The surface electronic structures of catalysts need to be carefully engineered in CO 2 reduction reaction (CO 2 RR), where the hydrogen evolution side reaction usually takes over under a significant overpotential, and thus dramatically lowers the reaction selectivity. Surface oxides can play a critical role in tuning the surface oxidation state of metal catalysts for a proper binding with CO 2 RR reaction intermediates, which may significantly improve the catalytic activity and selectivity. Here, we demonstrate the importance of surface-bonded oxygen on silver nanoparticles in altering the reaction pathways and improving the CO 2 RR performances. A comparative investigation on air-annealed Ag (Air-Ag) catalyst with or without the post-treatment of H 2 thermal annealing (H 2 -Ag) was performed. In Air-Ag, the subsurface chemically bonded O species (O− Ag δ+ ) was identified by angle resolved X-ray photoelectron spectroscopy and X-ray absorption spectroscopy techniques, and contributed to the improved CO selectivity rather than H 2 in CO 2 RR electrolysis. As a result, though the maximal CO Faradaic efficiency of H 2 -Ag is at ∼30%, the Air-Ag catalyst presented a high CO selectivity of more than 90% under a current density of ∼21 mA/cm 2 .

Here, we demonstrate atomic-resolution scanning transmission electron microscopy (STEM) imaging of light elements in small organic molecules on graphene. We use low-dose, room-temperature, aberration-corrected STEM to image 2D monolayer and bilayer molecular crystals, followed by advanced image processing methods to create high-quality composite images from ∼10 2 −10 4 individual molecules. In metalated porphyrin and phthalocyanine derivatives, these images contain an elementally sensitive contrast with up to 1.3 Å resolutionsufficient to distinguish individual carbon and nitrogen atoms. Importantly, our methods can be applied to molecules with low masses (∼0.6 kDa) and nanocrystalline domains containing just a few hundred molecules, making it possible to study systems for which large crystals cannot easily be grown. Our approach is enabled by low-background graphene substrates, which we show increase the molecules' critical dose by 2−7×. These results indicate a new route for low-dose, atomic-resolution electron microscopy imaging to solve the structures of small organic molecules.

Graphene substrates have been proposed as ultra-thin supports for dose-sensitive samples, where they increase dose resistance as a result of high thermal and electrical conductivity. For example, a single layer of graphene has provided a 9-fold protection factor when imaging single-layer MoS2 [1]. Unlike for inorganic crystals, graphene's protection factor for organic molecular crystals has yet to be systematically quantified. Here we use selected area electron diffraction (SAED) to measure the critical dose of both bulk and 2D bilayer organic crystals on either amorphous carbon (a-C) or single-layer graphene substrates. We find that the single layer of graphene offers a 2-7-fold improvement in the dose resistance. This improvement is significant: for example, increasing the critical dose by a factor of four should double the dose limited resolution for organic molecules [2]. Importantly, by comparing the improvement in critical dose for different d-spacings, we find that the protection factor provided by graphene is higher for higherorder diffraction spots. Our results indicate that graphene is especially useful for enabling near atomicresolution imaging of dose-sensitive samples, which we demonstrate by using aberration-corrected scanning transmission electron microscopy (ADF-STEM) to acquire images of cobalt(II) meso-tetrakis(4methoxyphenyl) porphyrin (CoTMPP).

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