Single-particle scanning electrochemical cell microscopy (SECCM) measurements on gold nanocubes ({100} facets) and nano-octahedra ({111} facets) showed a maximum hydrogen-evolution potential shift of roughly 100 mV between the two shapes, about twice the shift seen in macroscale ensemble voltammetry, yet the residual synthesis ligands (CTAB/CPC), the particle-substrate electrical contact, and the droplet-particle measurement geometry were all acknowledged as uncontrolled confounders. The open problem is to quantitatively partition the observed facet-dependent single-particle activity into crystallographic, residual-ligand, contact, and measurement-geometry contributions, to establish whether residual ligands persist at the buried particle-substrate and particle-droplet interfaces after standard cleaning, and thereby to reconcile single-particle and ensemble activity differences and determine how much of the reported facet dependence is genuinely crystallographic. The facet dependence remains quantitatively unexplained as of 2026.

Background

Colloidal synthesis can produce metal nanocrystals with well-defined shapes so that a chosen crystallographic facet dominates the surface: gold nanocubes (NCs) expose predominantly {100} planes and gold nano-octahedra (ODs) predominantly {111} planes. Because electrocatalytic rates depend on which crystal plane is exposed (through the adsorption energy of reaction intermediates), such facet-controlled particles are used as model catalysts for connecting surface structure to activity. Shape-controlled synthesis, however, requires surfactant ligands - here cetyltrimethylammonium bromide (CTAB, a cationic quaternary-ammonium surfactant) and cetylpyridinium chloride (CPC, a related pyridinium surfactant) - that adsorb on the particle surface and are notoriously difficult to remove completely; residual ligands can block active sites, impede electron transfer, and shift measured overpotentials (the extra potential beyond the thermodynamic requirement needed to drive the reaction at a given rate).

Scanning electrochemical cell microscopy (SECCM) is a scanning-probe technique in which an electrolyte droplet at the tip of a nanopipette forms a tiny electrochemical cell that contacts the sample at a series of positions, so that voltammetry (measurement of current as the potential is swept) can be performed on an individual nanoparticle, which is then imaged by electron microscopy to correlate its electrochemical response with its shape and facet. Choi, Siepser, Jeong, Wang, Jagdale, Ye, and Baker (Nano Letters, 2020) used correlative SECCM and scanning electron microscopy, together with macroscale linear-sweep and cyclic voltammetry (LSV/CV, ensemble measurements averaged over all deposited particles), to compare the hydrogen evolution reaction (HER, the electrochemical reduction of protons to H2) on individual gold NCs and ODs of matched surface area. They found NCs more active than ODs at the single-particle level, with a maximum potential difference of about 100 mV between particle types, while macroscale LSV showed only about a 50 mV difference - a discrepancy they attributed, tentatively, to sample inhomogeneity.

The paper itself flags the confounders that keep this from being a clean facet-activity measurement. Ligand removal by methanol rinsing and electrochemical cycling appeared harder on ODs than NCs (consistent with the expectation that CTAB binds more strongly to Au{111} than Au{100}), the authors postulate that CTAB remained at the buried interface between the particle's bottom facet and the glassy-carbon (a dense, chemically inert form of carbon widely used as an electrode material) support, where it could impede electron transfer and raise overpotentials, and their finite-element simulations of the droplet-particle geometry captured wetted-area effects but not interfacial kinetics. Later reviews from the same and adjacent groups (2024-2025) confirm that ligand-removal protocols for shaped nanocrystals are rarely validated across compositions and facets, and that ligand-density heterogeneity among ostensibly identical particles remains essentially uncharacterized, while single-entity methods have advanced mainly along other axes (surface strain, edge-versus-plane sites, CO2 reduction facet ordering on gold).

Problem Statement

Determine, quantitatively, the origin of the facet-dependent single-particle electrocatalytic activity measured at facet-controlled gold nanocrystals - specifically, for the hydrogen evolution reaction at {100}-dominant nanocubes versus {111}-dominant nano-octahedra as studied by correlative SECCM (Choi et al., Nano Lett. 2020). The problem has one overarching objective - establishing how much of the observed facet dependence (e.g., the ~100 mV single-particle potential shift between particle types) is genuinely crystallographic - and requires jointly: (1) establishing whether residual CTAB/CPC ligands persist, and with what coverage, at the buried nanoparticle-substrate interface and at the nanoparticle-droplet interface after standard cleaning (methanol rinse plus electrochemical cycling), given that removal appeared harder on {111}-dominant octahedra; (2) quantifying how residual ligand coverage alters single-particle electron-transfer kinetics and adsorption energetics; (3) quantifying the contributions of particle-substrate electrical contact and droplet-particle measurement geometry (beyond existing finite-element models that exclude interfacial kinetics); and (4) reconciling the single-particle potential shifts with the smaller shifts of ensemble (macroscale) voltammetry, including how ensemble inhomogeneity, incomplete ligand removal, and contact-geometry dispersion alter the ensemble-averaged response. All four contributions must be quantitatively separated for the facet dependence to be explained.

References

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