Al3+ Modification of Graphene Oxide Membranes: Effect of Al Source.

Graphene oxide (GO) membranes are promising materials for water filtration applications due to abundant nanochannels in the membrane structure. Because GO membranes are unstable in water, metal cations such as Al3+ are often introduced to the membrane structure to promote cross-linking between individual GO sheets. Here, we describe a simple yet versatile method to incorporate Al3+ into GO membranes formed via a slow self-assembly process. Specifically, we directly added aluminum to acidic GO sheet solutions from a variety of sources: Al2O3, AlCl3 and Al foil. Each species reacts differently with water, which can affect the GO solution pH and thus the density of carboxylate groups on the sheet edges available for cross-linking to the Al3+ cations. We demonstrate through characterization of the GO sheet solutions as well as the as-formed membranes' morphologies, hydrophobicities, and structures that the extent to which the Al3+ cross-links to the GO sheet edges vs. the GO sheet basal planes is dependent on the Al source. Our results indicate that greatest enhancements in the membrane stability occur when electrostatic and coordination interactions between Al3+ and the carboxylate groups on the GO sheet edges are more extensive than Al3+-π interactions between basal planes.

Using Al3+ to Tailor Graphene Oxide Nanochannels: Impact on Membrane Stability and Permeability.

Graphene oxide (GO) membranes, which form from the lamination of GO sheets, attract much attention due to their unique nanochannels. There is much interest in controlling the nanochannel structures and improving the aqueous stability of GO membranes so they can be effectively used in separation and filtration applications. This study employed a simple yet effective method of introducing trivalent aluminum cations to a GO sheet solution through the oxidation of aluminum foil, which modifies the nanochannels in the self-assembled GO membrane by increasing the inter-sheet distance while decreasing intra-sheet spacing. The Al3+ modification resulted in an increase in membrane stability in water, methanol, ethanol, and propanol, yet decreased membrane permeability to water and propanol. These changes were attributed to strong interactions between Al3+ and the membrane oxygenated functional groups, which resulted in an increase in membrane hydrophobicity and a decrease in the intra-sheet spacing as supported by surface tension, contact angle, atomic force microscopy, and X-ray photoelectron spectroscopy measurements. Our approach for forming Al3+ modified GO membranes provides a method for improving the aqueous stability and tailoring the permeation selectivity of GO membranes, which have the potential to be implemented in vapor separation and fuel purification applications.

Local transport properties, morphology and microstructure of ZnO decorated SiO2 nanoparticles.

We report on a novel, surfactant free method for achieving nanocrystalline ZnO decoration of an SiO(2) nanoparticle at ambient temperature. The size distributions of the naked and decorated SiO(2) nanoparticles are measured by means of dynamic light scattering, and a monodisperse distribution is observed for each. The morphology and microstructure of the nanoparticles are explored using atomic force microscopy and high resolution transmission electron microscopy. Investigation of the optical properties of the ZnO decorated SiO(2) nanoparticles shows absorption at 350 nm. This blue shift in absorption as compared to bulk ZnO is shown to be consistent with quantum confinement effects due to the small size of the ZnO nanocrystals. Finally, the local electronic transport properties of the nanoparticles are explored by scanning conductance atomic force microscopy. A memristive hysteresis in the transport properties of the individual ZnO decorated SiO(2) nanoparticles is observed. Optical absorption measurements suggest the presence of oxygen vacancies, whose migration and annihilation appear to contribute to the dynamic conduction properties of the ZnO decorated nanoparticles. We believe this to be the first demonstration of a ZnO decorated SiO(2) nanoparticle, and this represents a simple yet powerful way of achieving the optical and electrical properties of ZnO in combination with the simplicity of SiO(2) synthesis.