Electrochemical Investigations of 4‑Methoxypyridine Adsorption on Au(111) Predict Its Suitability for Stabilizing Au Nanoparticles

A thermodynamic analysis of the adsorption of 4-methoxypyridine (MOP) on Au(111) surfaces is presented in an effort to determine its propensity to stabilize metal nanoparticles. The adsorption of MOP is compared and contrasted to the adsorption of 4-dimethylaminopyridine (DMAP), the latter of which is well-known to form stable Au nanoparticles. Electrochemical studies show that MOP, like most pyridine derivatives, can exhibit two different adsorption states. The electrical state of the metal, the pH of the solution, and the surface crystallography determine whether MOP adopts a low-coverage, π-bonded orientation or a high-coverage, σ-type orientation. A modified Langmuir adsorption isotherm is used to extract free energies of adsorption which are roughly 10% stronger for DMAP compared to MOP at equivalent conditions when expressed on a rational basis. The higher adsorption strength is attributed to DMAP’s greater Lewis basicity. Qualitatively, MOP and DMAP adsorption are found to be completely analogous, implying that MOP-protected gold particles should be stable under conditions that favor the high-coverage adsorption state. Using a previously reported, single-phase synthesis, this is shown to be the case

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 p<i>K</i>_a. 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

Surface-Enhanced Infrared Spectroscopy and Neutron Reflectivity Studies of Ubiquinone in Hybrid Bilayer Membranes under Potential Control

Surface-enhanced infrared adsorption spectroscopy (SEIRAS) and neutron reflectometry (NR) were employed to characterize ubiquinone (UQ) containing hybrid bilayer membranes. The biomimetic membrane was prepared by fusing phospholipid vesicles on a hydrophobic octadecanethiol monolayer self-assembled on a thin gold film. Using SEIRAS, the assembly of the membrane is monitored <i>in situ</i>. The presence of ubiquinone is verified by the characteristic carbonyl peaks from the quinone ester. A well-ordered distal lipid leaflet results from fusion of vesicles with and without the addition of ubiquinone. With applied potential, the hybrid bilayer membrane in the absence of UQ behaves in the same way as previously reported solid supported phospholipid membranes. When ubiquinone is incorporated in the membrane, electric field induced changes in the distal leaflet are suppressed. Changes in the infrared vibrations of the ubiquinone due to applied potential indicate the head groups are located in both polar and nonpolar environments. The spectroscopic data reveal that the isoprenoid unit of the ubiquinone is likely lying in the midplane of the lipid bilayer while the head has some freedom to move within the hydrophobic core. The SEIRAS experiments show redox behavior of UQ incorporated in a model lipid membrane that are otherwise inaccessible with traditional electrochemistry techniques

Role of Au(I) Intermediates in the Electrochemical Formation of Highly Anisotropic Gold Nanostructures with Near-IR SERS Applications

In the presence of certain stabilizing ligands, such as pyridine derivatives, the reduction of Au­(III) ions has been speculated to generate Au­(I) intermediates that may play a key role in nanoparticle growth. Herein, the electrochemical behavior of Au­(III) in the presence of 4-methoxypyridine, Py, is reported in aqueous electrolytes. Voltammetric analysis reveals that a spontaneously formed Au­(III)–Py complex undergoes a two-step reduction process. The first reduction involves the transfer of two electrons and produces a Au­(I) species. A more cathodic one-electron transfer results in electrodeposited gold. Sustained generation of the Au­(I)–Py intermediate species produced from the first reduction step leads to disproportionation and the formation of aggregated nanoparticle meshes that loosely adhere to the ITO electrode. Conversely, application of more negative potentials leads to the formation of highly anisotropic nanodaggers from the electrodeposition of the Au­(I) species. The shape-directing properties of Py adsorbed on the nucleated gold result in preferential ⟨111⟩ growth. The length scale of the deposited dagger-like shapes is dependent on deposition potential and deposited charge, and arms extending several hundred nanometers are reported. Optical characterizations show extinction extending well into the near-infrared region, which is attributed to localized surface plasmonic resonances. Near-IR Raman sensing applications are demonstrated using FT-Raman with 1064 nm excitation. The nanodaggers provide SERS enhancement factors greater than 10⁶ for monolayers of 4-aminothiophenol

Electrochemical Studies of Capping Agent Adsorption Provide Insight into the Formation of Anisotropic Gold Nanocrystals

The ability of the 4-dimethylaminopyridine (DMAP) to stabilize and control the formation of anisotropic gold nanocrystals produced <i>via</i> the borohydride reduction of gold(III) salts is reported here. Electrochemical measurements of DMAP electrosorption on different low-index single crystal and polycrystalline electrodes is provided and shows a propensity for DMAP to preferentially adsorb on {100} facets. Measuring the electrochemical potential during nanocrystal formation shows that experimental conditions can easily be manipulated so that the growth of nanoseeds occurs at potentials that support preferential DMAP adsorption on {100} surfaces giving rise to highly anisotropic nanocrystals (nanorods, bipyramids, and nanopods). Nanopods with nearly 50 nm arm lengths are shown to form and produce a surface plasmon mode that extends well into the near IR (λ_max ≈ 1350 nm). Evidence is provided of the slow, partial reduction of tetrachloroaurate to a DMAP stabilized Au^I species. Shape control is achieved simply by varying the length of time, τ, that DMAP is allowed to partially reduce the Au^III ions prior to the addition of the strong reducing agent, NaBH₄. Thus the role of DMAP in producing anisotropic particle shapes is shown to be multifunctional. A mechanism accounting for the dependence of particle shape on τ is provided

Step-Scan IR Spectroelectrochemistry with Ultramicroelectrodes: Nonsurface Enhanced Detection of Near Femtomole Quantities Using Synchrotron Radiation

The result of interfacing step-scan spectroelectrochemistry with an IR microscope and synchrotron infrared (SIR) radiation is provided here. An external reflectance cell containing a 25 μm gold ultramicroelectrode is employed to achieve an electrochemical time constant less than one microsecond. The use of a prototypical electrochemical system, i.e., the mass-transport controlled reduction of ferricyanide, allows for a proof of principle evaluation of the viability of SIR for step-scan spectroelectrochemistry. An analysis of the importance of accounting for synchrotron source variation over the prolonged duration of a step-scan experiment is provided. Modeling of the material flux in the restricted diffusion space afforded by the external reflectance cell allows the quantitative IR results to be compared to theoretical predictions. The results indicate that only at very short times does linear diffusion within the cavity dominate the electrode response and the majority of the transient signal operates under conditions of quasi-hemispherical diffusion. The analytical information provided by the IR signal is found to be considerably less than that derived from the current response due the latter’s pronounced edge effects. The results provide a detection limit of 36 fmol for step-scan SIR measurements of ferrocyanide. Implications for future IR spectroelectrochemical studies in the microsecond domain are discussed

Electrochemical ATR-SEIRAS Using Low-Cost, Micromachined Si Wafers

Thin, micromachined Si wafers, designed as internal reflection elements (IREs) for attenuated total reflectance infrared spectroscopy, are adapted to serve as substrates for electrochemical ATR surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). The 500 μm thick wafer IREs with groove angles of 35° are significantly more transparent at long mid-IR wavelengths as compared to conventional large Si hemisphere IREs. The appeal of greater transparency is mitigated by smaller optical throughput at larger grazing angles and steeper angles of incidence at the reflecting plane that reduce the enhancement factor. Through use of the potential dependent adsorption of 4-methoxypyridine (MOP) as a test system, the microgroove IRE is shown to provide relatively strong electrochemical ATR-SEIRAS responses when the angle of incident radiation is between 50 and 55°, corresponding to refracted angles through the crystal of ∼40°. The higher than expected enhancement is attributed to attenuation of the reflection loss of p-polarized light and multiple reflections within the wafer-based IRE. The micromachined IREs are shown to outperform a 25 mm radius hemisphere in terms of S/N at wavenumbers less than ca. 1400 cm^–1 despite the weaker signal enhancement derived from the steeper angle incident on the IRE/sample interface. The high optical transparency of the new IREs allows the spectral observation of displaced water libration bands at ca. 730 cm^–1 upon solvent replacement by adsorbed MOP. The results are highly encouraging for the further development of low-cost, Si wafer-based IREs for electrochemical ATR-SEIRAS applications

Spatial Mapping of Methanol Oxidation Activity on a Monolithic Variable-Composition PtNi Alloy Using Synchrotron Infrared Microspectroscopy

The use of synchrotron-sourced infrared radiation to map the electrochemical activity of a binary metal (Pt and Ni) alloy is demonstrated. The alloy is created in such a way that its metal concentration varies along one of its dimensions thus creating a continuum of electrocatalyst compositions on a single electrode. Localized methanol oxidation activity is determined spectroscopically by measuring the rate of CO₂ production at variable positions along the alloy concentration gradient using an infrared microscope. Numerical simulations of the kinetically controlled reaction demonstrate that qualitative assessment of relative reaction rates is possible as long as the reaction is followed on time scales smaller than those that lead to diffusional broadening. Characterization of the alloy before and after electrochemical experiments reveals significant levels of base metal leaching. Highly dealloyed regions of the sample show the highest rates of methanol activity and have a final Ni atomic composition of approximately 5%. Surface roughening from the dealloying process is shown to be at least partially responsible for enhanced activity

Femtomole Infrared Spectroscopy at the Electrified Metal–Solution Interface

Characterization of surface adsorbed species using infrared (IR) spectroscopy provides valuable information concerning interfacial chemical and physical processes. However, <i>in situ</i> infrared studies of surface areas approaching the IR diffraction limit, such as micrometer scale electrodes, require a hitherto unrealized means to obtain high signal-to-noise (S/N) spectra from femtomole quantities of adsorbed molecules. A major methodological breakthrough is described that couples the high brilliance of synchrotron-sourced infrared microscopy with attenuated total reflection surface enhanced infrared spectroscopy (ATR-SEIRAS). The method is shown to allow the spectral measurement of a monolayer of 4-methoxypyridine (MOP) adsorbed on a surface enhancing gold film electrode under fully operational electrochemistry conditions. A factor of 15 noise improvement is achieved with small apertures using synchrotron IR relative to a thermal IR source. The very low noise levels allow the measurement of high quality IR spectra of 2.5 fmol of molecules confined to a 125 μm² beam spot

[1.1]Ferrocenophanes and Bis(ferrocenyl) Species with Aluminum and Gallium as Bridging Elements: Synthesis, Characterization, and Electrochemical Studies

Salt-metathesis reactions between dilithioferrocene (Li₂fc·2/3tmeda) and intramolecularly coordinated aluminum and gallium species RECl₂ [R = 5-Me₃Si-2-(Me₂NCH₂)­C₆H₃; E = Al (<b>2a</b>), Ga (<b>2b</b>); and R = (2-C₅H₄N)­Me₂SiCH₂; E = Al (<b>3a</b>), Ga (<b>3b</b>)] gave respective [1.1]­ferrocenophanes ([1.1]­FCPs). Those obtained from <b>2a</b> and <b>2b</b>, respectively, were isolated as analytically pure compounds and fully characterized including single-crystal X-ray structure determinations [<b>4a</b> (Al): 43%; <b>4b</b> (Ga): 47%]. Bis­(ferrocenyl) compounds of the type REFc₂ [R = 5-Me₃Si-2-(Me₂NCH₂)­C₆H₃; E = Al (<b>5a</b>), Ga (<b>5b</b>); and R = (2-C₅H₄N)­Me₂SiCH₂; E = Al (<b>6a</b>), Ga (<b>6b</b>)] and R₂SiFc₂ [R = Me (<b>7</b>^<b>Me</b>); Et (<b>7</b>^<b>Et</b>)] were prepared, starting from respective element dichlorides and lithioferrocene (LiFc). Molecular structures of <b>6a</b>, <b>7</b>^<b>Me</b>, and <b>7</b>^<b>Et</b> were solved by single-crystal X-ray analyses. One of the two Fc moieties of <b>6a</b> was bent toward the open coordination site of the aluminum atom. The measured dip angles α* of the two independent molecules in the asymmetric unit were 11.9(5) and 13.3(5)°, respectively. The redox behavior of [1.1]­FCPs <b>4</b> and bis­(ferrocenyl) species <b>5</b>, <b>6</b>, <b>7</b>, and (Mamx)­EFc₂ [Mamx = 2,4-<i>t</i>Bu₂-6-(Me₂NCH₂)­C₆H₂; E = Al (<b>8a</b>), Ga (<b>8b</b>)] were investigated with cyclic voltammetry. While all gallium and silicon compounds gave meaningful and interpretable data, all aluminum compounds were problematic with the exception of <b>8a</b>. Aluminum species, compared to respective gallium species, are more sensitive and, presumably, fluoride ions or residual water from the electrolyte and solvent are causing degradation. The splitting between the formal potentials for bis­(ferrocenyl) species was significantly smaller (<b>5b</b>, <b>6b</b>, and <b>8b</b>: Δ<i>E</i>°′ = 0.138–0.159 V) than that of the [1.1]­FCP <b>4b</b> (Δ<i>E</i>°′ = 0.309 V). These results were explained by assuming an electrostatic interaction between the two iron centers; differences between bis­(ferrocenyl) species and [1.1]­FCPs are likely due to a more effective solvation of Fe-containing moieties in the more flexible bis­(ferrocenyl) species