Applied Surface Science 327 (2015) 122–130 Contents lists available at ScienceDirect Applied Surface Science journal h om epa ge: www.elsevier.com/locate/apsusc Tuning the surface wettability of carbon nanotube carpets in multiscale hierarchical solids Anil K. Karumuri, Lvmeng He, Sharmila M. Mukhopadhyay ∗ Center for Nanoscale Multifunctional Materials, Department of Mechanical and Materials Science Engineering, Wright State University, Dayton, OH 45435, USA a r t i c l e i n f o Article history: Received 26 August 2014 Received in revised form 24 October 2014 Accepted 25 October 2014 Available online 24 November 2014 Keywords: Surface wettability Hierarchical hybrid porous materials Carbon nanotubes Plasma treatment Silica coating a b s t r a c t An attractive approach of increasing functionality of solid surfaces is to create hierarchical multiscale morphology by attaching tailored carpet-like arrays of Carbon nanotubes (CNT) on them. Such surfaces offer fractal morphology along with unprecedented increase in specific surface areas, and significantly boost the potency of porous materials used in surface-active applications. However, full utilization of these structures will require intimate interaction between the solid surface and its environmental fluid. CNT arrays tend to be hydrophobic, which limit their effectiveness in aqueous environments. In this research, we investigated two different surface modifications methods to induce hydrophilic property to CNT nano-carpets on graphitic substrates: dry oxygen plasma treatment and wet sol–gel oxide coating. Structure, morphology, composition and chemistry of these multiscale surfaces have been related to wettability and water flow properties. Plasma oxygen treatments did not alter the surface morphology, but induced temporary wettability, that could be reversed by heat treatment. On the other hand, sol–gel treatment permanently coated the nanotubes with a strongly bonded layer of amorphous SiO 2 . This coating imparts permanent alterations in surface chemistry, contact angle, wettability and water flow. Porous carbon foams were coated with CNT arrays and their water permeability measured before and after sol–gel silica coating. The hydrophilic coating was seen to increase flow rate and reduce pressure build-up. These results have important implications on all devices that utilize surface activity of porous solids, such as catalytic membranes, antimicrobial filters, and microfluidic sensors. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The rise in demand for catalysts, sensors, and environmental clean-up materials has fuelled a growing need for ultra-high surface area solids capable of increased interaction with the surrounding medium. One approach of addressing this is to synthesize mul- tiscale hierarchical materials comprising of high porosity solids whose surfaces are further enhanced with strongly attached car- bon nanotube (CNT) arrays [1]. These materials can offer many of the inherent nano-scale advantages while minimizing the envi- ronmental risks by anchoring nanomaterials on robust, easy to handle solids. Earlier studies [2] have shown that the specific sur- face area (SSA) of these types of CNT-modified structures can easily exceed that of the starting porous solid by several orders of magni- tude. These multiscale morphologies have been successfully used in many applications such as reinforcement for structural [3] and ∗ Corresponding author. Tel.: +1 937 775 5092 E-mail address: smukhopa@wright.edu (S.M. Mukhopadhyay). thermal management composites [4], liquid purifications devices [5], tissue scaffolding [6] and sensing [7]. In addition, these mate- rials offer excellent support for functional nanomaterials such as sensors [8], catalysts [9] and disinfection devices [5]. However, in addition to surface morphology and SSA, the addi- tional property that will be necessary for successful adaptation of these materials in future applications are their surface chem- istry and wettability. This will determine whether the nanotube enhanced surfaces can fully interact with the surrounding medium (air, water, biofluids, etc.) and utilize the increased surface area. One limitation of CNT arrays is that they tend to be hydropho- bic, which may prevent adequate infiltration of polar fluids such as water. Pure, graphite itself is non-polar and hydrophobic. Accord- ing to wetting models [10] for rough surfaces, water contact angle (CA) of inherently hydrophobic surfaces are expected to increase by a “roughness factor” in corrugated surfaces. CNT arrays, not sur- prisingly, will magnify such effects, resulting in super hydrophobic behavior (contact angle ≥160 ◦ ). This research is focused on investi- gating possible approaches that can improve the surface wettability of CNT grafted surfaces, while retaining their morphology. http://dx.doi.org/10.1016/j.apsusc.2014.10.154 0169-4332/© 2014 Elsevier B.V. All rights reserved.