Atomic Layer Electrodeposition of a Pt Monolayer on Au via Surface-Specific PtCl₄ ^2- Adsorption

Atomic layer deposition of Pt monolayer films on Au is demonstrated based on the irreversible adsorption of square planar PtCl₄ ^2- _aq in the flat configuration, followed by reduction at low overpotentials in NaCl-based electrolytes. The strong affinity of PtCl₄ ^2- _aq for Au at positive potentials, +0.40 V_SSCE, contrasts with its negligible interaction with Cl^- covered Pt. The saturated fractional surface coverage of PtCl₄ ^2- _ads on (111)-textured Au thin films is ≈0.13 in freshly prepared 0.1 mol/L HClO₄, 0.12 in 0.5 mol/L NaCl, and 0.104 in 4 mol/L NaCl, as determined by X-ray photoelectron spectroscopy. When the reduction of PtCl₄ ^2- _ads is restricted to potentials where adsorption is still substrate-selective, Pt nucleation and growth are limited to monolayer dimensions. Three different deposition modalities are examined: potentiostatic, voltammetric, and pulsed potential, and the specific parameters to deposit monolayer films are identified. Pulsed potential deposition enables the adsorption and reduction steps to be temporally separated and quantitatively evaluated.