Infrared Reflection Absorption Spectroscopy Research Articles

Deciphering the Orientation of the Aromatic Spacer Cation in Bilayer Perovskite Solar Cells through Spectroscopic Techniques.

Slowing the degradation of perovskite-based solar cells (PSCs) is of substantial interest. We engineered the surface by introducing a hydrophobic overlayer on a three-dimensional (3D) perovskite using fluorinated or nonfluorinated aryl ammonium cation spacers. The placement of a fluoroarene cation allows the formation of a bilayer structure, that is, layered/3D perovskites. By doing so, the surface hydrophobic character increases notably by the virtue of the perfluorinated benzene moiety. The fabricated devices thereof gave higher performance and longevity than control devices in addition to boosting reliability. The fluoro-phenethylammonium iodide (FPEAI)-based devices showed lower nonradiative carrier recombination. To decipher the orientation of the spacer cation in this bilayer structure, we probed the surface by polarization-modulated infrared reflection-absorption spectroscopy and noted substantial differences in the orientation due to the presence of fluorine substitution. We hypothesize that the stronger van der Waals interactions due to the higher electronegativity in FPEAI govern the orientation and performance enhancement and act as a barrier to moisture decomposition.

Cover Feature: Effect of Ectoine on the Conformation and Hybridization of dsDNA in Monolayer Films: A Spectroelectrochemical Study (ChemElectroChem 20/2021)

The Cover Feature shows double strand DNA monolayers on the gold electrode surface stored for a long time in aqueous solutions in the absence and presence of ectoine, compatible solute. The use of polarization modulation infrared reflection absorption spectroscopy allowed to follow both the electric potential and time dependent changes in the conformation, orientation and hybridization state of DNA helices. Our results demonstrate that in monolayer films, the addition of compatible solutes, such as ectoine, to the electrolyte solution may have a protective effect on dsDNA stability over time. More information can be found in the Article by J. Wittmar et al.

Zinc–Carboxylate Binding in Mixed Octadecanoic Acid and Octadecanol Monolayers on Proxy Seawater Solution Surfaces

Organic coatings on sea spray aerosol are largely comprised of fatty acids in addition to a vast array of other organic molecules including fatty alcohols. The seawater from which sea spray aerosol originates contains metal ions that interact with the organic coating at the air–seawater interface, resulting in transport of these metals into the atmosphere. Metal binding within single-substance monolayers on aqueous solutions has been previously studied. However, such binding events within mixed monolayers, especially those of ocean-relevant ratios, are not well-explored. Here, we examine four monolayer ratios of octadecanoic acid (stearic acid) to octadecanol (stearyl alcohol) on aqueous solutions with varying ZnCl2 concentrations and a seawater-relevant NaCl concentration of 0.465 M. Surface pressure–area (Π-A) isotherms and infrared reflection–absorption spectroscopy (IRRAS) are used to quantify the Zn2+–carboxylate surface binding affinities for each monolayer composition. We find that Zn2+–carboxylate binding is enhanced by ∼300 times at the surface when compared to the bulk solution. Addition of 10% octadecanol reduces the apparent surface binding affinity by over 50% from 3.6 × 103 to 1.4 × 103 M–1; this is significantly more than predicted from the slight reduction in viable 1:1 binding sites. Furthermore, 1:2 Zn2+–carboxylate binding is shown to only be viable for the 100 and 95% fatty acid film, whereas 1:1 binding is observed for all film ratios investigated.

Spectroelectrochemical and DFT approaches to the study of croconic acid adsorption at gold electrodes in acidic solutions

• Croconate anions adsorb at Au electrodes in croconic acid-containing solutions. • Croconate species bons in a bidentate configuration with O atoms on top positions. • Adsorption occurs with the molecular plane (quasi) normal to the metal surface. • Adsorption causes significant changes on the geometry and symmetry of croconate. • Collective vibrations broaden and split croconate bands at high coverage. The adsorption of species coming from croconic acid (4,5-dihydroxy-4-cyclopentene-1,2,3-trione, H 2 C 5 O 5 , CA) at gold electrodes was studied spectroelectrochemically in aqueous perchloric acid solutions. Voltammetric experiments suggest the existence of reversible adsorption processes in the potential range between 0.30 and 0.80 V vs RHE. Surface-Enhanced Infrared Reflection Absorption Spectroscopy experiments under Attenuated Total Reflection conditions (ATR-SEIRA) show potential-dependent adsorbate bands in the latter potential range that, according to Density Functional Theory calculations, can be assigned to adsorbed croconate. Calculated optimized geometry corresponds to bonding of croconate species to the gold surfaces involving two oxygen atoms in a bidentate configuration, with the molecular plane perpendicular to the metal surface for low and high crocoanate coverage. For intermediate coverage, lateral repulsions between neighbor croconate adsorbates give rise to the tilting of the molecular plane. The broadening and splitting at high electrode potentials of the C O stretching bands for adsorbed croconate can be explained by invoking the existence of collective vibrational modes appearing at high coverage. The cyclic voltammograms recorded in the CA-containing solutions also show the existence of reversible redox processes at potentials below 0.30 V involving adsorbed species, which are detected in the collected ATR-SEIRA spectra.

Mixed monolayer of a nucleolipid and a phospholipid has improved properties for spectroelectrochemical sensing of complementary nucleobases

A mixed monolayer of 1,2-dipalmitoyl-sn-glycero-3-(cytidine diphosphate) nucleolipid (DG-CDP) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was transferred from the air–water interface onto an gold (1 1 1) electrode surface using the Langmuir-Schaeffer (horizontal touch) technique. Compression isotherms were recorded for the monolayer spread at the air–water interface. These measurements demonstrated that the mixture composition 7:3 of DG-CDP:DPPC displayed reduced repulsive interactions between polar heads and ideal packing of acyl chains. Chronocoulometric experiments were performed to demonstrate stability of this film at the gold–solution interface. Photon polarization modulation infrared reflection absorption spectroscopy showed that the tilt angle of the acyl chains is slightly higher, and the chains are less twisted in the mixed than in the pure nucleolipid monolayer. In the mixed film, the tilt of the cytosine moiety (the sensing element of the monolayer) assumes a large angle with respect to the surface normal, even when the monolayer becomes detached from the gold surface. The mixed monolayer has improved properties for future applications for detection of guanine, its complementary base.

Adsorption of lysozyme on gold surfaces in the presence of an external electric potential

Adsorbed protein films consist of essential building blocks of many biotechnological and biomedical devices. The electrostatic potential may significantly modulate the protein behaviour on surfaces, affecting their structure and biological activity. In this study, lysozyme was used to investigate the effects of applied electric potentials on adsorption and the protein structure. The pH and the surface charge determine the amount and secondary structure of adsorbed lysozyme on a gold surface. In-situ measurements using polarization modulation infrared reflection absorption spectroscopy indicated that the concentration of both the adsorbed anions and the lysozyme led to conformational changes in the protein film, which was demonstrated by a greater amount of aggregated β-sheets in films fabricated at net positive charges of the Au electrode (Eads > Epzc). The changes in secondary structure involved two parallel processes. One comprised changes in the hydration/hydrogen-bond network at helices, leading to diverse helical structures: α-, 310- and/or π-helices. In the second process β-turns, β-sheets, and random coils displayed an ability to form aggregated β-sheet structures. The study illuminates the understanding of electrical potential-dependent changes involved in the protein misfolding process.

Zr-doped α-FeOOH with high faradaic efficiency for electrochemical nitrogen reduction reaction

Electrochemical nitrogen reduction reaction (NRR) is regarded as a promising route for ammonia synthesis, which still faces several challenges including NH3 yield and low selectivity. Herein, Zr-doped α-FeOOH (Zr/α-FeOOH) was developed as NRR electrocatalyst, which shows a high NH3 yield of 1.39 × 10−10 mol s−1 cm−2 and a significantly increased Faradaic efficiency (FE) of 35.63%. The Zr doping is found to induce more O vacancies as NRR active sites, which results in the increased NH3 yield. Besides, Zr species can suppress the competing side reaction effectively and have a positive effect on the high FE. Furthermore, the electrochemical in situ attenuated total reflection surface-enhanced infrared reflection absorption spectroscopy was conducted to investigate the NRR mechanism on Zr/α-FeOOH surface, which was revealed to follow an associative reaction pathway.

Temperature dependence of vibrational motions of thin polystyrene films by infrared reflection-absorption spectroscopy: A single measurement tool for monitoring of glass transition and temperature history

A method for monitoring of vibrational response and temperature region of glass transition by a single analysis is introduced with the use of an otherwise well-known spectroscopic method. The temperature-dependent changes of polystyrene (PS) thin films were investigated by polarization-modulated infrared reflection-absorption spectroscopy. The study revealed that changes in individual spectral regions with the temperature increase are specific and provide the relation to the processing and thermal history of the thin film. Temperature responses of infrared spectral region areas; an abrupt change of Car-H stretching, a step-like change of Car-H out-of-plane bending, a linear decrease of C–H stretching and C–H bending area, granted information on the temperature domain represented by local molecular motion change (β-transition) of PS. The different behavior of spectral regions is rationalized by consideration of vibrational free volume contribution to the total free volume of the system that is certainly influenced by functional groups nature, such as their size, geometry and involvement of non-bonding interaction.

Enhancing the surface sensitivity of in-situ/operando characterization of palladium membranes through polarization modulation and synthesis of optically smooth palladium thin films

Operando infrared reflection absorption spectroscopy (IRAS) is a powerful technique for elucidating the fundamental mechanisms by which reactive gases such as CO, C3H6 and H2S poison (inhibit) H2 permeation across palladium (Pd) and Pd-alloy H2 separation membranes, by integrating in-situ IRAS detection of chemical species adsorbed on the membrane surface with simultaneous measurement of H2 permeation rates on the same sample. Achieving high sensitivity to detection of surface-adsorbed species under realistic operating conditions is a significant challenge, however, because infrared absorption from gas-phase species can obscure the relatively weak absorption bands associated with surface-adsorbed species, and IRAS requires optically smooth Pd membrane surfaces to specularly reflect the infrared light at grazing angle incidence. This work addresses both of these challenges by (1) installing a photoelastic modulator in the infrared beam path to modulate the polarization of the infrared light between s- and p-polarization (PM-IRAS), and (2) by developing a new electroless plating method for synthesis of optically smooth Pd thin film membranes. The net result is a ~10× enhancement in the sensitivity to detection of surface-adsorbed CO in the presence of gas-phase CO, relative to IRAS characterization with commercial Pd foil. This work represents a significant advancement in the development and application of surface-sensitive operando membrane characterization, which is a relatively new research methodology that is expected to have a significant impact on membrane science in the coming years because of its unique ability to directly correlate membrane atomic/molecular structure to its performance under realistic operating conditions.

Electrostatics affects formation of Watson-Crick complex between DNA bases in monolayers of nucleolipids deposited at a gold electrode surface

Chronocoulometry was applied to determine charge in a monolayer of nucleolipid (1,2-dipalmitoyl-sn‑glycero-3-(cytidine diphosphate)) deposited at a gold electrode surface. The immersion method was used to measure the potential of zero charge of the interface (Epzci), which is a sum of charge on the monolayer of the nucleolipid and charge on the gold surface. Photon polarization infrared reflection absorption spectroscopy (PM-IRRAS) was used to determine formation of the Watson-Crick complex between terminal cytidine moiety of the nucleolipid and guanine (its complementary base) added to the solution. The combination of electrochemical and spectroscopic studies allowed one to demonstrate that the Watson-Crick complex is formed when the interface is positively charged. The potential applied to the electrode affects not only the complex formation but also orientation of the cytosine moiety. The complex is formed when the cytosine moiety is oriented assuming a small angle with respect to the electrode surface.

Experimental and Theoretical Insights into the Active Sites on WOx/Pt(111) Surfaces for Dehydrogenation and Dehydration Reactions

The catalytic upgrading of biomass is an important step toward realizing renewable chemical production. Pt/WOx and the inverse catalyst, WOx/Pt, have been shown to be highly active and selective for the C–O bond scission of various biomass-derived oxygenates. Yet, the nature of the active sites and detailed reaction mechanisms have not been well understood. In this study, carbon monoxide and isopropyl alcohol (IPA) have been used as probe molecules to study the active sites of WOx/Pt(111) model surfaces. Temperature-programmed desorption experiments identified two distinct active sites responsible for different reaction pathways: dehydrogenation over Pt sites and dehydration over WOx sites. High-resolution electron energy loss spectroscopy (HREELS) identified the surface reaction intermediates. In situ infrared reflection absorption spectroscopy and HREELS measurements confirmed the presence of hydroxyl groups on WOx, which were consumed upon the adsorption of IPA, suggesting that the hydroxyl groups were the active sites for IPA dehydration. Density functional theory (DFT) calculations further revealed the reaction mechanisms and activation barriers of the IPA dehydrogenation and dehydration reactions. The comparison of different in situ-generated hydroxyl groups on WOx by DFT calculations suggested that the most active sites for dehydration should be those with protons least strongly bound (site formation energy of −0.3 eV), with a dehydration activation barrier of 1.23 eV. The understanding from this study provides insights into the design of relevant metal/metal oxide catalysts for the upgrading of biomass-derived oxygenates.

Formation of a Ti-Cu(111) single atom alloy: Structure and CO binding.

A single atom Ti-Cu(111) surface alloy can be generated by depositing small amounts of Ti onto Cu(111) at slightly elevated surface temperatures (∼500 to 600K). Scanning tunneling microscopy shows that small Ti-rich islands covered by a Cu single layer form preferentially on ascending step edges of Cu(111) during Ti deposition below about 400K but that a Ti-Cu(111) alloy replaces these small islands during deposition between 500 and 600K, producing an alloy in the brims of the steps. Larger partially Cu-covered Ti-containing islands also form on the Cu(111) terraces at temperatures between 300 and 700K. After surface exposure to CO at low temperatures, reflection absorption infrared spectroscopy (RAIRS) reveals distinct C-O stretch bands at 2102 and 2050cm-1 attributed to CO adsorbed on Cu-covered Ti-containing domains vs sites in the Ti-Cu(111) surface alloy. Calculations using density functional theory (DFT) suggest that the lower frequency C-O stretch band originates specifically from CO adsorbed on isolated Ti atoms in the Ti-Cu(111) surface alloy and predicts a higher C-O stretch frequency for CO adsorbed on Cu above subsurface Ti ensembles. DFT further predicts that CO preferentially adsorbs in flat-lying configurations on contiguous Ti surface structures with more than one Ti atom and thus that CO adsorbed on such structures should not be observed with RAIRS. The ability to generate a single atom Ti-Cu(111) alloy will provide future opportunities to investigate the surface chemistry promoted by a representative early transition metal dopant on a Cu(111) host surface.

CO and H2 adsorption on Au-Ni bimetallic surfaces: a combined experimental and DFT theoretical study

Au-Ni bimetallic thin films were grown on refractory metal substrates. CO and H2 adsorption on Au-Ni bimetallic surfaces have been studied by a combination of in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. It is found CO desorption peak shifts from 413 K on pure Ni surfaces to 293 K on the isolated Ni atoms formed by alloying with Au atoms. The sharp decrease of CO desorption temperature on Au-Ni surfaces with increasing Au coverage is caused by the change of the favored CO adsorption sites from bridge/hollow sites on pure Ni surfaces to Ni top sites on Au-Ni bimetallic surfaces. In situ PM-IRRAS shows two CO adsorption bands on Au sites at 2119 cm−1 and 2103 cm−1 on Au-Ni surfaces at 80 K, which are due to CO bound on under-coordinated Au atoms and electron negatively charged Au sites modified with nearby Ni atoms, respectively. Even with the Au-Ni surface temperature at as low as 100 K, CO adsorption induced Ni surface segregation has been observed by in situ PM-IRRAS. Furthermore, DFT calculation results discover the adsorption energy of CO on Ni top sites continues to decrease with increasing Au coverage due to the geometric ensemble effect and the lowered d-band center after Ni alloying with Au. H2 desorption temperature decreases from 363 K on pure Ni thin films to 302 K with increasing Au coverage to 0.6 ML. A new H2 peak appears at around 170 K on the Au-Ni surfaces with Au coverages between 0.6 ML and 0.9 ML. This new H2 TPD peak is assigned to H2 desorption from the totally isolated Ni sites. With Au coverage above 1.5 ML, there is no any H2 desorption detected. The combined surface science studies and DFT calculations provide new insights into the surface structure-activity correlation of Ni-base bimetallic surface alloys.

Model electrocatalysts for the oxidation of rechargeable electrofuels - carbon supported Pt nanoparticles prepared in UHV

Isopropanol (IPA) can be used as a rechargeable electrofuel. In this approach, IPA is oxidized to acetone (ACE) in a direct alcohol fuel cell and the formed ACE is subsequently back-converted to IPA in a heterogeneously catalyzed process. To study the electrochemical reaction mechanisms of the IPA oxidation at the molecular level, appropriate and well-defined model electrocatalysts are necessary. In this work we prepare such model electrocatalysts by surface science methods in ultra-high vacuum (UHV). The catalysts consist of well-defined platinum nanoparticles on carbon supports. As carbon support, we use flat highly ordered pyrolytic graphite (HOPG) and thin (20 nm) magnetron sputtered carbon films on a polycrystalline gold substrate. In a first step, we characterize the model electrocatalysts and investigate their stability in-situ with complementary methods, i.e. by electrochemical scanning tunneling microscopy (EC-STM), electrochemical on-line inductively coupled plasma mass spectrometry (ICP-MS) and CO stripping experiments followed by electrochemical infrared reflection absorption spectroscopy (EC-IRRAS). We determined a stability window ranging from -0.65 VRHE to 1.15 VRHE for both sample types, independent of the presence or absence of IPA in the electrolyte. In the second step, we study the oxidation of IPA on tPt nanoparticles using differential electrochemical mass spectrometry (DEMS) and EC-IRRAS. The onset of IPA oxidation is observed at 0.3 VRHE. ACE is formed with high selectivity, while we identify traces of CO2 as the only side-product formed at higher potentials. However, we do not observe any formation of adsorbed CO. A direct comparison of these results with previous work on Pt(111) suggests that low coordinated Pt sites and size effects play a subordinate role for IPA oxidation on Pt electrocatalysts.

Effect of cavitation intensity control on self-assembling of alkanethiols on gold in room temperature ionic liquids

This study investigates the effect of cavitation intensity on self-assembling of alkanethiol molecules on gold in room temperature ionic liquids (RTILs) under low frequency ultrasound irradiation (20 kHz). The use of RTILs, with low vapor pressure, enabled cavitation activity to be controlled up to quenching through pressure decrease within an argon-saturated atmosphere. This control possibility was used to acquire deeper insights into the role of cavitation on self-assembling processes. It was shown by electrochemical, contact angles and Polarization Modulation - Infrared Reflection Absorption Spectroscopy (PM-IRRAS) measurements that cavitation activates orientation and organization of self-assembled monolayers (SAM). X-ray Photoelectron Spectroscopy (XPS) revealed that, even if chemical adsorption of molecules is highly activated under ultrasound irradiation, it is not dependent on acoustic cavitation intensity.

Electrochemical tools to disclose the electrochemical reduction mechanism of CO2 in aprotic solvents and ionic liquids

Carbon dioxide (CO 2 ) plays a key role in controlling the temperature of the Earth. But the increase in the concentration of CO 2 in the atmosphere brings with it a series of consequences, originating several environmental problems. The use of electrochemical, spectroscopic and molecular dynamics techniques are useful toolkits to valorize carbon dioxide, and to know the reduction mechanism as a function of CO 2 concentration, the cathode nature, and the electrolyte. This manuscript will be mainly centered in the use of ionic liquids (IL) for efficient CO 2 capture and valorization into different valuable products thanks to the CO 2 electrochemical reduction. In this sense, spectroelectrochemistry based on cyclic voltammetry coupled with Polarization Modulation-Infrared Reflection-Absorption Spectroscopy (PM-IRRAS) and Infrared Reflection-Absorption Spectroscopy (IRRAS) appear to be an efficient instrument to follow the CO 2 reactivity in imidazolium ionic liquids. Finally, we present molecular dynamics paired with cyclic voltammetry in order to calculate the diffusion coefficient of CO 2 and the number of electrons involved in its reduction process, respectively. Therefore, the current research opens the door to the use of theoretical-experimental approaches altogether to determine how is the CO 2 reduction mechanism. The CO 2 reduction products in function of the solvent and nature of the cathode is suggested, proving that the product obtained from the electrochemical reduction of CO 2 depends on the electrode material and the solvent.

Interaction of LL-37 human cathelicidin peptide with a model microbial-like lipid membrane

The only representative of cathelicidin peptides in humans is LL-37, a multifunctional antimicrobial peptide (AMP) that is a part of the innate immune response. Details of the LL-37 direct activity against pathogens are not well understood at the molecular level. Here, we present research on the mechanism of interaction between LL-37 and a model multicomponent bilayer lipid membrane (BLM), mimicking microbial cell membrane. Electrochemical impedance spectroscopy (EIS), high-resolution atomic force microscopy (AFM) imaging, and polarization-modulation infrared reflection–absorption spectroscopy (PM-IRRAS) were applied to study the peptide influence on a model microbial-like membrane. We show that LL-37 causes changes in the phospholipid molecules conformation and orientation, leading to membrane disintegration, significantly affecting the membrane electrical parameters, such as capacitance and resistance. High-resolution AFM imaging shows topographical and mechanical effects of such disintegration, while PM-IRRAS data indicates that introduction of LL-37 causes changes in the phospholipid acyl chains from all-trans to gauche conformations. Moreover, the presence of LL-37 significantly alters the value of the phospholipid tilt angle. Altogether, our results suggest a “carpet” membrane dissolution followed by a detergent-like membrane disruption mechanism upon LL-37 activity. This research gives a novel insight into the understanding of LL-37 influence on multicomponent model membranes and a promising contribution to the development of LL-37-derived therapeutic agents against drug-resistant bacteria.

The influence of palladium on the hydrogenation of acetylene on Ag(111).

We have used reflection absorption infrared spectroscopy (RAIRS) and temperature programmed reaction (TPR) to study the selective hydrogenation of acetylene on both a clean Ag(111) surface and on a Pd/Ag(111) single-atom-alloy surface. The partial hydrogenation of acetylene to ethylene is an important catalytic process that is often carried out using PdAg alloys. It is challenging to study the reaction with ultrahigh vacuum techniques because H2 does not dissociate on Ag(111), and while H2 will dissociate at Pd sites, H-atom spillover from Pd to Ag sites does not generally occur. We bypassed the H2 dissociation step by exposing the surfaces to atomic hydrogen generated by the hot filament of an ion gauge. We find that hydrogen atoms react with acetylene to produce adsorbed ethylene at 85 K, the lowest temperature studied. This is revealed by the appearance of a RAIRS peak at 950 cm-1 due to the out-of-plane wagging mode of adsorbed ethylene when acetylene is exposed to a surface on which H atoms are pre-adsorbed. The formation of both ethylene and ethane are detected with TPR, but no acetylene coupling products, such as benzene, were found. From quantitative analysis of the TPR results, the percent conversion and selectivities to ethylene and ethane were determined. Low coverages of Pd enhance the conversion but do so mainly by increasing ethane formation.

Formaldehyde Selectivity in Methanol Partial Oxidation on Silver: Effect of Reactive Oxygen Species, Surface Reconstruction, and Stability of Intermediates

Selective oxidation reactions on heterogeneous silver catalysts are essential for the mass production of numerous industrial commodity chemicals. However, the nature of active oxygen species in such reactions is still debated. To shed light on the role of different oxygen species, we studied the methanol oxidation reaction on Ag(111) single-crystal model catalyst surfaces containing two dissimilar types of oxygen (electrophilic, Oe and nucleophilic, On). X-ray photoelectron spectroscopy and low energy electron diffraction experiments suggested that the atomic structure of the Ag(111) surface remained mostly unchanged after accumulating low Oe coverage at 140 K. Temperature-programmed reaction spectroscopic investigation of low coverages of Oe on Ag(111) revealed that Oe was active for methanol oxidation on Ag(111) with a high selectivity toward formaldehyde (CH2O) production. High surface oxygen coverages, on the other hand, triggered a reconstruction of the Ag(111) surface, yielding Ag oxide domains, which catalyzes methanol total oxidation to CO2 and decreases the formaldehyde selectivity. This important finding indicates a trade-off between CH2O selectivity and methanol conversion, where 93% CH2O selectivity can be achieved for an oxygen surface coverage of θO = 0.08 ML (ML = monolayer) with moderate methanol conversion, while methanol conversion could be boosted by a factor of ∼4 for θO = 0.26 ML with a suppression of CH2O selectivity to 50%. Infrared reflection absorption spectroscopy results and density functional theory calculations indicated that Ag oxide contains dissimilar adsorption sites for methoxy intermediates, which are also energetically less stable than that of the unreconstructed Ag(111). The current findings provide important molecular-level insights regarding the surface structure of the oxidized Ag(111) model catalyst directly governing the competition between different reaction pathways in methanol oxidation reaction, ultimately dictating the reactant conversion and product selectivity.

Spectroscopic Analysis of Cu(II)-Complexed Thin Films to Characterize Molecular-Level Interactions and Film Behavior.

We report on the structure and dynamics of a Cu2+-complexed arachidic acid (AA) monolayer formed by Langmuir-Blodgett (LB) deposition. Infrared reflection-absorption spectroscopy (IRRAS) was used to characterize aliphatic chain -CH2 symmetric and asymmetric stretching modes and determine the chain tilt angle and order as a function of subphase pH. Monolayer structure is controlled by metal ion-amphiphile interactions. At low subphase pH (<5), film buckling at high surface pressure is observed, while for high subphase pH (≥5), monolayer buckling is not observed. This finding is correlated to monolayer structural mediation by metal ion-amphiphile interactions. Dynamics and mobility of a fluorophore incorporated into the monolayer were also affected by Cu2+-AA interactions, determined by fluorescence recovery after photobleaching (FRAP) measurements. These data are consistent with the formation of a rigid film due to Cu2+ coordination to AA headgroups, with the extent of headgroup protonation being determined by the pH of the subphase during monolayer deposition.