Nanoscale Insights into Enhanced Raman Spectroscopy

Enhanced Raman spectroscopies, such as surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS), are based on the amplification of intrinsically weak Raman signals of a molecule by metallic nanostructures. The main enhancement is attributed to electromagnetic enhancement. Chemical effects, such as formation of a surface complex, or a charge-transfer complex, co-adsorbed anion effect, also add to the enhancement of the signal. Using SERS, it has been difficult to study details of chemical enhancement and polarization effects due to limited optical resolution of the technique and usage of roughened metal surfaces. These obstacles were overcome with the development of the TERS technique. TERS has extended Raman spectroscopy into the nanoscale region. In this chapter, nanoscale insights into surface chemistry that lead to Raman signal enhancement are described. The effect of molecular binding and orientation as well as commonly used in SERS chloride activation of metal surfaces is discussed. Finally, we describe the future prospects of TERS and the challenges that keep us from harnessing the full potential of the technique

Surface-nitrogen Removal Studied from Desorption Dynamics

The removal of adsorbed nitrogen from catalyst surfaces is one of the key processes in proceeding efficient catalytic treatments of nitrogen oxides in exhaust gases from automotive and stationary sources. This review summarizes the typical characteristics of three removal processes of surface-nitrogen on metal surfaces, i.e., (i) 2N(a) → Nz(g), (ii) N(a) + NO(a) → N2O(a) → N2(g) + O(a) and (iii) NO(a) + O(a) → NO2(a) → NO3-(a) → N2(g). In the first process desorbing N2 is collimating along the surface normal direction and is vibrationally excited because a large amount of the binding energy is released in the associative process. N2 in the second process collimates into a highly inclined direction on flat surfaces because N2O lies on the surface immediately before dissociation. These characteristics are exemplified to be useful to examine the reaction pathway of NOx decomposition on catalysts

An Efficient Electrocatalyst for Oxygen Evolution Reaction in Alkaline Solutions Derived from a Copper Chelate Polymer via In Situ Electrochemical Transformation

Efficient oxygen evolution reaction (OER) electrocatalysts are highly desired in the field of water electrolysis and rechargeable metal-air batteries. In this study, a chelate polymer, composed of copper (II) and dithiooxamide, was used to derive an efficient catalytic system for OER. Upon potential sweep in 1 M KOH, copper (II) centers of the chelate polymer were transformed to CuO and Cu(OH)2. The carbon-dispersed CuO nanostructures formed a nanocomposite which exhibits an enhanced catalytic activity for OER in alkaline media. The nanocomposite catalyst has an overpotential of 280 mV (at 1 mA/cm2) and a Tafel slope of 81 mV/dec in 1M KOH solution. It has a seven-fold higher current than an IrO2/C electrode, per metal loading. A catalytic cycle is proposed, in which CuO undergoes electrooxidation to Cu2O3 that further decomposes to CuO with the release of oxygen. This work reveals a new method to produce an active nanocomposite catalyst for OER in alkaline media using a non-noble metal chelate polymer and a porous carbon. This method can be applied to the synthesis of transition metal oxide nanoparticles used in the preparation of composite electrodes for water electrolyzers and can be used to derive cathode materials for aqueous-type metal-air batteries

Bifunctional Catalytic Activity of γ-NiOOH toward Oxygen Reduction and Oxygen Evolution Reactions in Alkaline Solutions

Nickel oxyhydroxides (NiOOHs) are well-known for their superior activity toward oxygen evolution reaction (OER) in alkaline solutions. However, their activity toward oxygen reduction reaction (ORR) has been largely unexplored. There exist three NiOOH polymorphs: α-, β-, and γ-NiOOH, characterized by different interlayer spacing. Although still debated, γ-NiOOH with a large layer spacing has been indicated as the active phase for OER. Here, a highly crystalline γ-NiOOH was prepared in a carbon matrix by the in situ electrochemical transformation of nickel dithiooxamide Ni(dto) in 1 M KOH solution. The catalyst prepared in this way showed low overpotential not only for OER, but also for ORR in alkaline solutions. The onset potential for ORR is ~0.81 V vs. RHE, and the reaction proceeds via the 2e− transfer pathway. The high OER catalytic activity and relatively low ORR overpotential make this nanocomposite catalyst a good candidate for bifunctional OER/ORR catalyst, stable in alkaline solutions