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Title:Wettability of graphitic materials and their application in separation
Author(s):Ashraf, Ali
Director of Research:Nam, SungWoo
Doctoral Committee Chair(s):Nam, SungWoo
Doctoral Committee Member(s):Aluru, Narayana R; Jacobi, Anthony M; Dastgheib, Seyed A
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Graphitic carbon
Membrane distillation
Abstract:Graphitic carbon-water interaction or wettability of graphitic materials has received significant interest due to extraordinary thermal, physical, and electrical properties of different forms of graphitic carbon and influence of wettability in condensation, adhesion, separation, tribology, and adsorption. This important surface property is affected by change in topography, chemical heterogeneity, and surface charge. Graphene, which is a single atom thick sheet of graphitic carbon and building block of graphite, is an ideal model material to study this interaction. However, due to defect formation and contaminant adsorption from ambient atmosphere during the fabrication and use, wettability investigation using conventional means has been insufficient to investigate intrinsic wettability of graphene. Therefore, nanoscopic, microscopic and spectroscopic investigations under controlled conditions were performed to elucidate the relationships between surface defects, functionality, doping and wettability of graphene. First study on wettability focuses on the influence of surface defects and functional groups of multilayer graphene, used as a model for graphene surface. The investigation reveals that airborne hydrophobic contaminants, adsorbed on graphene surface in the form of functional groups, can influence the wettability of graphene and graphite significantly (as high as 50˚ in water contact angle (WCA) change), and were found to be stable even under harsh conditions (i.e., at 1000 °C temperature, and in hydrofluoric acid). A WCA value of 45 ± 3° was measured for a clean highly oriented pyrolytic graphite (HOPG) surface, which can serve as the intrinsic WCA for multilayer graphene. In addition, WCA for multilayer graphene changed from ~5° to ~80° based on the surface functional groups and defects remaining and subsequent surface treatment/cleaning methods (i.e., thermal, physical, and chemical treatment). To demonstrate the stable hydrophobic characteristics of single layer graphene, hydrophobic graphene-hydrophilic SiO2 patterns were fabricated by a graphene micro-patterning technique developed in our laboratory. These micron scale patterns show dropwise condensation on graphene and film-wise condensation of SiO2. In the second study, the doping-induced WCA tunability of graphene was investigated. As high as 13˚ modulation in WCA was observed for 300 meV chemical doping. For both n- and p-type doping using subsurface polyelectrolytes, graphene exhibits more hydrophilicity. Furthermore, adhesion force measurements using a hydrophobic self-assembled monolayer coated atomic force microscopy probe shows enhanced attraction towards undoped graphene, consistent with the wettability modulation. Such doping-induced wettability modulation is also achieved via subsurface metal doping. These measurements suggest for the first time that modulation of charge carrier density in graphene influences its wettability on the micro- and meso-scale. This tunable wettability due to doping opens the possibility of using graphene as a coating layer with tunable surface adhesion towards target chemicals/molecules. Inspired by the fundamental studies on wettability of graphitic materials, their potential as separation membranes was evaluated for membrane distillation (MD) and a transformative water-energy cogeneration technology operating at supercritical conditions of water. Based on the finding that hydrophobic contaminant layer is stable, different graphitic carbon membranes (i.e., graphite, carbon nanotube (CNT), pyrolytic carbon, and graphene) were fabricated and evaluated for their potential to work as a separation device for both room and high temperature MD application. In particular, a new class of robust CNT membranes is developed using a scalable chemical vapor deposition method by direct growth of the CNT on a nickel alloy (Hastelloy) mesh with micrometer-sized openings. The developed membrane is superhydrophobic, corrosion and oxidation resistant (up to 500 °C), and shows similar desalination performance as a commercial Teflon MD membrane. These robust carbon membranes are reusable and expected to be less susceptible to fouling because of their superhydrophobic properties. Furthermore, if fouled, they can be regenerated by heating in air or using an acid wash. This dissertation builds a framework to find a correlation between WCA of graphene with surface functional groups, doping level, and defects that can help design advanced graphitic carbon-based devices in the future. Moreover, development of graphitic carbon membranes suitable for high temperature applications opens the door of new possibilities for MD-based water desalination technology.
Issue Date:2016-03-09
Rights Information:Copyright 2016 Ali Ashraf
Date Available in IDEALS:2016-07-07
Date Deposited:2016-05

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