Zhiting Li

Effect of airborne contaminants on the wettability of supported graphene and graphite

It is generally accepted that supported graphene is hydrophobic and that its water contact angle is similar to that of graphite. Here, we show that the water contact angles of freshly prepared supported graphene and graphite surfaces increase when they are exposed to ambient air. By using infrared spectroscopy and X-ray photoelectron spectroscopy we demonstrate that airborne hydrocarbons adsorb on graphitic surfaces, and that a concurrent decrease in the water contact angle occurs when these contaminants are partially removed by both thermal annealing and controlled ultraviolet-O3 treatment. Our findings indicate that graphitic surfaces are more hydrophilic than previously believed, and suggest that previously reported data on the wettability of graphitic surfaces may have been affected by unintentional hydrocarbon contamination from ambient air.

Study on the surface energy of graphene by contact angle measurements.

Because of the atomic thinness of graphene, its integration into a device will always involve its interaction with at least one supporting substrate, making the surface energy of graphene critical to its real-life applications. In the current paper, the contact angle of graphene synthesized by chemical vapor deposition (CVD) was monitored temporally after synthesis using water, diiodomethane, ethylene glycol, and glycerol. The surface energy was then calculated based on the contact angle data by the Fowkes, Owens-Wendt (extended Fowkes), and Neumann models. The surface energy of fresh CVD graphene grown on a copper substrate (G/Cu) immediately after synthesis was determined to be 62.2 ± 3.1 mJ/m(2) (Fowkes), 53.0 ± 4.3 mJ/m(2) (Owens-Wendt) and 63.8 ± 2.0 mJ/m(2) (Neumann), which decreased to 45.6 ± 3.9, 37.5 ± 2.3, and 57.4 ± 2.1 mJ/m(2), respectively, after 24 h of air exposure. The ellipsometry characterization indicates that the surface energy of G/Cu is affected by airborne hydrocarbon contamination. G/Cu exhibits the highest surface energy immediately after synthesis, and the surface energy decreases after airborne contamination occurs. The root cause of intrinsically mild polarity of G/Cu surface is discussed.

Enhanced Room-Temperature Corrosion of Copper in the Presence of Graphene

This paper reports the enhancement of long-term oxidation of copper at room temperature by a graphene coating. Previous studies showed that graphene is an effective anticorrosion barrier against short-term thermal and electrochemical oxidation of metals. Here, we show that a graphene coating can, on the contrary, accelerate long-term oxidation of an underlying copper substrate in ambient atmosphere at room temperature. After 6 months of exposure in air, both Raman spectroscopy and energy-dispersive X-ray spectroscopy indicated that graphene-coated copper foil had a higher degree of oxidation than uncoated foil, although X-ray photoelectron spectroscopy showed that the surface concentration of Cu(2+) was higher for the uncoated sample. In addition, we observed that the oxidation of graphene-coated copper foil was not homogeneous and occurred within micrometer-sized domains. The corrosion enhancement effect of graphene was attributed to its ability to promote electrochemical corrosion of copper.

Photochemical oxidation of CVD-grown single layer graphene

CVD-grown single layer graphene undergoes rapid photochemical oxidation in the presence of ultraviolet light and oxygen. The oxidation results in a homogeneous decay of the graphitic material; no nanoscale line cracks or pits were observed with an atomic force microscope. The conductivity of the graphene film decreases with an increasing degree of oxidation. It is crucial to understand and enhance the photochemical stability of graphene for its long term use as a transparent conducting material.

Water Protects Graphitic Surface from Airborne Hydrocarbon Contamination

The intrinsic wettability of graphitic materials, such as graphene and graphite, can be readily obscured by airborne hydrocarbon within 5-20 min of ambient air exposure. We report a convenient method to effectively preserve a freshly prepared graphitic surface simply through a water treatment technique. This approach significantly inhibits the hydrocarbon adsorption rate by a factor of ca. 20×, thus maintaining the intrinsic wetting behavior for many hours upon air exposure. Follow-up characterization shows that a nanometer-thick ice-like water forms on the graphitic surface, which remains stabilized at room temperature for at least 2-3 h and thus significantly decreases the adsorption of airborne hydrocarbon on the graphitic surface. This method has potential implications in minimizing hydrocarbon contamination during manufacturing, characterization, processing, and storage of graphene/graphite-based devices. As an example, we show that a water-treated graphite electrode maintains a high level of electrochemical activity in air for up to 1 day.

Hierarchical Graphene/Metal Grid Structures for Stable, Flexible Transparent Conductors

We report an experimental study on the fabrication and characterization of hierarchical graphene/metal grid structures for transparent conductors. The hierarchical structure allows for uniform and local current conductivity due to the graphene and exhibits low sheet resistance because the microscale silver grid serves as a conductive backbone. Our samples demonstrate 94% diffusive transmission with a sheet resistance of 0.6 Ω/sq and a direct current to optical conductivity ratio σdc/σop of 8900. The sheet resistance of the hierarchical structure may be improved by over 3 orders of magnitude and with little decrease in transmission compared with graphene. Furthermore, the graphene protects the silver grid from thermal oxidation and better maintains the sheet resistance of the structure at elevated temperature. The graphene also strengthens the adhesion of the metal grid with the substrate such that the structure is more resilient under repeated bending.

Ultrafast Electron Transfer Kinetics of Graphene Grown by Chemical Vapor Deposition

High electrochemical reactivity is required for various energy and sensing applications of graphene grown by chemical vapor deposition (CVD). Herein, we report that heterogeneous electron transfer can be remarkably fast at CVD-grown graphene electrodes that are fabricated without using the conventional poly(methyl methacrylate) (PMMA) for graphene transfer from a growth substrate. We use nanogap voltammetry based on scanning electrochemical microscopy to obtain very high standard rate constants k(0) ≥25 cm s(-1) for ferrocenemethanol oxidation at polystyrene-supported graphene. The rate constants are at least 2-3 orders of magnitude higher than those at PMMA-transferred graphene, which demonstrates an anomalously weak dependence of electron-transfer rates on the potential. Slow kinetics at PMMA-transferred graphene is attributed to the presence of residual PMMA. This unprecedentedly high reactivity of PMMA-free CVD-grown graphene electrodes is fundamentally and practically important.

Origin of Asymmetry of Paired Nanogap Voltammograms Based on Scanning Electrochemical Microscopy: Contamination Not Adsorption

Formation of a nanometer-wide gap between tip and substrate electrodes by scanning electrochemical microscopy (SECM) enables voltammetric measurement of ultrafast electron-transfer kinetics. Herein, we demonstrate the advantage of SECM-based nanogap voltammetry to assess the cleanness of the substrate surface in solution by confirming that airborne contamination of highly oriented pyrolytic graphite (HOPG) causes the nonideal asymmetry of paired nanogap voltammograms of (ferrocenylmethyl)trimethylammonium (Fc(+)). We hypothesize that the amperometric response of a 1 μm-diameter Pt tip is less enhanced in the feedback mode, where more hydrophilic Fc(2+) is generated from Fc(+) at the tip and reduced voltammetrically at the HOPG surface covered with airborne hydrophobic contaminants. The tip current is more enhanced in the substrate generation/tip collection mode, where less charged Fc(+) is oxidized at the contaminated HOPG surface. In fact, symmetric pairs of nanogap voltammograms are obtained with the cleaner HOPG surface that is exfoliated in humidified air and covered with a nanometer-thick water adlayer to suppress airborne contamination. This result disproves a misconception that the asymmetry of paired nanogap voltammograms is due to electron exchange mediated by Fc(2+) adsorbed on the glass sheath of the tip. Moreover, weak Fc(+) adsorption on the HOPG surface causes only the small hysteresis of each voltammogram upon forward and reverse sweeps of the HOPG potential. Significantly, no Fc(2+) adsorption on the HOPG surface ensures that the simple outer-sphere pathway mediates ultrafast electron transfer of the Fc(2+/+) couple with standard rate constants of ≥12 cm/s as estimated from symmetric pairs of reversible nanogap voltammograms.

Raman Enhancement and Photo-Bleaching of Organic Dyes in the Presence of Chemical Vapor Deposition-Grown Graphene

Fluorescent organic dyes photobleach under intense light. Graphene has been shown to improve the photo-stability of organic dyes. In this paper, we investigated the Raman spectroscopy and photo-bleaching kinetics of dyes in the absence/presence of chemical vapor deposition (CVD)-grown graphene. We show that graphene enhances the Raman signal of a wide range of dyes. The photo-bleaching of the dyes was reduced when the dyes were in contact with graphene. In contrast, monolayer hexagonal boron nitride (h-BN) was much less effective in reducing the photo-bleaching rate of the dyes. We attribute the suppression of photo-bleaching to the energy or electron transfer from dye to graphene. The results highlight the potential of CVD graphene as a substrate for protecting and enhancing Raman response of organic dyes.

Substrate dependent photochemical oxidation of monolayer graphene

In this report we show that the photochemical oxidation of monolayer graphene is strongly dependent on its underlying substrate. It was found that chemical vapor deposition-grown monolayer graphene transferred onto a silicon wafer is more easily oxidized compared to graphene that was left on the copper substrate or transferred onto a H2-annealed copper foil. The differences in the degree of oxidation were tentatively attributed to the varying energies of adhesion between the graphene and the underlying substrate. Our result has significant implications for the outdoor use of graphene as a transparent conducting material.