Andrew Kozbial

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.

Understanding the Intrinsic Water Wettability of Molybdenum Disulfide (MoS2)

2D semiconductors allow for unique and ultrasensitive devices to be fabricated for applications ranging from clinical diagnosis instruments to low-energy light-emitting diodes (LEDs). Graphene has championed research in this field since it was first fabricated; however, its zero bandgap creates many challenges. Transition metal dichalcogenides (TMDCs), e.g., MoS2, have a direct bandgap which alleviates the challenge of creating a bandgap in graphene-based devices. Water wettability of MoS2 is critical to device fabrication/performance and MoS2 has been believed to be hydrophobic. Herein, we report that water contact angle (WCA) of freshly exfoliated MoS2 shows temporal evolution with an intrinsic WCA of 69.0 ± 3.8° that increases to 89.0 ± 3.1° after 1 day exposure to ambient air. ATR-FTIR and ellipsometry show that the fresh, intrinsically mildly hydrophilic MoS2 surface adsorbs hydrocarbons from ambient air and thus becomes hydrophobic.

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.

Characterization of the Intrinsic Water Wettability of Graphite Using Contact Angle Measurements: Effect of Defects on Static and Dynamic Contact Angles

Elucidating the intrinsic water wettability of the graphitic surface has increasingly attracted research interests, triggered by the recent finding that the well-established hydrophobicity of graphitic surfaces actually results from airborne hydrocarbon contamination. Currently, static water contact angle (WCA) is often used to characterize the intrinsic water wettability of graphitic surfaces. In the current paper, we show that because of the existence of defects, static WCA does not necessarily characterize the intrinsic water wettability. Freshly exfoliated graphite of varying qualities, characterized using atomic force microscopy and Raman spectroscopy, was studied using static, advancing, and receding WCA measurements. The results showed that graphite of different qualities (i.e., defect density) always has a similar advancing WCA, but it could have very different static and receding WCAs. This finding indicates that defects play an important role in contact angle measurements, and the static contact angle does not always represent the intrinsic water wettability of pristine graphite. On the basis of the experimental results, a qualitative model is proposed to explain the effect of defects on static, advancing, and receding contact angles. The model suggests that the advancing WCA reflects the intrinsic water wettability of pristine (defect-free) graphite. Our results showed that the advancing WCA for pristine graphite is 68.6°, which indicates that graphitic carbon is intrinsically mildly hydrophilic.

Manipulating the molecular conformation of a nanometer-thick environmentally friendly coating to control the surface energy

Long-chain fluorocarbons are the state-of-the-art materials as nanometer-thick coatings in growing nanotechnology industries. The low surface energy, resulting from the molecular nature of C–F bonds, is the key feature of perfluoro-materials that no other material can provide. However, research in the past decades has shown that long-chain fluorocarbons pose serious toxicological and environmental concerns because their degradation products, which have at least six fluorocarbons, are bioaccumulative, toxic and have high global warming potential. One possible solution for the dilemma is to develop hydrocarbons with short fluorocarbon side chains (HC-SFSCs), which have been demonstrated to be much more environmentally friendly. However, there have been few experimental studies on nanometer-thick HC-SFSCs on a solid substrate. Moreover, it is very challenging for HC-SFSCs to provide surface energy as low as fluorocarbons do. In the current paper, a nanometer-thick HC-SFSC has been deposited on a silica substrate by dip-coating. Ellipsometry and X-ray photoelectron spectroscopy (XPS) results indicated that the as-coated HC-SFSC molecules take a conformation between “random coil” and “flat chain”. Contact angle testing results showed that the as-coated HC-SFSC/silica has a relatively high surface energy and, interestingly, simple thermal annealing reduces the surface energy to a value close to that of Teflon. XPS studies suggested that the thermodynamic equilibrium conformation of HC-SFSCs on a solid substrate is actually in favor of low surface energy and the low mobility of HC-SFSCs induced by the solid confinement traps the molecule in a non-equilibrium conformation with higher surface energy. The finding here potentially provides a viable approach to make the surface energy of HC-SFSCs as low as those of the conventional perfluoro-materials.