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 SiOx 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.

Epitaxial Electrodeposition of Chiral Metal Surfaces on Silicon(643).

Surfaces of achiral materials exhibit two-dimensional chirality if they lack mirror symmetry. An example is the (643) surface of face-centered-cubic metals such as Au. The (643) and (6̅4̅3̅) surfaces are non-superimposable mirror images of each other. Chiral surfaces offer the possibility of serving as heterogeneous catalysts for chiral synthesis or providing a platform for chiral separation or crystallization. Here, we show the symmetry requirements for surface chirality, and we demonstrate that chiral surfaces can be produced by electrochemically depositing epitaxial films of Au onto commercially available Si(643) wafers. Au(643) is deposited onto one side of the wafer, and its enantiomer Au(6̅4̅3̅) is deposited on the other side of the wafer. In addition to the (643) orientation, the (8 14 17) orientation of Au is produced on the Si(643) wafers. The (8 14 17) orientation has a similar kinked surface to the (643) surface, but it has staggered kinks. Other metal films including Pt, Ni, Cu, and Ag that are electrodeposited onto the Au films exhibit strong in-plane and out-of-plane order. Hence, the method provides a pathway for producing chiral surfaces of a wide range of materials, and it obviates the need to work with expensive single crystals. The Ag/Au/Si(643) surface showed a preference for the electrochemical oxidation of d-glucose, whereas the Ag/Au/Si(6̅4̅3̅) surface showed preference for the oxidation of l-glucose.

An electrodeposited inhomogeneous metal-insulator-semiconductor junction for efficient photoelectrochemical water oxidation.

The photoelectrochemical splitting of water into hydrogen and oxygen requires a semiconductor to absorb light and generate electron-hole pairs, and a catalyst to enhance the kinetics of electron transfer between the semiconductor and solution. A crucial question is how this catalyst affects the band bending in the semiconductor, and, therefore, the photovoltage of the cell. We introduce a simple and inexpensive electrodeposition method to produce an efficient n-Si/SiOx/Co/CoOOH photoanode for the photoelectrochemical oxidation of water to oxygen. The photoanode functions as a solid-state, metal-insulator-semiconductor photovoltaic cell with spatially non-uniform barrier heights in series with a low overpotential water-splitting electrochemical cell. The barrier height is a function of the Co coverage; it increases from 0.74 eV for a thick, continuous film to 0.91 eV for a thin, inhomogeneous film that has not reached coalescence. The larger barrier height leads to a 360 mV photovoltage enhancement relative to a solid-state Schottky barrier.

Epitaxial Single-Domain Cu-BTC Metal-Organic Framework Thin Films and Foils by Electrochemical Conversion of Cuprous Oxide.

Metal-organic frameworks (MOFs) are an important class of crystalline porous materials with extensive chemical and structural merits. However, the fabrication of MOF thin films oriented along all crystallographic axes to achieve well-aligned nanopores and nanochannels with uniform apertures remains a challenge. Here, we achieved highly crystalline single-domain MOF thin films with the [111] out-of-plane orientation by electrochemical conversion of cuprous oxide. Copper(II)-benzene-1,3,5-tricarboxylate, Cu3(BTC)2 (referred to as Cu-BTC), is a well-known metal-organic open framework material with a cubic crystal system. Epitaxial Cu-BTC(111) thin films were manufactured by electrochemical oxidation of Cu2O(111) films electrodeposited on single-crystal Au(111). The Cu-BTC(111) shows an in-plane antiparallel relationship with the precursor Cu2O(111) with a -0.91% coincidence site lattice mismatch. A plausible mechanism was proposed for the electrochemical conversion of Cu2O into Cu-BTC, indicating formation of intermediate CuO, growth of Cu-BTC islands, and termination with coalesce into a dense film with a limiting thickness of about 740 nm. The Faradaic efficiency for the electrochemical conversion was 63%. In addition, epitaxial Cu-BTC(111) foils were fabricated by epitaxial lift-off following the electrochemical etching of residual Cu2O underneath the Cu-BTC. It was also demonstrated that Cu-BTC(111) films with two in-plane domains and textured Cu-BTC(111) films can be achieved on a large scale using electrodeposited Au/Si and Au-coated glass as low-cost substrates.

Room-temperature electrochemical reduction of epitaxial magnetite films to epitaxial iron films.

The electrochemical reduction of oxides to metals has been studied for decades. Earlier work produced polycrystalline bulk metals. Here, we report that pre-electrodeposited epitaxial face-centered cubic magnetite thin films can be electrochemically reduced to epitaxial body-centered cubic iron thin films in aqueous solution on single-crystalline gold substrates at room temperature. This technique opens new possibilities to produce special epitaxial metal/metal oxide heterojunctions and a wide range of epitaxial metallic alloy films from the corresponding mixed metal oxides.

Resistance switching in electrodeposited magnetite superlattices.

Defect-chemistry magnetite superlattices and compositional superlattices in the magnetite/zinc ferrite system are electrodeposited as epitaxial films onto single-crystal Au(111). The defect-chemistry superlattices have alternating nanolayers with different Fe(III)/Fe(II) ratios, whereas the compositional superlattices have alternating nanolayers with different Zn/Fe ratios. The electrochemical/chemical (EC) nature of the electrodeposition reaction is exploited to deposit the superlattices by pulsing the applied potential during deposition. The defect-chemistry superlattices show low-to-high and high-to-low resistance switching that may be applicable to the fabrication of resistive random access memory (RRAM).

Enantiospecific electrodeposition of a chiral catalyst.

Many biomolecules are chiral--they can exist in one of two enantiomeric forms that only differ in that their structures are mirror images of each other. Because only one enantiomer tends to be physiologically active while the other is inactive or even toxic, drug compounds are increasingly produced in an enantiomerically pure form using solution-phase homogeneous catalysts and enzymes. Chiral surfaces offer the possibility of developing heterogeneous enantioselective catalysts that can more readily be separated from the products and reused. In addition, such surfaces might serve as electrochemical sensors for chiral molecules. To date, chiral surfaces have been obtained by adsorbing chiral molecules or slicing single crystals so that they exhibit high-index faces, and some of these surfaces act as enantioselective heterogeneous catalysts. Here we show that chiral surfaces can also be produced through electrodeposition, a relatively simple solution-based process that resembles biomineralization in that organic molecules adsorbed on surfaces have profound effects on the morphology of the inorganic deposits. When electrodepositing a copper oxide film on an achiral gold surface in the presence of tartrate ion in the deposition solution, the chirality of the ion determines the chirality of the deposited film, which in turn determines the film's enantiospecificity during subsequent electrochemical oxidation reactions.

Response to Comment on "Spin coating epitaxial films".

Lu and Tang claim that the spin-coated films in our study are not epitaxial. They assume that all of the background intensity in the x-ray pole figures of the spin-coated materials is due to randomly oriented grains. There is no evidence for randomly oriented grains in the 2θ x-ray patterns. The background intensity in the pole figures is also comparable to the background from the single-crystal substrates, which is inconsistent with their assumption.

Spin coating epitaxial films.

Spin-coated films, such as photoresists for lithography or perovskite films for solar cells, are either amorphous or polycrystalline. We show that epitaxial films of inorganic materials such as cesium lead bromide (CsPbBr3), lead(II) iodide (PbI2), zinc oxide (ZnO), and sodium chloride (NaCl) can be deposited onto a variety of single-crystal and single-crystal-like substrates by simply spin coating either solutions of the material or precursors to the material. The out-of-plane and in-plane orientations of the spin-coated films are determined by the substrate. The thin stagnant layer of supersaturated solution produced during spin coating promotes heterogeneous nucleation of the material onto the single-crystal substrate over homogeneous nucleation in the bulk solution, and ordered anion adlayers may lower the activation energy for nucleation on the surface. The method can be used to produce functional materials such as inorganic semiconductors or to deposit water-soluble materials such as NaCl that can serve as growth templates.

Electrodeposited Epitaxial Cu(100) on Si(100) and Lift-Off of Single Crystal-like Cu(100) Foils.

A two-step potential electrodeposition technique is described which gives epitaxial films of Cu(100) on n-Si(100). Nucleation of epitaxial seeds occurs at -1.5 VAg/AgCl, whereas the film is grown at -0.5 VAg/AgCl. Cu deposition occurs with a Faradaic efficiency of 82.0% as determined spectrophotometrically. Epitaxy is achieved through a 45° in-plane rotation of Cu with respect to Si, which is shown by X-ray analysis. The 45° rotation reduces the lattice mismatch from -33.43% for an unrotated film to -5.86% for a 45° rotated film. Mosaicity, as determined via X-ray rocking curves, decreases with increasing thickness, going from a full width at half maximum of 3.99° for a 30 nm thick film to 1.67° for a 160 nm thick film. This translates to an increasing quality of epitaxy with increasing thickness. High resolution transmission electron microscopy imaging shows an amorphous SiO x interlayer between Cu and Si. Etching of SiO x with 5% HF allows epitaxial lift-off of the copper film, giving single crystal-like Cu(100) foils. Cu(100) films and single crystal-like foils have potential to be used as catalysts for CO2 reduction, substrates for technologically important materials like spintronic multilayer magnetic stacks and high temperature superconductors, and as active surfaces toward galvanic replacement by platinum group elements. Additionally, the foils could be used as single crystal-like substrates for flexible electronics.

Photoelectrochemistry of Ultrathin, Semitransparent, and Catalytic Gold Films Electrodeposited Epitaxially onto n-Silicon (111).

An ultrathin, epitaxial Au layer was electrochemically deposited on n-Si(111) to form a Schottky junction that was used as the photoanode in a regenerative photoelectrochemical cell. Au serves as a semitransparent contact that both stabilizes n-Si against photopassivation and catalyzes the oxidation of Fe2+ to Fe3+. In this cell, Fe2+ was oxidized at the n-Si(111)/Au(111) photoanode and Fe3+ was reduced at the Au cathode, leading to the conversion of solar energy into electrical energy with no net chemical reaction. The photocurrent was limited to 11.9 mA·cm-2 because of the absorption of light by the Fe2+/3+ redox couple. When a transparent solution of sulfite ion was oxidized at the photoanode, photocurrent densities as high as 28.5 mA·cm-2 were observed with AM 1.5 light of 100 mW·cm-2 intensity. One goal of the work was to determine the effect of the Au layer on the interfacial energetics as a function of the Au coverage. There was a decrease in the barrier height from 0.81 to 0.73 eV as the gold coverage was increased from island growth with 10% coverage to a dense Au film with a thickness of 11 nm. In all cases, the band-bending in n-Si was induced by the n-Si/Au Schottky junction instead of the energetic mismatch between the Fermi level of n-Si and the redox couple. The dense Au film gave the greatest stability. Although the photocurrent of the n-Si/Au photoanode with 10.2% island coverage dropped nearly to zero within 2 h, the photocurrent of the photoanode with a dense 11 nm thick Au film only decreased to 92% of its initial value after irradiation at open circuit with AM 1.5 light for 16 h. A 2.1 nm thick layer of SiO x formed between the Au film and n-Si. With further irradiation, the fill factor decreased because of the increase of series resistance as the SiO x layer thickness exceeded tunneling dimensions.

Epitaxial lift-off of electrodeposited single-crystal gold foils for flexible electronics.

We introduce a simple and inexpensive procedure for epitaxial lift-off of wafer-size flexible and transparent foils of single-crystal gold using silicon as a template. Lateral electrochemical undergrowth of a sacrificial SiO x layer was achieved by photoelectrochemically oxidizing silicon under light irradiation. A 28-nanometer-thick gold foil with a sheet resistance of 7 ohms per square showed only a 4% increase in resistance after 4000 bending cycles. A flexible organic light-emitting diode based on tris(bipyridyl)ruthenium(II) that was spin-coated on a foil exploited the transmittance and flexibility of the gold foil. Cuprous oxide as an inorganic semiconductor that was epitaxially electrodeposited onto the gold foils exhibited a diode quality factor n of 1.6 (where n = 1.0 for an ideal diode), compared with a value of 3.1 for a polycrystalline deposit. Zinc oxide nanowires electrodeposited epitaxially on a gold foil also showed flexibility, with the nanowires intact up to 500 bending cycles.

Nanometer-Thick Gold on Silicon as a Proxy for Single-Crystal Gold for the Electrodeposition of Epitaxial Cuprous Oxide Thin Films.

Single-crystal Au is an excellent substrate for electrochemical epitaxial growth due to its chemical inertness, but the high cost of bulk Au single crystals prohibits their use in practical applications. Here, we show that ultrathin epitaxial films of Au electrodeposited onto Si(111), Si(100), and Si(110) wafers can serve as an inexpensive proxy for bulk single-crystal Au for the deposition of epitaxial films of cuprous oxide (Cu2O). The Au films range in thickness from 7.7 nm for a film deposited for 5 min to 28.3 nm for a film deposited for 30 min. The film thicknesses are measured by low-angle X-ray reflectivity and X-ray Laue oscillations. High-resolution TEM shows that there is not an interfacial SiOx layer between the Si and Au. The Au films deposited on the Si(111) substrates are smoother and have lower mosaic spread than those deposited onto Si(100) and Si(110). The mosaic spread of the Au(111) layer on Si(111) is only 0.15° for a 28.3 nm thick film. Au films deposited onto degenerate Si(111) exhibit ohmic behavior, whereas Au films deposited onto n-type Si(111) with a resistivity of 1.15 Ω·cm are rectifying with a barrier height of 0.85 eV. The Au and the Cu2O follow the out-of-plane and in-plane orientations of the Si substrates, as determined by X-ray pole figures. The Au and Cu2O films deposited on Si(100) and Si(110) are both twinned. The films grown on Si(100) have twins with a [221] orientation, and the films grown on Si(110) have twins with a [411] orientation. An interface model is proposed for all Si orientations, in which the -24.9% mismatch for the Au/Si system is reduced to only +0.13% by a coincident site lattice in which 4 unit meshes of Au coincide with 3 unit meshes of Si. Although this study only considers the deposition of epitaxial Cu2O films on electrodeposited Au/Si, the thin Au films should serve as high-quality substrates for the deposition of a wide variety of epitaxial materials.

Electrodeposition of Epitaxial Lead Iodide and Conversion to Textured Methylammonium Lead Iodide Perovskite.

Applications for lead iodide, such as lasing, luminescence, radiation detection, and as a precursor for methylammonium lead iodide perovskite photovoltaic cells, require highly ordered crystalline thin films. Here, an electrochemical synthesis route is introduced that yields textured and epitaxial films of lead iodide at room temperature by reducing molecular iodine to iodide ions in the presence of lead ions. Lead iodide grows with a [0001] fiber texture on polycrystalline substrates such as fluorine-doped tin oxide. On single-crystal Au(100), Au(111), and Au(110) the out-of-plane orientation of lead iodide is also [0001], but the in-plane orientation is controlled by the single-crystal substrate. The epitaxial lead iodide on single-crystal gold is converted to textured methylammonium lead iodide perovskite with a preferred [110] orientation via methylammonium iodide vapor-assisted chemical transformation of the solid.

Electrodeposited germanium nanowires.

Germanium (Ge) is a group IV semiconductor with superior electronic properties compared with silicon, such as larger carrier mobilities and smaller effective masses. It is also a candidate anode material for lithium-ion batteries. Here, a simple, one-step method is introduced to electrodeposit dense arrays of Ge nanowires onto indium tin oxide (ITO) substrates from aqueous solution. The electrochemical reduction of ITO produces In nanoparticles that act as a reduction site for aqueous Ge(IV) species, and as a solvent for the crystallization of Ge nanowires. Nanowires deposited at 95 °C have an average diameter of 100 nm, whereas those deposited at room temperature have an average diameter of 35 nm. Both optical absorption and Raman spectroscopy suggest that the electrodeposited Ge is degenerate. The material has an indirect bandgap of 0.90-0.92 eV, compared with a value of 0.67 eV for bulk, intrinsic Ge. The blue shift is attributed to the Moss-Burstein effect, because the material is a p-type degenerate semiconductor. On the basis of the magnitude of the blue shift, the hole concentration is estimated to be 8 × 10(19) cm(-3). This corresponds to an In impurity concentration of about 0.2 atom %. The resistivity of the wires is estimated to be 4 × 10(-5) Ω·cm. The high conductivity of the wires should make them ideal for lithium-ion battery applications.

Superconducting filaments formed during nonvolatile resistance switching in electrodeposited δ-Bi(2)O(3).

We show that electrodeposited films of δ-Bi2O3 in a Pt/δ-Bi2O3/Au cell exhibit unipolar resistance switching. After being formed at a large electric field of 40 MV/m, the cell can be reversibly switched between a low resistance state (156 Ω) and a high resistance state (1.2 GΩ) by simply cycling between SET and RESET voltages of the same polarity. Because the high and low resistance states are persistent, the cell is a candidate for nonvolatile resistance random access memory (RRAM). A Bi nanofilament forms at the SET voltage, and it ruptures to form a 50 nm gap during the RESET step at a current density of 2 × 10(7) A/cm(2). The diameter of the Bi filament is a function of the compliance current, and can be tuned from 140 to 260 nm, but the current density in the RESET step is independent of the Bi diameter. An electromigration rupture mechanism is proposed. The Bi nanofilaments in the low resistance state are superconducting, with a Tc of 5.8 K and an Hc of 5 kOe. This is an unexpected result, because bulk Bi is not a superconductor.

Electrochemistry of free chlorine and monochloramine and its relevance to the presence of Pb in drinking water.

The commonly used disinfectants in drinking water are free chlorine (in the form of HOCl/OCl-) and monochloramine (NH2Cl). While free chlorine reacts with natural organic matter in water to produce chlorinated hydrocarbon byproducts, there is also concern that NH2Cl may react with Pbto produce soluble Pb(II) products--leading to elevated Pb levels in drinking water. In this study, electrochemical methods are used to compare the thermodynamics and kinetics of the reduction of these two disinfectants. The standard reduction potential for NH2Cl/Cl- was estimated to be +1.45 V in acidic media and +0.74 V in alkaline media versus NHE using thermodynamic cycles. The kinetics of electroreduction of the two disinfectants was studied using an Au rotating disk electrode. The exchange current densities estimated from Koutecky-Levich plots were 8.2 x 10(-5) and 4.1 x 10(-5) A/cm2, and by low overpotential experiments were 7.5 +/- 0.3 x 10(-5) and 3.7 +/- 0.4 x 10(-5) A/cm2 for free chlorine and NH2Cl, respectively. The rate constantforthe electrochemical reduction of free chlorine at equilibrium is approximately twice as large as that for the reduction of NH2Cl. Equilibrium potential measurements show that free chlorine will oxidize Pb to PbO2 above pH 1.7, whereas NH2Cl will oxidize Pb to PbO2 only above about pH 9.5, if the total dissolved inorganic carbon (DIC) is 18 ppm. Hence, NH2Cl is not capable of producing a passivating PbO2 layer on Pb, and could lead to elevated levels of dissolved Pb in drinking water.

Epitaxial electrodeposition of ZnO on Au(111) from alkaline solution: exploiting amphoterism in Zn(II).

The amphoteric nature of ZnO is used to produce the material from strongly alkaline solution. The solution pH is lowered globally to produce ZnO powder, and it is lowered locally at a Au(111) surface to produce epitaxial films. ZnO powder is precipitated from a solution of 10 mM Zn(II) in 0.25 M NaOH by simply adding 1 M HNO3 to the solution. For the film electrodeposition, the local pH at the electrode surface is decreased by electrochemically oxidizing the ascorbate dianion. The chemically precipitated ZnO powder grows with a sea urchin-like nanostructure, whereas the electrodeposited films have a columnar structure. ZnO electrodeposited onto a Au(111) single crystal has a ZnO(0001)[1011]//Au(111)[110] orientation relationship.