Observation of Weibull, Lognormal, and Gamma Distributions in Electrodeposited Cu and Cu-Ag Particles.

In this work, the nearest-neighbor distances and Voronoi cell features of Cu-Ag deposits were analyzed and fitted with Lognormal, Weibull, and Gamma distributions. The nearest-neighbor distance distributions of the samples were compared with those of complete spatially random points, showing spatial inhomogeneity due to the nucleation exclusion effect. The radial distribution function was calculated, showing both influences from the grain size and the nucleation exclusion effect. Voronoi cells were generated based on the shape of the grains. The size, occupancy, and coordination of the Voronoi cells were examined and fitted. The results show that although the Cu-Ag deposits seemed to be governed by the instantaneous nucleation mode, the spatial distribution of the nuclei was more impacted by the nucleation exclusion effect than the Cu-only samples. This behavior is also justified by the grain size distribution generated with Voronoi cell size and occupancy distributions.

Vibrational properties of metastable orthorhombic Bi2Se3.

Chalcogenide materials are being considered as some of the most promising systems for energy harvesting and energy conversion. Among them, the orthorhombic family of compounds X2Y3 (with X = Bi, Sb and Y = S, Se) has attracted special attention due to its interesting atomic structure and thermoelectric and optical properties. While Bi2S3 and Sb2Se3 have already been applied to solar cells, practical application of the new metastable Bi2Se3 is still a challenge due to the lack of data and knowledge on its properties. Here, the vibrational and structural properties of the orthorhombic metastable phase of Bi2Se3 are investigated by using Raman spectroscopy and ab initio calculations. We perform Raman spectroscopy measurements along with in situ thermal treatment on samples grown by electrochemical deposition. We show that by increasing the temperature an improved crystallization occurs in the orthorhombic structure, followed by recrystallization to the usual rhombohedral phase. Our results point out specific Raman modes of the orthorhombic phase. First principles computational results based on the density functional theory support the experimental data and describe three singlet Raman active vibrational modes, such as B(2)3g, B(2)2g and A(6)g.

Solvent Influence on the Magnetization and Phase of Fe-Ni Alloy Nanoparticles Generated by Laser Ablation in Liquids.

The synthesis of bimetallic iron-nickel nanoparticles with control over the synthesized phases, particle size, surface chemistry, and oxidation level remains a challenge that limits the application of these nanoparticles. Pulsed laser ablation in liquid allows the properties tuning of the generated nanoparticles by changing the ablation solvent. Organic solvents such as acetone can minimize nanoparticle oxidation. Yet, economical laboratory and technical grade solvents that allow cost-effective production of FeNi nanoparticles contain water impurities, which are a potential source of oxidation. Here, we investigated the influence of water impurities in acetone on the properties of FeNi nanoparticles generated by pulsed laser ablation in liquids. To remove water impurities and produce "dried acetone", cost-effective and reusable molecular sieves (3 Å) are employed. The results show that the Fe50Ni50 nanoparticles' properties are influenced by the water content of the solvent. The metastable HCP FeNi phase is found in NPs prepared in acetone, while only the FCC phase is observed in NPs formed in water. Mössbauer spectroscopy revealed that the FeNi nanoparticles oxidation in dried acetone is reduced by 8% compared to acetone. The high-field magnetization of Fe50Ni50 nanoparticles in water is the highest, 68 Am2/kg, followed by the nanoparticles obtained after ablation in acetone without water impurities, 59 Am2/kg, and acetone, 52 Am2/kg. The core-shell structures formed in these three liquids are also distinctive, demonstrating that a core-shell structure with an outer oxide layer is formed in water, while carbon external layers are obtained in acetone without water impurity. The results confirm that the size, structure, phase, and oxidation of FeNi nanoparticles produced by pulsed laser ablation in liquids can be modified by changing the solvent or just reducing the water impurities in the organic solvent.

Photoelectrochemistry of Self-Limiting Electrodeposition of Ni Film onto GaAs.

Gallium arsenide (GaAs) provides a suitable bandgap (1.43 eV) for solar spectrum absorption and allows a larger photovoltage compared to silicon, suggesting great potential as a photoanode toward water splitting. Photocorrosion under water oxidation condition, however, leads to decomposition or the formation of an insulating oxide layer, which limits the photoelectrochemical performance and stability of GaAs. In this work, a self-limiting electrodeposition method of Ni on GaAs is reported to either generate ultra-thin continuous film or nanoislands with high particle density by controlling deposition time. The self-limiting growth mechanism is validated by potential transients, X-ray photoelectron spectroscopy composition and depth profile measurements. This deposition method exhibits a rapid nucleation, forms an initial metallic layer followed by a hydroxide/oxyhydroxide nanofilm on the GaAs surface and is independent of layer thickness versus deposition time when coalescence is reached. A photocurrent up to 8.9 mA cm-2 with a photovoltage of 0.11 V is obtained for continuous ultrathin films, while a photocurrent density of 9.2 mA cm-2 with a photovoltage of 0.50 V is reached for the discontinuous nanoislands layers in an aqueous solution containing the reversible redox couple K3 Fe(CN)6 /K4 Fe(CN)6 .

Rational Compositional Control of Electrodeposited Ag-Fe films.

Some alloys are very difficult to electrodeposit, due to problems such as the large difference in the equilibrium potentials and/or deposition kinetics of the alloy components, as well as the bath instability due to the spontaneous reactions in the bulk electrolyte. The Ag-Fe system is one of those. In this work, a novel alkaline citrate-dimethylhydantoin (DMH) complex has been used to synthesize thermodynamically immiscible Ag-Fe alloy films. The large difference in standard potentials and deposition kinetics of Ag and Fe is partially resolved by complexing Ag(I) with DMH, while the instability caused from spontaneous reduction of Ag(I) by Fe(II) was partially resolved by adding 1% Fe3+, based on the philosophy of the mixed potential theory. Furthermore, a paradigm for control of film composition is developed, based on the mass-transfer coefficient ratio, tested with various bath constitutions. Uniform and high-quality films are obtained, with a composition error of <4 at. % comparing with the predicted values. Electrochemical reactions involved in the deposition baths were systematically investigated using cyclic voltammetry, showing satisfactory agreement with the predicted deposition potentials calculated by equilibrium thermodynamics. The films deposited at high overpotential are proven to be biphasic, containing Ag-fcc and Fe-bcc phases, with a trend of decreasing crystallinity at increasing overpotentials when deposited at constant deposition time. A narrow transition potential range (<0.05 V) from the onset of Fe deposition to its limiting current condition was observed.

Capillary transfer of soft films.

Existing transfer technologies in the construction of film-based electronics and devices are deeply established in the framework of native solid substrates. Here, we report a capillary approach that enables a fast, robust, and reliable transfer of soft films from liquid in a defect-free manner. This capillary transfer is underpinned by the transfer front of dynamic contact among receiver substrate, liquid, and film, and can be well controlled by a selectable motion direction of receiver substrates at a high speed. We demonstrate in extensive experiments, together with theoretical models and computational analysis, the robust capabilities of the capillary transfer using a versatile set of soft films with a broad material diversity of both film and liquid, surface-wetting properties, and complex geometric patterns of soft films onto various solid substrates in a deterministic manner.

High Selectivity Towards Formate Production by Electrochemical Reduction of Carbon Dioxide at Copper-Bismuth Dendrites.

The electrochemical reduction of CO2 provides an alternative carbon-neutral path for renewable synthesis of fuels and value-added chemicals. This work demonstrates that dendritic, bimetallic Cu-Bi electrocatalysts with nanometer-sized grains are capable of formate generation with a high selectivity. Optimizing composition of electrocatalyst could achieve a faradic efficiency of 90 % at -0.8 to -0.9 VRHE , and a partial current of more than 2 mA cm-2 . The combination of Cu with Bi enables modulation of the adsorption strength of intermediates. This leads to an increased selectivity and suppressed formation of spurious species, especially hydrogen and CO. Comparison of product distribution for Cu-In versus Cu-Bi indicated that Bi is essential to induce a favorable adsorption configuration of the intermediate species and to promote formate production.

Investigations on the Electrochemical Atomic Layer Growth of Bi2Se3 and the Surface Limited Deposition of Bismuth at the Silver Electrode.

The Electrochemical Atomic Layer Deposition (E-ALD) technique is used for the deposition of ultrathin films of bismuth (Bi) compounds. Exploiting the E-ALD, it was possible to obtain highly controlled nanostructured depositions as needed, for the application of these materials for novel electronics (topological insulators), thermoelectrics and opto-electronics applications. Electrochemical studies have been conducted to determine the Underpotential Deposition (UPD) of Bi on selenium (Se) to obtain the Bi2Se3 compound on the Ag (111) electrode. Verifying the composition with X-ray Photoelectron Spectroscopy (XPS) showed that, after the first monolayer, the deposition of Se stopped. Thicker deposits were synthesized exploiting a time-controlled deposition of massive Se. We then investigated the optimal conditions to deposit a single monolayer of metallic Bi directly on the Ag.

Guided Heterogeneous Nucleation of Sodium Chloride at Self-Assembled Monolayer-Modified Nanoporous Gold Films.

Drug delivery devices are generally inefficient when releasing the active compound at the targeted position. In this work, we investigate nanoporous gold (np-Au) as the drug eluting device, and we use the precipitation of NaCl as a model of drug sedimentation to evaluate the patterns of solute distribution. Hydrophilic and hydrophobic modifications of np-Au result in different, but both inhomogeneous, release patterns, with most of the precipitate forming outside the device. In contrast, the fabrication of a hydrophobic-hydrophilic-layered architecture allows full penetration through the bicontinuous np-Au network, resulting in a homogeneous release pattern. Similar architectures could be used to enhance the efficacy of drug delivery.

Performance and Reliability of Electrowetting-on-Dielectric (EWOD) Systems Based on Tantalum Oxide.

The electrowetting-on-dielectric behavior of Cytop/Tantalum oxide (TaOx) bilayers is studied by measuring their response vs applied voltage and under prolonged periodic cycling, below and above the threshold voltage VT corresponding to the breakdown field for the oxide. TaOx exhibits symmetric solid state I-V characteristics, with electronic conduction dominated by Schottky, Poole-Frenkel emission; conduction is attributed to oxygen vacancies (6 × 1016 cm-3), resulting in large currents at low bias. Electrolyte/Metal Oxide/Metal I-V characteristics show oxide degradation at (<5 V) cathodic bias; anodic bias in contrast results in stable characteristics until reaching the anodization voltage, where the oxide thickens, leading eventually to breakdown and oxygen production at the electrode. Electrowetting angle vs applied voltage undergoes three different stages: a parabolic variation of contact angle (CA) with applied voltage, CA saturation, and rebound of the CA to higher values due to degradation of the polymer layer. The contact angle remained stable for several hundred cycles if the applied voltage was less than VT; degradation in contrast is fast when the voltage is above VT. Degradation of the electrowetting response with time is linked to charge accumulation in the polymer, which inhibits the rebound of the CA when voltage is being applied.

Failure Modes during Low-Voltage Electrowetting.

Low-voltage electrowetting devices allow significant contact angle changes below a 50 V bias; however, operation under prolonged cycling and failure modes have not yet been sufficiently elucidated. In this work, the failure modes and performance degradation of Cytop (23-210 nm)/aluminum oxide (15-44 nm) bilayers have been investigated. Contact angle and leakage current were measured during stepped voltage measurements up to failure, showing three electrowetting response regimes: ideal Young-Lippmann behavior, contact angle saturation, and dielectric breakdown. The onset of ionic conduction in aluminum oxide and the resulting breakdown control when the layer would ultimately fail, but the thickness of the Cytop layer determined the achievable contact angle versus voltage characteristics. Cyclic electrowetting measurements studied the repeatability of contact angle change using an applied voltage above or below the voltage drop needed for polymer breakdown (VT). Results show repeatable electrowetting below VT and a rapidly diminishing contact angle response above VT. The leakage current and injected charge cannot be used to comprehensively assess the stability of the system during operation. The contact potential difference measured with a Kelvin probe provides an alternative means of assessing the extent of the damage.

Underpotential Co-deposition of Au-Cu alloys: switching the underpotentially deposited element by selective complexation.

Underpotential deposition and monolayer replacement processes are widely used for the synthesis of core/shell catalysts and heterointerfaces. Conventionally, only the more noble metal can be underpotentially deposited on or replace the less noble metal, limiting the number of accessible material configurations. We show here that the reverse process is possible, using the Au-Cu pair as a model system. By tuning the redox potentials of the two components via use of strong, selective metal ion complexes, Au-Cu alloys could be synthesized at will by (i) conventional underpotential co-deposition, whereby Cu is reduced at underpotential in parallel with the overpotential deposition of Au, or (ii) the reverse process, where Au is reduced at underpotential, while Cu is deposited at overpotential. Selective complexation also draws the redox potential of Au and Cu closer, resulting in co-deposition under activation control for the noble metal and precise alloy composition control by the applied potential, enabling in principle the formation of arbitrary metal or alloy interfaces. The alloys resulting from the two processes exhibit distinct enthalpy of mixing, suggesting different degrees of short-range order and dissimilar atomic configurations. These findings open new perspectives on underpotential deposition phenomena and possibly new synthetic opportunities in electrodeposition.

Three-phase contact force equilibrium of liquid drops at hydrophilic and superhydrophobic surfaces.

Hydrophilic and superhydrophobic surfaces were obtained by modifying a dendritic Au surface with carboxyl- or methyl-terminated self-assembled monolayers, respectively. The hydrophilic surface generates capillary forces which induce water flow through the grooves defined by the dendritic structure, resulting in a halo structure; the superhydrophobic surface on the other hand completely repels water drops. The contrasting behaviors exhibited by the two surfaces with nominally identical morphology but different surface chemistry are explained semi-quantitatively in terms of the equilibrium of surface forces developing at the three-phase (air-solid-water) contact lines.

Electrochemical impedance study of GaAs surface charge modulation through the deprotonation of carboxylic acid monolayers.

Modifications to the space charge region of p+ and p-GaAs due to surface charge modulation by the pH-induced deprotonation of bound carboxylic acid terminal monolayers were studied by electrochemical impedance spectroscopy and correlated to flat-band potential measurements from Mott-Schottky plots. We infer that the negative surface dipole formed on GaAs due to monolayer deprotonation causes an enhancement of the downward interfacial band bending. The space charge layer modifications were correlated to intermolecular electrostatic interactions and semiconductor depletion characteristics.

Tailoring morphology in free-standing anodic aluminium oxide: control of barrier layer opening down to the sub-10 nm diameter.

Free-standing, highly ordered porous aluminium oxide templates were fabricated by three-step anodization in oxalic, sulfuric or phosphoric acid solutions, followed by dissolution of the aluminium substrate in HgCl(2). Opening of the pore bottoms on the barrier layer side of these templates was carried out by using chemical or ion beam etching. Chemical etching is capable of achieving full pore opening, but partial pore opening occurs inhomogeneously. On the contrary, ion beam etching enables homogeneous and reproducible partial pore opening, with the pore size controlled through the etching time. By this method, pore openings as small as 5 nm can reliably be obtained.

Electrodeposition of Fe-Pt films with low oxide content using an alkaline complexing electrolyte.

Electrochemical deposition of equiatomic Fe-Pt from complexing electrolytes provides precise tuning of alloy stoichiometry, enables close control of the growth process, and results in limited oxygen incorporation. The films grow epitaxially on oriented substrates and the low oxygen content favors transformation from the as-deposited cubic to the high anisotropy L1(0) phase and magnetic hardening upon thermal annealing at temperatures (400-450 degrees C) much lower than previously achieved by other plating processes.

Charge storage and the oxygen evolution reaction in mixed Ni-Li oxides.

Electronic and catalytic properties of mixed oxides of Ni-Li are studied at room temperature with NMR and with electrochemical techniques in alkaline solutions. Our Li(x)Ni(1-x)O (0.04 < or = x < or = 0.2) samples have an f.c.c crystal structure, are in an anti-ferromagnetic phase, and exhibit a substantial conductivity due to a hole-hopping process with a characteristic rate which is essentially independent of composition. Several processes of reduction and oxidation occur in the voltage region between the hydrogen (HER) and oxygen evolution (OER) reactions, which are compared with related phenomena observed in pure Ni electrodes (Ni(OH)2 and NiO). Repeated cycles of reduction and oxidation yield electrodes with a somewhat improved OER activity, and a high charge-storage capacity. Heavy anodic treatments cause irreversible changes of these properties, particularly when coupled with months-long storage in air. The complex behavior of these electrodes is interpreted in terms of formation of a topotactic surface layer with hydrogenated species.

Electrolytic gold deposition on dodecanethiol-modified gold films.

The electrochemical nucleation and growth of Au from a Au sulfite electrolyte onto dodecanethiol-modified Au surfaces is investigated by a combination of microscopy and chronoamperometry methods. The self-assembled dodecanethiol monolayers are continuous but exhibit defects in correspondence of the Au grain boundaries and on top of Au terraces. Nucleation of Au films occurs initially at these defect sites, but only a small fraction of these nuclei survive an initial competition process. The remaining nuclei expand through three-dimensional progressive nucleation followed by diffusion-limited growth. The resulting Au films exhibit microstructures which are widely different from those observed in the electrochemical growth of Au on Au and that depend on the applied potential: while at low overpotentials the film grows as an assembly of hemispherical clusters, at intermediate overvoltages the films are smooth and at high overvoltages become dendritic. Metal growth onto self-assembled monolayer-modified substrates can thus provide an alternative method for controlling film morphology for a wide range of applications.

Effect of Pt particle size and distribution on photoelectrochemical hydrogen evolution by p-Si photocathodes.

In this work, we investigate the effect of the average size and density of Pt clusters on silicon on the photoelectrochemical production of hydrogen. The metallization of Si is performed via electroless deposition from aqueous HF solutions and from water-in-oil microemulsions. The first method enables control of the average diameter and density of Pt clusters by properly changing the deposition parameters like HF concentration and immersion times. However, on one hand, size dispersion is relatively wide and particles agglomeration may occur with this deposition technique. On the other hand, Pt islands with smaller dimensions at the nanoscale as well as with a narrower size distribution are deposited from reversed micellar solutions. Photoelectrochemical experiments show that the effect of Pt morphology on photoconversion efficiency strongly depends on light intensity. At low power of illumination (10 mW/cm2), Pt islands with a mean diameter of 100 nm and a density of 15 particles/microm2, which can be obtained via electroless deposition from a HF-based solution, provide the best photoelectrochemical performance. Nevertheless, this configuration of Pt clusters yields an abrupt collapse of photoconversion efficiency from 31% to 11.8% when the light power is increased up to 100 mW/cm2. At this light intensity, Pt islands with a mean size and density of approximately 40 nm and 75 particles/microm2, respectively, obtained via the microemulsion method, allow photoconversion efficiency as high as 20% to be achieved.

Electrodeposition of platinum on highly oriented pyrolytic graphite. Part I: electrochemical characterization.

The electrochemical deposition of Pt on highly oriented pyrolytic graphite (HOPG) from H2PtCl6 solutions was investigated by cyclic voltammetry and chronoamperometry. The effects of deposition overpotential, H2PtCl6 concentration, supporting electrolyte, and anion additions on the deposition process were evaluated. Addition of chloride inhibits Pt deposition due to adsorption on the substrate and blocking of reduction sites, while SO4(2-) and ClO4- slightly promote Pt reduction. By comparing potentiostatic current-time transients with the Scharifker-Hills model, a transition from progressive to instantaneous nucleation was observed when increasing the deposition overpotential. Following addition of chloride anions the fit of experimental transients with the instantaneous nucleation mode improves, while the addition of SO4(2-) induces only small changes. Chloride anions strongly inhibit the reduction process, which is shifted in the cathodic direction. The above results indicate that the most appropriate conditions for growing Pt nanoparticles on HOPG with narrow size distribution are to use an H2PtCl6 solution with HCl as supporting electrolyte and to apply a high cathodic overpotential.