CO 2 electroreduction facilitates the sustainable synthesis of fuels and chemicals 1 . Although Cu enables CO 2 -to-multicarbon product (C 2+ ) conversion, the nature of the active sites under operating conditions remains elusive 2 . Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques [3][4][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 completely oxidizing to single-crystal Cu 2 O nanocubes upon post-electrolysis air exposure. Operando analytical and four-dimensional (4D) electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) reveals the presence of metallic Cu nanograins under CO 2 reduction conditions. Correlated high-energy-resolution time-resolved Xray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C-C coupling. The quantitative structure-activity correlation shows a higher fraction of metallic Cu nanograins leads to higher C 2+ selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits 6 times higher C 2+ selectivity than the 18 nm counterpart with one-third of active Cu nanograins. The correlation of multi-modal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions.Editor's one-sentence summary: By investigation of structural dynamics during the lifecycle of Cu nanocatalysts, correlation of multimodal operando techniques was found to serve as a powerful platform to advance understanding of their complex structural evolution. Main text:1 Copper remains the only heterogeneous electrocatalyst to selectively catalyze CO 2 reduction reaction (CO 2 RR) to multicarbon (C 2+ ) products, including ethylene, ethanol, and propanol at appreciable rates 1,2 . Recent developments in operando/in situ methods, including advanced electron microscopy and synchrotron-based X-ray methods, provide powerful non-destructive tools to probe active sites and structural changes of electrocatalysts under reaction conditions 3-5 . However, there remains a lingering debate over the active state of Cu catalysts, regarding valence state or coordination environments under CO 2 RR. For instance, some reports have proposed Cu + species and subsurface oxide as possible active sites of oxide-derived Cu electrocatalysts [6][7][8][9] , while others suggested the active state of bulk Cu catalysts is metallic [10][11][12] as subsurface oxides are not stable under negative potentials [13][14][15] . Another possible structural descriptor of locally enhanced CO 2 RR activity has been reported to be micrometer-sized grain boundaries (GBs) on bulk metal electrodes [13][14][15][16][17][18] . Those studies probed the local activity at GBs with a µm-leve...

Electrochemically upgrading CO2 to carbon-neutral multicarbons (C2+) is a promising technology for CO2 recycling and utilization. Since such transformations involve multiple elementary steps, a tandem strategy becomes attractive as catalysts can be optimized for specific reaction steps independently. Related strategies have been demonstrated under low working current densities; however, the applicability of a tandem strategy towards high-rate CO2 electrolysis to C2+ is unknown. Here, we demonstrate that a Cu-Ag tandem catalyst can enhance the multicarbon production rate in CO2RR by decoupling high-rate CO2 reduction to CO on Ag and subsequent CO coupling on Cu. With the addition of Ag, the partial current towards C2+ over a Cu surface increased from 37 mA/cm 2 to 160 mA/cm 2 at -0.70 V vs RHE in 1M KOH while no mutual interference between two metals was observed. Moreover, the normalized intrinsic activity of C2H4 and C2H5OH in the tandem platform under CO2 reduction conditions is significantly higher than Cu alone under either pure CO2 or CO atmosphere. Our results indicate that the CO-enriched local environment generated by Ag can enhance C2+ formation on Cu beyond CO2 or CO feeding, suggesting possible new mechanisms in a tandem three-phase environment. Manuscriptprove CO2RR catalytic performance. Thus, we conducted post-electrolysis characterization of the tandem Cu500Ag1000 catalyst to determine whether the structure is maintained. Previous works have shown structural and electronic differences owing to strong Ag interactions with Cu: for example, up to 0.8 o Cu(111) peak shift in XRD could be found for a Cu-Ag alloy system 14 whereas up to 0.3 eV Cu 2p3/2 peak shift in XPS was reported for a Cu-Ag dimer. 23 In contrast, no peak shift of Cu or Ag could be observed for the tandem Cu500Ag1000 catalyst in either XRD, XPS or Cu LMM Auger peak after electrolysis, indicating the structural maintenance of this tandem catalyst and absence of electronic interactions between Ag and Cu throughout electrolysis. This absence is likely due to the bulk-like nature of Ag and Cu used, in addition to the mild conditions in which the electrode is fabricated, resulting in thermodynamically favored separation 52 . Importantly, this does not preclude the Ag-Cu surface and interfacial alloying observed in other reports which use more energetic fabrication conditions.2.2 Enhanced CO2RR catalytic performances toward C2+ products over tandem Cu-Ag catalysts. The polarization response curve of Cu500Ag1000 in Fig. 2a shows higher geometric current density than Cu500 or Ag1000 alone under the same potentials. Interestingly, partial current densities toward C2+ products over different catalysts are also observed to be substantially higher for Cu500Ag1000, which cannot be explained simply through the individual contributions of Cu500 and Ag1000 (Fig. 2b). Explicitly, Ag1000 does not contribute to C-C coupling reactions in the potential range from -0.5 V to -0.8 V vs RHE. Thus, all partial current toward C2+ products should come from the C...

Nitrogen fixation in a simulated natural environment (i.e., near ambient pressure, room temperature, pure water, and incident light) would provide a desirable approach to future nitrogen conversion. As the NN triple bond has a thermodynamically high cleavage energy, nitrogen reduction under such mild conditions typically undergoes associative alternating or distal pathways rather than following a dissociative mechanism. Here, we report that surface plasmon can supply sufficient energy to activate N 2 through a dissociative mechanism in the presence of water and incident light, as evidenced by in situ synchrotron radiationbased infrared spectroscopy and near ambient pressure X-ray photoelectron spectroscopy. Theoretical simulation indicates that the electric field enhanced by surface plasmon, together with plasmonic hot electrons and interfacial hybridization, may play a critical role in NN dissociation. Specifically, AuRu coreantenna nanostructures with broadened light adsorption cross section and active sites achieve an ammonia production rate of 101.4 μmol g −1 h −1 without any sacrificial agent at room temperature and 2 atm pressure. This work highlights the significance of surface plasmon to activation of inert molecules, serving as a promising platform for developing novel catalytic systems.

Mechanochromic response is of great importance in designing bionic robot systems and colorimetric devices. Unfortunately, compared to mimicking motions of natural creatures, fabricating mechanochromic systems with programmable colorimetric responses remains challenging. Herein, we report the development of unconventional mechanochromic films based on hybrid nanorods integrated with magnetic and plasmonic anisotropy. Magneticplasmonic hybrid nanorods have been synthesized through a unique space-confined seedmediated process, which represents an open platform for preparing next-generation complex nanostructures. By coupling magnetic and plasmonic anisotropy, the plasmonic excitation of the hybrid nanorods could be collectively regulated using magnetic fields. It facilitates convenient incorporation of the hybrid nanorods into polymer films with a well-controlled orientation and enables sensitive colorimetric changes in response to linear and angular motions. The combination of unique synthesis and convenient magnetic alignment provides an advanced approach for designing programmable mechanochromic devices with the desired precision, flexibility, and scalability.

Achieving perovskite-based high–color purity blue-emitting light-emitting diodes (LEDs) is still challenging. Here, we report successful synthesis of a series of blue-emissive two-dimensional Ruddlesden-Popper phase single crystals and their high–color purity blue-emitting LED demonstrations. Although this approach successfully achieves a series of bandgap emissions based on the different layer thicknesses, it still suffers from a conventional temperature-induced device degradation mechanism during high-voltage operations. To understand the underlying mechanism, we further elucidate temperature-induced device degradation by investigating the crystal structural and spectral evolution dynamics via in situ temperature-dependent single-crystal x-ray diffraction, photoluminescence (PL) characterization, and density functional theory calculation. The PL peak becomes asymmetrically broadened with a marked intensity decay, as temperature increases owing to [PbBr6]4− octahedra tilting and the organic chain disordering, which results in bandgap decrease. This study indicates that careful heat management under LED operation is a key factor to maintain the sharp and intense emission.