Metal nanoparticles (NPs) are widely used for electrocatalytic applications due to their large surface-to-volume ratio. Their structural features largely affect electrochemical activity, selectivity, and stability [1].

In this study, we report on the design and fabrication of nanostructured electrodes by solid-state dewetting (SSD), i.e., the heat-induced transformation of thin metal films into a “monolayer” of defined, spaced metal NPs (Fig. 1a) with tunable loading, composition, size and structure [2, 3]. Notably, dewetting occurs along with exposure of the substrate, and hence formation of the Pt-substrate-electrolyte interface, i.e., the triple-phase boundary (TPB). We investigate the role of the Pt structure and TPB on the hydrogen evolution reaction (HER) as model system.

For this, Pt films of various thicknesses were deposited by magnetron sputtering on fluorine-doped tin oxide (FTO) followed by heat treatment (500°C) to induce dewetting. As deposited and dewetted electrodes were characterized in view of their electrochemical performance and by various techniques such as X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS).

As-deposited Pt films as thin as 5 nm are closed and polycrystalline, with grain size in the range of the film thickness (i.e., 4-5 nm). Dewetting of these thin films leads to the formation of crystalline, faceted NPs with a mean size of 50 nm (Fig. 1b), and average crystalline domain size of 50 nm, indicating that NPs are mostly single crystals or twins. For 5 nm-thick films, the electrochemical surface area (ECSA, measured by hydrogen underpotential deposition) significantly decreases upon dewetting due to Pt agglomeration as the FTO becomes exposed. Interestingly, despite the decrease in ECSA, dewetted NP electrodes are clearly more active (> 3 times) in HER than as-deposited films (Fig. 1c, specific current density at – 50 mV vs. RHE). A similar increase in activity is also observed upon dewetting thinner Pt films into NPs. Differently, Pt films as thick as 30 and 50 nm do not agglomerate into NPs when heat-treated, show a minor change in grain size and surface area, and expose the FTO substrate to a small extent (i.e., the Pt film forms only few holes). Such films show a minor increase of HER activity after heat-treatment (Fig. 1c).

We rationalize these results proposing that while ECSA and grain size have a minor effect, a key factor governing the HER activity is the extension of the triple-phase boundary, that is, in this case the Pt-FTO-electrolyte triple interface. Heat-treated Pt films on diverse substrates, e.g., carbon paper, boron-doped diamond or Nb electrodes, shows similar activity trends as observed for Pt/FTO electrodes (Fig. 1d), suggesting that crucial for the increase in HER activity is the formation of the TPB due to exposure of the substrate upon dewetting.

In ongoing work, we are investigating further the role of NP faceting, and are quantifying with accuracy the extension of the TPB as a function of the Pt NPs size and how it affects the HER performance.