Crystallographic dependence of 4-dimethylaminopyridine electrosorption on gold

Template-Free Electroless Plating of Gold Nanowires: Direct Surface Functionalization with Shape-Selective Nanostructures for Electrochemical Applications

Metal nanowires (NWs) represent a prominent nanomaterial class, the interest in which is fueled by their tunable properties as well as their excellent performance in, for example, sensing, catalysis, and plasmonics. Synthetic approaches to obtain metal NWs mostly produce colloids or rely on templates. Integrating such nanowires into devices necessitates additional fabrication steps, such as template removal, nanostructure purification, or attachment. Here, we describe the development of a facile electroless plating protocol for the direct deposition of gold nanowire films, requiring neither templates nor complex instrumentation. The method is general, producing three-dimensional nanowire structures on substrates of varying shape and composition, with different seed types. The aqueous plating bath is prepared by ligand exchange and partial reduction of tetrachloroauric acid in the presence of 4-dimethylaminopyridine and formaldehyde. Gold deposition proceeds by nucleation of new grains on existing nanostructure tips and thus selectively produces curvy, polycrystalline nanowires of high aspect ratio. The nanofabrication potential of this method is demonstrated by producing a sensor electrode, whose performance is comparable to that of known nanostructures and discussed in terms of the catalyst architecture. Due to its flexibility and simplicity, shape-selective electroless plating is a promising new tool for functionalizing surfaces with anisotropic metal nanostructures.

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.