Systematic catalytic current enhancement for the oxidation of water at nanostructured iron(III) oxide electrodes

Nanoporous water oxidation electrodes with a low loading of laser-deposited Ru/C exhibit enhanced corrosion stability

For the oxidation of water to dioxygen, oxide-covered ruthenium metal is known as the most efficient catalyst, however, with limited stability. Herein, we present a strategy for incorporating a Ru/C composite onto a novel nanoporous electrode surface with low noble metal loading and improved stability. The Ru/C is coated on the pore walls of anodic alumina templates in a one-step laser-induced deposition method from Ru3(CO)12 solutions. Scanning electron microscopy proves the presence of a continuous Ru/C layer along the inner pore walls. The amorphous material consists of metallic Ru incorporated in a carbonaceous C matrix as shown by X-ray diffraction combined with Raman and X-ray photoelectron spectroscopies. These porous electrodes reveal enhanced stability during water oxidation as compared to planar samples at pH 4. Finally, their electrocatalytic performance depends on the geometric parameters and is optimized with 13 μm pore length, which yields 2.6 mA cm-2, or 49 A g-1, at η = 0.20 V.

Design Rules for Oxygen Evolution Catalysis at Porous Iron Oxide Electrodes: A 1000-Fold Current Density Increase

Nanotubular iron(III) oxide electrodes are optimized for catalytic efficiency in the water oxidation reaction at neutral pH. The nanostructured electrodes are prepared from anodic alumina templates, which are coated with Fe₂ O₃ by atomic layer deposition. Scanning helium ion microscopy, X-ray diffraction, and Raman spectroscopy are used to characterize the morphologies and phases of samples submitted to various treatments. These methods demonstrate the contrasting effects of thermal annealing and electrochemical treatment. The electrochemical performances of the corresponding electrodes under dark conditions are quantified by steady-state electrolysis and electrochemical impedance spectroscopy. A rough and amorphous Fe₂ O₃ with phosphate incorporation is critical for the optimization of the water oxidation reaction. For the ideal pore length of 17 μm, the maximum catalytic turnover is reached with an effective current density of 140 μA cm^-2 at an applied overpotential of 0.49 V.

Stimulus-Responsive Nanoporous System Based on a Redox-Active Molecular Self-Assembled Monolayer

Ordered arrays of electrically conducting cylindrical nanotubes are created by atomic layer deposition of a thin titanium dioxide layer onto the pore walls of an anodic alumina matrix. All geometric parameters (pore length and diameter and TiO₂ layer thickness) are defined and tunable experimentally. The titanium dioxide surface is subsequently functionalized with ferrocenylacetic acid. The chemisorbed ferrocene moieties are oxidized chemically and electrochemically. Monitoring the redox chemistry by UV-visible absorption spectroscopy allows for the quantification of the total density of redox-active units grafted to the surface, as well as the fraction of them oxidized at a given applied potential. The capillary properties of the surface can be adjusted by the applied potential, as quantified by contact angle measurements.

A model electrode of well-defined geometry prepared by direct laser-induced decoration of nanoporous templates with Au-Ag@C nanoparticles

We present an original type of model electrode system consisting of bimetallic Au-Ag nanoparticles embedded in an amorphous carbon matrix with an extremely well-defined geometry of parallel, straight, cylindrical macropores. The samples are prepared in one step by direct laser deposition of the metal/carbon composite onto the inner walls of a porous 'anodic' alumina matrix serving as a template. The coating is homogeneous from top to bottom of the pores, and the amount of material deposited can be tuned by the duration of the deposition procedure. As a test system, we demonstrate that a bimetallic Ag-Au@C system is catalytically active for the electrochemical oxidation of glucose in alkaline solution, the anodic reaction of a direct glucose fuel cell. Furthermore, the electrocatalytic current density increases with the amount of Ag-Au@C NPs deposited, up to a point at which the pores are clogged with it. This type of model system allows for the systematic study of geometric effects in fuel cell electrodes. It can be generalized to a number of different nanoparticle compositions, and thereby, to various electrocatalytic reactions.

Systematic tuning of segmented magnetic nanowires into three-dimensional arrays of ‘bits’

A method is presented for the preparation of a three-dimensional magnetic data storage material system.

Water Oxidation by Size-Selected Co27 Clusters Supported on Fe2 O3

The complexity of the water oxidation reaction makes understanding the role of individual catalytic sites critical to improving the process. Here, size-selected 27-atom cobalt clusters (Co₂₇ ) deposited on hematite (Fe₂ O₃ ) anodes were tested for water oxidation activity. The uniformity of these anodes allows measurement of the activity of catalytic sites of well-defined nuclearity and known density. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) characterization of the anodes before and after electrochemical cycling demonstrates that these Co₂₇ clusters are stable to dissolution even in the harsh water oxidation electrochemical environment. They are also stable under illumination at the equivalent of 0.4 suns irradiation. The clusters show turnover rates for water oxidation that are comparable or higher than those reported for Pd- and Co-based materials or for hematite. The support for the Co₂₇ clusters is Fe₂ O₃ grown by atomic layer deposition on a Si chip. We have chosen to deposit a Fe₂ O₃ layer that is only a few unit cells thick (2 nm), to remove complications related to exciton diffusion. We find that the electrocatalytic and the photoelectrocatalytic activity of the Co₂₇ /Fe₂ O₃ material is significantly improved when the samples are annealed (with the clusters already deposited). Given that the support is thin and that the cluster deposition density is equivalent to approximately 5 % of an atomic monolayer, we suggest that annealing may significantly improve the exciton diffusion from the support to the catalytic moiety.

Modification of Hematite Electronic Properties with Trimethyl Aluminum to Enhance the Efficiency of Photoelectrodes

The electronic properties of hematite were investigated by means of synchrotron radiation photoemission (SR-PES) and X-ray absorption spectroscopy (XAS). Hematite samples were exposed to trimethyl aluminum (TMA) pulses, a widely used Al-precursor for the atomic layer deposition (ALD) of Al2O3. SR-PES and XAS showed that the electronic properties of hematite were modified by the interaction with TMA. In particular, the hybridization of O 2p states with Fe 3d and Fe 4s4p changed upon TMA pulses due to electron inclusion as polarons. The change of hybridization correlates with an enhancement of the photocurrent density due to water oxidation for the hematite electrodes. Such an enhancement has been associated with an improvement in charge carrier transport. Our findings open new perspectives for the understanding and utilization of electrode modifications by very thin ALD films and show that the interactions between metal precursors and substrates seem to be important factors in defining their electronic and photoelectrocatalytic properties.

Preparation of electrochemically active silicon nanotubes in highly ordered arrays

Silicon as the negative electrode material of lithium ion batteries has a very large capacity, the exploitation of which is impeded by the volume changes taking place upon electrochemical cycling. A Si electrode displaying a controlled porosity could circumvent the difficulty. In this perspective, we present a preparative method that yields ordered arrays of electrochemically competent silicon nanotubes. The method is based on the atomic layer deposition of silicon dioxide onto the pore walls of an anodic alumina template, followed by a thermal reduction with lithium vapor. This thermal reduction is quantitative, homogeneous over macroscopic samples, and it yields amorphous silicon and lithium oxide, at the exclusion of any lithium silicides. The reaction is characterized by spectroscopic ellipsometry for thin silica films, and by nuclear magnetic resonance and X-ray photoelectron spectroscopy for nanoporous samples. After removal of the lithium oxide byproduct, the silicon nanotubes can be contacted electrically. In a lithium ion electrolyte, they then display the electrochemical waves also observed for other bulk or nanostructured silicon systems. The method established here paves the way for systematic investigations of how the electrochemical properties (capacity, charge/discharge rates, cyclability) of nanoporous silicon negative lithium ion battery electrode materials depend on the geometry.

Atomic layer deposition of a submonolayer catalyst for the enhanced photoelectrochemical performance of water oxidation with hematite

Hematite photoanodes were coated with an ultrathin cobalt oxide layer by atomic layer deposition (ALD). The optimal coating-1 ALD cycle, which amounts to <1 monolayer of Co(OH)2/Co3O4-resulted in significantly enhanced photoelectrochemical water oxidation performance. A stable, 100-200 mV cathodic shift in the photocurrent onset potential was observed that is correlated to an order of magnitude reduction in the resistance to charge transfer at the Fe2O3/H2O interface. Furthermore, the optical transparency of the ultrathin Co(OH)2/Co3O4 coating establishes it as a particularly advantageous treatment for nanostructured water oxidation photoanodes. The photocurrent of catalyst-coated nanostructured inverse opal scaffold hematite photoanodes reached 0.81 and 2.1 mA/cm(2) at 1.23 and 1.53 V, respectively.

Phase discrimination through oxidant selection in low-temperature atomic layer deposition of crystalline iron oxides

Control over the oxidation state and crystalline phase of thin-film iron oxides was achieved by low-temperature atomic layer deposition (ALD), utilizing a novel iron precursor, bis(2,4-methylpentadienyl)iron. This low-temperature (T = 120 °C) route to conformal deposition of crystalline Fe3O4 or α-Fe2O3 thin films is determined by the choice of oxygen source selected for the second surface half-reaction. The approach employs ozone to produce fully oxidized α-Fe2O3 or a milder oxidant, H2O2, to generate the Fe(2+)/Fe(3+) spinel, Fe3O4. Both processes show self-limiting surface reactions and deposition rates of at least 0.6 Å/cycle, a significantly high growth rate at such mild conditions. We utilized this process to prepare conformal iron oxide thin films on a porous framework, for which α-Fe2O3 is active for photocatalytic water splitting.

Hematite-based photo-oxidation of water using transparent distributed current collectors

High specific surface area transparent and conducting frameworks, fabricated by atomic layer deposition (ALD), were used as scaffolds for fabrication of equally high area, ALD-formed hematite structures for photo-oxidation of water to dioxygen. The frameworks offer high transparency to visible light and robust conductivity under harsh annealing and oxidizing conditions. Furthermore, they also make possible the spatially distributed collection of photocurrent from ultrathin coatings of hematite layers, enabling the formation of photoanodes featuring both large optical extinction and a hematite layer thickness nearly commensurate with the hole-collection distance. The distributed-current-collection approach increases the efficiency of water oxidation with hematite (by about a factor of 3 compared with an optimized flat electrode), is highly adaptable to future advances in thin film technology, and is further applicable to a multitude of nanostructures and optoelectronic applications that require ultrathin films without sacrificing optical thickness.