Electrocatalytic Behavior of Pd and Pt Nanoislands Deposited onto 4,4′-Dithiodipyridine SAMs on Au(111)

Probing surface properties of organic molecular layers by scanning tunneling microscopy

In view of the relevance of organic thin layers in many fields, the fundamentals, growth mechanisms, and dynamics of thin organic layers, in particular thiol-based self-assembled monolayers (SAMs) on Au(111) are systematically elaborated. From both theoretical and practical perspectives, dynamical and structural features of the SAMs are of great intrigue. Scanning tunneling microscopy (STM) is a remarkably powerful technique employed in the characterization of SAMs. Numerous research examples of investigation about the structural and dynamical properties of SAMs using STM, sometimes combined with other techniques, are listed in the review. Advanced options to enhance the time resolution of STM are discussed. Additionally, we elaborate on the extremely diverse dynamics of various SAMs, such as phase transitions and structural changes at the molecular level. In brief, the current review is expected to supply a better understanding and novel insights regarding the dynamical events happening in organic SAMs and how to characterize these processes.

Hydrogen Peroxide Oxidation Reaction on a 4-Mercaptopyridine Self-Assembled Monolayer on Au(111) Metallized by Platinum Nanoislands

A systematic investigation of the hydrogen peroxide oxidation reaction (HPOR) in phosphate buffer (pH = 7.3) on an Au(111) single crystal modified with a 4-mercaptopyridine self-assembled monolayer (SAM) has been conducted before and after metallization with Pt. While bare Au(111) shows considerable electrocatalytic activity towards the HPOR, the inhibition of the oxidation reaction after modification with the SAM implies that adsorbed 4-mercaptopyridine molecules do not catalyze the HPOR. However, SAM-modified Au(111) recovers catalytic activity for the HPOR already after a single metallization step fabricating Pt islands on-top. Hydrogen peroxide (HP) may then either react at the (non-metallic) Pt nanoislands or on reactivated Au sites, made accessible by structural changes of the SAM induced by the metallization. The shape of the voltammetric profiles for the HPOR on repeatedly metallized SAMs suggests that the contribution of Au to the total current density gradually diminishes with increasing Pt coverage while the contribution of the Pt islands increases. The electrochemical behavior is dominated by the Pt islands at a coverage of 0.5 ML obtained by three subsequent metallization steps.

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Chemical-Structure Evolution Model for the Self-Assembling of Amine-Terminated Monolayers on Nanoporous Carbon-Doped Organosilicate in Tightly Controlled Environments

Amine-terminated self-assembled monolayers are molecular nanolayers, typically formed via wet-chemical solution on specific substrates for precision surface engineering or interface modification. However, homogeneous assembling of a highly ordered monolayer by the facile, wet method is rather tricky because it involves process parameters, such as solvent type, molecular concentration, soaking time and temperature, and humidity level. Here, we select 3-aminopropyltrimethoxysilane (APTMS) as a model molecule of aminosilane for the silanization of nanoporous carbon-doped organosilicate (p-SiOCH) under tightly controlled process environments. Surface mean roughness (R_a) and the water contact angle (θ) of the p-SiOCH layers upon silanization at a 10% humidity-controlled environment behave similarly and follow a three-stage evolution: a leap to a maximum at 15 min for R_a (from 0.227 to 0.411 nm) and θ (from 25 to 86°), followed by a gradual decrease to 0.225 nm and 69^o, finally leveling off at the above values (>60 min). The -NH₃⁺ fraction indicating monolayer disorientation evolves in a similar fashion. The fully grown monolayer is highly oriented yielding an unprecedented low -NH₃⁺ fraction of 0.08 (and 0.92 of upright -NH₂ groups). However, while having a similar thickness of approximately 1.4 ± 0.1 nm, the molecular layers grown at 30% relative humidity exhibit a significantly elevated -NH₃⁺ fraction of 0.42, indicating that controlling the humidity is vital to the fabrication of highly oriented APTMS molecular layers. A bonding-structure evolution model, as distinct from those offered previously, is proposed and discussed.

Density Functional Theory Study of Pd Aggregation on a Pyridine-Terminated Self-Assembled Monolayer

By using density functional theory calculations, the initial steps towards Pd metal cluster formation on a pyridine-terminated self-assembled monolayer (SAM) consisting of 3-(4-(pyridine-4-yl)phenyl)propane-1-thiol on an Au(1 1 1) surface are investigated. Theoretical modelling allows the investigation of structural details of the SAM surface and the metal/SAM interface at the atomic level, which is essential for elucidating the nature of Pd-SAM and Pd-Pd interactions at the liquid/solid interface and gaining insight into the mechanism of metal nucleation in the initial stage of electrodeposition. The structural flexibility of SAM molecules was studied first and the most stable conformation was identified, planar molecules in a herringbone packing, as the model for Pd adsorption. Two binding sites are found for Pd atoms on the pyridine end group of the SAM. The strong interaction between Pd atoms and pyridines illustrates the importance of SAM functionalisation in the metal nucleation process. Consistent with an energetic driving force of approximately -0.3 eV per Pd atom towards Pd aggregation suggested by static calculations, a spontaneous Pd dimerisation is observed in ab initio molecular dynamic studies of the system. Nudged elastic band calculations suggest a potential route with a low energy barrier of 0.10 eV for the Pd atom diffusion and then dimerisation on top of the SAM layer.

Coordination controlled electrodeposition and patterning of layers of palladium/copper nanoparticles on top of a self-assembled monolayer

A scheme for the generation of bimetallic nanoparticles is presented which combines electrodeposition of one type of metal, coordinated to a self-assembled monolayer (SAM), with another metal deposited from the bulk electrolyte. In this way PdCu nanoparticles are generated by initial complexation of Pd²⁺ to a SAM of 3-(4-(pyridine-4-yl)phenyl)propane-1-thiol (PyP3) on Au/mica and subsequent reduction in an acidic aqueous CuSO₄ electrolyte. Cyclic voltammetry reveals that the onset of Cu deposition is triggered by Pd reduction. Scanning tunneling microscopy (STM) shows that layers of connected particles are formed with an average thickness of less than 3 nm and lateral dimensions of particles in the range of 2 to 5 nm. In X-ray photoelectron spectra a range of binding energies for the Pd 3d signal is observed whereas the Cu 2p signal appears at a single binding energy, even though chemically different Cu species are present: normal and more noble Cu. Up to three components are seen in the N 1s signal, one originating from protonated pyridine moieties, the others reflecting the SAM-metal interaction. It is suggested that the coordination controlled electrodeposition yields layers of particles composed of a Pd core and a Cu shell with a transition region of a PdCu alloy. Deposited on top of the PyP3 SAM, the PdCu particles exhibit weak adhesion which is exploited for patterning by selective removal of particles employing scanning probe techniques. The potential for patterning down to the sub-10 nm scale is demonstrated. Harnessing the deposition contrast between native and PdCu loaded PyP3 SAMs, structures thus created can be developed into patterned continuous layers.