Electrochemical defect-mediated thin-film growth

Epitaxial Electrodeposition of Ordered Inorganic Materials

ConspectusThe quality of technological materials generally improves as the crystallographic order is increased. This is particularly true in semiconductor materials, as evidenced by the huge impact that bulk single crystals of silicon have had on electronics. Another approach to producing highly ordered materials is the epitaxial growth of crystals on a single-crystal surface that determines their orientation. Epitaxy can be used to produce films and nanostructures of materials with a level of perfection that approaches that of single crystals. It may be used to produce materials that cannot be grown as large single crystals due to either economic or technical constraints. Epitaxial growth is typically limited to ultrahigh vacuum (UHV) techniques such as molecular beam epitaxy and other vapor deposition methods. In this Account, we will discuss the use of electrodeposition to produce epitaxial films of inorganic materials in aqueous solution under ambient conditions. In addition to lower capital costs than UHV deposition, electrodeposition offers additional levels of control due to solution additives that may adsorb on the surface, solution pH, and, especially, the applied overpotential. We show, for instance, that chiral morphologies of the achiral materials CuO and calcite can be produced by electrodepositing the materials in the presence of chiral agents such as tartaric acid.Inorganic compound materials are electrodeposited by an electrochemical-chemical mechanism in which solution precursors are electrochemically oxidized or reduced in the presence of molecules or ions that react with the redox product to form an insoluble species that deposits on the electrode surface. We present examples of reaction schemes for the electrodeposition of transparent hole conductors such as CuI and CuSCN, the magnetic material Fe₃O₄, oxygen evolution catalysts such as Co(OH)₂, CoOOH, and Co₃O₄, and the n-type semiconducting oxide ZnO. These materials can all be electrodeposited as epitaxial films or nanostructures onto single-crystal surfaces. Examples of epitaxial growth are given for the growth of films of CuI(111) on Si(111) and nanowires of CuSCN(001) on Au(111). Both are large mismatch systems, and the epitaxy is explained by invoking coincidence site lattices in which x unit meshes of the film overlap with y unit meshes of the substrate.We also discuss the epitaxial lift-off of single-crystal-like foils of metals such as Au(111) and Cu(100) that can be used as flexible substrates for the epitaxial growth of semiconductors. The metals are grown on a Si wafer with a sacrificial SiO_x interlayer that can be removed by chemical etching. The goal is to move beyond the planar structure of conventional Si-based chips to produce flexible electronic devices such as wearable solar cells, sensors, and flexible displays. A scheme is shown for the epitaxial lift-off of wafer-scale foils of the transparent hole conductor CuSCN.Finally, we offer some perspectives on possible future work in this area. One question we have not answered in our previous work is whether these epitaxial films and nanostructures can be grown with the level of perfection that is achieved in UHV. Another area that is ripe for exploration is the epitaxial electrodeposition of metal-organic framework materials from solution precursors.

Electrochemical Deposition of a Single-Crystalline Nanorod Polycyclic Aromatic Hydrocarbon Film with Efficient Charge and Exciton Transport

Electrochemical deposition has emerged as an efficient technique for preparing conjugated polymer films on electrodes. However, this method encounters difficulties in synthesizing crystalline products and controlling their orientation on electrodes. Here we report electrochemical film deposition of a large polycyclic aromatic hydrocarbon. The film is composed of single‐crystalline nanorods, in which the molecules adopt a cofacial stacking arrangement along the π–π direction. Film thickness and crystal size can be controlled by electrochemical conditions such as scan rate and electrolyte species, while the choice of anode material determines crystal orientation. The film supports exceptionally efficient migration of both free carriers and excitons: the free carrier mobility reaches over 30 cm² V⁻¹ s⁻¹, whereas the excitons are delocalized with a low binding energy of 118.5 meV and a remarkable exciton diffusion length of 45 nm.

Hierarchical nanotwins in single-crystal-like nickel with high strength and corrosion resistance produced via a hybrid technique

High-density growth nanotwins enable high-strength and good ductility in metallic materials. However, twinning propensity is greatly reduced in metals with high stacking fault energy. Here we adopted a hybrid technique coupled with template-directed heteroepitaxial growth method to fabricate single-crystal-like, nanotwinned (nt) Ni. The nt Ni primarily contains hierarchical twin structures that consist of coherent and incoherent twin boundary segments with few conventional grain boundaries. In situ compression studies show the nt Ni has a high flow strength of ∼2 GPa and good deformability. Moreover, the nt Ni has superb corrosion behavior due to the unique twin structure in comparison to coarse grained and nanocrystalline counterparts. The hybrid technique opens the door for the fabrication of a wide variety of single-crystal-like nt metals with unique mechanical and chemical properties.

Electroless Deposition of Pb Monolayer: A New Process and Application to Surface Selective Atomic Layer Deposition

The present work demonstrates an electroless (e-less) deposition of Pb monolayer on Au and Cu surface whose morphology and properties resemble its underpotentially deposited counterpart. Our results and analysis show that the e-less Pb monolayer deposition is a surface selective, surface controlled, self-terminating process. Results also show that the electroless Pb monolayer deposition is enabling a phenomenon for new deposition method called "electroless atomic layer deposition" (e-less ALD). Here, the e-less Pb monolayer serves as reducing agent and sacrificial material in surface limited redox replacement reaction with noble metal ions such as Pt ⁿ⁺, i.e., Pt deposition. The e-less ALD is highly selective to the metal substrates at which Pb forms the e-less monolayer. The full e-less ALD cycle leads to an overall deposition of a controlled amount of the noble metal. Repetition of the two-step e-less ALD cycle an arbitrary number of times leads to formation of a highly compact, smooth, and conformal noble metal thin film with applications spanning from catalyst synthesis to semiconductor technology. The process is designed for (but not limited to) aqueous solutions that can be easily scaled up to any size and shape of the substrate, deeming its wide applications.

Anion insertion enhanced electrodeposition of robust metal hydroxide/oxide electrodes for oxygen evolution

Electrochemical deposition is a facile strategy to prepare functional materials but suffers from limitation in thin films and uncontrollable interface engineering. Here we report a universal electrosynthesis of metal hydroxides/oxides on varied substrates via reduction of oxyacid anions. On graphitic substrates, we find that the insertion of nitrate ion in graphene layers significantly enhances the electrodeposit–support interface, resulting in high mass loading and super hydrophilic/aerophobic properties. For the electrocatalytic oxygen evolution reaction, the nanocrystalline cerium dioxide and amorphous nickel hydroxide co-electrodeposited on graphite exhibits low overpotential (177 mV@10 mA cm⁻²) and sustains long-term durability (over 300 h) at a large current density of 1000 mA cm⁻². In situ Raman and operando X-ray diffraction unravel that the integration of cerium promotes the formation of electrocatalytically active gamma-phase nickel oxyhydroxide with exposed (003) facets. Therefore, combining anion intercalation with cathodic electrodeposition allows building robust electrodes with high electrochemical performance.

Electrodeposition provides a facile fabrication means for electrochemical devices but weak substrate-deposit interactions cause poor performance. Here, authors utilize anion insertion within graphitic layers to improve the material interfaces and construct highly active O₂-evolving electrocatalysts.

Application of screen-printed carbon electrode modified with lead in stripping analysis of Cd(II)

In the work presented, a lead film electrode was prepared in situ on a screen-printed carbon support using a reversibly deposited mediator (Zn) and applied to the determination of Cd(II) by anodic stripping voltammetry. The electrochemical method for lead film formation is based on a co-deposition of a metal of interest (Pb), with a reversibly deposited zinc mediator, followed by oxidation of zinc, with additional deposition of lead at the appropriate potential. It serves to increase the density of lead particles, promoting lead film growth, and consequently helps to improve the electrochemical properties of the electrode. This was confirmed by microscopic and voltammetric studies. The obtained detection limit of Cd(II) is equal to 6.6 × 10−9 mol L−1 (−1.6 V for 180 s and then −0.95 V for 5 s). The presented procedure was successfully applied to cadmium determination in Bystrzyca River water samples.

Nanoscale electrodeposition of low-dimensional metal phases and clusters

The present status of the problem of electrochemical formation of low-dimensional metal phases is reviewed. The progress in this field achieved in the last two decades is discussed on the basis of experimental results obtained in selected electrochemical systems with well defined single crystal substrates. The influence of crystallographic orientation and surface inhomogeneities of foreign substrates on the mechanism of formation and the atomic structure of two-dimensional (2D) metal phases in the underpotential deposition range is considered. The localized electrodeposition of metal nanoclusters on solid state surfaces applying the STM-tip as a nanoelectrode is demonstrated.

2D ultrathin core-shell Pd@Pt(monolayer) nanosheets: defect-mediated thin film growth and enhanced oxygen reduction performance

An operational strategy for the synthesis of atomically smooth Pt skin by a defect-mediated thin film growth method is reported. Extended ultrathin core-shell structured d@Pt(monolayer) nanosheets (thickness below 5 nm) exhibit nearly seven-fold enhancement in mass-activity and surprisingly good durability toward oxygen reduction reaction as compared with the commercial Pt/C catalyst.

A hydrogel pen for electrochemical reaction and its applications for 3D printing

A hydrogel pen consisting of a microscopic pyramid containing an electrolyte offers a localized electroactive area on the nanometer scale via controlled contact of the apex with a working electrode. The hydrogel pen merges the fine control of atomic force microscopy with non-linear diffusion of an ultramicroelectrode, producing a faradaic current that depends on the small electroactive area. The theoretical and experimental investigations of the mass transport behavior within the hydrogel reveal that the steady-state current from the faradaic reaction is linearly proportional to the deformed length of the hydrogel pen by contact, i.e. signal transduction of deformation to an electrochemical signal, which enables the fine control of the electroactive area in the nanometer-scale regime. Combined with electrodeposition, localized electrochemistry of the hydrogel pen results in the ability to fabricate small sizes (110 nm in diameter), tall heights (up to 30 μm), and arbitrary structures, thereby indicating an additive process in 3 dimensions by localized electrodeposition.

Lead Underpotential Deposition on Pt-submonolayer Modified Au(111)

Lead underpotential deposition on Au(111) surface modified with submonolayer of Pt is studied using cyclic voltammetry and in situ scanning tunneling microscopy methods. The two-dimensional Pt submonolayers (nanoclusters) on Au(111) were obtained by spontaneous Pt deposition on Au(111) from × 103 M {PtCl6}2− + 0.1 M HClO4 solution. The in situ scanning tunneling microscopy data were analyzed using statistical image processing algorithm which enabled quantification of the morphology change on Pt-modified Au(111) surface as a function of applied underpotential. The results suggest that Pb underpotential deposition starts on Au steps and other surface defects, similar to Pb underpotential deposition on Au(111). The further process proceeds by Pb monolayer nucleation and growth on Au terraces into a complete layer. In parallel, the Pb monolayer starts to nucleate on top of the Pt nanoclusters. The final stage of the Pb underpotential deposition is formation of the compact Pb nanoclusters/layer on top of the pre-existing Pt nanoclusters. The scanning tunneling microscopy data suggests that morphology of underpotentially deposited Pb monolayer on Pt-modified Au(111) is similar to the starting surface in terms of the areal density of nanoclusters, their size and shape. The morphological changes of the Pt modified Au(111) surface during Pb underpotential deposition are correlated with cyclic voltammetry results.

Processing of nanoporous Ag layers by potential-controlled displacement (PCD) of Cu

A cementation-like process taking place under potential control and introduced in this work as a "potential-controlled displacement" (PCD) is developed as a new method for processing of nanoporous Ag structures with controlled roughness (porosity) length scales. Most of the development work is done in a deoxygenated electrolyte containing 1 x 10(-3) M AgClO(4 )+ 5 x 10(-2) M CuSO(4) + 1 x 10(-1) M HClO(4) using a copper rotating disk electrode at 50 rpm. At this electrolyte concentration, the Ag deposition is under diffusion limitations whereas the Cu dissolution displays a typical Butler-Volmer anodic behavior. Thus, a careful choice of the operational current density enables strict control of the ratio between the dissolving and depositing metals as ascertained independently by atomic absorption spectrometry (AAS). The roughness length scale of the resulting surfaces is controlled by a careful selection of the current density applied. The highest surface area and finest morphology is obtained when the atomic ratio of Ag deposition and Cu dissolution becomes 1:1. Preseeding of uniform Ag clusters on the Cu surface made by pulse plating of Ag along with complementary plating and stripping of Pb monolayer is found to yield finer length scale resulting in up to a 67% higher surface area. An electrochemical technique using as a reference value the charge of an underpotentially deposited Pb layer on a flat Ag surface is used for measuring the real surface area. Scanning electron microscopy (SEM) studies are conducted to examine and characterize the deposit morphology of Ag grown by PCD on Cu substrates.

Mechanism of film growth of tellurium by electrochemical deposition in the presence and absence of cadmium ions

The growth morphology and the kinetics of a thin film of Te on Au during electrochemical deposition at -62 mV (vs Ag/AgCl/3 M NaCl) have been studied. The deposition conditions are similar to those used previously by us to grow nanowires inside Au nanotubes by electrochemical deposition in the presence of Cd ions (Cd(2+)). By using electrochemical deposition on a planar Au electrode, we explored the growth of the Te film for two conditions: in the presence of Cd(2+) (0.1 mM TeO(2) + 1 mM CdSO(4) + 50 mM H(2)SO(4) solution) and in the absence of Cd(2+) (0.1 mM TeO(2) + 50 mM H(2)SO(4) solution). We used several surface investigation techniques to study the growth such as: in situ electrochemical atomic force microscopy (EC-AFM), in situ electrochemical surface plasmon resonance (EC-SPR), electrochemical methods, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). In the presence of Cd(2+), in situ electrochemical atomic microscopy showed that Cd(2+) acted as a mediator at the early deposition stage and caused smoothing of the Te deposit obtained. In the absence of Cd(2+), Te had an island growth. The electrochemical surface plasmon resonance showed that the deposit was characterized by a slower deposition rate in the presence of Cd(2+) than in the absence of Cd(2+). Additionally, the thickness of the film was evaluated using EC-AFM measurements, electrochemical stripping analysis, and EC-SPR. The results obtained from all three measurements agree well with the Te film obtained in the presence of Cd(2+), where a continuous and uniform film was formed. In the presence of Cd(2+), a Te film with a thickness of 1.04 nm and atomically flat surface was deposited on an ultraflat Au surface. The XPS spectrum showed no significant amount of Cd in the deposit, indicating that the Cd ion acted as a mediator and not as a co-deposition element.

Electrodeposition of Au monolayer on Pt(111) mediated by self-assembled monolayers

In-situ scanning tunneling microscopy (STM), cyclic voltammetry (CV), and infrared reflection-adsorption spectroscopy (IRRAS) have been used to examine the electrodeposition of gold onto Pt(111) electrodes modified with benzenethiol (BT) and benzene-1,2-dithiol (BDT) in 0.1 M HClO4 containing 10 microM HAuCl4. Both BT and BDT were attached to Pt(111) via one sulfur headgroup. STM and IRRAS results indicated that the other SH group of BDT was pendant in the electrolyte. Both BT and BDT formed (2 x 2) structures at the coverage of 0.25, and they were transformed into (square root(3) x square root(3))R30 degrees as the coverage was raised to 0.33. These two organic surface modifiers resulted in 3D and 2D gold islands at BT- and BDT-coated Pt(111) electrodes, respectively. The pendant SH group of BDT could interact specifically with gold adspecies to immobilize gold adatoms on the Pt(111) substrate, which yields a 2D growth of gold deposition. Molecular resolution STM revealed an ordered array of (6 x 2 square root(13)) after a full monolayer of gold was plated on the BDT/Pt(111) electrode. Since BDT was strongly adsorbed on Pt(111), gold adatoms only occupied free sites between BDT admolecules on Pt(111). This is supported by a stripping voltammetric analysis, which reveals no reductive desorption of BDT admolecules at a gold-deposited BDT/Pt(111) electrode. It seems that the BDT adlayer acted as the template for gold deposit on Pt(111). In contrast, a BT adlayer yielded 3D gold deposit on Pt(111). This study demonstrates unambiguously that organic surface modifiers could contribute greatly to the electrodeposition of metal adatoms.

Growth of ultrathin films of cadmium telluride and tellurium as studied by electrochemical atomic force microscopy

Time dependent, cathodic electrodeposition of ultrathin CdTe and Te films has been studied in 50 mM H(2)SO(4) + 1 mM CdSO(4) + 0.1 mM TeO(2) solutions at room temperature under potential control using electrochemical atomic force microscopy (EC-AFM). The films were also characterized electrochemically and with X-ray diffraction. The growth mechanism and the composition of the films depends on the applied potentials. Island-like growth mode was observed for CdTe films when the deposition potential was -0.35 V (SHE). At a more positive deposition potential of 0.138 V (SHE), Cd was not co-deposited into the film but affected the dynamic growth mode of the deposit. At this voltage smooth Te films were obtained. Depending on the applied potential, Cd acts either as a co-deposition element for CdTe film growth, or as a mediator for layer-by-layer growth of Te films.

Interface dynamics for copper electrodeposition: the role of organic additives in the growth mode

An atomistic model for Cu electrodeposition under nonequilibrium conditions is presented. Cu electrodeposition takes place with a height-dependent deposition rate that accounts for fluctuations in the local Cu2+ ions concentration at the interface, followed by surface diffusion. This model leads to an unstable interface with the development of protrusions and grooves. Subsequently the model is extended to account for the presence of organic additives, which compete with Cu2+ for adsorption at protrusions, leading to a stable interface with scaling exponents consistent with those of the Edwards-Wilkinson equation. The model reproduces the interface evolution experimentally observed for Cu electrodeposition in the absence and in the presence of organic additives.

Nonstandard roughness of terraced surfaces

We present a class of surfaces which are simultaneously flat and rough over the same range of lengths. They are self-affine, have well-defined roughness exponents, and consist of terraces of all sizes. The coexistence of flat and rough makes them respond to different external interactions with variable roughness. We demonstrate this for optical scattering (including x rays), two wetting situations, diffusion currents, and catalysis. A terraced Cu surface is a "self-assembled" experimental example, and designs for nano- and micromachined examples are presented.