The structure of Cu nanocrystals as catalysts for the electrochemical reduction of CO2 is a subject of considerable contemporary interest. Recent efforts have focused mainly on the preparation of Cu nanocrystals, but the question of their stability is equally relevant and has not been considered as extensively. Herein, we report on the reconstruction of Cu nanocrystals during CO2 reduction and discuss the factors influencing the observed changes with computerbased quantitative analysis and spectroscopic techniques. The timelines of opposing phenomena, sintering and declustering, previously reported separately, are detailed with a focus on two forces affecting the final morphology: applied potential and reaction intermediates. This intriguing system demonstrates the need for fundamental understanding of catalyst behavior preceding the ability to control its performance.
A large jump of proton transfer rates across solid‐to‐solid interfaces by inserting an ultrathin amorphous silica layer into stacked metal oxide nanolayers is discovered using electrochemical impedance spectroscopy and Fourier‐transform infrared reflection absorption spectroscopy (FT‐IRRAS). The triple stacked nanolayers of Co3O4, SiO2, and TiO2 prepared by atomic layer deposition (ALD) enable a proton flux of 2400 ± 60 s−1 nm−2 (pH 4, room temperature), while a single TiO2 (5 nm) layer exhibits a threefold lower flux of 830 s−1 nm−2. Based on FT‐IRRAS measurements, this remarkable enhancement is proposed to originate from the sandwiched silica layer forming interfacial SiOTi and SiOCo linkages to TiO2 and Co3O4 nanolayers, respectively, with the O bridges providing fast H+ hopping pathways across the solid‐to‐solid interfaces. Together with the complete O2 impermeability of a 2 nm ALD‐grown SiO2 layer, the high flux for proton transport across multi‐stack metal oxide layers opens up the integration of incompatible catalytic environments to form functional nanoscale assemblies such as artificial photosystems for CO2 reduction by H2O.
Coupling of robust, all-inorganic heterobinuclear light absorbers to metal oxide catalysts for water oxidation across an ultrathin product-separating silica membrane requires charge transfer through organic molecular wires embedded in the silica. A synthetic approach for assembling the bimetallic units on the silica surface is introduced that is compatible with the presence of encapsulated organic molecules. Accurate selection and fine tuning of the concentration of embedded conducting wires are enabled by a two-step method consisting of surface attachment of a tripodal anchor, trimethoxysilyl aniline, followed by attachment of p-oligo(phenylene vinylene) through amide linkage. Each step of the assembly process was monitored and characterized by a combination of Fourier transform infrared, Fourier transform-Raman, and UV-vis spectroscopy techniques. Hole transfer was observed from transient Co, formed by TiOCo → TiOCo charge transfer excitation of the chromophore, to p-oligo(phenylene vinylene) molecule within the 8 ns width of the photolysis laser pulse by transient optical absorption spectroscopy of the wire radical cation. The rectifying property of the light absorber-wire assembly enabled by appropriate selection of redox potentials of metals and embedded wire obviates the need for a molecularly defined linkage between the components. Combined with the previously observed ultrafast hole injection from the embedded wires to Co oxide catalyst, the result implies visible-light-induced hole transfer from visible-light-excited binuclear light absorber to water oxidation catalyst across the silica separation membrane in a few nanoseconds or faster. Demonstration and understanding of this interfacial charge-transfer step is critical for developing nanoscale core-shell architectures for complete photosynthetic cycles.
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