Shery L. Y. Chang

Highly active nickel oxide water oxidation catalysts deposited from molecular complexes

Nickel oxide (NiOx) water oxidation catalysts with high catalytic activity have been electrodeposited from [Ni(en)3]Cl2 (en = 1,2-diaminoethane, NiOx-en) in a 0.10 M borate buffer (NaBi) solution (pH = 9.2). Electrolysis experiments at a fixed applied potential of 1.1 V (vs. Ag/AgCl) established that the NiOx-en films sustain a stable current of 1.8 mA cm−2 for extended periods, compared with 1.2 mA cm−2 for films derived from [Ni(OH2)6](NO3)2 and [Ni(NH3)6]Cl2 when tested in a 0.60 M NaBi buffer. XAS studies indicate that the γ-NiOOH phase is formed in each case whereas SEM studies revealed significant differences in film morphology. The NiOx-en films were found to be more homogenous and to have a higher electroactive surface area, as determined from capacitance measurements. The results highlight the influence that the choice of molecular precursor can have on the activity and robustness of electrodeposited NiOx water oxidation catalysts.

Highly active screen-printed electrocatalysts for water oxidation based on β-manganese oxide

A versatile screen-printing method is applied for the preparation of efficient water oxidation catalysts based on a nanostructured β-MnO2 material prepared by a redox-precipitation method, and commercial β-MnO2. The catalyst films were tested for activity in water oxidation over a range of neutral to alkaline pH. The onset of water oxidation in case of the nanostructured MnO2 films is found at an overpotential (η) of 300 mV at pH 13.6 (1.0 M NaOH), with current densities reaching 10 mA cm−2 at η = 500 mV. The screen-printed MnO2 (nano) is one of the most active manganese oxide-based catalysts reported to date, despite consisting mostly of the common pyrolusite (β-MnO2) phase, so far generally found inactive in water oxidation. The films prepared from commercial β-MnO2 were found to be moderately active, with an onset of water oxidation at η = 500 mV (pH 13.6), and currents up to 5 mA cm−2 at η = 800 mV. At pH 6, the two samples exhibit similar activity and also match or surpass the performance of recent benchmark manganese oxides. X-ray absorption spectroscopy (XAS) studies suggest that the crystal phase is unchanged after prolonged electrochemical cycling. Scanning electron microscopy (SEM) analysis indicates very little corrosion of the surface morphology after prolonged catalyst operation at alkaline pH. However, high-resolution transmission electron microscopy (HRTEM) analysis shows the formation of a small amount of an amorphous phase on the surface of the nanorods after oxygen evolution over 12 hours in alkaline media.

Anodic deposition of NiOx water oxidation catalysts from macrocyclic nickel(II) complexes

Molecular complexes have been found to be excellent precursors for the deposition of catalytically active metal oxide films. Here, three macrocyclic Ni(II) amine complexes have been used for the electrochemical deposition of NiOx films from either a borate buffer solution (pH 9.2) or more basic conditions (pH = 12.9). The cyclic voltammetry of the complexes in both electrolytes shows very similar features and indicates the deposition of a catalytically active nickel oxide film. The NiOx films have been characterised using infrared (IR) and Raman spectroscopy complemented by scanning electron microscopy. Testing of the water oxidation activity at pH 9.2 and 12.9 showed that the films deposited from macrocyclic Ni(II) complexes exhibit similar catalytic activity to those derived from Ni2+ salts. The macrocyclic complexes offer the advantage of greater solubility and solution stability over a wider range of deposition conditions. At pH 9.2, the catalytic activity of the NiOx films was significantly higher when using a borate buffer and, in addition, the films were more active at pH 12.9 than at pH 9.2. The NiOx films deposited from molecular complexes were also found to show electrochromic properties. The oxidation of water by these films was enhanced by visible light. Water oxidation currents were observed to increase by ∼20% under simulated solar radiation.

Shape Control from Thermodynamic Growth Conditions: The Case of hcp Ruthenium Hourglass Nanocrystals

Recent successes in forming different shaped face centered cubic (fcc) metal nanostructures has enabled a greater understanding of nanocrystal growth mechanisms. Here we extend this understanding to the synthesis of hexagonally close packed (hcp) metal nanostructures, to form uniquely faceted ruthenium nanocrystals with a well-defined hourglass shape. The hourglass nanocrystals are formed in a three-step thermodynamic growth process with dodecylamine as the organic stabilizer. The hourglass nanocrystals are then shown to readily self-assemble to form a new type of nanocrystal superlattice.

Formation of a Nanoparticulate Birnessite‐Like Phase in Purported Molecular Water Oxidation Catalyst Systems

The fate of [MnIII/IV2(μ‐O)2(terpy)2(H2O)2]3+ (1) under conditions typically applied to test its ability to catalyze water oxidation was studied by X‐ray absorption spectroscopy and UV/Vis spectrophotometry by using [MnIII/IV2(μ‐O)2(bipy)4]3+ (2) and Mn2+ as controls (terpy=2,2′:6′,2“‐terpyridine, bipy=2,2′‐bipyridine). The sample matrix, pH and choice of oxidizing agent were found to have a significant effect on the species formed under catalytic conditions. At low range pH values (4–6), homogeneous catalysis testing in oxone implied that 1 remains intact, whereas in clay intercalate there is strong evidence that 1 breaks down to a birnessite‐like phase. In homogeneous solutions at higher pH, the results are consistent with the same birnessite‐like structure identified in the clay intercalate. The use of the molecular complexes, as a source of manganese instead of simple MnII salts, was found to have the effect of slowing down oxide formation and particle aggregation in solution. The original analytical results that implied the systems are molecular are discussed in the context of these new observations.

Electron-Beam Mapping of Vibrational Modes with Nanometer Spatial Resolution

We demonstrate that a focused beam of high-energy electrons can be used to map the vibrational modes of a material with a spatial resolution of the order of one nanometer. Our demonstration is performed on boron nitride, a polar dielectric which gives rise to both localized and delocalized electron-vibrational scattering, either of which can be selected in our off-axial experimental geometry. Our experimental results are well supported by our calculations, and should reconcile current controversy regarding the spatial resolution achievable in vibrational mapping with focused electron beams.

Performance of a direct detection camera for off-axis electron holography

The performance of a direct detection camera (DDC) is evaluated in the context of off-axis electron holographic experiments in a transmission electron microscope. Its performance is also compared directly with that of a conventional charge-coupled device (CCD) camera. The DDC evaluated here can be operated either by the detection of individual electron events (counting mode) or by the effective integration of many such events during a given exposure time (linear mode). It is demonstrated that the improved modulation transfer functions and detective quantum efficiencies of both modes of the DDC give rise to significant benefits over the conventional CCD cameras, specifically, a significant improvement in the visibility of the holographic fringes and a reduction of the statistical error in the phase of the reconstructed electron wave function. The DDCs linear mode, which can handle higher dose rates, allows optimisation of the dose rate to achieve the best phase resolution for a wide variety of experimental conditions. For suitable conditions, the counting mode can potentially utilise a significantly lower dose to achieve a phase resolution that is comparable to that achieved using the linear mode. The use of multiple holograms and correlation techniques to increase the total dose in counting mode is also demonstrated.

Catalytic Activity and Impedance Behavior of Screen‐Printed Nickel Oxide as Efficient Water Oxidation Catalysts

We report that films screen printed from nickel oxide (NiO) nanoparticles and microballs are efficient electrocatalysts for water oxidation under near-neutral and alkaline conditions. Investigations of the composition and structure of the screen-printed films by X-ray diffraction, X-ray absorption spectroscopy, and scanning electron microscopy confirmed that the material was present as the cubic NiO phase. Comparison of the catalytic activity of the microball films to that of films fabricated by using NiO nanoparticles, under similar experimental conditions, revealed that the microball films outperform nanoparticle films of similar thickness owing to a more porous structure and higher surface area. A thinner, less-resistive NiO nanoparticle film, however, was found to have higher activity per Ni atom. Anodization in borate buffer significantly improved the activity of all three films. X-ray photoelectron spectroscopy showed that during anodization, a mixed nickel oxyhydroxide phase formed on the surface of all films, which could account for the improved activity. Impedance spectroscopy revealed that surface traps contribute significantly to the resistance of the NiO films. On anodization, the trap state resistance of all films was reduced, which led to significant improvements in activity. In 1.00 m NaOH, both the microball and nanoparticle films exhibit high long-term stability and produce a stable current density of approximately 30 mA cm(-2) at 600 mV overpotential.

Cadmium oxide/alkali metal halide mixtures – a potential high capacity sorbent for pre-combustion CO2 capture

A series of cadmium oxide based materials were prepared by mixing cadmium carbonate with alkali metal halides. Subsequent heat treatment then transformed the cadmium carbonate into oxide to yield the active carbon dioxide sorbent. It was observed from thermogravimetric analysis that neat cadmium oxide does not sorb significant amounts of carbon dioxide, whereas doping the material with alkali halides facilitates conversion to cadmium carbonate. The cadmium oxide/sodium iodide mixture, in particular, was found to reversibly bind up to 24 wt% carbon dioxide in the temperature range of 250 to 300 °C, which is consistent with an almost stoichiometric conversion of the cadmium oxide to cadmium carbonate. The carbon dioxide could subsequently be released, in the same temperature range, when the gas supply was switched from carbon dioxide to an inert gas flow. The formation of the carbonate was separately verified by both infrared spectrometry and powder X-ray diffraction (XRD). In addition, XRD provided simultaneous detection of both the oxide and carbonate phases thus demonstrating their inter-dependency and is consistent with the absence of other cadmium phases. Le Bail refinement of the unit cell parameters did not reveal a significant change in the unit cell size of the cadmium oxide or carbonate due to mixing with alkali metal halides. Transmission electron microscopy on a 17.5% NaI sample indicated that the material consists of spherical particles of ∼250 nm diameter. Nitrogen physisorption experiments showed that the sodium iodide-enhanced material is non-porous and of a low surface area.

Stability of Porous Platinum Nanoparticles: Combined In Situ TEM and Theoretical Study.

Porous platinum nanoparticles provide a route for the development of catalysts that use less platinum without sacrificing catalytic performance. Here, we examine porous platinum nanoparticles using a combination of in situ transmission electron microscopy and calculations based on a first-principles-parametrized thermodynamic model. Our experimental observations show that the initially irregular morphologies of the as-sythesized porous nanoparticles undergo changes at high temperatures to morphologies having faceted external surfaces with voids present in the interior of the particles. The increasing size of stable voids with increasing temperature, as predicted by the theoretical calculations, shows excellent agreement with the experimental findings. The results indicate that hollow-structured nanoparticles with an appropriate void-to-total-volume ratio can be stable at high temperatures.