Surface enhanced infrared spectroscopy—Au(1 1 1-20nm)/sulphuric acid—new aspects and challenges

[2+1] Cycloadditions Modulate the Hydrophobicity of Ni-N4 Single-Atom Catalysts for Efficient CO2 Electroreduction

Microenvironment regulation of M-N4 single-atom catalysts (SACs) is a promising way to tune their catalytic properties toward the electrochemical CO2 reduction reaction. However, strategies that can effectively introduce functional groups around the M-N4 sites through strong covalent bonding and under mild reaction conditions are highly desired. Taking the hydrophilic Ni-N4 SAC as a representative, we report herein a [2+1] cycloaddition reaction between Ni-N4 and in situ generated difluorocarbene (F2C:), and enable the surface fluorocarbonation of Ni-N4, resulting in the formation of a super-hydrophobic Ni-N4-CF2 catalyst. Meanwhile, the mild reaction conditions allow Ni-N4-CF2 to inherit both the electronic and structural configuration of the Ni-N4 sites from Ni-N4. Enhanced electrochemical CO2-to-CO Faradaic efficiency above 98 % is achieved in a wide operating potential window from -0.7 V to -1.3 V over Ni-N4-CF2. In situ spectroelectrochemical studies reveal that a highly hydrophobic microenvironment formed by the -CF2- group repels asymmetric H-bonded water at the electrified interface, inhibiting the hydrogen evolution reaction and promoting CO production. This work highlights the advantages of [2+1] cycloaddition reactions on the covalent modification of N-doped carbon-supported catalysts.

Entropic contributions to the stability of electrochemically adsorbed anion layers on Au(111): a microcalorimetric study

We measure the entropy of formation of the interface upon anion adsorption (Cl^-, Br^- I^- and SO₄^2-) on Au(111) as an important indicator for the structure, order and composition of the interface. The entropy of formation of the interface exhibits a rather universal behaviour for all anions with a steep decrease upon initial adsorption followed by a shallow minimum at intermediate anion coverages and a strong increase close to the completion of the adsorbate adlayer. The strong variation of the entropy signals significant entropic contributions to the free enthalpy of the adsorption process and thus the stability of the adsorbed phase. At low anion coverages, close to the potential of zero charge, we attribute the entropy variations to the rearrangement of the interfacial water structure. At intermediate and high anion coverages, a comparison with the results of a lattice-gas model, considering pairwise repulsive interactions within the quasi-chemical approximation, shows that the entropy changes upon anion adsorption can be explained by the configurational entropy of the adsorbed phase. Thus, entropic contributions from both the solvent and the adsorbate are important for the stability of surface phases, particularly for disordered systems.

Electric Double Layer: The Good, the Bad, and the Beauty

The electric double layer (EDL) is the most important region for electrochemical and heterogeneous catalysis. Because of it, its modeling and investigation are something that can be found in the literature for a long time. However, nowadays, it is still a hot topic of investigation, mainly because of the improvement in simulation and experimental techniques. The present review aims to present the classical models for the EDL, as well as presenting how this region affects electrochemical data in everyday experimentation, how to obtain and interpret information about EDL, and, finally, how to obtain some molecular point of view insights on it.

Identification of electrochemically adsorbed species via electrochemical microcalorimetry: sulfate adsorption on Au(111)

We investigate compositional changes of an electrochemical interface upon polarization with electrochemical microcalorimetry. From the heat exchanged at a Au(111) electrode upon sulfate adsorption, we determine the reaction entropy of the adsorption process for both neutral and acidic solutions, where the dominant species in solution changes from SO₄²⁻ to HSO₄⁻. In neutral solution, the reaction entropy is about 40 J mol⁻¹ K⁻¹ more positive than that in acidic solution over the complete sulfate adsorption region. This entropy offset is explicable by a deprotonation step of HSO₄⁻ preceding sulfate adsorption in acidic solution, which shows that the adsorbing species is SO₄* in both solutions. The observed overall variation of the reaction entropy in the sulfate adsorption region of ca. 80 J mol⁻¹ K⁻¹ indicates significant sulfate‐coverage dependent entropic contributions to the Free Enthalpy of the surface system.

An In situ Raman study of intermediate adsorption engineering by high-index facet control during the hydrogen evolution reaction

An in situ Raman study of the mechanism of HER catalytic performance enhanced by high-index facets on Ti@TiO₂ nanosheets.

Optimization of a Commercial Variable Angle Accessory for Entry Level Users of Electrochemical Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS)

An evaluation of several experimental aspects that can optimize electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) performance using a commercially available, specular reflection accessory is provided. A comparison of different silicon single-bounce internal reflection elements (IREs) is made with emphasis on different face-angled crystal (FAC) options. Selection of optimal angle of incidence for maximizing signal and minimizing noise is shown to require consideration of the optical throughput of the accessory, reflection losses at the crystal surfaces, and polarization effects. The benefits of wire-grid polarizers and antireflective (AR) coatings on the IREs is discussed. High signal-to-noise ratios can be achieved by omitting polarizers, using an AR-coated FAC with a larger face angle, and working at angles of incidence close to the maximum throughput angle of the accessory.

Surface-Specific Spectroscopy of Water at a Potentiostatically Controlled Supported Graphene Monolayer

Knowledge of the structure of interfacial water molecules at electrified solid materials is the first step toward a better understanding of important processes at such surfaces, in, e.g., electrochemistry, atmospheric chemistry, and membrane biophysics. As graphene is an interesting material with multiple potential applications such as in transistors or sensors, we specifically investigate the graphene-water interface. We use sum-frequency generation spectroscopy to investigate the pH- and potential-dependence of the interfacial water structure in contact with a chemical vapor deposited (CVD) grown graphene surface. Our results show that the SFG signal from the interfacial water molecules at the graphene layer is dominated by the underlying substrate and that there are water molecules between the graphene and the (hydrophilic) supporting substrate.

In-Situ Infrared Spectroscopy Applied to the Study of the Electrocatalytic Reduction of CO2 : Theory, Practice and Challenges

The field of electrochemical CO₂ conversion is undergoing significant growth in terms of the number of publications and worldwide research groups involved. Despite improvements of the catalytic performance, the complex reaction mechanisms and solution chemistry of CO₂ have resulted in a considerable amount of discrepancies between theoretical and experimental studies. A clear identification of the reaction mechanism and the catalytic sites are of key importance in order to allow for a qualitative breakthrough and, from an experimental perspective, calls for the use of in-situ or operando spectroscopic techniques. In-situ infrared spectroscopy can provide information on the nature of intermediate species and products in real time and, in some cases, with relatively high time resolution. In this contribution, we review key theoretical aspects of infrared reflection spectroscopy, followed by considerations of practical implementation. Finally, recent applications to the electrocatalytic reduction of CO₂ are reviewed, including challenges associated with the detection of reaction intermediates.

Surface Modification on Cellulose Nanofibers by TiO2 Coating for Achieving High Capture Efficiency of Nanoparticles

Cellulose nanofibers were modified by TiO2 gel layer (~25 nm in thickness) via hydrolysis reaction on the surface of the cellulose nanofibers. After the TiO2 coating, the surface charge of the nanofiber dramatically changed from negative to positive. A high efficiency (~100%) of capturing negatively charged Au nanoparticles (5 nm) was successfully obtained by effectively utilizing the electrostatic interaction of surface charge between the TiO2-coated cellulose nanofibers and Au nanoparticles. Therefore, this technique of surface modification will be potentially used in improving filtration efficiency for membrane applications.

Qualitatively and quantitatively characterizing water adsorption of a cellulose nanofiber film using micro-FTIR spectroscopy

The efficient utilization of cellulose is very important for environmental protection and sustainability of resources.

Surface-Enhanced Infrared Spectroscopy Using Resonant Nanoantennas

Infrared spectroscopy is a powerful tool widely used in research and industry for a label-free and unambiguous identification of molecular species. Inconveniently, its application to spectroscopic analysis of minute amounts of materials, for example, in sensing applications, is hampered by the low infrared absorption cross-sections. Surface-enhanced infrared spectroscopy using resonant metal nanoantennas, or short "resonant SEIRA", overcomes this limitation. Resonantly excited, such metal nanostructures feature collective oscillations of electrons (plasmons), providing huge electromagnetic fields on the nanometer scale. Infrared vibrations of molecules located in these fields are enhanced by orders of magnitude enabling a spectroscopic characterization with unprecedented sensitivity. In this Review, we introduce the concept of resonant SEIRA and discuss the underlying physics, particularly, the resonant coupling between molecular and antenna excitations as well as the spatial extent of the enhancement and its scaling with frequency. On the basis of these fundamentals, different routes to maximize the SEIRA enhancement are reviewed including the choice of nanostructures geometries, arrangements, and materials. Furthermore, first applications such as the detection of proteins, the monitoring of dynamic processes, and hyperspectral infrared chemical imaging are discussed, demonstrating the sensitivity and broad applicability of resonant SEIRA.

Speciation of Adsorbed Phosphate at Gold Electrodes: A Combined Surface-Enhanced Infrared Absorption Spectroscopy and DFT Study

Despite the significance of phosphate buffer solutions in (bio)electrochemistry, detailed adsorption properties of phosphate anions at metal surfaces remain poorly understood. Herein, phosphate adsorption at quasi-Au(111) surfaces prepared by a chemical deposition technique has been systematically investigated over a wide range of pH by surface-enhanced infrared absorption spectroscopy in the ATR configuration (ATR-SEIRAS). Two different pH-dependent states of adsorbed phosphate are spectroscopically detected. Together with DFT calculations, the present study reveals that pKa for adsorbed phosphate species at the interface is much lower than that for phosphate species in the bulk solution; the dominant phosphate anion, H2PO4(-) at 2 < pH < 7 or HPO4(2-) at 7 < pH < 12, undergoes deprotonation upon adsorption and transforms into the adsorbed HPO4 or PO4, respectively. This study leads to a conclusion different than earlier spectroscopic studies have reached, highlighting the capability of the ATR-SEIRAS technique at electrified metal-solution interfaces.

Surface Enhanced Infrared Studies of 4-Methoxypyridine Adsorption on Gold Film Electrodes

This work uses electrochemical surface sensitive vibrational spectroscopy to characterize the adsorption of a known metal nanoparticle stabilizer and growth director, 4-methoxypyridine (MOP). Surface enhanced infrared absorption spectroscopy (SEIRAS) is employed to study the adsorption of 4-methoxypyridine on gold films. Experiments are performed under electrochemical control and in different electrolyte acidities to identify both the extent of protonation of the adsorbed species as well as its orientation with respect to the electrode surface. No evidence of adsorbed conjugated acid is found even when the electrolyte pH is considerably lower than the pKa. Through an analysis of the transition dipole moments, determined from DFT calculations, the SEIRA spectra support an adsorption configuration through the ring nitrogen which is particularly dominant in neutral pH conditions. Adsorption is dependent on both the electrical state of the Au film electrode as well as the presence of ions in the electrolyte that compete for adsorption sites at positive potentials. Combined differential capacitance measurements and spectroscopic data demonstrate that both a horizontal adsorption geometry and a vertical adsorption phase can be induced, with the former being found on negatively charged surfaces in acidic media and the latter over a wide range of polarizations in neutral solutions.

An improved sensor for electrochemical microcalorimetry, based on lithiumtantalate

We have developed a pyroelectric sensor for electrochemical microcalorimetry, based on LiTaO3, which provides unprecedented sensitivity for the detection of electrochemically induced heat effects. Deterioration of the heat signal by electrostriction effects on the electrode surface is suppressed by a multilayered construction, where an intermediate sapphire sheet dampens mechanical deformations. Thus, well textured thin metal films become viable candidates as electrodes. We demonstrate the sensor performance for Cu underpotential deposition on (111)-textured Au films on sapphire. The sensor signal compares well with a purely thermal signal induced by heating with laser pulses. The high sensitivity of the sensor is demonstrated by measuring heat effects upon double layer charging in perchloric acid, i.e., in the absence of electrochemical charge- or ion-transfer reactions.

SEIRAS Studies of Water Structure in a Sodium Dodecyl Sulfate Film Adsorbed at a Gold Electrode Surface

Surface-enhanced infrared reflection-absorption spectroscopy (SEIRAS) was used to investigate the structure of water that is incorporated within a film of sodium dodecyl sulfate (SDS) adsorbed at a thin gold nanoparticle film deposited onto a silicon substrate. Previous studies on a Au(111) electrode surface showed that SDS molecules form long-range ordered hemicylindrical hemimicelles (phase I) for potentials -0.2 ≤ E ≤ 0.45 V vs Ag/AgCl and a disordered bilayer (phase II) for potentials E ≥ 0.5 V vs Ag/AgCl. The SEIRA spectra demonstrated that the hemimicellar film is water-rich and contains both a network of hydrogen-bonded water and a disturbed network of hydrogen bonds consisting of monomeric and dimeric water in the hydrophobic region of the film. No network water was observed in phase II of the film. However, SEIRAS data showed that sulfate groups in the disordered bilayer are hydrated. The SEIRAS spectra of the film of SDS were compared to the previously measured spectra obtained using subtractively normalized interfacial Fourier transform IR spectroscopy (SNIFTIRS). The complementarity of the spectroscopic information obtained by these two techniques was demonstrated.

An in situ surface electrochemistry approach towards whole-cell studies: the structure and reactivity of a Geobacter sulfurreducens submonolayer on electrified metal/electrolyte interfaces

Characterisation of direct electron transfer processes between Geobacter sulfurreducens and the Au(111) surface was performed under electrochemical control.

Design, development, and demonstration of a fully LabVIEW controlled in situ electrochemical Fourier transform infrared setup combined with a wall-jet electrode to investigate the electrochemical interface of nanoparticulate electrocatalysts under reaction conditions

We present a detailed description of the construction of an in situ electrochemical ATR-FTIR setup combined with a wall-jet electrode to investigate the electrocatalytic properties of nanoparticulate catalysts in situ under controlled mass transport conditions. The presented setup allows the electrochemical interface to be probed in combination with the simultaneous determination of reaction rates. At the same time, the high level of automation allows it to be used as a standard tool in electrocatalysis research. The performance of the setup was demonstrated by probing the oxygen reduction reaction on a platinum black catalyst in sulfuric electrolyte.

Adsorption and assembly of ions and organic molecules at electrochemical interfaces: nanoscale aspects

We describe the history of electrochemical scanning tunneling microscopy (STM) and advances made in this field during the past 20 years. In situ STM allows one to monitor various electrode processes, such as the underpotential deposition of copper and silver ions; the specific adsorption of iodine and sulfate/bisulfate ions; electrochemical dissolution processes of silicon and gold single-crystal surfaces in electrolyte solutions; and the molecular assembly of metalloporphyrins, metallophthalocyanines, and fullerenes, at atomic and/or molecular resolution. Furthermore, a laser confocal microscope, combined with a differential interference contrast microscope, enables investigation of the dynamics of electrochemical processes at atomic resolution.

para-Mercaptobenzoic acid-modified silver nanoparticles as sensing media for the detection of ammonia in air based on infrared surface enhancement effect

To utilize the large signals provided by surface-enhanced infrared absorption (SEIRA) measurements for chemical sensing, a new sensing scheme was proposed and demonstrated for detection of ammonia in air samples. To increase the SEIRA effect, a sensing phase composed of multi-layers of silver nanoparticles (AgNPs) was prepared using a chemically controlled electroless deposition method. para-Mercaptobenzoic acid (pMBA) served as the controlling agent in formation of AgNPs, a surface modification agent of AgNPs for sensing, and a stabilizer to protect the AgNPs from coagulation and oxidation. The sensing approach utilized the interaction between pMBA and ammonia, which involves the formation of carboxylate-ammonium complex. After interaction, the enhanced IR absorption bands of pMBA on AgNPs were significantly changed and able to provide quantitative information on the ammonia concentrations. To optimize the conditions for preparing sensing elements, parameters used to form multi-layers of AgNPs were systematically varied and their corresponding sensitivities in detection of ammonia were recorded. The results indicate that AgNPs with diameters in the range of 100 nm provided the best performance in terms of detecting ammonia via the SEIRA effect. Also, the analytical signal generally increased as the number of layers of AgNPs increased, but was limited to certain layers, depending on the reaction conditions used in preparation of AgNPs. The sensing elements were found to be highly selective to ammonia and the detection limit approached 150 ppb with a linear range up to 25 ppm.