Materials Today Communications
Temporal evolution of wetting transitions of graphene oxide coated on roughened polyvinyl chloride surfaces
Graphical abstract
Introduction
Monolayer and few-layer oxygen-functionalized graphene, also called graphene oxide (GO), represent a well-known 2D form of carbon with exceptional mechanical, electronic, optical and thermal properties that has been exploited in a broad array of sensing, transporting, harvesting and actuating devices [[1], [2], [3], [4], [5]]. The technological importance of GO has been facilitated in part by the relative cost-effective scalability of its synthesis and functional deposition onto substrates compared with other carbon-based materials, making it well suited for large-scale commercial applications [[6], [7], [8]]. However, while the mechanical and electrical properties of GO have been well studied, elucidation of the wettability of GO and its apparent “transparency” and “opacity” upon deposition on a substrate remains a perplexing challenge [9,10]. It is well accepted that graphene wettability has a strong influence on its surface functionality, chemical and electrical properties. Hence, robust understanding of the wetting behavior of GO surfaces will have important implications for improving graphene-based device fabrication and operation.
The wettability of a material surface is strongly dependent on surface chemistry (composition) and physical topography (roughness). This property is generally defined by the intrinsic or apparent water contact angle (WCA). Molecular simulations have found that with oxygen-containing functional groups on the basal plane, graphene becomes hydrophilic and the water contact angle decreases with their concentration [9]. This forms the basis of using graphene wetting as a means to predict how GO wetting will develop. It has been shown that graphene films on Cu, Si and Au supports did not alter the intrinsic wetting behavior of the base substrate, suggesting wetting transparency of graphene [11]. However, only partial wetting transparency was observed for glass where wettability is dominated by short range interactions mediated by hydrogen bonding between water and glass. The same study also demonstrated that increasing the number of graphene layers on both Cu and glass substrates resulted in wetting transition towards hydrophobicity. Using a combination of theoretical analysis, molecular dynamics simulations and contact angle measurements, there have been assertions that monolayer graphene was not entirely wetting transparent when the substrate is either superhydrophobic or superhydrophilic [12]. In contrast, a later study reported the opacity of graphene wettability and that the underlying substrate exerts negligible effects [13]. Further confounding data revealed that wettability of graphene is not constant but can vary temporally under ambient air exposure. Adsorption of hydrocarbon contaminants and water from the ambient environment over time are responsible for the perceived hydrophobicity of graphene surfaces [[14], [15], [16]]. Epitaxial graphene on Cu and Ni supports displays substrate-dependent wetting that transitions from hydrophilicity towards increasing hydrophobicity as a result of long term physisorption of airborne hydrocarbon contaminants, saturating at WCA of 92−98° over a single year period [17].
The diverging conclusions drawn from the literature indicate that investigations of GO wettability should be done distinctly and separately from graphene wettability studies. Nonetheless the effects exerted by the underlying substrate is expected to be significant and warrants further clarification in both cases. While many studies have focused on substrates of different chemical composition, there is a dearth of information on the effects of surface roughness and the state of drying on GO wettability. In this work, the wetting transition characteristics of GO films deposited on roughened polyvinyl chloride (PVC) surfaces are investigated. A mechanistic explanation of the wetting behavior uncovered is revealed in comparison with that of unroughened PVC and glass substrates. Potential exploitation of the temporal evolution of the wetting transition of GO coated on rough surfaces for time-stamping is discussed.
Section snippets
Graphene oxide solution creation
One g of graphite powder (Sigma, 282,863) and 0.5 g of sodium nitrate (Sigma, S5022) were first mixed together in a beaker. Twenty-three mL of sulphuric acid (Sigma, 339,741) was added to the mixture with constant stirring. After 1 h, 3 g of potassium permanganate (Sigma, 223,468) was added gradually (to ensure that overheating did not occur) to the solution. The solution was then stirred for 12 h and then diluted with 500 mL of water followed by vigorous stirring. Five mL of 30 % hydrogen
Results and discussion
Fig. 1A and B show images of sessile water drops deposited on the unroughened and roughened PVC substrate. The former manifested WCA of 66°, indicating a hydrophilic characteristic (WCA < 90°). With the latter, the WCA was 102°, revealing a switch towards hydrophobicity (WCA > 90°) although not reaching the threshold for superhydrophobicity (WCA > 150°). In both cases, the WCA remained invariant with time. These results indicate that the shift towards increased hydrophobicity via surface
Conclusions
GO solution that was deposited on a PVC substrate that was roughened 7.8 times (in terms of its arithmetic average of the absolute values of the profile heights over the evaluation area) over its original unroughened state was found to exhibit time-dependent wetting change characteristics. The mechanistic model advanced to explain the wetting transition is based on the strong action of random micro-capillaries generated by the roughening process. These asperities, when filled with GO solution,
CRediT authorship contribution statement
Zhixiong Song: Conceptualization, Investigation, Methodology, Formal analysis, Writing - original draft. Eric Shen Lin: Investigation, Formal analysis, Writing - original draft. Md. Hemayet Uddin: Investigation, Formal analysis, Writing - original draft. Jian Wern Ong: Investigation, Formal analysis, Writing - original draft. Hassan Ali Abid: Investigation, Formal analysis, Writing - original draft. Zhiyuan Xiong: Validation, Writing - original draft. Dan Li: Validation. Oi Wah Liew:
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). The uniform surface roughening of PVC samples, carried out for us by Dextech Technologies, is acknowledged.
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