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Investigating the chemical nature of the adsorbed intermediate species on well-defined Cu single crystal substrates is crucial in understanding many electrocatalytic reactions. Herein, we systemati...

Plasmonic "hot spots" play a key role in surface-enhanced Raman scattering (SERS) enabling its ultrahigh surface sensitivity. Thus, precise prediction and control of the location of hot spots in surface nanostructures is of great importance. However, it is difficult to predict the exact location of hot spots due to complex plasmon competition and synergistic effects in three-dimensional (3D) multiparticle surface configurations. In this work, three types of Au@probe@SiO2 core-shell nanoparticles were prepared and a 3D hot spots matrix was assembled via a consecutive layer on layer deposition method. Combined with SERS, distinct probe molecules were integrated into different layers of the 3D multiparticle nanostructure allowing for the hot spots to be precisely located. Importantly, the hot spots could be controlled and relocated by applying different excitation wavelengths, which was verified by simulations and experimental results. This work proposes a new insight and provides a platform for precisely probing and controlling chemical reactions, which has profound implications in both surface analysis and surface plasmonics.

Understanding the role of the morphology and particle–particle interactions on the plasmonic properties is of significant importance for the development of nanomaterials with excellent optical properties. However, the preparation of precisely defined nanomaterials with sizes that span a large range and their controllable self-assembly still remain a great challenge. Here, a multistep seed-mediated method has been established for preparing uniform Ag nanocubes over a broad size range from nanoscale (50 nm) to microscale (1400 nm) and with different hierarchical nanostructures range from “zero-dimension” (“0D”) to “three-dimension” (“3D”). The influence of the size and the interactions between the Ag nanocubes on their surface-enhanced Raman scattering (SERS) properties have been systematically and quantitatively investigated. It is demonstrated through experiments and finite-difference time-domain (FDTD) calculations that the SERS activity is dependent on the matching of the nanocube size to the excitation wavelength. The optimal combinations are 80, 110 and 130 nm nanocubes with respect to 532, 638 and 785 nm excitation wavelength, respectively. Furthermore, the Raman enhancement of the Ag nanocube hierarchical nanostructures increases rapidly from “0D” to “3D”, due to the extra increase of the hot spots that is attributed to the out-of-plane plasmonic coupling realized in the “3D” hierarchical nanostructures. This work clearly illustrates the quantitative role of the size and dimension of Ag nanocubes on their SERS properties and provides fundamental information for the design of advanced nanomaterials with higher SERS sensitivity.

Here, we investigated the sealing quality between a microwire disk and the surrounding glass sheath of platinum disk ultramicroelectrodes (UMEs) using outer-sphere (ferrocene methanol, FcMeOH, oxidation) and inner-sphere electrochemical reactions (hydrogen underpotential deposition (HUPD) and the hydrogen evolution reaction (HER)) by the cyclic voltammetry (CV) approach. The tilt aspect in the CV curves is ascribed to the leakage of the electrolyte solution between the microelectrode wire and the glass sheath, causing an iR drop which shows the resistive nature of CV. The resistive nature of CV was analyzed by performing the HER using both poorly and well-sealed disk UMEs. Scan rate dependent double-layer capacitance (Cdl) data confirm the leak between a glass–wire interface in the UMEs. Further, we showed a quantitative treatment for the sealing assessment using analytical expressions. Overall, we demonstrate a rapid check procedure of the sealing quality in fabricating Pt disk UMEs. The simple procedure presented in this work can be used to evaluate the sealing quality of other types of micro/nanoelectrodes during their fabrication.

Controlling the size of nanoscale entities is important because many properties of nanomaterials are directly related to the size of the particles. Gold nanoparticles represent classic materials and are of particular interest due to their potential application in a variety of fields. In this study, hexanethiol-capped gold nanoparticles are synthesized via the Brust-Schiffrin method. Synthesized nanoparticles were characterized by various analytical techniques such as transmission electron microscopy, scanning tunneling microscopy (STM), UV-visible absorption spectroscopy and electrochemical techniques. We have varied the molar ratio of gold to the protecting agent (hexanethiol) to discover the effect of gold-to-hexanethiol ligand ratio on the size of gold particles. The clear correlation between particle size and molar ratio is found that the averaged particle size decreases from 4.28 ± 0.83 to 1.54 ± 0.67 nm as the gold-to-ligand molar ratio changes from 1:1 to 1:9. In contrast to a recent report that thiolated gold nanoparticles are under spontaneous disintegration when they are assembled on a gold substrate, our STM experiments proved that these gold nanoparticles can form a stable monolayer or multiple layers on the platinum electrode without observing disintegration within 72 h. Therefore, our STM experiments demonstrate that the disintegration behavior of gold nanoparticles is related to the type of ligands and the nature of substrate materials. In electrochemical experiments, these gold nanoparticles displayed an electrochemical quantized charging effect, making these nanoparticles useful in the device applications such as electrochemical or biological sensors.

The emerging field of plasmonics has promoted applications of optical technology, especially in plasmon-enhanced spectroscopy (PES). However, in plasmon-enhanced fluorescence (PEF), "metal loss" could significantly quench the fluorescence during the process, which dramatically limits its applications in analysis and high-resolution imaging. In this report, silver core silica shell-isolated nanoparticles (Ag@SiO2 NPs or SHINs) with a tunable thickness of shell are used to investigate the interactions between NPs and emitters by constructing coupling and noncoupling modes. The plasmonic coupling mode between Ag@SiO2 NPs and Ag film reveals an exceeding integrating spectral intensity enhancement of 330 and about 124 times that of the radiative emission rate acceleration for shell-isolated nanoparticle enhanced phosphorescence (SHINEP). The experimental findings are supported by theoretical calculations using the finite-element method (FEM). Hence, the SHINEP may provide a novel approach for understanding the interaction of plasmon and phosphorescence, and it holds great potential in surface detection analysis and singlet-oxygen-based clinical therapy.

Heterogeneous electrocatalytic reactions are believed to occur at a minority of coordination sites through a series of elementary reactions that are balanced by minor equilibria. These features mask changes in reaction sites, making it challenging to directly identify and analyze reaction sites or intermediates while studying reaction mechanisms. Systematic perturbations of a reaction system often yield systematic changes in material properties and behavior. Correlations between measurable changes in parameters describing the structure and behavior, therefore, serve as powerful tools for distinguishing active reaction sites. This review explores structure–property correlations that have advanced understanding of behavior and reaction mechanisms in heterogeneous electrocatalysis. It covers correlations that have advanced understanding of the contributions of the local reaction environment to reactivity, of structure and bonding within solid-state materials, of geometric or mechanical strain in bonding environments, and of the impact of structural defects. Such correlations can assist researchers in developing next generation catalysts by establishing catalyst design principles and gaining control over reaction mechanisms.

The consistent fabrication of high performance α-Fe2O3 photoanodes for the oxygen evolution reaction remains a challenge. We work towards resolving this issue by developing in situ variable temperature Raman spectroscopy as a means to better understand the formation of α-Fe2O3, using the conversion of γ-FeOOH to α-Fe2O3 under varied gaseous environments as a model case. The sensitivity of Raman spectroscopy to structural changes provides mechanistic insights that are not readily available in more conventional approaches, such as thermal gravimetric analysis and differential scanning calorimetry. The Raman spectra are combined with conventional thermal analyses to interpret the photoelectrocatalytic performance of a series of α-Fe2O3 photoanodes prepared by systematic variation of a three-stage annealing protocol. The combined results suggest that protohematite, a form of α-Fe2O3 where trapped hydroxyl ligands are balanced by Fe(III) vacancies, forms between 200 °C and 400 °C in a reaction environment-dependent fashion. This protohematite is shown to be remarkably persistent once formed, degrading photoelectrocatalytic performance. This research advances understanding of the γ-FeOOH to α-Fe2O3 structural transformation, illustrates a powerful method to study solid state phase transitions, and provides guidance for the synthesis of high quality α-Fe2O3 from a convenient precursor.

Described here is a semiquantitative theoretical treatment of the kinetics of outer sphere electrochemical reactions. The framework presented here, which is based on simple physical arguments, predicts heterogeneous rate constants consistent with previous experimental observations (k0 > 10 cm/s). This theory is applied to the analysis of voltammetry experiments involving ultramicroelectrodes modified with thin, insulating oxide films where electronic tunneling between the electrode and redox species in solution (metal–insulator–solution tunneling) is expected to play a prominent role. It is shown that analysis of the voltammetric response of an outer sphere redox couple can be used to track changes in the structure of the adsorbed insulating layer.

Single gold particles may serve as room temperature single electron memory units because of their size dependent electronic level spacing. Here, we present a proof-of-concept study by electrochemically controlled scanning probe experiments performed on tailor-made Au particles of narrow dispersity. In particular, the charge transport characteristics through chemically synthesized hexane-1-thiol and 4-pyridylbenzene-1-thiol mixed monolayer protected Au(144) clusters (MPCs) by differential pulse voltammetry (DPV) and electrochemical scanning tunneling spectroscopy (EC-STS) are reported. The pyridyl groups exposed by the Au-MPCs enable their immobilization on Pt(111) substrates. By varying the humidity during their deposition, samples coated by stacks of compact monolayers of Au-MPCs or decorated with individual, laterally separated Au-MPCs are obtained. DPV experiments with stacked monolayers of Au(144)-MPCs and EC-STS experiments with laterally separated individual Au(144)-MPCs are performed both in aqueous and ionic liquid electrolytes. Lower capacitance values were observed for individual clusters compared to ensemble clusters. This trend remains the same irrespective of the composition of the electrolyte surrounding the Au(144)-MPC. However, the resolution of the energy level spacing of the single clusters is strongly affected by the proximity of neighboring particles.