion concentration is orders of magnitude higher than that in the bulk. 26 The threshold concentration is lowered when the ionic size is made larger (that is, [PtCl 4 ] 2− with d − = 0.60 nm (d − is the anion diameter) is replaced by [PtBr 4 ] 2− with d − = 0.70 nm; the latter is more hydrophobic).…”

“…26 Using our statistical-mechanical theory for confined molecular liquids, we have shown that this change originates from a surfaceinduced phase transition: The space within a nanopore is abruptly filled with the second phase in which the ion concentration is orders of magnitude higher than that in the bulk. 26 The threshold concentration is lowered when the ionic size is made larger (that is, [PtCl 4 ] 2− with d − = 0.60 nm (d − is the anion diameter) is replaced by [PtBr 4 ] 2− with d − = 0.70 nm; the latter is more hydrophobic). 25,26 When the pore wall surface is made hydrophilic, by contrast, there is no such striking behavior observed: It seems that the enrichment of the ionic concentration near the hydrophilic surface does not occur and the electrochemical deposition within nanopores does not proceed irrespective of the ionic concentration in the bulk.…”

“…In recent works, 25,26 we have investigated platinum electrodeposition within nanoporous silicon electrode (the pore diameter is ∼3 nm on the average). The electrode is immersed in aqueous solution containing platinum complex ions (these are anions).…”

“…26 The threshold concentration is lowered when the ionic size is made larger (that is, [PtCl 4 ] 2− with d − = 0.60 nm (d − is the anion diameter) is replaced by [PtBr 4 ] 2− with d − = 0.70 nm; the latter is more hydrophobic). 25,26 When the pore wall surface is made hydrophilic, by contrast, there is no such striking behavior observed: It seems that the enrichment of the ionic concentration near the hydrophilic surface does not occur and the electrochemical deposition within nanopores does not proceed irrespective of the ionic concentration in the bulk. Near the hydrophilic surface, the concentration of the hydrophobic solute is not enriched but the number density of water is considerably heightened.…”

Lateral Growth of Polypyrrole Electropolymerized along Hydrophobic Insulative Substrates

“…Such a difference has been evaluated both experimentally and theoretically. [26][27][28] In aqueous solutions, it has been shown that weakly hydrated solutes are excluded from the bulk to a hydrophobic surface. The similar behavior, i.e., hydrophilic solutes are excluded to hydrophilic surface in hydrophobic solvent, has been reported, e.g., ethanol is concentrated on the surface of silica in cyclohexane.…”