Karina Abitaev

Abstract

ZnO inverse opals combine the outstanding properties of the semiconductor ZnO with the high surface area of the open-porous framework, making them valuable photonic and catalysis support materials. One route to produce inverse opals is to mineralize the voids of close-packed polymer nanoparticle templates by chemical bath deposition (CBD) using a ZnO precursor solution, followed by template removal. To ensure synthesis control, the formation and growth of ZnO nanoparticles in a precursor solution containing the organic additive polyvinylpyrrolidone (PVP) was investigated by in situ ultra-small- and small-angle X-ray scattering (USAXS/SAXS). Before that, we studied the precursor solution by in-house SAXS at T = 25 °C, revealing the presence of a PVP network with semiflexible chain behavior. Heating the precursor solution to 58 °C or 63 °C initiates the formation of small ZnO nanoparticles that cluster together, as shown by complementary transmission electron microscopy images (TEM) taken after synthesis. The underlying kinetics of this process could be deciphered by quantitatively analyzing the USAXS/SAXS data considering the scattering contributions of particles, clusters, and the PVP network. A nearly quantitative description of both the nucleation and growth period could be achieved using the two-step Finke–Watzky model with slow, continuous nucleation followed by autocatalytic growth.

Abstract

Abstract Microemulsions provide a unique opportunity to tailor the polarity and liquid confinement in asymmetric catalysis via nanoscale polar and nonpolar domains separated by a surfactant film. For chiral diene Rh complexes, the influence of counterion and surfactant film on the catalytic activity and enantioselectivity remained elusive. To explore this issue chiral norbornadiene Rh(X) complexes (X=OTf, OTs, OAc, PO2F2) were synthesized and characterized by X-ray crystallography and theoretical calculations. These complexes were used in Rh-catalyzed 1,2-additions of phenylboroxine to N-tosylimine in microemulsions stabilized either exclusively by n-octyl-β-D-glucopyranoside (C8G1) or a C8G1-film doped with anionic or cationic surfactants (AOT, SDS and DTAB). The Rh(OAc) complex showed the largest dependence on the composition of the microemulsion, yielding up to 59 \% (90 \%ee) for the surfactant film doped with 5 wt\% of AOT as compared to 52 \% (58 \%ee) for neat C8G1 at constant surfactant concentration. Larger domains, determined by SAXS analysis, enabled further increase in yield and selectivity while the reaction rate almost remained constant according to kinetic studies.

Abstract

The manifold applications of porous materials, such as in storage, separation, and catalysis, have led to an enormous interest in their cost-efficient preparation. A promising strategy to obtain porous materials with adjustable pore size and morphology is to use templates exhibiting the appropriate nanostructure. In this study, close-packed polystyrene (PS) nanoparticles, synthesized by emulsion polymerization, were used to produce porous PS and ZnO inverse opals. The size and distribution of the polystyrene nanoparticles, characterized by dynamic light scattering (DLS), small-angle neutron scattering (SANS), and scanning electron microscopy (SEM), were controlled via the concentration of sodium dodecyl sulfate (SDS). Systematic measurements of the water/styrene-interfacial tension show that the critical micelle concentration (CMC) of the ternary water--styrene--SDS system, which determines whether monodisperse or polydisperse PS particles are obtained, is considerably lower than that of the binary water--SDS system. The assemblies of close-packed PS nanoparticles obtained via drying were then studied by small-angle X-ray scattering (SAXS) and SEM. Both techniques prove that PS nanoparticles synthesized above the CMC result in a significantly unordered but denser packing of the particles. The polystyrene particles were subsequently used to produce porous polystyrene and ZnO inverse opals. While the former consists of micrometer-sized spherical pores surrounded by extended open-cellular regions of mesopores (Rpore ≈þinspace25 nm), the latter are made of ZnO-nanoparticles forming a structure of well-aligned interconnected pores.