Xuehua Zhang

Controllable corrugation of chemically converted graphene sheets in water and potential application for nanofiltration

A combination of AFM, SEM and permeation experiments suggests that the amplitude of corrugation of chemically converted graphene (CCG) sheets in water can be readily controlled by hydrothermal treatment, leading to a new class of permeation-tuneable nanofiltration membranes.

The length scales for stable gas nanobubbles at liquid/solid surfaces

We present results focusing on whether gas bubbles are stable at all length scales at liquid/solid surfaces. Tapping mode atomic force microscopy was used to observe nanoscale bubbles of air or hydrogen produced by two methods on hydrophilic and hydrophobic solid surfaces. The observed nanobubbles of air or hydrogen always have curvature radii of less than 2.0 μm and heights of less than 100 nm, suggesting the possible instability of these nanobubbles beyond these length scales. This is further supported by the observation of blank circular areas on the electrode surface, which are interpreted as footprints of departed large bubbles with curvature radii equal to or above 2.0 μm from the surface.

Evaporation-induced flattening and self-assembly of chemically converted graphene on a solid surface

Solvent evaporation is found to significantly affect the morphology and assembly of chemically converted graphene drop cast on certain solid substrates. On negatively charged substrates, drying could induce the flattening and self-assembly of a single layer of CCG in an edge-to-edge manner. A mechanism is proposed to explain these phenomena.

From transient nanodroplets to permanent nanolenses

Nanodroplets can be conveniently produced by the established protocol, called the solvent exchange. In this work, the transient nanodroplets were converted to permanent polymeric nanolenses by forming nanodroplets of monomers followed by in situ photopolymerization. This method could produce nanolenses with desired single or multiple components over a large area on both planar and non-planar surfaces. The morphology (average size and height/lateral size ratio) and the components of nanolenses could be controlled by the characteristics of the nanodroplets.

Surfactant-mediated formation of polymeric microlenses from interfacial microdroplets

Nano- and micro-scale lenses have a range of potential applications, such as in antireflective layers in photovoltaic or light emission devices, and in super resolution imaging in the near field modes. One of the protocols to mass produce polymeric microlenses is through the polymerization of microdroplets of a monomer precursor that are produced at solid–liquid interfaces by a solvent exchange technique. In this work, we have advanced this protocol by using surfactants. A cationic surfactant was added to the liquid phase for the control over the formation and morphology of polymerisable microdroplets and their resultant microlenses (i.e. the polymerized microdroplets). The results demonstrate that the surfactant could enable the production of polymerizable microdroplets on hydrophilic substrates by the solvent exchange technique, and eliminate the restriction by the substrate wettability on the microlens fabrication. Furthermore, the size distribution and aspect ratio of microlenses could be tuned by the surfactant concentration.

Assembling of graphene oxide in an isolated dissolving droplet

Controlling of the morphology and assembly of graphene oxide (GO) is important in the structure design of carbon materials using GO sheets as building blocks. In this work, we have studied GO assembling driven by the dissolution of a droplet immersed in a surrounding liquid phase. The as-assembled GO structures were highly crumpled with exotic morphology, which we refer to as GO snowballs. The detailed structure of GO snowballs was closely related to the dissolution dynamics of the droplet, which could be adjusted by the composition of the surrounding liquid.

Stitching chemically converted graphene on solid surfaces by solvent evaporation

The suspension of chemically converted graphene (CCG) provides a cost-effective and facile approach to construct graphene-based materials. However, wrinkles and aggregates usually occur when transferring graphene from suspension to solid-state, which significantly alter the optical, electrical, and electrochemical properties of deposited graphene. Our effort is devoted to the control of the morphology of individual graphene sheet deposited on solid surfaces by the solvent evaporation. Here we have studied the effects of additional components (e.g., organic solvent and electrolyte) in the CCG suspension on the CCG morphology. It was found that the CCG sheets could be stitched together and the graphene monolayer could be flattened by the addition of appropriate additives to the CCG suspension.

Formation, characterization and stability of oil nanodroplets on immersed substrates.

Nanoscale oil droplets locating at solid-liquid interfaces significantly impact the interfacial properties, which are concerned in both industry applications and fundamental studies. This review article presents an overview of the current progress in nanodroplet research. We will start from the characterization of interfacial nanodroplets and the formation of interfacial nanodroplets by direct adsorption from emulsions and by the solvent exchange protocol. Then we will review the experimental and theoretical studies on the evolution of oil nanodroplets including spreading, dissolution, and detachment. We will also cover the emerging applications of the interfacial nanodroplets in the fields of surface functionalization and nanostructure engineering, and particularly, highlight the potential application as capping agents to obtain architectures on microparticle surface. Finally we propose the challenges and the opportunities in this area. In our opinion, the nanodroplets have not only of high relevance to practical applications, but also serve as a model system for understanding many interfacial phenomena, such as phase separation and wetting on a microscopic scale.

Efficient photoinduced charge transfer in chemically-linked organic-metal Ag-P3HT nanocomposites

A novel nanocomposite of P3HT and Ag nanoparticles (Ag-P3HT) has been synthesized by laser ablation. The fluorescence wavelength of the nanocomposite can be tuned by varying the ablation duration. The steady-state emission maximum at 580 nm for pristine P3HT shows significant blueshift to wavelengths ranging from 520 to 550 nm for Ag-P3HT. NMR, FTIR and XPS spectroscopy confirm that chemical links are formed between P3HT and silver nanoparticles (NPs) in the nanocomposite. For the sample with an emission maximum at 530 nm, 1H NMR spectra indicate that 66% of the P3HT thiophene ring protons are replaced by Ag NPs. Time-resolved spectroscopy demonstrates that charge transfer efficiency increases for Ag-P3HT and this is attributed to the enhanced intimate interfacial contact between the chemically-bonded metal NPs and polymer chains. The synthetic route outlined provides a one-pot and green strategy for producing metal-organic polymeric materials, with controlled luminescence wavelengths and efficient photoinduced charge transfer that meet the requirement for developing high-performance organic light-emitting and photovoltaic devices.

Optical Characterisation of Non-Covalent Interactions between Non-Conjugated Polymers and Chemically Converted Graphene

Optical characterisation using dye molecules as probes was used to study the non-covalent interactions between chemically converted graphene (CCG) and non-conjugated, water soluble polymers in aqueous solution. The strong adsorption of non-conjugated polymers such as poly(ethylene oxide) (PEO) and poly(vinyl alcohol) (PVA) on CCG is observed by fluorescence and ultraviolet-visible spectroscopy and atomic force microscopy, and this leads to desorption of π-conjugated molecules from CCG. Such adsorption/desorption behaviour can be tailored by modifying the molecular weight of polymers and the chemistry of graphene. This finding provides a facile and non-covalent approach to the functionalisation of CCG and opens up new opportunities for the fabrication of graphene/polymer nanocomposites.