AmericanChemicalSociety.com

The development of more efficient and stable catalysts has been an increasingly important goal for chemists and materials scientists for both economic and environmental merits.  The significant progress in the synthesis of colloidal metal nanoparticles allows for the design of catalysts with superior performance by taking advantage of nanoparticles’ high surface-to-volume ratio and their shape-dependent surface structure.  However, because of their high surface energies, nanoparticles tend to coagulate and/or change shape when taking part in catalytic reactions and eventually lose their initial activity and selectivity.  With the support from ACS PRF grant, we have focused our research in the past year to improve the stability, recyclability, and catalytic selectivity of metal nanocatalysts. 

We have developed a general “surface-protected etching” strategy to create mesopores in the outer oxide shells so that mass transfer can occur but catalyst particles are remained isolated.  This process involves protection of the oxide particle surface by a layer of polymeric ligands and subsequent preferential etching of material from the interior of the particle.  Protection by the polymer allows the oxide particles to retain their original size while selective etching at the interior produces porous structures and eventually hollow spheres.  We have recently demonstrated the transformation of sol-gel derived silica particles into porous shells and hollow spheres by using PVP as the protecting ligand, and NaOH as the etching agent.  PVP is chosen as the surface protecting agent because of the strong hydrogen bonds that form between its carbonyl groups and the hydroxyls on a silica surface.  The presence of PVP on the surface dramatically decreases the overall etching rate of silica spheres in NaOH solution.  As the silica surface is protected by PVP, OH^- ions diffuse into the interior of silica spheres and lead to comparatively high etching rates at the central portions, yielding hollow spheres upon continued etching.  Simultaneously, the silica in the surface layer takes on a rougher, porous appearance in transmission contrast, which can be attributed to partial and localized etching of the remaining shell material.

Another process that allows the direct transformation of solid silica into porous shells involves the spontaneous dissolution and regrowth of silica in NaBH₄ solution.  We have recently discovered that silica colloids, when dispersed in an aqueous solution of NaBH₄, undergo a spontaneous morphology change from solid to hollow spheres.  Parallel but temporally/spatially separate core-dissolution and shell-growth processes appear to be responsible for the formation of the hollow structures.  Crucially, it is the unique chemistry of NaBH₄ (a high pH and slow decomposition into by-products) that provides the correct conditions for the growth of hollow shells.

We have applied the etching-based approaches for producing nanomaterials that can greatly improve the stability of catalyst nanoparticles against coagulation during reactions.  In this strategy, catalyst particles are first stabilized by encapsulating in a layer of oxides such as SiO₂.  Then, the “surface-protected etching” process is used to conveniently convert dense oxide coatings into porous shells so that chemical species can reach the core material to participate in reactions while the shells still act as physical barriers preventing the aggregation of the catalyst particles.  The permeation rate of chemical species through the shells can be controlled by varying the extent of etching.  To further increase the loading of catalyst particles, we have also developed a core-satellite nanocomposite catalyst system by first immobilizing a monolayer of metal nanocatalysts on the surface of silica supports, then overcoating the composite particles with another layer of silica to fix the position of metal nanoparticles, and finally exposing the catalysts to outside chemical species by creating mesopores in the outer shells through a “surface-protected etching” scheme.  

We have explored the introduction of superparamagnetic components to the catalyst supports to significantly improve the recycling efficiency.  Magnetically responsive hierarchical assemblies of silica colloids have been developed as recoverable supports of metal nanocatalysts for liquid phase reactions.  Such composite catalysts are expected to find use in many important industrial applications where separation and recycling are critically required to reduce the materials cost as well as waste production.  

We have also studied the stability of metallic nanoparticles such as Ag Nanoplates under UV Irradiation.  We have been able to tune the optical property and improve chemical stability of Ag nanoplates by precisely controlling the geometric shapes.  It has also been demonstrated the optical property of Au nanoparticles can be tuned by controlling their assembly form, for example, controlling the interparticle coupling.

In addition to nanocatalysts, we also expanded our research to the fabrication of magnetically tunable photonic structures by using the support partially from the ACS PRF grant.  A highly tunable photonic crystal system has been developed by self-assembling superparamagnetic colloidal particles of magnetite into ordered arrays by establishing a balance between the interparticle repulsive force and magnetically induced attractive force.  Diffraction occurs when the periodicity of the assembled structure and the wavelength of the incident light satisfy the Bragg condition.  A variation in the strength of the magnetic field changes the induced attractive force, and consequently the interparticle separation and the diffraction wavelength. 

We focused on developing methods for establishing repulsive and attractive interactions of comparable strengths in solvents of various polarities.  In particular, we have addressed the challenge of assembly in nonpolar solvents by using reverse micelles to help initiate the charge separation and create strong electrostatic interaction.  The success of assembly in various solvents allows us to develop a single-ink full-color printing system where the color was created by assembling superparamagnetic particles in UV curable polymer resin followed by color fixing using UV irradiation.  The polymer film containing superparamagnetic photonic crystal structures can be used as rewritable photonic paper by using hygroscopic salt solution as ink.  By rapid curing of emulsion droplets of magnetite/resin mixture, we have also developed a new type of tunable photonic crystal structure whose color can be tuned by rotating the relative orientation of these composite microspheres.