Hugh Geaney

Structuring materials for lithium-ion batteries: advancements in nanomaterial structure, composition, and defined assembly on cell performance

This review outlines the developments in the structure, composition, size, and shape control of many important and emerging Li-ion battery materials on many length scales, and details very recent investigations on how the assembly and programmable order in energy storage materials have not only influenced and dramatically improved the performance of some Li-ion batteries, but offered new routes toward improved power densities. This review also describes and discusses material aspects of hybrid and multiphasic materials including silicon, germanium, a wide range of metal oxides, alloys and crystal structures, carbons and other important materials. Methods including engineered porosity that offer the energy density of Li-ion batteries and the power density of pseudocapacitors are also highlighted. Recent developments in the analytical methods, electrochemical response, and the structure, composition, size, shape and defined assembly of active materials for a wide range of Li-ion cathodes and anodes are compared and assessed with respect to cell performance. Perspectives on the future development of energy storage materials based on structure as well as chemistry are also outlined.

Metal-assisted chemical etching of silicon and the behavior of nanoscale silicon materials as Li-ion battery anodes

This review outlines the developments and recent progress in metal-assisted chemical etching of silicon, summarizing a variety of fundamental and innovative processes and etching methods that form a wide range of nanoscale silicon structures. The use of silicon as an anode for Li-ion batteries is also reviewed, where factors such as film thickness, doping, alloying, and their response to reversible lithiation processes are summarized and discussed with respect to battery cell performance. Recent advances in improving the performance of silicon-based anodes in Li-ion batteries are also discussed. The use of a variety of nanostructured silicon structures formed by many different methods as Li-ion battery anodes is outlined, focusing in particular on the influence of mass loading, core-shell structure, conductive additives, and other parameters. The influence of porosity, dopant type, and doping level on the electrochemical response and cell performance of the silicon anodes are detailed based on recent findings. Perspectives on the future of silicon and related materials, and their compositional and structural modifications for energy storage via several electrochemical mechanisms, are also provided.

2D and 3D vanadium oxide inverse opals and hollow sphere arrays

High quality 2D and 3D inverse opals and hollow sphere arrays of vanadium oxide are grown on conductive substrates from colloidal polymer sphere templates formed by electrophoretic deposition or surfactant-assisted dip-coating. Inverse opals (IOs) are formed using variants of solution drop-casting, N2-gun assisted infiltration and high-rate (200 mm min−1) iterative dip-coating methods. Through Raman scattering, transmission electron microscopy and optical diffraction, we show how the oxide phase, crystallinity and structure are inter-related and controlled. Opal template removal steps are demonstrated to determine the morphology, crystallinity and phase of the resulting 2D and 3D IO structures. The ability to form high quality 2D IOs is also demonstrated using UV Ozone removal of PMMA spheres. Rapid hydrolysis of the alkoxide precursor allows the formation of 2D arrays of crystalline hollow spheres of V2O5 by utilizing over-filling during iterative dip-coating. The methods and crystallinity control allow 2D and 3D hierarchically structured templates and inverse opal vanadium oxides directly on conductive surfaces. This can be extended to a wide range of other functional porous materials for energy storage and batteries, electrocatalysis, sensing, solar cell materials and diffractive optical coatings.

2D and 3D photonic crystal materials for photocatalysis and electrochemical energy storage and conversion

Abstract This perspective reviews recent advances in inverse opal structures, how they have been developed, studied and applied as catalysts, catalyst support materials, as electrode materials for batteries, water splitting applications, solar-to-fuel conversion and electrochromics, and finally as photonic photocatalysts and photoelectrocatalysts. Throughout, we detail some of the salient optical characteristics that underpin recent results and form the basis for light-matter interactions that span electrochemical energy conversion systems as well as photocatalytic systems. Strategies for using 2D as well as 3D structures, ordered macroporous materials such as inverse opals are summarized and recent work on plasmonic–photonic coupling in metal nanoparticle-infiltrated wide band gap inverse opals for enhanced photoelectrochemistry are provided.

High performance inverse opal Li-ion battery with paired intercalation and conversion mode electrodes

Structured porous materials have provided several breakthroughs that have facilitated high rate capability, better capacity retention and material stability in Li-ion batteries. However, most advances have been limited to half cells or lithium batteries, and with a single mode of charge storage (intercalation, conversion, or alloying etc.). The use of dual-mode charge storage with non-traditional material pairings, while maintaining the numerous benefits of nanoscale materials, could significantly improve the capacity, energy density, stability and overall battery safety considerably. Here, we demonstrate an efficient, high capacity full inverse opal Li-ion battery with excellent cycle life, where both the cathode and anode binder-free electrodes are composed of 3D nanocrystal assemblies as inverse opal (IO) structures of intercalation-mode V2O5 IO cathodes and conversion-mode Co3O4 IO anodes. Electrochemically charged Co3O4 IOs function as Li-ion anodes and the full V2O5/Co3O4 cell exhibits superior performance compared to lithium batteries or half cells of either IO material, with voltage window compatibility for high capacity and energy density. Through asymmetric charge–discharge tests, the V2O5 IO/Co3O4 IO full Li-ion cell can be quickly charged, and discharged both quickly and slowly without any capacity decay. We demonstrate that issues due to the decomposition of the electrolyte with increased cycling can be overcome by complete electrolyte infiltration to remove capacity fading from long term cycling at high capacity and rate. Lastly, we show that the V2O5 IO/Co3O4 IO full Li-ion cells cycled in 2 and 3-electrode flooded cells maintain 150 mA h g−1 and remarkably, show no capacity fade at any stage during cycling for at least 175 cycles. The realization of an all-3D structured anode and cathode geometry with new mutually co-operative dual-mode charge storage mechanisms and efficient electrolyte penetration to the nanocrystalline network of material provides a testbed for advancing high rate, high capacity, stable Li-ion batteries using a wide range of materials pairings.

Doping controlled roughness and defined mesoporosity in chemically etched silicon nanowires with tunable conductivity

By using Si(100) with different dopant type (n++-type (As) or p-type (B)), we show how metal-assisted chemically etched (MACE) nanowires (NWs) can form with rough outer surfaces around a solid NW core for p-type NWs, and a unique, defined mesoporous structure for highly doped n-type NWs. We used high resolution electron microscopy techniques to define the characteristic roughening and mesoporous structure within the NWs and how such structures can form due to a judicious choice of carrier concentration and dopant type. The n-type NWs have a mesoporosity that is defined by equidistant pores in all directions, and the inter-pore distance is correlated to the effective depletion region width at the reduction potential of the catalyst at the silicon surface in a HF electrolyte. Clumping in n-type MACE Si NWs is also shown to be characteristic of mesoporous NWs when etched as high density NW layers, due to low rigidity (high porosity). Electrical transport investigations show that the etched nanowires exhibit ...

Mesoporosity in doped silicon nanowires from metal assisted chemical etching monitored by phonon scattering

Higher Education Authority (Irish Government’s Programme for Research in Third Level Institutions, Cycle 4, National Development Plan 2007–2013); Irish Research Council (Award No. RS/2011/797); Science Foundation Ireland (under the National Access Programme (NAP n417))

Solvent-less method for efficient photocatalytic α-Fe2O3 nanoparticles using macromolecular polymeric precursors

We report a method for solvent-less growth of single crystalline hematite Fe2O3 nanoparticles from metal-containing polymeric macromolecular complexes, and demonstrate their efficient photocatalytic degradation of persistent cationic dye pollutants under visible light. Macromolecular complexes such as chitosan·(FeCl2)y, chitosan·(FeCl3)y, PS-co-4-PVP·(FeCl2)y and PS-co-4-PVP·(FeCl3)y with controlled polymer:metal molar ratios of 1:1 and 5:1 were prepared by single reaction of the respective polymers and iron chloride salts in CH2Cl2. The stable insoluble compounds were characterized by elemental analysis, infra-red spectroscopy, EPR and diffuse reflectance spectroscopy, and confirm Fe salts with degrees of coordination of ∼60–70%. Pyrolysis of these macromolecular precursors under air and at 800 °C forms networked Fe2O3 nanoparticles, whose volumetric density, size and shape is controlled by the metal content and the nature of the macromolecular complex (chitosan or PS-co-4-PVP). For both polymers, the 1:1 molar ratio precursor produces nanoparticles ranging from 10–200 nm with a moderate superparamagnetic behavior and optical bandgap marginally larger than bulk Fe2O3. A matrix-incubated formation mechanism involving the carbonization of the organic matter, forming voids within the macromolecular complex wherein the Fe centres coalesce, oxidize and crystallize into nanoparticles is also proposed. The hematite Fe2O3 nanoparticle materials demonstrate very efficient photocatalytic degradation of persistent water pollutants such as the cationic dye methylene blue. The nanoparticulate material obtained from chitosan·(FeCl2)y 1:1 under the simulated sunlight (full visible spectrum) irradiation provides high rate degradation of MB by 73% in 60 min and >94% after 150 min, measured at 655 nm.

Fabrication of p-type porous GaN on silicon and epitaxial GaN

Porous GaN layers are grown on silicon from gold or platinum catalyst seed layers, and self-catalyzed on epitaxial GaN films on sapphire. Using a Mg-based precursor, we demonstrate p-type doping of the porous GaN. Electrical measurements for p-type GaN on Si show Ohmic and Schottky behavior from gold and platinum seeded GaN, respectively. Ohmicity is attributed to the formation of a Ga2Au intermetallic. Porous p-type GaN was also achieved on epitaxial n-GaN on sapphire, and transport measurements confirm a p-n junction commensurate with a doping density of ∼1018u2009cm−3. Photoluminescence and cathodoluminescence confirm emission from Mg-acceptors in porous p-type GaN.

Influence of binders and solvents on stability of Ru/RuOx nanoparticles on ITO nanocrystals as Li–O2 battery cathodes

Fundamental research on Li-O2 batteries remains critical, and the nature of the reactions and stability are paramount for realising the promise of the Li-O2 system. We report that indium tin oxide (ITO) nanocrystals with supported 1-2u2005nm oxygen evolution reaction (OER) catalyst Ru/RuOx nanoparticles (NPs) demonstrate efficient OER processes, reduce the recharge overpotential of the cell significantly and maintain catalytic activity to promote a consistent cycling discharge potential in Li-O2 cells even when the ITO support nanocrystals deteriorate from the very first cycle. The Ru/RuOx nanoparticles lower the charge overpotential compared with those for ITO and carbon-only cathodes and have the greatest effect in DMSO electrolytes with a solution-processable F-free carboxymethyl cellulose (CMC) binder (<3.5u2005V) instead of polyvinylidene fluoride (PVDF). The Ru/RuOx /ITO nanocrystalline materials in DMSO provide efficient Li2 O2 decomposition from within the cathode during cycling. We demonstrate that the ITO is actually unstable from the first cycle and is modified by chemical etching, but the Ru/RuOx NPs remain effective OER catalysts for Li2 O2 during cycling. The CMC binders avoid PVDF-based side-reactions and improve the cyclability. The deterioration of the ITO nanocrystals is mitigated significantly in cathodes with a CMC binder, and the cells show good cycle life. In mixed DMSO-EMITFSI [EMITFSI=1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide] ionic liquid electrolytes, the Ru/RuOx /ITO materials in Li-O2 cells cycle very well and maintain a consistently very low charge overpotential of 0.5-0.8u2005V.