The influence of steps on the deposition of Cu onto Au(111)

Unconventional Electrochemical Behaviors of Cu Underpotential Deposition in a Chloride-Based Deep Eutectic Solvent: High Underpotential Shift and Low Coverage

The (5 × 5) Moiré pattern resulting from coadsorption of Cu atoms and chloride ions on the Au(111) electrode is one of the most classical structures for underpotential deposition (UPD) in electrochemical surface science. Although two models have been proposed to describe the pattern, the details of the structure remain ambiguous and controversial, leading to a question that remains to be answered. In this work, we investigate the UPD behaviors of Cu on the Au(111) electrode in a chloride-based deep eutectic solvent ethaline by in situ scanning tunneling microscopy (STM). Benefiting from the properties of the ultraconcentrated electrolyte, we directly image not only Cu but also Cl adlayers by finely tuning tunneling conditions. The structure is unambiguously determined for both Cu and Cl adlayers, where an incommensurate Cu layer is adsorbed on the Au(111) surface with a Cu coverage of 0.64, while the Cl coverage is 0.32 (only half of the expected value); i.e., the atomic arrangement of the observed (5 × 5) Moiré pattern in ethaline matches neither of the models proposed in the literature. Meanwhile, STM results confirm the origin of the cathodic peak in the cyclic voltammogram, which indicates that the underpotential shift of Cu UPD in ethaline indeed increases by ca. 0.40 V compared to its counterpart in a sulfuric acid solution, resulting in a significant deviation from the linear relation between the underpotential shift and the difference in work functions proposed in the literature. The unconventional electrochemical behaviors of Cu UPD reveal the specialty of both the bulk and the interface in the chloride-based deep eutectic solvent.

Electro-Design of Bimetallic PdTe Electrocatalyst for Ethanol Oxidation: Combined Experimental Approach and Ab Initio Density Functional Theory (DFT)-Based Study

An alternative electrosynthesis of PdTe, using the electrochemical atomic layer deposition (E-ALD) method, is reported. The cyclic voltammetry technique was used to analyze Au substrate in copper (Cu²⁺), and a tellurous (Te⁴⁺) solution was used to identify UPDs and set the E-ALD cycle program. Results obtained using atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques reveal the nanometer-sized flat morphology of the systems, indicating the epitaxial characteristics of Pd and PdTe nanofilms. The effect of the Pd:Te ratio on the crystalline structure, electronic properties, and magnetic properties was investigated using a combination of density functional theory (DFT) and X-ray diffraction techniques. Te-containing electrocatalysts showed improved peak current response and negative onset potential toward ethanol oxidation (5 mA; -0.49 V) than Pd (2.0 mA; -0.3 V). Moreover, DFT ab initio calculation results obtained when the effect of Te content on oxygen adsorption was studied revealed that the d-band center shifted relative to the Fermi level: -1.83 eV, -1.98 eV, and -2.14 eV for Pd, Pd₃Te, and Pd₃Te₂, respectively. The results signify the weakening of the CO-like species and the improvement in the PdTe catalytic activity. Thus, the electronic and geometric effects are the descriptors of Pd₃Te₂ activity. The results suggest that Pd₂Te₂ is a potential candidate electrocatalyst that can be used for the fabrication of ethanol fuel cells.

In situ lattice tuning of quasi-single-crystal surfaces for continuous electrochemical modulation

The ability to control the atomic-level structure of a solid represents a straightforward strategy for fabricating high-performance catalysts and semiconductor materials. Herein we explore the capability of the mechanically controllable surface strain method in adjusting the surface structure of a gold film. Underpotential deposition measurements provide a quantitative and ultrasensitive approach for monitoring the evolution of surface structures. The electrochemical activities of the quasi-single-crystalline gold films are enhanced productively by controlling the surface tension, resulting in a more positive potential for copper deposition. Our method provides an effective way to tune the atom arrangement of solid surfaces with sub-angstrom precision and to achieve a reduction in power consumption, which has vast applications in electrocatalysis, molecular electronics, and materials science.

We reported a new method capable of adjusting the lattice structure of solid surfaces with sub-angstrom precision and achieved in situ and continuous control over electrochemical activity.

Effect of substrate symmetry on the orientations of MoS2 monolayers

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are promising platforms for developing next-generation electronic and optoelectronic devices due to their unique properties. To achieve this, the growth of large single-crystal TMDs is a critical issue. Unraveling the factors affecting the nucleation and domain orientation should hold fundamental significance. Herein, we design the chemical vapor deposition growth of monolayer MoS₂ triangles on Au(111) and Au(100) facets, for exploring the substrate facet effects on the domain orientations. According to multi-scale characterizations, we find that, the obtained triangular MoS₂ domains present two preferential orientations on the six-fold symmetric Au(111) facet, whereas four predominant orientations on the four-fold symmetric Au(100) facet. Using on-site scanning tunneling microscopy, we further reveal the preferred alignments of monolayer MoS₂ triangles along the close-packed directions of both Au(111) and Au(100) facets. Moreover, bunched substrate steps are also found to form along the close-packed directions of the crystal facets, which guides the preferential nucleation of monolayer MoS₂ along the step edges. This work should hereby deepen the understanding of the substrate facet/step effect on the nucleation and orientation of monolayer MoS₂ domains, thus providing fundamental insights into the controllable syntheses of large single-crystal TMD monolayers.

Probing the Surface of Noble Metals Electrochemically by Underpotential Deposition of Transition Metals

The advances in material science have led to the development of novel and various materials as nanoparticles or thin films. Underpotential deposition (upd) of transition metals appears to be a very sensitive method for probing the surfaces of noble metals, which is a parameter that has an important effect on the activity in heterogeneous catalysis. Underpotential deposition as a surface characterization tool permits researchers to precisely determine the crystallographic orientations of nanoparticles or the real surface area of various surfaces. Among all the work dealing with upd, this review focuses specifically on the main upd systems used to probe surfaces of noble metals in electrocatalysis, from poly‒ and single-crystalline surfaces to nanoparticles. Cuupd is reported as a tool to determine the active surface area of gold‒ and platinum‒based bimetallic electrode materials. Pbupd is the most used system to assess the crystallographic orientations on nanoparticles’ surface. In the case of platinum, Bi and Ge adsorptions are singled out for probing (1 1 1) and (1 0 0) facets, respectively.

Voltammetric study of conductive planar assemblies of Geobacter nanowire pilins unmasks their ability to bind and mineralize divalent cobalt

Geobacter bacteria assemble a helical peptide of the Type IVa pilin subclass as conductive pili decorated with metal binding and reduction sites. We used recombinant techniques to synthesize thiolated pilin derivatives and self-assembled them on gold electrodes as a monolayer that concentrated the metal traps at the liquid interface. Cyclic and step potential voltammetry demonstrated the conductivity of the pilin films and their ability to bind and reductively precipitate divalent cobalt (Co²⁺) in a diffusion-controlled reaction characterized by fast binding kinetics, efficient charge transfer, and three-dimensional nanoparticle growth at discreet sites. Furthermore, cobalt oxidation at the pilin film was slower than on bare gold, consistent with a peptide optimized for metal immobilization. These properties make recombinant pilins attractive building blocks for the synthesis of novel biomaterials for the immobilization of toxic cationic metals that, like Co²⁺, are sparingly soluble and, thus, less mobile and bioavailable as reduced species.

Underpotential deposition of Cu on Au(111) from neutral chloride containing electrolyte

The structure of a chloride terminated copper monolayer electrodeposited onto Au(111) from a CuSO₄/KCl electrolyte was investigated ex situ by three complementary experimental techniques (scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), X-ray standing wave (XSW) excitation) and density functional theory (DFT) calculations. STM at atomic resolution reveals a stable, highly ordered layer which exhibits a Moiré structure and is described by a (5 × 5) unit cell. The XSW/PES data yield a well-defined position of the Cu layer and the value of 2.16 Å above the topmost Au layer suggests that the atoms are adsorbed in threefold hollow sites. The chloride exhibits some distribution around a distance of 3.77 Å in agreement with the observed Moiré pattern due to a higher order commensurate lattice. This structure, a high order commensurate Cl overlayer on top of a commensurate (1 × 1) Cu layer with Cu at threefold hollow sites, is corroborated by the DFT calculations.

On a New Phase Transition in the Cu/Au(111)/H2SO4 System

A new pair of current spikes has been found in the cyclic voltammogram of Au(111) in an acidic copper sulphate solution, indicative of a phase transition in the Cu adlayer. The spike potential is well inside the bulk deposition region and hence, the new feature can be observed only at low Cu++ concentration, when bulk deposition is slow. The charge under the spike correlates linearly with extent of the (√3 × √3)R30° structure, up to 40μCcm-2 for the fully developed superstructure. The spike potential shifts with the sulphate concentration and hence, this feature is assigned to the desorption of sulphate from the well-known honey comb structure of the Cu upd layer.

Controlled growth of mesostructured crystalline iron oxide nanowires and Fe-filled carbon nanotube arrays templated by mesoporous silica SBA-16 film

The three-dimensional (3D) accessible pore structures (Imm space groups) of continuous mesoporous silica SBA-16 thin films have been prepared by a dip-coating technique in nonaqueous media under acidic conditions on indium-tin oxide glass (ITO). The films are oriented with the (111) crystal plane perpendicular to the surface of the film. On one hand, deposition of iron metal into the mesopores of SBA-16 films was achieved by using an electrochemical method. The Fe2O3 nanowire arrays were synthesized. The crystalline structures of porous Fe2O3 nanowires and nanorods were studied via TEM, SEM, and XRD. On the other hand, a small amount of Fe was deposited into the pores of the SBA-16 thin film as a catalyst, and carbon nanotube arrays formed inside the pores of SBA-16 film were fabricated by catalytic decomposition of acetylene at 700 degrees C. The second-order template synthesis method for preparing the ordered array of carbon nanotubes filled with Fe has been used. The carbon nanotubes are very uniform in diameter and length and are aligned vertically with respect to the SBA-16 film.

In Situ Study of the Growth Kinetics of Individual Island Electrodeposition of Copper

The growth kinetics for individual islands during electrodeposition of copper have been studied using in situ transmission electron microscopy. We show that for sufficiently large overpotentials, the growth kinetics approach the rate laws expected for diffusion-limited growth of hemispherical islands, characterized by two distinct regimes. At short times, the island growth exponent is 0.5 as expected for diffusion-limited growth of uncoupled hemispherical islands, while at longer times, the growth exponent approaches 1/6 as expected for planar diffusion to the growing islands. These results provide the first direct measurements of the growth of individual islands during electrochemical deposition. However, quantitative comparison with rate laws shows that the island radii are smaller than predicted and the island densities are much larger than predicted, and we suggest that this is related to adatom formation and surface diffusion, processes which are not included in conventional growth models.

Dynamic microscopy of nanoscale cluster growth at the solid-liquid interface

Dynamic processes at the solid-liquid interface are of key importance across broad areas of science and technology. Electrochemical deposition of copper, for example, is used for metallization in integrated circuits, and a detailed understanding of nucleation, growth and coalescence is essential in optimizing the final microstructure. Our understanding of processes at the solid-vapour interface has advanced tremendously over the past decade due to the routine availability of real-time, high-resolution imaging techniques yielding data that can be compared quantitatively with theory. However, the difficulty of studying the solid-liquid interface leaves our understanding of processes there less complete. Here we analyse dynamic observations--recorded in situ using a novel transmission electron microscopy technique--of the nucleation and growth of nanoscale copper clusters during electrodeposition. We follow in real time the evolution of individual clusters, and compare their development with simulations incorporating the basic physics of electrodeposition during the early stages of growth. The experimental technique developed here is applicable to a broad range of dynamic phenomena at the solid-liquid interface.