Solid-State Dewetting to Fabricate Defined Nanoparticles-Based Electrodes for Electrocatalysis

The design and fabrication of chemical-free nanoparticles (NPs) based electrodes with minimized material complexity are essential to obtain an interpretable electrochemical response that allows us to study electrodes and develop more active and stable materials. The traditional NPs fabrication process based on wet chemistry yields electrocatalysts based on NP “inks” (slurries) with complex and often unknown properties (composition, structure, mass transport properties). In our work, we develop binder-free NP electrodes by forming catalyst NPs directly on suitable electrodes using solid-state dewetting, i.e. the heat induced agglomeration of thin metal films into well-defined, spatially separated NPs with tunable loading, size, structure, and composition^[1,2].
We investigate first Pt NPs dewetted on fluorine-doped tin oxide (FTO) electrodes as a model electrode system for the Hydrogen evolution reaction (HER). Upon the dewetting of thin Pt films into NPs, the FTO surface underneath gets exposed along with the formation of Pt-FTO contact line and the reduction in the electrochemical surface area (ECSA) of Pt. Despite this decrease in ECSA, the Pt NPs exhibit an evident increase in the intrinsic activity of HER compared to the thin Pt film^[3].
Upon tuning the size of the Pt NPs, we reveal that the HER activity scale not only with the ECSA, but also with the Pt-FTO contact line length, and the second factor becomes dominant for nanoparticles smaller than ca. 10 nm. The smaller the Pt NPs, the larger the Pt-FTO contact line length and the higher the HER activity. Results of XPS coupled with other characterization techniques like XRD, TEM, and XAS, suggest that the enhanced HER activity is linked to the contact line length due to an electron transfer from FTO to Pt. The driving force for this phenomenon, referred to as electronic metal support interaction (EMSI), is due to the difference in the work function between the metal and the support (Pt ~5.5 eV and FTO ~4.4eV)^[4]. The excess electron on Pt affects its electronic structure causing a downshift of the Pt d-band center with respect to the Fermi level. This in turn results in a weaker and hence more thermoneutral adsorption of H* intermediate on the Pt sites near the Pt-FTO contact line, thereby accelerating the associative desorption step and accelerating the overall H2 kinetics. The effect is more pronounced when the dewetted Pt NPs become smaller than 10 nm because an increased mass fraction of Pt within each particle is near the Pt-FTO contact line, hence influenced by EMSI, and is surface exposed and consequently contributes to enhancing the overall HER performance.
Interestingly, Pt NPs dewetted on other electrically conductive substrates like boron-doped diamond (BDD) and Nb-SrTiO₃ (Nb: STO)^[5] single crystals also show significantly enhanced HER activity compared to thin Pt films, and spectroscopic evidence supports the electronic coupling and support interaction. These Pt-support combinations also show evidence of EMSI. In our ongoing research, we evaluate additional factors that, upon dewetting, can alter the Pt morphology and structure (e.g., exposed crystal planes), achieving a higher intrinsic HER activity.

[1] C. V. Thompson, Annu Rev Mater Res 2012, 42, 399.
[2] M. Altomare, N. T. Nguyen, P. Schmuki, Chem Sci 2016, 7, 6865.
[3] S. Harsha, R. K. Sharma, M. Dierner, C. Baeumer, I. Makhotkin, G. Mul, P. Ghigna, E. Spiecker, J. Will, M. Altomare, Adv Funct Mater 2024, 2403628.
[4] Y. Shi, Z. R. Ma, Y. Y. Xiao, Y. C. Yin, W. M. Huang, Z. C. Huang, Y. Z. Zheng, F. Y. Mu, R. Huang, G. Y. Shi, Y. Y. Sun, X. H. Xia, W. Chen, Nature Communications 2021 12:1 2021, 12, 1.
[5] R. K. Sharma, S. Harsha, G. Mul, M. Altomare, ECS Meeting Abstracts 2023, MA2023-02, 2059.