Patterning of highly oriented pyrolytic graphite and glassy carbon surfaces by nanolithography and oxygen plasma etching

We demonstrate a process for the fabrication of nanostructures on two types of carbon surfaces; glassy carbon (GC) and the basal surface of highly oriented pyrolytic graphite (HOPG). Using hole-mask colloidal lithography, etch masks with three different feature diameters were prepared on each of the two surface types. Oxygen reactive ion etching of different durations was then used to transfer the mask pattern onto the surfaces, yielding nanopillars with diameters ranging from ∼40 to 470 nm and heights between ∼30 and 430 nm. The structures were characterized using atomic force microscopy, scanning electron microscopy and optical spectrophotometry. Identical preparation schemes applied to the two materials yield nanostructures with remarkably different geometrical properties. In general GC structures are higher and narrower than HOPG structures prepared at the same plasma conditions. From the nanostructure dimensions and the corresponding etch times we have estimated etch rates in the forward and lateral directions to 0.19 and 0.015 nm/s for HOPG and 0.65 and 0.15 nm/s for GC. The different rates are attributed to different (an)isotropic etching behavior of the two materials in oxygen plasma. In addition, optical characterization reveals interesting changes in the surface reflectance as a result of the nanostructuring.     

Protein Adsorption on Hydroxyapatite Nanosensors with Different Crystal Sizes Studied In Situ by a Quartz Crystal Microbalance with the Dissipation Method

Hydroxyapatite (HAp) nanocrystals with different crystal sizes were deposited by the electrophoretic deposition method on the gold surface of a quartz crystal microbalance with a dissipation probe. The nanosensors formed this way were used to elucidate the adsorption mechanism of proteins with a similar pI value. The crystal sizes and the area of the a -plane affected only the adsorption amount of human serum albumin, but not that of bovine plasma fibrinogen. The viscoelastic property, Δ D/ Δ f , of each absorbed layer on the nanosensors was almost constant. The protein adsorption mechanism can be explained as follows: the dissociated carboxyl groups (negative charge) of albumin were interacted with calcium ions and the hydrated amine groups (positive charge) at the αC domain of fibrinogen were with phosphate ions on the HAp surface.     

Photophysics and photochemistry of ice films on graphite

We consider model systems consisting of submonolayer- to several monolayers-thick ice films grown at ultrahigh vacuum (UHV) and low (from ∼15 K) temperatures on an atomically clean graphite (0001) surface. They are characterized experimentally with temperature-programmed desorption-isothermal desorption spectroscopy (TPD-ITD) and high-resolution electron energy loss spectroscopy (HREELS). We provide examples of how ice structure and chemical properties are altered by incoming photons. The phenomena of “photon annealing” of pure amorphous films and their structure-selective laser ablation are discussed. In the case of water ice doped with alkali metals and coadsorbed with NO, we report on the observations of photoreactions taking place on the graphite/ice interface. Depending on the composition of the coadsorbed ice layer, irradiation leads to formation of H₂, CH₄, CO, NH₃, and CO₂. More specifically, we address the mechanisms of water photo-dissociation at surfaces and interfaces with emphasis on the role of (i) perturbation of water molecules by coadsorbates and/or substrate; (ii) the light-initiated electron (charge) transfer reactions leading to water splitting; and (iii) confinement of the precursor system and reaction products. In all examples the attention is on the operating mechanism for the transformations. The role of photo-excited charge carriers in the graphite substrate is underlined. Increased understanding of photo-stimulated reactions and processes involving water is needed to allow these processes to be effectively controlled and fully utilized.     

Graphene oxide and lipid membranes: interactions and nanocomposite structures

We have investigated the interaction between graphene oxide and lipid membranes, using both supported lipid membranes and supported liposomes. Also, the reverse situation, where a surface coated with graphene oxide was exposed to liposomes in solution, was studied. We discovered graphene oxide-induced rupture of preadsorbed liposomes and the formation of a nanocomposite, bio-nonbio multilayer structure, consisting of alternating graphene oxide monolayers and lipid membranes. The assembly process was monitored in real time by two complementary surface analytical techniques (the quartz crystal microbalance with dissipation monitoring technique (QCM-D) and dual polarization interferometry (DPI)), and the formed structures were imaged with atomic force microscopy (AFM). From a basic science point of view, the results point toward the importance of electrostatic interactions between graphene oxide and lipid headgroups. Implications from a more practical point of view concern structure-activity relationship for biological health/safety aspects of graphene oxide and the potential of the nanocomposite, multilayer structure as scaffolds for advanced biomolecular functions and sensing applications.     

Electrodeposition of selenium from 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate

Electrodeposition of Se films from air and water stable 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate ionic liquid was studied in open air conditions. The electrochemical behavior of H₂SeO₃ on gold and copper substrates was investigated in ionic liquid–water mixtures, and the influence of the deposition parameters such as time and bath temperature on the crystallinity of the film was examined. For the gold substrate, a reddish deposit containing amorphous, hexagonal and rhombohedral Se phases was identified at room temperature, while a grayish film of hexagonal and rhombohedral phases was formed at higher temperatures. The reddish Se film was found to exhibit a smoother surface with lower reflectivity in comparison to the grayish one. The band gap of the reddish film was close to that of pure amorphous red Se reported in literature, while the band gap of the grayish film was close to the pure hexagonal film. Photoelectron spectroscopy and energy dispersive X-ray spectroscopy show that both films consist of pure Se with only slight surface contaminations by remnants from the electrodeposition. In the case of a copper substrate, the electrodeposition of Se was accompanied by the formation of copper–selenide due to the reactivity of copper in H₂SeO₃.     

Photo-Assisted Processes for Improved Catalytic Activity and Selectivity of Environmentally Harmful Emissions

Photoenhanced activity of several supported noble metal/metal oxide combinations has been explored. Significant activity is observed for supported Pt and Pd samples. Specifically, the so called light-off temperatures for CO oxidation are found to decrease upon illumination. Evidence for a photocatalytic non-thermal mechanism rather than a photo-assisted thermal process is found through the wavelength dependence for CO oxidation on a Pt/alumina catalyst. The difficulty of separating real photocatalytic from other photo-assisted effects in the case of porous samples is discussed.