Films of Graphene Nanomaterials Formed by Ultrasonic Spraying of Their Stable Suspensions

<div><p>Graphene, a two-dimensional carbon allotrope, exhibits excellent optoelectronic properties. The assembly of graphene into films provides a platform to deepen the study of its interaction with varying surfaces, to engineer devices, and to develop functional materials. A general approach to produce graphene films consists of preparing a dispersion and laying it on a substrate of choice, followed by solvent evaporation. Here, we report the preparation of stable suspensions of new types of graphene nanomaterials namely, graphene nanoflowers (GNFs) and multi-layer graphene (MLG) flakes, in ethanol, <i>N</i>,<i>N</i>-dimethylformamide (DMF), and <i>N</i>-methyl-2-pyrrolidone (NMP). Sprayable suspensions of both GNFs and MLG were prepared in DMF/ethanol, which showed high stability, without addition of any surfactant. The stable suspensions were used to deposit micrometer-thick MLG/GNF films on glass substrates. Calculations of initial droplet size and of timescale of droplet evaporation are performed and possible thermophoretic effects on droplet deposition discussed as well. Coating glass substrates with a methacrylic acid–methyl methacrylate (MA) copolymer prior to the deposition significantly improved the adhesion of the nanomaterials to the substrate. With the MA coating, a substrate coverage of nearly 100% was achieved at 14-min spraying time for 0.05 wt% GNF and 0.1 wt% MLG suspensions. Raman spectra of the GNF and MLG films reveal that the films were made of MLG in which the individual graphene layers rotated from each other as in turbostratic graphene. This work provides a general approach to prepare graphene nanomaterial suspensions and to create films for a variety of applications. The spraying process applied in the current work is highly scalable and allows control of film characteristics through process parameters.</p><p>Copyright 2015 American Association for Aerosol Research</p></div

Amine Surface Modifications and Fluorescent Labeling of Thermally Stabilized Mesoporous Silicon Nanoparticles

Mesoporous silicon (PSi) has been shown to have extensive application opportunities in biomedicine, whereas it has frequently failed to produce complex systems based on PSi due to the lack of surface functional groups or the instability of the unmodified PSi surface. In the present study, PSi nanoparticles, stabilized by thermal oxidation or thermal carbonization, were successfully modified by grafting aminosilanes on the surface. The modifications were performed by covalently bonding 3-triethoxysilylpropylamine (APTES) or 3-(2-aminoethylamino) propyldimethoxymethylsilane (AEAPMS) on thermally oxidized PSi (TOPSi) and thermally carbonized PSi (TCPSi). These materials were systematically characterized with N₂ ad/desorption, TEM, contact angle, zeta potential, FT-IR, ²⁹Si CP/MAS NMR, and elemental analysis. To evaluate their application potentials, a fluorescent dye, fluorescein 5-isothiocyanate (FITC), was coupled on the surface of amine-modified nanoparticles. The effects of PSi matrix and surface amino groups on FITC coupling efficiency, fluorescent intensity, and the stability of fluorescence in simulated body fluid (SBF) were investigated. The nanoparticles modified with AEAPMS had higher FITC coupling efficiency than those modified with APTES. FITC-coupled TOPSi nanoparticles also possessed brighter fluorescence and better fluorescent stability in SBF. Furthermore, due to the protection caused by the mesoporous structure of PSi nanoparticles, the FITC-coupled TOPSi nanoparticles showed superior photostability in photobleaching experiment

Surface Chemistry, Reactivity, and Pore Structure of Porous Silicon Oxidized by Various Methods

Oxidation is the most commonly used method of passivating porous silicon (PSi) surfaces against unwanted reactions with guest molecules and temporal changes during storage or use. In the present study, several oxidation methods were compared in order to find optimal methods able to generate inert surfaces free of reactive hydrides but would cause minimal changes in the pore structure of PSi. The studied methods included thermal oxidations, liquid-phase oxidations, annealings, and their combinations. The surface-oxidized samples were studied by Fourier transform infrared spectroscopy, isothermal titration microcalorimetry, nitrogen sorption, ellipsometry, X-ray diffraction, electron paramagnetic resonance spectroscopy, and scanning electron microscopy imaging. Treatment at high temperature was found to have two advantages. First, it enables the generation of surfaces free of hydrides, which is not possible at low temperatures in a liquid or a gas phase. Second, it allows the silicon framework to partially accommodate a volume expansion because of oxidation, whereas at low temperature the volume expansion significantly consumes the free pore volume. The most promising methods were further optimized to minimize the negative effects on the pore structure. Simple thermal oxidation at 700 °C was found to be an effective oxidation method although it causes a large decrease in the pore volume. A novel combination of thermal oxidation, annealing, and liquid-phase oxidation was also effective and caused a smaller decrease in the pore volume with no significant change in the pore diameter but was more complicated to perform. Both methods produced surfaces that were not found to react with a model drug cinnarizine in isothermal titration microcalorimetry experiments. The study enables a reasonable choice of oxidation method for PSi applications

Endogenous Stable Radicals for Characterization of Thermally Carbonized Porous Silicon by Solid-State Dynamic Nuclear Polarization ¹³C NMR

As with all nanomaterials, characterization of the surface chemistry of mesoporous silicon (PSi) is crucial for the development in its diverse applications. Nuclear magnetic resonance (NMR) is one of the most powerful methods to study the chemistry of nanomaterials, but it is currently underutilized with PSi due to low signal-to-noise ratios achieved with this material which lead to very long measurement times. Here we show that endogenous radicals exist in thermally carbonized PSi and demonstrate the feasibility of solid-state dynamic nuclear polarization (DNP) NMR without addition of organic radicals. Use of DNP NMR is demonstrated to highly improve the signal-to-noise ratio while significantly reducing the measurement times. This technique opens new possibilities for the use of more advanced NMR techniques allowing the detailed characterization of complex materials such as PSi. Furthermore, the chemical structure of thermally carbonized PSi is studied by complementary techniques, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy