Macroporous materials—electrochemically grown photonic crystals

Macroporous Silica Foams Fabricated via Soft Colloid Templating

Macroporous materials with controlled pore sizes are of high scientific and technological interest, due to their low specific weight, as well as unique acoustic, thermal, or optical properties. Solid foams made of titania, silica, or silicon, as representative materials, have been previously obtained with several hundred nanometer pore sizes, by using sacrificial templates such as spherical emulsion droplets or colloidal particles. Macroporous structures in particular are excellent candidates as photonic materials with applications in structural coloration and photonic bandgap devices. However, whereas using spherical building blocks as templates may provide tight control over pore shape and size, it results in materials with an often unfavorable local topology. Templating dry-foam or compressed-emulsion structures appear as attractive alternatives, but have not been demonstrated so far for submicron pore sizes. Herein, the use of soft, flexible microgel colloids decorated with silica nanoparticles as templates of macroporous foams is reported. These purposely synthesized core-shell colloids are assembled at ultra-high effective volume fractions by centrifuging and thermal swelling, thereby resulting in uniform disordered materials with facetted pores, mimicking dry foams. After removal of the polymer component via calcination, lightweight pure silica structures are obtained with a well-defined cellular or network topology.

Macroporous conductive polymer films fabricated by electrospun nanofiber templates and their electromechanical properties

We demonstrate a facile method to fabricate macroporous poly (3,4-ethylenedioxythiophene)/poly (4-styrene sulfonate) (PEDOT/PSS) films with empty channels by using electrospun nanofiber as a sacrificial template. The channels within the PEDOT/PSS films were prepared by depositing PEDOT/PSS aqueous dispersion onto poly (vinyl pyrrolidone)/poly(methyl methacrylate) (PVP/PMMA) nanofiber template, and then the nanofibers were removed by solvent extraction. The average diameter of the channels is 313±45 nm, which is almost the same as the parent PVP/PMMA nanofibers. The macroporous PEDOT/PSS film with the empty channels showed an enhancement of electromechanical properties compared to the nonporous PEDOT/PSS film.

Mimicking the colourful wing scale structure of the Papilio blumei butterfly

The brightest and most vivid colours in nature arise from the interaction of light with surfaces that exhibit periodic structure on the micro- and nanoscale. In the wings of butterflies, for example, a combination of multilayer interference, optical gratings, photonic crystals and other optical structures gives rise to complex colour mixing. Although the physics of structural colours is well understood, it remains a challenge to create artificial replicas of natural photonic structures. Here we use a combination of layer deposition techniques, including colloidal self-assembly, sputtering and atomic layer deposition, to fabricate photonic structures that mimic the colour mixing effect found on the wings of the Indonesian butterfly Papilio blumei. We also show that a conceptual variation to the natural structure leads to enhanced optical properties. Our approach offers improved efficiency, versatility and scalability compared with previous approaches.

Controlling the electrodeposition of mesoporous metals for nanoplasmonics

We have investigated the mechanism for formation of large-scale ordered structures of copper and silver by direct electrodeposition through colloidal crystal templates. It is demonstrated that the morphology is controlled by the diffusion of reactant species (metal cations and oxygen molecules) from the bulk solution to the electrode surface. The Cu system can be tuned to fill or not to fill the original drying cracks in the template in order to produce isolated or interconnected domains. This has great potential for the formation of individual or interconnected antennae in nanoplasmonic structures.

Orientationally correlated colloidal polycrystals without long-range positional order

We probe the local and global structure of spin-coated colloidal crystals via laser diffraction measurements and scanning electron and atomic force microscopies, and find that they are unique three-dimensional orientationally correlated polycrystals, exhibiting short-range positional order but long-range radial orientational correlations, reminiscent of-but distinct from-two-dimensional colloidal hexatic phases. Thickness and symmetries are controllable by solvent choice and spin speed. While the polycrystallinity of these colloidal films limits their applicability to photonics, we demonstrate their feasibility as templates to make crack-free magnetic patterns.

Electrochemical preparation of distinct polyaniline nanostructures by surface charge control of polystyrene nanoparticle templates

A family of nanostructured polyaniline (PANI) materials including polystyrene (PS)/PANI core/shell particles, PANI hollow spheres, PANI/PS nanocomposite and nanoporous PANI, were conveniently prepared by surface charge control of PS nanoparticle templates which resulted in different polymer growth mechanisms when PANI was electropolymerized around the templates.

Fabrication of hollow colloidal crystal cylinders and their inverted polymeric replicas

We fabricated colloidal crystals on a fiber by a dip-coating method. The self-assembly of monodisperse colloidal particles was affected by the curvature of the fiber (the reciprocal of the fiber radius). As the fiber became smaller in diameter, fewer layers of the colloidal spheres were coated for a given lift-up speed. The hollow colloidal crystal cylinders were used as a template for creating macroporous structure having three-dimensionally interconnected air cavities. Specifically, the polymer precursor was infiltrated into the colloidal crystal template and the macroporous polymer structures were obtained after the selective etching of colloidal particles.

Nonenzymatic Glucose Detection by Using a Three-Dimensionally Ordered, Macroporous Platinum Template

A three-dimensionally ordered, macroporous, inverse-opal platinum film was synthesized electrochemically by the inverted colloidal-crystal template technique. The inverse-opal film that contains platinum nanoparticles showed improved electrocatalytic activity toward glucose oxidation with respect to the directly deposited platinum; this improvement is due to the interconnected porous structure and the greatly enhanced effective surface area. In addition, the inverse-opal Pt-film electrode responds more sensitively to glucose than to common interfering species of ascorbic acid, uric acid, and p-acetamidophenol due to their different electrochemical reaction mechanisms. Results showed that the ordered macroporous materials with enhanced selectivity and sensitivity are promising for fabrication of nonenzymatic glucose biosensors.

Optical properties of nanoparticle-based metallodielectric inverse opals

Metallodielectric inverse opals were prepared by co-crystallizing silica-coated gold nanoparticles and polymer spheres, followed by removal of the crystal template. The inverse opals exhibit a distinct reflectance peak, which results from Bragg diffraction due to the highly ordered 3D macroporous structure. Photonic band-structure calculations indicate that the characteristic reflectance peaks observed are signatures of the directional gap at the L point. It is demonstrated that the optical properties (the position and magnitude of the electromagnetic bandgaps) of the gold-silica nanocomposite inverse opals can be engineered by varying the nanoparticle morphology (core size and shell thickness) and/or the nanoparticle volume-filling ratio of the composite. The use of metallodielectric nanoparticles to form inverse opals offers a versatile approach to prepare photonic materials that may exhibit absolute bandgaps.

Cluster size analysis of two-dimensional order in colloidal gold nanoparticle arrays

A protocol for cluster size distribution analysis was developed in order to parametrize local two-dimensional (2D) order in a quantitative manner, using mean cluster sizes and fractional hcp cluster formation (fhcp). Cluster size analysis was performed on 2D arrays of Au nanoparticles encapsulated in resorcinarene tetrathiol, which were organized into close-packed films at aqueous interfaces. The degree of monolayer formation and 2D order within the self-assembled nanoparticle arrays was observed to be strongly dependent on the amount and type of electrolyte (chloride and/or citrate) adsorbed on the nanoparticle surface, prior to encapsulation and extraction to the solvent interface. Increasing the concentration of adsorbed electrolyte could promote monoparticulate film formation but had a variable effect on local 2D order.