Surface sensitive infrared spectroelectrochemistry using palladium electrodeposited on ITO-modified internal reflection elements.

Palladium nanoparticles have been electrodeposited on the surfaces of conductive indium tin oxide (ITO) modified silicon internal reflection elements. The resulting films are shown to be excellent platforms for attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) studies of palladium surfaces. Monitoring the mid-infrared reflectivity of the interface during the constant potential electrodepostion of a Pd2+ precursor reveals a distinct and reproducible minimum that corresponds to the onset of the electronic percolation threshold of the deposited metal islands as confirmed by scanning electron microscopy. Effective medium theory (EMT) is used to model the reflectivity of the Si/ITO/Pd interface as a function of the volume fraction of the deposited metal and to calculate the ATR-SEIRA spectra of an adsorbed monolayer of organic molecules. EMT calculations are in qualitative agreement with most aspects of the experimental spectra which show that the intensities and spectral line shapes are highly dependent on the amount of deposited palladium. The methodology is applied to the potential dependent adsorption of 4-methoxypyridine on palladium. The experimental results show that the pyridine derivative adopts an edge-tilted orientation on oxide covered Pd and undergoes an orientation change in the hydrogen adsorption region that increases both the degree of edge-tilt and the extent of π-bonding between the pyridine ring and the metal.

Microsecond Resolved Infrared Spectroelectrochemistry Using Dual Frequency Comb IR Lasers.

A dual infrared frequency comb spectrometer with heterodyne detection has been used to perform time-resolved electrochemical attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). The measurement of the potential dependent desorption of a monolayer of a pyridine derivative (4-dimethylaminopyridine, DMAP) with time resolution as high as 4 µs was achieved without the use of step-scan interferometry. An analysis of the detection limit of the method as a function of both time resolution and measurement coadditions is provided and compared to step-scan experiments of an equivalent system. Dual frequency comb spectroscopy is shown to be highly amenable to time-resolved ATR-SEIRAS. Microsecond resolved spectra can be obtained with high spectral resolution and fractional monolayer detection limits in a total experimental duration that is 2 orders of magnitude less than the equivalent step-scan experiment.

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.

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.

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.