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Advances in electrocatalysis research rely heavily on building a thorough mechanistic understanding of catalyst active sites under realistic operating conditions. Only recently have techniques emerged that enable sensitive spectroscopic data collection to distinguish catalytically relevant surface sites from the underlying bulk material under applied potential in the presence of an electrolyte layer. Here, we demonstrate that operandohigh-energy-resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) is a powerful spectroscopic method which offers critical surface chemistry insights in CO2electroreduction with sub-electronvolt energy resolution using hard X-rays. Combined with the high surface area-to-volume ratio of 5 nm copper nanoparticles, operandoHERFD-XAS allows us to observe with clear evidence the breaking of chemical bonds between the ligands and the Cu surface as part of the ligand desorption process occurring under electrochemical potentials relevant for the CO2reduction reaction (CO2RR). In addition, the dynamic evolution of oxidation state and coordination number throughout the operation of the nanocatalyst was continuously tracked. With these results in hand, undercoordinated metallic copper nanograins are proposed to be the real active sites in the CO2RR. This work emphasizes the importance of HERFD-XAS compared to routine XAS in catalyst characterization and mechanism exploration, especially in the complicated electrochemical CO2RR.

The design of new plasmonic materials is essential for continued progress in light manipulation at nanometer length scales.Often, multimetallic nanoparticles exhibit superior catalytic, mechanical, or corrosion-resistant behavior compared with their unary counterparts. Despite these advantages, designing multimetallic plasmonic materials remains challenging because, except for Au, Ag, and Al, most metallic elements exhibit poor plasmonic behavior. Here, we describe a strategy for manipulating the plasmon resonances of noble metal nanoparticles by post-transition metal alloying. We show how the metallic properties of post-transition metals—Bi, Ga, In, and Sn—can be imparted onto noble metals, enabling tunable higher-energy plasmon resonances that maintain high extinction coefficients and enter the UV. Theoretical modeling of simulated metallic mixtures and idealized Drude free-electron materials shows how this tunability is generalizable. Consequently, post-transition metal alloying provides a powerful strategy for realizing a new chemically diverse class of tunable plasmonic materials.

Carbon dioxide electroreduction facilitates the sustainable synthesis of fuels and chemicals1. Although Cu enables CO2-to-multicarbon product (C2+) conversion, the nature of the active sites under operating conditions remains elusive2. Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques3–5. Here, we present a comprehensive investigation of the structural dynamics during the life cycle of Cu nanocatalysts. A 7 nm Cu nanoparticle ensemble evolves into metallic Cu nanograins during electrolysis before complete oxidation to single-crystal Cu2O nanocubes following post-electrolysis air exposure. Operando analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy shows the presence of metallic Cu nanograins under CO2reduction conditions. Correlated high-energy-resolution time-resolved X-ray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C–C coupling. Quantitative structure–activity correlation shows that a higher fraction of metallic Cu nanograins leads to higher C2+selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C2+selectivity than the 18 nm counterpart with one-third of active Cu nanograins. The correlation of multimodal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions.

We use classical electrodynamics calculations to investigate the plasmonic properties of the post-transition metals Al, Bi, Ga, In, and Sn, which are active in the ultraviolet, focusing in particular on the material- and resonance-dependent origins of plasmon broadening. Analytic Mie theory, the modified-long wavelength approximation, and the quasistatic dipole approximation together show that radiative processes dominate plasmon dephasing and damping in small (5–25 nm radius) Al, Bi, Ga, In, and Sn spheres. For Al, Ga, In, and Sn, the radiative contribution (∼0.5–2 eV) to the plasmon linewidth is more than 100-fold greater than the nonradiative contribution (0.001–0.02 eV) derived from the bulk dielectric function. This is significantly different than what is observed for Ag spheres, where nonradiative contributions (∼0.1 eV) are the primary source of broadening up to a radius of 25 nm. Overall, these data suggest that the plasmonic properties, dephasing, and lifetimes for Al, Ga, In, and Sn—and to a lesser extent Bi—spheres are qualitatively similar. To develop a more general understanding of the relationship between plasmon energy and linewidth, we use a model for ideal free-electron Drude metals. It is seen that the linewidth increases at higher energies even for lossless Drude metals, suggesting that the increased broadening observed in UV-active metals is a generalizable observation. These data have important implications for the use of these metals for ultraviolet plasmonics. The increased importance of radiative damping for post-transition metals could influence the ability to harvest photons, generate hot carriers, and enhance spectroscopy in the ultraviolet while providing new opportunities for manipulating high-energy photons.

Plasmonic nanoparticles and nanocrystalline materials have broad applicability in catalysis, optoelectronics, sensing, and sustainability. Below, we detail a robust protocol for the synthesis of bimetallic Au–Sn nanoparticles in mild, aqueous conditions. This protocol describes the steps for synthesizing gold nanoparticle seeds, diffusing Sn into the seeds by chemical reduction, and the optical and structural analysis by UV-visible spectroscopy, X-ray diffraction, and electron microscopy.