Plasmon Enhanced Two-Photon Probing with Gold and Silver Nanovoid Structures

Nonlinear optical signals benefit greatly from the enhanced local optical fields in the vicinity of plasmonic nanostructures. Gold and silver nanovoid arrays of varying size and thickness, fabricated by electrochemical deposition are shown here to act as stable plasmonic nanostructures and to enhance the weak, incoherent two‐photon excited process of surface‐enhanced hyper Raman scattering (SEHRS) with high microscopic homogeneity and reproducibility that typical SEHRS experiments have not been addressing so far. Silver nanovoids yield stronger enhancement than gold voids, but gold nanovoid arrays show improved stability at high laser excitation intensities. Combined screening experiments using SEHRS and second‐harmonic generation (SHG) reveal a dependence of the enhancement of both signals on void structural parameters and similar optimum geometries for both two‐photon processes. The results confirm the suggested important role for the enhancement of the near‐infrared excitation field in SEHRS and suggest SHG as a fast screening tool to identify nanostructures that can support high SEHRS enhancement.Peer Reviewe

Operando Electrochemical Raman Spectroscopy

dvances in nanoscience and particularly in the fabrication of sophisticated nanostructures have led to substantial progress in the design of catalytic materials. Probing species at an aqueous interface during electrochemical reactions poses a great challenge to experimentalists and has raised considerable attention over the past decades, which is especially intriguing in the field of electrocatalysis. Several approaches for coupling electrochemical measurements to spectroscopic characterization of the electrode itself have been established and such experimental setups now represent state-of-the-art analytical techniques. Alongside with such developments a substantial improvement of already existing technologies for the analysis of nanostructures has been achieved, rendering them suitable to address the aforementioned challenges. A prime example is the case of Raman spectroscopy, which has been substantially developed in recent years toward the study of adsorbed molecules and/or the state of a material during catalytic reactions.Boosted by the discovery of the significant signal enhancement upon using nanostructured metals as substrates, Raman spectroscopy has evolved to become an appropriate tool for allowing the detection of Raman-active species in very small concentrations. Moreover, its versatility regarding operating conditions led to the development of operando electrochemical Raman spectroscopy (OERS), namely, Raman spectroscopic investigations of the electrochemical interfaces while electrochemical conversion under a controlled reaction rate is proceeding. Being electrochemists by training, we focus in this contribution on reactions linked to challenges in modern electrochemistry such as water electrolysis and CO2 conversion, and we highlight recent studies providing a comprehensive overview of developments important to the field. We aim to familiarize the reader with theoretical background of electrochemistry and Raman spectroscopy, and we broadly illustrate the implications of OERS for the understanding of fundamental electrochemical reactions and the elucidation of reaction mechanisms

High-performance iridium thin films for water splitting by CVD using new Ir(I) precursors

Thin films of iridium can be utilized in a wide range of applications and are particularly interesting for catalytic transformations. For the scalable deposition of functional Ir thin films, metal-organic chemical vapor deposition (MOCVD) is the method of choice, for which organometallic precursors that embody a high volatility and thermal stability need to be specifically tailored. Herein, we report the synthesis, analysis, and evaluation of new volatile Ir(I)-1,5-cyclooctadiene complexes bearing all-nitrogen coordinating guanidinate (DPDMG), amidinate (DPAMD) and formamidinate (DPfAMD) ligands. The amidinate-based Ir complex [Ir(COD)(DPAMD)] together with O2 was implemented in MOCVD experiments resulting in highly crystalline, dense, and conductive Ir films on a variety of substrate materials. The Ir deposits achieved an outstanding electrochemical performance with overpotentials in the range of 50 mV at –10 mA‧cm-2 for catalytic hydrogen evolution reaction (HER) in acidic solution. The ability to deposit Ir layers via MOCVD exhibiting promising functional properties is a significant step towards large scale applications

Probing the local reaction environment during high turnover carbon dioxide reduction with Ag-based gas diffusion electrodes

Discerning the influence of electrochemical reactions on the electrode microenvironment is an unavoidable topic for electrochemical reactions that involve the production of OH$^{−}$ and the consumption of water. That is particularly true for the carbon dioxide reduction reaction (CO$_2$RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH$^{−}$ and H$_2$O activity that in turn can possibly affect activity, stability, and selectivity of the CO$_2$RR. We determine the local OH$^{−}$ and H$_2$O activity in close proximity to a CO$_2$-converting Ag-based gas diffusion electrode (GDE) with product analysis using gas chromatography. A Pt nanosensor is positioned in the vicinity of the working GDE using shear-force-based scanning electrochemical microscopy (SECM) approach curves, which allows monitoring changes invoked by reactions proceeding within an otherwise inaccessible porous GDE by potentiodynamic measurements at the Pt-tip nanosensor. We show that high turnover HER/CO$_2$RR at a GDE lead to modulations of the alkalinity of the local electrolyte, that resemble a 16 m KOH solution, variations that are in turn linked to the reaction selectivity

In situ carbon corrosion and Cu leaching as a strategy for boosting oxygen evolution reaction in multimetal electrocatalysts

The number of active sites and their intrinsic activity are key factors in designing high-performance catalysts for the oxygen evolution reaction (OER). The synthesis, properties, and in-depth characterization of a homogeneous CoNiFeCu catalyst are reported, demonstrating that multimetal synergistic effects improve the OER kinetics and the intrinsic activity. In situ carbon corrosion and Cu leaching during the OER lead to an enhanced electrochemically active surface area, providing favorable conditions for improved electronic interaction between the constituent metals. After activation, the catalyst exhibits excellent activity with a low overpotential of 291.5 (\pm\) 0.5 mV at 10 mA $cm^{−2}$ and a Tafel slope of 43.9 mV $dec^{−1}$. It shows superior stability compared to $RuO_{2}$ in 1 $\tiny M$ KOH, which is even preserved for 120 h at 500 mA $cm^{−2}$ in 7 $\tiny M$ KOH at 50 °C. Single particles of this CoNiFeCu after their placement on nanoelectrodes combined with identical location transmission electron microscopy before and after applying cyclic voltammetry are investigated. The improved catalytic performance is due to surface carbon corrosion and Cu leaching. The proposed catalyst design strategy combined with the unique single-nanoparticle technique contributes to the development and characterization of high-performance catalysts for electrochemical energy conversion

Redox replacement of silver on MOF-derived Cu/C nanoparticles on gas diffusion electrodes for electrocatalytic CO2CO_{2} reduction

Bimetallic tandem catalysts have emerged as a promising strategy to locally increase the CO flux during electrochemical $CO_{2}$ reduction, so as to maximize the rate of conversion to C−C-coupled products. Considering this, a novel Cu/C−Ag nanostructured catalyst has been prepared by a redox replacement process, in which the ratio of the two metals can be tuned by the replacement time. An optimum Cu/Ag composition with similarly sized particles showed the highest $CO_{2}$ conversion to $C_{2+}$ products compared to non-Ag-modified gas-diffusion electrodes. Gas chromatography and in-situ Raman measurements in a $CO_{2}$ gas diffusion cell suggest the formation of top-bound linear adsorbed *CO followed by consumption of CO in the successive cascade steps, as evidenced by the increasingνC−H bands. These findings suggest that two mechanisms operate simultaneously towards the production of $HCO_{2}H$ and C−C-coupled products on the Cu/Ag bimetallic surface

Hollow CeO2_2@Co2_2N Nanosheets Derived from Co-ZIF-L for Boosting the Oxygen Evolution Reaction

Rational design of highly active electrocatalysts for the oxygen evolution reaction (OER) is critical to improving overall electrochemical water splitting efficiency. This study suggests hollow CeO$_2$@Co$_2$N nanosheets synthesized using Co-ZIF-L as a precursor, followed by a hydrothermal reaction and a nitridation process as very attractive OER catalysts. The increased activity is supposed to be due to nitridation and strong electronic interaction between CeO$_2$ and Co$_2$N that contribute to the formation of active CoOOH phase. The synthesized CeO$_2$@Co$_2$N exhibits low overpotentials of 219 and 345 mV at OER current densities of 10 and 100 mA cm$^{–2}$, respectively, as well as a long-term durability of 30 h at a comparatively high current density of 100 mA cm$^{–2}$−2

B-Cu-Zn Gas diffusion electrodes for CO2_2 Electroreduction to C2+_{2+} Products at High Current Densities

Electroreduction of CO$_2$ to multi-carbon products has attracted considerable attention as it provides an avenue to high-density renewable energy storage. However, the selectivity and stability under high current densities are rarely reported. Herein, B-doped Cu (B-Cu) and B-Cu-Zn gas diffusion electrodes (GDE) were developed for highly selective and stable CO2 conversion to C$_{2+}$ products at industrially relevant current densities. The B-Cu GDE exhibited a high Faradaic efficiency of 79 % for C$_{2+}$ products formation at a current density of −200 mA cm$^{−2}$ and a potential of −0.45 V vs. RHE. The long-term stability for C$_{2+}$ formation was substantially improved by incorporating an optimal amount of Zn. Operando Raman spectra confirm the retained Cu$^{+}$ species under CO$_2$ reduction conditions and the lower overpotential for *OCO formation upon incorporation of Zn, which lead to the excellent conversion of CO$_2$ to C$_{2+}$ products on B-Cu-Zn GDEs

Energy conversion at nanointerfaces: general discussion

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